Three-dimensional graphene-TiO₂ hybrid nanomaterial for high efficient photocatalysis

1 Introduction

Hybrid materials of graphene and semiconductor nanomaterials have received much attention due to their high potential in enhancing the photocatalytic efficiency of semiconductor nanomaterials [1], [2], [3], [4], [5], [6]. There are a variety of materials that show photocatalytic properties, among which titanium dioxide (TiO₂) is one of the most widely used heterogeneous photocatalysts [7]. TiO₂ has excellent physical and chemical stabilities and electronic and optical properties and is inexpensive. In the hybrid system, graphene acts as an electron acceptor to ensure the fast charge transfer to the current collector. Effective charge separation can therefore be achieved to reduce the electron-hole recombination, and as a result, the photocatalytic activity of the hybrid materials is enhanced [8], [9], [10], [11], [12]. Like in many other graphene-based applications, the quality of graphene is important for making high-efficiency hybrid materials. Many methods have been developed to prepare graphene; however, it remains challenging to make high-quality graphene on a large scale [13].

Three-dimensional (3D) graphene in this context represents a new format of graphene material. 3D graphene can be fabricated by self-assembly [14], templating [15], or chemical vapor deposition (CVD) [16] (Figure 1). For example, the self-assembled graphene hydrogel was fabricated via the hydrothermal reduction of an aqueous solution of graphene oxide (GO) [14]. The obtained 3D graphene hydrogel was light and strong, and the size of materials can be controlled by the reaction vessel. In an example of the template-assisted synthesis of 3D graphene, closely packed polystyrene (PS) beads were used as the template [15]. An aqueous GO solution was infiltrated to coat the PS beads, which were then removed by heat treatment. The 3D structure can be controlled by adjusting the size of the template. The CVD method was also applied to prepare 3D graphene [16]. In this protocol, a nickel (Ni) or copper (Cu) foam was used as the substrate to develop the 3D structure. This method produces pristine graphene without using GO as the starting material.

Figure 1:

Fabrication of 3D graphene by (A) self-assembly, (B) template assistance, and (C) CVD. Adapted with permission from Ref. [17]. © Copyright 2015 Royal Society of Chemistry.

Compared to two-dimensional (2D) graphene sheets, 3D graphene has a number of unique features [14], [15], [16], [17], [18]. 3D graphene has high surface area resulting from the interconnected graphene network. Whereas 2D graphene sheets have conducting pathway restricted to the planar direction, 3D graphene provides multiple channels for fast electron transport. Its excellent electrical conductivity is also due to the absence of intersheet contact resistance. Moreover, 3D graphene materials reduce the possibility of agglomeration of 2D graphene sheets caused by the strong intersheet π-π stacking. In addition, whereas 2D graphene sheets are difficult to handle or reuse because they are usually dispersed in liquid phase or deposited on a substrate, 3D graphene can be easily manipulated and controlled with regard to size, shape, and density while at the same time maintaining its superior conductivity as well as mechanical and thermal stabilities. All these properties make 3D graphene promising candidates for applications in catalysis, energy storage devices, and solar cells [17], [18]. In this chapter, we will focus on 3D graphene-TiO₂ hybrid materials and summarize the recent progress on these materials in photocatalysis.

2 Hybrid materials fabricated from CVD 3D graphene

3D graphene-TiO₂ hybrid material was developed by Park et al. for the fabrication of electrochemical photovoltaic cells [19]. Monolayer 3D graphene was prepared on Cu foam using the CVD technique. The commercial TiO₂ paste was applied to the 3D structure followed by annealing and sintering. The authors stated that TiO₂ was physically absorbed on the outer surface of graphene and also acted as support to sustain the continuous 3D graphene structure (Figure 2). Without their support, monolayer 3D graphene would easily collapse and lost their 3D structure [16]. Prepared 3D graphene-TiO₂ hybrid material was used as the photoanode in dye-sensitized solar cell (DSSC) and their electrochemical properties were studied. No significant enhancement was observed in photocurrent density-voltage (J-V) curves or impedance spectra in the presence of graphene. This unexpected result originated from an unfunctionalized inner face, which was directly exposed to the electrolyte causing electron leakage to the electrolyte. The enhanced performances were observed after the inner face of graphene was functionalized with 4-nitrobenzenediazonium and further embedded with TiO₂ nanoparticles. The continuous 3D graphene provided a direct pathway for photoexcited electrons to the fluorine-doped tin oxide (FTO) current collector as shown in Figure 2D. It exhibited 9.2% energy conversion efficiency, which was higher than the reported TiO₂ hybrid materials with reduced GO.

Figure 2:

(A) Schematic of 3D graphene-TiO₂ hybrid material before postfunctionalization. SEM images of (B) inner and (C) outer surfaces of the hybrid material. (D) J-V curves of DSSC. Adapted with permission from Ref. [19]. © Copyright 2015 American Chemical Society.

Huang et al. prepared flexible DSSC with CVD grown 3D graphene on plastic indium-tin oxide (ITO)-coated poly(ethylene terephthalate) (PET) substrate [20]. A small amount of 3D graphene (0.5–1.5 wt.%) was added into commercial P25 TiO₂ suspension to enhance the electron transport property. The 3D graphene-TiO₂ mixture was then deposited on the ITO-PET substrate by a multistep doctor blading protocol. The authors found that a moderate amount of 3D graphene was important to achieve the enhanced DSSC performance. A higher amount of 3D graphene gave higher surface area, but too much graphene led to light harvesting competition with the dye and increased charge recombination by the exposed graphene to electrolyte [21], [22]. On the contrary, too little graphene did not provide efficient charge separation and electron transport. With the moderate amount of graphene, photoexcited electrons can be easily transferred to current collectors through 3D graphene before they are recombined (Figure 3). This formulation resulted in up to 6.41% of power conversion efficiency, which was the highest value for plastic-based flexible DSSC.

Figure 3:

Electron transport path in DSSC with (A) TiO₂ only and (B) 3D graphene-TiO₂ hybrid material. (C) J-V curves of prepared DSSC. Adapted with permission from Ref. [20]. © Copyright 2015 Elsevier.

3 Hybrid materials fabricated from template-assisted 3D graphene

Kim et al. fabricated 3D graphene-embedded TiO₂ inverse opal (IO) electrode for DSSC by the templating method [23]. Self-assembled monodispersed PS colloids (~750 nm in diameter, 15 μm thick) were used as the template, and an aqueous suspension of GO and commercial TiO₂ nanoparticles (~15 nm) was infiltrated into the PS colloidal template. Because the size of GO sheets were mostly larger (from 100 nm to a few micrometers) than the cavities in the PS template (~200 nm), GO sheets were mostly situated on the top layers of the IO structure. The localized graphene minimized the light absorption by graphene itself and consequently increased current density by reducing the light harvesting competition with dye. To prevent graphene from forming direct contact with the electrolyte, the IO structure was treated with TiCl₄ to grow ~30 nm rutile TiO₂ nanoparticles such that the graphene layer was embedded inside the TiO₂ layer. Consequently, the energy conversion efficiency value of 7.5% was achieved, which was 55% higher compared to TiO₂-based IO electrode.

4 Hybrid materials fabricated from self-assembled 3D graphene

Self-assembled 3D graphene-TiO₂ hydrogel was applied in reusable photocatalysis and energy application. In the work of Zhang et al., the hydrogel was synthesized by the hydrothermal reaction of GO and P25 TiO₂ at 180°C for 2 h [24]. The conjugated TiO₂ nanoparticles were thought to prevent the agglomeration of graphene sheets and contribute to increased surface area. The photocatalytic test was performed by monitoring the decomposition of methylene blue (MB) dye. The 3D graphene-TiO₂ hydrogel showed high dye adsorption capacity and degraded 10 ppm of MB completely within 30 min under UV irradiation (Figure 4). The enhanced photocatalytic efficiency was attributed to the transfer of the photogenerated electrons from TiO₂ to interconnected 3D graphene network. The authors also used the hydrogel as an electrode in supercapacitor. Improved electrochemical capacitance and cycling stability were demonstrated at more than 2000 cycles.

Figure 4:

3D graphene-TiO₂ hydrogel and its photocatalytic activity, measured by monitoring the decomposition of MB. C and C₀ are the actual and initial concentrations of MB, respectively. TGH: 3D graphene-TiO₂ hydrogel. Adapted with permission from Ref. [24]. © Copyright 2013 American Chemical Society.

Zhang et al. demonstrated that a strong interaction of TiO₂ with graphene aerogel and its uniform distribution led to higher performance in photocatalysis [25]. The hybrid graphene aerogel was prepared by an in situ hydrothermal process from GO, Ti(SO₄)₂, and glucose. The authors proposed that the hydroxyl groups on glucose interacted strongly with both TiO₂ and graphene and hindered the agglomeration of TiO₂ nanoparticles. As a result, improved electron transport efficiency and cycling stability of 3D graphene-TiO₂ were observed compared to the material without glucose (Figure 5).

Figure 5:

(A) Preparation of TiO₂ nanocrystals grown in situ on graphene aerogels. (B) Cycling photocatalytic activity, measured by monitoring the decomposition of methyl orange under 300 W Xe lamp (C and C₀ are the actual and initial concentrations of dye, respectively; GA: graphene aerogel). (C) Cycling performance at different current densities. Adapted with permission from Ref. [25]. © Copyright 2014 American Chemical Society.

5 Covalently conjugated hybrid materials fabricated from CVD 3D graphene

Several conclusions can be drawn from the results described above. The high conductivity of graphene reduces the electron-hole recombination, and the 3D structure offers multiple pathways for electron transfer [19], [20], [23], [24], [25]. The amount and the environment of graphene are important to prevent the direct exposure of graphene to electrolyte and to reduce the light harvesting competition with the dye in DSSC [19], [20], [23]. Lastly, strong interactions between TiO₂ and graphene can produce enhanced electron transport efficiency as well as high cycling stability [25]. In the examples discussed above, except for CVD grown 3D graphene, reduced GO has been used. It has been demonstrated that reduced GO has much worsened electronic properties than pristine graphene due to the presence of remaining oxygen species and defects even after reduction [26], [27]. Therefore, it can be anticipated that, by conjugating TiO₂ nanoparticles on pristine 3D graphene and through strong interactions such as covalent bonds, the resulting hybrid 3D graphene-TiO₂ system may lead to even higher photocatalytic activity and stability.

We have developed a general method to covalently conjugate nanoparticles on pristine graphene [28]. Due to the low chemical reactivity of graphene, highly reactive intermediate species such as radical, carbene, nitrene, and aryne are often used to functionalize graphene [29]. In our method, perfluorophenyl azide (PFPA) was used, and when it is activated by UV, heat, or microwave [30], [31], it gives singlet perfluorophenyl nitrene that reacts with graphene through [1+2] cycloaddition reaction [32], [33], [34], [35]. Moreover, the two functional groups in PFPA, azide and R, allows the functionalization reaction to be carried out selectively and sequentially (Figure 6A). Various functional group (R) such as carboxylic acid, thiol or disulfide [36], silane [37], and phosphate [38] can be applied to conjugate different nanoparticles, including TiO₂, gold, silica, iron oxide, quantum dots, silver nanoparticles, and polymeric materials. We have successfully conjugated silica and gold nanoparticles on pristine graphene using PFPA-functionalized silica or gold nanoparticles [28]. Using this technique, we also covalently conjugated TiO₂ nanoparticles on 3D graphene fabricated by CVD (Figure 6B). The resulting hybrid material is expected to show an enhanced photocatalytic activity resulting from the strong interactions between TiO₂ and 3D graphene. Work on this material is in progress, and the results will be presented in a future account.

Figure 6:

(A) Synthesis of covalently conjugated 3D graphene-TiO₂ hybrid material and (B) SEM image of free standing 3D graphene-TiO₂ hybrid material. Inset, high magnification.

6 Conclusions

In summary, 3D graphene-TiO₂ hybrid materials have shown their potential as highly efficient photocatalysts. Compared to 2D graphene, 3D graphene-based hybrid materials possess higher surface area, more conductivity pathways, and higher cycling stability. These properties contribute to the enhanced efficiency of 3D graphene-TiO₂ hybrid nanomaterials in photocatalysis, water and air purification, DSSC, and energy storage applications. With the development of new strategies to improve the synthesis and material design, further improvement in these properties is possible for these novel 3D graphene-TiO₂ hybrid nanomaterials.