Mesoporous TiO2 modified with carbon quantum dots as a high-performance visible light photocatalyst

https://doi.org/10.1016/j.apcatb.2016.01.070Get rights and content

Highlights

  • Carbon quantum dots (CQDs) modified mesoporous TiO2 was made by sol-gel method.

  • Remarkable visible light photocatalytic activity was observed for dye degradation.

  • The composite material exhibits high stability and recyclability.

  • The up-conversion properties of CQDs and mesoporosity of TiO2 were investigated.

  • The dye degradation mechanism is illustrated.

Abstract

We report a preparation method for visible light responsive Carbon Quantum Dots (CQDs) embedded in mesoporous TiO2 materials. The as-prepared mesoporous TiO2 (meso-Ti-450) material is a member of the recently designed University of Connecticut (UCT) mesoporous materials family. The UCT materials were synthesized based on sol-gel chemistry. The nanoparticles are randomly packed in inverse surfactant micelles and mesopores are formed by interconnected intraparticles. To achieve full usage of the visible region of sunlight (>400 nm), CQDs were introduced without destroying the mesopores. The photocatalytic performance of the CQDs/meso-Ti-450 was investigated by the degradation of methylene blue. Due to the up-conversion property and electron withdrawing property of CQDs, the photocatalytic activity of the composite material was largely enhanced under visible light irradiation. The highest photocatalytic activity was achieved by 5% CQDs/meso-Ti-450 in an hour. Compared to commercial P25, which is capable of removing 10% methylene blue (MB) under visible light conditions, the 5% CQDs/meso-Ti-450 can mostly remove MB (98%) under the same conditions. To date, the usage of mesoporous titanium oxide and carbon material composites for dye degradation under visible light has not been reported.

Introduction

Environmental and energy concerns have attracted significant attention on the development of highly efficient heterogeneous photocatalysts for the degradation of various kinds of organic contaminants [1], [2], [3]. Semiconductors have been widely studied as photocatalysts due to their wide absorbance range. Among the common semiconductors, such as iron oxide [4], copper oxide [5], and zinc oxide [6], titanium dioxide (TiO2) has been widely studied and utilized in many photocatalytic applications. As a result of its long-term chemical and optical stability, strong oxidizing ability, nontoxicity, and low cost, TiO2 is considered to be the most efficient photocatalyst to date [7]. The photocatalytic properties of TiO2 are attributed to the production of photogenerated electrons in the conduction band (CB) and holes in the valence band (VB), which occur upon the radiation of ultraviolet (UV) light (10–400 nm). The photogenerated electrons and holes diffuse to the TiO2 surface and form highly reactive radicals (OHradical dot, O2), which are capable of oxidizing nearby organic molecules [8], [9], [10], [11]. Among various TiO2 materials reported, commercial TiO2 (Degussa P25) is a benchmark photocatalyst and has been widely studied. P25 is a mixed phase TiO2 material with 70–80% anatase and 30–20% rutile [12], [13]. The remarkable photocatalytic efficiency contributes to the synergistic effect between anatase and rutile phases [14], [15]. However, due to the relatively large band gap (3.0–3.2 eV), TiO2 can only absorb short wavelength light, which falls in the UV region [16], [17]. Given that the entire solar light consists of less than 5% ultraviolet light, the photocatalytic activity of TiO2 under natural sunlight is largely limited [18]. In addition, commercial P25 is nonporous and has a relatively low surface area, which impedes the adsorption of target molecules, thus lowering the photocatalytic efficiency [19]. Therefore, the design of a high surface area and visible light responsive photocatalytic system has become an urgent task [16], [20].

Mesoporous materials are considered as excellent catalysts for applications requiring high surface area and large amount of active sites, due to their tunable structural properties such as surface area, pore volume, size, and nanocrystallinity [21], [22]. Mesoporous titanium dioxide that combines a photoactive framework and an open porous structure is of great interest [23]. Mesoporous TiO2 applied in the photocatalysis area has been extensively reported. Li et al. [24] synthesized gold nanoparticles embedded in a mesoporous titania photocatalyst and showed that gold nanoparticles are well dispersed in the mesoporous TiO2 networks. The mesoporous channels offer a larger surface area and enhanced accessibility than P25. Feng et al. [25] reported that mesoporous titanium dioxide exhibits an enhanced photoreactivity relative to P25 due to the synergetic effects of the mesoporosity and light-transmittance ability of the catalysts. Zhu et al. [26] prepared flowerlike hierarchical TiO2 materials with a high surface area and mesoporous channels, which exhibit good photocatalytic performance towards degradation of methylene blue. Both the mesoporous structure and unique morphology contributed to the enhanced activity compared to commercial TiO2. However, the main drawbacks of conventional mesoporous TiO2 materials are their poorly ordered mesoporous structure, low crystallinity, and low thermal stability. In 2013, our group has developed a general inverse micelle sol-gel approach for the preparation of a series of mesoporous metal oxide materials, which are called UCT materials (University of Connecticut mesoporous materials) [27]. The inverse micelle sol-gel method used Pluronic P123 as a surfactant species, which formed inverse micelles in a low pH condition. The presence of nitrate ions facilitates the penetration of positively charged metal nitro-clusters into the inverse micelle followed by a thermal decomposition of nitrate ions into nitric oxide species, which controlled the sol-gel chemistry. The 1-butanol, which served as an interface modifier and solvent, further prevented the undesirable aggregation. The subsequent calcination treatment generates thermally stable and crystalline mesoporous materials with monomodal and tunable pore sizes. The UCT materials have been used in various applications, such as photocatalytic reactions [28], water oxidation [29], methane oxidation [30], and CO oxidation [31]. Based on the inverse micelle sol-gel process, mesoporous TiO2 (UCT-62) has been synthesized in this study.

In an attempt to improve the visible light photocatalytic activity of metal oxides, carbon nano-species have been utilized as modifying materials recently [32], [33], [34]. In terms of photophysical properties, carbon quantum dots have been found to possess excellent up-conversion photoluminence (UCPL). Up-conversion is a process where lower-energy light (near-infrared or infrared) is converted to higher-energy light (ultraviolet or visible) through multiple photon absorption. That is usually achieved via the use of lanthanide, actinide ions, transition metals, and semiconductor quantum dots [35], [36], [37], [38]. Compared to down-conversion materials, materials with an up-conversion property are less common and more versatile, which are able to utilize the visible spectrum of sunlight, resulting in an improved photocatalysis efficiency [39], [40]. Due to their unique optical properties such as bright fluorescence and strong absorption, CQDs have received much interest. Compared to the traditional metallic quantum dots, which are limited in use due to environmental hazard issues [41], [42], CQDs possess low toxicity, good biocompatibility, high photostability, high aqueous solubility, strong emission, high natural abundance, and also are an electron reservoir [39], [43], [44], [45], [46]. Therefore, CQDs have gradually become promising carbon nanomaterials. Considerable work has been done on photocatalytic dye degradation with quantum dot-based materials, but only a few researchers are using CQDs on mesoporous materials. Yu et al. [2] developed CQDs embedded in a mesoporous hematite complex photocatalyst, which led to a high degradation efficiency of 97% for MB with the assistance of H2O2 under visible light. The intimate contact between CQDs and mesoporous hematite facilitated the electron transfer, and thus inhibited the electron-hole recombination. However, this photocatalyst requires coexistence of a strong oxidizing agent during the photocatalytic tests.

The CQDs embedded in mesoporous TiO2 composite photocatalytic systems have not been reported previously. In this work, we demonstrated the design of a CQDs/mesoporous TiO2 (UCT-62) composite to exploit full usage of the visible spectrum of sunlight based on the up-conversion property of CQDs. The CQDs modified mesoporous TiO2 maintained the pure anatase phase and mesoporous structure, and exhibited high adsorption capacity and excellent visible light photocatalytic activity under ambient conditions.

Section snippets

Chemicals

Titanium (IV) isopropoxide (≥97%). 1-Butanol (anhydrous, 99.8%), poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) PEO20-PPO70-PEO20 (Pluronic P123), l-ascorbic acid were purchased from Sigma-Aldrich. Concentrated nitric acid (HNO3, 68–70%) was purchased from J. T. Baker. All chemicals were used as received and used without further purification.

Preparation of Meso-Ti-X and CQDs/meso-Ti-X

The mesoporous TiO2 was synthesized by the recently developed inverse micelle template sol-gel approach [27]. In a typical

Physiochemical properties of mesoporous TiO2

Fig. 1 shows both the wide angle and low angle diffraction lines for titanium oxide materials calcined at different temperatures. All materials exhibit a typical anatase TiO2 crystalline pattern according to JCPDS card (No. 00-021-1272). Increasing calcination temperature leads to a better crystallinity (Fig. 1a). The low angle diffraction peak is an indicator of a regular mesopore structure in soft-template prepared mesoporous materials. Since most mesoporous materials are amorphous on the

Discussion

In this study, the mesoporous TiO2 was synthesized by a recently developed inverse micelle sol-gel method [27]. This approach involves sol-gel reactions of the transition metal oxo-clusters with surfactant species in hydrated inverse micelle nanoreactors. The reaction was controlled by the unique thermal decomposition of nitrate ions. The formed NOx species are attached to metal oxo-clusters, followed by an increased acidity and the formation of mesoporous metal oxides [63]. The formed

Conclusions

In summary, CQDs embedded mesoporous TiO2 composites were first successfully synthesized by a low cost, environmentally friendly sol-gel and ultrasonic-hydrothermal method. The mesoporous structure was preserved with the introduction of CQDs. The photocatalytic activity was investigated using methylene blue degradation under visible light irradiation. Compared to commercial Degussa P25, pure mesoporous TiO2, and CQDs/P25, CQDs/meso-Ti-450 composites showed enhanced catalytic performance. The

Acknowledgments

This work was supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Chemical, Biological, and Geological Sciences under Grant DE-FGO2-86ER13622.A000. The authors would like to thank Dr. Lichun Zhang for his assistance in TEM imaging in the Institute of Materials Sciences, UConn, Dr. Angeles-Boza, and Dr.Kumar for allowing us to use their instruments, and Dr. Frank Galasso for helpful discussions.

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