Facile Approach for the Syntheses of Ultrafine TiO2 Nanocrystallites

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A Facile Approach for the Syntheses of Ultrafine TiO2 Nano-crystallites with Defects and C Heterojunction for Photocatalytic Water Splitting Xiaole Weng, Qingshan Zeng, Yili Zhang, Fan Dong, and Zhongbiao Wu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.6b00828 • Publication Date (Web): 27 Jun 2016 Downloaded from http://pubs.acs.org on June 27, 2016

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ACS Sustainable Chemistry & Engineering

A Facile Approach for the Syntheses of Ultrafine TiO2 Nano-crystallites with Defects and C Heterojunction for Photocatalytic Water Splitting Xiaole Weng1, Qingshan Zeng1, Yili Zhang1, Fan Dong2, Zhongbiao Wu1* 1. Key Laboratory of Environment Remediation and Ecological Health, Ministry of Education, College of Natural Resources and Environmental Science, Zhejiang University, Zhejiang Provincial Engineering Research Center of Industrial Boiler & Furnace Flue Gas Pollution Control, 388 Yuhangtang Road, Hangzhou, 310058, P. R. China. Fax/Tel: 0086 571 8795308; E-mail: [email protected]. 2. Chongqing Technology& Business University, College of Environmental& Biological Engineering, Chongqing, 400067,P. R. China.

Abstract: In this paper, a supercritical water (sc-H2O)reaction medium was employed for the syntheses of ultrafine TiO2 nano-crystallites (at ca. 5 nm) that were linked with lactate species at surface. The resulting hybrid material was then subjected to an ageing at ca. 300 °C for 2 h under N2 atmosphere. After subjected to spherical aberration corrected STEM and EPR analyses, it was noted that the aged sample was shown with highly distorted crystal lattice with oxygen vacancies at surface and Ti3+ in the bulk. The anoxic ageing also caused incomplete combustion for lactate species, leading to the formation of C heterojunction with TiO2. UV-vis, PL and Transient photocurrent (TP) measurements indicated that the resulting surface oxygen vacancies and C heterojunction conferred a combination of advantages in enhancing visible light absorption and promoting electron-hole pair separation for aged sample, which led to *

Corresponding author: Tel.: +86-571-87953088; Fax: +86-571-87953088;

E-mail: [email protected] (Zhongbiao Wu). 1 / 27

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significantly promoted hydrogen production efficiency in photocatalytic water splitting under a full-spectrum irradiation (i.e. the aged TiO2 had yielded ca. 4 folds higher hydrogen production rate than the non-aged one and ca. 40-50 folds higher than commercial Degussa P25). We expected that the work conducted herein could provide a facile and controllable approach to simultaneously produce defects and C heterojunction for ultrafine TiO2nano-crystallites, which might lead to scale-up production of them for industry. Keywords: TiO2; Photocatalytic; Water splitting; Heterojunction; Defect; Hydrogen production; Supercritical Water.

Introduction Worldwide energy crisis has forced many countries to search for alternative sources for energy generation. This process has recently been facilitated by the occurrence of serious air pollutions as resulted from traditional fuels (e.g. coal and petroleum) burning in many developing countries. Hydrogen is a clean, renewable and sustainable source that has been considered as a promising alternative for energy generation.1, 2 However, the ineffective production of hydrogen has been a “bottle-neck” to enforce them into practical application. In 1972, Fujishima and Honda developed a photocatalytic route to produce hydrogen via photo-electrochemical water splitting on a TiO2 electrode.3 This finding opens up new focus on developments of advanced photocatalysts for photocatalytic water splitting under either visible or UV irradiation.4, 5Amongst these developed catalysts, 2 / 27


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TiO2 has attracted the most interest due to its abundant raw material source, simple processing technique and low production cost. However, the wide band gap (at 3.2 eV) and rapid electron-hole recombination make the TiO2 generally with low hydrogen production efficiency in water splitting. As such, modifications via either narrowing the band gap or separating the electron and hole are demanded, which are generally achieved by introducing effective dopants or combining with other semiconductors to form heterojunction.6-11 Defects are reported also able to improve the photocatalytic activity of TiO2. The associated oxygen vacancies and Ti3+ can act as “acceptor” and “donor” to enhance the light absorption and separate the electron-hole pairs during irradiation.12, 13 In literature, the defective TiO2 is generally synthesized via partial reduction of TiO2 at elevated temperatures and/or very high pressures under reducing atmosphere (e.g. H2, NaBH4, CO, etc.).14-16 This approach however tends to cause safety issue and could lead to contamination and variation of products batch in batch due to the insufficient control on process parameters, which is hence difficult for scale-up production. In recent decades, supercritical water (hereafter referred as sc-H2O) has offered tremendous benefits for controllable syntheses of nano-crystallites owning to its unique crystallizing environment.17-19 The sc-H2O can provide a highly hydrolysing and dehydrating environment in which rapid precipitation of nano-crystallites from appropriate precursors can occur under controlled conditions.20, 21 In this paper, an organic Ti precursor was used, which was 3 / 27



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subjected to precipitation in sc-H2O and led to the production of ultrafine TiO2 nano-crystallites (at ca. 5 nm) linked with residual organic species at surface. The hybrid material was then subjected to a heat-treatment under an anoxic environment, which led to reduction of TiO2 and formation of C heterojunction. A host of analytical techniques, including Cs-STEM, EPR, Transmission IR, XPS, PL, etc. were used to get insights into the defect formation mechanism and electron-hole transmission and separation behaviours. The photocatalytic activity of resulting catalyst in water splitting was also measured.

Materials and methods Materials Commercial P25 was purchased from Degussa Company (Germany).Titanium (IV) bis (ammonium lactate) dihydroxide [CH3CH(O-)CO2NH4]2Ti(OH)2 (50 wt% in water, TiBALD) was supplied from Alfa Aesar company (UK).All chemicals were used as obtained. Deionised water was used in all experiments. sc-H2O syntheses The ultrafine TiO2 nano-crystallites (hereafter referred as sc-TiO2) were prepared by using a three-pump sc-H2O flow reactor as described in supplementary Figure S1. Briefly, an aqueous solution of TiBALD (0.1M, 7.5 mL/min) was pumped to meet a flow of deionised water (pumping rate at 7.5 mL/min) at room temperature. This mixture was then pumped to meet a superheated water (pumping rate at 30 mL/min) at a mixing point (a SwagelokTMconfined jet mixer), whereupon rapid precipitation of crystalline TiO2 occurred. The temperature set for water heating was at ca. 450 ºC. An 4 / 27


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adjustable length band heater set to 450 ºC was also added to maintain the reactor temperature. The TiO2precipitates formed in the confined jet mixer were then cooled down to room temperature, passed through a 7 µm filter to remove large aggregates and then collected from the exit of back-pressure regulator (that was used to maintain the system pressure at ca. 23.1 MPa). Solids were recovered by centrifuging the suspension and then freeze-dried. The driedsc-TiO2powders were then subjected to a heat-treatment at 300 °C (heating rate at 3 °C/min) for 2 h under N2 atmosphere to yield defective TiO2 with C heterojunction(hereafter referred as sc-TiO2-N2). Characterizations Freeze-drying was performed using a Vacuum Freeze Dryer, Model LGJ-10C, supplied from Beijing Boyikang Laboratory Instruments Co., Ltd; the solids were frozen for 4 h and then freeze-dried for 24 h at 10 Pa.X-ray powder diffraction was obtained by using a Rigaku D/Max RA diffractometer with Cu Kα radiation (λ = 0.15418 nm) at 40 kV and 150 mA and at an angle of 2θ from 20° to 80°. The morphology of crystallites was evaluated using a model JEM-ARM200F (JEOL Company, Japan) high-resolution transmission electron microscope (200 kV accelerating voltage) instrument equipped with a spherical aberration corrected STEM. BET was determined by using N2 physisorption (Micromeritics ASSP 2020)at 77 K. Pre-treatment was conducted at 80 °C for 2 h under vacuum. EPR spectra were collected at room temperature on a Bruker ER200-SRC EPR spectrometer. PL spectra were measured by using a fluorospectrophotometer (PL, Fluorolog-3-Tau, France) with a Xe lamp as excitation source at room temperature. Surface elemental analysis 5 / 27



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was conducted by using a Thermo ESCALAB 250 XPS instrument with a Al-Kα radiation (hν = 1486.6 eV) operated at 150W. The signal of adventitious carbon (a binding energy of 284.6 eV) was used to calibrate the binding energy scale. Curve fits were performed via using a Shirley background and a Gaussian peak shape with 30 % Lorentzian character. Transmission IR was conducted using a ZnSe windows coupled to Nicolet 6700 FTIR spectrometers at 4 cm-1 resolution with 64 co-added scans. Samples were mixed with KBr (weight ratio at 1:50) and pressed to form thin disks for analysis. UV-Vis diffuse reflection spectra were obtained using a Scan UV-Vis spectrophotometer (UV-Vis DRS, TU-1901, China) equipped with an integrating sphere assembly. BaSO4 was used as the reflectance sample. The spectra were recorded at room temperature in air in the range from 230 to 800 nm. Activity measurements Photocatalytic hydrogen generation experiments were performed in a top-irradiation quartz container filled with 50 mL of 30 % methanol solution. A 300W Xe lamp with or without UV-cut optical filter (λ > 420 nm) was placed above the reactor as light source. The temperature was maintained using a flow of water in the jacket around the reactor. In a typical reaction, 0.1 g sample without loading any cocataltysts was dispersed in the solution via magnetic stirring. Before illumination, a purge of high purity Ar gas was bubbled for 15 min to completely remove the residual oxygen. The amount of hydrogen was measured by using GC analyzer (Agilent Technologies 7890A) equipped with a thermal conduct detector (TCD).

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Results and discussion X-ray powder diffraction and BET surface area analyses Phase identity and purity of TiO2 nano-crystallites were evaluated by using X-ray powder diffraction (XRD), which were directly referenced with JCPDS patterns for peak assignments. As shown in Figure 1, the sc-TiO2 sample revealed the characteristic reflections of anatase TiO2 at ca. 25.3, 37.9, 48.1 55.1, 62.7 and 75.1°, which were similar to those of sc-TiO2-N2 sample, implying that the TiO2 anatase phase was effectively retained after ageing. The ageing had slightly increased the crystallinity of sc-TiO2-N2 sample but slightly decreased the surface area from ca. 268.3 g/m2 (for sc-TiO2 sample) to ca. 206.4 g/m2 (for sc-TiO2-N2 sample). HR-TEM and Cs-STEM analyses After subjected to HR-TEM analyses (see supplementary Figure S2), the sc-TiO2 sample revealed a mean particle size at ca. 5.2±1.9 nm whilst that for sc-TiO2-N2 was at ca. 6.4±2.2 nm, indicating that ultrafine nano-crystallites were effectively retained after ageing. It was noted that both of the samples had revealed the lattice spacing for two adjacent planes at ca. 0.357 nm, corresponding to the (101) plane of anatase TiO2,22 which was consistent with the XRD analyses. In view of the defects in TiO2 lattice, a spherical aberration corrected STEM (Cs-STEM) was then employed. As shown in Figure 2, the sc-TiO2 sample had revealed regularly ordered Ti or O atoms in TiO2 lattice. In contrast, the sc-TiO2-N2 sample had shown highly distorted feature with trace 7 / 27



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amounts of point and line defects present in the lattice. This result is promising as it directly evidenced that defective TiO2 nano-crystallites were successfully obtained via our developed approach, which avoided the high temperature and pressure ageing under reducing atmospheres.14, 15 EPR measurements In general, the formation of defects would be accompanied by the generation of Ti3+ and oxygen vacancies in TiO2 lattice. Indeed, after subjected to electron paramagnetic resonance (EPR) measurements (see Figure 3), the sc-TiO2-N2 sample had revealed distinct EPR signals at g = 1.998 and 1.972, which were corresponded to oxygen vacancy and Ti3+, respectively.23 In contrast, the sc-TiO2 sample had shown much weaker EPR signals, revealing few oxygen vacancies and Ti3+ in the sample, consistent with the Cs-STEM analyses (see Figure 2). From the EPR analyses, it can be also deduced that the Ti3+ in the sc-TiO2-N2 sample was mainly existed in the bulk as surficial Ti3+ would tend to interact with O2 from air, producing O2− initially and then O− species.24 The O− species were generally shown with the EPR signal at g ≈ 2.02,25 which was however absent in the sc-TiO2-N2 sample. Transmission IR analyses To get insight into the defect formation mechanism in sc-TiO2-N2 sample, transmission IR was then employed to evaluate the surficial organic ligands at surface. As shown in Figure 4, the sc-TiO2 sample revealed a series of IR peaks at ca. 3401, 1630, 1406, 1384, 1120 and 1050 cm-1. The peaks at ca. 3401 and 8 / 27


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1630 cm-1 were originated from the stretching vibrations of water molecules, including hydroxyl groups and molecular water.26 The peak at ca. 1406 cm-1 was ascribed to N-H stretching from residual ammonia. The peak at ca. 1384 cm-1 was ascribed to COO- band and peaks at ca. 1120 and 1050 cm-1 were from C-CH3 and C-O stretching.27 Since the TiBALD precursor consisted of bidentate lactate ligand linked to Ti atom, the organic species at sc-TiO2 surface were proposed to be ammonium lactate species.28 After aged in N2 atmosphere, only the characteristic peaks of water molecules were retained, implying that lactate species had been mostly oxidized. The oxidation products were evaluated by using N2-TPSR measurement in which the sc-TiO2 was subjected to an ageing under N2 atmosphere at elevated temperatures with the gaseous products being simultaneously monitored by using a Mass Spectrometry. As shown in supplementary Figure S3, distinct CO and CO2 production peaks were appeared at the temperature of ca. 300 °C, implying that the majority of oxidation products of lactate species were CO and CO2. It is expected that the oxidation of lactate species under anoxic environment would induce reduction of TiO2 where the surficial active oxygen species of TiO2 would react with lactate species to form oxygen vacancies at surface (see Figure 2b for Cs-STEM image). Furthermore, such oxidation might also lead to the formation of elemental C at surface as the anoxic ageing would cause incomplete oxidation for lactate species. This had been evidenced by the presence of black colour in the sc-TiO2-N2 sample (see supplementary Figure S4). The amounts 9 / 27



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of residual C were measured by using elemental analyses in which the sc-TiO2-N2 sample was subjected to a heat-treatment to ca. 800 °C under a purge of O2 with the gaseous products being simultaneously monitored. Based on the measurement, ca.1.3 wt% C was detected in the sc-TiO2-N2 sample. XPS analyses One might argue that the residual C or N (these were from the ammonia lactate species) might dope into the TiO2 lattice during ageing. To verify this, XPS was then conducted to evaluate the surficial elemental species. As shown in Figure 5a, in C1s spectra, the sc-TiO2-N2 sample revealed three peaks with binding energies (BE) at ca. 284.8, 286.3 and 288.6 eV. The peak at ca. 284.8 eV was assigned to adventitious elemental carbon from the XPS instrument itself 29, which should also involve the residual elemental C as resulted from the incomplete oxidation of lactate species. Peaks at ca. 286.3 and 288.6 eV were assigned to C-O and C=O, respectively,30 which implied that there were few lactate species retained in the sc-TiO2-N2 sample. In general, the C doping intoTiO2 lattice would induce a characteristic BE at ca. 282 eV,31 the absence of which indicated that there was no C doping in the sc-TiO2-N2 sample. This was also the case for N doping as a characteristic BE at ca. 396 eV was also not observed6 (see Figure 5b). Furthermore, it was reported that the non-metal doping could generally lift the valance band of TiO2.32 However, our VB-XPS result (see Figure 5c) revealed that the valance bands for sc-TiO2 and sc-TiO2-N2 samples were similar, which further confirmed the absence of C or 10 / 27


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N doping in the sc-TiO2-N2 sample. In Ti2p spectra, it was noted that the Ti2p3/2 peak of sc-TiO2-N2 sample had shifted towards lower binding energy (see supplementary Figure S5), which suggested the existence of Ti3+ in the sample,33, 34 consistent with the Cs-STEM and EPR results. Photocatalytic activity measurements After subjected to photocatalytic water splitting measurements under a full-spectrum irradiation (where methanol was used as sacrificial agent), the sc-TiO2 had revealed a hydrogen generation rate at ca. 48.0 µmol h-1 g-1, which was much lower than that of sc-TiO2-N2 (at ca. 196.0 µmol h-1 g-1, see Figure 6a). To evaluate the effect of calcination temperature, we also measured the photocatalytic activities of sc-TiO2 that were aged at 200 and 400 °C. The two samples had yielded the hydrogen generation rate at ca. 145.1 and 160.6 µmol h-1 g-1, respectively, both lower than the sc-TiO2-N2 sample, indicating that 300 °C ageing could an optimized calcinations condition. Since direct comparison in hydrogen generation rate to literature was difficult (due to the varied measuring conditions and sacrificial agents), a commercial Degussa P25 was then selected for comparison. It was noted that the P25 only yielded ca. 4 µmol h-1 g-1 production rate under our measuring condition, which was ca. 40-50 folds lower than that of sc-TiO2-N2 sample. This confirms that the sc-TiO2-N2 sample was indeed with remarkable activity in photocatalytic water splitting. A repeated test was also conducted for the sc-TiO2-N2 sample. In each test, sacrificial agent was re-loaded to compensate the loss in former run. A high 11 / 27



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purity Ar gas was purged for 15 min prior to each test with an aim to completely remove the residual oxygen in solution. As shown in Fig. 6b, the photocatalytic activity of sc-TiO2-N2 sample was effectively retained after 5 runs, revealing a good stability for the sample. To further confirm the promotion function of defects in photocatalytic water splitting, we then prepared a non-defective sc-TiO2/C sample by dry blending the sc-TiO2 powders with activated carbon (at ca. 1 wt%). The resulting sample had shown a hydrogen generation rate at ca. 132.4 µmol h-1 g-1 (see supplementary Figure S6), which was lower than that of defective sc-TiO2/C sample (at ca. 196.0 µmol h-1 g-1), indicating that the defects in the sc-TiO2-N2 sample indeed promote the photocatalytic activity in water splitting. Furthermore, a P25/C sample was also prepared via an identical route to non-defective sc-TiO2/C, which only showed ca. 24.8 µmol h-1 g-1 hydrogen generation rate, much lower than that of sc-TiO2/C sample, which revealed the significant advantage of sc-H2O route in the syntheses of advanced photocatalysts. To probe into the visible light activity of sc-TiO2-N2 sample, a 400 nm UV-cut filter was applied, which however induced negligible hydrogen production over the sample, indicating that the sc-TiO2-N2 sample did not have visible light activity in photocatalytic water splitting. Cronemeyer35 indicated that the localized oxygen vacancies could induce a state ca. 0.75 to 1.18 eV below the conduction band minimum of TiO2. This state was however lower 12 / 27


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than the redox potential for hydrogen evolution, hence making the visible light photocatalytic activity of defective TiO2 negligible. However, the introduction of this state could enhance the light absorption for sc-TiO2-N2 sample where UV-Vis DRS measurement (see supplementary Figure S7a) had revealed a red shift absorption in the sample. Further toluene oxidation also demonstrated that the sc-TiO2-N2 sample could effectively oxidize the gaseous toluene under visible light irradiation (> 420 nm) whilst the sc-TiO2 did not (see supplementary Figure S7b and dataS7). PL and transient photocurrent measurements To get insight into the photo-induced electron-hole recombination and transmission behaviours over the sc-TiO2 and sc-TiO2-N2 samples, both of them were subjected to PL measurements with the excitation wave length at 280 nm. As shown in Figure7a, the sc-TiO2 sample had revealed an intense PL signal in the range of 400 to 550 nm where the sc-TiO2-N2 did not. In general, the PL signals are resulted from the recombination of photo-induced charge carriers where the lower the PL intensity, the lower the recombination rate of electron-hole pairs.36 Based on the PL result, it was deduced that the sc-TiO2-N2 sample was with a much lower electron-hole recombination rate as compared to the sc-TiO2 sample, revealing significantly promoted electron-hole separation ability for the sample. Such improvement was believed due to the presence of oxygen vacancies that could serve as charge carrier traps to capture the photoelectrons37 and the conductive C heterojunction that could effectively 13 / 27



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transfer the electrons.38 From the PL results, it was also deduced that the oxygen vacancies in the sc-TiO2-N2 sample were mainly at surface as the bulk oxygen vacancies could only act as electron-hole recombination centres, which were unable to separate the electrons and holes.39 The captured electrons by surface oxygen vacancies could be attacked by adsorbed O2 species to produce superoxide radical groups, which would enhance the photocatalytic oxidation ability for sc-TiO2-N2 sample40-42 (see toluene oxidation in supplementary Figure S7b). Transient photocurrent (TP) measurements were also conducted to measure the amounts of photo-induced electrons. As shown in Figure7b, the current as generated by sc-TiO2 (under a 300W Xe light irradiation) was initial at ca. 0.64 µA, which was gradually decreased to ca. 0.45 µA after 75s. The reason was due to that some carriers were recombined at the surface of sc-TiO2 due to the lack of surface oxygen vacancies and C heterojunction. In contrast, the sc-TiO2-N2 sample had shown a stabilized photocurrent at ca. 0.64 µA, revealing that the oxygen vacancies and C heterojunction had conferred a combination of advantages in effectively separating the electron-hole pairs over the sample.

Conclusion In summary, we have developed a facile route for the syntheses of ultrafine TiO2 nano-crystallites with defects and C heterojunction. The approach involved the binding of organic species onto the TiO2 surface in sc-H2O flow reactor. The resulting 14 / 27


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hybrid material was then aged under N2 atmosphere, leading to the reduction of TiO2 and the formation of C heterojunction. The presences of oxygen vacancies and C heterojunction were favorable to inhibit the electron-hole pair recombination, which led to greatly improved hydrogen generation efficiency in photocatalytic water splitting. We expect that the work conducted herein could provide a controllable and scalable approach for the production of ultrafine TiO2 nano-crystallites with defects and C heterojunction for industry.

Supporting Information Additional experimental results had been provided in supporting information. This material

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Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 51478418) and the Program for Zhejiang Leading Team of S&T Innovation (Grant No. 2013TD07).

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Figure captions Figure 1 XRD patterns of sc-TiO2 and sc-TiO2-N2 samples. Figure 2 Cs-STEM images of (a) sc-TiO2and (b) sc-TiO2-N2 samples. Figure 3 EPR spectra of sc-TiO2 and sc-TiO2-N2 samples. Figure 4 Transmission IR spectra of sc-TiO2 and sc-TiO2-N2 samples. Figure 5 XPS spectra of (a) C 1s and (b) N 1s of sc-TiO2-N2 sample; (c) VB-XPS spectra of sc-TiO2 and sc-TiO2-N2 samples. Figure 6 (a) Photocatalytic activities for hydrogen generation under a 300 W Xe light for P25, sc-TiO2 and sc-TiO2-N2 (aged at 200, 300 and 400 °C, respectively) samples and (b) the repeated hydrogen generation test for sc-TiO2-N2 sample. 21 / 27



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Figure 7 (a) PL spectra of sc-TiO2 and sc-TiO2-N2samplesunder the irradiation of 280 nm; (b) Photocurrents of sc-TiO2 and sc-TiO2-N2 electrodes irradiated with a 300 W Xe lamp.

Figure 1 XRD patterns of sc-TiO2 and sc-TiO2-N2 samples.

Figure 2 Cs-STEM images of (a) sc-TiO2and (b) sc-TiO2-N2 samples. 22 / 27


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Figure 3 EPR spectra of sc-TiO2 and sc-TiO2-N2 samples.

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Figure 4 TransmissionIR spectra of sc-TiO2 and sc-TiO2-N2 samples.

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(b)

(a)

(c)

Figure 5 XPS spectra of (a) C 1s and (b) N 1s of sc-TiO2-N2 sample and (c) VB-XPS spectra of sc-TiO2 and sc-TiO2-N2 samples.

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Figure 6 (a) Photocatalytic activities for hydrogen generation under a 300 W Xe light for P25, sc-TiO2 and sc-TiO2-N2(aged at 200, 300 and 400 °C, respectively)samples and (b) the repeated hydrogen generation test for sc-TiO2-N2 sample.

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(a)

(b)

Figure 7 (a) PL spectra of sc-TiO2 and sc-TiO2-N2samplesunder the irradiation of 280 nm; (b) Photocurrents of sc-TiO2 and sc-TiO2-N2 electrodes irradiated with a 300 W Xe lamp.

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A Facile Approach for the Syntheses of Ultrafine TiO2 Nano-crystallites with Defects and C Heterojunction for Photocatalytic Water Splitting Xiaole Weng1, Qingshan Zeng1, Yili Zhang1, Fan Dong2, Zhongbiao Wu1*

The table of contents entry A facile approach was developed to produce advanced catalysts for photocatalytic water splitting to produce sustainable energy source of hydrogen.

*

Corresponding author: Tel.: +86-571-87953088; Fax: +86-571-87953088;

E-mail: [email protected] (Zhongbiao Wu).


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Facile Approach for the Syntheses of Ultrafine TiO2 Nanocrystallites

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