Over 1% Efficient Unbiased Stable Solar Water Splitting Based on a

Subscriber access provided by Kaohsiung Medical University

Letter

Over 1% Efficient Unbiased Stable Solar Water Splitting Based on a Sprayed CuZnSnS Photocathode Protected by a HfO Photocorrosion-resistant Film 2

4

2

Dingwang Huang, Kang Wang, Le Yu, Thi Hiep Nguyen, Shigeru Ikeda, and Feng Jiang ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b01005 • Publication Date (Web): 11 Jul 2018 Downloaded from http://pubs.acs.org on July 11, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

Over 1% Efficient Unbiased Stable Solar Water Splitting Based on a Sprayed Cu2ZnSnS4 Photocathode Protected by a HfO2 Photocorrosion-resistant Film Dingwang Huanga, Kang Wanga, Le Yua, Thi Hiep Nguyenb，Shigeru Ikedac and Feng Jianga*

a. Institute of Optoelectronic Materials and Technology, South China Normal University, Guangzhou 510631, China b. Research Center for Solar Energy Chemistry, Osaka University, 1-3 Machikaneyama, Toyonaka, Osaka 560-8531, Japan c. Department of Chemistry, Konan University, 9-1Okamoto, Higashinada, Kobe, Hyogo 658-8501, Japan *email address: [email protected]

ABSTRACT: Surface passivation of the CdS/Cu2ZnSnS4 photocathode by a HfO2 layer was found to be very effective for enhancing the photoelectrochemical stability. The dependence of the photoelectrochemical performance, especially the stability of the Cu2ZnSnS4-based photocathode, on the thickness of the HfO2 film was systematically investigated. The thickness of the HfO2 layer obviously influenced the PEC stability and efficiency of the Cu2ZnSnS4-based photocathode. The CdS/Cu2ZnSnS4 photocathode modified with a 6-nm-thick HfO2 layer showed a long-term PEC photocurrent stability of over 10 hours while still retaining high half-cell solar to hydrogen efficiency (HC-STH) of 2.7%. Finally, we fabricated an unbiased solar water splitting device based on the Pt-HfO2/CdS/Cu2ZnSnS4 photocathode in tandem with a BiVO4 photoanode, and this tandem device not only exhibited an unassisted STH conversion efficiency of 1.046% but also showed a high long-term stability of over 10 hours.

1

ACS Paragon Plus Environment

ACS Energy Letters 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 20

Solar energy with its associated abundant resource potential and contribution to reducing costs has motivated an obvious increase in research on its large-scale applications in recent years.1-6 Solar water splitting to produce hydrogen is a promising way for solar energy harvesting, as hydrogen can be produced from water by solar light irradiation and then be stored, transported and consumed without any harmful consequences. Previously, many photovoltaic materials such as Si, Cu(InxGa1-x)Se2 (CIGS) and CdTe were successfully applied in photoelectrochemical (PEC) solar water splitting and achieved remarkable results.7-11 However, the large amount of energy consumed during the complicated Si fabrication process, the rarity of the metals Ga and In in CIGS, and the toxicity of Cd in CdTe have restricted the development of their large-scale application. Thus, researchers including us are searching for new low-cost and environmentally friendly materials that retain a high efficiency.12-21 Cu2ZnSnS4 (CZTS) is considered a promising semiconductor for PEC solar water splitting due to its earth-abundant elements, low toxicity and similar absorption properties as CIGS.22-27CZTS has a wide band gap (1.5 eV), which results in an ideal open-circuit voltage (Voc) and photocurrent for solar cells or solar water splitting cells.28 The first report of a CZTS-based photocathode for hydrogen production was presented by Yokoyama et al. They claimed that Pt-TiO2/CdS/CZTS stacked photocathode is very suitable for efficient solar water splitting.

29

Followed

that, Rovelli et al. found that adding oxide coatings could enhance the durability of cathode.30 Moreover, Ma and Wang et al. reported that the CZTS film deposited onto Mo mesh also generated efficient PEC photocurrent for hydrogen production under visible light irradiation.31,32 The highest photocurrent density of 11 mA/cm2 (0 VRHE) 2


Page 3 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

was reported by Yang et al. using a stacked Pt/TiO2/CdS/CZTS electrode, but the photocurrent of this electrode exhibited obvious degradation within 1 hour.33 Previously, we found that adding an In2S3 layer onto CdS/CZTS could efficiently improve the stability of the overall solar water splitting properties; however, the photocurrent still decreased within 3 hours, and thus, we were not able to extend the performance much longer than that of the initial report.34 Improving both the efficiency and stability of a photoelectrode has a large impact on the cost of the resulting hydrogen. The largest problem associated with PEC water splitting using a CZTS photocathode is degradation/photocorrosion of the photocathode, resulting an obvious reduction in the photocurrent during photoirradiation.35 However, the stability issue of CZTS-based PEC water splitting has been discussed in very few reports.30 Even though several efforts for improving the PEC stability of CZTS-based photocathodes have been reported, the obtained results are still not ideal, and no CZTS photocathodes with long-term (over 10 hours) durability have been reported to date. In this study, we found that the deposition of a HfO2 overlayer efficiently enhanced the stability of the CZTS photocathode. Due to the photocorrosion-resistant property of HfO2, the stacked Pt/HfO2/CdS/CZTS photocathode showed high PEC stability over 10 hours and a record half-cell solar-to-hydrogen efficiency (HC-STH) of 2.7% among CZTS-based photocathodes. Furthermore, by connecting to a BiVO4 photoanode, a tandem device of Pt/HfO2/CdS/CZTS with BiVO4 was prepared and exhibited a stable unbiased solar to hydrogen (STH) conversion efficiency of 1.046%. In this work, the CZTS film was prepared by spraying the precursor film on a Mo-coated glass substrate followed by sulfurization.36,37 The obtained CZTS film crystallized in a kesterite structure without any other obvious secondary phases, as confirmed by X-ray diffraction (XRD) and Raman analyses (Fig. S1). However, it is difficult to accurately discern the secondary phases with simple XRD and Raman data.38 From the results of energy dispersive X-ray spectroscopy (EDS), we know that the thus-obtained CZTS film possessed a relatively Cu-poor and Zn-rich composition, which imparted p-type characteristics. As shown in Fig. 1a and 1b, scanning electron 3



microscopy (SEM) images of the CZTS film surface and cross-section indicated that well-grown crystallites with grain sizes of over 1μm were densely packed on the substrate.

Fig. 1. Surface (a) and cross-sectional (b) SEM images of CZTS and surface SEM images of CdS/CZTS (c) and HfO2/CdS/CZTS (d). By reference to our previous works on CZTS photovoltaics/PEC cells, an n-type CdS film was introduced as a surface modification layer onto a CZTS film to form a pn junction for the separation of photoexcited carriers.39 Fig. 1c shows the surface morphology of the CdS-covered CZTS film: the surface of each CZTS grain was fully covered with small CdS particles based on comparison to the surface morphology of the bare CZTS film shown in Fig. 1a. In this study, we deposited a continuous, novel HfO2 layer onto the CdS-covered CZTS film by atomic layer deposition (ALD), which completely changed the appearance of the surface morphology, as shown in Fig. 1d. The HfO2 layer looks like not only covered the CdS/CZTS grains but also passivated their grain boundaries, achieving full protection without exposing the underlying CdS film of the stacked HfO2/CdS/CZTS electrode. The CdS layer was prepared by chemical bath deposition according to the method of many previous works,40-43 and the HfO2 layer was prepared by atomic layer deposition. The preparation details are provided in the experimental section in supporting information. Linear sweep voltammetry (LSV) results for the Pt-modified CZTS (Pt-CZTS) film, 4


Page 4 of 20

Page 5 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

the Pt-CdS/CZTS film and the Pt-HfO2/CdS/CZTS photocathode under simulated solar light irradiation were shown in Fig. S2. The detected photocurrent of the Pt-CZTS film was not obvious, which is consistent with our previous results.34 After deposition of the CdS layer, the photocurrent was dramatically enhanced compared with that of the Pt-CZTS film, suggesting that appreciable separation of the photoexcited carriers of the CZTS photoabsorber occurred due to the formation of a pn junction at the CdS/CZTS interface. The observed photocurrent density at 0 VRHE and the photocurrent onset potential of the present Pt/CdS/CZTS electrode were about 5 mA cm-2 and 0.65 VRHE, respectively. Similar results were reported previously.33,34 In this work, a HfO2 layer was used as a protecting film to enhance the PEC stability. After deposition of the HfO2 layer (6nm-thick, as estimated from the number of deposition cycles), the photocurrent density of the Pt-HfO2/CdS/CZTS electrode reached 11.9 mA/cm2 (0 VRHE) with a very low dark current while retaining an onset potential at about 0.65 VRHE (Fig. S2). The CZTS photocathode with such 6nm-thick HfO2 layer showed appreciable better PEC performance. Note that the thickness of the HfO2 layer should significantly influence the PEC photocurrent and stability of the Pt-HfO2/CdS/CZTS photocathode, and thus, investigation of the HfO2 layer thickness dependence of the PEC performance and the stability of the Pt-HfO2/CdS/CZTS photocathode is necessary.

5



Fig. 2. Time course photocurrent density of H0 (a), H3(b), H6(c) and H10(d) during the long time of 10hours durability test. The LSV performance of H0(e), H3(f), H6(g) and H10(h) before and after durability test were shown at the right side correspondingly. Fig. 2 shows the photocurrent densities of Pt-HfO2/CdS/CZTS photocathodes with HfO2 protection layers of different thickness (0-10 nm, where 0 nm represents the Pt-CdS/CZTS film) at 0 VRHE under 10 hours of continuous solar light irradiation (AM 1.5G) in a Na2HPO4 + NaH2PO4 buffer solution at pH 6.5. For simplicity, we define the photocathodes Pt-CdS/CZTS, Pt-HfO2(3nm)/CdS/CZTS, Pt-HfO2(6 nm)/CdS/CZTS and Pt-HfO2(10 nm)/CdS/CZTS as H0, H3, H6 and H10, respectively. From Fig. 2a, we observed that the PEC stability of H0 was very poor, as the photocurrent density started at approximately 6.5 mA/cm2 upon illumination and rapidly decreased to 1.95 mA/cm2 over a period of 4 hours, and then continuously to decreased to only about 1.25mA/cm2 over 10hours. The LSV curve of H0 after the durability test (Fig. 2e) is consistent with the results shown in Fig. 2a: after the test, the photocurrents of H0 across the whole range of applied potentials are only 1/3~1/4 of the initial photocurrent. After surface modification by a thin layer of HfO2 (3 nm) 6


Page 6 of 20

Page 7 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

to form H3, the photocurrent still exhibited notable degradation (Fig. 2b), although the rate of decrease was obviously slower, indicating effective protection of the CdS/CZTS surface by the HfO2 layer. The corresponding LSV curve of H3 after the 10-hour stability test presented in Fig. 2f maintained approximately 50% of the initial photocurrent across the whole range of applied potentials, which is consistent with the results shown in Fig. 2b. The observed degradation in photocurrent is likely caused by insufficient coverage of the CdS/CZTS surface by the HfO2 film with such a low thickness (3 nm). Partial damage/dissolution of the exposed CdS during the long-term PEC reaction was still occur: we have checked the XPS of H3 before and after stability test, Cd 3d peaks was observed in H3 before durability test and the CdO peaks were observed after the durability test, data were shown in Fig.S3. Therefore, we further increased the thickness of the HfO2 layer and found that the deposition of a 6-nm-thick HfO2 film significantly improved the PEC stability: the photocurrent density of H6 shown in Fig. 2c started at approximately 6 mA/cm2, rapidly increased to 12 mA/cm2 in the first 30 min, and then stabilized at about 12 mA/cm2 for 10 hours without exhibiting an obvious decrease. The initial photocurrent increase was always observed in our samples. Several researchers also reported similar initial photocurrent increase. By reference to their previous papers, we think that the photocurrent increase may be caused by the filling and passivation of traps in the materials during the test. Such an increase has also been observed in CuBaSn(S,Se) and halide treated CIGSe photocathodes.13,44 After the 10-hour stability test, we performed LSV again and did not find any obvious differences between the LSV results of H6 before and after the 10-hour stability test (Fig. 2g). When we further increased the thickness of the HfO2 layer to 10 nm, the photocurrent of the resulting H10 photoelectrode was also very stable over 10 hours (Fig. 2d), and the LSV curve of H10 after the stability test did not change relative to the initial performance (Fig. 2h). However, the observed photocurrent density of H10 was approximately only 2.3 mA/cm2, which is much lower than that of H6. Although the thicker HfO2 film effectively protected the CdS/CZTS photoelectrode, the induced increase in the series resistance of the stacked photoelectrode dramatically decreased the photocurrent density. 7



Page 8 of 20

The PEC stabilities of H0, H6 and H10 were also confirmed by investigation of their microstructures. Fig. S3 shows the surface and cross-sectional SEM images of H0 and H6 before and after the 10-hour stability test. As shown in FigS4 a,c, a dense ca. 100nm thick CdS layer homogeneously covered the original CZTS photocathode to form a layered CdS/CZTS structure. After the 10-hour durability test, the CdS layer of the Pt/CdS/CZTS sample showed appreciable changes, i.e., loose particles were dispersed on the CZTS film’s surface (Fig. S4 b, d); thus, dissolution of the CdS layer likely occurred during the PEC reaction. While, we did not observe any obvious changes in the morphology and structure of the HfO2/CdS double layer of before and after the reaction, as shown in Fig. S4 e-h. The thicker HfO2 film entirely passivated the CdS/CZTS surface in the original H6 photocathode before the durability test, and no changes in the cross-section and surface morphology of H6 were observed after the durability test, consistent with the long-term high PEC stability of H6 observed in Fig. 2 c, d. The photocurrent density of H6 at 0 VRHE reached about 12 mA/cm2, whereas that of sample H0 reached only about 5 mA/cm2. Hence, the HfO2 layer in the Pt/HfO2/CdS/CZTS electrode achieved sufficient passivation and favorable electronic contact with both the CdS layer and the Pt deposits. Specifically, this modification likely provided better electronic contact at the Pt/HfO2 interface than at the Pt/CdS interface, and such an ultrathin HfO2 passivation film probably acted as a tunneling layer to facilitate the direct transfer of more carriers to the surface to split water, detail could be reference to the band diagram shown in Fig.S5. In addition, tunnel mechanism not only has the direct tunneling but also includes trap assisted tunneling and Fowler-Nordheim tunneling, the defects of HfO2 induced trap assisted tunneling may also be possible for the carrier penetration cross the HfO2 passivation film.45 Photoexcited carriers tunneling effect at the ultra-thin insulating oxide film under the surface of semiconductor photoelectrode that facilitate the transfer of carriers for the improvement in photoelectrochemical current were already observed in SiO2/Si and Al2O3/CdS/CIGS

previously.46,47

The

photocurrent

enhancement

of

our

HfO2/CdS/CZTS photoelectrode probably also be attributed to the tunneling of photo-excited carriers at the ultra-thin HfO2 passivation layer. In addition, electronic 8


Page 9 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

leaky of the oxide passivation film was also the feasible reason, this allowed the use of relatively thicker overlayer for minimization of the pinhole density while providing conformality and acceptable barrier layer properties and a considerable increase in stability compared to ultrathin tunneling layers.48 However, thicker HfO2 film significantly enhanced the series resistance and blocked carrier transport, which explains the low photocurrent density of only 2 mA/cm2 obtained for H10. The protection role and tunneling effect of ultrathin insulating layer i.e. SiO2 on crystalline Si for PEC water splitting were already reported. The effect of ultrathin HfO2 layer in our case is very similar to their report.

46-49

Crystalline Si has a flat surface

morphology with much less defects than that of compound semiconductor of CdS and CZTS, thus 1-2nm thick SiO2 layer could efficiently protected/passivated the surface of monocrystalline Si, while CZTS/CdS based photocathode needs relatively thicker HfO2 film (~6nm) to fully protection/ passivation considering its surface condition. CdS

in

a

photocatalytic

device

is

well-known

to

undergo

oxidative

self-photocorrosion by the photogenerated positive holes. We previously observed similar results from the hole-induced oxidization mechanism of CdS, the XPS results were shown in Fig.S6.34,50-52 Since X-ray photoelectron spectroscopy (XPS) is a surface-sensitive technique that usually probes the top ultra-thin part of the material, we can claim that at least the surface of CdS was oxidized during the photodegradation process, leading to the rapid reduction in photocurrent shown in Fig. 2a, b. In this study, we also observed obvious decomposition of the CdS film through oxidation but do not give the XPS results again, similar results could be reference to Fig.S6. However, the effects of the HfO2 layer on these electrodes were investigated by XPS to understand the underlying physical and electrochemical aspects that were responsible for providing a stable photocathode performance during the PEC reaction. Passivation by the HfO2 layer could efficiently inhibit the corrosion of CdS by preventing direct contact between the CdS layer and the electrolyte. The XPS spectra of H6 before and after the durability test are shown in Fig. S7; we did not find any differences between them. No obvious Cd or Zn peaks were observed in H6 before and after the durability test as shown in Fig. S7 c, d, indicating that a dense HfO2 film 9



effectively covered the CdS/CZTS photocathode both before and after the durability test. The positions and intensities of the peaks in the Hf 4f and O 1s spectra of the sample after the durability test were almost the same as those in the spectra collected before the durability test (Fig. S7 a, b), indicating that the HfO2 film is a photocorrosion-resistant material and that the photocorrosion of CdS was suppressed by coverage of its surface with an HfO2 layer to prevent direct contact between the CdS layer and the electrolyte. This is the first report on a CZTS-based photocathode with a HfO2 passivation layer, and the stability observed over 10 hours is also the highest obtained so far with CZTS-based photocathodes. This facile strategy should be applicable to various photocathode systems for PEC water splitting, though the stability of H6 is less than the crystal Si/TiO2 based photocathode which presented impressive hundreds hour stabilities. 53 The best stable photocurrent density of 12.8 mA/cm2 achieved by H6 in this work is the highest value obtained for CZTS photocathodes to date. Calculation of the HC-STH using the current density-potential curves gave a value of ca. 2.7% at approximately 0.36 VRHE for the present best H6 electrode, as shown in Fig. 3a, which is comparable with that of CIGS (6.6%), CuBaSnSSe (1.09%) and Cd substituted CZTS (4%) based photocathodes.13,28,47

Fig. 3: (a) HC-STH efficiency curves of H6 photocathodes; (b)Time course curves of H2 and O2 evolution of the H6 photocathode in the conventional three-electrode configuration under simulated sunlight (AM 1.5G) radiation at 0VRHE. The solid line denotes time course curve of one-half of the electrons passing through the outer 10


Page 10 of 20

Page 11 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

circuit (e-/2). Fig. 3b shows typical time course curves of H2 evolution from the H6 photocathode where one-half of the electrons passed through the outer circuit (e-/2). When the reaction was performed at 0 VRHE, H2 monotonically accumulated at a constant rate of 0.85 μmol min-1 throughout the detection period of 3 hours, indicating the high PEC stability of the sample. The faradaic efficiency determined from the ratio of the H2 evolution rate to e-/2 was higher than 95% indicating that no other reduction/oxidation processes occurred. Fig. S8 shows a photo of a typical working sample, in which obvious small H2 bubbles formed across the whole surface of the H6 photocathode.

Fig. 4: (a) I–V curves of H6 photocathode and BiVO4 photoanode under AM1.5G; (b) typical unbiased photocurrent generated from H6/BiVO4 tandem device under AM 1.5G solar light irradiation over 10hours durability test. (c) hydrogen (black square) and oxygen evolution (red circle) produced from the tandem device detected by gas chromatography, the solid line denotes the time course curve of e-/2; (d) the diagram 11



of the H6/BiVO4 tandem device for unbiased solar water splitting; (e) photo of the two electrode H6/BiVO4 tandem device. All measurements were carried out in a 0.2 mol dm-3 Na2HPO4/NaH2PO4 solution (pH 6.5) under AM1.5 G. A stand-alone unassisted PEC device is the ultimate target for H2∕O2 evolution in water splitting. Thus, the highly stable and efficient PEC performance of the Pt-HfO2/CdS/CZTS photocathode, i.e., H6, motivated us to fabricate a bias-free solar water splitting device by connecting a BiVO4 photoanode to the H6 photocathode (Fig. 4d). The PEC performance of the tandem device was evaluated using a two-electrode electrochemical configuration (see Fig. 4e). For the two-electrode measurements, before fabrication of the tandem device, the performances of the BiVO4 photoanode and the H6 photocathode were evaluated independently. The I–V curves of the BiVO4 photoanode and CZTS-based photocathode were obtained under 1 sun illumination, as shown in Fig. 4a. An operating current of about 0.64 mA (Iop) and an operating potential of 0.57 VRHE (Uop) were obtained for the tandem device at the crossing point between the individual two-electrode I-V curves (Fig. 4a).54,55 The long-term unbiased PEC current density versus time (I–t) performance of the tandem device is shown in Fig. 4b; the unbiased photocurrent of 0.68 mA (i.e. Jop= 0.68mA/0.8cm2 = 0.85 mA/cm2) matched well with the Iop value (Fig. 4a) and was highly stable for 10 hours. The unassisted STH efficiency of the tandem device was estimated to be 1.046% according to the equation η

STH

= Jop*1.23/P (where P is the power of the

illuminating light).56 This is the first report of an unbiased CZTS-based photoelectrode with such a high long-term durability, and the obtained stable efficiency is the highest obtained for CZTS-BiVO4-based photocathode/photoanode tandem devices for unbiased overall solar water splitting. Furthermore, the efficiency obtained in this report is almost 3 times higher than our previously reported best efficiency for a CZTS-based photocathode modified by an In2S3/CdS double film. 24 The amount of H2 evolved from the Pt/HfO2/CdS/CZTS photocathode-BiVO4 photoanode tandem device during PEC water splitting was measured in the two-electrode system without bias under irradiation by a 300 W xenon lamp (AM 12


Page 12 of 20

Page 13 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

1.5G). Fig. 4c shows the typical time course curve of H2 evolution where half of the electrons passed through the outer circuit (e-/2). H2 monotonically accumulated at a constant rate of 0.178 μmol min-1, indicating the stable PEC property of this tandem device. The faradic efficiency determined from the ratio of the H2 evolution rate to e-/2 (i.e., ca. 0.195 μmol min-1) was close to unity (91.3%). Finally, the total amount of H2 evolved from the tandem device after 3 hours of photoirradiation was approximately 32 µmol. In summary, a high-quality dense CZTS thin film prepared by facile spraying of the precursor followed by sulfurization was used as the base photocathode in this work. Protection of the CdS-covered CZTS photocathode surface by an atomic layer-deposited HfO2 passivation film efficiently enhanced the PEC stability of the CZTS photocathode. The effects of the HfO2 layer thickness on the PEC performance, especially the stability, were systematically studied. The thinnest HfO2 layer did not fully cover the CdS surface, resulting in photocorrosion of the exposed CdS. Though the thickest HfO2 layer fully protected the CdS layer and achieved very good PEC stability of the CZTS-based photocathode, the series resistance of this photocathode was dramatically enhanced and blocked the transfer of carriers to the surface of the HfO2 layer for water splitting. The CdS/CZTS photocathode modified by a 6-nm-thick HfO2 layer showed a long-term PEC photocurrent stability of over 10 hours while still retaining an appreciable HC-STH of 2.7%. An unbiased solar water splitting

device

was

subsequently

prepared

from

the

Pt/HfO2/CdS/CZTS

photocathode in tandem with a BiVO4 photoanode, and this tandem device not only exhibited an unassisted STH conversion efficiency of 1.046% but also showed high long-term stability for over 10 hours. This is the first CZTS-based photocathode to achieve this long-term stability, and the obtained unbiased solar water splitting efficiency of 1.046% is the highest value achieved by CZTS-BiVO4-based tandem devices so far. Therefore, the present Pt/HfO2/CdS/CZTS electrode is promising for application in the water reduction part of stable tandem PEC solar water splitting devices. Further systematic optimization and studies are currently underway. 13



Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: XXXX Experiment detail, XRD and Raman spectrum of the as prepared CZTS/Mo/glass samples. Current density-potential curves of Pt/CZTS, Pt/CdS/CZTS, and Pt-HfO2 /CdS/CZTS photocathodes in a 0.2 mol dm-3 Na2HPO4/NaH2PO4 solution (pH 6.5) under chopped solar simulated AM 1.5G light irradiation. Surface and cross-sectional morphology of H0 and H6 before and after durability test. Band diagram of Pt/HfO2/CdS/CZTS for solar water splitting. XP spectra of Pt/CdS/CZTS before and after durability test. Typical XP spectrum from Hf 4f (a), O1s (b), Zn 2p (c) and Cd 3d (d) of H6 photocathode before and after 10 hours durability test. Typical photo of the Pt/HfO2/CdS/CZTS photocathode for solar water splitting of H2 evolution

Author Information Corresponding Author *e-mail: [email protected] Notes The authors declare no competing financial interest.

Acknowledgements This work was supported by National Natural Science Foundation of China (No. 61704060) and “Outstanding Young Talent Project” of South China Normal University.

References (1)

Larcher, D.; Tarascon, J. Towards Greener and More Sustainable Batteries for

Electrical Energy Storage. Nature Chemistry 2015, 7, 19-29. (2)

Lewis, N. S.; Nocera, D. G. Powering the planet: Chemical Challenges in Solar

Energy Utilization. PNAS 2006, 103, 15729-15735. (3) Goswami, D. Y.; Vijayaraghavan, S.; Lu, S.; Tamm, G. New and Emerging 14


Page 14 of 20

Page 15 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

Developments in Solar Energy. Solar Energy 2004, 76, 33-43. (4) Kamat, P. V. Meeting the Clean Energy Demand: Nanostructure Architectures for Solar Energy Conversion. J. Phys. Chem. C. 2007, 111, 2834-2860. (5)

Lewis, N. S. Toward Cost-Effective Solar Energy Use. Science 2007, 315,

798-801. (6)

Chopra, K. L.; Paulson, P. D.; Dutta, V.

Thin-film Solar Cells: an Overview.

Prog. Photovolt.: Res. Appl. 2004, 12, 69-92. (7)

Maeda, K.; Domen, K. Photocatalytic Water Splitting: Recent Progress and

Future Challenges. J. Phys. Chem. Lett. 2010, 1, 2655-2661. (8)

Walter, M. G.; Warren, E. L.; McKone, J. R.; Boettcher, S. W.; Mi, Q.; Santori,

E. A.; Lewis, N. S. Solar Water Splitting Cells. Chem. Rev. 2010, 110, 6446-6473. (9)

Oh, J.; Deutsch, T. G.; Yuan, H.; Branz, H. M. Nanoporous Black Silicon

Photocathode for H2 Production by Photoelectrochemical Water Splitting. Energy Environ. Sci. 2011, 4, 1690-1694. (10) Yokoyama, D; Minegishi, T.; Maeda, K.; Katayama, M.; Kubota, U.; Yamada, A.; Konagai, M.; Domen, K. Photoelectrochemical Water Splitting Using a Cu(In,Ga)Se2 Thin Film. Electrochemistry Communications 2010, 12, 851-853. (11)

Brown, K. A.; Dayal, S.; Ai, X.; Rumbles, G.; King, P. W. Controlled Assembly

of Hydrogenase-CdTe Nanocrystal Hybrids for Solar Hydrogen Production. J. Am. Chem. Soc. 2010, 132, 9672-9680. (12) Septina, S.; Tilley, S. D. Emerging Earth-Abundant Materials for Scalable Solar Water Splitting. Curr. Opin. in Electrochem. 2017, 2, 120-127. (13)

Zhou, Y.; Shin, D.; Ngaboyamashina; Han, Q.; Parker, C. B. ; Mitzi, D. B.;

Glass, J. T. Efficient and Stable Pt/TiO2/CdS/CuBaSn(S,Se) Photocathode for Water Electrolysis Applications. ACS Energy Lett. 2018, 3, 177-183. (14) Ge, J.;

Yu, Y.;

Yan, Y. Earth-abundant Orthorhombic BaCu2Sn(SexS1-x)4 (x

≈ 0.83) Thin-Film for Solar Energy Conversion. ACS Energy Lett. 2016, 1, 583-588. (15) Ge, J.; Yan, Y. Synthesis and Characterization of Photoelectrochemical and Photovoltaic Cu2BaSnS4 Thin Films and Solar Cells. J. Mater. Chem. C 2017, 5, 6406-6419. 15



(16)

Page 16 of 20

Kim, J.; Yang, W.; Oh, Y.; Lee, H.; Lee, S.; Shin, H.; Kim, J.; Moon, J.

Self-Oriented Sb2Se3 Nanoneedle Photocathodes for Water Splitting Obtained by Simple Spin-Coating Method. J. Mater. Chem. A 2017, 5, 2180-2187. (17)

Prabhakar, R. R.; Septina, W.; Siol, S.; Moehl, T.; Joliat, R. W.; Tilley, S. D.

Photocorrosion-Resistant Sb2Se3 Photocathodes with Earth Abundant MoSx Hydrogen Evolution Catalyst. J. Mater. Chem. A 2017, 5, 23139-23145. (18)

Zhang, L.; Li, Y.; Li, C.; Chen, Q.; Zhen, Z.; Jiang, X.; Zhong, M.; Zhang, F.; Zhu,

H. Scalable Low-Band-Gap Sb2Se3 Thin-Film Photocathodes for Efficient Visible−NearInfrared Solar Hydrogen Evolution. ACS Nano 2017, 11, 12753–12763. (19)

Yang, W.; Ahn, J.; Oh, Y.; Tan，J.; Lee, H.; Park, J.; Kwon, H.; Kim, J.; Jo, W.;

Kim, J. et al. Adjusting the Anisotropy of 1D Sb2Se3 Nanostructures for Highly Efficient Photoelectrochemical Water Splitting. Adv. Energy Mater. 2018, 8, 1702888-1702899. (20)

Tan, J.; Yang, W.; Oh, Y.; Lee, H.; Park, J.; Moon, J. Controlled

Electrodeposition of Photoelectrochemically Active Amorphous MoSx Co-Catalyst on Sb2Se3 Photocathode. ACS Appl. Mater. Interfaces 2018, 10, 10898–10908. (21) Ros, C.; Andreu, T.; Giraldo, S.; Izquierdo-Roca, V.; Saucedo, E.; Morante, J. R. Turning Earth Abundant Kesterite-based Solar Cells into Efficient Protected Water Splitting Photocathode. ACS Appl. Mater. Interfaces 2018, 10(16), 13425-13433. (22) Ren, Y.; Richter, M.; Keller, J.; Redinger, A.; Unold, T.; Donzel-Gargand, O.; Scragg, J.; Bjorkmans, C.P. Investigation of the SnS/Cu2ZnSnS4 Interface in Kesterite Thin Film Solar Cells. ACS Energy Lett. 2017, 2(5), 976-981. (23) Jiang, F.; Ozaki, C.; Gunawan; Harada, T; Tang, Z.; Minemoto, T; Nose, Y.; Ikeda, S. Effect of Indium Doping on Surface Optoelectrical Properties of Cu 2ZnSnS4 Photoabsorber

and

Interfacial/Photovoltaic

Performance

of

Cadmium

Free

In2S3/Cu2ZnSnS4 Heterojunction Thin Film Solar Cell. Chem. Mater. 2016, 28, 3283–3291. (24) Jiang, F.; Ikeda, S.; Harada, T.; Matsumura, M. Pure Sulfide Cu2ZnSnS4 Thin Film Solar Cells Fabricated by Preheating an Electrodeposited Metallic Stack. Adv. Energy Mater. 2014, 4, 1301381. 16


Page 17 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

(25)

Jiang, F.; Ikeda, S.; Tang, Z.; Minemoto, T.; Septina, W.; Harada, T.;

Matsumura, M. Impact of Alloying Duration of an Electrodeposited Cu/Sn/Zn Metallic Stack on Properties of Cu2ZnSnS4 Absorbers for Thin-film Solar Cells. Prog. Photovoltaics Res. Appl. 2015, 23, 1884–1895. (26)

Xu, J.; Yang, X.; Yang, Q.; Wong, T.; Lee, C. Cu2ZnSnS4 Hierarchical

Microspheres as an Effective Counter Electrode Material for Quantum Dot Sensitized Solar Cells. J. Phys. Chem. C 2012, 116, 19718-19723. (27) Suryawanshi, M.; Shin, S. W.; Ghorpade, U.; Song, D.; Hong, C. W.; Han, S.; Heo, J.; Kang, S. H.; Kim, J. H. A Facile and Green Synthesis of Colloidal Cu2ZnSnS4 Nanocrystals and Their Application in Highly Efficient Solar Water Splitting. J. Mater. Chem. A 2017, 5, 4695-4709. (28) Tay, Y. F.; Kaneko, H.; Chiam, S. Y.; Lie, S.; Zheng, Q.; Wu, B.; Hadke, S. S.; Su, Z.; Bassi, P. S.; Bishop, D. et al. Solution-Processed Cd-Substituted CZTS Photocathode for Efficient Solar Hydrogen Evolution from Neutral Water. Joule 2018, 2, 537-548. (29) Yokoyama, D.; Minegishi, T.; Jimbo, K.; Hisatomi, T.; Ma, G.; Katayama, M.; Kubota, J.; Katagiri, H.; Domen, K. H2 Evolution from Water on Modified Cu2ZnSnS4 Photoelectrode under Solar Light. Appl. Phys. Express 2010, 3, 101202. (30) Rovelli, L.;

Tilley, S. D.;

Sivula, K. Optimization and Stabilization of

Electrodeposited Cu2ZnSnS4 Photocathodes for Solar Water Reduction. ACS Appl. Mater. Interfaces 2013, 5, 8018-8024. (31) Wang, P.; Minegishi, T.; Ma, G.; Takanabe, K.; Satou, Y.; Maekawa, S.; Kobori, Y.; Kubota, J.; Domen, K. Photoelectrochemical Conversion of Toluene to Methylcyclohexane as an Organic Hydride by Cu2ZnSnS4-Based Photoelectrode Assemblies. J. Am. Chem. Soc. 2012, 134, 2469-2472. (32)

Ma, G.;

Minegishi, T. ;

Yokoyama, D.;

Kubota, J.;

Domen, K.

Photoelectrochemical Hydrogen Production on Cu2ZnSnS4/Mo-mesh Thin-film Electrodes Prepared by Electroplating. Chem. Phys. Lett. 2011, 501, 619-622. (33) Yang, W.; Oh, Y.; Kim, J.; Jeong, M. J.; Park, J. H.; Moon, J. Molecular Chemistry-Controlled Hybrid Ink-Derived Efficient Cu2ZnSnS4 Photocathodes for 17



Photoelectrochemical Water Splitting. ACS Energy Lett. 2016, 1, 1127−1136. (34) Jiang, F.; Gunawan; Harada, T.; Kuang, Y.; Minegishi, T.; Domen, K.; Ikeda, S. Pt/In2S3/CdS/Cu2ZnSnS4 Thin Film as an Efficient and Stable Photocathode for Water Reduction under Sunlight Radiation. J. Am. Chem. Soc.2015, 137, 13691-13697. (35)

Seger, B.; Pedersen, T.; Laursen, A. B.; Vesborg, P. C. K.; Hansen, O.;

Chorkendorff, I. Using TiO2 as a Conductive Protective Layer for Photocathodic H2 Evolution. J. Am. Chem. Soc. 2013, 135, 1057-1064. (36) Kamoun, N.; Bouzouita, H.; Rezig, B. Fabrication and Characterization of Cu2ZnSnS4 Thin Films Deposited by Spray Pyrolysis Technique. Thin Solid Films 2007, 515, 5949-5952. (37) Emrani, A.; Vasekar, P.; Westgate, C. R. Effects of Sulfurization Temperature on CZTS Thin Film Solar Cell Performances. Solar Energy 2013, 98, 335-340. (38) Kumar, M.; Dubey, A.; Adhikari, N.; Venkatesan, S.; Qiao, Q. Strategic Review of Secondary Phases, Defects and Defect-complexes in Kesterite CZTS-Se Solar Cells. Energy Environ. Sci. 2015, 8, 3134-3159. (39)

Gunawan; Septina, W.; Ikeda, S.; Harada, T.; Minegishi, T.; Domen, K.;

Matsumura, M. Platinum and Indium uSlfide-modified CuInS2 as Efficient Photocathodes for Photoelectrochemical Water Splitting. Chem. Commun. 2014, 50, 8941-8943. (40) Yan, C.; Liu, F.; Song, N.; Ng, B. K.; Stride, J. A.; Tadich, A.; Hao, X. Band Alignments of Different Buffer Layers (CdS, Zn(O,S), and In2S3) on Cu2ZnSnS4. Appl. Phys. Lett. 2014, 104, 173901. (41)

O’Brien, P.; McAleese, J. Developing an Understanding of the Processes

Controlling the Chemical Bath Deposition of ZnS and CdS. J. Mater. Chem. 1998, 8, 2309-2314. (42) Moualkia, H.; Hariech, S.; Aida, M. S. Structural and Optical Properties of CdS Thin Films Grown by Chemical Bath Deposition. Thin Solid Films 2009, 518, 1259-1262. (43)

Li, J. Preparation and Properties of CdS Thin Films Deposited by Chemical

Bath Deposition. Ceramics International 2015, 41, S376–S380. 18


Page 18 of 20

Page 19 of 20 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Energy Letters

(44) Guijarro, N.; Prévot, M. S.; Yu, X.; Jeanbourquin, X. A.; Bornoz, P.; Bourée, W. S.; Johnson, M.; Le Formal, F.; Sivula, K. A Bottom-Up Approach toward All-Solution-Processed High-Efficiency Cu(In,Ga)S2 Photocathodes for Solar Water Splitting. Adv. Energy Mater. 2016, 6, 1501949. (45) Winqvist, E. Leakage Current and Breakdown of HfO2/InGaAs MOS Capacitors. [D] Bachelor Thesis of Lund University 2015, page9-10. (46) Esposito, D. V.; Levin, I.; Moffat, T. P.; Talin, A. A. H2 Evolution at Si-based Metal-insulator-semiconductor Photoelectrodes Enhanced by Inversion Channel Charge Collection and H Spillover. Nature Materials 2013, 12, 562-568. (47)

Chen, M.; Liu, Y.; Li, C.; Li, A.; Chang, X.; Liu, W.; Sun, Y.; Wang, T.;

Gong, J. Spatial Control of Cocatalysts and Elimination of Interfacial Defects towards Efficient and Robust CIGS Photocathodes for Solar Water Splitting. Energy Environ. Sci. 2018, DOI: 10.1039/C7EE03650G (48) Hu, S.; Shaner, M.; Beardslee, J.: Lichterman, M.; Brunschwig, B.; Lewis, N. Amorphous TiO2 Coatings Stabilize Si, GaAs, and GaP Photoanodes for Efficient Water Oxidation. Science, 2014, 344, 1005-1009. (49) Bae, D.; Seger, B.; Vesborg, P. C. K.; Hansen, O.; Chorkendor, I. Strategies for Stable Water Splitting via Protected Photoelectrodes. Chem. Soc. Rev. 2017, 46, 1933-1954. (50) Jaime-Acuña, O.E.; Villavicencio, H.; Díaz-Hernández, J. A.; Petranovskii, V.; Herrera, M.; Raymond-Herrera, O. Atomic and Electronic Structure of Quaternary CdxZnySδOγ Nanoparticles Grown on Mordenite. Chem. Mater. 2014, 26, 6152−6159. (51)

Chen, X.;

Shen, S.;

Guo, L.;

Mao, S.

Semiconductor-based

Photocatalytic Hydrogen Generation. Chem. Rev. 2010, 110, 6503−6570. (52)

Ohtani, B.; Pal, B.; Ikeda, S. Photocatalytic Organic Syntheses: Selective

Cyclization of Amino Acids in Aqueous Suspensions. Catal. Surv. Asia 2003, 7, 165-176. (53) Yin, Z.; Fan, R.; Huang, G.; Shen, M. 11.5% Efficiency of TiO2 Protected and Pt Catalyzed n+np+-Si Photocathodes for Photoelectrochemical Water Splitting: Manipulating the Pt Distribution and Pt/Si Contact. Chem. Comm. 2018, 54, 543-546. 19



(54) Kim, T. W.; Choi, K. Nanoporous BiVO4 Photoanodes with Dual-Layer Oxygen Evolution Catalysts for Solar Water Splitting. Science 2014, 343, 990-994. (55) Ng, Y. H.; Iwase, A.; Kudo, A.; Amal, R. Reducing Graphene Oxide on a Visible-Light BiVO4 Photocatalyst for an Enhanced Photoelectrochemical Water Splitting. J. Phys. Chem. Lett. 2010, 1, 2607-2612. (56)

Li, Z.; Luo, W.; Zhang, M.; Feng, J.; Zou, Z. Photoelectrochemical Cells for

Solar Hydrogen Production: Current State of Promising Photoelectrodes, Methods to Improve Their Properties, and Outlook. Energy Environ. Sci. 2013, 6, 347-340.

20


Page 20 of 20

Over 1% Efficient Unbiased Stable Solar Water Splitting Based on a

Recommend Documents