Promoting the spatial charge separation by building porous ZrO2@TiO2 heterostructure toward photocatalytic hydrogen evolution

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Abstract

The robust photocatalytic hydrogen evolution (PHE) from water needs an effective photogenerated charge spatial separation and enough contact between reactant and catalyst, but the synthesis of catalysts with the characteristics remains a challenge. Herein, we report the design of core-shell heterostructure consisted of thin TiO2 layer uniformly coated on porous ZrO2 polyhedron for effective PHE. In this system, UiO-66-NH2, one of popular MOF with Zr as metal node, has been chosen as the precursor template due to its plentiful pores, uniform morphology, as well as the rich NH2 groups. Our results show that Ti precursor can uniformly coat on UiO-66-NH2, by means of interaction of tetrabutyl titanate (TBT) with -NH3 in UiO-66-NH2. Followed by the calcination, the Ti precursor and UiO-66-NH2 can be converted into ZrO2 and TiO2, respectively, thus leading to the formation of ZrO2@TiO2 core-shell heterostructure. The ZrO2@TiO2-500 has the high specific surface area of 52.4 m2 g−1. Besides, the intimate contact of TiO2 shell with ZrO2 core facilitates the separation and migration of photoinduced carriers, exposing more active sites for the surface photocatalytic hydrogen evolution reaction. The spectrum and electrochemical characterization further exhibit the extended life of photon-generated carrier and easy mass transfer. The optimized ZrO2@TiO2-500 shows enhanced photocatalytic rate of 39.7 mmol h−1 g−1, much higher than those of ZrO2 (0.8 mmol h−1 g−1) and TiO2 (7.6 mmol h−1 g−1).

Graphical abstract

The design of core-shell heterostructure consisted of thin TiO2 layer uniformly coated on porous ZrO2 polyhedron for effective PHE.

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Introduction

The solar photocatalytic water splitting has attracted more and more attention in current world due to its ability for the production of clean hydrogen energy with many advantages of mild condition, simple operation, direct utilization of sunlight and no secondary pollution [1], [2], [3]. The development of effective photocatalysis with good activity and stability is constantly pursued, but remains a severe challenge. TiO2, as a stable, nontoxic photocatalyst, is promising in the photocatalytic hydrogen evolution field [4], [5]. However, the practical application of TiO2 has been limited due to high recombination probability of charge carriers and low quantum efficiency caused by weak electron mobility and short minority carrier diffusion [6], [7]. In this regard, many methods have been developed to improve the photocatalytic performance of TiO2, such as doping [8], [9], surface sensitization [10], [11], and the construction of the heterostructure [12], [13], [14]. By integrating TiO2 with ZrO2 [15], CdS [16], Fe2O3 [17], g-C3N4 [18], Cu2O [19] or BiVO4 [20], a new carrier transport path can be established. The corresponding heterostructure can accelerate charge separation during the photocatalytic process, thus improving the catalytic performance largely. Notably, a core-shell heterostructure has possessed many advantages for the effective separation of photogenerated charge carriers, thus being interesting in the photocatalytic filed [21], [22], [23], [24]. In that case, the photogenerated electrons and holes in heterostructure will transfer toward different direction, so their recombination can be suppress. As a kind of wide band gap n-type semiconductor, ZrO2 with the band gap of about 5.0 eV is useful in hydrogen generation with strong resistance to corrosion [25]. Coupling of TiO2 with ZrO2 is effective for the degradation of organic compounds, such as copper supported TiO2/ZrO2, ZrO2/TiO2-xNx and S-doped TiO2/ZrO2 [26], [27], [28]. The self-assembly, colloidal aggregation, and precursor templates are the most common approaches for ZrO2@TiO2 core-shell structure [29], [30], [31]. Nevertheless, these syntheses are difficult to generate the uniform coating and regular structure with high specific areas, which is not conductive to bring enhanced photocatalytic activity.

In order to overcome the above issues, the porous and regular ZrO2 should be chosen as template to support TiO2 uniformly. The selection of the suitable precursor for ZrO2 is a key to realize the goal. The metal organic framework (MOF) is a class of crystal material with micro- and mesoporous structure and large specific surface area, which can be used as a template or precursor for the synthesis of semiconductor oxides [32], [33]. The UiO-66-NH2, a typical MOF with Zr ion as the metal node and 2-aminoterephthalic acid as the organic ligand, has high porosity, large specific surface area, and network topology [34]. These characterizations make the UiO-66-NH2 promising to produce the Zr-based composites with regular morphology such as C-ZrO2/g-C3N4 [35]. However, up to now, the preparation of ZrO2@TiO2 core-shell heterostructure from UiO-66-NH2 as Zr source has not been reported yet.

In this work, we report a synthetic method to obtain porous core-shell heterostructure consisted of ZrO2 and TiO2. UiO-66-NH2 (Zr-MOF) is applied as precursor template and Zr source. Then, the hydrolyzed tetrabutyl titanate (Ti source) is absorbed on its surface through the interaction with the rich single bondNH2 group in UiO-66-NH2. After thermal treatment, ZrO2@TiO2 core-shell heterostructure has been fabricated, in which thin TiO2 shell covers the ZrO2 core evenly. In the core-shell structure, the matched bang gap of TiO2 and ZrO2 facilitate the separation of photoexcited electron-hole pairs and their migration toward the inner and outer surfaces, thereby decreasing their recombination. As a result, the ZrO2@TiO2 exhibit enhanced photocatalytic activity of 39.7 mmol h−1 g−1, much higher than those of ZrO2 (0.8 mmol h−1 g−1) and TiO2 (7.6 mmol h−1 g−1). This work opens a new avenue to design MOF-derived core-shell heterostructure photocatalysts for the hydrogen evolution.

Section snippets

Experimental section

Zirconium chloride (ZrCl4, 99.9%), 2-aminotetrephthalic acid (H2ATA, 98%) and acetic acid (CH3COOH, 99.5%) were purchased from Sigma-Aldrich Trading Co. Ltd. C16H36O4Ti (99%) (TBT) was purchased from Tianjin Kermel Co. Ltd. Ethanol (99.7%) and N, N-Dimethylformamide (DMF, 99.7%) were purchased from Tianjin Fuyu fine chemical industry Co. Ltd. All chemical reagents were obtained from commercial reagents company without further purification. The deionized water was used in the experiment process.

Results and discussion

A schematic procedure for the porous ZrO2@TiO2 core-shell heterostructure is shown in Scheme 1. Firstly, UiO-66-NH2 with plentiful single bondNH2 group and pores was employed as a template and Zr source. The tetrabutyl titanate was adsorbed on the surface of UiO-66-NH2 and hydrolyzed to form Ti precursor layer. A precursor contained both Ti and Zr (UiO-66-NH2@Ti) was formed. Finally, a calcination under air (500 ℃) of UiO-66-NH2@Ti results in the formation of the porous ZrO2@TiO2 core-shell

Conclusion

In summary, we have prepared porous ZrO2@TiO2 core-shell heterostructure photocatalytic catalyst by utilizing UiO-66-NH2 (Zr-MOF) as precursor and Zr source. Porous ZrO2@TiO2 heterostructure inherits the characteristics of UiO-66-NH2. Compared with ZrO2 and TiO2, ZrO2@TiO2-500 displays the higher photocatalytic hydrogen evolution rate of 39.7 mmol h−1 g−1. The superior photocatalytic hydrogen evolution properties of ZrO2@TiO2-500 can be attributed to its core-shell heterostructure, large

Declaration of Competing Interest

The authors declare no competing financial interests.

Acknowledgment

This work was supported by the National Natural Science Foundation of China (21771061, 21601055) and University Nursing Program for Young Scholars with Creative Talents in Heilongjiang Province (UNPYSCT-2017118)

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