Controlled Electrodeposition of Photoelectrochemically Active

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Controlled Electrodeposition of Photoelectrochemically Active Amorphous MoSx Co-Catalyst on Sb2Se3 Photocathode Jeiwan Tan, Wooseok Yang, Yunjung Oh, Hyungsoo Lee, Jaemin Park, and Jooho Moon ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00305 • Publication Date (Web): 16 Mar 2018 Downloaded from http://pubs.acs.org on March 16, 2018

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ACS Applied Materials & Interfaces

Controlled Electrodeposition of Photoelectrochemically Active Amorphous MoSx Co-Catalyst on Sb2Se3 Photocathode Jeiwan Tan, Wooseok Yang, Yunjung Oh, Hyungsoo Lee, Jaemin Park, and Jooho Moon*

Department of Materials Science and Engineering, Yonsei University, 50 Yonsei-ro Seodaemungu, Seoul 03722, Republic of Korea

KEYWORDS photoelectrochemical hydrogen evolution, amorphous molybdenum sulfide, cocatalyst, cyclic voltammetry, antimony triselenide photocathode

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ABSTRACT Amorphous molybdenum sulfide (a-MoSx) is a promising hydrogen evolution catalyst owing to its low cost and high activity. A simple electrodeposition method (cyclic voltammetry) allows uniform formation of a-MoSx films on conductive surfaces. However, the morphology of aMoSx, deposited on a TiO2/Sb2Se3 photocathode could be modulated by varying the starting potential. The cathodically initiated a-MoSx showed conformal film-like morphology, while anodic initiation induced inhomogeneous particulate deposition. The film-like morphology of aMoSx was subjected to catalyst activation, which improved the photocurrent density and reduced the charge transfer resistance at the semiconductor/electrolyte interface as compared to its particulate counterpart. X-ray photoelectron spectroscopy confirmed that different chemical states of a-MoSx (photoelectrochemically active sites) were developed based on the electrodeposited a-MoSx morphology. The research provides an effective approach for uniformly depositing cost-effective a-MoSx on nanostructured photoelectrodes, for photoelectrochemical water splitting.


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INTRODUCTION The generation of hydrogen from water using sunlight through photoelectrochemical (PEC) cells is an attractive method to resolve the current environmental threats, while reducing the dependence on fossil fuels.1 A key building block of a PEC device is a semiconducting material for absorbing sunlight and inducing water splitting reactions.2 Typically, PEC water splitting reactions involve three major processes including absorption of light to generate electron-hole pairs, charge separation and migration to the surface of semiconductor, and finally electrochemical charge transfer (water reduction for hydrogen evolution or oxidation for oxygen evolution).3,4 A light-harvesting semiconductor initiates the multiple electron-transfer processes driven by thermodynamic potential difference, however the sluggish charge transfer kinetics at the semiconductor/liquid interface generally pose an obstacle for achieving high conversion efficiency.5 In order to improve the kinetics of semiconductor/liquid interface, the implementation of a co-catalyst that is capable of reducing the overpotential of electrochemical reaction is imperative. In addition, economical issues on both semiconductor and co-catalyst should be carefully considered for the realization of future PEC device based solar hydrogen production.6,7 Although various low-cost semiconductor materials, such as metal oxides,3 sulfides,8 and selenides9,10 have been well developed, inexpensive co-catalyst systems for hydrogen evolution reaction (HER) should be further investigated in order to satisfy the commercialization issues.7,11 Platinum (Pt) is currently the best-known HER catalyst with extremely low overpotential, however its scarcity and high cost limit the large scale utilization for PEC devices.12,13 In this regard, various noble metal-free HER electrocatalysts, composed of earthabundant elements, have been explored to accomplish cost-competitive photoelectrode system,


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including transition metal alloys,14 oxides,15 chalcogenides,16 carbides,17 and polymers.18 Among them, molybdenum sulfide is considered as one of the most promising HER catalysts because of its low cost, nontoxicity, high catalytic activity, and excellent chemical stability.19,20 Pioneering work revealed that only unsaturated sulfur atoms at the edge sites of crystalline MoS2 are catalytically active, while the basal planes are inert.21,22 In this respect, several stoichiometric MoS2 have been developed in an attempt to reducing the catalyst size in the form of nanoparticles,23 nanoclusters,24 nanowires,25 nanosheets26 and nanoporous structures,27 thereby effectively exposing active edge sites. However, these crystalline MoS2 catalysts are mostly produced by high temperature sulfurization reactions or hydrothermal reactions, which impede widespread utilization on labile substrates such as photoelectrodes. By contrast, Merki et al. reported that amorphous MoSx (a-MoSx, x = 2 – 3) film is also highly active toward HER comparable to nanocrystalline counterpart.28 The a-MoSx films are readily obtainable by simple aqueous solution phase deposition via electrochemical method (e.g., potentiostatic electrolysis, cyclic voltammetry).29 This solution-processability makes a-MoSx more suitable as a co-catalyst for the cost-effective photoelectrode. Although most of the researches on a-MoSx are restricted to the evaluation of its electrocatalytic activity on conductive substrates (e.g., glassy carbon, fluorine-doped tin oxide (FTO) etc.), only a few attempts have been directed to the application to the photoelectrodes as a co-catalyst.20,30–33 For instance, n+p-silicon was employed as semiconducting light absorber while thin metallic protection layer was necessary to avoid the surface oxidation of silicon electrode during electrodeposition, hindering the light absorption to silicon.30 Morales-Guio et al. developed TiO2-protected Cu2O photocathode employing a-MoSx film as a co-catalyst.20 They demonstrated that a conformal layer of a-MoSx effectively isolates the inner layer (i.e., TiO2)


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from the electrolyte, achieving higher stability as compared to Pt-decorated photocathode. However, so far researches on a-MoSx decorated photoelectrode suffer from the lack of in-depth understanding

of

a-MoSx

electrodeposited

onto

the

photoelectrode

in

terms

of

photoelectrochemical properties. In this regard, detailed analysis on electrochemical behavior during the electrodeposition is required to control the morphology and surface chemical states of amorphous co-catalyst, enabling efficient photoelectrode system. Antimony triselenide (Sb2Se3) has been considered as a potential light absorbing material due to its low-cost, low-toxicity and promising optoelectrical properties,34,35 which are suitable for PEC applications.33,36 Our group for the first time reported solution-processed Sb2Se3 photoelectrode using simple spin-coating method and its nanostructure was well-optimized for efficient PEC water splitting through controlling the solution chemistry. However, Pt surface decoration still remained a challenge to achieve ultimate low-cost hydrogen production.9,10 Recently, Prabhakar et al. employed a-MoSx catalyst layer directly deposited onto Sb2Se3, leading to high-performance and cost-effective photocathode with ~ 16 mA cm-2 at 0 V versus a reversible hydrogen electrode (RHE, VRHE).33 Despite the promising performance, they only focused on the effect of post-sulfurization after deposition of a-MoSx co-catalyst. The systematic study on the deposition behavior of a-MoSx on semiconductor surface is still elusive. Herein, we employ a-MoSx as a co-catalyst for TiO2-surface-protected Sb2Se3 in order to provide an indepth understanding of electrodeposited a-MoSx layer with sufficient photoelectrochemically active sites can be obtained by controlling the reaction sequence of the cyclic voltammetry (CV). By conducting the CV profile and monitoring microstructure evolution depending on the number of cycles, we observed that only cathodically initiated MoSx (denoted as CI-MoSx), i.e., prepared under the potential starting at -0.3 versus a reversible hydrogen electrode (RHE, VRHE) where the


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reductive deposition of MoSx takes place initially, allows uniformly deposited thin film type MoSx. On the other hands, the anodically initiated MoSx (denoted as AI-MoSx), i.e., produced under the potential starting at 0.6 VRHE where the oxidative deposition of MoSx initially occurs, leads

to

inhomogeneous

deposition

of

particulate

catalysts

(Scheme

1).

The

photoelectrochemical properties were characterized via sweep voltammetry in conjunction with electrochemical impedance spectroscopy (EIS). These characterizations allow us to understand that the CI-MoSx enables effectively reducing the charge transfer kinetics for HER as compared to AI-MoSx, providing a simple technique to develop a cost-effective photoelectrode system.

RESULTS AND DISCUSSION The electrodeposition of a-MoSx co-catalyst was carried out on the TiO2 surface-protected Sb2Se3 photocathode from an aqueous solution containing 1 mM (NH4)2MoS4 as the source of aMoSx under two different conditions as explained in Scheme 1. Detailed preparation methods are described in Experimental Section. Figure 1 shows the top-view scanning electron microscopy (SEM) images of a-MoSx decorated on photocathodes as a function of the cycle numbers of electrodeposition. The microstructures of bare Sb2Se3 and TiO2/Sb2Se3 before the CV process (i.e., deposition of a-MoSx) are shown in Figure S1. As the cycle number of CV deposition process increases, the CI-MoSx grows continuously while fully covering the nanorod shaped Sb2Se3, eventually leading to conformal film-like morphology with well-defined nanorod characteristics (Figure 1a-c). On the other hand, relatively non-uniform particulate attachments are observable for the AI-MoSx with the increasing cycle numbers of CV deposition, resulting in a raspberry-like nanorods (Figure 1d-f). To further characterize the morphological features of aMoSx, transmission electron microscopy (TEM) as well as scanning transmission electron


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microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDS) were performed for both the CI-MoSx and AI-MoSx samples after 80 cycles of electrodeposition. Figure 2a shows a crosssectional TEM image of representative region of CI-MoSx sectioned by a focused ion beam, which reveals the film-like uniform structure of a-MoSx with thickness of ~ 20 nm on top of nanorod TiO2/Sb2Se3 photoelectrode. This is in a good agreement with SEM images. High angle annular dark field (HAADF) and STEM-EDS elemental mapping images serve as supportive evidence for conformal deposition of CI-MoSx around the TiO2/Sb2Se3 nanorods. By contrast, the AI-MoSx exhibits non-uniform island structures of MoSx with the size of ~ 10 ̶ 20 nm deposited on TiO2/Sb2Se3 surfaces (Figure 2b). Although the a-MoSx electrodeposition on photoelectrode has been previously demonstrated,20 there is no report on the influence of the starting potential applied at the first cycle of voltage sweep on the varying morphologies of a-MoSx. To elucidate the formation mechanisms depending upon the starting potential, the current density vs. potential curves during the CV deposition process were investigated. Figure 3a show cyclic voltammograms for selected cycles monitored during CV process of CI-MoSx. A reductive current is observed in the range of potentials from 0.1 to -0.3 VRHE as the cycle iteration increases. In oxidation region, relatively smaller current density is observed as compared to the reduction region. However, the magnified view reveals a noticeable oxidation occurred at around 0 VRHE (marked by yellow region, Figure 3b). According to the previous electrochemical deposition of a-MoSx, the initial reaction under the application of cathodic potential (i.e., CI-MoSx) yields the deposition of molybdenum disulfide (MoS2) through the reduction of MoS42- as described in eq 1:37

MoS4

2-

+ 2H2 O + 2e- → MoS2 + 2HS- + 2OH-

(1)


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We ascribe the oxidative currents pronounced in yellow region to a re-oxidation of deposited MoS2 through the reaction with surface adsorbed bisulfide (HS-) ions during the potential sweep from -0.3 to 0.6 VRHE as described in eq 2: MoS2 + 2HS- → MoS3 + H2 S + 2e-

(2)

This is an electrochemical topotactic-like oxidation in which surface MoS2 is transformed to MoS3 without morphological alternation. MoS3 provides further reaction sites for the next reduction by reacting with MoS42- during the returning potential sweep from 0.6 to -0.3 VRHE, leading to deposition of MoS2 as described in eq 3: 2-

MoS3 + MoS4 + 3H2 O + 4e- → 2MoS2 + 3HS- + 3OH-

(3)

Equations 2 and 3 repeatedly occur during the subsequent CV process, resulting in film-like conformal morphology of a-MoSx around TiO2/Sb2Se3. Interestingly, the current density vs potential profiles during the deposition of AI-MoSx show different behavior (Figure 3c) as compared to CI-MoSx even though the applied potential window is identical. An oxidative current is predominantly observed in the range of positive potentials (around 0.4 to 0.6 VRHE) as the cycle iteration increases. The initial reaction under the application of anodic potential yields the deposition of MoS3 as well as sulfur through the oxidation of MoS42- as described in eq 4:38

MoS4

2-

→ MoS3 +

1 8

S8 + 2e-

(4)


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During the potential sweep from 0.6 to -0.3 VRHE, we ascribe these reductive currents to either a reductive deposition of MoS2 (eq 3) or a reductive corrosion consuming elemental sulfur as expressed in eq 5: 1 8

S8 + 2H+ + 2e- → H2 S

(5)

Larger current density in the reduction region of AI-MoSx (Figure 3c) is observed as compared to the reduction region of CI-MoSx (Figure 3a). This likely implies simultaneous occurrence of reductive corrosion as well as MoS2 deposition.29 This corrosive dissolution of sulfur will give rise to inhomogeneous morphology of the electrodeposited phase, exposing TiO2 surface. During the returning potential sweep from -0.3 to 0.6 VRHE, two possible oxidation reactions (represented by eq 2 and 4) can occur, leading to either the formation of MoS3 on the preelectrodeposied MoS2 (eq 2) or the co-deposition of MoS3 and S on the exposed TiO2 surface (eq 4). The magnified view reveals a noticeable oxidation occurred at around 0.6 VRHE (marked by blue region, Figure 3d), which is not observed in Figure 3b. This oxidation current indicates that oxidative deposition of MoS3 and S predominately occurs on the extensively corroded and exposed TiO2 surface during the CV process, eventually leading to non-uniform electrodeposited structure as shown in Figure 1f and 2b. To the best of our knowledge, this is the first report of the influence of starting potential to the final morphology of a-MoSx deposited on semiconductor surface (e.g., TiO2). In contrast to the deposition onto semiconductor surface, a-MoSx electrodeposited on conductive substrate (e.g., FTO) revealed almost no difference in deposition behavior of a-MoSx during CV process (Figure S2a, b); a reduction current of both CI-MoSx/FTO and AI-MoSx/FTO is observed in the range of potential from 0.1 to -0.3 VRHE (equation 1) while an oxidation current is observed


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around 0.4 to 0.6 VRHE (equation 4). The corresponding morphologies of CI-MoSx/FTO and AIMoSx/FTO both similarly showed film-type microstructures after 80 cycles of CV process (Figure S2c, d). Furthermore, to demonstrate the HER performance, the linear sweep voltammetry has been additionally performed on both CI-MoSx/FTO and AI-MoSx/FTO, which exhibited almost identical electrochemical behavior (Figure S2e). Our results on conductive substrate are consistent to the previous report.39 This clearly demonstrates that our strategy is substrate-dependent in which different morphology and HER activity of MoSx are observable only when deposited onto semiconductor surfaces (i.e., TiO2) rather than conductive surfaces. Moreover, the main role of TiO2 is a protective layer that has been widely adopted to other p-type photocathodes, such as Si and Cu2O.20,30 Although a-MoSx layer can be directly deposited onto Sb2Se3 (without TiO2) by the galvanostatic method with constant current density of –50 µA cm-2,33 the resulting MoSx quality is poor due to the lack of controllability (i.e., only one type of cathodic reaction occur), while CV process provides an efficient way to achieve high-quality MoSx by controlling the deposition reactions. Moreover, an electrochemical corrosion of Sb2Se3 might accelerate during CV process without TiO2 protection layer.40 Because the main purpose of this work is to provide a novel technique to achieve high-quality MoSx by CV process, it is necessary to deposit TiO2 protection layer prior to MoSx deposition. The PEC performance of Sb2Se3 based photocathode with TiO2 as a protective layer and a-MoSx as a co-catalyst are demonstrated in Figure 4. The n-type TiO2 deposition also facilitates band bending by formation of p-n junction, so that the photogenerated charge carriers are effectively separated without recombination enhancing the photocurrent density.9,10 Figure 4a and 4b show the PEC performances depending on the iteration of linear voltammetry sweep from +0.3 to -0.2 VRHE for the 80 cycles electrodeposited CI-MoSx and AI-MoSx, respectively. For


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better comparison, the photocurrent densities at 0 VRHE for both samples are recorded as a function of scan iterations (Figure 4c). Interestingly, CI-MoSx/TiO2/Sb2Se3 shows the improvement in the photocurrent from first to second scan and reaches a saturation at ~ 4.8 mA cm-2 after third scan. By contrast, AI-MoSx/TiO2/Sb2Se3 exhibits a slight increase in the photocurrent followed by almost no variation at ~ 1.1 mA cm-2 with the increasing scan iterations. This can be considered that CI-MoSx/TiO2/Sb2Se3 is well activated by photo-assisted reductive voltammetry sweep as generally observed in HER electrocatalysts such as a-MoSx20,33 and RuOx41 whereas activation process does not work for AI-MoSx/TiO2/Sb2Se3. Reductive activation involves the electron transfer from the electrode to a-MoSx, followed by reduction of either Mo or S species, thereby generating HER active sites.29,42 CI-MoSx is in a conformal contact with TiO2 layer, enabling facile electron transport across the entire TiO2/a-MoSx interface for reductive activation as schematically illustrated in Figure S2a. On the other hand, isolated particulate morphology of AI-MoSx only permits the limited electron transport, losing the electrons delivered to the exposed TiO2 surfaces. Therefore, CI-MoSx is capable of being well-activated, achieving high photocurrent density that is slightly lower than Pt catalyst (Figure S3) and comparable to other a-MoSx deposited on Cu2O photocathode (5.7 mA cm-2 at 0 VRHE).20 In addition, our photoelectrochemical performances agree with recently reported aMoSx/Sb2Se3 photocathode (5 mA cm-2 at 0 VRHE),33 which have a great potential for improvements after further optimization of surface treatment of Sb2Se3 such as sulfurization or selenization.33,36 To elucidate the difference in HER activity between CI-MoSx and AI-MoSx, we performed electrochemical impedance spectroscopy (EIS) in a frequency range of 300 kHz to 0.05 Hz under simulated solar light illumination at 0 VRHE after the activation process of as-


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deposited a-MoSx (i.e., repeating linear voltammetry sweep from +0.3 to -0.2 VRHE until the photocurrent density at 0 VRHE reaches a saturation). Figure 5a and 5b show the Nyquist plots for CI-MoSx/TiO2/Sb2Se3 and AI-MoSx/TiO2/Sb2Se3, respectively. The resistance between high and low frequency intercepts represents the total polarization of the electrode, which can be further deconvoluted into several semicircles in terms of frequency regions. We employed a simple equivalent circuit consisting of serial connected three resistors–capacitors model in which each represents a resistance (R) and a constant phase element (CPE) at three different frequency ranges (Figure 5c) because three separated frequency regions in a Bode plot for CIMoSx/TiO2/Sb2Se3 are clearly shown (Figure 5d).31,43 For convenience, we classify highfrequency resistance (RHF), middle-frequency resistance (RMF) and low-frequency resistance (RLF) which are represented by summit frequencies of 20 kHz, 200 Hz and 3 Hz, respectively. The fitted results are summarized in Table S1. According to recent EIS analysis of a PEC cell for water splitting,44 RHF is influenced by the transport of the charge carriers within the electrode, whereas RLF represents the charge transfer reaction at the electrode/electrolyte interface. The CIMoSx/TiO2/Sb2Se3 exhibits RLF four times smaller than AI-MoSx/TiO2/Sb2Se3 (Table S1), which is in agreement with the observed difference in the photocurrent density, indicating that the PEC performance is mainly governed by the characteristics of a-MoSx co-catalyst that determines the charge transfer resistance of HER. Interestingly, only CI-MoSx/TiO2/Sb2Se3 shows additional arc at middle frequency region (~ 200 Hz). It has been suggested that additional arc at this frequency region is related to electron trapping at surface state prior to charge transfer to electrolyte.45 This surface state represents an additional electronic state positioned at forbidden state within the band gap of the material (Figure S4).46 The RMF is only observed in film-like CI-MoSx due to temporally trapped electrons at the surface state. It is speculated that this surface state induces


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temporal charge accumulation followed by facilitating the charge transfer reaction at semiconductor/electrolyte interface because surface state position is closely located to the thermodynamic potential of HER.20,47,48 Furthermore, the stability test was performed for both CI-MoSx and AI-MoSx at 0 VRHE (i.e., low overpotential region) and -0.2 VRHE (i.e., high overpotential region) as shown in Figure S6. Interestingly, both CI-MoSx and AI-MoSx showed degradation at low overpotential region, whereas CI-MoSx exhibited stable photocurrent over 1 h without degradation at high overpotential region. It is speculated that HER catalytic activity of MoSx is insufficient to rapidly extract the photogenerated electrons at the electrode/electrolyte interface, so that the additional overpotential is required to enhance the charge transfer kinetics of MoSx for hydrogen evolution. However, temporal charge accumulation might occur in low overpotential region, leading to partial reduction of TiO2 and the accompanying detachment of MoSx co-catalyst. To understand large difference in both photocurrent density and charge transfer kinetics between CI-MoSx and AI-MoSx, the chemical state of a-MoSx should be elaborately analyzed. Figure 6a show X-ray photoelectron spectroscopy (XPS) spectra of Mo 3d region before (i.e., asdeposited a-MoSx) and after the activation process of CI-MoSx. Since the Mo 3d region overlaps with the S 2s region, specific fitting criteria is necessary as described in Table S2. Briefly, the Mo 3d spectra can be deconvoluted into three doublets (each doublet includes Mo 3d3/2 and Mo 3d5/2 peak) of Mo(VI), Mo(V) and Mo(IV) states represented by blue, green and red colored peaks. In addition, S 2s can be also fitted into three singlets including S0 (purple), S22- (dark blue), and S2- (orange).20,49 The binding energy and full width half maximum (FWHM) for each peak are summarized in Table S2. Before the activation, Mo(IV) 3d5/2 peak (at 229.16 eV, red dotted line) is attributed to MoS2 electrodeposited under cathodic potential (eq 1 and 3), while


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Mo(VI) 3d5/2 peak (at 232.36 eV, blue dotted line) represents MoS3 electrodeposited by reoxidation of MoS2 (eq 2). In addition, Mo(V) 3d5/2 peak (at 231.23 eV, green dotted line) is originated when Mo(VI) state is reduced by an electron transfer from S2- in the MoS3. Simultaneously, the S22- state, well-known as bridging disulfide ligands,50 is produced as described in eq 6: 2-

2Mo(VI)(S2- )3 → Mo(V)2 (S2 )(S2- )4

(6)

After the surface activation, the peaks for Mo(IV) 3d5/2 (at 228.6 eV, red) and Mo(V) 3d5/2 (at 230.56 eV, green) are shifted to lower energy direction (as marked by arrows) with increased intensity, whereas the peak for Mo(VI) 3d5/2 (at 232.36 eV) maintains at identical position with significantly decreased intensity. The variation of peak intensity indicates that Mo(VI) state of MoS3 is reduced to Mo(IV) and Mo(V) states (i.e., reductive activation). These reduced Mo species have been considered as HER active sites because they can play as a proton absorption site.42 The lowered intensity of Mo(VI) peak after the activation likely implies the presence of MoO3 surface oxide layer. It is believed that MoS3 predominantly exists as manifested by Mo(VI) state before the activation, and residual MoS3 after the activation is then oxidized during the sample preparation for XPS measurement.29 The binding energy shift to lower level can be explained as lowered fermi level of a-MoSx possibly due to the formation of additional surface states in CI-MoSx.51,52 In addition, no detectable S0 state (purple peak) indicates the elimination of elemental S on the surfaces of photoelectrode through a reductive corrosion during the activation process. Furthermore the increased area ratio of S2- state (orange single peak) to S22- state (dark blue single peak) is indicative of the consumption of bridging disulfide ligands accompanying the reduction of neighboring Mo atoms as expressed by the following:42


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2-

Mo(V)2 (S2 )(S2- )4 + 4H+ + 4e- → Mo(IV)2 (S2- )4 + 2H2 S

(7)

On the other hand, AI-MoSx exhibits only small variation in both the position and intensity of Mo 3d5/2 peaks after the activation process (Figure 6b). The positions of all three Mo 3d5/2 energy (dotted line) are negligibly changed during the activation and the binding energy is similar to pre-activated CI-MoSx (Table S2). The Mo(VI) peak intensity marginally decreases, while the intensities of Mo(V) and Mo(IV) responsible for HER active sites slightly increase. While the S0 peak intensity representing elemental S is to some extent lowered, the ratio of S2- to S22- remains almost identical even after the activation process. In this regard, the AI-MoSx undergoes less activation as compared to well-activated CI-MoSx, which agrees with PEC performance difference (Figure 4). Our electrochemical analyses and XPS results clearly demonstrate that the film-like microstructure is beneficial to the activation of a-MoSx co-catalyst, improving PEC performances as compared to particulate structure.

CONCLUSIONS In summary, a noble-metal free a-MoSx/TiO2/Sb2Se3 photocathode was successfully fabricated via simple electrodeposition, for PEC hydrogen evolution. The PEC performance was significantly influenced by the morphological difference in electrodeposited a-MoSx co-catalysts, during the CV process. Starting from the cathodic potential, the resulting CI-MoSx exhibited conformal film-like morphology, whereas the AI-MoSx, starting from anodic potential, showed non-uniform particulate structure, after 80 CV-cycles. This film-like morphology (CIMoSx/TiO2/Sb2Se3) was more suitable for facile catalytic activation, demonstrating four-times higher photocurrent density (~ 4.8 mA cm-2 at 0 VRHE) as compared with AI-MoSx/TiO2/Sb2Se3


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(~ 1.1 mA cm-2 at 0 VRHE). The enhanced PEC performance was mainly attributed to highly developed HER active sites within the a-MoSx co-catalyst, which led to a reduced chargetransfer resistance at the semiconductor/electrolyte interface. This simple and effective deposition of a-MoSx can be readily applied to various nanostructured photoelectrodes, particularly on semiconductor surfaces. Moreover, microstructure-dependent PEC performance variation, can provide in-depth understanding of cost-effective amorphous co-catalysts, for further development of highly efficient PEC water splitting systems. EXPERIMENTAL SECTION Preparation of TiO2 Surface-Modified Sb2Se3 Photocathode. First, the Sb2Se3 nanostructure was fabricated onto 70-nm-thick Au-coated FTO glass, using a controlled Sb2Se3 precursor solution, by a spin coating method inside a N2-filled glovebox, as reported previously.9 The Sb solution was obtained by dissolving 0.1 M SbCl3 (99.99%, Alfa Aesar, Heysham, UK) in 12 mL of 2-methoxyethanol (2ME, 99.5%, Sigma Aldrich, St. Louis, MO, USA). The Se solution was obtained by dissolving 0.385 g of Se powder (99.5%, Sigma Aldrich), in a co-solvent of thioglycolic acid (TGA, 98%, Sigma Aldrich) and ethanolamine (EA, 99.5%, Sigma Aldrich), at a molar ratio of 5:95. The mixed Sb-Se solution was spin-coated at 2000 rpm for 30 s, and immediately dried at 300°C for 3 min, under N2 atmosphere. This spin-coating process was repeated six times, the film was subsequently annealed at 350°C for 20 min in a N2-filled glove box, and post-annealed at 200°C for 30 min in air. An n-type TiO2 layer was deposited on the Sb2Se3 via an atomic layer deposition (ALD) system (NCS Inc., Daejeon, Korea), employing titanium tetra-isopropoxide (TTIP, Easychem, Korea) and H2O, as Ti and O sources, respectively. The deposition temperature was 160°C and the TTIP was evaporated at 75°C. Each ALD cycle consisted of a 3 s TTIP pulse, 10 s N2 purge, 2 s H2O pulse, and 10 s N2 purge. Four-hundred


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deposition cycles were followed by post-annealing at 200°C for 1 h in air. Finally, a copper wire and silver paste were employed to form a contact with the device, and nonessential parts of the electrode were covered with an epoxy resin (HYSOL 9642, Henkel, Düsseldorf, Germany). Deposition of a-MoSx Co-Catalyst. A 1 mM solution of (NH4)2MoS4 (99.5%, Sigma Aldrich), in 0.1 M NaClO4 (99.5%, Sigma Aldrich) aqueous solution (400 mL), was prepared (measured pH was 5.4) for the electrodeposition of a-MoSx. Prior to immersion of the as-prepared electrode into the solution, sufficient Ar purging, with magnetic stirring, was carried out to eliminate the oxygen. Eighty, consecutive, potential-cycles were conducted with a potentiostat (1287A, Solartron, UK), using a typical three-electrode configuration of a Ag/AgCl/KCl (4 M) reference electrode, with a Ti wire counter electrode, to prevent Pt contamination of the a-MoSx. Cyclic voltammetry was performed from +0.1 to -0.8 V vs. Ag/AgCl (i.e., +0.6 to -0.3 VRHE) for the AIMoSx, and from -0.8 to +0.1 V vs. Ag/AgCl (i.e., -0.3 to +0.6 VRHE) for the CI-MoSx, at a fixed scan-rate of 50 mV s-1. Finally, the a-MoSx deposited electrode was rinsed in DI water, before PEC characterization. In addition, FTO substrate was also employed for identical electrodeposition of a-MoSx to see if there was the effect of the substrate type. Physical and Chemical Characterization. The surface-morphology evolution of the asprepared photocathode (CI- or AI-MoSx/TiO2/Sb2Se3), was studied as a function of cycle iterations, using field emission SEM (JSM-7001F, JEOL, Tokyo, Japan). The FIB technique was used to prepare electron-transparent foils of 80-cycle processed CI-MoSx and AI-MoSx samples. The microstructures of the a-MoSx were confirmed by TEM (JEM-F200, JEOL, Tokyo, Japan), and corresponding HAADF and STEM-EDS mapping images were obtained (Talos F200X, FEI, Hillsboro, OR, USA), at a 200 kV acceleration voltage. Surface chemical information from the a-MoSx was analyzed by XPS (K-alpha, Thermo Scientific Inc., UK), and the C 1s peaks were


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calibrated with a binding energy of 284.6 eV. For reliable analyses, pre- and post-activation samples were stored in vacuum, immediately after the electrodeposition of a-MoSx, and PEC measurements (after photocurrent density saturation). Photoelectrochemical Measurements. The PEC measurements were conducted in a typical three-electrode system with a Ag/AgCl/KCl (4 M) reference electrode, and a Pt wire as the counter electrode. The PEC cell was submerged in a 0.5 M aqueous H2SO4 solution, and the measurements used simulated solar light illumination (AM 1.5G, Newport Corporation). For the sample of a-MoSx/FTO, the electrochemical measurements were conducted without illumination. For all measurements, the applied potentials were referred to the RHE, to enable comparison with other reports. The following equation was employed to convert the potential: ERHE = EAg/AgCl + 0.059 pH + 0.197 EIS measurements were also performed in the same configuration, using a potentiostat combined with a frequency analyzer (1260, Solartron, UK). Polarization of the PEC cells was analyzed in the frequency range 300 kHz to 0.05 Hz, with an AC amplitude of 10 mV, under simulated solar light illumination at 0 VRHE.


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Scheme 1. (a) Schematic illustration showing the growth process of the nanostructured TiO2/Sb2Se3-based photocathode. (b) Electrodeposition of a-MoSx, as the starting potential was changed from cathodically (-0.3 VRHE) initiated, to anodically (0.6 VRHE) initiated condition. Both a-MoSx catalysts were continuously electrodeposited on TiO2/Sb2Se3, during the cyclic voltammetry.


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Figure 1. SEM images of a-MoSx electrodeposited on the TiO2/Sb2Se3 photoelectrodes. The morphological evolution of the photoelectrodes, observed as a function of the number of voltage sweep cycles: (a) 20 cycles, (b) 40 cycles, and (c) 80 cycles, for the CI-MoSx, and (d) 20 cycles, (e) 40 cycles, and (f) 80 cycles, for the AI-MoSx.


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Figure 2. TEM images and EDS elemental mapping showing microstructures after 80 electrodeposition cycles of a-MoSx on TiO2/Sb2Se3 nanorods. (a) CI-MoSx exhibiting film-like morphology with a thickness of ~ 20 nm, and (b) AI-MoSx showing non-uniform particulate structures, with a size of ~ 10 − 20 nm.


21


(b) 0.025

0

e ctiv du Re

-0.05

-0.3

-0.1

CI starts here

(c)

0.1

0.3

0.5

(d)

-2

O

p ve de xidati

n ositio

0

c ve ucti Red

orro

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-0.3

0.005

0.0

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0.3

0.6

0.3

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AI-MoSx

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Potential scan direction changed

0.010

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0.06

Cycle 20 Cycle 40 Cycle 60 Cycle 80

0.015

0 -0.3

0.7

0.12

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0.3

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sion

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-2

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Cycle 1 Cycle 5 Cycle 10 Cycle 20 Cycle 40 Cycle 60 Cycle 80

ion sit po de

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on

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R

ati xid e-o


0.05

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(a)


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

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0.04

Cycle 1 Cycle 5 Cycle 10 Cycle 20

Cycle 40 Cycle 60 Cycle 80

Potential scan direction changed

0.02

0 -0.3

AI starts here

0.0

0.3 0.6 Potential / VRHE

0.3

Figure 3. (a) Cyclic voltammograms of 1, 5, 10, 20, 40, 60, and 80 electrodeposition cycles of CI-MoSx onto a Sb2Se3 based photocathode. (b) Magnified oxidation region of the CI-MoSx electrodeposition. Yellow region exhibits re-oxidation of deposited MoS2, leading to the formation of MoS3. (c) Identically selected cyclic voltammograms during electrodeposition of AI-MoSx, and (d) magnified oxidation region. Blue region shows oxidative deposition of both MoS3 and S.


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(a)

off

(b)

1st scan

0 -5

-5

-1

-2

2nd scan


-2

off

0 -5

-2

5 mA cm

on off

3rd scan

0 -5 -10

on off

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on

-2

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-10 off

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on

-10

off 4th scan

0

0 -5

-5

on

on

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-10 -0.2

1st scan

-10

-10

-10

off

0

5 mV s

on


AI-MoSx/TiO2/Sb2Se3

CI-MoSx/TiO2/Sb2Se3 -0.1

0.0

0.1

0.2

0.3

-0.2

-0.1

-2

(c)

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J at 0 VRHE / mA cm

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


-5 CI-MoSx

-4 -3 -2 -1 0

AI-MoSx

1

2

3

4

Number of scans Figure 4. PEC performance of (a) CI-MoSx and (b) AI-MoSx, as a function of scan iteration of the linear voltammetry sweep. (c) The photocurrent density at 0 VRHE for both samples, plotted as a function of scan iterations, showing different activation behavior between CI-MoSx and AIMoSx.


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Figure 5. EIS spectra of (a) CI-MoSx and (b) AI-MoSx, after activation. Scatter points represent the original experimental data, whereas solid lines relate to the fitted curves. (c) The Bode plot clearly showing three different frequency regions of CI-MoSx. (d) A simple equivalent circuit model employed for deconvolution of EIS spectra.


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(a)

Exp Fit

CI-MoSx before activation

(b)

Mo(VI) Mo(V) Mo(IV)

AI-MoSx before activation

Exp Fit Mo(VI) Mo(V) Mo(IV)

0

0

S 2S2

S 2S2

2-

2-

S

CI-MoSx after activation

238

236

Mo(VI) 3d5/2 Mo(V) 3d5/2 Mo(IV) 3d5/2

234

232

230

228

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224

S

Intensity / a.u.

Intensity / a.u.

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


AI-MoSx after activation

238

236

Mo(VI) 3d5/2 Mo(V) 3d5/2 Mo(IV) 3d5/2

234

232

230

228

226

224

Binding energy / eV

Binding energy / eV

Figure 6. X-ray photoelectron spectra of Mo 3d and S 2s regions, before and after activation of (a) CI-MoSx and (b) AI-MoSx. The three doublets of Mo 3d are Mo(VI), Mo(V), and Mo(IV) states, represented by blue, green, and red colored peaks, respectively. Each of the Mo 3d5/2 peak positions are denoted by dotted vertical lines, showing that only the Mo(V) and Mo(IV) 3d5/2 peaks of CI-MoSx were shifted to a lower level, after activation (arrows). The three peaks of S 2s are S0, S22-, and S2- states, represented by purple, dark blue, and orange colored peaks, respectively.


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ASSOCIATED CONTENT Supporting Information Additional SEM images and PEC performance data, schematic illustrations of activation mechanism and band alignment. This Supporting Information is available free of charge on the ACS Publications website.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interests.

ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIP) (2012R1A3A2026417).

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TABLE OF CONTENTS


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Controlled Electrodeposition of Photoelectrochemically Active

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