More than protection: the function of TiO 2 interlayers in hematite functionalized Si photoanodes †

Worldwide significant eﬀorts are ongoing to develop devices that store solar energy as fuels. In one such approach, solar energy is absorbed by semiconductors and utilized directly by catalysts at their surfaces to split water into H 2 and O 2 . To protect the semiconductors in these photo-electrochemical cells (PEC) from corrosion, frequently thin TiO 2 interlayers are applied. Employing a well-performing photoanode comprised of 1-D n-Si microwires (MWs) covered with a mesoporous (mp) TiO 2 interlayer fabricated by solution processing and functionalized with a -Fe 2 O 3 nanorods, we studied here the function of this TiO 2 interlayer by high-energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) spectroscopy, along with X-ray emission spectroscopy (XES) and standard characterization techniques. Our data reveal that the TiO 2 interlayer not only protects the n-Si MW surface from corrosion, but that it also acts as a template for the hydrothermal growth of a -Fe 2 O 3 nanorods and improves the photocatalytic eﬃciency. We show that the latter eﬀect correlates with the presence of stable oxygen vacancies at the interface between mp-TiO 2 and a -Fe 2 O 3 , which act as electron traps and thereby substantially reduce the charge recombination rate at the hematite surface.


Introduction
Solar-driven photo-electrochemical (PEC) water splitting is one of the most promising routes for producing green fuels, such as hydrogen. 1,2Efficient PEC devices frequently employ co-catalyst decorated semiconductor photo-electrodes to harvest solar energy and to split water into hydrogen and oxygen.In the search for the most efficient photoelectrode for PEC applications, semiconductors like TiO 2 , 3,4 a-Fe 2 O 3 , 5-7 p/n Si, 8-10 WO 3 , 11 CuO 2 , 12,13 BiVO 4 [14][15][16][17] and Ta 3 N 5 18,19 have been studied since Fujishima and Honda 4 first demonstrated the approach in 1972.
To address potential scalability requirements of the PEC water splitting system, photo-electrodes comprising earth-abundant elements are frequently studied.Manufacturing the components employing solution-based techniques may contribute to further lower the costs of the production process.Examples for earth abundant components employed are silicon (Si) and hematite (a-Fe 2 O 3 ), which under one sun illumination have theoretical solar conversion efficiencies of B43% and B15%, respectively. 20,21However, the stability of Si in the aqueous condition is low, 22 and thus protective layers of, for instance, Ti, 23 Ni, 24 NiO x , 25 or TiO 2 8,22,26,27 have been applied.The thickness of the most frequently used TiO 2 layers varies from B2 to 130 nm. 28It was initially believed that thick TiO 2 films might act as insulators.However, comprehensive studies by the groups of Lewis, 27,28 Tilly, 29,30 and Chorkendorff 31,32 demonstrate that even TiO 2 layers exceeding 100 nm thickness efficiently transfer charges. 29Interestingly, our recent study of variously functionalized p-Si microwires photoelectrodes indicates that their performance in seawater splitting was even enhanced by a B50 nm thick mesoporous TiO 2 interlayer. 8Apparently, the role of the TiO 2 protective layer seems to be more versatile and sophisticated then previously assumed.So far, the role of the TiO 2 protective layer has been primarily studied by photo-electrochemical techniques, 33,34 such as Mott-Schottky measurements 35 and incident photon-to-current efficiency measurements (IPCE), 36 which both give an integral view of the TiO 2 layer.However, there is still a lack of understanding from a fundamental atomic level electronic structure perspective.
Using an all solution-based approach, we prepared an n-Si MWs/mp-TiO 2 /a-Fe 2 O 3 photoanode with highly competitive photo-electrochemical performance and employed it to understand the multiple roles of the TiO 2 interlayer.Our findings suggest that the TiO 2 interlayer not only stabilizes the photoanode, but that the mesoporous (mp) morphology of TiO 2 also acts as a template for the uniform and high surface area hydrothermal growth of the a-Fe 2 O 3 co-catalyst.Furthermore, the presence of the mp-TiO 2 interlayer helps to suppress the exciton charge recombination between n-Si MWs photoelectrodes and co-catalysts, which results in better charge transfer kinetics at the electrode/electrolyte interface for water oxidation.High-energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES), and valence-to-core X-ray emission spectroscopy (vtc-XES) at the Ti K-edge demonstrate significant electronic structural changes.At the same time, the Fe K-edge shows little change indicating that the Fe electronic structure is hardly affected by mesoporous TiO 2 .Our data suggest that the modification of the electronic structure in the TiO 2 interlayer leads to the exceptional photo-electrochemical performance of our functionalized n-Si photoanode during water oxidation.

Results and discussion
Preparation, morphology, and elemental characterization of functionalized n-Si MWs A 1-D microwire structure (n-Si MWs) on the surface of a planar n-Si was obtained by treating small pieces of n-Si wafers (2 Â 2 cm 2 ) with an etching electrolyte comprised of HF and AgNO 3 following a previously published procedure (see ESI, † for further details). 37The resulting n-Si MWs were then spin-coated with a TiO 2 sol-gel, 8,38,39 resulting in n-Si MWs/mp-TiO 2 photoelectrodes.In this process, the block-co-polymer formed spherical micelles that supported the formation of a mesoporous structure of the protective TiO 2 layer (Fig. SI 1, ESI †).The cross-sectional SEM image in Fig. 1a shows that the MW's have a length of about 25 mm and that the TiO 2 coating was complete, yet thin enough not to remove the spacing between the MW.A magnified view of the TiO 2 coating is presented in Fig. 1b.Onto this mesoporous TiO 2 surface, hematite nanorods were hydrothermally grown 40 (see ESI † for further details).The morphology of the resulting n-Si MWs/mp-TiO 2 /a-Fe 2 O 3 is shown in Fig. 1c and d.It can be seen that despite the acidic conditions during the hydrothermal treatment, the MWs remained mostly intact and that a uniform coverage with ca.200 nm long Fe 2 O 3 nanorods were obtained.The excellent coverage of the MW with small, similarly sized a-Fe 2 O 3 nanorods suggests that the mesopores of mp-TiO 2 acted as templates for nanorod formation by serving as nucleation sites (see also below).After the synthesis, the samples were annealed at 380 1C, so that the Fe 2 O 3 nanorods were converted majorly to alpha hematite (a-Fe 2 O 3 ), as confirmed by X-ray photoelectron spectroscopy (see details in Fig. 1e shows that in the absence of the TiO 2 layer, the n-Si MWs were partially dissolved by the acidic iron precursor and that about 5 times larger a-Fe 2 O 3 rods (ca. 1 mm) were obtained that lacked good contact to the n-Si MWs.The increased size of a-Fe 2 O 3 nanorod/crystallites is likely due to the significantly fewer nucleation sites provided by n-Si MWs as compared to n-Si MWs/mp-TiO 2 .
To test if the surface morphology of the TiO 2 interlayer is indeed essential for an even growth of a-Fe 2 O 3 nanorods, we also prepared a photoanode covered by an amorphous TiO 2 layer (n-Si MWs/a-TiO 2 /Fe 2 O 3 ).For this, a commercial titanium diisopropoxide bis(acetylacetonate) solution (diluted with ethanol) was applied using a similar spin-coating cycle as for making mp-TiO 2 . 6,8Due to the absence of a block co-polymer like pluronic P123, which is known to promote mesopore formation, 38,39 and due to the lower viscosity of the commercial precursor solution, the resulting TiO 2 film on the n-Si MWs was amorphous (a-TiO 2 ) and less-even compared to the sol-gel approach.The electron microscopic image in Fig. 1f shows that although the a-TiO 2 interlayer largely protected the n-Si MW during the hydrothermal treatment, fewer Fe 2 O 3 crystallites formed on its surface.In addition, the Fe 2 O 3 crystallites were of varying size and shape, with less good contact to the a-TiO 2 surface as compared to mp-TiO 2 (Fig. 1d).These results further confirm that mp-TiO 2 acts, via its mesopores, as a template for an even and dense hydrothermal growth of wellconnected Fe 2 O 3 nanorods on top of the MWs.This is important for providing a large catalytic surface and for minimizing the charge carrier transport pathways within Fe 2 O 3 . 41,42ased on this structural inspection, we confirmed that mesoporous TiO 2 protects n-Si MWs in acidic media, 27,43 and propose that it also provides essential nucleation sites for the growth of uniform and well-connected Fe 2 O 3 nanorods.

Photo-electrochemical characterization
The PEC performance of the n-Si MWs/mp-TiO 2 /a-Fe 2 O 3 photoanode was systematically compared to various control samples, such as bare planar n-Si, bare n-Si MWs, n-Si MWs/mp-TiO 2 , n-Si MWs/a-Fe 2 O 3 and n-Si MWs/a-TiO 2 /a-Fe 2 O 3 .The photocurrent densities (J ph ) obtained in aqueous 1 M NaOH electrolyte at one sun illumination and an applied potential of 1.23 V RHE (using a Pt counter electrode) are shown in Fig. 2 (see ESI † for further details).The corresponding I/V curves are displayed in Fig. SI 3 and SI 4 (ESI †).Fig. 2 shows that for bare planar n-Si the J ph was 0.5 mA cm À2 , and that J ph increased to 0.8 mA cm À2 for bare n-Si MWs, reflecting the expected improved light absorption and charge-transfer capabilities of the microwire morphology. 8Interestingly, mp-TiO 2 sol-gel coating of the n-Si MWs led to a further increase of J ph to 1.01 mA cm À2 .Most importantly, we noted a significant rise in J ph to 4 mA cm À2 (AE0.02mA cm À2 ) when the n-Si MWs/mp-TiO 2 were additionally decorated with a-Fe 2 O 3 nanorods.In the absence of the protective TiO 2 layer, the photocurrent dropped to 1.2 mA cm À2 for n-Si MWs/a-Fe 2 O 3 , while the n-Si MW/a-TiO 2 /a-Fe 2 O 3 photoanode reached a photocurrent of 2.5 mA cm À2 , still clearly below the values achieved with a mp-TiO 2 protection layer.
To confirm solar-assisted water splitting, the photogenerated gases were analyzed by sampling the headspace with gas chromatography during a chronoamperometry (at 1.23 V RHE ; Fig. SI 5, ESI †).After a short onset period, a nearly stable photocurrent of about 4 mA cm À2 was observed for the n-Si MW/mp-TiO 2 /a-Fe 2 O 3 photoanode over the entire sampling period of 2 hours.The accumulated amounts of photogenerated hydrogen and oxygen collected from the headspace of the PEC cell were 4.33 and 2.2 mmol cm À2 (Fig. SI 5b, ESI †), corresponding to faradaic efficiencies of 94% and 97%, respectively.
The TiO 2 interlayer also significantly lowered the onset potential, which we defined here as the potential at which the net photocurrent density reached 0.1 mA cm À2 .The value for n-Si MW/mp-TiO 2 /a-Fe 2 O 3 was only 0.27 V RHE (Fig. SI 3b, ESI †), which is, to the best of our knowledge, the lowest reported onset potential for an undoped a-Fe 2 O 3 nanorod based photoanode.In the absence of the mp-TiO 2 interlayer, the onset potential was 540 mV higher (0.81 V RHE ; Fig. SI 3b, ESI †).The onset potential for an electrode comprising similar a-Fe 2 O 3 nanorods grown on FTO substrate with an a-TiO 2 interlayer was found to be 0.77 V RHE . 6,44The above experiments indicate that the low onset potential and high J ph reported here may come from having a dual absorber PEC system made of uniformly  sized Fe 2 O 3 nanorods that are well connected to the n-Si MWs via mp-TiO 2 .
To test if the bandgaps of both Si and Fe 2 O 3 contribute to the light absorption and thus J ph of the n-Si MW/mp-TiO 2 /a-Fe 2 O 3 photoelectrode, this multilayer system was selectively excited at either 455 nm, to probe the contribution of the a-Fe 2 O 3 nanorods, or at 625 nm for testing the performance if only the n-Si MWs absorbed light (Fig. SI 6, ESI †).Employing 455 nm illumination at 100 mW cm À2 , we found that at 1.23 V RHE J ph was 2.4 mA cm À2 , which was almost 60% of the photocurrent obtained at 1 sun illumination (Fig. SI 6, ESI †).This indicates that under these experimental conditions the a-Fe 2 O 3 nanorods were not simply dark OER catalyst, but instead acted as photo absorbers, while n-Si MWs functioned mainly as a conductive substrate.However, we cannot exclude the possibility that some photons were absorbed also by Si at 455 nm. 45Importantly, essentially no dark currents were observed for the n-Si MWs/ mp-TiO 2 /a-Fe 2 O 3 photoanode up to at least a potential of 2.5 V RHE (Fig. SI 3c, ESI †).Next, we illuminated our photoanode at 625 nm (100 mW cm À2 ) to selectively excite Si.At 1.23 V RHE J ph was found to be 3.7 mA cm À2 (Fig. SI 6a and b, ESI †), which is 93% of the photocurrent obtained with 1 sun illumination (4.0 mA cm À2 ).
The capacity of the n-Si MWs/mp-TiO 2 /a-Fe 2 O 3 photoanode to act as a dual solar absorber system became more apparent when the same experiment was repeated at 2.5 V RHE .Here, the J ph reached 20 mA cm À2 under 1 sun illumination.The photocurrent was 20% lower if only the n-Si MWs were excited, and nearly 40% lower if predominantly the a-Fe 2 O 3 nanorods were excited (Fig. SI 6, ESI †).We thus conclude that our n-Si MWs/ mp-TiO 2 /a-Fe 2 O 3 photoanode acts as dual absorber system, which is in line with a recent study on another hematite/ Si nanowire PEC system. 45This is schematically illustrated in Fig. 3.
For comparison, Xiaopeng Qi et al. 46 reported an n-Si/ a-Fe 2 O 3 core-shell structure with 25 mm long Si MWs with a J ph of 5.28 mA cm À2 at 1.23 V RHE and an onset potential of 0.5 V RHE recorded with a scan rate of 50 mV s À1 .For the scan rate of 20 mV s À1 used in this study, Xiaopeng Qi et al. 46 reported a J ph of B3.8 mA cm À2 at 1.23 V RHE with an onset potential of 0.5 V RHE , which is almost twice the onset potential reported in this study.Recently, Zhongyuan Zhou et al. 47 achieved a maximal J ph of 3.12 mA cm À2 at 1.23 V RHE and an onset potential of 0.15 V RHE for a Sn-doped hematite film grown on Si MWs, whereas their pristine n-Si/a-Fe 2 O 3 photoanode had an onset potential of 0.85 V RHE .
Insight into the mechanism by which TiO 2 improves the performance of photoanodes can be gained by studying the transient photo-response of a-Fe 2 O 3 functionalized n-Si MWs without (Fig. 4a and b) and with (Fig. 4c and d) the mp-TiO 2 interlayer.The transient photocurrent density at 1.5 V RHE (2-electrode set-up) was nearly three-fold higher for n-Si MW/ mp-TiO 2 /a-Fe 2 O 3 (6 mV cm À2 ) than for n-Si MW/a-Fe 2 O 3 (2.2 mV cm À2 ), which is fully consistent with the data in Fig. 2. The sharp spikes visible for both samples in the anodic (positive-spike) and cathodic (negative-spike) regions arise due to the light-induced generation and the dissipation of charge carriers at the electrode/electrolyte interface.When zooming into the regions of the anodic spikes, it is seen that nearly twice as many charges dissipated in the device lacking the mp-TiO 2 layer, and that this process occurred over a significantly longer time period (Fig. 4b and d).Thus, the presence of a mp-TiO 2 interlayer appears to block the mobility of the holes photogenerated in a-Fe 2 O 3 towards the n-Si MWs, since the valence band of mp-TiO 2 lies at lower energy than in a-Fe 2 O 3 (Fig. 5b).Assuming the same exciton generation with and without the TiO 2 layer (as the thin TiO 2 layer is optically transparent), the addition of the wide bandgap TiO 2 interlayer thus reduced unproductive charge recombination and thereby allowed  In summary, besides the well-known function of the TiO 2 protection layer in passivating the surface defects on Si MWs, 8,32,33 we tentatively assign the superior performance of n-Si MW/mp-TiO 2 /a-Fe 2 O 3 as compared to n-Si MW/a-Fe 2 O 3 to a better interfacial connection between mp-TiO 2 and a-Fe 2 O 3 , the creation of stable oxygen vacancies (electron traps) in the mp-TiO 2 interlayer, and to the low band edge positioning of TiO 2 that likely suppresses the hole mobility from a-Fe 2 O 3 towards n-Si. 32

Oxygen vacancies
9][50][51] They establish that oxygen vacancies are generated by high-temperature annealing (B400 1C) of TiO 2 in oxygen-deficient atmospheres.This process can be described using the Kro ¨ger-Vink notations: [51][52][53] Eqn ( 1) is a reversible reaction, meaning that the surface oxygen vacancies disappear gradually in ambient conditions (in the presence of molecular oxygen).Early studies from Gra ¨tzel and co-workers, 49,54 and Hoffmann and co-workers 50 suggested stabilization of surface oxygen vacancies in the nanoparticulate TiO 2 lattice by the introduction of acceptor type dopants such as Fe 3+ as ionic charge-compensating species represented by eqn (2): Previous studies have already correlated the presence of oxygen vacancies in TiO 2 with enhanced photocatalytic properties of TiO 2 . 48,52Additionally, increasing the active surface area of TiO 2 particles by reducing their size has also been employed for enhancing photocatalytic properties of TiO 2 . 55,56As mp-TiO 2 thin films are known for having an about 800 times larger effective surface area than planar TiO 2 thin films, 48,57,58 the incorporation of Fe ions onto mp-TiO 2 thin films may prove to be an effective strategy for stabilizing oxygen vacancies, and to thereby enhance photocatalytic water oxidation efficiency.In our study, the Fe ions were incorporated onto the surface layers of mp-TiO 2 during the hydrothermal growth of a-Fe 2 O 3 nanorods onto the surface of mp-TiO 2 and the subsequent annealing at 380 1C in an oxygen-deficient atmosphere.
High energy resolution fluorescence detected X-ray absorption near edge structure (HERFD-XANES) and valence to core X-ray emission spectroscopy (vtc-XES) To provide further evidence for the oxygen vacancies and to study what electronic structure changes they cause in TiO 2 , we performed HERFD-XANES and X-ray emission spectroscopy at beamline ID26 of the European Synchrotron Radiation Facility (ESRF).HERFD-XANES spectra were obtained at the Ti K-edge for bare mp-TiO 2 , mp-TiO 2 coated on planar n-Si (planar n-Si/ mp-TiO 2 ), n-Si MWs/mp-TiO 2 , and n-Si MWs/mp-TiO 2 /a- By contrast, the Ti K-edge HERFD-XANES data reveal that the hydrothermal growth of a-Fe 2 O 3 nanorods onto mp-TiO 2 strongly influences the electronic structure of mp-TiO 2 , as shown in Fig. 6a and b.For Ti, the XANES pre-edge peaks in the energy range of B4966 to B4976 eV reflect core electron transition to Ti-3d4p hybridized states, which are highly sensitive to the local geometry of the Ti centers. 59,60The pre-edge peak A1 corresponds to the transition of a Ti 1s electron into a t 1g band-like state.The small low energy shoulder on the A2 peak (A2*) is due to 1s-e g excitation, while the 1s electron This journal is © the Owner Societies 2020 excitation to crystal field split d-states on the neighboring Ti atoms results in the main A2 and the A3 peaks. 61,62The A3 peak corresponds to an e g band-like state, which is due to pure non-local excitation.The non-local excitations are mediated by oxygen nearest neighbors. 63,64][67][68] Comparison of the pre-edge features of the n-Si MWs/mp-TiO 2 photoelectrodes with (red line) and without (grey line) a-Fe 2 O 3 nanorods (Fig. 6b) reveals that the A2* peak becomes significantly more prominent when a-Fe 2 O 3 nanorods are present.At the same time, the other features are unchanged, indicating no significant structural changes.As the A2* peak arises from a 1s-e g excitation, the presence of a stronger A2* peak in n-Si MWs/mp-TiO 2 /a-Fe 2 O 3 is attributed to the creation of oxygen vacancies at the mp-TiO 2 and a-Fe 2 O 3 interface that leads to a distortion of the local coordination which increases the mixing of metal p and d orbitals. 48The absence of a shift of the main K-edge shows that the Ti-O distances, on average, do not change, 68,69 indicating that the bulk of the Ti remains in oxidation state Ti 4+ .
In line with our results, Ghaffari et al. 70 reported that the increase in the Ti K-edge A2* pre-edge peak intensities correlates with improved photocatalytic properties of their SrTi (1Àw) Fe w O (3Àd) system, which are caused by oxygen vacancies or defects.As the vacancies were created during the hydrothermal growth of a-Fe 2 O 3 nanorods and subsequent annealing at 380 1C, we propose that they are restricted to the interface between mp-TiO 2 and a-Fe 2 O 3 .The band-bending physics presented in Fig. 5b shows that these oxygen vacancies could trap part of the electrons photogenerated in the a-Fe 2 O 3 nanorods.This would then effectively inhibit the recombination of charge-carriers in the a-Fe 2 O 3 nanorods, leaving the photogenerated holes available for enhanced water oxidation.
To comprehend further the changes in the electronic structure of the mp-TiO 2 interlayer caused by growing Fe 2 O 3 nanorods onto it, X-ray emission spectroscopy (XES) was performed at the Ti and Fe K-edges.The Kb fluorescence lines for Ti are shown in Fig. 7, while the Fe Kb-XES is displayed in Fig. SI 10 (ESI †).Fig. 7b shows that the 3p to 1s core-shell-tocore-shell (Kb 1,3 ) line of Ti is invariant to the presents of Fe 2 O 3 nanorods, and the same is observed for the corresponding Fe signal (Fig. SI 10b, ESI †).As Kb 1,3 lines are known to be sensitive to the local spin and thus the oxidation state, the absence of a detectable change indicates that the local distortion of the oxygen environment does not involve redox changes of Ti 4+ or Fe 3+ , in line with the observation in the XANES data.The valence-to-core emission lines at higher energies (Fig. 7c and d) arise from metal-ligand mixed orbitals and directly probe the ligand environment.Here we do observe changes for Ti, in agreement with the K absorption pre-edge spectral changes.It is important to note that the maximum amount of TiO 2 that can interact with a B20 nm diameter a-Fe 2 O 3 nanorods is B20%; for detailed calculation, see the ESI.† The percentage of Ti atoms changing the local coordination as derived from the spectral changes in Fig. 6 is in approximate agreement with the estimated interaction of mp-TiO 2 with a-Fe 2 O 3 nanorods.Thus, we suggest that the oxygen vacancies created in mp-TiO 2 are restricted to the interfacial boundary between mp-TiO 2 and the a-Fe 2 O 3 nanorods.][73][74] We found that electronic structural changes take place in the mp-TiO 2 when sandwiched between n-Si MW and a-Fe 2 O 3 , while the Fe-edge hardly changed.This indicates that the improved photocatalysis arises from improved carrier transport within the device rather than a lowered barrier for water splitting at a-Fe 2 O 3 .This is consistent with our recent finding that p-Si MWs/TiO 2 photocathodes undergo significant changes when decorated with NiO x or CoO x . 8Thus, our study paves the way towards understanding the crucial role of the TiO 2 interlayer by providing X-ray spectroscopic evidence for the stabilization of oxygen vacancies by the application of

Conclusions
In conclusion, we report here a dual solar photon absorber system comprised of n-Si MWs and a-Fe 2 O 3 .The n-Si MWs and the a-Fe 2 O 3 were connected by a thin mp-TiO 2 layer, which acted as a template for the growth of a-Fe 2 O 3 nanorods.In the absence of the mp-TiO 2 interlayer, the overall photoelectrochemical performance of a-Fe 2 O 3 functionalized n-Si MWs was significantly reduced.Favorable band bending in the n-Si MWs/mp-TiO 2 /a-Fe 2 O 3 photoanodes was found to be the critical factor for decreasing the charge recombination and thus increasing the availability of holes for water oxidation.A thorough investigation using X-ray spectroscopy suggests that the hydrothermal growth of a-Fe 2 O 3 nanorods on n-Si MWs/mp-TiO 2 with subsequent annealing at 380 1C led to favorable changes in the electronic structure of mp-TiO 2 .By contrast, the electronic structure of a-Fe 2 O 3 was hardly affected by mp-TiO 2 .In summary, the mp-TiO 2 interlayer presented in this study acts (i) as a protective layer for n-Si MWs, (ii) as a template for the uniform and high surface area hydrothermal growth of the a-Fe 2 O 3 nanorods, and (iii) in combination with Fe 2 O 3 , it suppresses charge recombination at the mp-TiO 2 /a-Fe 2 O 3 interface via stabilized oxygen vacancies (electron traps).
We expect that, in the future, this multi-functionality of the TiO 2 interlayer can be utilized in developing further improved photoelectrodes.As exemplified here, the detailed X-ray spectroscopic characterization of the photoelectrodes will be a crucial analytical tool in this process.

Fig. 2
Fig. 2 Net photocurrent responses for bare and functionalized n-Si photoelectrodes at 1.23 V RHE .

Fig. 3 A
Fig. 3 A schematic representation of energy diagrams and charge-carrier mobility in the n-Si MW/mp-TiO 2 /a-Fe 2 O 3 photoelectrode.
Fe 2 O 3 (Fig. SI 7, ESI †).HERFD-XANES spectra at the Fe K-edge were recorded for bare a-Fe 2 O 3 , n-Si MWs/a-Fe 2 O 3 , and n-Si MWs/ mp-TiO 2 /a-Fe 2 O 3 (Fig. SI 8 and SI 9, ESI †).All spectra were normalized to their total area.The spectra were not corrected for over-(or self-) absorption effects that may significantly distort the spectra of the bare oxides.The spectra are thus also presented as first derivatives in order to facilitate comparison.The Fe K-edge HERFD-XANES taken of all samples are essentially superimposable (Fig. SI 8 and SI 9, ESI †), indicating that neither the deposition of Fe 2 O 3 onto mp-TiO 2 nor Si MWs had a significant effect on the electronic structure of a-Fe 2 O 3 .Similarly, the first derivatives of the Ti K-edge HERFD-XANES spectra of bare mp-TiO 2 and n-Si MWs/mp-TiO 2 are nearly identical (Fig. SI 7, ESI †).This indicates that the contact with the n-Si MWs did not induce significant electronic structural changes in the TiO 2 lattice.