Chinese Journal of Chemical Physics  2018, Vol. 31 Issue (4): 370-374

The article information

Shan Zhou, Tung-Han Yang, Ming Zhao, Younan Xia

Quantitative Analysis of the Reduction Kinetics of a Pt(Ⅱ) Precursor in the Context of Pt Nanocrystal Synthesis

Chinese Journal of Chemical Physics, 2018, 31(4): 370-374

http://dx.doi.org/10.1063/1674-0068/31/cjcp1805121

Article history

Accepted on: June 30, 2018
Quantitative Analysis of the Reduction Kinetics of a Pt(Ⅱ) Precursor in the Context of Pt Nanocrystal Synthesis
Shan Zhoua, Tung-Han Yangb, Ming Zhaoa, Younan Xiaa,b
Dated: Received on May 29, 2018; Accepted on June 30, 2018
a. School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332, USA;
b. The Wallace H. Coulter Department of Biomedical Engineering, Georgia Institute of Technology and Emory University, Atlanta, Georgia 30332, USA
*Author to whom correspondence should be addressed. Younan Xia, E-mail:younan.xia@bme.gatech.edu
Abstract: In this letter, we report a quantitative analysis of how a Pt(Ⅱ) precursor is reduced to atoms at different temperatures for the formation of Pt nanocrystals with different morphologies and sizes. Our results suggest that in the early stage of a synthesis, the Pt(Ⅱ) precursor is reduced to atoms exclusively in the solution phase, followed by homogeneous nucleation to generate nuclei and then seeds. At a relatively low reaction temperature such as 22℃, the growth of the seeds is dominated by autocatalytic surface reduction that involves the adsorption and then reduction of the Pt(Ⅱ) precursor on the surface of the just-formed seeds. This particular growth pathway results in relatively large assemblies of Pt nanocrystals. When the reaction temperature is increased to 100℃, the dominant reduction pathway will be switched from surface to solution phase, producing much smaller assemblies of Pt nanocrystals. Our results also demonstrate that a similar trend applies to the seed-mediated growth of Pt nanocrystals in the presence of Pd nanocubes.
Key words: Kinetic model    Nanocrystal synthesis    Precursor reduction

Noble-metal nanocrystals have received ever increasing interests owing to their applications in catalysis, photonics, electronics, sensing, and medicine [1-3]. Platinum nanocrystals, in particular, are among the best choice of catalytic materials toward a number of reactions, including oxygen reduction and (de)hydrogenation [4-7]. Many research efforts have been devoted to improving the catalytic performance of Pt nanocrystals by controlling their surface structure or morphology [8-10]. Thanks to the progress over the last two decades, it is now possible to design and rationally produce Pt nanocrystals with diversified shapes using either one-pot synthesis or seed-mediated growth [8, 9]. However, current understanding of the nucleation and growth of Pt nanocrystals is mainly derived from qualitative observations and it remains unsolved how the Pt(Ⅱ) or Pt(Ⅳ) precursor is reduced in a typical synthesis [11]. To achieve a better control over the synthesis, it is of critical importance to elucidate the correlation between the outcome of a synthesis and the reduction kinetics involved [11].

Herein, we report a quantitative analysis of the kinetics at which a Pt(Ⅱ) precursor is reduced to atoms during the formation of Pt nanocrystals. The synthesis involves the mixing of PtCl$_4$$^{2-} with ascorbic acid (AA, reducing agent) and poly(vinylpyrrolidone) (PVP, colloidal stabilizer) in an aqueous solution held at a temperature in the range of 22-100 ℃. FIG. 1 (a, b) shows transmission electron microscopy (TEM) images of the products obtained at 22 and 100 ℃, respectively, in the absence of pre-formed seeds. The products obtained at 22 ℃ consisted of assemblies (with an average size of 31.8 nm) of small Pt nanocrystals that had an average size of about 2.5 nm. In contrast, the small Pt nanocrystals (also about 2.5 nm in size) formed at 100 ℃ tended to form assemblies with a smaller size of 17.4 nm. When pre-formed Pd nanocubes were introduced into the reaction system to serve as seeds, the small Pt nanocrystals were formed exclusively on the surface of the seeds at 22 ℃ (FIG. 1(c)). The Pd-Pt nanocrystals had an average size of 33.9 nm, which is similar to the size of the assemblies of Pt nanocrystals obtained in the absence of pre-formed seeds. This is reasonable considering the relatively high amount of Pt(Ⅱ) precursor (0.0102 mmol) used for seed-mediated growth compared to the amount of Pd seeds (0.0034 mmol). As such, the volume occupied by the Pd seeds can be more or less neglected. When the reaction temperature was raised to 100 ℃, however, the small Pt nanocrystals were observed to assemble both in the solution phase and on the surface of the seeds (FIG. 1(d)). The assemblies of Pt nanocrystals in the solution phase had a much smaller size compared to the assemblies obtained in the absence of pre-formed seeds (FIG. 1(b)). This can be attributed to the fact that some Pt atoms were deposited on the surface of the Pd nanocubes when pre-formed seeds were introduced, resulting in a smaller number of Pt for the formation of assemblies.  FIG. 1 (a, b) TEM images of the products obtained when PtCl_4$$^{2-}$ was reduced by AA at (a) 22 ℃ and (b) 100 ℃, respectively, in the absence of pre-formed seeds. (c, d) TEM images of the products obtained by reducing PtCl$_4$$^{2-} with AA at (c) 22 ℃ and (d) 100 ℃, respectively, in the presence of pre-formed Pd nanocubes as seeds To understand why products with different morphologies were formed at different reaction temperatures, we conducted a quantitative analysis of the reduction kinetics to elucidate the reduction pathway involved in the formation of Pt nanocrystals by following the methodology recently developed for the Pd system [12, 13]. In general, the reduction of a Pt(Ⅱ) precursor can also undergo two different pathways: solution-phase (Eq.(1)) versus surface-based (Eq.(2)):  \text{Solution}\ \text{reduction}:\text{Pt}(\text{II})+2{{\text{e}}^{-}}\xrightarrow{{{k}_{1}}}\text{P}{{\text{t}}^{0}} (1)  \text{Surface}\ \text{reduction}:\text{Pt}(\text{II})+\text{Pt}_{n}^{0}+2{{\text{e}}^{-}}\xrightarrow{{{k}_{2}}}\text{Pt}_{n+1}^{0} (2) where Pt(Ⅱ) is the precursor and Pt_n^0 represents the surface atoms on the nuclei or seeds. For solution reduction, the precursor ions are directly reduced to atoms in the solution phase. Surface reduction is an autocatalytic process, in which the precursor ions first adsorb onto the surface of the just-formed nuclei or pre-formed seeds and are then reduced to atoms. It is worth mentioning that the reducing agent can be assumed to take a more or less fixed concentration because it was used in large excess relative to the Pt(Ⅱ) precursor. The total reduction rate for Pt(Ⅱ) precursor can thus be expressed as:  \begin{eqnarray} \textrm{rate}=k_1 [\textrm{Pt(II})]+k_2 [\textrm{Pt(II)}][\textrm{Pt}_n^0] \end{eqnarray} (3) To quantitatively understand which reduction pathway was in dominance during the formation of Pt nanocrystals in the absence of pre-formed seeds, we used inductively-coupled plasma mass spectrometry (ICP-MS) to measure the concentration of Pt(Ⅱ) remaining in the reaction solution at different time points (FIG. 2 (a) and (b)). As can be seen from FIG. 2(a), when the reduction was conducted at room temperature, the concentration of Pt(Ⅱ) remained almost unchanged in the first 150 min and then underwent a sudden decrease. By fitting the experimental data with the Finke-Watzky model (Eq.(3), see the supplementary materials for details) [14, 15], we obtained a value of 2.97\times10^{-6} min^{-1} for the rate constant k_1 (solution reduction), whereas the rate constant for surface reduction (k_2) was 2.89\times10^{-2} min^{-1}(mmol/L)^{-1}. When the reaction temperature was raised to 100 ℃, the concentration of Pt(Ⅱ) started to drop as soon as the reagents were mixed, and fitting to the experimental data gave values of 1.44 min^{-1} for k_1 and 3.05 min^{-1}(mmol/L)^{-1} for k_2. Similar fittings were also carried out for the experimental data collected at other temperatures in the range of 22-100 ℃, as shown in FIG. S1 in supplementary materials. After acquiring the rate constants at each reaction temperature, we were able to calculate the reduction rate of each reduction pathway throughout the synthesis. As plotted in FIG. 2(c, d), both reduction pathways occurred at very slow rates in the early stage of a synthesis conducted at 22 ℃. Around 150 min into the synthesis, surface reduction was accelerated to take the dominance. In comparison, solution reduction and surface reduction occurred at much faster rates when the reaction temperature was raised to 100 ℃, and solution reduction was in dominance during almost the entire span of the synthesis. Only in a late stage did surface reduction become slightly faster than solution reduction.  FIG. 2 Quantitative analysis of the kinetic parameters for the reduction of Pt(Ⅱ) precursors by AA at different reaction temperatures. (a, b) The concentration of PtCl_4$$^{2-}$ precursor remaining in the reaction solution as a function of reaction time at two temperatures: (a) 22 ℃ and (b) 100 ℃, respectively. The Finke-Watzky (F-W) model was used to fit the data for the reduction of PtCl$_4$$^{2-} precursor. (c, d) The rates of solution reduction and surface reduction for PtCl_4$$^{2-}$ precursor as a function of reaction time corresponding to the data points in (a, b)

Based on the kinetic analysis, we could now understand why the Pt nanocrystals took distinct morphologies when the syntheses were conducted at different temperatures. A schematic of the mechanism for the formation of Pt nanocrystals is shown in FIG. 3(a, b). When no pre-formed seeds were present, solution reduction followed by homogeneous nucleation, was the only possible pathway to generate Pt$_n^0$ nuclei and then seeds in the stage of nucleation (FIG. 3(a)). After the formation of nuclei/seeds, the precursor could still be reduced in the solution and then deposited onto the nuclei/seeds for the growth. At the same time, the precursor could adsorb onto the surface of the nuclei/seeds and were then reduced through the autocatalytic surface reduction (FIG. 3(b)). The proportions of these two reduction pathways are largely determined by the kinetics, which is highly dependent on the reaction temperature. When the reaction was conducted at a relatively low temperature such as 22 ℃, both reduction pathways occurred at very slow rates and the majority of the reduction of the Pt(Ⅱ) precursor was dominated by surface reduction (FIG. 2(c)). Therefore, the number of nanocrystals formed was greatly reduced due to the limited homogeneous nucleation. Once the nuclei/seeds had been formed, the following reduction would preferentially occur on the surface of the nuclei/seeds for the nanocrystal to grow into larger sizes instead of generating additional nuclei/seeds. This led to the formation of relatively large assemblies of small Pt nanocrystals as the final products (FIG. 1(a)). When the reaction temperature was increased to 100 ℃, solution reduction was in dominance. The acceleration in solution reduction rate at the beginning of a synthesis resulted in more nuclei/seeds and thereby smaller assemblies of small nanocrystals as the final products (FIG. 1(b)). Additional TEM images showing assemblies of Pt nanocrystals obtained at other temperatures again confirm that the size of the assemblies decreased with the increase in reaction temperature (FIG. S2 in supplementary materials). In the stage of growth, deposition of atoms derived from both surface and solution reduction can take place on the surface of the just-formed seeds, resulting in the formation of additional small nanocrystals attached to the surface of the seeds. Therefore, the final products were always assemblies of Pt nanocrystals no matter which reduction pathway was in dominance, but the size of the assemblies was significantly reduced when the contribution from solution reduction was increased and more nuclei/seeds were produced at the nucleation stage. The possible involvement of random aggregation of small Pt nanocrystals can be ruled out, otherwise the assemblies should take a larger size at a higher temperature as more small Pt nanocrystals were produced in the nucleation stage.

 FIG. 3 (a, b) Schematics showing the reduction pathways of a Pt(Ⅱ) precursor in (a) nucleation and (b) growth stages, respectively, in the absence of pre-formed seeds. During nucleation, the Pt(Ⅱ) precursor follows the solution reduction pathway to generate Pt atoms, followed by homogeneous nucleation to generate Pt$_n^0$ nuclei or seeds. During growth, the Pt(Ⅱ) precursor undergoes either surface reduction or solution reduction to generate Pt atoms for their deposition onto the nuclei/seeds. (c) Potential energy diagrams corresponding to the conversion of Pt(Ⅱ) ions to Pt atoms via two different pathways. (d) Temperature-dependent percentages of contributions of solution reduction and surface reduction, respectively, to the total reduction when the concentration of the Pt(Ⅱ) precursor dropped from 1.02 mmol/L to 0.5 mmol/L

When we switched to seed-mediated growth, a similar trend was also observed (FIG. 1(c, d)). The small Pt nanocrystals were found to be exclusively attached to the surface of the Pd nanocube seeds at 22 ℃, indicating limited homogeneous nucleation due to the suppression of solution reduction at a relatively low temperature. At 100 ℃, small Pt nanocrystals were observed on the surface of the Pd nanocube seeds, but there was a relatively large proportion of assemblies of Pt nanocrystals in the solution, indicating the significant role of homogeneous nucleation caused by the acceleration in solution reduction kinetics.

Knowing the values of $k_1$ and $k_2$ at different reaction temperatures allowed us to calculate the activation energy of each reduction pathway [12]. By plotting ln$k_1$ and ln$k_2$ as a function of 1/$T$, the slope of the linear regression line could be used to calculate the activation energy of the reduction pathways using the Arrhenius equation (FIG. S3 in supplementary materials). FIG. 3(c) shows a potential energy diagram to illustrate the transformation of the Pt(Ⅱ) precursor into elemental Pt through two different reduction pathways. The surface reduction has an activation energy of 56.5 kJ/mol, which is much lower than that (140.4 kJ/mol) of the solution reduction. Therefore, at low reaction temperatures, the reduction of the Pt(Ⅱ) precursor prefers to undergo surface reduction due to the insufficient thermal energy to overcome the high energy barrier to solution reduction. Once the precursor ions have gained adequate thermal energy at high reaction temperatures, solution reduction will be preferred since the precursor ions can collide more frequently with the reducing agents than with the surface of nuclei/seed.

The transition from surface reduction to solution reduction at elevated temperatures could be quantitatively compared by analyzing the contributions of these two reduction pathways to the total reduction of the Pt(Ⅱ) precursor at different temperatures (FIG. 3(d)). At low reaction temperatures (22-80 ℃), more than 85% of the total reduction of the Pt(Ⅱ) precursor underwent the surface reduction pathway. When the reaction temperature approached 90 ℃, the contributions from surface reduction (60%) and solution reduction (40%) became comparable. The order was eventually reversed when the temperature was further raised to 100 ℃. At this temperature, surface reduction only contributed 33% to the total reduction while solution reduction dominated the total reduction (67%). As can be seen from the plot, the temperature needed to switch the dominance of reduction pathway was around 93 ℃.

In summary, we have quantitatively analyzed the reduction kinetics/pathway of PtCl$_4$$^{2-}$ by AA in the absence/presence of pre-formed seeds at different reaction temperatures. Our results indicate that the Pt(Ⅱ) precursor was exclusively reduced in the solution phase in the early stage of a synthesis for the generation of nuclei and then seeds through homogeneous nucleation regardless of the reaction temperature. During growth at a relatively low temperature (22-80 ℃), the Pt(Ⅱ) precursor preferentially adsorbed onto the surface of the seeds and was then reduced to atoms. When the temperature was greater than 90 ℃, the Pt(Ⅱ) precursor was preferentially reduced in the solution to generate Pt atoms for their subsequent deposition onto the seeds. Similar transition was also observed when pre-formed Pd nanocubes were introduced into the reaction system to serve as seeds, in which surface reduction was in dominance at a low reaction temperature and solution reduction prevailed at a high reaction temperature. A quantitative understanding of the reduction kinetics and pathways of the Pt(Ⅱ) precursor would offer insightful guidelines for the rational synthesis of Pt nanocrystals with controlled morphologies, to optimize their performance in various catalytic applications.

Supplementary materials: Experimental details, detailed methods for analyzing the reaction kinetics, ICP-MS results and curve fittings based on the Finke-Watzky model, TEM images, and linear fitting based on the Arrhenius equation.

ACKNOWLEDGMENTS

This work was supported in part by a grant from National Science Foundation of the United States (DMR-1505441) and startup funds from the Georgia Institute of Technology.

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a. 美国佐治亚理工学院 化学与生物化学系, 亚特兰大, 佐治亚 30332;
b. 美国佐治亚理工学院与埃默里大学生物医学工程系, 亚特兰大, 佐治亚 30332