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Kinetics, Catalysis, and Reaction Engineering

The enhanced removal of toxic Cr(VI) in wastewater by synthetic TiO2/ g-C3N4 microspheres/rGO photocatalyst under irradiation of visible light Guoying Li, Yaohui Wu, Meng Zhang, Bingxian Chu, Wen-Yi Huang, Minguang Fan, Lihui Dong, and Bin Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05990 • Publication Date (Web): 07 May 2019 Downloaded from http://pubs.acs.org on May 8, 2019

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Industrial & Engineering Chemistry Research

The enhanced removal of toxic Cr(VI) in wastewater by synthetic TiO2/g-C3N4 microspheres/rGO photocatalyst under irradiation of visible light Guoying Lia, Yaohui Wua, Meng Zhanga, Bingxian Chua, Wenyi Huangc, Minguang Fan*ab, Lihui Donga, Bin Li*a aKey

Lab of Petrochemical resource processing and the process strengthening technology, College

of Chemistry and Chemical Engineering, Guangxi University, Nanning 530004, PR. China bGuangxi

Colleges and Universities Key Lab of Applied Chemistry Technology and the Resource

Development, Guangxi University, Nanning 530004, PR. China cCollege

of Biological and Chemical Engineering, Guangxi University of Science and Technology,

Guangxi Key Lab of Green Processing of Sugar Sources, Liuzhou 545006, China

* Co-author: E-mail address: [email protected] (Minguang Fan) and [email protected] (Bin Li)

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Abstract Herein, the cost-effective photocatalyst of T-CNS-G was synthesized through a simplified two-pot hydrothermal procedure and its structure and characteristics were researched by various characterizations. The as-prepared T-CNS-G nanohybirds exhibited obviously enhanced photocatalytic activities (about 90% after 4 hour) for Cr(VI) removal and good cycle stability (5 cycles), which was irrsdiation by visible light. The increased photocatalytic activity of T-CNS-G photocatalysts is the result of the positive synergy of TiO2 nanoparticle, g-C3N4 microspheres and rGO nanosheets. The synergy made the composite catalyst have a larger specific surface area, stronger visible light absorption capacity, lower fluorescence intensity and higher photocurrent intensity, thereby increasing the active site of the catalysts and the mobility of photoinduced carriers. In addition, the possible photocatalytic reaction mechanism for a high-efficiency removal of Cr(VI) was proposed by combining the


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theoretical calculation with experimental test, which show that photoginduced electrons reduced Cr(VI) to Cr(III) through photocatalytic reaction.

Keywords: TiO2/g-C3N4 microspheres/rGO; Cr(VI) removal; Photocatalysis; Synergetic effect.

1. Introduction The problem of heavy metal pollution has received extensive attention because it can accumulate in living organisms and seriously endanger human and animal health [1-3]. As a heavy metal, Hexavalent chromium (Cr(VI)) can lead to a variety of diseases, such as lung cancer, chromate ulcer, nasal septum perforation, which are also toxic to other organisms [4,5]. Compared with toxic Cr(VI), trivalent chromium (Cr(III)) has reduced toxicity and fluidity as micronutrients in living organisms [6]. Therefore, it is a very ideal idea to convert Cr(VI) into Cr(III) in wastewater. The removal of Cr(VI) by semiconductor photocatalytic technology is increasingly favored by researchers. Because of its simple operation, lower cost, high efficiency and high repeatability, it is considered a promising method for removing Cr(VI) [7,8].



Currently, researchers are still looking for effective and low-cost visible light photocatalysts to remove Cr(VI) by photocatalytic technology. Semiconductor TiO2 has been known as a promising photocatalyst due to some of its qualities; abundance, lower cost, large surface area, non-toxicity, superior photostability and so on [9,10]. However, previous studies have shown that due to its wide band gap (about 3.2 eV), titanium dioxide can only absorb ultraviolet light (about 4% of solar energy) [10,11]. Therefore, most of the light sources used in the current research on TiO2 photocatalytic removal of Cr(VI) are ultraviolet or sunlight. In recent years, in order to improve the utilization of TiO2-based catalysts for solar energy, researchers have made considerable efforts in extending the photoresponse of TiO2-based catalysts to the visible region, such as non-metal doping, metal doping, precious metal deposition, structural heterojunction and so on [12-16]. Studies have shown that forming a heterojunction by combining TiO2 with other semiconductors with narrow band gaps is a resultful method to increase the solar utilization of TiO2based catalysts [11,16,17]. At present, g-C3N4 has been widely researched and concerned for its low toxicity, low cost, visible-light driven band gap, long-term stability [18-20]. Unfortunately, the photoreaction activity of pure g-C3N4 is still unsatisfactory because it is affected by the weak van der Waals force between two adjacent planes, making the transfer process of photogenerated carriers slow down, resulting to the electron-hole pairs recombining at a high rate [21,22]. At present, a lot of attempts have been made by researchers to design the required g-C3N4-TiO2 composites, since they have a structure overlapping with a matching, which facilitates the separation and transfer of photoginduced carriers, improving the photocatalytic activity of visible light. [23,24]. However, the photocatalytic reaction efficiency of the binary catalyst still needs to be enhanced for practical applications because the photogenerated electron-hole pairs are still partially recombined at the two semiconductor interfaces [25,26]. Hence, it is urgent to improve the transfer of charges kinetics of g-C3N4-TiO2 binary reduction of the accelerated photoreaction system to increase the photoreaction efficiency. To further promote photo-generated carrier separation and transfer in binary system, it has been developed to construct a ternary system by combining a conductive material, a cocatalyst and another semiconductor [27-29]. In these ternary nanocomposites, synergetic effects induced multi-step charge transfer, which


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significantly improved photocatalytic activity. Graphene(G) has received extensive attention because of its qualities of surface area which is high, electrical conductivity which is quite good and electron mobility [30]. In theory, graphene (G) or redox graphene (rGO) can not only increase the system’s s urface area, but also increase its surface adsorption and reaction kinetics, thereby significantly improving reduction of the accelerated photoreaction activity [31,32]. Unfortunately, up to now, photocatalytic removal of Cr(VI) by TiO2/g-C3N4/rGO composites has hardly been reported. Therefore, our research is focused on the photocatalytic removal of Cr(VI) to less toxic Cr(III) using TiO2/g-C3N4/rGO photocatalyst. The TiO2/g-C3N4 microsphere/rGO (T-CNS-G) hybrids with high photocatalytic removal of Cr(VI) activity were fabricated through a simplified two-step hydrothermal method. g-C3N4 microsphere (CNS) was obtained via hydrothermal reaction. The g-C3N4 microsphere and the precursor of TiO2 (T) were simultaneously distributed on the platform of graphene nanosheet by the second hydrothermal step. We predict g-C3N4 has more suitable band gap energy in the whole system, which can widen the range of light response of the entire system, thereby improving photocatalytic reduction of chromium activity of the system. Besides, for the spherical g-C3N4 3D three-dimensional structure, can serve as a framework in the system, so that TiO2 and rGO do not accumulate agglomerate, the TiO2 particles with better dispersion in the rGO and g-C3N4 microsphere surface. Due to the high electron mobility of rGO which provide conductive pathways, the multi-step electron transfer process can be successfully induced.

2. Experimental section 2.1. Preparation of g-C3N4 microsphere (CNS) The synthesis of CNS refered to the method of Cui [33]. 15 mmol of 1,3,5trichlorotriazine and 7.5 mmol of dicyandiamide were placed in a 100 mL autoclave of polytetrafluoroethylene, then poured 60 mL of acetonitrile. After stirring the mixture for 12 hours, then transferred to a blast drying oven at 200 ºC for 48 hours. After the reaction vessel was naturally cooled, the solid after the reaction was washed several times with ethanol and distilled water, dried in a blast oven at 80 °C for 12 hours, and then ground to obtain g-C3N4 microsphere, which was designated as CNS.



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2.2. Preparation of graphene oxide (GO) GO was prepared refered to Hummers’ method [34]. Concretely, weigh 1 g of NaNO3 and 2 g of graphite powder into the beaker, and then poured 46 mL of 98% H2SO4 and stirred. Next, 6 g of KMnO4 was slowly added under stirring to control the solution temperature below 15 ° C. Further, the mixture was heated to 38 ° C and stirred for 6 hours, and then slowly added with 92 mL of distilled water. The mixture was then transferred to an oil bath at 98 ° C for 15 minutes. After the end of the previous operation, 280 mL of water and 6 mL of 30% H2O2 were added to the mixture and stirred. Finally, it was washed three times with 5% HCl and water. After drying at 60 °C for 24 hours in a blast drying oven, it was ground to obtain GO.

2.3. Synthesis of TiO2/g-C3N4 microsphere/rGO (T-CNS-G) hybrids This method has been improved compared to the study by Huang et al [100]. Typically, 1 mg of GO was sonicated for 3 hours in 40 mL of absolute ethanol, then 10 mg of CNS and 0.445 mL of tetrabutyl titanate were weighed into the above solution, and stirred vigorously for 30 minutes. Thereafter, 0.5 mL of 40wt% hydrofluoric acid was added to the mixture under continuous stirring. The mixture was poured into a 100 mL dry Teflon autoclave and incubated at 180 ° C for 12 hours in an oven. After the reaction was completed, the reaction vessel was allowed to cool, and the sample was washed three times with deionized water and ethanol. The sample was collected by grinding after drying in a blast drying oven at 80 °C for 12 hours. A similar hydrothermal process was used to prepare Pure rGO (G), TiO2 (T), TiO2/rGO

(T-G),

g-C3N4/rGO

(CNS-G)

and

g-C3N4/TiO2/rGO

(T-CNS-G)

complexes.

2.4. Characterization In order to investigate the composition and the structure of internal atoms or molecules of materials, X-ray diffraction spectroscopy (XRD) was performed on the catalysts. The XRD instrument is a Max-2600 X-ray diffractometer from Japan, Rigaku, with a range of 10 to 80° in 2θ and monochromatic Cu Kα radiation. In this paper, the surface morphology of the catalysts were observed by a Scanning Electron Microscope (SEM) of Hitachi S-3400, and the test voltage was 10 kV. The submicroscopic structure of the catalysts were investigated by Transmission Electron


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Microscopy (TEM) on a FEI TF20 type device. To investigate the atomic valence state and elemental composition of the catalysts, X-Ray photoelectron spectroscopy (XPS) measurement was performed on a Thermo ESCALAB 250XI X-ray photoelectron spectroscope. The pore size distribution and Brunauer-Emmett-Teller (BET) specific surface area of the catalysts were tested using a TriStar II Model 3020 nitrogen adsorption/desorption apparatus from Micron, USA. Nitrogen was the adsorbate and the pretreatment temperature was 200°. Ultraviolet-visible diffuse reflectance spectroscopy (UV-vis/DRS) was performed on a Shimadzu’s TU-1901 ultraviolet spectrophotometer. The fluorescence spectrum (PL) was carried out by using an excitation wavelength of 315 nm in a F-4600 FL spectrophotometer. The photocurrents of catalysts were tested by using a standard three-electrode cell with Pt wire as the counter electrode and Ag/AgCl as reference electrode in NaSO4 solution. Electrochemical

impedance

spectroscopy

(EIS)

was

obtained

using

an

electrochemical workstation.

2.5. Photocatalytic activity test The effects of different photocatalysts on the concentration of Cr(VI) ions under irradiation of visible light were tested to understand their photocatalytic activity. A 50 mg of photocatalyst was weighed and added to 100 mL of a 100 mg·L-1 K2Cr2O7 aqueous solution (pH=3). The solution was continuously stirred for 1 hour in a lightfree environment to achieve adsorption equilibrium, which was then irradiated with a 300 W Xe lamp in a photoreaction apparatus with a UV cut filter. Every 1 hour, 10 ml was withdrawn from the reaction solution using syringe, and the reaction system was in room temperature (25 °C). All samples were made on the high-speed centrifuge to 8000 r/min and centrifuged 5 min, to diphenylcarbazide hydrazine color developing agent, with Cr(VI) ion concentration was surveyed at a wavelength of 540 nm by spectrophotometry. In the cycle test experiment, after each photocatalytic reaction, the solution was allowed to stand apart from the catalyst by standing, deionized water was used to wash the separated photocatalyst carefully to use it again. In this paper, a photocatalyst with excellent photocatalytic activity was subjected to a masking experiment of active factors to identify the main active factors in the photocatalytic reaction process, aimed at exploring the reaction mechanism of decrease Cr(VI). In addition to adding hole and electron scavengers to the reaction, the procedure were same to the reduction of the accelerated photocatalytic activity measurements.



3. Discussion of the Results 3.1. Catalysts characterization results In order to investigate the composition and structure of internal atoms or molecules of photocatalysts, Figure 1 shows the XRD patterns of the synthesized photocatalysts. For rGO, a broad diffraction peak around 24.5º is observed, which means that GO has been reduced to rGO [36].The strong peak of g-C3N4 at 27.5º belongs to the interlayer stack reflection of the conjugated aromatic system, and the graphite material is assigned to the (002) plane [37]. For pure TiO2, the characteristic peaks at about 25.3, 37.8, 48.1, 54.4, 55.0, 62.7 and 75.0º can be easily specified as (101), (004), (200), (105), (211), (204), (215) crystal faces of anatase phase TiO2, respectively, and the results are consistent with the data of the standard card number JCPDS#21-1272. The T-CNS, T-G and T-CNS-G composites have similar diffraction patterns to TiO2. Due to its low content, no rGO peak was observed in the T-G and T-CNS-G photocatalysts, and the absence of g-C3N4 peak in the T-CNS and T-CNS-G photocatalysts is due to its low crystallinity and high dispersion on the rGO platform. The results suggest that the loading of rGO and g-C3N4 does not affect the orientation and structure of anatase TiO2. In addition, the intensity of (004) peaks in T-CNS and T-CNS-G are distinctly broader and weaker than pure TiO2. The results show that gC3N4 microsphere act as a dispersant for the construction of the T-CNS-G system, which allowed the TiO2 nanoparticles to be better dispersed on the surface of rGO flakes and g-C3N4 microspheres. A similar situation occurred in T-G and CNS-G, indicating that the reduced graphene also has the function of dispersing TiO2 particles and g-C3N4 spheres. In order to certify the existence of reduced graphene in the T-CNS-G photocatalyst, the Raman spectroscopy (Figure S1) of GO, rGO and T-CNS-G composite catalysts was carried out in this paper. For GO, the D and G peaks at 1339 and 1583 cm-1 can be found, where the D peak is designated as the vibration of the sp3 carbon atom of the disordered graphite, and the G peak is due to the plane vibration of the sp2 carbon atom. From the data, the intensity ratio of the D/G peak of the GO nanosheet was calculated to be 1.01, and the ID/IG values of rGO and T-CNS-G were increased to 1.11 and 1.08, respectively. The increase in the ID/IG value of rGO and T-CNS-G is related to the reduction of GO to rGO, which further indicates that GO has been


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reduced to rGO by hydrothermal method in this study and GO in synthetic T-CNS-G complex Restored to rGO.

Figure 1 XRD patterns for all samples

To further explore the morphologies and microstructures of the synthesized photocatalysts, Figure 2 show the low magnification SEM images of TiO2, g-C3N4, T-CNS and T-CNS-G. As can be seen from Figure 2(a), TiO2 processes small nanoparticle agglomeration which obtained by a simplified one-step hydrothermal procedure. In Figure 2(b), the SEM image indicates that the g-C3N4 product obtained from the solvothermal process without template participation are well-defined microspheres with smooth surface, but does not exclude a small part of irregular morphology. As shown in Figure 2(c), the TiO2 portion can be attached to the g-C3N4 microsphere and partially dispersed in the g-C3N4 gap, which show that the agglomerated TiO2 is dispersed by the g-C3N4 microsphere, and the result is similar to the XRD results. The experimental results show that there is no effect on the g-C3N4 microspheres after the second hydrothermal reaction. In addition, from the SEM image of T-CNS-G photocatalyst, it can be observed that there are a large number of uniformly dispersed TiO2 nanoparticles and g-C3N4 microspheres on the rGO platform, indicating that the loading of graphite by the second step of hydrothermal treatment does not affect morphology of the T-CNS photocatalyst. The simultaneous construction of TiO2 nanoparticle, rGO nanosheets and g-C3N4 microspheres in the TCNS-G system facilitates the smooth transfer of photoinduced carriers on the surface of the semiconductors, accelerating the progress of photocatalytic oxidation-reduction



reactions.

Figure 2 SEM images for TiO2 (a), g-C3N4 (b), T-CNS (c) and T-CNS-G (d)

Figure 3 show the TEM of as-prepared TiO2, g-C3N4 microspheres, T-CNS and TCNS-G. From Figure 3(a), It is obviously demonstrates that TiO2 consists of many nearly transparent and irregular nanoparticles. In Figure 3(b), g-C3N4 is of a solid spherical shape with different sizes, and did not exclude impurities. In Figure 3(c), coexistence of TiO2 nanoparticles and g-C3N4 microspheres can be observed simultaneously in the T-CNS composites. From Figure 3(d), it can be observed that a large number of TiO2 are loaded onto the surface of g-C3N4 microspheres and rGO nanosheets, and the results are similar to the SEM results. From the HRTEM image of T-CNS-G composite (Figure 3(e)), the edges of the rGO and g-C3N4 layers were observed, and most of the g-C3N4 was covered with a large amount of TiO2 nanoparticles. It is also observed in the figure that the lattice fringes having a interplanar spacing of 0.35 nm is belong to the interplanar spacing of the (101) crystal plane of TiO2. The above results indicate that the T-CNS-G photocatalyst has been synthesized through a simplified two-pot hydrothermal procedure in this paper.


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Figure 3 TEM images for TiO2 (a), g-C3N4 (b), T-CNS (c) and T-CNS-G (d), HRTEM image for TCNS-G (e)

To investigate the atomic valence state and elemental composition of the T-CNS-G photocatalyst, the XPS spectrum of the T-CNS-G composite was measured, as shown in the Figure 4. The spectrum of C 1s (Figure 4(a)) can be divided into four peaks with binding energy (BE) of 283.5, 284.2, 285.3 and 287.1 eV. Among them, the peak at BE=283.5 eV generally belongs to a chemical bond connecting C and Ti atoms (CTi) in the TiO2 lattice, whereas the peak appearing at 284.2 eV can be attributed to sp2 C-C bonds of rGO [39]. The other two peaks locate at 285.3 and 287.1 eV are ascribed to C-N and C-O-C backbone in the g-C3N4 [40], respectively. In Figure 4(b),



the three peaks at the BE of 397.9, 398.7 and 400.2 eV in the N 1s spectrum correspond to the sp2 hybrid aromatic N (C-N=C), the tertiary nitrogen N-(C)3 and CN-H groups, respectively[41].The strong peaks appear at the BE of 457.6 and 463.3 eV in the Ti 2p spectrum (Figure 4(c)) are attributed to Ti 2p3/2 and Ti 2p1/2 respectively, indicating that the valence of the Ti element in the T-CNS-G composite is tetravalent [42]. Form Figure 4(d), the O 1s high-resolution spectrum showed obvious three peaks at BE of 528.8, 530.3 and 531.1 eV, which assign to C-O-Ti groups, N-C-O groups and O-H bond [27,43], showing that during the hydrothermal treatment is when the oxidation reactions occur. The XPS spectrum of T-CNS-G indicates that g-C3N4, TiO2 and rGO coexist, and there are some strong interactions between them.

Figure 4 high-resolution XPS spectrum of the T-CNS-G sample: C1s (a), N1s (b), Ti 2p (c) and O1s (d)

In Figure 5, the pore size distribution curves and N2 adsorption isotherms are presented in the study of the structural characteristics of the materials preparedas. Form Figure 5(a), the N2 adsorption isotherms curves of TiO2, CNS, T-CNS and T-


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CNS-G composites exhibit the type IV adsorption characteristic and H3 type hysteresis loops, corresponding to the mesoporous structure [44]. However, g-C3N4 nanospheres exhibit typical type II adsorption characteristics, indicating that nonporous features are combined with morphological results [45]. Figure 5(b) is a plot of the pore size distribution of the obtained photocatalyst, but we could not observe the BJH pore size distribution peak of g-C3N4 nanospheres may be related to its nonporous layered structure characteristic. The figure shows that TiO2, T-CNS and TCNS-G have significant pore size distribution peaks between 2 and 10 nm, which further indicates the formation of mesopores. Figure 5(b) shows the BET surface areas of the pristine CNS, TiO2, T-CNS, and T-CNS-G are determined to be ca. 8.9, 149.2, 152.7, and 178.6 m2/g, respectively. Obviously, the BET surface area of TiO2 composed of nanoparticles is much larger than that of g-C3N4 microspheres. The BET surface area of T-CNS is larger than one-component catalyst due to the presence of TiO2 in the composites which are dispersed by the g-C3N4 microspheres. The maximum surface area exhibited by the T-CNS-G composites is mainly the result of the interaction between the rGO nanosheets, the g-C3N4 nanospheres and TiO2 nanoparticles.

Figure 5 N2 adsorption-desorption isotherms curves (a) and the pore size distribution curves (b) of TiO2, CNS, T-CNS and T-CNS-G composite, respectively

The UV-vis/DRS is shown in Figure 6 to investigate the light absorption properties of the prepared materials. The pure TiO2 exhibits an absorption wavelength at around 405 nm, which is related to the band gap of anatase TiO2, indicating that it can only respond to ultraviolet [46]. For g-C3N4 prepared by hydrothermal method, it has



strong absorption in visible light, which is similar to other studies [33,37]. , The absorption edges of T-CNS and T-CNS-G photocatalysts red-shifted apparently to visible light region, compared with pure TiO2. Moreover, compare with T-CNS photocatalysts, the absorption edge of the T-CNS-G photocatalysts is red-shifted toward a higher wavelength direction. The above phenomenon indicates that both rGO and g-C3N4 contribute to the visible light absorption of the TiO2-based catalyst.

Figure 6 UV-vis/DRS of all samples

In Figure S2, the Eg of g-C3N4 and TiO2 were 1.56 and 3.20 eV, respectively, which can be estimated from the intercept of the tangents to the plots of ( hv)1/2 vs photon energy [47-50]. The results indicate that the Eg of g-C3N4

prepared by the hydrothermal process is much narrower than that obtained by calcination. PL spectra were obtained (Figure 7(a)) to research the recombination of photoinduced carriers in the photocatalyst. Theoretically, the photocatalyst has a low PL intensity and its photo-generated carrier recombination rate is low, so it has the high photoreaction activity [51,52]. In Figure 7(a), all photocatalysts show strong emission peaks centered on the wavelength of light of about 450 nm. Compared with CNS, T-CNS and T-CNS-G exhibit PL emission peaks with lower intensities, indicating that the combination with graphene and g-C3N4 microsphere plays a important role in inhibiting photoinduced carrier recombination. The T-CNS-G


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photocatalysts exhibits the lowest PL intensities because of the rGO with high electrical conductivity and electron mobility, thus exhibiting the highest photocatalytic activity [53,54].

Figure 7 Photoluminescence spectra (a), EIS spectra (b) of T, CNS, T-CNS and T-CNS-G

We performed a photocurrent response test (Figure S3) and electrochemical impedance spectra (EIS) (Figure 7(b)) of T, CNS, T-CNS and T-CNS-G under irradiation of visible light to further understand the recombination of photoinduced carriers. The photocurrent increases rapidly after the start of the visible light irradiation, and the photocurrent quickly disappears when the light source is turned off. Both pure TiO2 and pure g-C3N4 samples show very low photocurrent intensity, which is because the TiO2 wide band gap affects its absorption of visible light., while the fast recombination of g-C3N4 photoinduced carriers leads to short electron survival time. However, compared TiO2 and g-C3N4 with T-CNS photocatalysts , TCNS photocatalysts showed a significant increase in photocurrent intensity, which confirmed the efficient transfer of photoelectrons between TiO2 and g-C3N4 and the longer lifetime of photoinduced carriers. The T-CNS-G composite exhibited the strongest photocurrent intensity, showing that loading rGO can further suppress photoinduced carriers recombination. Furthermore, the EIS in Figure 7b indicates that the radius of the Nyquist circle of T-CNS-G is smaller than the radius of g-C3N4 and T-CNS, indicating that T-CNS-G photocatalysts has a lower charge transfer resistance. The above results all indicate that the interactions between TiO2 nanoparticle, g-C3N4 microspheres and rGO nanosheets reduce the interface charge transfer resistance, so that photoinduced carriers are efficiently transferred and separated.



3.2. Photocatalytic activity and stability of the photocatalysts The prepared materials were used for photocatalytic removal of Cr(VI) under the irradiation of visible-light to evaluate their photocatalytic performance, as shown in Figure 8(a). C/C0 value of the sample over time has decreased to some extent, especially the C/C0 value of T-CNS-G composite decreases the maximum amplitude, which shows that the removal rate is highest in the presence of T-CNS-G photocatalysts. The photocatalytic removal of Cr(VI) rates for TiO2, CNS, T-CNS, TG, and CNS-G samples were 21% ,40%, 63%, 45%, 69% at 4 hour, respectively. Apparently, both g-C3N4 and rGO promoted the photocatalytic reaction under irradiation of visible light as we expected. Furthermore, the photocatalytic removal rates of Cr(VI) for T-CNS-G photocatalyst increased to 97% after 4 h visible-light irradiation. In Figure S4, the photoreaction activity comparison test of the hydrothermally prepared T-CNS-G samples and the physically mixed T-CNS-G(M) samples is shown. From the Figure S4, the photocatalytic reaction activity of T-CNSG samples prepared by hydrothermal method is obviously higher than that of T-CNSG samples prepared by physical mixing. The results clearly show that the increase of photocatalytic reaction activity is mainly because of the positive synergy of rGO, TiO2 and g-C3N4, which broadens the range of visible-light response and improves the efficiency of photoinduced carriers separation.

Figure 8 Reaction profiles of reduction of the photoreaction removal of Cr(VI) under irradiation of visible light (a) and photodegradation kinetics over the as-prepared samples (b)


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Study on the kinetics of photocatalysts removal of Cr(VI) by apparent first-Order C  model [55]: ln  0   Kt Where K represents rate constant. Figure 8(b) shows the C  . C  ln  0  linear relationship between  C  and t, in which the K values of the pristine TiO2,

CNS, T-CNS, T-G, CNS-G and T-CNS-G photocatalysts are 0.047, 0.11, 0.215, 0.110, 0.246 and 0.689 h-1, respectively. This result reveals that the T-CNS-G exhibited the highest removal efficiency of Cr(VI) in all photocatalysts, which is approximately 10 times and 3 times in contrast with pristine TiO2 and CNS, respectively. Photocatalysts stability is also an important indicator of practical application. Therefore, Figure 9 shows the recycling of photocatalytic removal of Cr(VI) for TCNS-G composite under the irradiation of visible light. The photocatalytic removal activity of Cr(VI) in the presence of T-CNS-G composite was not significantly reduced after repeated for 5 times, indicating that the T-CNS-G composite has excellent stability in photocatalytic reaction to remove Cr(VI). In addition, Figure S5 shows the XRD patterns of T-CNS-G composite before and after the photocatalytic reaction cycle for five times. The results showed that the XRD spectra of T-CNS-G did not change obviously before and after the cyclic reaction, which indicated that TCNS-G composites possess excellent stability which it becomes the cost-effective sunlight-driven photocatalyst as we are looking for.

Figure 9 Reusability study of T-CNS-G nanocomposite for reduction of the accelerated photoreaction removal of Cr(VI) under irradiation of visible light



3.3. Possible photoreaction mechanism In order to identify the product of Cr(VI) removal in T-CNS-G composite after visible light irradiation, the XPS spectrum of the T-CNS-G composite after photocatalytic reaction was tested. In Figure S6, the Cr 2p signal can be divided into three peaks at BE = 577.0, 579.3 and 581.9 eV. The peaks appearing at BE = 577.0 and 578.7 eV correspond to the surface Cr(III) 2p3/2 [56], while the peak locates at 581.9 eV correspond to Cr(VI)2p3/2. The Cr 2p spectrum indicates that the surface of the T-CNS-G photocatalysts is mainly Cr(III) adsorbed by visible light photocatalytic reaction in the presence of T-CNS-G photocatalysts. Therefore, the removal of Cr(VI) by photocatalytic technology is mainly achieved by the reduction of Cr(VI) to Cr(III) on the surface of photocatalysts. To determine the reaction mechanism of photocatalytic reduction of Cr(VI) by TCNS-G Compositesof, we explored possible active substances based on theoretical and experimental analysis. We determined the active species of T-CNS-G composites photocataytic reduction Cr(VI) process by adding e- scavengers (K2S2O8) and h+ scavengers (formic acid, FA) in the light experiment. As shown in Figure 10(a), It can be observed that the addition of FA further improved the removal efficiency of Cr(VI), while the photocataytic removal activity decreases largely as K2S2O8 were added (Figure 10(b) also gives this result). The comparison results demonstrate that e- are the main active factors in the photocataytic removal of Cr(VI) by T-CNS-G composites.

Figure 10 Reduction of the accelerated photoreaction removal of Cr(VI) by T-CNS-G with the addition of electron and hole scavengers and their photodegradation kinetics curves (b)


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The conduction band (CB) potentials of TiO2 and g-C3N4 were calculated to be 0.29 and -0.56 eV, respectively, and their valence band (VB) potentials were 2.91 and 1.00 eV, respectively. In the T-CNS-G integrated system, g-C3N4 has more suitable conduction band energy for TiO2 according to the theoretical calculation. g-C3N4 can be excited by visible light because it has a narrow band gap. When visible light is irradiated on the T-CNS-G composite photocatalyst, photoinduced electrons are generated on the g-C3N4 CB, and photoinduced holes are generated on the VB. Since the Fermi level of rGO (-0.08 eV [60]) and the CB potential of TiO2 are more positive than g-C3N4, the e- in the g-C3N4 CB are rapidly transferred to the TiO2 CB by the action of the built-in electric field. Since rGO having high electron mobility and conductivity provides a conductive path, the e- in the TiO2 CB can be further migrated to the surface of rGO, enabling multi-step electron transfer. Furthermore, according to the conclusions of SEM and TEM, e- in the g-C3N4 CB can also be directly transferred to the surface of RGO. These two ways of transferring electrons are beneficial to accelerate the transfer of photoinduced electrons, thereby accelerating the reduction of Cr(VI) to Cr(III). In addition, since the VB potential of TiO2 is more positive than the potential of g-C3N4, h+ remains on the g-C3N4 VB, and h+ interacts with water to form O2 and H+. Therefore, the reaction mechanism of photocatalytic removal of Cr(VI) in T-CNS-G system is shown in Figure 11 and the corresponding photocatalytic removal of Cr(VI) reaction equation may be expressed by the following equation (5) (7) [61,62]. (5) (6) (7)



Figure 11 Mechanism diagram of photocatalytic removal of Cr(VI) in T-CNS-G system

4. Conclusions In this paper, a cost-effective visible-light responsive T-CNS-G photocatalyst has been fabricated using a simplified two-pot hydrothermal procedure. The obtained TCNS-G photocatalysts exhibited superior photocatalytic activity and good reusability even after 5 cycles for removal of Cr(VI) under irradiation of visible-light. XPS analysis after photocatalytic reaction show that Cr(VI) was reduced to less harmful Cr(III) on the surface of T-CNS-G catalyst after irradiation of visible light. Accroding to other characterization results, the T-CNS-G nanocomposite exhibited the largest BET surface area, wider visible light response range and lower PL intensity, compared with other samples. These results demonstrate that the improvement of photoreaction efficiency of T-CNS-G composites attribute to the positive synergetic effects of three components: (a) g-C3N4 microspheres acted as a dispersant for the construction of the T-CNS-G system, which allowed the TiO2 nanoparticles to be better dispersed on rGO flakes and the surface of g-C3N4 microspheres; (b) rGO and g-C3N4 enhanced visible light absorption performance of TiO2-based composites and improved the separation efficiency of photoinduced carriers; (c) due to the high electron mobility of rGO which provided conductive pathways, two electron transfer processes was successfully induced. This study shows that designing TiO2-based composites by combining multiple semiconductors with narrow band gaps can improve the photoreaction activity under irradiation of visible light.

Support Information Supporting information provided by the http://pubs.acs.org or author.


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Acknowledgement This work was greatly supported by the Natural Science Foundation of Guangxi Province (No.2015GXNSFAA139027) and State Key Laboratory of Guangxi Petrochemical Resource Processing and Processing Strengthening Technology (No. 2017k007).

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TiO2/g-C3N4 microspheres/graphene composite (T-CNS-G) with highly photocatalytic activity has been prepared via a simple two-pot hydrothermal method. Benefiting from the positive synergetic effect, 90% of Cr(VI) can be reduced within 4 h by using T-CNS-G as the photocatalyst under visible light.


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