Journal of Catalysis 317 (2014) 83–90

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Journal of Catalysis

journal homepage: www.elsevier .com/locate / jcat

Increased silver activity as a result of controllable reaction-drivenreconstruction for high-index facets

http://dx.doi.org/10.1016/j.jcat.2014.05.0210021-9517/� 2014 Elsevier Inc. All rights reserved.

⇑ Corresponding author. Fax: +86 22 27892340.E-mail address: gzli@tju.edu.cn (G. Li).

Guozhu Li a,⇑, Yujiao Jiang a, Danlei Chen a, Jian Li b, Shuxiang Lu b

a Key Laboratory for Green Chemical Technology of Ministry of Education, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, People’s Republic of Chinab College of Material Science and Chemical Engineering, Tianjin University of Science and Technology, Tianjin 300457, People’s Republic of China

a r t i c l e i n f o

Article history:Received 15 April 2014Revised 28 May 2014Accepted 29 May 2014

Keywords:Silver nanoparticlesCatalytic activityRestructuringHigh-index facet

a b s t r a c t

Metal nanoparticles (NPs) have been studied extensively as catalysts due to their high activity. However,the application of metallic nanostructures in catalytic reactions is hampered by their uncontrollable per-formance due to their reconstruction in reactive environments. Here, controllable restructuring of silverNPs in the reactive environment has been achieved to increase their activity. Surface restructuring of theas-synthesized silver NPs was monitored in situ by UV–vis spectra. Kinetic analysis, transmission electronmicroscopy (TEM), and molecular simulation were conducted to investigate the mechanism of this pro-cess. Results indicate that this reaction-driven restructuring leads to the formation of reactive high-indexfacets on silver NPs. In the reduction of 4-nitrophenol by sodium borohydride, silver NPs with extremelyhigh catalytic activity (surface-area-normalized rate constant of 13.22 s�1 m�2 L) were obtained via accu-rately controlled surface restructuring of silver NPs obtained by UV irradiation.

� 2014 Elsevier Inc. All rights reserved.

1. Introduction

Metal nanoparticles (NPs) have been studied extensively as cat-alysts due to their high activity, promoting many different types ofreactions [1–4]. However, the application of metallic nanostructuresin catalytic reactions is hampered by their uncontrollable perfor-mance due to the changes of their size or shape in reactive environ-ments. Previous studies showed that the surfaces of noble metalswould restructure to adapt their geometrical and electronic struc-ture to reactive environments. Such processes have profound effectson catalytic performance [5]. It has also been shown that some nano-particles and bimetallic surfaces undergo structural rearrangementdepending on reaction conditions [6,7]. For example, silver nano-structures are sensitive to several conditions, e.g., light [8–10], heat[11], ions [12–14], and templates [15,16]. However, from anotherperspective, the sensitivity of silver makes it easily restructure tocertain shapes in the reactive environment. Therefore, to increaseand control silver activity in a reaction, the mechanism of silverrestructuring in reactive environment, which is unclear now, is adesirable and challenging target in the catalysis community.

This paper describes a reaction-driven restructuring of silverNPs, which leads to the dramatic promotion of silver activity

during the reduction of 4-nitrophenol (4-NP) by borohydride inwater at room temperature. Primarily, silver NPs obtained by ultra-violet (UV) irradiation were used to catalyze the reaction. To oursurprise, the kinetic data are different from those for reactions cat-alyzed by silver NPs obtained via traditional methods as reportedbefore, as shown in Fig. 1. As reported previously, the reaction pro-files follow a first-order rate law with a linear correlation of �ln(A/A0) with time t when an excess of borohydride is used, as shown inFig. 1A [17]. Notably, two kinetic stages happened following theinduction period at the initial stage in our result, as shown inFig. 1B. Thus, this paper focuses on understanding the mechanismof this interesting process and utilizes the discovery to increase sil-ver activity for catalytic applications.

2. Experimental

2.1. Synthesis of initial silver nanoparticles

A mixture of 200 lL of 0.375 mmol L�1 AgNO3 and 60 mL of0.1 mmol L�1 4-nitrophenol (4-NP) was preprepared in a three-necked round-bottomed flask wrapped in aluminum foil thatensures mixture in the dark. The mixture was stirred for 15 minunder N2. The synthesis of silver nanoparticles was conducted inaqueous solution under vertical irradiation by a 300-W high-pressure xenon lamp (PLS-SXE-300UV, Beijing Trusttech. Co.

Fig. 1. Plots of �ln(A/A0) against the reaction time for the reduction of 4-NP by NaBH4 catalyzed by silver nanoparticles obtained by NaBH4 reduction (A) and UV irradiation(B) (A0 is the initial absorbance of 4-NP at 400 nm; A is the absorbance of 4-NP at 400 nm at time t).

84 G. Li et al. / Journal of Catalysis 317 (2014) 83–90

Ltd.). The UV–vis (200–800 nm) was separated by vis-ref opticalfilters. The intensity is 37.0 mW cm�2 at 365 nm for UV–vis light.The irradiation area of the light source was ca. 20 cm2. The reactorwas sealed with connected nitrogen airbags. The reaction temper-ature was room temperature. During the reaction, mixture waswithdrawn every 5 or 10 min.

2.2. Synthesis of silver NPs by ascorbic acid and NaBH4

Silver NPs were prepared in water at room temperature follow-ing a similar procedure reported by Chiu et al. [21]. A mixture of1 mM AgNO3, and 2 mM ascorbic acid was prepared in a vial, andone shot of NaBH4 with a final concentration of 0.8 mM wasinjected. The final volume of all reaction solutions was 5 mL. Thereaction mixture was stirred vigorously for 30 min.

2.3. Synthesis of silver NPs in PAMAM dendrimer

The preparation of metal nanoparticles in aqueous solution wascarried out by the chemical reduction of a metal salt–dendrimermixture with sodium borohydride [9]. For a typical experiment,0.2 cm3 of freshly prepared 20 mmol L�1 metal salt was added to19.7 cm3 of G4 PAMAM dendrimer aqueous solutions of10 mmol L�1, and the solutions were stirred for 1 h. Then 0.1 cm3

of 0.4 mol L�1 freshly prepared sodium borohydride was quicklyadded to the solutions under stirring for 30 min.

2.4. Catalytic activity tests

A mixture of 3 mL of the above as-synthesized Ag nanoparticlesand 4-NP solution and 10 lL of 3 M NaBH4 solution was added intoa quartz cuvette, followed by addition of catalyst solution to themixture. The final concentration of Ag is 1.25 � 10�6 mol L�1. Theconcentration of residual was determined using a Hitachi U-3010UV–vis spectrometer by monitoring the characteristic absorptionwavelength of 400 nm for 4-NP. The color of the solution changedgradually from yellow to transparent as the reaction proceeded.UV–vis spectrometry was used to record the change in absorbanceat a time interval of 1 s at k = 400 nm. It has been found that theconcentration of 4-NP (pH 9–11) is linear with its absorbance at400 nm, as shown in Fig. S1. Therefore, absorbance at 400 nm wasmonitored and used directly. The rate constant k (s�1) was deter-mined from the slope of the linear correlation of �ln(A/A0) withtime t. The surface area-normalized rate constants k (s�1 m�2 L)were calculated based on the rate constants k (s�1) and the total

surface area of metal nanoparticles normalized to the unit volumeof the solution, S (m2 L�1). The specific surface area of the nanopar-ticles (ellipsoid or spherical shape) was calculated from the sizesmeasured from the TEM micrographs. The product of this reactionis 4-aminophenol, as indicated by high performance liquid chroma-tography analysis.

2.5. Simulation details

2.5.1. ModelsModels of 4-NP and various silver surfaces were prepared at all

atom resolutions with geometry optimization using Materials Stu-dio (MS, Accelrys Inc., San Diego, CA) [18,19]. The dynamic processwas performed with the Discover module in MS. Models of the Agsurfaces were constructed from multiples of the unit cell. The Ag{111} surface was of dimensions 30.229 � 30.229 � 7.405 Å, theAg {100} surface of dimensions 30.2294 � 30.2294 � 7.405 Å,the Ag {110} surface of dimensions 30.229 � 30.229 � 6.429 Å,the Ag {210} surface of dimensions 30.229 � 31.415 � 6.421Å, the Ag {310} surface of dimensions 30.229 � 29.619 � 6.299 Å,the Ag {510} surface of dimensions 30.229 � 37.392 � 6.222 Å,and the Ag {610} surface of dimensions 30.229 � 30.229 �7.306 Å.

2.5.2. Force fieldWe employed the COMPASS force field [20]. It is the ab initio

force field that enables accurate and simultaneous prediction fora broad range of molecules and polymers. It is also the high-qualityforce field to consolidate parameters of organic and inorganicmaterials.

2.5.3. Setup and simulationFor each 4-NP-surface combination, simulation boxes were pre-

pared using models of the respective silver surface. Both visualanalysis of 4-NP structure on the surface and computation of theadsorption energy were conducted based on these systems. Eachof these boxes for a given surface-4-NP combination was con-structed two times with independent start structures to carry outmultiple simulations and verify convergence of the time-averagedconformations as well as of the energy. The 4-NP molecule was ini-tially positioned close to the surface, using different initial orienta-tions relative to the surface to enhance conformation sampling.The x and y dimensions of the seven simulation boxes were similarfor a better comparison. Equilibration of 4-NP on the Ag surfaces,especially on the high-index Ag surfaces, was convergent at room

Fig. 2. Plots of �ln(A/A0) against the reaction time for the reduction of 4-NP by NaBH4 catalyzed by silver nanoparticles obtained by UV irradiation for 5 min (A), 30 min (B),50 min (C), and 60 min (D).

Fig. 3. Rate constant k of the reactions before and after surface restructuring ofsilver nanoparticles obtained after various times of UV irradiation.

Fig. 4. XPS of the Ag(3d5/2) core level region of silver NPs.

G. Li et al. / Journal of Catalysis 317 (2014) 83–90 85

temperature relative to significant adsorption of 4-NP. Moleculardynamics simulation was carried out in the canonical ensemble(NVT). We employed a time step of 1 fs, a temperature of 298 Kand temperature control by the Anderson thermostat, a sphericalcutoff of van der Waals interactions at 12 Å, and a summation ofCoulomb interactions using the particle mesh Ewald method. Everysystem was equilibrated for 0.5 ns, followed by 0.5 ns simulationtime at 298 K to collect thermodynamic data and snapshots foranalysis.

2.5.4. AnalysisWe analyzed visually all trajectories and identified representa-

tive conformations of 4-NP on each Ag surface. Close contactsbetween 4-NP and Ag surfaces were calculated using a scaleddistance criterion. A tolerance factor of 0.891 was selected. Adsorp-tion energies were determined by calculating the differencebetween the total energy of the relaxed Ag-4-NP chemisorbedcomplex and the total energies of the two constituent pieces, theAg surface and the 4-NP molecule:

Fig. 6. HRTEM image of a silver nanoparticle showing the atomic steps.

86 G. Li et al. / Journal of Catalysis 317 (2014) 83–90

Einteraction ¼ Etotal � Esurface � Eorgan;

where Einteraction is the adsorption energy of 4-NP on the Ag surface,Etotal is the total energy of the relaxed Ag-4-NP complex, Esurface isthe total energy of the Ag surface, and Eorgan is the total energy ofthe 4-NP molecule. Thus, the adsorption energy represents theenergy difference between taking the 4-NP molecule from an infi-nite distance and allowing it to adsorb onto the Ag surface.

3. Results and discussion

Catalytic reduction of 4-NP by borohydride ions is one of themost often used reactions to test the catalytic activity of metalnanoparticles in aqueous solution. After an induction time t0 inwhich no reduction takes place, the reaction starts and can bedescribed by a first-order rate law with the linear correlation of�ln(A/A0) with time t [17]. Figs. 1A and S2 show the regular kineticdata of 4-NP reduction catalyzed by silver NPs synthesized inascorbic acid [21] and in G4-PAMAM dendrimers [22], respectively.After an induction period t0, high linearity between �ln(A/A0) andtime t was monitored. Previously, surface restructuring of the Aunanoparticles was discovered by the analysis of the induction per-iod t0 [23]. Recently, Kalekar et al. [24] also found that the nonlin-ear behavior in the kinetic curve could be attributed to the changein the platinum nanostructure framework. They demonstrated thatthe shape of platinum changed after around 10 min and its activitybecame lower. Herein, kinetic data were collected for the reductionof 4-NP by NaBH4 catalyzed by silver nanoparticles obtained by UVirradiation for various periods of time, as depicted in Fig. 2. Thetwo linear parts of the kinetic curve indicate the reconstructionof silver in the reaction. Interestingly, the activity of silver NPs isincreased after restructuring under certain conditions. Overall,the reshaping period of silver here is much shorter than that ofplatinum, indicating the easily controllable restructuring of as-synthesized silver NPs in the reactive environment. In addition,

Fig. 5. TEM images of silver nanoparticles obtained by UV irradiation for 2

the protection by N2 is necessary during the UV process for thepreparation of initial silver NPs. Because Ag2O rather than Agwas obtained by UV irradiation in the aerobic environment, asshown in Fig. S3.

The systematic analysis of kinetic data for the reactions cata-lyzed by silver NPs obtained after different irradiation times isillustrated in Fig. 3. The catalytic activities of initial silver NPs justafter irradiation for various times are similar, with a rate constant kof around 0.01 s�1. However, after restructuring, the rate constantsof the second stage show drastic changes with the UV irradiationtime. k Increases steadily when the irradiation time is shorter than20 min, reaches a maximum at a UV irradiation time of 20 min, anddecreases with further increase of irradiation time. It can bededuced that the state of initial silver NP will strongly affect its

0 min (A), during low-rate reaction (B), and in high-rate reaction (C).

Fig. 7. A plot of the calculated adsorption energy of 4-NP on various facets of silver.

G. Li et al. / Journal of Catalysis 317 (2014) 83–90 87

reshaping behavior and final activity. The above finding indicates anew avenue to increase silver activity in situ in the reaction envi-ronment via controlling the exposure time of the initial silver solu-tion to UV light.

X-ray photoelectron spectroscopy (XPS) was used to character-ize the state of silver after UV irradiation. XPS of the Ag(3d5/2) corelevel region of Ag NPs obtained by UV irradiation for 20 min isshown in Fig. 4. There are two components after deconvolution,attributed to Ag2O (367.7 eV) and Ag0 (368.2 eV), respectively[25,26]. Results indicate that both Ag0 and Ag+ ions coexist in the

Fig. 8. Plots of �ln(A/A0) against the reaction time for the reduction of 4-NP by NaBH4 ca20 min (C), and 30 min (D) in the presence of 4-NP.

solution. From the area of the respective XPS peaks, 38% of Ag isin the form of Ag0 estimated. We hypothesize that the unreducedsilver ion will be reduced to Ag0 in the environment of sodiumborohydride, and deposit onto the silver nanoparticles via Ostwaldripening.

To further gain insight into the process, transmission electronmicroscopy (TEM) was conducted to characterize the size, shape,and structure of the samples. Fig. 5 displays the TEM images ofthe silver NPs immediately after UV irradiation for 20 min(Fig. 5A), during low-rate reaction (Fig. 5B), and in high-rate reac-tion (Fig. 5C). In the period of low-rate reaction, dramatic sizeincrease of silver NPs was found (Fig. 5A and B). The particle sizewas increased from 1.63 ± 0.51 nm to 5.40 ± 3.04 nm. Results ver-ify our hypothesis based on the XPS analysis that reduction anddeposition of unreduced silver ions occurred. But in this periodmost silver NPs were covered by facets of low index. Before reac-tion, the small silver NPs were mostly truncated octahedra coveredwith (111), (110), and (100) facets. During the low-rate reaction,the larger silver NPs were mostly cubic or tetrahedral covered with(111) and (100) facets. However, the shape of the NPs was chan-ged from polyhedra to irregular shapes in the high-rate reaction, asshown in Fig. 5C. Therefore, it can be concluded that reconstructionof silver NPs indeed happened during the reaction. The direct evi-dence from structural studies by TEM further validates the conclu-sion deduced from kinetic data.

Based on the statistical results on particle size from TEMimages, the surface-area-normalized rate constant k before andafter restructuring is 0.37 s�1 m�2 L and 3.86 s�1 m�2 L, respec-tively, with more than 10 times increase.

talyzed by silver nanoparticles obtained by UV irradiation for 5 min (A), 10 min (B),

88 G. Li et al. / Journal of Catalysis 317 (2014) 83–90

Fig. 6 presents a HRTEM image of the silver NP after restructur-ing, in which the border atoms can be resolved. The crystal is ori-ented along the h100i axis. Similarly to the results reported byZhou et al. [27], various small facets were found on the particle,which can be assigned to a series of low-coordinate atomic steps,such as {210}, {310}, {510}, and {610}. The structure is obviouslydifferent from that of silver NPs obtained by chemical reduction ofascorbic acid and NaBH4 (Section 2), which is octahedral coveredwith (111) facets (Fig. S4). Previous studies also demonstrated thatnoble metals with high-index planes usually exhibit very high cat-alytic activity for structure-sensitive reactions [28]. Therefore, wehypothesize that the great boost of silver activity is attributableto the generation of many high-index facets on silver NPs afterrestructuring in the reaction environment.

To prove our hypothesis, molecular simulation was carried outfor the adsorption of 4-NP on various facets of silver. Fig. S5 showsthe top views and side views of binding configurations of 4-NP onvarious Ag surfaces. The adsorbed molecule lies parallel to the Agsurfaces of low-index facets ({111}, {100}, and {110}) at a dis-tance of �3–4 Å, in agreement with previous study by the Cormagroup [29]. Analysis of the final geometry shows that the nitrogroup and the C@C double bond interact moderately with low-coordinated Ag atoms situated at the edge.

Fig. 7 shows the calculated energy (eV) of adsorption of 4-NPonto various silver facets. Obviously, adsorption energy of somehigh-index facets, (310), (510), and (610), is higher than that oflow-index facets, (111), (100), and (110). Therefore, in the envi-ronment of 4-NP, it is apt to generate more high-index facets dueto their stronger adsorption of 4-NP during the restructuring pro-cess. This is consistent with the TEM results of Fig. 5. In previousstudies of heterogeneous catalysis, Nørskov and co-workers [30]

Fig. 9. TEM images of silver nanoparticles after restructuring in the reaction obtained by U4-NP.

took the ammonia kinetics as a guideline, pointing out that theoptimum catalyst should be one with an adsorbate binding energyin the range from �1.4 to �0.8 eV. In principle, overstrong bindingrenders the adsorbate catalyst so strongly bound that a reactioncannot occur. In the regime of overly weak binding, the catalysthas no interaction with the reactant. Specifically, by a systematicinvestigation of 4-NP reduction by bimetallic dendrimer encapsu-lated NPs, Pozun et al. [31] demonstrated that catalytic propertiesof metal NPs are also dependent on the adsorbate’s binding energy.They showed the direct dependence of the reaction rate constant kon the binding energy; the optimum adsorption energy is in therange from �0.8 to �1.1 eV. Here, the adsorption energy between4-NP and high-index facets of silver, (310), (510), and (610), isin the range from �1.0 to �1.1 eV, which is in the optimum rangeand responsible for their high activity. Early works have alreadyreported that the bonding of adsorbates may induce changes inlocal atomic composition and surface structure, changing the activ-ity and selectivity of the catalyst [32]. The above finding hints thatthe interaction between 4-NP and facets of silver can increase theproportion of high-index facets during restructuring, and thusincrease silver activity.

Inspired by this idea, we synthesized silver NPs in the presenceof 4-NP by UV irradiation for more accurate control of facet gener-ation in the later restructuring process. The kinetic behaviors aresimilar to that summarized in Fig. 2, with two obvious linear parts,as displayed in Fig. 8. Furthermore, TEM images of the samplesafter UV irradiation and after reaction are presented in Figs. S6and 9, respectively. It can be seen from Fig. S6 that the size of Agnanoparticles increased as the time of UV irradiation increased.The obtained Ag nanoparticles were mainly polyhedral, coveredwith low-index facets. Ag nanoparticles with irregular shapes were

V irradiation for 5 min (A), 10 min (B), 20 min (C), and 30 min (D) in the presence of

Fig. 10. Rate constant k of the reactions before and after surface restructuring ofsilver nanoparticles obtained by UV irradiation for various times in the presence of4-NP.

G. Li et al. / Journal of Catalysis 317 (2014) 83–90 89

obtained after restructuring, as shown in Fig. 9A and B. Whenthe irradiation time was more than 20 min, the obtained Agnanoparticles were mainly spherical (Fig. 9C and D). Based on thestatistical results of particle size from TEM images, the surface-area-normalized rate constants k before and after restructuringwere calculated.

The systematic analysis of kinetic data affected by the irradia-tion time is illustrated in Fig. 10. The catalytic activities of initialsilver NPs had been already as high as 0.5–2.0 s�1 m�2 L, indicatingthat 4-NP indeed has an effect on particle synthesis. Moreover,after restructuring, the rate constant of the second stage showeddifferent levels of increase with various UV irradiation times. A vol-cano plot was also obtained for the rate constant after restructur-ing. The peak in the volcano occurs where the UV irradiation timeis 10 min, where k = 13.22 s�1 m�2 L. As expected, this reshapingprocess has been further accelerated and more active sites areobtained by the direction of 4-NP.

To evaluate the catalytic performance of silver NPs obtainedhere, the catalytic activities of silver in this work and reported inthe literature are summarized in Table S1 for comparison. More-over, catalytic activities of the other metal nanoparticles (exceptsilver) for the reduction of 4-NP by sodium borohydride aresummarized in Table S2. As compared to other noble metals (e.g.,Pt, Pd, and Au), silver showed relatively low activity in the previousstudies. This can be attributed to the lack of control of its size andshape in the reactive environment. However, the highest rate con-stant k ever reported (7.61 s�1 m�2 L) was obtained by silver in thework of Fernandez-Merino et al. [33], showing the high potential ofsilver for catalytic applications. Now this study further pursued sil-ver activity to a k value of 13.22 s�1 m�2 L by the accurate controlof restructuring in the reactive environment based on kinetic dataanalysis, TEM characterization and molecular simulation.

4. Conclusions

In summary, silver NPs with extremely high activity(k = 13.22 s�1 m�2 L) have been obtained via an accurately con-trolled reshaping process in the reduction of 4-NP by borohydride.From the kinetic data, the surface restructuring of silver NPs wasdiscovered and monitored in situ by UV–vis spectra during thereaction. TEM results showed that the obtained silver NPs afterrestructuring were rich in reactive high-index facets. Molecularsimulation further confirmed that the generation of high-indexfacets is attributable to their relatively higher adsorption energyfor 4-NP than that of the low-index facets. Moreover, the moderatevalues of adsorption energy (�1.0 to �1.1 eV) between 4-NP and

high-index facets of (310), (510), (610) are responsible for theirhigh activity. This work provides a new example of engineeringthe nanostructure of silver NPs in the reactive environment formore accurate regulation of catalytic processes. Deeper insight intosimilar processes will lead to a number of promising routes thathave the potential to control performance of catalysts under morecomplex reaction conditions and for more practical applications.

Acknowledgments

This work was supported by the research fund of the NationalNatural Science Foundation of China (No. 21306132), the DoctoralProgram of Higher Education (No. 20120032120008), and theNatural Science Foundation of Tianjin (No. 12JCQNJC06000).

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.jcat.2014.05.021.

References

[1] L. Lloyd, Handbook of Industrial Catalysts, Springer, 2011.[2] P. Serp, K. Philippot, Nanomaterials in Catalysis, Wiley-VCH, 2013.[3] J.M. Thomas, W.J. Thomas, Principles and Practice of Heterogeneous Catalysis,

VCH, 1997.[4] Y. Lei, F. Mehmood, S. Lee, J. Greeley, B. Lee, S. Seifert, R.E. Winans, J.W. Elam,

R.J. Meyer, P.C. Redfern, D. Teschner, R. Schlögl, M.J. Pellin, L.A. Curtiss, S. Vajda,Increased silver activity for direct propylene epoxidation via subnanometersize effects, Science 328 (2010) 224–228.

[5] F. Tao, M. Salmeron, In situ studies of chemistry and structure of materials inreactive environments, Science 331 (2011) 171–174.

[6] Feng Tao, Michael E. Grass, Yawen Zhang, Derek R. Butcher, James R. Renzas,Zhi Liu, Jen Y. Chung, Bongjin S. Mun, Miquel Salmeron, G.A. Somorjai,Reaction-driven restructuring of Rh–Pd and Pt–Pd core shell nanoparticles,Science 322 (2008) 932–934.

[7] A.F. Lee, C.V. Ellis, J.N. Naughton, M.A. Newton, C.M. Parlett, K. Wilson,Reaction-driven surface restructuring and selectivity control in allylic alcoholcatalytic aerobic oxidation over Pd, J. Am. Chem. Soc. 133 (2011) 5724–5727.

[8] R. Jin, Y. Cao, C.A. Mirkin, K.L. Kelly, G.C. Schatz, J.G. Zheng, Photoinducedconversion of silver nanospheres to nanoprisms, Science 294 (2001) 1901–1903.

[9] B. Tang, S. Xu, J. An, B. Zhao, W. Xu, Photoinduced shape conversion andreconstruction of silver nanoprisms, J. Phys. Chem. C 113 (2009) 7025–7030.

[10] Q. Zhang, J. Ge, T. Pham, J. Goebl, Y. Hu, Z. Lu, Y. Yin, Reconstruction of silvernanoplates by UV irradiation: tailored optical properties and enhancedstability, Angew. Chem. Int. Ed. 48 (2009) 3516–3519.

[11] B. Tang, S. Xu, X. Hou, J. Li, L. Sun, W. Xu, X. Wang, Shape evolution of silvernanoplates through heating and photoinduction, ACS Appl. Mater. Interfaces 5(2013) 646–653.

[12] C.-M. Tsai, M.-S. Hsu, J.-C. Chen, C.-L. Huang, Mechanistic study of shapeevolution of silver nanoprisms in the presence of KSCN, J. Phys. Chem. C 116(2012) 461–467.

[13] J. Zhang, M.R. Langille, C.A. Mirkin, Photomediated synthesis of silvertriangular bipyramids and prisms: the effect of pH and BSPP, J. Am. Chem.Soc. 132 (2010) 12502–12510.

[14] M. Kim, Y.W. Lee, D. Kim, S. Lee, S.-R. Ryoo, D.-H. Min, S.B. Lee, S.W. Han,Reshaping nanocrystals for tunable plasmonic substrates, ACS Appl. Mater.Interfaces 4 (2012) 5038–5043.

[15] P. Wang, H. He, Y. Jin, Shape transformation and visible region plasmonicmodulation of silver nanoplates by graphene oxide, Small 8 (2012) 3438–3442.

[16] X. Xia, J. Zeng, Q. Zhang, C.H. Moran, Y. Xia, Recent developments in shape-controlled synthesis of silver nanocrystals, J. Phys. Chem. C 116 (2012) 21647–21656.

[17] P. Herves, M. Pérez-Lorenzo, L.M. Liz-Marzán, J. Dzubiella, Y. Lu, M. Ballauff,Catalysis by metallic nanoparticles in aqueous solution: model reactions,Chem. Soc. Rev. 41 (2012) 5577–5587.

[18] J. Feng, R.B. Pandey, R.J. Berry, B.L. Farmer, R.R. Naik, H. Heinz, Adsorptionmechanism of single amino acid and surfactant molecules to Au {111}surfaces in aqueous solution: design rules for metal-binding molecules, SoftMatter 7 (2011) 2113–2120.

[19] L. Ruan, H. Ramezani-Dakhel, C.-Y. Chiu, E. Zhu, Y. Li, H. Heinz, Y. Huang,Tailoring molecular specificity toward a crystal facet: a lesson frombiorecognition toward pt {111}, Nano Lett. 13 (2013) 840–846.

[20] H. Sun, COMPASS: an ab initio force-field optimized for condensed-phaseapplications overview with details on alkane and benzene compounds, J. Phys.Chem. B 102 (1998) 7338–7364.

90 G. Li et al. / Journal of Catalysis 317 (2014) 83–90

[21] C.-Y. Chiu, Y. Li, L. Ruan, X. Ye, C.B. Murray, Y. Huang, Platinum nanocrystalsselectively shaped using facet-specific peptide sequences, Nat. Chem. 3 (2011)393–399.

[22] K. Esumi, R. Isono, T. Yoshimura, Preparation of PAMAM- and PPI-metal (silver,platinum, and palladium) nanocomposites and their catalytic activities forreduction of 4-nitrophenol, Langmuir 20 (2004) 237–243.

[23] S. Wunder, Y. Lu, M. Albrecht, M. Ballauff, Catalytic activity of faceted goldnanoparticles studied by a model reaction: evidence for substrate-inducedsurface restructuring, ACS Catal. 1 (2011) 908–916.

[24] A.M. Kalekar, K.K.K. Sharma, A. Lehoux, F. Audonnet, H. Remita, A. Saha, G.K.Sharma, Investigation into the catalytic activity of porous platinumnanostructures, Langmuir 29 (2013) 11431–11439.

[25] B. Xin, L. Jing, Z. Ren, B. Wang, H. Fu, Effects of simultaneously doped anddeposited Ag on the photocatalytic activity and surface states of TiO2, J. Phys.Chem. B 109 (2005) 2805–2809.

[26] J.T. Wolan, G.B. Hoflund, Surface characterization study of AgF and AgF2

powders using XPS and ISS, Appl. Surf. Sci. 125 (1998) 251–258.[27] Z.Y. Zhou, Z.Z. Huang, D.J. Chen, Q. Wang, N. Tian, S.G. Sun, High-index faceted

platinum nanocrystals supported on carbon black as highly efficient catalystsfor ethanol electrooxidation, Angew. Chem. Int. Ed. 49 (2010) 411–414.

[28] Z. Quan, Y. Wang, J. Fang, High-index faceted noble metal nanocrystals, Acc.Chem. Res. 46 (2012) 191–202.

[29] M. Boronat, P. Concepción, A. Corma, S. González, F. Illas, P. Serna, A molecularmechanism for the chemoselective hydrogenation of substitutednitroaromatics with nanoparticles of gold on TiO2 catalysts: a cooperativeeffect between gold and the support, J. Am. Chem. Soc. 129 (2007) 16230–16237.

[30] J.K. Nørskov, T. Bligaard, A. Logadottir, S. Bahn, L.B. Hansen, M. Bollinger, H.Bengaard, B. Hammer, Z. Sljivancanin, M. Mavrikakis, Y. Xu, S. Dahl, C.J.H.Jacobsen, Universality in heterogeneous catalysis, J. Catal. 209 (2002) 275–278.

[31] Z.D. Pozun, S.E. Rodenbusch, E. Keller, K. Tran, W. Tang, K.J. Stevenson, G.Henkelman, A systematic investigation of p-nitrophenol reduction bybimetallic dendrimer encapsulated nanoparticles, J. Phys. Chem. C 117(2013) 7598–7604.

[32] K.J. Andersson, F. Calle-Vallejo, J. Rossmeisl, I. Chorkendorff, Adsorption-drivensurface segregation of the less reactive alloy component, J. Am. Chem. Soc. 131(2009) 2404–2407.

[33] M.J. Fernandez-Merino, S. Villar-Rodil, J.I. Paredes, P. Solis-Fernandez, L.Guardia, R. Garcia, A. Martinez-Alonso, J.M.D. Tascon, Identifying efficientnatural bioreductants for the preparation of graphene and graphene-metalnanoparticle hybrids with enhanced catalytic activity from graphite oxide,Carbon 63 (2013) 30–44.