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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Exploratory Study of ZnxPbOy Photoelectrodes for Unassisted Overall Solar Water Splitting He Lin,† Xia Long,†,‡ Jue Hu,† Yongcai Qiu,†,‡ Zilong Wang,† Ming Ma,† Yiming An,† and Shihe Yang*,†,‡ †
Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China Guangdong Key Lab of Nano-Micro Material Research, School of Chemical Biology and Biotechnology, Shenzhen Graduate School, Peking University, Shenzhen 518055, China
‡
S Supporting Information *
ABSTRACT: A complete photoelectrochemical (PEC) water splitting system requires a photocathode and a photoanode to host water oxidation and reduction reactions, respectively. It is thus important to search for efficient photoelectrodes capable of full water splitting. Herein, we report on an exploratory study of a new photoelectrode family of ZnxPbOyZnPbO3 and Zn2PbO4similarly synthesized by a simple and economical method and shown to be a promising photocathode (p-type semiconductor) and photoanode (n-type semiconductor), respectively. From PEC measurements, the bare ZnPbO3 photocathode achieved a photocurrent density of −0.94 mA/cm2 at 0 V versus reversible hydrogen electrode (RHE), whereas the pristine Zn2PbO4 photoanode delivered a photocurrent density of 0.51 mA/ cm2 at 1.23 V versus RHE. By depositing suitable cocatalysts onto the photoelectrodes established above, we also demonstrated unassisted overall PEC water splitting, a rare case, if any, wherein a single material system is compositionally engineered for either of the photoelectrodes. KEYWORDS: ZnxPbOy photoelectrodes, photoelectrochemical, solar water splitting, unassisted overall, cocatalyst
1. INTRODUCTION Because of the exponential growth of energy demand and environment pollution, it is urgent to develop environmentally friendly, renewable energy resources, and this has prompted broad research activities over the past few decades.1,2 Hydrogen energy, because of its greenness, recyclability, and high energy conversion efficiency, has been recognized as an auspicious substitute to fossil fuels.3 The idea of sunlight-driven water splitting to produce hydrogen as a zero-carbon technology has inspired many researchers to pursue along the line.4 PEC cells, dating back to the innovative work of Honda and Fujishima,5 are widely researched as a neat approach to utilizing solar energy and then directly converting it to chemical energy by using semiconductors.6−8 Much attempt has been committed to exploring new materials, optimizing the fabrication methods, and modifying the surfaces of the existed materials to create more active sites. Till present, large band gaps (3.2 eV for TiO2)9−11 and photocorrosion (CdS12,13 and Ta3N514,15) are still some of the problems that need to be addressed. Because of their comprehensive availability, easy synthesis, superior stability, and low price, metal oxides have been widely used as photoelectrodes in PEC water splitting. However, most of the single metal oxides studied so far do not meet all of the requirements to serve as an efficient photoelectrode. For © XXXX American Chemical Society
instance, they may have too large band gaps, or even with a suitable band gap, they may fail to exhibit their theoretical maximum efficiencies because of low carrier densities, high barriers for interfacial charge transfer, fast carrier recombination, small grain sizes, interfacial defects, and so forth.16−18 In this regard, multimetal oxides may have potential to address some of the problems. ZnO and PbO2 are well-known metal oxide materials with n-type semiconducting characteristics.19,20 ZnO is easily processible and has been used as photoanodes for PEC water splitting, but its band gap is too large to cover an appreciable range of the solar spectrum and it has stability issues as well. On the other hand, PbO2 is thermally unstable, strongly oxidizing, and not easily administrable. It is tempting to envision mixing ZnO and PbO2 and tuning their semiconducting characteristics for PEC electrodes. Surprisingly, such a ternary system, ZnxPbOy, has been rarely, if ever, reported for PEC water spitting. Nevertheless, there have been many prior studies on the synthesis and optical, electrical, and dielectric properties of the ZnxPbOy systems. It was found that reaction conditions such as reaction temperature and pressure not only are crucial for the Received: January 9, 2018 Accepted: March 14, 2018
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DOI: 10.1021/acsami.8b00421 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
then stirred at 75 °C for 3 h. Later, the precipitate was collected and washed three times with ethanol after cooling down to RT.37 Ethyl cellulose (0.35 g) and the precipitate were dispersed in terpineol (5 mL) and ethanol (20 mL), and then ZnO nanoparticle pastes with a concentration of 20 mg/mL were obtained. Third, the ZnO nanoparticle pastes were coated onto the PbO2 films, which had been ultraviolet−ozone (UVO) treated for 0−120 min, at 3000 rpm for 30 s (preparing for ZnPbO3). In addition, after 5 min of drying at 50 °C in air, the procedure was repeated again to obtain a double-layer of the ZnO on the top of the PbO2 layer (preparing for Zn2PbO4). Finally, these thin films were first heated to 125−150 °C (at the rate of 5 °C/min) for 0−240 min in a sealed muffle furnace (10 cm × 10 cm × 10 cm) and then transferred into the tube furnace and raised to 500−575 °C at a rate of 5 °C/min, insulating 120−240 min in the air and 480−600 min in the argon, followed by natural cooling down to RT in argon. Finally, the thin films were immersed into 25 wt % NH3· H2O, as well as applied with a constant potential (+0.3 V vs Ag/AgCl) to remove any excess ZnO. In addition, soaking in 1 M NaOH solution was followed to remove any excess PbOx. The oxygen evolution catalysts (Co−Pi) were deposited on the Zn2PbO4 photoanode by photoassisted electrodeposition with a constant potential (0.3 V vs Ag/AgCl) under 1 sun AM 1.5 simulated sunlight illumination for 600 s in a solution of 0.5 mM cobalt nitrate in 0.1 M potassium phosphate buffer at pH = 7.38 However, the deposition of MoS2 catalyst on the ZnPbO3 photocathode was done by drop-cast. The preparation of MoS2 was based on the previous work in our group.39 Briefly, 110 mg of (NH4)2MoS4 was dissolved in 30 mL H2O by stirring at RT. Then, 40 mL of DMF (the volume ratio of DMF to H2O is 4:1) and 0.5 mL of N2H4·H2O were added in the solution, which was further stirred for 20 min and then transferred to a 100 mL Teflon-lined autoclave and kept in an oven for 12 h at 200 °C. After the reaction, the product was washed repeatedly with distilled water. Then, 10 mg of MoS2 and 0.1 mL of 50 wt % polytetrafluoroethylene solution were ultrasonically dispersed in 10 mL of ethanol to form a homogeneous ink. Finally, 10 μL of the catalyst ink was dropcast onto the ZnPbO3 photocathode. Details on the characterization and PEC performance measurement of the ZnPbO3 and Zn2PbO4 photoelectrodes can be found in the Supporting Information.
formation of ZnxPbOy but also highly affect the electric and dielectric properties of the oxides. The interest in the ZnxPbOy system can be traced back to Keester and White,21 who, by using X-ray powder diffraction, studied the phase relations of PbO−ZnO−O at 4 kPa oxygen partial pressure and successfully synthesized crystalline ternary phases of Zn2PbO4 and ZnxPb1−xO. Zhou et al.22 reported lead-doped zinc oxide nanowires and investigated the roles of doped Pb on its optical properties, showing a strong red shift with the increase of the Pb content. In addition, Yousefi et al.23 synthesized Pb-doped ZnO nanowires by thermal evaporation and by using photoluminescence (PL) and Raman spectra; they found that with increasing Pb concentration, both optical properties and crystallinity of Pb-doped ZnO nanowires decreased. These studies on the synthesis and physical properties of Zn−Pb−O are revealing the high electrical and dielectric constant and coexistence of various phases, which promise various applications.24 In the present report, photoelectrodes of ZnPbO3 and Zn2PbO4 were synthesized by a similar, simple and economical method, which were ascertained to be p-type and n-type semiconductors, respectively. When applied in PEC water splitting, the pristine ZnPbO3 photocathode and Zn2PbO4 photoanode achieved a photocurrent density of −0.94 and 0.51 mA/cm2 at 0 and 1.23 V versus reversible hydrogen electrode (RHE) under AM 1.5G in potassium phosphate solution (pH = 7), correspondingly. The ability to compositionally engineer the photoanode and photocathode, using MoS2 and CoPi to modify the surfaces of ZnPbO3 and Zn2PbO4, respectively, allowed us to devise and demonstrate an overall PEC water splitting cell with a single material system. Unassisted overall water splitting has attracted much interest in recent years.25−27 However, there have been only scattered reports on using a single material or a single material system for both photocathode and photoanode to split water without an applied bias, such as La5Ti2CuS5O728 and silicon with different dopants or surface capping layers.29−35 The discovery of unassisted overall water splitting with the ZnxPbOy system shall enlarge the family of photoelectrodes, which is predicted to exposit extensive applications of the potential of employing transition metal plumbate as semiconductor photoelectrodes.
3. RESULTS AND DISCUSSION 3.1. Material Synthesis. The synthesis process of the ZnPbO3 and Zn2PbO4 photoelectrodes is laid out in Scheme S1 in the Supporting Information. First, the β-PbO2 layer was electrodeposited on FTO in a standard three-electrode system at RT with the electrolyte containing 0.1 M Pb(CH3COO)2, 0.2 M NaNO3, and 0.1 M HNO3. Figures S1A and S1B in the Supporting Information shows the top and cross-sectional view scanning electron microscopy (SEM) images of PbO2 thin film with around 300 nm thickness on the FTO substrate, respectively. The next step was spin-coating of the ZnO nanoparticles paste. To ensure the uniformity of ZnO nanoparticles, the PbO2 thin film need to be treated by UVO, which can effectually remove the surface organic species and thus drastically reduce the contact angle of the polar solvents.40 The size of the as-synthesized ZnO nanoparticles is ca. 10 nm in diameter, as can be seen from transmission electron microscopy images (Figure S2A,B). It is worth noting that the ZnO thin film is almost flat (Figure S2C,D), indicating good wetting and spread ability of the paste.41,42 Third, the thin films were heated to 125−150 °C and left standing for 0−240 min. Then, the temperature was increased to 500−575 °C, at which the thin films were treated in air for 120−240 min and then in argon for 480−600 min, followed by natural cooling down to RT in argon (the detailed reasons will be explained below). Finally, the thin films were immersed into 25 wt %
2. EXPERIMENTAL SECTION 2.1. Chemicals. All chemicals were used as received without further purification if not otherwise indicated. Ultrapure deionized water (18.2 MΩ/cm) was used in all experiments. 2.2. Materials Synthesis and Photoelectrode Preparation. The ZnPbO3 and Zn2PbO4 thin films are based on electrodeposition (see Scheme S1 in the Supporting Information). First, the electrodeposition technique was applied to deposit PbO2 on the FTO substrate, which were cleaned in deionized water, next in ethanol, and finally in acetone for 30 min, respectively, with an ultrasonic cleaner. The aqueous electrolyte, which contained 0.1 M lead acetate (Pb(CH3COO)2), 0.2 M sodium nitrate (NaNO3), and 0.1 M nitric acid (HNO3), was quite stable in ambient environment. Electrochemical oxidation (Pb2+ + 2H2O = PbO2 + 4H+ + 2e−) was used to deposit PbO2, which was handled in a standard three-electrode system under room temperature (RT) with a constant potential (1.5−1.8 V vs Ag/AgCl) for 40−60 s. During electrodeposition, the FTO substrate gradually changed to bright brown color, suggesting the deposition of β-PbO2.36 Second, the ZnO nanoparticles paste were prepared by using zinc acetate dihydrate (0.18−0.2 M) which was dissolved in ethanol under stirring at 75 °C. Then, KOH/ethanol solution (0.35−0.4 M) was added drop wise at 65−75 °C in 15 min. The mixture solution was B
DOI: 10.1021/acsami.8b00421 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 1. Morphology and structure characterizations of the as-synthesized ZnxPbOy samples prepared at 550 °C. (A) XRD patterns of ZnPbO3 and Zn2PbO4. (B,C) Top-view SEM images of Zn2PbO4 and ZnPbO3. XPS spectra of ZnPbO3 and Zn2PbO4. (D) 4f peaks of Pb, (E) 2p peaks of Zn, and (F) 1s peak of O.
vertical stripes lying flat on the entire surface of the FTO substrate, and the thickness of the film was around 300 nm (as shown in Figure S5A). This observation indicates that the surface of the Zn2PbO4 film is almost smooth. The SEM image in Figure 1C shows that the ZnPbO3 thin film consists of rice grain-like particles with an identical thickness around 300 nm (Figure S5B). These dissimilarities of morphological and size between Zn2PbO4 and ZnPbO3 are due to the different crystal growth rates and mechanism. In addition, the chemical states of the as-obtained samples were examined by X-ray photoelectron spectroscopy (XPS), with the binding energies calibrated with the C 1s peak of aliphatic carbon at 285.0 eV. The binding energy of Pb 4f, Zn 2p, and the O 1s in the samples of ZnPbO3 and Zn2PbO4 are shown in Figure 1D−F. In detail, Figure 1D shows two symmetric peaks centered at 138.9 and 143.7 eV (ZnPbO3) and 138.6 and 143.4 eV (Zn2PbO4), corresponding to Pb 4f7/2 and Pb 4f5/2, respectively; these data corroborate that the lead of our samples exists nominally in the Pb4+ valence state.47,48 Figure 1E indicates the representative 2p region of Zn. It shows two sharp peaks located at 1021.2 and 1044.2 eV for ZnPbO3 and 1021.7 and 1044.7 eV for Zn2PbO4, which are attributed to the binding energy of Zn 2p3/2 and Zn 2p1/2, respectively, consistent with previous reports.49,50 Figure 1F shows the O 1s core level spectrum of ZnPbO3 (531.3 eV) and Zn2PbO4 (531.1 eV). The differences in XPS spectral features are noteworthy. Conceivably, the small size of the wurtzite hexagonal crystal structured ZnO facilitates its diffusion into the tetragonal β-PbO2, forming the LN-type ZnPbO3 and orthorhombic structured Zn2PbO4. The largely changed crystal structure of the products may account for the differences in the XPS spectral features. Literally, the different peak locations reflect the electron density changes surrounding the various atoms, which also evidenced the successful synthesis of ZnxPbOy photoelectrodes after the high-temperature annealing.
NH3·H2O, as well as applied with a constant potential (+0.3 V vs Ag/AgCl) to remove any excess ZnO. In addition, soaking in 1 M NaOH solution was followed to remove any excess PbOx. 3.2. Morphology and Structure Characterizations. After sintering, the ZnPbO3 and Zn2PbO4 thin films could be obtained from the spin-coated single-layer and double-layer of ZnO on the top of the PbO2 layer, respectively, as shown in the X-ray diffraction (XRD) patterns in Figure 1A. To our delight, majority of the diffraction peaks of the ZnPbO3 (red line) and Zn2PbO4 (black line) are in good agreement with the standard data [JCPDS card no. PDF 231498 (ZnPbO3) and 231496 (Zn2PbO4)], indicating that lithium niobate-type (LN-type) structured ZnPbO 3 4 3 and orthorhombic structured Zn2PbO424,44 crystals have been successfully synthesized. However, some very low intensity XRD peaks are located at 14.25° and 43.85° for Zn2PbO4 and 26.25°, 27.10°, and 45.35° for ZnPbO3 because of probably the trace amount of Pb3O4 phases. This was caused by a small amount of Pb atoms that diffused and reacted during the high-temperature calcination, which nevertheless do not contribute to the photoactivities and have little impact on chemical and physical properties of the photoelectrodes. To quantitatively determine the amount of impurities contained in the samples, thermogravimetric analysis (TGA) was measured (Figure S3) for the ZnPbO3 and Zn2PbO4 in air at a heating rate of 10 °C/min. The presence of nanometric Pb3O4 resulted in a slight weight loss (ca. 0.1%) at 350 °C, whereas a large scale of Pb3O4 remained fairly stable until 550 °C. A trace weight loss (ca. 0.6%) was observed at the temperature higher than 550 °C, and the resulting product might be PbO.45,46 This weight loss was negligible, proving that the ZnPbO3 and Zn2PbO4 films were successfully synthesized in high purity. In addition, the as-synthesized ZnPbO3 and Zn2PbO4 phases are quite stable because of their formation in the high-temperature processes (Figure S4). The morphologies of ZnPbO3 and Zn2PbO4 were examined by SEM. As shown in Figure 1B, the Zn2PbO4 thin film shows C
DOI: 10.1021/acsami.8b00421 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 2. Band gap and band position determination of the ZnxPbOy photoelectrodes. (A) UV−vis absorption spectra of ZnPbO3 and Zn2PbO4. Inset: Optical photographs of ZnPbO3 and Zn2PbO4. (B) Tauc plots of ZnPbO3 and Zn2PbO4. (C) PL spectra of ZnPbO3 and Zn2PbO4. (D) Ultraviolet photoelectron spectra (UPS) of ZnPbO3 and Zn2PbO4 films.
Figure 3. PEC characterization of the ZnxPbOy photoelectrodes. Mott−Schottky plots collected at 1 kHz for ZnPbO3 (A) and Zn2PbO4 (D). Chopped-light linear sweep voltammetry curves of ZnPbO3, MoS2/ZnPbO3 (B) and Zn2PbO4, CoPi/Zn2PbO4 (E) recorded under AM 1.5G irradiation in potassium phosphate solution (pH = 7) at a scan rate of 25 mV/s. ABPE of ZnPbO3 (/MoS2) (C) and Zn2PbO4 (/CoPi) (F) obtained using a three-electrode system.
in which α is the measured absorption coefficient, hν is the photon energy, Eg is the optical band gap energy, and A is a constant. Theoretically, n equal to 1/2 or 2 for an indirect and direct band transitions, respectively. In this case, the linearity of plots of (αhν)1/2 or (αhν)2 against hν are used to determine the characteristics of the transitions, also the x-axis intercept provides the information on the band gap energy. The DFT calculation of ZnPbO3 in a previous report43 indicated a direct transition, which was revealed at 2.78 eV according to the Tauc
3.3. Band Gap and Band Position Determinations. Figure 2A shows the absorption spectra of ZnPbO3 and Zn2PbO4. Both samples had absorption in the visible-light region. The optical photographs of ZnPbO3 and Zn2PbO4 are shown in the inset of Figure 2A. The band gaps of ZnPbO3 and Zn2PbO4 were determined by ultraviolet−visible (UV−vis) optical absorption spectroscopy by following Tauc eq 1 (αhν)n = A(hν − Eg )
(1) D
DOI: 10.1021/acsami.8b00421 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 4. Photon to current quantum efficiency. APCE and IPCE spectra of bare ZnPbO3 (A) and Zn2PbO4 (B). Gas evolution profiles, faradaic efficiencies and long-term I−t curves of ZnPbO3/MoS2 (C) and Zn2PbO4/CoPi (D), respectively.
yield a straight line, and the flat band potential (Vfb) is decided from the intercept on the x-ordinate. Using this approach, the flat band potentials of ZnPbO3 and Zn2PbO4 were estimated to be 0.81 and 0.38 V, respectively. Besides, the p-type characteristic of ZnPbO3 is further confirmed by the negative slope of the Mott−Schottky plot, whereas the positive slope indicates that Zn2PbO4 is an n-type semiconductor. As expected, in the ZnxPbOy ternary system, while Zn 3d and O 2p orbitals form the valence band, the top of valence band is determined by O 2p orbitals. In addition, the conduction band is mainly made up of Pb 6s and O 2p orbitals, but it is the Pb 6s orbitals that make up the bottom of the conduction band.43 In our experiments, no intentional doping was introduced, so the source of the different carrier types should be intrinsic to the ZnxPbOy itself. Hence, the p-type characteristics might be induced by the formation of zinc and/or lead vacancies (VZn and/or VPb), whereas the n-type characteristics may result from the oxygen vacancies (VO).58 To enhance the catalytic kinetics for hydrogen evolution reaction (HER) or oxygen evolution reaction (OER) on the surfaces as well as to achieve high water splitting activity and stability, powerful electro-cocatalysts are in a great need for both photocathodes and photoanodes. Herein, cobalt− phosphate (Co−Pi) was applied as the cocatalyst deposited on the surface of Zn2PbO4 photoanode by photo-electrodeposition with a constant potential.38 In addition, molybdenum disulfide (MoS2) with abundant exposed reactive sulfur sites at the edges as a HER catalyst was deposited on the ZnPbO3 by drop-casting.39 PEC measurements were carried out in a three-electrode system, which contains the Pt wire and Ag/AgCl as the counter and reference electrode, respectively, as well as the ZnxPbOybased photoelectrodes as the working electrode. In Figure 3B, the onset potential of pristine ZnPbO3 and ZnPbO3/MoS2 at potassium phosphate (KH2PO4) buffer (pH = 7) can be estimated to be 0.71 and 0.89 V versus RHE, respectively,
plots (Figure 2B). However, the Tauc plots of Zn2PbO4 show a slightly lower indirect band gap of 2.27 eV (Figure 2B) and a fundamental direct transition at 2.3 eV (Figure S6). PL spectroscopy of ZnxPbOy photoelectrodes (Figure 2C) was performed by using 514 nm lasers. A strong emission peak at 584 nm and the negligible band edge emission (at around 545 nm) are detected in Zn2PbO4, supporting the conclusion that Zn2PbO4 tends to have an indirect fundamental band gap.51,52 Besides, defects may exist in the Zn2PbO4, possibly resulting in the red-shifted and broadened PL features. On the other hand, no obvious emission peaks were observed in the 525−700 nm range from ZnPbO3, suggesting that the residual absorption at wavelengths large than 514 nm played a little role in the photoresponse of ZnPbO3. UPS was utilized to analyze the energy levels of ZnPbO3 and Zn2PbO4 (Figures 2D and S7). In general, UPS with its highenergy resolution (
