Direct Observation of Yolk–Shell Transforming to Gold Single Atoms

www.acsnano.org

Direct Observation of Yolk−Shell Transforming to Gold Single Atoms and Clusters with Superior Oxygen Evolution Reaction Efficiency Downloaded via UNIV FRANKFURT on July 26, 2019 at 01:31:26 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Chao Cai,# Shaobo Han,# Qi Wang,# and Meng Gu* Department of Materials Science and Engineering, Southern University of Science and Technology, No. 1088 Xueyuan Boulevard, Shenzhen, Guangdong 518055, China S Supporting Information *

ABSTRACT: Noble metal particles and atoms tend not to react with other media even at elevated temperatures due to their inert nature. Herein, we report surprising findings of Au diffusion and dissolution in a Ni2P matrix, forming single Au atoms and tiny clusters. The dynamic atomic diffusion process was directly probed by in situ heating scanning transmission electron microscopy (STEM). The Au yolks can diffuse and dissolve completely into the Ni2P shell at temperatures of 350 °C or higher, resulting in an inward volume expansion of the Ni2P shell and formation of single Au atoms and tiny Au clusters in Ni2P. The resulting structure exhibited outstanding oxygen evolution reaction (OER) efficiency that is 16-fold that of commercial IrO2. The in situ STEM finding explains its drastically improved catalytic performance after annealing, confirming Au single atoms and clusters on a Ni2P surface as active OER sites. Our finding emphasizes the role of dynamic structural changes during simple heat treatment to enhance catalysis efficiency. KEYWORDS: electrocatalyst, nickel phosphides, single-atom catalyst, yolk−shell, OER

G

atom catalyst can increase the efficient loading and reduce the usage of precious metals by a factor of at least 10.4,5 A typical way to make a single-atom catalyst requires only a very low loading of active materials on the surface of a substrate. Then the samples go through a high-temperature heating, which can cause a bonding of the surface atoms with the underlying substrate or replacing some cations in the substrate.4 A stable bonding environment is required for stable electrochemical cycling of catalysts. Due to the high-temperature annealing process, coalescence of neighboring clusters or migration of atoms between nanoparticles can cause sintering and deactivation of catalysts. Therefore, the as-obtained samples may contain clusters or nanoparticles, which need additional acid etching to eliminate these particles. However, if we can reduce the surface and interface energy through coordinating single metal atoms with ligands or a substrate that has very strong affinity, we can likely stabilize these single

reen energy and a cleaner environment call for the production of sustainable energy resources, such as hydrogen gas, which can react with oxygen to produce power and only water as product. Industrial hydrogen production needs efficient catalysts to reduce the energy cost for water splitting. Oxygen evolution reaction (OER) catalysis involves a four-electron process and its resulting kinetics is quite slow, which is the determining step for overall water splitting to produce hydrogen gas.1−3 However, the commercial IrO2 catalyst for OER is very expensive due to usage of precious metals. In order to reduce the cost of clean hydrogen, we must reduce the usage of precious metal catalysts. Recently, a single-atom catalyst turned out to be an efficient catalyst for chemical reactions. In addition, a single-atom catalyst has high selectivity due to the universal coordinating environment of these single atoms. All single atoms can be efficient active sites and counted as efficient loading. In contrast, the active sites reside only at the surface of bulk catalysts or nanoparticlebased catalysts, and the atoms inside the particles do not contribute to the whole catalysis reaction. Therefore, a single© XXXX American Chemical Society

Received: March 19, 2019 Accepted: July 23, 2019

A

DOI: 10.1021/acsnano.9b02135 ACS Nano XXXX, XXX, XXX−XXX

Article

Cite This: ACS Nano XXXX, XXX, XXX−XXX

Article

ACS Nano

Figure 1. (a) HAADF image of the fresh Au@Ni2P yolk−shells; (b) overlaid Au and Ni EDS map; (c) Au map; (d) Ni map; (e) P map; (f) high-resolution HAADF image and (g) high-resolution bright field STEM and FFT analysis of one Au@Ni2P yolk−shell; (h) atomic-scale analysis of the orientation relationship between the yolk and shell of another particle.

Figure 2. (a) OER catalyst performance before and after heating at 350 °C compared with other samples; arrows mark the oxidation peak of Ni-containing species. (b) Tafel plot of different samples; (c) measured current density at 1.47 V vs RHE; (d) stability of Au@Ni2P-350 °C at 1.48 V vs RHE. Inset shows the stability test of Au@Ni2P and Ni2P at 1.56 V vs RHE.

metal atoms.6−8 Li et al. reported unexpected stable singleatom (Pd, Pt, Au) formation through heating noble metal nanoparticles.9 Zheng et al. utilized an ultraviolet photo-

chemical route to synthesize well-dispersed Pd single atoms on ultrathin TiO2 as an outstanding catalyst for hydrogenation of aldehydes.10 These single atoms exhibited better catalytic B

DOI: 10.1021/acsnano.9b02135 ACS Nano XXXX, XXX, XXX−XXX

Article

ACS Nano

Figure 3. (a) XRD with Ni2P and Au peaks indexed by blue and pink arrows; (b) overall STEM imaging of the annealed sample at 350 °C for 1 h; (c) line profile of the red arrow region in panel d; (d, e) atomic-scale imaging of the Au single atoms in the Ni2P; the single Au atoms are circled in white; the little Au clusters are circled in red; (f) line profile across the Au dimer in circled region “1” in panel e; (g) line profile across the Au cluster in circled region “2” in panel e; in both panels f and g, STEM images are included in the upper right corners to show the place where the line profile is taken.

yolk−shell structure to about 350 °C and kept it at this temperature for about 1 h. We can clearly observe the atomic diffusion and single-atom formation in Ni2P using aberrationcorrected in situ high-resolution scanning transmission electron microscopy (STEM). In the ex situ annealing control experiment, similar atomically dispersed Au single atoms on Ni2P were also clearly visible, which is consistent with in situ

activity and selectivity than nanoparticles. The local substrate surface chemistry and defects also play a vital role in securely anchoring these single atoms.5,10 Herein, we report another effective way to convert noble gold nanoparticles into single atoms or tiny clusters by a simple annealing process. First, we synthesized nanoscale Au@Ni2P yolk−shell structures. Then we simply heated the Au@Ni2P C

DOI: 10.1021/acsnano.9b02135 ACS Nano XXXX, XXX, XXX−XXX

Article

ACS Nano TEM results. The Au atoms can diffuse and reach the surface the Ni2P, forming efficient active sites for OER. The annealed Au−Ni2P sample exhibited drastically improved OER performance, surpassing a commercial IrO2 sample by 16-fold and fresh yolk−shell by 12-fold.

RESULTS AND DISCUSSION Figure 1 shows the STEM high-angle annular dark field (HAADF) Z-contrast image of the fresh Au@Ni2P yolk−shell structure. In such Z-contrast STEM images, the Au yolk exhibits brighter contrast due to higher atomic number than the Ni2P shell. The chemical identity of the yolk and shell is further proved by EDS mapping in Figure 1(b−e). Figure 1(f) is the magnified high-resolution Z-contrast image of Au@ Ni2P. Meanwhile, the simultaneously acquired bright field STEM image is shown in Figure 1(g). The fast Fourier transform (FFT) of the shell and Au yolk indicate that the Au(111) planes are epitaxial with the Ni2P(301) crystal plane, as shown in Figure 1(g). In addition, we can clearly observe that the Au yolk is polycrystalline rather than single-crystalline. The yellow line points out a grain boundary that separates the two parts of Au with different crystal orientations. Another atomic-scale bright field STEM image is shown in Figure 1(h), which clearly shows the epitaxial relationship of Au [200]// Ni2P [021]. As demonstrated in Figure 2, the fresh Au@Ni2P particles exhibited superior performance compared with commercial IrO2, RuO2 (Figure S1), and pure Ni2P. In comparison, the pure Au nanoparticles exhibited very poor current density even at very large overpotentials. Through this comparison, we can safely draw the conclusion that the Au@Ni2P yolk−shell together can enhance the OER efficiency, which indicated that the interfaces or some sort of structural combination played an important role in OER. Even more surprisingly, we found that the Au@Ni2P yolk−shell nanoparticles showed much better performance after annealing at 350 °C for about 1 h. Arrows in Figure 2a mark the blue shift of Ni-containing species’ oxidation peak, demonstrating the evolved Ni-containing species before electrochemical oxidation. Figure 2b summarizes the specific Tafel slopes of a series of samples. Ni2P (71 mV/dec) has much better OER performance than pure Au (124 mV/dec), demonstrating that Ni2P is a more active species. The efficacious charge transport in Au@Ni2P yolkshell nanoparticles is determined by the structural and electronic state of Ni2P. The Tafel slope has not shown an evident decrease after coupling Au into Au@Ni2P yolk−shells (70 mV/dec) compared to pure Ni2P (71 mV/dec), indicating the bulk Au yolks did not change the electronic state of Ni2P significantly. This feature corresponds well to the reported Au@Co3O4 core−shell nanostructures.11 The Tafel slope decreased substantially after annealing Au@Ni2P structures (58 mV/dec) compared to fresh Au@Ni2P. This change points to a highly promoted charge transport efficiency on the Ni2P surface after annealing.9 The decreased Tafel slope suggests that the activated state of Ni2P originates from the Au single atoms and clusters dynamically formed during annealing as found by STEM in Figures 3 and 4. The performance of the annealed sample at 1.47 V vs RHE has a 12-fold increase compared with fresh Au@Ni2P, which is a 40-fold increase of pure Ni2P and 16-fold of commercial IrO2, as shown by Figure 2(c), which is far better than most reported Au/Ni2P-based materials for OER (Table S2). In addition, this is compared with commercial RuO2 in Figure S1. RuO2 possesses an

Figure 4. (a) Fresh sample, (b) after heating at 350 °C for 64 min, and (c) after additional heating at 500 °C for 5 min; (d) atomicscale image and FFT of the structure after the Au dissolved in the Ni2P lattice at the square region in panel c.

overpotential of 323 mV at 10 mA/cm2. The Au@Ni2P annealed at 350 °C for 1 h showed significantly higher OER activity than RuO2, which is about 13-fold the current density of RuO2 at 1.47 V vs RHE. Because the Au state is sensitive to the annealing temperature,12 we check the OER activity of Au−Ni2P with different annealing temperatures (Figure S3). The relative data reveal that 350 °C is the optimal temperature to achieve the highest OER activity. In addition, we tested the OER performance of pure Ni2P hollow spheres without Au yolks after annealing at 350 °C (Ni2P-350) in Figure S2. The Ni2P-350 exhibited even slightly lower OER activity than as-prepared Ni2P. The decreased OER activity may result from the aggregation of Ni2P nanoparticles and decreased surface areas after annealing. On the contrary, Au@Ni2P-350 showed dramatically improved OER performance, as shown in Figure 2a. Previous literature reported that Au alone might not be a good catalyst for OER, while Aubased composites are efficient OER catalysts due to outstanding synergistic effects with the supports.11,13 Through this comparison, we believed that the active sites may originate from the synergistic effect between Au single atoms and the Ni2P support. In addition, the Au single-atom-doped Ni2P alloy phase may result in a changed electronic structure, which may also benefit the OER. Inspired by the increased OER performance of the annealed Au@Ni2P sample, we conducted the stability test of Ni2Pcontaining samples, as shown in Figure 2d. The pure Ni2P shows a low stability in alkaline solution at 1.56 V vs RHE, which is consistent with previously reported literature.14 Further X-ray photoelectron spectroscopy (XPS) is used to characterize the surface chemistry of Au−Ni2P before and after annealing in Figure S4. The annealed Au−Ni2P nanoparticles possess very different electronic states for Au, Ni, and P. These results suggest that the synergistic effect between Au single atoms/clusters and Ni2P matrix may be one significant D

DOI: 10.1021/acsnano.9b02135 ACS Nano XXXX, XXX, XXX−XXX

Article

ACS Nano

Figure 4(e) exhibited elongation and splitting, indicating lattice distortion by diffused Au atoms in the Ni2P lattice. The diameter of the inner hole of the nanoparticles changed from 16 nm to about 11 nm after the whole heating process. In comparison, we found that the shell on the bottom left region expanded from 8.5 nm to about 13.4 nm. The hole shrank, but the shell thickness increased significantly due to the dissolution of Au in Ni2P. In conclusion, initiated by annealing the heterogeneous Au@Ni2P catalyst at elevated temperatures, gold atoms can diffuse through the Ni2P matrix and form single atoms and tiny clusters. With controlled temperature and time, the gold atoms can diffuse into the Ni2P shell and form gold single atoms well dispersed in the Ni2P matrix and on surfaces. With further heating at elevated temperature up to 500 °C, all gold atoms can dissolve in Ni2P and form a uniform solid-solution phase. The above structural changes can correlate well with the performance of Au@Ni2P catalysts in OER as shown by our comparative study. Going beyond our study, previous literature also reported enhanced performance of catalysts after annealing as-synthesized nanostructures in an oven for a period of time at elevated temperature.17−21 The resulting structural transformation is often a major contributing factor for improved performance in catalysis. Here, diffusion of Au in Ni2P resulted in an intermediate phase with single gold atoms in a Ni2P matrix and on the surface. This single-atom catalyst synthesized via a heating-yolk−shell approach may provide a high selectivity in various chemical reactions.

contributor to such high OER activity. The stability of those samples increased dramatically after coupling Au into Ni2P. Noble metal atoms can promote the structural stability of their composites, which was also observed in the case of Au@ Ni3S2.15,16 To find out what structural changes have taken place, we performed XRD and HAADF STEM imaging analysis of the Au@Ni2P sample after ex situ heat treatment at 350 °C for 1 h. As shown in the X-ray diffraction pattern in Figure 3(a), it reveals a hexagonal structure of the Ni2P shell with a = b = 0.586 nm and c = 0.337 nm. Surprisingly, the Au characteristic (111) peak disappeared after annealing at 350 °C for 1 h. As shown in Figure 3(a) and Table S2, the 2-theta value of the characteristic peaks of Ni2P have shifted to a slightly smaller value after inclusion of diffused Au atoms. Correspondingly, the plane spacing of (111), (201), and (210) also increased slightly. The overall STEM imaging in Figure 3(b) shows that the Au yolks were completely gone after annealing. Detailed atomic-scale imaging analysis in Figure 3(c−e) revealed that the Au atoms all diffused into the Ni2P. As shown by the line profile in Figure 3(c), the Au showed up as much brighter dots than Ni atomic columns in such Z-contrast imaging conditions. The full width at half-maximum of the Au peaks is ∼0.16 nm, which is less than 0.2 nm and indicates a likely single-atom Au state in Ni2P. In addition, more single Au atoms are circled in white in Figure 3(d,e). In the meantime, we also observed little Au clusters, such as dimers and trimers, as circled in red in Figure 3(d,e) with aberration-corrected STEM. We carefully measured the distance between the Au dimers in the red circle “1” in Figure 3(e) and plotted the line profile in Figure 3(f), in which a Au−Au spacing of 0.17 nm is identified between the center of two Au peaks. In addition, Figure 3(g) plots the line profile across the Au cluster region in the red circle “2” in Figure 3(e), in which a Au−Au spacing of 0.14 nm is identified. These small Au−Au spacings indicated the likely Au−Au bonding and the presence of dimers and clusters in the annealed sample. In order to clearly visualize the structural changes, we did in situ TEM at elevated temperatures to probe the dynamical structural changes in real time. As shown by Figure 4, at room temperature, the Au yolk shows a bright color due to a heavier atomic number and presents itself as the yolk in the Ni2P shell. The size of the original vacant hole in Ni2P is around 16 nm. At 350 °C, the Au atoms began to diffuse into the Ni2P and a lot of bright clusters and single atoms appear in the Ni2P shell as circled in red in Figure 4(b). Please also note that the inner surface of the Ni2P showed up with a brighter color than the original state at room temperature due to the agglomeration of Au single atoms as pointed by the red arrow in Figure 4(b). This picture clearly maps out the diffusion pathway for Au. At high temperature (350 °C or higher), the gold yolk became unstable, and Au atoms began to diffuse into Ni2P. These Au atoms quickly diffused to cover the inner surface of Ni2P due to a much larger surface diffusivity than bulk diffusivity. With a longer time at elevated temperature, these Au atoms began to diffuse into the Ni2P, forming Au single atoms and bright clusters inside the Ni2P shell and on its surface, as labeled in red in Figure 4(b). With further heating to 500 °C for 5 min, the yolk totally disappeared and the contrast of the whole Ni2P became evenly bright for the whole particle. With highresolution STEM in Figure 4(d), we can observe the crystalline lattice spacing of 2.89 Å, which corresponds to the (110) surface of Ni2P. However, the diffraction spots in the FFT of

CONCLUSIONS In situ TEM analysis and ex situ heating experiments revealed surprising Au single-atom diffusion in Ni2P at elevated temperatures. A simple annealing procedure greatly changed the morphology and atomic structural configuration of the Au@Ni2P yolk−shell sample. We found that Au yolks can diffuse into and even dissolve completely in the Ni2P shell at 350−500 °C. The resulting Au single atom/clusters on the Ni2P catalyst showed superior efficiency in OER, surpassing the commercial IrO2 and fresh Au@Ni2P yolk−shells by 16and 12-fold, respectively. In addition, the annealed Au−Ni2P exhibited much better stability than pure Ni2P in the harsh OER environment. Our findings may be a direction of making efficient and stable single-atom catalysts through simply controlled heating of a core-shell or yolk-shell structure. EXPERMENTAL SECTION Chemicals. Oleylamine (>95%), ammonia borane (>90%), octadecylene (>98%), chloroauric acid (50% metal base), nickel acetylacetone (>95%), triphenylphosphine (>98%), hexane (>97%), ethanol (>99%), IrO2 (>99.9%), potassium hydroxide (99.999%), sulfuric acid (98%), and isopropanol (99%) were purchased from Aladdin. Nafion solution (4.5 wt %) was purchased from Sigma. Synthesis of Au Nanoparticles. In a Schlenk tube, 50 mg of chloroauric acid was dissolved into 10 mL of oleylamine (OAm) and 10 mL of octadecylene (ODE) at 120 °C with magnetic stirring for 1 h. The prepared transparent solution was heated to 168 °C and kept at this temperature for 1 h. The red solution was washed several times by hexane and ethanol at 10 000 rpm for 5 min. The Au powder was collected and dispersed in ethanol. Synthesis of Ni2P Nanoparticles. In a Schlenk tube, 2 mmol of nickel acetylacetone was dissolved into 10 mL of OAm and 10 mL of ODE at 120 °C in an Ar-filled glovebox. The prepared transparent solution was heated to 220 °C and kept at this temperature for 1 h. Then 8 mmol of triphenylphosphine was added into the solution, and E

DOI: 10.1021/acsnano.9b02135 ACS Nano XXXX, XXX, XXX−XXX

Article

ACS Nano the solution was kept at 320 °C for 2 h before cooling to room temperature. The black powder was collected and washed in the same way as Au nanoparticles. Synthesis of Au@Ni2P Nanoparticles. Au@Ni2P was synthesized in the same way as Ni2P, but we added 50 mg of chloroauric acid at 120 °C to the starting solution. Heat Treatment. A 20 mg amount of as-prepared Au@Ni2P powder was put into a tube furnace under Ar flow at atmospheric pressure. Then the powder was heated to 300, 350, or 500 °C for 1 h, respectively. Electrochemical Characterization. The as-prepared samples were loaded on carbon (EC 300) with a mass percentage of 20 wt %. Isopropanol, water, and Nafion solution (with a volume ratio of 1:3:0.01) was used as a solution for preparing the catalyst ink (2 mg of catalyst/mL). The ink was deposited on a polished glassy carbon electrode surface as the working electrode with a catalyst loading of 0.013 mg/cm2. In a three-electrode system, 1 M KOH (or 0.5 M H2SO4) was used as electrolyte; Ag/AgCl and carbon rod were used as the reference electrode and counter electrode, respectively. Before the test, the working electrode was immersed into the electrolyte for 2 h. Line scan voltammetry (0.005 V/s) was used to characterize the catalytic performance. Structural and Chemical Analysis. A Bruker D8 X-ray diffractometer with Cu Kα radiation was utilized to probe the crystal structure. XPS measurements were conducted on an Axis Ultra DLD multitechnique surface analysis system, and relative curves were calculated by C1s (284.8 eV). The aberration-corrected STEM was performed using a double Cs-aberration corrected Themis G2 microscope at 300 kV. The EDS mapping was acquired using a Super-X EDS detector.

REFERENCES (1) Zheng, X.; Zhang, B.; De Luna, P.; Liang, Y.; Comin, R.; Voznyy, O.; Han, L.; García de Arquer, F. P.; Liu, M.; Dinh, C. T.; Regier, T.; Dynes, J. J.; He, S.; Xin, H. L.; Peng, H.; Prendergast, D.; Du, X.; Sargent, E. H. Theory-Driven Design of High-Valence Metal Sites for Water Oxidation Confirmed Using In Situ Soft X-ray Absorption. Nat. Chem. 2018, 10, 149. (2) Bergmann, A.; Martinez-Moreno, E.; Teschner, D.; Chernev, P.; Gliech, M.; de Araújo, J. F.; Reier, T.; Dau, H.; Strasser, P. Reversible Amorphization and The Catalytically Active State of Crystalline Co3O4 During Oxygen Evolution. Nat. Commun. 2015, 6, 8625. (3) Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y. A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science 2011, 334, 1383− 1385. (4) Qiao, B.; Wang, A.; Yang, X.; Allard, L. F.; Jiang, Z.; Cui, Y.; Liu, J.; Li, J.; Zhang, T. Single-Atom Catalysis of CO Oxidation Using Pt1/FeOx. Nat. Chem. 2011, 3, 634. (5) Chen, Y.; Ji, S.; Sun, W.; Chen, W.; Dong, J.; Wen, J.; Zhang, J.; Li, Z.; Zheng, L.; Chen, C.; Peng, Q.; Wang, D.; Li, Y. Discovering Partially Charged Single-Atom Pt for Enhanced Anti-Markovnikov Alkene Hydrosilylation. J. Am. Chem. Soc. 2018, 140, 7407−7410. (6) Chen, Y.; Ji, S.; Chen, C.; Peng, Q.; Wang, D.; Li, Y. SingleAtom Catalysts: Synthetic Strategies and Electrochemical Applications. Joule 2018, 2, 1242−1264. (7) Yang, X.-F.; Wang, A.; Qiao, B.; Li, J.; Liu, J.; Zhang, T. SingleAtom Catalysts: A New Frontier in Heterogeneous Catalysis. Acc. Chem. Res. 2013, 46, 1740−1748. (8) Liu, J.-C.; Wang, Y.-G.; Li, J. Toward Rational Design of OxideSupported Single-Atom Catalysts: Atomic Dispersion of Gold on Ceria. J. Am. Chem. Soc. 2017, 139, 6190−6199. (9) Wei, S.; Li, A.; Liu, J.-C.; Li, Z.; Chen, W.; Gong, Y.; Zhang, Q.; Cheong, W.-C.; Wang, Y.; Zheng, L.; Xiao, H.; Chen, C.; Wang, D.; Peng, Q.; Gu, L.; Han, X.; Li, J.; Li, Y. Direct Observation of Noble Metal Nanoparticles Transforming to Thermally Stable Single Atoms. Nat. Nanotechnol. 2018, 13, 856−861. (10) Liu, P.; Zhao, Y.; Qin, R.; Mo, S.; Chen, G.; Gu, L.; Chevrier, D. M.; Zhang, P.; Guo, Q.; Zang, D.; Wu, B.; Fu, G.; Zheng, N. Photochemical Route for Synthesizing Atomically Dispersed Palladium Catalysts. Science 2016, 352, 797−800. (11) Zhuang, Z.; Sheng, W.; Yan, Y. Synthesis of Monodispere Au@ Co3O4 Core-Shell Nanocrystals and Their Enhanced Catalytic Activity for Oxygen Evolution Reaction. Adv. Mater. 2014, 26, 3950−3955. (12) Wang, Q.; Zhao, Z. L.; Cai, C.; Li, H.; Gu, M. Ultra-Stable 4HGold Nanowires at up to 800 °C in Vacuum. J. Mater. Chem. A 2019, DOI: 10.1039/C9TA01306G. (13) Xu, Y.; Duan, S.; Li, H.; Yang, M.; Wang, S.; Wang, X.; Wang, R. Au/Ni12P5 Core/Shell Single-Crystal Nanoparticles as Oxygen Evolution Reaction Catalyst. Nano Res. 2017, 10, 3103−3112. (14) Popczun, E. J.; McKone, J. R.; Read, C. G.; Biacchi, A. J.; Wiltrout, A. M.; Lewis, N. S.; Schaak, R. E. Nanostructured Nickel Phosphide as an Electrocatalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2013, 135, 9267−70. (15) Cai, C.; Han, S.; Caiyang, W.; Zhong, R.; Tang, Y.; Lawrence, M. J.; Wang, Q.; Huang, L.; Liang, Y.; Gu, M. Tracing the Origin of Visible Light Enhanced Oxygen Evolution Reaction. Adv. Mater. Interfaces 2019, 6, 1801543. (16) Cai, C.; Mi, Y.; Han, S.; Wang, Q.; Liu, W.; Wu, X.; Zheng, Z.; Xia, X.; Qiao, L.; Zhou, W.; Zu, X. Engineering Ordered DendriteLike Nickel Selenide as Electrocatalyst. Electrochim. Acta 2019, 295, 92−98. (17) Edwards, J. K.; Pritchard, J.; Piccinini, M.; Shaw, G.; He, Q.; Carley, A. F.; Kiely, C. J.; Hutchings, G. J. The Effect of Heat Treatment on the Performance and Structure of Carbon-Supported Au−Pd Catalysts for the Direct Synthesis of Hydrogen Peroxide. J. Catal. 2012, 292, 227−238.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b02135. Additional electrochemical results and XRD, TEM, and XPS data (PDF)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Chao Cai: 0000-0002-3695-3247 Meng Gu: 0000-0002-5126-9611 Author Contributions #

C.C., S.H., and Q.W. contributed equally.

Author Contributions

C.C. completed the catalytic experiments. S.H. and Q.W. conducted the in situ STEM experiments. M.G. designed the experiment and wrote the paper. All authors contributed to the revision of the manuscript. Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21802065), Development and Reform Commission of Shenzhen Municipality 2017 (no. 1106), Shenzhen Peacock Plan (KQTD2016022620054656), Guangdong Innovative and Entrepreneurial Research Team Program (2016ZT06N500), and Guangdong Provincial Key Laboratory of Energy Materials for Electric Power with contract no. 2018B030322001. F

DOI: 10.1021/acsnano.9b02135 ACS Nano XXXX, XXX, XXX−XXX

Article

ACS Nano (18) Li, H.; Nakajima, H.; Ito, K. Optimization of Annealing Catalyst Powder for High Temperature PEMWE. ECS Trans. 2016, 75, 1095−1105. (19) Sui, M.; Li, M.-Y.; Kunwar, S.; Pandey, P.; Zhang, Q.; Lee, J. Effects of Annealing Temperature and Duration on the Morphological and Optical Evolution of Self-Assembled Pt Nanostructures on cPlane Sapphire. PLoS One 2017, 12, No. e0177048. (20) Feng, T.; Ding, L.; Chen, L.; Di, J. Deposition of Gold Nanoparticles upon Bare and Indium Tin Oxide Film Coated Glass Based on Annealing Process. J. Exp. Nanosci. 2019, 14, 13−22. (21) Figuerola, A.; Huis, M. v.; Zanella, M.; Genovese, A.; Marras, S.; Falqui, A.; Zandbergen, H. W.; Cingolani, R.; Manna, L. Epitaxial CdSe-Au Nanocrystal Heterostructures by Thermal Annealing. Nano Lett. 2010, 10, 3028−3036.

G

DOI: 10.1021/acsnano.9b02135 ACS Nano XXXX, XXX, XXX−XXX