Controllable fabrication of nanostructured materials for photoelectrochemical water splitting via atomic layer deposition.

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Controllable fabrication of nanostructured materials for photoelectrochemical water splitting via atomic layer deposition Tuo Wang, Zhibin Luo, Chengcheng Li and Jinlong Gong* Photoelectrochemical (PEC) water splitting is an attractive approach to generate hydrogen as a clean chemical fuel from solar energy. But there remain many fundamental issues to be solved, including inadequate photon absorption, short carrier diffusion length, surface recombination, vulnerability to photo-corrosion, and unfavorable reaction kinetics. Owing to its self-limiting surface reaction mechanism, atomic layer deposition (ALD) is capable of depositing thin films in a highly controllable manner, which makes it an enabling technique to overcome some of the key challenges confronted by PEC water splitting. This tutorial review describes some unique and representative applications of ALD in fabricating high performance PEC electrodes with various nanostructures, including (i) coating conformal thin films on three-dimensional scaffolds to facilitate the separation and migration of photocarriers and enhance light trapping, as well as realizing controllable doping for bandgap engineering

Received 16th October 2013

and forming homojunctions for carrier separation; (ii) achieving surface modification through deposition

DOI: 10.1039/c3cs60370a

of anti-corrosion layers, surface state passivation layers, and surface catalytic layers; and (iii) identifying the main rate limiting steps with model electrodes with highly defined thickness, composition, and

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interfacial structure.

Key learning points (1) (2) (3) (4)

Nanostructured ALD coatings for improved carrier separation and transportation, as well as enhanced light trapping Doping realized by ALD for bandgap engineering and forming homojunctions Surface modification achieved through deposition of anti-corrosion layers, surface state passivation layers, and surface catalytic layers Model electrodes with defined composition and structure for probing the mechanisms in PEC water splitting systems

1 Introduction Photoelectrochemical (PEC) water splitting of water into H2 and O2 by using sunlight is a clean, renewable approach to convert intermittent solar energy into stable and storable fuel. Photoelectrodes used for PEC water splitting are typically semiconductors immersed in electrolytes. When the photon energy of the incident light is larger than the bandgap energy of the semiconductor, electrons are excited to the conduction band of the semiconductor, leaving holes in the valence band. With suitable energetic positioning of the bands, water molecules in contact with the semiconductor are reduced to H2 by Key Laboratory for Green Chemical Technology of Ministry of Education, School of Chemical Engineering and Technology, Tianjin University, Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, China. E-mail: [email protected]; Fax: +86 22 87401818

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photoelectrons and oxidized to O2 by photoholes. The bottom of the conduction band should be more negative than the H+/H2 redox potential (0 V vs. normal hydrogen electrode, NHE), and the top of the valence band should be more positive than the O2/OH redox potential (+1.23 V vs. NHE); thus the theoretical minimum bandgap of a semiconductor for water splitting is 1.23 eV.1 In addition to this minimum thermodynamic requirement, additional driving force has to be included to overcome the overpotential due to the electron transfer processes at semiconductor–liquid junctions with unfavorable kinetics. Hence, a bandgap of about 1.6–2.4 eV is required for practical devices.2 For semiconductors with a conduction band edge more positive than the H+/H2 redox potential, or a valence band edge more negative than the O2/OH redox potential, complete water splitting cannot be achieved. This problem can be solved by applying external bias, or integrating a photovoltaic (PV) cell. Alternatively, high-performance photoelectrodes that

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work only for half reactions of water splitting are also desired to construct ‘‘Z-scheme’’ systems composed of an H2-evolving photoelectrode and an O2-evolving photoelectrode. In a PEC water splitting cell, an O2-evolving photoanode is an n-type semiconductor that drives holes toward the semiconductor–electrolyte interface owing to the electric field generated by band bending. This photoanode is wired with the counter-electrode via external circuit so that electrons can flow to the counter electrode to reduce water to form H2. Thus O2 and H2 are produced separately. Similarly, the p-type semiconductor serving as the H2-evolving photocathode facilitates

electron transfer to the electrolyte interface for the reduction reaction. A more integrated design is the back-to-back photoelectrodes with no external wiring. Different from PEC cells, semiconductor particles or colloids dispersed in aqueous solution can serve as self-supported photocatalysts for water splitting for the purpose of simplicity and low cost; however the extra separation process of H2 and O2 is required and photocatalyst particles cannot be wired up to apply external bias. The scope of this tutorial review is limited to PEC water splitting, so we restrict our discussion to systems with separate photoelectrodes connected via external wiring.

Tuo Wang received his BS from Tianjin University and PhD from the University of Texas at Austin, both in chemical engineering. After gaining another year of research experience as a postdoctoral associate, he joined Novellus Systems, Inc. (currently Lam Research Corp.) as a process development engineer in Tualatin, OR. Since August 2012, he has been an associate professor in chemical engineering in Tianjin Tuo Wang University. His research included ALD of high-k dielectrics and epitaxial growth of perovskite thin films, as well as PECVD of carbon-based etching hardmasks. His current research focuses on nanostructured materials for energy conversion and storage systems.

Zhibin Luo obtained his BS (2012) degree in chemical engineering and technology from the South China University of Technology. He is currently a graduate student at Tianjin University under the supervision of Professor Jinlong Gong. He works in the Energy & Environmental Catalysis Laboratory at the School of Chemical Engineering and Technology. His research interests are in the design and synthesis of metal oxides for solar water splitting.

Chengcheng Li obtained his BS degree in chemistry from Nankai University in 2011. He is currently a graduate student at Tianjin University under the supervision of Professor Jinlong Gong. He is interested in atomic layer deposition (ALD) and nanostructured materials for solar water splitting.

Chengcheng Li

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Zhibin Luo

Jinlong Gong studied chemical engineering and received his BS and MS degrees from Tianjin University and his PhD degree from the University of Texas at Austin under the guidance of C. B. Mullins. After a stint with Professor George M. Whitesides as a postdoctoral research fellow at Harvard University, he joined the faculty of Tianjin University, where he currently holds a Pei Yang Professorship in chemical Jinlong Gong engineering. He was a visiting scientist at the Pacific Northwest National Laboratory in 2007. He has served on the editorial boards of several journals including Chemical Society Reviews. He is an elected Fellow of the Royal Society of Chemistry (FRSC). He has published more than 100 papers in peer-refereed journals. His research interests in surface science and catalysis include catalytic conversions of green energy, novel utilizations of carbon dioxide, and synthesis and applications of nanostructured materials.


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An ideal semiconductor photoelectrode for water splitting is expected to meet the following requirements: (i) a suitable bandgap width to absorb photons in the visible region, (ii) high electronic conductivity to facilitate carrier transfer, (iii) favorable band positions for unassisted water splitting reaction and efficient injection of photogenerated carriers, (iv) excellent stability in aqueous electrolyte solutions, and (v) must contain earthabundant and environmentally friendly elements.3 Unfortunately, it is not easy for a material to meet all these requirements. Although most intensively investigated, the wide bandgap of TiO2 (3.2 eV) limits its absorption to the UV range, which accounts for only B5% of the solar radiation reaching the earth.1 Narrow bandgap semiconductors, such as Si, CdS, and Cu2O, are capable of absorbing visible light which accounts for B44% of solar radiation, but they are vulnerable to photo-corrosion under water splitting conditions.2 a-Fe2O3 (2.2 eV) and WO3 (2.6 eV) have attractive visible light activities, but they suffer from low conductivity, short carrier diffusion length, and relatively high overpotentials, which limits their saturate photocurrents and current onset potentials.4,5 Moreover, surface and interfacial properties of the semiconductor electrodes need to be carefully tuned to suppress the recombination at surface trapping sites.1 Recently, atomic layer deposition (ALD) has attracted much attention in the area of PEC water splitting due to its unique nature for precisely controlling film thickness and conformal growth of complex nanostructures (Fig. 1a). ALD is a self-limiting

Fig. 1 Schematic of (a) complex nanostructures coated by a conformal ALD-deposited thin film and (b)–(e) an example of one ALD cycle (molecules are oversized for clarity purpose).


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deposition technique for producing thin films based on the sequential exposure of two gaseous precursors that can be adsorbed on the surface of the pristine substrate and the growing film, separated by inert gas purging that removes unadsorbed precursor molecules and by-products.6 Although sharing similar chemistry with chemical vapor deposition (CVD), ALD involves a pure gas–solid reaction where the adsorbed molecules in each reaction cycle is one monolayer at the maximum, which causes the layer-by-layer growth mechanism of ALD. The ALD process is composed of repeated ALD cycles. A typical ALD cycle consists of four steps: dosing and purging of the first and the second precursors (Fig. 1b–e), where the dosing–purging of one of the two precursors is sometimes referred to as a half cycle. For example, for the deposition of Al2O3, the first and second precursors are metal and oxidant precursors, respectively. Starting from a hydroxyl group terminated substrate, trimethylaluminum (TMA) molecules are introduced into an ALD chamber during the metal precursor dosing step, and they react with the hydroxyl groups on the surface of the substrate through ligand exchange7 (Fig. 1b, molecules are oversized for clarity purpose), producing gaseous CH4 as the by-product. It is suggested that the oxygen atom of the surface hydroxyl first interacts with the empty p-orbital of the aluminum atom of the TMA molecule, followed by a sigma-bond metathesis to liberate CH4. This surface reaction will automatically stop once all active surface hydroxyl sites are consumed by the reaction with TMA molecules, since the –CH3 groups will not further react with TMA in the gas phase, leaving behind a new surface with only one layer of TMA molecules which are chemically adsorbed. This is the origin of the self-limiting growth mechanism, which ensures that ALD-deposited films are extremely conformal and uniform in thickness. Then the by-product (CH4) and excessive precursor (TMA) are purged out of the chamber with an inert gas (Fig. 1c). In the oxidant dosing step, H2O hydrolyzes the –CH3 groups of adsorbed TMA (Fig. 1d), converting the substrate back to the hydroxyl group terminated surface, the same as the initial surface (Fig. 1b) but with one layer of Al2O3 deposited. Purging away the excessive H2O and by-products (Fig. 3e) completes an entire ALD cycle, which can be repeated a certain number of times to reach the targeted film thickness. The amount of material added in each ALD cycle is referred to as growth per cycle (GPC). It is important to point out that GPC in ALD is typically less than one monolayer because of the steric hindrance of ligands on precursor molecules, while the GPC of some ALD processes (e.g., tetrakis(dimethylamido)-titanium/NH3, diethylzinc/H2O) may exceed a monolayer, likely due to the readsorption of gaseous by-products or the decomposition of precursors.7 Owing to the unique surface self-limiting growth mechanism, ALD can be used as an enabling tool to address the aforementioned problems facing PEC water splitting systems. This tutorial review focuses on the application of ALD for PEC water splitting, where ALD-deposited layers can be either photoactive layers that effectively absorb photons and generate photocurrents or performance enhancing layers that play a key role in achieving efficient and endurable photoelectrodes. This article starts with a discussion of the conformal coating on complex

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nanostructures, a key feature of ALD used for compensating the short carrier diffusion length, improving carrier separation, and enhancing light trapping. ALD-based doping is then introduced. The accurately controlled doping enables bandgap engineering for visible-light absorption and forming homojunctions for carrier separation. Using ALD-deposited thin films as the corrosionresistant layer is then described, which stabilizes many semiconductors that suffer from photo-corrosion in harsh electrolytes. Next, the ability of ALD-deposited films to passivate surface charge trapping sites is explained, followed by a discussion of ALD-grown films as catalytic layers. Finally, model photoelectrodes with clearly defined structures fabricated by ALD are presented for the purpose of probing the mechanisms in PEC water splitting systems. More details about the ALD mechanism, precursor selection, and deposition condition optimization were systematically reviewed by George and Puurunen.6–8 Complex nanostructures prepared by ALD can be found in the work of Knez and Kim.9,10

2 Three-dimensional (3D) coating One of the most unique features that make ALD a promising technique for fabrication of PEC electrodes is its ability to deposit conformal coatings on 3D nanostructures in a highly controllable manner. With precisely controlled thickness and composition, coating thin films onto 3D substrates provides a new strategy to solve some of the key issues in PEC water splitting systems including (i) compensating short carrier diffusion length, (ii) promoting carrier separation, and (iii) enhancing light trapping. Many complex nanostructures, such as nanorods, nanowires (NWs), and nano-trenches, have been fabricated by ALD.9,10 Owing to the surface self-limiting reaction nature of ALD, conformal thin films can be coated, or filled into/onto nanostructures, as long as the precursor molecules used for ALD can diffuse into the nanopores to saturate the surface and then be efficiently purged away after each half cycle. Gordon et al. have calculated the minimum amount of precursor exposure required for conformal coatings inside holes and trenches with regular shapes. The required exposure for holes with aspect ratios ranging from 1 to 2000 was calculated,11 which would be useful when predicting precursor dosing time for coating irregular 3D scaffolds. Methods for coating the interior surface of nanoporous structures using ALD were reviewed by Detavernier et al.12 2.1

Conductive scaffolds to overcome short diffusion length

A common contradictory characteristic of many attractive photoelectrode materials, such as TiO2, WO3, and a-Fe2O3, is the long optical penetration depth combined with the short carrier diffusion length, which leads to a dilemma situation that a thickness of several micrometers is needed to completely absorb the incident light, but only carriers generated in the topmost layer would be able to diffuse to the space charge layer (or depletion layer), and then drift to the semiconductor– electrolyte interface for water splitting. That is, a huge portion of

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photons absorbed deep in the bulk are simply wasted through the generation of heat and/or emission of luminescence. One strategy to address this dilemma is to limit the thickness of the semiconductor layer to a value comparable to the sum of the carrier diffusion length and carrier drift length of the semiconductor in contact with the electrolyte, so that the built-in potential within the semiconductor would drive the carriers that diffuse to the depletion layer to drift to the semiconductor– electrolyte interface and thus suppress the recombination of electron–hole pairs. The incomplete light absorption due to the limited film thickness can be compensated by nano-tailoring the semiconductor layer to enlarge the surface area as well as enhance the light trapping effect. A straightforward method to prepare such electrodes would be coating thin semiconductor layers on highly conductive 3D nano-scaffolds with a large surface area. The scaffold can be either highly oriented single crystals or oxides with high conductivity. Single crystals are well-known for high electron mobility and absence of defects and grain boundaries. Wang and co-workers creatively synthesized single crystalline TiSi2 nanonets coated with thin conformal TiO2, WO3, and a-Fe2O3 films by ALD to fabricate PEC electrodes.4,5,13 Serving as a highly conductive scaffold, TiSi2 nanonet is capable of efficient charge transport. Moreover, the morphology of the TiSi2 nanonets provided an excellent growth template for the coated semiconductor layer to enhance light absorption, as well as enlarge the semiconductor– electrolyte interface where the depletion layer was developed. When anatase TiO2 (B27 nm) was deposited on TiSi2 nanonets by ALD using Ti(i-PrO)4 and water as precursors, a photocurrent of 0.6 mA cm 2 was achieved at 0 V vs. Ag/AgCl in 0.05 M KOH solution (pH B 12.7) illuminated by a 150 W xenon lamp (Fig. 2a and b), with a peak overall power conversion efficiency of 16.7% under monochromic UV illumination.13 A heterostructured electrode composed of WO3 thin films coated on single crystalline TiSi2 nanonet was also fabricated by ALD. (tBuN)2(Me2N)2W was used as the W precursor to avoid forming corrosive by-products. Aided by a Mn over-layered catalyst, the WO3 electrode was found to be stable in neutral solution.4 The authors proposed that the built-in electric field drove the electrons and holes away from and toward the solid–liquid interface, respectively. Thus, the intrinsic low conductivity of WO3 could be compensated by the highly conductive TiSi2 scaffold. In the case of a-Fe2O3—an attractive PEC material that suffers from notoriously short carrier diffusion length—a similar strategy could be applied.5 A record-high incident photon-to-current conversion efficiency (IPCE) of 46% at 400 nm was achieved by coating 25 nm a-Fe2O3 on TiSi2 nanonets using ALD (Fig. 2c and d). Without adding any water oxidation catalyst or intentionally doping Fe2O3 to increase the charge diffusion length, high photocurrents of 1.6 and 2.7 mA cm 2 at 1.23 and 1.53 V vs. reversible hydrogen electrode (RHE) were achieved under air mass 1.5 (AM1.5) illumination in a 1 M NaOH electrolyte. Transparent conducting oxides (TCOs) can be used as superior conductive nano-scaffolds in some cases, because their wide bandgap makes them less likely to compete with the coated photoactive layer for the incident photons. Noh et al. reported


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Fig. 2 (a) TEM image of TiO2 coated on TiSi2, (b) J–V plots of the TiO2 coated TiSi2 electrode in 0.05 M KOH under different illumination conditions. Reproduced with permission from ref. 13. (c) TEM image of Fe2O3 coated on TiSi2 nanonet, (d) comparison of the IPCEs of Fe2O3 with and without the TiSi2 nanonet scaffold (measured at V = 1.53 V vs. RHE). Reproduced with permission from ref. 5. Copyright American Chemical Society, 2009 and 2011, respectively.

the performance of 3D TiO2–TCO core–shell electrodes.14 Single crystalline indium tin oxide (ITO) NWs were synthesized on flexible stainless steel mesh (SSM) to serve as efficient charge transport paths while providing a large surface area. Highly crystalline anatase TiO2 was deposited onto the ITO NWs using titanium tetraisopropoxide (TTIP) and water as the ALD precursors at 250 1C (Fig. 3a). The photoelectrode composed of a 20 mm long ITO NW core and a 36 nm TiO2 shell increased the photocurrent by four times compared with the SSM electrode at 0 V vs. saturated calomel electrode (SCE) (B1.04 V vs. RHE) in 1 M KOH aqueous solution (pH B13.5) and illuminated by a 450 W mercury arc lamp, and the current was saturated above an ITO NW length of 20 mm (Fig. 3b). The enhanced photocurrent was attributed to the effect that photoexcited electrons

Fig. 3 (a) Pictorial representation and cross-sectional TEM image (inset) of the ITO core–TiO2 shell NW photoelectrode structure, (b) J–V curves of the core–shell photoelectrode as a function of ITO NW length, orange line: dark currents of all the specimens. Reproduced with permission from ref. 14. Copyright 2012 American Institute of Physics.


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within the TiO2 shell were efficiently collected and transported by the conductive ITO core. The authors also pointed out an optimal TiO2 shell thickness of 36 nm experimentally, and calculated the depletion layer thickness of TiO2 to be B36.8 nm; this agreement verified that the built-in potential in TiO2 was a key factor in driving the photogenerated electrons and holes into the ITO core and the electrolyte, respectively. In addition to high conductivity, the light trapping effect of the ITO NW was also a significant factor for the enhanced photocurrent, as bare SSM would lose 80% of incoming photons via reflection and transmission, while ITO NW with a length of 20 mm was found to effectively reduce reflection and transmission. The light trapping effect will be discussed later on. Porous TCO nanopores also showed great potential as conductive scaffolds for coating conformal photoactive layers. Porous antimony tin oxide (ATO) films (B2 mm in thickness) can be prepared on FTO substrates by spin coating of colloidal ATO nanoparticles (nanoATO). Even though nanoATO possessed less defined conduction pathways compared with single crystalline TiSi2, a photocurrent of 0.58 mA cm 2 at 0 V vs. Ag/AgCl (1.0 M KOH electrolyte, pH B 13.5) was achieved under AM1.5 when porous ATO was coated by 9 nm rutile TiO2 from 200 ALD cycles (Fig. 4a and b),15 which was comparable to that achieved by anatase TiO2 deposited on TiSi2 (Fig. 2b).13 The enhanced photocurrent, compared to that for planar TiO2 on FTO substrates, was attributed to the unique core–shell structure, where the highly conductive ATO scaffold quickly drained the photogenerated electrons, thus inhibiting the electron–hole recombination in the TiO2 layer. The optimized thickness was 9 nm for TiO2. Further increasing the TiO2 thickness resulted in

Fig. 4 (a) Schematic diagram for synthesizing 3D FTO/nanoATO/TiO2 photoelectrodes by ALD without sealing the nanopores inside the nanoATO scaffold, (b) J–V curves for planar FTO/TiO2 (red line) and 3D FTO/nanoATO/TiO2 electrodes (blue line) with 200 ALD cycles of TiO2 (B9 nm). Reproduced with permission from ref. 15. (c) Schematic of the 3D porous conductive NTO scaffold with a-Fe2O3, (d) J–V curves for a-Fe2O3 on the 3D NTO scaffold, compared with a-Fe2O3 only and NTO only electrodes. Reproduced with permission from ref. 16. Copyright American Chemical Society, 2013 and 2012, respectively.

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lower photocurrent as thicker TiO2 started to seal the nanopores in the conductive scaffold. Without the nanopores in contact with the electrolyte, the hole travel distance increased from several nanometers (i.e., thickness of the TiO2 coating) to several micrometers (i.e., thickness of the nanoATO film), since a significant fraction of photoholes must travel to the top surface of the electrode to react with the electrolyte. Therefore, the ability to deposit thin films with accurately controlled thickness in 3D structures is highly desirable in order to exploit the full potential of a well-studied photoactive material such as TiO2. ALD is also capable of fabricating high-quality 3D porous TCO scaffolds with the assistance of another porous template. Using porous TiO2 (composed of TiO2 particles with a nominal particle size of B150 nm) as the template, very thin porous TCO scaffolds were fabricated by depositing 8 3 nm thick niobium tin oxide (NTO) conformally on the porous TiO2 template.16 NTO was deposited by inserting a single Nb2O5 ALD layer into a certain number of SnO2 ALD layers, which resulted in highquality TCO with an optimized electrical conductivity up to 37 S cm 1 and a low optical absorption coefficient of 0.99 mm 1 at 550 nm. Importantly, the high transparency NTO films exhibited excellent stability between pH 1 and 9 over a one-week period, a critical feature for stable PEC electrode operation. The porous NTO scaffold was further coated by 11 3 nm a-Fe2O3 using atmospheric pressure CVD (APCVD) to form a photoelectrode (Fig. 4c) with a high photocurrent of 2.26 mA cm 2 at 1.4 V (vs. RHE) under simulated AM1.5 illumination (Fig. 4d), comparable to that of 25 nm ALD-grown a-Fe2O3 on TiSi2 nanonets.5 The Fe2O3–TiO2 film without a NTO current collector (the ‘‘a-Fe2O3 only’’ curve in Fig. 4d) showed negligible photocurrent due to the poor electronic contact. 2.2

3D heterojunctions for carrier separation

The conformal coating ability of ALD can be used to construct 3D heterojunctions by coating one material onto another 3D nanostructured one, where both materials are semiconductors with staggered band-edge positions (type-II band alignment) that is favorable for the separation of photogenerated carriers. Ji et al. coated 17 nm ZnO shells on rutile TiO2 nanorod cores using ALD, forming a 3D TiO2–ZnO heterojunction electrode structure.17 Upon ZnO coating, the electrode showed higher photocurrent than the bare TiO2 nanorod array electrode, which was attributed to improved carrier separation at the heterojunction, as well as the quick electron transport in the single crystalline rutile TiO2 core that facilitated the injection of additional electrons from ZnO to TiO2. ALD-based 3D heterojunction structures composed of semiconductors with different bandgaps also enabled the optical absorption at complementary parts of the solar spectrum, known as the ‘‘Z-scheme’’, as demonstrated by Wang and co-workers.18 By conformally coating hematite shells on Si NWs, 3D heterojunction electrodes were formed, where hematite and Si can be excited by UV (365 nm) and IR (980 nm) light, respectively (Fig. 5a). But no appreciable photocurrent was observed until a sufficient anodic bias was applied when illuminated with UV or IR alone, since electrons in hematite

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Fig. 5 (a) Energy band schematics and (b) J–V curves of the Fe2O3/Si NW electrode under UV, IR and combined monochromatic illumination, (c) energy band schematics of the Fe2O3/Si NW electrode under AM1.5, (d) J–V plots for Fe2O3 on Si NWs (red) and Fe2O3 on planar FTO (black) under AM1.5 in 1 M NaOH. Reproduced with permission from ref. 18. Copyright 2012 American Chemical Society.

or holes in Si cannot flow through the entire electrode. Photocurrent was detected only when hematite and Si were excited simultaneously (Fig. 5b). Under AM1.5 illumination, photogenerated electrons from hematite and holes from Si could recombine at the hematite–Si interface, whereas holes from hematite are transferred to the electrolyte for the O2-evolving oxidation reaction, and electrons from Si flow to the Pt counter electrode for the H2-evolving reduction reaction (Fig. 5c). This heterojunction electrode exhibited a low photocurrent turn-on potential of 0.6 V (vs. RHE) without adding any catalyst (Fig. 5d), which was enabled by utilizing low energy photons absorbed by Si. ALD-deposited layers can also be used as a sacrificial template to prepare 3D heterojunction structures. When toxic ALD precursors are needed to deposit the shell material of heterojunctions, such as CdS or CdSe, an ALD-grown sacrificial layer could be deposited first and then an ion exchange method could be used to convert the conformal ALD shell to the target material with well-defined thickness. Fan and co-workers nicely demonstrated the fabrication of a TiO2 inverse opal (IO)/CdSe photoelectrode using ALD thin film coating followed by ion exchange. Sacrificial ZnO was coated onto TiO2 IO first, and then two consecutive ion exchange reactions converted the ZnO layer to ZnSe, and finally CdSe. The TiO2/CdSe electrode was tested as the PEC photoanode in a three-electrode system for hydrogen generation (Fig. 6a). In such a heterostructured photoelectrode, CdSe acts as an external visible-light absorber, sometimes called a sensitizer, to absorb visible light owing to its narrow bandgap. Then the photogenerated carriers in the CdSe sensitizer will migrate to TiO2 because of the appropriate band alignment between CdSe and TiO2. The conduction band position of CdSe is higher than that of the TiO2, and thus the photogenerated electrons within CdSe are transferred to TiO2, and then flow to the Pt counter electrode for H2 evolution


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Fig. 7 (a) Schematic presentation and (b) cross-sectional SEM image of the ZnO NA/TiO2 IO bilayer structure. Reproduced with permission from ref. 23. Copyright 2012 Elsevier.

Fig. 6 (a) Schematics of the PEC cell, with TiO2/CdSe as the photoanode, Pt as the counter electrode, and Ag/AgCl as the reference electrode, (b) band energy levels of TiO2 and CdSe: photogenerated electrons within CdSe are transferred to TiO2, while the holes (not drawn) are scavenged by the Na2SO3/Na2S electrolyte solution, (c) J–V curves of the TiO2/CdSe: five samples were prepared by ALD plus ion exchange with different ALD cycles, and one sample by 6 SILAR cycles, (d) stability test under chopped light at 0 V vs. Ag/AgCl in the same solution. Reproduced with permission from ref. 19. Copyright 2012 Nature Publishing Group.

through the water reduction reaction (Fig. 6b). The holes are scavenged by the sulfide electrolyte. Successive ionic layer adsorption and reaction (SILAR) is one of the commonly used methods to coat the sensitizing materials. Compared with SILAR, the ALD plus ion exchange method exhibited superior control of the thickness and the size of the sensitizer by controlling the thickness of the ZnO layer. With an optimized ZnO thickness of 60 ALD cycles and an optimized ion exchange reaction condition, the TiO2 IO/CdSe electrode reached a high photocurrent of 15.7 mA cm 2 under AM1.5 at 0 V (vs. Ag/AgCl) in an electrolyte solution of 0.24 M Na2S and 0.35 M Na2SO3, whereas the sample prepared from an optimized SILAR of 6 cycles exhibited a photocurrent of B6.5 mA cm 2 (Fig. 6c). The photo-corrosion is one of the major concerns for CdSe as well as other chalcogenide sensitizers. To improve the stability of CdSe, photogenerated holes need to be transferred away from CdSe by forming an appropriate heterojunction, or using electrolytes composed of a hole scavenger. Using Na2SO3/Na2S as the electrolyte solution, the photocurrent of the TiO2/CdSe electrode showed negligible degradation over a 400-second stability test (Fig. 6d).19 2.3

3D nanostructures for enhanced light trapping

Inadequate light absorption is an issue facing many semiconductor materials. When the inadequacy is originated from limited absorption range in the light spectrum, this issue can be solved by tailoring the intrinsic material property, such as narrowing the bandgap, or coupling with other materials (e.g., adding a sensitizer as an external light absorber,1 or utilizing the plasmonic effect of Au nanoparticles20). On the other hand,


to minimize the photon loss due to reflection or scattering, enhancing light trapping would be another effective way to improve the overall efficiency of solar energy conversion systems, which has been demonstrated by ITO NWs14 and Si nanocones.21 IO has been widely used as the replica shell of opal that can be facilely prepared by the bottom-up self-assembly of submicron spheres into face centered cubic (fcc) multilayers. It can be synthesized in a controllable manner through ALD due to its ability to deposit conformal thin films into narrow pores while retaining the morphology of the pristine surface. Fan and co-workers reported the PEC performance of anatase TiO2 IOs fabricated by coating TiO2 on self-assembled close packed fcc polystyrene (PS) spheres, followed by removal of the PS sphere via thermal decomposition.22 Upon sensitization by CdS quantum dots (QDs) for enhanced visible light absorption, IOs with 288 nm diameter exhibited a photocurrent of 4.84 mA cm 2 at 0 V (vs. Ag/AgCl) in a 0.25 M Na2S and 0.35 M Na2SO3 mixed solution under simulated AM1.5, as well as a peak IPCE of 30% at B480 nm with a two electrode configuration under 0 V bias. The light harvesting efficiency of the TiO2 IO electrode can be further improved by forming a bilayer structure where ZnO NW arrays are grown directly on the TiO2 IOs. Upon TiO2 infiltration of fcc PS spheres by ALD, a ZnO seeding layer could be deposited by ALD to allow the following thermal growth of ZnO NW arrays (NAs) (Fig. 7a and b). Upon CdS sensitization, the ZnO NA/TiO2 IO bilayer electrode showed a photocurrent of B5 mA cm 2 at 0.75 V (vs. Ag/AgCl) in the same solution, two times higher than that of the pure ZnO NW array electrode. The authors ascribed the improved photocurrent to the TiO2 IO photonic crystal serving as a ‘‘dielectric mirror’’ that increased the optical path and improved light-matter interactions, thus enhancing the overall light harvesting efficiency.23

3 Doping Doping is a well-recognized method for tailoring the properties of materials. Owing to the layer-by-layer growth mechanism, ALD-based doping can be easily achieved by periodically inserting one (or multiple) ALD layer(s) of dopant elements into repeated ALD cycles of the host material. The dopant concentration, as well as the distribution of the dopants (e.g., in the growth direction), can be controlled by altering the ALD cycle ratio of the two materials.24 These two features, plus the versatile 3D

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coating capability, made ALD a powerful tool for bandgap engineering and carrier separation enhancement.


3.1

Bandgap engineering

To extend the optical absorption to the visible-light range for a semiconductor with a relatively large bandgap, such as TiO2, doping has been considered as an effective approach to narrow the bandgap.1 In terms of ALD-deposited TiO2, nitrogen doping has been demonstrated by several groups.25–27 Pore et al. achieved N-doping by growing TiN and TiO2 alternately on a Si substrate, in an [x(TiCl4 + NH3) + y(TiCl4 + H2O)] n manner, where x and y represented the ALD cycle number of TiN and TiO2, respectively, and n determined the overall film thickness.25 Although N-doped TiO2 films prepared by this method could be excited by the visible light, increased carrier recombination, as evidenced by sharp transient spikes during switching on and off of the lamp in the linear sweep voltammogram, occurred due to the generation of an undesirable amount of oxygen vacancies. Alternatively, N-doping can be realized by introducing NH3 and H2O simultaneously, using ammonia water vapor as the precursor. Cheng et al. prepared N-doped TiO2 using TiCl4 and ammonia water with an NH3-to-H2O volume ratio of 420 : 1.26 A 1.6 at% of N-doping was achieved, where the substitutional N was 70% in terms of total N according to X-ray photoelectron spectroscopy (XPS) analysis, reducing the bandgap energy from 3.2 to 2.25 eV (Fig. 8a). IPCE confirmed that the photoactivity of N-doped TiO2 was extended to B500 nm, with a saturated IPCE of 4% at 380–410 nm (Fig. 8b). Similar to the results reported by Pore et al.,25 sharp transient spikes were observed when the lamp was switched on and off in the linear sweep voltammogram, indicating the existence of a high carrier recombination rate upon N-doping. To enhance visible-light absorption without

Fig. 8 (a) The bandgap energies of N-doped and undoped ALD-grown TiO2 films extracted from the Tauc plot, (b) IPCE for N-doped and undoped TiO2 films at 0 V (vs. Ag/AgCl) in a 0.1 M KOH aqueous electrolyte (pH B13). Reproduced with permission from ref. 26. Copyright 2008 The Electrochemical Society. (c) XPS N 1s spectra and (d) wavelengthdependent photocurrents of N-doped TiO2 films grown with different NH3-to-H2O injection volume ratios. Reproduced with permission from ref. 27. Copyright 2011 Elsevier.

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introducing successive oxygen vacancies that may act as holetrapping sites, a moderate N-doping level is desired for TiO2. Cheng et al. also prepared a series of N-doped films with an NH3-to-H2O volume ratio varying from 350 to 500 : 1.27 The substitutional N at% could be adjusted by changing the NH3-toH2O ratio, and was determined by XPS (Fig. 8c). As illustrated in Fig. 8d, the photocurrent response is extended to B550 nm, with increased photocurrent in the visible light range as N concentration increases from 0.2 to 0.7 at%. However, when the N concentration is higher than 1.2 at%, the photoactivity of N-doped TiO2 rapidly decreases along with an abrupt increase in the oxygen vacancy level. Thus, an optimal N-doping concentration should lie between 0.7 and 1.2 at%. It is worth mentioning that the gaseous HCl by-product formed in the TiCl4–H2O and TiCl4–NH3 processes may readsorb on the surface and affect the main adsorption process, as well as etch the film constituents or the ALD chamber. Moreover, metal oxide particles may be generated when using TiCl4 as the Ti precursor, through the so-called agglomeration process. While details on how the agglomeration occurs have not yet been clarified, Puurunen proposed that surface hydroxyl groups are chlorinated to form intermediate hydroxychloride molecules. Then the hydroxychloride reacts, through its hydroxyl groups, with adsorbed surface chloride groups to form metal oxide particles.7 Despite the nonideal ALD behaviors of the TiCl4–H2O and TiCl4–NH3 processes, ALD-based doping has proved to be an effective method for bandgap engineering, and may find applications for many other photoelectrochemically important materials. 3.2

Forming homojunctions for carrier separation

ALD-based doping can form p–n homojunctions. Semiconductors such as hematite have suffered from high overpotentials, which require external bias to drive the reaction. While adding catalysts can partially overcome this problem, forming a buried homojunction to build internal fields is effective and practical. Lin et al. doped Mg into Fe2O3 by inserting one cycle of a bis(ethylcyclopentadienyl)magnesium (Mg(CpEt)2) precursor into every 5 cycles of an Fe precursor (iron tert-butoxide), resulting in 3% Mg-doped Fe2O3.28 Without changing the crystal structure and optical properties of hematite, Mg-doped Fe2O3 was proved to be indeed p-type as evidenced by a negative slope in the Mott–Schottky plot. Upon the formation of n–p homojunctions by directly growing 5 nm Mg-doped Fe2O3 on 20 nm planar Fe2O3 (intrinsic n-type due to oxygen vacancies), a 200 mV turn-on voltage reduction toward the cathodic direction was observed (Fig. 9a), which was caused by the built-in field that facilitated carrier separation. This is a significant improvement comparable to the result achieved by adding surface catalysts that improved carrier transfer. The existence of built-in field was verified by the high frequency semicircle in the Nyquist plot, as well as the accelerated open circuit decay after switching off of the lamp because the junction was a preferable site for electron– hole recombination. It is noteworthy that the p-type layer on top of the n-type Fe2O3 should be very thin to avoid introducing a potential well in the electronic structure which is detrimental to the PEC performance, and ALD is an excellent technique to


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Fig. 9 (a) J–V curves of n-type Fe2O3 with and without p-type coating (25 nm total thickness for both) under AM1.5 illumination, where a 200 mV reduction of the turn-on voltage was observed for the sample with 5 nm p-Fe2O3, (b) IPCEs of these two samples at 1 V (vs. RHE). Reproduced with permission from ref. 28. Copyright 2012 American Chemical Society.

meet this requirement because of the precise control of growth and doping.29

4 Anti-corrosion protective layers Narrow bandgap semiconductors as PEC electrodes are capable of absorbing a large portion of the solar spectrum. But the main issue of this group of materials is their poor stability in aqueous electrolytes. One strategy to improve their anti-corrosion ability is to coat a corrosion-resistant overlayer that protects the underlying semiconductors while not inhibiting charge transfer and light absorption. If the corrosion-resistant layer is extremely thin (o2 nm), electrons will tunnel through the protective layer to finish the reaction. Thicker overlayers without direct electron tunnelling have also been successfully demonstrated as anti-corrosion layers. 4.1

Ultrathin tunnelling protective layers

The tunnelling effect has been widely used in metal–insulator– semiconductor (MIS)-based microelectronic devices. In nonvolatile flash memories, the memory state can be written by direct tunnelling of electrons across the tunnel oxide layer, a thin oxide layer deposited on a Si substrate, into the floating gate that serves as a charge trapping layer. This motivates transfer of the state-of-the-art ALD technology to solar conversion systems, where high quality pinhole-free protective layers could be attained to protect corrosion vulnerable photoelectrodes. The MIS photoelectrode design was proposed as an effective architecture to allow stable and efficient PEC water splitting with narrow bandgap semiconductors.30 The key idea of MIS photoelectrodes is to use a thin insulating layer, usually an oxide, to protect the semiconductor against the corrosive electrolyte, while allowing carriers to tunnel through. Silicon is an attractive semiconductor for PEC water splitting because of its earth-abundance and narrow bandgap (B1.1 eV), but Si is chemically unstable in electrolytes, especially when biased. To overcome this drawback, Chen et al. deposited an ultrathin (2 nm) pinhole-free, corrosion-resistant TiO2 protective layer on pristine Si substrates using ALD, and showed that the underlying Si could remain intact for continuous operation in


Fig. 10 (a) Schematic and (b) cross-sectional TEM image of the TiO2protected Si anode with an Ir catalytic overlayer; constant current stability tests at 5.1 mA cm 2 (1 mA on a 0.196 cm2 sample area) under AM1.5 in (c) 1 M acid and (d) 1 M base, with and without the TiO2 protection layer. Reproduced with permission from ref. 31. Copyright 2011 Nature Publishing Group.

highly corrosive solutions.31 A 3 nm Ir catalytic overlayer—thin enough for light transmission—was also deposited on TiO2 to catalyze water oxidation. The schematic and transmission electron microscopy (TEM) image of the composite anode can be found in Fig. 10a and b. Under AM1.5 illumination, the n-Si sample generating a constant photocurrent of 5.1 mA cm 2 could remain functional in both 1 M acid and 1 M base solutions for at least 8 hours when protected with a 2 nm ALD-grown TiO2 layer, but failed within 0.5 hour without TiO2 (Fig. 10c and d). At a constant potential of 1.7 V (vs. NHE in 1 M NaOH, pH B13.5) in the dark, the p+-Si sample could generate a stable photocurrent around 15.3 mA cm 2 for 24 hours when protected with a 2 nm TiO2 layer in a 1 M NaOH solution, and failed within 0.5 hour without TiO2. When protected with 2 nm TiO2, the interfacial SiO2—resulting from the native oxide on the as-received Si substrate—remained of the same thickness before and after the stability test, while a thick insulating SiO2 layer was grown without the protection of TiO2. The facile electron tunnelling through the 2 nm TiO2 layer was confirmed by current–voltage (I–V) measurements, which also revealed that the electronic conduction across the TiO2 layer was highly dependent on the thickness of TiO2. Thus, using ALD to deposit a conformal protective film with accurately controlled thickness is of vital importance in stabilizing an otherwise corrosion vulnerable semiconductor. A similar tunnel oxide protected Si-based photoelectrode was also demonstrated by Esposito et al., although rapid thermal oxidation, rather than ALD, was used to prepare the ultrathin protective layer.30 ZnO also suffers from photo-corrosion in aqueous solutions under UV illumination. Zhang et al. deposited ultrathin (1 nm) TiO2 protective layers on ZnO NW arrays using ALD, and the resulting core–shell electrode exhibited stable PEC water splitting performance in a pH 13 alkaline solution.32 Although the authors did not claim a direct tunnelling mechanism, they pointed out

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that the protective layer should be very thin to shorten the diffusion path for minority carriers.


4.2

Non-tunnelling protective layers

In addition to ultrathin insulating layers that allow electron tunnelling, thicker corrosion-resistant overlayers with suitable band energy alignments with respect to the semiconductor can also efficiently promote charge transfer while protecting the underlying substrate. Notably, Si NWs were protected by B35 nm ALD-deposited TiO2 in a 1 M KOH electrolyte, as reported by Yang and co-workers.33 Under 100 mW cm 2 illumination, planar Si and Si NWs without TiO2 coating were etched and H2 was generated vigorously. In contrast, the Si NW–TiO2 core–shell structure showed a constant photocurrent of B0.28 and B0.18 mA cm 2 for n- and p-type Si NWs, respectively, at 1.23 V vs. RHE under AM1.5, over a 1 hour stability test. Unlike the Si–TiO2 system in ref. 31 where electrons tunnel through the ultrathin TiO2 layer (o2 nm), a 35 nm thick TiO2 will form a junction with Si, which facilitates the separation and transportation of the photogenerated carriers. TiO2 is intrinsically n-type, and different band bending will be formed with n-type and p-type Si NWs. Under illumination, electron–hole pairs are generated within both TiO2 and Si since the TiO2 shell is almost transparent in the visible light region. In an n-Si/n-TiO2 junction (Fig. 11a), the photogenerated holes in TiO2 move to the n-TiO2/ electrolyte interface for the oxidation reaction to produce O2 due to the band bending, while photogenerated electrons move to the n-Si/n-TiO2 interface and recombine with the photogenerated holes from n-Si. Meanwhile, photogenerated electrons within n-Si will flow to the counter electrode for the reduction reaction and release H2. In the case of a p-Si/n-TiO2 junction (Fig. 11b), the photogenerated holes in TiO2 can move either to the electrolyte or to the p-Si/n-TiO2 interface, and the photogenerated electrons in TiO2 will come across an undesirable potential barrier before reaching the p-Si/n-TiO2 interface. Thus, the unfavorable band bending in the p-Si/n-TiO2 junction would result in a photocurrent much less than that for the n-Si/n-TiO2 junction. Although ultrathin tunnelling TiO2 and thicker non-tunnelling TiO2 are both capable of protecting the underlying Si as anti-corrosion layers, one should keep in mind that the carrier transfer mechanisms are different.

Fig. 11 Schematic band diagrams and charge carrier transfer of (a) n-Si/ n-TiO2 and (b) p-Si/n-TiO2 under illumination. Reproduced with permission from ref. 33. Copyright 2008 American Chemical Society.

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Fig. 12 (a) SEM image of the p-InP nanopillar arrays, inset: schematic of the nanopillar p-InP/TiO2/Ru photoelectrode, (b) photocurrent stability test of the planar and nanopillar p-InP/TiO2/Ru photoelectrodes under AM1.5 in 1 M HClO4 (pH 0.51) at 0.23 V (vs. NHE). Reproduced with permission from ref. 34. Copyright 2012 Wiley-VCH. Photocurrent stability tests of (c) a MnO-coated n-Si sample under AM1.5 at 0.6 V (vs. Ag/AgCl) in 1 M KOH (pH B13.5), and (d) bare n-Si and MnO-coated n-Si samples under AM1.5 at 0 V (vs. solution potential) in 0.35 M K4Fe(CN)6–0.05 M K3Fe(CN)6. Reproduced with permission from ref. 35. Copyright 2013 American Chemical Society.

Lee et al. deposited 3–5 nm TiO2 on p-type InP nanopillars using ALD, and studied the electrode stability in acid media for H2 evolution.34 The planar InP substrates were nanostructured into nanopillars for enhanced anti-reflection and improved wettability to suppress H2 bubble formation. A top Ru catalytic layer was added to improve the open-circuit potential of InP (Fig. 12a). When protected with a 5 nm ALD-grown TiO2 layer, the nanopillar electrode demonstrated stable operation in 1 M HClO4 solution (pH 0.51) for more than 4 hours with a constant photocurrent of B37 mA cm 2 at 0.23 V (vs. NHE) under AM1.5 illumination (Fig. 12b). Unlike the ultrathin TiO2 films (o2 nm), the authors considered that 2–5 nm is too thick for electrons in the InP substrate to tunnel through, and that the electrons may be transported through tail or defect states of amorphous TiO2, or even pinholes. Even without direct tunnelling, the measured photocurrent of 37 mA cm 2 and the onset potential of 730 mV (vs. NHE) for the 5 nm-TiO2-protected p-InP verified the effectiveness of ALD-deposited protective layers for facile carrier transfer. Without over-layered noble-metal catalysts, metal-oxide protective layers have also proved to efficiently prohibit the corrosion of the underlying narrow bandgap semiconductor, while maintaining a high charge transfer rate. Strandwitz et al. demonstrated that a 10 nm ALD-grown MnO layer can provide sufficient protection against the corrosion of n-Si, which displayed a stable photocurrent of B28 mA cm 2 at 0.6 V (vs. Ag/AgCl) under AM1.5 illumination in 1 M KOH solution (pH B 13.5) for 10–30 min (Fig. 12c).35 The spikes in photocurrent were due to the formation of O2 bubbles. When a 0.35 M K4Fe(CN)6–0.05 M K3Fe(CN)6 solution was used, the MnO-protected n-Si electrode


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exhibited a stable photocurrent of B36 mA cm 2 at 0 V (vs. solution potential) under AM1.5 (Fig. 12d). In both cases, bare n-Si electrodes without MnO were passivated quickly and exhibited negligible photocurrents. Compared with ultrathin TiO2,31 MnO protective layers are less effective in preventing the formation of the insulating oxide on the Si surface, which was attributed to the large resistivity of MnO, as well as the change of the ionic radius of Mn over redox cycling that changed the structure of MnO, introducing porosity in the protective layer, thus rendering the electrode to be passivated. Cuprous oxide (Cu2O), with a direct bandgap of 2 eV and composed of an earth-abundant element, has been considered as a promising p-type oxide semiconductor for PEC water splitting. The main problem of Cu2O is its low stability in electrolyte solutions, because the redox potential of Cu2O lies within its bandgap. Paracchino et al. used ALD to deposit n-type Al-doped ZnO (Al:ZnO) and TiO2 hybrid layers to protect Cu2O films in a 1 M Na2SO4 electrolyte buffered at pH 4.9.36 Pt nanoparticles were also electrodeposited on the electrode to catalyze the water reduction reaction that reduces H+ ions in water to generate H2 (Fig. 13a). The staggered band offset between the p-type substrate and the n-type protective layer could facilitate the transfer of photogenerated electrons from the Cu2O substrate through the protective layer to the electrolyte to complete the reduction reaction. Optimized electrodes showed a photocurrent of 7.6 mA cm 2 at 0 V (vs. RHE) at mild pH, and the Faradaic efficiency was estimated to be close to 100%. When protected with Al:ZnO and TiO2, the Cu2O electrodes remained active after 1 hour of testing during the water reduction reaction. Without a protective layer, bare Cu2O electrodes under AM1.5 illumination at 0 V (vs. RHE) quickly formed metallic Cu that was visually observable as black circles, and confirmed by scanning electron microscopy (SEM) and XPS. TiO2 alone of a thickness up to 30 nm still could not protect the Cu2O electrode for more than 20 min, which was attributed to the formation of pinholes and non-uniform TiO2 growth. A 20 nm ZnO layer was necessary to contribute more hydroxyl groups on the Cu2O surface to enhance uniform and

Fig. 13 (a) Schematic of the Cu2O electrode protected by ALD-grown Al:ZnO and TiO2, with Pt nanoparticles deposited as catalyst. Reproduced with permission from ref. 36. Copyright 2011 Nature Publishing Group. (b) A 10 hour stability test for a Cu2O/Al:ZnO(20 nm)/TiO2(20 nm)/Pt electrode at 0 V (vs. RHE) under chopped AM1.5 illumination at pH 5, where the TiO2 top-layer was deposited at 200 1C and annealed at 200 1C for 2 hours in oxygen. Reproduced with permission from ref. 3. Copyright 2012 The Royal Society of Chemistry.


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dense adsorption of Ti precursor molecules during the ALD process. The incorporation of Al2O3 into ZnO was believed to stabilize the ZnO layer, rather than increase the electron concentration. Paracchino et al. also showed that the deposition temperature used in ALD is one of the most determinant factors influencing the band energy positions of the Al:ZnO and TiO2 protective layers, a critical factor for favorable band alignments to enhance carrier separation.3 Etching away of the protective TiO2 layer was observed during the stability test, and TiO2 could be stabilized by Nb incorporation. Similar to the Al incorporation into ZnO, this was ascribed to a structural stabilization phenomenon rather than electron doping. When the TiO2 toplayer was deposited at 200 1C and annealed at 200 1C for 2 hours in oxygen for improved crystallinity, 62% of the initial photocurrent value at 0 V (vs. RHE) can be retained over a 10 hour stability test under AM1.5 illumination at pH 5 (Fig. 13b). One should note that when protecting nanostructured substrates such as Si NWs, InP nanopillars, and cubic Cu2O with irregular structures, ALD is an ideal, if not the only, technique in sealing the complex surface topologies uniformly owing to its unique surface self-limiting growth mechanism. When the tunnel oxide concept is used to protect the narrow bandgap semiconductor, the atomic-scale thickness control of ALD makes it well suited to deposit ultrathin films with minimum pinholes and defects.

5 Passivation of surface states Surface recombination has been a primary concern for PVrelevant devices for decades. Nanostructured surfaces may not be an appealing electrode design in terms of reducing surface recombination, although nanostructures are beneficial to enhance the absorption of incident light by directing the propagation of light along a specific path, or forming an anti-reflective layer that can be considered as an effective medium with a gradient refractive index profile developed between the substrate and air. There exists a trade-off between nanotexturing the electrode surface and suppressing the surface recombination, unless an effective strategy is introduced to passivate the surface states (e.g., carrier trapping or recombination sites due to crystal defects) for nanostructured surfaces. In the field of PEC water splitting, Yang and co-workers deposited TiO2 thin films on rutile TiO2 NW cores by ALD using TiCl4 and H2O at 300 1C.37 With 150 ALD cycles, a 13–15 nm rutile TiO2 shell was epitaxially grown on the rutile NW core, as verified by high-resolution TEM (HRTEM) imaging and selected area electron diffraction (SAED) (Fig. 14a), while 60 and 300 ALD cycles resulted in amorphous and anatase phases of TiO2, respectively. When coated with a 150-ALD-cycle epitaxial rutile shell, the photocurrent of the 1.8 mm long TiO2 NW electrode was improved to 1.1 mA cm 2 at 1.5 V (vs. RHE), 50% higher than that of the bare TiO2 NW electrode (Fig. 14b), because the epitaxial layer could passivate the surface trapping states of NW and suppress the surface recombination rate,

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In addition to realizing stable PEC water splitting in a strong alkaline solution, the ZnO NWs coated with a ultrathin (1 nm) TiO2 shell, as demonstrated by Liu et al., exhibited a photocurrent of 0.5 mA cm 2 at 1.23 V vs. RHE under AM1.5 illumination, 25% higher than that of bare ZnO NW electrodes, which was attributed to the passivation of surface trapping states on ZnO NW, thus improving the carrier transfer efficiency.32

Fig. 14 (a) HRTEM image and SAED patterns (insets) of 150 ALD cycles of a rutile TiO2 thin film epitaxially grown on a rutile TiO2 nanowire, (b) photocurrent densities for 1.8 mm long TiO2 nanowire array electrodes coated with various ALD cycles of TiO2 (60, 100, 150, 200, 250, 300, and 450 cycles), where 60, 150, and 300 cycles correspond to 5–7, 13–15, and 25–30 nm TiO2, respectively. Reproduced with permission from ref. 37. Copyright 2012 American Chemical Society.

thus improving the charge collection efficiency. In contrast, an anatase shell decreased the photocurrent compared with bare rutile NW, since anatase films were unable to be epitaxially deposited on the rutile core, and the grain boundaries formed in the polycrystalline anatase shell served as extra charge trapping sites. Also, it has been recently demonstrated that the electron affinity of anatase is higher than that of rutile, and both the valence and conduction bands of rutile are higher than those of anatase.38 Thus, the holes generated in the rutile core would have to overcome an energy barrier to transfer to the anatase shell, which largely reduced the hole transfer efficiency. For a-Fe2O3 with a bandgap (2.0–2.2 eV) close to the ideal requirement for water splitting, high electron–hole recombination and surface trapping states are among the major factors limiting the performance of a-Fe2O3.39 Sivula and co-workers demonstrated that the surface trapping states can be partially alleviated by coating an extremely thin (0.1 to 2 nm) Al2O3 passivation layer by ALD.40 The overpotential of Si-doped a-Fe2O3 electrodes was reduced by 100 mV after ALD Al2O3 passivation, and the photocurrent was increased from 0.24 to 0.85 mA cm 2 at 1 V (vs. RHE) under AM1.5 illumination. After the addition of a Co2+ catalyst, a record photocurrent of 0.42 mA cm 2 was achieved at 0.9 V (vs. RHE). Despite the thermodynamic instability of Al2O3 in base solution, the photocurrent of the Al2O3 coated a-Fe2O3 electrode was stable for the entire duration of the PEC measurement (ca. 30 min) in the 1 M NaOH electrolyte (pH 13.6). However, long term exposure (ca. 1 h) in 1 M NaOH at 1.03 V vs. RHE under AM1.5 illumination caused the dissolution of the Al2O3 layer. A catalytic effect of the Al2O3 coating was ruled out by carefully examining the transient photocurrents, electrochemical impedance, and photoluminescence spectra, and it was confirmed that the enhanced PEC performance was due to surface state passivation of the ALD-grown ultrathin overlayer. Indeed, Al2O3 deposited via ALD has a stoichiometric composition over a wide temperature range,41 which could fill the oxygen vacancies on the surface of a-Fe2O3 resulting from a disordered crystalline surface acting as the origin of surface trapping states.42 The surface passivation coating can also act as a multifunctional layer to prevent photo-corrosion and aid charge transfer.

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6 Catalytic layer Besides anti-corrosion layers and surface state passivation layers, ALD is also capable of depositing conformal surface catalytic layers on complex 3D electrodes to increase the charge transfer kinetics and lower the overpotential in the PEC process. Bent and co-workers reported the first investigation of highly active non-noble MnOx/glassy carbon catalysts using ALD for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR).43 In the OER, hydroxyl groups in water can be oxidized to O2, which is particularly important for PEC water splitting. Using bis(ethylcyclopentadienyl) manganese (Mn(EtCp)2) and water as the Mn and O precursors, the authors deposited 500 cycles of MnOx (B42 nm) and demonstrated that the as-deposited films were MnO and transformed into Mn2O3 after annealing. In terms of the OER, MnO and Mn2O3 required 1.84 V and 1.81 V (vs. RHE) to reach a current density of 10 mA cm 2, respectively, in the anodic-going sweep of the first cycle of the cyclic voltammetry measurement in an O2 saturated 0.1 M KOH electrolyte. This result indicated a high OER catalytic activity for both MnO and Mn2O3, and their similar activity was likely because the surfaces of the MnO and Mn2O3 catalysts were electrochemically oxidized to similar active states at the high anodic potentials required for water oxidation. Wang and colleagues confirmed that ALD-grown MnOx films as thin as one ALD cycle exhibited catalytic activities for the OER in the dark for 20 nm hematite (on 100 nm Pt/Ti foil substrates) photoanodes. Surprisingly, when MnOx coated hematite samples were exposed to AM1.5 illumination for PEC measurements in 1.0 M NaOH aqueous electrolyte (pH 13.5), the photocurrent onset potential for 0.1, 0.5, and 2 nm MnOx coated hematite was 1.37, 1.53, and 1.52 V (vs. RHE), respectively, significantly more positive than 1.03 V for bare hematite. Moreover, a significant decrease of photocurrent for MnOx coated samples was observed compared to that for bare hematite (Fig. 15a). The worse PEC performance upon MnOx deposition was attributed to a high density of surface states introduced by MnOx, which may result in a potential drop within the Helmholtz layer, causing the Fermi-level pinning effect that hindered the carrier separation process within the semiconductor. Thus, when trying to coat a catalytic layer on a photoelectrode, great care should be taken to avoid creating a high density of surface states at the interface.44 Co-based catalyst is another attractive alternative to Ir- or Ru-based OER catalysts when hematite is used as the photoanode material. Using a CoCp2/O3 ALD process, Riha et al. coated submonolayer Co(OH)2/Co3O4 coatings on 13 nm planar


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Fig. 15 (a) J–V plots for 20 nm hematite (on 100 nm Pt/Ti foil substrates) planar photoanodes with and without ALD-deposited MnOx catalytic layers in a 1.0 M NaOH aqueous electrolyte (pH 13.5) under AM1.5 illumination. Reproduced with permission from ref. 44. Copyright 2012 Elsevier. (b) J–V plots for 13 nm hematite photoanodes on a planar FTO-ITO substrate (black), on an ITO-coated IO scaffold (green), on an equivalent planar electrode further coated with 1 CoCp2/O3 ALD cycle (blue, dashed), and on an equivalent IO scaffold further coated with 1 CoCp2/O3 ALD cycle (purple, dashed). Performance was measured at pH 13.1 under AM1.5 illumination. Reproduced with permission from ref. 45. Copyright 2013 American Chemical Society.

hematite and nanostructured hematite IO. The as-deposited film was 90% Co(II), in the form of either Co(OH)2 or CoO, and B10% Co3O4. Tested at pH 13.1 (0.1 M KOH electrolyte) under AM1.5 illumination, just one ALD cycle of CoCp2/O3, corresponding to a thickness of 0.06 nm, significantly reduced the photocurrent onset potential by B100 and 200 mV for planar and nanostructured IO hematite electrodes, respectively. The photocurrent of planar and IO hematite with this submonolayer Co(OH)2/Co3O4 catalyst reached 1.4 and 2.3 mA cm 2 at 1.53 V vs. RHE, respectively (Fig. 15b).45 Moreover, Ekerdt and co-workers demonstrated that CoO thin films can be epitaxially grown on SrTiO3 (STO)-buffered Si(001) by ALD, and the formed CoO(12 nm)/STO(13.6 nm)/Si photoelectrode showed visible light water photooxidation activity, while no PEC response was observed for the TiO2/STO/Si sample, which was attributed to the Co t2g states positioned inside the bandgap of STO, allowing the absorption of visible light by exciting electrons from the Co t2g states to the empty Ti 3d states at the CoO/STO interface.46

7 Planar model electrodes The major goal of studying model electrodes is to understand the charge separation/accumulation/transfer mechanisms and identify the main rate limiting steps in the PEC water splitting systems. With well defined thickness, composition, and interfacial structure, planar thin films deposited by ALD could serve as excellent model electrodes to identify the structure–function relationship of the PEC system and ensure the validity of electrochemical measurement results, such as fitting the impedance spectroscopy (IS) data and applying the Mott–Schottky equation. Owing to the conformal coating ability of ALD, optimized planar films could be directly coated onto 3D nanostructures to form high performance nanostructured electrodes with the strategies discussed earlier. In addition, information extracted


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from the well-defined model electrode would be of great value in assisting in the design of new PEC systems. IS is a technique to examine the energy storage (capacitance) and energy dissipation (resistance) properties of an electrochemical system, but the main issue is the extraction of reliable information while avoiding ambiguous over-parameterization. One approach is to construct simplified and well-established equivalent circuits to interpret the IS results.47 When applying the Mott–Schottky equation to a semiconductor in contact with an electrolyte, it is important to note that this equation is based on the assumption that the semiconductor–electrolyte interface is perfectly planar, except when the surface roughness is either smaller or larger than the thickness of the space-charge layer.48 ALD can be used to fabricate well-defined photoelectrodes with planar interfaces. As a model material, TiO2 was deposited as a planar electrode by ALD to explore its PEC properties. Cheng et al. found that amorphous TiO2 was formed initially and then crystallized into anatase phase when the film thickness exceeded a critical value at low deposition temperatures. The grain size was affected by the deposition temperature, and had a minimum at B400 1C. The increase of grain size and decrease of defect density could enhance the PEC properties of the TiO2 thin films.49 Using the capacitances by fitting the IS data with the equivalent circuit, ¨ et al. calculated the flat band potential and donor Heikkila density of planar TiO2 from the Mott–Schottky equation, and estimated that the depletion layer thickness of TiO2 should be less than 50 nm.50 The authors noted that the optimal film thickness is larger than the sum of the depletion layer thickness and the carrier diffusion distance of TiO2, and attributed this to increased crystallinity and grain size of thicker films, which enhanced the development of the depletion layer and reduced the recombination centers during the hole transfer. Hamann and co-workers systematically studied ALD-deposited planar hematite (a-Fe2O3) thin films as simplified models to isolate the mutually related factors determining the PEC performance of a-Fe2O3.39,47,51–54 Using the planar a-Fe2O3 model film deposited by ALD, it was pointed out that when fast redox shuttles, such as [Fe(CN)6]3 /4 and [IrCl6]3 /4 solutions, were used as the electrolyte, the rate of hole collection was not the bottleneck of the photocurrent. When an aqueous electrolyte without a fast redox shuttle was used, the photocurrent was largely reduced, indicating that water oxidation was the rate limiting step, which confirmed the necessity of adding a oxidation catalyst to the a-Fe2O3 system.51 Owing to the atomic level thickness control of ALD, a-Fe2O3 with thickness from 7 to 56 nm was used as an electrode in aqueous [Fe(CN)6]3 /4 solution for photocurrent density measurements.52 Under AM1.5 illumination at 0.19 V vs. SCE (1.14 V vs. RHE) at pH 12, the photocurrent density first increased linearly with a-Fe2O3 thickness from 7 to 20 nm, and saturated at around 0.25 mA cm 2 (Fig. 16a). The initial increase of photocurrent could be partially attributed to the increase of Fe2O3 thickness. However, the 20 nm a-Fe2O3 absorbed about twice as many photons as the 7 nm a-Fe2O3, but generated more than four times the photocurrent. Films thicker than 20 nm

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Fig. 16 (a) Photocurrent density from J–V curves with different a-Fe2O3 thicknesses, (b) impedance spectra for a-Fe2O3 with 1200 ALD cycles (50 nm) for capacitance extraction (inset, simplified equivalent circuit). Reproduced with permission from ref. 52. Copyright 2011 American Chemical Society.

absorbed more photons but did not generate higher photocurrent. These phenomena can be explained as follows: only holes generated in the depletion region of the film can be driven to the electrolyte interface for water oxidation, while holes generated in the bulk (deeper than 20 nm) would not be able to drift to the surface to contribute to photocurrent. Since a simple planar system was used as the electrode, the capacitances could be determined by fitting the impedance data to a simple equivalent circuit (Fig. 16b), and the Mott–Schottky equation could be safely applied to calculate dopant density and flat-band voltage. The thickness of the depletion region of a-Fe2O3 was determined to be B17 nm, with a maximum hole drift length of B5.8 nm. The 20 nm optimal a-Fe2O3 thickness determined experimentally was approximately equal to the calculated depletion region, which implies that the initial thickness increase below 20 nm would increase the electric field that developed across the film. Thus, the carrier recombination in the bulk was identified as the main factor limiting the performance of a-Fe2O3 electrodes. Only charge carriers generated in the depletion region can be transferred to the interface. Owing to the clearly defined thin film electrode structure prepared by ALD, Hamann and co-workers also cleverly constructed a general physical model to describe the surface states at the a-Fe2O3/solution interface.39 A simplified equivalent circuit (Fig. 17a) was used to interpret the IS data for an electrode with 60 nm planar a-Fe2O3 on the FTO substrate. As shown in Fig. 17b, the strong correlation between the peak of Ctrap (capacitance due to charging surface states) and the valley of Rct,trap (charge transfer of holes to the solution affected by surface states), as well as the onset position of photocurrent, suggested that the water oxidation reaction was predominantly aided by holes trapped by surface states, rather than holes directly from the valence band. It was clearly shown that photogenerated holes were trapped in surface states prior to the onset of photocurrent, and it was deduced that reversible oxidation of the surface hydroxide species on a-Fe2O3 electrodes was the first step in water oxidation. Suppressing the surface electron concentration was pointed out as a strategy to facilitate the formation of surface intermediates to assist the water oxidation process.47 Electrodes with 60 nm ALD-deposited a-Fe2O3 were also coated with cobalt phosphate (Co–Pi) catalyst, and a simplified equivalent circuit was used to interpret the IS behavior (Fig. 17c).53 The capacitance of the Co–Pi layer (CCo–Pi)

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Fig. 17 (a) Simplified equivalent circuit and (b) J–V curve, Ctrap (capacitance due to charging surface states) and Rct,trap (charge transfer of holes to the solution affected by surface states) extracted from fitting the IS data for a 60 nm planar a-Fe2O3 electrode, under AM1.5 at pH 6.9; reproduced with permission from ref. 39; (c) simplified equivalent circuit and (d) extracted CCo–Pi (capacitance of the Co–Pi layer) for 60 nm planar a-Fe2O3 electrodes coated with Co–Pi catalyst with different thicknesses, under AM1.5 at pH 6.9. Reproduced with permission from ref. 53. Copyright 2012 American Chemical Society.

increased with the increasing thickness of Co–Pi (Fig. 17d), suggesting that holes were stored in the Co–Pi catalyst. Current transients showed spike features when illumination was turned on and off, corresponding to the oxidation and reduction of Co(III) and Co(IV) in the Co–Pi catalyst. a-Fe2O3 electrodes with a thicker Co–Pi layer required a larger amount of charge to pass through to complete the oxidation or reduction reaction before reaching a steady state current (data not shown). This is consistent with the trend of CCo–Pi in Fig. 17d, implying that holes were efficiently collected and stored in the Co–Pi catalyst, which reduced the surface state recombination and increased the water oxidation efficiency. When the model electrode is fabricated with doped thin films with controllable dopant concentration and distribution, the real function of the incorporated elements could be identified, allowing the deconvolution of surface effects (catalysis, surface passivation, etc.) from bulk effects (e.g., electric doping and crystallinity) in the PEC process. Hamann and co-workers prepared planar a-Fe2O3 thin films with controlled amounts of Ti dopants to investigate the effect of Ti-doping.54 By depositing very thin TiO2 films on top of a-Fe2O3, the identical photocurrents with and without an TiO2 overlayer indicated that no surface catalytic or passivation effects were associated with Ti atoms. Mott–Schottky analysis applied on a homogeneously Ti-doped hematite electrode with 1 ALD cycle of Ti inserted into every 15 ALD cycles of Fe2O3 ruled out the possibility that Ti acted as electric dopants since the electric dopant density was found to be unchanged upon Ti-doping. By fitting the IS data to the equivalent circuit, the authors concluded that


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Ti dopants produced a hematite surface with a higher density of active sites for water oxidation. Moreover, due to the ease of thickness control of ALD, the photocurrents of the 300 and 1200 cycles Ti-doped and undoped hematite electrodes were compared; it was found that the thicker hematite films were not significantly improved upon the incorporation of Ti atoms, both in aqueous solutions and with a fast hole collector, which implied that the incorporation of Ti could alleviate the dead layer effect that had resulted from an imperfect crystal layer at the hematite–FTO and hematite–electrolyte interfaces.

8 Conclusions and outlook ALD offers two significant advantages over many other competing techniques such as CVD, pulsed laser deposition (PLD), e-beam evaporation, and sputtering for thin film deposition: (i) controllable thickness at the atomic level, and (ii) the ability to coat complex 3D nanostructures. Additionally, ALD is capable of composition control and doping foreign elements, as many other deposition techniques can do. These features make ALD an excellent tool to fabricate both photoactive layers and performance enhancing layers for photoelectrodes. Many major obstacles in realizing high performance PEC water splitting can be overcome by the assistance of ALD. The short carrier diffusion length of a semiconductor can be addressed by depositing it as a thin film on conductive scaffolds and/or forming homoor heterojunctions. Inadequate visible light absorption can be enhanced by bandgap engineering and/or forming light trapping structures. Surfaces of the photoelectrodes can be modified by coating conformal layers with accurate thickness and composition for photo-corrosion prevention, surface state passivation, and catalytic purposes. Rate limiting steps for PEC water splitting can be identified with electrodes with clearly defined composition, morphology, thickness, and interface. The major disadvantage of ALD is the slow deposition rate, since usually less than a monolayer of material is deposited in each ALD cycle. Depositing thin films on 3D complex structures may make this problem even more severe, because it requires a longer time for dosing and purging compared to coating on planar substrates. Thus, growing micrometer-thick films by ALD may not be very much practical. However, the slow deposition rate can be compensated by batch processing (i.e., depositing on multiple samples simultaneously), which is a feasible solution for ALD since its surface controlled reaction relies less on the flow type of the precursors and the carrier gas. Other novel designs such as rotary ALD, spatial ALD, roll-to-roll ALD, and atmospheric pressure ALD have also been developed for higher throughput and industrial scaling-up. The ALD technique is expected to achieve greater impact in the area of PEC water splitting as long as the capability of ALD continues to advance. Although the range of materials that could be grown by ALD is wide,8 many photoelectrochemically important materials, such as Au and stoichiometric Ta3N5 (without metallic TaN), still remain difficult to deposit. Catalytic metals and metal oxides, such as Pt, Pd, Ir, and IrO2, have been


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successfully deposited by ALD, but their precursors are prohibitively expensive. Toxic ALD precursors, such as Si2H6 and H2S, have limited the application of important photoactive materials such as Si, Ce2S3, etc. Thus, new ALD precursors and chemistries are desired to expand the usage of ALD in PEC water splitting systems, which requires extensive future investigations. Furthermore, ALD-based doping is always achieved by inserting one ALD layer of doping material into several layers of the host material, which makes it difficult to achieve homogeneous doping in extremely low concentrations. Instead of introducing dopant elements as discrete ALD cycles, pulsing the precursors for doping and host materials simultaneously with precisely controlled vapor pressures may be an alternative but requires more research studies on ALD methodology as well as the theory for the competing adsorption of two kinds of precursor molecules. Moreover, enhancing the film crystallinity, or even realizing epitaxial growth with defect-free interfaces on various 3D nanostructures, would greatly improve the quality of the ALD-deposited layers for superior PEC performances. In order to better understand the reaction chemistry, growth behavior, and surface/interface evolution during the ALD process, state-of-the-art analysis modules, such as in situ spectroscopic ellipsometry, Fourier transform infrared spectroscopy, and angle resolved XPS, need to be interfaced with ALD systems to monitor and predict the bulk and interface properties of the deposited film.

Acknowledgements We acknowledge the National Natural Science Foundation of China (21222604, 51302185), the Program for New Century Excellent Talents in University (NCET-10-0611), the Scientific Research Foundation for the Returned Overseas Chinese Scholars (MoE), the Seed Foundation of Tianjin University (60303002, 60302042), the Program of Introducing Talents of Discipline to Universities (B06006), and the Natural Science Foundation of Tianjin City (13JCYBJC37000) for financial support.

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Improving hematite-based photoelectrochemical water splitting with ultrathin TiO2 by atomic layer deposition.

BiVO4 photoanodes for efficient photoelectrochemical water splitting.

TiO2 nanorods for photoelectrochemical water splitting.

Complex Materials by Atomic Layer Deposition.

Fabrication of Thin Films of α-Fe2O3 via Atomic Layer Deposition Using Iron Bisamidinate and Water under Mild Growth Conditions.

Perovskite thin films via atomic layer deposition.

Atomic layer deposition of metastable β-Fe₂O₃ via isomorphic epitaxy for photoassisted water oxidation.

Interface induce growth of intermediate layer for bandgap engineering insights into photoelectrochemical water splitting.

Gyroidal mesoporous multifunctional nanocomposites via atomic layer deposition.

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Fabrication of transferable Al(2)O(3) nanosheet by atomic layer deposition for graphene FET.

Activation of Ultrathin Films of Hematite for Photoelectrochemical Water Splitting via H2 Treatment.

Integrating atomic layer deposition and ultra-high vacuum physical vapor deposition for in situ fabrication of tunnel junctions.

Controllable photonic mirror fabricated by the atomic layer deposition on the nanosphere template.

Photoelectrodes based upon Mo:BiVO4 inverse opals for photoelectrochemical water splitting.

Deposition of uniform Pt nanoparticles with controllable size on TiO2-based nanowires by atomic layer deposition and their photocatalytic properties.

Atomic layer deposition in nanostructured photovoltaics: tuning optical, electronic and surface properties.

Fabrication of nanofluidic diodes with polymer nanopores modified by atomic layer deposition.

Atomic layer deposition of a MoS₂ film.

Synergistic Effect of Surface Plasmonic particles and Surface Passivation layer on ZnO Nanorods Array for Improved Photoelectrochemical Water Splitting.

Upscaling of integrated photoelectrochemical water-splitting devices to large areas.

An alumina stabilized ZnO-graphene anode for lithium ion batteries via atomic layer deposition.

Fabrication of highly stable silica coated ZnCuInS nanocrystals monolayer via layer by layer deposition for LED application.

Logic gates operated by bipolar photoelectrochemical water splitting.