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TiO2 nanotubes with laterally spaced ordering enable optimized hierarchical structures with significantly enhanced photocatalytic H2 generation

In the present work we grow self-organized TiO2 nanotube arrays with a defined and controlled regular spacing between individual nanotubes. These defined intertube gaps allow to build up hierarchical 1D-branched structures, conformally coated on the nanotube walls using a layer by layer nanoparticle TiO2 decoration of the individual tubes, i.e. having not only a high control over the TiO2 nanotube host structure but also on the harvesting layers. After optimizing the intertube spacing, we build host-guest arrays that show a drastically enhanced performance in photocatalytic H2 generation, compared to any arrangement of conventional TiO2 nanotubes or conventional TiO2 nanoparticle layers. We show this beneficial effect to be due to a combination of increased large surface area (mainly provided by the nanoparticle layers) with a fast transport of the harvested charge within the passivated 1D nanotubes. We anticipate that this type of hierarchical structures based on TiO2 nanotubes with adjustable spacing will find even wider application, as they provide an unprecedented controllable combination of surface area and carrier transport.

In the present work we grow self-organized TiO 2 nanotube arrays with a defined and controlled regular spacing between individual nanotubes. These defined intertube gaps allow to build up hierarchical 1D-branched structures, conformally coated on the nanotube walls using a layer by layer nanoparticle TiO 2 decoration of the individual tubes, i.e. having not only a high control over the TiO 2 nanotube host structure but also on the harvesting layers.
After optimizing the intertube spacing, we build host-guest arrays that show a drastically enhanced performance in photocatalytic H 2 generation, compared to any arrangement of conventional TiO 2 nanotubes or conventional TiO 2 nanoparticle layers. We show this beneficial effect to be due to a combination of increased large surface area (mainly provided by the nanoparticle layers) with a fast transport of the harvested charge within the passivated 1D nanotubes. We anticipate that this type of hierarchical structures based on TiO 2 nanotubes with adjustable spacing will find even wider application, as they provide an unprecedented controllable combination of surface area and carrier transport.
Since the groundbreaking work of Fujishima and Honda [1] in 1972, photocatalytic H 2 generation by water splitting based on TiO 2 materials has received immense research interest.
Over the past decades, not only TiO 2 but also hundreds of other semiconductors have been investigated for their H 2 generation performance (e.g. ranging from Si to group II-VI, III-V compound semiconductors and to various transition metal oxides, and so on). [2][3][4] In spite of some intrinsic deficits of TiO 2 , such as a wide band gap and a sluggish electron transfer kinetics, it still remains the most studied photocatalytic material as it has an almost unique (photo)corrosion resistance and comparably low cost. [5][6][7][8][9] The photoelectrochemical principle to hydrogen generation is that suitable light irradiation promotes electrons from the semiconductor (TiO 2 ) valence band to the conduction band, thus generating electron hole pairs that then can react at the semiconductor surface with redox couples in the surroundingshydrogen (H 2 ) generation occurs from ejected conduction band electrons to H 2 O or H + .
In order to maximize the efficiency of the H 2 photocatalytic reactions on TiO 2 , typically TiO 2 nanoparticles, either free floating in the solution or compacted to electrodes, are used. [10][11][12][13] Nanoscale photocatalysts provide not only a high specific surface area but also a short diffusion path for the excited carriers to reach the particle surface (minimizing recombination). More recently, one-dimensional (1D) structures such as nanotubes, nanowires and nanorods have attracted significant interest as they can provide, except for a large surface area, a directional carrier transporter (faster), and in many cases intrinsically support an orthogonal electron-hole separation. [5,7,8,14] These features further aid in minimizing the recombination of charge carriers and thus minimize the efficiency loss. [5,8,9] In the past years, particularly highly ordered TiO 2 nanotube layers (NTs) grown by self-organizing electrochemical anodization of titanium have attracted wide interest. [8] For these tubes, which are vertically aligned on the substrate, geometry and structure can be easily controlled by the anodization parameters and by a post formation thermal annealing. [15] In some cases, such nanotube layers were reported to outperform comparable nanoparticle layers in their photocatalytic performance. [16,17] This was mainly ascribed to a more efficient charge separation and transport which, in fact, is not surprising considering that the electron mobility of nanoparticle layers has been reported to be orders of magnitude lower than that of nanotubes. [16,[18][19][20] Nevertheless, in terms of specific surface area, such NTs provide clearly lower values (30-50 m 2 g -1 ) than comparable nanoparticle layers where values of 100 m 2 g -1 (and more) can be reached.
In order to combine the benefits of NTs (electron transport properties) and nanoparticles (large surface area), hierarchical structures have been explored where TiO 2 particles are decorated onto the TiO 2 NTs. [21] However, up to now, optimized hierarchical structures require the ability to space the 1D conducting material in desired distances on a substrate, e.g. as in the case of nanowires, [22] nanorods [23] or nanotubes grown by template method. [24] However, using classic anodic TiO 2 nanotube layers does not allow this, as conventional anodization leads to a hexagonally close-packed arrangement of nanotubes, and this geometry allows only a limited decoration with defined secondary nanoparticle layers.
In the present work we introduce the use of spaced, self-ordered nanotube layers with controllable regular intertube gaps to build defined hierarchical structures, as shown in Fig. 1.
In contrast to the classic (hexagonal close-packed) NTs in Fig. 1c, spaced nanotubes can be precisely coated, layer by layer with nanoparticles, up to an optimized number of layers without clogging the tube's mouth. These structures when used for photocatalytic H 2 production can exhibit a strongly enhanced efficiency compared with any conventional TiO 2 NTs or compared to a plain TiO 2 nanoparticle layer.
The formation of self-organized nanotubular layers with defined intertube gaps takes place in some electrolytes under anodization conditions (outlined in the SI) that establish a controlled spacing, in fact, independent control of tube diameter and intertube distance, can be established by control over the water content in the electrolyte and the applied voltage. After a set of preliminary experiments (see SI) where we evaluated the photocurrent response of nanotubes that have different spacing, we selected nanotubes with spacing in the range of 150-200 nm in order to build up hierarchical structures (see Fig. S1, S2).
The spaced TiO 2 NTs were grown by anodization of Ti in hot triethylene glycol (TEG) based electrolyte, while the reference nanotubes (R-NTs) are grown in ethylene glycol based electrolyte, see Fig. 1 (for more details, see SI). After anodization, these spaced TiO 2 NTs (S-NTs) uniformly cover the surface (Fig. 1a), and have a 220 nm diameter with an average intertube space (wall-to-wall) of approx. 150-200 nm (Fig. 1b). Such a geometry was found to be ideal for a highly conformal layer by layer decoration with TiO 2 nanoparticles (NPs), using a TiCl 4 -hydrolysis approach, [25] as shown in Fig. 1e and Fig. 2: it allows a controlled decoration with up to 5 layers. Each decorated layer is ≈17 nm thick and consists of ≈5-10 nm diameter TiO 2 nanoparticles that homogeneously coat both the interior and exterior surfaces of the nanotube walls, as evident from the SEM images in Fig. 1d and e. After annealing the decorated NTs at 450°C in air, high resolution TEM images (Fig. 1f,g) show that the TiO 2 NPs are interconnected and fully crystallized (anatase (101) crystallographic planes, 3.46 Å lattice spacing) can be identified. Moreover, XRD confirms the anatase phase of the NTs (see Fig. 2.c), while XRD and TEM results together indicate that not only the tubes but also the decorated particles were converted to anatase. [26,27] In order to assess the photocatalytic H 2 evolution performance, layers were decorated with a Pt co-catalyst (1 nm Pt thick, SI) and illuminated with continuous UV light (325 nm) in an ethanol-water mixture (Fig. 1h,i). Fig. 1h shows a comparison of the H 2 amounts evolved from spaced NTs with and without nanoparticle decoration, reference NTs and a nanoparticle (P) film. The H 2 evolution amount from plain S-NTs is already slightly higher than R-NTs, however both are clearly lower than for the defined TiO 2 nanoparticle film on FTO substrate (the nanoparticle film is shown in Figure S4).
The detailed evaluation of the spaced NTs hierarchical structures, with layer by layer TiO 2 nanoparticle decoration, is shown in the SEM images of Fig. 2a. With every additional layer, not only the inner diameter of NTs decreases but also the outer diameter expands. More importantly, the photocatalytic performance increases and maximizes after three layers, then decreases (Fig. 2b); higher amounts of NP loadings led to a drop in H 2 generation as NTs open geometry decreases, thus reducing the surface area as well as light penetration.
However, for S-NTs, after the optimal three times layer by layer TiO 2 NP decoration, a seven times improvement of the photocatalytic H 2 generation is obtained and this hierarchical structure clearly outperforms the nanoparticle layer. If the same multiple decoration treatment is attempted with classic NTs (R-NTs), not only does the second decoration already result in an inhomogeneous morphology but also the nanotubes are blocked with TiO 2 NPs (Fig. S5).
For these R-NTs, the generated H 2 amount is ≈16 µmol h -1 cm -2 , which is half of the hierarchical S-NT structure produced under similar conditions.
When investigating the effect of the length of nanotubes for the three times decorated spaced NTs, a maximum H 2 production rate is registered for a 7 µm tube length (Fig. 1i, Fig.   S6). This is in line with literature, where a thickness of 6-7 µm is found to be an optimum between light absorption and electron diffusion length (previous reports show for NP UV light penetration (325 nm) of 1-3 µm, and diffusion length in TiO 2 NTs of several 10 µm while in particle layers this is only some few µm). [20,28,29] Additionally, a 1nm thick Pt decoration is optimal in view of photocatalytic H 2 generation (see Fig. S7, S8). For these optimized S-NTs, the H 2 generation is linear over extended time, indicating that the structures are stable, see Usually, as-formed S-NTs are amorphous (only titanium peaks are detected) and by 450 °C air annealing, crystallization induces anatase formation (XRD patterns in Fig 2.c). [30] S-NTs were layer by layer coated with TiO 2 nanoparticles and air annealed again at 450 °C (for 10 min) to crystallize the nanoparticles and the hierarchical structures crystallography remains unchanged after the three layers decoration. conventional tubes, several aspects were investigated which include their photocurrent spectra (Fig. 3a), their relative active surface area (Fig. 3b), the relative electron transport time from intensity modulated photocurrent spectroscopy (IMPS) measurements (Fig. 3c) and their relative ability to photocatalytically generate OH radicals (Fig. 3d).
From photocurrent spectra measurements (Fig. 3a), most apparent is that the nanoparticle layer on FTO shows a very low photon to current conversion efficiency compared with the nanotube layers -this is indicative of the high recombination rate of electron-hole pairs in the nanoparticle structure. Clearly, S-NTs show a significantly higher photocurrent magnitude than R-NTs, with the highest values for the three times loading, and nanoparticle decorated S-NTs show also a shift of the wavelength of the maximum IPCE.
In order to determine the surface area of the structures, dye loading measurements ( Fig. 3b) for R-NTs, TiO 2 nanoparticle films, spaced NTs and three times loading spaced NTs (S-NTs-T3) were performed and resulted in 41, 101, 24 and 131 nmol cm -2 , respectively. This shows that the NPs decoration of S-NTs strongly increases their specific surface area, up to 5 times (from dye loadings of 24 to 131 nmol cm -2 ), thus enhancing the active surface area. In fact, the average particle size in the nanoparticle layers (10-15 nm) is larger than the particles obtained from the TiCl 4 layers (5-10 nm), therefore the hierarchical structure shows a higher surface area than the nanoparticle layer. [31] To examine the electronic properties of the structures, IMPS measurements were performed for these selected structures, see Fig. 3c. Clearly, electron transport in the TiO 2 particle layer is significantly slower than that for either bare nanotubes -reference or spaced.
While both types of non-decorated nanotubes (hexagonally-packed and spaced) have a similar electron transport rates, one layer NP decoration of the spaced NTs improves the electron transport time by a decade and the optimal three times decoration shows only slightly faster transport time compared to one layer, while additional NP layers deposition leads to no further improvement. These findings can be ascribed to the fact that the TiCl 4 treatment not only increases the specific surface area but also passivates the defects on TiO 2 NTs, thus enhancing the electronic properties. [32] A classic photocatalytic test for the formation of OH• radicals, i.e. using fluorescence from terephthalic acid under UV light (λ=325 nm) illumination -see Fig. 3d, confirms that under the same conditions in spaced NTs (S-NTs) twice the rate of OH• radicals can be formed than with classical hexagonally packed NTs (R-NTs) -this confirms that not only a strongly improved H 2 formation rate can be achieved using hierarchical NTs but also that in classic photocatalytic reactions (photodegradation, photosynthesis) hierarchical structures are able to generate strongly enhanced amounts of active species.
In summary, the present work shows the fabrication of a photocatalytic platform consisting of hierarchical TiO 2 nanostructures. The approach is based on using highly ordered spaced TiO 2 NTs as a scaffold for a controlled layer by layer TiO 2 nanoparticle deposition.
After optimizing the nanoparticle decoration, hierarchical TiO 2 nanostructures show a significant enhancement of the photocatalytic performance in comparison to the hexagonally close-packed TiO 2 NTs and conventional TiO 2 nanoparticle films. This improvement is ascribed to the combination of fast electron transfer due to the 1D structure and to the high surface area owing to nanoparticle decoration. This also opens a practical route for the deposition of other materials in the spaced TiO 2 NTs, thus using to full advantage such morphology for the development of other state-of-the art applications.