All-Solid-State Z-Scheme α-Fe2O3/Amine-RGO/CsPbBr3 Hybrids for Visible-Light-Driven Photocatalytic CO2 Reduction
Lead halide perovskite (PVK) has been deemed as a promising photocatalyst alternative because of its remarkable photoelectrical properties; however, the severe charge recombination has limited its catalytic activity. Herein, we report a PVK-based Z-scheme heterojunction, a-Fe 2 O 3 /Amine-RGO/CsPbBr 3 , for highefficiency CO 2 reduction in the presence of H 2 O. By delicately controlling the interfacial interaction, effective Z-scheme electron transfer from a-Fe 2 O 3 to CsPbBr 3 is built, leading to boosted charge separation and prolonged carrier lifetime, as confirmed by electron spin resonance (ESR), transient absorption (TA) spectra, etc. The impactful spatial separation of photo-generated carriers in Z-scheme system finally enables an 8.3-fold enhancement in photocatalytic performance as compared to CsPbBr 3 . A stable product yield of 469.16 mmol g À1 and an electron consumption yield of 3,132.46 mmol g À1 are achieved. This work is expected to provide deep insights into boosting the photocatalytic performance of PVK by modulating the charge carrier dynamics.
INTRODUCTION<!>The Bigger Picture<!>RESULTS AND DISCUSSION<!>ESR Measurement<!>Photocurrent Measurement<!>Ultrafast Spectra Measurements<!>Photocatalytic CO 2 Reduction<!>CONCLUSIONS<!>EXPERIMENTAL PROCEDURES<!>Synthesis of CsPbBr 3 nanocrystal<!>Materials and Device Characterization<!>TAS Characterizations<!>Photoelectrochemical Measurements
<p>Ongoing substantial consumption of fossil fuel together with concomitant CO 2 emission have exacerbated the well-known energy crisis and global warming issue, thus focalizing enormous scientific attention in the field of CO 2 neutralization techniques. Artificial photosynthesis is regarded as the most promising strategy to recycle CO 2 sustainably since it employs renewable sunlight to power the catalytic reduction of CO 2 , which can not only reduce the atmospheric concentration of CO 2 but also produce solar fuels simultaneously, such as CH 4 , CO, CH 3 OH, etc. [1][2][3] In pursuit of high CO 2 conversion efficiency, various semiconductor photocatalysts have been explored in the past decades, but the low visible-light-harvesting capability and photoconversion efficiency remain great challenges. 4,5 Recently, lead halide perovskite (PVK) material has attracted enormous attention for its remarkable optoelectronic properties and potential optoelectronic applications, especially in solar cells, [6][7][8][9] light-emitting diodes (LEDs), 10,11 and photodetectors. 12,13 Benefiting from its large extinction coefficient, excellent visible-light-harvesting ability, and high charge carrier mobility, recently, PVK materials have been applied for photocatalytic CO 2 reduction. [14][15][16][17] However, rapid charge recombination tendency corresponding to the high photoluminescence quantum yield (PLQY) of PVK nanocrystal has seriously hindered its catalytic performance. To tackle this issue, some elegant strategies such as coupling PVK with graphene, 14 porous g-C 3 N 4 nanosheets, 18 metalorganic framework, 19,20 Pd metal nanosheet, 21 metal dichalcogenide, 22 etc.,</p><!><p>Although lead halide perovskite (PVK) has been deemed as a promising photocatalyst alternative because of its remarkable photoelectrical properties, the severe charge recombination has limited its catalytic activity. To solve this problem, previous scientific efforts have been widely laid on the fabrication of PVK-based type II heterojunctions or Schottkyjunctions. Unfortunately, this was at the cost of the weakened reduction ability of the photogenerated electrons due to the electron transfer. Herein, a PVKbased Z-scheme photocatalyst is delicately designed to not only boost the charge separation but also preserve the high redox potential of semiconductors. By delicately controlling the interfacial interaction, effective Z-scheme electron transfer is built, which enables the largely enhanced visible-light-driven overall conversion of CO 2 and H 2 O to CH 4 and O 2 . This work is expected to provide a new way to rational design-effective PVKbased photocatalyst.</p><p>have been reported to effectively boost the charge separation and thus the CO 2 reduction catalytic performance.</p><p>However, the facilitated charge separation in these PVK-based type II heterojunctions or Schottky-junctions is at the cost of weakened reduction ability of the photo-generated electron due to the electron transfer (Figure S1), which has significantly spoiled the superiority of PVK as a CO 2 reduction photocatalyst. In contrast, bio-inspired all-solid-state Z-scheme photocatalyst was found to not only boost the spatial separation of photo-induced electron-hole pairs but also afford a maximum redox potential with a high conduction band edge (E CB ) from photocatalyst I (PC I) and a low valence band edge (E VB ) from photocatalyst II (PC II), which has laid a solid foundation for simultaneously reducing CO 2 to solar fuels and oxidizing H 2 O to O 2 , the two ultimate reactions of artificial photosynthesis (Figure S1). 23,24 The key bottleneck of such Z-scheme technique lies in the exploration of suitable PC I that can well match with the band positions of PC II, and more importantly, constructing efficient interfacial charge transfer path between them. [25][26][27][28][29] In consideration of the excellent photoelectrical properties of PVK, it is anticipated that a glorious photocatalytic CO 2 reduction performance will be yielded by integrating it with appropriate semiconductor to construct Z-scheme heterojunction. Unfortunately, as far as we know, the fabrication of PVK-based Z-scheme photocatalyst has not been reported yet.</p><p>Herein, we report a PVK-based all-solid-state Z-scheme photocatalyst for efficient photocatalytic CO 2 reduction and water oxidation. CsPbBr 3 and a-Fe 2 O 3 are elaborately selected as the PC I and PC II, respectively, considering their visible-light-absorption ability and outstanding reduction and oxidation potential. CsPbBr 3 nanocrystal has a band gap of $2.3 eV with negative enough E CB toward CO 2 reduction. 14,19 a-Fe 2 O 3 shows a band gap of $2.2 eV and is known as a robust photocatalyst for water oxidation to O 2, although its E CB is too positive to reduce CO 2 . 30,31 The matched energy band structure enables the formation of a direct Z-scheme charge transfer from a-Fe 2 O 3 to CsPbBr 3 , which can inhibit the radiative combination of photo-induced charge carrier effectively. Moreover, the Z-scheme work efficiency can be accelerated forward when amine functionalized reduced graphene oxide (Amine-RGO) is introduced to regulate the interfacial interaction of CsPbBr 3 and a-Fe 2 O 3 , which is authenticated by electron spin resonance (ESR), photoelectrochemical, and ultrafast spectroscopy measurement. As an encouraging result, the well-designed a-Fe 2 O 3 /Amine-RGO/CsPbBr 3 composite delivers a significantly advanced catalytic activity for photocatalytic CO 2 reduction to CH 4 and CO, accompanied by almost stoichiometric amount of O 2 generation. To the best of our knowledge, it is the first PVK-based Z-scheme heterojunction toward photocatalytic CO 2 reduction.</p><!><p>Synthesis and Characterization of the a-Fe 2 O 3 /Amine-RGO/CsPbBr 3 Z-Scheme Heterostructure The a-Fe 2 O 3 /Amine-RGO/CsPbBr 3 hybrid was fabricated through a multistep fabrication process (Figure 1A): a-Fe 2 O 3 single-crystal nanorod array grown on fluorinedoped tin oxide (FTO) substrate was prepared first by hydrothermal method, followed by surface coating of amine-functionalized RGO nanosheets and subsequent CsPbBr 3 nanocrystal deposition through a solvent evaporation-deposition process (see the Experimental Procedures section for details). Here, Amine-RGO was introduced between a-Fe 2 O 3 and CsPbBr 3 to work as an electron mediator. [32][33][34][35] It is well known that a high-quality interface is of great significance in ensuring effective charge transfer in heterojunctions. Here, the heterogeneous interfaces in triple hybrid was delicately regulated. First, the surface of a-Fe 2 O 3 nanorods was modified with ionic liquid molecules ([BMIM]BF 4 ), because it can facilitate the homogeneous and close wrapping of graphene oxide (GO) nanosheets onto the surface of nanorods through p-p interaction, 36,37 which is expected to facilitate the electron extract from a-Fe 2 O 3 . Second, the GO shell was reduced and functionalized with amine groups via subsequent treatment by hydrazine vapor, which can further serve as active growth sites for CsPbBr 3 nanocrystals by use of the interaction between amine and Pb 2+ , ensuring the uniform and close deposition of CsPbBr 3 nanocrystals. 38,39 As shown in Figures 1B and S2, the as-grown a-Fe 2 O 3 were smooth nanorods with a height of $780 nm and an average diameter of $40 nm. After surface treatment of Amine-RGO, an ultrathin coating layer can be found on the surface of a-Fe 2 O 3 nanorods, indicating the successful wrapping of RGO to form a core/shell structure (Figures 1C, 1D, and S3). Besides, additional RGO nanosheets were also observed to bridge neighboring nanorods together to form an interlaced network. In contrast, RGO nanosheets suspending only on the top of a-Fe 2 O 3 nanorods can be observed in the control sample prepared in the absence of ionic liquid molecules (Figure S4). This is reasonable because the liquid drop of GO solution cannot penetrate well into the nanorod film without the assistance of [BMIM]BF 4 molecules before spin coating process, demonstrating the surface modification of [BMIM]BF 4 played a key role in achieving such homogeneous wrapping. Undoubtedly, sufficient and close contact between a-Fe 2 O 3 and RGO would contribute a lot to the interface charge transfer. The successful reduction of GO and the formation of superficial amine groups were verified by Raman spectrum and Fourier transform infrared (FTIR) spectroscopy (Figure S5). Compared with GO, an increased D/G intensity ratio was observed after hydrazine treatment. Moreover, signals assigned to amine groups (N-H, 1,564 cm À1 ; -NH 2 , 3,334 cm À1 ) were observed in FTIR spectrum, indicating that GO was effectively reduced to highly conductive amine-functionalized RGO. 40,41 After CsPbBr 3 deposition, no obvious changes could be observed in the scanning electron microscopy (SEM) image (Figure 1E), but nanoparticles with diameter of about 6 nm dispersed separately on the surface of either the composite nanorods or the RGO bridges could be clearly observed in transmission electron microscopy (TEM) image (Figure 1F). Without the assistance of Amine-RGO, CsPbBr 3 nanoparticles were dispersed unevenly on the surface of a-Fe 2 O 3 nanorods; furthermore, severe aggregation of CsPbBr 3 happened in the gap between a-Fe 2 O 3 nanorods (Figure S6), which revealed that the surface amine groups on RGO would provide more growth sites for CsPbBr 3 and thus obviously prevent it from excessive growth and aggregation during solvent evaporation-deposition process. Moreover, the high-resolution TEM image (Figure 1G) demonstrated the clear lattice spacing of 0.41 and 0.25 nm, corresponding to the (110) planes of CsPbBr 3 and the (110) plane of a-Fe 2 O 3 , respectively. The high-angle annular dark field (HAADF) image and energy dispersive X-ray spectroscopy (EDX) mapping (Figures 1H and S7) further verified the dispersed decoration of CsPbBr 3 nanocrystals and their relatively homogeneous distribution on the whole film. In addition, the X-ray diffraction (XRD) patterns of a-Fe 2 O 3 /Amine-RGO/CsPbBr 3 sample matched well with the hematite a-Fe 2 O 3 and orthogonal CsPbBr 3 (Figure 1I). Stronger peaks at 15.0 , 21.5 , 30.7 , 34.2 , 37.7 , 43.7 , and 46.4 indicated the high crystallinity of CsPbBr 3 . 42 No typical diffraction pattern of RGO can be detected, which may be attributed to its low loading amount. According to the thermogravimetric analysis (TGA), the mass ratio of RGO was calculated to be $3.4% relative to a-Fe 2 O 3 (Figure S8). In addition, the mass fraction of CsPbBr 3 was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis, which was 11.3% and 25.0% in a-Fe 2 O 3 / CsPbBr 3 and a-Fe 2 O 3 /Amine-RGO/CsPbBr 3 , respectively.</p><p>To further investigate the surface chemical interaction in a-Fe 2 O 3 /Amine-RGO/ CsPbBr 3 composite sample, X-ray photoelectron spectroscopy (XPS) was performed. For comparison, pristine CsPbBr 3 nanocrystals (with similar size to those in composite sample, synthesized via anti-solvent method, Figure S9), a-Fe 2 O 3 nanorods, and a-Fe 2 O 3 /Amine-RGO, a-Fe 2 O 3 /CsPbBr 3 samples were also examined as a control (Figures 2 and S10). Pristine a-Fe 2 O 3 showed typical peaks at 725.07 and 711.65 eV, assigned to the binding energy of Fe 2p 3/2 and Fe 2p 1/2 , respectively. Obviously, these peaks shifted positively (by 0.52 eV) after coating with Amine-RGO, indicating a largely decreased electron cloud density of Fe due to the strong interaction between a-Fe 2 O 3 and Amine-RGO. However, further deposition of CsPbBr 3 onto a-Fe 2 O 3 /Amine-RGO led to the elimination of such positive shift. On the contrary, large positive shift was observed for the binding energies of both Pb 4f (by 0.45 eV, 4f 7/2 , and 4f 5/2 ) and Cs 3d (by 0.54 eV, 3d 5/2 , and 3d 3/2 ) compared with pristine CsPbBr 3 , suggesting an electron transfer from CsPbBr 3 to a-Fe 2 O 3 via Amine-RGO. It should be noted that both negative shift of Fe 2p (by 0.49 eV) and positive shift of Pb 4f (by 0.18 eV) relative to the pristine counterpart is witnessed for the a-Fe 2 O 3 /CsPbBr 3 sample, but the shift value is smaller than those of a-Fe 2 O 3 /Amine-RGO/CsPbBr 3 , demonstrating a stronger chemical and electronic coupling interactions among ternary components happened with the introduction of Amine-RGO.</p><p>It is well perceived that the shift of binding energy in heterojunctions is always an indicative of strong interfacial interaction and charge transfer. Therefore, the energy band structure of composite samples was determined by ultraviolet-visible (UV-vis) absorption spectra and ultraviolet photoelectron spectra (UPS). A distinct absorption edge at about 582 nm was displayed in a-Fe 2 O 3 film, agreeing with a band gap of 2.14 eV (Figure S11). After decorating with Amine-RGO and CsPbBr 3 , the absorption capability in whole detected range was enhanced with the band gap of CsPbBr 3 calculated to be 2.36 eV. According to the UPS spectra (Figure S12), the E VB of a-Fe 2 O 3 and CsPbBr 3 was located at À6.97 and À6.07 eV (versus vacuum), thus the E CB was estimated to be À4.83 and À3.71 eV, respectively. Additionally, the work functions (f) of a-Fe 2 O 3 and CsPbBr 3 were calculated to be 5.10 and 4.34 eV, respectively, which correspond to a fermi level (E F ) of À5.10 eV and À4.34 eV, respectively. [43][44][45] Therefore, a staggered energy band structure is formed between a-Fe 2 O 3 and CsPbBr 3 with the E F position of a-Fe 2 O 3 much lower than that of CsPbBr 3 (Figure 2C, I), which suggests that they are inclined to form a valid Z-scheme system. 46,47 Due to the big difference of E F , spontaneous electron transfer from CsPbBr 3 to a-Fe 2 O 3 would occur when they contact intimately, 48,49 which had been confirmed by above XPS results. Hence an interfacial built-in electric field (BIEF) with the direction from CsPbBr 3 to a-Fe 2 O 3 could be constructed after achieving the fermi level alignment between each other (Figure 2C, II). 50 Upon visible light irradiation, it is speculated that the internal electric field would guide the photo-generated charge carriers toward Z-scheme transfer mode. That is the photoelectrons in the CB of a-Fe 2 O 3 would recombine with the holes in the VB of CsPbBr 3 , leaving holes in the VB of a-Fe 2 O 3 while electrons in the CB of CsPbBr 3 (Figure 2C, III). 51 Furthermore, when Amine-RGO interlayer with high conductivity and electron affinity, which can facilitate the electron extract from a-Fe 2 O 3 and provide ''high way'' for subsequent electron transport, was elaborately introduced to interface of a-Fe 2 O 3 and CsPbBr 3 , the Z-scheme charge transfer should be strengthened (Figure 2C, IV). Fortunately, this kind of electron transfer can be probed in microscale by using Kelvin probe force microscopy (KPFM) (Figure S13). 52,53 As a result, the spatially separated holes and electrons with high redox potentials could be further utilized to participate in the oxidation and reduction reaction, respectively.</p><!><p>To further validate the formation of Z-scheme system, ESR analysis was carried out to detect the spin active ,O 2 À and ,OH À species using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as the trapping agent. As displayed in Figure 3A, only signals attributing nor DMPO-,OH could be detected. Therefore, based on the above analysis, the formation of Z-scheme charge transfer process in a-Fe 2 O 3 /CsPbBr 3 and a-Fe 2 O 3 / Amine-RGO/CsPbBr 3 hybrids can be confirmed. Additionally, the signal intensity of hybrid samples was much higher than that of the individual counterpart, and the ternary a-Fe 2 O 3 /Amine-RGO/CsPbBr 3 exhibited the strongest signals. Higher signal intensity means more electrons and holes accumulated on the CB of CsPbBr 3 and the VB of a-Fe 2 O 3 , respectively. It can be inferred that the introduction of Amine-RGO has efficiently enhanced the work efficacy of Z-scheme toward charge carrier separation.</p><!><p>To further probe the Z-scheme charge transfer and separation behaviors, photoelectrochemical test was performed to monitor the anodic and cathodic photocurrent of composite films, respectively. The chopped light anode photocurrent-time curves (Figure 4A) showed that the highest anodic photocurrent density was yielded for a-Fe 2 O 3 (17.4 mA/cm À2 ), 1.8-and 7.6-folds higher than that of a-Fe 2 O 3 /CsPbBr 3 (9.8 mA/cm À2 ) and a-Fe 2 O 3 /Amine-RGO/CsPbBr 3 (2.3 mA/cm À2 ). The dramatically decreased current density in composite samples should be attributed to the formation of Z-scheme structure at the interfaces. That is the photo-generated electrons of a-Fe 2 O 3 would transfer to the VB of CsPbBr 3 via RGO, and this electron transfer direction was opposite to the electron running direction of external circuit, thus leading to a prominent decrease in anodic current of a-Fe 2 O 3 (Figure 4B). Correspondingly, when cathodic current of electrode was monitored, an obviously increased photocurrent was obtained in composite samples since the interfacial electron transfer direction was identical to extra electric field (Figure S14). These results, in combination with ESR results, provide solid evidences for the formation of the Z-scheme heterogeneous structure.</p><!><p>Furthermore, the Z-scheme charge transfer dynamics were investigated in depth by the ultrafast spectrum technologies, which have been considered as powerful methods to disclose the charge transport mechanism in semiconductors. First, both a-Fe 2 O 3 /CsPbBr 3 and a-Fe 2 O 3 /Amine-RGO/CsPbBr 3 exhibited photoluminescence (PL) peak of CsPbBr 3 centered at 520 nm (Figure 5A). However, compared with pure CsPbBr 3 , the PL emission intensity was dramatically quenched. Therefore, timeresolved photoluminescence (TRPL) spectra was conducted to quantitatively understand the carrier dynamics. Figure 5D clearly presents an accelerated PL decay for a-Fe 2 O 3 /CsPbBr 3 and a-Fe 2 O 3 /Amine-RGO/CsPbBr 3 , and the latter exhibits the fastest decay rate. After fitting the decay curves with a model of two exponential functions (Table S1), the average PL lifetimes of CsPbBr 3 , a-Fe 2 O 3 /CsPbBr 3 , and a-Fe 2 O 3 /Amine-RGO/CsPbBr 3 were calculated to be 25.38, 16.61, and 1.73 ns, respectively. The quenched PL intensity and accelerated PL decay indicated the effective charge transfer between CsPbBr 3 and a-Fe 2 O 3 , which can be accelerated with the presence of Amine-RGO.</p><p>Furthermore, femtosecond transient absorption (TA) spectroscopy was performed.</p><p>As depicted in Figures 6A-6C, a pronounced negative peak at 520 nm and a broad  S2). Corresponding to the TRPL results, the shortened decay time in composite samples can be interpreted as the efficient interfacial charge transfer. In contrast, no obvious degradation of the photoexcited absorption signal of a-Fe 2 O 3 can be observed within nanosecond timescale, indicating the surface-trapped holes should be consumed in much longer time scale. Therefore, TA test was conducted further in a long-time scale extending to microsecond (Figures 6E-6G). Interestingly, it can be found the lifetime of the trapped holes in a-Fe 2 O 3 has been prolonged after coupling with CsPbBr 3 , agreeing well with the sluggish decay dynamics curves (Figure 6H). Specifically, the average lifetime increased largely from 3.72 ms for pure a-Fe 2 O 3 to 6.34 and 6.61 ms for a-Fe 2 O 3 /CsPbBr 3 and a-Fe 2 O 3 /Amine-RGO/CsPbBr 3 , respectively (Table S3), indicating a timely and spatial charge separation happened in composite films, which led to the suppressed electron-hole recombination in a-Fe 2 O 3 and thus the prolonged lifetime. This phenomenon also supported the auxiliary formation of Z-scheme charge transfer between CsPbBr 3 and a-Fe 2 O 3 , given the fact that this absorption signal would be annihilated quickly if type II heterojunction was formed, due to the fast hole extract from a-Fe 2 O 3 to CsPbBr 3 . It is worth noting that with the presence of Amine-RGO, retarded decay at 570 nm and accelerated decay at 520 nm superior than a-Fe 2 O 3 /CsPbBr 3 were observed, confirming the electron transfer from a-Fe 2 O 3 to the CsPbBr 3 had been promoted by Amine-RGO once again, which suppressed the charge recombination in both CsPbBr 3 and a-Fe 2 O 3 simultaneously. Based on these analyses, enhanced photocatalytic activity in a-Fe 2 O 3 /Amine-RGO/CsPbBr 3 can be expected because of the timely and spatial Z-scheme charge separation. The excitation wavelength is 400 nm.</p><!><p>Visible-light-driven photocatalytic CO 2 reduction performances of samples were investigated in the atmosphere of CO 2 and H 2 O vapor (AM 1.5G, l > 420 nm, 150 mW cm À2 ). The corresponding data were summarized in Figures 7 and S15; Table S4. For a-Fe 2 O 3 /Amine-RGO/CsPbBr 3 , CH 4 was detected as the main reduction product, accompanied by small amount of CO and H 2 . The total yield of reduction products is 181.68 mmol g À1 after 15 h of continuous illumination, which is much higher than that of a-Fe 2 O 3 /CsPbBr 3 (66.64 mmol g À1 ) and CsPbBr 3 nanocrystals (30.55 mmol g À1 ). No reduction product was detected for pure a-Fe 2 O 3 because its E CB is too positive to reduce CO 2 . As far as the photoelectron participating the reduction action was concerned, the total electron consumption yield (Yield electron ) of a-Fe 2 O 3 /Amine-RGO/CsPbBr 3 , calculated by Yield electron = 2Yield CO + 8Yield CH4 + 2Yield H2 , was 1,214.22 mmol g À1 , and the corresponding electron consumption rate (R electron ) was determined to be 80.95 mmol g À1 h À1 , which was 8.3-and 3.1-fold of pure CsPbBr 3 (9.80 mmol g À1 h À1 ) and a-Fe 2 O 3 / CsPbBr 3 (25.95 mmol g À1 h À1 ), respectively. The electron selectivity for CH 4 was up to 93.4%. In addition, O 2 was also detected as the oxidation product of photoexcited holes thanks to the good oxidation ability of a-Fe 2 O 3 . Of particular note is that the yield of O 2 is nearly close to one fourth of total yield of electron (Figure S16). Therefore, the main involved gas-solid photocatalytic reaction probably followed the pathway CO 2 (g) + 2H 2 O(g) = CH 4 (g) + 2O 2 (g). That is to say the proton source for the reduction of CO 2 originated from the water.</p><p>In addition, such Z-scheme composite exhibited good stability during long-time photocatalytic reaction. The time courses of reduction products on a-Fe 2 O 3 / Amine-RGO/CsPbBr 3 (Figure 7B) showed that the evolution of CH 4 , CO and H 2 increased almost linearly during the continuous irradiation of 40 h. A high overall yield of 469.16 mmol g À1 and electron consumption yield of 3,132.46 mmol g À1 were achieved. Meanwhile, no obvious change was found in XRD pattern and UVvis absorption spectra, implying a superior long-term photocatalytic stability (Figure S17). This conclusion can be further supported by the recycling tests, in which nearly constant photocatalytic performance was monitored within 5 cycles (Figure 7C). Furthermore, control photocatalytic experiments were conducted to confirm the origin of catalytic products. First, after replacing CO 2 with N 2 , the photocatalytic responses of samples were almost annihilated (Table S5). Second, replacing CO 2 (m/z = 44) with 13 CO 2 (m/z = 45) as the reactant, obvious 13 CH 4 (m/z = 17) and 13 CO (m/z = 29) signals can be observed in mass spectrum (Figure S18A). These results verify that the obtained carbonaceous compounds (CO and CH 4 ) were generated from the catalytic reduction of added CO 2 . 58 Moreover, the isotope experiment by replacing H 2 O (vapor, m/z = 18) with H 2 18 O (vapor, m/z = 20) led to the formation of 18 O 2 (m/z = 36) (Figure S18B), further confirming that the product molecular oxygen was derived from added H 2 O molecules.</p><!><p>In this work, a PVK-based Z-scheme heterojunction, a-Fe 2 O 3 /Amine-RGO/CsPbBr</p><!><p>Preparation of a-Fe 2 O 3 /Amine-RGO a-Fe 2 O 3 nanorod array film was synthesized following a hydrothermal procedure as reported previously, 23 which was then immersed into 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM]BF 4 )/ethanol (0.1mg/mL) solution for 15 min. GO nanosheets were prepared via a modified Hummer's method from graphite powders. After that, 100 mL of GO dispersion solution (1 mg/mL) was spin-coated onto the [BMIM]BF 4 modified a-Fe 2 O 3 film with a speed of 3,000 rpm for 30 s and then washed with ethanol. Subsequently, an annealing process at 450 C was conducted in argon atmosphere for 30 min to reinforce the adhesion between the GO sheets and a-Fe 2 O 3 nanorod. To functionalize the as-prepared a-Fe 2 O 3 /GO films with amine groups, the films were exposed to hydrazine vapor (35 wt %) at 100 C for 12 h and then washed with distilled water.</p><p>Fabrication of a-Fe 2 O 3 /CsPbBr 3 and a-Fe 2 O 3 /Amine-RGO/CsPbBr 3 Hybrids The a-Fe 2 O 3 /Amine-RGO/CsPbBr 3 hybrid was prepared by a solvent evaporationdeposition method. The PVK precursor was firstly prepared by dissolving CsBr and PbBr 2 in dimethyl sulfoxide (DMSO) with a concentration of 0.3 M. After that, 1 mL of the prepared PVK precursor was dropped onto the a-Fe 2 O 3 /Amine-RGO film and kept for 10 min before spin coating at 3,000 rpm for 30 s. Subsequently, the film was dried at 60 C for 4 h to facilitate the crystallization of CsPbBr 3 nanocrystals. Similarly, the a-Fe 2 O 3 /CsPbBr 3 composite was prepared under the same experimental procedure.</p><!><p>CsPbBr 3 nanocrystal was prepared by a procedure modified from a published work. First, the Cs-oleate precursor solution was prepared by dissolving Cs 2 CO 3 (81.4 mg) in the mixture of octadecene (10 mL) and oleic acid (2.5 mL) in a 100 mL 3-neck flask.</p><p>The mixture was dried for 1 h at 120 C, followed by heating to 150 C under N 2 atmosphere until the complete dissolution of all Cs 2 CO 3 . Then, PbBr 2 (18.3 mg) was dissolved into a mixture solution of dimethylformamide (DMF, 1 mL), hexanoic acid (11.6 mg), and n-octyl amine (12.9 mg) in air at room temperature. A homogeneous mixture of isometric PbBr 2 precursor (0.5 mL) and Cs-oleate solution (0.5 mL) was then quickly injected into 20 mL of toluene to induce the immediate crystallization of CsPbBr 3 . CsPbBr 3 precipitate was obtained by centrifugation and then purified by washing with ethyl acetate which was further re-dispersed into ethyl acetate for further application. The CsPbBr 3 films were fabricated by drop-coating method and dried further at 200 C for 90 min in a vacuum drying oven to remove the surface organic ligands.</p><!><p>The morphology of various samples was observed by scanning electron microscopy (SEM, SU8010), transmission electron microscopy (TEM), and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) (JEOL ARM-200F). XRD patterns of all samples were collected in the range of 10-80 (2q) using MiniFlex X-ray diffractometer. Raman spectra were obtained on a J-Y T64000 Raman spectrometer with 532 nm wavelength incident laser light. The Fourier transform infrared (FTIR) spectrum in the region of 500-4,000 cm À1 was examined on Nicolet 330 spectrometer with KBr pellets. The absorption behaviors of the samples were studied by the UV-vis spectrometer (Shimadzu UV-3600). Inductively coupled plasma atomic emission spectroscopy (ICP-AES) analysis was carried out by TJA IRIS (HR). TGA was performed under Ar environment using a NetzschTG209F3 TGA system. The XPS and UPS experiments were performed on a photoelectron spectrometer (ESCALAB 250, Thermo Fisher Scientific). KPFM experiments were performed under ambient conditions using a Bruker Dimension Fastscan. PL spectra and TRPL spectra were measured using FLS980 spectrometer (Edinburgh Instruments Ltd.). The ESR investigations were conducted on a JESFA200 ESR spectrometer.</p><!><p>Transient absorption spectroscopy (TAS) was performed based on a Ti: sapphire laser source (Coherent Legend) to generate pulses centered at 800 nm with a full width at half maximum (FWHM) of 150 fs and 1 kHz repetition rate, and the signal on fs-ns timescales were collected by Helios spectrometers (Ultrafast Systems) while that on fs-us timescales were collected by EOS spectrometers (Ultrafast Systems). The 400 nm pump beam was formed from part of the 800 nm output pulse from the amplifier, and its power was adjusted by a range of neutral-density filters. The pump beam was focused at the sample with a beam waist of about $360 mm and the power intensity was fixed at 200 mJ cm À2 in this experiment. TA changes were measured in the visible region from 420 to 700 nm.</p><p>Examination of Photocatalytic CO 2 Reduction Performance Photocatalytic reduction of CO 2 with H 2 O vapor was carried out in a 56 mL sealed Pyrex reaction vessel under atmospheric pressure at ambient temperature ($298 K). As shown in Figure S19, the as-prepared catalyst films were loaded into a Pyrex reaction cell filled with CO 2 and saturated H 2 O vapor. A 150 W Xe lamp (Zolix) with an AM1.5G and 420 nm optical filter was used as the visible light source, and the light intensity was adjusted to 150 mW cm À2 by calibrating with an NRELcalibrated Si solar cell. The gaseous products were analyzed by gas chromatography (GC9790 plus, Fuli Analytical Instrument Co., Ltd.) equipped with a thermal conductive detector (TCD) and a flame ionization detector (FID). Isotope-labeling experiments using 13 CO 2 and H 2 18 O molecules were performed under the same conditions, and the gaseous products were analyzed using gas chromatographymass spectrometry (GCMS-QP2010). The catalytic performances of samples were calculated according to the total weight of hybrid materials.</p><!><p>Photocurrent measurements were performed on a standard three electrode cell with asprepared sample films as the working electrode, a platinum foil as the counter electrode, and a saturated Ag/AgCl as the reference electrode. The 0.05 M tetrabutylammonium hexafluoro phosphate (TBAPF 6 )/ethyl acetate solution was filled in the quartz cell as the electrolyte. The anodic and cathodic photocurrents were recorded on a Zennium electrochemical workstation (Zahner) under irradiation of a 150 W Xe lamp (AM1.5G, l > 420 nm, and 100 mW cm À2 ) at a potential of +0.4 V and À0.4V, respectively. In order to make a better comparison, the photocurrent in this work was calculated by subtracting the current value under dark from the current value under light.</p>