Hematite Microwire Photoanode by the

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Regulating the Silicon/Hematite Microwire Photoanode by Conformal Al2O3 Intermediate Layer for Water Splitting Zhongyuan Zhou, Shaolong Wu, Liujing Li, Liang Li, and Xiaofeng Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18681 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 22, 2019

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ACS Applied Materials & Interfaces

Regulating the Silicon/Hematite Microwire Photoanode by Conformal Al2O3 Intermediate Layer for Water Splitting Zhongyuan Zhou,†,‡ Shaolong Wu,*,†,‡ Liujing Li,†,‡ Liang Li,†,‡ and Xiaofeng Li*,†,‡ †School

of Optoelectronic Science and Engineering & Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215006, China ‡Key Lab of Advanced Optical Manufacturing Technologies of Jiangsu Province & Key Lab of Modern Optical Technologies of Education Ministry of China, Soochow University, Suzhou 215006, China Corresponding Author: *E-mail: [email protected]; [email protected]

ABSTRACT The dual-absorber photoelectrodes have been proved to possess greater potential than the single-absorber systems in the applications of photoelectrochemical (PEC) cells (e.g., solar-driven water splitting); however, the mismatching of the energy bands and substantial carrier recombinations at the two absorber interfaces are normally subsistent. Here, we introduce an intermediate layer of conformal Al2O3 into the silicon/hematite (Si/α-Fe2O3) microwire photoanode for enriching the understanding of the interaction among the interlayer, the inner and outer absorbers. Our results show that the Si/Al2O3/α-Fe2O3 microwire photoanode with the thickness-optimized Al2O3 can lead to a substantially increase in the photocurrent from 0.83 mA/cm2 to 2.08 mA/cm2 at 1.23 VRHE (under one sun irradiation) and an obvious decrease in the onset potential relative to the counterpart without Al2O3. Through analyzing the PEC responses under various monochromatic lights, photoelectrochemical impedance spectroscopy and 1 ACS Paragon Plus Environment

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intensity-modulated photocurrent spectroscopy, we ascribe the improvements to that the suitable-thickness Al2O3 can passivate the Si microwire surfaces and the bottom surfaces of α-Fe2O3 film and give rise to Al doping into the post-synthesized α-Fe2O3. These essential causes promote the carrier separation in α-Fe2O3, diminish the photoanode surface recombination rate, and then increase the surface charge transfer efficiency.

KEYWORDS dual-absorber photoanode, solar water splitting, interfacial layer, carrier dynamics, surface kinetics

INTRODUCTION Photoelectrochemical (PEC) water splitting is regarded as an attractive way for harvesting solar energy in the form of hydrogen;1–3 nevertheless, the photoanode is the bottleneck for solar-driven water splitting due to that the water oxidation with a 4electron process is the rate-limiting step of the two half-reactions.4–6 Numerous materials have been employed to construct the photoanode (e.g., Si, BiVO4, WO3 and α-Fe2O3),7–10 however, the photo-voltage generated from a single absorber layer is mostly lower than the required voltage (i.e., 1.23 V) for thermodynamic water splitting. Therefore, a wide-bandgap semiconductor coupled with a narrow-bandgap semiconductor has been proposed to reach the PEC water splitting with a relatively low (or even no) applied bias.11–13 For example, WO3/BiVO4 hybrid photoanode have been 2 ACS Paragon Plus Environment

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intensively investigated due to the type II alignment of energy band formed between the two absorbers; however, the reported onset potentials for the WO3/BiVO4 photoanodes were still much larger than 0 VRHE.14–16 Lewis et al. prepared np+-Si/WO3 core/shell microwire array photoelectrodes, which achieved the unbiased water splitting due to the additional photo-voltage from the embedded Si photovoltaic cell.17 Recently, dual-absorber silicon/hematite (Si/α-Fe2O3) system was proposed considerably as a potential candidate for the PEC water splitting photoanode due to the following 3 factors.18–23 First, Si and α-Fe2O3 are low-cost and environment-friendly photoactive materials. Second, the conduction band edge of Si is higher than the H2O/H2 potential in energy band diagram, and the valence band edge of α-Fe2O3 is lower than the H2O/O2 potential, so it is thermodynamically possible to realize the overall light-driven water splitting.18 Third, the embedded Si/α-Fe2O3 heterojunction is able to produce a photo-voltage which improves the hole separation efficiency of αFe2O3 and charge transfer kinetics at photoanode surfaces.20 However, these interfacial issues (e.g., the mismatching of the energy bands and the charge recombination or trapping via the interface defects) are inherent in the Si/α-Fe2O3 photoanode, as yet not been specially addressed in literatures.19 Significant improvement in PEC performance of the dual-absorber photoanodes would be anticipated if these unfavorable factors can be satisfactorily resolved.21–23 The electronic, structural, and morphological characteristics of the photoanodes play a vital role in the PEC water splitting, and the efficient charge transport and transfer among the conductive layer, the light absorption layers, and the electrolyte are 3 ACS Paragon Plus Environment


decisive.24–27 Consequently, the investigations on interface engineering of photoanodes was extensively performed, such as using a catalyst layer to increase the surface charge transfer kinetics,28–30 a passivation layer to suppress the surface charge recombination rate,31–33 and an underlayer to block the undesired carrier transport.34–36 Wang et al. systematically investigated the working mechanisms of NiFeOx catalyst coated on αFe2O3 photoanode, and deemed that the catalyst can improve the PEC performance from the reduced surface recombination rate, rather than the increased charge transfer rate.28 Gratzel et al. introduced an oxide underlayer (e.g., Nb2O5 and Ga2O3) between the αFe2O3 and conductive substrate, and found that the underlayer was functioned to suppress the electron back injection, which results in charge recombination around the α-Fe2O3/substrate interfaces.34 Currently, there are a few studies on engineering the intermediate interface for dual-absorber photoelectrode. For instance, Lewis et al. designed and constructed the np+-Si/FTO/TiO2 microwire photoanode, where the fundamental function of the used FTO intermediate layer has been verified to supply low resistance and ohmic behavior between p-Si and TiO2.37 Gong et al. uncovered that an ultrathin Al2O3 layer inserted between the protective layer of TiO2 and the CdS absorber can passivate the CdS/TiO2 interfacial defects.38 Although two absorbers (i.e., CdS and Cu(In,Ga)Se2) are employed in Gong’s report, the Al2O3 interlayer is not located between the two absorbers, and the influence of the Al2O3 interlayer on the Cu(In,Ga)Se2 film is not included. According to our knowledge, the investigations on an intermediate layer between two absorbers are rare, and the understanding of the interaction among the intermediate layer, the upper and lower absorbers is also rather 4 ACS Paragon Plus Environment

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limited.39–42 In this work, we introduced an intermediate layer of conformal Al2O3 between the prepared Si microwire (MW) and the post-synthesized α-Fe2O3 film. We observed that the photocurrent density of the prepared Si/Al2O3/α-Fe2O3 MW photoanode with the thickness-optimized Al2O3 at 1.23 VRHE can be enhanced to 2.08 mA/cm2 from 0.83 mA/cm2 relative to the original Si/α-Fe2O3 MW photoanode, along with the onset potential decrease of 0.21 V. The influences of various-thickness Al2O3 on the carrier transport of the two absorbers are discussed via the PEC measurements under the ultraviolet and red monochromatic lights, PEC impedance spectroscopy (PEIS) and intensity-modulated photocurrent spectroscopy (IMPS) kinetics analysis. We find that a suitable-thickness Al2O3 can passivate the Si MW surfaces and the bottom surfaces of α-Fe2O3 film, as well as promote the carriers transport in α-Fe2O3 due to Al doping. Moreover, the surface charge transfer efficiency of the Si/Al2O3/α-Fe2O3 MW photoanode can be increased mainly by reducing the surface charge recombination rate.

METHODS Silicon microwire preparation. All the corresponding reagents and experimental materials were of analytical grade and were used without any further purifications. The SiMWs were etched from single crystalline n-Si (100) wafers (0.01–0.02 •cm). The main sequential progresses of the fabrication include: 1) obtaining photoresist patterns by lithography, 2) duplicating the reverse patterns of the photoresist with Au/Ti bilayer, and 3) metal-assisted chemical etching. The related mechanism and detailed conditions 5 ACS Paragon Plus Environment


can be found elsewhere.43,44 Conformal Al2O3 coating. The intermediate layer of conformal Al2O3 was coated on the prepared SiMWs at 250 °C by atomic layer deposition (ALD, Labnano 9100, Ensure Nanotech) method. Trimethylaluminium (TMA) and H2O were used as the precursors in a 30 mbar N2 flow atmosphere. The typical dose durations for TMA and H2O were 0.025 s and 0.025 s, the stay durations were 5 s and 5 s, and the purge durations were 15 s and 15 s, respectively. The Al2O3 thickness for each ALD cycle is around 0.08 nm. The final thickness of Al2O3 was exactly controlled by setting the number of ALD cycles. α-Fe2O3 synthesis and photoanode preparation. The α-Fe2O3 film was grown on the ordered SiMWs with or without Al2O3 coating by thermal decomposition of Fe(NO3)3, detailed experimental conditions can be found elsewhere.22,23 The photoanode was prepared as follows. 1) The rear side of the SiMW substrate was first coated by In-Ga thick film. 2) Then a conductive wire was attached to the In-Ga layer. 3) Silica gel (Nanda 703) was finally used to wrap the entire samples except for the front side with an active region (~0.5 cm2). Structural, material, and optical characterizations. Material and structural characteristics were carried out by field-emission scanning electron microscope (SEM) on Hitachi S4800. Transmission electron microscope (TEM) and energy dispersive Xray spectroscopy (EDS) were performed on a FEI Tecnai G2 F20 S-Twin. The samples for cross-sectional TEM analysis were prepared via ultrathin section processing (Leica EM UC7-FC7, LEICA). The phase purity was performed by X-ray diffraction (XRD, 6 ACS Paragon Plus Environment

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MRD X‘Pert-Pro) with Cu-Kα radiation. The molecular structure analyses were carried out by X-ray photoelectron spectroscopy (XPS, 250Xi ESCALAB) and Raman spectroscopy (HR800 LabRAM). Reflectance spectra (UV-3600, SHIMADZU) and photoluminescence (PL) spectra (FLS920, Edinburgh Analytical instrument) were also performed. Photoelectrochemical response measurements. PEC measurements and analysis were performed in a three-electrode configuration with an electrochemical workstation (CIMPS, Zennium Zahner). The working electrode, counter electrode and reference electrode are the freshly-prepared Si/Al2O3/α-Fe2O3 MW photoanodes, the Pt mesh and the Ag/AgCl, respectively. 1.0 M NaOH was employed as the electrolyte. The potential versus reversible hydrogen electrode (RHE) can be get based on the Nernst equation.45 All the current-potential (J-U) behaviors were measured with a scan rate of 20 mV/s in the anodic direction. One sun irradiation was obtained via a xenon light equipped with an AM 1.5G filter (SS-F7-3A, Enlitech). Besides, the ultraviolet (s/n Ls 1272, Zennium Zahner) and red (s/n Ls 1273, Zennium Zahner) monochromatic lights with modulating power density were used. The corresponding central wavelengths were 365 nm and 620 nm, respectively. Electrochemical analysis. The Mott-Schottky curves under the fixed frequency of 1 kHz were acquired under no irradiation. PEIS plots was acquired under one sun irradiation. The alternate current (AC) perturbation is 10 mV in amplitude, and the frequency is swept from 100 KHz to 0.1 Hz under the selected constant-potential conditions. The numerical fitting of the Nyquist plots was carried out using the Z7 ACS Paragon Plus Environment


viewer software. The charge-extraction measurement was based on the same electrochemical workstation. A LED (WLC02, s/n Ls 1271, Zennium Zahner) with a power intensity of 800 W/m2 was used as the irradiation source. The target voltage and discharge current were 0.25 V (vs. Ag/AgCl) and 100 μA, respectively. The light-on time, light-off time and discharge time were 2 s, 1 s, and 7 s, respectively. IMPS were conducted in the same three-electrode configuration as that of the PEC response measurements to probe surface kinetics of photoelectrodes with multi-step charge transfer reaction. A 365 nm wavelength LED (s/n Ls 1272, Zennium Zahner) was employed as the irradiation source with the power intensity of 45 W/m2. For IMPS measurements, ±5 % modulation intensity around 45 W/m2 was used, and the frequency range was from 3 KHz to 0.1 Hz. The surface-charge transfer and recombination rate constants are extracted from the Nyquist plots (imaginary vs. real components of the normalized photocurrent, IIM vs. IRE) based on the technique pioneered by Peter et al.46

RESULTS AND DISCUSSION

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Figure 1. Typical SEM (a–b) and TEM (c–g) images of the prepared Si/Al2O3-300/αFe2O3 MWs. (a) is the oblique-view SEM image. (b) is the cross-section image of the ground floor of the microwires, and the inset image is the top-view high magnification SEM image with a scale bar of 200 nm. (c) TEM shows the thicknesses of Al2O3 and α-Fe2O3. (d) and (e) are the HRTEM of the boundaries of Si/Al2O3 and Al2O3/α-Fe2O3, respectively. The STEM area was marked in orange box of (f) and EDS elements mapping were shown in (g). The ordered SiMWs are vertically aligned with rough surfaces, and the diameter, period and length are ~4 m, ~8 m and ~25 m, respectively (Figure S1a and b). The overall morphology of the Si/α-Fe2O3 MWs is the same as that of the bare SiMWs. There are some pinholes in the α-Fe2O3 surfaces (Figure S1c and d), which are from the employed growth method.23 In contrast, the surfaces of the Al2O3-coated SiMWs are relatively smooth simultaneously sustaining the original shape of the bare SiMWs



(Figure S1e and f), and the surfaces of Si/Al2O3/α-Fe2O3 MWs are also relatively smooth (Figure S1g and h) compared to the Si/α-Fe2O3 MWs. Figure 1a shows that the original shape of SiMWs is sustained after sequentially coating Al2O3 and α-Fe2O3 films. The distinct multi-layers of Si, Al2O3 and α-Fe2O3 can be confirmed by Figure 1b, and the insert image shows that the synthesized α-Fe2O3 film is formed by the compacted nanoparticles. XRD pattern (Figure S2), Raman spectrum (Figure S3) and XPS spectrum (Figure S4) jointly verify the achievement of α-Fe2O3 film, and the introduction of Al2O3 interlayer doesn’t cause the phase change of hematite. In order to view the layered structure more clearly, we employed the TEM analysis. Figure 1c–g shows the TEM images of Si/Al2O3/α-Fe2O3 MWs, where the Al2O3 film was obtained with 300 ALD (atomic layer deposition) cycles. One can see that the Al2O3 interlayer is dense with a nominal thickness of 23 nm, and the outer layer of α-Fe2O3 is formed by accumulation of nano-crystalline with a thickness of 76 nm. High-resolution TEM (HRTEM) images show the obscure boundaries between Si and Al2O3 (Figure 1d) as well as between Al2O3 and α-Fe2O3 (Figure 1e). Note that the morphology of the αFe2O3 nanocrystal grown on the SiMWs with or without Al2O3 has some differences. There is a uniform α-Fe2O3 film (62 nm) for the Si/α-Fe2O3 MWs from the TEM analyze.23 The crystallinity of the coated hematite is obviously deteriorated in micro zones. The layered structure of the Si/Al2O3/α-Fe2O3 can be verified by STEM images (Figure 1f and g; Figure S5). One may note that the core of the MW in Figure 1f is fragmentary, which is from the ultrathin section processing for the preparation of the TEM-analysis samples. 10 ACS Paragon Plus Environment

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Figure 2a shows the J-U curves for the Si/Al2O3/α-Fe2O3 MW photoanodes with different thicknesses of Al2O3. The advantages of using dual absorbers (i.e., SiMWs and α-Fe2O3 film) over the single absorber of SiMWs or α-Fe2O3 film have been demonstrated in our previous work.23 The Si/α-Fe2O3 MW photoanode shows a significant PEC response, and the photocurrent density at 1.23 VRHE ([email protected] V) and onset potential (Uon) are 0.83 mA/cm2 and 0.81 VRHE, respectively. With introducing the Al2O3 interlayer, the J-U curves show substantial and various shifts, i.e., [email protected] V first increases and then decreases with the increasing thickness of Al2O3; meanwhile Uon shifts first to the cathodic direction and then to the anodic direction. Among various Al2O3 thicknesses, the case with 100 ALD cycles is the champion with [email protected] V = 2.08 mA/cm2 and Uon = 0.60 VRHE, and the case with 300 ALD cycles shows the worst performance (i.e., [email protected] V = 0.22 mA/cm2 and Uon = 1.13 VRHE). Another thing to note is that the champion Al2O3 with an apparent thickness of ~7 nm is grown prior to coating the hematite film. However, during the anneal treatment for the hematite film, the Al and Fe elements can diffuse to the layers of α-Fe2O3 and Al2O3, respectively. So the actual thickness of pure Al2O3 layer is less than the nominal thickness.34,36 To confirm the superiority of the Al2O3 interlayer in the dual-absorber configuration, the Al2O3 film from 100 ALD cycles was grown on the SiMWs without α-Fe2O3 (i.e., Si/Al2O3 MW photoanode). The observed PEC response was shown in Figure S6, one can see that the corresponding [email protected] V is 0.65 mA/cm2 and Uon is 0.85 VRHE, which are much inferior to those of the Si/Al2O3/α-Fe2O3 MW photoanode with 100 ALD cycles. To understand the effect of the Al2O3 interlayer, we examined the properties of 11 ACS Paragon Plus Environment


three selected cases with 0, 100, and 300 ALD cycles, which are denoted as Si/α-Fe2O3, Si/Al2O3-100/α-Fe2O3, and Si/Al2O3-300/α-Fe2O3 MW photoanodes, respectively. Time-resolved currents of the Si/α-Fe2O3, Si/Al2O3-100/α-Fe2O3, and Si/Al2O3300/α-Fe2O3 MW photoanodes were obtained at 1.23 VRHE under ON-OFF cycle of one sun irradiation (Figure S7). Transient photocurrent for the Si/Al2O3-100/α-Fe2O3 MW photoanode is always the largest, and the photocurrent relationship among the three cases is consistent with the results in Figure 2a. When the light is switched ON, the photocurrent shows a sharp spike, which can be explained by the rapid charging of surface or interface states.47 A reduction of the photocurrent with continuous irradiation can be attributed to that these carriers at surface or interface states not totally transfer into electrolyte and some are recombined or trapped by defects. The photocurrent attenuation ratio (defined as (Jhole-Jss)/Jhole,48 where Jhole is the instantaneous photocurrent and Jss is the steady-state photocurrent) with the same duration of irradiation for the Si/α-Fe2O3, Si/Al2O3-100/α-Fe2O3 and Si/Al2O3-300/α-Fe2O3 MW photoanodes are 16.7%, 7.3% and 22.8%, respectively. One can see that Si/Al2O3100/α-Fe2O3 MW photoanode has the minimal attenuation ratio, meanwhile, the Si/Al2O3-100/α-Fe2O3 MW photoanode has the maximal charge accumulation densities (Figure S8), which implies that the thin Al2O3 can efficiently suppress the carrier recombination at the surfaces and interfaces of the hybrid photoanode, whereas the thick Al2O3 would increase the carrier recombination. The photostability performance of the Si/Al2O3/α-Fe2O3 MW photoanode was shown in Figure S9. The anodic overshoot and longer current fall are mainly from the surface rapid charging and charge 12 ACS Paragon Plus Environment

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separation in light absorbers, and the continuous decay (over 60 min) is mostly due to the gradual corrosion of Al2O3 and Si along with the resulting lift-off of the hematite.23

Figure 2. (a) J-U curves of Si/Al2O3/α-Fe2O3 MW photoanodes with different thicknesses of Al2O3 under the one sun irradiation (solid lines) and in the dark (dashed line for Si/α-Fe2O3 MW photoanode). The dark J-U curves for all the involved samples are almost overlapped in the present coordinate system, so only one dark J-U curve is shown for simplicity. (b) Schematic diagram of energy band and charge transfer in Si/Al2O3/α-Fe2O3 MW photoanode with an appropriate thickness of Al2O3 under irradiation. Upward purple and red arrows mark that the α-Fe2O3 and Si region absorb the relatively short and long wavelength lights, respectively. The solid and dashed arrows indicate that the carrier transport in the corresponding process are major and minor, respectively. The black and red arrows represent that the carrier transport in the corresponding process are desirable and undesirable, respectively. The conceivable energy band diagram of the Si/Al2O3/α-Fe2O3 MW photoanode with 13 ACS Paragon Plus Environment


an appropriate thickness of Al2O3 under irradiation is plotted in Figure 2b. Once the photo-carriers are generated from the light absorption, the band bending (i.e., built-in electric field) facilitates the separation and transfer of the photogenerated electrons/holes, that is, the photogenerated holes result in water oxidation at hematite/electrolyte interface, and the photogenerated electrons result in water reduction at counter electrode surface.22,23 And most of the photo-electrons in α-Fe2O3 are annihilated with the photo-holes in Si via tunneling effect through the thin Al2O3 interlayer. In addition, the Al2O3 interlayer can passivate the SiMW surfaces and the bottom surfaces of the α-Fe2O3 film, leading to the reduced interfacial recombination and trapping effect.38 These favorable factors lead to that the Si/Al2O3/α-Fe2O3 MW photoanode can possess much enhanced PEC performance relative to the Si/α-Fe2O3 MW photoanode. To confirm the above-mentioned conjectures in analysis of energy band diagram and figure out the primary causes of the PEC performance changes from the thicknessvarying Al2O3. The advantage and essential distinctions for using the controlled SiMWs (relative to using the Si nanowire arrays) have been discussed in our previous publication.23 Optical reflectance spectra were firstly examined (Figure S10). Reflectance spectra show that both of the Si/α-Fe2O3 and Si/Al2O3/α-Fe2O3 MWs have low reflectances (i.e., < 5% in the wavelength range of 300–1100 nm), which is much lower than that of the polished Si wafer. Since the SiMWs are atop of a thick Si substrate (~500 μm) and the rear side is coated by a metallic film, the optical transmittance for the prepared photoanodes can be ignored. So we deem that the difference in the PEC 14 ACS Paragon Plus Environment

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performance for the Si/Al2O3/α-Fe2O3 MW photoanodes with various thicknesses of Al2O3 is mainly from carrier transport and transfer (rather than generation) processes.49,50 For better understanding the respective effect of intermediate Al2O3 layer on the carrier transports inside the Si and -Fe2O3 layers, we employed two monochromatic lights with wavelengths (λ) of 365 nm and 620 nm. For the prepared -Fe2O3, the experimental band gap is 2.1 eV (corresponding to the λ = 590 nm) from Figure S11. If an absorption coefficient of 3.8×105 cm-1 at λ = 365 nm for -Fe2O3 is used, the light intensity is reduced to be ~6% of the original value after penetrating 76 nm-thick Fe2O3 according to the Beer-Lambert law.51 So the absorption for λ = 365 nm (620 nm) light is dominantly from the -Fe2O3 (Si) absorber. Figure 3a and d show the net photocurrent density vs. potential curves under λ = 365 nm and λ = 620 nm irradiation, respectively. Compared to the Si/α-Fe2O3 MW photoanode, the case with 100 ALD cycles under the two monochromatic lights possesses the obviously improved [email protected], while the case with 300 ALD cycles shows substantially reduced value. The magnitude relationship of the net photocurrent is consistent with that under one sun irradiation.



Figure 3. PEC response performances of Si/α-Fe2O3, Si/Al2O3-100/α-Fe2O3 and Si/Al2O3-300/α-Fe2O3 MW photoanodes under different monochromatic lights (i.e., λ = 365 nm for (a–c) and λ = 620 nm for (d–f), respectively). (a and d) are net photocurrent density vs. potential curves under light intensity of 45 and 65 W/m2, respectively. (b and e) are light intensity dependent net photocurrent density. (c and f) are transient current density under circular ON-OFF irradiation at 1.23 VRHE. Net photocurrent density vs. light intensity (J-I) curves at 1.23 VRHE (Figure 3b and e) and transient current intensity under circular ON-OFF irradiation (Figure 3c and f) under the two monochromic irradiations were further examined. From the light intensity dependent net photocurrent density, one can see that: 1) the Si/Al2O3-100/αFe2O3 MW photoanode possesses the largest slope, meaning the greatest responsivity; 2) the J-I relationship under λ = 365 nm irradiation is linear, while under λ = 620 nm is non-linear. It suggests that the IPCE (incident photon-to-current conversion efficiency) values under λ = 365 nm is almost constant as the light intensity increases (Figure S12), but under λ = 620 nm irradiation they gradually decrease. The main reason for the non16 ACS Paragon Plus Environment

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linear J-I behavior may be ascribed to the poor crystallization of the prepared -Fe2O3, which results in the transport of photo-holes (from Si absorber via tunneling Al2O3 interlayer) inside the -Fe2O3 layer to be frustrated. In contrast, the transport of photoelectrons (from hematite absorber via tunneling Al2O3 interlayer) inside the Si layer is accessible due to the single-crystalline and relatively good conductivity.52,53 Transient current intensity (Figure 3c and f) show that the photoanode with 100 ALD cycles exhibits the best photocurrent responses, and the photocurrent magnitudes are consistent with the J-U and J-I behaviors. The photocurrent attenuation ratio during the same duration of irradiation for the Si/Al2O3-100/α-Fe2O3 MW photoanode is 15.8% (13.5%) for λ = 365 nm (620 nm) irradiation, i.e., obviously smaller than those of the other two photoanodes. The decreased attenuation ratio is attributed to the Al2O3 passivation on Si and hematite. One may note that the photocurrent attenuation ratios under the one sun irradiation for all of the three photoanodes are much smaller, which can be attributed to that the photo-carriers in Si and hematite are excited simultaneously (i.e., the result of synergy effect). The passivation effect of the introduced Al2O3 was further verified by steady-state photoluminescence (PL) spectroscopy (Figure S13). Whether the -Fe2O3 film was grown on SiMW or glass substrate, once the Al2O3 interlayer was coated on the substrate prior to -Fe2O3 growth, the PL peak intensity was obviously suppressed, specifically for the SiMW substrate. It demonstrates that the intermediate layer of conformal Al2O3 has well passivated the SiMW surfaces and the bottom surfaces of -Fe2O3.38 PEIS was measured to characterize the dynamics of carriers both in bulk and on 17 ACS Paragon Plus Environment


surfaces of the photoelectrode under one sun irradiation using equivalent circuit,54,55 which includes carrier trapping on the photoelectrode surfaces and carrier transferring from surface states into electrolyte. Figure 4a shows the typical Nyquist plots at 0.7 VRHE. Each sample shows two semicircles, and the case with 100 ALD cycles presents the minimum-radius semicircles, which suggests that the resistances for carrier trapping on the surface states and for carrier transferring from the surface states to electrolyte are minimal.56

Figure 4. PEIS of Si/α-Fe2O3, Si/Al2O3-100/α-Fe2O3 and Si/Al2O3-300/α-Fe2O3 MW photoanodes under one-sun irradiation. (a) shows the Nyquist plots at 0.7 VRHE. The insert in (a) shows the equivalent circuit diagram. (b–d) represent the fitting parameters of RS+Rtrap+Rct, Cbulk and Css, respectively. The employed equivalent circuit for fitting the PEIS data was inserted in Figure 4a, where the RS is the series resistance, Rtrap the surface charge trapping, Rct the charge 18 ACS Paragon Plus Environment

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transfer in the solid/liquid surface, Cbulk and Css the charge accumulation in photoanode and on the surface states, respectively.57 In order to reveal the effect of the Al2O3 interlayer, PEIS were measured from 0.6 VRHE to 1.3 VRHE, and the presentative Nyquist plots were shown in Figure S14. A summary of RS, Rtrap, Cbulk, Rct, and Css are given in Figures 4b–d and S15. The RS values of the three photoanodes are comparable and much smaller than the values of Rtrap and Rct, suggesting that the latter two resistances play the decisive roles. As the applied potential is larger than the corresponding Uon and continuously increases, both of Rtrap and Rct are decreased. It can be attributed to that the increased potential enlarges the gradients of energy band bending (i.e., facilitating carrier separation) and promote the chemical action at the photoanode surface (i.e., facilitating carrier transfer).58 Among the three photoanodes, the Si/Al2O3-100/α-Fe2O3 MW photoanode has the smallest Rtrap and Rct as a whole. Besides, the total resistance (i.e., summation of RS, Rtrap and Rct) is plotted in Figure 4b, from which one can see that Si/Al2O3-100/α-Fe2O3 MW photoanode still shows the minimal total resistance, and the total resistance of the Si/Al2O3-300/α-Fe2O3 photoanode is a little smaller than that of the counterpart without Al2O3. The relationship of the total resistances for the three photoanode is not completely consistent with the PEC performances of Figure 2a, which can be ascribed to that a large capacitance is also required for achieving a large photocurrent.59,60 Figure 4c shows that the overall Cbulk of the Si/Al2O3-300/α-Fe2O3 MW photoanode is the smallest, and overall Cbulk of the Si/Al2O3-100/α-Fe2O3 MW photoanode is slightly smaller than that of the Si/α-Fe2O3 MW photoanode. The Cbulk relationship can be explained by that the tunneling process for carrier transport between 19 ACS Paragon Plus Environment


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the α-Fe2O3 and the Si is substantially blocked by the thick Al2O3 interlayer, while it is weakly hindered for the thin Al2O3 interlayer. Figure 4d shows that Si/Al2O3-100/αFe2O3 MW photoanode possesses the largest Css value, and the Si/α-Fe2O3 MW photoanode has the smallest Css values. We deem that the minimal total resistance and maximal Css together account for the largest photocurrent of the Si/Al2O3-100/α-Fe2O3 MW photoanode. Although the general behavior of the Al2O3 interlayer in the dual-absorber photoanode has been interpreted with the PEIS, further understanding of the surface charge transfer processes (i.e. the extracted carriers at the photoanode/electrolyte interfaces transfer or be recombined) is required to elucidate the interaction among the upper absorber, intermediate layer, and lower absorber. Hence we did the IMPS analysis, which is used as a tool to probe surface kinetics of photoelectrodes with multistep charge transfer reaction.61,62 It has been successfully applied to hematite photoanode systems.63 Surface charge transfer efficiency (ηct) can be obtained according to ηct = Ktran / (Ktran + Krec)

(1)

where Ktran and Krec represent the surface charge transfer and recombination rate constants, respectively.28 Figure 5a displays the IMPS complex curves of the Si/αFe2O3, Si/Al2O3-100/α-Fe2O3 and Si/Al2O3-300/α-Fe2O3 MW photoanodes under λ = 365 nm irradiation, through which the charge transfer efficiency can be calculated from the intercept ratio of the photocurrents between the low-frequency intercept and the high-frequency, and the sum of rate constants of Ktran and Krec can also be determined 20 ACS Paragon Plus Environment

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(i.e., the corresponding angular frequency at the apex of the semicircle). The smaller radii of the semicircles in Figure 5a indicate larger surface charge transfer efficiencies, which are consistent with the observed transient current behaviors in Figure 3c.28 We further examine the full IMPS responses of the three photoanodes in the potential range of 0.6 VRHE to 1.3 VRHE (Figure S16). Figure 5b shows that the charge transfer efficiency gradually increases with the potential for all the three photoanodes. The improved charge transfer efficiency is in agreement with the increased photocurrent at the potential of interest, which verifies the IMPS analysis. A comparison of the Ktran and Krec rate constants is shown in the Figure 5c and d. The overall values of Krec are much larger than those of Ktran, suggesting that most of the carriers at the photoanode surface do not efficiently transfer into the electrolyte. It can be explained by the sluggish kinetics of hematite for oxygen evolution reaction (OER) and can be significantly improved by using an OER catalyst (e.g., RuO2).64 As the applied potential increases, the Krec shows a remarkable drop, while Ktran nearly keeps constant. It agrees with Peter’s model, and similar results are also reported in literatures.65 Among the three photoanodes, the difference in Ktran is relatively small (i.e., the three cases in the same order of magnitude), while the difference in Krec is significant (i.e., the Si/Al2O3-100/αFe2O3 MW photoanode has the smallest Ktran values). The kinetics results estimated from IMPS analysis indicate that the appropriate-thickness Al2O3 interlayer can effectively improve surface charge transfer efficiency from reducing surface carrier recombination rate (rather than increasing surface charge transfer rate).



Figure 5. IMPS of Si/α-Fe2O3, Si/Al2O3-100/α-Fe2O3 and Si/Al2O3-300/α-Fe2O3 MW photoanodes under monochromic irradiation with λ = 365 nm and P = 45 W/m2. (a) is the IMPS complex plots at 1.2 VRHE. (b–d) represent the surface-charge transfer efficiency, transfer rate constant and recombination rate constant at different applied potentials, respectively. The primary cause for the influenced surface carrier recombination rate constant can be concluded as a combination of the carrier tunneling through the Al2O3 interlayer and the Al doping into hematite.66–68 The carrier tunneling between Si and hematite has been demonstrated in Figure 3, and the effects of Al2O3 thickness on carrier tunneling are revealed by PEIS analysis. To confirm the assumption of Al doping, we performed Mott-Schottky plot and XPS investigations. From the Mott-Schottky plots (Figure S17), we get that the bulk donor density (Nd) for the Si/Al2O3-300/α-Fe2O3 MW photoanode (4.1×1018 cm-3) is over 2 times larger than that for the Si/α-Fe2O3 MW photoanode (1.8×1018 cm-3). The doping in hematite has been widely employed and regarded as an 22 ACS Paragon Plus Environment

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efficient way to improve the hematite conductivity and then obtain improved PEC performances.36 The enhanced Nd is mainly derived from the Al element injection into the hematite via the annealing treatment during the hematite growth process, which is also observed elsewhere.36,69,70

Figure 6. XPS spectra of Fe 2p (a) and O 1s (b) from the Si/α-Fe2O3, Si/Al2O3-100/αFe2O3 and Si/Al2O3-300/α-Fe2O3 MW photoanode. Al 2p peak in XPS spectrum has been detected from the Si/Al2O3/α-Fe2O3 MW photoanode (Figure S4). Here we examine the fine-scan XPS of Fe and O elements from the Si/α-Fe2O3 MW photoanodes with or without Al2O3 interlayer.67 Figure 6a shows that the Fe 2p XPS spectra of the three photoanodes are similar, and no Fe2+ 23 ACS Paragon Plus Environment


satellite peaks (ca. 715 eV) are observed, which confirms the -Fe2O3 formation.23 By means of XPS-peak-differentiating analysis, Fe 2p XPS spectra is decomposed into four peaks, and these fingerprint features are well consistent with previous reports for -Fe2O3, implying that Fe3+ was the main iron state in the as-prepared outermost film.67 Note that the Fe 2p3/2 has an obvious shift (~0.6 eV for the fitting peak indicated by the blue line in Figure 6a) to a lower bending energy after introducing the Al2O3 interlayer, which is caused by Al doping. As a result, the Al element with a smaller radius takes place of some Fe-element sites in the as-prepared -Fe2O3, leading to strain the lattice and decrease the Fe-O-Fe bond distance.67,70 As illuminated in Figure 6b, four fitting peaks are obtained in O 1s XPS spectra for these cases with Al2O3 interlayer, and they can be regared as be from FeOOH, Fe(OH)O, -Fe2O3 and Al2O3, respectively.67 The peaks of FeOOH (usually regarded as a catalytic and passivation layer) and Fe(OH)O show a noteworthy redshift and the fraction of FeOOH peak over the whole O 1s peak is enhanced, which are favorable for reducing the surface carrier recombination rate of the photoanode.30 The presence of Al2O3 peak hints that the surface of the as-prepared hematite film may be passivated by the Al2O3 interlayer, which also contributes to the enhanced PEC performance for the Si/Al2O3-100/α-Fe2O3 MW photoanode. Furthermore, the oxygen vacancy peak was got from the O 1s XPS spectrum (Figure S18). The observed peak at ~529.9 eV is due to oxygen atoms bounded to metal atoms, while the peak at ~532.4 eV represents the defect sites, (i.e. oxygen vacancies).23 From the fitted O 1s XPS spectra, we can get that the ratio the peak at ~532.4 eV and the peak at ~529.9 eV are 87% and 228% for the Si/Al2O3-100/α-Fe2O3 MWs and Si/Al2O324 ACS Paragon Plus Environment

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300/α-Fe2O3 MWs, respectively.

Figure 7. Schematic illustration of the carrier transport for (a) Si/α-Fe2O3, (b) Si/Al2O3100/α-Fe2O3 and (c) Si/Al2O3-300/α-Fe2O3 MW photoanodes under one sun irradiation. The black and red arrows represent that the carrier transport in the corresponding process are desirable and undesirable, respectively. The solid and dashed arrows indicate that the carrier transport in the corresponding process are major and minor, respectively. Ktran and Krec represent rate constants of charge transfer and recombination at the photoanode surface, respectively. The larger thickness of the arrow indicating Krec means the larger value. The fundamental functions of the intermediate layer of various-thickness Al2O3 inserted into the Si/-Fe2O3 MW photoanode has been insightfully understood from the above discussions, and the carrier dynamics and surface kinetics can be schematically summarized in Figure 7. For the dual-absorber Si/α-Fe2O3 MW photoanode, the ideal situation is that the photo-holes in α-Fe2O3 layer move toward the hematite/electrolyte interface for water oxidation, the photo-electrons in Si layer move toward to counter electrode, and the photo-electrons in α-Fe2O3 layer are recombined with the photo-holes in Si layer.18,22,23 However, the prepared SiMW surfaces usually have substantial 25 ACS Paragon Plus Environment


surface defects and there are numerous interfacial states for the obtained Si/α-Fe2O3 MW photoanode.23 As a result, a significant fraction of photo-holes in α-Fe2O3 would be trapped or recombined for the case interfacial states; analogously, a significant fraction of photo-electrons in Si would also be trapped or recombined (as shown in Figure 7a). Besides, most of these holes extracted on the photoanode surface do not efficiently transfer into electrolyte due to a combination of the sluggish OER kinetics of hematite and the low-efficiency separation of the photo-electrons in α-Fe2O3 layer. When a thin Al2O3 interlayer is introduced (as shown in Figure 7b), the interfacial states are efficiently restrained from the passivation effect, and carrier recombination or trapping at the interfaces of Si/Al2O3 and Al2O3/α-Fe2O3 are substantially decreased relative to the counterpart without Al2O3 interlayer. Fortunately, the photo-electrons in α-Fe2O3 layer and photo-holes in Si layer can tunnel through the thin Al2O3 interlayer, so both in Si and α-Fe2O3 absorbers the efficient separation of electrons and holes can be guaranteed. Moreover, Al doping effect (facilitating the carrier separation in α-Fe2O3 layer and passivating the photoanode surface) together with Al2O3 passivation effect lead to that the surface charger recombination rate of the Si/Al2O3-100/α-Fe2O3 MW photoanode is much smaller than that of the counterpart without Al2O3 interlayer.36,38 When the Al2O3 interlayer is thick (as shown in Figure 7c), the carrier tunneling effect would be seriously hindered, resulting in the photo-electrons in α-Fe2O3 layer or photoholes in Si layer cannot be extracted out as desired.57,59 Hence, the accumulated photoelectrons would diffuse from the bottom surfaces of α-Fe2O3 to the top surfaces, which gives rise to the increased bulk or surface recombination and then a larger surface 26 ACS Paragon Plus Environment

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charge recombination rate as compared to the case with thick Al2O3 interlayer. In addition to the suppression of electron-hole recombination at the FTO/α-Fe2O3 interface, the influence of the underlayer on the individual-hematite photoanode has been also studied, which implies the doping into hematite can be achieved via the Sn diffusion from the FTO substrate.34,36 The doping effect can increase the electron density, meanwhile it can intensify the band bending, leading to the increased charge separation and the reduced recombination at the hematite/solution interface. These effects are also applicable in Si/α-Fe2O3 dual-absorber photoanode, and the distinguished difference for the Al2O3 layer inserted into the FTO/Fe2O3 and the Si/Fe2O3 photoanodes is that, the Al2O3 in the Si/α-Fe2O3 photoanodes can also passivate the Si microwire surfaces and the thickness-varying Al2O3 can lead to different effects. CONCLUSION The effect of Al2O3 interlayer on the Si/α-Fe2O3 MW photoanode for PEC water splitting have been investigated. After inserting a conformal Al2O3 interlayer with an optimized thickness, the photocurrent density at 1.23 VRHE can be enhanced to 2.08 mA/cm2 from 0.83 mA/cm2 and the onset potential has a cathodical shift of 0.21 V. We have revealed that the interfacial states and the resultant carrier recombination or trapping at the two absorber interfaces are efficiently restrained from the passivation effect of the intermediate layer; meanwhile, the efficient separation of electrons and holes both in Si and α-Fe2O3 absorbers can be guaranteed by tunneling effect unless the Al2O3 is too thick. Electrochemical analysis suggests that when the Al2O3 interlayer is 27 ACS Paragon Plus Environment


thin, the surface charge recombination rate can be substantially reduced due to a combination of the Al doping into the α-Fe2O3 layer and the passivation effect of Al2O3, enabling the increased surface charge transfer efficiency. While the Al2O3 interlayer is thick, the surface charge recombination rate is significantly increased due to the suppressed carrier tunneling through the interlayer, resulting in a low extraction efficiency of photo-electrons in the α-Fe2O3 layer and the serious carrier recombination. Our work promotes the understanding of the interaction between the dual absorbers and the interlayer, and provides a new way to improve the performance of multi-absorber photoelectrodes.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Details of characterizations (including SEM, XRD, Raman, XPS, TEM, PL spectra and UV-Vis), PEC response measurements (including transient current intensity, charge density and photocurrent density), electrochemical analysis (including PEIS, IMPS and Mott-Schottky plots).

AUTHOR INFORMATION Corresponding Author [email protected]; [email protected] 28 ACS Paragon Plus Environment

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Notes The authors declare no competing financial interest

ACKNOWLEDGMENTS We are very grateful for the financial support from National Natural Science Foundation of China (61504088, 61675142, 61875143), Natural Science Foundation of Jiangsu Province (BK20181169, BK20180042), Natural Science Foundation of the Jiangsu Higher Education Institutions of China (17KJA480004), China Postdoctoral Science Foundation (2017M611898, 2018T110549), and Priority Academic Program Development (PAPD) of Jiangsu Higher Education Institutions.

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Table of Contents


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Figure 1. Typical SEM (a–b) and TEM (c–g) images of the prepared Si/Al2O3-300/α-Fe2O3 MWs. (a) is the oblique-view SEM image. (b) is the cross-section image of the ground floor of the microwires, and the inset image is the top-view high magnification SEM image with a scale bar of 200 nm. (c) TEM shows the thicknesses of Al2O3 and α-Fe2O3. (d) and (e) are the HRTEM of the boundaries of Si/Al2O3 and Al2O3/αFe2O3, respectively. The STEM area was marked in orange box of (f) and EDS elements mapping were shown in (g). 82x114mm (300 x 300 DPI)

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Figure 2. (a) J-U curves of Si/Al2O3/α-Fe2O3 MW photoanodes with different thicknesses of Al2O3 under the one sun irradiation (solid lines) and in the dark (dashed line for Si/α-Fe2O3 MW photoanode). The dark J-U curves for all the involved samples are almost overlapped in the present coordinate system, so only one dark J-U curve is shown for simplicity. (b) Schematic diagram of energy band and charge transfer in Si/Al2O3/α-Fe2O3 MW photoanode with an appropriate thickness of Al2O3 under irradiation. Upward purple and red arrows mark that the α-Fe2O3 and Si region absorb the relatively short and long wavelength lights, respectively. The solid and dashed arrows indicate that the carrier transport in the corresponding process are major and minor, respectively. The black and red arrows represent that the carrier transport in the corresponding process are desirable and undesirable, respectively. 82x129mm (300 x 300 DPI)


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Figure 3. PEC response performances of Si/α-Fe2O3, Si/Al2O3-100/α-Fe2O3 and Si/Al2O3-300/α-Fe2O3 MW photoanodes under different monochromatic lights (i.e., λ = 365 nm for (a–c) and λ = 620 nm for (d–f), respectively). (a and d) are net photocurrent density vs. potential curves under light intensity of 45 and 65 W/m2, respectively. (b and e) are light intensity dependent net photocurrent density. (c and f) are transient current density under circular ON-OFF irradiation at 1.23 VRHE. 149x182mm (300 x 300 DPI)



Figure 4. PEIS of Si/α-Fe2O3, Si/Al2O3-100/α-Fe2O3 and Si/Al2O3-300/α-Fe2O3 MW photoanodes under one-sun irradiation. (a) shows the Nyquist plots at 0.7 VRHE. The insert in (a) shows the equivalent circuit diagram. (b–d) represent the fitting parameters of RS+Rtrap+Rct, Cbulk and Css, respectively. 82x109mm (300 x 300 DPI)


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Figure 5. IMPS of Si/α-Fe2O3, Si/Al2O3-100/α-Fe2O3 and Si/Al2O3-300/α-Fe2O3 MW photoanodes under monochromic irradiation with λ = 365 nm and P = 45 W/m2. (a) is the IMPS complex plots at 1.2 VRHE. (b– d) represent the surface-charge transfer efficiency, transfer rate constant and recombination rate constant at different applied potentials, respectively. 82x105mm (300 x 300 DPI)



Figure 6. XPS spectra of Fe 2p (a) and O 1s (b) from the Si/α-Fe2O3, Si/Al2O3-100/α-Fe2O3 and Si/Al2O3300/α-Fe2O3 MW photoanode. 82x135mm (300 x 300 DPI)


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Figure 7. Schematic illustration of the carrier transport for (a) Si/α-Fe2O3, (b) Si/Al2O3-100/α-Fe2O3 and (c) Si/Al2O3-300/α-Fe2O3 MW photoanodes under one sun irradiation. The black and red arrows represent that the carrier transport in the corresponding process are desirable and undesirable, respectively. The solid and dashed arrows indicate that the carrier transport in the corresponding process are major and minor, respectively. Ktran and Krec represent rate constants of charge transfer and recombination at the photoanode surface, respectively. The larger thickness of the arrow indicating Krec means the larger value. 170x82mm (300 x 300 DPI)


Hematite Microwire Photoanode by the

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