Rare earth metal Gd influenced defect sites in N doped TiO2: Defect mediated improved charge transfer for enhanced photocatalytic hydrogen production

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Abstract

In this experimental studies, we report the synthesis of TiO2 co-doped by both cationic and anionic sites by simple sol-gel based method. All the prepared samples exhibit the anatase crystalline morphology however, showed lattice distortion caused by the displacement of Ti4+ sites by Gd3+. The improved visible absorption is witnessed by the Gd and N co-doping with an assured redshift in the absorption edge. The N and Gd displacement inside TiO2 lattice accompanied by the creation of Osingle bondTisingle bondN and Gdsingle bondOsingle bondTi bonds are characterized by the X-ray photoelectron spectra. The strong resonance signal by Gd4f electrons in the electron paramagnetic resonance spectroscopy further substantiate the displacement of lattice cites of TiO2 by Gd3+ ions. The longevity of the photo produced charges observed in fluorescence spectra of Gd and N co-doped TiO2 is because of the effective transfer of charges to the defect sites. The aforementioned catalysts are tested for their capacity for the H2 production from water splitting. The 2 wt% gadolinium and nitrogen co-doped TiO2 has shown 10764 μmol g−1 H2 production which is 26 times higher than the commercial Degussa P-25 catalyst. The enhanced activity for hydrogen production can be attributed to factors such as increased absorptivity under visible light and effective charge carrier separation.

Graphical abstract

Schematic representation of excitations in N-doped TiO2, Gd-doped TiO2 and Gd, N co-doped TiO2.

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Introduction

Semiconductor oxide based photocatalysis is one of the extensively studied research in the recent years [1], [2]. This technology has extensively studied due to its applications in the treatment of pollutants, disinfection and hydrogen production so on [3], [4], [5]. Amongst various semiconductor oxides that are been studied such as CdS, ZnO, CeO2, Bi2O3 etc. [6], [7], [8], [9], TiO2 has proven to exhibit better efficiency due to its stability, low cost, non-toxicity. On this note, titania based photocatalysis is extensively explored for its applications. Although water splitting technology has huge promise as a potential viable option for the renewable energy of the future, it suffers from major limitations that thwart its commercialization. Amongst those major limitations, the high recombination and lower visible light activity are the major impediments for the efficacy of this technology [10]. In an attempt to address the problem, various TiO2 based modifications such as metal doping, dye sensitization, non-metal doping, heterostructures are been carried out [11], [12], [13], [14], [15], [16].

Amongst various modifications, doping is easier and effective approach. In the case of metal doping, the intermediate levels are formed in the lattice which would decrease the electron recombination whereas the nonmetal doping helps the upward movement of the valence band possibly improving the visible light absorption of the developed photocatalyst [17], [18], [19]. In an attempt to obtain the either benefits, the synergy effect of metal and nonmetal based co-doping has been extensively explored in the recent years [20], [21], [22]. For instance, Zhuang et al. has carried out a synergetic co-doping by using Sn, N on titania. According to the experimental results, the Rhodamine B dye degradation and hydrogen production capacity of the photocatalyst was higher in comparison to the pristine TiO2. They attributed the improved activity to the efficient transfer and migration of photogenerated charge carriers [23]. Although the transition metal based co-doping is explored enough, there are still some knowledge gaps about the nonmetal, and rare metal doped titania performance in water splitting. Considering the promise and potential of co-doping, in this study, we have prepared N and Gd co-doped TiO2 photocatalyst. The hydrogen evolution from water-methanol mixture is studied at various concentrations both under the natural as well as the simulated sun light irradiation. The role of methanol is to act as a hole capture agent on the TiO2 that will indeed delay the recombination of the photocatalyst.

Section snippets

Experimental

The chemicals used, method of catalyst preparation, characterization techniques and the procedures of photocatalytic experiments in the present study are described in the supplementary section.

X-ray diffraction (XRD)

The XRD patterns of the prepared GdNTi photocatalysts are presented in the Fig. 1. All the samples exhibited the XRD peaks related to the anatase phase with 2θ values at 25.2, 37.8, 48.5, 53.7, 55.1, 62.8, 68.7, 70.5 and 75.0 corresponding to the (101), (004), (200), (105), (211), (204), (116), (220) and (215) planes respectively. Interestingly, the intensities of the XRD peaks are found to reduce with increasing the doping amount of gadolinium in to NTi, accompanied by the broadening of the

Conclusions

The present study demonstrated the metal and nonmetal (Gd and N) doped TiO2 ensuring high activity for photocatalytic hydrogen production under sun light radiation. It is observed that the nitrogen doping has extended the TiO2 absorption into visible light whereas, the Gd doping resulted in the formation of defect levels in the TiO2 structure. Photoluminescence study confirmed that these defects sites are playing vital role in improving the charge carrier transportation and separation in TiO2.

Acknowledgements

This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (No. 2015R1A1A3A04001268 & 2017R1A2A2A05000770). This research was supported by the 2017 Research Fund (1.170013.01) of UNIST (Ulsan National Institute of Science & Technology). This research was also financially supported by the Energy Technology Development Program of the Korea Institute of Energy Technology Evaluation

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