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

The fabrication of heterojunction between different crystalline phases has been considered to be an effective strategy for  promoting charge separation during photocatalytic process.  Herein, the mixed-crystalline-phase (MC), spindle-like TiO2 was prepared with a simple hydrothermal method, which was  followed by a series of calcination processes. The final products  are composed of two crystalline phases including anatase and  brookite. The anatase/brookite ratio of the TiO2 is tuned by  varying the calcination temperature. The MC TiO2 that consisted of 85.5% anatase and 14.5% brookite has the highest rate of  photocatalytic hydrogen evolution (290.2 μmol h−1) compared  to the purely anatase TiO2. This is attributed to the mixedphase heterojunction structure that improves electron-hole separation, and therefore, enhances the photocatalytic hydrogen  production.

INTRODUCTION

With the increasing consumption of the world’s limited  reserves of fossil fuels, there has been a growing interest  in finding renewable and clean energy sources. Hydrogen  is believed to be the most promising source of clean energy, and the production of hydrogen from water via semiconductor-based photocatalysis is an ideal route [1−5]. Of  the potential semiconductors, TiO2 is considered to be the  most suitable for photocatalytic water splitting because of  its low cost, non-toxicity, and long-term stability [6−9].  However, TiO2-based photocatalysts currently show low  photocatalytic activity for splitting water into H2 and O2.  One of the key issues related to this problem is the limited  charge-separation efficiency upon photoexcitation, which  largely depends on the intrinsic electronic and structural  properties of TiO2 [10−12]. To improve the photocatalytic  activity of TiO2, it is highly desirable to develop approaches  that can efficiently promote charge separation.

The fabrication of a heterojunction between different  semiconductors has been demonstrated to be an effective  strategy for promoting charge separation during photocatalytic processes [13,14]. For TiO2-based photocatalysts,heterojunction structures, such as Ag-TiO2 [13], C3N4- TiO2 [14], and TiO2-graphene [15], can provide a driving  force for separating the photoinduced charge carriers by  changing the band gap structure. A fully formed heterojunction could also lead to enhanced photocatalytic activity in either the production of hydrogen or degradation  of pollutants. According to previous studies, composite  semiconductor heterojunctions with a mixed phase have  been demonstrated excellent photocatalytic performance  [16,17]. TiO2 commonly exists in three phases: rutile  (tetragonal, P42/mnm), anatase (tetragonal, I41/amd), and  brookite (orthorhombic, Pbca). Of these three phases,  brookite nanocrystals exhibit higher photocatalytic activity  than the rutile and anatase phases. Furthermore, compared  to single-phase TiO2, mixed-phase TiO2, such as anatase/ brookite, rutile/brookite, and anatase/rutile, has been proven to have higher photocatalytic activity [9,10]. For example, anatase/brookite TiO2 nanocrystals were synthesized  with a sonochemical sol-gel method, and the nanocrystals  exhibited higher photocatalytic activity than single-phase  anatase TiO2 [18]. A brookite/rutile mixture of TiO2 was  synthesized through the thermolysis of TiCl4, and a brookite/anatase mixture of TiO2 was obtained via the hydrolysis  of titanium isopropoxide in the presence of nitric acid [19].  However, it remains a challenge to synthesize mixed-phase  TiO2 with a simple one-step process and tunable phase  composition. More importantly, the essential relationship  between the crys tal phase of the heterojunction and photocatalytic activity is not well understood. An in-depth understanding of how the crystal phase of the heterojunction  affects photocatalytic activity would be a great aid in the  design and preparation of efficient semiconductor-based  photocatalysts.

In this study, we investigated the synthesis of mixed-crys talline-phase (MC), spindle-like, anatase/brookite TiO2 with a simple hydrothermal method. During the experimental process, titanium oxysulfate was selected as the Ti  precursor to produce the spindle-like TiO2. The TiO2 was then subjected to various calcination processes to obtain a  series of products. The anatase/brookite ratio of the TiO2 was tuned by varying the calcination temperature.

EXPERIMENTAL SECTION

Synthesis of MC TiO2

The MC TiO2 was fabricated with a hydrothermal method.  Titanium oxysulfate (TiOSO4, 0.1 g) was dissolved in 30  mL of distilled water. Then, 4 mL of 80% hydrazine hydrate  (H4N2 · H2O) was added dropwise to the TiOSO4-containing solution under mechanical stirring. After being stirred  for 0.5 h, the suspension was transferred to a Teflon-lined  autoclave and heated to 150°C for 24 h. After the hydrothermal process, the resulting white products were separated with centrifugation, washed with distilled water and  ethanol for three times, and dried for 24 h at 60°C in an  oven. To improve the crystallinity and control the phase  composition, the MC TiO2 was heated at various temperatures (from 500 to 900°C) under N2 atmosphere for 1 h.  The samples obtained were denoted as MC-T, where T represents the calcination temperature. For example, the sample calcinated at 500°C was denoted as MC-500.

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.