CoSe2 and NiSe2 Nanocrystals as Superior Bifunctional Catalysts for

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CoSe and NiSe Nanocrystals as Superior Bifunctional Catalysts for Electrochemical and Photoelectrochemical Water Splitting In Hye Kwak, Hyung Soon Im, Dong Myung Jang, Young Woon Kim, Kidong Park, Young Rok Lim, Eun Hee Cha, and Jeunghee Park ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b12093 • Publication Date (Web): 05 Feb 2016 Downloaded from http://pubs.acs.org on February 6, 2016

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ACS Applied Materials & Interfaces

CoSe2 and NiSe2 Nanocrystals as Superior Bifunctional Catalysts for Electrochemical and Photoelectrochemical Water Splitting

In Hye Kwak,† Hyung Soon Im,† Dong Myung Jang,† Young Woon Kim,‡ Kidong Park,† Young Rok Lim,† Eun Hee Cha,‡ and Jeunghee Park*,† †

Department of Chemistry, Korea University, Jochiwon 339-700, Korea

‡

Graduate School of Green Energy Engineering, Hoseo University, Asan 336-795, Korea

*Corresponding author: E-mail address [email protected] KEYWORDS: NiSe2, CoSe2, water splitting, bifunctional, Si nanowire, photoelectrochemical cell

ACS Paragon Plus Environment

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ABSTRACT Catalysts for oxygen evolution reactions (OER) and hydrogen evolution reactions (HER) are central to key renewable energy technologies, including fuel cells and water splitting. Despite tremendous effort, the development of low-cost electrode catalysts with high activity remains a great challenge. In this study, we report the synthesis of CoSe2 and NiSe2 nanocrystals (NCs) as excellent bifunctional catalysts for simultaneous generation of H2 and O2 in water-splitting reactions. NiSe2 NCs exhibit superior electrocatalytic efficiency in OER, with a Tafel slope (b) of 38 mV dec–1 (in 1 M KOH), and HER, with b = 44 mV dec–1 (in 0.5 M H2SO4). In comparison, CoSe2 NCs are less efficient for OER (b = 50 mV dec–1), but more efficient for HER (b = 40 mV dec–1). It was found that CoSe2 NCs contained more metallic metal ions than NiSe2, which could be responsible for their improved performance in HER. Robust evidence for surface oxidation suggests that the surface oxide layers are the actual active sites for OER, and that CoSe2 (or NiSe2) under the surface act as good conductive layers. The higher catalytic activity of NiSe2 is attributed to their oxide layers being more active than those of CoSe2. Furthermore, we fabricated a Si-based photoanode by depositing NiSe2 NCs onto an n-type Si nanowire array, which showed efficient photoelectrochemical water oxidation with a low onset potential (0.7 V versus reversible hydrogen electrode) and high durability. The remarkable catalytic activity, low cost, and scalability of NiSe2 make it a promising candidate for practical water-splitting solar cells.


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Water splitting is currently an important research topic as it can utilize both water and solar (or electric) energy for a cleaner, recyclable, and cheaper approach to hydrogen generation. The water-splitting reaction can be divided into two half-reactions: the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), both of which are crucial for the overall efficiency of water splitting. However, water splitting is mainly hampered by the kinetically sluggish fourelectron OER (4OH– → O2 + 2H2O + 4e– in alkaline media). Catalyst development is critical in addressing this challenge in order to efficiently couple multiple proton and electron transfers with low overpotentials. To date, the most efficient electrochemical OER catalysts are ruthenium oxide (RuO2) and iridium oxide (IrO2), despite their limited availability and high cost.1–4 Consequently, robust and efficient alternative nanocatalysts based on cost-effective earthabundant 3d metals have been vigorously pursued, but substantial progress is still needed.5–13 Pt-carbon materials have been proven to be state-of-the-art HER catalysts. However, they suffer from scarcity and high cost, limiting their widespread use. Numerous work has focused on finding new non-noble metals or metal-free materials to replace expensive Pt-carbon catalysts.14– 20

Indeed, great progress has been made in the last few years in developing earth-abundant metal

chalcogenides and phosphides with high activity in strong acidic solutions. However, HER catalysts may be inactive or even unstable in strong basic electrolytes, while OER catalysts are usually unstable in acidic solution. Therefore, developing a bifunctional catalyst for both O2 and H2 generation using the same electrolyte is a challenging issue that has been scarcely reported, although it is crucial for designing overall water-splitting catalysts.11,21–32 Moreover, the use of a bifunctional catalyst is highly advantageous in simplifying the system and reducing the costs. Herein, we synthesized CoSe2 and NiSe2 nanocrystals (NCs), which were simultaneously applied to HER and OER. The selection of these materials was inspired by the excellent


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electrocatalytic activity of metal selenides (CoSe2, NiSe, and NiSe2) toward either HER or OER.27,33–43 However, bifunctional catalytic performance has only been reported for NiSe by Tang et al.27 In the present work, we compared the bifunctional catalytic activity of CoSe2 and NiSe2 NCs, including their tolerance to acid and alkaline environments, and showed that, overall, NiSe2 NCs are a superior bifunctional catalyst for water splitting. To the best of our knowledge, the bifunctional catalytic power of CoSe2 and NiSe2 has not been previously examined by this approach. Furthermore, to prove whether they can act as a catalyst for solar water splitting, we deposited NiSe2 NCs onto a silicon nanowire (Si NW) array, and investigated the photoelectrochemical (PEC) cell performance. Si has long been considered a good candidate material for solar watersplitting photoelectrodes to produce H2 or O2.44–51 Remarkable progress has been made in recent years toward Si nanostructures that can improve the light absorption capability and increase electrode/electrolyte interfacial charge carrier collection. Moreover, research efforts have also been devoted to combining Si with earth-abundant HER or OER catalysts. Recently, CoSe2 coupled with a Si microwire array has been shown to act as a promising photocathode in the solar-driven water-splitting reaction.51 To our knowledge, NiSe2 has been never used in a watersplitting photoanode. The synthesis and characterization of free-standing CoSe2 and NiSe2 NCs, and their hybrid structures with the Si NW array, are described in the Supporting Information. A photo-induced cation-exchange reaction of germanium chalcogenide (GeSe2) NCs in water was used to synthesize free-standing CoSe2 and NiSe2 NCs, as described elsewhere.52 This method had many benefits including high yield, short reaction time, and excellent reproducibility. The XRD pattern showed a pyrite cubic phase for both NCs, with peaks matching those of the references: CoSe2


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(JCPDS No: 09-0234, Pa3, a = 5.858 Å) and NiSe2 (JCPDS No: 41-1495, Pa3, a =5.991 Å), as shown in Figure S1 (Supporting Information). High-resolution transmission electron microscopy (HRTEM) images revealed the spherical morphology of CoSe2 and NiSe2 NCs. The average NC size was 20 nm. Lattice-resolved TEM and the corresponding fast Fourier-transformed (FFT) images of CoSe2 and NiSe2 NCs at a zone axis of [111] and [012] are shown in Figures 1a and 1b, respectively. The d-spacing of the (220) and (200) planes was 2.0 and 2.9 Å for CoSe2 and NiSe2, respectively. Energy-dispersive X-ray fluorescence (EDX) spectra with scanning TEM (STEM) images showed that the ratio of metal to Se was 1:2, with negligible oxygen. The STEM image and EDX mapping of individual NiSe2 NCs confirmed homogenous distribution over whole NC, with a Ni/Se ratio of 1:2. To evaluate electrocatalytic performance in OER and HER, we performed linear sweep voltammetry (LSV) using a rotating disk electrode (RDE). The same quantity of each catalyst (1 mg cm–2) was loaded onto a glassy carbon RDE. The results are summarized in Table 1. Figure 2a shows LSV curves for CoSe2 and NiSe2 NCs in O2-saturated 1 M KOH solution (scan rate: 2 mV s–1) at a rotation speed of 1600 rpm. A typical three-electrode setup was used, with a Ag/AgCl reference electrode. The potentials reported in our work were referenced to the reversible hydrogen electrode (RHE) through standard calibration. IrO2 (Sigma-Aldrich, 99.9% trace metals basis) was chosen as a reference and LSV curves and Tafel plots were obtained for each catalyst under the same conditions. The LSV curves of CoSe2, NiSe2, and IrO2 had onsets at 1.55, 1.43, and 1.46 V, respectively. The overpotential (η) that delivered a current density (J) of 10 mA cm-2 was 0.43, 0.25, and 0.32 V for CoSe2, NiSe2, and IrO2, respectively, where η was defined as potential (vs. RHE) – 1.229 V. The chronoamperometric response demonstrated the


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slight anodic current attenuations of 5% and 0.3% within 6 h for CoSe2 and NiSe2, respectively (inset). The data was collected at a constant potential of 1.65 V, with intervals of 0.2 s. The results showed the higher OER catalytic activity of NiSe2 relative to CoSe2, with a higher stability. The kinetics of OER catalysis were examined using Tafel plots of η (V) vs. log J (mA cm-2), based on the Tafel equation, η = b log(J/J0) (Figure 2b). The Tafel slope (=b) and exchange current density (J0), obtained from the linear portion at the low-potential region, corresponding to activation-controlled current density regions. CoSe2, NiSe2, and IrO2 had b = 50, 38, and 53 mV dec-1 (dec = decade), respectively. The lower overpotential values and Tafel slopes (with higher exchange currents) highlighted better catalytic activity for NiSe2 than CoSe2. Although NiSe2 exhibited a lower exchange current than IrO2, known to be the best catalyst, it showed exceptionally high performance in terms of a higher current density and lower Tafel slope. To better understand the OER mechanism, in situ Raman spectroscopy measurements were conducted (discussed later). Table S1 (Supporting Information) summarizes the previous works that reported by other groups. The Tafel slope of NiSe2 was lower than NiSe (64 mV dec-1), as reported by Tang et al.27 In order to make further comparisons with the other works that were often performed in 0.1 M KOH, we obtained LSV data in 0.1 M KOH, as shown in Figure S2 (Supporting Information), and the results are summarized in Table 1. The Tafel slope of CoSe2 (67 mV dec-1 in 0.1 M KOH) was comparable with 51 and 66 mV dec-1, as reported by the Yu group.33,37 Electrocatalytic HER performances were assessed in both H2-saturated acid (0.5 M H2SO4) and H2-saturated base (1 M KOH). A standard calomel electrode (SCE) was used as the reference electrode in acidic solution. The LSV recorded for CoSe2 and NiSe2 in acid showed a


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small onset value (~0.1 V) (Figure 2c). The cathodic current rose rapidly under more negative potentials. The overpotential that delivered a current density of 10 mA cm-2 was 0.16 and 0.19 V for CoSe2 and NiSe2, respectively. As a reference point, we also performed measurements using a commercial Pt catalyst (20 wt% Pt on Vulcan carbon black), which exhibited high HER catalytic performance (onset occurs near zero potential). Correcting the raw data for iR losses revealed clearly more impressive performance of CoSe2 compared to that of NiSe2 (see Supporting Information, Figure S3). Linear portions of the Tafel plots at low-potential regions were fit to the Tafel equation, yielding Tafel slopes of about 30, 40, and 44 mV dec-1 for Pt/C, CoSe2, and NiSe2, respectively (Figure 2d). Generally, two mechanisms are involved in the HER process, namely the Volmer step (electrochemical hydrogen adsorption; H3O+ + e– → Hads), followed by either a Heyrovsky (electrochemical desorption; Hads + H3O+ + e– → H2 + H2O) or Tafel process (chemical desorption; Hads + Hads → H2). A Tafel slope of 120, 40, or 30 mV dec–1 would be expected for the Volmer, Heyrovsky, or Tafel steps, respectively, as the rate-determining step. Thus, the Heyrovsky process was the rate-determining step for HER for both CoSe2 and NiSe2. The sufficiently low η and b indicated they were near state-of-the-art non-noble catalysts. Comparison with previous work on the HER performance of CoSe2 and NiSe2 (Table S1, Supporting Information) indicated that our values were similar to the averages, 40 mV dec-1 for CoSe2 (48, 40.2, 41, 42.2, 31.2, 32, 48, and 39.6 mV dec-1) and 39 mV dec-1 for NiSe2 (56.9, 37.3, 31, and 31.1 mV dec–1). To assess their activity in strong basic solution, LSV curves were measured in 1 M KOH. The Tafel slopes of CoSe2 and NiSe2 became 126 and 139 mV dec–1, respectively, which are about the same as that (120 mV dec–1) of NiSe reported by Tang et al.27


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Therefore, both CoSe2 and NiSe2 NCs exhibit excellent HER activity comparable to that of the previous studies. To prove that CoSe2 and NiSe2 NCs were active and stable catalysts toward both OER and HER in 1 M KOH, we investigated whole water splitting in a two-electrode configuration using them as both anode and cathode (Figure 2e). The scan direction was from positive (0–2 V) to negative potential (back to 0 V) for OER and from negative (–0.5 V) to positive potential (back to 0 V) for HER. This water-splitting cell exhibited high performance, requiring a cell voltage of 1.6 V to afford the 10 mA cm–2 water-splitting current with vigorous gas evolution at both electrodes. Durability testing was then performed chronoamperometrically at 1.8 V (OER) and 0.18 V (HER), showing that the current attenuation is 0% and 6%, respectively, for 6 h (Figure 2f). The OER and HER catalytic activities of NiSe2 NCs were stable over long-term testing. The XRD pattern confirmed that the phase of NiSe2 NCs is unchanged, as shown in Figure S4 (Supporting Information). This stable catalytic performance in strong basic solutions was probably critical for realistic applications. We analyzed the electronic states of samples using X-ray photoelectron spectroscopy (XPS). Figure 3a shows fine-scanned Co 2p1/2 and 2p3/2 peaks for CoSe2, which are blue-shifted by 0.7 eV from neutral Co (metal Co0), at 793.3 and 778.3 eV, respectively. These peaks were asymmetric, and thus resolved into three C1-C3 bands using the Voigt function, with C1 at 779.0 (0.7), C2 at 780.6 (2.3), and C3 at 784.1 (5.8) eV for 2p3/2 peak. The values in parentheses represent the blue shift of each band from the neutral position. Referring to the data for CoO (2p3/2 peak at 780.4 eV), as shown in Figure S5 in Supporting Information, the C1 and C2 bands (with a ratio of 3:2) were assigned to the Co-Se and Co-O bonding structures of CoSe2, and native (amorphous) oxide layers at the surface, respectively. The C3 band corresponded to the


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shake-up satellite peak. The smaller blue shift of the Co-Se band indicated that the Co oxidation number was less than 2. Figure 3b shows the Ni 2p1/2 and 2p3/2 peaks of NiSe2, blue-shifted by 1.2 eV from neutral Ni at 870.0 and 852.7 eV, respectively. The 2p3/2 peak was resolved into three bands (N1-N3) using the Voigt function, with N1 at 853.9 (1.2), N2 at 856.1 (3.4), and N3 at 860.3 (7.6) eV. The values in parentheses represent the blue shift of each band. The areas of the N1 and N2 bands had a 1:1 ratio. The XPS data for NiO shows two peaks at 853.7 and 855.5 eV due to Ni2+ and Ni3+ ions, respectively (see Figure S3, Supporting Information). Therefore, the N1 band was assigned, not only to the Ni-Se bonding structure of NiSe2, but also the Ni-O bonding structures that possibly exist at surface. The N2 band most likely originated from Ni3+ ions in the surface oxide phase. Shake-up satellite peaks (N3) were observed on the 7.6 eV higher binding energy side. Figure 3c shows the Se 3d peaks of CoSe2 and NiSe2. The peaks for CoSe2 and NiSe2 at 54.8 eV (S1 band) are red-shifted by 0.8 eV from neutral (55.6 eV for 3d5/2). The Se peaks of CoSe2 and NiSe2 corresponded to Se anions that bind with Co and Ni cations, respectively. The weak peak at 59 eV (S2 band) was assigned to Se-O bonding structures at the surface. We performed X-ray absorption spectroscopy (XAS) measurements at the Co and Ni L2,3 edges to obtain the electronic structure of CoSe2 and NiSe2 underneath the oxide layers, taking advantage of the longer probe depth compared with XPS (Supporting Information, Figure S6). The data unveiled that Ni ions in NiSe2 had a similar oxidation number (+2) to those of NiO. CoSe2 contained Co ions that are consistently less oxidized than those in CoO. Based on the XPS and XAS data, we concluded that the Co ions in CoSe2 were more metallic than Ni ions in NiSe2.


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Using density functional theory calculations, Liu and Rodriguez predicted that Ni2P could be an excellent catalyst for HER. They suggested that proton-acceptor (P sites) and hydrideacceptor sites (Ni sites) work in a cooperative manner.53 The same model can be used to rationalize CoSe2 and NiSe2 studied here. Both Co (and Ni) and Se are active sites for HER, functioning as hydride-acceptor and proton-acceptor centers, respectively. The negative Se anions are favorable proton-acceptor sites and metal cations form the hydride-acceptor center. It was expected that Co ions in CoSe2, being more metallic than Ni ions in NiSe2, produced favorable hydride-acceptor sites. We also noted that CoSe2 NCs had a higher ratio for Co-Se/CoO (3:2) than that of Ni-Se/Ni-O (1:1) in NiSe2 NCs. Therefore, the lower amount of oxide phase at the surface could form more favorable hydride-acceptor centers, making CoSe2 a more active HER electrocatalyst. To understand the OER mechanism, we monitored the in situ Raman spectrum while the potential applied, as shown in Supporting Information, Figure S7. We identified the formation of metal oxyhydroxide (M-OOH) such as Co-OOH and Ni-OOH at voltages where the OER occurs. The XPS data already revealed the existence of the oxide outerlayers for the as-grown NCs. As the potential is applied, M2+→M3+ oxidation takes place at the outerlayers and the M3+ ions produce M-OOH, which is a crucial species for OER. In basic solution, the metal ions of CoSe2 and NiSe2 existed as metal hydroxide and oxidized to form the oxide layers under the applied positive potential. The oxide layers become actual OER catalytic sites, as suggested for NiSe and Ni5P4 by other groups.27,28 The NiSe2 (or CoSe2) underneath the oxide layers maintained the electrical conductivity between back electrode and the active oxide layers, and thus enhanced the catalytic activity.


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For OER, the Tafel slope (38 mV dec-1 in 1 M KOH) of NiSe2 implied that the ratedetermining step was a second electron-transfer reaction (ca. b = 39 mV dec-1), where one electrochemical pre-equilibrium step preceded a rate-determining electrochemical step. Therefore, the rate-determining electrochemical step would involve the reaction of OH– with an adsorbed O atom to form the OOH species (Ni-O + OH– → Ni-OOH + e–), with an electrochemical pre-equilibrium step (e.g., Ni-OH + OH– → Ni-O + e– + H2O or Ni-OH → Ni-O + H+ + e–). For CoSe2, b = 58 mV dec-1 was consistent with the value of Co3O4, suggesting a mechanism (corresponding to b = 59 mV dec-1) with an electrochemical pre-equilibrium step (similar as that of Ni-OH) that precedes a rate-determining chemical step (e.g., 2Co-O + H2O → Co-OOH + Co-OH).54 The Markovic group reported that the OER reactivity of hydroxide monolayers of first-row transition metals (M) followed the order Fe < Co < Ni, as governed by the M-OH bond strength.10 The Boettcher group also found a similar trend of Co < Ni for metal oxide thin films.55 Therefore, the higher catalytic activity of NiSe2 was ascribed to the Ni oxide or hydroxide layer being more active than the Co equivalent. In addition, the higher population of Ni-O bonding structures in NiSe2 favors the formation of the Ni-OOH intermediate, compared with the lower population of Co-O bonding structures in CoSe2. Further studies may be needed to clarify changes in the electrode surface. Nevertheless, we suggest that the coexistence of NiSe2 (or CoSe2) and amorphous oxide phases is responsible for the excellent bifunctional electrocatalytic powers. Now the NiSe2 NCs have been utilized in water-splitting Si NW-based photoelectrodes as follows. The detailed experimental procedure for the preparation of the electrode is described in detail in the Supporting Information. Briefly, Si NWs were fabricated using a Ag metal-assisted


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etching method, and NiSe2 NCs were synthesized using Ni(NO3)2 solution-coating and subsequent chemical vapor deposition of Se powders at 300 °C. Figure 4a shows typical SEM images of NiSe2 NCs deposited on the vertically aligned Si NW array, with a Si NW length of 7 µm. Figure 4b shows the corresponding HRTEM images. NiSe2 NCs deposited homogenously on the Si NW had an average size of 20 nm. The lattice-resolved TEM and corresponding FFT images for NiSe2 NCs showed the single-crystalline nature of individual NCs. The adjacent (211) and (210) planes were separated by 2.4 and 2.7 Å, respectively. EDX mapping confirmed the presence of Ni and Se elements in NCs and Si for NWs (Figure 4c). The ratio of Ni to Se was about 1:2. The XRD pattern showed a pyrite cubic phase NiSe2 (Supporting Information, Figure S1). Figure 5a shows the current density (mA cm-2) vs. applied potential (vs. RHE) curves of the PEC fabricated using the Si-NiSe2 NW array as the photoanodes of water splitting PEC cell in 1 M KOH electrolyte under Xe lamp illumination with 100 mW cm-2 AM1.5G. The electrodes showed photo-responses upon the linear voltage sweep between 0.6 and 2.5 V. Photocurrent curves were also measured under chopped illumination with a time interval of 2 s. Remarkable photo-response was observed from on/off light cycles and the current level increased quickly under irradiation. After extended cycles, the photocurrent could still be changed distinctly by repeatedly turning the light on/off, demonstrating that the photoanode has excellent reversible and stable activity in OER. The dark current was very low (10–7 A cm-2). The onset potential or open-circuit voltage (VOC) was estimated to be 0.7 V. The photocurrent density at water oxidation potential E0 (vertical dotted line) of 1.23 V was about 5.8 mA cm-2 (corresponding to short-circuit current, JSC). The data from only bare Si NW arrays was also measured, but no obvious photo-response could be seen in this potential range. The photocurrent onset (0.7 V)


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shifted to significantly more negative potentials than the electrocatalytic OER of NiSe2 (1.43 V). Reduction of the required applied potential stemmed from the photovoltage (=0.67 V) generated by the Si-NiSe2 NW photoanode. Figure 5b shows the chronoamperometric photo-response data upon 2s interval on/off irradiation cycles at a constant applied potential of 1.23 V. Few current spikes were observed upon turning the light on/off, indicating that surface recombination was negligible. The chronoamperometric photo-response for 13 h demonstrated the stability of the photoanode; an anodic current attenuation was only 6% (Figure 5c). HRTEM images and XPS revealed the gradual degradation of NiSe2 into its oxide form during photocatalytic water spitting, which was responsible for photocurrent reduction (Figures S8 and S9, Supporting Information). Photon-to-oxygen conversion efficiency (η) was calculated using the following equation:

η=

, where J is photocurrent density, VOC is the onset potential of OER, E0 is the

water oxidation potential (=1.23 V vs. RHE), FF is the fill factor, and Iph is the incident photon density (=100 mW cm–2 in this work). The efficiency, calculated from the measurement in Figure 5a, was 0.8%, where J = 5.8 mA cm-2 (at E0), VOC = 0.7 V, and FF = 0.26. The notably low VOC value of Si-NiSe2 NW photoanodes promised a high-performance photoelectrode for PEC devices. The good catalytic activity of NiSe2 NCs would contribute to increasing the photovoltage and conversion efficiency. The photocatalysis principle of photoanodes has been suggested as follows: as soon as light absorption and photocarrier generation occur within the Si NW (presumably an electron in the valence band jumped into conduction band), the active Ni oxide layers of NiSe2 NCs capture the holes by oxidizing OH– to generate O2. In summary, CoSe2 and NiSe2 NCs (average size = 20 nm) were synthesized using a novel photo-induced cation-exchange reaction of GeSe2 NCs in aqueous solution. They were suitable


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as bifunctional electrocatalysts for both OER and HER. NiSe2 exhibited superior catalytic power in OER, with b = 38 mV dec-1 in 1 M KOH. In contrast, CoSe2 were less efficient in the OER (b = 50 mV dec-1). For HER, CoSe2 exhibited higher electrocatalytic activity than NiSe2 (b = 40 and 44 mV dec-1 in 0.5 M H2SO4, and 126 and 139 mV dec-1 in 1 M KOH, respectively). They both have excellent stability in strong acidic and basic environments. XPS and XAS data showed that the Co ions of CoSe2 were more metallic than the Ni ions of NiSe2, which was responsible for their higher HER catalytic power. In situ Raman spectroscopy provided evidence for oxidation to Co-OOH and Ni-OOH, suggesting that the Co (or Ni) oxide layers were the actual OER catalytic sites and that NiSe2 (or CoSe2) underneath the oxide layers maintained the electrical conductivity, thus enhancing the catalytic activity. The higher OER catalytic activity of NiSe2 was attributed to the oxide layer, which was more active than that of CoSe2. We also showed that NiSe2 NCs could function as OER catalysts in a Si NW-based photoanode in the water-splitting PEC. The Si-NiSe2 NW array exhibited a remarkably low onset potential (0.7 V vs. RHE) and high durability under 100 mW cm-2 AM1.5G irradiation. Considering the abundance of the materials and the active, stable catalytic power, NiSe2 NCs are attractive candidates for practical solar fuel production.

ASSOCIATED CONTENT Supporting Information. Experimental details, Table S1, Figure S1-S9. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author


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*[email protected] ACKNOWLEDGMENT This study was supported by NRF (20110020090; 2014R1A6A1030732; 2009-0082580). The HVEM (Daejeon) and XPS (Pusan) measurements were performed at the KBSI. The experiments at the PLS were partially supported by MOST and POSTECH.

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Table 1. Experimental overpotential and Tafel plot parameters of CoSe2 and NiSe2 NCs for OER and HER. electrolyte OER

HER

η (V)b b (mV dec-1)c J0 (µA cm-2)c η (V) b (mV dec-1) J0 (µA cm-2) η (V) b (mV dec-1) J0 (mA cm-2)

1 M KOH

0.1 M KOH

0.5 M H2SO4

η (V) -1

b (mV dec ) J0 (mA cm-2)

1 M KOH

CoSe2 0.43 50 1.9 0.51 67 1.4 0.16 40

NiSe2 0.25 38 5.1 0.41 50 1.6 0.19 44

5.7×10-2 0.52

2.0×10-3 0.54

126 8.6×10-3

139 5.0×10-3

Referencea 0.32 53 45.8 0.47 72 26.3 0.015 30 3.23 0.090 53 0.87

a

References for OER and HER are IrO2 and Pt/C, respectively. bOverpotential at J = 10 mA cm-2. Tafel equation: η = b log(J/J0), where η is the overpotential (measured), defined as E (vs. RHE) − 1.229 V for OER and E (vs. RHE) for HER; b is the Tafel slope (mV dec-1); J is the current density (measured); J0 is the exchange current density. c


24

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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


Figures

Figure 1. HRTEM and corresponding FFT images of (a) CoSe2 and (b) NiSe2 NCs. The distances between the adjacent planes were close to those of the corresponding bulk phase. EDX spectrum and mapping of CoSe2 and NiSe2 NCs (with STEM images) show Co:Se = 1:2 and Ni:Se = 1:2.


25


0

CoSe2

50

NiSe2

-10

40

IrO2

-20

NiSe2

-30

10

1

2

3

4

5

-50

6

Time (h) 0 1.0

1.2

1.4

1.6

1.8

-60 -0.8

-0.6

Potential vs. RHE (V)

ec Vd m 0 =5

Se 2 Co

b

b

0.20 -1.0

NiS

3m =5

-1

ec Vd

0.5 -2

log J (mA cm )

0.0

0.1

CoSe 2 (0.5

0.0 1.0

1M

H KO

e CoS 2

-0.5

0.0

)

) KOH (1 M

) KOH (1 M Pt/C SO ) Pt/C (0.5 M H 2 4

M H 2SO 4)

0.5

1.0

0.5

1.0

1.5

2.0


) H SO 4 (0.5 M 2 NiSe 2

0.2

2

0.0

e ( N iS 2

0.3

-1 IrO 2 dec V m 8 3 e b=

-0.5

-0.5 50

0.4

OER o E (O2/H2O)

Pt/C IrO2

-90

0.0

0.5 (d) -1

0.30

NiSe2

-60


(b)

0.25

KO H

-0.2

CoSe2

-30

-2

0.35

-0.4

HER 0 E (H2/H2O)

0

Current Density (mA cm )

0 0

1 M KOH

-40

CoSe2

(1 M

20

Pt/ C

20

(e)

30

0.5 M H2SO4

Pt/C

NiSe2

40

60

CoSe2

)

30

(c)

Pt/C (0.5 M H SO ) 2 4

(a)

-2

Current Density (mA cm )

60

η (V)

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 26 of 30

1.5

-2

log J (mA cm )

(f) NiSe2

40

OER

30 0

HER

-10

-20

0

1

2

3

4

5

6

Time (h)

Figure 2. (a) LSV curves of CoSe2, NiSe2, and IrO2 on an RDE (1600 rpm) for OER in an O2saturated 1 M KOH solution (scan rate: 2 mV s–1), and corresponding (b) Tafel plots. The potentials were referenced to the RHE. Inset of (a) shows the chronoamperometric response (current density vs. time) for CoSe2 and NiSe2 at a constant potential of 1.65 V. (c) LSV curves for CoSe2, NiSe2, and high-quality commercial Pt/C as catalysts for HER in 0.5 M H2SO4 and 1 M KOH (scan rate: 2 mV s–1). (d) Tafel plots derived from LSV curves for HER. (e) LSV curves of CoSe2, NiSe2, Pt/C, and IrO2 for water splitting (in 1 M KOH) in a two-electrode configuration with a scan rate of 2 mV s–1. (f) Chronoamperometric responses recorded on NiSe2 at a constant applied potential of 1.8 V (for OER) and –0.18 V (for HER). Catalyst loading was 1 mg cm–2 for all samples.


26

Page 27 of 30

(a) Co 2p (CoSe2)

0

Co

2p3/2

(c) Se 3d 3d5/2 Se0

2p1/2 0

Intensity (arb. units)

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


Co

CoSe2 C3 C2

800

795

790

785

C1

S2

780

775 0

(b) Ni 2p (NiSe2)

Ni 2p3/2

2p1/2

NiSe2

0

Ni

N3 N2 885

880

875

870

S1

865

860

N1

855

S2 850

60

C1

S1 55

Binding Energy (eV)

Figure 3. Fine-scanned XPS of (a) Co and (b) Ni 2p peaks, and (c) the Se 3d peak for CoSe2 and NiSe2. The data points (open circles) are fitted by Voigt functions, and the sum of the resolved bands is represented with black lines. The position of the neutral peak (Co0, Ni0, and Se0) is marked by a dotted line to delineate the blue shift.


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Page 28 of 30

Figure 4. (a) SEM micrograph of NiSe2 NCs deposited on the vertically aligned Si NW array. (b) HRTEM and its corresponding FFT images showing NiSe2 NCs attached to the Si NW. The adjacent (211) and (210) planes of NiSe2 are separated by 2.4 and 2.7 Å, respectively. (c) EDX mapping confirms the presence of Ni and Se elements in the NCs, and Si in the NW.


28

-2

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


Current Denisity (mA cm )

Page 29 of 30

10

(a)

6

Light

(b)

4

8

2 0

0

6

E

0

10

20

30

40

50

60

Time (s) 4

6

(c)

4

2

VOC

2

Dark

0

0

1.0

1.5

2.0

2.5 0

2


4

6

8

10 12

Time (h)

Figure 5. (a) Current density (mA cm-2) vs. potential (vs. RHE) for Si-NiSe2 NW array photoanode measured in 1 M KOH electrolyte, under 100 mW cm–2 AM1.5G irradiation. Scan rate is 20 mV s–1. A schematic diagram for the water splitting PEC is shown in the inset. Chronoamperometric photo-responses for (b) on/off irradiation cycles and (c) 13 h irradiation, recorded at a constant applied potential of 1.23 V (E0).


29


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Table of Contents Graphic


30

CoSe2 and NiSe2 Nanocrystals as Superior Bifunctional Catalysts for

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