Selectivity on Etching: Creation of High-Energy Facets on Copper

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Selectivity on Etching: Creation of High-Energy Facets on Copper Nanocrystals for CO2 Electrochemical Reduction Zhenni Wang, Guang Yang, Zhaorui Zhang, Mingshang Jin, and Yadong Yin ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.6b00602 • Publication Date (Web): 14 Mar 2016 Downloaded from http://pubs.acs.org on March 15, 2016

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Selectivity on Etching: Creation of High-Energy Facets on Copper Nanocrystals for CO2 Electrochemical Reduction Zhenni Wang,† Guang Yang,‡ Zhaorui Zhang,† Mingshang Jin,†,* and Yadong Yin§,* †Frontier Institute of Science and Technology and State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an, Shaanxi, 710049, China. ‡

Electronic Materials Research Laboratory, Key Laboratory of the Ministry of Education &

International Center for Dielectric Research, Xi’an Jiaotong University, Xi’an, Shaanxi, 710049, China §

Department of Chemistry, University of California, Riverside, California 92521, USA.

KEYWORDS. High-energy facets • copper • rhombic dodecahedron • chemical etching • surface energy

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ABSTRACT. Creating high-energy facets on the surface of catalyst nanocrystals represents a promising method for enhancing their catalytic activity. Herein we show that crystal etching as the reverse process of crystal growth can directly endow nanocrystal surface with high-energy facets. The key is to avoid significant modification to the surface energies of the nanocrystal facets by capping effects from solvents, ions, and ligands. Using Cu nanocubes as the starting material, we have successfully demonstrated the creation of high-energy facets in metal nanocrystals by controlled chemical etching. The etched Cu nanocrystals with enriched highenergy {110} facets showed significantly enhanced activity toward CO2 reduction. We believe the etching-based strategy could be extended to the synthesis of nanocrystals of many other catalysts with more active high-energy facets.

Creation of high-energy facets on the surface of catalyst nanocrystals has recently been regarded as an efficient way for improving their catalytic activity, selectivity and stability.1-17 A very general and useful construct for thinking about the exposure of high-energy facets is “surface energy”.18-20 In the growth mode of nanocrystals, the facets with higher surface energies usually grow at a higher rate than those with lower ones; therefore, the fast-growing facets will eventually disappear, resulting in nanocrystals terminated with low-energy surfaces which are typically inactive in catalytic reactions.18-20 It is thus imperative to modify the surface energy by surface capping, making high-energy facets stable under the reaction conditions. However, by far only a few capping agents have been reported to be effective for reducing the energy of certain high-energy facets, with a typical example of amine toward Pt{411} facets, in which the “surface energy” of Pt{411} facets is reduced by the adsorption of amine.9 As a result, a high percentage


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of {411} facets are retained on the surface of the synthesized Pt nanocrystals, which overcome the thermodynamic limitations of crystal growth habits. Generally, it remains an open question whether there are other effective synthetic strategies for achieving high-energy facets so as to facilitate their applications. Crystal etching represents the reverse path of crystal growth. Theoretically, both crystal growth and etching are directly related to “surface energy”. Those facets with higher surface energies can grow/etch at higher rates. Different from the growth, however, the facets with higher etching rate will tend to be exposed on the surface of crystals after etching. Not surprisingly, without the need of capping agents for varying the “surface energy”, etching of nanocrystals can directly create high-energy facets. This etching process could also take advantage of many of the same chemical principles used in direct growth mode for controlling the shape of nanostructures, by considering the relationship between these seemingly opposite transformations. With these in mind, here we report a simple etchant system based on an oxidation reaction for metal nanocrystals, where the metal atoms on particular facets were oxidized into ions. Governed by thermodynamics, facets with higher surface energies will be etched at higher rates and eventually exposed on the surface of the metal nanocrystals. We reveal that the etching rates of different facets could be greatly affected by the reaction parameters including the choice of solvent, temperature, etchant concentration, and time. RESULTS AND DISCUSSION


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Figure 1. (a) Schematic illustration of nanocrystal syntheses through two different ways: growth and etching.

Figure 1 highlights the difference between crystal growth and etching of a nanocrystal, where growth results in the exposure of low-energy surfaces (surfaces AB and CD) while in contrast etching leads to the formation of high-energy surfaces (surface BC). In this work, copper (Cu) was chosen for proof of concept by considering its high activity toward oxidation. Figure 2a shows the work functions and surface energies of different facets of Cu according to the simulation results reported previous.21 Among the three basic facets of {100}, {111}, and {110}, {110} exhibits the lowest work function and the highest surface energy.21 The highest surface energy allows {110} facets adsorb more etchant molecules, while the lowest work function suggests that the atoms on {110} facets would be easier to be oxidized into ions and dissolved into the reaction solution, suggesting a higher etching rate along the direction than that along the or directions. One would therefore expect to obtain Cu rhombic dodecahedrons (RDs) with a high percentage of high-energy {110} facets through a simple etching process. Cu nanocubes (NCs) were firstly prepared using a seed mediated overgrowth process that we reported previously, by overgrow Cu on small Pd nanocube seeds (Figure S1).22 The well-


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defined cubic shape of the nanocrystals allows relatively easy identification of the morphology change during the etching process. Selenium (Se) was then dissolved in tri-n-octylphosphine (TOP) and served as an oxidant for etching Cu NCs. Compared with other etchants such as oxygen gas, the TOP-Se is easier to handle due to its liquid form and the etching rate could be finely tuned in a more controllable way. Figure 2b illustrates the possible etching paths of Cu NCs, with highly selective etching on {110} facets against {100} and {111} facets due to the differences in surface energy and work function.

Figure 2. (b) Surface energies and work functions of the three facets in Cu metal nanocrystals. (c) Schematic illustration showing that the preferred etching along leads to shrinkage of low energy {100} facets and exposure of more high energy {110} facets.

As shown in Figure 3, Cu NCs gradually evolved into Cu RDs in the presence of the TOPSe etchant. Theoretically, Cu atoms on {110} surfaces of Cu NCs would be oxidized by Se, resulting in the formation of Cu2+ ions. Meanwhile, Se could accept two electrons in this process thus turn into Se2- ions.23 With short reaction time (t = 4 h), Cu NCs were slightly truncated at their edges due to the fast-etching by TOP-Se along direction, while its side faces were still mostly covered by {100} facets. It should be pointed out that the volume of the obtained nanocrystals is slightly smaller than that of the original Cu NCs. More detailed characterizations,


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including images of scanning electron microscopy (SEM), transmission electron microscopy (TEM), and high-resolution transmission electron microscopy (HRTEM) of the product obtained after 4-h etching can also be found in Figure S2. When the reaction proceeded to t = 8 h (Figures 3b, S3), the coverage of {110} facets gradually increased due to the enlargement of {110} facets and the shrinkage of {100} facets. Eventually, after 12 h of etching, all of Cu NCs evolved into Cu RDs with their surface enclosed exclusively by {110} facets (Figure 3c). Further etching would only reduce the size of the Cu RDs rather than altering their shape as their surfaces have been fully enclosed by {110} facets (Figures 3d, S4). This result indicates that the etching is not due to the selective capping of Cl- ions on the {100} facets of Cu nanocrystals, but mainly resulted from the fast-etching of high-energy {110} facets of Cu by TOP-Se.

Figure 3. TEM images of as-etched Cu NCs subjected to different periods of etching: (a) 4 h, (b) 8 h, (c) 12 h, and (d) 24 h.

The rhombic dodecahedral shape of Cu nanocrystals after etching was further characterized by SEM, TEM, HRTEM, and selected area electron diffraction (SAED). As can be seen in Figures 4a and 4b, all nanocrystals surveyed were high-quality Cu RDs, demonstrating the


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excellent control over the product shape. The size of RDs ranged narrowly from 80 to 100 nm, smaller than that of the starting Cu NCs which is about 110 nm. Both HRTEM and SAED on individual RDs indicated the single-crystalline nature of these Cu nanocrystals. HRTEM image also showed lattice fringes with interplanar spacing of 0.13 nm, corresponding to {220} fringes of face-centered cubic (fcc) Cu (Figures 4c and 4d).

Figure 4. Cu RDs prepared by selective etching. (a-c) representative SEM and TEM images. (d) HRTEM image of the region indicated by the box in (c), and (e-j) TEM images and selected-area-electron diffraction patterns of individual rhombic dodecahedrons oriented along the (e-f) [111], (g-h) [001], and (i-j) [011] directions.

We further analyzed the Cu RDs oriented along [111], [001], and [011] directions by TEM imaging and recording the SAED patterns, as shown in Figures 4e-4j. These directions can also be confirmed by the projection shapes of Pd cubic seeds inside the Cu nanocrystals, which show the hexagonal, cubic, and rectangular along [111], [001], and [011], respectively. The ideal projections of rhombic dodecahedral model along above three directions are hexagonal, cubic,


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and hexagonal (insets to Figure 4e-4j), in good agreement with the outline of TEM images of the Cu nanocrystals. We further deduced the general expressions of projection angles of fcc metal nanocrystals as a function of Miler indices. We found that the surface structure of rhombic dodecahedral nanocrystals can be best revealed by measuring the projection along [011], where two projection angles (α and β) are associated with Miller indices. In the TEM image (Figure 4i), the value of α and β are 125º and 109º, respectively, very close to the theoretical values (125.16 º and 109.28 º) for rhombic dodecahedral model with {110} facets. Moreover, the projection angles of Cu RDs along [111] and [001] direction also coincide with those of rhombic dodecahedral models with {110} facets. Based on the above analyses, we can conclude that high-energy {110} stepped facets have been successfully created on the surface of Cu nanocrystals through crystal etching.


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Figure 5. (a) Plot of the length of {110} facets versus etching time demonstrating the etching rate by TOP-Se at the standard concentration and temperature. (b) Plot of the length of {110} facets versus the concentration of TOP-Se and etching temperature, showing the strong relationship between the etching rate and the concentration of the etchant and the etching temperature.

The etching rate, as determined by the area of {110} facets exposed on the surface of Cu nanocrystals, can be obtained by measuring the length of {110} facets (indexed as “L”) of nanocrystals when viewed along the direction in the TEM images of the samples prepared with different reaction times. Figure 5a shows the plot of L versus the etching time, while Figure S5 illustrates the corresponding TEM images. It is clear that L increases with the reaction time. When the etching proceeded for 4 h, L was 31 nm. The increasing of the etching time to 8 and 12 h resulted in the increase of L to 42 and 48 nm. As the reaction progressed considerably slowly, it is reasonable to expect that one can obtain nanocrystals with precisely controlled surface structure (proportion of {110} facets) by simply stopping the etching at appropriate time. Compared with other etchants such as oxygen gas, TOP-Se allows convenient adjustment of its concentration and subsequently control over the etching rate. As can be seen in Figure 5b, black line, and Figure S6, L increases with the concentration of the etchant with the same reaction time (4 h), consistent with the expectation that the higher concentration of the etchant can accelerate the etching process of Cu. Another important factor of the reaction parameters, temperature, can also significantly affect the etching rate, as shown in Figure 5b, blue line, and Figure S7. L also increases with the reaction temperature. These results indicate unambiguously that chemical etching as a synthesis process could be well controlled by varying the reaction parameters. In order to better understand the etching mechanism of Cu nanocrystals, the influences of reaction solvent, type of the etchant, and the addition of the specific capping agent were further


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examined. Firstly, we note that in the absence of TOP-Se, the Cu nanocrystals were very stable with a well-maintained cubic shape (Figure S8), confirming the etchant role of TOP-Se. The surface of the etched Cu nanocrystal was analyzed by X-ray photoelectron spectroscopy (XPS) as shown in Figure S9. The spectrum in the Cu 2p region shows two peaks at 932 and 952 eV, corresponding to Cu 2p3/2 and Cu 2p1/2. No Se and P signals were detected for etched Cu RDs, implying a clean surface without the binding of TOP or Se species, which is beneficial for the catalytic applications. The dissolution of Cu2+ ions in TOP-Se can be also proved by the production of CuSe nanoplates when the clear solution is heated in a box furnace (see Figure S10 for corresponding SEM, XRD and energy dispersive spectroscopy analyses).

Figure 6. TEM images of Cu NCs after being etched in the presence of {100}-capping agent (oleylmine).

The key to the creation of high-energy facets via chemical etching is to avoid any significant modification to the surface energies of the nanocrystal facets. Such surface modification may come from the binding effect of solvents, ions, and organic capping ligands that are present during etching. Interestingly, nearly all of the previously reported processes for etching of metal nanocrystals resulted in the exposure of low-energy facets.24,25 This was mainly due to the presence of strong capping agents, such as bromide ions (Br-) and poly(vinylpyrrolidone) (PVP). These capping agents, indeed, can preferentially adsorb onto specific facets and create a protective barrier that blocks chemical attacks to these facets, often


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leading to the formation of thermodynamically stable products that are enclosed with low-energy facets. For example, Yang et al. have previously reported that Ag nanocubes can be obtained by simply etching Ag octahedrons in the presence of PVP, which can provide specific capping effect toward {100} facets of Ag nanocrystals.24 In the current system, the weak binding of TOP to Cu surface is believed to benefit the formation of high energy facets during etching, as otherwise a strong binding might modify the surface energies and lead to specific capping affinity toward certain facets so that the etching behavior would be different. In addition to TOP, we notice that N, N-dimethyl formamide (DMF) does not show strong affinity to the Cu surface, and therefore can also act as a suitable solvent for the creation of high-energy surfaces and yield Cu RDs, as shown in Figure S11. In contrast, if Cu NCs were etched in oleylamine (OAm), which has been proven to be an effective capping agent for the {100} facets of Cu,26 etching would proceed along the and directions due to the blockage of OAm on the {100} facets. As a result, the corners and edges of Cu NCs were etched away, leaving hexapod-shaped nanocrystals with cubic arms (enclosed by {100} facets), as shown in Figure 6. In conclusion, ideal operation conditions to achieve highenergy surfaces via etching require a fine selection of capping ligand which doesn't possess specific capping effect on the selected metal nanocrystals. The enrichment of high-energy {110} facets makes the etched Cu nanocrystals active catalysts for some important reactions. As Cu nanocrystals are known to be efficient catalysts for the electrochemical conversion of CO2 into hydrocarbons, we have evaluated the catalytic performance of Cu RDs in this reaction. Figure 7a compares the CO2 electroreduction currents corrected for corresponding currents in CO2-free solutions of Cu NCs, truncated Cu NCs, and Cu RDs. In order to avoid changes in surface oxidation, as well as surface roughening of the


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copper,27 the excursion of the electrode potential for all cyclic voltammetry (CV) measurements was maintained between -0.7 and -1.5 V. Plotting the CV measurements against the reference electrode (Ag/AgCl) enabled comparison of the onset potential for the reduction process, such as the hydrogen evolution reaction (HER) in the N2-saturated environment, and the two simultaneous processes, HER and CO2 reduction, in the CO2-saturated environment.28

Figure 7. (a) CO2 electroreduction currents corrected for currents in CO2-free solutions for Cu nanocrystals with different shapes: Cu NCs, truncated Cu NCs, and Cu RDs. The electrochemical evaluation was conducted in a solution containing 0.25 M K2CO3 at a sweep rate of 20 mV s-1. (b) The comparison of the faradaic efficiency between Cu NCs and Cu RDs toward CO2 electroreduction.


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Compared with the starting Cu NCs, both the etched nanocrystals exhibit much higher catalytic activities in the CO2 electroreduction reaction. For example, the onset potentials of the etched nanocrystals were more positive than that of Cu NCs. In particular, Cu RDs show the most positive onset potential, which was only -1.1 V. The onset potentials were similar for etched nanocrystals with different ratios of {110} facets due to the same energy barrier of CO2 reduction on the same exposed facets. Moreover, at -1.4 V, the current density of the RDs is nearly 3 times higher than that of the Cu NCs, indicating the enhanced catalytic activity towards CO2 of Cu RDs. The current density of the Cu RDs increased with an increase in the ratio of {110} facets, since the Cu atoms on {110} can exhibit higher catalytic ability than that on {100} facets. Theoretically, the surface areas of the Cu NCs and RDs are estimated to be 71 and 140 cm2/mg. Based on these values, it is possible to obtain the specific activities of Cu nanocrystals toward electrochemical reduction of CO2, which are -14.2 mA/cm2 for Cu NCs and -33.6 mA/cm2 for Cu RDs at -1.4 V, as shown in Figure S12. The durability of Cu RDs in the catalytic reactions was further evaluated by the chronoamperometric measurements performed at -1.4 V for 4000 s. As can be seen in Figure S13, no significant decrease of the current density could be found during the detection, implying an excellent stability of Cu RDs. The sample after the durability test was further characterized by TEM analysis, which suggests that the rhombic dodecahedral shape of Cu nanocrystals could be well maintained (Figure S14). Accordingly, the catalytic activity of Cu nanocrystals is Cu(110) > Cu(100). To summarize, Cu NCs enclosed by {110} facets not only show the rich redox behavior as seen in the CV curves, but also promoted the surface reactivity of Cu metals due to the exposed high-energy facets. Not only the activity, but also the selectivity of Cu nanocrystals after etching was significantly enhanced. As can be seen in Figure 7b, the selectivity toward CH4, C2H4, C2H6 and C3H8 is higher on Cu(110) facets


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than on the original Cu(100) facets. In comparison, CO produced on Cu(110) is much less than those on Cu(100) facets. This result indicate that the production of C1 and C2 are favored on the {110} facets of Cu RDs. Therefore, the Cu RDs, which was enclosed by {110} stepped facets, should be an excellent candidate catalyst for the CO2 electroreduction reaction. CONCLUSION In summary, using Cu nanocubes as a typical example, we have successfully demonstrated the creation of high-energy facets in metal nanocrystals by controlled chemical etching. We reveal that the key to the creation of high-energy facets in nanocrystals by etching is to avoid significant modification to the surface energies of the nanocrystal facets by capping effects from solvents, ions and ligands. The etched Cu nanocrystals with enriched high-energy {110} facets show significantly enhanced activity toward CO2 reduction. This etching based strategy could be potentially extended to the creation of high-energy facets on the surfaces of nanocrystals of many other materials.

Methods Experimental Section Synthesis of Pd Nanocubes. The Pd nanocubes were synthesized by adding a Na2PdCl4 solution into a mixture of AA, PVP, and KBr according to our previous report. In a typical synthesis, 8.0 mL of an aqueous solution containing 105 mg of PVP, 60 mg of AA, and 600 mg of KBr was placed in a vial and preheated to 80 °C in an oil bath under magnetic stirring for 10 min. Subsequently, 3.0 mL of an aqueous solution containing 57 mg of Na2PdCl4 was added with a pipette. After the vial had been capped, the reaction was allowed to continue at 80 ℃ for 3 h.


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The product was collected by centrifugation, washed three times with water to remove excess PVP, and redispersed in 11 mL of water. Synthesis of Pd@Cu core-shell Nanocubes. The Pd@Cu core-shell nanocubes were synthesis using seed-mediated overgrowth according to our previous report. In a standard synthesis, 10 mL of an aqueous solution containing 21 mg of CuCl2·2H2O, 90 mg of hexadecylamine (HDA), 50 mg of glucose, and 0.02mL of the aqueous suspension of Pd seeds was placed in a vial. After the vial had been capped, the solution was magnetically stirred at room temperature overnight and then heated to 100 ºC in an oil bath for another 6 h. The product was collected by centrifugation, washed three times with water to remove excess HDA. Etching of Pd@Cu nanocubes. In a standard procedure, the obtained Pd@Cu nanocubes were dispersed in 6 mL Tri-n-octylphosphine (TOP), and 0.88 mL TOP-Se (0.5 mol/L) was added with pipette. The mixture was heated to 60 ºC in an oil bath for 4 h without stirring. The product suspension was transferred into a 1.5 mL centrifuge tube using a pipette, followed by washing with cyclohexane three times. Electrochemical measurements. Cu nanocrystals modified working electrodes were fabricated by depositing a hexane dispersion of Cu nanoparticles onto a glassy carbon electrode. Upon drying under air at room temperature, the electrode was covered with 5 µL of 5 wt% Nafion dispersed in water. Electrochemical experiments were conducted on an Autolab electrochemistry station, which was equipped with a rotating disk electrode (RDE) system (No.1154-R65WHPL) in a thermostatic glass cell. Cyclic voltammetry (CV) was carried out in N2 saturated solution. A sheet of Platinum was used as the counter electrode. The reference electrode was an Ag/AgCl. The electrochemical reduction reaction of CO2 was carried out in a CO2-saturated 0.25 M K2CO3


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solution. The CV curves were recorded by scanning the potential from -0.7 V to -1.5 V at a rate of 20 mV s-1. All measurements were conducted at room temperature. Characterizations. SEM images were obtained with an FEI field-emission scanning electron microscope (Sirion XL) operating at an accelerating voltage of 30 kV and a HITACHI fieldemission scanning electron microscope (S4800) operating at an accelerating voltage of 3 kV. TEM and HRTEM images were performed at a transmission electron microscopy (TEM, JEM2100F), and EDX mapping images were performed with an EDX system attached to a JEM2100F TEM. The adsorption spectra were obtained with an ocean optic UV/Vis spectrometer. Electrochemical measurements were conducted on AutoLab (PGSTAT302N) electrochemical station. ASSOCIATED CONTENT Supporting Information. SEM and TEM images of Cu NCs and the nanocrystals obtained with variation of reaction parameters, include reaction time, concentration of the etchant, reaction temperature or addition of the specific capping agent; XPS of etched nanocrystals, and SEM, XRD, and energy dispersive spectroscopy of CuSe nanocrystals. “This material is available free of charge via the Internet at http://pubs.acs.org.” AUTHOR INFORMATION Corresponding Author Jin, M.* ([email protected]), Yin, Y.* ([email protected]) Author Contributions


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The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources Financial support from National Natural Science Foundation of China (No. 21403160, 21471123, and 51202180). Financial support from the U.S. National Science Foundation (CHE-1308587). ACKNOWLEDGMENT We are grateful for the funding from the National Natural Science Foundation of China (Grant No. 21403160, and 21471123) and Xi’an Jiaotong University (“the start-up fund” and “the Fundamental Research Funds for the Central Uni-versities”). G.Y. acknowledges the funding from the National Natural Science Foundation of China (Grant No. 51202180) and the Scientific Research Foundation for the Returned Overseas Chinese Scholars, State Education Ministry. Y.Y. acknowledges support from the U.S. National Science Foun-dation (CHE-1308587). ABBREVIATIONS NCs, nanocubes; RDs, rhombic dodecahedrons; Cu, copper; Se, selenium; TOP, tri-noctylphosphine; SEM, scanning electron microscopy; TEM, transmission electron microscopy; HRTEM, high-resolution transmission electron microscopy; SAED, selected area electron diffraction; fcc, face-centered cubic; XPS, X-ray photoelectron spectroscopy; PVP, poly(vinylpyrrolidone); DMF, N, N-dimethylformamide; OAm, oleylamine; CV, cyclic voltammetry; HER, hydrogen evolution reaction; REFERENCES


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Insert Table of Contents

Chemical etching as the reverse process of growth can directly create high-energy facets on nanocrystalline catalysts for enhanced catalytic performance.


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Selectivity on Etching: Creation of High-Energy Facets on Copper

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