The fabrication and the characterization of a TiO₂/titanate nanohybrid for efficient hydrogen evolution

Abstract

A TiO₂/titanate nanojunction photocatalyst was synthesized by a one-step hydrothermal process. Scanning electron microscopy and transmission electron microscopy were employed to characterize the morphology and structure, and to further elucidate the morphological evolution of the resulting products. X-ray diffraction and X-ray photoelectron spectroscopy were used to generally assess the crystallite phase composition of the samples and the phase transition behaviour. The TiO₂/titanate nanojunction with excellent structure is of benefit for mass transfer and especially for photon-generated electron–hole separation. As a result, the nanojunction is anticipated to exhibit good photocatalytic activities for hydrogen evolution. The H₂ evolution of TiO₂/titanate achieves a production rate of 230.1 μmol h⁻¹. Moreover, this report will offer a new promising strategy to improve photocatalytic hydrogen evolution efficiency with low-cost.

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1. Introduction

The photocatalytic hydrogen evolution from water splitting is an important process to solve the energy crisis and environmental issues.^1–5 Since the initial report of a photoelectrochemical cell using Pt–TiO₂ electrodes for hydrogen evolution by Fujishima and Honda in 1972, considerable studies have been focused on the development of highly efficient and stable photocatalysts.^6–10 To date, the TiO₂-based semiconductor has undoubtedly proven to be one of the excellent photocatalysts for its easy preparation, high stability, low cost, and non-toxicity.^11–14 However, their practical applications were hindered by the short lifetimes of photogenerated electron–hole pairs. Thus, it is still necessary to design novel photocatalyst systems to improve photocatalytic efficiency. Among a variety of strategies that employed to enhance the photocatalytic efficiencies of photocatalysts, the construction of semiconductor heterojunction has attracted a lot of attention due to its perfect effectiveness in improving the photocatalytic activity.^15–18

Recently, the layered titanate with ion-exchangeable and highly photoactivated nanostructures was widely used because of its excellent photoelectronic, catalytic, and mechanical properties.¹⁹ In general, different morphology titanates including nanotubes, nanofibers, and nanosheets were synthesized through two routes: alkali wet method and solid-state method.²⁰ While the excessive aggregation of titanate photocatalysts causes substantial reduction of active surface area; as a result, the density of active sites is reduced, and photocatalytic ability is decreased. However, if proper nanoparticles can intercalate interlayer of titanate, this would inhibit the aggregation of titanates to some extent and form heterojunction composite. This is beneficial to photocatalytic activity for such architecture would provide an enhancement for light harvesting, short diffusion distance for excellent charge transport, as well as a large contact area for fast interfacial charge separation and photocatalytic reactions.

Herein, we present the fabrication and the characterization of TiO₂/titanate nanohybrid, which has excellent photocatalytic hydrogen production activity. Photocatalysts were synthesized by one-step hydrothermal process. During the typical experimental process, a certain volume of tetrabutyl titanate and hydrazine hydrate was added into ethanol under continuous stirring. Subsequently, the resulting mixture was transferred to a Teflon-lined autoclave and heated at 150 °C for 24 h. Then the final white products were obtained for the following photocatalytic reaction. The morphological and the structure of TiO₂/titanate nanohybrid, as well as their photocatalytic performance, was presented. The results show that TiO₂/titanate nanohybrid possess the highest photocatalytic activity among different products in the photocatalytic hydrogen evolution under UV irradiation.

2. Experimental section

2.1 Preparation for TiO₂/titanate nanohybrid

The chemical reagents were obtained from commercial sources as guaranteed-grade reagents, and they were used without further purification. Tetrabutyl titanate (98%), and hydrazine hydrate were purchased Sigma-Aldrich Trading Co., Ltd. The ethanol, anatase TiO₂, and P25 were obtained from Beijing Chemical Co. Ltd.

In a typical synthetic procedure, a tetrabutyl titanate (TBOT, 3.4 mL) with different volume of hydrazine hydrate (varied from 1 mL, 2 mL, and 4 mL) were directly added into 30 mL ethanol. After stirring for a short time the resulting suspension was transferred to a Teflon-lined autoclave (50 mL) and heated to 150 °C for 24 h. After hydrothermal treatment, the white precipitation was recovered and purified by centrifugation three times and redispersion cycles with ethanol, and dried under vacuum at 60 °C for 4 h to obtain the final samples. They were denoted as TTN-x, where x (x = 1, 2 and 3) refers to the volume of hydrazine hydrate for 1 mL, 2 mL, and 4 mL, respectively. The pure titanate was obtained by similar experiment process including 0.68 mL TBOT and 4 mL hydrazine hydrate.

2.2 Photocatalytic hydrogen production

The photocatalytic hydrogen production experiment was conducted in an online photocatalytic hydrogen production system (Aulight, Beijing, CEL-SPH₂N) at ambient temperature (20 °C). 0.1 g catalyst was suspended in a mixture of 80 mL distilled water and 20 mL methanol in the reaction cell by using a magnetic stirrer. 1 wt% Pt loaded photocatalysts were prepared by known standard method of in situ photodeposition method using H₂PtCl₆ aqueous solution. Prior to the reaction, the mixture was deaerated by evacuation to remove O₂ and CO₂ dissolved in water. The reaction was carried out by irradiating the mixture with UV light from a 300 W Xe lamp with a 200–400 nm reflection filter which means the wavelength of light is approximately 200–400 nm. Gas evolution was observed only under irradiation, being analyzed by an on-line gas chromatograph (SP7800, TCD, molecular sieve 5 Å, N₂ carrier, Beijing Keruida Limited).

2.3 Characterization

X-ray powder diffraction (XRD) patterns were obtained by Bruker D8. Scanning electron microscopy (SEM) micrographs were taken using a Hitachi S-4800 instrument operating at 5 KV. The transmission electron microscopy (TEM) experiment was performed on a JEM-2100 electron microscope (JEOL, Japan) with an acceleration voltage of 200 kV. The specific surface area was determined according to the Brunauer–Emmett–Teller (BET) method using a Tristar II 3020 surface area and porosity analyzer (micromeritics). UV-visible absorption spectroscopy was recorded using a UV-visible spectrophotometer (SHIMADZU UV-2550). The surface chemical composition were examined by X-ray Photoelectron Spectroscopy (XPS) using a Kratos-AXIS ULTRA DLD apparatus with Al(Mono) X-ray source, and the binding energies were calibrated with respect to the signal for adventitious carbon (binding energy = 284.6 eV). The photoluminescence spectra (PL) of samples were recorded with a FLS920 fluorescence spectrophotometer.

3. Results and discussion

Firstly, the morphology of as-synthesized typical sample (TTN-1) was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As SEM image shown in Fig. 1a, the photocatalyst was made up of a large number of nanosheets that stack on each other. The layered structure of titanate nanosheets can be observed in TEM image (Fig. 1b) as well. It is noted that a large amounts of nanoparticles adhered on the surface of layered titanate.

Fig. 1 (a) SEM image, (b) TEM image, and (c and d) HRTEM images of TiO₂/titanate nanohybrid (TTN-1) from the area labeled by the frame.

The size of the nanoparticle was about 5 nm, and the HRTEM image exhibits the lattice spacing of 0.351 nm (Fig. 1c), which can be attributed to the (101) lattice planes of anatase TiO₂.²¹ The results indicate that the TiO₂ and titanate has formed heterojunction. Fig. 1d is the HRTEM image of the corresponding marked area, which exhibits the interlayer spacing of about 0.81 nm that corresponds with the (200) lattice plane of H₂Ti₂O₅·H₂O. Meanwhile, the lattice spacing of 0.324 nm corresponds with its (310) lattice plane.²² The possible formation mechanism for TiO₂/titanate nanohybrid was that NH₄⁺ ions pyrolysis from hydrazine hydrate act as weak electrolyte provide positive charge to stabilize the negatively charged titanate layers. While part of tetrabutyl titanate hydrolyze into particle and adhered on titanate layers. Then the photocatalyst was obtained as a result of the ammonia liberation after drying. The unique morphology of TiO₂/titanate photocatalyst would lead to change on its crystalline structure and photocatalytic performance.

The structure was further investigated by X-ray diffraction (XRD). As shown in Fig. 2, the diffraction peaks of TiO₂/titanate nanohybrid were weak which was mainly caused by the relatively low crystallinity. The diffraction peak at 9.6° for (200) reflection reveals layered titanate was obtained after hydrothermal treatment and the interlayer spacing is about 0.90 nm.²³ Meanwhile, the diffraction peaks at 2θ = 48.0° and 61.8° correspond well with the (020) and (002) lattice planes of protonic titanate H₂Ti₂O5·H₂O (JCPDS 47-0124), respectively. Furthermore, the diffraction peak at 25.3° corresponds well with the (101) lattice planes of anatase TiO₂.²⁴ These results showed good agreement with the TEM analysis, indicating that a nanojunction might be formed, which could result in better separation of photoinduced charge carriers and more efficient electron transfer within the composite structure. The XRD pattern of other products obtained including TTN-2 and TTN-3 is also added as Fig. S1 in ESI.† Additionally, the proper nanoparticles growth would inhibit the aggregation of titanates to some extent, which can be confirmed by BET analysis (Fig. S2 in ESI†). TiO₂/titanate nanohybrid possesses much larger surface area (329.5 m² g⁻¹) than titanate (146.2 m² g⁻¹). This is beneficial to photocatalytic activity for such nanohybrid would provide a large contact area for fast interfacial charge separation and photocatalytic reactions.

Fig. 2 X-ray diffraction (XRD) patterns of TiO₂/titanate (TTN-1).

To gain insight into more structure information for TiO₂/titanate nanohybrid, the surface chemical composition and atom structure states were further investigated by X-ray photoelectron spectroscopy (XPS) analysis. Fig. 3 shows the high-resolution Ti 2p and O 1s spectra of both the titanate and the TiO₂/titanate nanohybrid. For titanate in Fig. 3a, the binding energy at 458.3 eV and 464.2 eV are attributed to Ti 2p3/2 and Ti 2p1/2, respectively. Interestingly, for TiO₂/titanate nanohybrid in Fig. 3a, Ti 2p slightly shift toward lower binding energy compared to that of the titanate. Normally, this kind of shift is attributed to the change of chemical state possibly from the formation of typical nanohybrid structure.²⁵ The interaction between the TiO₂ and titanate will also enhance the separation rate of photon-generated carrier. Meanwhile, the high resolution O 1s spectra can be also deconvoluted into two peaks locating at 530.1 eV and 532.2 eV. The O 1s peak is at 530.1 eV is mainly attributed to the oxygen of titanate crystal lattice, agreeing with previous reports.²⁶ Meanwhile, the O 1s peak is at 532.2 eV, which is closely related to the significant surface hydroxyl groups. However, it is clear that the peak of the –OH group (532.2 eV) in TiO₂/titanate nanohybrid is much higher than that in titanate, which originates from the surface enlargement of TiO₂/titanate nanohybrid. All these results can further confirm the successful incorporation of TiO₂ and titanate with intense interaction.

Fig. 3 XPS spectra of titanate (a and b) and TiO₂/titanate nanohybrid (c and d) (TTN-1).

The optical properties of the TiO₂/titanate semiconductor were altered after the formation of layered structure. In Fig. 4, TiO₂/titanate composite has a band edge at about 370 nm corresponding to 3.43 eV, indicating slight blue shift compared to pure TiO₂ and titanate (3.33 eV). This reflects the quantum confinement effects of the thinner nanosheet–TiO₂ nanohybrid structure are evidenced. Furthermore, the electronic band structure of TiO₂/titanate nanohybrid with higher energy level, which is expected that TiO₂/titanate nanohybrid would possess the thermodynamically enhanced reduction and oxidation power in photocatalytic reactions. The charge-transfer rate between semiconductor catalyst and redox species in solution also depends on such energy level correlation. So it is expected that TiO₂/titanate would possess a better photocatalytic performance for hydrogen evolution.

Fig. 4 UV-vis diffuse reflectance spectra of titanate and TiO₂/titanate nanohybrid (TTN-1), inset is the band gap energies.

Photoluminescence (PL) emission spectrum has been widely used to investigate the lifetime of electron–hole pairs in solid semiconductor materials, and can provide information on separation/recombination of photo-induced charge carriers.²⁷ In other words, a lower recombination rate or a higher transfer of those electrons and holes can result in a lower PL intensity. The PL spectra of titanate and TiO₂/titanate excited at 320 nm were detected (Fig. 5). The PL spectrum for the titanate is characterized by a broad band at around 375 nm, exhibiting strong emission intensity. As expected, it could be observed that a significant decrease in emission intensity for TiO₂/titanate in comparison to that of titanate, showing that photo-generated electrons and holes recombination rate is reduced. A TiO₂/titanate nanojunction where is a region of photo-generated carriers separated formed at the interface of TiO₂ and titanate. Thus, it could suppress the photo-generated carriers' recombination and enhance photocatalytic activity.

Fig. 5 Photoluminescence spectra of titanate and TiO₂/titanate (TTN-1) under the excitation of 320 nm.

In order to demonstrate the structure–function correction of TiO₂/titanate nanohybrid, the rate of hydrogen production was performed to evaluate the photocatalytic activity under UV light irradiation. Fig. 6 shows the time-dependent photocatalytic hydrogen evolution on the TiO₂/titanate hybrid catalysts together with other catalysts for comparison. The anatase TiO₂ shows negligible activity with an average hydrogen production rate of merely 9.01 μmol h⁻¹, which indicate that anatase TiO₂ has poor hydrogen evolution kinetics. Similarly, the hydrogen production rate of titanate only reached 62.29 μmol h⁻¹. However, the TiO₂/titanate sample shows a dramatic increase in hydrogen evolution, achieving production rate of 230.1 μmol h⁻¹. The hydrogen evolution is closer to that of commercially available P-25 photocatalyst. The high performance of TiO₂/titanate hybrid indicates that engineering the composite structure can indeed induce synergistic effects. This finding also underlines the importance of multiple shape engineering for photocatalyst design. Additionally, the photocatalytic H₂ evolution results of other products including TTN-2 and TTN-3 are listed as Fig. S3 in ESI.†

Fig. 6 Photocatalytic H₂ evolution performance of different products with 1 wt% Pt deposited 0.1 g catalyst under UV light irradiation.

The enhancement of hydrogen production rate can be ascribed to the fact that the unique geometry of the TiO₂/titanate. Because the combination of TiO₂ and titanate significantly increased its specific surface area, as a result, the density of active sites is augmented, and photocatalytic ability enhanced. Meanwhile, nanosheet shortens the distance that photogenerated charges diffuse to reaction sites as well. What is more, the TiO₂/titanate heterojunction would improve the rapid transfer of the excited electron between TiO₂ and titanate, which can react with H⁺ or H₂O to create H₂ or O₂. Thus, the TiO₂/titanate heterojunction facilitate the spatial separation of charge carriers and suppress the recombination of photogenerated electron–hole pairs, enhancing the efficiency in the utilization of photogenerated electrons for hydrogen production.

4. Conclusions

In summary, TiO₂/titanate photocatalyst was synthesized by one-step hydrothermal process. With excellent structural benefits for mass transfer and especially for charge separation, The H₂ evolution of 1 wt% Pt/TiO₂/titanate achieve production rate of 230.1 μmol h⁻¹. Moreover, this report will offer a new promising strategy to improve photocatalytic hydrogen evolution efficiency and reduce the cost. As a result, the nanojunction is anticipated to exhibit good photocatalytic activities for hydrogen evolution.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (51372071, 21201059), the Foundation of Heilongjiang Province of China (B201412), the Application Technology Research and Development Project of Harbin City (2013AA7BG025). Heilongjiang University Fund for Distinguished Young (JCL201302).

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Footnote

† Electronic supplementary information (ESI) available: XRD pattern, N₂ adsorption–desorption isotherms, and photocatalytic H₂ evolution performance. See DOI: 10.1039/c4ra14544e

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