Synthesis of Cubic-Shaped Pt Particles with (100) Preferential

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Synthesis of Cubic-Shaped Pt Particles with (100) Preferential Orientation by a Quick, One-Step and Clean Electrochemical Method Jie Liu,† Xiayue Fan,† Xiaorui Liu,† Zhishuang Song,† Yida Deng,‡ Xiaopeng Han,‡ Wenbin Hu,†,‡ and Cheng Zhong*,†,‡ †

Key Laboratory of Advanced Ceramics and Machining Technology (Ministry of Education) and ‡Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China ABSTRACT: A new approach has been developed for in situ preparing cubic-shaped Pt particles with (100) preferential orientation on the surface of the conductive support by using a quick, one-step, and clean electrochemical method with periodic square-wave potential. The whole electrochemical deposition process is very quick (only 6 min is required to produce cubic Pt particles), without the use of particular capping agents. The shape and the surface structure of deposited Pt particles can be controlled by the lower and upper potential limits of the square-wave potential. For a frequency of 5 Hz and an upper potential limit of 1.0 V (vs saturated calomel electrode), as the lower potential limit decreases to the H adsorption potential region, the Pt deposits are changed from nearly spherical particles to cubic-shaped (100)-oriented Pt particles. High-resolution transmission electron microscopy and selectedarea electron diffraction reveal that the formed cubic Pt particles are single-crystalline and enclosed by (100) facets. Cubic Pt particles exhibit characteristic H adsorption/desorption peaks corresponding to the (100) preferential orientation. Ge irreversible adsorption indicates that the fraction of wide Pt(100) surface domains is 47.8%. The electrocatalytic activities of different Pt particles are investigated by ammonia electro-oxidation, which is particularly sensitive to the amount of Pt(100) sites, especially larger (100) domains. The specific activity of cubic Pt particles is 3.6 times as high as that of polycrystalline spherical Pt particles, again confirming the (100) preferential orientation of Pt cubes. The formation of cubicshaped Pt particles is related with the preferential electrochemical deposition and dissolution processes of Pt, which are coupled with the periodic desorption and adsorption processes of O-containing species and H adatoms. KEYWORDS: cubic platinum particles, square-wave potential, (100) preferential orientation, electrochemical deposition, electrocatalyst

1. INTRODUCTION Due to their superior catalytic properties, supported Pt particles on conductive supports have been the most extensively studied electrocatalysts for a wide variety of important electrochemical reactions, e.g., electro-oxidation of methanol,1−3 ethanol,4 ammonia,5,6 and hydrazine7 as well as oxygen reduction reaction.8−11 These reactions play a vital role in the energyand environmental-related applications, such as fuel cells10 and the treatment of pollutants.6 Both experimental and theoretical investigations have demonstrated that many electrochemical reactions on Pt are highly sensitive to the surface structure (atomic arrangement of exposed crystal surface).12−14 Therefore, the shape of the Pt particles is a critical parameter in regulating their electrocatalytic properties because the shape determines facets terminating the surface and the corresponding surface atomic arrangement and coordination.12−14 Therefore, tremendous effort has been devoted to the synthesis of Pt particles with desired shape in order to tailor their properties such as the activity and selectivity.12−18 For instance, Pt cubes enclosed by six (100) facets have been found to be much more active compared to polycrystalline Pt particles for a wide variety of electrochemical reactions such as the electrooxidation of ammonia,17,19,20 methanol,21 and dimethyl ether22 and the oxygen reduction reaction in H2SO4 solution.23 It has been © 2017 American Chemical Society

generally accepted that the ammonia electrooxidation takes place almost exclusively on Pt(100) sites.19,20,24,25 As a result, Pt cubes with (100) preferential orientation exhibit 3−5 times higher activity than that of the polycrystalline Pt particles.17,26−28 Lu et al.22 found that the catalytic activity of Pt cubes with (100) preferential surfaces for the dimethyl ether electrooxidation was nearly 3 times higher than that of commercial Pt black catalyst. Han et al.21 demonstrated that cubic Pt particles with dominant (100) facets showed lower onset potential and higher current density for the electrooxidation of methanol and ethanol than those of the polycrystalline Pt catalyst, suggesting the higher catalytic activity of cubic Pt particles. Since the pioneering work from El-Sayed’s group,16 in which cubic Pt nanoparticles were prepared by H2 reduction with the presence of sodium polyacrylate, a large variety of wet chemical approaches have been developed for the shape-controlled synthesis of Pt particles.15,17,28−32 To date, significant progress has been made in this field, and the majority of synthesis methods require the introduction of organic or inorganic Received: March 26, 2017 Accepted: May 18, 2017 Published: May 18, 2017 18856

DOI: 10.1021/acsami.7b04267 ACS Appl. Mater. Interfaces 2017, 9, 18856−18864

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ACS Applied Materials & Interfaces

Figure 1. Schematic diagram of the periodic square-wave electrodeposition with the lower potential limit of (a) −0.3 and (b) −0.1 V (vs SCE) and (c) the potentiostatic electrodeposition at −0.3 V (vs SCE).

formation of Pt cubes enclosed by (100) facets was confirmed by the scanning electron microscopy (SEM), transmission electron microscopy (TEM), selected-area electron diffraction (SAED), and structure-sensitive reactions. The fraction of wide Pt(100) surface domains was estimated by the Ge irreversible adsorption. The electrooxidation of ammonia was used as a typical reaction since it is particularly sensitive to the amount of Pt(100) sites.17,20,24,28,44 In addition, the ammonia electrooxidation has received great interest since it addresses both clean energy supply and the environmental protection issues.6,46 To the best of our knowledge, this is the first time that cubic-shaped Pt particles with (100) preferential orientation have been prepared by the electrodeposition method.

capping agents that can be adsorbed selectively on the Pt(100) surfaces to prepare cubic (100)-oriented Pt particles.29 Typical capping agents include organic ones such as sodium polyacrylate,16,33 polyvinylpyrrolidone (PVP),34 tetradecyltrimethylammonium bromide (C14TABr),35 oleic acid/oleylamine,36 1-adamantanecarboxylic acid (ACA),37 and inorganic ones such as CO,38 KI,39 and Cu ions.30,31 For instance, Qu et al.31 and Niu et al.30 reported the preparation of cubic Pt particles with the size of 100−500 nm under the presence of Cu2+ ions in the synthesis solution. However, the use of these capping agents may introduce heterogeneous impurities and requires post-treatment to remove them.13 Besides, additional transfer (deposition) process of formed Pt particles onto the conductive support is required to form an electrode for electrocatalysis applications. This makes the preparation process relatively complicated and time-consuming. Previous studies including work from our group have demonstrated that the electrochemical deposition is a facile and one-step technique to prepare Pt electrocatalysts with controlled surface morphology.3,24,40−45 Unfortunately, to the best of our knowledge, there has yet to be a report of preparing cubic Pt particles with (100) preferential orientation by the electrodeposition method without the use of capping agents. In the present work, a one-step, clean, and quick electrochemical method was developed to prepare cubic (100)oriented Pt particles directly on the conductive support. The whole preparation process is extremely clean since the deposition solution is very simple, and it contains only H2PtCl6 and HCl. Pt cubes were prepared by the electrodeposition with the periodic square-wave potential mode with controlled lower and upper potential limits. The successful

2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. H2PtCl6 and GeO2 were purchased from Sigma-Aldrich. H2SO4, HCl, (NH4)2SO4, NaOH, and KOH were obtained from Sinopharm Chemical Reagent Co., Ltd. All the solutions were prepared using deionized water with 18.2 MΩ cm from a Milli-Q water purification system. The working electrode for the electrodeposition and electrochemical tests was a polished glassy carbon electrode (GCE) with a diameter of 5 mm. 2.2. Electrode Preparation and Characterizations. Pt particles were prepared on the surface of GCE by the electrochemical deposition with the periodic square-wave potential in 5 mM H2PtCl6 + 0.5 M HCl aqueous solution. The electrochemical deposition experiment was carried out using a PARSTAT 2273 electrochemical workstation with a three-electrode configuration. A GCE with the diameter of 5 mm served as the working electrode, Pt foil (4 cm2) as the counter electrode, and a saturated calomel electrode (SCE) as the reference electrode. The frequency and the upper potential limit of the periodic square-wave potential were 5 Hz and 1.0 18857


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ACS Applied Materials & Interfaces V (vs SCE), respectively. The lower potential limit was changed to investigate its effect on the surface morphology/preferential orientation of the deposited Pt particles (Figure 1a,b). For comparison, conventional potentiostatic electrodeposition was also used for the preparation of Pt particles in the same solution (Figure 1c). The shape/surface morphology of the formed Pt particles on the GCE was characterized by a SEM (S-4800, Hitachi, Japan). The surface structure of the Pt particles was investigated by a JEOL-2100F TEM and the SAED on the TEM. 2.3. Electrochemical Measurements. Electrochemical measurements were also performed in a three-electrode configuration with a PARSTAT 2273 electrochemical workstation. Pt particles electrodeposited on the GCE, a Pt foil (4 cm2), and SCE were used as the working electrode, the counter electrode, and the reference electrode, respectively. The potential measured by the SCE was referenced to a reversible hydrogen electrode (RHE) for the convenience of comparison. The preferential orientation and the electrochemically active surface area (ECSA) of prepared Pt particles were characterized by cyclic voltammograms (CVs) in 0.5 M H2SO4 solution at a scan rate of 0.05 V s−1. The ECSA of Pt particles is estimated from the steady CVs by the H desorption charge.47 The fraction of Pt(100) terrace sites or larger (100) domains on Pt cubes was calculated by the irreversible adsorption of Ge.48 The GCE with deposited cubic Pt particles on its surface was immersed for 1 min in GeO2 solution (0.01 M GeO2 + 1 M NaOH) at 0.85 V (vs RHE). It was subsequently rapidly transferred to a 0.5 M H2SO4 solution with a droplet of the GeO2 solution on the GCE surface. Voltammogram was recorded from 0.04 to 0.64 V (vs RHE) at 0.05 V s−1, and the charge related to the oxidation of Ge adatoms can be used to estimate the amount of Pt(100) sites. The electrocatalytic activity of Pt particles for the ammonia oxidation was investigated by CV measurements in 1 M KOH + 0.1 M ammonia solution at a scan rate of 0.01 V s−1. All solutions were deaerated by purging Ar gas (99.999%). All experiments were performed at controlled temperature of 25 ± 1 °C.

Figure 2. SEM images of the surface morphology of Pt particles prepared by the periodic square-wave deposition at the lower potential limit of (a) −0.3 and (c) −0.1 V (vs SCE) and (e) those prepared by the potentiostatic electrochemical deposition at −0.3 V (vs SCE). (b), (d), and (f) Magnification images of (a), (c), and (e), respectively. The inset shows the corresponding particle size distribution.

3. RESULTS AND DISCUSSION Figure 2 shows the SEM images of the surface morphology of Pt particles electrodeposited by different parameters. It is obviously seen that many cubic Pt particles are formed and randomly distributed on the substrate under the square-wave electrodeposition condition with the lower/upper potential limits of −0.3 and 1.0 V (vs SCE) and the frequency of 5 Hz (Figure 2a). High-magnification SEM image clearly reveals the formation of well-defined Pt cubes with straight edges and sharp corners (Figure 2b). This suggests the effectiveness of using square-wave potentials to control the shape of deposited Pt particles. The shape distribution of Pt particles is determined over 100 particles in the SEM images, which shows that the particle shape is dominated by the cubic-like shape with a yield of about 71%, and the particle size is about 175 nm (Figure 2a). For those irregular Pt particles, some of them exhibit a cubiclike shape to some extent but with dendrite growth on the surface, as can be seen more clearly in Figure 2b. The lower potential limit of the square-wave potential plays an important role in the shape of the deposited Pt particles. As the lower potential limit increases from −0.3 to −0.1 V (vs SCE), nearly spherical Pt particles with relatively rough surface are formed on the substrate (Figure 2c,d). Our previous work showed that the electrochemical deposition potential greatly affects the electro-reduction and diffusion processes of Pt ions during the electrodeposition process, which has a profound effect on the surface morphology of Pt deposits.3,40,41,43 Such spherical Pt particles are generally formed when the electrodeposition process is activation-controlled under relatively small deposition overpotential.40−42 As shown in Figure 2e,f, conventional

potentiostatic electrochemical deposition at −0.3 V (vs SCE) results in a totally different surface morphology of the Pt particles. The formation of prickly Pt particles with sharp tips on surface can be identified.20 Figure 3a shows the TEM image of a Pt cube, clearly demonstrating the nearly cubic shape. This is fully consistent with the SEM observations (Figure 2b). Figure 3b shows the high-resolution TEM (HRTEM) image of a cubic Pt particle, revealing the highly crystalline structure of the cubic Pt particle with clear lattice fringes. The interplanar spacing is 1.96 Å, which corresponds well to the lattice spacing of the Pt(200) plane (refer to Powder Diffraction File 04−0802, Joint Committee on Powder Diffraction Standards, 2004).20 The inset in Figure 3b shows the SAED pattern corresponding to this HRTEM image of the cubic Pt particle, indicating the single-crystalline nature. The angle between OA and OB is 45°, and the distance ratio between OA and OB is 1.414, indicating the [001] zone axis. The above results indicate that the formed cubic Pt particles are enclosed by (100) facets. Although SEM and TEM can provide direct information about the shape and atomic structure of the Pt particles, they can only reflect a limited number of particles and fail to represent the entire batch of particles. Therefore, structuresensitive electrochemical reactions are further used to characterize the overall information on the surface structure of Pt particles. It has been widely reported that the H adsorption/desorption on Pt in H2SO4 solution is highly 18858


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Figure 3. (a) TEM image of a cubic Pt particle and (b) corresponding HRTEM image of this cubic Pt particle. Inset is the corresponding SAED pattern.

confirming that cubic Pt particles have dominant (100) facets. Similar CV feature has also been extensively reported for cubic Pt particles synthesized by wet chemical methods.17,28,50 Prickly Pt particles also exhibit preferential (100) orientation to some extent. Furthermore, we note that the height of the h3 peak of cubic Pt particles is higher than that of prickly Pt particles and also much higher than that reported in the previous studies.20,21 This suggests the larger amount of wide Pt(100) terraces. On the contrary, for nearly spherical Pt particles, the height of the h2 peak is lower than that of the h1 peak, and there is no h3 peak, typical of polycrystalline Pt.44,51 Table 1 shows the value

sensitive to the surface structure of Pt, which can qualitatively characterize the type and density of various surface sites (preferential orientation) on Pt particles.17,24,44 Figure 4 shows

Table 1. Value of h2/h1 and h3/h1 of Pt Particles with Different Morphology

Figure 4. CVs recorded on cubic Pt particles, spherical Pt particles, and prickly Pt particles, respectively, in 0.5 M H2SO4 solution at 0.05 V s−1.

samples

h2/h1

h3/h1

spherical Pt particles prickly Pt particles cubic Pt particles

0.86 1.28 1.34

0.91 1.23

of h2/h1 and h3/h1 of Pt particles with different morphology. The value of h2/h1 peak current ratio for nearly spherical Pt particles is 0.86, corresponding to the typical value for polycrystalline Pt.24,44,51 Such spherical Pt particles have also been well-characterized by previous work from our group, demonstrating that they are featured by the polycrystalline feature without showing any preferential orientation.3,40−42 This is fully consistent with the present work. Compared with that of nearly spherical Pt particles, the value of h2/h1 for cubic Pt particles and prickly Pt particles is higher, suggesting a higher amount of Pt(100) sites. Also, the higher value of h3/h1 for cubic Pt particles in comparison to that of prickly Pt particles indicates the larger amount of wide Pt(100) terraces. A quantitative analysis of the amount of Pt(100) surface sites was performed by Ge irreversible adsorption. Figure 5 shows the voltammogram of Ge irreversible adsorption on cubic Pt particles, nearly spherical Pt particles, and prickly Pt particles, respectively, in 0.5 M H2SO4 solution. After Ge irreversible adsorption, peaks related to hydrogen adsorption/desorption processes on Pt are absent, indicating blockage of the Pt surface by the Ge coverage. There is a broad oxidation peak at around

the CVs recorded on the Pt cubes, nearly spherical Pt particles, and prickly Pt particles, respectively, in 0.5 M H2SO4 solution. All CVs show characteristic potential regions for the H adsorption/desorption region between 0.04 and 0.34 V (vs RHE), the double-layer region between 0.14 and 0.64 V (vs RHE), and the Pt oxidation/reduction region between 0.64 and 1.24 V (vs RHE).49 We note that the H desorption region in the anodic branch shows distinct difference for different Pt particles, indicating their different surface structures. Depending on the shape/morphology of the deposited Pt particles, the H desorption region exhibits two or three characteristic peaks, h1, h2, and/or h3, at about 0.09, 0.19, and 0.26 V (vs RHE). It has been generally accepted that h1, h2, and h3 peaks are related to the H desorption on (110) sites, (100) step sites and terrace borders, and wide (100) terrace sites or larger (100) domains, respectively.7,17,24,48 The CV of cubic Pt particles shows a higher h2 peak compared with the h1 peak and also exhibits a well-defined h3 peak. This is characteristic of (100)-oriented Pt, 18859


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reveal the intrinsic activity. All these CVs exhibit a characteristic oxidation peak at about 0.69 V (vs RHE), corresponding to the ammonia electrooxidation.6,24,41,44,54 Our previous work found that such oxidation peak was not observed in the KOH aqueous solution without ammonia.55 Furthermore, the peak oxidation current density depends strongly on the shape of the electrodeposited Pt particles. Generally, the specific (intrinsic) activity of electrocatalysts can be determined by the peak oxidation current density normalized by the Pt ECSA.6,56 The specific activity of the cubic Pt particles is 1.24 mA cm−2, which is 3.6 times higher than that of the nearly spherical Pt particles (0.34 mA cm−2) and 1.8 times higher than that of the prickly Pt particles (0.70 mA cm−2). Table 2 compares specific activity of Pt particles prepared in this work with those of previous work using electrochemical deposition methods for ammonia electrooxidation. The result shows that Pt cubes prepared in this work exhibit higher specific activity for ammonia electrooxidation due to the presence of higher amount of Pt(100) sites. The mechanism of ammonia electro-oxidation and intermediates formed during the reaction on Pt have been investigated in detail by previous studies.57−63 Both experimental17,19,62,64 and theoretical studies61,65 have confirmed that the ammonia electro-oxidation is extremely sensitive to the surface structure of Pt, and it takes place almost exclusively on Pt(100) sites especially on larger (100) domains. For example, on the basis of the electrochemical characterizations and online electrochemical mass spectrometry, Rosca et al.62,64 found that the enhanced activity of Pt(100) surface is attributed to its ability to stabilize active NH2 intermediates, favoring the formation of hydrazine (N2H4) that is then dehydrogenated quickly to N2. With the use of surface-enhanced Raman spectroscopy (SERS), Vidal-Iglesias et al.63 also found the formation of N2H4 during the ammonia electrooxidation on Pt. By contrast, Pt(111) surface results in a deeper dehydrogenation of NH3,ads to strongly adsorbed nitrogen atoms, which poisons the Pt surface.62 These experimental finds have been supported by theoretical studies such as molecular dynamics and density functional theory (DFT) calculations.61,65 This (100) structure-sensitive activity has been widely demonstrated not only on bulk Pt single crystal surfaces19,25 but also on Pt particles with (100) preferential orientation prepared by different methods.17−20,26,28,32,38,66 Previous studies have extensively reported that as the amount of Pt(100) sites, especially wide Pt(100) terraces increases, the specific activity of Pt for the ammonia electrooxidation increases.17,19,20,28 In this work, the corresponding Pt(100) proportion and specific activity from Pt particles with various morphology were summarized in Table 3, and the result shows that the specific activity of different Pt particles increases following the order nearly spherical Pt particles < prickly Pt particles < cubic Pt particles, suggesting an increase of Pt(100) sites as the shape of Pt particles changes from the nearly spherical shape to the cubic shape. This trend agrees well with that revealed by the CV measurements in H2SO4 solution (Figure 4) and the quantitative characterizations by the Ge irreversible adsorption (Figure 5). For Pt particles with high amount of Pt (100) sites, especially the cubic Pt particles, it has been proved theoretically and experimentally that the improved specific activity for ammonia oxidation was mainly attributed to the large amount of Pt (100) sites instead of the possible presence of surface structural defects.4,20,51 Besides, our previous work found that for polycrystalline Pt particles with similar morphology

Figure 5. CVs of Ge irreversible adsorption on cubic Pt particles, prickly Pt particles, and spherical Pt particles, respectively, in 0.5 M H2SO4 solution.

0.49 V (vs RHE) and a reduction peak at around 0.39 V (vs RHE) during the reverse scan, which is the characteristic redox peak for the Ge adatoms on Pt(100) sites.48,52 The value of the fraction of the Pt(100) sites can be estimated by the Ge oxidation charge.48 Only wider (100) surface domains that are large enough for Ge adsorption can be detected by using this method.17,53 The proportion of wide Pt(100) sites decreases in the order of cubic Pt particles (47.8%) > prickly Pt particles (26.9%) > nearly spherical Pt particles (13.8%). Previous work from our group prepared (100)-oriented Pt particles with prickly surface morphology by galvanostatic electrodeposition, and the highest obtained value of the fraction of wide Pt(100) surface domains was about 25%.20 This value is much lower than that of the cubic-shaped Pt particles in this work, indicating a much higher proportion of wide Pt(100) surface domains on the cubic Pt particles. This is also consistent with the H desorption behavior which exhibits stronger h3 peak (Figure 4) compared to that reported in our previous work.20 Another (100) structure-sensitive electrochemical reaction, i.e., electrooxidation of ammonia, has been used to evaluate the surface structural information on the Pt particles with different shape. Figure 6 shows CVs measured on cubic, prickly, and nearly spherical Pt particles in ammonia-containing alkaline solution. The current is normalized by the Pt ECSA so as to

Figure 6. CVs measured on cubic, prickly, and spherical Pt particles in 1 M KOH and 0.1 M ammonia aqueous solution at 10 mV s−1. 18860


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Table 2. Comparison of Specific Activity of Pt Particles Prepared in the Present Work with Previous Work Using Electrochemical Deposition Methods for Electro-Oxidation of Ammonia4 test protocola

catalyst type cubic Pt particles (present work) flower-like Pt particles spherical Pt particles sheet-like particles well-dispersed Pt nanosheets flower-like Pt particles Pt nanoparticles with preferential (100) orientation Pt electrodeposited at −0.2 V (vs Ag/AgCl) a

0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1

M M M M M M M M

NH3 NH3 NH3 NH3 NH3 NH3 NH3 NH3

+ + + + + + + +

specific activity (mA cm−2)b

ref

1.24 0.54 0.28 0.51 0.32 0.30 0.79 0.58

42 42 42 43 43 20 44

−1

1 M KOH, at 10 mV s 1 M KOH, at 10 mV s−1 1 M KOH, at 10 mV s−1 1 M KOH, at 10 mV s−1 1 M KOH, at 10 mV s−1 1 M KOH, at 10 mV s−1 0.1 M NaOH, at 10 mV s−1 1 M NaOH, at 20 mV s−1

using CV measurements. bThe specific activity was the peak oxidation current normalized by the Pt ECSA.

are formed instead. This again suggests that H adatoms are essential for the formation of cubic Pt particles during the electrodeposition by the periodic square-wave potential. It should be noted that this is not the only factor that contributes to the achievement of cubic Pt particles since the potentiostatic electrodeposition at the H-electroadsorption potential only results in the prickly Pt deposits without the formation of welldefined cubic Pt particles (Figure 2e,f). The periodic change of the electrode potential and the selection of the upper potential limit also play critical roles in controlling the shape of the Pt particles. For the successful formation of cubic Pt particles, an upper potential limit of 1.0 V (vs SCE) is required, which attains the potential range of OH-electroadsorption (surface oxide formation). Periodic electroadsorption and electrodesorption of OH-species is responsible for the selective electrodissolution of Pt, which promotes the disruption and the rearrangement of surface Pt atoms and also plays a significant role in the development of (100) preferential orientations.67 This together with the H-adtoms assisted formation of Pt(100) sites during the cathodic half-cycle expands the Pt(100) sites and results in the formation of cubic Pt particles. Although more experimental work is required and currently underway to give a more comprehensive mechanistic explanation of the development of cubic (100)-oriented Pt particles, this does not lower the practical importance of the electrochemical method developed in this work for preparing cubic Pt particles with preferential (100) orientation. Compared to commonly reported methods which require specific additives to prepare cubic Pt particles and also need subsequent post-treatment and electrocatalysts transfer process to form an electrode, this method has a great advantage of in situ growing Pt cubes on the conductive support in a very simple and clean aqueous solution.

Table 3. Results of Pt(100) Proportion and Specific Activity from Pt particles with Different Morphology samples

Pt(100) proportion

specific activity (mA cm−2)

spherical Pt particles prickly Pt particles cubic Pt particles

13.8% 26.9% 47.8%

0.34 0.70 1.24

although the Pt particle size could markedly influence the mass activity it had almost no influence on the specific activity.41,43 In addition, typical reported values of the specific activity of Pt particles with well-defined preferential (100) orientation for ammonia electrooxidation range from 1.1 to 1.3 mA cm−2.27,28,66 The value of the specific activity of cubic Pt particles (1.24 mA cm−2) obtained in the present work agrees well with that reported in previous studies.27,28,66 As mentioned previously, the intermediates formed during the ammonia electro-oxidation on Pt and the corresponding reaction mechanism have been extensively investigated in detail54,57−63 and therefore are out of the scope of the present work. The purpose of the present work is to demonstrate whether it is possible to develop a one-step and clean electrochemical method to prepare cubic (100)-oriented Pt particles. All the above results are fully consistent and support the formation of cubic Pt particles with preferential (100) orientation, confirming that we have developed a simple and effective approach to produce (100)-oriented Pt cubes without any capping agents. In this work, periodic square-wave potential was used to prepare Pt particles. During the anodic half-cycle, Pt is selectively electrodissolved, while during the cathodic halfcycle, Pt atoms are selectively electrodeposited on the surface. The preferential electrochemical dissolution and electrochemical deposition of Pt atoms are coupled with the electroadsorption and electrodesorption of O-containing species, e.g., OH, and H adatoms (depending on the lower and upper potential limits). All these factors contribute to the formation of cubic Pt particles. As shown in Figures 2 and 3, cubic Pt particles with preferential (100) orientation are formed when the lower potential limit is −0.3 V (vs SCE). Since the electroadsorption of H adatoms takes place at this potential, it suggests the presence of H adatoms plays an important role in the formation of Pt cubes. Previous studies found that the surface free energies of different Pt crystallographic planes could be changed by the H adsorption.49 When H atoms are present, Pt(100) has a lower surface free energy than Pt(111) surface, favoring the development of Pt(100) sites.7,20,49,51 However, as the lower potential limit shifts positively from −0.3 to −0.1 V (vs SCE), which is out of the H-electroadsorption/ electrodesorption potential range, nearly spherical Pt particles

4. CONCLUSION Pt cubes with (100) preferential orientation were in situ electrodeposited on the GCE surface by using periodic squarewave potential. As the lower potential limit of the square-wave shifts negatively to the H adsorption potential region, the shape of deposited Pt particles is changed from the nearly spherical shape to the cubic one. The formed cubic Pt particles are single-crystalline and enclosed by (100) facets, as characterized by HRTEM and SAED. Compared to nearly spherical Pt particles, cubic Pt particles show much more intense H desorption peaks corresponding to (100) step sites and terrace borders, and larger (100) domains, indicating the presence of the preferential (100) orientation of cubic Pt particles. The proportion of Pt(100) sites of cubic Pt particles is 47.8%, as determined by the Ge irreversible adsorption. The specific activity of cubic Pt particles for electrooxidation of ammonia is 18861


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ACS Applied Materials & Interfaces 1.24 mA cm−2, which is 3.6 times higher than that of the nearly spherical Pt particles without preferential (100) orientation. This also suggests the higher amount of Pt(100) sites especially larger (100) domains on the cubic Pt particles.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yida Deng: 0000-0002-8890-552X Xiaopeng Han: 0000-0002-7557-7133 Cheng Zhong: 0000-0003-1852-5860 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Key Research and Development Program (2016YFB0700205), National Natural Science Foundation of China (51571151), National Natural Science Foundation of China and Guangdong Province (U1601216), and Tianjin Natural Science Foundation (16JCYBJC17600).

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Synthesis of Cubic-Shaped Pt Particles with (100) Preferential

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