CdTe-Based Photoanode for Oxygen Evolution from Water under

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A CdTe-Based Photoanode for Oxygen Evolution from Water Under Simulated Sunlight Jin Su, Tsutomu Minegishi, Yosuke Kageshima, Hiroyuki Kobayashi, Takashi Hisatomi, Tomohiro Higashi, Masao Katayama, and Kazunari Domen J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.7b02526 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 9, 2017

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The Journal of Physical Chemistry Letters

A CdTe-based Photoanode for Oxygen Evolution from Water under Simulated Sunlight Jin Su a, Tsutomu Minegishi a,b, Yosuke Kageshima a, Hiroyuki Kobayashi c,d, Takashi Hisatomi a, Tomohiro Higashi a, Masao Katayama a, and Kazunari Domen a, * a

Department of Chemical System Engineering, School of Engineering; The University

of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113–8656, Japan. b

c

PRESTO-JST, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113–8656, Japan. FUJIFILM Corporation, 577, Ushijima, Kaisei-Machi, Ashigarakami-gun,

Kanagawa, 258-8577, Japan d

Japan Technological Research Association of Artificial Photosynthetic Chemical

Process (ARPChem), The University of Tokyo, 2-11-9 Iwamotocho, Chiyoda-ku, Tokyo 101-0032, Japan.

Email: [email protected]

ACS Paragon Plus Environment


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ABSTRACT This study investigated the properties of a photoanode fabricated by depositing a p-type CdTe thin film on a CdS-coated FTO substrate (CdTe/CdS/FTO) via close space sublimation. This CdTe/CdS/FTO electrode was found to work as a photoanode with a long absorption edge wavelength of 830 nm. In a CdTe-based photoanode such as this, the p-n junction formed at the CdTe/CdS interface promotes charge separation of photoexcited carriers and forces photo-generated holes to move toward the photoanode surface to promote oxidation reactions on the electrode surface. A MoOx buffer layer was also found to play a crucial role in facilitating the transfer of photo-generated holes to surface reaction sites through decreasing the energy barrier at the interface between the CdTe and a surface protective layer. A bi-photoelectrode photoelectrochemical cell composed of a CdTe-based photoanode and a CdTe-based photocathode exhibited a solar-to-hydrogen conversion efficiency of 0.22% without an external voltage in response to illumination by AM 1.5G light. TOC GRAPHIC


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Photoelectrochemical (PEC) water splitting represents a promising means of converting solar energy into chemical energy in the form of hydrogen.1-7 Photoelectrodes play a central role in a PEC cell, with n-type and p-type semiconductors typically serving as the photoanode and photocathode, respectively.8-15,

35-37

Both photoanode and photocathode materials have been

widely researched over recent years, with the aim of achieving efficient PEC water splitting. However, the products developed to date have failed to satisfy strict requirements, such as having a long absorption edge, an optimal band structure

and

durability.

Chalcogenides

are

potential

candidates

for

photoelectrode materials because they exhibit narrow band gaps combined with band edge potentials suitable for water splitting,16-18 although their lack of chemical stability is problematic. Some chalcogenide photocathodes have been found to be relatively stable and to allow highly efficient hydrogen evolution from water, but there have been few reports regarding robust oxygen evolution from water using chalcogenide-based photoanodes.19 One approach to utilizing materials with inadequate stability such as chalcogenides as durable, efficient photoanodes for water oxidation is to apply a coating of a more stable substance while introducing functional structures, such as multilayers including a p-n junction.8,11,13,19 To date, PEC cells for solar hydrogen production from water have been fabricated using photoelectrodes incorporating buried p-n junctions. As an example, Lin et al. reported an amorphous silicon-based photocathode that showed a remarkable onset potential



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as well as a superior photocurrent and good durability.20 Cadmium telluride (CdTe) has a long absorption edge wavelength of 830 nm and a high light absorption coefficient.21,22 Because of these advantageous properties, there

have

been

many

investigations

of

applications

of

CdTe

to

photoelectrochemistry and photocatalysis.23-27 Recently, we reported PEC properties of CdTe based photocathodes including flat-band potential, Faradaic efficiency of about 100% and extremely high incident photon-to-current conversion efficiency (IPCE) of >95% at 550-660 nm.18, 27 However, there have been few reports concerning oxygen evolution from water using CdTe-based photoanodes. Lichterman et al. demonstrated stable water oxidation using a TiO2 and Ni-coated n-type CdTe single crystal wafer as a photoanode under simulated sunlight.19 The present study investigated oxygen evolution from water under simulated sunlight using a CdTe-based photoanode. As discussed above, CdTe has attractive properties and, based on its absorption edge, is expected to generate a high photocurrent of 29 mA cm-2 under illumination by a standard AM 1.5G sunlight spectrum. CdTe also has practical applications as a photovoltaic (PV) material in polycrystalline thin films with superstrate structures. In this work, a CdTe-based photoanode with a structure similar to that of a CdTe-based PV device was prepared using a SnO2:F coated glass (FTO) substrate as an optical window. This device was fabricated by depositing a CdTe thin film on the FTO substrate following the chemical bath deposition (CBD) of a CdS layer. The CdTe/CdS interface formed a p-n junction that played a crucial role in providing


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the driving force for the reaction. To allow stable oxygen evolution from water, a protective Ti layer was deposited on the CdTe and CdS-coated FTO (CdTe/CdS/FTO). In addition, to facilitate the oxygen evolution reaction (OER) and to promote charge transfer to the Ti layer, a Co species and a MoOx layer were introduced to the photoanode surface and the Ti/CdTe interface, respectively. The detailed sample preparation procedures are provided in the Supporting Information (SI). The resulting CdTe-based photoanode exhibited anodic photocurrents of 0.85 and 3.8 mA cm-2 at 0.6 and 1.2 VRHE, respectively, under AM 1.5G illumination. The maximum half-cell solar-to-hydrogen (HCSTH) conversion efficiency of the CdTe-based photoanode was 0.85% at 0.8 VRHE. A bi-photoelectrode PEC cell composed of a CdTe photocathode and a CdTe photoanode without an external voltage and under AM 1.5G irradiation demonstrated overall water splitting utilizing light up to 830 nm while generating a photocurrent of 0.17 mA cm-2, indicating an STH efficiency of 0.22%.

Prior to the examination of PEC properties, structural characterization was performed using scanning electron microscopy (SEM). A cross-sectional SEM image of the 200 nm-thick Ti protective layer and the 30 nm-thick MoOx layer on CdTe/CdS/FTO (Ti/MoOx/CdTe/CdS/FTO) is presented in Figure 1A. This image shows that the 2 μm-thick CdTe layer had a polycrystalline structure and that the upper Ti layer completely covered the CdTe. In contrast, neither the 80 nm-thick CdS layer nor the MoOx layer can be clearly seen. The top-view SEM images in Figures 1B-F indicate



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variations in the surface morphology during the Ti/MoOx/CdTe/CdS/FTO preparation procedure. As shown in Figure 1E, the CdS/FTO surface was composed of small grains, even though CdTe/CdS/FTO synthesized in previous work exhibited large polygonal grains with columnar shapes.28 It should be noted that annealing of the CdS/FTO at 400 °C can improve the crystallinity of the CdS layer.29

Figure 1. Cross-sectional SEM image of (A) Ti/MoOx/CdTe/CdS/FTO, top-view SEM images of (B) Ti/MoOx/CdTe/CdS/FTO, (C) MoOx/CdTe/CdS/FTO, (D) CdTe/CdS/FTO, (E) CdS/FTO and (F) FTO surfaces, as well as a cross-sectional SEM image of (G) the CdTe/CdS/FTO interface.

As shown in Figures 1B and 1C, the surface morphology of the CdTe/CdS/FTO was dramatically changed by the deposition of the MoOx and Ti layers. Specifically, the MoOx-coated CdTe/CdS/FTO (MoOx/CdTe/CdS/FTO) had an amorphous surface in


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which the grain boundaries in the CdTe/CdS/FTO were partly filled with MoOx. The top-view SEM images also demonstrate that the surface morphology of the Ti/MoOx/CdTe/CdS/FTO specimen incorporating the 200 nm-thick Ti protective layer was clearly different from that of the MoOx/CdTe/CdS/FTO. The X-ray diffraction (XRD) patterns obtained from the CdTe/CdS/FTO, CdS/FTO and bare FTO substrate are shown in Figure S1. No CdS peaks were generated by the CdS/FTO because the CdS layer was relatively thin (80 nm) and the grain size was small. The XRD peaks obtained from the CdTe/CdS/FTO were assignable to CdTe (PDF #15-0770) and FTO. The optical properties of CdTe/CdS/FTO with 80 nm- and 2 μm-thick CdS and CdTe layers were investigated using UV-vis transmission spectroscopy, as shown in Figure S2. The absorption edge was unclear because of the presence of an interference fringe, although the spectrum was consistent with a band gap energy of 1.5 eV. Following the above characterizations, the PEC properties of the prepared electrodes were examined. The current-potential curves for CdS/FTO and CdTe/CdS/FTO electrodes under simulated AM 1.5G light are presented in Figure S3. The CdS/FTO electrode showed a clear anodic photocurrent, reflecting its n-type semiconducting nature. The photocurrent produced by the CdTe/CdS/FTO electrode at approximately 1.23 VRHE was much larger than that for the CdS/FTO electrode due to the long absorption edge wavelength of CdTe. The CdTe monolayer prepared by the close space sublimation (CSS) method would be expected to exhibit a cathodic photo-response,18,23 while the CdTe/CdS/FTO showed an anodic photo-response because of the formation



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of a p-n junction at the CdTe/CdS interface. This junction promotes the charge separation of photo-excited carriers and drives the oxidation reaction in response to light irradiation. However, it should be noted that the anodic photocurrent from the CdS/FTO and CdTe/CdS/FTO likely contributed to the photo-corrosion of both the CdS and CdTe in addition to the oxidation of water.

Figure

2.

(A)

Current-potential

curves

for

Co(OH) x/Ti/CdTe/CdS/FTO

and

Co(OH)x/Ti/MoOx/CdTe/CdS/FTO under chopped simulated sunlight, and (B) IPCE values at 0.6 VRHE for Co(OH)x/Ti/MoOx/CdTe/CdS/FTO in a 0.5 M aqueous KH 2PO4 solution with pH adjusted to 8.

To facilitate the water oxidation reaction while suppressing photo-corrosion of the photoelectrode, the effects of surface modifications with an OER catalyst and a surface


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protective layer were examined. The current-potential curve for a CdTe/CdS/FTO photoelectrode with an OER catalyst, Co(OH)x, and a Ti protective layer (that is, Co(OH)x/Ti/CdTe/CdS/FTO) Co(OH)x/Ti/CdTe/CdS/FTO

is

provided

photoanode

in

generated

Figure anodic

2A.

photocurrents

The of

approximately 0.07 and 2.3 mA cm-2 at 0.6 and 1.2 VRHE, respectively, under simulated sunlight. Co(OH)x is expected to enhance the OER. However, the anodic photocurrent of the Co(OH)x/Ti/CdTe/CdS/FTO photoanode was comparable to that of the CdTe/CdS/FTO. One possible reason for the comparable photocurrent values is that the CdTe/CdS/FTO photoanode may have contributed to both the OER from water and photo-corrosion, as discussed above, while the Co(OH)x/Ti/CdTe/CdS/FTO photoanode promoted only the OER because of the suppression of photo-corrosion by the 200 nm-thick Ti layer. Considering the long absorption edge for CdTe, it was evident that the photocurrent generated by this photoanode could be increased significantly. To facilitate the OER on the photoanode surface, the effect of electrical contact between the Ti layer and the CdTe layer was investigated. As can be seen from Table S1, the work function of Ti (4.3 eV) is clearly lower than that of CdTe. Because of the p-type semiconducting character of CdTe and the small work function of Ti, an undesirable Schottky barrier would be expected to form at the Ti/CdTe interface, as illustrated in Figure S4A. Therefore, to obtain the desirable Ohmic contact between the CdTe and Ti layers, a MoOx layer was introduced between the two.30-33 MoOx has been reported to have a work function of 5.6 eV and to act as an insulator.30,31 Because of its higher work function value, the MoOx was expected to undergo sufficient contact with



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the CdTe layer, as in Figure S4B. Electrochemical impedance spectra for the photoanodes revealed that the introduction of MoOx layer decrease the series resistance through the formation of ohmic contact as shown in Figure S11 in the SI. Furthermore, the insulating nature of this material was anticipated to contribute to mitigating the shunt issue between the Ti layer and the FTO through the pinholes in the CdTe layer. In fact, a CdTe/CdS/FTO photoanode surface modified with a 30 nm-thick MoOx buffer layer, a 200 nm-thick Ti layer and Co(OH)x (Co(OH)x/Ti/MoOx/CdTe/CdS/FTO) generated a clear anodic photocurrent under simulated sunlight above 0.4 VRHE, as shown in Figure 2A. The photocurrents obtained from the photoanode were increased from 0.07 and 2.3 mA cm-2 to 0.85 and 3.8 mA cm-2 at 0.6 and 1.2 VRHE, respectively, subsequent to introducing the MoOx layer. The HC-STH determined from the currentpotential curve of the Co(OH)x/Ti/MoOx/CdTe/CdS/FTO photoanode shown in Figure S5 had a maximum value of 0.85% at 0.8 VRHE. The IPCE values for the Co(OH)x/Ti/MoOx/CdTe/CdS/FTO at 0.6 VRHE are plotted in Figure 2B, and demonstrate that the photoanode was responsive to photons up to approximately 900 nm. The maximum IPCE value of 5.4% was obtained at 620 nm, although the IPCE clearly decreased with wavelength below 560 nm due to light absorption by the CdS layer, which had an absorption edge of 520 nm. These data indicate that photons absorbed by the CdS layer were not utilized for PEC reactions, similar to results obtained with PV devices.34 It should be noted that the predicted photocurrent under AM 1.5G light based on the IPCEs in Figure S6, 1.0 mA cm-2, is in very good agreement with the observed photocurrent of 0.85 mAcm-2 in the current potential curve shown in


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Figure 2A at 0.6 VRHE. The time course of the photocurrent generated by a Co(OH)x/Ti/MoOx/CdTe/CdS/FTO and Co(OH)x/CdTe/CdS/FTO at 0.6 VRHE under simulated sunlight are shown in Figure S7. However, the anodic photocurrent from Co(OH)x/CdTe/CdS/FTO can be originated by the oxidation of the CdTe layer.38 To reveal the reason of decreased photocurrent of Co(OH)x/Ti/MoOx/CdTe/CdS/FTO photoanode, the surface analysis using SEM and XPS were conducted onto the photoanode. SEM images shown in Figure S8 revealed that the surface morphology of the photoanode before and after the durability test were almost identical. The XPS spectra shown in Figure S9 revealed that only the Ti species were detected before the durability test, and Cd and Te species were detected in addition to the Ti species after the test. These experimental results indicate that the photoanode surface was fully covered by the Ti layer before the durability test, however, the Ti layer was partly removed during the test. For future improvement of the durability, development of the surface protection layer can be the key. The results of gas product analysis by GC, used to assess the Faradaic efficiency, are presented in Figure 3. It should be noted that there was a lag time of approximately 10 min following the start of the reaction prior to the detection of both hydrogen and oxygen because of the requirement to homogenize the gas phase in the PEC cell. During the span of a 60 min PEC reaction, the Co(OH)x/Ti/MoOx/CdTe/CdS/FTO photoanode showed stable oxygen evolution from water. Furthermore, the quantity of evolved oxygen was consistent with the amount expected from the integrated charge passing through the external circuit. The amount of hydrogen evolved simultaneously was also



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approximately two times that of the evolved oxygen, in agreement with the expected stoichiometric

ratio.

These

experimental

data

demonstrate

that

the

Co(OH)x/Ti/MoOx/CdTe/CdS/FTO was able to drive the stoichiometric water splitting reaction with a Faradaic efficiency of approximately 100%. To confirm the potential applications of a CdTe PEC device, a bi-photoelectrode PEC cell composed of a CdTe-based photoanode and photocathode was examined. The photocathode was fabricated by preparing a CdTe thin film on a Cu and Au-coated FTO substrate by the CSS method. This was done because a CdTe-based photocathode treated via post-deposition calcination using CdCl2 as the sintering reagent and surface modified with a CdS layer and Pt (Pt/CdS/CdTe(CdCl2)/Cu/Au/FTO) has been reported to exhibit high IPCE values (>95%) in response to irradiation at 560-660 nm.18 The current-potential curves for the Co(OH)x/Ti/MoOx/CdTe/CdS/FTO photoanode and the Pt/CdS/CdTe(CdCl2)/Cu/Au/FTO photocathode are plotted in Figure 4A. Here, cathodic current densities are plotted for the photocathode. These current-potential curves cross one another at 0.55 VRHE, indicating that the PEC cell was capable of generating a photocurrent without an external bias voltage under simulated sunlight in conjunction with a working potential of 0.55 VRHE for both electrodes. The time course of the photocurrent from the bi-photoelectrode PEC cell without an external bias voltage under simulated sunlight is displayed in Figure 4B. The photocurrent value 20 s after the beginning of irradiation was 0.17 mA cm-2, indicating that the initial STH was 0.22%, although this value gradually decreased to 0.08 mA cm-2 over the course of 30 min. The successful construction of a bi-photoelectrode PEC cell composed of a


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CdTe-based photoanode and photocathode reflects the potential of CdTe as a photoelectrode material. The gradual decrease in the photocurrent and the low photocurrent also indicate that the stabilization of CdTe-based photoelectrodes and further enhancement of water splitting through surface modifications with stable and functional materials, such as TiO2, will be the primary challenges related to this technology in future.

Figure 3. Hydrogen and oxygen evolution from a Co(OH)x/Ti/MoOx/CdTe/CdS/FTO photoanode and a counter electrode under simulated sunlight in a three-electrode configuration with an applied potential of 0.6 VRHE in a 0.5 M phosphate solution (pH 8). The theoretical amounts of hydrogen and oxygen as calculated from the obtained photocurrent are shown as dashed curves.



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Figure

4.

(A)

Current-potential

(Co(OH)x/Ti/MoOx/CdTe/CdS/FTO)

curves and

for a

a

CdTe-based

CdTe-based

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photoanode photocathode

(Pt/CdS/CdTe(CdCl2)/Cu/Au/FTO) under chopped simulated sunlight in a 0.5 M aqueous KH2PO4 solution with pH adjusted to 8, and (B) the time course of the photocurrent generated by a CdTebased photoanode and CdTe-based photocathode under simulated sunlight without the application of an external voltage. The surface areas of the photocathode and photoanode were 0.14 cm2 and 0.07 cm2, respectively.

In summary, a CdTe-based photoanode with a long absorption edge of 830 nm and a low onset potential of approximately 0.4 VRHE was successfully fabricated in the present work. Comparison with previously reported photoanodes is made in Table S2 in the SI. The electrode prepared by the CSS deposition of a p-type CdTe thin film on a CdScoated FTO substrate followed by surface modification with a Ti protective layer and a Co(OH)X OER catalyst functioned as a photoanode because of the formation of a p-n junction at the CdTe/CdS interface. The introduction of a MoOx buffer layer to the


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CdTe-based photoanode also increased the photocurrent from 0.07 and 2.3 mA cm-2 to 0.85 and 3.8 mA cm−2 at 0.6 and 1.2 VRHE, respectively, under AM 1.5G illumination. The significantly improved photocurrent for this CdTe photoanode is attributed to the formation of an Ohmic contact following the introduction of the MoOx buffer layer, resulting in improved charge transfer between the reaction sites and the CdTe layer. The maximum HC-STH for the optimized photoanode was 0.85% at 0.8 VRHE. We believe the application of post-deposition annealing treatment with existence of CdCl2 as a sintering reagent should enhance the anodic photocurrent from the CdTe based photoanode similar to the case of photocathode18. The analysis of gaseous products confirmed that the Faradaic efficiency of this CdTe-based photoanode during the PEC water oxidation process was almost 100%. A bi-photoelectrode PEC cell composed of a CdTe photoanode and CdTe photocathode was successfully constructed and generated a 0.17 mA cm-2 photocurrent 20 s after the beginning of irradiation without an external bias voltage under AM 1.5G light irradiation, indicating that the initial STH was 0.22%. We note that the clear decrease of bi-photoelectrode PEC cell during the reaction can be because of the insufficient durability of both photoanode and photocathode. Future challenges include improving both the stability and photocurrent through surface modification with functional materials. This study indicates that highly efficient charge separation at an internal p-n junction structure, the use of a protective layer, surface modification with co-catalysts, and the introduction of a high work function buffer layer all improve the performance of photoanodes intended for PEC water oxidation. The use of p-type semiconducting materials as light absorbers demonstrated in the present work



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could have applications to the development of photoanodes capable of absorbing a wider range of wavelengths so as to obtain more efficient solar hydrogen production.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at the internet http://pubs.acs.org. Experimental details, X-ray diffraction patterns, a UV-vis transmission spectrum, Current-potential curves, a HC-STH curve, Table of work functions of various materials. (PDF)

AUTHOR INFORMATION Corresponding Author E-mail: [email protected].

ACKNOWLEDGMENTS This work was supported by Grants-in-Aid for Scientific Research (A) (nos. 16H02417 and 17H01216) from the Japan Society for the Promotion of Science (JSPS), as well as by the Precursory Research for Embryonic Science and Technology (PRESTO) program (no. JPMJPR1543) of the Japan Science and Technology Agency (JST), and the Artificial Photosynthesis Project of the New Energy and Industrial Technology


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Development Organization (NEDO). J. Su also wishes to acknowledge the support of the China Scholarship Council (CSC) (no. 201406060032).

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