Spontaneous Pt Deposition on Defective Surfaces ... - ACS Publications

Research Article www.acsami.org

Spontaneous Pt Deposition on Defective Surfaces of In2O3 Nanocrystals Confined within Cavities of Hollow Silica Nanoshells: Pt Catalyst-Modified ITO Electrode with Enhanced ECL Performance Young Shin Cho,†,‡ Soo Min Kim,‡ Youngwon Ju,§ Junghoon Kim,‡ Ki-Wan Jeon,†,‡ Seung Hwan Cho,‡ Joohoon Kim,*,§ and In Su Lee*,†,‡ †

National Creative Research Initiative Center for Nanospace-confined Chemical Reactions and ‡Department of Chemistry, Pohang University of Science and Technology (POSTECH), Pohang, Gyeongbuk 790-784, Korea § Department of Chemistry, Research Institute for Basic Sciences, KHU-KIST Department of Converging Science and Technology, Kyung Hee University, Seoul 130-701, Korea S Supporting Information *

ABSTRACT: Although the deposition of metallic domains on a preformed semiconductor nanocrystal provides an effective pathway to access diverse hybrid nanocrystals with synergistic metal/semiconductor heterojunction interface, those reactions that take place on the surface of semiconductor nanoscrystals have not been investigated thoroughly, because of the impediments caused by the surface-capping organic surfactants. By exploiting the interfacial reactions occurring between the solution and nanoparticles confined with the cavities of hollow nanoparticles, we propose a novel nanospace-confined strategy for assessing the innate reactivity of surfaces of inorganic semiconductor nanoparticles. This strategy was adopted to investigate the newly discovered process of spontaneous Pt deposition on In2O3 nanocrystals. Through an in-depth examination involving varying key reaction parameters, the Pt deposition process was identified to be templated by the defective In2O3 surface via a unique redox process involving the oxygen vacancies in the In2O3 lattice, whose density can be controlled by high-temperature annealing. The product of the Pt-deposition reaction inside the hollow silica nanoparticle, bearing In2O3-supported Pt catalysts inside the cavity protected by a porous silica shell, was proved to be an effective nanoreactor system which selectively and sustainably catalyzed the reduction reaction of small-sized aromatic nitro-compounds. Moreover, the surfactant-free and electroless Pt deposition protocol, which was devised based on the surface chemistry of the In2O3 nanoparticles, was successfully employed to fabricate Pt-catalyst-modified ITO electrodes with enhanced electrogenerated chemiluminescece (ECL) performance. KEYWORDS: hollow nanoparticle, surface reaction, In2O3, Pt, electrogenerated chemiluminescence

1. INTRODUCTION Over the past decade, the chemical and morphological changes occurring during surface-involving reactions have been exploited to devise postsynthetic transformation strategies for complex hybrid nanoparticles, which render presynthesized nanoparticles of more complicated structures.1−3 They have also been employed to form heterojunction interfaces between dissimilar materials. In particular, redox reactions between metal oxides and dissolved metal ions, which involve the growth of in situ reduced metallic species on the surface of a noncolloidal semiconductor material, were recently exploited to produce intimately contacting heterojunction interfaces such as semiconductor/noble metal (e.g., TiO2/M (M = Au, Pt, Ag) and WO3/M (M = Rh, Au, Pt, Ag)), which exhibited improved activity and stability as photocatalysts.4−6 Notwithstanding this extendible applicability, the innate reactivity of such surfaces has not been investigated thoroughly, owing to the impediments caused by the surface-capping organic surfactants that are required to form the colloidal dispersion. On the other hand, attempts made using surfactant-free nanoparticles, which undergo sintering or aggregation in suspensions, have mostly © XXXX American Chemical Society

led to inhomogeneous reactivity among nanoparticles or between regions of their surfaces.7,8 Herein, we propose a nanospace-confined strategy for ensuring an interfacial reaction between inorganic surfaces confined within the interior space of hollow nanoparticle, which was adopted in the present study to investigate the newly discovered process of spontaneous Pt deposition on In2O3 nanocrystals (Figure 1). The solution confined within the protected cavity is expected to provide an undisturbed and consistent environment for reactions, thus allowing for the entire surface area of the immersed nanocrystals to exhibit homogeneous reactivity.9−14 The use of a porous and selectively permeable nanoshell allows one to distinguish the surface-involving reactions occurring within the cavity from those occurring in the outer solution.15−18 Moreover, the encapsulated nanocrystals can remain in the isolated state even during treatment under harsh conditions or high temperatures, Received: February 24, 2017 Accepted: May 3, 2017

A

DOI: 10.1021/acsami.7b02757 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

h to obtain sf-In2O3. These colloids were rinsed with deionized water by centrifugation three times to remove the residual silica remnants. 2.4. Attempt To Immobilize the Pt Nanocrystals on the Oleic Acid-Coated In2O3. For the experiment in ethyl acetate solvent system, synthesized oleic acid-coated In2O3 (3 mg) in cyclohexane was added to a mixture of 15 mg of Pt(acac)2 and 3 mL of ethyl acetate. The colloidal suspension was then stirred vigorously at 70 °C for 12 h. For the experiment in ethyl acetate/water two-phase solvent system, synthesized oleic acid-coated In2O3 (2 mg) colloidal solution dispersed in 1 mL of ethyl acetate is mixed with 1 mL of 10 mg/mL Na2PtCl4 aqueous solution. The obtained suspension was then stirred vigorously at 70 °C for 12 h. 2.5. Pt Deposition Nanoparticles on the sf-In2O3. sf-In2O3 (3 mg) was treated with 3 mL of aqueous Na2PtCl4 solution (5 mg/mL) at 70 °C for 12 h and the resulting particles were isolated by centrifugation and purified with repeating dispersion in deionized water by centrifugation three times. The color of the final product was dark-brown. For testing the reactivity of the micron-sized In2O3 particle, commercialized In2O3 (Aldrich, 3 mg) was used instead of the sf-In2O3. 2.6. Preparation of In2O3@h-SiO2. The confinement of In2O3 on the hollow silica shell was performed through the modification of the previously reported method.26 Igepal CO-520 (1.4 mL) was dispersed in a 40 mL vial containing 20 mL of cyclohexane with vigorous stirring. Next, 10 mg of In2O3 nanoparticles dispersed in cyclohexane was added to the solution. Then 160 μL of NH4OH (28−30%) solution was added dropwise to the mixture with vigorous stirring. Finally, the mixture of TEOS (240 μL) and APTES (18 μL) was then introduced and the mixed solution was reacted for 24 h causing the low-density silica shell followed by addition of TEOS (180 μL), which causes sequential hydrolysis and condensation reaction for another 24 h, building the high-density silica outer shell. The solution was then diluted with DW to 0.5 mg/mL and etched inner low-density silica shell selectively with gentle stirring at 70 °C for 1.5 h. Color of the solution was light-blue during the first 15 min and subsequently turned into colorless. The resulting In2O3@h-SiO2 was purified with redispersion in water by centrifugation three times. 2.7. Deposition Pt Nanoparticles on the In2O3@h-SiO2 (Preparation of In2O3/Pts@h-SiO2). One mg of 500 °C annealed In2O3@h-SiO2 in DW was added in 1 mL of Na2PtCl4 (5.0 mg/mL) solution at 70 °C for 12 h and washed with water by repeating dispersion in an aqueous suspension and centrifugation three times. The color of obtained aqueous colloidal suspension was transparent light-brown. 2.8. Examination of the Influences of Annealing Temperature and Reaction Temperature and pH. For the Pt growth experiment conducted on different reaction temperature, 1 mg of In2O3@h-SiO2 annealed at 500 °C in DW was added in 1 mL of Na2PtCl4 (5.0 mg/mL) solution and then stirred at room temperature, 40 °C, 70 and 100 °C for 12 h, respectively. As the control experiment for elucidating the role of annealing temperature for Pt growth on In2O3 surface, lyophilized In2O3@h-SiO2 was calcined at different temperatures varying from 100 to 800 °C for 5 h. One mg of the annealed In2O3@h-SiO2 was treated with 1 mL of Na2PtCl4 (5.0 mg/ mL) solution at 70 °C for 12 h. For the control experiment with the different pH condition, the pH of the mixture prepared by mixing 1 mg of In2O3@h-SiO2 annealed at 500 °C with 1 mL of Na2PtCl4 (5.0 mg/mL) solution was adjusted to 1.8, 2.4, and 3.4 using diluted HCl solution. 2.9. Time-Course Study of Pt Growth Phenomenon. Two mg of In2O3@h-SiO2 annealed at 500 °C was mixed in 2 mL of Na2PtCl4 (5.0 mg/mL) solution at 70 °C. 250 μL of reaction mixture was extracted after 10 min, 1, 2, 4, 8, and 12 h, respectively. The obtained samples were rinsed with DW for three times. 2.10. Sample Preparation for XPS Analysis. In2O3@SiO2 was prepared through the same reverse microemulsion procedure with that applied for the synthesis of the sf-In2O3. As-obtained In2O3@SiO2 was dried via freeze-dry and annealed at different temperature varying from 100 to 800 °C for 5 h. Annealed samples were treated with NaOH solution (3 M) for 15 h at RT and rinsed with DW for three times.

Figure 1. Confinement strategy of Pt deposition within cavity of hSiO2.

thus allowing for an in-depth examination of the effects of variations in the nanoparticle surface parameters such as the curvature, crystallinity, and defect density.19−21 Accordingly, this hollow nanoreactor-based investigation allowed us to determine that the Pt deposition process is templated by the defective In2O3 surface via a unique redox process involving the oxygen vacancies in the In2O3 lattice, whose density can be manipulated by changing the annealing temperature. The controllable reactivity of the In2O3 surface allowed us to devise a surfactant-free and electroless protocol for decorating In2O3 nanoparticles with a high density of Pt nanocrystals. This protocol was successfully employed to modify an ITO electrode with Pt-catalyst-decorated In2O3 nanoparticles, which led to the ECL performance of the modified electrode being much higher than that of an unmodified one.22,23

2. EXPERIMENTAL SECTION 2.1. Material and Chemicals. All reagents, including In(CH3COO)3 (Alfa Aesar), Indium acetylacetonate (Alfa Aesar), oleic acid (Aldrich), 1-hexadecene (Alfa Aesar), 1-octadecane (Alfa Aesar), platimum(II) acetylacetonate (Aldrich), ethyl acetate (Samchun Chem.), Trimethylamine N-oxide (Aldrich), Igepal CO-520 (Aldrich), tetraethyl orthosilicate (TEOS, Acros), Indium oxide (Aldrich), Na2PtCl4·xH2O (Strem), NaOH (Samchun Chem.), (3aminopropyl)triethoxysilane (APTES, Aldrich), Ru(bpy)3Cl2·6H2O (Ru(bpy)32+, Sigma-Aldrich), tripropylamine (TPrA, Sigma-Aldrich) and phosphate-buffered saline (PBS, Sigma-Aldrich) were used as received. 2.2. Characterizations. Transmission electron microscopy (TEM) was conducted using JEOL JEM-2100 and JEOL JEM2100F for STEM-EDS elemental mapping and high-angle annular dark-field imaging, respectively. Scanning electron microscopy (SEM) was carried out with XL30S FEG (Philips). Powder X-ray diffraction patterns were obtained by an X-ray diffractometer (18 kW, Rigaku, Japan). X-ray photoelectron spectra (XPS) were acquired on a Kα Xray photoelectron spectrometer (Thermo Fisher, UK) with Al Kα (hυ = 1496.0 eV) as the excitation source. The binding energies in the XPS spectral analysis were calibrated for specimen charging by referencing C 1s to 285.0 eV. The Ultraviolet Photoelectron Spectra (UPS) was performed at the 4D beamline at the Pohang Accelerator Laboratory. 2.3. Preparation of sf-In2O3. The synthesis of sf-In2O3 was prepared through the modification of the previously reported method.24 The 10 mg of as-synthesized In2O3−oleic acid nanoparticles stabilized by oleic acid and oleyl amine in cyclohexane.25 was added to the mixture of 1.4 mL of Igepal CO-520 and 20 mL of cyclohexane. Subsequently, 160 μL of NH4OH (28−30%) was added dropwise to the reaction mixture with vigorous stirring. The mixture gradually increased in turbidity when a 180 μL of TEOS was mixed while stirring. The reaction was terminated after 6 h. White colloid of the silica encapsulating In2O3 (In2O3@SiO2) was precipitated by adding MeOH to the reaction suspension, which was collected by centrifugation and purified with redispersion in EtOH by centrifugation and sonication twice and then rinsed with deionized water by centrifugation. The as-obtained In2O3@SiO2 was annealed at 500 °C for 5 h to remove the organic surfactants and immersed in an aqueous NaOH (3 M) solution with gentle stirring at room temperature for 15 B


Research Article

ACS Applied Materials & Interfaces The resulting colloidal suspension was then dried with freeze drier again. 2.11. UPS Measurement of sf-In2O3 NPs Annealed at Different Temperatures in Air. The work function value of each sample was determined with respect to the work function of the clean Au sample with 4.7 eV as a reference and each value was estimated by linear extrapolation along the tangent line of the spectrum onset. During the measurement, a negative bias of −5 V was applied. 2.12. Synthesis of 16 nm Sized In2O3. In a typical experiment of synthesizing the bigger In2O3 nanoparticles, 5 mmol of indium acetylacetonate, 10 mmol of oleyamine (85%), and 10 mmol of oleic acid (90%) were mixed with 17.5 mL of 1-octadecane in a three-neck flask equipped with a condenser. After the mixture was vacuumed at 110 °C for 1 h, the temperature was increased to 210 °C at a rate of 8 °C/min under N2 protection and remained for 2 h with agitation. The temperature was then maintained at 295 °C for an additional 1 h. The resultant colloids were cooled down to room temperature radiantly. With mixing 40 mL of acetone, 5 mL of chloroform, and 1 mL of ethanol with 5 mL of product solution, the resultant nanoparticles were isolated by centrifugation and washed with hexane and acetone mixture for three times. The purified nanoparticles were dispersed in cyclohexane, forming stable colloidal suspension. 2.13. Preparation of Modified ITO Electrode. Thin films of oleic acid-stabilized In2O3 (In2O3-oleic acid) were prepared on ITO glass through the modified spin-coating technique reported previously.27 Briefly, ITO electrodes (10 × 20 mm2) were rinsed with acetone and ethanol, subsequently. The rinsed ITOs were spincoated with the In2O3−oleic acid dispersed in cyclohexane (1 mg/mL) in a spin-coater at a rotation speed of 5000 rpm for 35 s. The spincoated ITOs were then baked at 500 °C in air for 5 h to produce oxygen defects on the surface of In2O3, and subsequently immersed in 20 mL of Na2PtCl4 solution (5 mg/mL) at 70 °C for 12 h. After the treatment in the Pt precursor solution, the modified ITOs were washed in DI water for 1 min. 2.14. Electrogenerated Chemiluminescence (ECL) Measurements. All electrochemical measurements were carried out with a potentiostat (model 440, CH Instruments, USA) using a conventional three-electrode electrochemical cell with ITO working electrodes (area: 0.126 cm2). A Pt wire and a Ag/AgCl (3 M, NaCl) electrode were used as a counter and a reference electrode, respectively. Especially, for ECL measurements, the three-electrode cell was connected to the slit of a monochromator (Acton Standard SP2150, Princeton Instruments, USA) equipped with a charge-coupled device (CCD) (PIXIS 100B, Princeton Instruments, USA) as previously reported.22 All ECL data were obtained by cycling the potential of the working electrodes between 0.0 and 1.7 V (vs Ag/AgCl) at a scan rate of 100 mV/s. 2.15. Evaluation of Catalytic Activity of the Reduction of In2O3/Pts@h-SiO2 in the Reduction Reaction of Nitroarenes. One millimole of nitroarene and 3.0 mL of THF were mixed in bomb reactor with a magnetic stir bar. To this bomb reactor 0.05 mol % Pt catalyst (based on Pt contents) was added. The bomb reactor was purged three times with hydrogen gas and then pressurized to 1 MPa. The reaction was allowed to stir at 50 or 25 °C for 3−24 h and then filtered through Celite to give the corresponding arylamine. The conversion yield was determined by 1H NMR spectroscopy using dibromomethane as an internal standard. For the investigation of the recyclability of the catalyst, the nanoparticles were retrieved after the catalytic reaction by the centrifugation, washed with THF three times, and used for consecutive reaction.

(sf-In2O3), which had been prepared by modifying previously reported procedures, under constant stirring at 70 °C, the initial colorless suspension gradually darkened with time, and a darkbrown suspension was generated within 12 h.24,25 Analyses of the resulting solids through transmission electron microscopy (TEM) and high-resolution TEM (HR-TEM), and X-ray diffraction (XRD) measurements suggested the deposition of Pt nanocrystals around the aggregated sf-In2O3s (Figure 2).

Figure 2. HRTEM and TEM images of the sf-In2O3 after the reaction in a Na2PtCl4 solution. (a) HRTEM image of the Pt NCs deposited on the surface of sf-In2O3, (b, c) exhibit highly agglomerated and In2O3surface detached Pt nanocrystals, respectively, in the reaction product.

This indicated the spontaneous reduction of the Pt2+ species through an unexpected electroless process. On the other hand, the same reaction when performed using commercialized In2O3 particles (Sigma-Aldrich) composed of micrometer-sized granules did not yield any evidence of Pt growth, which excited our curiosity to explore any distinctive reactivity of the surface of the nanometer-sized In2O3 particles (Figure S1). However, the deposition of Pt on sf-In2O3 did not occur uniformly over the surfaces of the highly aggregated nanocrystals and even resulted in surface-detached Pt nanocrystals and a mass of their agglomerates, which hinders the reasoning behind the observed Pt deposition process, such as the involvement of the In2O3 surface (Figure 2). Alternative attempts using homogeneously dispersed oleic acid-coated In2O3 nanocrystals (In2O3-oleic acid) either in ethyl acetate or a two-phase ethyl acetate/water solvent system did not result in a reaction, probably because of the interference from the organic surfactants (Figure S2). 3.2. Nanospace-Confined Pt Deposition on In2O3 Nanoparticle Encapsulated Inside the Cavity of Hollow Silica Nanoparticle. To study the unexpected Pt deposition phenomenon in depth, we came up with a confinement strategy involving the synthesis and utilization of sf-In2O3 encapsulated within the cavity of hollow silica nanoparticles (h-SiO2). For this purpose, In2O3−oleic acid were coated successively with aminosilane-enriched and nearly pure silica shells, yielding In2O3@NH2-SiO2@SiO2 with an outer diameter of 44 (±5) nm, in which an In2O3 core was covered by two silica layers with different cross-linking densities (Figure 3b). The lesscross-linked inner shell was easily hydrolyzed and could be selectively etched in an aqueous suspension at 70 °C, resulting in the formation of targeted In2O3@h-SiO2, which contained a single In2O3 nanocrystal (In2O3) composed of, on average, three fused grains 7 (±1) nm in size in an empty space with a diameter of 26 (±5) nm (Figure 3c).26,29 The resulting In2O3@ h-SiO2 were lyophilized and then calcined at 500 °C in air; this removed the interfering organic surfactants without causing any discernible changes in the size, shape, or aqueous dispersibility (Figure 3d). When the 500 °C-annealed In2O3@h-SiO2 were immersed in an aqueous solution of Na2PtCl4 and stirred at 70 °C and pH of

3. RESULTS AND DISCUSSION 3.1. Spontaneous Deposition of Pt Nanocrystals on the Surfactant-Free In2O3 Nanoparticle. The phenomenon of Pt deposition on the In2O3 surface, which was the subject of the investigation, was first encountered during an attempt to prepare electrocatalytic In2O3/Pts nanocomposites.28 When a Na2PtCl4 solution was added to an aqueous suspension containing 7 (±1) nm-sized surfactant-free In2O3 nanocrystals C


Research Article


Figure 3. TEM and HRTEM (inset) images of (a) In2O3-oleic acid, (b) In2O3@NH2-SiO2@SiO2, (c) In2O3@h-SiO2, and (d) 500 °Cannealed In2O3@h-SiO2 and (e) their XRD patterns. Insets of c and d: images showing well-dispersed aqueous suspensions of (c) In2O3@hSiO2 and (d) 500 °C-annealed In2O3@h-SiO2.

4.4, tiny Pt nanocrystals grew exclusively on In2O3 within the cavity. TEM and HRTEM images of the product, In2O3/Pts@ h-SiO2, obtained after a reaction time of 12 h suggested that a number of reduced Pt nanocrystals with an average diameter of 1.4 (±0.4) nm were dispersively immobilized around In2O3, creating a satellite-type semiconductor/noble metal hybrid nanocrystal, In2O3/Pts, within the hollow silica nanoparticle (Figure 4a).30,31 The nonexistence of any reduced Pt species at

Figure 5. (a) Time-course TEM images and (b) relative amounts of the In and Pt element estimated by STEM-EDS during Pt deposition with In2O3@h-SiO2.

change was observed in the size and morphology of In2O3 over the reaction period. The time-course study and an EDS elemental analysis also revealed that the Pt growth did not occur at the expense of In2O3; this was consistent with the XRD pattern of In2O3/Pts@h-SiO2, which indicated that the initial In2O3 phase was retained after the Pt deposition reaction (Figures 3e and 5b). 3.3. Investigation of the Mechanism of Pt Deposition on the Surface of In2O3 Nanoparticle. A noteworthy trend was observed in a series of experiments during which the annealing temperature (Tanneal) was varied; these experiments were performed before the Pt deposition reaction with In2O3@ h-SiO2. When the In2O3@h-SiO2 powders were treated in air at different Tanneal (100−800 °C) and then immersed in a Na2PtCl4 solution at 70 °C, Pt was deposited on the In2O3 surface only when Tanneal ≥ 400 °C (Figure 6). In addition, the reactivity improved with an increase in Tanneal from 400 to 800 °C. Thus, the highest density of the Pt nanocrystals on In2O3 was observed in the case of the 800 °C-annealed In2O3@hSiO2. The relative amount of Pt NCs deposited on the surface of the sf-In2O3 by changing annealing temperature was tabulated (Table S1). The relative amount of Pt NPs on sfIn2O3 to In is similar above 400 °C annealing temperature. However, one thing we should point out is that the estimated amount of Pt deposited on sf-In2O3 at low annealing temperature from RT to 300 °C was much larger than that deposited on sf-In2O3 at high annealing temperature. It is because the amine functional groups on the surface of the SiO2

Figure 4. (a) TEM image and HRTEM image (inset), (b) STEM-EDS Pt mapping image and HAADF and Pt mapping images (inset) of In2O3/Pts@h-SiO2.

the outside of h-SiO2 confirmed the involvement of the intact In2O3 surface in the Pt growth process. The Pt was deposited exclusively on In2O3, which was encapsulated within the cavity, was also confirmed by scanning transmission electron microscopy/energy-dispersive X-ray spectroscopy (STEM/ EDS) and high-angle annular dark field (HAADF) analyses (Figure 4b). Moreover, in keeping with the objective of the confinement strategy, Pt deposition occurred homogeneously over the entire In2O3 surface of all the In2O3@h-SiO2s. This allowed us to systematically examine the reaction by varying the time as well as the reaction parameters. A time-course TEM study revealed that a small number of Pt nanocrystals began to appear on a few of the In2O3s inside the cavity as early as at 10 min. For longer reaction times, further nucleation occurred, spreading to the entire In2O3 surface for reaction times of 10 min to 12 h, leading to the deposition of an increasing number of Pt nanocrystals with a constant size of 1.4 (±0.4) nm evenly on every In2O3s (Figure 5a). In the meantime, no discernible D


Research Article


Figure 6. TEM images STEM-HADDF images(inset in b−d) of products of the immersion of the In2O3@h-SiO2s in a Na2PtCl4 solution at 70 °C for 12 h, which were air-annealed at (a) 100, (b) 300,(c) 400, (d) 500, (e) 600, and (f) 700 °C with corresponding XPS spectra of O 1s energy level. Numbers in the spectra indicate the area ratios of peaks at 530.4 eV (In−O−In) and 531.7 eV (V0) in the deconvoluted spectra.

well as a peak located around 533 eV can be ascribed to In− OH.37,38 This indicated that the thermal annealing process promoted the formation of the oxygen vacancies (V0) in In2O3, with the V0 density increasing with Tanneal, as has been observed previously in oxide materials.39,40 The fact that the high-temperature annealing treatment was indispensable and had a significant effect on the Pt deposition process, as determined based on the confinement strategy using In2O3@h-SiO2, led us to the conclusion that the spontaneous deposition of Pt proceeds via a unique mechanism that involves the reduction of PtCl42− at the defective In2O3 surface by the oxygen vacancy electrons whose energy is known to increase with an increase in the V0 density.28,41 Therefore, the dependence of the Pt deposition process on Tanneal can be explained based on the assumption that, when In2O3@h-SiO2 are treated at Tanneal ≥ 400 °C, the increase in the V0 density also increases the energy of the captured electrons to a level high enough to allow for electron transfer from the V0 to the PtCl42−. In other words, the reduction potential of the redox reaction between the defective In2O3 and PtCl42− becomes

still remained at low annealing temperature, which caused deposition of Pt NP on the SiO2, resulting in a high relative amount of Pt to In. On the other hand, all amine functional groups on SiO2 was decomposed around 300 °C32 and no Pt NPs observed on SiO2, which caused relatively low amount of Pt to In with respect to low annealed samples. The increase in Pt deposition at higher Tanneal was also confirmed by comparing the HRTEM and HAADF images of the reaction products in Figure 6, which shows that Pt was deposited only in the case of the 400 °C- and 500 °C-annealed In2O3@h-SiO2, with a greater number of Pt nanocrystals being deposited in the case of the latter one. In order to examine whether the thermal annealing process had any effect on the In2O3 phase, X-ray photoelectron spectroscopy (XPS) was performed on the samples annealed at the various Tanneal. The deconvolution analysis of the O 1s core level spectra indicated that the area ratio of the peaks at 531.7 and 530.4 eV, which were attributable to the oxygen defects in the In2O3 matrix and the lattice oxygen of the In−O−In bond,33−36 respectively, increased steadily with the Tanneal, changing from 0.54 at 100 °C to 1.45 at 800 °C (Figure 6) as E


Research Article


the Fermi energy level.41 According to the aforementioned, the amount of the oxygen defects on the surface of the sf-In2O3 can be increased with increasing Tanneal, generating upward shift of the Fermi energy level and donor states. Therefore, In2O3 with enough oxygen defects can spontaneously give electrons to [PtCl4]2− ions to be reduced to form Pt, resulting in spontaneous deposition of the Pt NPs on the surface of the In2O3. According to our careful observation of TEM images and STEM-EDS mapping images, although SiO2 has large amounts of oxygen defects on its surface, SiO2 cannot reduce Pt cations and there is no Pt NPs deposited on the surface of the SiO2 (Figure 4b). It may be because that SiO2 is insulating material, its band gap is very large, and all electrons in SiO2 can be well-localized in their s and p orbitals. In another series of experiments during which the Pt deposition temperature (Tdeposit) was changed, the Na2PtCl4 treatment of the 500 °C-annealed In2O3@h-SiO2 at 100 °C resulted in greater Pt deposition than that in the case of the treatment at 70 °C. On the other hand, no Pt growth was detected at lower temperatures (i.e., 40 °C and room temperature) (Figure S4). This observed effect of Tdeposit can be ascribed to the variation in the reduction potential of the PtCl42−(aq)/Pt(s) pair, which is more positive at higher temperatures.49 In addition, the extent of Pt deposition decreased with a lowering of the pH of the reaction suspension, with no deposition occurring at pH values lower than 1.8, presumably owing to the lowered PtCl42−(aq)/Pt(s) reduction potential under more acidic conditions. The effect of particle size on the surface reactivity was also investigated by introducing a larger In2O3 nanocrystal with a size of 16 (±3) nm within h-SiO2. When the synthesized In2O3(16 nm)@hSiO2 were subjected to calcination at Tanneal of 500 °C and a subsequent Na2PtCl4 treatment at Tdeposit of 70 °C, which resulted in substantial Pt deposition on the 7 nm sized In2O3, no Pt deposition was not detected on the 16 nm sized In2O3. A comparison of the XPS spectra of the 500 °C-annealed In2O3 samples revealed that the V0/In−O−In peak area ratio was much lower for the 16 nm-sized In2O3 (0.49) than for the 7 nm-sized In2O3 (0.60), indicating that the lower V0 density in the case of the larger nanoparticles was probably because of the smaller surface area (Figure S5). Accordingly, the lower activity of the larger In2O3 nanoparticles during Pt deposition can be ascribed to the V0 energy of the V0-deficient In2O3 being lower and insufficient for inducing the spontaneous reduction of Pt2+ at a Tdeposit of 70 °C. During continued attempts to determine the right conditions for inducing the reaction, it was revealed that annealing at 800 °C or performing the Pt deposition reaction at 100 °C resulted in Pt deposition even on the 16 nm sized In2O3 (Figure 8). This indicates that the larger In2O3 requires higher Tanneal and Tdeposit for Pt deposition, as these lift the V0 level of the defective In2O3 and the reduction potential of the PtCl42−(aq)/Pt(s) pair, respectively, to the levels required for the spontaneous redox reaction. 3.4. Evaluation of Effectiveness of the In2O3/Pts@hSiO2 as a Catalytic Nanoreactor System. The structure of the resultant In2O3/Pts@h-SiO2, bearing oxide-supported catalysts inside the protected cavity enclosed by a porous shell, led us to expect the utility as a nanoreactor system which can exert the selective and sustainable catalysis in transforming small organic substrate molecules.9−11,15−17,19,20 Therefore, to investigate the catalytic activity of In2O3/Pts@h-SiO2 catalyst in the reduction of nitroarenes, we choose four different substrates varying the size of molecules. The conversion yield of the

more positive, leading to the spontaneous reduction of the PtCl42− species and the deposition of Pt (Figure 7). The

Figure 7. Schematic band structure change of In2O3 annealed at different temperature (Ec, bottom of the conduction band; Ev, top of the valence band; blue line, donorlike state; red dashed line, Fermi energy level; black dashed line, standard reduction potential of [PtCl4]2−/Pt vs NHE).

increase in Pt deposition with the increase in Tanneal from 400 to 800 °C can also be attributed to increase in the energy of the V0 of the In2O3 annealed at the higher Tanneal. To support our hypothesis, we performed ultraviolet photoelectron spectroscopy (UPS) on sf-In2O3 at different annealing temperature, which allows us to estimate a relative position of Fermi energy level by measuring work function and top of the valence band from tangent line of the spectrum onset in valence band region as well as the bottom of the conduction band determined by using known band gap of the In2O3.42 Although it needs more research for the mechanism to be fully understood, the increase of a number of oxygen vacancies of the sf-In2O3 during the air-annealing process might be attributed to thermal desorption of the oxygen atom behind two electrons in the vacancy, whose phenomenon is similar to the previous reports.39,40,43−45 In Figure S3a, secondary electron cutoff region shows that the work function of the sfIn2O3 was decreased with increasing Tanneal, which directly indicates that Fermi energy level moves up to the conduction band and trend of the work function change exhibits an rapid decrease in work function at relatively low heating temperature (from 300 to 500 °C). This result shows a good agreement with previous report.46 Additionally, In2O3 has a filled valence band by 2p and 3d orbital of O2− and In3+, respectively as well as In2O3 has a conduction band with 5s orbital of In3+. In our system, In2O3 is somewhat reduced by generated oxygen vacancies after annealing in air and it gives rise to donor-like state near the Fermi energy level. It is because that the oxygen vacancy is surrounded by In3+ ion (In: 5s orbitals) which is stabilized from In: 5s band by a lack of covalent bonding with the missing O2− ion and hence symmetrized 5s orbital in In3+ at each oxygen vacancy form shallow donor states just below the conduction band that trap two electrons per oxygen vacancy.47,48 When increasing the number of oxygen defects produced on the surface of the In2O3, donor-like states close to the conduction band will ionize, inducing increase in density of the carriers in the conduction band, resulting in upward shift of F


Research Article


Table 1. Reduction of Nitroarenes with In2O3/Pts@h-SiO2 as a Catalysta

conversion (%)b

Figure 8. TEM and HRTEM(inset) images of samples obtained by treating 500 °C-annealed In2O3(16 nm)@h-SiO2 in a Na2PtCl4 solution for 12 h at (a) 70 and (b) 100 °C and (c) 800 °C-annealed In2O3(16 nm)@h-SiO2 immersed in a Na2PtCl4 solution at 70 °C for 12 h.

catalyst

1a

1b

1c

1d

Pt/In2O3@h-SiO2 (at 50 °C) Pt/In2O3@h-SiO2 (at 25 °C)d Pt/C (at 25 °C)e

>99(>99)c 98 95

38 7 73

16 9 >99

70 30 86

a Condition A: nitroarene (1.0 mmol), In2O3/Pts@h-SiO2 (0.05 mol %, based on Pt contents), THF (3.0 mL), H2 (1 MPa) at 50 °C for 3 h. bDetermined by 1H NMR based on nitroarenes. cRecovered catalyst was used in iterative 3th cycles. dRuns for 24 h. eRuns for 12h.

reaction was determined by 1H NMR spectroscopy based on the amount of nitroarene. Whereas the reaction of nitrobenzene (1a) catalyzed by 0.05 mol % of In2O3/Pts@h-SiO2 resulted in a full conversion to form aniline (1b) after 3 h in THF at 50 °C under 1 MPa pressure of H2, the same reactions performed with larger substrates, such as 1-nitronaphthalene (1b) and 9-nitroanthracene (1c), exhibited moderate to poor conversion yields of 38% and 16%, respectively, to afford the corresponding 1-aminonaphthalene (2b) and 9-aminoanthracene (2c). Compared with the reactions performed at 50 °C, the dramatic size-selectivity was observed when the reactions run at 25 °C. Although the substrate 1a still showed a nearly full conversion (98%) to give arylamine 1b in the presence of In2O3/Pts@h-SiO2 catalyst at 25 °C after prolonged reaction time (24 h), substrates 1b and 1c exhibited negligible conversion yields of 7% and 9%, respectively to form arylamines 2b and 2c. In the meanwhile, the conversion of 1nitropyrene (1d) was found to deviate somewhat from the tendency among substrates 1a−1c in the nitro reduction reaction. Accordingly, when substrate 1d was subjected in the presence of In2O3/Pts@h-SiO2 catalyst under the reaction conditions at temperatures ranging from 25 and 50 °C, 1aminopyrene (2d) was formed in 70 and 30% conversion yields, respectively. Although the interpretation have yet to be completely elucidated, this result may imply that the shape of substrate molecules is another influential factor besides the size in determining the diffusion rate through the porous silica shell of the hollow nanoreactor. Control experiments with the Pt/C catalyst showed high conversion yields of 1a−1d (73−99%) at 25 °C after 12 h, confirming that the size selectivity was mainly attributed to the porous-silica shell. In order to test the reusability of the catalyst, a nitro reduction reaction was carried out employing 1a as a substrate and the recovered catalyst was found to maintain its reactivity to three consecutive runs. The above results were tabulated in Table 1. 3.5. Employment of the Pt Deposition Process for Modifying Electrode Surface with Pt Catalysts; Enhanced ECL Performance of the In2O3/Pt-Modified ITO Electrode. Thus, far, we elucidated the unexpected reactivity of In2O3 nanoparticle surfaces, which allowed a surfactant-free and electroless method for the functionalization of the In2O3 surfaces via creating the intimately contacting heterojunctions

with a high density of Pt nanocrystals. An interesting application of the newly discovered Pt deposition process, creating the intact In2O3/Pt heterojunctions, is the modification of ITO electrodes with the Pt catalyst-immobilized In2O3 nanoparticles (In2O3/Pts). Because it has been reported that the modification of ITOs with Pt nanoparticles provides enhanced ECL generation due to the electrocatalytic activity of the Pt nanoparticles, we expected highly enhanced ECL of Ru(bpy)32+ (bpy = 2,2′-bipyridyl) with tripropylamine (TPrA) coreactant on the In2O3/Pts-modified ITOs.22,23 As illustrated in Figure 9a, a cyclohexane dispersion of In2O3-oleic acid was

Figure 9. (a) Schematic illustration for showing the preparation of the modified ITO electrodes. (b) SEM image of a In2O3/Pts-modified ITO and (c) TEM image of the In2O3/Pts detached from the In2O3/ Pts-modified ITO.

first spin-cast on a piece of ITO glass (Figure S6). The In2O3modified ITO glass substrate was then calcinated in air at 500 °C for 5 h and subsequently treated with a Na2PtCl4 solution at 70 °C for 5 h. Figure 9b, c show a SEM image of the resultant In2O3/Pts-modified ITO and a TEM image of the In2O3/Pts, which were detached from the modified ITO electrode, respectively. The SEM image suggests that the In 2 O 3 G


Research Article


compare the ECL emissions quantitatively. The much less ECL obtained with the Pt NP-modified ITO is attributable to the adverse effect of the surface-capping organic surfactants on the chemically synthesized Pt nanoparticles, which clearly demonstrates the merits of the surfactant-free deposition of Pt nanoparticles on the In2O3 surfaces as suggested in the present study.

nanoparticles were randomly distributed on the ITO substrate and the assembly remained intact after the treatment with the aqueous Na2PtCl4 solution. The TEM image also clearly exhibits the deposited Pt nanocatalysts on the surface of the assembled In2O3. We also confirmed the presence of Pt on the In2O3 nanoparticles using inductively coupled plasma mass spectrometry. After the characterization, we investigated the ECL characteristics of the Ru(bpy)32+/TPrA system on the In2O3/Pts-modified ITOs. For comparison, the ECL of Ru(bpy)32+/TPrA was also studied with In2O3(not decorated with Pt catalyst)-modified ITOs. Figure 10 compares the

4. CONCLUSION In summary, by exploiting the interfacial reactions that occur between the interior solution and nanoparticles encapsulated within the cavity of hollow nanoparticles, we devised a novel nanospace-confined strategy for assessing the innate reactivity of the surfaces of inorganic nanoparticles, which could not be done using colloidal nanoparticles. The effectiveness of this strategy was demonstrated through an in-depth examination of the spontaneous deposition of Pt on the cavity-captured In2O3 nanoparticles by varying a number of reaction parameters. This suggested that the mechanism is mediated by oxygen defects, which are abundant on the high-temperature annealed In2O3 surface. In addition, we also found a use for this newly discovered and interpreted surface chemistry of In2O3 and the resultant In2O3/Pts hybrid nanocrystals for fabricating selective and recyclable catalytic nanoreactor system and Pt-catalystmodified ITO electrodes with enhanced ECL performance.

■

Figure 10. ECL-potential curves and corresponding CVs (inset) of 75 μM Ru(bpy)32+ and 100 mM TPrA in 0.15 M phosphate-buffered saline (PBS) solution (pH 7) obtained on (i) a In2O3/Pts-modified ITO and (ii) a In2O3-modified ITO. Scan rate: 100 mV/s.

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b02757. Experimental details and additional analyses (PDF)

representative ECL-potential curves of the Ru(bpy)32+/TPrA on the In2O3/Pts- and In2O3-modified ITOs. The In2O3/Ptsmodified ITO displayed much enhanced ECL emission compared to the emission obtained with the In2O3-modified ITO, which clearly demonstrates that the ECL was significantly enhanced by the Pt nanocatalysts deposited on the In2O3. It is also notable that the electroactive surface area of the In2O3/Ptsmodified ITO electrode was only ∼1.1 times larger than that of the In2O3-modified ITO (Figure S7), which suggested that the substantial enhancement in ECL emission on the In2O3/Ptsmodified ITO could not be solely ascribed to the increased surface area of the In2O3/Pts-modified ITO. Therefore, the enhanced ECL emission is primarily attributable to the catalytic activity of the deposited Pt nanoparticles on the In2O3/Ptsmodified ITO. The catalytic Pt nanoparticles allowed facile oxidation of Ru(bpy)32+/TPrA on the In2O3/Pts-modified ITO as demonstrated in the cyclic voltammograms (CVs) of the Ru(bpy)32+/TPrA, which is in consistent with the previous reports (Inset in Figure 10).22,23 As a control experiment, we also prepared chemically synthesized Pt NPs with similar size (∼4 nm) using surface-capping organic surfactants (i.e., oleic acid and oleylamine). ITO electrodes were modified with the chemically synthesized Pt NPs and then ECL tests were carried out. The resulting Pt NP-modified ITOs were compared with the In2O3/Pts-modified ITOs in terms of their ECL characteristics. Specifically, we compared the ECL-potential curves of the Ru(bpy)32+/TPrA on the both Pt NP- and In2O3/Pts-modified ITOs. The Pt NP-modified ITOs (curve (ii) in Figure S8) exhibited much less ECL emission compared to the emission obtained with the In2O3/Pts-modified ITOs (curve (i) in Figure S8). Because we could measure the surface areas of the both modified ITOs, we normalized the peak ECL intensities with the surface areas of the Pt NP- and In2O3/Pts-modified ITOs, which are 17.5 and 226.9 A.U./mm2, respectively, to

■

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (I.S.L.). *E-mail: [email protected] (J.K.). ORCID

Seung Hwan Cho: 0000-0001-5803-4922 In Su Lee: 0000-0002-2588-1444 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (MSIP) (NRF-2016R1A3B1907559) (I. S. L.) and (NRF-2014S1A2A2028540) (J. K.).

■

REFERENCES

(1) De Trizio, L.; Manna, L. Forging Colloidal Nanostructure via Cation Exchange Reactions. Chem. Rev. 2016, 116, 10852−10887. (2) Buck, M. R.; Schaak, R. E. Emerging Strategies for the Total Synthesis of Inorganic Nanostructures. Angew. Chem., Int. Ed. 2013, 52, 6154−6178. (3) Sun, Y. Metal Nanoplates on Semiconductor Substrates. Adv. Funct. Mater. 2010, 20, 3646−3657. (4) Pan, X.; Xu, Y.−J. Efficient Thermal- and Photocatalyst of Pd Nanoparticles on TiO2 Achieved by an Oxygen Vacancies Promoted Synthesis Strategy. ACS Appl. Mater. Interfaces 2014, 6, 1879−1886. (5) Pan, X.; Xu, Y.-J. Defect-Mediated Growth of Noble-Metal (Ag, Pt, and Pd) Nanoparticles on TiO2 with Oxygen Vacancies for H


Research Article

ACS Applied Materials & Interfaces Photocatalytic Redox Reactions under Visible Light. J. Phys. Chem. C 2013, 117, 17996−18005. (6) Xi, G.; Ye, J.; Ma, Q.; Su, N.; Bai, H.; Wang, C. In Situ Growth of Metal Particles on 3D Urchin-like WO3 Nanostructrues. J. Am. Chem. Soc. 2012, 134, 6508−6511. (7) Elliott, E. W., III; Glover, R. D.; Hutchison, J. E. Removal of Thiol Ligands from Suface-Confined Nanoparticles without Particle Growth or Desorption. ACS Nano 2015, 9, 3050−3059. (8) Cargnello, M.; Chen, C.; Diroll, B. T.; Doan-Nguyen, V. V. T.; Gorte, R. J.; Murray, C. B. Efficient Removal of Organic Ligands from Supported Nanocrystals by Fast Thermal Annealing Enables Catalytic Studies on Well-Defined Active Phase. J. Am. Chem. Soc. 2015, 137, 6906−6911. (9) Vaz, B.; Salgueiriño, V.; Pérez-Lorenzo, M.; Correa-Duarte, M. A. Enhancing the Exploitation of Functional Nanomaterials through Spatial Confinement: The Case of Inorganic Submicrometer Capsules. Langmuir 2015, 31, 8745−8755. (10) Li, Y.; Shi, J. Hollow-Structured Mesoporous Materials: Chemical Synthesis, Functionalization and Applications. Adv. Mater. 2014, 26, 3176−3205. (11) Lee, J.; Kim, S. M.; Lee, I. S. Functionalization of Hollow Nanoparticles for Nanoreactor Applications. Nano Today 2014, 9, 631−667. (12) Xiao, M.; Zhao, C.; Chen, H.; Wang, J. "Ship-in-a-Bottle” Growth of Noble Metal Nanostructures. Adv. Funct. Mater. 2012, 22, 4526−4532. (13) Yeo, K. M.; Choi, S.; Anisur, R. M.; Kim, J.; Lee, I. S. SurfactantFree Platinum-on-Gold Nanodendrites with Enhanced Catalytic Performance for Oxygen Reduction. Angew. Chem., Int. Ed. 2011, 50, 745−748. (14) Gao, C.; Zhang, Q.; Lu, Z.; Yin, Y. Templated Synthesis of Metal Nanorods in Silica Nantubes. J. Am. Chem. Soc. 2011, 133, 19706−19709. (15) Liu, J.; Yang, H. Q.; Kleitz, F.; Chen, Z. G.; Yang, T.; Strounina, E.; Lu, G. Q.; Qiao, S. Z. Yolk-Shell Hybrid Materials with a Periodic Mesoporous Organosilica Shell: Ideal Nanoreactors for Selctive Alcohol Oxydation. Adv. Funct. Mater. 2012, 22, 591−599. (16) Sanlés-Sobrido, M.; Pérez-Lorenzo, M.; Rodríguez-González, B.; Salgueiriño, V.; Correa-Duarte, M. A. Highly Active Nanoreactors: Nanomaterial Encapsulation Based on Confined Catalysis. Angew. Chem., Int. Ed. 2012, 51, 3877−3882. (17) Kim, S. M.; Jeon, M.; Kim, K. W.; Park, J.; Lee, I. S. Postsynthetic Functionalization of a Hollow Silica Nanoreactor with Manganese Oxide-Immobilized Metal Nanocrystals Inside the Cavity. J. Am. Chem. Soc. 2013, 135, 15714−15717. (18) Lian, J.; Xu, Y.; Lin, M.; Chan, Y. Aqueous-Phase Reactions on Hollow Silica-Encapsulated Semiconductor Nanoheterostructures. J. Am. Chem. Soc. 2012, 134, 8754−8757. (19) Wang, G.-H.; Hilgert, J.; Richter, F. H.; Wang, F.; Bongard, H.J.; Spliethoff, B.; Weidenthaler, C.; Schuth, F. Platinum-Cobalt Bimetallic Nanoparticles in Hollow Carbon Nanospeheres for Hydrogenolysis of 5-Hydroxymethylfurfural. Nat. Mater. 2014, 13, 293−300. (20) Qiao, Z.-A.; Zhang, P.; Chai, S.-H.; Chi, M.; Veith, G. M.; Gallego, N. C.; Kidder, M.; Dai, S. Lab-in-a-Shell: Encapsulating Metal Clusters for Size Seiving Catalysis. J. Am. Chem. Soc. 2014, 136, 11260−11263. (21) Ding, S.; Chen, J. S.; Qi, G.; Duan, X.; Wang, Z.; Giannelis, E. P.; Archer, L. A.; Lou, X. W. Formation of SnO2 Hollow Nanospheres inside Mesoporous Silica Nanoreactors. J. Am. Chem. Soc. 2011, 133, 21−23. (22) Kim, Y.; Kim, J. Modification of Indium Tin Oxide with Dendrimer-Encapsulated Nanoparticles To Provide Enhanced Stable Electrochemiluminescence of Ru(bpy)32+/Tripropylamine While Preserving Optical Transparency of Indium Tin Oxide for Sensitive Electrochemiluminescence-Based Analyses. Anal. Chem. 2014, 86, 1654−1660.

(23) Fan, F. F.; Bard, A. J. Observing Single Nanoparticle Collisions by Electrogenerated Chemiluminescence Amplification. Nano Lett. 2008, 8, 1746−1749. (24) Kim, K. W.; Kim, S. M.; Choi, S.; Kim, J.; Lee, I. S. Electroless Pt Deposition on Mn3O4 Nanoparticles via the Galvanic Replacement Process: Electrocatalytic Nanocomposite with Enhanced Performance for Oxygen Reduction Reaction. ACS Nano 2012, 6, 5122−5129. (25) Liu, Q.; Lu, W.; Ma, A.; Tang, J.; Lin, J.; Fang, J. Study of QuasiMonodisperse In2O3 Nanocrystals: Synthesis and Optical Determination. J. Am. Chem. Soc. 2005, 127, 5276−5277. (26) Wu, S.-H.; Tseng, C.-T.; Lin, Y.-S.; Lin, C.-H.; Hung, Y.; Mou, C.-Y. Catalytic Nano-Rattle of Au@hollow Silica: towards a PositionResistant Nanocatalyst. J. Mater. Chem. 2011, 21, 789−794. (27) Lee, J.; Lee, S.; Li, G.; Petruska, M. A.; Paine, D. C.; Sun, S. A Facile Solution-Phase Approach to Transparent and Conduction ITO Nanocrystal Assemblies. J. Am. Chem. Soc. 2012, 134, 13410−13414. (28) Liu, Y.; Mustain, W. E. High Stability, High Activity Pt/ITO Oxygen Reduction Electrocatalysts. J. Am. Chem. Soc. 2013, 135, 530− 533. (29) Tan, L.; Chen, D.; Liu, H.; Tang, F. A Silica Nanorattle with a Mesoporous Shell: An Ideal Nanoreactor for the Preparation of Tunable Gold Cores. Adv. Mater. 2010, 22, 4885−4889. (30) Zhang, B.; Li, Y. H.; Zhong, J. H.; Yang, X. H.; Zhang, H. M.; Zhao, H. J.; Yang, H. G. Platinum@regular Indium Oxide Nanooctahedra as Difunctional Counter Electrodes for Dye-Sensitized Solar Cells. J. Mater. Chem. A 2014, 2, 6331−6336. (31) Wang, X.; Lin, K. S. K.; Chan, J. C. C.; Cheng, S. Direct Synthesis and Catalytic Applications of Ordered Large Pore Aminopropyl-Functionalized SBA-15 Mesoporous Materials. J. Phys. Chem. B 2005, 109, 1763−1769. (32) Wang, Y.; Liu, B.; Cai, D.; Li, H.; Liu, Y.; Wang, D.; Wang, L.; Li, Q.; Wang, T. Room-Temperature Hydrogen Sensor based on Grain-Boundary Controlled Pt Decorated In2O3 Nanocubes. Sens. Actuators, B 2014, 201, 351−359. (33) Golovanov, V.; Mäki-Jaskari, M. A.; Rantala, T. T.; Korotcenkov, G.; Brinzari, V.; Cornet, A.; Morante, J. Experimental and Theoretical Studies of Indium Oxide Gas Sensors Fabricated by Spray Pyrolysis. Sens. Actuators, B 2005, 106, 563−571. (34) Bermudez, V. M.; Berry, A. D.; Kim, H.; Piqué, A. Functionalization of Indium Tim Oxide. Langmuir 2006, 22, 11113−11125. (35) Forget, A.; Limoges, B.; Balland, V. Efficient Chemisorption of Organophosphorous Redox Probes on Indium Tin Oxide Surfaces under Mild Conditions. Langmuir 2015, 31, 1931−1940. (36) Lei, F.; Sun, Y.; Liu, K.; Gao, S.; Liang, L.; Pan, B.; Xie, Y. Oxygen Vacancies Confined in Ultrathin Indium Oxide Porous Sheets for Promoted Visible-Light Water Splitting. J. Am. Chem. Soc. 2014, 136, 6826−6829. (37) Wang, K.; Chang, Y.; Lv, L.; Long, Y. Effect of Annealing Temperature on Oxygen Vacancy Concentration of Nanocrystalline CeO2 Film. Appl. Surf. Sci. 2015, 351, 164−168. (38) Sivaram, V.; Crossland, E. J. W.; Leijtens, T.; Noel, N. K.; Alexander-Webber, J.; Docampo, P.; Snaith, H. J. Observation of Annealing-Induced Doping in TiO2 Mesoporous Single Crystals for Use in Solid State Dye Sensitized Solar Cells. J. Phys. Chem. C 2014, 118, 1821−1827. (39) Sabri, M. M.; Jung, J. H.; Yoon, D. H.; Yoon, S. H.; Tak, Y. J.; Kim, H. J. Hydroxyl Radical-Assisted Decomposition and Oxidation in Solution-Processed Indium Oxide Thin-film Transistors. J. Mater. Chem. C 2015, 3, 7499−7505. (40) Nguyen, M. C.; Jang, M.; Lee, D. H.; Bang, H. J.; Lee, M. J.; Jeong, J. K.; Yang, H. C.; Choi, R. Li-Assisted Low-Temperature Phase Transition in Solution-Processed Indium Oxide Films for HighPerformance Thin Film Transistor. Sci. Rep. 2016, 6, 25079−25091. (41) Greiner, M. T.; Chai, L.; Helander, M. G.; Tang, W.-M.; Lu, Z.H. Transition Metal Oxide Work Functions: Influence of Cation Oxidation State and Oxygen Vacancies. Adv. Funct. Mater. 2012, 22, 4557−4568. I


Research Article

ACS Applied Materials & Interfaces (42) Vasilopoulou, M.; Douvas, A. M.; Georgiadou, D. G.; Palilis, L. C.; Kennou, S.; Sygellou, L.; Soultati, A.; Kostis, I.; Papadimitropoulos, G.; Davazoglou, D.; Argitis, P. The Influence of Hydrogenation and Oxygen Vacancies on Molybdenum Oxides Work Function and Gap States for Application in Organic Optoelectronics. J. Am. Chem. Soc. 2012, 134, 16178−16187. (43) Ginstrup, O. The Redox System Platinum(0)/Platinum(II)/ Platinum(IV) with Chloro and Bromo Ligands. Acta Chem. Scand. 1972, 26, 1527−1541. (44) Chiu, Y. H.; Hsu, Y. J. Au@Cu7S4 Yolk@Shell NanocrystalDecorated TiO2 Nanowires as an All-day-Active Photocatalyst for Environmental Purification. Nano Energy 2017, 31, 286−295. (45) Fujii, K.; Sato, Y.; Takase, S.; Shimizu, Y. Effects of Oxygen Vacancies and Reaction Conditions on Oxygen Reduction Reaction on Pyrochlore-Type Lead-Ruthenium Oxide. J. Electrochem. Soc. 2015, 162, F129−F135. (46) Zhang, H.; Xie, C.; Zhang, Y.; Liu, G.; Li, Z.; Liu, C.; Ma, X.; Zhang, W. F. Effects of Thermal Treatment under Different Atmospheres on the Spectroscopic Properties of Nanocrystalline TiO2. J. Appl. Phys. 2008, 103, 103107−103110. (47) Diebold, U. The Surface Science of Titanium Dioxide. Surf. Sci. Rep. 2003, 48, 53−229. (48) Fan, J. C. C.; Goodenough, J. B. X-ray Photoemission Spectroscopy Studies of Sn-doped Indium Oxide Films. J. Appl. Phys. 1977, 48, 3524−3531. (49) Neamen, D. A. Semiconductor Physics and Devices; McGraw-Hill: New York, 2003.

J


Spontaneous Pt Deposition on Defective Surfaces ... - ACS Publications

Recommend Documents