DSpace at VNU: Effects of heat treatment and poly(vinylpyrrolidone) (PVP) polymer on electrocatalytic activity of polyhedral Pt nanoparticles towards their methanol oxidation

DSpace at VNU: Effects of heat treatment and poly(vinylpyrrolidone) (PVP) polymer on electrocatalytic activity of polyhe...

Colloid Polym Sci (2011) 289:1373–1386 DOI 10.1007/s00396-011-2460-6 ORIGINAL CONTRIBUTION Effects of heat treatment and poly(vinylpyrrolidone) (PVP) polymer on electrocatalytic activity of polyhedral Pt nanoparticles towards their methanol oxidation Nguyen Viet Long & Michitaka Ohtaki & Masayuki Nogami & Tong Duy Hien Received: 15 January 2011 / Revised: 27 February 2011 / Accepted: April 2011 / Published online: 16 June 2011 # Springer-Verlag 2011 Abstract In this paper, the polyhedral Pt nanoparticles under control were prepared by polyol method using AgNO3 and poly(vinylpyrrolidone) (PVP) in the reduction of H2PtCl6 with ethylene glycol (EG) Transmission electron microscopy (TEM) and high resolution (HR) TEM measurements were used to investigate their characterization In the case of the previous removal of PVP by N V Long Department of Education and Training, Posts and Telecommunications Institute of Technology, Km 10 Nguyen Trai, Hanoi, Vietnam N V Long (*) : M Nogami Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan e-mail: nguyenviet_long@yahoo.com M Nogami e-mail: mnogami@mtj.biglobe.ne.jp M Nogami e-mail: nogami@nitech.ac.jp N V Long : M Ohtaki Department of Molecular and Material Sciences, Interdisciplinary Graduate School of Engineering Sciences, Kyushu University, 6-1 Kasugakouen, Kasuga, Fukuoka 816-8580, Japan T D Hien Laboratory for Nanotechnology, Vietnam National University, Ho Chi Minh, Vietnam washing and heating at 300°C, the specific morphologies of polyhedral Pt nanoparticles were still observed However, the removal of PVP only by heat treatment at 300°C without washing causes the significant variation of their morphology The large Pt particles were observed in the self-aggregation and assembly of the as-prepared polyhedral Pt nanoparticles The pure Pt nanoparticles by washing and heat treatment showed the electrocatalytic property better than PVP-Pt nanoparticles by heat treatment due to the incomplete removal of PVP and by-products from AgNO3 Therefore, the removal modes of PVP without changing their characterization are required to obtain the good catalytic performance Keywords Pt nanoparticles Polyhedral Pt nanoparticles Pt nanocrystals Heat treatment Poly(vinylpyrrolidone) (PVP) polymer Fuel cells Pt-based catalysts AgNO3 Introduction At present, the pure Pt nanoparticles-based catalysts are of great interest owing to their specific features derived from size and morphology characterizations that are exploited as efficient catalysts in both homogeneous and heterogeneous catalytic technologies [1, 2] In addition, these have led to investigate the dependence of catalytic activity on the size and shape of Pt nanoparticles as well as the surface-to-volume ratio and quantum effect because of their potential applications in electronics, catalysis, and biology [1–5] Therefore, many considerable works were focused on controlling their morphology and 1374 size for the catalytic and electrocatalytic reactions [4–6] In catalysis, Pt nanoparticles and Pt-based catalysts catalyze most of hydrogenation, and oxidation hydration reactions for the synthesis of important compounds [1–6] In addition, they exhibit the electrocatalytic activity for CO, methanol oxidation reaction in fuel cells [1] However, there are the significant difficulties in achieving the good performance of Pt-based catalysts [1–7] Most recently, Pt nanoparticles have been used as catalysts for methanol and ethanol electrooxidation [4, 8–11] depending on their definite size and shape Nowadays, their uses with supports, for example carbon nanotubes, oxide matrices (CeO2, ZrO2, and CeO2–ZrO2 etc.), and functional hybrid materials become very necessary [1–6] As a result, the electrocatalytic behavior and ability of catalytic enhancement are much higher and better than Pt nanoparticles only In addition, Pt-based catalysts are also the core components of fuel cell technologies [1] Recently, the role of metal-support interaction on the catalytic activity of carbon-supported Pt nanoparticles towards oxygen reduction and methanol oxidation has also been shown [1–3] On the enormous demands of catalytic materials of high quality and performance, researchers have focused on the modifications and improvements of the main factors that control both the size and shape of Pt nanoparticles for the synthesis of Pt-based bimetallic and supported nanoparticles [12, 13] In this paper, our effort is made in order to control the size and shape of Pt nanoparticles using AgNO3 and achieve their desirable catalytic characterizations Here, we present the preparation of Pt nanoparticles with the polyhedral shape using the addition of AgNO3 in the reduction of H2PtCl6 in extra EG at 160°C Especially, the important role of PVP polymer as a ligand in the stabilization of Pt nanoparticles is discussed in a comparison of the electrocatalytic activity of PVP-protected Pt nanoparticles respective to the different PVP removals on the surfaces of Pt nanoparticles to the electrocatalytic enhancement of pure Pt nanoparticles On the other hand, the important role of heat treatment is meaningfully considered in obtaining the better catalytic activity of Pt nanoparticles Interestingly, we find that the nucleation, growth, and formation mechanisms of large and irregular Pt nanoparticles due to their crystal overgrowth and assembly were observed by the attachments of PVP-protected Pt nanoparticles in their heat treatment at high various temperatures Furthermore, their electrocatalytic behaviors for methanol oxidation indicated that the pure Pt nanoparticle-based catalysts exhibit the higher electrocatalytic activity than PVPprotected Pt nanoparticles in the same conditions of heat treatment We concluded that PVP must be removed from the surface of Pt nanoparticles before the experiments of their catalytic activity Colloid Polym Sci (2011) 289:1373–1386 Experimental methods Chemicals In our typical preparation process, chemicals for the chemical synthesis of polyhedral Pt nanoparticles were poly(vinylpyrrolidone) (PVP, Mw =55,000) as a stabilizer (Sigma-Aldrich), chloroplatinic acid hexahydrate as Pt precursor (Sigma-Aldrich), ACS reagent (Fw = 517.9, H 2PtCl 6·6H2O), ethylene glycol (EG) (M w = 62.07 g mol−1, 95.5 %) (Aldrich) as both solvent and reducing agent, silver nitrate (AgNO3) as a modifying agent, metals basis (Mw =169.88 gmol−1, 99.9999%) (Aldrich) All the solvents of analytical grade and without further purification were ethanol, acetone, and hexane (Aldrich or Sigma-Aldrich) In addition, ionized water and distilled water were also prepared by a Narnstead nanopure H2O purification system Synthesis of polyhedral Pt nanoparticles In order to make polyhedral Pt nanoparticles, we used the stock solutions of mL of EG and 0.5 mL of 0.04 M AgNO3, mL of the solution of 0.0625 M H2PtCl6, and mL of 0.375 M PVP polymer First, every a small volume of 0.0625 M H2PtCl6 of the total volume of mL and every small volume of PVP of 0.375 M are simultaneously added into the volumetric flask in many times (first the addition of 20 μL of H2PtCl6 solution in the flask, and then the addition of 40 μL of PVP each time via a syringe), every 60 s per one time very quickly so that a volume of the solution of 0.375 M PVP was two times more than a volume of the solution of 0.0625 M H2PtCl6 [14] until mL of H2PtCl6 and mL of 0.375 M PVP were thoroughly used The reduction of [PtCl6]−2 by EG occurred and finished for 10–30 min, which was necessary for achieving their sharp and polyhedral morphologies The resultant mixture was heated and refluxed at 160°C for the chemical reduction of [PtCl6]−2, and the color of the resultant solution became dark-brown For the sharp Pt nanoparticles (cubic, octahedral, and tetrahedral main morphologies), the reduction of [PtCl6]−2 by EG occurred and finished for 10–30 min, in contrast to their unsharp shapes, the reduction of [PtCl6]−2 by EG occurred for a long time Then, the product was centrifuged at 15,000 rpm for 15 using Sigma 3K30C-Kubota centrifuge The supernatant was separated and precipitated by adding a triple volume of acetone to remove any impurities from outside and continuing to the centrifugation step at 12,000 rpm for 30 again The precipitate was collected and diluted in mL of ethanol with sonication for 15 to generate the solution of colloidal Pt nanoparticles by an ultrasound generator (200 W/37 kHz) Then, mL volume of hexane was added to wash and make the dispersion Colloid Polym Sci (2011) 289:1373–1386 adequately, and the solution was centrifuged at 3,000 rpm for 10 Noticeably, the precipitate was washed several times with the same mixture of ethanol and hexane to remove the impurities Finally, the precipitate of pure Pt nanoparticles was redispersed in mL of ethanol to obtain colloidal Pt nanoparticles (sample 1) To study the effects of heat treatment to PVP-protected Pt nanoparticles, we used the resultant solution of the product of PVP-protected Pt nanoparticles without the removal of PVP by the procedure of centrifuging and washing with the mixture ethanol and n-hexane (sample 2) In cyclic voltammetry experiment, we prepared sample in the same procedure as sample but Pt nanoparticles were dispersed in mL of mili-Q water In the case of PVP-protected Pt nanoparticles, we also prepared sample in order to investigate the effect of PVP as well as the remaining AgNO3 reagent on the electrocatalytic activity of Pt nanoparticle catalysts Preparation of Pt nanoparticles catalyst In order to obtain the pure Pt nanoparticles for electrochemical measurements, mL of the resultant solution of as-prepared nanoparticle was washed by using a triple volume of acetone and followed by centrifugation at 5,000 rpm Next, the solid product was re-dispersed in ethanol and a triple volume of hexane The resultant mixture was centrifuged at 3,000 rpm The procedure of washing Pt nanoparticles in the mixture of ethanol/ hexane was done three times After washing, the nanoparticles were dispersed in milli-Q water in order to achieve the fixed density of colloidal Pt nanoparticles at mg/mL with the aid of ICPS analyzer The working electrode was a glassy carbon rod (RA5, Tokai Carbon Co., Ltd.) with a diameter of 5.2 mm First, the electrode surface was cleaned and activated by using a kind of polishing-cloth (Buehler Textmet) with alumina slurry (Aldrich, particle size about 50 nm), followed by copious washing with milli-Q water This procedure was repeated until the surface looked like a mirror Then, μg of the Pt loading was set onto the surface of the polished electrode (samples and 4) The loaded electrodes were dried in air for h at 25°C and heated with the heating rate of 1°C/min up to 450°C in air and a keeping time of h in order to remove organic species We used a very slow heating rate to avoid the problem of sintering of the nanoparticles [15–17] The electrodes were allowed to cool normally and then exposed into the flow of the mixture of H2/N2 gases (20%, 80%) at 100°C for h to reduce the existence of PtO and ensure a pristine catalyst surface In order to improve the mechanical stability of electrode surfaces, more 10 μL of wt.% Nafion® solution was added onto the electrode and followed by drying in air overnight before the electrochemical measurements 1375 Material characterization UV–vis spectra and XRD methods The volumes of the reaction mixture were collected and used in an appropriate time during the synthesis They were investigated by UV–vis–NIR spectroscopy (Ubest 570 UV–vis–NIR spectrometer) for the comparison of kinetics and mechanisms of the formation of Pt nanoparticles Meanwhile, mL of ethanol of Pt nanoparticles was set onto the slides of special glass for the XRD method with a small area ∼1 cm The drops of ethanol of Pt nanoparticles were poured on glass and were dried at 80°C and 100°C for h prior to use Moreover, these glass slides were treated with ethanol to remove any impurities The X-ray diffraction patterns were recorded by a diffractometer (X'PertPhillips) operating at 45 kV/45 mA and using Cu-Kα radiation (1.54056 Å) Transmission electron microscopy In order to characterize Pt nanoparticles, 30 μL of ethanol of Pt nanoparticles (sample 1) were prepared by using a copper grid At the same time, 30 μL of PVP-protected colloidal Pt nanoparticles (sample 2) was set onto a copper grid The solvent of ethanol was slowly dried and evaporated in air at the room temperature Then, two copper grids has heated and annealed at 573 K for h to remove all solvents and PVP from the surfaces of Pt nanoparticles The images of Pt nanoparticles were obtained by a transmission electron microscopy (TEM) (JEOL-JEM-2010XII) operated at 200 kV The TEM data of particle shape and size was analyzed, and the HRTEM images were taken at 200 kV with a magnification up to 1,500,000 times Electrochemical measurements Cyclic voltammetry experiment (samples and 4) was performed at room temperature using a typical setup of three-electrode electrochemical system connected to a potentiostat (SI 1287 Electrochemical Interface, Solartron) The cell was a 50-mL glass vial, which was carefully treated with the mixture of H2SO4 and HNO3, and then washed generously with milli-Q water A leak-free AgCl/ Ag/NaCl electrode (RE-1B, ALS) served as the reference and all potentials were reported vs Ag/AgCl The counter electrode was a Pt coil (002234, ALS) The electrolyte solution was bubbled with N2 gas for 30 before every measurement and a N2 blanket was kept during the actual course of potential sweeping For the base voltammetry, the electrolyte was the solution of 0.1 M HClO4 that was 1376 a 4.0 3.5 3.0 Intensity(a.u.) diluted from 70% concentrated solution (Aldrich) using milli-Q water The potential window between −0.2 to 1.0 V with a sweep rate of 50 mV/s was used For the methanol oxidation, the electrolyte was added with 1.0 M methanol in milli-Q water The system measurements were cycled until the stable voltammograms were achieved The electrochemical surface areas (ECA) was estimated by considering the area under the curve in the hydrogen desorption region of the forward scan and using 0.21 mC/cm [17] for the monolayer of hydrogen adsorbed on a Pt surface Colloid Polym Sci (2011) 289:1373–1386 PVP in EG AgNO3 in EG under stirring (2 h) at 160oC PVP-protected Pt nanoparticles with AgNO3 AgNO3 in EG H2PtCl6 in EG 2.5 2.0 1.5 1.0 0.5 Results and discussion 0.0 200 400 600 800 Wavelength(nm) Pt(111) (Sample 2) with PVP (Sample 1) without PVP 30 45 60 Pt(311) 75 Pt(222) Pt(220) Pt(200) Figure 1a shows the typical UV–vis absorption spectra of the samples of Pt nanoparticle without using the centrifugation, mL of ethanol, 30 μL of 0.0265 M H2PtCl6 solution (also, 30 μL of 0.375 M PVP, 30 μL of the product of the solution containing Pt nanoparticles) The comparison is carried out in the UV–vis absorption spectra of PVP, AgNO3, and H2PtCl6 in EG and the product of PVPprotected Pt nanoparticles with AgNO3 There are the peaks at the centered range ∼252–259 nm due to the ligand-tometal charge-transfer transition of [PtCl6]−2 ions We found that Pt nanoparticles were stabilized by PVP It was proven that there are the sharp peaks at 251–259 nm because of the fact that the ligand field splitting of Pt5d orbital expands to the coordination of N and/or O atoms of PVP to Pt4+ ions and Pt nanocrytals [14] The sample with only PVP exhibited the weak absorbance comparable to those of other samples of Pt4+ ions and Pt nanocrytals It was stated that the initial Pt seeds leading to Pt clusters, and leading to that ultra-small size nanocrytals were formed in this process for a short time of 1–3 min, which mainly related to their nucleation from Pt4+ ions into Pt0 clusters and nanoclusters [18–20] Then, the nucleation and growth processes happened simultaneously Finally, we could obtain our final product of PVP-protected Pt nanoparticles in the extra EG solvent As illustrated in Scheme 1, three models of tetrahedral, cubic, and octahedral Pt NPs are stabilized by chains or monolayers or the layers of PVP polymer because of the fact that PVP strongly, tightly binds to the surface of the nanoparticles and sterically blocking them from contacting each other [1] In this case, PVP can bind to a metal surface through either the carbonyl or the tertiary amine of the pyrrolidone ring due to the covalent interactions at the surface At relative high PVP concentration, PVP monolayers will be formed by the PVP chains Consequently, PVP monolayers will surround and cover the surfaces of b Intensity (a.u.) UV–vis spectra and XRD patterns of Pt nanoparticles 90 o 2Theta( ) Fig a UV–vis spectra of PVP, AgNO3, H2PtCl6 in EG, and the product of PVP-protected Pt nanoparticles with AgNO3 b Two XRD patterns of Pt nanoparticles: S1 for the pure Pt nanoparticles before the heat treatment (sample 1) and S2 for PVP-protected Pt nanoparticles containing AgNO3 by heat treatment (sample 2) Pt nanoparticles The coverage of PVP monolayers or the thick or thin layer of PVP was explained in ref [49] Figure 1b shows two typical XRD patterns of Pt nanoparticles (pure Pt nanoparticles with heat treatment and PVP-protected Pt nanoparticles with heat treatment) The results represent the property of the crystalline Pt face centered cubic (fcc) phase The peaks are obviously characterized by a (111) peak, (200), (220), (311), and (222) peaks respective to 2θ values of about 39.52°, 45.96°, 67.40°, 81.10°, and 85.58°, respectively Two XRD patterns of Pt nanoparticles are nearly similar in their appearance and shape The XRD peaks of Pt nanoparticles are broad and comparable to those of the corresponding bulk Pt material We can use one of XRD peaks and calculate the average size of Pt nanocrystallites on the basic of the width of the reflection according to the Debye–Scherrer equation: Colloid Polym Sci (2011) 289:1373–1386 Scheme a The structure of polyvinylpyrrolidone (PVP) polymer The surface atoms of Pt metal nanoparticle strongly coordinate with O atoms of PVP chain [49] b–d Models of cubic, tetrahedral, and octahedral Pt nanoparticles with the protection of PVP polymer e A description of one short chain of PVP polymer 1377 (b) (a) (c) (d) (e) D=0.9λ/(βcosθ), where β is the full width at half maximum (FWHM) of the peak, θ is the angle of diffraction, and λ is the wavelength of the X-ray radiation Here, the (220) reflections of Pt nanoparticles can be used to calculate the average size according to the above formula Thus, the crystallite size of Pt particle was estimated about nm The effects of AgNO3 and heat treatment on the size and morphology of Pt nanoparticles Figures and show the highly polyhedral Pt nanoparticles synthesized with the well-controlled size of 7–15 nm (sample 1) and very sharp shapes through the introduction of the low contents of AgNO3 as size and shape-controlling reagent and the gradual addition of the precursors of PVP and H2PtCl6 in the 12:1 volume ratio [14] In addition, the appearances of the sharp cubic, octahedral, and tetrahedral shapes of Pt nanoparticles with short reduction time of H2PtCl6 due to their controlled growth of (100) and (111) selective surfaces are very good for their applications in catalytic activity and reactions, especially a lot of consideration of sharp tetrahedral Pt nanocrystals EG acts as a solvent and reducing agent at high temperature [9] around 160°C PVP uses as a particle stabilizer and controls the shape of the Pt particles, leading to cubic, tetrahedral, and octahedral Pt nanoparticles [1] It shows that every 1378 Fig TEM and HRTEM images of polyhedral Pt nanoparticles with scale bars: a–b 50 nm and c–d 10 nm The existence of sharp and tetrahedral Pt nanoparticles in our preparation procedure Sample was heated at 300°C after PVP was removed by the mixture of ethanol and n-hexane The temperature was kept at 300°C for h (sample 1) Colloid Polym Sci (2011) 289:1373–1386 (a) (b) 50 nm (c) (d) 10 nm octahedral nanoparticle has the fixed very sharp corners and the oppositely fixed truncated corners These remarks directly related to the nucleation and growth of single nanoparticles leading to determine the competitive directions for their sharply polyhedral morphologies between cube and octahedra It was known that polyhedral Pt nanoparticles have been formed in the homogenous growth by modified polyol method using AgNO3 as structurecontrolling agent [9] Here, we observed that the morphology and size of tetrahedral Pt nanoparticles were very good for the catalytic activity Interestingly, the crystal overgrowths of Pt nanostructures after heat treatment were observed at the same concentrations of H2PtCl6 and AgNO3 precursors when we removed PVP without going through the processes of washing and cleaning these nanoparticles together with the sonication processes The aggregation of Pt nanoparticles in Fig was observed in the face-to-face and one particle-by-one particle behavior located at the surfaces of Pt nanoparticles by their orientated attachment and random self-assembly leading to the short chains of Pt nanoparticles in their orders at the nanoscale degree The results showed their morphology with the sharp surfaces, edges, and corners indicating the perfections of Pt surfaces and their crystal structure for their synthesis for a short time of 5–15 These good morphologies would be important for solid-catalysts 50 nm 10 nm engineering, especially in the areas of industrial catalysts when the demands of Pt nanoparticles of nanocube and octahedra with their size-controlled synthesis have increased more It has been proven that Ag species (Ag42+ or Ag0) can adsorb preferentially on the facets of Pt (100) planes, and Pt-based cations can replace Ag species by a favorable electrochemical reaction of 4Ag+H2PtCl6 →4AgCl+Pt(0)+ 2HCl [4, 14] The role of Ag+ enhanced the (100) growth and/or suppresses (111) growth of Pt nanoparticles However, Ag+ was not reduced by EG at high temperature at 160°C Thus, it is strongly believed that AgNO3 plays the role of the structure-directing agent to Pt nanoparticles [14] In addition, they were possibly adsorbed on the surface of (100) facets, and cations based on surfaces of Pt nanoparticles were easily be removed by organic solvents [23, 24] Most of Pt nanoparticles in our results are so-called single crystalline structures (such as polyhedrons of cubes, cuboctahedrons, octahedrons, tetrahedrons, and truncated polyhedrons of cubes, cuboctahedrons, octahedrons, tetrahedrons), so-called twinned nanostructures (such as decahedron and icosahedron), and various irregular nanostructures [21–28] Here, the distance between the adjacent lattice fringes was estimated, and the interplanar distance was about 0.190 nm, corresponding to the interplanar distance of the (111) plane (Fig 3a) The TEM image of the assembled Pt nanoparticles Colloid Polym Sci (2011) 289:1373–1386 1379 (b) (a) (g) {111} d1 d2 {100} d3 nm nm {111} (c) (d) (h) {100} {100} {111} nm nm (f) (e) (i) {111} {111} {100} nm nm Fig HRTEM images of polyhedral Pt nanoparticles with scale bars: a–f nm g–i Models of octahedral, cubic, cuboctahedral, and tetrahedral Pt nanoparticles and the truncated octahedral and tetrahe- dral Pt nanoparticle a Lattice fringe for d1 =0.193 nm Sample was heated at 300°C after PVP was removed by the mixture of ethanol and n-hexane This temperature was kept at 300°C for h (sample 1) leads to the formation of a porous and large particle about 100 nm even without any control method when removing PVP polymer in Fig Therefore, their assembly of Pt nanoparticles can be controlled in order to use the specific templates (polymers or solvents) This offers their potential applications in catalysis and biology The morphology of large and porous particle looks like a cubic Pt nanoparticle The phenomenon of self-assembly was illustrated in Scheme Therefore, the selfassembled templates are employed for the orientation of colloidal nanocrystals obtained by self-assembly This approach is considered as the template-directed assembly The assembly techniques of particles around droplets (polymers or solvents) will open new routes to produce 1380 Fig TEM of polyhedral Pt nanoparticles with scale bar of 50 nm (the removal of PVP by the mixture of ethanol and hexane without heat treatment) The phenomena of surface attachments, selfaggregation, and assembly were confirmed functionalized ordered materials [48], potentially useful in biological and catalytic applications Figures and show the typical TEM and HRTEM images of Pt nanoparticles by polyol method at 160°C with 0.5 mL of 0.04 M AgNO3 in extra EG solvent (sample 2) Most of Pt nanoparticles have their various morphologies The overgrowths of polyhedral Pt nanoparticles annealed at 300°C for h were observed Clearly, the morphology of Pt nanoparticles was significantly changed by heat treatment During their synthesis, PVP is used to stabilize and control the size and morphology of Pt nanoparticles against their aggregation In addition, the amount of PVP polymer can bind the nanoparticle surface after the catalyst synthesis Therefore, PVP should be removed in the minimal content before the catalytic reactions Obviously, PVP polymer plays an important role to stabilize their morphology and size of these Pt nanoparticles Despite the fact that these Pt nanoparticles were put on copper grids, they still have their interfacial interactions to their surface attachments, aggregation, and self-assembly leading to form the larger Pt particles ∼20 nm and up to 60 nm in our research through their aggregation that was introduced in the final formation of Pd nanostructures by the heat treatment [29] The mechanism of their surface attachment, self-aggregation, and assembly is illustrated in Scheme Colloid Polym Sci (2011) 289:1373–1386 (110) sites, the current response in the potential range between −0.046 and −0.112 V denotes the weakly bounded state of hydrogen On the other hand, the manifestation for Pt(100) sites as governed by Volmer–Heyrovsky mechanism is observed at −0.05 V, which is referred as the strongly bounded hydrogen [30] Beyond 0.5 V in the forward sweep, the PtO formation is observed The negative peak in the reverse scan at around similar potential gives its reduction Moreover, a reduction current from −0.60 to 0.15 V can be attributed to a (100) terraces with long-range adsorption state [31] The relative peak height for the two aforementioned Pt sites can give some information to the shape and yield of the as-prepared nanoparticles [9] Typical polycrystalline Pt electrode has the peak height for (110) to be relatively higher than that for the (100) sites [32] For the polyhedral shape Pt nanoparticles, a similar case is noted where the (100) peaks show smaller height than that of (100) in the voltammetric response suggesting the morphologies of Pt nanoparticles that are mainly of (111) facets and (100) minor facets, i.e., polyhedral shapes of octahedrons, tetrahedrons, and cubes The catalytic activity of nanoparticles of well-defined edges and large surface boundaries are said to be greatly dependent on the bounding planes where catalytic activity follows the (110)>(100)> (111) order, as in the case of hydrogen-related reactions [32, 33] By calculating the charge transfer for the hydrogen adsorption and desorption of the catalysts in (a) (b) Electrocatalytic property of Pt nanoparticles Figure shows the cyclic voltammograms of polyhedral Pt nanoparticles with the different removal of PVP polymer acquired at 50 mV/s in 0.1 M HClO4 solution The voltammograms display the typical shape for the base voltammetry According to Volmer–Tafel mechanism for Pt Scheme The self-aggregation and assembly mechanism of various Pt nanoparticles without using any methods of assembling is illustrated in the case of pure Pt nanoparticles after the PVP removal with ethanol and hexane Colloid Polym Sci (2011) 289:1373–1386 Fig TEM and HRTEM images of polyhedral Pt nanoparticles with scale bars: a–b 50 nm, c–d 10 nm, and e–f nm Insets in c illustrates the overgrowths of [111] directions Lattice fringes: d4 = 0.261 nm and d5 =0.191 nm Sample was heated at 300°C without removing PVP by the mixture of ethanol and n-hexane This temperature was kept at 300°C for h (sample 2) 1381 (b) (a) 50 nm 50 nm (d) (c) [111] [111] [111] 10 nm (e) 10 nm (f) d5 d4 nm the voltammogram, the electrochemical surface area of Pt nanoparticle can be determined The specific charge transfer (QH) due to hydrogen adsorption and desorption has the equation: QH =(QTotal −Qdl)/2, where QTotal denotes the total amount of charge during electro-adsorption and desorption of hydrogen on Pt sites, and the QDL represents the capacitive charge due to the double layer capacitance [34] The area under the curve in the relevant region can give the value of QTotal, and the value of Qdl can be obtained by taking the area under the same region but with upper and lower boundaries as horizontal lines, each passing on a data point just outside the hydrogen desorption/ adsorption waves The electrochemical surface area (ECA) nm is then calculated as follows: ECA=QH/(0.21×LPt), where 0.21 served as the conversion factor (in mC/cm2) for a monolayer of hydrogen and LPt is the catalyst loading on the glassy carbon surface (in milligrams per square centimeter) [35] The ECA was approximately 6.75 m2/g for washedonly Pt nanoparticles, 8.56 m2/g for directly heated only-Pt nanoparticles, and 10.53 m2/g for washed and heated Pt nanoparticles It can be deduced from the hydrogen desorption/adsorption region that there exists a significant difference in the effective surface area for catalytic reactions depending on the removal scheme implemented for PVP molecules When Pt nanoparticles are simply washed (with acetone, ethanol and hexane), the removal 1382 Fig HRTEM images of polyhedral Pt nanoparticles with scale bars: a–f nm (the removal of PVP only by the heat treatment at 300°C for h) Lattice fringe: d6 =0.191 nm (sample 2) Colloid Polym Sci (2011) 289:1373–1386 (b) (a) d6 nm (c) a structural variation from the corner (d) nm nm (e) (f) nm of PVP is said to be incomplete When the sample is heated without washing, the hydrogen desorption improves and is slightly better than washed samples with an increase in ECA about 27% Even so, it is still suspected that some residual PVP and carbonaceous species are not completely removed from the Pt surface It is considered that Pt nanoparticles subjected to both washing and heating demonstrated the highest ECA value with an increase about 56%, in contrast with washedonly samples We have attempted to perform cyclic voltammetry on unwashed and untreated samples, but the corresponding voltammograms were extremely poor due to severe blocking by EG and PVP The further nm nm studies indicated that the onset of PVP decomposition by heat treatment is observed at a minimum of 200°C where an improved catalytic reaction is confirmed for ethylene hydrogenation [36] Our heating temperature of 450°C was conducted at a very slow rate of 1°C/min in order to prevent particle sintering [37], thereby preserving the original shape and facets of the Pt nanoparticles The effect of PVP is said to be more of steric in nature, impeding the progress of catalytic reaction from occurring by preventing incoming reactants from reaching on the Pt nanoparticle surface and hindering the exit for reaction products and intermediates To demonstrate the effect of the three removal schemes for PVP to catalytic Colloid Polym Sci (2011) 289:1373–1386 heating 0.00010 washing 0.00005 0.00000 -0.00005 -0.00010 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 E (V) vs Ag/AgCl Fig Cyclic voltammograms of polyhedral Pt nanoparticles under the different PVP removals Electrolyte solution is 0.1 M HClO4 and scan rate is 50 mV/s activity during a typical catalyst-assisted reaction, cyclic voltammetry in methanol electrooxidation was conducted In the removal, an amount of PVP that can remain on the surfaces of Pt nanoparticles will cover them by a coating layer Necessarily, the thermal decomposition of an amount of remaining polymer of PVP-stabilized Pt nanoparticles is the good PVP removal without using solvents (hexane and ethanol), but the size and morphology of Pt nanoparticles are significantly changed which influences their catalytic property The removal of PVP from the surface of Pt nanoparticles is very crucial and important to achieve their good catalytic activity with the high surface-to-volume ratio Metal Pt nanoparticles have been intensively studied because of their excellent catalytic properties derived from their size and morphology and quantum effect [4, 5, 8–11] Specially, the active sites of Pt-based nanocatalysts of {111}, {100}, and {110} planes were very important to their catalytic property [50, 51] Both experimental and theoretical atomic structures of surface monatomic steps on industrial Pt nanoparticles were confirmed in their edges of nanoparticles They significantly alter the atomic positions of monatomic steps in their proximity, which can lead to substantial deviations in the catalytic properties compared with the extended surfaces In our results, it is certain that the as-prepared tetrahedral, octahedral, and cubic Pt nanoparticles and the truncated polyhedral Pt nanoparticles show their rich and diverse characterizations of {111}, {100}, and {110} specific facets or planes Figure shows the cyclic voltammograms of polyhedral Pt nanoparticles in the mixture of 0.1 M HClO4 and M methanol The stable voltammogram was attained after 20 cycles of sweeping between −0.2 to 1.0 V potential range, and two oxidation peaks are observed The first one is between 0.6 and 0.7 V in the forward scan, and the other at around 0.50 V in the reverse scan The two peaks are directly related to the methanol oxidation and its associated intermediate species In addition, the shape of the curves is in excellent agreement with other reports [38–40] The shoulder near 0.8 V refer to the complete oxidation of intermediate species causing an increase of the current even at a higher potential The anodic current decreased significantly when the voltage was raised beyond the peak current potential because of the blocking on surface active sites as induced by the adsorbed intermediate species Afterwards, the anodic current increases where an increase in the available active sites progressed and the methanol molecules are again being oxidized In the backward scan, the peak at 0.50 V is associated with the removal of the carbonaceous products from the forward scan [41] The peak current density in the forward scan (if) serves as benchmark for the catalytic activity of Pt nanoparticles during methanol dehydrogenation For the prepared catalyst samples, if values are 7.62×10−4, 8.75×10−4, and 9.90× 10−4 A/cm2 for washed-only, heated-only, and washed and heated samples, respectively The obtained values, in reference to other papers, are comparable considering there is no carbon support for the Pt nanoparticles, leaving some inaccessible catalyst sites [9, 38, 41] With the implemented heating procedure, it is clear that heating alone is not sufficient for complete elimination of PVP and a longer heating time should have been carried out Recently, it has been shown that the behavior of PVP decomposition on Pt is quite different from PVP where they found out that PVP decomposes less when coated on Pt nanoparticles at a temperature of 500°C and heating time of 30 [42] Even though this is the case, we avoid heating the nanoparticles at a higher temperature and at longer periods 0.0010 Current density (mA/cm2) washing and heating 0.00015 Current density (mA/cm2) 1383 washing and heating heating 0.0008 washing 0.0006 0.0004 0.0002 0.0000 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 E (V) vs Ag/AgCl Fig Cyclic voltammograms towards methanol electro-oxidation of polyhedral shape Pt nanoparticles under different PVP removals Electrolyte solution is 0.1 M HClO4 +1.0 M CH3OH and scan rate is 50 mV/s 1384 Colloid Polym Sci (2011) 289:1373–1386 Current density (mA/cm2) to block their severe particle growth There is an optimum heating condition without sacrificing the particle morphology This has been realized by washing the nanoparticles with organic solvents before heating them at a specific temperature Based on the results, it is notable that PVP can seriously degrade the catalytic activity of Pt nanoparticles in methanol oxidation reaction if they are not effectively removed Washing or heating alone does not effectively remove PVP molecules Figure shows the chronoamperometry results taken from polyhedral Pt nanoparticles with different removal schemes for PVP The polarization potential was 0.5 V and held over a period of h Base from the graph, washed and heated Pt nanoparticles have a higher current response throughout the measurement This can be an indication of the availability of accessible catalytic sites where intermediate species like Pt = C = O are being stripped The percentage of current left after h are determined to be 3.74%, 8.35%, and 1.78% for washed Pt nanoparticles, heated Pt nanoparticles, and washed and heated Pt nanoparticles, respectively The latter exhibits a slightly faster decay of current over time where residual PVP is apparently in lowest concentration This signifies the protective contribution of PVP towards the particle stability due to the long working operation of Pt nanopaticle catalysts, i.e., in comparison with the case of washed Pt nanoparticles and another case of heated Pt nanoparticles The presence of PVP can be summarized into two opposing influences on catalytic performance It prevents Pt nanoparticle degradation leading to higher tolerance to poisoning species At the same time, it blocks active surface sites which eventually decreases the conversion rate of chemical reactants In Figs 7, and 9, we supposed that the complete removal of PVP from the Pt nanoparticle surface by washing and heating is so important for obtaining the maximal catalytic activity and catalytic properties In washing and heating heating washing 0.0004 0.0002 addition, PVP polymer was also studied in details for protecting Pt nanoparticles for hydrogenation reactions after a high temperature oxygen heat pre-treatment and assumed that PVP was completely removed during calcination at 623–723 K in flowing O2 [43] In a related study, the complete PVP removal was heated at 673 K for PVPprotected Au/Pd bimetallic nanoparticles supported on SiO2 for CO oxidation [43] and Pt/SBA-15 materials [44] Certainly, the decomposition of PVP by heat treatment method was related to the possible decompositions First, the side chain (pyrrolidone) group was released and followed by decomposition of the vinyl chain or vinyl chain fragmenting and decomposition followed by pyrrolidone decomposition The PVP decomposition by heat treatment resulted in the products such as CO, CO2, H2O, and by-products [42–44] Importantly, the removal of PVP cannot maintain the size and morphology of Pt nanoparticles One of the important properties of PVP polymer was specified in the glass transition temperature The glass transition temperature increases with molecular weight and reaches the stabilization at about 175°C corresponding to high molecular weight (for example 55,000) [45–47] Thus, it was suggested that the temperature of synthetic reaction of Pt nanoparticles by the reduction of H2PtCl6 with EG at 160°C is very convenient to control their morphology In addition, we can explain that most of AgNO3 used in our synthesis of Pt nanoparticles become Ag and retain in the surfaces of Pt nanoparticles during their calcinations by heat treatment that influences the catalytic activity of Pt nanoparticles The possibilities of Pt–Ag alloy particles generated during the heat treatment were suggested Here, we did not wash products made with hexane and ethanol PVP was removed at 300°C, but it is certain that Ag species remained on the surface of Pt nanoparticles When PVP was not removed, the electrostatic stabilization of metal colloid particles existed despite of the fact that steric stabilization of metal colloidal Pt particles was held by polymers or surfactant molecules Our investigations show the importance of the complete PVP removal from the Pt catalyst surface by organic solvents before the accurate catalytic reaction is measured This correlation suggests that the importance of the removal of polymer was done from the surfaces of Pt nanoparticles by heat treatment and the removal of Ag (or AgNO3) from the surface of Pt nanocrystals for their practical applications in catalysis 0.0000 1000 2000 3000 4000 5000 6000 7000 Conclusions t (s) Fig Chronoamperometry data of polyheral Pt nanoparticles under the different PVP removals In this paper, Pt nanoparticles with their polyhedral shapes are successfully prepared and evaluated in terms of catalytic Colloid Polym Sci (2011) 289:1373–1386 activity towards methanol electrooxidation The electrochemical data reveal the superior catalytic performance for sharply polyhedral Pt nanoparticles owing to their surfaces and morphologies that exhibit at a higher concentration of surface steps, kinks, islands, terraces, and corners The first removal of PVP by using the mixture ethanol and n-hexane is much better than that of PVP by the heat treatment leading to an increase in the catalytic activity The differences of Pt 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