Rong Hao, Baojiang Jiang* , Mingxia Li, Ying Xie and Honggang Fu*

Photocatalytic hydrogen production experiments 

The photocatalytic hydrogen production experiments were conducted in an online photocatalytic hydrogen production system (AuLight, Beijing, CEL-SPH2N) at ambient temperature (20°C). The catalyst (0.1 g) was suspended in a mixture of distilled water (80 mL) and methanol (20 mL) in the reaction cell under magnetic stirring. The 1 wt.%- Pt-loaded photocatalysts were prepared with the standard in situ photodeposition method using an H2PtCl6 aqueous solution. Prior to the reaction, the mixture was deaerated by evacuation to remove any O2 and CO2 dissolved in the water. The reaction was initiated by irradiating the mixture with UV light from a 300 W Xe lamp equipped with a 200– 400 nm reflection filter, i.e., the wavelength of light used was approximately 200–400 nm. The gas evolution was only observed under irradiation, and was analyzed with an online gas chromatograph (SP7800, TCD, 5 Å molecular sieve, N2 carrier gas, Beijing Keruida Limited). To evaluate the photocatalytic stability, after the first 2.5 h hydrogen production run, the photocatalyst was separated from the suspension, washed with water, and dried at 60°C. The recovered photocatalyst was then used for the next hydrogen production run under the same conditions.

RESULTS AND DISCUSSION 

The crystal structure and composition of the synthetic products were confirmed with XRD (Fig. 1). As can be seen, the diffraction peaks of the MC TiO2 at 2θ = 25.3°, 37.8°, 48.2°, 53.9°, 55.1°, and 62.7° belong to the anatase phase of TiO2 [20,21]. It is worth noting that the small peak at 2θ = 30.8° is the (121) reflection of the brookite phase of TiO2 [22]. As the calcination temperature increases, the intensity of the characteristic brookite peak decreases. Once the calcination temperature reaches 900°C, t he (121) peak has completely disappeared, and thus, the sample is composed of only the anatase phase. Furthermore, as the calcination temperature increases, the ratio of brookite to anatase changes. The phase composition of each sample is summarized in Table S1. The ratio of brookite is initially 8% for the as-prepared MC TiO2, but the ratio increases to approximately 14.5% for the MC-600 sample, which can be attributed to the calcination temperature. This further proves that both anatase and brookite phases were formed in the final products. Usually, the brookite is a metastable phase of TiO2 that is converted into anatase or rutile structures upon calcination. However, in this work, the high ratio of brookite observed in the final products is because the amino group of the hydrazine hydrate may inhibit the crystal phase change, leaving the brookite structure unchanged. Interestingly, the surface area of the samples also changes from 47.3 m2 g−1 for the as-prepared MC TiO2 to 59.4 m2 g−1 for the MC-500 sample (Table S1). The surface area then decreases as the calcination temperature increases. Raman spectroscopy was also used to confirm the presence of the mixed anatase/brookite phases in the samples; the detailed Raman spectra are shown in Fig. S1. For reference purposes, the peak at 153 cm−1 co rresponds to the E1g mode of the anatase phase of TiO2. The other peaks of TiO2 are also present at 405, 525, and 648 cm−1. These peaks are all characteristic peaks of the anatase phase of TiO2 [23]. However, for the MC-T samples, it is clear that there are some changes in their Raman spectra after the thermal treatment. Even though increasing the calcination temperature does not change the intensity of all of the peaks, it changes the positions of all of the peaks. This shifting of the peaks towards lower wavenumbers is evidence of the presence of brookite in the calcined products. We then studied the microscopic morphology and structure of the samples with TEM. TEM images of the as-prepared MC TiO2 and MC-600 samples are shown in Fig. 2. The low-magnification image (Fig. 2a) demonstrates that the as-prepared MC TiO2 has a unique spindle-like morphology. After the calcination process, the morphology of the MC-600 sample has slightly changed, resulting in the aggregation of the nanoparticles with a mean size of 40–70 nm (Fig. 2b). Highly detailed images of the structures were obtained with high-resolution TEM (HRTEM), as shown in Fig. 2c. It should be noted that the interplanar distance of 0.35 nm is close to the d-spacing of the (101) plane of anatase TiO2 reported in the previous studies [24]. The other region was also analyzed, and the lattice fringes were found to be separated by 0.23 nm, which is close to the d-spacing of the (121) plane of brookite TiO2 [25]. Thus, these experimental results confirm that brookite-phase TiO2 is in contact with the anatase-phase TiO2 to form two-phase heterojunction nanostructures, which is in good agreement with the XRD results. TEM images of the MC-900 sample are shown in Fig. S2. In addition, a possible structure of the brookite/anatase heterojunction is shown in Fig. 2d. To investigate the composition of the MC TiO2 in the final products, we performed XPS analysis on the MC-600 (brookite/anatase TiO2) and MC-900 (anatase TiO2) samples (Fig. 3). In Fig. 3a, the two peaks of the MC-900 sample at 458.72 and 464.45 eV are assigned to the Ti 2p3/2 and Ti 2p1/2 spin-orbit-split states of the photoelectrons in the Ti4+ chemical state, respectively [26]. Interestingly, in the MC-600 XPS spectrum, the Ti 2p states slightly shift towards lower binding energies compared to that of the MC900. Normally, this kind of shift is attributed to the changes in the chemical environment, such as oxygen vacancies in this case. The oxygen vacancies are beneficial to improving the photocatalytic performance. The existence of oxygen vacancies is further validated by the differences in the O 1s XPS spectra of the samples (Fig. 3b). In the O 1s XPS

spectrum of the MC-900 sample, the O 1s peak centered at 529.9 eV is mainly attributed to the oxygen in the TiO2 crystal lattice, which agrees with the previous studies [27]. However, in the O 1s XPS spectrum of the MC-600 sample, the O 1s peak has slightly shifted by 0.3 eV from 529.9 to 529.6 eV. The origin of this downward shift is the wellknown band bending effect, which is caused by the extra electrons from the oxygen defects in the TiO2 crystal lattice. In addition, the oxygen vacancies will also influence the O2 absorption performance of the composites. The O 1s signal at 531.5 eV is closely related to the hydroxyl groups of oxygen species adsorbed by the composites. A pronounced increase in this peak can be observed in the XPS spectrum of the MC-600 sample, which indicates the adsorption of additional oxygen species because of the high chemical reactivity of the surface. Therefore, the presence of an MC heterojunction can greatly enhance the adsorption of O2, promoting the photogenerated electrons captured by the adsorbed O2, and leading to a large increase in the charge separation. In addition, the Ti 2p XPS spectra, SPV spectra, and photocurrent responses of the other samples are shown in Fig. S3. The brookite/anatase heterojunction could change the energy band structure of the final products. Therefore, the UV-vis absorption spectra of the samples were recorded to investigate the energy band structure and light-harvesting capabilities of the photocatalysts. Fig. S2 shows the UV-vis absorption spectra of the MC-600 and MC900 samples. The results indicate that the MC-600 sample has a strong absorption band in the short wavelength/high energy region of the light spectrum. The band gap energies are calculated by extrapolating the absorption edges with the equation (hν F(R∞))2 = A(hν-Eg). The band gap energies of the samples were estimated to be approximately 3.4 eV (MC-900) and 3.5 eV (MC-600), with the slight difference caused by the different ratios of the brookite and anatase phases. Furthermore, with the higher energy of the electronic band structure of the MC-600 sample, it is expected that the MC-600 sample will possess thermodynamically enhanced reduction and oxidation abilities in photocatalytic reactions. The charge-transfer rate between the semiconductor catalyst and redox species in solution also depends on the energy level correlation. Therefore, it is expected that the MC-600 sample will possess a better photocatalytic performance for hydrogen evolution. The rate of hydrogen production under a UV light was measured to evaluate the photocatalytic activity of the final products. Fig. 4a shows the photocatalytic hydrogen evolution of the different catalysts as a function of time. The as-prepared MC TiO2 shows negligible activity, with an average hydrogen production rate of only 25 μmol h−1, which indicates that it has poor hydrogen evolution kinetics. Similarly, the hydrogen production rate of MC-900 sample is only 20 μmol h−1. However, the MC-600 sample shows a dramatic increase in the rate of hydrogen evolution, achieving a production rate of 290.2 μmol h−1. The high performance of MC-600 sample indicates that engineering the composite structure can induce synergistic effects. These results show that the MC-600 TiO2, which consists of 85.5% anatase and 14.5% brookite, has the highest rate of photocatalytic hydrogen evolution when compared to the purely anatase TiO2. A recycling test was performed to investigate the stability and reactivity of the MC-600 sample as a photocatalyst. Fig. 4b shows the measured hydrogen production under a UV light during five consecutive cycles. As can be seen from the first run to the fifth run, the MC-600 sample can be effectively recycled and repeatedly used without any obvious decrease in activity. This is attributed to the effective electron-hole separation that results from the transfer of charges from the brookite conduction band to the anatase conduction band [19,20]. The 1 wt.%-Pt-loaded photocatalysts were characterized after the photocatalytic measurements with TEM micrographs (Fig. S5), which show a uniform distribution of particles that contribute to the enhanced hydrogen evolution stability. The introduction of heterojunctions between the anatase and brookite phases is the main reason for the high photocatalytic activity. Therefore, the SPV spectra of the samples were measured to understand the heterojunction effect. SPV spectroscopy is a rapid and non-destructive measurement technique for obtaining information about the separation rate of photoinduced charge carriers, and is based on the differences in the SPV before and after the sample is irradiated [28]. Fig. 5a shows the SPV responses of the MC-600 and MC-900 samples in air. According to the principles of SPV, the SPV signal mainly results from the photogenerated charges, followed by the separation of charges under the built-in electric fiel d or diffusion processes. Interestingly, compared to the MC-900 sample, the MC-600 sample has a weak SPV response. It is thought that the oxygen vacancies in the MC-600 sample can easily capture the photoinduced electrons, inhibiting the recombination of photogenerated charges, resulting in a low net surface charge, and therefore, a weak SPV response. These experimental results also show that the mixture of crystalline phases allows photogenerated electrons to be extracted and stored, reducing the electron-hole pair recombination rate, and thus, improves the quantum yield and lifetime of the charge carriers. Another type of photocurrent measurement was conducted to understand the conductivity and charge transport properties of the MC-600 and MC900 samples (Fig. 5b). As can be seen, a fast and uniform photocurrent response is obtained for each switch-on and switch-off event of both electrodes. However, the photocurrent of the MC-600 electrode is approximately 7-time higher than that of the MC-900 electrode, indicating that the enhanced photocurrent and good stability is achieved through the electronic interactions between the brookite and anatase phases. Thus, for the MC-600 sample, the photoinduced electron-hole pairs are easily transferred to the sample’s surface because of the interfacial interactions between the brookite and anatase phases. Furthermore, the MC-600 sample has the highest concentration of the brookite phase and one of the highest surface areas compared to that of the other products, which are possibly responsible for the excellent photocatalytic performance for hydrogen evolution.

Conclusions 

In summary, the spindle-like MC TiO2 samples were prepared with a simple method. During the experimental process, titanium oxysulfate was selected as the Ti precursor to produce the spindle-like MC TiO2. A mixture of the anatase and brookite phases of TiO2 produced higher rates of photocatalytic hydrogen evolution compared to that of purely anatase TiO2. The mixed-phase heterojunction structures decreased the electron-hole pair recombination rate, and subsequently increased the lifetime of the charge carriers.