Photocatalysis using a semiconductor and solar light is widely regarded as an ideal green technique for dealing with the global energy crisis and environmental issues [1-5]. In recent decades, many researchers in the field of photocatalysis have focused their attention on improving the photocatalytic efficiency to enable practical applications [6-10]. The photocatalytic efficiency is influenced by many factors, among which the separation and transportation of photoexcited charge carriers (electrons/holes) play crucial roles. Previous studies have shown that constructing composite semiconducting materials is an effective way of achieving good separation of photoexcited charge carriers and subsequent redox reactions on the catalyst surface, thus improving the photocatalytic efficiency [11-15].

Based on the above considerations, we developed a novel graphene/tourmaline/TiO₂ (G/T/TiO₂) composite system. TiO₂ has been widely studied as a photocatalyst for water splitting and photodegradation of organic pollutants [1-4, 16, 17] because of its chemical inertness, cost effectiveness, environmental friendliness, and stability against light and chemical corrosion [18]. However, bare TiO₂ usually has the drawbacks of limited light-absorption ability (UV only) and rapid recombination of photoinduced charge carriers, leading to a poor photocatalytic performance when the entire solar spectrum is taken into account. Because of their intrinsic stoichiometry, graphene oxide (GO) exfoliated nanosheets are anionic two-dimensional materials with large surface areas [19, 20]. This unique two-dimensional structure makes graphene an excellent catalyst support with good electronic conductivity/mobility. The use of semiconductor/graphene materials as photocatalysts with enhanced activities has been reported [21, 22]. Graphene has also been used as an efficient cocatalyst with high activity in the photodegradation of organic pollutants [23] and photocatalytic hydrogen evolution [24]. Tourmaline is a borosilicate mineral with the R3m space group [25]. The general chemical formula of tourmaline is XY₃Z₆Si₆O₁₈(BO₃)₃W₄, where X is K⁺, Na⁺, Ca²⁺, or a vacancy; Y is Mg²⁺, Fe³⁺, Al³⁺, Cr³⁺, V³⁺, or Ti⁴⁺; Z is Al³⁺, Fe³⁺, Mg²⁺, Cr³⁺, V³⁺, or Fe²⁺; and W is OH^-, O^-, or F^-. Tourmaline has a single-symmetry polar axis and shows both pyroelectric and piezoelectric properties [26]. The electrostatic field on the surface of tourmaline arises from silicon-oxygen octahedral distortion [26, 27] and its direction is parallel to the c axis, i.e., opposing charges are present at different ends of tourmaline particles. The electric field strength increases with decreasing tourmaline particle size. When the particles are micron sized, the tourmaline surface has an electric field of strength 10⁶-10⁷ V/m [25]. It is supposed that the electric field formed on the tourmaline surface plays an important role in the separation/transportation of photoexcited charge carriers, and therefore affects the photocatalytic activity of a composite semiconductor system containing tourmaline [28].

Here, we report the fabrication of G/T/TiO₂ composites using a two-step hydrothermal method. The mass percentage of each component in the composite was optimized based on the composite's performance in the photocatalytic degradation of 2-propanol (IPA). We found that the usually negatively charged surface of GO obtained by chemical delamination [20] could be modified to become positively charged [29, 30]. We therefore investigated and compared the effects of the surface charged state of GO on the photocatalytic performances of the composites. We found that the electrostatic field on the tourmaline surface played an important role in the photocatalytic activity, and the activity of the composite with negatively charged GO was higher than that of the composite with positively charged GO. The related photophysical and photochemical mechanisms were also investigated. The results of our study provide an effective method for the development of composite photocatalytic systems in which the charge carrier separation, and therefore the photocatalytic properties, can be modified by tailoring the surface charged state (e.g., GO) and/or by introducing an external field (electric or magnetic) arising from a specific component (e.g., tourmaline) in the composite.

GO was prepared from natural graphite powder using a modified Hummers method. Typically, graphite powder (3.0 g) was dropped into a beaker of concentrated sulfuric acid (120 mL), which was cooled in an ice-water bath. Then potassium permanganate (15 g) was gradually added to the mixture. After stirring in the ice-water bath for 2 h, the mixture was transferred to water at 308 K and stirring was continued for 1 h. The temperature was raised to 338 K, water (250 mL) was added, and the mixture was stirred gently for 2 h. The mixture was diluted with water to 1400 mL, H₂O₂ aqueous solution (30 wt%, 30 mL) was added, and the reaction was continued for 20 min. The suspension was centrifuged and washed with aqueous hydrochloric acid solution (10 wt%) until no sulfate ion was detected. The obtained GO was freeze dried [31].

GO was modified with poly(diallyldimethylammonium chloride) (PDDA; 20 wt% in water, M_w = (2-3) × 10⁵) as follows. GO (100 mg) and water (100 mL) were placed in a 200 mL beaker and the mixture was ultrasonicated for 2 h until the dispersion became clear, without any visible particles. The PDDA (30 mg) solution was mixed with water (100 mL). The GO suspension was slowly added dropwise, at a rate of 2 mL/min, to the PDDA solution under stirring and the mixture was stirred overnight. Excess polymer was removed by repeated centrifugation (1 × 10⁴ r/min, 10 min) with deionized (DI) water and the PDDA-modified GO (denoted by P-GO) was dried in a vacuum oven for 10 h [30].

The ternary composites were synthesized using a two-step hydrothermal method. Briefly, a mixture of GO (50 mg) or P-GO (50 mg) and tourmaline (schorl, 500 mg) was dispersed in water (26 mL). The dispersion was transferred to a 50 mL Teflon-lined stainless-steel autoclave; the autoclave was sealed tightly, and heated at 453 K for 10 h. After cooling naturally, the black-gray precipitates were collected by centrifugation, washed alternately with DI water and ethanol several times, and dried in a vacuum oven at 343 K for 4 h. The same method was used with different amounts of GO or P-GO (0, 0.1%, 0.5%, and 1% by mass) and different amounts of tourmaline (0, 1%, 5%, and 10% by mass) in the starting material solution. Graphene/tourmaline powders were obtained after washing and drying at 353 K for 10 h. The prepared graphene/tourmaline powders were mixed with tetrabutyl titanate (TBT; 1.28 mL) and dispersed in water (26 mL). The aqueous solution was transferred to a 50 mL Teflon-lined stainless-steel autoclave and heated at 453 K for 10 h. Finally, the products were washed alternately with ID water and ethanol several times. The samples were denoted by GX/TY/TiO₂ and P-GX/TY/TiO₂, where X (0-1) and Y (0-10) are the mass percentages of graphene and tourmaline, respectively. For comparison, TiO₂ powders were also synthesized using the hydrothermal method under the same conditions but without adding GO and tourmaline.

Crystal structures were determined by X-ray diffraction (XRD; D8 Advanced, Bruker, Germany) using Cu-K_aradiation (l = 0.154178 nm) at a scanning rate of 0.02°/s. Ultraviolet-visible (UV-vis) diffuse reflectance spectra were recorded at room temperature (UV-2700, Shimadzu, Japan) using BaSO₄ as a reference and converted to absorption spectra using the Kubelka-Munk method. The Brunauer-Emmett-Teller surface areas were determined using a surface area analyzer (BET-BJH-AsiQcovoo 2-4, Quantachrome, USA). The morphologies and microstructures of the samples were examined using scanning electron microscopy (SEM; S4800, Hitachi, Japan) and transmission electron microscopy (TEM; Technai G2 F20, FEI, the Netherlands). The surface charges were determined using a zeta potential analyzer (Nano ZS, Malvern, UK). During the measurements, the samples were suspended in DI water at low concentrations and the pH was kept constant at 6.5. Fourier-transform infrared (FTIR) spectroscopy was performed using an FTIR spectrometer (Nicolet6700, Thermo Electron Corporation, USA).

In the degradation of gaseous IPA, acetone is usually produced as an intermediate before complete oxidation to CO₂. IPA is therefore commonly used as a model organic pollutant for photocatalytic decomposition [32]. The photocatalytic decomposition of IPA was performed in a cylindrical static Pyrex glass vessel (total volume 500 mL). A certain amount of catalyst powder (the amount of TiO₂ in all samples was 50 mg to enable activity comparisons) was evenly dispersed on a circular glass dish to give a uniform area and the dish was mounted in the vessel, which was sealed with a quartz cover and a rubber O-ring. After carefully washing with artificial air, a drop of liquid IPA was injected into the vessel, which was kept in the dark for 3 h to achieve adsorption-desorption equilibrium. The reaction was initiated by light irradiation using a xenon lamp operated at 140 W. Gaseous samples (5 µL) were periodically extracted from the reaction vessel and analyzed using a gas chromatography system (GC-2014, Shimadzu) equipped with a flame ionization detector and a methanizer.

Fig. 1 shows the XRD patterns of TiO₂, and the G0.5/T5/TiO₂ and G0.5/T1/TiO₂ composites. The patterns all clearly show the characteristic peaks of the hexagonal anatase phase (JCPDS No.71-1166), and the introduction of graphene and/or tourmaline had almost no effect on the XRD pattern and TiO₂ crystallinity. No peaks from either graphene or tourmaline were identified because of the relatively small mass percentages of these two components in the composites.

The microstructure of G0.5/T5/TiO₂ was examined using SEM and TEM. As shown in Fig. 2(a), the SEM image of the G0.5/T5 binary composite before loading with TiO₂ nanoparticles shows that graphene was evenly decorated with tourmaline particles. The TEM image of the G0.5/T5/TiO₂ composite (Fig. 2(b)) shows that the TiO₂ nanoparticles were uniformly loaded on the graphene/tourmaline support. The high-resolution TEM image of the ternary composite in Fig. 2(c) shows a lattice spacing of 0.35 nm, corresponding to the (101) plane of anatase TiO₂ (JCPDS No. 71-1166) [24, 33], and a lattice spacing of 0.398 nm, corresponding to the (220) plane of tourmaline (JCPDS No. 85-1811). These results confirm the formation of a G0.5/T5/TiO₂ ternary composite with good crystallinity and an interfacial structure.

As mentioned above, clear diffraction peaks from graphene and tourmaline were not detected because of their relatively small percentages in the composite. However, Raman scattering is a more sensitive tool and to detect each component in the G/T/TiO₂ composite. Fig. 3 shows Raman spectra of the synthesized GO, TiO₂, and the G0.5/T5/TiO₂ composite. The spectra contain bands at 150, 397, 515, and 638 cm^-1, which can be assigned to the E_g(1), B_1g(1), A_1g + B_1g(2), and E_g(2) modes, respectively, of anatase TiO₂ [34]. Two bands, at about 1342 (D band) and 1599 cm^-1 (G band), can also be observed, proving the presence of graphene in the G0.5/T5/TiO₂ composite [35]. The ratio of the D and G band intensities (I_D/I_G) for G0.5/T5/TiO₂ was larger than that for GO, indicating reduction of GO in the composite.

Fig. 4 shows the UV-vis diffuse reflectance spectra of TiO₂, and the G0.5/T1/TiO₂ and G0.5/T5/TiO₂ composites. The main absorption edge for all three samples appears at around 370 nm, corresponding to the typical energy band gap of TiO₂ (E_g: 3.35 eV). This implies that the introduction of graphene/tourmaline into the composite did not affect the absorption range of TiO₂. However, the enlarged spectra (inset) clearly show enhanced absorption in the visible-light region for the absorption ranges of graphene ( > 500 nm) [26, 36] and tourmaline (600-800 nm) [28].

The FTIR spectra of GO and PDDA-functionalized GO in Fig. 5 show vibrations from carboxylic groups in both positively and negatively charged GO at around 1627-1631 cm^-1. This can be attributed to the absorption of water and skeletal vibrations of unoxidized graphite [37]. The absorption peak at around 1400 cm^-1 corresponds to the carboxyl bending vibration [29], and the new peak, at around 1472 cm^-1, in the PDDA-modified GO spectrum arises from N-H bond bending [30].

The surface charge properties of a material can be determined based on the zeta potential. We used the electrokinetic method to measure the zeta potentials of GO, PDDA-GO, G0.1/T1, PDDA-G0.1/T1, and TiO₂ samples in water. The zeta potentials of PDDA-GO and GO were 60 and −35 mV, respectively, implying that GO was successfully functionalized with PDDA. The measured zeta potential of GO (-35 mV) is close to that previously reported in the literature [38]. The results for G0.1/T1 (-15.3 mV) and PDDA-G0.1/T1 (26 mV) show electrostatic attraction between tourmaline and oppositely charged graphene. The zeta potential of anatase TiO₂ in water was ~28 mV, which is consistent with the previously reported value [39].

The graphene and tourmaline contents in the composite were optimized by performing photocatalytic IPA decomposition to acetone over various samples (i.e., G0.1/T1/TiO₂, P-G0.1/T1/TiO₂, G0.5/T1/TiO₂, G0.5/T5/TiO₂, G0/T5/TiO₂, and G0.5/T0/TiO₂), in designed orthogonal experiments; the results are shown in Fig. 6. After irradiation for 1 h, the amount of acetone evolved over G0.1/T1/TiO₂ (187 mmol) was higher than that over P-G0.1/T1/TiO₂ (139 mmol) (Fig. 6(a)). An electrostatic field is present on the surfaces of tourmaline particles as a result of spontaneous polarization [27] therefore the positively charged end of tourmaline can attract negatively charged graphene, and the negatively charged end electrostatically attracts TiO₂ nanoparticles, which usually have a positive surface charge [40, 41]. Conversely, when the graphene surface is positively charged, the negative end of tourmaline is attracted by graphene, leaving the positive end of tourmaline exposed. As a result, the positive TiO₂ particles are repelled. Apparently, a much closer neighborhood of the G/T/TiO₂ composite can be achieved when the graphene is negatively charged directly by the hydrothermal process. It is believed that this composite configuration favors the separation/transfer of photogenerated electrons and holes, and this enhances the photocatalytic activity in IPA degradation.

Fig. 6(b) shows IPA degradation over ternary composites with different amounts of graphene. The amount of tourmaline was 1 wt%. Pure TiO₂ gave a low acetone evolution rate (148 mmol/h) because of easy electron-hole pair recombination. The inclusion of graphene, i.e., in the G/T1/TiO₂ hybrid, improved the IPA degradation activity. The optimum composition was G0.5/T1/TiO₂, which gave the maximum acetone evolution (209 mmol) after irradiation for 1 h. Further increases in the amount of graphene led to decreased activity because the introduced graphene blocked the light incident on TiO₂. We also adjusted the weight content of tourmaline in the composite from 0 to 10 wt%, with a graphene content of 0.5 wt%. Fig. 6(c) shows that the composite with 5.0 wt% tourmaline gave the highest acetone evolution rate (223 mmol/h). The acetone evolution rate decreased with increasing tourmaline content because too much gray tourmaline reduced light absorption by TiO₂.

Generally, the photocatalytic activity of a semiconductor is affected by several parameters, e.g., the surface area, crystallinity, absorbance, and the presence of an electric field. The full width at half maximum (FWHM) of the XRD pattern can be used as a semi-quantitative index of the crystallinity. Our measurements showed that all the composite samples had similar surface areas (~160 m²/g) and FWHM values (~1.26), regardless of their compositions. These results together with the UV-vis spectra (see Fig. 4) imply that the surface area, crystallinity, and absorbance were not the key factors accounting for the different activities in IPA photodegradation. Fig. 6(d) shows that the activity of the ternary composite G0.5/T5/TiO₂ was higher than that of the binary systems G0.5/TiO₂ and T5/TiO₂, or TiO₂ alone. It should be noted that no obvious decrease in the activity was observed during long time periods and repeated reactions, confirming that the composites were stable in photocatalytic reactions under the current experimental conditions.

The enhanced photocatalytic activity of the G/T/TiO₂ composite can be attributed to the synergetic effect of tourmaline and graphene, which facilitates separation and transportation of the electrons and holes photogenerated in TiO₂. This is supported by the photoluminescence (PL) intensity decrease and the slight blue shift of the photoluminescence peak for the composite compared with those for TiO₂ (see Fig. 7), indicating photoexcited electron transfer from TiO₂ to graphene and/or tourmaline. A similar phenomenon has also been observed for two nanocomposite systems consisting of PbS quantum dots and carbon nanotubes [12] or TiO₂ nanobelts [13].

To investigate the mechanism of IPA photodegradation over G0.5/T5/TiO₂, we used t-butyl alcohol (TBA), benzoquinone (BQ), and ammonium oxalate (AO) as scavengers of the species for •OH, O₂^•-, and holes, respectively. Fig. 8 shows that the introduction of TBA does not affect the degradation rate, but the presence of BQ and AO significantly decreases the IPA photodegradation rate. These observations imply that O₂^•- and holes are the main reactive oxygen species in IPA photodegradation, and the holes are less important than O₂^•- in the photocatalytic process. Based on these results, we propose that IPA degradation over the G0.5/T5/TiO₂ composite mainly involves the following three processes: (1) electrons and holes are excited from the semiconductor upon irradiation; (2) electrons combine with O₂ molecules to produce O₂^•- radicals, which oxidize IPA to acetone; and (3) holes directly oxidize IPA to acetone [42, 43].

Scheme 1 shows the proposed reaction mechanism for IPA photodegradation over the G0.5/T5/TiO₂ composite. On irradiation, electrons are generated in TiO₂ and transferred to the surface. In a G/TiO₂ system, the electrons generated in TiO₂ are rapidly injected into graphene, which has a slightly lower potential. The superior electronic conductivity of graphene effectively suppresses the recombination of electrons and holes generated in TiO₂ because the electrons and holes migrate in opposite directions, driven by the electric field on tourmaline [26], i.e., the electrons diffuse to the positively charged end of tourmaline and the holes migrate toward the negatively charged end of tourmaline. During this process, electrons are transferred to graphene and combine with a molecule of O₂ to produce the O₂^•- radical, and then the O₂^•- radical oxidizes IPA to acetone [41]. In addition, some TiO₂ nanoparticles are present on the tourmaline surface, and the photogenerated electrons can reduce O₂ to form O₂^•- radicals, which oxidize IPA on the surfaces of TiO₂ and tourmaline. The composite can effectively suppress the recombination of electron-hole pairs generated in TiO₂ in three ways, and can therefore prolong the lifetimes of the charge carriers and improve the photocatalytic activity.

G/T/TiO₂ composites and PDDA-G/T/TiO₂ composites were prepared using a simple two-step hydrothermal method. In the photocatalytic degradation of IPA, the performance of the ternary composite (G/T/TiO₂) was better than that of the binary systems (G/TiO₂, T/TiO₂) or TiO₂ alone. The optimum ternary composite composition was G0.5/T5/TiO₂ (0.5 wt% graphene and 5 wt% tourmaline), over which the highest acetone evolution rate (223 mmol/h) was achieved. The enhanced photocatalytic activity of the G/T/TiO₂ composites can be ascribed to the synergetic effect of tourmaline and graphene, which facilitates separation and transportation of electrons and holes photogenerated in TiO₂. A mechanistic study indicated that O₂^•- and holes were the main reactive oxygen species in photocatalytic degradation of IPA. The results of this work show that for a composite system, the charge carrier separation, and hence the photocatalytic properties, can be effectively modified by tailoring the surface charged state and/or by introducing an external field (electric or magnetic) via material design.