Insights into the effects of surface/bulk defects on photocatalytic hydrogen evolution over TiO2 with exposed {001} facets

https://doi.org/10.1016/j.apcatb.2017.08.046Get rights and content

Highlights

  • Surface defect is more beneficial for increasing the charge separation efficiency.

  • Surface defects induce a conduction band tail located above the H+/H2 potential.

  • Bulk defects introduce a conduction band tail located below the H+/H2 potential.

  • TiO2 with surface defects shows enhanced photocatalytic activity for H2 evolution.

Abstract

This paper describes the effects of defect distribution on energy band structure and the subsequent photocatalytic activity over TiO2 with exposed {001} facets as the model catalyst. Our results show that only surface oxygen vacancies (Vo’s) and Ti3+ centers in TiO2 can be induced by hydrogenation treatment, whereas the generation of bulk Vo’s and Ti3+ species depends on the thermal treatment in nitrogen. Both the surface and bulk defects in TiO2 can promote the separation of electron-hole pairs, enhance the light absorption, and increase the donor density. The presence of surface and bulk defects in TiO2 can not change the valence band maximum, but determine the conduction band minimum. Surface defects in TiO2 induce a tail of conduction band located above the H+/H2 redox potential, which benefits the photocatalytic performance. However, bulk defects in TiO2 generate a band tail below the H+/H2 potential, which inhibits hydrogen production. Thus, the change of band gap structure by defects is the major factor to determine the photocatalytic activity of TiO2 for hydrogen evolution. It is a new insight into the rational design and controllable synthesis of defect-engineered materials for various catalytic processes.

Introduction

Photocatalysis is a promising strategy for clean hydrogen production from water [1]. With a suitable band structure, TiO2 is regarded as the benchmark material in photocatalytic reactions, and makes hydrogen evolution reaction thermodynamically possible [2], [3]. Thus, enormous strategies have been developed to improve the photoactivity of TiO2 nanomaterials, for example, the energy band engineering [4], [5], the surface/interface control [6], [7], and the morphology fabrication [8], [9]. Chen et al. first reported that defect-engineered TiO2 nanomaterials can be obtained at 200 °C for 5 days in high-pressure hydrogen atmosphere, and the as-prepared black TiO2 showed enhanced photocatalytic activity [10]. The defects lead to the changes of optical property and energy band structure of TiO2 and act as the key role on the enhanced photocatalytic performance [10]. To overcome the disadvantages of the rigorous reaction conditions, for example, the long hydrogenation time, high pressure, and explosion risks resulting from the utilization of pure H2, we developed a concept of hydrogen spillover enhanced hydrogenation by adding a small amount of Pt to the pristine TiO2 [11]. With the aid of hydrogen spillover on Pt, the TiO2 nanomaterials can be efficiently hydrogenated in 8 vol% H2/N2 at normal pressure. Additionally, a Schottky barrier can be formed at the Pt/TiO2 interface, which facilitates charge separation, resulting in the enhanced photoactivity [12].

Generally, defects are deemed as the recombination and trapping centers between photo-generated charge carriers, leading to a decrease in the photocatalytic activity [1]. However, other researchers thought that the induced defects in TiO2 improved charge carrier separation and transportation, and suppressed the recombination of photogenerated carriers [13], [14]. Presently, the function of defect distribution of TiO2 is still under debate in photocatalytic process. Kong et al. found that increasing the relative concentration ratio of surface defects to bulk defects could improve the photocatalytic activity because of the enhanced separation efficiency of photo-generated electrons and holes [15]. Yan et al. elucidated that surface oxygen vacancy clusters in TiO2 could promote the separation of charge carriers and increase the activity during photocatalytic reaction [16]. Furthermore, Leshuk et al. demonstrated that the hydrogenated TiO2 exhibited the worse photocatalytic activity compared with the pristine TiO2, and attributed it to the formation of bulk defects after hydrogenation [17]. However, it is reported that the disordered TiO2 with a high concentration of bulk defects showed the better photocatalytic activity than the surface defect dominant TiO2 [18]. Pei et al. reported that the bulk defect-engineered TiO2-x showed an enhanced H2 production rate, and the authors elucidated that bulk trapping could suppress the recombination of charge carriers [19]. Based on the above analysis, the role of defects in TiO2 for photocatalytic reactions is still unclear. In this sense, a bridge between defect engineering and the tunable properties of TiO2-based nanomaterials must be established.

Yang et al. first synthesized anatase TiO2 single crystals with a large percentage (47%) of the high energy {001} facets [20]. Previous studies elucidated that the catalytic performance of TiO2 {001} facets is higher than that of the thermodynamically stable {101} facets. The higher surface energy of TiO2 {001} facets can benefit the dissociative adsorption of reactant molecules compared with {101} facets due to the high density of unsaturated coordination Ti atoms, low coordination numbers of exposed atoms and active oxygen atoms with wide bond angles of Ti-O-Ti in {001} facets [7]. Subsequently, the {001}-faceted TiO2 nanomaterials have attracted a broad interest for their superior physicochemical properties [7], [21], [22]. Wang et al. demonstrated that the enhanced photocatalytic activity of the hydrogenated TiO2 with exposed {001} facets for methylene blue (MB) decomposition resulted from the induced Ti3+ species and oxygen vacancies [23]. Yu et al. elucidated that the distribution of the Ti3+ irons and Vo’s was critical in enhancing the photoactivity of MB degradation [24]. Liu et al. reported that the Ti3+/Vo’s sites and the coexposed {101} and {001} facets of the TiO2 nanocrystals could promote CO2 photoreduction activity [6]. Previous discussion about defect-engineered TiO2 with exposed {001} facets mainly focused on MB degradation and CO2 reduction. Recently, Chen et al. reported that the hydrogenated {001}-facet-dominant TiO2 nanocrystals were more active in photocatalytic hydrogen evolution than the hydrogenated TiO2 nanocrystals with {101} and {100} facets [25]. The enhanced activity could be ascribed to the electric field generated between stoichiometric surface and reduced subsurface [25].

In spite of the promising photocatalytic features of defect-engineered TiO2 with exposed {001} facets, little attention has been paid concerning the explanation of the relationship between defect distribution (surface or bulk) and photocatalytic activity of H2 production. Additionally, the hydrogenation treatment is usually conducted at high temperatures [18], [26]. No attempt has been made to elucidate the defect formation mechanism during the simultaneous thermal treatment and hydrogenation. In this work, we chose the TiO2 single crystal with exposed high energy {001} facets as the model catalyst, and the concept of hydrogen spillover enhanced hydrogenation on Pt was utilized for defect fabrication. We revealed the formation mechanism for the surface and bulk defects in TiO2 during the thermal hydrogenation process and distinguished the effects of the surface and bulk defects on energy band structure of TiO2 for photocatalytic hydrogen evolution from water. This work may provide a deep insight into the rational design and fabrication of defect-engineered nanomaterials for various photocatalytic processes.

Section snippets

Materials synthesis

Pristine TiO2 with exposed {001} facets were synthesized by a solvothermal method. In a typical procedure, 25 g of tetrabutyl titanate (TBT) was mixed with 3.5 mL of 40 wt% HF solution, under stirring for 20 min in a 100 mL Teflon-lined autoclave. Thereafter, the autoclave was kept at 180 °C for 24 h. After solvothermal reaction, the sample was cooled down to room temperature. The resulting white precipitates were collected and washed with absolute ethanol and distilled water for several times, and

Results and discussion

Fig. S2 shows the profiles of photocatalytic hydrogen production, the constant and stable amounts of H2 were produced over all of the as-prepared samples under simulated solar light irradiation. The hydrogen evolution rates and the apparent quantum efficiencies were summarized in Table 1. The sample of TH700 shows a photocatalytic hydrogen evolution rate of 15130.8 μmol g−1 h−1, which is 1.7-fold and 3.7-fold than the samples of TW and TC, respectively. Additionally, the specific surface areas of

Conclusions

In summary, we have successfully revealed the relationship between defect distribution (surface and bulk) and photocatalytic activity of hydrogen evolution using TiO2 with exposed {001} facets as the model catalyst. With the aid of EELS, XPS, XAFS, EPR and UV/Visible Raman, we found that surface Ti3+ centers and Vo’s generated through the hydrogenation process, and bulk Ti3+ species and Vo’s could be introduced by the thermal treatment in nitrogen. The induced surface and bulk defects can both

Acknowledgements

This work was financially supported by the National Key Research and Development Program of China (No. 2016YFB0600901), the National Natural Science Foundation of China (No. 21476159, No. 21525626, No. U1463205), the 973 program (No. 2014CB932403), and the Natural Science Foundation of Tianjin (No. 15JCZDJC37400). Authors are also grateful to the Program of Introducing Talents of Disciplines to China Universities (No. B06006). The staff members of BSRF and SSRF are acknowledged for their

References (57)

  • Z. Zhao et al.

    Reduced TiO2 rutile nanorods with well-defined facets and their visible-light photocatalytic activity

    Chem. Commun.

    (2014)
  • M.M. Khan et al.

    Band gap engineered TiO2 nanoparticles for visible light induced photoelectrochemical and photocatalytic studies

    J. Mater. Chem. A

    (2014)
  • L. Liu et al.

    Engineering coexposed {001} and {101} facets in oxygen-deficient TiO2 nanocrystals for enhanced CO2 photoreduction under visible light

    ACS Catal.

    (2016)
  • W.J. Ong et al.

    Highly reactive {001} facets of TiO2-based composites: synthesis, formation mechanism and characterization

    Nanoscale

    (2014)
  • W. Zhou et al.

    Ordered mesoporous black TiO2 as highly efficient hydrogen evolution photocatalyst

    J. Am. Chem. Soc.

    (2014)
  • J.H. Pan et al.

    Hierarchical N-doped TiO2 hollow microspheres consisting of nanothorns with exposed anatase {101} facets

    Chem. Commun.

    (2011)
  • X. Chen et al.

    Increasing solar absorption for photocatalysis with black hydrogenated titanium dioxide nanocrystals

    Science

    (2011)
  • Y. Zhu et al.

    H2 spillover enhanced hydrogenation capability of TiO2 used for photocatalytic splitting of water: a traditional phenomenon for new applications

    Chem. Commun.

    (2014)
  • J.H. Yang et al.

    Roles of cocatalysts in photocatalysis and photoelectrocatalysis

    Acc. Chem. Res.

    (2013)
  • K.F. Zhang et al.

    Black N/H-TiO2 nanoplates with a flower-like hierarchical architecture for photocatalytic hydrogen evolution

    ChemSusChem

    (2016)
  • Y. Liu et al.

    Vacancy engineering for tuning electron and phonon structures of two-dimensional materials

    Adv. Energy Mater.

    (2016)
  • M. Kong et al.

    Tuning the relative concentration ratio of bulk defects to surface defects in TiO2 nanocrystals leads to high photocatalytic efficiency

    J. Am. Chem. Soc.

    (2011)
  • J. Yan et al.

    Understanding the effect of surface/bulk defects on the photocatalytic activity of TiO2: anatase versus rutile

    Phys. Chem. Chem. Phys.

    (2013)
  • T. Leshuk et al.

    Photocatalytic activity of hydrogenated TiO2

    ACS Appl. Mater. Interfaces

    (2013)
  • X. Liu et al.

    Progress in black titania: a new material for advanced photocatalysis

    Adv. Energy Mater.

    (2016)
  • H.G. Yang et al.

    Anatase TiO2 single crystals with a large percentage of reactive facets

    Nature

    (2008)
  • J.G. Yu et al.

    Enhanced photocatalytic CO2-reduction activity of anatase TiO2 by coexposed {001} and {101} facets

    J. Am. Chem. Soc.

    (2014)
  • J. Pan et al.

    On the true photoreactivity order of {001} {010}, and {101} facets of anatase TiO2 crystals, Angew. Chem

    Int. Ed.

    (2011)
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