Article (Special Issue on Photocatalytic H2 Production and CO2 Reduction)
Nitrate-group-grafting-induced assembly of rutile TiO2 nanobundles for enhanced photocatalytic hydrogen evolution

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Abstract

In this study, an acid-induced assembly strategy for a rutile TiO2 photocatalyst was proposed on the basis of the treatment of lamellar protonated titanate with a concentrated HNO3 solution. Nitrate groups were successfully grafted onto a TiO2 surface and induced the assembly of rutile TiO2 nanorods into uniform spindle-like nanobundles. The resulting TiO2 product achieved a photocatalytic hydrogen evolution rate of 402.4 μmol h−1, which is 3.1 times higher than that of Degussa P25-TiO2. It was demonstrated that nitrate group grafting caused the rutile TiO2 surface to become negatively charged, which is favorable for trapping positive protons and improving charge carrier separation, thereby enhancing photocatalytic hydrogen production. Additionally, surface charges were crucial to structural stability based on electrostatic repulsion. This study not only developed a facile surface modification strategy for fabricating efficient H2 production photocatalysts but also identified an influence mechanism of inorganic acids different from that reported in the literature.

Graphical Abstract

Nitrate group grafting induced the assembly of rutile TiO2 nanorods into spindle-like nanobundles. The grafted photocatalyst exhibited a significantly enhanced H2 evolution rate based on the improved proton trapping efficiency of the negatively charged surface.

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Introduction

The rapid consumption of fossil fuels has accelerated the global energy crisis and various environmental issues that threaten societal development. Photocatalytic hydrogen production has been recognized as a promising technique based on its direct utilization of clean and sustainable solar energy to realize solar-to-chemical fuel conversion [1, 2, 3, 4]. Among various semiconductor photocatalysts, titanium dioxide (TiO2) has been extensively investigated as one of the most promising candidates for photocatalytic water splitting based on its several advantages, including matched band energy, nontoxicity, low cost, and stability against photocorrosion [5, 6, 7, 8, 9, 10, 11]. TiO2 crystals have three main polymorphs of rutile, anatase, and brookite. It is widely recognized that anatase TiO2 generally possesses higher photocatalytic activity than that of rutile TiO2 because of its high reactivity [12]. Recently, nanoscale rutile crystals have also been developed as promising photocatalysts since it was discovered that they play an active role in certain cases [13, 14]. Furthermore, rutile TiO2 is a more thermodynamically stable state compared to the anatase and brookite phases [15]. However, rutile TiO2 photocatalysts suffer from the same major issue as anatase photocatalysts, namely high recombination loss of photogenerated electron-hole pairs. Many attempts have been made to improve the separation efficiency of charge carriers to reduce recombination loss, including functional group grafting, construction of surface heterojunctions, and co-catalyst loading [16, 17, 18, 19, 20, 21, 22, 23, 24, 25].

Since photocatalytic reactions typically occur on the surface active sites of photocatalysts, surface properties have a significant influence on photocatalytic activity based on their modification of charge transfer pathways [26, 27]. It has been well established that the grafting of inorganic acids, such as phosphoric, sulfuric, boric, and hydrochloric acid, can modify the surface properties of photocatalysts to enhance their photocatalytic activity [28, 29, 30, 31, 32]. The influence mechanisms of these inorganic acids have been examined from two main perspectives. One generally accepted mechanism is that acidification enhances the surface protonation of oxygen atoms to promote the formation of hydroxyl groups and oxygen adsorption, thereby improving charge carrier separation to boost reactivity [28, 29, 30]. For example, Cao et al. [28] reported that phosphoric acid treatment modifies phosphate groups on the surface of TiO2 nanocrystals to form –Ti–O–P–OH, which improves photocatalytic degradation efficiency. He et al. [29] treated TiO2 nanosheets with sulfuric acid to form surface hydroxyl groups and oxygen vacancies/Ti3+ species, which extended the lifetime of electron-hole pairs. Wu et al. [30] introduced phosphoric and boric acids to co-modify rutile TiO2 nanorods using residual chlorine, leading to the formation of –Ti–O–P–OH and –Ti–Cl:B–OH group ends. Their results confirmed that the promotion of adsorption of O2 facilitates the capture of photogenerated electrons based on acid co-modification. Dhandole et al. [31] proposed a synergistic effect between the OH-rich surfaces of HCl-treated rutile TiO2 nanorods and metal oxide co-catalysts that contributes to enhanced charge transfer. The second mechanism is based on electrostatic interactions between surface functional groups and reactive species. Li et al. [32] demonstrated that modification using phosphate groups creates a negative field effect on the surface of BiOCl, which effectively traps positive holes to suppress charge recombination. Based on the research above, it can be concluded that inorganic acid treatment has a significant effect on the enhanced photocatalytic degradation of organic pollutants based on the generation of surface hydroxyl groups and inorganic oxyanions. Despite the remarkable merits of such surface modification strategies, their effects on the photocatalytic production of clean solar fuels have not been extensively studied thus far. Therefore, it remains a considerable challenge to develop efficient photocatalysts for hydrogen production using acid treatments.

In this paper, we report the facile synthesis of spindle-like rutile TiO2 nanobundles via nitric-acid-induced assembly. First, lamellar protonated titanate (LPT) was prepared as a TiO2 precursor based on our previous research [33, 34]. Subsequently, concentrated nitric acid was introduced to induce the assembly of rutile TiO2 nanorods. The first three hours of light irradiation revealed an average hydrogen evolution of 402.4 μmol h−1 for the as-prepared TiO2 photocatalysts, which is 3.1 times higher than that of a Degussa P25-TiO2 reference. The mechanism for enhanced hydrogen production was determined to be related to the loss and recovery of photocatalytic efficiency. The synthesis strategy and the role of surface-grafted nitrate groups in hydrogen production are illustrated in Scheme 1. The proposed surface modification strategy with inorganic acid was confirmed to be an effective method to improve photocatalytic hydrogen production based on a novel mechanism of pollutant degradation reactions.

Section snippets

Synthesis of LPT as a precursor

LPT was prepared as a TiO2 precursor based on our previous studies [33, 34]. In a typical procedure, 9 mL of tetrabutyl titanate is first dispersed into 66 mL of absolute ethanol. This solution is then added into 90 mL of deionized water dropwise under vigorous stirring. The resulting suspension is then heated to 70 °C for 2 h under mechanical stirring to evaporate most of the solvent. Next, 300 mL of 1-M NaOH aqueous solution is added to the suspension. After continuous stirring for another 12

Structural characterization

The crystallinities and phase compositions of a series of TiO2 samples were characterized using XRD. As shown in Fig. 1, the R-TiO2 and R-TiO2-1M samples present distinct diffraction peaks at 2θ = 27.5°, 36.1°, 41.2°, 43.8°, 54.3°, 56.4°, 62.9°, and 69.1°, corresponding to the (110), (101), (111), (210), (211), (220), (002), and (301) reflection planes of rutile TiO2 (JCPDS 21-1276), respectively. In contrast, the A-TiO2 product obtained from the conventional hydrothermal method is a typical

Conclusions

In summary, we developed a facile nitric-acid-induced assembly strategy for the synthesis of spindle-like rutile TiO2 nanobundles. Structural characterizations demonstrated that the proposed strategy is effective for surface modification using nitrate groups. The R-TiO2 sample exhibited the highest photocatalytic H2 evolution rate of 402.4 μmol h−1, which is 3.1 times higher than that of Degussa P25-TiO2. In contrast, the anatase and rutile TiO2 photocatalysts without sufficient nitrate group

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    Published 5 January 2020

    We thank the Analysis and Testing Centre, Huazhong University of Science and Technology for the characterization of materials. This study was supported by the National Natural Science Foundation of China (21771070, 21571071) and the Fundamental Research Funds for the Central Universities (2018KFYYXJJ120, 2019KFYRCPY104).

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