Polyelectrolyte Brush Templated Multiple Loading of Pd Nanoparticles

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J. Phys. Chem. C 2009, 113, 7677–7683

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Polyelectrolyte Brush Templated Multiple Loading of Pd Nanoparticles onto TiO2 Nanowires via Regenerative Counterion Exchange-Reduction Qian Ye,†,‡ Xiaolong Wang,‡ Haiyuan Hu,‡ Daoai Wang,‡ Shaobai Li,*,† and Feng Zhou*,‡ State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Lanzhou UniVersity, Lanzhou 730000, China and State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China ReceiVed: February 12, 2009; ReVised Manuscript ReceiVed: March 14, 2009

In this paper, a novel strategy was reported to produce ternary nanocomposite containing TiO2 nanowires, poly[(dimethylamino)ethyl methacrylate] (PDMAEMA) brush and Pd nanoparticles. PDMAEMA brushes were grafted from catecholic initiator anchored on TiO2 nanowires via surface-initiated atom transfer radical polymerization (SI-ATRP) at ambient conditions. The PDMAEMA uniformly covered the surface of TiO2 nanowires and the two component of PDMAEMA and TiO2 shows clear boundary interface. PDMAEMA brushes were derived with CH3I to form quaternized-PDMAEMA (Q-PDMAEMA) so as to facilitate anion exchange with PdCl42-, which was followed by in situ reduction with NaBH4 to obtain the TiO2 nanowiresQ-PDMAEMA/Pd(0) ternary nanocomposite. Pd nanoparticles have very small size of about 2-4nm and monodisperse, uniformly distributed among polymer brush network. Interestingly, after reduction reaction, concurrent with generation of Pd nanoparticles, the ammonium cations were largely recovered, which allows further breath-in PdCl42- counterions and uploads more Pd nanopaticles by repeated steps. The responsivity of both PDMAEMA and Q-PDMAEMA decorated TiO2 nanowires was studied in response to pH and salt solution. The electrocatalytic behavior of the ternary nanocomposite was investigated. Introduction Titanium oxide (TiO2) as an n-type semiconductor material with special photoresponse has been a key material of interest for many years.1 It have been widely used in dye-sensitized solar cells,2-5 photocatalysts,6-8 electrochemical sensors,9,10 selfcleaning coating,11 and water-splitting catalysts for hydrogen generation.12,13 In particular, low-dimensional TiO2 materials, such as nanowires and nanotubes, have received considerable attention recently.14 In addition, TiO2 can be an effective support for a catalyst (noble metal) specifically in hydrogenation reactions, promoting the catalytic reaction. For example, TiO2 strongly interacts with Pd upon reduction at high temperatures via “the strong metal-support interaction” (SMSI).15 For example, an improved selectivity for ethylene production in selective acetylene hydrogenation over Pd/TiO2 catalysts compared to Pd/SiO2 was reported by Moon et al.16 Pd/TiO2 can also increase the efficiency of photocatalytic water splitting.17 For these applications, the size, distribution, and density of Pd nanoparticles have a key impact on the catalytic activities. Pd/ TiO2 composites have been synthesized by various methods such as photodeposition,18 chemical reduction19 method, and cosputtering method.20 However, control over the size, distribution, and morphology of Pd nanoparticles still remains a challenge since metal particle nucleation and growth occur randomly.21 In the present study, we used a polyelectrolyte brush matrix to template the synthesis of Pd nanoparticles on the surface of TiO2 nanowires to obtain Pd nanoparticles with a fairly small size of about 3 nm and nearly monodisperse. Moreover, the ability of polyelectrolyte brushes to allow for regenerative counterion * To whom correspondence should be addressed. E-mail: zhouf@ lzb.ac.cn. † Lanzhou University. ‡ Chinese Academy of Sciences.

exchange makes it possible for multiple nanoparticle loading.22 As a consequence, the nanoparticle density significantly increased. The surface-initiated polymerization (SIP) provides a facile approach to decorate the surface of inorganic materials with chemically tethered polymer chains on one end.23 The vast majority of reported SIPs have utilized silane or thiol initiator to initiate polymerization.24 However, silane chemistry is not effective in the case of TiO2, probably due to the limited free hydroxyl group on the TiO2 surface, especially for anatase TiO2, which are obtained via high-temperature annealing up to 500 °C. Recently, the catecholic amino acid L-3,4-dihydroxyphenylalanine (L-DOPA) was found in the adhesive pad proteins in marine mussels.25 Although its function is not fully understood, the higher concentration of the L-DOPA unit at the adhesive-substrate interface26 suggests a possible role in promoting adhesion. The discovery has stimulated great interest in exploiting catechol to enhance interfacial adhesion of materials.27 Catechol derivatives have recently been used to anchor small functional biomolecules onto ferromagnetic nanoparticles (Fe2O3) for protein separations28 and for linking of DNA and dyes to surfaces of semiconductor nanoparticles (TiO2).29 It was also used to attach initiator onto titanium or steel plate followed by SIP of polyethylene glycol containing monomer to realize antibiofouling treatment of these biomedical materials.30 There is no report on using the strategy to grow polymer brushes from nanomaterials (nanoparticles, nanowires) to produce hybrid composite materials. Presently, we report on growing polyelectrolyte brushes onto TiO2 nanowires from catecholic initiator and polyelectrolyte brush templated Pd nanoparticle loading. More importantly, charged groups in polyelectrolyte brushes can be recovered after nanoparticle deposition and still possess the ion-exchange capability, which allows multiple loading of nanoparticles, opening up a novel

10.1021/jp901301t CCC: $40.75  2009 American Chemical Society Published on Web 04/09/2009

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way for controllable particle uploading. The catalytic activity of the TiO2/brush/Pd was preliminarily studied. Experimental Section Reagent. Monomer 2-(dimethylamino)ethyl methacrylate was obtained from Aldrich. Initiator 2-bromo-2-methyl-N-[2-(3,4dihydroxyphenyl)ethyl]propionamide was synthesized according to the literature.30 Copper(I) bromide was purified by reflux in acetic acid. (NH4)2PdCl4 was obtain from the Chemical Reagent Co. of Shanghai (Shanghai, China). 3-Hydroxytyramine hydrochloride was obtained from Aldrich. Ultrapure water used in all experiments was obtained from a NANOpure Infinity system from Barnstead/Thermolyne Corp. Other reagents were used as received. 1. Synthesis of the Initiator 2-Bromo-2-methyl-N-[2-(3,4dihydroxyphenyl)ethyl]propionamide. A 250 mL round-bottomed flask was charged with borax (Na2B4O7 · 10H2O, 3.83 g, 10 mmol) and 100 mL water. The solution was degassed with Ar for 30 min, and dopamine · HCl (1.9 g, 10 mmol) was added. The reaction mixture was stirred for 15 min, and the pH was adjusted to pH 9-10 with Na2CO3 · H2O (3.99 g, 32 mmol). The resulting solution was cooled in an ice bath, and 2-bromoisobutyl bromide (1.24 mL, 2.30 g, 10 mmol) was added dropwise via a syringe. The reaction mixture was allowed to reach room temperature and stirred for 24 h under Ar. The pH of the solution was maintained at 9-10 with Na2CO3 · H2O during the reaction. The reaction solution was then acidified to pH ) 2 with 6 M aqueous HCl solution and extracted with EtOAc (3 × 100 mL). The combined organic extracts were dried over anhydrous MgSO4, and the solvent was evaporated under reduced pressure to give a brown liquid. The crude product was purified by silica gel column chromatography (5% MeOH in CHCl3) to give a colorless viscous liquid (1.06 g, 3.5 mmol, yield 35%) of compound 2. 1H NMR (300 MHz, CDCl3), (ppm): 6.81 (d, J ) 8.1 Hz, 1H), 6.74 (d, J ) 1.8 Hz, 1H), 6.60 (dd, J ) 8.1, 1.8 Hz, 1H), 3.48 (dd, J ) 13.5, 6.9 Hz, 2H), 2.72 (t, J ) 6.9 Hz, 2H), 1.90 (s, 6H). 13C NMR (300 MHz, CDCl3), (ppm): 172.81, 144.02, 142.80, 130.58, 120.90, 115.60, 115.35, 62.64, 41.89, 34.68, 32.45. 2. Substrate Preparation and Modification. TiO2 nanowires were synthesized from commercial TiO2 granule in a Teflonlined autoclave, as described in our previous work.31 TiO2 nanowires (100 mg) were immersed in a 1.5 mg/mL solution (ultrapure water:ethanol ) 1:4) of initiator and stirred for 18 h in the dark at room temperature. The modified TiO2 nanowires were isolated and purified by repeated washing with ultrapure water and ethanol using centrifugation. 3. Surface-Initiated Polymerization. In a typical SI-ATRP, 1 mL of DMAEMA monomer and 10 mL of 1:1 (v:v) MeOH/ H2O mixture were placed in a flask under Ar flow for 20 min; then, 60.8 mg of bipyridyl and 30.4 mg of CuBr were charged into a flask and purged with Ar flow again; 20 min later 100 mg of initiator-modified TiO2 nanowires was added. The polymerizations were performed at room temperature under Ar protection. The mixture remained maroon and stable throughout the reaction. At various times, the substrates were taken out of the polymerization solution and washed with copious ultrapure water and ethanol using centrifugation. The thickness of the PDMAEMA brush was controlled by changing the polymerization time. The polymer-grafted samples were further dried under vacuum overnight before further analysis. 4. Preparation of PDMAEDM Brush Containing Pd Nanoparticles on TiO2 Nanowires. First, quaternization of PDMAEMA was carried out in iodomethane/CH3NO2 (1:5) at room

Ye et al. temperature for 24 h. After the quaternized PDMAEMA (denoted as Q-PDMAEMA) on TiO2 nanowires surface was thoroughly rinsed with solvent using centrifugation, it was then immersed in 0.1 M (NH4)2PdCl4 solution for 1 h to exchange anion, and finally, the resulted material was immersed in fresh 0.1 M NaBH4 for 30 min to reduce Pd(II). The obtained PDMAEMA/Pd composite brushes were isolated and purified by repeated washing with ultrapure water using centrifugation. 5. Preparation of Pd Nanoparticles-TiO2 Nanowires/ITO Electrode. TiO2 nanowires-Q-PDMAEMA/Pd(0) were dispersed in Nafion ethanol solution. The solution was sonicated for 15 min, and then the catalyst solution was applied by a micropipet to the ITO electrode surface. Electrochemical measurements were made using a three-electrode cell at 25 °C. Pt foil and SCE were used as the counter and reference electrodes, respectively. The ITO electrode was the working electrode, which was brushed with the catalyst ink. All potentials are reported vs SCE in the paper. Solutions of 1 M H2SO4 and 1 M CH3OH in 1 M H2SO4 were purged with N2. To identify the properties of the Pd-based catalyst in H2SO4 and to measure the onset potential of methanol oxidation, CV was performed in the potential range between -0.2 and 1.2 V. Characterization. 1H NMR 13C NMR spectra were recorded on a 300 MHz spectrometer (Bruker AM-300) using CDCl3 as the solvent. Chemical composition information about the samples was obtained by X-ray photoelectron spectroscopy (XPS); the measurement was carried out on a PHI-5702 multifunctional spectrometer using Al KR radiation, and the binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon. The morphology was investigated by transmission electron microscopy (TEM) (Hitachi model H-900). Thermal stability was determined with a thermogravimetric analyzer (TGA) (Perkin-Elmer, PET) over a temperature range of 25-900 °C at a heating rate of 10 °C/min under a N2 atmosphere. FT-IR was recorded on a TENSOR 27 instrument (BRUCKER). Electrochemical experiments were performed by a CHI 600 electrochemical workstation (Chenhua, Shanghai). Results and Discussion Depiction of the general approach to grow polymer brushes onto TiO2 via SIP32 from biomimetic initiator is shown in Scheme 1. The bifunctional initiator contains a catechol end group for surface anchoring and an alkyl bromine to activate SIP of 2-(dimethylamino)ethyl methacrylate (DMAEMA).33 PDMAEMA-grafted-TiO2 nanowires (denoted as TiO2 nanowires-g-PDMAEMA) was then quaternized to give TiO2 nanowires-Q-PDMAEMA. The TiO2 nanowires-Q-PDMAEMA nanocomposites can entrap transition-metal ions by the coordinating segments, and subsequently, the metal ions can be reduced in situ to construct novel metallic nanoparticles.34 Successful initiator immobilization and polymer grafting were first ascertained by XPS. Figure 1A displays the XPS survey spectra of TiO2 nanowires, TiO2 nanowires-initiator, and TiO2 nanowires-g-PDMAEMA (4 h). Surface chemical composition data of these surfaces with different polymer growth time are summarized in Table 1. The TiO2 nanowires were mainly composed of Ti and O as well as a small amount of C resulting from adventitious hydrocarbon contamination. Successful anchoring of initiator-modified TiO2 nanowires was confirmed by the presence of N and Br signals not detected for the TiO2 nanowires. After 4 h SI-ATRP of DMAEMA, polymer grafting was verified by a substantial increase in C concentration (from 40.67% to 72.89%) and the evident decrease of Ti signal. XPS provided evidence for the systematic increase in the amount of

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Figure 1. (A) XPS survey spectra of TiO2 nanowires (a), initiator-modified TiO2 nanowires (b), and TiO2 nanowires-g-PDMAEMA (4 h) (c). The insert is the high-resolution spectrum of the C 1s region of TiO2 nanowires-g-PDMAEMA. (B) XPS survey spectra of TiO2 nanowires-g-PDMAEMA (4 h) (a), TiO2 nanowires-Q-PDMAEMA (b), TiO2 nanowires-Q-PDMAEMA/Pd(II) (c), and TiO2 nanowires-Q-PDMAEMA/Pd(0) (d). (C) XPS survey spectra of TiO2 nanowires-g-PDMAEMA in the N (1s) level regions before (curve a) and after (curve b) quaternization. (D) XPS spectra of TiO2 nanowires-Q-PDMAEMA-PdCl4 in the Pd (3d) level regions before (curve a) and after (curve b) reduction reaction.

SCHEME 1: Preparation of Functional TiO2 Nanowires

TABLE 1: Surface Analysis Results of Initiator-Modified TiO2 Nanowires and Grafted TiO2 Nanowires (different polymerization time SI-ATRP) Samples XPS atomic concentration (%) sample initiator-modified-TiO2 TiO2-g-PDMAEMA-

0.5 h 1h 2h 4h

[Br]

[N]

[O]

[Ti]

[C]

1.63 0.65 0.41 0.30 0.20

1.82 5.61 6.06 6.46 6.18

41.57 27.50 22.96 21.03 19.90

14.30 3.83 1.61 1.85 0.61

40.67 62.41 68.96 70.35 72.89

PDMAEMA with the polymerization time (Table 1). Furthermore, high-resolution XPS spectral analysis demonstrated good agreement with the structure of the grafted polymer coating. As shown in the inset of Figure 1A, the C 1s high-resolution scan of TiO2 nanowires-g-PDMAEMA shows the peak components at binding energies of about 284.6 and 285.3 eV,

attributed to the C-H/C-C, C-O/C-Br. The C 1s core-level spectra evidently added a peak at 288.5 eV, attributed to OdC-O after PDMAEMA was formed. Successful quaternization and formation of ternary nanocomposite were first ascertained by XPS. Figure 1B displays the XPS survey spectra of TiO2 nanowires-g-PDMAEMA (4 h), TiO2 nanowires-QPDMAEMA, TiO2 nanowires-Q-PDMAEMA/Pd(II), and TiO2 nanowires-Q-PDMAEMA/Pd(0). Successful quaternization was indicated by the presence of I signals not detected for the TiO2 nanowires-g-PDMAEMA. Moreover, as shown in Figure 1C, the N1s peak of TiO2 nanowires-g-PDMAEMA is at 339.0 eV. After quaternization, the peak shifts to 402.2 eV with complete disappearance of ternary amine. This is in contrast with what we reported before that quaternization was low to a maximum of 50% on planar substrate.33a The FT-IR spectra as shown in Figure 2 were employed to further confirm the presence of

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Figure 2. FT-IR spectra of (a) TiO2 nanowires, (b) TiO2 nanowiresinitiator modified, and (c) TiO2 nanowires-g-PDMAEMA (polymerization time ) 4 h).

grafted coatings by observation of the absorptions of typical functional groups containing in the polymer. Figure 2c demonstrates the FT-IR spectrum of composite after 4 h SI-ATRP of DMAEMA. Typical features of the PDMAEMA include the absorptions at 2942 cm-1 due to C-H symmetric and asymmetric stretching of methyl and -CH2- groups, 2819 and 2766 cm-1 from C-H stretching of the -N(CH3)2 groups, a sharp peak at 1727 cm-1 from CdO stretch in the ester group, a peak at 1454 cm-1 from CH2 bending, and a peak at 1150 cm-1 from C-N stretching of -N(CH3)2 groups. The anion exchange with PdCl42- was confirmed by the appearance of signals of Pd(II) and Cl. XPS measurements were made to confirm the chemical reduction of PdCl42- and the formation of Pd(0) on TiO2 nanowires-Q-PDMAEMA/Pd. As depicted in Figure 1D, the Pd (3d5/2) and Pd (3d3/2) peaks are present at 337.4 and 342.8 eV, respectively, prior to reduction. After reduction, the peaks shift to 340.0 and 334.5 eV, respectively, which is consistent with the change in oxidation state from +2 to 0. TGA was carried out in order to evaluate the change of the organic portion during polymer grafting on TiO2 nanowires at different polymerization times. As seen in Figure 3A, we observe a two-step degradation profile for TiO2 nanowires-gPDMAEMA samples. For instance, a typical sample (TiO2 nanowires-g-PDMAEMA, 4 h polymerization) exhibited two thermal decomposition processes at ∼251 and ∼327 °C. Similar shapes of TGA profiles were also observed for samples with different polymerization times. TGA of TiO2 nanowires had a 12.1% weight loss in the range between 100 and 600 °C. Upon addition of ATRP initiator, the weight loss increased to 13.2%; so, the 1.1 wt % difference was ascribed to the portion of attached initiator. According to TGA results of TiO2 nanowiresg-PDMAEMA at different polymerization times, the residual mass percents are 78.2% (30 min), 71.9% (1 h), 65.7% (2 h), and 62.6% (4 h) at 600 °C, which shows the grafting percentage of PDMAEMA is 7.5% (30 min), 13.8% (1 h), 20.0% (2 h), and 23.1% (4 h). As shown in Figure 3A, TGA also provided evidence for the systematic increase in the amount of PDMAEMA with the polymerization time, indicating the living nature of polymer growth. Thermogravimetric analysis was used to study the decomposition pattern and the thermal stability of TiO2/polymer/Pd composites. As seen in Figure 3B, the thermal decomposition of TiO2 nanowires-Q-PDMAEMA (curve d) also exhibits a twostep mechanism. The TGA curve of TiO2 nanowires-QPDMAEMA (curve d) displayed lower a decomposition temperature than TiO2 nanowires-g-PDMAEMA (curve c). Moreover, TiO2 nanowires-Q-PDMAEMA (curve d) displayed more weight loss than TiO2 nanowires-g-PDMAEMA (curve c) due to the added decomposable methyl group and iodide ion and enlarged

Ye et al. polymer molecular weight. After anion exchange using (NH4)2PdCl4, the TGA curve of TiO2 nanowires-Q-PDMAEMA(II) and TiO2 nanowires-Q-PDMAEMA(0) showed less weight loss than TiO2 nanowires-Q-PDMAEMA due to nondecomposable inorganic element. PDMAEMA is a pH-responsive polymer.33 The TiO2 nanowires-g-PDMAEMA can be dispersed in HCl aqueous media of pH 2.0 at room temperature. PDMAEMA chains of the TiO2 nanowires-g-PDMAEMA can be selectively precipitated by adding NaOH aqueous alkaline media to increase the pH to 11.0. As demonstrated in this study, TiO2 nanowires-g-PDMAEMA solutions exhibit pH-responsive behavior due to protonation/ deprotonation of N,N-dimethylaminoethyl groups in PDMAEMA component. In acidic environment the tertiary amines get protons to form quaternary ammonium and become positively charged polyelectrolyte. The TiO2 nanowires-g-PDMAEMA brush are highly stretched along the radial direction because of the geometrical constraint, the electrostatic repulsion between PDMAEMA brushes, and strong chain-solvent interactions. However, in a basic environment, the amine groups of PDMAEMA are deprotonated and the high grafting density limits the available space that PDMAEMA brushes can occupy. PDMAEMA brushes chain-chain interactions are stronger than chain-solvent interactions, and PDMAEMA brushes gradually shrink and precipitate from solution.35 The result indicates that TiO2 nanowires-g-PDMAEMA brushes have a good pHresponsive property. Figure 4B show photographs of dispersity of TiO2 nanowiresQ-PDMAEMA in 0.1 M NaCl, 0.1 M NaClO4, 0.1 M NH4PF6, and 0.1 M CF3SO3Na solutions. Transformation to other forms was realized via anion exchange of the I- counteranions with an excess of other target anions such as Cl-, ClO4-, PF6-, and CF3SO3-. Derivation of PDMAEMA with iodomethane retains its water solubility. The same situation was found for TiO2 nanowires-Q-PDMAEMA-Cl. In contrast, TiO2 nanowires-QPDMAEMA substituted with other lipophilic anions ClO4-, PF6-, and CF3SO3- are non-water soluble. This indicates that the solubility of modified TiO2 nanowires can be altered by anion exchange with relatively hydrophobic anions ClO4-, PF6-, and CF3SO3-. The association constants and hydrophobicities of ClO4-, PF6-, and CF3SO3- are generally larger than hydrophilic I- and Cl-. The anions in TiO2 nanowires-Q-PDMAEMACl are fully dissociated in water because of their hydration. Stable dispersion in water comes from charge repulsion of the cations that counterbalance the van der Waals interactions between TiO2 nanowires. In contrast, the dissociation of hydrophobic anions (ClO4-, PF6-, and CF3SO3-) is unfavorable because of poor solvation of the anions in water.36 The tunable solubility in aqueous media is very useful in its application of either solution processed device fabrication or the recycling of TiO2 as catalyst. The success of the strategy was further verified by TEM characterization. Figure 5 shows high-resolution transmission electron microscopic (HRTEM) images of nongrafted TiO2 nanowires, TiO2 nanowires-g-PDMAEMA (4 h), and TiO2 nanowires-Q-PDMAEMA/Pd(0) in ethanol prepared by dropping their dispersion onto carbon-coated copper grid. Nongrafted TiO2 nanowires were observed to be aggregates of 20 to over 100 nm. In contrast, TiO2 nanowires-g-PDMAEMA was observed to have fine dispersion as one or two wires. These findings clearly revealed that the PDMAEMA-g-TiO2 nanowires allow dispersion and stability in appropriate solvents. Figure 5B shows the TEM evolution of surface morphology of TiO2 nanowires-g-PDMAEMA (4 h). In fact, all the TiO2 nanowires

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Figure 3. (A) TGA traces of TiO2 nanowires (a), TiO2 nanowires-initiator modified (b), TiO2 nanowires-g-PDMAEMA (30 min) (c), TiO2 nanowiresg-PDMAEMA (1 h) (d), TiO2 nanowires-g-PDMAEMA (2 h) (e), and TiO2 nanowires-g-PDMAEMA (4 h) (f). (B) TGA curves of TiO2 nanowires (a), TiO2 nanowires-initiator modified (b), TiO2 nanowires-g-PDMAEMA (polymerization time ) 4 h) (c), TiO2 nanowires-Q-PDMAEMA (d), TiO2 nanowires-Q-PDMAEMA/Pd(II) (e), and TiO2 nanowires-Q-PDMAEMA/Pd(0) (f).

Figure 4. (A) Photographs of PDMAEMA brush-modified TiO2 nanowires in water with different pH values: (a) pH ) 11 and (b) pH ) 2. (B) Photographs of Q-PDMAEMA brush-modified TiO2 nanowires in different electrolyte solutions: (a) 0.1 M NaCl, (b) 0.1 M NaClO4, (c) 0.1 M NH4PF6, and (d) 0.1 M CF3SO3Na.

Figure 6. Histogram of size distribution of Pd nanoparticles.

Figure 7. XPS survey spectra of TiO2 nanowires-Q-PDMAEMA-PdCl4 in the Pd (3d) level regions before (a) and after (b) the first chemical reduction, before the second chemical reduction but reloaded with PdCl42- (c), after the second reduction, and (d) after the third chemical reduction (e).

Figure 5. TEM images of TiO2 nanowires (A), TiO2 nanowires-gPDMAEMA (polymerization time ) 4 h) (B), and TiO2 nanowiresQ-PDMAEMA/Pd(0) (C).

were homogeneously coated with a very uniform PDMAEMA layer, and the interface boundary was very clear as shown in Figure 5B. Increasing polymerization time, the thickness of surface coatings increased with time (data not shown). After TiO2 nanowires-g-PDMAEMA (4 h) was formed, the shell thickness increased 15 nm (Figure 5B). The polymer coatings reported here are significantly thicker than those formed by adsorption of DOPA-functionalized polymers from solution (“graft-to” approach) onto similar substrates.37 However, the

nature of the chemical interaction between grafted polymer and underlying substrate is expected to be similar for both “graftfrom” and “graft-to” approaches. On TiO2 nanowires, the interaction is likely to take the form of a bidentate coordination complex between the catechol oxygens and a TiO2 nanowires surface.38 Binding energies between TiO2 and dopamine have recently been estimated by density functional theory to be on the order of 25-30 kcal/mol,39 suggesting that grafted polymer chains are strongly bound to TiO2 nanowires surface. From TEM of TiO2 nanowires-Q-PDMAEMA/Pd(0), it can be seen that high-density dark particles were generated and adhered individually on TiO2 nanowires (Figure 5C); the sizes the Pd nanoparticles were measured to be 2-4 nm with near monodispersity (Figure 6). Imaginably, the metal nanocrystals were

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SCHEME 2: Repeated Uploading of Pd Nanoparticles via Regenerative Counterion Exchange and Chemical Reduction

formed with adjacent ion precursor. The absence of external ion resource prevents further crystal growth of metal clusters and allows formation of narrowly distributed metal nanocrystal. The presence of polymer matrix also stabilizes newly formed metal nucleates and prevents their further collision. The uniformity and good coverage come from the uniform coverage of polymer brushes. If we look into the involved chemistry in detail, an interesting question will be raised: whether the positive charges within polyelectrolyte brushes are freed upon reduction of metal complex so that they resume the ion exchange capacity. If so, the uploading of metal nanoparticles can be reasonably proceeded another time, possibly more times. The assumption was first simply tested visually by observing the solubility. TiO2 nanowires-Q-PDMAEMA/Pd(II) have poor dispersivity in water medium due to the strong ion pairing effect to screen charges

Ye et al. and cross-linking effect by divalent counterions; however, after reduction the composite turned a slightly dark color, indicating generation of metal particles, but exhibited good dispersivity in water again, just like TiO2 nanowires-Q-PDMAEMA, indicating charged groups assumed the dissociated state. The possible mechanism of counterion exchange is shown in Scheme 2. As depicted in Figure 7, the Pd(II) (3d5/2) and Pd(II) (3d3/ 2) peaks are present at 337.5 and 342.5 eV, respectively, prior to reduction. After first reduction, both peaks shift to 340.2 and 334.8 eV, respectively, which implies the change in oxidation state from +2 to 0. After the second loading of (NH4)2PdCl4, the Pd (3d5/2) and Pd (3d3/2) peaks are present at 337.5, 342.5, 341.0, and 335.6 eV, which shows the coexistence of Pd(+2) and Pd(0), verifying that TiO2 nanowires-Q-PDMAEMA/Pd(0) ternary nanocomposites can exchange anion PdCl42-. After the second reduction, the Pd peaks exhibit pure Pd(0) but with increased intensity in the normalized spectra with the times of loading. The multiple uploading of Pd nanoparticles was also confirmed by TGA, showing systematic weight loss decrease in TiO2 nanowires-Q-PDMAEMA/Pd(0) vs the loading cycles (Figure 8). TGA of TiO2 nanowires-Q-PDMAEMA/Pd(0) (first) had a 28.3% weight loss in the range between 100 and 600 °C; after the second and third reduction the weight loss is 21.2% and 18.2%, so the 7.1 and 3.0 wt% decreases of weight loss were attributed to the additional Pd nanoparticles uploaded in second and third cycle. It needs to be pointed out that the particle uploading procedure cannot proceed all the times, since the already existing nanoparticles would occupy large space within polymer brushes, restrict mobility of polymer chains, and shield the accessibility of the polymer segment to an external environment. Eventually the ion-exchange capacity will be lost. Pd dispersed on TiO2 nanostructure, which leads to high surface area substrates, showed excellent catalytic activities.19 Cyclic voltammograms (CVs) were collected for Pd-TiO2 nanowires electrocatalyst in nitrogen purged 1 M H2SO4 (Figure 9A), which is used to electrochemically characterize and stabilize the catalyst materials, before methanol oxidation was undertaken. In the case of Pd-TiO2 nanowires materials, anodic current due to Pd oxide formation is also seen, commencing already at 0.59 V, with its reduction occurring in a peak at 0.3 V under these conditions. Figure 9B shows the first scan cyclic voltammograms for Pd-catalyzed TiO2 nanowires on ITO electrode in 1 M CH3OH + 1 M H2SO4. On the scan to positive potentials from -0.2 V, the onset of the methanol oxidation was around +0.54 V, and a large methanol oxidation peak was observed at +0.83 V on the positive irreversible scan. Conclusions

Figure 8. TGA traces of TiO2 nanowires-Q-PDMAEMA/Pd(0) 1st (a), 2nd (b), and 3rd (c).

We demonstrated a facile and effective approach to modify TiO2 nanowires with functional polymers via surface-initiated

Figure 9. (A) Cyclic voltammograms of the Pd-TiO2 nanowires materials/ITO electrode in 1 M H2SO4 at room temperature at a scan rate of 50 mV/s. (B) CVs (50 mV/s) of Nafion-dispersed Pd-TiO2 nanowires catalysts deposited on ITO electrode in 1 M CH3OH + 1 M H2SO4.

Loading of Pd Nanoparticles onto TiO2 Nanowires polymerization using catechol-based initiator. The solubility/ precipitation of the TiO2 nanowire/polymer can be manipulated by altering the external solution conditions, such as pH, electrolyte, etc. The functional polymers can be used to template metal Pd nanoparticle synthesis on TiO2 nanowires. Nanoparticles synthesized in this way have a very uniform diameter of 2-4 nm, narrow dispersity, good dispersibility, and high coverage on nanowires. Most interestingly, polyelectrolytetemplated nanoparticle uploading can be repeated several times by regenerative ion-exchange-reduction so that the amount of Pd nanoparticles can be to some extent controlled. The preparation method of TiO2 nanowires-Q-PDMAEMA/Pd(0) is not limited to Pd nanoparticles; it may be used to prepare a variety of metal and semiconductor particles/polymer on the surface of various metal and metal oxide nanostructures. The TiO2 nanowires/polymer/Pd(0) ternary composites might find applications in (electro)catalysis. Acknowledgment. The work was financially supported by the “Top Hundred Talents” Program of CAS and Key Project of NSFC (50835009) and “973” (2007CB607601). References and Notes (1) Chen, X. B.; Mao, S. S. Chem. ReV. 2007, 107, 2891. (2) O’Regan, B.; Gra¨tzel, M. Nature (London) 1991, 353, 737. (3) Bach, U.; Lupo, D.; Comte, P.; Moser, J. E.; Weisso¨rtel, F.; Salbeck, J.; Spreitzer, H.; Gra¨tzel, M. Nature (London) 1998, 395, 583. (4) Du¨rr, M.; Schmid, A.; Obermaier, M.; Rosselli, S.; Yasuda, A.; Nelles, G. Nat. Mater. 2005, 4, 607. (5) Mor, G. K.; Shankar, K.; Paulose, M.; Varghese, O. K.; Grimes, C. A. Nano Lett. 2006, 6, 215. (6) Heller, A. Acc. Chem. Res. 1995, 28, 503. (7) Tang, J.; Wu, Y.; McFariand, E. W.; Stucky, G. D. Chem. Commun. 2004, 1670. (8) Fei, H. L.; Liu, Y. P.; Li, Y. P.; Sun, P. C.; Yuan, Z. Z.; Li, B. H.; Ding, D. T.; Chen, T. H. Microporous Mesoporous Mater. 2007, 102, 318. (9) Meier, K. R.; Gratzel, M. ChemPhysChem 2002, 371. (10) Topoglidis, E.; Cass, A. E. G.; O’Regan, B.; Durrant, J. R. J. Electroanal. Chem. 2001, 517, 20. (11) Fujishima, A.; Honda, K. Nature (London) 1972, 238, 37. (12) Khan, S. U. M.; Al-Shahry, M.; IngLer, W. B. Science 2002, 297, 2243. (13) Park, J. H.; Kim, S.; Bard, A. J. Nano Lett. 2006, 6, 24. (14) (a) Xiong, C. R.; Balkus, K. J., Jr. Chem. Mater. 2005, 17, 5136. (b) Wu, C. W.; Ohsuna, T.; Kuwabara, M.; Kuroda, K. J. Am. Chem. Soc. 2006, 128, 4544. (c) Macak, J. M.; Tsuchiya, H.; Schmuki, P. Angew. Chem., Int. Ed. 2005, 44, 2100. (15) Tauster, S. J.; Fung, S. C.; Garten, R. L. J. Am. Chem. Soc. 1978, 100, 170. (16) Kang, J. H.; Shin, E. W.; Kim, W. J.; Park, J. D.; Moon, S. H. J. Catal. 2002, 208, 310. (17) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Chem. ReV. 1995, 95, 735.

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