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Efficient and Stable Pt/TiO/CdS/CuBaSn(S,Se) Photocathode for Water Electrolysis Applications Yihao Zhou, Donghyeop Shin, Edgard Ngaboyamahina, Qiwei Han, Charles B. Parker, David B. Mitzi, and Jeffrey T. Glass ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.7b01062 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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ACS Energy Letters

Efficient and Stable Pt/TiO2/CdS/Cu2BaSn(S,Se)4 Photocathode for Water Electrolysis Applications Yihao Zhou1#, Donghyeop Shin1,2,4#, Edgard Ngaboyamahina3#, Qiwei Han1,2, Charles B. Parker3, David B. Mitzi1,2*, Jeffrey T. Glass1,3* 1

Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC

27708, USA 2

Department of Chemistry, Duke University, Durham, NC 27708, USA

3

Department of Electrical & Computer Engineering, Duke University, Durham NC, 27708 USA

4

Photovoltaic Laboratory, Korea Institute of Energy Research, Daejeon, 34129, Korea

AUTHOR INFORMATION Corresponding Authors *D. B. Mitzi: Email: [email protected] *J. T. Glass: Email: [email protected]

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ABSTRACT

The Cu2BaSnS4-xSex (CBTSSe) system has attracted remarkable attention as an emerging chalcogenide semiconductor with desirable electronic and optical properties for solar energy conversion applications. The current study combines sputtered bandgap-tailored CBTSSe films with TiO2/CdS protective overlayers to significantly improve photoelectrochemical (PEC) properties as well as device stability in aqueous solutions under AM 1.5G simulated sunlight. The Pt/TiO2/CdS/CBTSSe(x≈3) photocathode exhibits relatively high photocurrent (~12.08 mA/cm2 at 0 V/RHE, i.e. the highest value reported for CBTSSe-based PEC devices) and stable hydrogen evolution for more than 10 hours. The applied TiO2/CdS layers protect the underlying CBTSSe and create desirable band alignment for efficient charge extraction at the heterointerfaces. The present results highlight the opportunities that CBTSSe materials provide as efficient and stable photocathodes for water electrolysis.

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ACS Energy Letters

Fossil fuels are an integral component of meeting the current energy demands of human activity. In 2016, around 65% of the total electricity consumed in the United States was generated from fossil fuels1. However, fossil fuel-based power generation is vulnerable to increasing cost and finite fuel reserves. Additionally, byproducts such as greenhouse gases have negative environmental impacts. Compared to fossil fuels, hydrogen is a clean and efficient fuel2. However, 95% of current hydrogen production derives from fossil fuel-based processes3 and thus continues to negatively impact the environment. In contrast, light-induced water electrolysis represents a safe, sustainable and environmentally friendly route to produce hydrogen. Since Honda and Fujishima discovered a way to produce hydrogen using a TiO2 anode in a photoelectrochemical (PEC) cell4, various materials and systems for water electrolysis have been investigated to achieve higher efficiency5-7. One of the most promising device configurations is the two-electrode PEC system in which hydrogen and oxygen are electrogenerated at the photocathode and photoanode, respectively. Specifically, numerous semiconductor materials8-9 such as GaAs7, Cu2O10 and Si11 have been investigated as photocathodes for hydrogen evolution. In recent years, zinc-blende-related chalcogenides (e.g., CuGaSe2 (CGSe), CuInxGa1-xSe2 (CIGSe) and CuZnSnS4 (CZTS)) have also been considered as efficient photocathode materials, due to their excellent properties including high absorption coefficient, suitable band structure and tunable bandgap (1.0~2.4 eV)

12-15

. It has been reported that a CdS/CGSe photocathode has

yielded a photocurrent of 7 mA/cm2 at 0 V vs Reversible Hydrogen Electrode (RHE)12 and a PEC device with Pt/Mo/Ti/CdS/CIGSe structure has demonstrated a photocurrent of 30 mA/cm2 at 0 V/RHE13. Although CIGSe shows the highest photocurrent among chalcogenide-based PEC devices, the use of scarce/expensive materials (In and Ga) may impact scalability and fabrication cost, leading to longer pay-back time and limited adoption. To overcome scarcity of the raw


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materials utilized in PEC devices, CZTS has been investigated and a CZTS photocathode with In2S3/CdS coating has achieved a photocurrent of 9 mA/cm2 at 0 V/RHE14 and one with TiO2/CdS coatings has yielded 13 mA/cm2 at −0.2 V/RHE15. Beyond photocurrent, operational stability in the aqueous media represents another important PEC device requirement. Bare CIGSe-based PEC cells show rapid device degradation during hydrogen evolution16. To address this issue, protection layers (e.g., ZnS, TiO2/Al-doped ZnO (AZO)/CdS)17-18 have typically been employed to prevent device degradation. However, even for the most promising results with a ZnS coating as the protection layer, the photocurrent for the CIGSe-based PEC device drops more than 60% after a 10-hour test.17 Similarly, a TiO2/AZO/CdS-coated CZTS photocathode showed substantially decreased performance within 15 minutes under AM 1.5G illumination18. Most recently, Cu2BaSnSxSe4-x (CBTSSe) has attracted considerable attention as an emerging chalcogenide material with desirable electronic and optical properties19 for solar energy applications. While CBTSSe maintains tetrahedral coordination for the Cu and Sn atoms within the structure (as for the zinc-blende-related systems), the Ba adopts a much larger coordination due to the large ionic size and electropositive nature of Ba, which discourages formation of detrimental cationic disorder. As a proof-of-concept, pure sulfide Cu2BaSnS4 (CBTS) was utilized in photovoltaic devices, successfully demonstrating 1.6% power conversion efficiency (PCE)19. In contrast to CZTS20, the narrow width of the photoluminescence peak and abrupt transition near the absorption edge for CBTS suggest reduced cationic disordering and decreased band tailing19—i.e., considered important targets for efficient solar energy conversion application. By incorporating Se to reduce the band gap of CBTS (2 to 1.55 eV)19, a higherefficiency 5.2% PCE CBTSSe-based PV device has been achieved, in part as a result of the smaller bandgap and increased light absorption21. In addition to PV applications, PEC devices


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ACS Energy Letters

with CBTS as a photocathode were reported in 2016, showing photocurrent of as high as 4 mA/cm2 at 0 V/RHE22-23. With the assistance of a Pt/TiO2/ZnO/CdS overlayer, the photocurrent of a pure-sulfide CBTS-based PEC device was recently increased up to 7 mA/cm2 at 0 V/RHE.24 Furthermore, as Se content in this material system increased, the photocurrent with CBTSSe(x≤2.4) photocathode was improved, due to the smaller band gap (i.e. increase in light absorption) and larger grain size (reduced recombination at the grain boundaries)23. However, with or without Se incorporation, PEC devices displayed photocorrosion issues23, indicating poor device stability, as was observed for other chalcogenide PEC devices18,

25

. When

Pt/TiO2/ZnO/CdS layers were introduced in the pure-sulfide CBTS device structure, device performance stability improved, but long term stability has not been demonstrated24. In this work, we fabricate Pt/TiO2/CdS/CBTSSe(x≈3) PEC devices with chemical-bathdeposited (CBD) CdS and atomic-layer-deposited (ALD) TiO2. In comparison to the previous report for CBTS PEC devices with overlayer stack24, the current study employs Se-alloyed CBTSSe with x≈3 to lower the bandgap, and therefore maximize light absorption, while maintaining trigonal crystal structure21. The current structure was simplified by removing the ZnO layer. Moreover, it has also been reported that ZnO is sensitive to moisture26 and chemically unstable in electrolyte27. Due to the improved absorption, better band alignment and reduced interfaces, the photocurrent of our PEC photocathodes now exceeds ~12 mA/cm2 at 0 V/RHE, which corresponds to the highest photocurrent among reported PEC devices for this materials class, yielding a half-cell solar-to-hydrogen (HC-STH) efficiency of 1.09% at 0.183 V/RHE. In addition to high performance, the CBTSSe-based PEC devices exhibit excellent stability, with no significant photocurrent decrease over 10 hours at 0 V/RHE—i.e., far better than the aforementioned CZTS and CIGSe-based PEC devices17-18. Finally, a dissolved hydrogen


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measurement was performed to verify the photocathode’s ability to generate hydrogen gas at 0 V/RHE.

The

significant

improvement

in

photocurrent

and

stability

offered

by

Pt/TiO2/CdS/CBTSSe(x≈3) electrodes suggest a promising future for CBTSSe-based photocathodes in photoelectrochemical applications. Figure 1(a) shows the X-ray diffraction (XRD) pattern of a CBTSSe film on a Mo-coated glass substrate. All peaks, except for those associated with the Mo and Mo(S,Se)2 layers, are assigned to the trigonal CBTSSe phase with Miller indices19, 28-29. Given the dependence of the unit cell parameters on the S:Se ratio19 and using a Pawley-fitting method to refine the lattice parameters (a=b=6.590(3) Å and c=16.457(5) Å), it was found that x≈3 in the current Cu2BaSnSxSe4-x film, consistent with our previous results21. As shown in the top-view SEM image (Figure 1(b)), the CBTSSe film consists of large grains—i.e., greater than 1 µm across. This large grain microstructure is ideal for high-performance PEC devices, due to the reduced number of grain boundaries. A cross sectional SEM image of the very thin CdS/TiO2 double layer, deposited on the surface of the CBTSSe film, appears in Figure 1(c). Room-temperature photoluminescence (PL) measurements were carried out to estimate the band gap of the CBTSSe(x≈3), which was determined to be approximately ~1.54 eV as depicted in Figure 1(d). This value is consistent with direct band gap fit of the Tauc plot obtained from the absorbance data for an analogous CBTSSe(x≈3) film on glass as seen in Figure S1. Note that we previously established that the PL peak position is within 10 meV of the bandgap position, as determined from the quantum efficiency data of an associated solar cell device19, 21. The presence of a narrow PL peak (fullwidth-at-half-maximum=60 nm) indicates reduced cationic disordering in this material system, compared to CZTSSe kesterite materials, and thereby reduced antisite-mediated band tailing and recombination19, 21, 30.


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ACS Energy Letters

Figure 1. (a) X-ray diffraction (XRD) pattern for Cu2BaSnS4-xSex films (x≈3; film composition estimated by a unit cell refinement through a Pawley-fitting method, yielding a=b=6.590(3) Å and c=16.457(5) Å) deposited on Mo-coated glass. Miller indices for the XRD peaks of the film with a trigonal structure (PDF no. 03-065-7569) are also displayed. Peaks associated with the Mo and Mo(S,Se)2 interfacial layer are noted. (b) SEM top-view image of CBTSSe(x≈3) film deposited on Mo-coated glass. (c) SEM cross-sectional image of CBTSSe(x≈3) film with TiO2/CdS layers. (d) Room-temperature photoluminescence (PL) spectrum for CBTSSe(x≈3) film, obtained using a 442 nm laser excitation source. Linear sweep voltammetry (LSV) was used to characterize the prepared CBTSSe(x≈3) electrodes with different overlayers (Pt/CdS and Pt/TiO2/CdS) as shown in Figure 2(a). Both


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electrodes with and without TiO2 layers showed cathodic photocurrents, confirming the p-type nature of CBTSSe(x≈3). The onset potential and photocurrent density were significantly increased by the ALD-TiO2 overlayer, with the Pt/TiO2/CdS/CBTSSe(x≈3) electrode yielding a photocurrent density of 12.08 mA/cm2 at 0 V/RHE and an onset potential of 0.56 V/RHE (defined as the potential where photocurrent reaches 0.05 mA/cm2 under AM 1.5G illumination31). The Pt/CdS/CBTSSe(x≈3) electrode exhibited a photocurrent density of only 0.33 mA/cm2 at 0 V/RHE and an onset potential of 0.3 V/RHE. During the ALD process, the CdS/CBTSSe(x≈3) was heated at 200oC for 2 hours and 36 minutes. Such an extended annealing time is expected to improve the CdS/CBTSSe(x≈3) junction quality and influence its PEC performance. To decouple the effect of annealing from that of the TiO2 overlayer, CdS/CBTSSe(x≈3) samples were annealed at 200oC in an ALD chamber for the same amount of time and tested with Pt catalyst. From Figure 2(a), it can be seen that annealing significantly increases the photocurrent density of Pt/CdS/CBTSSe(x≈3), which reaches a value of 7.93 mA/cm2. The onset potential is found to be approximately 0.49V/RHE. The PEC performance improvement of annealed Pt/CdS/CBTSSe(x≈3) could be ascribed to a better p/n junction quality. Compared to unannealed Pt/CBTSSe(x≈3) electrodes (with ~0.1 mA/cm2 photocurrent density at 0 V/RHE in Figure S2),23 the annealed Pt/CdS/CBTSSe(x≈3) electrode showed an improved onset potential and increased photocurrent density at 0 V/RHE, most likely as a result of the enhanced charge separation at the CdS/CBTSSe(x≈3) p/n junction. Also, the deeper valence band maximum (VBM) of CdS may block holes generated in the CBTSSe(x≈3) film from diffusing to the solid/electrolyte interface. The addition of a 30 nm-thick continuous and pin-hole free TiO2 layer32, deposited using an ALD method, further enhances the charge separation, leading to photocurrent improvement (from 7.93 mA/cm2 to 12.08 mA/cm2) and


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ACS Energy Letters

increased onset potential (from 0.49 V/RHE to 0.56 V/RHE). According to a recent study of the CBTS band structure33, CBTS has a high conduction band minimum (CBM) of approximately 3.7 eV vs vacuum, compared to other chalcogenide materials. Additionally, as for the Cu2ZnSn(S,Se)4 system (which offers similar elemental contributions to the CBM and valence band minimum or VBM relative to CBTSSe)34, Se incorporation is only expected to shift the position of the CBM by no more than ~0.35 eV35, leading to the estimated band diagram of the Pt/TiO2/CdS/CBTSSe(x≈3) structure shown in Figure 2(b). The high CBTSSe CBM and type II band alignment36 with TiO2/CdS make Pt/TiO2/CdS/CBTSSe(x≈3) a promising photocathode to facilitate electron transfer from CBTSSe to the hydrogen evolution reaction37.

Figure 2. (a) Linear Sweep Voltammetry (LSV) of Pt/TiO2/CdS/CBTSSe(x≈3),annealed Pt/CdS/CBTSSe(x≈3) and unannealed Pt/CdS/CBTSSe(x≈3) structures in 0.5 M KH2PO4/0.5 M


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Na2SO4, with scan rate of 10 mV/s from anodic to cathodic potential. Dash line-LSV in light, solid line-LSV with light chopped on and off every 50mV. (b) Estimated band structure of the Pt/TiO2/CdS/CBTSSe(x≈3) photocathode and Pt anode. (c) Half-cell solar-to-hydrogen (HCSTH)

efficiency

from

the

LSVs

of

the

Pt/TiO2/CdS/CBTSSe(x≈3),

annealed

Pt/CdS/CBTSSe(x≈3) and unannealed Pt/CdS/CBTSSe(x≈3) electrodes. The HC-STH efficiency was calculated from the LSV curve of the Pt/TiO2/CdS/CBTSSe(x≈3) device under AM 1.5G illumination (Figure 2(c)), with highest efficiency of 1.09% achieved at 0.183 V/RHE. In comparison, the maximum HC-STH efficiency of the annealed Pt/CdS/CBTSSe(x≈3) and unannealed Pt/CdS/CBTSSe(x≈3) devices are 0.58% at 0.154 V and 0.021% at 0.174 V, respectively. The illuminated open circuit potential was also significantly improved by the ALD-TiO2 (see Figure S3(a)). The photopotential of the unannealed Pt/CdS/CBTSSe(x≈3) device was 0.16 V and much higher than without the CdS coating (~0.015 V, Figure S3(b)). With the ALD-TiO2 introduced into the device configuration, the photopotential increased to 0.46 V. Furthermore, for the Pt/CdS/CBTSSe(x≈3) electrode, the photopotential slightly decreased under illumination, indicating a photocorrosion process38, whereas the photopotential of the Pt/TiO2/CdS/CBTSSe(x≈3) device remained constant, presumably due to the TiO2 protection layer. The excellent TiO2 coverage provided by the ALD process prevents the electrolyte from contacting the CBTSSe(x≈3) layer, which also reduces recombination at the solid-electrolyte interface. The incident photon-to-current efficiency (IPCE) measurement was used to characterize the photocurrent at different wavelengths from 900 to 300 nm. An IPCE spectrum for the Pt/TiO2/CdS/CBTSSe(x≈3) electrode at 0 V/RHE appears in Figure 3(a). The IPCE value increases significantly when the wavelength is below 810 nm, with the inflection point of the


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ACS Energy Letters

IPCE derivative at 1.53 eV (≈810 nm as shown in Figure S4), in agreement with the PL measurement (Figure 1(d)). From 800 to 530 nm the IPCE value increases, reaching a highest value of 62.9% at 530 nm. The smooth drop of IPCE from 520 to 400 nm likely arises due to absorption from the CdS window layer. From 400 to 300 nm, the IPCE value decreases rapidly, since the ALD-TiO2 layer significantly absorbs photons and multiple interface recombination pathways exist between TiO2 and the external circuit. Integrating the photocurrent under the AM 1.5G spectrum from the IPCE data (Figure 3(a)) yields 12.29 mA/cm2, similar to the value obtained from the LSV technique.

Figure 3. (a) IPCE spectrum of the Pt/TiO2/CdS/CBTSSe(x≈3) electrode from 300 to 900 nm, measured at 0 V vs RHE, and corresponding integrated photocurrent density under AM 1.5G solar illumination. (b) Chronoamperometry (CA) of the Pt/TiO2/CdS/CBTSSe(x≈3) electrode at 0 V/RHE under AM 1.5G illumination for 10 hours, showing the stability of Pt/TiO2/CdS/CBTSSe(x≈3) electrode. To examine the stability of the Pt/TiO2/CdS/CBTSSe (x≈3) electrodes, chronoamperometry (CA) at 0 V/RHE was carried out for 10 hours (Figure 3(b)). During illumination, the photocurrent gradually increased (in absolute value) from 12 to 13.2 mA/cm2 for the first 3


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hours. The photocurrent increase may be due to the filling and passivation of traps in the materials during the test. Such an increase has also been observed in halide treated CIGSe photcathodes39. After 3 hours, the photocurrent remained constant without significant decrease for the rest of the test, indicating excellent stability of the Pt/TiO2/CdS/CBTSSe(x≈3) electrode. The light was chopped off for 8 min after 4 hours and 10 hours to monitor the dark current density. The dark current density remained ~0 mA/cm2 as shown in Figure 3(b), indicating that the surface composition of the electrode did not change significantly. Top-view SEM images of the film before and after the test (Figure S5) supports the absence of photocorrosion-related degradation. After the stability test, LSV (Figure S6(a)) was performed again and it was found that the HC-STH efficiency improved significantly compared to the pre-test value, increasing from 1.09% at 0.183 V/RHE to 1.85% at 0.25 V/RHE (Figure S6(b)). However, the J-V curve gradually returned to the initial level after multiple LSV scans in light, indicating that the passivated states slowly revert to their original configuration (Figure S7).


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ACS Energy Letters

Figure 4. Dissolved hydrogen measurement in the three-electrode configuration under AM 1.5G illumination at 0 V/RHE and the corresponding photocurrent density-time curve of a Pt/TiO2/CdS/CBTSSe(x≈3) electrode. The illuminated area is 0.687 cm2. Finally, dissolved hydrogen was measured for an hour (Figure 4). Due to its low solubility and diffusion from the photoelectrode to the hydrogen microprobe, dissolved hydrogen is only detected around 10 minutes after light was turned on. The dissolved hydrogen amount increases linearly and reaches 93 µmol from 10 to 70 minutes after the experiment starts. This indicates approximately 63% of the generated hydrogen are dissolved in the electrolyte (assuming columbic efficiency is 100%, ~147 µmol hydrogen should be converted). After light is turned off, the dissolved hydrogen amount begins to decrease, as hydrogen gas evolves out of the electrolyte and no more hydrogen is generated. In conclusion, a high-performance Pt/TiO2/CdS/CBTSSe(x≈3) photocathode with improved stability was prepared by a two-step method (co-sputtering and post-annealing). A bilayer of CBD-CdS and ALD-TiO2 on the CBTSSe (i.e., Pt/TiO2/CdS/CBTSSe(x≈3)) significantly improved the photocurrent and onset potential of CBTSSe-based PEC cells. A photocurrent of 12.08 mA/cm2 was obtained at 0 V/RHE, which is the highest value reported for CBTSSe-based PEC devices. The Pt/TiO2/CdS/CBTSSe(x≈3) electrode generated a stable photocurrent at 0 V/RHE for more than 10 hours under AM1.5G illumination. Compared to the best performance CZTS-based photocathodes14-15, the current CBTSSe(x≈3)-based photocathode shows higher photocurrent (e.g., 15.52 mA/cm2 at -0.15 V/RHE vs 13 mA/cm2 at -0.2 V/RHE for the CZTSbased photocathode15) and similar HC-STH (1.09% and 1.85% for the CBTSSe(x≈3)-based photocathode before and after the stability test, respectively, vs 1.63% for the CZTS-based photocathode14). Although the PEC performance of the CBTSSe(x≈3)-based photocathode is


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currently lower than that for the best CIGS based photocathode13 (30 mA/cm2 at 0 V/RHE and 8.5% HC-STH), the CBTSSe films are composed of earth abundant metals, whereas the scarce/expensive elements (In and Ga) in CIGS films may limit the scalability and fabrication cost for this technology. Additionally, the 10 hours stability of the CBTSSe(x≈3)-based photocathode outperforms the above mentioned CZTS- and CIGS-based photocathodes. Furthermore, the synthesis of CIGS and CZTS materials has been optimized for decades, whereas CBTSSe as a photocathode has been studied for less than 3 years. Future work focused on the effects of electrolyte pH, catalyst loading and interface engineering should further improve PEC performance.

EXPERIMENTAL METHODS To fabricate PEC devices, CBTSSe films (~1 µm thickness) were co-sputtered on molybdenum (Mo)-coated glass substrates, followed by sequential post annealing in excess sulfur and selenium vapor21, 23. To form a p/n junction, CdS layers (~50 nm thickness) were grown via a CBD method19 followed by ALD-TiO2 films (~30 nm thickness) to improve electron charge extraction and protect devices from photocorrosion. For comparison, bare CBTSSe devices (i.e., without CdS or TiO2 layers) and CdS/CBTSSe devices (i.e., without a TiO2 layer) were also fabricated. Finally, Pt was electrochemically deposited on the surface of the TiO2/CdS/CBTSSe stack as a hydrogen evolution catalyst. The details of device fabrication and characterization are provided in the Supporting Information.

ASSOCIATED CONTENT


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ACS Energy Letters

Supporting Information. Preparation of the CBTSSe(x≈3) films, CBD-CdS, ALD-TiO2, Pt catalyst, fabrication of electrodes for PEC measurement, results of LSV of Pt/CBTSSe(x≈3), results of photopotential measurement, results of IPCE derivative, SEM before and after the stability, J-V curves before and after stability test, and photocurrent ratio vs LSV cycles after the stability test.

AUTHOR INFORMATION Author Contributions #Y. Zhou, D. Shin and E. Ngaboyamahina contributed equally.

Yihao Zhou: [email protected] Donghyeop Shin: [email protected] Edgard Ngaboyamahina: [email protected] Qiwei Han: [email protected] Charles B. Parker: [email protected] David B. Mitzi: [email protected] Jeffrey T. Glass: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors thank Trisha Dupnock and Professor Marc A. Deshusses for use of the Unisense hydrogen microprobe. D.S., Q.H. and D.B.M. acknowledge support by the National Science


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Foundation under Grant No. 1511737. Y.Z., C.P., and J.G. acknowledge support under award ECCS-1344745. This work was performed in part at the Duke University Shared Materials Instrumentation Facility (SMIF), a member of the North Carolina Research Triangle Nanotechnology Network (RTNN), which is supported by the National Science Foundation (Grant ECCS-1542015) as part of the National Nanotechnology Coordinated Infrastructure (NNCI). All opinions expressed in this paper are the authors’ and do not necessarily reflect the policies and views of the NSF.

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