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Hole-selective CoO/SiO/Si Heterojunctions for Photoelectrochemical Water Splitting Seungtaeg Oh, Soonyoung Jung, Yong Hwan Lee, Jun Tae Song, Tae Hyun Kim, Dip K. Nandi, Soo-Hyun Kim, and Jihun Oh ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b03520 • Publication Date (Web): 12 Sep 2018 Downloaded from http://pubs.acs.org on September 12, 2018

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ACS Catalysis

Hole-selective CoOx/SiOx/Si Heterojunctions for Photoelectrochemical Water Splitting Seungtaeg Oh†, Soonyoung Jung§, Yong Hwan Lee†, Jun Tae Song‡,⊥, Tae Hyun Kim§, Dip K. Nandi§, Soo-Hyun Kim*,§, and Jihun Oh*,†,‡,⊥

†Department

of Materials Science and Engineering, Korea Advanced Institute of Science and

Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea ‡Graduate

School of Energy, Environment, Water, and Sustainability (EEWS), KAIST, 291

Daehak-ro, Yoseong-gu, Daejeon 34141, Republic of Korea ⊥

KI Institute for NanoCentury, KAIST, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic

of Korea §School

of Materials Science and Engineering, Yeungnam University, Gyeongsangbuk-do

38541, Republic of Korea

ACS Paragon Plus Environment

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ABSTRACT

Cobalt oxide (CoOx), an earth-abundant and low-cost oxygen evolving catalyst (OEC), has notable advantages as a top protection layer of photoanodes for solar-driven water oxidation due to its desirable durability. However, cobalt oxides exist as various phases, such as Co(II)O, Co2(III)O3, Co3(II,III)O4, and the (photo)electrochemical properties of CoOx are significantly governed by its phase. Atomic layer deposition (ALD) is a suitable method to form a multifunctional layer for photoelectrochemical (PEC) water splitting because it allows direct growth of a conformal high-quality film on various substrates as well as facile control over its chemical phases by adjusting the deposition conditions. Here, a well-controlled CoOx/SiOx/n-Si heterojunction prepared by ALD is demonstrated for solar-driven water splitting. The phase of the ALD CoOx films can be easily controlled from CoO to Co3O4 by varying the deposition temperature. In addition, this systematic study reveals that its energetic as well as electrochemical properties are changed significantly with the phase. Whereas CoO grown at 150 o

C produces high photovoltage by building desirable hole-selective heterojunctions with n-Si,

Co3O4 formed at 300 oC has a better catalytic property for water oxidation. To address this competitive correlation, we developed a double-layered (DL) ALD CoOx film that has advantages of both CoO and Co3O4. The DL ALD CoOx/SiOx/Si heterojunction photoanode produces photocurrent density of 3.5 mA/cm2 without a buried junction and maintains saturating current density of 32.5 mA/cm2 without noticeable degradation during 12 hours in 1M KOH under a simulated 1 sun illumination.

KEYWORDS photoelectrochemical water splitting, oxygen evolution reaction (OER), CoOx, silicon, atomic layer deposition (ALD), carrier-selective contact, heterojunction.


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ACS Catalysis

INTRODUCTION Photoelectrochemical (PEC) water splitting is considered an ideal renewable energy technology because it is able to produce hydrogen using only water and solar energy.1-3 Since silicon is earth-abundant, low-cost, and capable of absorbing most of the solar spectrum, it is a promising material for realizing commercialization of PEC water splitting.4-6 In addition, it has an optimum band gap (~1.1 eV) as the bottom cell in part of a tandem structure that can produce theoretical maximum solar-to-hydrogen (STH) efficiency.7 However, bare silicon requires high overpotential for water reduction or oxidation reactions due to its poor surface property and therefore it is necessary to use a cocatalyst for reducing such overpotential.8-9 Moreover, with respect to the oxygen evolution reaction (OER), since Si is easily oxidized or even corrodes in the water oxidation conditions, a protection layer is indispensable for sustainable PEC water splitting of Si-based photoanodes.10-11 In conventional Si-based photoanodes, a metal cocatalyst is usually used.11-12 Although these metal cocatalysts generally have desirable catalytic properties and durability, the high opacity of the metal blocks the sunlight incident to Si. Therefore, the design of a metal cocatalyst on a photoanodes should be determined carefully for maximizing its efficiency.11,

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As an

alternative protection strategy for Si-based photoanodes, metal-oxide semiconductors, such as TiO2, NiOx, and SrTiO3, have recently garnered intense attention.14-18 Since these metal-oxide semiconductors have a wide bandgap allowing high penetration of sunlight and are chemically stable under water oxidation condition, they are considered to be a suitable material for a Si protective layer.


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Thus far, three strategies for protecting the Si photoanode using a metal-oxide semiconductor have been proposed. First, Chen et al. presented ultra-thin (~2 nm) TiO2/Si photoanodes prepared using atomic layer deposition (ALD). This thin layer allows tunneling of holes from Si to electrolyte and also protects the underlying Si for 24 hours under water oxidation conditions.19 However, exquisite control is required for deposition because a tiny difference in the thickness can remarkably affect the stability and efficiency of Si photoanodes. Second, the Lewis group used a defective thick metal-oxide layer as a protective layer. In this case, the thick metal-oxide layer permits transportation of holes from Si to the electrolyte through a defect-band and this strategy demonstrates high durability over 100 hours.20 Finally, a hole-selective, multi-functional heterojunction using a chemically stable high band gap material is considered a promising strategy for Si photoanodes.15 Recently, the carrier-selective heterojunctions attract great attention in Si photovoltaics due to their outstanding solar conversion efficiency.21-22 Therefore, if the (electro)chemically robust high band gap material is able to transport holes selectively from Si to electrolyte, this heterojunction can give excellent PEC performance and durability. For instance, in case of the photoanodes, the valence band edge of the protective layer should be energetically aligned with or higher than that of Si to build the desirable hole-selective heterostructure. Then, valence band holes generated in Si can move spontaneously to the electrolyte in absence of any energy barrier at the interface. In contrast, conduction band electrons in Si are not able to transport to electrolyte. In this case, it is possible to produce a top metal-oxide layer that is thick enough to protect the underlying Si and this hetero-junction can produce high photovoltage due to its excellent charge collection property with suppressed carrier recombination. In addition, if this protective material has a catalytic property for water oxidation, it can serve as a multi-functional layer acting as a cocatalyst.


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ACS Catalysis

Therefore, to design a PEC cell having hetero-structures, it is critical to develop a multifunctional layer with excellent electrocatalytic properties and proper band alignments for Si that maximize charge transport to the co-catalysts from Si. Cobalt oxide (CoOx) is a promising oxygen evolution catalyst (OEC) because it is earthabundant and inexpensive with a high catalytic property for the water oxidation reaction in high pH electrolyte.23-25 Since CoOx possesses both high bandgap and chemical stability, it can been applied as cocatalysts atop photoanodes.26-28 For instance, Yang et al. presented a CoOx/p+n-Si photoanode using plasma-enhanced ALD. This photoanode produced photocurrent density of 17 mA/cm2 at water oxidation potential and operated for 24 hours without degradation.29 The Sharp group reported a multi-functional Co3O4/Co(OH)2 bilayer on p+n-Si with excellent stability and PEC OER performance.30 However, the buried Si pn homojunction used in the previous works can limit the maximum obtainable photovoltage about 630 mV from the carrier recombination while photovoltage of 740 mV is achieved in Si heterojunction solar cells.31-32 Moreover, CoOx exists in various phases, such as Co(II)O, Co2(III)O3, and Co3(II,III)O4, and their optical, electrical, and electrochemical properties are dramatically changed with their phase.25,

33-36

The ALD

CoOx/Si heterojunction photoanodes are introduced by Zhou et al. but the correlation between PEC performance and the CoOx phase on Si photoanodes has not yet been clarified.37 Therefore, a study on the effect of CoOx phase on PEC performance is needed for the desirable design of hetero-structured CoOx/Si photoanodes. However there have been few studies systematically shown the importance of the electrochemical properties and band structures of various phases of CoOx films on Si photoanode for PEC OER. Cobalt oxides can be prepared using various deposition methods, including hydrothermal synthesis,38-39 electrochemical (EC) deposition,40-41 and chemical vapor deposition (CVD).42-43


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However, it is difficult to precisely control the thickness and phase of the film with these deposition strategies. Therefore, a deposition technique that is capable of fine-tuning the deposited material is needed to develop a high performance multi-functional protective layer. As an advanced chemical deposition method, ALD has many advantages for PEC applications. The thickness of the film can be precisely controlled and high-quality films on a planar or even nanostructured substrate can be prepared on a large scale.44-45 Furthermore, with ALD it is possible to adjust the phase and composition of the deposited material easily through control of the deposition conditions.46-47 Therefore, it is a suitable tool to perform the systematic study of cobalt oxides having various phases which decide the electrical, optical, and catalytic properties. Herein, we fabricated a hole-selective CoOx/SiOx/Si heterojunction having various phases of cobalt-oxide using ALD. In addition, we carried out a systematic study to unveil the relationship between the phase and various properties of CoOx film on n-Si, and found the crucial obstacle restricting the PEC performance of CoOx/SiOx/Si hetero-structured photoanodes. Through this study, we identify the contradictory coupling in charge transfer and injection properties of CoO and Co3O4 at CoOx/Si and CoOx/water interfaces, respectively, which determine the electrochemical and photoelectrochemical performance of the CoOx/Si cells. In order to overcome this limitation and to obtain high efficiency, we developed a double-layered (DL) ALD CoOx film having both high catalytic activity and excellent charge collection property. Our DL ALD CoOx/SiOx/n-Si photoanode exhibited both an excellent OER catalytic property and high photovoltage. In addition, it produces photocurrent density of 3.5 mA/cm2 at 1.23 V versus a reversible hydrogen electrode (vs. RHE), which is a compatible performance with previously reported CoOx/Si photoanodes with a buried junction,29-30 and maintains a


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ACS Catalysis

saturating photocurrent density for 12 hours without degradation under simulated 1 sun illumination in 1 M KOH.

RESULTS AND DISCUSSIONS Characterization of ALD CoOx Films. Figure 1 shows physical and chemical characterizations of the ALD-grown CoOx films. The films were grown at four different temperatures of 150, 200, 250, and 300 oC and analyzed accordingly in detail by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscope (SEM), and transmission electron microscopy (TEM). All ALD CoOx films were deposited to about 30 nm thickness. Further detailed information about the material characterization can be found in the experimental section. The XRD results of these ALD grown films on Si are shown in Figure 1a. The distinct peaks observed for the films grown at 150 oC could be clearly assigned to the CoO phase (JCPDS no. 48-1719). However, because the peak at 37.24o also could be assigned to the (311) plane of the cubic spinel-Co3O4 phase (JCPDS no. 42-1467), the possibility of the existence of a small amount of Co3O4 phase in the film cannot be excluded. But we believe that the dominant growth of rock-salt CoO phase at the lowest deposition temperature of this study is evident because other XRD peaks related to the spinel phase were not shown. The XRD result of the sample deposited at 200 oC appears to be similar to that deposited at 150 oC while the peak intensity from the (200) plane of the rock-salt CoO phase was slightly increased, indicating improved crystallinity. An interesting feature was noticed when the deposition was carried out further at higher temperature of 250 oC. While the (200) peak for the CoO phase appears to decrease, the peak at 37.2o related to the (311) plane of Co3O4 and the (111) plane of


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CoO rather increased. In addition, new XRD peaks corresponding to the cubic spinel-Co3O4 phase (JCPDS no. 42-1467) are identified at 59.59 and 65.4o, which could be assigned to the (511) and (440) planes of the spinel-Co3O4 phase. This phenomenon clearly reveals that the higher deposition temperature suppresses the formation of CoO and at the same times facilitates the growth of more Co3O4 phase in the films. This effect further stands out when the film was grown at 300 oC. The XRD analysis at this temperature shows the complete elimination of the peaks assigned to (200) and (220) planes of the rock-salt CoO phase. On the contrary, we observed other new intensive peaks at 19.12 and 31.59o which can be assigned to the (111) and (220) planes of the spinel-Co3O4 phase and the intensities of peaks at 59.59 and 65.4o of Co3O4 were significantly increased. In addition, the peak corresponding to the (311) plane of this Co3O4 phase became stronger in intensity at 300 oC, becoming the most prominent signature of this spinel phase. Therefore, the XRD analysis confirms that the ALD grown CoOx material can be deposited in the CoO-dominant phase or a mixed phase of CoO and Co3O4 or a single phase of Co3O4 depending on the deposition temperature. Moreover, the ratio of these two individual phases in the mixed phase CoOx could also be varied upon controlling the temperature at which it is grown. Thus, our ALD process provides an exquisite tool to study the role of the phase of CoOx on solar driven water splitting, as discussed later tin this article. XPS measurement were carried out to understand the surface chemistry of these films and to complement the XRD results. Figure 1b and c exhibit the XPS spectra of the Co 2p and O 1s orbitals in the ALD CoOx films deposited at four different temperatures. CoOx has various sets of Co 2p orbitals according to the oxidation states of Co in the film.48 In XPS spectra of Co 2p on our ALD CoOx films, we could easily classify the spectra into the two different shapes. First, the strongest peaks of Co 2p3/2 and 2p1/2 appeared at binding energy (BE) of 780.35 and


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ACS Catalysis

795.9 eV respectively in the ALD CoOx films deposited at 150 and 200 oC. When the growth temperature increased more than 250 oC, Co 2p peaks were located at lower BE. These shift of BE locations of Co 2p spectra corresponds to different oxidation states of CoO and Co3O4. The satellite peaks of Co 2p, indicated by black arrows, in ALD CoOx films deposited at 150 and 200 o

C also suggests that its phase consisted mainly of Co2+.29, 49 The O 1s spectra of the ALD CoOx

film deposited at low temperatures show two distinctive peaks located at 529.8 and ~531.6 eV that are attributed to the bulk O2- of metal-oxide (Co-O) bond and the adsorbed O (O-H), respectively.50 On the contrary, the peaks related to bulk O2- in the O 1s spectra of ALD CoOx film deposited at high temperature appeared at higher binding energy compared to those of the ALD CoOx film deposited at low temperature. In addition, when the CoOx film deposited at temperature higher than 250 oC, the peaks indicating adsorbed O almost disappeared and the O 1s spectra of Co3O4 remained.50-51 Thus, the XPS analysis of the films is also in agreement of the growth temperature dependent ALD CoOx phase formation. Figure 1d and e show the cross-sectional view TEM images of the films grown at 150 and 300 oC, respectively, on the Si substrates. In previous XRD and XPS analyses, it was found that the phases of the cobalt-oxides grown at 150 and 300 oC are clearly different from each other. The TEM analysis of these CoOx films also reveals the temperature-dependent phase formation of the ALD grown materials. Uniform and smooth deposition by ALD could be recognized from these TEM images. However, a relatively rougher surface was found for the film grown at 300 o

C. A similar conclusion can be drawn from the tilted SEM images (Figure S2e and f). In the

insets of Figure 1d and e, the high-resolution TEM (HRTEM) images show the corresponding lattice array of these two films. The inter-planar d-spacing calculated from the lattice fringes of the film grown at 150 oC was found to be ~ 2.13 Å, which corresponds to the (200) planes for


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rock-salt CoO phase.38,

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On the other hand, approximately 2.45 Å d-spacing for the films

deposited at 300 °C can be matched with the inter-planer distance of (311) planes of spinelCo3O4 which is the most prominent signature of this phase.53 The fast-Fourier-transform (FFT) patterns of ALD CoOx films deposited at 150 and 300 oC confirm that two films are mainly CoO and Co3O4, respectively (Figure S3). Thus, the HRTEM analysis results are completely in line with the XRD results. The HRTEM images also indicate the presence of the interfacial SiOx layer between CoOx films and Si substrates. This interfacial SiOx layer is believed to form during the initial stage of CoOx deposition. The thicknesses of interfacial SiOx are 1.81 and 1.56 nm for ALD CoOx films grown at 150 and 300 oC, respectively. It means that the SiOx growth during our ALD process is not significant and would not affect the PEC and EC performance of ALD CoOx films.54 We additionally conducted electron energy loss spectroscopy (EELS) to confirm the phase of these two CoOx films, as shown in Figure 1f and g. Since EELS is able to precisely identify the oxidation states of metal-oxide material with high-resolution, it is considered one of the most useful techniques to check the phase of 3d transition metal-oxides.55-56 First, we can identify the phase of CoOx films using the L3/L2 ratio of Co-L edge in Figure 1e. The L3/L2 ratio of ALD CoOx deposited at 150 oC is almost twofold greater than that of ALD CoOx grown at 300 o

C and this result is consistent with the previous EELS analysis for determining the different

phases of cobalt-oxides.43, 55 Furthermore, the different position of peaks in the O-K edge of ALD CoOx deposited at 150 and 300 oC also reveals that their phases are matched to CoO and Co3O4, respectively.43 Based on these analyses, we denoted the CoOx films deposited at 150, 200, 250, and 300 oC as 150-CoO, 200-CoxOy, 250-CoxOy, and 300-Co3O4, respectively. In addition, the denotation ALD CoOx is used hereafter when we generally refer to cobalt oxides deposited by ALD.


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ACS Catalysis

EC and PEC Properties of ALD CoOx on Si with Various Deposition Temperatures. Figure 2a exhibits EC j-V curves of ALD CoOx/SiOx/p++-Si with various deposition temperatures in 1M KOH under dark. In this case, all ALD CoOx films were deposited to a thickness of about 30 nm, as shown in Figure S2. The catalytic property of the ALD CoOx film continually enhanced with deposition temperature and consequentially, 300-Co3O4 shows 110 mV less overpotential than 150-CoO. Interestingly, although CoOOH forms at both surfaces of 150-CoO and 300-Co3O4 films after prolonged water oxidation reaction, 300-Co3O4 films still shows enhanced OER characteristics, as shown by Figure S4 and 5. This indicates that the electrochemical property of CoOx is governed significantly by its bulk as well as surface property.57-58 Surprisingly, the PEC performance of ALD CoOx/SiOx/n-Si photoanodes exhibits a completely different behavior from the EC performance of the ALD CoOx electrocatalyst. Figure 2b shows PEC j-V curves of ALD CoOx/SiOx/n-Si photoanodes in 1M KOH under a simulated 1 sun illumination. The thickness of all ALD CoOx grown with various deposition temperatures is also set to about 30 nm. The PEC performance of the 150-CoO/SiOx/n-Si photoanode is much higher than that of the 300-Co3O4/SiOx/n-Si photoanode. The 150-CoO/SiOx/n-Si photoanode requires 190 mV less overpotential to produce photocurrent density of 10 mA/cm2 than the 300Co3O4/SiOx/n-Si photoanode. In addition, the performance of the 250-CoxOy/SiOx/n-Si photoanode is slightly higher than that of the 200-CoxOy/SiOx/n-Si. We believe that this is caused by different photovoltages from ALD CoOx/SiOx/Si heterojunction grown at various temperatures. For instance, the 150-CoO/SiOx/n-Si heterojunction shows the highest photovoltage of 560 mV. The photovoltage of the ALD CoOx/SiOx/n-Si heterojunction continuously decreased with increasing deposition temperature and, as a result, the photovoltage


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of the 300-Co3O4/SiOx/n-Si heterojunction is only about 260 mV, as shown in Figure 2c. Here, the photovoltage of the photoanodes is measured by the potential difference at 10 mA/cm2 between ALD CoOx/SiOx/n-Si and ALD CoOx/SiOx/p++-Si in 1M KOH. Therefore, the phase of cobalt-oxides affects not only the electrocatalytic property of the ALD CoOx but also the photovoltage of the ALD CoOx/SiOx/n-Si heterojunction; there is a competitive coupling between these two properties of the ALD CoOx/SiOx/n-Si heterojunction, as shown in Figure 2c. Therefore, a systematic analysis that distinctly exhibits the relationship between phases and PEC performance is required to develop a high performance CoOx/SiOx/n-Si heterostructure for PEC water splitting. Energy Band Diagrams of ALD CoOx/SiOx/Si Heterostructure with Various Deposition Temperatures. Figure 3a shows the energy band diagrams of two cobalt-oxides and Si.59-60 The bandgap and energy band position of cobalt-oxides are significantly changed depending on their phases. In particular, since the valence band edge position of ALD CoOx determines the charge separation property of the CoOx/SiOx/Si heterostructure, it is the key factor for producing high photovoltage in this heterojunction. The rock-salt CoO has a bandgap of 2.4 eV and its valence band position is nearly equal to that of Si. On the other hand, the spinel-Co3O4 has a relatively small bandgap of 1.6 eV and its extremely lower valence band position may interrupt the transportation of holes generated in Si. Figure 3b shows Tauc plots of 150-CoO and 300-Co3O4 films on quartz obtained by transmittance measurement. We confirmed that the optical bandgaps of 150-CoO and 300-Co3O4 films having 30 nm thickness are 2.7 and 1.83 eV, respectively. While a much higher bandgap at lower deposition temperature indicates predominant CoO phase formation, the lower value of the same confirms the Co3O4 phase. These observations regarding the optical band-gaps of different


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ACS Catalysis

CoOx phases are also in good agreement with the existing literatures.61 Two distinguished slopes is seen from the Tauc plot of 300-Co3O4.The first slope at higher eV corresponds to its optical bandgap (charge transfer from O2--Co2+), the second arises due to charge transfer from O2--Co3+ where the Co3+ level lies below the conduction band edge.61 Figure 3c presents the valence band edge and Fermi level of positions of the ALD CoOx/SiOx/n-Si heterostructure measured by UV photoelectron spectroscopy (UPS) [For detailed information about the UPS spectra of ALD CoOx films on n-Si, see Figure S7]. As shown in Figure 3c, the valence band edge of 150-CoO is nearly identical to that of theoretical CoO. In addition, the valence band edge and Fermi level positions of ALD CoOx films continually decreased with increasing deposition temperature. As a result, the valence band edge of 300Co3O4 on n-Si lies at a much lower position compared to that of Si. Note that the obtained valence band edge position of 300-Co3O4 is quite different from the theoretical position of spinel-Co3O4. This discrepancy is caused by band bending caused in the heterojunction. Since the Fermi energy levels of two materials in a heterojunction must be equal in equilibrium, the change in their energy band positions is proportional to the difference in the Fermi level between the two materials. As the Fermi level position of 300-Co3O4 is much lower than that of CoO, its energy band is shifted more dramatically after forming a heterojunction with n-Si. In addition, the XPS analysis of ALD CoOx films and Si indicates the detailed band edge offsets of CoOx/Si heterostructure that are key parameters to decide the charge separation property of a heterostructure, as shown by Figure S8.62 The band edge offsets of CoOx/Si heterostructures are constructed using Anderson’s rule which only considers electron affinity of each materials in vacuum. We note that the actual band edge offset could be different from the values in Figure S8 as Anderson’s rule ignores the effects such as chemical bonding between two materials, dipole


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formation and interfacial defects at the heterojunction.63-64 Nevertheless, this approach has been commonly used to determine the band edge offset of a heterojunction in a PEC system.65-67 We also note that our heterojunctions locate about 30 nm below the CoOx/water interface and, therefore,

the electrolyte is believed not to affect the band edge offset of the CoOx/Si

heterojunction. Based on XPS, UPS, and transmittance, we constructed the energy band diagram of 150CoO and 300-Co3O4/SiOx/n-Si heterojunctions under an equilibrium condition in 1M KOH, as shown in Figure 3d. From this band diagram of heterojunction structures, we anticipate that the holes photo-generated in n-Si can move easily through 150-CoO. Furthermore, transport of electrons from n-Si to the electrolyte is restricted due to the high conduction band barrier of 150CoO. Thus, holes and electrons produced in the 150-CoO/SiOx/n-Si heterojunction readily can be collected at the front and backside of the photoanode, respectively. On the contrary, in the case of the 300-Co3O4/SiOx/n-Si heterojunction, the large valence band edge offset (~ 1.06 eV) between 300-Co3O4 and n-Si interrupts transportation of holes from Si to the electrolyte. This is consistent with the electrochemical impedance spectroscopy (EIS) in Figure S9, which clearly shows the greater interfacial resistance at 300-Co3O4/n-Si with the large band edge offset. The Mott-Schottky plot indicates that the flat-band potential of the 300-Co3O4/SiOx/n-Si is about 0.16 V negative than that of the 150-CoO/SiOx/Si (Figure S10). This means that n-Si in the 300Co3O4/SiOx/n-Si has more bent band as can be seen in Figure 3d. Interestingly, the 300Co3O4/SiOx/n-Si photoanode produces photocurrent density of 1 mA/cm2 at 1.41 V vs. RHE although its valence band barrier blocks holes, as shown in Figure S11. This means that the hole transport mechanism is different from that of the 150-CoO/SiOx/n-Si photoanode. We believe that 300-Co3O4 has a defect-band that can allow the transport of holes from Si to the electrolyte


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through hopping. Nevertheless, we speculated that the hole transportation of 300-Co3O4 would be poorer than that of 150-CoO in our Si heterojunction photoanodes. Through this systematic analysis, it can be concluded that the phase of ALD grown CoOx determines the charge collection as well as the catalytic property of the CoOx/SiOx/n-Si photoanode, and there is unfavorable coupling between these two properties of ALD CoOx when applied on n-Si photoanodes. A Multi-functional Double-layered ALD CoOx Film. In order to obtain both the high photovoltage of the 150-CoO/SiOx/n-Si heterostructure and the excellent surface nature for the OER of the 300-Co3O4 surface simultaneously, we devised a double-layered (DL) ALD CoOx film wherein 150-CoO and 300-Co3O4 layers are deposited sequentially on n-Si. Figure 4a shows the band diagram of the DL ALD CoOx structure on n-Si as a solution to surpass the limitation of single-phase CoOx for efficient solar-driven water splitting. In this DL ALD CoOx/SiOx/n-Si heterojunction, a 150-CoO layer directly above Si builds desirable band alignment to generate high photovoltage and a top 300-Co3O4 layer serves as cocatalyts for the OER. We note that our DL ALD CoOx/SiOx/n-Si heterostructure differs from the previously reported Co3O4/Co(OH)2 bilayer on p+n-Si because photovoltage of Co3O4/Co(OH)2 bilayer on p+n-Si is generated from the buried pn homo-junction not from the Co3O4/Si heterostructure and Co3O4 serves only to protect Si surface against corrosion during OER in the previous work.30 We also note that Si heterojunction solar cells can have higher solar conversion efficiency than Si homojunction, reaching 26.3% efficiency.32 Figure 4b exhibits a cross-sectional TEM image of our DL ALD CoOx on n-Si. In this DL ALD CoOx layer, the thicknesses of the 150-CoO and 300-Co3O4 films were set to 20 and 10 nm, respectively. As shown in Figure 4b, we found 0.245 and 0.21 Å lattice fringe spacings on the bottom and top layers of the DL ALD CoOx film, which can be


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easily assigned to CoO (200) and Co3O4 (311), respectively. In addition, the phase difference in DL CoOx appeared more distinctly in the EELS analysis, as shown in Figure 4c and d. We performed a line scan of EELS with 2 nm width from the Si substrate to the top DL ALD CoOx. The spectra of cobalt-L2/L3 and oxygen-K in the bottom and top layers of ALD CoOx are matched to those of 150-CoO and 300-Co3O4, respectively. In particular, the peak at 536.6 eV for Co3O4 phase is detected from 16 nm height and appeared distinctly at 18 nm height of DL CoOx. This also suggests that the high temperature during the second deposition step for DL CoOx films has negligible effect on the bottom 150-CoO layer. Figure 5 shows the PEC performance of DL ALD CoOx/SiOx/n-Si photoanodes in 1M KOH under a simulated 1 sun illumination. The DL ALD CoOx/SiOx/n-Si photoanode consisting of both 15 nm of 300-Co3O4 and 150-CoO layers exhibits about 480 mV of photovoltage and produces 3.5 mA/cm2 photocurrent density at 1.23 V vs. RHE, which is two-fold higher than that of the ALD 150-CoO/SiOx/n-Si photoanode. The EIS analysis of a DL CoOx/SiOx/n-Si photoanode indicates that the DL/CoOx/SiOx/n-Si has much lower charge transfer resistances than the single-layered CoOx/SiOx/n-Si (Figure S12). We also investigated the PEC OER characteristics of DL ALD CoOx/SiOx/n-Si photoanodes with various thicknesses of the top and bottom layers: all DL ALD CoOx/SiOx/n-Si have about a 30 nm DL CoOx layer (Figure S13). As clearly seen in Figure 5a, the DL ALD CoOx/SiOx/n-Si photoanode with the thinner top Co3O4 layer exhibits better PEC performance. For example, the DL ALD CoOx/SiOx/n-Si with 4 nmthick Co3O4 produces photocurrent density of 9 mA/cm2 at the water oxidation potential. This PEC performance corresponds to about 50% of photocurrent at 1.23 V and similar voltage onset, compared to the previously reported CoOx/SiOx/n-Si photoanode by the Lewis group.37 But we also note that our photoanode shows nearly comparable performance to Co3O4 on nanotextured


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Si with p+n buried junction,29 despite our photoanode does not contain any p+n buried junction. This highlight importance of tailoring electrocatalytic property as well as the energetics of the CoOx/Si heterostructures. The DL ALD CoOx film with 4 nm-thick was formed by postannealing of the 150-CoO at 300 oC [For detailed fabrication and characterization of the DL ALD CoOx film with 4 nm-thick Co3O4 layer, see Figures S14-S17 in the SI]. On the other hand, DL CoOx/SiOx/n-Si photoanode having thicker top 300-Co3O4 layer exhibits high overpotential for OER, even worse than that of the single-layered ALD CoOx/SiOx/n-Si photoanode. We attribute this to the charge transport properties of the top Co3O4 layer. The electrochemical performance of a metal-oxide catalyst could be significantly influenced by its thickness because of its poor conductance. Indeed, as shown in Figure S18, when the thickness of 300-Co3O4 is decreased from 50 to 5 nm, the overpotential of 300-Co3O4/SiOx/p++-Si is gradually reduced. Therefore, the precise thickness control of the top Co3O4 in the DL ALD CoOx film is important to maximize the PEC performance of a heterojunction photoanode. The thickness of the protective layer is also closely related to the durability of the heterostructured photoanode. An excessively thin protective layer may be insufficient to protect the bottom Si from the high pH electrolyte during long time test. As shown in Figure S19, the 150CoO/SiOx/n-Si photoanode having 10 nm ALD CoOx thickness showed rapidly decreasing photocurrent density during a chronoamperometry stability test. Since the phase transition of a metal-oxide catalyst is accompanied by a volume change, flaws can be generated during the film transition and these flaws can cause exposure of vulnerable Si.68-69 In addition, it was also noticed that the exposed Si surface was etched by KOH during water oxidation as depicted in the plan-view SEM images of Figure S20a. Such etched rough Si surface then can weaken the adhesion between CoOx and the Si surface. Eventually, it can also result in a partial peeling of


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CoOx. Consequently, detachment of the cocatalyst film causes a decrease of the catalytic performance. On the other hand, we confirmed that 30 nm thickness of 150-CoO is enough to protect the underlying Si, as shown by Figure S19 and 20b. Furthermore, the overpotential of the 150-CoO/SiOx/n-Si photoanode having 30 nm 150-CoO thickness is rather slightly reduced after the 12 hour test, as shown by Figure S19b. We assume that this enhancement is caused by the rough surface of the ALD CoOx film in Figure S20b, which is produced by the phase transition of ALD CoOx for long time water oxidation reaction. The phase transition of CoOx films can affect the performance stability of the DL ALD CoOx/SiOx/Si photoanode when it has a thin Co3O4 top layer. As shown in Figure 5b, the photocurrent density of the DL ALD CoOx/SiOx/n-Si photoanode with 4 nm 300-Co3O4 and 26 nm 150-CoO layers continuously decreased during the 12 hour stability test. We believe that the 4 nm-thick Co3O4 layer in the DL ALD CoOx film changes completely to CoOOH during extended period of water oxidation reaction (Figure S22) and eventually becomes the singlelayered 150-CoO/SiOx/n-Si heterojunction. In fact, the PEC performance of this DL ALD CoOx/SiOx/n-Si become similar to that of a 150-CoO/SiOx/n-Si photoanode after stability test, as shown in Figure S21. On the contrary, the DL ALD CoOx/SiOx/n-Si photoanode consisting of both 15 nm 150-CoO and 300-Co3O4 layers maintains high current density during the 12 hour stability test, as shown by Figure 5b. In addition, the overpotential of the DL ALD CoOx/SiOx/nSi photoanode also slightly decreased after the stability test. We believe that this also originates from the change of the surface morphology caused by the phase transition. Therefore, the thickness of top and bottom CoOx layers in multi-functional DL ALD CoOx film must be selected carefully to achieve both high performance and stability during PEC OER. In addition,


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we believe that the PEC performance of DL CoOx/SiOx/n-Si will be more improved with high durability, if the top and bottom thickness of DL CoOx film is optimized.

CONCLUSION In summary, we demonstrate hole-selective CoOx/SiOx/n-Si heterojunctions for PEC OER. Various phases of cobalt oxide films were directly grown on n-Si using the ALD process, and the role of the electronic and electrochemical properties of various CoOx films on the PEC characteristics of hole-selective Si heterojunction photoanodes is investigated. CoO, formed at low temperature, exhibits the superior hole-selective contact with n-Si for the OER but its EC property for water oxidation is relatively poor compared to Co3O4. On the other hand, although Co3O4, deposited at high temperature, is a superior OEC material, it has significant valence band offset with n-Si, which blocks hole transport from n-Si to electrolyte. In order to eliminate this competitive correlation between CoO and Co3O4 for PEC water splitting, we proposed a multifunctional DL ALD CoOx/SiOx/n-Si heterojunction photoanode that can have both high photovoltage and an excellent OEC property. Our DL ALD CoOx/SiOx/n-Si photoanode produced about 3.5 mA/cm2 photocurrent density without a buried junction, and operated for more than 12 hours without any corrosion and degradation. We believe that our work provides a desirable framework to construct Si and other photoanodes heterostructured with multifunctional transition metal-oxides such as NiO, and NiFeOx. Finally, since the ALD technique can be applied to not only planar substrates but also to high-aspect ratio structures, we believe that our ALD CoOx can achieve higher OER efficiency using Si nanostructures to expand the electrochemical active area and enhance light absorption. In addition, we also believe that this


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multi-functional DL structure can be applied to other important PEC reactions such as hydrogen evolution and CO2 reduction reaction because this architecture maximizes both the catalytic property and the photovoltage of photoelectrodes.

EXPERIMENTAL SECTION Atomic Layer Deposition of CoOx on Si Substrates. Cobalt oxide (CoOx) thin films were deposited by ALD on n-type Si (100) wafers (1 – 3 Ω·cm) to study the properties of these films and to apply them in solar driven water splitting. A travelling-wave type ALD reactor (Lucidia-D100, NCD Technology) was adopted to deposit these CoOx films using bis(1,4-di-isopropyl-1,4-diazabutadiene)cobalt [C16H32N4Co, Co(dpdab)2] and oxygen molecules (O2) within a temperature window of 150-300 °C. The Co-precursor delivery line was kept at 100 °C to prevent any possible condensation of the precursor during the reaction. 100 standard cubic centimeters per minute (sccm) N2 was used as a purging gas after both precursor and reactant pulsing to carry out the residual chemicals and the reaction by-products out of the ALD chamber. To ensure self-limiting ALD growth of the films, the reaction was set as 10s precursor pulsing10s purging-10s reactant pulsing-10s purging. This reaction sequence was repeated to deposit every CoOx thin films with a desired thickness in this study. Materials Characterization. In order to analyze the properties of ALD CoOx, various characterization tools were used. X-ray diffraction (XRD) measurement was carried out to confirm the phase and crystallinity of ALD CoOx (RIGAKU D/MAX-2500 with Cu-Kα radiation, λ = 0.154 Å). X-ray and UV photoelectron spectroscopy (XPS and UPS) measurements were carried out to check the phase and energy band position of CoOx (K-alpha


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and Sigma Probe, Thermo VG Scientific). Transmission electron spectroscopy (TEM) and electron energy loss spectroscopy (EELS) were conducted for a more precise analysis of the CoOx phase using double Cs corrected transmission electron microscopy at 300 keV (TEM, Titan cubed G2 60-300, FEI Co.). In addition, scanning electron microscopy (SEM) were performed to analyze the morphology of CoOx films using a field emission scanning electron microscope (FE-SEM, SU5000, Hitachi and Magellan400, FEI Co.). The optical properties and band gap of CoOx films were analyzed by UV–Vis-NIR spectrophotometry (Cary 5000, Agilent Technologies). Preparation of ALD CoOx/SiOx/n-Si Photoanodes. An ohmic contact was formed using an In-Ga eutectic alloy (Sigma Aldrich) to the backside of a Si wafer piece coated by ALD CoOx and a copper wire was attached to this ohmic contact. In order to protect the rear connected area of photoanodes and expose only the effective area in front of the photoanodes, samples were sealed using an industrial epoxy (Loctite 9460). The active area was calculated by scanning an image (a scanner having a resolution of 300 dpi) and with an image processing program (image J). EC and PEC Measurements. In order to perform PEC and EC measurement, 3electrode measurement configuration was employed in this work. A Pt wire and an Ag/AgCl (sat KCl) were used as a counter and a reference electrode, respectively. Note that a Pt counter electrode does not affect the electrocatalytic properties of CoOx films since negligible Pt deposition on CoOx films is found by XPS even after prolonged OER (Figure S6). A 1M KOH electrolyte (pH 14) was made using KOH pellets (Sigma Aldrich) and a Pyrex glass vessel with a flat window was used to construct the PEC cell. A 300 W Xe lamp (Oriel Instrument, Model 6258) equipped with AM1.5G and water infra-red filters (Newport, Liquid filter 1.5 IN ALUM


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body model 61945) was used as the light source. The light intensity was then calibrated to 1 sun using a Si reference (Oriel Instrument, 91150V). All electrochemical measurements were carried out using a potentiostat (BioLogic, SP-150). Although we did not measure the evolved O2 using a gas chromatography, it is widely-known that CoOx produces only O2 by water oxidation reaction in KOH.24, 37, 70 In EIS analysis, the working electrode was biased at a constant potential while the frequency was swept from 200 kHz to 1 Hz with a 10mV AC dither. The results were fitted by using an EC lab software V11.10 (BioLogic).

AUTHOR INFORMATION Corresponding Author * [email protected] and [email protected]

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: SEM and TEM analysis, UPS, XPS and XPS spectra, EIS and Mott-Schottky plot, and additional electrochemical measurements (PDF)

ACKNOWLEDGMENT


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This work was supported by National Research Foundation (NRF) of Korea (No. 2014R1A4A1003712 (BRL Program) and No. 2017R1A2B4008736) funded by the Korea government (Ministry of Science and ICT), and the Korea Institute of Energy Technology Evaluation and Planning (KETEP) and the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea (No. 20163030014020), and the Priority Research Centers Program through the NRF funded by the Ministry of Education (2014R1A6A1031189). This work was supported by the Advanced Technology Center Program (#10077265) funded by the MOTIE of the Republic of Korea. The precursor used in this study was provided by UP Chemical Co. Ltd., Korea.


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50. Ressnig, D.; Shalom, M.; Patscheider, J.; Moré, R.; Evangelisti, F.; Antonietti, M.; Patzke, G. R. Photochemical and electrocatalytic water oxidation activity of cobalt carbodiimide. J. Mater. Chem. A 2015, 3 , 5072-5082. 51. Yang, J.; Liu, H.; Martens, W. N.; Frost, R. L. Synthesis and characterization of cobalt hydroxide, cobalt oxyhydroxide, and cobalt oxide nanodiscs. J. Phys. Chem. C 2009, 114 , 111119. 52. Ryu, W.-H.; Shin, J.; Jung, J.-W.; Kim, I.-D. Cobalt (II) monoxide nanoparticles embedded in porous carbon nanofibers as a highly reversible conversion reaction anode for Liion batteries. J. Mater. Chem. A 2013, 1, 3239-3243. 53. Liu, Y.; Zhu, G.; Ge, B.; Zhou, H.; Yuan, A.; Shen, X. Concave Co3O4 octahedral mesocrystal: polymer-mediated synthesis and sensing properties. CrystEngComm 2012, 14 , 6264-6270. 54. Scheuermann, A. G.; Kemp, K.; Tang, K.; Lu, D.; Satterthwaite, P.; Ito, T.; Chidsey, C.; McIntyre, P. Conductance and capacitance of bilayer protective oxides for silicon water splitting anodes. Energy Envrion. Sci. 2016, 9, 504-516. 55. Ahn, C. C., Application of EELS to Ceramics, Catalysts and Transition Metal Oxides. Transmission electron energy loss spectrometry in materials science and the EELS atlas, 2nd ed.; John Wiley & Sons: Damstadt, 2006; pp 271-316. 56. Tan, H.; Verbeeck, J.; Abakumov, A.; Van Tendeloo, G. Oxidation state and chemical shift investigation in transition metal oxides by EELS. Ultramicroscopy 2012, 116, 24-33. 57. Jung, S.; McCrory, C. C.; Ferrer, I. M.; Peters, J. C.; Jaramillo, T. F. Benchmarking nanoparticulate metal oxide electrocatalysts for the alkaline water oxidation reaction. J. Mater. Chem. A 2016, 4 , 3068-3076. 58. Chou, N. H.; Ross, P. N.; Bell, A. T.; Tilley, T. D. Comparison of cobalt‐based nanoparticles as electrocatalysts for water oxidation. ChemSusChem 2011, 4, 1566-1569. 59. Chen, S.; Wang, L.-W. Thermodynamic oxidation and reduction potentials of photocatalytic semiconductors in aqueous solution. Chem. Mater. 2012, 24, 3659-3666. 60. Otto, S.; Fauster, T. Two-photon photoemission from CoO layers on Ir (100). J. Phys.: Condens. Matter 2016, 28, 055001. 61. Matsuda, A.; Yamauchi, R.; Shiojiri, D.; Tan, G.; Kaneko, S.; Yoshimoto, M. Roomtemperature selective epitaxial growth of CoO (111) and Co3O4 (111) thin films with atomic steps by pulsed laser deposition. Appl. Surf. Sci. 2015, 349, 78-82. 62. Chen, S.; Pan, X.; Xu, C.; Huang, J.; Ye, Z. X-ray photoelectron spectroscopy study of energy-band alignments of ZnO on buffer layer Lu2O3. Phys. Lett. A 2016, 380 , 970-972. 63. Borisenko, V. E.; Ossicini, S., From Abbe's Principle to Azbel'–Kaner Cyclotron Resonance. What is what in the Nanoworld: A Handbook on Nanoscience and Nanotechnology, 3rd ed.; John Wiley & Sons: Weinheim, 2013; pp 10-11. 64. Neamen, D. A., Additional Semiconductor Devices and Device Concepts. An introduction to semiconductor devices, Int. ed.; McGraw-Hill: New York, 2006; pp 560-564. 65. May, K. J.; Fenning, D. P.; Ming, T.; Hong, W. T.; Lee, D.; Stoerzinger, K. A.; Biegalski, M. D.; Kolpak, A. M.; Shao-Horn, Y. Thickness-dependent photoelectrochemical water splitting on ultrathin LaFeO3 films grown on Nb: SrTiO3. J. Phys. Chem. Lett. 2015, 6, 977-985. 66. Liu, H.; Antwi, K. A.; Chua, C.; Huang, J.; Chua, S.; Chi, D. Epitaxial synthesis, band offset, and photoelectrochemical properties of cubic Ga2S3 thin films on GaAs (111) substrates. ECS Solid State Lett. 2014, 3, 131-135.


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Figure 1. Material characterization of ALD CoOx with various deposition temperatures. (a) XRD, (b) Co 2p and (c) O 1s XPS spectra of ALD CoOx films on n-Si with deposition temperature from 150 to 300 oC. Cross-sectional TEM images of ALD CoOx films on n-Si deposited at (d) 150 and (e) 300 oC. Each inset image in (d) and (e) present high-resolution TEM (HRTEM) images of ALD CoOx films deposited at 150 and 300 oC, respectively. (f) Co-L2/L3 and (g) O-K of EELS spectra of ALD CoOx films on n-Si grown at 150 and 300 oC.


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Figure 2. EC and PEC properties of ALD CoOx with various thicknesses. (a) EC j-V curves of ALD CoOx/SiOx/p++-Si under dark and (b) PEC j-V curves of ALD CoOx/SiOx/n-Si photoanodes under a simulated 1 sun illumination in 1M KOH. The thickness of all ALD CoOx film is set to about 30 nm. (c) The photovoltage of ALD CoOx/SiOx/n-Si heterojunction and overpotential of ALD CoOx/SiOx/p++-Si for producing 10 mA/cm2 of current density. The ALD CoOx films are deposited at 150, 200, 250, and 300 oC.


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Figure 3. Energy band diagrams of ALD CoOx/SiOx/n-Si heterojunction with various deposition temperatures. (a) Energy band position of rock salt-CoO, spinel-Co3O4, and n-Si. (b) Tauc plot of 150-CoO and 300-Co3O4 films having 30 nm thickness on quartz wafer. (c) The valence band edge and Fermi level positions of ALD CoOx films on n-Si with deposition temperature from 150 to 300 oC. (d) Energy band diagram of 150-CoO and 300-Co3O4/SiOx/n-Si heterojunction under equilibrium condition in 1M KOH.


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Figure 4. Double-layered (DL) ALD CoOx on n-Si. (a) Energy band diagram of a DL ALD CoOx/SiOx/n-Si photoanode under illumination in 1M KOH. (b) Cross-sectional TEM image of the DL ALD CoOx on n-Si. The yellow and green inset images indicate the HRTEM images in part of bottom and top layer in the DL ALD CoOx film, respectively. (c) Co-L2/L3 and (d) O-K of EELS spectra of DL ALD CoOx with various heights of DL ALD CoOx film.


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Figure 5. PEC performance and stability of the DL ALD CoOx/SiOx/n-Si photoanodes. (a) PEC j-V curves and (b) chronoamperometry stability test of DL ALD CoOx/SiOx/n-Si photoanodes having about a 30 nm DL CoOx layer consisting of top and bottom layers with various thickness (top/bottom) under a simulated 1 sun illumination in 1M KOH. Stability tests of two DL ALD CoOx/SiOx/n-Si photoanodes are carried out at 1.51 (4/26 nm) and 1.61 V (15/15 nm) vs. RHE, respectively, which are the potentials producing saturating photocurrent density of these Si photoanodes.


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ToC figure


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Si Heterojunctions for ... - ACS Publications

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