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Size- and Density-Controllable Fabrication of Platinum Nanoparticles/ITO Electrode by Pulse Potential Electrodeposition for Ammonia Oxidation Siyuan Li, Haiyan Chen, Jie Liu, Yida Deng, Xiaopeng Han, Wenbin Hu, and Cheng Zhong ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b08604 • Publication Date (Web): 02 Aug 2017 Downloaded from http://pubs.acs.org on August 7, 2017

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ACS Applied Materials & Interfaces

Size- and Density-Controllable Fabrication of Platinum Nanoparticles/ITO Electrode by Pulse Potential Electrodeposition for Ammonia Oxidation Siyuan Li,†,‡ Haiyan Chen,§,‡ Jie Liu,† Yida Deng,# Xiaopeng Han,# Wenbin Hu,†,# and Cheng Zhong*,†,# †

Key Laboratory of Advanced Ceramics and Machining Technology (Ministry of Education),

School of Materials Science and Engineering, Tianjin University, Tianjin 300072, China §

Department of Echocardiography, Zhongshan Hospital, Fudan University; Shanghai Institute of

Medical Imaging, Shanghai Institute of Cardiovascular Diseases, Shanghai 200032, China #

Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science

and Engineering, Tianjin University, Tianjin 300072, China ‡

These authors contributed equally to this work.

KEYWORDS: platinum nanoparticles, indium tin oxide, pulse potential electrodeposition, ammonia oxidation, electrocatalysis

ABSTRACT:

The

pulse

potential

electrodeposition

was

successfully

utilized

to

electrochemically fabricate platinum (Pt) nanoparticles on indium tin oxide (ITO) conductive

ACS Paragon Plus Environment


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glass substrate for catalysis towards ammonia electro-oxidation. The effect of deposition parameters (lower potential El, lower potential duration tl and upper potential duration tu) on the size and number density of Pt nanoparticles was investigated by the scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The electrocatalytic activity of the Pt nanoparticle/ITO electrode for ammonia oxidation was characterized by the cyclic voltammetry (CV) method. The results showed that lower El and longer tl accelerate the formation of Pt nuclei while longer tu favors the growth of grain size to some extent, as El mainly tunes electrochemical overpotential while tl and tu affect the activation and mass transfer process. By tuning the deposition parameters, Pt nanoparticle/ITO electrodes with polycrystalline nature and 5-nm-scale primary particles, could be easily modified in Pt particle size and number density. Furthermore, the Pt nanoparticle/ITO electrode shows high mass specific catalytic activity (MA) towards ammonia oxidation (1.65 mC µg-1), much higher than that of commercial Pt/C electrode (0.32 mC µg-1). And the high catalytic performance results not only from nanosize effect of Pt nanoparticles, but also from the special morphology formed during electrodeposition process. 1. INTRODUCTION Platinum nanoparticles (Pt nanoparticles) supported on appropriate conductive substrates have received significant attention due to their distinguished electrocatalytic performance and various applications such as electrocatalysis,1 electrochemical sensors,2 solar cells3 and energy conversion systems.4-5 To date, among various kinds of conductive substrates, carbon-based materials,3,

6-8

metals like Au9 and metal oxides such as TiO2 and CeO2,10-11 are typical and

traditionally used matrix materials. Recently, indium tin oxide (ITO) transparent glass has attracted considerable and increasing interest as a conductive support12-14 because of its high


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electrical conductivity, excellent light transmittance, relatively low cost15 and chemical stability. As a result, several methods have been developed to fabricate Pt nanoparticles on the ITO substrate, such as sputtering,15 chemical precipitation,14 cluster-beam deposition,16 chemical reduction17 and electrodeposition. Among various preparation methods, the method of electrochemistry was greatly applied in traditional fields,18-19 and the electrodeposition method has special advantages such as low cost, rapid deposition rate, easy-to-control procedure, high purity and in-situ formation of the product.20-22 Therefore, considerable work has focused on the electrodeposition of Pt nanoparticles on the ITO substrate. For instance, Zhao et al.23 utilized galvanic displacement technique to prepare Pt on ITO for oxygen reduction reaction catalysis. Zhang et al.24 fabricated Pt nanoflowers with particle size ranging from 200 nm to 2 µm using potentiostatic deposition for methanol electro-oxidation. In the previous of our work, Pt particles with the particle size from about 100 to 400 nm were prepared by the electrodeposition methods.25-26 To fully utilize rare resource of the noble metal, great efforts have been made to fabricate nano-scale Pt particles with high catalytic performance. It is critical to note that the particle size and number density have fundamental influence on the catalytic performance, and thus they should be controlled properly. However, conventional electrodeposition methods, including galvanostatic and potentiostatic electrodeposition,27 have limitations in controlling the particle size and number density due to limited controllable parameters. By comparison, pulse electrodeposition has the capability to reduce cathodic concentration polarization by enhancing mass transfer during the anodic potential. And through modifying the parameters, it provides us a promising approach to control nucleation-growth process of Pt nanoparticles and further to control the number density as well as the particle size. It is reported that pulse potential was



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successfully carried out in deposition of Pt nanoparticles supported on different substrates including ITO and improved performance was obtained.7, 28-29 For example, Hsieh et al.30 utilized the pulse electrodeposition technique to prepare flower-like Pt particles with approximate 700 nm in diameter on ITO for highly efficient dye-sensitized solar cell. Nevertheless, the influence of deposition parameters, such as lower potential (El), lower potential duration (tl) and upper potential duration (tu), on the particle size and number density of fabricated Pt nanoparticle/ITO electrodes, has remained unclear. Furthermore, Pt nanoparticles with small particle size below 10 nm, which are of great significance for practical applications, have been rarely achieved. In the present work, we deposited Pt nanoparticles on the ITO electrodes by pulse potential electrodeposition. And we systemically investigated the influence of the deposition parameters (lower potential limit, lower potential duration and upper potential duration) on the size and number density of Pt nanoparticles on ITO in the H2PtCl6 + HCl solution system. The size and number density of Pt particles were characterized by the scanning electron microscopy (SEM) and structural analysis was carried out by the transmission electron microscopy (TEM). The amount of the Pt loading was determined by an inductively coupled plasma-optical emission spectrometry (ICP) method. The electrocatalytic activity of the fabricated Pt nanoparticle/ITO electrodes towards ammonia oxidation was characterized by the cyclic voltammetry. For comparison, the electrocatalytic activity of commercial Pt/C catalysts for the ammonia oxidation was also measured under the same condition. 2. EXPERIMENTAL SECTION 2.1. Reagents and Materials. Hexachloroplatinic acid (H2PtCl6) was purchased from SigmaAldrich. H2SO4, KOH and (NH4)2SO4 were purchased from Beijing Chemicals (Beijing, China). All chemicals were of analytical grade and used as received. Indium tin oxide (ITO) glass


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substrates were purchased from Southern Glass Co., Ltd., Shenzhen, China. Commercial Pt/C catalysts (10 wt.% Pt) with Pt particle size below 3.5 nm were purchased from Johnson Matthey Company. All the solutions were prepared from ultrapure water (18.2 MΩ cm) purified by using a Milli-Q water purification system (Millipore, Billerica, MA, USA) and deaerated by purging a high-purity nitrogen (N2) gas (99.999%) throughout the test. 2.2. Electrode preparation. Prior to use, the ITO glass substrates were cleaned in acetone and then ultrapure water by the sonication and dried with a nitrogen stream. The Pt particles were deposited on an ITO substrate by the pulsed potential electrodeposition method in an aqueous solution containing 5 mM H2PtCl6 and 0.5 M HCl. The electrodeposition was performed on an electrochemical workstation (PARSTAT 2273, Princeton Applied Research, USA) with a threeelectrode cell. A sheet of ITO with an exposed geometry area of 1 cm2 was used as the working electrode. The reference electrode and the counter electrode were a saturated calomel electrode (SCE) and a Pt plate. Among deposition parameters, upper potential (Eu) was kept at 0.5 V (vs. SCE) as higher Eu may cause anodic degradation of ITO.31 And the investigated parameters in the experiment are lower potential (El), lower potential duration (tl) and upper potential duration (tu) (Figure 1). The total time of tl under different deposition conditions was kept for 40 seconds. And mean currents of various deposition conditions were obtained by mathematical integration of current-time curves and then divided by total deposition time (listed in Table 1).



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Figure 1. Diagram of pulsed electrodeposition parameters.

2.3. Characterizations. The particle size and number density of the deposited Pt nanoparticles were investigated by the SEM (FEI Nova 200, NanoLab SEM). High-resolution morphology and structure of Pt nanoparticles were characterized by the TEM (JEM-2100F). To obtain the amount of the deposited Pt (Pt loading), the prepared Pt deposits were fully dissolved by 3 mL aqua regia at 70 °C for 30 min. The aqua regia solution was then diluted with ultrapure water to 10 mL. The Pt concentration of in this solution was analyzed by an inductively coupled plasma-optical emission spectrometry (Thermo elemental, IRIS intrepid).32 Based on the concentration of the dissolved Pt in the solution and volume of the solution, the amount of Pt can be determined.32 All electrochemical tests were carried out on the electrochemical workstation. A threeelectrode system was also used, where a Pt nanoparticle/ITO electrode prepared at different electrodeposition conditions was used as the working electrode, a SCE and a Pt plate were used as the counter electrode and the reference electrode, respectively. The electrochemical active surface area (ECSA) of the Pt nanoparticle/ITO electrode was calculated from the steady CV at 50 mV s −1 in 0.5 M H2SO4 solution.33 The electrocatalytic activity was characterized by CV in


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0.1 M ammonia and 1 M KOH at a scan rate of 10 mV s-1.34 All the solutions were deaerated with high-purity N2 gas (99.999%) for 30 min before use and throughout the test. 3. RESULTS AND DISCUSSION All deposition parameters in the experiments, results obtained from SEM images and data of mean deposition current were listed in Table 1. First the effect of lower potential (El) on the formed Pt nanoparticles was investigated. Figure 2 shows the particle size and number density of Pt nanoparticles deposited at various lower potential from −0.2 to −1.0 V (vs. SCE). From Figure 2a-e, the mean particle size of Pt nanoparticles decreases dramatically with the decreasing of the El from −0.2 to −0.8 V (vs. SCE), from about 90 to 30 nm in diameter. And according to the standard deviations of particle size in Table 1, Pt nanoparticles shows a more uniform size distribution as the El decreases. As for the number density, Figure 2a-d showed an obvious increase in density, from 3.0×109 cm−2 to 4.5×1010 cm−2. However, when the El is further decreased to −1.0 V (vs. SCE) in Figure 2e, the increasing trend of number density is not apparent as that of Figure 2a-d, and the Pt loading declines. Therefore it is worth noticing that at the El of −0.8 V (vs. SCE), the Pt nanoparticles exhibit fine particle size, well size distribution and proper Pt loading.



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Figure 2. SEM images of Pt nanoparticle/ITO electrodes electrodeposited at tl = 1 s, tu = 0.1 s and El = (a) −0.2 V, (b) −0.4 V, (c) −0.6 V, (d) −0.8 V and (e) −1.0 V (vs. SCE). The insets show SEM images at higher magnification.

Table 1. Parameters and results related to pulse electrodeposition Sample Number

El (V)

tl (s)

tu (s)

Mean particle size (nm)

Standard Deviation of particle size

Mean number density (cm-2)

Mean current (mA)

#1

−0.2

1

0.1

89.5

17.1

3.0×109

−0.68

#2

−0.4

1

0.1

49.5

20.7

5.1×109

−1.32


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#3

−0.6

1

0.1

42.6

13.8

1.7×1010

−6.04

#4

−0.8

1

0.1

36.6

10.1

4.5×1010

−17.33

#5

−1.0

1

0.1

24.8

5.8

5.5×1010

−22.68

#6

−0.8

0.01

0.1

68.3

16.6

4.4×109

−1.18

#7

−0.8

0.1

0.1

58.8

18.2

1.4×1010

−2.40

#8

−0.8

2

0.1

34.5

9.2

5.7×1010

−24.36

#9

−0.8

0.1

0.01

34.1

7.6

2.6×1010

−25.25

#10

−0.8

0.1

1

68.2

15.7

8.4×109

−1.34

#11

−0.8

0.1

2

88.6

23.1

6.6×109

−0.65

#12

−0.8

0.01 0.01

66.0

16.4

7.4×109

−0.66

#13

−0.8

0.01

94.7

17.0

1.3×109

−0.12

1

Figure 3 shows the influence of the lower potential time (tl) on the particle size and number density of Pt particles deposited at El = −0.8 V (vs. SCE) and tu = 0.1 s. When the tl is set as 0.01 s, the average particle size of the deposited Pt nanoparticles is about 68.3 nm (Figure 3a). As the tl increases to 0.1 s, the refinement of the Pt particle size is noticeably observed, with the average particle size of about 58.8 nm (Figure 3b). With the tl continuously increasing to 1 s, a significant decline of the particle size of Pt nanoparticles occurs and the number density of Pt particles is also greatly enhanced (Figure 3c). And when the tl increases to a higher value of 2 s, deposited Pt nanoparticles agglomerate into a nearly continuous Pt film, instead of being individual particles (Figure 3d).



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Figure 3. SEM images of Pt nanoparticle/ITO electrodes electrodeposited at El = −0.6 V, tu = 0.1 s and tl = (a) 0.01 s, (b) 0.1 s, (c) 1 s, (d) 2 s.

Figure 4 and Figure 5 further investigate the effect of upper potential time (tu) on the Pt nanoparticles. Figure 4 shows the particle size and number density of Pt nanoparticles deposited under tl = 0.1 s and different tu ranging from 0.01 s to 2 s. It can be seen that the tu in the pulsed electrodeposition has a remarkable effect on particle size of Pt nanoparticles. As tu increases from 0.01 to 2 s, the mean particle size of Pt nanoparticles increases noticeably from 34.1 (Figure 4a) to 88.6 nm (Figure 4d). When longer tu is applied, Pt particles contact with each other, showing a tendency of particle agglomeration. Simultaneously, the number density of Pt nanoparticles decreases apparently with the increment of the tu, from 2.6×1010 cm−2 to 6.6×109 cm−2. In addition, larger standard deviation of particle size in Table 1 indicates decentralized particle size distribution at longer tu. Figure 5 shows SEM images of Pt nanoparticles deposited under tl = 0.01 s (shorter than Figure 4) and different tu ranging from 0.01 s to 1 s. Apart from the


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increase of particle size and great decline of number density with increment of the tu, it is clearly seen that the tu play a significant role in the surface morphology of the deposited Pt nanoparticles. When the tu is 0.01 s (Figure 5a), the surface morphology of Pt is characterized by sharp-edged polygonal particles and pricky particles. As the tu increases to 0.1 s, the pricks attached to Pt nanoparticles and sharp edges begin to disappear (Figure 5b). Furthermore, as the tu increases to 1 s, the Pt particles are featured with spherical-like morphology (Figure 5c).

Figure 4. SEM images of Pt nanoparticle/ITO electrodes electrodeposited at El = −0.8 V (vs. SCE), tl = 0.1 s and tu = (a) 0.01 s, (b) 0.1 s, (c) 1 s, (d) 2 s.



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Figure 5. SEM images of Pt nanoparticle/ITO electrodes electrodeposited at El = −0.8 V (vs. SCE), tl = 0.01 s and tu = (a) 0.01 s, (b) 0.1 s, (c) 1 s. The insets show SEM images at higher magnification.

Understanding the parameters in the electrodeposition process is essential to realize the parametric control of the particle size, number density and further to improve the catalytic performance. When pulse potential is applied, two main process occur simultaneously in aqueous electrochemical system: the activation process and the mass transfer process.35 When El is applied in the cathodic half-cycle of the pulsed potential electrodeposition, the reduction of Pt ions and the nucleation and growth of Pt nuclei occur. And at anodic half-cycle (tu), depletion of metal ions near the cathode is supplemented by diffusion of ions from bulk solution,36 and therefore concentration polarization on electrodes decreases, which makes pulse deposition different from galvanostatic and potentiostatic deposition. By modifying the deposition parameters, the above processes could be influenced and therefore be controlled. Herein the effect of El, tl and tu could be interpreted as follows. (1) El. In our study, electrochemical overpotential (η) could be directly determined by the El applied on cathode. During the electrodeposition, overpotential is the parameter of major importance in providing driving force to nucleation-growth process and further influencing size and number density of Pt nanoparticles. In electro-nucleation, only those nuclei that reach or exceed the critical radius could have their Gibbs free energy decreasing by crystal growth and could continue growing.37 And according to previous literature, the critical radius is inversely proportional to overpotential.38 Thus at higher overpotential, more nucleation site would be


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activated as critical radius decreases. Furthermore, the nucleation rate is determined by the following equation:36

v = K1 exp(

− K2

η

)

(1)

where K1 is constant and K2 is related to the energy for nucleation. According to equation (1), higher cathodic current in Table 1 (corresponding to sample #1−#5) caused by increasing |η|, favors the formation of Pt nuclei on the substrate. Meanwhile, the increasing |η| decelerates the growth of Pt nanoparticles, which leads to the decrease of the particle size, as shown in Figure 2. However, it is unrealistic to raise |η| without limit for higher nucleation rate and smaller particle size. As Pt is considered as one of the metals that have high catalytic performance for hydrogen evolution reaction,39 hydrogen is relatively easier to generate at the surface of Pt nanoparticle/ITO electrode in aqueous solution and leads to decreased deposition current efficiency. Figure 2e illustrates the phenomenon that the formation of Pt is possibly prevented by competitive hydrogen evolution. Furthermore, ultrahigh |η| would result in the electrochemical reduction of ITO,40 which destroys the conductive substrate and has negative effects on the electrodeposition process. (2) tl. Other things being equal, with longer tl (sample #4 and #6 – #8 in Table 1), the mean cathodic current increases dramatically. Therefore, the Pt nanoparticles are featured with higher number density as more nuclei are formed (Figure 3). At shorter tl, reduced concentration of PtCl62- ions near the electrode surface caused by consumption at cathodic half-cycle would obtain timely supplement at anodic half-cycle and the deposition process is controlled mainly by activation process.41 With increased tl, the growth of Pt nanoparticles would be limited by the diffusion rate of ions and therefore the process gradually switches to mass transfer control, which causes the decline of the mean particle size of Pt nanoparticles at longer tl. Similarly, inadequate



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supplement of ions from bulk solution at longer tl would improve proportion of competitive reactions and decrease the deposition current efficiency. In experiments more vigorous hydrogen evolution was observed at relatively longer tl accompanied by higher cathodic mean current. (3) tu. The tu affects the deposition process mainly by influencing the mass transfer process. As shown separately by sample #6, #12, #13 and #7, #9, #10 in Table 1, the decrease of number density of Pt nanoparticles with longer tu could be illustrated by the smaller mean current, which is in agreement with above discussion. And the application of Eu causes anodic current that leads to the preferential dissolution of nuclei with smaller radius and thus with longer tu the number density decreases. As of morphology, selective dissolution exists at sharp edge of Pt nanoparticles and therefore more regular spherical particles form at longer tu (Figure 5c). In addition, the growth of Pt particles resulting from relatively longer tu, could be explicated by adequate supply of PtCl62- ions from bulk solution and the deposition is controlled by the activation process.41 In previous study, Pt nanoparticle/ITO electrodes featured with large particles and spherical particles performed inferior mass specific catalytic activity.16 Hence in consideration of the effect of tu, to obtain well-dispersed nano-sized Pt nanoparticles for catalysis, insufficient Pt supply caused by a relatively shorter tu, or “ion choke”, is needed. To satisfy certain needs, the best electrodeposition condition provides optimal balance between those parameters discussed above.


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Figure 6. Pt loading-normalized CVs measured on Pt nanoparticle/ITO electrode samples (#4, #7) in 0.5 M H2SO4 solution at 0.05 V s-1.

To measure the electrochemically active surface area (ECSA, cm2) of fabricated Pt nanoparticle/ITO electrodes, two samples (#4 and #7) with proper Pt particle size, number density and Pt loading were tested by a cyclic voltammetry (CV) method in N2-saturated aqueous solution containing 0.5 M H2SO4 at a scan rate of 0.05 V s-1. As shown in Figure 6, the regions related to hydrogen adsorption/desorption, double-layer effect and construction/destruction of Pt oxide layer could be clearly observed in CV process, and polycrystalline feature of deposited Pt is obviously identified from specific region of hydrogen adsorption.42 From CVs, the ECSA of electrodes could be estimated by following equation:43

ECSA = QH / QH0

(2)

where QH refers to the charge related to hydrogen adsorption/desorption process (mC) and QH0 is the theoretical quantity of charge needed for a hydrogen adsorption monolayer on the assumption that one Had for one Pt surface atom (0.21 mC cm-2).43 QH is calculated by integration of the



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region which is corresponding to hydrogen adsorption/desorption in CVs (from −0.2 to 0.2 V vs. SCE), excluding the effect of double-charge layer and then divided by scan rate of CVs.41 Furthermore the ECSA of each electrode is respectively normalized by deposited Pt loading (mPt, µg) and the results are defined as mass specific active surface area (SSA, cm2 µg-1). Calculated results listed in Table 2 shows that the SSA of Pt/C and Pt/ITO electrodes are similar. And sample #4, which was deposited at longer tl and featured with finer Pt nanoparticles, exhibits a relatively higher SSA when compared with #7. The higher SSA of sample #4 suggests that Pt nanoparticles deposited on electrode #4 has higher surface-to-volume ratio than those on electrode #7, which is in consistence with SEM results shown in Figure 3b (#7) and c (#4). Table 2. Results related to CVs measured in H2SO4 solution. QH (mC)

ECSA (cm2)

mPt (µg)

SSA (cm2 µg-1)

Pt nanoparticle/ITO #4

0.26

1.24

10.0

0.12


0.24

1.14

15.4

0.08

Pt/C

0.06

0.29

2.5

0.11


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Figure 7. Pt loading-normalized CVs measured on Pt nanoparticle/ITO electrodes (#4, #7), a commercial 10% Pt/C electrode and a pure ITO substrate in 0.1 M NH3 + 1 M KOH solution at 0.01 V s-1.

The catalytic performance of Pt nanoparticle/ITO electrodes was tested by electro-oxidation reaction of ammonia with the technique of CV in N2-saturated solution containing 0.1 M NH3 and 1 M KOH at 0.01 V s-1. Pt loading-normalized CVs are shown in Figure 7. Here a commercially available Pt/C (10% Pt, mass fraction) electrode is tested with the same method for comparison. In previous literature, it was shown that there was no characteristic current peak at potential of around −0.37 V (vs. SCE) in aqueous solution without ammonia44-45 and pure ITO showed no oxidation peak in ammonia (inset of Figure 7). Therefore, the noticeable peak at −0.37 V (vs. SCE) could be ascribed to ammonia electro-oxidation on Pt nanoparticles. It is clear that Pt nanoparticle/ITO electrodes exhibit ultrahigh catalytic activity in contrast with that of commercial Pt/C. Moreover, sample #4 shows higher catalytic performance than #7, and therefore to some extent the increment of tl during electrodeposition leads to enhanced catalytic activity of Pt nanoparticle/ITO electrodes. This is in agreement with above discussion. To quantification the catalytic performance of each electrode, QAOR, defined as the charge for ammonia oxidation (mC), is calculated by integration of current curve under the oxidation peak and then divided by the scan rate.25 And when the QAOR is normalized by Pt loading mPt, mass specific activity (MA, mC µg-1), a parameter that reveals intrinsic catalytic activity, is obtained:41

MA=

QAOR mPt

(3)

Herein the mass activity is defined as the catalytic activity per mass of Pt. And specific activity (SA, mC cm-2) could also be obtained when normalizing the QAOR by the ECSA of each



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electrode. Table 3 lists calculated results corresponding to catalytic performance towards ammonia electro-oxidation. It is obvious that a magnificent increase in the catalytic performance is observed on Pt nanoparticle/ITO electrodes, in contrast to Pt/C. In Table 3, sample #4 shows approximately 5 times the MA of Pt/C and 3.6 times the peak current of Pt/C, suggesting enhanced catalytic performance of Pt/ITO electrodes. Furthermore, #4, deposited at longer tl than #7, shows higher performance in both the MA and SA, indicating that catalytic activity of Pt nanoparticle/ITO can be successfully and easily altered by modifying the electrodeposition parameters. Table 3. Results related to CVs measured in ammonia solution. QAOR (mC)

MA (mC µg-1)

SA (mC cm-2)


19.63

1.65

15.83


16.52

1.27

14.49

Pt/C

0.80

0.32

2.75


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Figure 8. (a) TEM and (b) high-resolution TEM (HRTEM) image of Pt nanoparticles of sample #4. (c) FFT results of selected areas in red square.

From CV results, sample #4, with the highest catalytic performance, was chosen from Table 1 for further investigation by the TEM. Figure 8 shows the TEM and the high-resolution TEM (HRTEM) images of the Pt nanoparticles of sample #4. As shown in Figure 8a, small primary nanocrystals with about 5 nm in size aggregate together to form bigger Pt particles. The high catalytic performance of Pt nanoparticle/ITO electrodes could be ascribed to the nano-size effect of those primary nanocrystals. According to previous literature, the mechanism of ammonia electro-oxidation on Pt metal proposed by Gerischer and Mauerer is shown as follows:46 NH3(aq) → NH3,ads

(4)

NH3,ads + OH− → NH2,ads + H2O + e−

(5)

NH2,ads + OH− → NHads + H2O + e−

(6)

NHx,ads + NHy,ads → N2Hx+y,ads

(7)

N2Hx+y,ads + (x + y)OH− → N2 + (x + y)H2O + (x + y)e−

(8)

NHads + OH− → Nads + H2O + e−

(9)

where x = 1 or 2 and y = 1 or 2. It is reported that when particle size of Pt decreases to around 5 nm, a stronger adsorption capacity for OH- might occur on Pt surface in alkaline solution and therefore the amount of adsorbed poisonous intermediates Nads decreases simultaneously due to competitive adsorption.17, 47 Thus as sample #4 consists of finer Pt nanoparticles than that of #7, the higher catalytic performance results not only from higher SSA but also from enhanced catalytic activity per active surface area (SA). Furthermore, in previously reported literature, the Pt (100) sites have been proved to exhibit ultrahigh catalytic performance towards ammonia oxidation48-49 while Martinez-Rodriguez et al. prepared Pt nanoparticles with high fraction of (100) sites and



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obtained an oxidation peak current of 2.0 mA cm-2.50 Nevertheless, sample #4 shows peak current of 0.9 mA cm-2 according to Figure 7 although without clear evidence of preferential (100) orientation in Figure 8c. Thus the specific morphology such as protruding edges and pricky structures of Pt nanoparticles formed during tl is an important factor that must be taken into consideration and is believed to boast high catalytic activity.27, 51-53 Thanks to those specific structures, in comparison with commercial available Pt/C electrode, Pt nanoparticle/ITO electrodes provide higher performance due to higher SA. In addition, during longer tl, the formation and growth of Pt nuclei are controlled by the mass transfer process and Pt nanoparticles tend to grow in 2-D structure to produce more protruding morphology, leading to higher MA of sample #4 than that of sample #7. In conclusion, the cause of improved performance of Pt nanoparticle/ITO electrode lies in nano-size effect and specific morphology of Pt nanoparticles, while by modifying the pulse electrodeposition parameters and further the size and number density of Pt nanoparticles, the catalytic activity of Pt nanoparticle/ITO electrodes could be improved successfully and easily. 4. CONCLUSION Pt nanoparticles were successfully fabricated on ITO glass substrate using the pulse potential electrodeposition method. The deposition parameters (lower potential El, lower potential duration tl and upper potential duration tu) have significant effect on the particle size and number density of Pt nanoparticles. Lower El, longer tl or shorter tu have similar influence in accelerating the formation of Pt nuclei and preventing growth of Pt nanoparticles, as the El mainly tunes electrochemical overpotential while the tl and tu affect the activation and mass transfer process. By modifying the deposition parameters, Pt particle size and number density could be easily modified. Furthermore, the selected Pt nanoparticle/ITO samples show high catalytic


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performance towards ammonia oxidation, much higher than that of commercial 10 wt.% Pt/C electrode. Although lacking preferential (100) orientation, the Pt nanoparticle/ITO electrodes exhibit ultrahigh mass specific activity (MA) compared with that of Pt/C. And the high catalytic performance results not only from nano-size effect of Pt nanoparticles, but also from specific morphology of Pt nanoparticles formed during deposition process. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (C. Zhong) Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Key Research and Development Program (2016YFB0700205), National Natural Science Foundation for Distinguished Young Scholars (51125016), Joint Funds of the National Natural Science Foundation of China (U1601216), and Tianjin Natural Science Foundation (16JCYBJC17600).

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Figure 1. Diagram of pulsed electrodeposition parameters. 297x209mm (150 x 150 DPI)


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Figure 2. SEM images of Pt nanoparticle/ITO electrodes electrodeposited at tl = 1 s, tu = 0.1 s and El = (a) −0.2 V, (b) −0.4 V, (c) −0.6 V, (d) −0.8 V and (e) −1.0 V (vs. SCE). The insets show SEM images at higher magnification. 1014x928mm (72 x 72 DPI)



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Figure 3. SEM images of Pt nanoparticle/ITO electrodes electrodeposited at El = −0.8 V, tu = 0.1 s and tl = (a) 0.01 s, (b) 0.1 s, (c) 1 s, (d) 2 s. 595x426mm (119 x 119 DPI)


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Figure 4. SEM images of Pt nanoparticle/ITO electrodes electrodeposited at El = −0.8 V (vs. SCE), tl = 0.1 s and tu = (a) 0.01 s, (b) 0.1 s, (c) 1 s, (d) 2 s. 191x132mm (300 x 300 DPI)



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Figure 5. SEM images of Pt nanoparticle/ITO electrodes electrodeposited at El = −0.8 V (vs. SCE), tl = 0.01 s and tu = (a) 0.01 s, (b) 0.1 s, (c) 1 s. The insets show SEM images at higher magnification. 1352x463mm (72 x 72 DPI)


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Figure 6. Pt loading-normalized CVs measured on Pt nanoparticle/ITO electrode samples (#4, #7) in 0.5 M H2SO4 solution at 0.05 V s-1. 297x209mm (150 x 150 DPI)



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Figure 7. Pt loading-normalized CVs measured on Pt nanoparticle/ITO electrodes (#4, #7), a commercial 10% Pt/C electrode and a pure ITO substrate in 0.1 M NH3 + 1 M KOH solution at 0.01 V s-1. 297x209mm (150 x 150 DPI)


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Figure 8. (a) TEM and (b) high-resolution TEM (HRTEM) image of Pt nanoparticles of sample #4. (c) FFT results of selected areas in red square. 1049x462mm (72 x 72 DPI)



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Table of content 807x587mm (72 x 72 DPI)


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ITO

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