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Solar hydrogen production from zinc telluride photocathode modified with carbon and molybdenum sulfide Youn Jeong Jang, Jaehyuk Lee, Jinwoo Lee, and Jae Sung Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07575 • Publication Date (Web): 24 Feb 2016 Downloaded from http://pubs.acs.org on February 27, 2016

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ACS Applied Materials & Interfaces

Solar Hydrogen Production from Zinc Telluride Photocathode Modified with Carbon and Molybdenum Sulfide Youn Jeong Jang,a Jaehyuk Lee,a,† Jinwoo Lee,a and Jae Sung Lee b *

a

Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, South Korea

b

School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 689-798, South Korea

KEYWORDS: photoelectrochemical hydrogen production; zinc telluride; molybdenum sulfide; carbon protection layer; photocathode

ABSTRACT. A zinc telluride (ZnTe) film modified with MoS2 and carbon has been studied as a new photocathode for solar hydrogen production from photoelectrochemical (PEC) water splitting. The modification enhances PEC activity and stability of the photocathode. Thus the MoS2/C/ZnTe/ZnO electrode exhibits highly improved activity of -1.48 mAcm-2 at 0 VRHE with a positively shifted onset potential up to 0.3 VRHE relative to bare ZnO/ZnTe electrode (-0.19 mAcm-2, 0.18 VRHE) under the simulated 1sun illumination. This represents the highest value ever reported for ZnTe-based electrodes in PEC water splitting. The carbon densely covers the

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surface of ZnTe to protect it against photocorrosion in aqueous electrolyte and improves charge separation. In addition, MoS2 further enhances the PEC performance as a hydrogen evolution cocatalyst.

Introduction Hydrogen is a clean, storable, and transportable fuel, which can be combusted or converted into electricity using fuel cells.1 Sun light is the most abundant and sustainable energy source and water is the most abundant natural resource available on earth. 2 Hence, photoelectrochemical (PEC) water splitting is the ideal process to produce renewable hydrogen in a sustainable manner from these natural resources. 3,4 In spite of the rapid progress in the past decade, the process is still not efficient enough for practical applications. The most important element for such a solar hydrogen generator is the efficient semiconductor photoelectrodes that have to meet many requirements including; i) a small band gap to absorb a wide range of solar light, ii) suitable band alignment to produce hydrogen from water, iii) facile charge transport to facilitate faradaic reaction, and iv) durability in aqueous solutions. Zinc telluride (ZnTe) is an attractive candidate for photocathode of PEC water splitting device. It has a suitable band gap (2.26 eV) for effective light harvesting, and its most negative conduction band edge position (−1.63 VRHE) among p-type semiconductors offers a large driving force for interfacial electron transfer from semiconductor to acceptors in electrolyte. 5,6 Recently, ZnTe photocathodes have been studied mostly in PEC CO2 reduction reaction to produce CO, H2 and HCOOH from water and CO2 under light illumination in order to take advantage of its highly negative conduction band edge. 7,8 There are fewer studies on PEC water splitting to produce hydrogen and oxygen. 9-11 In both cases, the pristine ZnTe photocathode shows modest


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activity, high overpotential, and low stability. These shortcomings have to be overcome for the material to become a viable photocathode in a practical device. Here we report that combined modification with carbon and MoS2 of ZnTe-based photocathode improves its PEC performance with reduced overpotential, enhanced photoactivity, and improved stability. Nanocarbons such as graphene, fullerene, or carbon nanotubes (CNT) are known for their high conductivity required for electrocatalytic materials. They could also be used as a protective layer for unstable semiconducting photoelectrode materials. 12,13 MoS2 has been studied as an efficient non-precious metal electrocatalyst or a co-catalyst on a semiconductor photocathodes to reduce overpotential for hydrogen evolution reaction (HER). 1,14,15 Thus we fabricated a thin carbon layer by soaking the catalyst in an aqueous glucose solution followed by carbonization under inert condition. Then MoS2 was deposited on the photocathode by dropping the precursor solution on the carbon-coated ZnTe and annealing under a reductive gas condition. The effects of the dual modification were quite dramatic. Thus MoS2/C/ZnTe/ZnO photocathode recorded a water reduction photocurrent of -1.48 mAcm-2 at 0 VRHE with a positively shifted onset potential up to 0.3 VRHE under the simulated 1 sun illumination, while the bare ZnTe/ZnO composite photocathode recorded a meager photocurrent of -0.19 mAcm-2 and an onset potential of 0.18 VRHE. The carbon coating layer also tremendously increased its stability by minimizing photocorrosion of ZnTe. We have demonstrated the great potential of the ZnTe-based photocathode for solar fuel production by a smart dual modification strategy to address two serious drawbacks of the material, i.e. slow kinetics of the water reduction and photocorrosion.

Results and Discussion Fabrication of MoS2/C/ZnTe/ZnO photocathode


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The procedures to fabricate doubly-modified ZnTe/ZnO with carbon and molybdenum sulfide (MoS2/C/ZnTe/ZnO) and a operating three-electrode PEC cell are schematically depicted in Figure 1. The ZnTe/ZnO composite (or ZT) was formed in two steps; ZnO nanowire (NW) formation by a hydrothermal method and its conversion to ZnTe by the dissolutionrecrystallization method.7,16,17 Here the ZnO was a sacrifical precursor to convert ZnTe and charge trasport medium to improve charge separation.7,17 Then, ZT was first shielded with carbon (C/ZnTe/ZnO or ZTC) by a wet chemical method.13 Thus, ZT was soaked into an aqueous glucose solution, and annealed at 350 °C for 5 h in an electric furnace under N2 flow. The thickness of carbon layer on ZT was controlled by glucose concentration (1– 10 mg ml-1) as confirmed by SEM and HRTEM images as shown in Figure S1 (Supporting Information). At low glucose concentrations (1-3 mg ml-1), the carbon particles covered ZT particles sparsely. Above 5 mg ml-1, the dense and continuous carbon layer was formed as shown in Figure S1a. The average thickness of carbon layer was determined by HRTEM as summarized in Figure S1b

and c. In addition, the formed carbon layer from glucose showed both crystalline and amorphous phases in HRTEM images of ZTC surfaces in Figure S2.


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Figure 1. (a) Synthetic strategies of MoS2/C/ZnTe/ZnO (ZTCMS) photocathode and SEM morphologies at each stage of modifications. (b) Illustration of photoelectrochemical water splitting system with ZTCMS photocathode, reference (RE) and Pt counter electrode (CE).

Figure 2. Photocurrent densities (measured at - 0.7 V vs. RHE) and photostability of the carbon covered ZnTe/ZnO photocathodes depending on glucose concentration as precursor of carbon layer. To optimize thickness of carbon layer, PEC performances were measured for the ZTC electrodes prepared with different glucose concentrations as in Figure 2. The linear sweep potential measurements of ZTC were carried out from -0.7 to 0.2 VRHE in 0.5 M Na2SO4 with Ar purging under 1 sun illumination. The stability test of ZTC was done by chronoamperometry for 20 min at -0.7 VRHE under the same operating conditions. The stability value was defined as the photocurrent density of ZTC at 20 min of chronoamperometry (J) divided by the initial photocurrent density (J0). In Figure S3, the much higher photocurrent density for optimal ZTC-5 (-18.276 mA cm-2 at -0.7 VRHE) than that for bare ZT (-10.336 mA cm-2) shows a significant


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effect of the carbon layer on the enhanced photocurrent generation. The conductive property of carbon facilitates transfer of photo-generated charges resulting in improved photo-activity as discussed in detail later. In addition, the stability of ZTC-5 was enhanced to 82.0 % from 39.4 % of bare ZT. The effect of carbon layer was further studied by chronoamperometry for 3 h and XPS measurements for both ZT and ZTC-5. The Te and Se based chalcogenide materials usually have critical durability problems due to chemical instability and photocorrosion. 7,18,19 During ZnTe crystal formation in aqueous solution, a small amount of TeO2 is formed due to superficial oxidation. 7 The existence of both Te4+ due to TeO2 and Te2- due to ZnTe was observed in Te core-level XPS as in Figure S4. The Te4+ peaks were found at 586.98 and 576.78 eV for Te 3d3/2 and 3d5/2, and Te2- peaks at 583.07 and 572.68 eV. The ratio of Te4+/Te2- was almost 0.8 (Figure S4a.) After chronoamperometry test at 0.0 VRHE for 3 h under 1 sun illumination, XPS revealed all the Te 3d3/2 and 3d5/2 peaks without any critical binding energy shift. Interestingly, however, the Te4+/Te2- ratio for used ZT was much enhanced to ca. 4.0, whereas that of used-ZTC remained almost the same at ca. 0.8. Thus, it is clear that the thin carbon layer (< 20nm) protects the ZnTe surface from oxidation and thus contributes to enhanced durability.


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Figure 3. (a) X-ray diffraction (XRD) patterns of ZnTe/ZnO (ZT) and MoS2/C/ZnTe/ZnO (ZTCMS), and X-ray photoelectron spectroscopy (XPS) of Zn 2p (b), Te 3d (c), C 1s (d), Mo 3d (e), and S 2p (f) for the ZTCMS photocathode. To further enhance the PEC performance of the ZTC photocathode, we introduced a hydrogen evolution reaction (HER) electocatalyst. Although Pt is the best known and most active electrocatalyst, MoS2 has received great attention recently as a low-cost, non-noble metal catalyst for HER with a high activity. 20,21 Thus, MoS2 was deposited on ZT and ZTC by the simple drop-reduction process15 to obtain MoS2/ZnTe/ZnO (ZTMS) and MoS2/C/ZnTe/ZnO (ZTCMS) cathodes. The ammonium tetrathiomolybdate was converted into MoS2 at 300 ℃ under N2/H2 reductive gas flow as confirmed by XRD patterns in Figure S5. The XRD patterns for ZTCMS and ZT presented in Figure 3a included Zn (JCPDS no. 03-065-3358), ZnO (JCPDS no. 01-089-0511) and ZnTe (JCPDS no. 01-065-0385), but no peaks related to carbon or MoS2 were observed.


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The carbon and MoS2 depositions were confirmed by XPS, energy dispersive spectroscopy analysis on scanning electron microscopy (SEM-EDS), and electron energy loss spectroscopy results in scanning TEM (TEM-EELS). In Figure 3b-c, XPS indicates the presence of Zn2+ and Te2- indicating ZnTe. 7 The XPS of C 1s core level in Figure 3d implies the formation of carbon layer with strong C-C bonding peak at 285.0 eV, while other peaks of low intensities are observed related to CO32- at 289.5 eV, C-C=O at 288.7 eV and C-O at 286.4 eV. 22.23 The two intense peaks at 232.5 and 229.5 eV in Figure 3e is attributed to core-level Mo 3d3/2 and 3d5/2, respectively, indicating Mo4+ oxidation state. 24 For the S 2p XPS in Figure 3f, two doublet peaks are observed due to S2- (163.6 and 162.9 eV) and S2- (162.4 and 161.8 eV). 21 Thus, MoS2 was formed thermally from (NH4)2MoS4 via MoS3 under reductive gas flow. 15 The actual form of molybdenum sulfide is MoS2+x with dominant MoS2 and some remaining MoS3. The SEMEDS data of ZTCMS shown in Figure S6 are consistent with the previous XPS analyses. These results demonstrate that MoS2+x and carbon uniformly covered the ZnTe photocatalyst by the simple methods.


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Figure 4. High resolution transmission electron microscopy (HRTEM) images with low magnitude (a) and an enlarged image of the marked spot in a (b). Electron energy loss spectroscopy (EELS) mapping results in scanning HRTEM at the same spot for Zn (c), C (d), S (f), and Mo (g) and an overlapped image of all elements (h). The HRTEM image of ZnTe showing crystallinity in its fast Fourier transform diffraction pattern (i), and that of C/MoS2 showing both amorphous and crystalline nature of C and MoS2 in the Fourier transform diffraction pattern (j). The scale bars represent 50 nm in Figure 4(b)-(h).


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Figure 4b shows the magnified HRTEM image of a ZnTe grain covered with carbon and MoS2 (ZTCMS) on the area marked in Figure 4a. TEM-EELS mapping for Zn, Te, C, S, and Mo elements and overlapped element images are presented in Figure 4b-h. The images indicate that C and MoS2 cover ZnTe uniformly. The high magnification HRTEM images and fast Fourier transform (FT) diffraction patterns (shown in insets) were obtained focusing on ZnTe (Figure 4i) and C/MoS2 (Figure 4j) sides of ZTCMS. The (111) lattice plane of zinc blende ZnTe is observed as confirmed by 0.352 nm lattice spacing and hexagonal FT diffraction pattern in Figure 4i. 25 On the other hands, C and MoS2 in C/MoS2 side are mixed together in Figure 4j. MoS2 on ZT (ZTMS) was also analyzed by HRTEM in Figure S7, which shows the lattice fringe with a spacing of 0.61 nm corresponding to the (002) planes of MoS2.26,27 The thermal reduction temperature affects the crystallinity of MoS2 during transformation of (NH4)2MoS4 to MoS2. 15 Due to limited annealing temperature (< 300 °C), both amorphous and crystalline phases are present together.

Photoelectrochemical hydrogen evolution To evaluate PEC performance of ZnTe-based photocathodes, linear sweep voltammetry (LSV) was carried out in a 0.5 M Na2SO4 solution under simulated 1 sun illumination (100 mWcm-2) using a three-electrode configuration with working (ZT-based photocathodes), reference (Ag/AgCl) and counter electrodes (Pt wire). As shown in Figure 5a, ZT recorded -0.19 mAcm-2 of photocurrent at 0.0 VRHE (the theoretical redox potential to produce H2 from H+) and 0.18 VRHE onset potential. On the other hand, doubly-modified ZTCMS showed a much enhanced photocurrent (-1.48 mAcm-2) with a positively shifted onset potential of 0.3 VRHE. The


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singly modified ZTC and ZTMS photocathodes recorded -0.25 mAcm-2/ 0.21 VRHE and -0.33 mAcm-2/ 0.24 VRHE of photocurrents/onset potentials, respectively.

Figure 5. Photoelectrochemical performances of ZnTe/ZnO (ZT), C/ZnTe/ZnO (ZTC), MoS2/ZnTe/ZnO (ZTMS) and MoS2/C/ZnTe/ZnO (ZTCMS): (a) Photocurrent generation. (b) Photoelectrochemical conversion efficiencies as a function of applied bias (ABPE). (c) Incident photon conversion efficiency (IPCE). The inset illustrates the integrated photocurrent at 0.0 VRHE from the IPCE results. (d) Surface charge separation efficiencies. (e) Nyquist plots and fitted results of electrochemical impedance spectroscopy. (f) Schematic illustration of ZTCMS showing photon absorption and photogenerated electron-hole pair separation. All measurements were carried out in 0.5 M Na2SO4 electrolyte (10 mV s-1 as a scan rate) under 1 sun illumination.

Thus, when both C and MoS2 were loaded on ZnTe/ZnO, the photocathode shows the highest onset potential as well as the best photocurrent. To the best of our knowledge, the performance represents the most positively shifted onset potential and the highest photocurrent among the


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previously reported values for ZnTe-based photocathodes listed in Table S1 of SI.

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7-11

The

measurements under chopped irradiation were presented in Figure S8 to monitor currents under dark and light conditions simultaneously. The applied bias photo-current efficiency (ABPE) of ZT-based electrodes was determined as a function of applied bias potential by the equation of ηABPE [%] = JPmax VPmax / Pin * 100, where JPmax, VPmax and Pin are photocurrent density [mAcm-2], applied potential [VRHE] at maximum power point and incident light flux [mWcm-2] under 1 sun. 13,28 As shown in Figure 5b, ZTCMS exhibits the highest ABPE 0.12 % at 0.175 VRHE, while ZT records ABPE of 0.009 % at 0.145 VRHE. The ZTC and ZTMS exhibit ABPE values of 0.018 and 0.033 %, respectively. Incident photon-to-current conversion efficiency (IPCE) was evaluated under illumination with a 300 W Xe lamp coupled with a monochromator.29,30 The IPCE was calculated by the equation of ηIPCE [%]) = (1240 * J) / ( Pmono * λ) * 100, where J, Pin, and λ are photocurrent density [mAcm-2] at 0.0 VRHE, light power density [mWcm-2] at λ, the wavelength of incident light [nm], respectively. All photocathodes were biased at 0.0 VRHE under irradiation in wavelengths of 300-700 nm. As shown in Figure 5c, IPCE for all the samples are observed below 570 nm in coincidence with the light absorption spectra of the materials in Figure S9. It indicates that photocurrents and water reduction activity originate from the photons absorbed by band gap transition in ZnTe (< 570 nm or 2.26 eV). In particular, doubly-modified ZTCMS exhibits unprecedentedly high IPCE value over 30 % among ZnTe based photocathodes and much higher than those of ZT (7.5 %), ZTC (11.8 %), and ZTMS (12.6 %) photocathodes. A synergistic effect between two modifiers (carbon and MoS2) is evident. The inset image in Figure 5c illustrates the integrated photocurrent at 0.0 VRHE from the IPCE results. The calculated photocurrents are slightly smaller than the measured currents but show an excellent correspondence.


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Let us consider the origin of improved PEC water reduction performance by the dual modification. We propose that C and MoS2 may facilitate charge carrier diffusion and enhance charge separation efficiency. For the study of surface property in detail, charge injection efficiency (ηsurface) was estimated based on the ratio photocurrent of water reduction over photoreduction currents of electron acceptors, sacrificial agent (O2) and water. Nevertheless the negligible electrochemical oxygen reduction activities of the components; ZnTe, C, and MoS2, the photogenerated electron is facilely transferred into O2 electron acceptor relative to water due to its more positive redox potential than that of water. Following the convention, we assume that the photoreductions by the electron acceptors were so facile that the interfacial charge recombination or the electron injection barrier to electrolyte is negligible.31,32 The photocurrent (J)-applied voltage (V) curves from LSV for all electrodes are presented in Figure S10 of SI for all photocathodes. The ratio of photocurrents with and without electron scavenger O2 gives ηsurface as summarized in Figure 5d. Thus the ηsurface values of ZT, ZTC, ZTMS, and ZTCMS were about 36, 52, 58, and 81 % at 0.0 VRHE, respectively. Hence, C and MoS2 deposited on ZnTe enhance surface charge separation before photogenerated charge recombination and their effects are complementary to one other. Electrochemical impedance spectroscopy (EIS) measurements of the photocathodes were performed at 0.0 VRHE under 1 sun irradiation in Figure 5e. Generally, the smaller impedance value (proportional to the radius of the semicircle in the Nyquist plot) indicates better charge transfer within electron pathway reflecting improving electron transfer followed the order ZT

Solar Hydrogen Production from Zinc Telluride ... - ACS Publications

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