reduced graphene oxide prepared by gamma irradiation

reduced graphene oxide prepared by gamma irradiation

Radiation Physics and Chemistry 165 (2019) 108371 Contents lists available at ScienceDirect Radiation Physics and Chemistry journal homepage: www.el...

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Radiation Physics and Chemistry 165 (2019) 108371

Contents lists available at ScienceDirect

Radiation Physics and Chemistry journal homepage:

Photocatalytic degradation of ethylene by TiO2 nanotubes/ reduced graphene oxide prepared by gamma irradiation


Xueting Xie, Liqin Li, Shengying Ye*, Quan Zhang, Xuemei Chen, Xun Huang College of Food Science, South China Agricultural University, Wushan, Guangzhou, Guangdong, 510640, PR China



Keywords: Photocatalysis Ethylene 60 Co-radiolysis Titanium dioxide nanotubes Reduced graphene oxide

A nanocomposite photocatalyst of TiO2 nanotubes/reduced graphene oxide (rGO-TNTAs) were prepared by γray radiolysis. The rGO-TNTAs were studied for photocatalytic decomposition of ethylene (C2H4) in a refrigerated environment. Degradation efficiency of ethylene by these materials was described by the apparent first-order rate constant (K). The composite was characterized by Raman spectroscopy (RS), X-ray photoelectron spectroscopy (XPS), Atomic force microscopy (AFM) and Field emission electron microscopy (FESEM). The preparing parameters for materials affecting the degradation efficiency in terms of the rate constant were studied, including the GO addition and the irradiation dose. The results showed that (1) gamma irradiation can induce GO to partial rGO, the rGO surface of rGO-TNTAs was smooth and loosely lamellar structure. (2) The K of the rGO-TNTAs is highly dependent on the GO addition and the irradiation dose. With a GO addition of 0.1 g and an irradiation dose of 20 kGy, the maximum K value of rGO-TNTAs could be obtained under the experimental conditions. (3) From analysis of the Raman spectroscopy spectrum, optimum changes had occurred in the intensity ratio of D-band to G-band of the GO that had been γ-ray-irradiated with a dose of 20 kGy, resulting in the K of rGO-TNTAs increased by 40.9% compared with that of TNTAs.

1. Introduction Fruits and vegetables can produce ethylene(C2H4) after harvest. The accumulation of ethylene in a closed storage environment would accelerate the ripening and aging of fruits and vegetables. Therefore, effective control the content of ethylene in the storage environment of fruits and vegetables is of extreme importance to improve the quality of its storage (Saltveit, 1999; Haji et al., 2008; Yang et al., 2016). Due to its highly ordered and controllable dimensions, TiO2 nanotubes arrays (TNTAs) have better performance in the field of photocatalysis (Hu et al., 2017; Tong et al., 2014; Yuan et al., 2014). TNTAs fabricatrd by potentiostatic anodization of pure Ti in fluoride-based baths are easily obtained with large surface area in comparison to other methods of synthesis (Kalbacova et al., 2008; Kang and Chen, 2010). Moreover, the thermal annealing method involving the use of a muffle furnace at varying temperatures and times is needed to convert them from the amorphous state to anatase/rutile phase. However, major disadvantage of using TNTAs, which like powdered TiO2, is its high rate recombination of photogenerated electron(e−)-hole pairs(h+). This factor leads to shorten the average carrier lifetime and limits widespread application of TNTAs (Tomova et al., 2015; Neppolian et al., 2012).


In order to overcome the limitation various approaches have been developed, e.g. it was reported that loading of graphene may enhance the overall photocatalytic efficiency (Pu et al., 2017; Gong et al., 2017; Huang et al., 2013). Graphene, a sp2-bonded carbon sheet with a thickness of single atom as a two-dimensional (2-D) carbonaceous material, has been widely used for preparing nanohybrids with semiconductors because of its unique structure and physico-chemical properties (Zhu et al., 2010). There are various techniques to obtain individual graphene sheets like micro-mechanical exfoliation of highly ordered pyrolytic graphite, chemical vapor deposition and epitaxial growth (Novoselov et al., 2004; Kim et al., 2009). All these techniques show their own advantages. However, it is difficult to realize high yields of single-layer graphene since these techniques are limited by the high cost and particularly complex process. Therefore, reduced graphene oxide (rGO) obtained from partial reduction of graphene oxide (GO) in solution has been widely used as an inexpensive substitute for graphene (Zhang et al., 2012). rGO is similar in properties to pristine graphene, e.g. a perfect electron mediator and transporter to efficiently suppress recombination of photoexcited carriers (Chen et al., 2010; Bell et al., 2011; Gu et al., 2013). Even though there are many chemical routes available for production of rGO, it is still desirable to design a rapid and large-scale synthesis for carbonaceous material. γ-ray

Corresponding author. Tel.: +86 13622762682. E-mail address: [email protected] (S. Ye). Received 21 October 2018; Received in revised form 25 March 2019; Accepted 15 June 2019 Available online 05 July 2019 0969-806X/ © 2019 Elsevier Ltd. All rights reserved.

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degrade ethylene in simulated refrigerated environment of horticultural products (3 °C ± 1 °C, relative humidity 85% ± 3%). The reactor primarily consists of two rGO-TNTAs composite material, three cold cathode ultraviolet lamps with a power of 12 W and a wavelength of 254 nm which do not generate ozone, and three fans for gas balance and circulation, as well as an inductive hygrometer. Moreover, the reactor provided with an ethylene inlet and a sampling port respectively. In the experiment of photocatalytic degradation of ethylene, the ethylene was injected into the reactor through the inlet, and the gas in the reaction system reached the adsorption equilibrium after 4 h. Then, ethylene concentration was detected by gas chromatography-flame ionization detector and area external standard method every 15 min, the mean value of ethylene concentration obtained from three tests was taken as the initial concentration. Meanwhile, three UV lamps were turned on, and tested every 30 min for 10 times. Operation conditions of gas chromatography were shown as follows: column temperature 100 °C, inlet temperature 150 °C, FID detector temperature 200 °C, nitrogen as the carrier gas and the flow rate of 27 mL/min. The initial concentration of ethylene was 30 mg/m3. When the concentration of ethylene was low, its photocatalytic decomposition conformed to a pseudo-first-order reaction, and the kinetics can be expressed as ln (Ct/C0) = K·t, in which K was the apparent rate constant and Ct was the ethylene concentration at time t, and when t = 0, Ct = C0. In order to investigate the influence of preparation parameters on photocatalytic activity, we designed two sets of experiments, the experimental conditions were shown in Table 1. The first group of experiments included changes in the amount of GO addition and γ-radiation dose in aqueous solution used to obtain rGO. G and D were used to represent experiments carried out at different GO additions and irradiation doses, respectively. G1-5 corresponded to five experiments which GO addition was 0.02, 0.06, 0.10, 0.14 and 0.18 g, respectively. Similarly, D1-5 represented the experiments at five irradiation doses (0, 10, 20, 30 and 40 kGy). The purpose was to study the relationship between the preparing parameters and the K of the photocatalytic process. The second group of experiments included the comparison of decomposition experiments using the TNTAs without γ radiation annealing, the bare rGO of TNTAs and the TNTAs doped with rGO, in order to compare the K of a TNTAs doped with rGO with that of an undoped TNTAs.

irradiation reduction technology, which is of high energy, environmental protection, safety and reliability, and strong operability (Li et al., 2014), is considered one of the most promising strategies for the preparation and modification of materials. This technology, which combines physics and chemistry closely, provides a new path for the preparation of new functionalized photocatalytic materials. In the present work, we summarize the results obtained in our laboratory regarding the use of an TiO2 nanotubes-doped reduced graphere oxide prepared by gamma irradiation for photocatalytic degradation of ethylene. The GO, prepared rGO and rGO-TNTAs were characterized by Raman spectroscopy, X-ray photoelectron spectroscopy, Atomic force microscopy and Field emission electron microscopy. The specific objectives have been to identify the effects of the preparation parameters of the rGO-TNTAs, such as the GO addition and the irradiation dose, on the photocatalytic degradation of C2H4, as well as to compare the photocatalytic efficiency of a TNTAs doped with rGO with that of an undoped TNTAs. 2. Materials and methods 2.1. Materials and


Co-γ ray source

Titanium sheet (TA1, 99.5%) was provided by Baoji metal materials and equipment manufacturing Co., Ltd. Graphene oxide (GO) powder was provided by Nanjing Xianfeng nano material technology Ltd. Isopropyl alcohol (AR) was obtained from Guangzhou Chemical Reagent Factory. Perchloric acid (70%, AR) was obtained from Tianjin Xinyuan Chemical Co., Ltd. Ethanol (AR)and Ethanediol (AR) were provided by Tianjin Fuyu Fine Chemical Co., Ltd. Ammonium fluoride (AR) was provided by Tianjin Fuchen Chemical Reagent Factory. Polyvinylpyrrolidone (PVP, K30) was obtained from Aladdin Reagent Co., Ltd. The cobalt-60 γ ray source (Nordion International Co. Ltd., Ontario, Canada) was obtained from Guangdong Radiation Technology Center and its activity was approximately 1.31 × 1015Bq. 2.2. Preparation of the photocatalytic materials Pretreatment of titanium sheet: 1000#, 2500# and 5000# sandpaper was used to polish titanium sheets in sequence. The electropolishing solution of 500 mL was prepared according to the volume ratio of perchloric acid to ethanol (1:9) and then cooled down to 10 °C in ice water. Afterwards, the titanium sheet and graphite sheet were fixed on the anode and cathode respectively, the front and back side of titanium sheet were electrolyzed for 30s in the pre-prepared electropolishing solution at voltage of 10 V and temperature of 10 °C. Finally, titanium sheet was rinsed by deionized water and dried. Preparation of TNTAs by anodic oxidation method: the pre-treated titanium sheet as anode and the graphite sheet as cathode, both of them were dipped into the NH4F organic electrolyte solution (800 g) which prepared using 0.5 wt % of NH4F, 2 wt% of H2O and ethanediol with a depth of 65 mm, after that, the TNTAs were obtained by reacting for 5 h under the conditions of 40 V voltage, 30 mm bipolar distance and 20 °C reaction temperature. The amorphous TNTAs were then irradiated by 60 Co-γ ray with a dose of 20 kGy for annealing treatment. Preparation of rGO solution: 100 mg of graphite oxide powder and 1 wt% of free radical scavenger (isopropyl alcohol) were added to 50 mL of deionized water to form GO dispersion solution under ultrasonication at 750 W for 2.5 h, and then the treated dispersion solution was irradiated by 60Co-γ ray to get the rGO solution. The rGO solution was dripped on the surface of TNTAs to construct the composite rGO-TNTAs.

2.4. Characterization and data processing The structure of GO and rGO were tested by Raman spectra (RS, inVia, Renishaw, England). Changes of C1s oxygen-containing functional groups on the surface of GO before and after irradiation were obtained using X-ray photoelectron spectroscopy (XPS, ESCALAB 250). Atomic force microscope (AFM, BioScope Catalyst, Veeco) and Field emission scanning electron microscope (FESEM, MeiLin, Zeiss) were used to get the morphology of rGO and rGO-TNTAs. Data processing and analysis of this experiment was based on Origin 9.0 (Origin Lab, USA), XPS PEAK4.1 (Raymund W.M. Kwok, Taiwan), NanoScope Analysis (Veeco, USA) and Excel (Microsoft, USA). 3. Results and discussion 3.1. Characterization Raman spectra in the 1000–2000 cm−1 wavenumber range of GO before and after gamma irradiation were presented in Fig. 2. It is obvious in all samples that the two characteristic peaks corresponding to D band and G band respectively. Table 2 summarizes the shift of peaks band, the ratio of the D to G intensities (ID/IG), the full-width at half maximum of G band (FWHM) and the distance between defects(nD) from the data obtained concerning Raman spectra patterns (Fig. 2). According to Tab 2, the position of the G peak of samples treated

2.3. Reaction device and procedures in the C2H4 degradation experiment Fig. 1 showed the photocatalytic reaction device, which devised to 2

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Fig. 1. Schematic diagram of photocatalytic reactor.

display a blue shift, compared with GO before gamma irradiation. It is well known that the D band originates from the disorder-induced mode of C atoms, whereas the G band is attributed to the E2g vibrational mode of graphitic planes corresponding to the in-plane vibration of sp2 carbon atoms in a 2D hexagonal lattice. The intensity ratio of D-band to G-band (ID/IG) is used to evaluate the density of the defect of carbon crystals [Ferrari and Robertson, 2000]. It is evident from the table that the value of ID/IG and nD increased with increasing radiation dose to the GO solution in the range up to 20 kGy. However, ID/IG and nD of samples treated at the higher radiation doses (30, 40 kGy) presented a slight decline, which may be ascribed to the fact the recovery of sp2, carboncarbon bonds of the graphitic lattice and connection of new sp2 clusters in the films during gamma irradiation treatment. The FWHM of G band was used to assist to evaluate defects [Ammar et al., 2015]. The FWHM of G band, became a narrower from 81.48 to 69.23 cm−1 with increasing radiation dose from 10 kGy to 20 kGy. The sharper may be caused by a certain extent increase of higher density of defects. These results confirm that GO is successfully reduced to rGO in the process of radiation. A further insight into the carbon atom layers in the two samples before gamma irradiation and after gamma irradiation with dose of 20 kGy obtained arose from Raman spectroscopy in the 750–3500 cm−1 wavenumber range (Fig. 3). The 2D Raman peak frequency was twice the frequency of the D peak. The intensity ratio of 2D to G (I2D/IG) and the FWHM of the 2D peak directly reflected the electron band structure of carbon materials, and these electron band structures were related to the number of carbon atom layers. The process of radiation with dose of 20 kGy yielded rGO with larger I2D/IG and a smaller 2D peak FWHM values, compared with the GO Fig. 3. This result suggests that gamma irradiation could make GO reduce the thickness. The XPS spectrums of C1s of GO and as-prepared rGO were shown in Fig. 4 (black line). After the C1s peaks of GO and rGO were studied by means of XPS-peak-differentiation-imitating analysis, four peaks located at about 284.5, 286, 287.6 and 288.2 eV were observed, it meant that there were four chemical states of the carbon atoms in GO and rGO, which correspond to C=C/C-C, C-O, C=O and COOH groups

Fig. 2. Raman spectra in the 1000–2000 cm−1 wavenumber range of GO treated at various γ-radiation doses.

respectively. And the red line represented the effect of peak fitting. It was noticed that the C = O peak area was large (Fig. 4(a)), indicating that a large amount of carbonyl groups were contained, and after irradiation, the peak areas of C=O and COOH decreased obviously (Fig. 4(b)), which indicated that the oxygen-containing organic functional groups (carbonyl and carboxyl groups) on the surface of GO were removed by reduction. By calculating the ratio of area of peak 1 and the sum area of peak 2,3,4, it can be seen that the ratio of sp2/sp3 increased from 1:1 to 2.38:1 after the GO was reduction by gamma irradiation. The analysis of X-ray photoelectron spectroscopy further confirmed that rGO can be prepared by gamma irradiation. Fig. 5 exhibited AFM image of rGO after gamma irradiation with

Table 1 Summary of experimental conditions. Name of experiment

1st set 2nd set

a b c


G1-5 D1-5 EXP 1b EXP 2c EXP 3

rGO-TNTAs rGO-TNTAs TNTAs no-annealing TNTAs rGO-TNTAs

Preparing parameters for rGO

Light intensity

Amount of GO addition (g)


0.02; 0.06; 0.10; 0.14; 0.18 0.10 ~ ~ 0.10

10 0; 10; 20; 30; 40 ~ ~ 20

The UV light intensity was measured by means of a photometer on the surface of the photocatalyst. The experiment was conducted under TNTAs without γ radiation annealing. The experiment was photocatalytic degradation of C2H4 using a bare rGO photocatalyst as a reference. 3

1.95 1.95 1.95 1.95 1.95



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were ordered and vertically arranged with diameter of 60–80 nm and thickness of 5–10 nm. Moreover, the loaded material was dispersedly spread on the surface of the TNTAs, and a small amount of material covered the nozzle but did not enter the inside of tube wall.

Table 2 Summary of Raman spectroscopies about samples. Sample

Raman shift of D band (cm−1)

Raman shift of G band (cm−1)


FWHM(b) (cm−1)

nD(c) (1011 cm2)

0 kGy 10 kGy 20 kGy 30 kGy 40 kGy

1358.82 1367.93 1376.87 1379.75 1370.08

1589.29 1582.49 1581.35 1577.69 1578.03

0.7660 0.8489 0.9174 0.8697 0.8468

82.62 81.48 69.23 71.04 79.15

1.97 2.18 2.35 2.23 2.17

3.2. Effects of parameters for rGO formation on photocatalytic degradation of C2H4 Table 2 lists the ratio of the intensity of the D-band to G-band peaks (ID/IG) obtained at each radiation dose. No marked reduction in the ID/ IG was observed for samples irradiated at 30 or 40 kGy compared with that irradiated at 20 kGy. The ID/IG value intensity was used to evaluate the density of defects on graphene sheets. The ID/IG was found to increase by approximately 16.5% between the D1 and D3 samples (Table 2), suggesting that the irradiation reduction process would destroy the carbon bonds while removing functional groups, leading to growth of disorder. The sites of the previously present oxygen rich groups were therefore only partially re-graphitized and new vacancies were likely introduced both on the edges and across the graphene sheets (Li et al., 2008; Ji et al., 2013). The transformation was ascribed to an increase in defects which was attributable to evolve CO and/or CO2 species specially from epoxy groups (Mattevi et al., 2009). However, the ID/IG gradually decreases beyond this point up to a 20 kGy, which was attributable to the recovery of sp2, carbon-carbon bonds of the graphitic lattice and connection of new sp2 clusters. In addition, the ID/IG is related to the stability of hydroxyl groups. Isopropanol plays an important role as oxidative radical scavenger which helps to eliminate oxidative radicals in water and convert into reductive radicals during the γ-ray irradiation process. As the irradiation dose increased over 20 kGy, high irradiation intensities probably minimize the action of isopropyl alcohol. Figs. 7 and 8 showed the kinetic curves of ethylene reduction under the conditions that mentioned in the first group of experiments in Table 1. The linear relationships affirmed that the photocatalytic decomposition of C2H4 can be rationally described by first-order kinetics because of its good fitting, with R2 > 0.992 in all situations (Table 3). The rate constant (K) calculated from the slope of the first-order reaction plot was presented in Table 3. Compared with the degradation efficiency of Bi2WO6–TiO2/starch nanocomposite films (Wang et al., 2019), rGO-TNTAs has great advantages in ethylene degradation. It is noted from Table 3 that when the GO addition amount was 0.1 g, the maximum K value can be obtained, however, the K value decreased as the GO addition continued to increase. A possible reason for the decline of photocatalytic degradation ability was that excessive amount of GO can lead to the stacking of reduced graphene oxide, which weakens the efficiency of graphene in promoting electron separation, and thus degrades the degradation ability of the composite photocatalyst. Meanwhile, the excessive amount of rGO loaded on the surface of TNTAs will also reduce the absorption and utilization efficiency of TNTAs to external ultraviolet light, resulting in the decrease of its ability to promote carrier separation efficiency. It is evident from Table 3 that with the increase of irradiation dose, the K value increased first and then decreased, and the maximum value was obtained when the dose was 20 kGy. It can be seen that as the ID/IG decreased (in Table 3, the ID/IG decrease from 0.9174 to 0.8468), the photocatalytic degradation rate (K) decreases. Raghavan et al. (2018) suggested that defect in rGO, which is dependent on the ratio of the intensity of the D-band to G-band peaks, could have directed a strong interaction between TiO2 and rGO, as a result achieved enhanced photocatalytic activity. Raghavan’s suggestion is a good candidate for an explanation of our experimental results.


All spectra are collected at room temperature using the laser energy 2.41 eV (λ = 514.5 nm). b The FWHM is the full width at half maximum for the G peak. c The distance between defects was calculated from expression nD = (1.8 ± 0.5) × 1022 × (ID/IG)/λ4 by Ref [Cancado et al., 2012].

Fig. 3. Raman spectra in the 750–3500 cm−1 wavenumber range of GO and GO treated at 20 kGy dose.

Fig. 4. GO(a) and as-prepared rGO (b) of X-ray photoelectron spectroscopy for C1s.

dose of 20 kGy, it can be clearly seen that rGO was flaky(Fig. 5(a)), and the height of the slice of the sample was measured in the range of 3.31 nm ± 0.167 nm (Fig. 5(b)). Fig. 6 showed the FESEM of as-prepared rGO-TNTAs, it indicated that the nanotubes, which presents on the surface of titanium-based,

3.3. Comparison of C2H4 degradation under different photocatalyst Kinetic curves of ethylene reduction under the different photocatalyst described for the second set experiments in Table 1 are 4

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Fig. 5. Morphology (a)and height of different color line along morphology(b) of atomic force microscopic diagram for the GO after gamma irradiation with dose of 20 kGy. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

Fig. 6. FESEM images of the as-prepared rGO-TNTAs.

Fig. 8. Kinetic curves of ethylene reduction for an aqueous solution containing 0.1 g GO subjected to various γ-radiation doses. Table 3 The ratio of the intensity of the D and G peaks and rate constants (K) calculated based on the assumption that the photocatalytic processes follow first-order reaction kinetics.

Fig. 7. Kinetic curves of ethylene reduction for an aqueous solution containing various amount of GO at a γ-radiation dose of 10 kGy.

Name of experiment


Regression equation


Rate constant, K a (min−1)

G1 G2 G3 G4 G5 D1 D2(G3) D3 D4 D5

0.8489 0.8489 0.8489 0.8489 0.8489 0.7660 0.8489 0.9174 0.8697 0.8468

ln(Ct / C0) ln(Ct / C0) ln(Ct / C0) ln(Ct / C0) ln(Ct / C0) ln(Ct / C0) ln(Ct / C0) ln(Ct / C0) ln(Ct / C0) ln(Ct / C0)

0.997 0.992 0.993 0.997 0.995 0.998 0.993 0.994 0.993 0.993

3.63 × 10−4 3.70 × 10−4 3.52 × 10−4 3.28 × 10−4 3.06 × 10−4 3.45 × 10−4 3.52 × 10−4 3.91 × 10−4 3.39 × 10−4 3.08 × 10−4

= = = = = = = = = =

−0. −0. −0. −0. −0. −0. −0. −0. −0. −0.

000363⋅t 000370⋅t 000352⋅t 000328⋅t 000306⋅t 000345⋅t 000352⋅t 000391⋅t 000339⋅t 000308⋅t

a Rate constant was determined from the slope of a plot of the relationship between ln(Ct/C0) and time (min); slope = –K.

presented in Fig. 9. The apparent rate constants for the processes using TNTAs no-annealing, TNTAs and rGO-TNTAs as photocatalyst were 1.47 × 10−4 min−1, 2.31 × 10−4 min−1 and 3.91 × 10−4 min−1, respectively. The value of K for rGO-TNTAs increases about 40.9% compared with that of TNTAs. It is noteworthy that rGO played a significant role in the photocatalytic activity of TNTAs. Apparently, at the optimal level of rGO doping and irradiation dose, rGO-TNTAs is a better photocatalyst compared to TNTAs which can be attributed to the following reasons: (i) the defects in rGO, as a result achieved subsequently improved the active sites for photocatalytic activity. (ii) GO on TNTAs, the photogenerated electrons (ecb−) flow towards the rGO at the interface. This leads to the increase of charge separation efficiency and hence improves the photocatalytic activity of

TNTAs. (iii) High specific surface area promotes increased C2H4 adsorption. 4. Conclusions In our work, titanium dioxide nanotube arrays (TNTAs) was prepared by anodic oxidation method, followed by irradiation under 60Coγ radiation annealing, the reduced graphene oxide (rGO) was prepared by ultrasonic stripping and irradiated reduction, and finally the composite photocatalyst RGO-TNTA was obtained by combining TNTA with 5

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Fig. 9. Kinetic curves of ethylene reduction under different photocatalyst.

RGO. The as-prepared rGO-TNTAs were studied for photocatalytic degradation of ethylene (C2H4) in simulated refrigerated environment and the following conclusions made: (1) γ-ray irradiation is an efficient way to reduce GO to partial rGO. The reduction effect of GO was confirmed by different characterizations. (2) The K of the rGO-TNTAs is highly dependent on the GO addition and the irradiation dose. With a GO addition of 0.1 g and an irradiation dose of 20 kGy, the maximum K value can be attained for the rGO-TNTAs. (3) From analysis of the Raman spectroscopy spectrum, optimum changes had occurred in the intensity ratio of D-band to G-band (ID/ IG) of the GO that had been γ-ray-irradiated with a dose of 20 kGy, resulting in the K of rGO-TNTAs increased by 40.9% compared with that of TNTAs. Acknowledgment Financial support from the National Natural Science Foundation of China (grant no. 31171449) is gratefully acknowledged by the authors. References Ammar, M.R., Galy, N., Rouzaud, J.N., Toulhoat, N., Vaudey, C.E., Simon, P., 2015. Characterizing various types of defects in nuclear graphite using Raman scattering: Heat treatment, ion irradiation and polishing. Carbon 95, 364–373. Bell, N.J., Yun, H.N., Du, A., Coster, H., Smith, S.C., Amal, R., 2011. Understanding the enhancement in photoelectrochemical properties of photocatalytically prepared tio2reduced graphene oxide composite. J. Phys. Chem. C 115, 6004–6009. Cancado, L.G., Jorio, A., Martins Ferreira, E.H., Stavale, F., Achete, C.A., Capaz, R.B., 2012. Ferrari, Quantifying defects in graphene via Raman spectroscopy at different excitation energies. Nano Lett. 11, 3190–3196 2011. Chen, C., Cai, W., Long, M., Zhou, B., Wu, Y., Wu, D., 2010. Synthesis of visible-light