Enhancing Photocatalytic Performance through Tuning the Interfacial Process between TiO 2-Assembled and Pt-LoadedMicrospheres

1 School of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot 010021, China 2 State Key Lab of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China 3 Key Lab of Optoelectronic Materials Chemistry and Physics, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences, Fuzhou 350002, China


Introduction
Degradation of organic pollutants by photocatalytic technology has attracted increasing attention [1,2].In all kinds of photocatalysts, simple oxide semiconductors (e.g., TiO 2 , ZnO, and SnO 2 ) have been extensively studied [3][4][5][6], among which TiO 2 nanomaterials characterized by the advantages of nontoxicity, stability, cheapness, and high activity have become the basis of promising photocatalysts [7,8].However, due to the wide bandgap and low quantum efficiency, the practical applications are always restricted [9][10][11].Therefore, some endeavors (like doping foreign elements [12,13] and coupled semiconductors or composites [5]) have been implemented, and some gains are obtained.Even so, efficient methods in enhancing photocatalytic activity are still challenging, since the elements introduced into the semiconductors often act as the recombination of photogenerated electrons and holes that would decrease the quantum efficiency.
One has to execute some other routes such as imputing external fields or depositing some noble metals.For example, when H 2 [14] and magnetic field [15] are involved into the systems, the photocatalytic activities can be significantly improved, while the reaction systems became complicated, seemly not favorable for comprehension of photocatalytic process.In addition, loading noble metal (e.g., Au, Ag, Pt, Pd) onto semiconductors has been observed to greatly improve the photocatalytic activity [9,[16][17][18], in which the efficient utility of photogenerated electrons in the photodegradation process is critical but often ignored.
In this work, we synthesized TiO 2 -assembled microspheres with different Pt loading content through microsphere formation with a subsequent photoreduction deposition of Pt nanoparticles.With tuning the loading quantity of  metal Pt, an optimum photocatalytic activity was achieved at a loading of 0.85 wt% Pt.Further, the impacts of Pt loading content were investigated as a measure for the interfacial process between microspheres and Pt nanoparticles responsible for the enhanced photocatalytic activities.

Experimental Section
2.1.Sample Preparation.All reagents employed in the experiments were analytical grade and used without further purification.TiO 2 microspheres were of fabricated according to the solution chemistry described in our previous work [19].Briefly, 20.5 mL TiCl 4 (Alpha, 99%) was added dropwise into 60 mL distilled water at 0 • C in an ice-water bath under vigorously magnetic stirring to form a given concentration aqueous solution.After stirring for about 1 h, the solution 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 residue solution.The resulting product was dried at 80 • C for 4 h in air.Pt-deposited TiO 2 microspheres were prepared via a photoreduction process.Generally, 0.5 g of the as-prepared TiO 2 microspheres was immersed into 100 mL solution of H 2 PtCl 4 (Sino-Platinum Co., Ltd.) in distilled water.0.1 M acetic acid (Sinopharm Chemical Reagent Co., Ltd.) was used to adjust the pH of suspension to about 3.Then, highpurity nitrogen passes through the suspension for 15 min to remove the oxygen in suspension.After stirring for 3 h, the mixture solution was irradiated for 3.5 h by a 300 W highpressure xenon lamp while stirring.Finally, the specimen was centrifuged and washed with distilled water until no Cl − was detected in the rinsing water.The quantity of H 2 PtCl 4 needed was set according to the mass ratio of metal Pt to TiO 2 as 0.1 wt%, 0.5 wt%, 1.0 wt%, and 3 wt%, and the corresponding samples were named as 0.1-Pt-TiO 2 , 0.5-Pt-TiO 2 , 1.0-Pt-TiO 2 , and 3.0-Pt-TiO 2 .After being dried in vacuum at 80 • C, the resulting products were obtained.

Sample Characterization.
Powder X-ray diffraction (XRD) patterns of the samples were collected on Rigaku MinFlex benchtop X-ray diffractometer with Cu K α irradiation.The crystallite sizes for the samples were calculated according to Scherrer formula, D = 0.89λ/β cos θ, where λ is the X-ray wavelength employed, θ is the diffraction angle matches along with diffraction peak (110), and β is defined as the peak width at half height after the instrumental broadening is subtracted.The lattice parameters of the samples were calculated by a least-squares method using Rietica Rietveld software.Ni powder serves as an internal standard for peak position determination.Morphologies of the samples were observed by field emission scanning electron microscopy (FE-SEM) (JEOL JSM-6700).The specific loading contents of Pt for all samples were measured by Inductively Coupled Plasma OES spectrometer (ICP) (Ultima2).UV-vis diffuse reflectance spectra of the samples were obtained using a Varian Cary 500 UV-vis-NIR spectrometer.Infrared spectra of the samples were measured on a Perkin-Elmer IR spectrophotometer at a resolution of 4 cm −1 using the KBr pellet technique.

Photocatalytic Activity Evaluation.
Photocatalytic activities test of samples was implemented on a UV light photoreactor, which was constructed by a quartz tube (4.6 cm in inner diameter and 17 cm in length) surrounded by four UV lamps (λ = 254 nm, 4 W).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 MO (10 mg/L) solution and magnetically stirred for 5 h to establish 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 power and UV-vis absorption spectra of the supernatant were measured with a Perkin-Elmer UV lambda 35 spectrophotometer.

Results and Discussion
Figure 1 shows the SEM images of pure TiO 2 and 1.0-Pt-TiO 2 samples.It is explicit to found that pure TiO 2 sample exhibits a microsize spherical morphology as constructed by bundles of nanowires, in which nanowires grew from the center of microsphere to the outward surface along with the direction [001], exposing the outer surface (110) of nanowire and tip (001) of nanowire while keeping a roughly parallel configuration [19].Upon photoreduction deposition, very small Pt particles are dispersed uniformly onto the microspheres, since there are no obvious changes in the microsphere morphology.Further, Pt deposited onto the microspheres is in metallic state as indicated by XPS and TEM [20].
The exact loading contents of metal Pt for all samples were determined by ICP.For sample 0.1-Pt-TiO 2 , the Pt content measured was 0.1 wt%, which is close to the initial one.With increasing the initial Pt content, the corresponding measured Pt increased, such as from 0.43 wt% for 0.5-Pt-TiO 2 to 0.85 wt% for 1.0-Pt-TiO 2 and further to 1.46 wt% for 3.0-Pt-TiO 2 .
XRD patterns for TiO 2 microspheres with and without Pt deposition were shown in Figure 2. Without Pt deposition, TiO 2 microspheres were highly crystallized, as indicated by strong diffraction peaks.All diffraction peaks matched well with the standard diffraction data for rutile phase (JCPDS, No. 21-1276), which demonstrated the formation of a pure rutile phase for TiO 2 microspheres.After Pt deposition, no significant changes were seen in XRD patterns.Based on the Scherrer formula' calculation, the primary particle sizes for TiO 2 samples with and without Pt modification were all about 6.2 nm.Therefore, Pt modification has no apparent influence on the particle size of TiO 2 microspheres.Besides this, no traces of metal Pt were detected from the XRD patterns in the Pt-TiO 2 series samples, regardless of Pt content.One of the primary reasons for this phenomenon is that Pt content in Pt-TiO 2 samples is too low to be detected.
Further, Pt deposition has no apparent effects on the lattice structure of rutile TiO 2 .As indicated by structural refinements, the lattice parameters of all samples almost kept the same within the experimental errors.For instance, the lattice parameters for 3.0-Pt-TiO 2 were a = 0.46143 (±0.00009) nm and c = 0.29582 (±0.00006), respectively, which are closer to that of a = 0.46160 (±0.0005), c = 0.29622 (±0.0004) of pure TiO 2 .Combining the analytic results of SEM and XRD, it can be concluded that the structure of rutile TiO 2 microspheres remained unchanged after Pt loading.
It is well known that the premise of photocatalytic reaction is the adsorption of incidence light by catalysts.Therefore, UV-visible diffuse spectrum measurements appear to be particularly important.Figure 3 shows the UV-visible diffuse reflectance spectrum (UV-Vis DRS) of all samples prepared.PureTiO 2 microspheres reflect above 90% visible light and all samples showed intense absorbance in the UV region (200 < λ < 400 nm).Differently, as the content of Pt loading increases, strong plasma response was observed in the visible region.Pt deposition is thus playing a critical role in the visible light adsorption because highly dispersed Pt nanoparticles can absorb nearly all the incident light [21].It is worth emphasizing that this absorbance of visible light does not contribute a lot to the UV-light activity likely because visible light with a low energy does not allow an efficient light excitation.
Degradation of MO molecules is employed for the photocatalytic property evaluation of the samples.Specific degradation rate of MO molecule in the solution is monitored by examining the intensity variation of characteristic absorption peak of MO at 464 nm.As shown in Figure 4, all Pt-loaded microspheres exhibited a fast degradation of MO molecules.Particulary in the starting 30 min, all Ptloaded microspheres showed a high removal of MO molecule beyond 67%, while less than 27% MO molecules were degraded by the pure TiO 2 microspheres in this period of irradiation (Figure 4(c)).Moreover, the photocatalytic performance of 1.0-Pt-TiO 2 sample is comparable with that of P25.Further, with increasing the Pt loading content, the photocatalytic activities became better, as indicated by the observation that the complete degradation time of MO became shortened from 80 min to 40 min as the loading quantity increased from 0.1 wt% to 0.85 wt%.Loading content to 0.85 wt% led to an optimum photocatalytic activity as indicated by a fast degradation of MO molecule in 40 min.However, higher Pt loading does not always mean high photocatalytic efficiency.For instance, the photocatalytic efficiency of 3.0-Pt-TiO 2 was only half of that for 1.0-Pt-TiO 2 in the whole photocatalytic process.
The optimized photocatalytic activities with Pt loading described above might be interpreted as follows.Firstly, loading metal Pt onto surfaces of TiO 2 microspheres is beneficial for electronic transfer that decreases the recombination of photoinduced electrons and holes and thereby can greatly enhance the photoquantum efficiency.Nevertheless, too much metal Pt loadings may hinder the direct interaction between the incident light and microspheres, which leads to an incomplete excitement of photocatalysts.On the other hand, Pt nanoparticles loaded may attract holes to serve as recombination centers through recombining with electrons, which leads to a decrease in quantum efficiency [22].As proposed in Scheme 1, metal Pt loaded was mainly deposited onto the face (110) and photogenerated electrons migrated predominantly along the direction [110] to metal Pt [20].However, too much Pt loading led to the covering of the external surface of TiO 2 microsphere with Pt nanoparticles (Scheme 1(b)), which contribute negatively to the photocatalytic activity because of the not enough direct interaction between incident light and semiconductor.Consequently, overfull Pt loading does not help to increase the photocatalytic activity.Also, the above phenomenon indicates that metal Pt loading only favored to the transfer of electrons and does not possess photocatalytic activity inherently or the activity is far less than TiO 2 sample prepared in this work.
FT-IR spectra of PureTiO 2 and 1.0-Pt-TiO 2 samples after MO degradation were given in Figure 5.For comparison, the FT-IR spectrum for PureTiO 2 microspheres was also shown.The surface chemistry of the microspheres is characterized by a band observed around 1625 cm −1 for the H-O flexural vibration [23] and the bands centered at 1250 and 1155 cm −1 for the vibrations of -C-O-bonds.The lattice nature for the microspheres is represented by the bands observed between 700 and 500 cm −1 for the stretching bending vibrations of TiO 2 , the band around 500 cm −1 for the stretching vibration of Ti-O-Ti, and that around 600 cm −1 for bending vibration band of Ti-O and O-Ti-O.It is striking that some new weak bands appeared at 1250 and 1155 cm −1 for both PureTiO 2 and 1.0-Pt-TiO 2 after photocatalytic reaction, which are not observed before the photocatalytic reaction as for PureTiO 2 .These new bands are probably due to the residual segments of the organic molecules degraded.Moreover, the vibration signals of these new bands for PureTiO 2 sample are stronger than those for 1.0-Pt-TiO 2 sample, which indicates that the adsorbed residual organic pieces after MO degradation by PureTiO 2 are much more.Therefore, Pt loading is in favor of the complete degradation of organic molecules.
Based on the previous experimental results, one can see that loading metal Pt onto TiO 2 microspheres greatly improved the photocatalytic efficiency.This finding can be explained in terms of the special assembly structure of TiO 2 microspheres and the interfacial interactions between metal Pt and microspheres.As shown in Scheme 2, excitement of microspheres by ultraviolet light leads to the separation of photogenerated electron-hole pairs.Since the work function of metal Pt is ϕ m = 5.36 eV, larger than that of 4.2 eV for the face (110) of TiO 2 [24], the Femi energy level for microspheres locates above that of metal Pt.Therefore, electrons can easily transfer from the conduction band of TiO 2 to metal Pt.Continuous transfer of electrons leads to the formation of Schottky barriers at the interfaces between Pt and TiO 2 microspheres.Due to the existence of Schottky barriers [7], reverse process for electrons migration from metal Pt to TiO 2 is hard to occur.Therefore, recombination of photogenerated electrons and holes can be efficiently inhibited and photoquantum efficiency increased spontaneously.It is well known that the separation of light-induced electron-hole pairs is critical in the photocatalytic reaction.However, some photochemical processes except for the enhancement of photoquantum efficiency are hardly discussed.For example, the highly efficient utility of photogenerated electrons and holes in photo-degradation process plays an important role in enhancing the photocatalytic efficiency.In our recent work, self-assembled TiO 2 microspheres sample has been found to exhibit a giant dielectric property [25], which is closely related to the cavities structure in between the parallel adjacent nanowires within the microspheres that can be taken as microcapacitors.These microcapacitors possess a giant dielectric constant and therefore might provide a superior electronic storage capacity.Because the production of light-induced electrons is faster than the degradation of MO molecule in photocatalytic reaction, compensating the time difference of photogenerated electrons and degradation reaction participated by electrons is pretty necessary.Due to the existences of microcapacitors, the accumulation and releasing of electrons can enhance the utility of electrons, while avoiding the recombination of electrons and holes.Therefore, dual roles from Pt deposition and special structure of TiO 2 microspheres contribute to the efficient separation of light-induced electrons and holes that greatly enhances the utility of electrons in favor of the optimum photocatalytic activities.

Conclusions
We have fabricated the Pt-TiO 2 microspheres by a solution chemistry with a subsequent Pt loading through a photoreduction.Pt loading led to an efficient separation of photogenerated electron-hole pairs.Meanwhile, electrons stored in the "microcapacitor" of TiO 2 microspheres contributed to the enhancement of photocatalytic activity under UV light irradiation.Varying the Pt loading content gives rise to an optimum catalytic efficiency.By monitoring the surface chemistry of Pt-TiO 2 microspheres before and after photocatalytic reactions, we found that the degradation of MO molecules was more complete in the presence of Pt loading.

Figure 4 :
Figure 4: (a) Typical time-dependent absorption spectra of MO solution in the presence of 3.0-Pt-TiO 2 under ultraviolet light irradiation; (b) C/C 0 as a function of irradiating time of MO solution for different samples in 90 min under UV light irradiation; (c) degradation rate for different Pt loading content of TiO 2 after irradiation for 30 min under UV light with a wavelength of 254 nm irradiation.The photocatalytic performances of PureTiO 2 and commercial P25-TiO 2 were measured in the same conditions for comparison.

Scheme 2 :
Scheme 2: Reaction mechanism proposed for the photocatalytic removal of MO molecules.