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Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Hydrogen Evolution Enhancement over a Cobalt-Based Schottky Interface Hao-Zheng Yu,†,○ Yong Wang,†,○ Jie Ying,*,§,⊥ Si-Ming Wu,† Yi Lu,† Jie Hu,† Ji-Song Hu,# Ling Shen,† Yu-Xuan Xiao,† Wei Geng,† Gang-Gang Chang,‡ Christoph Janiak,⊥ Wei-Hua Li,§ and Xiao-Yu Yang*,†,∥,∇

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State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, and School of Materials Science & Engineering and ‡School of Chemistry, Chemical Engineering and Life Science, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, China § School of Chemical Engineering and Technology, Sun Yat-sen University, Zhuhai 519082, China ∥ School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, United States ⊥ Institut für Anorganische Chemie und Strukturchemie, Heinrich-Heine-Universität Düsseldorf, 40204 Düsseldorf, Germany # School of Science, Hubei University of Technology, Wuhan 430068, China ∇ Southern Laboratory of Ocean Science and Engineering (Guangdong, Zhuhai), Zhuhai 519000, China S Supporting Information *

ABSTRACT: A proof-of-concept strategy for significant enhancement of hydrogen evolution reaction (HER) performance of transition metals via construction of a metal/ semiconductor Schottky junction is presented. The decoration of low-cost commercial TiO2 nanoparticles on the surface of microscale Co dendrites causes a significant charge transfer across the Co/TiO2 Schottky interface and enhances the local electron density at the Co surface, confirmed by X-ray photoelectron spectroscopy results and density functional theory calculations. The Co/TiO2 Schottky catalyst displays superior HER activity with a turnover frequency of 0.052 s−1 and an exchange current density of 79 μA cm−2, which are about 4.3 and 4.0 times greater than that of pristine Co, respectively. Moreover, the Co/TiO2 Schottky catalyst displays excellent electrochemical durability for long-term operation in both alkaline solution and high saline solution. Theoretical calculations suggest that the Schottky junction plays an important role to optimize hydrogen adsorption free energy (ΔGH*) by tuning the electronic structure, which enhances the performance for HER of the Co/TiO2 Schottky catalyst. This study of modulating the electronic structure of the catalysts via the Schottky junction could provide valuable insights for designing and synthesizing low-cost, high-performance electrocatalysts. KEYWORDS: Schottky junction, hydrogen evolution reaction, electronic structure, DFT, cobalt-based electrocatalysts

1. INTRODUCTION Hydrogen with high gravimetric energy density has attracted great interest as a renewable, clean, and carbon-neutral energy source.1−4 Electrochemical water splitting in alkaline media is pursued as one of the most feasible and environmental-friendly technologies to prevent the acid-fog contamination and produce high-purity hydrogen.5 However, the requirement of high overpotential for hydrogen evolution reaction (HER) is the main challenge of electrochemical water splitting in alkaline media.6 Pt-based electrocatalysts, as the currently most efficient catalysts for HER, still meet many critical problems, such as the high-cost and limited resource of Pt, that immensely impede their large-scale implementation.1,7−9 Non-Pt materials, such as transition metals,10,11 phosphides,12,13 sulfides,14,15 nitrides,16,17 oxides,18,19 and carbon materials,20,21 have therefore been investigated. Among them, transition-metal-based materials have received a lot of © XXXX American Chemical Society

attention as very important alternative catalysts due to their abundance, low cost, low toxicity, and abundant redox chemistry.22 It is therefore of technical importance to develop transition-metal-based electrocatalysts with high HER activity and durability in alkaline media. The surface electronic structure, as the nature of transition metals of HER electrocatalysts, needs to be an electron donor for producing hydrogen. There have been many successful attempts to optimize the electronic structure, such as nanoscaled size effects,23−25 alloying strain effects,26−28 shape facet effects,29−31 heteroatom doping effects,32−34 and support hybrid effects.35−38 Note that the modulation of the interface junction has been a very promising approach to a highReceived: February 22, 2019 Accepted: June 6, 2019 Published: June 6, 2019 A

DOI: 10.1021/acsami.9b03368 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces performance design of photoelectromaterials.39−43 Few examples on the Schottky metal−semiconductor junction have been reported to enhance the surface charge density of interfacial metals for improving the catalytic activity, such as metal-like nanomaterials/C3N444 and metal/N-doped C.45 However, it is still rare for the transition-metal-based Schottky catalysts to achieve a significant enhancement of HER performance. Cobalt has emerged in the last decade as the most universal transition metal for the development of hydrogen-evolving catalysts,46−48 while TiO2 is one of the most attractive semiconductors for photo-electrotechnologies.49,50 Hierarchically fractal Co with a combination of microstability and nanoactivity51 are therefore chosen to be integrated with nanosized commercial TiO2 for the construction of a Schottky junction. Herein, we present a proof-of-concept strategy to remarkably enhance the HER activity of cobalt in alkaline media by a Schottky junction with commercial TiO2. Tuning of the surface electronic structure of Co via the Schottky junction has been proven by both experimental results and theoretical calculations. More importantly, our strategy can be easily explored to largely improve the HER activity of various Schottky catalysts constructed by transition metals and semiconductors, such as Ni/TiO2 and Co/MoS2.

the inductively coupled plasma-atomic emission spectrometry (ICPAES, Prodigy 7, Leeman Labs Inc). 2.5. Electrochemical Measurements. All electrochemical experiments were conducted using an electrochemical station (Autolab PGSTAT302N) with a three-electrode system. A glassy carbon (GC) electrode, standard Hg/HgO electrode, and graphite rod were used as the working, reference, and counter electrode, respectively. The electrolytes (1.0 M KOH) were deaerated by continuous N2 bubbles through the experiments. Typically, 10 mg of catalyst and 30 μL of 5 wt % Nafion were dispersed in 970 μL of isopropanol and ultrasonicated for 30 min. Then, 10 μL of the ink was added dropwise onto the GC electrode (5 mm diameter) and dried at ambient conditions. For the HER test, the linear sweep voltammetry (LSV) used a scan rate of 5 mV s−1. Electrochemical impedance spectroscopy (EIS) data were collected at −0.2 V vs reversible hydrogen electrode (RHE) from 100 kHz to 0.1 Hz with an AC amplitude of 10 mV. The accelerated degradation tests (ADTs) were measured by performing cyclic potential sweeps between 0.07 and −0.47 V vs RHE with 100 mV s−1 sweep rate for 2000 cycles. The chronopotentiometric measurements were conducted for 10 and 6 h at 10 mA cm−2 current density in an alkaline solution (1.0 M KOH) and a highly saline solution (3.0 g L−1 NaCl and 1.0 M KOH solution), respectively. 2.6. Density Functional Theory (DFT) Calculation. DFT calculation was conducted using the Cambridge Sequential Total Energy Package (CASTEP) code in the Materials Studios package.15,54,55 The lattice parameters of the crystal were optimized as a = b = 2.489 Å, c = 4.013 Å for Co, in good line with experimental values of a = b = 2.503 Å, c = 4.061 Å (ICDD no. 05-0727 for Co). For anatase TiO2, the lattice parameters of the crystal were optimized as a = b = 3.802 Å, c = 9.755 Å, in good line with the experimental values of a = b = 3.784 Å, c = 9.515 Å (ICDD no. 71-1166 for TiO2). There is a small rutile in the commercial TiO2 nanoparticles (Figure S1b). The Co (002) and TiO2 (101) plane, as the dominant planes for these materials, were modeled by a two-layer plate with reduplicated 6 × 3√3 and 3 × √14 surface space cell at a 15 Å vacuum region between the plates along the z-axis. The convergence tolerances for energy, maximum displacement, and maximum force were 5.0 × 10−5 eV per atom, 5.0 × 10−3 Å, and 0.1 eV Å−1, respectively. While optimizing the geometry, the surface atoms in the topmost layer and adsorbates were relaxed, but the atoms in the bottom layer were constrained. The hydrogen adsorption free energy (ΔGH*) is calculated as follows:55 ΔGH* = E(Cat*H) − E(Cat) − 1/2E(H2). Cat represents the catalyst. E(Cat*H) represent the total energy of the catalyst plus the adsorbed hydrogen atom. E(Cat) and E(H2) represent the total energies of the catalyst and gas H2, respectively.

2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Cobalt chloride hexahydrate (CoCl2·6H2O), nickel chloride hexahydrate (NiCl2·6H2O), sodium hydroxide (NaOH), commercial TiO2 nanoparticles (particle size around 25 nm), hydrazine hydrate (N2H4·H2O), N,N-dimethylformamide (DMF), ammonium tetrathiomolybdate ((NH4)2MoS4), and absolute ethanol (EtOH, AR) were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemical reagents were used without any treatment. All deionized water used was purified by a Wuhan pinguan instrument water purification system (18.25 MΩ·cm resistivity). 2.2. Synthesis of Co/TiO2. Co/TiO2 was prepared in our laboratory according to a previous report.52 In a typical synthetic procedure, 238 mg of CoCl2·6H2O, 5.0 mg of commercial TiO2, 50 mg of NaOH, and 1.5 mL of N2H4·H2O were dispersed in 10 mL of deionized water by constantly stirring for 30 min. Then, the solution was put in an autoclave at 200 °C for 1.5 h. The black solid sample was washed with deionized water and EtOH for several times and collected by centrifugal separation at 3000 rpm for 1 min. Finally, the sample was dried by vacuum freeze-drying equipment for 12 h, and then Co/TiO2 was obtained. 2.3. Synthesis of Co, Co + TiO2, Ni/TiO2, and Co/MoS2. Co metal particles were synthesized using the same synthetic procedure except in the absence of commercial TiO2. Co + TiO2 was prepared as a physical mixture of 98.75 wt % Co metal particles and 1.25 wt % commercial TiO2. Ni/TiO2 was synthesized by the same procedure as Co/TiO2 by the substitution of CoCl2·6H2O. Co/MoS2 was synthesized according to a previous report.53 Typically, 30 mg of the above Co metal particles and 10 mg of (NH4)2MoS4 were dispersed in 30 mL of DMF by stirring for 0.5 h. Then, the mixture was put into an autoclave at 210 °C for 20 h. The products were washed with deionized water and EtOH for three times and collected by centrifugation at 3000 rpm for 1 min; then, the obtained sample was dried by vacuum freeze-drying equipment for 12 h. 2.4. Characterization. The crystal structures were examined by powder X-ray diffraction (XRD; D8 Advance). The morphologies were investigated by scanning electron microscope (SEM; Hitachi S4800) and transmission electron microscope (TEM; JEOL-2100F). The elemental mapping was obtained by energy-dispersive X-ray spectroscopy (EDS) attached to the SEM instrument. The chemical compositions were examined by X-ray photoelectron spectroscopy (XPS; PHI Quantera II). The elemental analysis was performed by

3. RESULTS AND DISCUSSION A schematic illustration of the synthesis and hydrogen evolution procedure of the Co/TiO2 Schottky catalyst is shown in Figure 1a. Hierarchically fractal Co/TiO2 was obtained by a one-pot hydrothermal synthesis of Co-metal dendrites in the presence of commercial TiO2 nanoparticles (Figure S1). Due to the different Fermi levels between metal and semiconductor, the electron charge will flow from TiO2 to Co at the contacted interface, which would increase the electron density of Co, further resulting in the enhancement of the HER performance. The morphology of Co/TiO2 is shown in Figures 1b and S2. Co/TiO2 displays a dendrite-like fractal structure with the size of about 15 μm assembled from uniform rods with an average diameter of about 300 nm and the lengths of about 3 μm. To confirm the distributions of commercial TiO2 nanoparticles, EDS element mapping was conducted as shown in Figure 1c− e, which verifies that Co/TiO2 is composed of Co, Ti, and O elements and shows the uniform Ti coverage of Co surface, indicating the highly dispersed TiO2 nanoparticles over the B


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size and low content (the mass fraction of TiO2 is 1.25 wt % in Co/TiO2 from ICP-AES measurement result in Table S1). The electrocatalytic activities of catalysts for HER were evaluated by using a three-electrode system in a N2-saturated 1.0 M KOH solution (see Experimental Section for details). The polarization curves of Co/TiO2 with different TiO2 mass ratios are displayed in Figure S5. When the mass fraction of TiO2 in Co/TiO2 is 1.25 wt %, Co/TiO2 can reach the highest HER activity. Besides Co/TiO2, Co + TiO2 (physical mixture of Co metal and commercial TiO2 with 1.25 wt % mass fraction of TiO2), Co metal particles (prepared by using the same procedure but in the absence of commercial TiO2), and commercial TiO2 were also investigated as control samples. As shown in Figure 2a, Co/TiO2 exhibits a significantly improved

Figure 1. (a) Schematic illustration of the synthesis and hydrogen evolution procedure of Co/TiO2 Schottky catalyst. (b−e) SEM and the corresponding EDS element mapping images of dendritic Co/ TiO2. (f) TEM image of dendritic Co/TiO2, the inset in (f) is a highmagnification image of the surface of dendritic Co/TiO2. (g) HRTEM image of TiO2 particle at the Co/TiO2 surface. Region I in (g) shows the interface between TiO2 and Co. (h) HR-TEM image of region I. Region II in (h) is a crystal fusion region between TiO2 and Co. Inset is the inverse FFT of the interface between TiO2 and Co as indicated in region II. The white, orange, and blue dots represent TiO2, nanofusion phase, and Co, respectively. The original inverse FFT image and interpreting atomic models are given in Figure S3.

whole microscale Co dendrites and their close mutual contacts. The TEM image of Co/TiO2 confirms the fractal structure (Figure 1f) and shows that the surface of the dendrites is covered with nanoparticles with the same size of pristine TiO2 nanoparticles (inset of Figure 1f), which can be possibly attributed to TiO2. The high-resolution TEM images shown in Figure 1g,h present the interfacial structure between the microscale dendrite and its surface nanoparticles. The obvious lattice fringes interlaced with the interplanar distances of 0.355 and 0.202 nm can be assigned to the (101) and (002) planes of the anatase TiO2 and the hexagonal Co, respectively, demonstrating these surface nanoparticles as TiO2. More importantly, the inverse fast Fourier transform (FFT) image shown in region II of Figure 1h exhibits a clear contacted interface with nanofusions between TiO2 and Co (inset of Figures 1h and S3), indicating the formation of the strong interaction at the metal−semiconductor interface, demonstrating the formation of Schottky junction. The structure of Co/TiO2 is characterized by XRD. Four obvious peaks at 41.7, 44.8, 47.6, and 75.9° can be assigned to (100), (002), (101), and (110) planes of the hexagonal Co (ICDD no. 05-0727) (Figure S4). No characteristic peak for TiO2 can be found due to their small

Figure 2. (a) LSV curves of Co/TiO2, Co + TiO2, Co, and TiO2, respectively. The inset is the overpotential at a current density of 10 mA cm−2. (b) Tafel plots derived from the LSV curves. (c) J0 derived from the Tafel plots. (d) Cdl obtained from the plots of ΔJ versus scan rates. (e) Nyquist plots at an overpotential of 200 mV. (f) LSV curves of Co/TiO2 before and after 2000 CV cycles; the inset is the chronopotentiometric tests of Co/TiO2 at a current density of 10 mA cm−2 for 10 h in 1.0 M KOH solution.

current density at the same operating potential and much lower onset potential in comparison with Co + TiO2, Co, and TiO2. At the current density of 10 mA cm−2, Co/TiO2 displays an overpotential of 229 mV, which is much more positive than that of Co + TiO2 (299 mV), Co (356 mV), and TiO2 (462 mV). Notably, the HER activity of Co/TiO2 is also superior to that of the previously reported Co-based electrocatalysts (Table S2). To gain insights into the mechanism in the reaction process, the Tafel slope of Co/TiO2 is derived to be 88 mV dec−1, which is lower than that of Co + TiO2 (128 mV dec−1), Co (135 mV dec−1), and TiO2 (141 mV dec−1). The Tafel slope of 88 mV dec−1 reveals that the rate-limiting step for the HER process in Co/TiO2 is the Heyrovsky process, i.e., the Volmer−Heyrovsky mechanism.4,44 As shown in Figure 2b, C


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ACS Applied Materials & Interfaces the exchange current density (J0) of Co/TiO2 is 79 μA cm−2, which is much higher than other Co + TiO2 (38 μA cm−2), Co (20 μA cm−2), and TiO2 (16 μA cm−2), demonstrating the large catalytically active surface area and fast electron transfer.4,56 The electrochemically active surface area was also collected by measuring the CV curves in the potential range from −0.02 to 0.08 V vs RHE (Figure S6). The electrochemical double-layer capacitance (Cdl) of Co/TiO2 is 787 μF cm−2, which is much higher than that of Co + TiO2 (326 μF cm−2), Co (148 μF cm−2), and TiO2 (47 μF cm−2). The large values of Cdl suggest a high exposure of active sites for the outstanding HER activity.4,55 At 400 mV overpotential, the turnover frequency (Table S3) for Co/TiO2 is 0.052 s−1, which is about 2.3, 4.3, and 17.3 times greater than that of Co + TiO2 (0.023 s−1), Co (0.012 s−1), and TiO2 (0.003 s−1), respectively. Furthermore, the EIS measurements were conducted to investigate the intrinsic charge-transfer kinetics of these four catalysts. Figure 2e displays the Nyquist plots of Co/TiO2, Co + TiO2, Co, and TiO2. All Nyquist plots show an arc or semicircles in the frequency region from 100 kHz to 0.1 Hz at an overpotential of 200 mV vs RHE. The fitted charge-transfer resistance (RCT) values for Co/TiO2, Co + TiO2, Co, and TiO2 are 82, 131, 322, and 1780 Ω, respectively (Table S3). Co/TiO2 shows the smallest RCT value among these four catalysts, revealing the excellent charge-transfer kinetics at the interface of the Co/TiO2 Schottky junction for HER catalysis.44 ADTs and chronopotentiometric measurements were conducted to study the long-term stability of Co/TiO2 under HER conditions. The ADTs of Co/TiO2 were conducted by performing 2000 potential cycles sweeping between +0.07 and −0.47 V (vs RHE) in 1.0 M KOH aqueous solution. It is found that Co/TiO2 displays a small increased overpotential of 39 mV at the current density of 10 mA cm−2 after ADTs. Moreover, the overpotential exhibits only a negligible change after 10 h at the current density of 10 mA cm−2 in 1.0 M KOH solution, further indicating its outstanding electrochemical long-term durability. The morphology of Co/ TiO2 remained intact after 10 h of chronopotentiometric measurements (Figure S7), demonstrating their superior structural stability. In addition, the stabilities of Co/TiO2 and pristine Co in highly saline solution (3.0 g L−1 NaCl and 1.0 M KOH solution) were also compared. Co/TiO2 exhibited an increase of only 8 mV in the overpotential at the current density of 10 mA cm−2 (Figure S8), which is much lower than that of pristine Co (64 mV increase), demonstrating their potential application of splitting seawater in hydrogen production. To investigate the electronic structure of metal and semiconductor before and after the formation of Schottky junction, XPS was conducted (Figures 3a,b and S9). Compared to that of pristine Co, the Co 2p3/2 binding energy of Co/TiO2 shows an obvious negative shift of 0.7 eV, demonstrating that the surface cobalt atoms are electron collectors in this Schottky catalyst, which increases the electron density of Co and, in turn, enhances the HER activity. This result demonstrates that the electron charge has been transferred from TiO2 to Co via the metal/semiconductor interface because of their different Fermi levels, namely, the Schottky effect.57 Moreover, the binding energy of Ti 2p3/2 in Co/TiO2 shows an obvious positive shift in comparison to that of pristine TiO2, indicating again the formation of the Schottky junction. To further support the change in electronic structure

Figure 3. (a) XPS spectra of Co 2p of Co/TiO2 and Co. (b) XPS spectra of Ti 2p of Co/TiO2 and TiO2. (c) DOS plots of d-orbital contribution to Co/TiO2 and Co. (d) DOS plots of total orbital contribution to Co/TiO2 and Co.

before and after the Schottky contact, the density-of-state (DOS) plots were calculated by DFT. Considering that Co(002) and TiO2(101) are the dominant facets in Co/TiO2 as determined by both TEM and XRD (Figures 1h, S1b and S4), we thus chose these facets in the DFT calculations. Figure S10 shows the periodic slab model of Co(002)/TiO2(101), individual Co(002), and TiO2(101), respectively. As shown in Figure 3c, the d-band centers have been found to be a negative shift from −1.16 eV (Co) to −1.38 eV (Co/TiO2) after the Schottky contact, suggesting the weaker chemical adsorption capability of Co/TiO2 Schottky catalyst, which is favorable for hydrogen desorption.15 Furthermore, the calculated DOS plots of total orbital shows that electron density of Co/TiO2 is higher than that of Co at the Fermi level (Figure 3d), indicating the improved electrical conductivity of Co/TiO2 due to the Schottky effect.15,58 We thus chose these facets in the DFT calculations. The charge density difference at the Co/TiO2 Schottky junction was calculated as shown in Figure 4a−c. The green region and the yellow region represent the charge accumulation and charge depletion, respectively. Charge redistribution occurred at the Co(002)/TiO2(101) interface region due to the Schottky effect between Co and TiO2. Figure 4d exhibits a planar-averaged charge density difference on the Co/TiO2 interface. The positive value and the negative value denote electron accumulation and electron depletion, respectively. The change in the interfaces indicates that the electrons are expectedly transferred from the TiO2 side to the Co side across the interface, and the net charge accumulation led to form a built-in electric field at the interface, which is the Schottky junction, to increase the charge separation and transportation, and subsequently promote HER.22,41,44 The change of the surface electronic structure of the catalyst can efficiently optimize ΔGH*, which has a well-established correlation to HER performance.59 The more negative value of ΔGH* suggests a stronger binding of hydrogen. The HER activity is higher when ΔGH* is closer to 0 eV. As shown in Figure 4e, ΔGH* of Co/TiO2 is calculated to −0.38 eV, which is much closer to 0 eV in comparison with Co (−0.65 eV) and TiO2 (−0.81 eV). Thus, compared with ΔGH* for the reaction at the Co and TiO2 surface, the Co/TiO2 Schottky catalyst exhibits a D


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electronic properties of Co/TiO2 and increase the local electron density at the Co surface. This appears to achieve a weaker, hence optimized, hydrogen adsorption free energy, and accordingly increased HER activity. Moreover, our strategy can be easily extended to the synthesis of other metal/semiconductor Schottky catalysts (Ni/TiO2 and Co/MoS2) with largely improved HER activity. Our study opens up the general principle to efficiently design Schottky catalysts between a transition metal and a semiconductor with excellent catalytic performance.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b03368. Figures including SEM images, XRD patterns, inverse FFT image, interpretating atomic models, LSV curves, CV curves, XPS spectrum, periodic slab models, stability test, and tables (PDF)

Figure 4. (a) Top view, (b) side view (a axis), and (c) front view (b axis) of the charge density difference for Co/TiO2 Schottky catalyst. (d) Planar-averaged electron density difference Δρ(z) of Co/TiO2 Schottky catalyst. The green and yellow areas indicate electron accumulation and depletion, respectively. (e) ΔGH* of the Co/TiO2 interface, Co surface, and TiO2 surface, respectively.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected], [email protected] (J.Y.). *E-mail: [email protected], [email protected] (X.Y.Y.).

weaker, more favorable hydrogen atom binding on the surface of the catalyst and thus can promote HER. To further apply this facile strategy of the Schottky junction construction to other commonly used metal−semiconductor combinations, we obtained Ni/TiO2 and Co/MoS2 Schottky catalysts (Figures S11 and S12) under similar fabrication process as that for Co/TiO2. In comparison with the original metal, the overpotentials of Ni/TiO2 and Co/MoS2 at the current density of 10 mA cm−2 are dramatically decreased by 55 and 162 mV, respectively (Figure 5). In a word, introducing

ORCID

Gang-Gang Chang: 0000-0002-8085-6575 Christoph Janiak: 0000-0002-6288-9605 Xiao-Yu Yang: 0000-0003-3454-3604 Author Contributions ○

H.Z.Y. and Y.W. equally contributed to this work.

Author Contributions

H.Z.Y. and Y.W. carried out the experiments of the synthesis and electrochemical performance. X.Y.Y. conceived the project and provided the idea. H.Z.Y. and Y.W. designed the experiments. S.M.W. helped with the XRD and SEM test technologies and the corresponding analysis. Y.L. and J.H. helped with the XPS and TEM data analysis. X.Y.Y., J.Y., S.M.W., Y.L., J.H., L.S., Y.Y.X., W.G., G.G.C., and W.H.L. helped with the electrochemical test and the corresponding mechanism. X.Y.Y. and J.Y. proposed the mechanisms. J.Y. and Y.X.X. helped with the pictures drawing. J.S.H. performed the DFT calculation and analysis. H.Z.Y., J.Y., C.J., and X.Y.Y. wrote and revised the paper. All authors have given approval to the final version of the manuscript.

Figure 5. (a) LSV curves of Ni/TiO2 and Ni. (b) LSV curves of Co/ MoS2 and Co.

a Schottky junction in electrocatalysts is a good solution to increase the surface electron density of the metal; subsequently, the proton access to active sites is promoted, charge transfer becomes fast, and the kinetic barrier is decreased, significantly enhancing the HER.44

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by PCSIRT (IRT_15R52), NSFC (5181101338, U1662134, 21706199, and 51525903), ISTCP (2015DFE52870), HPNSF (2016CFA033), and JPSTDP (20180101208JC).

4. CONCLUSIONS In summary, we present an efficient and low-cost strategy to fabricate a transition-metal-based Schottky catalyst with largely enhanced HER activity in alkaline media. The microscale Co dendrites were decorated with commercial TiO2 nanoparticles to form a Schottky junction, which exhibited a significantly enhanced HER activity in comparison with pristine Co. The XPS results and the DFT calculations consistently agree that the interfacial Schottky junction can effectively modulate the

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REFERENCES

(1) Ying, J.; Jiang, G.; Paul Cano, Z.; Han, L.; Yang, X.; Chen, Z. Nitrogen-Doped Hollow Porous Carbon Polyhedrons Embedded with Highly Dispersed Pt Nanoparticles as a Highly Efficient and Stable Hydrogen Evolution Electrocatalyst. Nano Energy 2017, 40, 88−94.

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