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6407337ccc600523a3cd2921 | 7 | To the best of our knowledge, studies of growth mechanisms for similar systems tend to be carried out using kinetic Monte Carlo (kMC) or mean field approximation methods, with some studies choosing to use classical molecular dynamics. However, as the growth mechanism is governed by the competition between the adatom-substrate and the adatom-adatom interaction, we can use these in combination with the strength of adhesion and selected ab initio molecular dynamics (aiMD) simulations, which account for temperature effects, to predict the likely growth mechanism and avoid expensive activation energy calculations. |
6407337ccc600523a3cd2921 | 8 | For this paper we built on our previous approach to extract the maximum amount of information on the interaction between Cu and the substrate, while keeping the computational cost manageable. Figure ) gives an overview of the different models used and the information so obtained. Our aim is to use this approach for TaN surfaces modified with different metal dopants with the aim of studying how the interaction between Cu and modified TaN is affected by different dopants to identify new barrier/liner materials for advanced interconnects. |
6407337ccc600523a3cd2921 | 9 | Additionally, this allows us to evaluate the limitations of our approach for predicting the thin film morphology. For the purposes of this study we have selected W and Co as the dopants. These are more sustainable alternatives to currently used liner materials like Ru. Co was of particular interest to us, as it is known as a suitable liner material for Cu and its incorporation into TaN gives a likely candidate for a single, combined barrier + liner film. Co and W also allow us to further investigate the effects of the difference in ionic radius between Ta and the dopant, because Co has the smallest ionic radius of 0.55 Å (high-spin) or 0.61 Å (low-spin) and W, with an ionic radius of 0.66 Å, is very similar in size to Ta, with an ionic radius of 0.72 Å. Co is magnetic, therefore we tested the effects of high-spin vs low-spin on geometry and adsorption energy; this is discussed in the Supporting Information, Section S2. To the best of our knowledge, while neither W nor W x N y have been studied as a liner material, W x N y is a known barrier material. Finally, atomic layer deposition (ALD) process chemistries are known for Co, W and their respective nitrides, permitting their introduction into TaN through ALD. |
6407337ccc600523a3cd2921 | 10 | Similarly to the approach described in our previous work, we have doped the top layer of a ε-TaN(1 1 0) slab with W and Co. To do this, 25%, 50% or 100% of the Ta atoms in the top layer were replaced with the dopant. The surfaces will be labelled as W dopant-concentration or Co dopant-concentration , giving for example W 25 for 25% W doping or Co 100 for 100% Co doping. There are two coordination environments for metal atoms in the TaN (1 1 0) surface. The S-site forms a six coordinate structure with N, while the F-site is three coordinate with N. In our previous work, we found that Ru doping is more favourable at the F-site than the S-site, and that the number of dopants at each site affects the stability of the surface. This is also the case for Co and W doping. For 50% doping, we chose two different doping distributions , namely the most and least favourable 50% dopant distributions. |
6407337ccc600523a3cd2921 | 11 | These will be distinguished as W 50-S /W 50-F and Co 50-S /Co 50-F , as they contain the maximum number of S-site and F-site dopants, respectively. The resulting doped TaN surfaces are shown in Figure for W doping and in Figure for Co doping. The selected sites for single Cu adsorption are also highlighted. |
6407337ccc600523a3cd2921 | 12 | Four more potential combined barrier/liner materials, in addition to those Ru-doped surfaces identified in our previous work, were identified using the above approach. These are 25% and 50% Co-doped TaN and 25% and 100% W-doped TaN, although the latter could also be considered as a layer of WN passivation on TaN. This shows clearly that it is possible to tailor the properties of a material through doping in the surface, which in the case of a barrier material like TaN would reduce the overall volume occupied by the barrier/liner without compromising the barrier properties. The selection of potential combined barrier/liner materials identified through our studies is based on materials that are already used in current manufacturing processes and should therefore be simple to include into a process recipe. Some material properties, such as resistivity and reliability, still need to be tested and optimised experimentally. This study also allowed us to determine the limitations of the different Cu models used and the information they can provide about the thin film morphology. Combined with insights on the effect of dopant ionic radius on the thermal stability of the material, we present a more computationally efficient approach to predicting the thin film morphology compared to our work in reference , which is useful for understanding growth morphology at material interfaces. |
6407337ccc600523a3cd2921 | 13 | Here, E total is the total energy of the doped, relaxed surface, E clean is the total energy of the bare TaN (1 1 0) surface, n is the number of dopants in the system, and E dopant is the energy of the bulk dopant in vacuum per atom. Similarly, E T a is the energy of bulk Ta in vacuum per atom. |
6407337ccc600523a3cd2921 | 14 | To evaluate the competition between Cu-surface and Cu-Cu interactions which play a large role in controlling thin film morphology, we calculate the Cu-Cu interaction energy. For this we first calculate E Cu-substrate , which is the binding energy calculated with reference to a gas phase Cu n cluster instead of a single Cu atom: |
6407337ccc600523a3cd2921 | 15 | where E surf * is the single point energy of the surface after it relaxes due to Cu adsorption and E Cu cluster is the single point energy of the Cu n in the structure it takes after adsorption on the surface. By isolating the Cu-substrate interaction we can now calculate the metal interaction energy: |
6407337ccc600523a3cd2921 | 16 | Smearing with σ = 0.1 eV, and an energy cut-off of 400 eV. The convergence criteria were 1x10 -4 eV for the energy minimisation and 0.02 eV/ Å for the forces in the ionic relaxations. We also ran ab initio molecular dynamics (aiMD) calculations for Cu 4 and Cu 29 adsorbed on doped TaN in the NVT ensemble (constant number of particles, N, constant volume, V, and constant temperature, T) with the following inputs: the kinetic energy cutoff is 250 eV; temperature is 500 K. A time step of 5 fs and a total of 1000 steps were used to achieve a total simulation time of 5 ps. The starting geometries used were the optimised structures from the static relaxations, however the atoms in the bottom two layers of the surface slab were frozen for all aiMD calculations. |
6407337ccc600523a3cd2921 | 17 | Due to the smaller ionic radius of Co (0.55 or 0.61 Å, depending on high or low spin) compared to Ta (0.72 Å), extremely strong surface rearrangements were observed on the Co 50-S , Co 50-F and Co 100 surfaces. For this reason, the adsorption energy and the Cu-Cu interaction energy for these surfaces were computed by using E surf , the single point energy of the rearranged surface after removing Cu instead of E bare , the total energy of the relaxed surface before metal adsorption, as a reference. This is the same energy used to compute E Cu-substrate , which has the consequence that the Cu-Cu interaction energies are based solely on the difference in the Cu reference energies. |
6407337ccc600523a3cd2921 | 18 | The relaxed W-and Co-doped TaN surfaces and their unique adsorption sites are shown in Figures and. The most favourable adsorption site for each surface was chosen for the adsorption of Cu 2 and Cu 4 species. The binding energies for each single Cu atom adsorption are detailed in the Supporting Information, Table (W-doped TaN) |
6407337ccc600523a3cd2921 | 19 | and Table (Co-doped TaN). All Co-doped surfaces rearrange to some degree upon Cu adsorption, creating gaps on the surface that trap the Cu atom, depending on the adsorption site. This is due to the difference in ionic radius between Co (0.55 or 0.61 Å, depending on spin) and Ta (0.72 Å). While for TaN and all W-and Ru-doped surfaces the 5 coordinate site A is most favourable, only Co 50-S shares this most favourable adsorption site. On |
6407337ccc600523a3cd2921 | 20 | Co atom) and two N atoms. This site is likely more favourable than site A as the difference in ionic radius creates a gap between the Co and Ta atom that Cu can partially occupy, as shown in Figure and. On Co 50-S , there are two Co atoms at site B and a surface rearrangement occurs. This rearrangement leaves a gap for Cu to occupy (Figure ). The result is a strong adsorption energy of -3.82 eV/atom. However site A on this surface forms a larger gap that allows Cu to fully incorporate into the surface layer leading to a particularly large adsorption energy of -4.61 eV/atom, as shown in Figure . |
6407337ccc600523a3cd2921 | 21 | The Co 100 surface rearranges to the point where it becomes difficult to describe an adsorption site after relaxation using our labelling. This is similar to Ru 100 in ref . This is most likely caused by the difference in the ionic radius of Co and Ru compared to Ta. Figure compares the Ru, Co and W 100% doped surfaces before and after Cu adsorption. Here, the effect of the difference in ionic radius becomes clear. From largest to smallest the ionic radius of the relevant atoms are: Ta ≳ W > Ru > Co. No rearrangements are observed for W 100 due to the similar ionic radius of W and Ta. A clear rearrangement is observed for Ru 100 , while Co 100 shows the most significant rearrangement due to the large difference in ionic radius. Additionally, without the presence of Ta atoms this "CoN-like" layer begins to distort significantly as illustrated in Figure . Here, we observe extremely large gaps that expose the underlying TaN slab. Additionally, several Co and N atoms migrate away from TaN and adsorb atop the remaining Co and N atoms to form a second layer of CoN (see Figure ). These rearrangements are most likely caused by the lattice mismatch and different unit cells for CoN (cubic) and TaN (hexagonal). More details on this are included in Section S6 of the Supporting Information. |
6407337ccc600523a3cd2921 | 22 | For the study of small Cu n clusters, we have selected five typical Cu 2 and Cu 4 geometries. All geometries are initially adsorbed at the most favourable single atom adsorption site for each surface. There are two Cu 2 models, "close" and "apart" where two Cu atoms are adsorbed either at neighbouring sites or separated by two unoccupied sites. This is a very basic model to determine an initial preference towards association or separation. For Cu 4 , we chose three different models. These are (i) a line along equivalent adsorption sites, (ii) a flat rhombus configuration or (iii) a 3D tetrahedron. This lets us determine any initial preference towards 2D or 3D structures and shows how Cu-Cu interactions compare to Cu-surface interactions (see Equations 3 and 4). Analysis of the competition between these two interactions allows us to select the surfaces most likely to promote wetting of Cu for further study. |
6407337ccc600523a3cd2921 | 23 | The competition between Cu-substrate and Cu-Cu interaction for the four Co-doped TaN surfaces shows that the Cu-substrate interaction is stronger than the Cu-Cu interaction for all adsorption geometries. This indicates that Cu should wet on all four surfaces, since wetting is promoted when the adatom-substrate interaction is stronger than the adatom-adatom interaction. However, the rearrangements of the Co-doped surfaces could impede or complicate the growth process. The most significant differences between the two energy contributions occurs for Co 25 and Co 100 , while the magnitude of the interactions is somewhat more competitive for Co 50-S and Co of the Supporting Information). |
6407337ccc600523a3cd2921 | 24 | Figure shows that the binding energies of Cu on W-doped TaN are weaker than those on Co-doped TaN. We also observe stronger Cu-Cu interactions compared to Co-doped TaN. The Cu-Cu interaction energies are strongest for the rhombus structures on all but the W 25 surface. This is most likely due to the difference in the shape of the rhombus on W 25 which upon relaxation, resembles the line configuration and thus has fewer interactions between the Cu atoms. The tetrahedral configuration has a favourable adsorption energy on all of the surfaces. On W-doped surfaces, the Cu-substrate and Cu-Cu interaction energies are competitive, while on Co-doped surfaces the Cu-substrate interaction is stronger than the Cu-Cu interaction. Although the tetrahedron is 3D, as the film grows, further tetrahedrons could join into a 2D film, based on this relationship between Cu-substrate and Cu-Cu interaction energies. On the W 50-F and W 100 surfaces where the 2D rhombus is most favourable, a 2D film could form based on this preference, as the Cu-substrate interactions remain favourable despite the strong Cu-Cu interaction energy. Additionally, the W 25 surface may promote wetting, as this is the only surface where all geometries have a Cu-substrate interaction that is stronger than the Cu-Cu interaction. Based entirely on the competition between Cu-substrate and Cu-Cu interaction Cu should wet on this surface. However, the overall binding energy of Cu on this surface is weaker than that computed for Cu on TaN (see reference ). Given that Cu does not wet on TaN, but has a stronger adhesion compared to W 25 , it is not possible to make a conclusive prediction with these models based on the computed adsorption energies and structures alone. In contrast, the Cusubstrate interaction is more favourable than the Cu-Cu interaction energy for all structures on Co-doped surfaces. |
6407337ccc600523a3cd2921 | 25 | Both Co 50-S and Co 100 are thermally unstable in this stoichiometry during the aiMD simulation. Co 50-S loses N 2 after 0.5 ps at 500 K, while Co 100 loses five N atoms after 0.5 ps at 500 K. Geometries at the relevant snapshot are shown in the Supporting Information, Figure . |
6407337ccc600523a3cd2921 | 26 | CoN is the cobalt nitride stoichiometry with the most favourable formation energy, closely followed by Co 2 N. Given that CoN is the most favourable stoichiometry, the distortion of the Co 100 surface and its thermal instability likely do not arise from the particular stoichiometry. However, bulk CoN has a cubic unit cell and bulk show the same instabilities as Co 100 . Additional analysis on the stable metal nitrides of Ta, Ru, Co and W and the effect this has on the 100% doped surfaces is detailed in Section S6 of the Supporting Information. Table in the Supporting Information shows formation energies, types of unit cell and lattice parameters for selected Ta, Co, Ru and W nitrides. |
6407337ccc600523a3cd2921 | 27 | Co 25 and Co 50-F are both thermally stable and the geometry of the initial rhombus structure of Cu 4 before and after the aiMD run are shown in Figure . The results from the aiMD simulations support our initial hypothesis that these surfaces should promote wetting of Cu. On Co 25 , one of the Cu atoms separates from the cluster, as shown in Figure . This is accompanied by some surface distortions, which cause a Ta atom be pushed partially out of the surface layer. The effect such distortions would have on surface roughness and stability require a much larger model and are outside the scope of this study. The other three atoms remain at their original positions from the relaxed starting geometry, however a single atom migrating away from the cluster supports our idea that there is a preference for fewer Cu-Cu interactions on this surface. Similarly, on Co 50-F , the rhombus rearranges into a line configuration along a row of Co dopants, as shown in Figure . This is likely caused by the strong interactions between Co and Cu, allowing the Co dopant to act as a nucleation site in the surface, similarly to Ru dopants as On W 25 the less favourable rhombus with three atoms in a line rearranges into a square after 5 ps at 500 K. Each atom is adsorbed at a site A, thus increasing the number of Cu-Cu interactions compared to the 0 K geometry (see Figure ). The Cu 4 structure on W 50-S remains unchanged after 5 ps at 500 K (see Figure ). On W 50-F , it appears that the atoms migrate across the surface toward the nearest A sites and begin forming into the same square structure observed for W 25 (see Figure ). On W 100 , the rhombus rearranges into a tetrahedron, even though this geometry was not as favourable during the standard geometry relaxations (see Figure ). The tetrahedron was most favourable on W 25 , however the fact that the rhombus structure did not rearrange into a 3D structure at 500 K, as well as the favourable Cu-substrate interaction, indicates that 25% W could be enough to prevent Cu island formation, while 100% W is potentially too high of a doping concentration, in that it will be similar to TaN. |
6407337ccc600523a3cd2921 | 28 | In our previous work , we used a 29 atom model of Cu in order to gain better insight into the behaviour of a Cu model with larger number of atoms at a different stage of the film growth process. We use this model on the most promising Co-and W-doped TaN surfaces from Sections 3.2 and 3.3 and assess our predictions of 2D vs 3D Cu morphology based on Cu-Cu and Cu-substrate interactions. The surfaces selected are Co 25 , Co 50-F , W 25 , W 50-F and W 100 . Similar to Ru-doped TaN surfaces, we observe a transition from the initial single layer of Cu 29 to a two-layered structure on all surfaces. On Co 25 , 7 Cu atoms move to the second layer, while on Co 50-F 6 atoms migrate to form a second layer, as shown in Figure . On both W 50-F and W 100 , the Cu structure resembles facets on the Cu(1 1 1) surface. This is the desired crystal structure for Cu in interconnects as it has the best electromigration reliability. On W 25 , 6 Cu atoms migrate into the second layer and the overall geometry continues to resemble the original Cu 29 geometry, as shown in Figure . On W 50-F there are 10 atoms in the second Cu layer and on W 100 there are 9 atoms in the second Cu layer. During the aiMD calculation the atoms rearrange to be more close packed. There are no transitions to a third layer observed over the course of 5 ps on W 100 , however a Cu atom migrates to the third layer on W 50-F after approximately 2 ps. Further upward migration occurs, creating a 3D cluster structure shown in Figure . The reduced number of atoms in the second layer of Cu 29 on Co-doped TaN compared to W-doped TaN shows that Co doping is better at preventing upward migration, as expected given the more favourable adhesion between Cu and Co as compared to Cu and W. |
6407337ccc600523a3cd2921 | 29 | Cu atoms begin to migrate into the second layer on Co 25 and compared to Co 50-F the number of Cu atoms that migrates is larger. However no Cu migration to form a third layer is seen. Additionally, we observe a Cu atom detaching from the Cu cluster and migrating towards a Co dopant, while another migrates along the edges of the cluster, but not upwards to form a 3D structure. This indicates good mobility of Cu atoms over the surface. Such Cu atom detachment from the Cu 29 structure was only observed on two other surfaces, the 1 ML Ru passivated surface discussed in references and the W 25 surface. The favourable Cu-substrate interactions, along with the lack of formation of further Cu layers and the Cu atoms that migrate away from Cu 29 suggest that 25% Co-doped TaN is a strong candidate for a combined barrier/liner material that limits 3D island growth. |
6407337ccc600523a3cd2921 | 30 | One such structure begins to incorporate into the surface gaps and resembles the geometry of the Cu 4 rhombus on Co 50-F , shown in Figure . There is an equivalent surface gap underneath Cu, however atoms do not incorporate here. This indicates that the regular Cu structure formed here is favoured compared to incorporation into the surface when there is a large number of Cu atoms. |
6407337ccc600523a3cd2921 | 31 | These results additionally show that Cu preferentially migrates towards the areas of higher Co concentration in the surface, which is consistent with Cu 2 and Cu 4 adsorption on Co-doped TaN. In our previous work, we found that Ru also acted as a nucleation site for Cu atoms, however we did not observe such preferential migration during the aiMD runs on the equivalent Ru-doped surface in ref . While a smooth film can likely be achieved with 50% Co doping, for interconnect applications a surface like Co 25 , where the dopants are more distributed should have an advantage in promoting growth of a 2D Cu film. However, it is possible that area selective growth of Cu on CoN could be achieved by combining CoN and TaN. Based on our results, atoms should selectively migrate towards the Co-rich areas. The stronger adhesion of Cu on CoN compared to TaN also means that it could be possible to selectively remove any unwanted Cu atoms in the Ta-rich areas of the surface. The literature around selective atom removal tends to focus on using irradiation techniques to change the composition of polyatomic materials. Other work on selective removal targets specific surface sites based on localised electronic excitations in the surface with the aim to smooth rough metal surfaces. Meanwhile, processes for selective atomic layer etching are still in development and tend to be material-selective rather than truly area-selective. Although the rhombus rearranged into a tetrahedron during the Cu 4 aiMD simulation in Section 3.3, the results of the Cu 29 aiMD simulation and the fact that the Cu 4 rhombus structure was preferred during the geometry relaxation suggest that W 100 acts as a liner. As we see no exchange of Cu and surface atoms combining WN and TaN like this should yield a thermally stable material that can act as a barrier, despite the possible inferior barrier reliability of W x N y compared to TaN. On W 25 , the Cu atoms rearrange into a wire-like structure. Additionally, a single Cu atom breaks away from the cluster and migrates across the surface, indicating enhanced atom mobility. We also observe several transient states throughout the aiMD run-time where the Cu 29 structure separates into smaller clusters, before rearranging into the wire-like structure. This is shown in Figure . As the W dopants are uniformly distributed throughout the surface, meaning that there are no W-rich areas, this rearrangement is clearly not caused by enhanced interaction between Cu and W. Given that the only difference between the interactions of Cu with W 25 compared to the other W-doped surfaces is an increased Cu-substrate energy, it is possible that the distribution of W-dopants in TaN can strengthen the interaction with Cu. Even though there are no clear indications in our analysis why W 25 behaves differently in this respect, provided that these wire-like structures would join into a smooth thin-film, 25% W-doped TaN may be a candidate for a combined barrier/liner material. While the weaker overall adhesion of Cu atoms is of some concern, due to the lack of rearrangements and surface gaps in W 25 this composition may have superior ), Cu-substrate interaction energies (Equation ) and Cu-Cu interaction energies (Equation ) for Cu 29 interacting on doped TaN surfaces. stability compared to Ru-and Co-doped TaN. |
6407337ccc600523a3cd2921 | 32 | The Cu-Cu interaction energy for Cu 29 on both W 50-F and W 100 was calculated (see Table ). We find that the Cu-Cu interaction energy is significantly larger than the the Cu-substrate interaction and that the magnitude of these interactions are similar on both surfaces. For W 50-F this is consistent with migration of Cu atoms into a third layer. However, there are no Cu atoms migrating into a third layer on W 100 . The Cu-Cu interaction is much more favourable than the Cu-substrate interaction for Cu 29 on Co-and W-doped TaN (Table ). Given the large number of possible Cu-Cu interactions compared to the Cu-substrate interactions, it is likely that this is directly reflected by the large Cu-Cu interaction energy. This means that for very large Cu structures, the competition between these two interactions becomes essentially meaningless and is no longer necessarily a predictor of film morphology. |
6407337ccc600523a3cd2921 | 33 | Discussion of the electronic properties of these surfaces based on Bader charges and density of states (DOS) plots are included in the Supporting Information, Section S7. The DOS is metallic for all structures studied. Given the comparatively smaller number of Cu, Co and W atoms compared to the TaN slab, the DOS is dominated by metallic TaN. We find that the contributions of Co to the total DOS are stronger than those of W and Cu. The Bader charges show, that Cu atoms tend to be more oxidised on the Co-doped surfaces compared to Ru-and W-doped TaN. On all thermally stable surfaces, as for systems we previously studied, Cu atoms are only oxidised if they interact directly with the surface, while all other Cu atoms remain metallic. These electronic properties should yield reasonable Cu resistivity, which is essential for Cu interconnects. |
6407337ccc600523a3cd2921 | 34 | In this paper, we showed how incorporation of Co and W into the surface layer of TaN can be used to control morphology of deposited Cu. In particular we show how a preference for 2D vs 3D stability is driven by the competition between Cu-substrate and Cu-Cu interactions. Using static geometry relaxations and aiMD calculations at finite temperature, we determined that 3D growth of Cu should be inhibited and wetting of Cu should be promoted in the following compositions: 25% Co-doping, 25% W-doping, 50% Co doping (in the more favourable dopant distribution) and 100% W-doping. Interestingly, two Co-TaN compositions are thermally unstable: the 100% and the less favourable 50% Co-TaN TaN surfaces as a result of the ionic radius mismatch between Co and Ta. |
6407337ccc600523a3cd2921 | 35 | The favourable Cu-substrate interactions on the 25% Co-doped surface, along with minimal surface rearrangements due to the lack of possible Co-Co interactions in the surface layer, make it a promising candidate for a combined barrier/liner material -we see migration of only a small number of Cu atoms into a second layer during the relaxation and the aiMD at 500 K, indicating that it should inhibit formation of 3D islands. Meanwhile, 25% W doping does not improve the binding energy of Cu compared to TaN, but it improves the Cu-substrate interaction significantly compared to the other W-doped TaN surfaces. This leads to 2D Cu structures, however the inferior adhesion of Cu and the wire-like structures formed may not be favourable for the manufacturing process. |
6407337ccc600523a3cd2921 | 36 | Additionally, the interactions between Cu and W-doped TaN with W concentrations above 25% are quite strong, indicating that 100% doping, or essentially passivation with WN can create a combined barrier/liner material. To the best of our knowledge this is also the first time that possible liner properties of WN have been studied. |
6407337ccc600523a3cd2921 | 37 | Due to the gaps created in the surface by Ru and Co doping and the subsequent incorporation of Cu into this layer, these combined barrier/liner materials must be at minimum made up of two atomic layers (one layer of TaN and one layer of doped TaN) to retain their integrity as diffusion barriers. While this is not an issue for W doping, tungsten nitrides have been shown to have inferior barrier properties. For this reason, the "100% W-doped" layer of WN likely cannot be used on its own and will require an additional layer of TaN. |
6407337ccc600523a3cd2921 | 38 | Comparing the conclusions drawn based on results from different sizes of Cu clusters we are able to evaluate the limits of our models. We find that Cu-Cu vs Cu-substrate interactions can be a useful indicator of film morphology for small Cu clusters, however they are no longer useful larger models where many more Cu-Cu interactions are present. Given this information, we now know that Cu 4 models are useful for predicting the film morphology and that a large model such as the Cu 29 cluster is best used to confirm results obtained by comparing metal-metal and metal-substrate interactions of smaller clusters. This significantly reduces the computational cost and overall time needed to predict thin film morphology. This provides a rapid method to screen a material selection for specific applications. |
627357e943d1f02a2820332a | 0 | Global climate changes represent a serious threat to humans. The Intergovernmental Panel on Climate Change (IPCC) report expected that the mean global temperature will rise by 1.9 °C in 2100 (). It is estimated that the concentration of CO2 in the atmosphere will be increased to 950 ppm by 2100 (the present value of 400 ppm; ). The changes in the climate temperature are irreversible, causing a real threat to the environment and humankind. The emission of gases is one of the most causes of global climate change . Gases can be classified as incondensable inorganic gases (i.e., hydrogen (H2), carbon dioxide (CO2), carbon monoxide (CO), nitrogen (N2), oxygen (O2), and noble gases such as He-Kr) and condensable organic gases (i.e., methane (CH4), ethane (C2H6), ethene (C2H4)). Greenhouse gases, e.g., CO2, and NOx, are mainly responsible for global climate changes. Among these gases, CO2 emission to the atmosphere from human activities such as breathing, industrial processes, and the burning of fossil fuels is one of the leading causes of global warming . Thus, several methods were reported for CO2 capture and utilization (CCU), including adsorption and sequestration . |
627357e943d1f02a2820332a | 1 | Metal-organic frameworks (MOFs) are organic-inorganic crystalline porous materials . They have well-defined pore structures with high porosity as high as 50% of the crystal volume, offering low density (0.2-1 g/cm ) and high specific surface areas (> 10,000 m 2 /g for some cases) . They can be produced via a reticular synthesis procedure, creating ordered networks or framework structures with strong bonds between organic and organic moieties . The construction networks between both moieties tune the geometry of MOFs via designing the secondary building units (SBUs) with suitable organic linkers . The functional groups and the porosity of MOFs can be changed via methods such as post-synthetic modification (PSM) . Multivariate MOFs (MTV-MOFs) with different metal nodes or clusters and other organic functionalities can also be synthesized . Many MOF materials have strong bonds between their moieties, offering high chemical and thermal stability in the temperature range of 250 o C to 500 o C . The high chemical stability of MOFs, especially against water molecules, is usually required for applications such as gas adsorption of CO2 from the atmosphere or hot-flue gases . MOFs were applied for several applications, including CO2 adsorption , chemical conversion/fixation of CO2 , catalysis , photovoltaic devices , sensors , hydrogen production , dye sensistizing solar cells (DSSCs) , water treatment , energy , and osmotic power generators . |
627357e943d1f02a2820332a | 2 | ZIFs crystals are porous materials with high chemical and thermal stabilities (up to 400 o C). They have similar properties to porous MOFs with a full exposition of imidazole-based ligand edges and faces . ZIFs have the potential to improve several applications such as biomedical applications , gene delivery , biomedicine , environmental , and energybased applications . They are also promising materials for gas separation/storage. ZIF-8 (Zn-based ZIFs) was reported as a colorimetric sensor for the simple detection of CO2 . |
627357e943d1f02a2820332a | 3 | Herein, ZIFs-based applications were reviewed to remove CO2 via adsorption, fixation, and chemical conversion (Figure ). This review address most of the current opportunities and challenges for the topic. We highlighted the innovative synthesis methodologies and discussed the challenges of the existing synthesis procedures. Adsorption of CO2 from pure gas and mixed gas was summarized using various forms of ZIFs materials, including powder, membrane, thin-film, foams, and three-dimensional (3D) objects. ZIFs-based materials exhibit high adsorption capacities and excellent selectivity compared to other porous materials. These features open the door for industrial applications and commercialization. |
627357e943d1f02a2820332a | 4 | The 3D structures of ZIFs crystals consist of transition metal cations (M) such as Zn 2+ , and Co 2+ , which are coordinated to an imidazole-based (Im) linker (Figure ). The coordination bonds are formed between the tetrahedral metal centers and nitrogen atoms in the 1,3-positions of the imidazolate linker . The nature between the N atom and the metal node is not well known. However, the crystal combines both types of covalent and coordination bonds. The bond angle of M-Im-M is 145 o, similar to the Si-O-Si bond angle ) in zeolites. Thus, the name was 'Zeolitic Imidazaolte Frameworks', meaning 'zeolite-like materials based on imidazolate. ZIFs exhibit zeolite topology such as SOD, RHO, or LTA. ZIFs crystals show the framework's flexibility concerning gas adsorption. More than 105 ZIF topologies are in the literature (Figure ) . |
627357e943d1f02a2820332a | 5 | However, several studies reported the instability of ZIFs materials. A study stated that ZIF-8 crystallites were unstable in water under ambient conditions . ZIF-8 crystals were dissolved in water, forming zinc and imidazolate ions . They undergo a phase shift to a dense structure and a leaf-like morphology . However, it is essential to mention that the conversion depends on the ratio of ZIF-8/water . The degradation of ZIF-8 is higher in culture media than in deionized water . This observation could be due to the reaction between the released zinc ions with phosphate ions in phosphate-buffered saline (PBS) or with organic moieties in Luria Bertani (L.B.) media. The degradation of ZIF-8 in L.B. and PBS media was 70-80 wt.%, which is higher than in pure water (22 wt.%) . The stability of ZIFs materials can be enhanced by shielding the metalligand bond from the attack of water molecules . separation of gas, as well as for size-selective catalysis due to their pore size (< 5 Å) . A study using thermodynamically corrected diffusivities of probe molecules at 35 °C showed that the adequate aperture size of ZIFs materials such as ZIF-8 was 4.0-4.2 Å . These values are significantly larger than pore apertures estimated using X-ray diffraction (XRD), showing an aperture size of 3.4 Å . This observation indicates that ZIF-8 showed an aperture size similar to well-known porous materials such as zeolite 4A (3.8 Å) and 5A (4.3 Å). The structures of most of the reported ZIFs are flexible. The aperture diameter of ZIF-8 increases from 3.4 to 4.0 Å after gas adsorption . |
627357e943d1f02a2820332a | 6 | ZIFs materials were synthesized via solvothermal methods. The synthesis procedure involved using an organic solvent such as dimethylformamide (DMF) at a temperature above 100 o C for a few hours. The amide solvent such as DMF is decomposed to generate amines that deprotonate the imidazole, forming imidazolate. The resultant materials are pure phases with big crystal sizes. |
627357e943d1f02a2820332a | 7 | A water-based method was reported to synthesize ZIFs materials at room temperature. The procedure involved the addition of chemical precursors such as zinc salts and organic linkers into water. The crystallization can be accelerated via deprotonation using a base that deprotonates the organic linker to initiate the coordination. The organic base plays a dual role; deprotonation and creates a metal oxide or metal hydroxide that offers sacrificial templates for crystal formation. |
627357e943d1f02a2820332a | 8 | Mechanochemical methods advanced the synthesis of MOFs . ZIF-8 was synthesized via one-step mechanochemical processing using a stoichiometric ratio of ZnO nanoparticles and Hmim . The procedure showed the complete conversion of ZnO nanopowders into ZIF-8 with a particle size of ca. 80 nm in diameter with minimal agglomeration. The materials have a surface area of 1885 m 2 /g . The grinding process can be assisted with different chemicals such as NaOH and ionic liquids . The synthesis involved adding a small amount of NaOH powder before grinding at a temperature of 343 K for 24 h . The mechanochemical synthesis procedure is simple, fast, and can be used for large-scale production. However, the products lack high purity and contain residual inorganic materials such as ZnO. |
627357e943d1f02a2820332a | 9 | ZIFs crystal can be grown via in-situ procedure using modified and unmodified supports. The insitu method involves the growth of ZIFs crystals into porous support. This technique includes the immersion of support into the solution of ZIFs precursors. After a period at a specific temperature, the nucleation of ZIFs crystals starts producing layers of ZIFs crystals. However, the use of unmodified support lacks a high nucleation rate due to functional groups that serve as nucleation sites. |
627357e943d1f02a2820332a | 10 | The in-situ synthesis of ZIF-8 membranes on the support of α-alumina disks was reported using a contra-diffusion method . The contra-diffusion procedure involves immersing α-alumina in a solution of zinc metal followed by immersion in a solution containing Hmim and HCOONa. The growth of ZIF-8 crystal was achieved under energy sources such as solvothermal or microwave conditions. The contra-diffusion method can be used for the defective heal membranes because they can be recycled using ligand and metal solutions . It is a cost-effective method compared to conventional synthesis methods. |
627357e943d1f02a2820332a | 11 | An automatic procedure such as electrospinning was also reported using a high voltage electrical charge to draw a seed from a syringe containing a ZIF's seed solution. In the electrospinning procedure, droplets on the capillary tip can be formed by inducing a surface charge on the solution by the applied potential. The electrostatic forces of the charged droplet overcome the surface tension of the solution producing charged jets of the solution into a substrate. The solvent of the deposited droplet undergoes rapid evaporation producing ZIFs crystals on the substrates. The electrospinning method was used to create ZIF-8 crystals from a polyvinyl pyrrolidone (PVP) solution containing seed crystals, followed by a solvothermal procedure on the SiO2 substrate . |
627357e943d1f02a2820332a | 12 | PVP polymer allows uniform dispersion of the ZIF-8 seeds on the substrate, offering a strong adhesion between the substrate and the membrane. Electrospinning procedures produce pure crystals of ZIFs materials. It has been recognized as a reliable method for producing ZIFs-based membranes. It can be applied for large-scale production and industrial scale. |
627357e943d1f02a2820332a | 13 | Chemical attachment of ZIFs seeds on substrates can be achieved using several methods such as dip coating , slip-coating , thermal seeding, reactive seeding, and microwave-induced thermal deposition (MITD) . The dip-coating procedure involves the dipping of a substrate such as α-alumina disks in a solution of ZIFs/PEI seeding solution for the fabrication of ZIF-7 |
627357e943d1f02a2820332a | 14 | The method involved the secondary or seeded growth of ZIFs crystal into the support that contained attached seeding crystals (Figure ). A seed crystal of ZIFs materials can be attached to the membrane supports via physical or chemical treatments. The method is different from the insitu synthesis method. There is no crystal nucleation, growth, and intergrowth in secondary growth synthesis. The choice of ex-situ procedure is critical for the membrane's performance. Generally speaking, ZIF8-based membranes synthesized using rubbing seeding produce higher gas permeability than the electrospinning seeding procedure. |
627357e943d1f02a2820332a | 15 | The rapid thermal deposition (RTD) method is based on the evaporation-induced crystallization concept. It consists of immersing the porous support in the precursor solution of ZIFs materials followed by heating the soaked support in an oven at temperatures of 180-200 o C for a short time (15 min). This step promotes the flow of the precursor solution inside the support. The precursor solution is then evaporated for crystallization homogeneously inside and heterogeneously outside the support. |
627357e943d1f02a2820332a | 16 | Electrospray deposition offers a uniform distribution of minute droplets containing ZIFs precursors generated via an electrostatic force applied on a capillary. This method forms fine-charged precursor droplets that travel through a thermal gradient zone due to an electrical potential gradient. The droplets land and spread on the substrate surface before evaporation-induced nucleation and crystallization of ZIFs crystals to form continuous membranes. This method offers a significant reduction in the synthesis time and precursor consumption. It can be used for easy control of the membrane's thickness. It also simplifies the activation process of the membrane for gas adsorption. The rate of evaporation is a crucial feature during membrane fabrication. It is essential to obtain continuous pore channels in ZIF membranes. The temperature of evaporation for the hot plate should be optimized based on the solvent's boiling point (bp.). At high temperatures, most of the solvent in the electrospray droplets was dried quickly before the successive droplets reached the substrate surface. Thus, the deposited droplet will form single crystals rather than creating a continuous layer of ZIFs materials. On the other hand, low evaporation temperature makes the precursor solution wet the substrate surface. The droplet smears into the interior of the substrate without considerable formation of ZIF crystal. |
627357e943d1f02a2820332a | 17 | ZIFs materials exhibit higher adsorption properties toward CO2 . The diffusivity of CO2 is two times higher than that of CH4 . However, there are strong interactions between CO2 molecules and the functional groups of ZIFs, including hydrogen bond formation. These properties lead to high adsorption capacity and selectivity toward CO2 over other gases. Furthermore, modification can further enhance adsorption capacity and selectivity via an amendment (Table ) . ZIFs can be synthesized using an environmentally friendly solvent such as water. They exhibit high |
627357e943d1f02a2820332a | 18 | Adsorption of CO2 using ZIFs can be improved using ionic liquids (ILs) . Ionic liquids (ILs) such as 1-Butyl-3-methylimidazolium Acetate [Bmim][Ac] and 1-Ethyl-3-methylimidazolium Acetate [Emim][Ac] were impregnated into ZIF-8 for CO2 adsorption . [Bmim][Ac]@ZIF-8 with a loading of 30 wt.% IL exhibited high selectivity toward CO2 adsorption. [Bmim][Ac]@ZIF-30 showed higher CO2 uptake with excellent selectivity for all investigated temperatures (303, 313, and 323 K). It offered up to 7 times higher CO2 capacity than the pristine ZIF-8 at 0.2 bar and 303 K . CO2 can be dissolved into ILs. Thus, ZIFs materials containing ILs provided high adsorption capacity. The use of binary ILs enhanced the CO2 selectivity by 3.5-fold compared to simple ILs@ZIF-8 . A binary ILs with both hydrophobic-hydrophobic properties exhibited higher CO2 selectivity. This observation can be attributed to the strong interactions between CO2 and IL anions. Adsorption using simple and binary mixtures of ILs in ZIF-8 was investigated via density functional theory (DFT) and grand canonical Monte Carlo (GCMC) methods. Data analysis revealed that ILs with fluoride and hydrophobic properties exhibited high adsorption of CO2 . ILs encapsulated ZIFs materials exhibited higher adsorption capacity . |
627357e943d1f02a2820332a | 19 | Dye encapsulated ZIFs materials were reported for CO2 adsorption. Dye molecules enhanced the adsorption capacity of CO2 . Dye@ZIF-8 showed a superior capacity for CO2 of 1.46 mmol•g -1 at 273 K. It provided a 42 % higher adsorption capacity than that of ZIF-8 (0.84 mmol•g - 1 ) under the same conditions . The guest molecules such as dyes improved the adsorption capacity of CO2. |
627357e943d1f02a2820332a | 20 | Membrane-based technologies are an attraction for the adsorption of CO2 (Table ). They offer a cost-effective solution with a high ability for CO2 adsorption. They circumvent some of the challenges of other technologies, such as multiple stages procedures that include pressurizing, depressurizing, and purging. They provide high efficiencies compared to other methods such as conventional pressure swing adsorption systems or cryogenic separation methods. The membranes should exhibit pore continuity (interconnectivity) with defect-free structure and high adherence to the substrate surface for high-adsorption performance. |
627357e943d1f02a2820332a | 21 | The high stability of ZIFs-based membranes ensures reliability and reproducibility in the separation performance for CO2, which are necessary for industrial applications. The chemical and thermal stability of ZIFs-based membranes are not widely investigated compared to ZIF crystals . There are only a few reports on the strength of ZIFs-based membranes . ZIF-69 membrane exhibits high chemical stability . It showed good composure under boiling with methanol and benzene . However, the membranes were unstable in boiling water. |
627357e943d1f02a2820332a | 22 | Membrane-based technologies are not widely used due to the challenges in manufacturing. The current fabrication methods produce small and medium-scale membrane units. There is a high need for throughput methods for large-scale production. The membrane fabrication requires dense polymeric materials that usually exhibit high selectivity but with low permeability. Several materials were reported to improve the gas adsorption using membrane technologies, including MOF such as MOF-5 and ZIF (Table ). There are several fabrication methods for synthesizing ZIFs-based membranes (Figure ). There is no general fabrication method that can be suitable for all membranes. In-situ and ex-situ growth of ZIFs materials on a substrate was reported. Modern or innovative technologies such as electrospray and three-dimensional (3D) printing were investigated (Figure ). The general fabrication method facilitates the scaled-up processes. The in-situ growth c-oriented ZIF-69 membranes use α-alumina substrates for CO2/CO separation . |
627357e943d1f02a2820332a | 23 | Electrospray deposition was used to prepare ZIF-8 and ZIF-7 membranes for H2/CO2 separation. The membranes were synthesized via preparing two solutions of the metal salts and the linker separately and then mixing them to obtain ZIFs precursor solutions. The mixed solution was then fed into a nozzle with an applied potential for electro-spraying on a hot α-alumina substrate (Table ). |
627357e943d1f02a2820332a | 24 | Methods such as RTD and electrospray deposition require high temperatures to promote crystallization and shorten the synthesis time. The synthesis procedures should meet some requirements, making it standard for most ZIFs-based membranes. They should offer conditions such as control structures, thickness, and ZIFs-loadings. They should have high reproducibility for effective gas separation. |
627357e943d1f02a2820332a | 25 | Adsorption of CO2 can be improved using the hard structure of ZIF-90 via sequential reduction using NaBH4 and lithiation reaction (i.e., formation of ZIF-91-OLi) . ZIFs materials with high rigid structures ensure that the pore size will be preserved without change during the fabrication process . The rigid membranes can be achieved through post-synthetic modification and functionalization with imine and APTES . However, it is essential to note that there is no diffusive motion for CO2 and CH4 in rigid ZIF-8 . MMM with tiny defects in ZIF-8 enhanced the CO2/N2 separation . |
627357e943d1f02a2820332a | 26 | The functional groups of the organic linker in ZIFs materials play an important role in the adsorption capacity and the selectivity of the materials (Figure ) . Adsorption properties of ZIF materials can be tuned via a mixed linker . ZIFs of a linker with electron-withdrawing and donating groups have enhanced CO2 adsorption . Density functional theory (DFT) calculation was used to quantify the binding ability of CO2 for ZIF materials for 137 linkers . The presence of asymmetrical functional groups, e.g., NO2/OH, CN/OH, and Cl/OH in imidazolate improved the CO2 adsorption . Amino-functionalized ZIF-8 materials using 2,5-diimidazolyl aniline (ZIF-DIA) exhibited superior CO2 adsorption . It can be used for CO2 capture in humid flue gas . Grand Canonical Monte Carlo (GCMC) simulations for ZIF-69 revealed that the electronegative -Cl functional group in the organic linker of ZIF-69 displayed an inductive effect. |
627357e943d1f02a2820332a | 27 | The effect of the functional groups is correlated to the heat of adsorption and selectivity . The electronegative functional group such as Cl offers electrostatic interactions with the polar gas molecules such as CO2 molecules that can be trapped at the pore opening. On the other side, nonpolar gas such as (CH4) undergoes penetration through the pore without adsorption. Thus, the materials offer high selectivity. Using DFT and GCMC, theoretical calculation revealed that the terminal group's rotation tunes the linker swing motion ZIF-90 . The Nitro group of 2nitroimidazole linkers offers a strong Lewis acid-base interaction . On the other side, the interaction of H atoms of the benzimidazole and the oxygen atoms of CO2 enabled high selectivity . Adsorption of CO2 was enhanced two-fold after solvent assisted ligand exchange (SALE) of ZIF-8 with an imidazole linker containing electron withdrawing groups such as -NO2 and -SH . Electron withdrawing groups in the linker of ZIF crystal enhanced the acidity for the hydrogen atoms on linker offering hydrogen bond interactions with the oxygen atoms of CO2 (Figure ) . |
627357e943d1f02a2820332a | 28 | Besides the functional groups of the linker, the ligand dipole moment is also a key factor for CO2 adsorption (Figure ) . Data analysis showed an exponential relationship between the ligand dipole moments and the isosteric heat of adsorption (qst) of CO2 (Figure ). The high ligand dipole moment showed a 5-to 7-fold improvement in the selectivity for CO2 adsorption for CO2/CH4, CO2/N2, and CO2/CO mixtures (Figure ). |
627357e943d1f02a2820332a | 29 | The porosity of ZIFs is a key parameter in gas adsorption. The substrates shouldn't block the porosity of ZIFs crystals. The pore aperture size of ZIFs crystals can be modified using a postsynthetic modification to increase the gas separation performance. Post-synthetic procedures can constrict the pore aperture size, reduce intercrystalline defects, increase the crystal's homogenous distribution inside the membrane, or create active adsorption sites. It is crucial to balance reducing the pore aperture size and the permeability of the gases. Small pore size apertures significantly decrease the gas permeability. On the other side, selectivity is usually enhanced by the pore aperture size reduction and the reduction of grain boundary defects. Consequently, non-selective transport pathways and the ''gate opening effect'' can be prevented. |
627357e943d1f02a2820332a | 30 | Moreover, thermal and hydrothermal stability can be improved via the post-synthetic functionalization step, which may facilitate membrane activation. A study on the porosity effect (windows and cavity) was reported using positron lifetime for ZIFs; ZIF-90, ZIF-93, and ZIF-94 . Data analysis revealed that the smaller pore size (micropore or ultramicropores) of ZIFs is optimal for CO2 adsorption under ultralow pressure . ZIFs crystals with the smaller pores (e.g., ZIF-7 and ZIF-94) showed higher CO2 adsorptions than ZIFs materials with larger pores (e.g., ZIF-11 and ZIF-93) at low pressures (<1 bar) . In contrast, the opposite is true at higher pressures, i.e., the larger-pore structures showed significantly higher adsorption . Hollow ZIF-8 containing meso/macro pores showed good adsorption of CO2 with an adsorption capacity of 1.05 mmol/g at 1 bar . |
627357e943d1f02a2820332a | 31 | Fluorine containing 1-butyl 3-methylimidazolium ([BMIM] + ) ILs (i.e., [BMIM] + with counter ions such as [BF4] -and [PF6] -anions) exhibited high adsorption of CO2 (Figure ) . (Figure ) . This could be also due to the high dispersion of fluorine-containing anions on the ZIF-8 surface compared to the non-fluorinated anion . The incorporation of ILs in ZIFs materials enhances the adsorption and selectivity of CO2 . a b Figure a) structure of the ionic liquid, and b) gas-phase optimized geometries of CO2@ILs, with shortest interaction distances (in Å). Figure reprinted with permission from Ref. . Copyrigt belongs to Elsevier. |
627357e943d1f02a2820332a | 32 | Ethylenediamine (ED) treated ZIF-8 nanoparticles enhanced the selectivity of CO2/CH4 by 37% compared to untreated ZIF-8 . It can remove H2S and CO2 from natural gas . The use of nanoparticles such as carboxylated carbon nanotubes (CNTs) composite prevented the aggregation of ZIF-8 in the membrane, increased the free volume of the MMM, and enhanced the CO2 adsorption performance and CO2/N2 separation performance . ZIF-8-PEI@IL (15 wt.%) increased the CO2 permeability of MMMs by 123%, and the selectivity of CO2/N2 and CO2/CH4 from 17 and 12 to 76 and 25, respectively . |
627357e943d1f02a2820332a | 33 | The morphology of ZIF materials exhibits an insignificant effect on CO2 adsorption . The low and high aspect ratio ZIF-78 showed similar CO2 adsorption behavior . However, leaf-like ZIF (ZIF-L) with 2D morphology exhibited high CO2 adsorption. This observation is due to the cushion-like pore of ZIF-L (Figure ). The vibrational properties of ZIFs materials such as ZIF-68 and ZIF-69 . The high CO2 uptake causes high free-breathing vibrations of the ligands, causing the large ZIF pores . |
627357e943d1f02a2820332a | 34 | The fabrication of ZIFs-based membranes requires support or substrate. The judicious choice of substrate is important during the synthesis of ZIFs-based membrane. Several substrate were reported including ceramic , ionic liquids membrane , and polymeric-based materials. Ceramic supports including α-alumina , titania , yttria-stabilized zirconia (YSZ) , ZnO , and SiO2 were reported. α-alumina was the commonly used ceramic substrate. It offers excellent mechanical stability, and for this reason, it is the most commonly preferred ceramic-based substrate. Some of these ceramics show excellent mechanical and thermal stability. Ceramic such as titania can promote heterogeneous nucleation. However, titania is expensive and brittle leading to broken during the gas adsorption . Support of two different substrates such as α-alumina and titania was also described to combine the high mechanical stability of α-alumina and the smooth appearance of titania . |
627357e943d1f02a2820332a | 35 | The support should promote the heterogeneous nucleation of ZIFs crystals. The bare support lacks heterogeneous nucleation. Thus, surface modification of the support is usually required to create active sites for crystal nucleation. In fact, α-alumina supports were modified with organic linkers , polymer , or ZnO . ZnO or organic linkers can be used as sacrificial templates for the nucleation and growth of ZIFs crystals . The sacrificial templates enable the formation of smooth layers of ZIFs crystals. The ceramic substrate can also be modified chemically with 3aminopropyltriethoxysilane (APTES) . APTES molecules act as a binder between the membrane layer, the support, and the ZIF-22 crystals . It forms a covalent bond between the ethoxy groups of APTES react and the surface hydroxyl groups of cerium support, such as Al2O3 . The presence of functional groups of the support and the surface modification ensure effective heterogeneous sites for developing continuous layers of ZIFs materials on the substrate . |
627357e943d1f02a2820332a | 36 | and Torlon . The functional groups of these polymeric materials strengthen the interaction between the polymeric support and the organic linkers. Polyaniline enhanced thermal stability and CO2 adsorption property of ZIF-8 . 20 wt.% ZIF-68 MMM increased CO2 permeability by 116% and 122% for CO2/N2 and CO2/CH4, respectively, for Matrimid MMMs . |
627357e943d1f02a2820332a | 37 | The well-intergrown ZIF crystals are required to obtain high pore continuity, i.e., good interconnectivity in membranes. The homogenous growth of ZIFs layers into the substrate can be promoted using deprotonation. A base such as triethylamine (TEA), sodium formate (HCOONa), ammonium hydroxide (NH4OH), or sodium hydroxide (NaOH) can be used to deprotonate the organic linker and promotes crystal growth . Thus, the ZIFs crystals can be grown in all substrate directions. The base can also play a dual role; deprotonation and the formation of a sacrificial template via conversion of Zn salts into ZnO. It was also reported that Hmim exhibits a higher degree of deprotonation in water than in organic solvents . Besides the base, polymers such as polyethyleneimine (PEI) were also reported for ZIF7-based membranes . The functional groups of PEI promote the linkage between the seeds and the support. PEI can also deprotonate the organic ligand .The adsorption using ZIF-67 can be improved via the in-situ growth of the adsorbent inside hollow carbon nanospheres (ZIF-67@HCSs) . The CO2 adsorption capacities of ZIF-67@HCS-40, ZIF-67@HCS-50, and ZIF-67@HCS-60 were 4.6 mmol/g, 3.9 mmol/g, and 3.0 mmol/g, respectively at 273K . |
627357e943d1f02a2820332a | 38 | Solvothermal and hydrothermal methods are widely used for the synthesis procedures of ZIFs materials. Solvents such as dimethyl formamide (DMF), dimethyl formamide (DEF), methanol, and water were commonly used to synthesize ZIFs materials. They are critical for the synthesis of the desired materials. Water offers several advantages being a low-cost solvent and benign solvents for health and environmental concerns. However, few ZIF-membrane, such as ZIF-8 membranes, were synthesized using water as the solvent. |
627357e943d1f02a2820332a | 39 | The adsorption of gases such as CO2 requires the presence of an empty pore. The pore structure can be evacuated using a different method. The process is called an activation step that aims to remove all occluded solvent molecules. The activation with low boiling point solvents such as water or ethanol removes solvents such as DMF or DEF present in the pore of the materials. The solvent exchange method requires soaking the materials in a solvent such as ethanol or methanol for a long time to ensure the full exchange of the occlude solvents. The solvent exchange process may cause damage to the ZIF crystals due to the high mass transfer rate. Thus, it is highly recommended to use a solvent mixture of DMF-methanol to activate ZIF78-based membranes to slow down the diffusion rate of DMF . The process requires several steps. For example, the solvent exchange process can be started by immersing the membranes in a low methanol content of DMF-methanol solvent to reduce the concentration gradient of DMF. Further steps were also performed by increasing the methanol content and ended with using pure methanol at the end of the activation process . The solvent exchange with a highly volatile solvent such as methanol enables the fast removal via the drying process . The synthesis procedures such as electrospray deposition simultaneously synthesize and activate ZIF-based membranes for ZIF-7 and ZIF-8 membranes. |
627357e943d1f02a2820332a | 40 | The loading effect for gases, e.g., CH4 and CO2, was investigated . The amount of the gases affect the adsorption and selectivity. The gases are usually present as a mixture with different pressure. The interaction energies between CH4 molecules and ZIF-10 are almost constant regardless of the gas's loading due to the absence of interaction forces such as hydrogen bonds (HBs). On the other side, the interaction energies between CO2 and ZIF-10 decreased rapidly with the increase of the gas's loading . Gibbs ensemble Monte Carlo (GEMC) and molecular dynamics (MD) simulations were investigated for CO2 adsorption using ZIF-87 via diffusion . |
627357e943d1f02a2820332a | 41 | Data analysis revealed that low concentrations showed high selectivity as significant as 140. The authors also found that the selectivity value can be increased further with decreasing CO2 concentration . It was reported that both CO2 and CH4 are preferentially located proximal to the -C=C-bond of the Hmim linker, i.e., near the aperture at a low loading of gas molecules. |
627357e943d1f02a2820332a | 42 | CO2 gas molecules significantly promote both the translational and rotational motions of the n-C4H10/CO2 mixture in ZIF-10. CO2 molecules exhibit strong hydrogen bonds with the imidazolate rings leading to weakened interactions between alkanes and ZIF-10. Furthermore, both the translational and rotational motions are small compared to other gases. Thus, there was selective adsorption toward CO2 molecules compared to alkanes . |
627357e943d1f02a2820332a | 43 | Non-equilibrium dynamic Monte Carlo (DMC) simulations combined with dual control volume (DCV), denoted as DCV-DMC, were explored for the separation selectivity of a gas mixture of CH4/CO2 gas mixtures in ZIF-8 membrane (thickness of 20 nm) . Data analysis revealed that the parts near membrane surfaces at both ends play a crucial role in determining separation selectivity . |
627357e943d1f02a2820332a | 44 | The diffusion of CO2 gases inside ZIF-11 crystals was investigated using multinuclear pulsed field gradient (PFG) nuclear magnetic resonance (NMR) . Data analysis revealed that the intracrystalline diffusivity of CO2 decreased with an increasing diffusion time. This observation is due to the reflections of diffusing CO2 molecules from the external crystal surface. |
627357e943d1f02a2820332a | 45 | Several mechanisms have been reported for gas adsorption, including the gate-opening mechanism , the flip motion of the linkers , and the linker swing motion . The low-temperature heat capacity measurements of ZIF-8 confirm the structural change of the framework upon adsorption . Data analysis revealed CO2 rearrangement and lattice expansion . The adsorption mechanism depends on several parameters, including temperature . A study reported no gate opening related in the temperature range of 133 K and 227 K . There are three sites for the binding of CO2 and ZIF-68 (at 298 K and 1 kPa); the center of the HPR cages (Site I), the corners of the GME cages (Site III), and the nearby bIM linkers of the KNO cages (Site III), as shown in Figure 10 . Another study was reported using the van der Waals density functional model to study the binding of CO2 into ZIFs materials . The data analysis reveals the presence of three distinct binding sites for CO2 . with permission from Ref. . Copyright belongs to Elsevier. |
627357e943d1f02a2820332a | 46 | A computational study using GCMC simulations were reported to study the adsorption of CO2 and H2O using the Zinc Triazolate-based Framework (ZTF) . Data analysis reveals that the interactions between ZTF and CO2 occur via two sites; Lewis acid-Lewis base interactions and hydrogen bonding, together with apparent electrostatic interactions . In situ Raman investigation was reported to study the interactions of ZIFs with CO2 under pressure (0-10 bar) |
627357e943d1f02a2820332a | 47 | and temperature regimes (0-64 °C) . Raman spectra show a significant shift (159 cm -1 ) in the vibration peaks of phenyl bending mode, revealing the presence of hydrogen bond interaction (Figure ) . The presence of electron-withdrawing groups enhanced the acidity of the linker's hydrogen atoms, improving the hydrogen bond interactions with the oxygen atoms of CO2 . |
627357e943d1f02a2820332a | 48 | Carbon dioxide (CO2) can be removed via the conversion of CO2 into valuable chemicals (Figure ) . It can be converted via chemical reaction (Figure ), photocatalysis, and electrochemical methods (Figure ). The chemical transformation of CO2 can be achieved via cycloaddition with epoxides to produce cyclic carbonates, hydrogenation, N-formaylation , and synthesis of dimethyl carbonate (DMC) from CO2 and methanol (Figure ). The following sections summarize most of the findings for these topics. |
627357e943d1f02a2820332a | 49 | ZIF-8 has been considered the first catalyst based on ZIFs materials for CO2 fixation via cycloaddition (Table ) . It catalyzed the cyclic carbonate formation, i.e., styrene carbonate from CO2 and styrene oxide, with a yield of 53.0% at 100 °C (Table ). Besides, the catalytic activity of the ZIF-8 catalyst was declined after recycling. After the reaction, the material lost its distinctive crystalline nature and catalytic performance. ZIF-68 was reported to remove CO2 via cyclic carbonate formation using styrene oxide under mild reaction conditions (120 °C and 1.00 MPa) . It yielded > 93.3% after 12 h . Dual-ligand ZIF (ZIF-8-90) was used for CO2 removal via the formation of five-membered cyclic carbonate with epichlorohydrin (ECH) . ZIF-8-90 exhibited high selectivity (> 99%) compared to ZIF-8 . |
627357e943d1f02a2820332a | 50 | ZIFs materials (ZIF-7, ZIF-8, ZIF-9, and ZIF-67) were used as precursors for the synthesis of metal-based catalysts embedded into N-doped nanoporous carbons via carbonization at elevated temperature for cycloaddition reaction (Table ) . Among the carbonized materials, carbonized ZIF-9 at 600 °C (denoted as C600-ZIF-9) under an inert gas (Ar) showed the highest catalytic activity (yield of 90%) at 80 o C under 0.6 MPa of CO2 . The increased activity could be due to the uniformly distributed acidic and basic sites of partly oxidized cobalt nanoparticles and nitrogenous species . ZnO nanoparticles encapsulated into N-doped porous carbon were synthesized via the carbonization of ZIF-8 followed by oxidation treatment with sodium hypochlorite (NaClO) . The oxidation of the carbonized materials offered several oxygencontaining functional groups such as carboxyl, lactone, and alkoxy. ZnO@NPC-Ox-700 (NPC: Ndoped porous carbon, Ox oxidized, 700 refers to carbonized temperature ( o )) exhibited the highest catalytic conversion, selectivity, and recyclability for CO2 cycloaddition . Bimetallic ZIF containing Co and Zn can be used to synthesize Zn/Co@C magnetic nanoparticles via carbonization process at 900 o C, denoted as N-doped magnetic porous carbon (NPC-900) . NPC-900 is a recyclable catalyst with a simple separation procedure via an external magnet . ZIF-L-derived catalyst offered high catalytic activities for CO2 cycloaddition with 83% yield under mild conditions (70 °C and 0.1 MPa CO2) . |
627357e943d1f02a2820332a | 51 | A nucleophile attacks the coordinated epoxide to activate the reaction. A co-catalyst e.g., tetrabutylammonium bromide, (TBAB) forms a Bromo-alkoxy intermediate . The authors suggested the Zn-N bond dissociation and creation of active sites of Zn-OH species (lowcoordinated Zn species) that act as Lewis acid/base for CO2 cycloaddition. They also claimed that the dissociated N-species (pyrrolic or pyridinic) were inactive sites for the reaction (Figure ) . |
627357e943d1f02a2820332a | 52 | A yolk-shell Cu2O@ZIF-8 material was synthesized via the in-situ growth of ZIF-8 on Cu2O modified polyvinyl pyrrolidone (PVP) . It was used to convert CO2 via the reaction with propargylic alcohols and propargylic amines under mild conditions, offering turnover numbers (TONs) of 12.1 and 19.6, respectively (Figure ). It is a noble metal-free catalyst for synthesizing valuable α-alkylidene cyclic carbonates and oxazolidinones using CO2 (Figure . |
627357e943d1f02a2820332a | 53 | The optimal loading of Cu-Ni was 5 wt.%. The highest DMC yield was only 6.39%, with a MeOH conversion of 12.79% at 20 bar of CO2 . The catalyst can be recycled for 4 cycles. However, derived FeZnK-NC was also reported for CO2 hydrogenation in the production of C2-C4 olefins with high stability . It showed C2+ selectivity of 63 mol.% . The hydrogenation of CO2 using ZIF67-derived materials can be promoted using sodium via impregnation with NaNO3 . |
627357e943d1f02a2820332a | 54 | framework (Cu/N-C) was synthesized via the carbonization of Cu(acac)2@ZIF-8 . Cu/N-C served as an excellent electrocatalyst for electrocarboxylation of styrene with CO2. It contains two different valence states of Cu + and Cu 2+ , offering a higher charge density. The electron-rich singleatomic Cu sites exhibited effective activation of CO2 into CO2 -that attack styrene to produce phenyl succinic acid (Figure ). Cu/N-C offered high Faradaic efficiency, good selectivity, and a high production rate of 92%, 100%, 216 mg•cm -2 •h -1 , respectively . |
627357e943d1f02a2820332a | 55 | Electrochemical conversion of CO2 can be improved using ZIFs-based catalysts with high electric conductivity (Table ). The performance of the electrocatalyst depends on the metal centers of the ZIF materials . The performance can also be enhanced under light using photoactive components in the investigated electrocatalysts . [292] CsPbBr3 quantum dots. Co centers in ZIF-67 accelerated the charge separation process and activated the adsorbed CO2 molecules leading to high catalytic CO2 reduction activity . ZIF-67 was used as a precursor for synthesizing ultrathin holey Co3O4 nanosheets (thickness of 1.8 nm) . The synthesized Co3O4 nanosheets was used as a co-catalyst and support for photosensitizer [Ru(bpy)3]Cl2•6H2O. [Ru(bpy)3]Cl2•6H2O/Co3O4 nanosheets showed CO generation rate and selectivity of 4.52 μmol/h and 70.1%, respectively. Co3O4 nanosheets offered a large specific surface area, high affinity to promote CO2 adsorption, enhanced the separation and transport of charge carriers, and offered several active sites for CO2 photoreduction . The Zn2GeO4/ZIF-8 with 25 wt.% ZIF-8 showed 3.8 times higher CO2 adsorption capacity than the bare Zn2GeO4 nanorods, offering a 62% enhancement in the photocatalytic reduction of CO2 into CH3OH . ZIF-8 prevent photocorrosion of CdS enable good photocatalysis . TiO2/Co-ZIF-90 showed 2.1 times higher CO2 photoreduction than pure TiO2 . ZIF-based catalyst can be combined with Au nanoparticles and assembled into 3D Ni foam (Figure ) . NF@ZnO/Au@ZIF-8 photocatalyst showed 270.02 μmol/g with high selectivity of 89.72%. It exhibited high stability. The main product using ZnO, NF@ZnO, and NF@ZnO@ZIF-8 were CO. While, in the presence of Au, CO is further reduced, forming -CHO, -CH2O, finally leading to the production of CH3OH and CH4. The reduction of Au is due to the high adsorption performance and high surface electron density of gold. Then -CHO gained protons and photogenerated electrons to form -CH2O. Afterward, the -CH2O further obtains protons and photogenerated electrons to form -OCH3. Finally, the -OCH3 reacts with protons and photogenerated electrons to CH4 (Figure ) . |
627357e943d1f02a2820332a | 56 | In-situ FT-IR spectra of a mixture of CO2 and H2O vapor mixture under dark (0-60 min) and illumination (180 min) using NF@ZnO/Au@ZIF-8, (b) GC-MS analysis of the products generated using 13 CO2 isotope, (c) pathway for the reduction to CO and CH4 on the surface of NF@ZnO/Au@ZIF-8, (d) the band structure and charge transfer process of NF@ZnO/Au@ZIF-8 catalyst under UV-Vis light, and (e) a diagram for CO2 photocatalytic reduction using NF@ZnO/Au@ZIF-8 catalyst. Figure reprinted with permission from Ref. . Copyright belongs to Elsevier. |
627357e943d1f02a2820332a | 57 | ZIF-8, ZIF-67, and Co/Zn-ZIF showed yield of 51%, 44%, and 73%, respectively. On the other side, carbonization (at a temperature of 600 o C, 700 o C, 800 o C, and 900 o C, NPC-X, X refers to the carbonization temperature) of these materials offered metal oxide embedded carbon with high light absorbance. On the other side, NPC-600 and NPC-900 showed high catalytic activities of 93-94% yield, while NPC-700 and NPC-800 exhibited a yield of 84-86% . At low carbonization temperature (600°C), N-C layers can well wrap the Co NPs leading to the formation of smaller grains and high metallic Co 0 content due to the effective inhibition of surface oxidation. In contrast, there is a high probability of surface oxidation at high carbonization temperature leading to a surface with CoOx oxides that inhibit the charge separation and transfer. Thus, the N-C/Co-600 sample exhibited the best photocatalytic activity . |
627357e943d1f02a2820332a | 58 | Missing linkers during the co-assembly process offer open metal sites, which facilitate cycloaddition catalysis. The defects inside ZIF can be determined using several techniques such as temperature-programmed desorption (TPD), high-resolution transmission electron microscopy (HR-TEM), and thermogravimetric analysis (TGA). ZIF-8 with uncoordinated N-sites on the framework showed a high chemical fixation of CO2 . Unfortunately, so far, the defects in the frameworks are uncontrollable and lack high reproducibility. |
627357e943d1f02a2820332a | 59 | The cycloaddition of CO2 depends on several parameters. The catalytic performance can be improved via the addition of a co-catalyst, e.g., TBAB , or gold (Au) . Co-catalysts such as n-Bu4NBr (TBAB), n-Pr4NBr, n-Pr4NBr, Me4NBr showed a carbonate yields of 91%, 68%, 33%, and 6%, respectively, using hollow-structured Zn-Co based ZIF . The catalytic performance of these co-catalysts can be ordered in the sequence of nBu4N + >n-Pr4N + >Et4N + >Me4N + . The high performance of TBAB is due to the low interaction between nBu4N + and Br -. |
627357e943d1f02a2820332a | 60 | The cycloaddition of CO2 and ECH can be performed without solvents . Ionic metalloporphyrin encapsulated ZIF-8 offered a solvent-free synthesis of cyclic carbonates from CO2 (1 atm) and epoxides without co-catalyst . The presence of dehydrating agents is vital for the reaction that produces water . MgCO3 and CH3CN are the common dehydrating agents. The activation energy of using MgCO3 (5.4 kJ/mol) is lower than that of using CH3CN (7.8 kJ/mol) . The electrolytes for the electrochemical method are essential. NaCl showed the highest electrochemical reduction of CO2 to the CO selectivity . |
627357e943d1f02a2820332a | 61 | Photocatalysis CO2 cycloaddition requires a catalyst with good light absorption. High CO2 concentration is usually required for high conversion and yield. The CO2 cycloaddition reaction can be accelerated under solar-driven cycloaddition using a Xenon lamp with 320 mW/cm 2 [196]. The N-doped carbon catalyst is an effective photocatalyst for photothermallydriven CO2 cycloaddition . |
627357e943d1f02a2820332a | 62 | ZIFs-based adsorbents offer several advantages. ZIFs-based membranes exhibit good thermal stability for gas separation . ZIFs materials such as ZIF-78, ZIF-79, ZIF-80, ZIF-81, and ZIF-82, showed high selectivity for separation of CO2 and CH4 with selectivity of 10.6:1 to 9.1:1 for ZIF-78 to ZIF-82, respectively . They retain high CO2 gas compared to other materials such as zeolite gmelinite (GME) series and BPL-activated carbon . ZIFs-based materials exhibit high adsorption efficiency toward CO2 over other gases. They can be used as powder and membranes. The adsorption performance can be enhanced via several methods ensuring high capacity and better selectivity. The synthesis of ZIF-based membranes on tubular support is promising for research and pilot-scale production before their actual deployment in industrial and commercialization applications. Most of the current methods for fabricating ZIFs-based membranes lack high reproducibility. This is a significant challenge that should be circumvented to enable consistent performance. Solving the challenges such as reproducibility and lab-scale production will enable commercialization and application on an industrial scale. |
627357e943d1f02a2820332a | 63 | The conversion of CO2 into valuable compounds is promising. Several reactions were reported, including photocatalytic reduction, the transformation of CO2, electrocatalytic conversion, formation of dimethyl carbonate, and hydrogenation. The products of these reactions are valuable industrial compounds. For example, cyclic carbonates are essential intermediates for producing other substances and polymers. They can be used as aprotic polar solvents or electrolytes for lithium-ion batteries (LIBs). Further investigation should be carried out to improve the catalytic performance of ZIFs-based materials enabling large-scale production. |
627357e943d1f02a2820332a | 64 | Processing ZIFs materials into the custom design are essential. Most of the reported synthesis procedure produces powder materials limiting their applications. The uses of biopolymers such as cellulose and chitosan enable to processing of ZIFs materials into commercial forms using well-established technologies. Biopolymers enable processing MOFs into forms such as foam . |
629df71e97e76a377cc7f06e | 0 | With roots in ancient civilizations, sustainability has gained public attention in recent decades with a series of landmark reports, events, and treaties. These efforts recognize the necessity to promote equitable and prosperous communities while simultaneously maintaining the ecosystems that support them. This need is particularly relevant for water technologies as they safeguard basic human rights (water and sanitation ) and are essential components of broader initiatives to re-envision other systems designed to meet societal needs (e.g., resource recovery, green chemistry, regenerative agriculture ). In line with this need, the field of sustainability science and sustainability engineering continue to evolve in support of the society's transition toward sustainability. To apply the concept of sustainability in the research, development, and deployment (RD&D) of technologies, implementable and systematic methodologies are necessary to quantitatively evaluate technologies as they mature. To this end, multiple sets of guiding principles (e.g., the Green Chemistry 39 and Green Engineering 21 Principles) and frameworks (e.g., the UN Inclusive Wealth framework 40 ) have been proposed. These principles and frameworks often represent aspirational goals for technologies, engineered systems, or national entities, and are not intended to provide structured guidance to inform the prioritization of RD&D for specific technologies. Similarly, methodologies such as Life Cycle Sustainability Assessment have been proposed to address the "whole-picture" of sustainability, but methodologies such as this often lack quantitative, transparent strategies to navigate tradeoffs across dimensions of sustainability and technology-specific indicators of engineering performance (e.g., contaminant removal). More critically, there is a growing recognition of the importance of uncertainty, especially for earlystage technologies associated with higher levels of uncertainty. Nonetheless, the results of sustainability analyses are often presented as single values. This "false precision" overlooks aleatory (due to randomness) and epistemic (due to the lack of knowledge) uncertainty that is Ultimately, these limitations undermine the accessibility and utility of sustainability analyses to inform decision-making for the RD&D of technologies, which is crucial in the society's pursuit of sustainability. In this paper, we review and synthesize published literature related to sustainability analyses to present a tutorial review on Quantitative Sustainable Design (QSD). QSD integrates concepts associated with sustainability science and engineering to expedite and support the RD&D of technologies (Figure ). Through the lens of QSD, we establish a shared lexicon as the foundation for interdisciplinary communication and to support methodological transparency and consistency (Table ). In presenting this methodology, we discuss published studies using the shared lexicon to put them in the context of QSD. Specifically, we begin by defining the problem space, which includes specifying the system of interest as well as relevant decision variables, technological parameters, and contextual parameters (Section 2). Next, we establish design and process algorithms to generate the system inventory (mass and energy flows that enter and leave the system). Sustainability indicators can then be quantified using techniques that span economic, environmental, human health, and social dimensions (Section 4). By compiling all algorithms used in system simulation and sustainability characterization, a system model can be created to quantify uncertainty and generate the desired types of insight (Section 5). Through the review of existing tools, we discuss the status of tools for sustainability analyses and how the different steps of QSD can be executed under uncertainty. With literature examples and representative figures, we further illustrate how QSD can be used (i) to characterize sustainability indicators under uncertainty, (ii) to identify sustainability drivers, (iii) to set RD&D targets, (iv) to understand uncertainty drivers, (v) to explore alternative scenarios (i.e., distinct combinations of decision variables, technological parameters, and contextual parameters), and (vi) to inform practical deployment. Ultimately, QSD can be used to elucidate the complex and intertwined connections among design decisions, technology characteristics, contextual factors, and sustainability indicators, thus enabling transparent and agile planning and design processes. Finally, we identify future research needs for the continued development and application of sustainability analyses for technology RD&D, with the goal of supporting society's pursuit of sustainable development. |
629df71e97e76a377cc7f06e | 1 | The first step of QSD is to define the problem space. In essence, this is the process of specifying the technology of interest, what will be included or excluded from the analysis (i.e., the system boundary), and the ranges of assumptions that can be made for QSD inputs. QSD inputs comprise any assumptions that may influence the performance and sustainability of the system, including decisions about the system design or operation of the system (i.e., decision variables), characteristics of the technology or its components (i.e., technological parameters), and the context in which the technology can be deployed (i.e., contextual parameters). The QSD inputs form an N-dimensional "problem space" encompassing all possible combinations of values of the QSD inputs (N is the number of QSD inputs), and the sustainability of the system will be evaluated within this space through QSD. To help familiarize the reader with these terms and their meaning, a summary of terminology definitions can be found in Table and specific examples from the literature can be found in Table and Table (Electronic Supplementary Information, ESI). |
629df71e97e76a377cc7f06e | 2 | This conceptual system should contain the technologies of interest and their interactions with upstream, downstream, and parallel unit processes. Notably, the system does not need to be limited to the boundary of any specific physical system, but it can include any related processes that may be impacted by changes to QSD inputs. In selecting the system boundary, an "allinclusive" approach would involve all upstream and downstream unit processes that interact directly or indirectly with the technologies. However, this approach may result in the inclusion of unit processes that are inconsequential to system sustainability, add unnecessary layers of complexity, and introduce sources of uncertainty that demand additional resources to evaluate. |
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