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Cuprous/cupric heterojunction photocathodes with optimal phase transition interface via preferred orientation and precise oxidation Seung Ki Baek, Joo Sung Kim, Young Dae Yun, Young Been Kim, and Hyung Koun Cho ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b01715 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on June 29, 2018

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ACS Sustainable Chemistry & Engineering

Cuprous/cupric heterojunction photocathodes with optimal phase transition interface via preferred orientation and precise oxidation Seung Ki Baek†‡, Joo Sung Kim†‡, Young Dae Yun†, Young Been Kim†, and Hyung Koun Cho†* †

School of Advanced Materials Science and Engineering, Sungkyunkwan University, 2066,

Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do 16419, Republic of Korea * Corresponding author. Tel.: +82 31 299 4733; fax: +82 31 290 7410 E-mail address: [email protected] (H.K. Cho)

Abstract. To effectively transport photogenerated charges within a cuprous oxide (Cu2O) photocathode and to transfer them to the electrolyte, cupric oxide (CuO) as a heterojunction material enable to provide the additional built-in electric field is perfectly suitable band position and can be produced from the original Cu2O films by simple oxidation process. However, their different crystal structures and lattice constants inevitably induce crystal distortions and the formation of a defective interface, resulting in the reduction of the photocurrent and photovoltage. To alleviate these intrinsic problems and improve its photoelectrochemical (PEC) performance, we fabricated a Cu2O/CuO interface with high crystallinity and a well-aligned atomic arrangement by preparing a Cu2O absorber underlying layer preferentially oriented with [111] direction and forming a thin CuO overlayer (20–30 nm) via oxidation process at precisely controlled reduced oxygen partial pressure. At 5 Torr, the resultant Cu2O double-layer structure exhibited smooth crystallographic phase transition boundary from cubic Cu2O to monoclinic CuO and well-aligned lattice fringes. This exhibited considerable improvement in photocurrent 1 ACS Paragon Plus Environment

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density (2.8 mA/cm2 at 0 V vs. RHE) and onset potential (0.83 V), compared with those of pristine Cu2O. Importantly, these enhancements were achieved without coating of photocatalytic materials on the photoelectrodes.

KEYWORDS: Cu2O, CuO, Photocathode, Electrodeposition, Electron transport

INTRODUCTION Energy conversion from sunlight to electrical or chemical energy has been considered as a future-oriented approach for clean energy systems due to its large capacity, cleanliness, and sustainability. Among several energy sources based on sunlight, hydrogen produced from solar water splitting has attracted great attention, as it can be easily used a chemical fuel without producing waste. Furthermore, water splitting is a simple and clean process, which ideally only requires water and sunlight for energy conversion.1 Since solar water splitting system typically consists of two half-cell reactions, namely, oxygen evolution reaction (OER) and hydrogen evolution reaction (HER), it is necessary to investigate the performance of these half-cell reactions on individual photoelectrodes separately.2 TiO2, the common n-type photo-absorber for OER, is naturally abundant and has a long-term stability. However, it has an unsuitable band gap (3.0 eV) for the absorption of the solar spectrum, which would result in poor solar-to-hydrogen (STH) conversion efficiency.3 Thus, new n-type absorbers for PECs, such as Fe2O3 (2.3 eV), BiVO4 (2.4 eV), and WO3 (2.8 eV), have been proposed as a photoanode for OER.4-6 On the contrary, p-type photocathodes for the HER have been fabricated with conventional InP, Si, and CuInGaSe semiconductors which have intrinsic disadvantages, such as inadequate band gap (< 1.23 eV), high cost, and corrosivity.7-9 Alternatively, cuprous oxide (Cu2O) is 2 ACS Paragon Plus Environment

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considered as a suitable material for PEC photocathode, owing to its favorable band gap of 2.1 eV and natural abundance. Its theoretical STH efficiency is approximately 18% under the AM 1.5 spectrum.10 However, Cu2O-based photocathodes have a low water-reduction photocurrent due to inefficient charge transport of photogenerated electrons. This is related to shorter minority carrier diffusion length than the absorption length of light, thereby inducing bulk recombination loss.11 Furthermore, despite the conduction band (CB) of Cu2O being -0.7 V away from the hydrogen evolution potential, a favorable energy band position for HER, the surface defects of Cu2O hinder the enhancement of onset potential and lead to surface recombination loss. This loss can be effectively suppressed via various approaches: nanostructuring of the Cu2O (nanowires and nanorods), reduction of film thickness, application of p-type dopants, and formation of heterojunction with adequate semiconductors such as TiO2, cupric oxide (CuO), and Ga2O3.12-17 Additionally, the transfer efficiency of photogenerated charges to the electrolyte has been remarkably improved by the deposition of HER catalysts and surface passivation layers.18, 19

The construction of heterojunction with Cu2O absorbers can provide the additional built-in

electric field that can overcome inefficient charge transport and transfer, thereby increasing both photocurrent and photovoltage. Binary oxides such as TiO2, CuO, ZnO, and Ga2O3 have been actively investigated as overlayer films for the Cu2O photocathodes. The film should have a conduction band that is more positively positioned relative to that of Cu2O. Therefore, the conduction band edge of the overlayer materials should be positioned between that of Cu2O (+0.7 V vs. RHE) and the hydrogen evolution potential (0 V vs. RHE) to transfer the photogenerated electrons seamlessly from the Cu2O absorber to the overlayer and electrolyte. Among the previously reported overlayer materials, the p-type CuO, with chemical components identical to that of Cu2O, has appropriate conduction band edge for HER. Thus, it satisfies

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minimum requirements for electron transport, regardless of the pH of electrolytes. Nevertheless, achieving high-performance PECs from the Cu2O/CuO heterojunction is hindered by several factors. First, the incongruent crystal structures and lattice constants of Cu2O (cubic system with 4.27 Å) and CuO (monoclinic system with 4.68 Å × 3.42 Å × 5.13 Å) induce compressive stress and crystal distortion in the CuO films.20 Consequently, a high density of defect states developed at the interface in response to the strain from the Cu-O bonds under thermal oxidation. These defects act as trapping sites for charge carriers, leading to degraded photocurrent and photovoltage values. Furthermore, they promote severe chemical and photochemical corrosion when the photoelectrodes is unstable in the aqueous media. Therefore, these inevitable defect states at the Cu2O/CuO interface limit both charge transfer and transport between the Cu2O/interface and bulk Cu2O layer. Second, upon CuO layer formation on the surface, CuO, owing to its deficient photoactive and electrical conduction properties, intensifies recombination loss and self-reduction process, which severely degrades the PEC performances.21 Thus, CuO as an overlayer in the Cu2O photocathodes produced positive and negative effects in different studies. However, the origin of these reported discrepancies was not established. Our suspected checkpoints are followed: i) the oxidation of Cu2O into CuO is an equilibrium process, which means the oxidation rate is high at an oxygen-rich atmosphere; and ii) the interface is a crystallographic transition boundary from cubic to monoclinic structure, suggesting that the formation of defective interfaces is inevitable. Thus, we propose that heterojunction oxide PECs with high performances can be realized through the formation of precisely and artificially controlled CuO layers on oriented Cu2O absorbers, resulting in sufficient suppression of the defective sites at the Cu2O/CuO interface. In this study, we produced optimally aligned Cu2O/CuO heterostructure photocathodes by oxidizing the


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orientation-controlled Cu2O layers under reduced oxygen partial pressure. When the Cu2O films were preferentially oriented along the [111] direction by using the Cu2O:Sb buffer layer, the exposed facets mostly consisted of {200} crystal planes with three-faced pyramids.22 Subsequent thermal oxidation of the Cu2O:Sb/Cu2O double layer mitigated the defect formation at the Cu2O/CuO interface by minimizing lattice mismatch. The resultant Cu2O:Sb/Cu2O/CuO photocathodes experienced simultaneous improve in photocurrent and photovoltage.

EXPERIMENTAL SECTION The copper oxide layers were electrochemically deposited using a solution containing 0.4 M copper sulfate anhydrous (CuSO4, Junsei, > 98%) and 3 M lactic acid (C3H6O3), buffered to pH 11 by a sodium hydroxide (4 M NaOH) solution. The electrodeposition process was performed with a potentiostat (Princeton Applied Research Versatate 4) using a three-electrode system consisting of ITO/glass substrate as the working electrode, and Ag/AgCl (saturated NaCl) and platinum sheets as the reference and counter electrodes, respectively. The single Cu2O layers were potentiostatically deposited at -0.3 V until 3 C/cm2 electric charge density is attained, while the double layers consisted of 0.3 C/cm2 Cu2O:Sb seed and 2.7 C/cm2 Cu2O absorber layers. The seed layers were potentiostatically deposited at -0.5 V in a Cu2O deposition bath with additional 3 mM antimony sulfate (Sigma Aldrich, > 98%). The solution temperature during deposition was maintained at 60 °C. The CuO overlayers on the Cu2O single and the Cu2O:Sb/Cu2O double layers were produced by thermal oxidation in the furnace at 400 °C with O2 partial pressures ranging from 0 to 10 Torr for 1 h. Here, the single and double layers annealed at O2 partial pressures of 5 and 10 Torr are labeled SL-5 and SL-10, and DL-5 and DL-10, respectively. For the ALD of chemically durable Al2O3 and TiO2, trimethylaluminum and titanium tetraisopropoxide precursors were contained in stainless steel canisters at 10 °C and 60 °C, 5 ACS Paragon Plus Environment


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respectively, to maintain appropriate vapor pressures. The flow of the counter reactant (H2O) was controlled by a needle valve, and N2 and Ar (99.9999%) were used as purging gases. For deposition with better conformity and uniformity, an infiltration step was added after the precursor and reactant exposure step. The deposition temperatures were 150 °C and 200 °C for Al2O3 and TiO2, respectively. The morphological properties of thermally-oxidized Cu2O films were characterized by field-emission scanning electron microscopy (FE-SEM, JSM-6700F, 10 kV). X-ray diffraction (XRD, Bruker AXS D8 Discover with a Cu Kα radiation source) was performed to examine the crystal structure and crystallinity of oxidized samples. The interface between the Cu2O and oxidized CuO layers were analyzed by transmission electron microscopy (TEM, Titan G2, 200 kV) for high-resolution TEM (HRTEM) images. Optical absorbance and transmittance spectra were

characterized

using

an

ultraviolet-visible

light-near

infrared

(UV-Vis-NIR)

spectrophotometer (Varian Cary 5000). All photoelectrochemical measurements were investigated using the Princeton Applied Research Versastat 4 system with a three-electrode system under a 150-W xenon lamp calibrated by AM 1.5 filter. Here, the components of the three-electrode system are similar with the deposition process. Photoresponse and capacitancevoltage measurements were conducted in 1.0 M sodium sulfate (Na2SO4) solution buffered with 0.1 M K2HPO4 for pH 5. Mott-Schottky plots were obtained at 500 Hz and 10 mV to estimate the flat band potential and carrier concentration.

RESULTS AND DISCUSSION The main challenges faced in the design of Cu2O-based photoelectrodes with high conversion efficiency include increasing charge transport within them and improving charge


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transfer to the electrolyte. Figure 1a shows the key factors influencing bulk recombination loss during charge transfer to the electrolyte and surface charge recombination loss at the Cu2O/electrolyte interface. The PEC properties of Cu2O photoelectrodes cannot be improved owing to their intrinsic material limitations: i) low electrical conductivity of p-type Cu2O causes the bulk recombination of photogenerated electrons and holes during charge transport into the electrolyte and back into the electrode, respectively; and ii) large offset between the conduction band of Cu2O and electrolyte redox potential induces recombination loss from surface defects and degradation of charge transfer efficiency. Figure 1b illustrates how these can be resolved by the structural alteration of Cu2O photoelectrodes by using overlayers. The deposition of an adequate overlayer with a conduction band edge that is more positive than that of Cu2O, is expected to facilitate the separation of photogenerated electron-hole pairs, and surface recombination due to the large offset between the conduction band of Cu2O and electrolyte redox potential is prevented. The p-type CuO layer has a more positive conduction band than Cu2O and more negative potential with respect to the hydrogen evolution potential, indicating its suitability as overlayers for the enhancement of charge transport and transfer efficiencies. However, since Cu2O and CuO crystals have different crystal systems and lattice constants, the lattice mismatch at the Cu2O/CuO interfaces can generate defect states, inducing non-radiative recombination. Furthermore, the CuO layers with sufficient thickness to act as absorber can cause bulk recombination owing to their relatively low conductivity and corrosivity, even in dark condition, compared with the Cu2O layer. Therefore, to ameliorate charge transfer efficiency of Cu2O-based photoelectrodes using CuO overlayer, three strategies should be considered: i) using controlled Cu2O crystals with in-plane orientation which can minimize the Cu2O/CuO lattice mismatch, ii) precisely regulating the CuO thickness to eliminate bulk recombination in the CuO layer, and iii)



forming excellent crystal transition interface region. Previous studies fabricated Cu2O-based photocathodes by thermal annealing process at ambient atmosphere to form CuO film and revealed improved charge transport efficiency.23, 24 However, the bulk recombination in CuO and dark current generated from its self-reduction resulted in unsatisfactory performance. The CuO is reduced to Cu with hydrogen ions and electrons coming from water decomposed by the PEC reaction. Thus, CuO is corroded to metallic Cu, resulting in immediate performance degradation, and the corresponding chemical reaction that can occur during PEC water splitting is as follows. CuO + 2H+ + e- → Cu + H2O (equation 1) Thus, the reported results revealed simultaneous positive and negative impacts on PEC performances. Consequently, the effectiveness of the top CuO layers in the Cu2O photocathodes is still unvalidated and their optimal structure requires further studies. We attributed these to the polycrystalline characteristics of Cu2O, which consist of large size facet crystals and completely different crystal structure from CuO. Furthermore, based on the phase-diagram, the CuO phase is more stable in ambient conditions. Hence, the oxidation of Cu2O to CuO will be significantly and non-uniform under the oxygen-rich atmosphere. When the surface lattices of the cubic Cu2O crystals encounter oxygen molecules during heat treatment, the oxygen atoms induce atomic reconstruction by forming monoclinic CuO crystals. To minimize the structural distortion caused by incongruent crystal systems at the interface, the discrepancy between the lattice parameters of these crystals and the tilting of copper atoms should both be reduced. According to international center for diffraction data (ICDD), the ideal lattice matching condition for Cu2O and CuO involves the parallel epitaxial alignment of Cu2O (111) and CuO (−111) lattice planes because their interplanar spacings are very close [d(111)Cu2O= 0.2465 nm and d(111)CuO= 0.2522 nm], as shown in Fig. 1d. Thus, 8 ACS Paragon Plus Environment

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Svintsitskiy et al. proposed the model of surface restructuring along the {−111}CuO → {202}Cu4O3 → {111}Cu2O planes during reduction process to diminish lattice mismatch.25 However, this can only be realized on flat surfaces such as epitaxial thin films. Furthermore, the crystallographic orientation at the interfaces formed on faceted side walls is complex. The typical electrodeposited-Cu2O layers were synthesized with various preferred orientations towards [111], [110], and [200] directions depending on the competitive growth between kinetic and thermodynamic mechanisms, which depend on the pH of electrolytes. Nonetheless, the formation of layers oriented along a single direction is improbable. Thus, Cu2O films with less preferred orientation revealed surface facets with various angles and morphologies, which imply that the formation of sufficiently aligned homogenous Cu2O/CuO heterojunction at the interface is impossible. Moreover, despite the production of Cu2O with uniform facet morphology oriented along the (111) direction, the Cu2O/CuO interfaces are inclined by 54.7o, as shown in Fig. 1e. To overcome this complexity, the use of underlying layers as buffer layer for highly preferred (111) orientation of Cu2O absorbers was considered. Furthermore, if the underlying layer has sufficiently high mobility and electrical conductivity, the hole transport efficiency of photogenerated charges from the absorber can be also improved. Hence, our proposed photoelectrode structure is based on three-layer copper oxides. As shown in Fig. 1c, bulk recombination can be resolved by enhancing charge transport efficiency. Therefore, we designed a thermal treatment process of Cu2O:Sb/Cu2O double layer structure to obtain highly preferred orientation with the exposed {200} facets. Finally, the product revealed homogenous CuO films with significantly diminished interface defects by exhibiting remarkable lattice matching as shown Fig. 1e.



The samples of the Cu2O single layer and Cu2O:Sb/Cu2O double layer were oxidized as shown in Fig. 2a. Here, a double layer sample was designed with highly preferred orientation towards the [111] direction. To control the oxidation rate (or thickness) during the CuO formation on top of the Cu2O, ambient gas condition of low O2 partial pressure between 0 and 10 Torr was applied in the vacuum furnace. The Cu2O crystal has a symmetric cubic structure, while the CuO crystal has an asymmetric monoclinic structure. Consequently, the solid-state phase transformation, which the change in crystal structure, occurs as Cu2O is oxidized to CuO. At different molar volumes between Cu2O and CuO, gradient compressive stress is applied by the surface on the bulk of Cu2O. This leads to the migration of Cu atoms to the surface via grainboundary diffusion. Accordingly, the oxidation process of 2 + → 4 proceeds. Typical Cu2O films have considerably rough faceted surfaces with triangular or rectangular shapes depending on preferred orientation. Consequently, the change in crystal structure during oxidation process and the rough Cu2O surface causes non-uniform oxidation thickness and local stress in the oxidized CuO overlayers. The latter interfere with the formation of the Cu2O/CuO heterojunction with ideally high crystallinity and abrupt interface. As illustrated in Figs. 2b-e, the morphological properties can be considerably changed by the existence of Cu2O:Sb underlying layer and the control of O2 partial pressure. Higher O2 pressure promotes oxidation reaction, resulting in the production of thick CuO layer, irrespective of the presence of Cu2O:Sb layer. Consequently, the oxidized surfaces produce porous grains, which crack the Cu2O/CuO interface by internal stress from the different crystal structure (Figs. 2c and 2e). In contrast, there are no porous grains and cracks in the samples with very thin CuO overlayer oxidized at low O2 partial pressure (Figs. 2b and 2d).


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The XRD patterns of Cu2O single and double layers oxidized at various O2 partial pressures and 400 °C were characterized to investigate the variation of crystal phases during oxidation process (Fig. 3). All of the XRD peaks were well indexed to the standard Cu2O (JCPDS 65-3288) and CuO (JCPDS 65-3288) crystals, regardless of the existence of the Cu2O:Sb buffer layer. For the oxidation at less than 300 °C, only the Cu2O phase was observed (no crystal change). However, the oxidation process above 500 °C produced excessive Cu on the surface. This evidenced the ability of heat treatment to interfere with the formation of homogeneous thin CuO overlayers for effective electron transfer. It is reported that the Cu2O photocathode films exhibit preferred orientation towards the [111] direction upon deposition of Cu2O:Sb seed layer, compared with the Cu2O single layer which showed the diffraction peaks corresponding to (111), (200), and (220) planes at 36.5°, 42.4°, and 61.5°, respectively. When the O2 gas was injected to a partial pressure of 5 Torr at 400 °C, both Cu2O single and double layer patterns manifested CuO (002) and (111) planes with weak intensities at 35.5° and 38.4°, respectively. Interestingly, upon oxidation, the peak of (111) Cu2O significantly intensified for the double layer sample but remained unchanged for the single layer. Furthermore, for the oxidation at higher oxygen partial pressure of above 10 Torr, the intensity of the (111) Cu2O peak considerably decreased, and the diffraction peaks from the CuO became more apparent, implying the formation of sufficient CuO crystal phases. Consequently, the oxidation process of the Cu2O photocathodes yielded various results depending on oxygen partial pressure: i) the crystallinity of photoabsorber Cu2O films improved with the external heat treatment, and ii) controlled CuO overlayers were produced from the Cu2O. Figures 4a and b show the plots of current density versus potential for the oxidized Cu2O photocathodes prepared at various O2 partial pressures in 1.0 M Na2SO4 solution at pH 5.0



(buffered with KHPO4) under AM 1.5 light. As shown in Fig. 4a, SL-5 provided photocurrent density of 1.0 mA/cm2 at 0 V (vs. RHE) analogous to the pristine Cu2O (SL), while the onset potential is slightly larger 0.68 V vs. RHE (Cu2O SL= 0.6 V). However, these performance gains are insufficient to explain the increase in photovoltage from the oxidation of Cu2O surface. Moreover, the SL-10 exhibited an undesirably low photocurrent density of 0.5 mA/cm2 at 0 V (vs. RHE) together with the presence of definite dark current. Based on Cu-related Pourbaix diagram, CuO can be easily reduced to Cu2O or Cu phases at more negative potentials, indicating severe electro-corrosion. This reduction reaction explains the considerable decrease in the photocurrent density of the SL-10 sample and demonstrates the negative effect of potential on the formation of the CuO overlayers by oxidation process, as reported previously.16, 23, 24 Thus, the positive gain in charge transfer efficiency provided by the Cu2O/CuO heterostructure may be hindered by the large lattice mismatch between Cu2O and CuO, as interface defects result in significant strengthening of defect-related recombination. Subsequently, DL-5 exhibited considerably improved photocurrent density (2.8 mA/cm2 at 0 V vs. RHE), compared with the pristine Cu2O samples, DL (1.7 mA/cm2) and SL (1.0 mA/cm2) samples, as shown in Fig. 4b. In addition, in contrast with those in the SL samples, the onset potential of DL-5dramatically increased from 0.67 V (pristine DL) to 0.83 V as shown in the inset of Fig. 4b. These enhancements in both photovoltage and photocurrent of DL-5 were achieved without the coating of photocatalytic materials on the photoelectrodes. Furthermore, these values exceed those reported from bare Cu2O photocathodes. However, DL-5 exhibited slight dark current due to the intrinsic corrosivity of thin CuO overlayers. Furthermore, DL-10 shows seriously decreased photocurrent together and evident dark current due to the formation of excessive CuO layer, which was also confirmed in SL-10 sample. These results can be confirmed


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by the top-view SEM images, which reveal a distinct conformal thin CuO layer on the side facets of DL-5 and an absence of pyramidal-crystals in DL-10 owing to the excess CuO oxidation. To quantitatively analyze the photoresponse efficiency of photocathodes under electric bias over the total irradiation, the applied bias photon-to-current efficiency (ABPE) of the thermally oxidized Cu2O double layer samples was estimated based on the current versus potential (J-V) curves using the following equation: ABPE % =

1.23 − × 100 % 1 ! "

where Jph is the photocurrent density under applied bias, Vb is the potential between the hydrogen evolution potential and applied bias, and Pin is the incident light power density (1 sun = 100 mW/cm2). As shown in Fig. 4c, the DL-5 displayed a maximum ABPE of 0.28% at 0.2 V (vs. RHE), while the solar conversion efficiencies of pristine DL and SL-5 samples are 0.05% (at 0.05 V vs. RHE) and 0.18% (at 0.3 V vs. RHE), respectively. Therefore, it is obvious that the DL-5 sample with a thin CuO overlayer utilizes the light more efficiently for PEC working than the other photocathodes in this study, Furthermore, the ability of the Cu2O:Sb/Cu2O/CuO heterostructure to form appropriate junctions and suppress recombination was confirmed. As previously mentioned, minimizing the lattice mismatch between CuO and Cu2O should be emphasized and the strengthening of the [111] orientation via Cu2O:Sb underlying layer produces the Cu2O/CuO with optimal interfaces and band alignment. Thus, in order to illustrate clear explanation, more detailed microstructural characterization was implemented focusing on the interface region to investigate the lattice mismatch between Cu2O and Cu2O, and the direction of layer orientations. The crystallinity and interface analyses of the controlled CuO films on the Cu2O SL and DL samples were performed by TEM and HRTEM, as shown in Fig. 5. The most favorable lattice 13 ACS Paragon Plus Environment


alignment with the lowest lattice mismatch involves parallel planes of Cu2O (111) and CuO (111). However, the thermal oxidation for the Cu2O with (111) preferred orientation prompted phase transition to monoclinic CuO with slight (002) orientation, effectively prohibiting the formation of Cu2O (111) // CuO (111) in the side facets. Since the angle between Cu2O {111} planes is 70.5o and that between CuO (111) and (002) is 52.8 o, a Cu2O (111) // CuO (111) interface is impossible. Hitherto, most studies on the orientation between Cu2O and CuO crystal domains have been conducted under the assumption of ideal flat surface or random orientation. However, the heterojunction of the oxidized Cu2O/CuO is also produced on the side walls of the faceted crystals with the strongly preferred orientation. Hence, Cu2O/CuO heterojunction with favorable interface and crystal orientation with minimal mismatch could be realized. Therefore, we produced a heterojunction of Cu2O (200) (0.214 nm) // CuO (111) (0.232 nm) planes with acceptable lattice mismatch, as shown in Fig. 1e. The angle between Cu2O (111) and (002) planes is 54.7o and that between CuO (002) and (111) planes is 52.8 o, both are comparable. Correspondingly, Cu2O (002) and CuO (111) planes can be aligned with similar angles from the preferred orientations of Cu2O and CuO crystals, as demonstrated in Fig. 5a. The top CuO layer in DL-5 was approximately 20–40 nm thick. Figures 5b and 5e are HRTEM images of the heterojunction in the sidewall of the crystal facets of DL-5 and SL-5. Observably, the CuO overlayer in the DL-5 resembles crystal continuity with the underlying Cu2O. However, that in SL-5 manifested discontinuous crystal orientation and defective transition region at the CuO/Cu2O interface (indicated by two dotted lines, Fig. 5f) due to its irregular crystal orientations. In contrast, Moiré fringe contrasts with periodic distance are observed in the interface region of the DL-5 sample. Typically, these Moiré fringes are interference patterns most commonly seen when two crystals with well-ordered lattice fringes overlap along the


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electron beam direction. Thus, CuO crystal phase on Cu2O is attained via homogeneous transition of well-aligned lattice fringes.26 If the crystals were randomly oriented or high density of dislocations exists, these Moiré fringes should not be produced with periodic distance. Accordingly, HRTEM images verify that CuO overlayers with remarkably improved crystallinity were achieved via highly oriented grain growth of Cu2O films from the Sb-doped bilayer structure and slow oxidation by controlling oxygen partial pressure. The d-spacings of 0.214 and 0.247 nm measured in the Cu2O region (Fig. 5c) exactly accord with the (200) and (111) planes of cubic Cu2O, respectively. Here, the Cu2O (200) plane oriented towards the in-plane direction perfectly aligns with the (111) plane of monoclinic CuO with lattice spacings of 0.232 nm. This leads to a strain of ~10% in the interface of two crystals. However, based on the periodicity of the observed Moiré fringes, we can infer that one period is composed of 11 CuO (111) planes and 12 Cu2O (200) planes as shown in Fig. 5c. Therefore, the residual strain between these two layers is considerably reduced to under 1% by periodically generating an extra half plane in the Cu2O underlayer. In addition, two diffraction patterns (DPs) from each layer in the DL-5 revealed diffraction spots consistent with domain matching structure, while the SL-5 exhibited randomly rotated polycrystalline DPs. Here, the (111) CuO and (200) Cu2O diffraction spots in Fig. 5b1 and 5b2 are perfectly parallel to the zone axis of [11%0] Cu2O. These results suggest that Cu2O double layer samples with strongly preferred orientation and a remarkable amount of {200} facets exposed towards the [111] direction generated crystalline CuO layers with reduced interfacial strains and well-aligned crystal orientation. Consequently, these microstructural improvements were accountable for the significantly improved PEC performance presented in Fig 4b.



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To understand how these microstructural differences from SL-5 and DL-5 samples affects PEC performance; we analyzed both charge transport and transfer efficiencies based on electrochemical measurements under various environments. The photocurrent generation of PEC photoelectrodes was evaluated based on the following factors: (i) light harvesting efficiency (&' which describes the absorbance capability of photoactive materials, (ii) charge transport efficiency ( &() governed by the charge separation and electrical properties of the photoelectrodes, and (iii) charge transfer efficiency (&*( at the electrode-electrolyte interface.27 Among these factors, &() and &*( are closely related with bulk and surface recombination, respectively, and both significantly influence the hydrogen evolution onset potential (Von). Figure 6a shows the onset potential values from the SL and DL samples under different oxidation conditions. The increase in Von of the double layer structure was attributed to enhanced crystallinity by the Cu2O:Sb buffer layer. The thin CuO overlayer produced on the Cu2O photoelectrodes

further

enhanced

the

Von

by

suppressing

recombination

near

the

Cu2O/electrolyte junction. The Von change was more evident in the double layer before and after oxidation (DL and DL-5) than in the single layer sample (SL and SL-5), which was explained by the high-quality Cu2O/CuO junction with reduced strain and low defects in the double layer. Generally, the critical factors for high photocurrents, &() and &*( , are estimated upon the addition of electron scavenger H2O2 to the electrolyte. This induces extremely fast charge transfer efficiency, i.e., a nearly 100% value for &*(+, -, . Consequently, the photocurrent for H2O reduction with H2O2 can be estimated by: +, - = ./0 × &' × &() × &*( 2 +, -, = ./0 × &' × &() 3


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Since the &*( of H2O2 is ≈1, the &*(+, - can be calculated by +, - /+, -, , as shown in Fig. 6b. Correspondingly, the &*( of the DL-5 is around 50% at 0 V (vs. RHE), which is higher than that of the pristine DL (30%). This implies that surface defects, which induce the recombination loss at the electrode-electrolyte interface, were diminished upon passivation by the CuO overlayer with good crystallinity. This improvement in &*( was also detected in the single layer, but with minimal gain (from 11 to 15% in the SL-5). Furthermore, the high-quality Cu2O-CuO heterojunction provides enhanced charge transport efficiency. Hence, we evaluated the &() of SL-5 and DL-5 with the following equation: &() = +, -, /(./0 × &' ) (4) The optical absorbance of SL-5 and DL-5 was first measured to distinguish their &' . As shown in Fig. 6c, the absorbance of SL-5 is observably higher than that of DL-5 at the long wavelength of > 500 nm. This is attributed to the strongly preferred orientation and vertically aligned grain boundaries in the double layer samples, as reported in our previous study.22 Even though general fact that high absorbance leads to the high photocurrent, but our experiment yielded opposite results. Consequently, the &() of DL-5 suggested significantly enhanced PEC performance (Fig. 6d) with an improved charge transport efficiency >10%. Although CuO film with relatively narrower band gap has higher absorbance than the Cu2O, thermal oxidation degrades PEC performance regardless of the existence of Cu2O:Sb layer, as shown in Figs. 4a and b. The Mott-Schottky plots of Cu2O thermally-oxidized under various O2 partial pressures were obtained (Fig 7a). The carrier concentration (NA) and the flat band potential (Vfb) were extracted from following equation; 1 2 :; < = 8 − 9 − = 5 5 34 3) 567



where, C is the capacitance between the semiconductor and electrolyte; 34 3) is the permittivity of the semiconductor, V (vs. RHE) is the applied bias, and T is the absolute temperature. Here, the Mott-Schottky results were acquired not from the electrical properties in the bulk of Cu2O absorber semiconductor but from the electrochemical reaction between the CuO surface and electrolyte. The DL-10 sample, 350 nm thick, underwent considerable oxidation and exhibited higher flat band potential (0.83 V vs. RHE) obtained from the intercept of the Mott-Schottky plots. The calculated NA of DL-10 from equation (5) is 8.9 x 1015 cm-3. Moreover, DL-5 has similar carrier density due to comparable slopes of C-V curves with that of DL-10. These values allowed the approximation of depletion width (W) in the semiconductor/electrolyte interface using W=@

AB AC DEF

GV − 9 IJ

KL

(6)

The depletion width in the CuO layer was obtained at 43 nm for the quasi-Fermi level at 0 V (vs. RHE). The schematic band diagrams of DL-10 and DL-5 in contact with the electrolyte was shown in Figs. 7b and c, respectively. Based on their transmittance spectra, the band gaps of Cu2O and CuO were confirmed to be 2.1 eV and 1.55 eV, respectively. Furthermore, if the thickness of the top CuO overlayer is formed exceeding the depletion width, the additional recombination loss can be negatively generated due to the inefficient separation of electron charges photogenerated from Cu2O and CuO and bypath consumption of electrons via active corrosion, resulting in the deterioration of PEC performance. In addition, the front illumination hinders absorbance in the Cu2O absorber film due to partial absorption from its narrow band. In contrast, the formation of very thin CuO overlayers which can be fully depleted or band bending effectively alleviates recombination loss in the junction region. Thus, both photocurrent and photovoltage of DL-5 were enhanced. It was reported that ultrathin Al2O3 is very effective in 18 ACS Paragon Plus Environment

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growing conformal thin TiO2 layers, and thus we applied thin passivation layers consisting of ALD grown Al2O3/TiO2 bilayers for surface passivation towards photocorrosion. Although the overall photocurrent is slightly reduced owing to the use of the Al2O3 and TiO2 layers with low electrical conductivity, these results prove that the formed CuO definitely contributes to the improvement of the PEC performance.

CONCLUSION Although Cu2O-based photo-absorber materials have adequate band gap and band position for PECs, the recombination loss in the Cu2O bulk and Cu2O/electrolyte interface incapacitated charge transport and transfer, effectively diminishing the PEC performances. Among potential materials for the heterojunction with effective charge transport, the effectiveness on the use of the binary CuO overlayer is still debating. We attributed this difficulty to rapid oxidation of Cu2O to CuO at oxygen-rich atmosphere and the formation of defective interfaces due to abrupt crystal transformation from cubic to monoclinic structure. To prove this, we controlled the oxidation rate of Cu2O to CuO by regulating the oxygen partial pressure and reinforcing the preferred orientation of the Cu2O underlying layers. The strongly preferred orientation of the Cu2O films towards the [111] direction was achieved by adding Cu2O:Sb buffer layer. Subsequently, the {200} facets of the double-layer structure was completely exposed for the thermal oxidation process. The reduced oxidation rate produced optimal CuO overlayers with 20-30 nm thickness. Simultaneously, the Cu2O/CuO interfaces produced well-aligned lattice fringes with smooth crystallographic transition. These Cu2O/CuO heterojunctions displayed periodic Moiré fringes and diffraction patterns with domain matching, implying highly crystalline interface. From various PEC analyses, we found that photochemical properties



improved with i) enhanced charge transport efficiency owing to less interface defects and ii) improved charge transfer efficiency from decent conduction band offset and fully depleted CuO. Furthermore, excessively oxidized Cu2O absorbers displayed deteriorated photochemical performance as CuO suffer from self-reduction and bulk recombination. This implied that CuO overlayers should be fabricated at optimum thickness and with highly crystalline interfaces.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.xxxxxxx. LSV measurements from the samples annealed at different temperatures AUTHOR INFORMATION Corresponding Author * Tel: +82 31 299 4733; fax: +82 31 290 7410; e-mail: [email protected]. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ‡These authors contributed equally to this work. ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (Grant No. 2018R1A2B2004050) and “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea (No. 20174030201800).

REFERENCES 21 ACS Paragon Plus Environment


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Table of Contents Graphic and Synopsis

High performance photocathode for water splitting with crystallographically well-aligned Cu2O/CuO structure via precisely controlled oxidation process


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Figure captions Figure 1. Schematic band diagrams of the photocathode cell showing recombination loss in the bulk electrode and electrode/electrolyte interface. The photocathodes are (a) Cu2O, (b) Cu2O/CuO, and (c) Cu2O:Sb/Cu2O/CuO. (d) Atomic arrangement of Cu2O and CuO viewed along the [01%1] and [11%0] (projection direction), respectively, (e) Atomic arrangement of the possible lattice matching between faceted Cu2O and CuO interface.

Figure 2. (a) Thermal oxidation prepared at different processes and their cross-sectional SEM images: (b) SL-5, (c) SL-10, (d) DL-5, and (e) DL-10.

Figure 3. XRD spectra of single and double layers thermally oxidized at various oxygen partial pressures, suggesting the presence of Cu2O and CuO crystals.

Figure 4. Fundamental photoelectrochemical properties of thermally oxidized (a) single layers and (b) double layers, and (c) ABPE spectra of SL-5, DL, and DL-5 samples. Top-view SEM images (d-f) of the SL and DL samples annealed at different oxygen partial pressures.

Figure 5. Schematic diagram showing crystal facet morphology: (a) DL-5 and (d) SL-5. Brightfield TEM, and SADP (b) DL-5 and (e) SL-5. These data were obtained from b1, b2 (DL-5) and e1, e2 (SL-5) at the Cu2O/CuO interface region [marked by rectangles in (b) and (e)]. HRTEM 27 ACS Paragon Plus Environment


images of (c) DL-5 and (f) SL-5. Crystallographic orientation of the Cu2O and CuO phases with interface lattice matching (g and h).

Figure 6. (a) Onset potential values of thermally oxidized single and double layers and (b) charge transfer efficiency (&*( ) calculated from the J-V curves in the Na2SO4 electrolytes with H2O2 electron scavenger. (c) Optical absorbance of SL-5 and DL-5 samples and (d) calculated charge transport efficiency (&() ).

Figure 7. (a) Mott-Schottky plots for the bare and thermally oxidized double layers. Schematic band diagrams of (b) DL-10 and (c) DL-5, with energy positions estimated from Mott-Schottky plots.

Figure 8. Stability tests of the (a) DL (no protection) and (b) SL-5 and DL-5 samples covered with Al2O3/TiO2 protection layers.


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Figure 1. Schematic band diagrams of the photocathode cell showing recombination loss in the bulk electrode and electrode/electrolyte interface. The photocathodes are (a) Cu2O, (b) Cu2O/CuO, and (c) Cu2O:Sb/Cu2O/CuO. (d) Atomic arrangement of Cu2O and CuO viewed along the [0-11] and [1-10] (projection direction), respectively, (e) Atomic arrangement of the possible lattice matching between faceted Cu2O and CuO interface. 240x190mm (300 x 300 DPI)

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Figure 2. (a) Thermal oxidation prepared at different processes and their cross-sectional SEM images: (b) SL-5, (c) SL-10, (d) DL-5, and (e) DL-10. 150x53mm (300 x 300 DPI)


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Figure 3. XRD spectra of single and double layers thermally oxidized at various oxygen partial pressures, suggesting the presence of Cu2O and CuO crystals. 84x149mm (300 x 300 DPI)



Figure 4. Fundamental photoelectrochemical properties of thermally oxidized (a) single layers and (b) double layers, and (c) ABPE spectra of SL-5, DL, and DL-5 samples. Top-view SEM images (d-f) of the SL and DL samples annealed at different oxygen partial pressures. 192x102mm (300 x 300 DPI)


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Figure 5. Schematic diagram showing crystal facet morphology: (a) DL-5 and (d) SL-5. Bright-field TEM, and SADP (b) DL-5 and (e) SL-5. These data were obtained from b1, b2 (DL-5) and e1, e2 (SL-5) at the Cu2O/CuO interface region [marked by rectangles in (b) and (e)]. HRTEM images of (c) DL-5 and (f) SL-5. 247x190mm (300 x 300 DPI)



Figure 6. (a) Onset potential values of thermally oxidized single and double layers and (b) charge transfer efficiency (ηct) calculated from the J-V curves in the Na2SO4 electrolytes with H2O2 electron scavenger. (c) Optical absorbance of SL-5 and DL-5 samples and (d) calculated charge transport efficiency (ηtr). 150x113mm (300 x 300 DPI)


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Figure 7. (a) Mott-Schottky plots for the bare and thermally oxidized double layers. Schematic band diagrams of (b) DL-10 and (c) DL-5, with energy positions estimated from Mott-Schottky plots. 177x64mm (300 x 300 DPI)



Figure 8. Stability tests of the (a) DL (no protection) and (b) SL-5 and DL-5 samples covered with Al2O3/TiO2 protection layers. 124x164mm (300 x 300 DPI)


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Cupric Heterojunction Photocathodes ... - ACS Publications

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