Selective Pt Deposition onto the Face (110) of TiO2 Assembled

ARTICLE pubs.acs.org/JPCC

Selective Pt Deposition onto the Face (110) of TiO2 Assembled Microspheres That Substantially Enhances the Photocatalytic Properties Jun Zhang,†,§ Liping Li,‡ Tingjiang Yan,† and Guangshe Li*,† †

State Key Laboratory of Structural Chemistry and ‡Key Lab of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, People’s Republic of China § School of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, People’s Republic of China

bS Supporting Information ABSTRACT: This work reports on a two-stage strategy toward the selective Pt deposition onto the face (110) of TiO2 assembled microspheres for excellent photocatalytic activities. This approach includes the assembly of nanowires with exposed faces (110) and tips (001) to form microspheres, which is followed by Pt deposition onto the face (110) under photoreduction. Systematic sample characterizations indicate that there exist abundant microcavities in between the roughly parallel nanowires of the microspheres which act as the microcapacitors for electronic storage, while Pt deposition featured by surface plasmon resonance under irradiation may function to donate or accept electrons. Therefore, under ultraviolet-light irradiation, Pt deposition accelerated the electronic migration from the conduction band of the microspheres to Pt, while under visible-light irradiation, electrons from Pt nanoparticles would transfer to the conduction band of the microspheres with a simultaneous transfer of the compensable electrons from a donor in reaction solution to the deposited Pt nanoparticles. As a consequence, Pt deposition onto TiO2 assembled microspheres is proved to yield an excellent photocatalytic performance superior over that without Pt deposition under ultraviolet or visible light irradiation.

1. INTRODUCTION Photocatalysis has become one of the most promising technologies in removing organic pollutants, especially those at relatively low concentrations but with high toxicity.1 Among the various photocatalysts reported, TiO2 has shown merit with such features as high stability, cheapness, and environmental friendliness, no secondary pollution, and rather strong ability to remove a broad class of organic pollutants. However, there are two disadvantages that limit its practical applications. The first one is represented by its wide band gap energy (e.g., 3.0 eV for rutile or 3.2 eV for anatase), which allows only the ultraviolet light absorption. The second one is associated with the relatively low overall quantum efficiency because of the fast recombination of photogenerated electrons and holes during the photocatalytic reactions. As a result, great efforts have been made to improve the photocatalytic activities of TiO2.2 For instance, structural modifications of TiO2 based on doping foreign ions (e.g., metals3
6 or nonmetals7
9) or coupling with other semiconductors (e.g., WO3)10 have been investigated to extend the absorption threshold to visible light, and some gains have already been obtained. Although the band gap of TiO2 can be tailored to a certain extent, the defect levels closely relevant to doping or semiconductor coupling would also act as the recombination centers of photogenerated electrons and holes, which would r 2011 American Chemical Society

lead to the significant lowering of the quantum efficiency. Consequently, many researchers started to explore other techniques such as those based on the introduction of magnetic field11 or H2 input12 into the photochemical reaction systems, which indeed has effectively improved the quantum efficiency of TiO2. However, the reaction systems have thus become complicated and may also lead to some safety problems. Deposition of noble metals onto TiO2 could be a promising approach to meet such problems for excellent photocatalytic activities, since, as reported by others, deposition of Pt, Au, Ag, or Pd onto TiO2 can result in Schottky barriers on interfaces between noble metals and TiO2, yielding an efficient separation of photogenerated electron
hole pairs as well as an increased absorbance to visible light due to the surface plasmon resonance of noble metals under light irradiation.10,13 Therefore, photocatalytic efficiency is expected to be substantially improved by depositing noble-metals onto TiO2. In this regard, there has been some progress, focusing on the photocatalytic activities under UV-light irradiation, while it is still unattainable as to the efficient Received: April 15, 2011 Revised: June 7, 2011 Published: June 09, 2011 13820

dx.doi.org/10.1021/jp203511z | J. Phys. Chem. C 2011, 115, 13820–13828

The Journal of Physical Chemistry C utilization of the visible light. Therefore, it is highly necessary to develop a fabrication method for photocatalysts that can efficiently respond to the ultraviolet light or visible light for high activities. To reach such a goal is still very difficult when using the conventional noble-metal deposition methods, since noble-metal particles are often randomly deposited onto the surfaces of semiconductors13 and the crystal face selectivity for noble-metal deposition is almost not expected. A very recent work14 by Jiao et al. demonstrates the formation of TiO2 hollow spheres that consist of a highly active {116} plane with Pt doping, which implies the achievement of highly photocatalytic activities through noble metal deposition onto the specific crystal faces of TiO2. In this work, we designed a two-step method to prepare the Pt deposition TiO2 microspheres for highly photocatalytic activities. Namely, rutile TiO 2 microspheres assembled by nanowires were first synthesized to show almost 100% exposure faces (110) and tips (001) through a solution chemistry. Then Pt nanoparticles were selectively deposited onto the faces (110) under the designed photoreduction. Pt deposition is indicated to significantly enhance the photocatalytic performance under UV or visible light irradiation. The strategy reported in this work is fundamentally important, which may be extended to other metal
semiconductor hybrids with tailored interfacial processes for highly catalytic reactions.

2. EXPERIMENTAL SECTION 2.1. Sample Preparation. All reagents employed in the experiments were analytical grade and used without further purification. Self-Assembled Microspheres of Rutile TiO2. All samples were synthesized according to the solution chemistry described in our previous work15 with a few modifications. In a typical experiment, 20.5 mL of TiCl4 (Alpha, 99%) was added dropwise into 60 mL of distilled water at 0 °C in an ice
water bath under vigorous magnetic stirring to form a given concentration of aqueous solution. After the solution was stirred for about 1 h, it was transferred to a Teflon-lined stainless steel autoclave (100 mL). After reaction at 160 °C for 2 h, the autoclave was allowed to cool to room temperature naturally, and the final product was washed carefully with distilled water to remove Cl
in the residual solution. The rinsing process was repeated until the pH of the solution reached 7, and then the resulting product was dried at 80 °C for 4 h in air. Pt Deposition onto the Self-Assembled Microspheres of Rutile TiO2. All deposited samples were synthesized via a photoreduction process. Namely, 0.5 g of the as-prepared self-assembled microspheres was immersed into a 100 mL solution of H2PtCl4 (0.0256 mmol, Sino-Platinum Co., Ltd.) in distilled water. Acetic acid (0.1 M, Sinopharm Chemical Reagent Co., Ltd.) was used to adjust the pH of suspension to about 3. Then high-purity nitrogen passed through the suspension for 15 min to remove the oxygen in the suspension. After the mixure was stirred for 3 h, the solution was irradiated for 3.5 h by a 300 W high-pressure xenon lamp while stirring. Finally, the specimen was centrifuged and washed with distilled water until no Cl
was detected in the rinse water. After being dried in vacuum at 80 °C, the resulting product was obtained and the sample denoted as Pt-TiO2. 2.2. Sample Characterization. Powder X-ray diffraction (XRD) patterns of the samples were collected on Rigaku MinFlex II benchtop X-ray diffiractometer with Cu KR irradiation. The

ARTICLE

crystallite sizes for the samples were calculated according to the Scherrer formula, D = 0.89λ/β cos θ, where λ is the X-ray wavelength employed, θ is the diffraction angle for diffraction peak (110), and β is defined as the half-width after subtracting the instrumental broadening effect. Morphologies of the samples were observed by field emission scanning electron microscopy (FESEM) (JEOL JSM-6700). Pt-TiO2 microsphere samples were determined by transmission electron microscope (TEM), highresolution transmission electron microscopy (HRTEM), and energy-dispersive X-ray spectroscopy (EDX) on JEM-2010. UV
vis diffuse reflectance spectra of the samples were obtained using a Varian Cary 500 UV
vis
NIR spectrometer and were converted from reflection to absorption by the Kubelka
Munk method. Specific surface areas of the samples were determined from the nitrogen adsorption data at liquid nitrogen temperature using the Barrett
Emmett
Teller (BET) technique on a Micromeritics ASAP 2000 surface area and porosity analyzer. Photoluminescence spectra (PL) of the sample were obtained using an Edinburgh Analytical Instruments FL/FSTCSPC920 coupled with a time correlated single-photo counting system. Chemical compositions of the samples were studied by X-ray photoelectron spectroscopy (XPS) on an ESCA-LAB MKII apparatus. Alternating current (ac) impedance measurements were carried out over a frequency range from 40 Hz to 5 MHz at room temperature and an oscillation voltage of 0.5 V using a precision LCR meter (Agilent 4294A) with an assistant clamp of Agilent 16334A to obtain the real (Z0 (ω)) and imaginary(Z00 (ω)) parts. 2.3. Photocatalytic Activity Evaluation. There are two sets employed for the activity measurements: (i) a UV light photoreactor, which was constructed of a quartz tube (4.6 cm in inner diameter and 17 cm in length) surrounded by four UV lamps (λ = 254 nm, 4 W); (2) a visible light photoreactor, which was constructed of a quartz tube cooled by water and irradiated by a 200 W halogen lamp. This lamp was located at a distance of 6 cm away from the surface of the solution in the reactor. A 400 nm cutoff glass filter was used to remove the radiation in the UV region emitted by the halogen lamp. Methyl orange (MO) was taken as a probe molecule for evaluating the photocatalytic activity. All experiments were performed at room temperature. Namely, 100 mg of the sample was dispersed in 100 mL of MO solution and magnetically stirred for 5 h to establish the adsorption/desorption equilibrium of MO solution on the sample surfaces before illumination. Then the suspension solution was irradiated by light and collected at a regular time interval. Finally, the collected solution was centrifuged at a rate of 8500 rpm to remove the solid powders, and UV
vis absorption spectra of the supernatant were measured with a Perkin
Elmer UV lambda 35 spectrophotometer.

3. RESULTS Photocatalytic performances of pure-TiO2 and Pt-TiO2 were comparatively evaluated by degradation of MO solution under UV and visible light irradiation, respectively, in which the degradation rate of MO molecules in the solution was monitored by examining the intensity variation of the characteristic absorption peak of MO at 464 nm. For comparison, the photocatalytic activities of the commercial P25-TiO2 (Degussa) were also measured under similar conditions. Under UV light irradiation, pure-TiO2 showed certain photocatalytic activity with a period of time of about 2.5 h to completely decompose MO molecules (Figure 1a). When pure-TiO2 was 13821

dx.doi.org/10.1021/jp203511z |J. Phys. Chem. C 2011, 115, 13820–13828

The Journal of Physical Chemistry C

ARTICLE

Table 1. Properties of TiO2 Microspheres before and after Pt Depositiona adsorption of kapp (λ = 254 nm) kapp (λ >400 nm) surface

a

Figure 1. Photocatalytic activities of the samples under UV light irradiation: (a) degradation rate of MO solution as a function of irradiation time; (b) ln(Co/C) versus irradiation time.

deposited by Pt, photocatalytic activity was significantly improved, as indicated by the shortened degradation duration of 40 min, comparable to that for commercial P25-TiO2. Such an activity improvement is not the consequence of the adsorption ability of the samples for MO molecules, since at the adsorption balance, pure-TiO2 exhibited an adsorption ability of 0.935 mg/g of catalyst, which is much larger than that of 0.594 mg/g of catalyst for Pt-TiO2 or 0.700 mg/g of catalyst for the commercial P25TiO2 (Table 1). Under visible-light irradiation, photocatalytic activities for these samples became totally different. As indicated in Figure 2a, Pt-TiO2 still maintained a high degradation rate under visiblelight irradiation. After irradiation for 7 h, only 34% of the MO molecules were residual. Comparatively, without Pt deposition, the degradation rate was only 21%, and almost no photocatalytic activity was shown as in the case for commercial P25-TiO2. The above photocatalytic degradation processes could be described by the Langmuir
Hinshelwood rate equation as derived from a simple saturation kinetic mechanism. For a relatively high concentration of organic pollutant solution, Langmuirian kinetic expression follows a pseudo-first-order equation described as lnðCo =CÞ ¼ kapp t

ð1Þ

catalyst

MO (mg/g of

(wt %/min 3 g of

(wt %/h 3 g of

sample

catalyst)

catalyst)

catalyst)

(m2/g)

pure-TiO2

0.935

0.06528

0.29416

63

Pt-TiO2

0.594

0.24797

0.94111

58

P25-TiO2

0.700

0.39266

0.41763

50

area

Relevant data for commercial P25-TiO2 are also given for comparison.

where C is the concentration of MO as a function of reaction time, Co is the initial concentration of MO after reaching the adsorption/desorption equilibrium, kapp is the first-order rate constant, and t is the reaction time. The first-order rate constant kapp could be obtained from the linear time dependences of ln(Co/C) as shown in Figures 1b and 2b. The relevant data are listed in Table 1. It is seen that (1) under UV light irradiation, the rate constants of MO degradation follow such a sequence, P25-TiO2 > Pt-TiO2 . pure-TiO2, while under the visible-light irradiation, the degradation rate sequence changes to Pt-TiO2 > P25-TiO2 > pureTiO2, and moreover (2) Pt-TiO2 under UV light irradiation showed a first-order rate constant of kapp = 0.248 wt %/min 3 g, about 15 times larger than that of 0.016 wt %/min 3 g under visible light irradiation. These results demonstrate that TiO2 is an excellent photocatalyst under UV-light irradiation regardless of Pt deposition and that Pt deposition could also stimulate the activity under visible light irradiation. Closely related to the photocatalytic activities are several microstructural factors such as specific surface area, crystallite sizes, crystallinity, morphology, and defect features. The specific surface areas of the samples were first examined by BET measurements. It is found that pure-TiO2 is microporous, showing a surface area of 63 m2/g (Table 1). Upon Pt deposition, surface area slightly reduced to about 58 m2/g, consistent with the relatively small adsorption ability. Therefore, Pt deposition significantly reduced the adsorption sites of MO molecules, as proposed for other systems by Iliev et al.16 Crystallinity and crystallite sizes of the samples were also comparatively examined for studying the impacts of Pt deposition on photocatalytic activities. Shown in Figure 3 are XRD patterns of the samples. It is indicated that both pure-TiO2 and Pt-TiO2 crystallized in the pure phase of rutile structure, which is apparently different from the mixture of rutile with anatase for commercial P25-TiO2. The crystallite sizes for both samples were relatively small, as indicated by the broadened diffraction peaks. The constituent particles were also highly oriented, since the full widths at half-height for peaks (101) and (002) were significantly narrowed comparing to the main peak (110). The average crystallite sizes of both pure-TiO2 and Pt-TiO2 were calculated by the Scherrer formula to be approximately 6 nm. Therefore, TiO2 crystallinity does not change at all before and after Pt deposition. It is noted that no traces of Pt were observed by XRD in Figure 3, possibly because of its too low content or very small particle size when deposited uniformly onto the surfaces of rutile TiO2 microspheres. Consistent with the high orientation revealed by XRD, pureTiO2 was observed by SEM to be micrometer-sized spheres that are constructed by bundles of nanowires (Figure 4). Each nanowire was a tiny single crystal of rutile TiO2 that radiates from the inner nucleus of the microspheres toward the outer 13822

dx.doi.org/10.1021/jp203511z |J. Phys. Chem. C 2011, 115, 13820–13828

The Journal of Physical Chemistry C

ARTICLE

Figure 3. XRD patterns of Pt-TiO2 and pure-TiO2. For comparison, XRD data for P25-TiO2 along with the standard diffraction data for bulk anatase TiO2, rutile TiO2, and Pt are also given.

Figure 2. Photocatalytic activities of the given samples as indicated by (a) degradation rate of MO solution as a function of irradiation time under visible light irradiation and (b) ln(Co/C) versus photodegradation time of MO under visible-light irradiation.

surfaces, while retaining the roughly parallel arrangements as reported in our recent work.17 As a result, all nanowires grew along [001] and exposed their tips (001) to form the microsphere outer surfaces, and the adjacent nanowires exposed the face (110) and arrayed with a little interval to show the nanoscale boundary cavities as the microporous structure as determined by BET. The special structure of the constituent nanowires for rutile TiO2 microspheres provides two possible directions, [110] and [001], for electrons to migrate: First, face (110) can be enriched by electrons,18 since it is an efficient reduction face for rutile TiO2 as indicated by the reduction of Fe3+ to Fe2+ in the photoreaction process. Thus, it is believed that photogenerated electrons could predominantly be spread along the [110] direction (rather than [001] direction) toward surface (110) of each nanowire due to its higher energy required for the electrons to spread from nucleation center to microsphere surfaces (001). In addition, it is well-known that Ti atoms are 6-fold coordination and oxygen atoms are 3-fold coordination in rutile TiO2. The termination (110) of the nanowires may break the Ti
O bonds which lie normal to the surface plane and result in 5-fold Ti and 2-fold O atoms on surfaces.19 The undercoordinated bridging oxygen atoms at surface (110) may form a minimum energy position for a single Pt atom.19 Therefore, the present microspheres were assembled

by bundles of nanowires with special growth direction and terminal plane, which could be suitable for Pt deposition. The successful Pt deposition onto TiO2 microspheres was further confirmed by TEM and EDX. As indicated in Figure 5, Pt nanoparticles with a diameter of about 2
3 nm were well deposited on surfaces of TiO2 nanowires within the microspheres. High-resolution TEM was used to further analyze the deposition of Pt particles. The d-spacings at 0.325 and 0.2487 nm observed in Figure 5b corresponded well to those of (110) and (101) planes for rutile TiO2, respectively, while the lattice fringes with an interplanar spacing of 0.227 nm were associated with the (111) plane of Pt nanoparticles deposited. Further EDX analysis demonstrates the presence of Pt deposition on TiO2 assembled microspheres (Figure 5c). The impacts of Pt deposition on the chemical states, defects, and electronic structure of the microspheres were investigated by XPS. A survey spectrum of pure-TiO2 (not shown) indicates the presence of chemical species Ti, O, and C. Upon Pt deposition, signals associated with Pt were clearly seen (Figure 6a). For instance, the signal for Pt4f7/2 is located at 71.2 eV (Figure 6b), and the spin split between Pt4f7/2 and Pt4f5/2 is approximately 3.2 eV, which matches well the standard data for Pt0.20 Therefore, Pt particles were deposited onto the surfaces of TiO2 nanowires in the form of the metallic state. Even so, Pt deposition also affects the valence electron densities of Ti and O. As indicated in Figure 7, the chemical shift of Ti 2p3/2 for Pt-TiO2 is about 458.3 eV, which is slightly smaller than that of 458.5 eV for pure-TiO2. In addition, there are no shoulder peaks at lower binding energies before and after Pt deposition. In combination with the EPR measurement (Supporting Information), it is thus concluded that Ti ions are mainly in +4 valence state and that there are no traces of Ti3+ species stably existing in lattice or the relevant defects (e.g., oxygen vacancies) regardless of Pt deposition. A slight shift of Ti 2p signals toward lower binding energies indicated the increase of valence electron densities around the 13823

dx.doi.org/10.1021/jp203511z |J. Phys. Chem. C 2011, 115, 13820–13828

The Journal of Physical Chemistry C

Figure 4. (a) SEM and (b) enlarged SEM image for a singular particle of pure-TiO2.

Figure 5. (a) TEM image, (b) HRTEM image, and (c) EDX spectrum of Pt-TiO2. Pt nanoparticles were characterized by crystal plane (111) with a d spacing of 0.227 nm, while rutile nanowires were indicated by planes (110) and (101) at d spacings of 0.325 and 0.2487 nm, respectively.

titanium nuclear, which can be the consequence of either an electronic transfer from Pt particle to TiO2 microspheres or the formation of Ti3+. Since our EPR measurements excluded the presence of Ti3+ species (Supporting Information), there may exist an electronic transfer from Pt to TiO2 microspheres. Pt deposition also imposed impacts on the relative amounts of the surface oxygen species for photocatalytic activities. As shown in Figure 7b, O 1s signals for the samples before and after Pt deposition are broadened and could be fitted in terms of two components from lattice oxygen and chemisorbed oxygen (OH). For pure-TiO2, the OH content estimated from the signal located at 531.5 eV is 39.9%, which increased up to 49.1% upon Pt deposition. Such an increase in OH concentration indicates that Pt deposition could significantly improve the capture ability of hydroxyls necessary for the enhanced photocatalytic efficiency because of the highly efficient interactions between hydroxyls and photoinduced holes during the photocatalytic processes.21

ARTICLE

The impacts of Pt deposition on the defects and charge separation efficiency were investigated by PL spectra. As shown in Figure 8, both pure-TiO2 and Pt-TiO2 exhibited one broad and asymmetric PL emission band with a maximum centered at about 420 nm, which corresponds to the near band edge emission of TiO2. This observation further confirms the absence of lattice defects like oxygen vacancies, since defects like Ti3+ ions or oxygen vacancies may introduce a deep level in the band gap between the conduction band and valence band that would give rise to a visible light emission at longer wavelengths.22 It should also be noted that upon Pt deposition, PL intensity became almost 2 times weaker. For almost all semiconductors with an exciton emission, the higher PL intensity represents a higher recombination rate of electrons and holes. Therefore, Pt deposition has substantially improved the lifetime of photoinduced electrons and holes, which is helpful to understand why Pt-TiO2 has a photocatalytic activity much better than pure-TiO2. The impacts of Pt deposition on the electronic structures of the microspheres were also investigated by UV
vis diffuse reflectance spectrum and PL emission. Figure 9 shows the UV
vis diffuse reflectance spectra of the samples before and after Pt deposition. It is seen that Pt deposition does not make any significant difference in the ultraviolet absorption band edges, despite a shift of band edge toward lower energies when compared to that for commercial P25-TiO2. Further, Pt deposition also led to a higher absorbance in the visible region, which differs from pure-TiO2 or P25-TiO2. For the latter cases, there is an absorption of more than 95% ultraviolet light. It is well documented that many factors such as transition metal ions with d electrons, oxygen vacancies, and surface plasmon resonance of noble metal deposited onto TiO2 could produce a visible absorption.22a,23 For the present work, the visible absorption upon Pt deposition does not come from other transition metal ions since our EDX and XPS measurements detect only one kind of cation signal from Ti. The contribution of oxygen vacancies to the visible absorption can also be ruled out because (1) no oxygen vacancies were observed by EPR and (2) only one band edge emission was detected when excited under 307 nm irradiation. It is thus concluded that the higher visible absorbance upon Pt deposition is the consequence of the highly uniformed deposition of Pt nanoparticles, which has been proved by others to absorb nearly all incident light.24 Therefore, the enhanced visible light absorption upon Pt deposition improves the photocatalytic activity. The above experimental results demonstrate that Pt deposition onto the self-assembled microspheres led to an electronic transfer from Pt to TiO2. Concerning the self-assembled microspheres of TiO2, our recent work15,17 has indicated that there are a great number of nanoscale boundary cavities formed by the roughly parallel nanowires which can act as microparallel capacitors. It is well established that microcapacitors can accumulate electrons when charged. We proposed that Pt deposition may improve the storage capability of electrons through enhancing the dielectric property. To confirm this assumption, we compared the dielectric constants of the microspheres before and after Pt deposition through measuring the corresponding impedance data at room temperature. As indicated in Figure 10, pure-TiO2 exhibits a dielectric constant much higher than that of P25-TiO2, just like what we reported for the similarly prepared microspheres.15,17 Upon Pt deposition, the dielectric permittivity was further improved, as more clearly indicated by the data at lower frequencies. Therefore, Pt deposition is highly helpful to 13824

dx.doi.org/10.1021/jp203511z |J. Phys. Chem. C 2011, 115, 13820–13828

The Journal of Physical Chemistry C

ARTICLE

Figure 6. (a) XPS survey spectrum and (b) core level of Pt 4f of the sample Pt-TiO2.

Figure 7. Core level XPS of (a) Ti 2p and (b) O 1s for pure-TiO2 and Pt-TiO2.

Figure 8. PL emission spectra of pure-TiO2 and Pt-TiO2 excited under 307 nm irradiation.

accumulate more electrons once in the presence of an electron transfer.

Figure 9. UV
vis diffuse reflectance spectra of Pt-TiO2 and pure-TiO2. The relevant data for P25-TiO2 were also given for comparison.

4. DISCUSSION On the basis of the above experimental results, the question of why Pt deposition leads to a better photocatalytic activity of TiO2 hierarchical microspheres becomes clear. The first reason is associated with the exposed face (110) that is highly beneficial for Pt deposition. The face (110) of rutile TiO2 is a most efficient reduction face, as proved by Murakami et al.18 who found Fe3+ ions adsorbed on the (110) face can be reduced effectively. Therefore, under UV-light irradiation, face (110) of the microspheres can be enriched by electrons. As a result, Pt4+ ions in the solution adsorbed on faces (110) are expected to accept electrons, which result in the stable deposition of Pt nanoparticles onto the faces (110) under UV irradiation. Such a photoreduction process for Pt deposition is illustrated in Scheme 1. Namely, under UV-light irradiation, electrons in the valence band of TiO2 were excited and then moved along the [110] direction to combine with Pt4+ ions adsorbed on surfaces of the nanowires. Since the photogenerated electrons would easily spread along the [110] direction, Pt4+ ions adsorbed on face (001) may move to the plane (110) to accept electrons. Eventually once the reduction process of Pt4+ to Pt0 is accomplished, Pt nanoparticles are expected to 13825

dx.doi.org/10.1021/jp203511z |J. Phys. Chem. C 2011, 115, 13820–13828

The Journal of Physical Chemistry C

ARTICLE

Figure 10. Frequency dependences of dielectric permittivity, (a) ε0 and (b) ε00 for given samples at room temperature. The relevant data for P25 with and without Pt deposition are also given for comparison.

Scheme 1. Scheme of Photoreduction Process Designed for Pt Deposition onto the Microspheres of TiO2

selectively anchor on the specific face (110) of each nanowire, meanwhile the site-selective modification by Pt nanoparticles favors the photogenerated carrier separation and migration when excited, contributing the enhanced photocatalytic activity. The consequence of Pt deposition is the improved photocatalytic activity under UV-light or visible-light irradiation, though the relevant photocatalytic mechanisms can be quite different. Under UV-light irradiation, almost all semiconductor photocatalysts show an electronic transfer from valence band (VB) to conduction band (CB), leaving holes in the valence band. Then some of these photoinduced electrons or holes would migrate to the surfaces to participate in the photocatalytic

Scheme 2. Mechanisms Proposed for Photocatalytic Reactions of Pt-TiO2: (1) under UV-Light Irradiation and (2) under Visible-Light Irradiation

reactions, while others at CB would jump back to the VB via a radiation process to yield a light emission or by nonradiation process to give heat. In this regard, many possible factors like surface area, particle size, defects, or band gap have been dismissed by the above experiments for influencing the photocatalytic activity. Instead, one has to consider the separation of 13826

dx.doi.org/10.1021/jp203511z |J. Phys. Chem. C 2011, 115, 13820–13828

The Journal of Physical Chemistry C photoinduced electrons and holes upon Pt deposition, in which the work function governs the electronic transfer or/and the trapping or recombination of photogenerated electrons and holes. The work function for Pt metal is ϕm = 5.36 eV,25 which is larger than that of 4.2 eV for surface (110) of rutile TiO2.26 Therefore, the schematic level diagram for Pt-deposited microspheres can be constructed, as shown in Scheme 2(1). It is clear that the CB of TiO2 microspheres is located above the Ef of the Pt nanoparticles and that Pt deposition led to the appearance of a Schottky barrier at Pt metal
semiconductor contact. Obviously, photogenerated electrons would transfer from CB of TiO2 to Pt nanoparticles, while the reversible process can be extremely difficult. That is to say, Pt deposition may effectively prevent the recombination of photogenerated electrons and holes, accounting for the much improved photocatalytic activity. Although Pt deposition has led to a decrease in the surface area and adsorption ability for MO molecules, the relevant photocatalytic activity under UV light irradiation is still comparable with commercial P25. In this regard, the special assembled nanostructure of the microspheres may play a crucial role, since the nanoscale boundary cavities existing among the roughly parallel rutile nanowires may act as the microparallel capacitors that could accumulate more electrons. When electrons were excited by UV light from VB to CB, the photogenerated electrons can be trapped by these microparallel capacitors and then transfer to Pt or migrate to the microsphere surfaces. So, it can be concluded that the existence of microparallel capacitors and the transfer of photogenerated electrons from TiO2 to Pt metal particle are the dominant reasons for improving the photocatalytic performance under UV light. Naya et al. had concluded that the selectivity of photocatalytic reactions can be achieved through tuning the energy of photons.27 Nevertheless, for the present work, photocatalytic mechanism under UV-light irradiation appears not applicable for that under visible-light irradiation, since (1) the band gap energy before or after Pt deposition is larger than 3.1 eV (Figure 9) and therefore visible-light irradiation with λ >400 nm is not able to excite the VB electrons and (2) there are no deep impurity levels within the band gap because of the absence of defects like Ti3+ ions or oxygen vacancies (Figure 8). In addition, it has also been experimentally proved that the depositing noble metal can induce the photogenerated electron localization which favors the enhanced photon absorption efficiency.28 Similar to what is reported elsewhere,29 the photocatalytic mechanism upon Pt deposition under visible-light irradiation can be described in terms of Scheme 2(2). That is, Pt nanoparticles were first excited under visible-light irradiation because the surface plasmon responsiveness occurred when receiving a continuous energy supply. Then excited electrons may transfer from the full band of Pt below the Fermi level to its higher energy level. These activated electrons at higher energy levels may facilitate to migrate from Pt to the CB of TiO2 microspheres. Subsequently, electrons accepted by CB of TiO2 microspheres could be directly captured by adsorbing the O2 molecules in the vicinity of microsphere surfaces to form the O2
• species for the photocatalytic reactions. It should be mentioned that the time for producing the photoexcited electrons is much shorter than that for the reaction to form the O2
• species and furthermore to decompose MO molecules. Therefore, accumulation of extra electrons is highly necessary for compensating the time difference. In this regard, the microcapacitors formed by adjacent

ARTICLE

nanowires within microspheres (Figures 4 and 5) and the enhanced dielectric constant upon Pt deposition (Figure 10) could provide a huge electronic storage. Additionally, Pt deposition also facilitates the uptake of photogenerated electrons by molecular oxygen to form O2
•. Owing to the continuous electronic migration from Pt nanoparticles to TiO2 microspheres, Pt nanoparticles excited under visible-light irradiation may exert some degree of positive charges. The electronic donors (e.g., H2O or HO
) in MO solution would readily provide electrons to Pt metal to form the radicals (•OH), since the potential of the above donor is more negative than that of the holes on Pt nanoparticles. These highly activated radicals (O2
• and •OH) could participate in the photocatalytic reaction for effective removal of MO species.

5. CONCLUSIONS Self-assembled microspheres of rutile TiO2 were first constructed by bundles of nanowires that grow along the direction [001] to show the exposed face (110) and tips (001). The exposed surfaces (110) were then selectively deposited by Pt nanoparticles. Pt deposition does not significantly alter the microsphere characteristics, surface area, or defects, while slightly reducing the adsorption ability toward MO molecules and increasing the dielectric constant of TiO2 microspheres. Even so, under either UV-light or visible-light irradiation, Pt deposition has led to a much improved photocatalytic activity toward the removal of MO molecules, which is superior over that without Pt deposition. Such an activity enhancement is closely related to the self-assembled microsphere features and Pt deposition. For self-assembled microspheres, the roughly parallel nanowires within them formed a great deal of nanoscale boundary cavities which could act as the microparallel capacitors for storage of the excited electrons. Upon Pt deposition, visible-light irradiation led to surface plasmon resonance over Pt nanoparticles, and therefore there may occur an enhanced migration and separation of photogenerated electrons and holes, which accounts for the significantly improved photocatalytic performance. ’ ASSOCIATED CONTENT

bS

Supporting Information. EPR signals of pure TiO2 and Pt-TiO2. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was financially supported by NSFC (No. 21025104, 91022018, 20831004), and National Basic Research Program of China (2007CB613306, 2011CBA00501). ’ REFERENCES (1) (a) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Chem. Rev. (Washington, DC, U.S.) 1995, 95, 735. (b) Chen, C.; Ma, W.; Zhao, J. Chem. Soc. Rev. 2010, 39, 4206. 13827

dx.doi.org/10.1021/jp203511z |J. Phys. Chem. C 2011, 115, 13820–13828

The Journal of Physical Chemistry C

ARTICLE

(2) (a) Su, Y.; Xiao, Y.; Fu, X.; Deng, Y.; Zhang, F. Mater. Res. Bull. 2009, 44, 2169. (b) Zhao, Q.; Li, M.; Chu, J.; Jiang, T.; Yin, H. Appl. Surf. Sci. 2009, 255, 3773. (c) Zhang, D. Transition Met. Chem. 2010, 35, 933. (3) Jamalluddin, N. A.; Abdullah, A. Z. Ultrason. Sonochem. 2011, 18, 669. (4) Yao, Z. P.; Jia, F. Z.; Tian, S. J.; Li, C. X.; Jiang, Z. H.; Bai, X. F. ACS Appl. Mater. Interfaces 2010, 2, 2617. (5) Dholam, R.; Patel, N.; Santini, A.; Miotello, A. Int. J. Hydrogen Energy 2010, 35, 9581. (6) Liu, Y.; Xie, L.; Li, Y.; Qu, J. L.; Zheng, J.; Li, X. G. J. Nanosci. Nanotechnol. 2009, 9, 1514. (7) Tojo, S.; Tachikawa, T.; Fujitsuka, M.; Majima, T. J. Phys. Chem. C 2008, 112, 14948. (8) Long, M. C.; Cai, W. M.; Wang, Z. P.; Liu, G. Z. Chem. Phys. Lett. 2006, 420, 71. (9) Tian, H. J.; Hu, L. H.; Zhang, C. N.; Chen, S. H.; Sheng, J. A.; Mo, L.; Liu, W. Q.; Dai, S. Y. J. Mater. Chem. 2011, 21, 863. (10) Iliev, V.; Tomova, D.; Rakovsky, S.; Eliyas, A.; Puma, G. L. J. Mol. Catal. A: Chem. 2010, 327, 51. (11) Zhang, W.; Wang, X. X.; Fu, X. Z. Chem. Commun. 2003, 2196. (12) Chen, Y. L.; Li, D. Z.; Wang, X. C.; Wu, L.; Wang, X. X.; Fu, X. Z. New J. Chem. 2005, 29, 1514. (13) (a) Mizukoshi, Y.; Sato, K.; Konno, T. J.; Masahashi, N. Appl. Catal., B 2010, 94, 248. (b) Wang, C.; Yin, L.; Zhang, L.; Liu, N.; Lun, N.; Qi, Y. ACS Appl. Mater. Interfaces 2010, 2, 3373. (c) Chuang, H. Y.; Chen, D. H. Nanotechnology 2009, 20. (14) Jiao, Y. Z.; Peng, C. X.; Guo, F. F.; Bao, Z. H.; Yang, J. H.; Schmidt-Mende, L.; Dunbar, R.; Qin, Y.; Deng, Z. F. J. Phys. Chem. C 2011, 115, 6405. (15) Hu, W.; Li, L.; Tong, W.; Li, G. Chem. Commun. 2010, 46, 3113. (16) Iliev, V.; Tomova, D.; Bilyarska, L.; Tyuliev, G. J. Mol. Catal. A: Chem. 2007, 263, 32. (17) Hu, W. B.; Li, L. P.; Tong, W. M.; Li, G. S.; Yan, T. J. J. Mater. Chem. 2010, 20, 8659. (18) Murakami, N.; Ono, A.; Nakamura, M.; Tsubota, T.; Ohno, T. Appl. Catal., B 2010, 97, 115. (19) Celik, V.; Unal, H.; Mete, E.; Ellialtioglu, S. Phys. Rev. B 2010, 82. (20) Liu, Z. M.; Xie, Y.; Ding, K. L.; Tao, R. T.; Sun, Z. Y.; Zhang, H. Y.; An, G. M. J. Am. Chem. Soc. 2009, 131, 6648. (21) Jing, L. Q.; Wang, D. J.; Wang, B. Q.; Li, S. D.; Xin, B. F.; Fu, H. G.; Sun, J. Z. J. Mol. Catal. A: Chem. 2006, 244, 193. (22) (a) Li, L. P.; Li, G. S.; Xu, J. X.; Zheng, J.; Tong, W. M.; Hu, W. B. Phys. Chem. Chem. Phys. 2010, 12, 10857. (b) McHale, J. L.; Knorr, F. J.; Zhang, D. Langmuir 2007, 23, 8686. (c) Nedeljkovic, J. M.; Abazovic, N. D.; Comor, M. I.; Dramicanin, M. D.; Jovanovic, D. J.; Ahrenkiel, S. P. J. Phys. Chem. B 2006, 110, 25366. (23) (a) Karunakaran, C.; Abiramasundari, G.; Gomathisankar, P.; Manikandan, G.; Anandi, V. J. Colloid Interface Sci. 2010, 352, 68. (b) Carneiro, J. T.; Savenije, T. J.; Mul, G. Phys. Chem. Chem. Phys. 2009, 11, 2708. (24) Vorontsov, A. V.; Savinov, E. N.; Jin, Z. S. J. Photochem. Photobiol., A 1999, 125, 113. (25) Oatley, C. W. Proc. Phys. Soc., London 1939, 51, 318. (26) Imanishi, A.; Tsuji, E.; Nakato, Y. J. Phys. Chem. C 2007, 111, 2128. (27) Naya, S.-i.; Teranishi, M.; Isobe, T.; Tada, H. Chem. Commun. 2010, 46, 815. (28) Carneiro, J. T.; Savenije, T. J.; Mul, G. Phys. Chem. Chem. Phys. 2009, 11, 2708. (29) Tian, Y.; Tatsuma, T. J. Am. Chem. Soc. 2005, 127, 7632.

13828

dx.doi.org/10.1021/jp203511z |J. Phys. Chem. C 2011, 115, 13820–13828