THE PHYSICOCHEMICAL ANALYSIS & PHOTOCATALYST PERFORMANCE
Equation 3: Scherrer’s equation
Table 1. The crystallite size of the TiO2 particles in rGO-TiO2 (S), rGO-TiO2 (H1) and rGO-Anatase TiO2 (H2) composites.
Samples 2θ (degree) λ (nm) 𝛽 𝑐𝑜𝑠 𝑐𝑜𝑠 𝜃 D (nm)
rGO-TiO2 (S) 25.3o 0.15406 0.0016 86.57
rGO-TiO2 (H1) 25.3o 0.15406 0.0013 107.05
rGO-Anatase TiO2 (H2) 25.3o 0.15406 0.0033 42.14
Table 1 shows the crystallite size of samples which were calculated by using a Scherrer’s equation to answer the objective one in this article. The small crystallite size will provide a large surface area but in contrast, it will cause the rapid aggregation which decreases its effective surface area and absorbance power [14]. Table 1 clearly shows that the smallest crystallite size is rGO- Anatase TiO2 (H2) with 42.14 nm and then followed by rGO-TiO2 (S) with 86.57 nm and lastly is rGO-TiO2 (H1) with 107.05 nm. Based on this evaluation, we found that the smallest crystallite size is not necessarily the best quality of crystalline phase and growth. The rGO-TiO2 (H1) had the biggest crystallite size which might reduce its surface area and absorbance power. However, as reported in our previous article, the small crystallite size was affecting the growth morphology [19]. The sharpest diffraction peak in rGO-TiO2 (H1) at 25.3 as in Figure 1 was to prove that it has the optimum crystallite size compared to the other samples. It can be assumed that this optimum crystallite size helped the TiO2 to smoothly spread with low agglomeration on the rGO sheets.
In conclusion, the XRD results as shown in Figure 1 and Table 1 were successfully achieved by answering the first research question in this article. The effect of the synthesis method in the sample's physicochemical properties was successfully confirmed in this result. The crystallinity and crystallite size of rGO-TiO2 (H1) is obviously better compared to the rGO-TiO2 (S) and rGO-Anatase TiO2.
63 Figure 2. Kubelka-Munk function plot of TiO2, rGO-TiO2 (S), rGO-TiO2 (H1) and rGO-Anatase
TiO2 (H2)
Figure 2 shows the results of UV-Vis analysis to answer the objective two in this article.
According to M. Pelaez et al., the lower the band gap energy, the wider the range of visible light can be absorbed which is the important criteria to perform the photoactivity [25]. The lower the elemental band gap absorption leads to the better photoactivity performance because the electrons at the Valence Band (VB) do not need to absorb a high energy from the UV light to excite and escape to the Conduction Band (CB). The results in Fig. 2 shows the Tauc plot of the modified Kubelka-Munk (KM) which determines the band gap energy of each sample. We can see the approximated band gaps of TiO2, rGO-Anatase TiO2 (H2), rGO-TiO2 (S) andrGO-TiO2 (H1) are 3.40eV, 3.10eV, 2.90eV, 2.60eV respectively. Notably, the rGO-TiO2 (H1) had the lowest band gap compared to the other samples. In conclusion, the UV-Vis results were successfully achieving objective two and answering the first research question in this article. The second objective to investigate the band gap size is clearly shown in Figure 2 while the effect of the synthesis method in the sample's physicochemical properties was also successfully confirmed in this result. It can be concluded that the doped samples have a better band gap size compared to the undoped TiO2. According to some researchers, the band gap narrowing occurs because of the formation of strong Ti-O-C bonds between the TiO2 and rGO composites [26]. The smaller the band gap, the better the formation of Ti-O-C bonds due to the strong contact between TiO2 and rGO. Based on this result, the formation of Ti-O-C bonds in rGO-TiO2
(H1) is the strongest compared to the other samples. The formation of Ti-O-C bonds leads to improved absorption edge and resulting in a better electron transition from the VB to the CB for a better photocatalytic performance. Due to M. B. Suwarnkar et. al., the lowest band gap of rGO-TiO2
(H1) with 2.60 eV has led to enhanced photoactivity performance of TiO2 [27]. Besides that, according to A. Mills and S. Le Hunte, the sufficient decreased band gap energy of TiO2 photocatalyst can help to absorb the visible light range energy and might be possible to utilise the solar spectrum up to 40% instead of less than 1% only before the band gap alteration [28]. Therefore, it can be
64 concluded that, the results in Fig 3 and Fig.4 were successfully answered by the research question one and these samples will be further examined to answer the research question two in this article by using the Gas Chromatograph (GC) testing.
Photocatalytic testing
Figure 3 shows the results of GC testing under visible light irradiation to answer the objective three in this article. The GC testing is to analyse the photoactivity performance of TiO2, 1AgTiO2, 3AgTiO2 and 5AgTiO2 were irradiated under visible light irradiation for 8 hours. The yield of methane, CH4 gas was calculated by using the formula in Eq. 2 and plotted in Figure 3. The rGO- TiO2 (H1) shows the highest CH4 yield with 722 nmol/gcat compared to TiO2 with 63 nmol/gcat, rGO- Anatase TiO2 (H2) with 288 nmol/gcat and rGO-TiO2 (S) with 380 nmol/gcat. Notably, the photoactivity performance of rGO-TiO2 (H1) was increased dramatically compared to the TiO2 which aligns with the finding reported by L.-L. Tan et. al. [15]. Based on these results, it was proven that the intimate contact between the TiO2 and rGO may accelerate the transfer of photogenerated electrons on TiO2 to rGO and decelerate the recombination rate of charge carriers same as reported by [16,17].
Figure 3. Photocatalytic testing over TiO2, rGO-TiO2 (S), rGO-TiO2 (H1) and rGO-Anatase TiO2
(H2) under visible light irradiation CONCLUSION
In summary, the three objectives to answer the two research questions in this article were successfully achieved. The effect of the synthesize method in the sample's physicochemical properties such as crystallinity phase, crystallite size and band gap energy were successfully proved. The five sharp diffraction peaks and 107.05 nm optimum crystallite size of rGO-TiO2 (H1) were successfully proven that this sample was the best compared to the rGO-TiO2 (S) and rGO-Anatase TiO2. Besides that, the rGO-TiO2 (H1) also had the lowest band gap energy with 2.60 eV compared to rGO-Anatase TiO2 (H2) and rGO-TiO2 (S) with 3.10 eV and 2.90 eV respectively. Secondly, the effect of the synthesis method in photocatalytic performance was also achieved in this article. The results show
65 that the rGO-TiO2 (H1) has the best performance under visible light irradiation. The synthesis method and the raw material obviously affect the physicochemical properties of rGO-TiO2 and indirectly enhance its photoactivity performance. The intimate contact between the TiO2 and rGO may accelerate the transfer of photogenerated electrons on TiO2 to rGO and decelerate the recombination of charge carriers. The hydrothermal synthesis method by using a TBT as a raw material are the novel and valuable findings in this article. Several characterizations are suggested in further work in order to strengthen these findings such as FESEM and TEM to analyze the growth morphology and SBET to confirm the surface área. It can be concluded that, this simple method is potentially used to synthesize a high grade of rGO–TiO2 at the large scale in the future due to its green and environmentally friendly raw material, low equipment cost, and simple and efficient lab works which were considered as the others beneficial in this research work.
ACKNOWLEDGEMENT
A special thanks goes to Heriot Watt University Malaysia (HWUM) for this collaboration with Monash University Malaysia. The laboratory work was conducted at chemical engineering research lab, Monash University Malaysia. The software to analyze raw data such as Origin 9.1 and excel had been provided by HWUM and PPST, Universiti Malaysia Sabah (UMS).
REFERENCES
[1] Chen, G., Waterhouse, G.I.N., Shi, R., Zhao, J., Li, Z., Wu, L.Z., Tung, C.H. and Zhang, T. G. From solar energy to fuels: Recent advances in light-driven C1 chemistry. Angew. Chemie - Int. Ed. 58(49) (2019) 17528–17551.
[2] Najafabadi, A.T. Emerging applications of graphene and its derivatives in carbon capture and conversion: Current status and future prospects. Renew. Sustain. Energy Rev., 41 (2015), 1515–1545.
[3] Yi, N., Unruangsri, J., Shaw, J. and Williams, C.K. Carbon dioxide capture and utilization: using dinuclear catalysts to prepare polycarbonates. Faraday Discuss. (2015), 67–82.
[4] Miao, R., Luo, Z., Zhong, W., Chen, S.Y., Jiang, T., Dutta, B., Nasr, Y., Zhang, Y. and Suib, S.L. Mesoporous TiO2 modified with carbon quantum dots as a high-performance visible light photocatalyst. 189 (2016), 26-38 [5] Abu, S. and Ribeiro, C. Nitrogen-doped titanium dioxide : An overview of material design and dimensionality
effect over modern applications. Journal Photochem. Photobiol. C Photochem. Rev., 27 (2016), 1–29.
[6] PYaashikaa, P.R., Kumar, P.S., Varjani, S. J.and Saravanan, A. A review on photochemical , biochemical and electrochemical transformation of CO2 into value-added products. J. CO2 Util., 33(5) (2019), 131–147.
[7] Han, X., Li, S., Peng, Z., Al-Yuobi, A.O., Bashammakh, A.S.O., El-Shahawi, M.S. and Leblanc, R.M.
Interactions between carbon nanomaterials and biomolecules. J. Oleo Sci. 7(1) 2016, 1–7.
[8] Chai, B., Peng, T., Zhang, X., Mao, J. Li, K. and Zhang, X. Dalton transactions. (2013), 3402–3409.
[9] Wang, B. Chen, W. Song, Y. Li, G. Wei, W.and Fang, J. Recent progress in the photocatalytic reduction of aqueous carbon dioxide. Catal. Today (2017), 0–1.
[10] Chen, J. Qiu, F. Xu, W. Cao, S. and Zhu, H. Recent progress in enhancing photocatalytic efficiency of TiO2- based materials. Appl. Catal. A Gen. 495, (2015), 131–140.
[11] Cue´llar-Franca, A. A. Rosa M.. Carbon capture, storage and utilisation technologies: A critical analysis and comparison of their life cycle environmental impacts. J. CO2 Util., 9, (2015), 82–102, 2015.
[12] Li, J., Lin, Y., Pan, X., Miao, D., Ding, D., Cui, Y., Dong, J. and Boa, X. Enhanced CO2 Methanation activity of Ni/anatase catalyst by tuning strong metal-support interactions. ACS Catal., 9(7) (109), 6342–6348, 2019.
[13] Jiun, S. Tan, L. and Tan, L. Recent advances in carbon quantum dot (CQD)- based two dimensional materials for photocatalytic applications. Catalysis Sci. & Tech. (2019), 5882–5905.
[14] Awang, H.and Infaza, N. Synthesis of reduced graphene oxide-titanium (rGO-TiO2) composite using a solvothermal and hydrothermal methods and characterized via XRD and UV-Vis. Natural Resources. 10(02)
66 (2019), 17–28.
[15] Tan, L.L. Ong, W.J., Chai, S.P. and Mohamed, A. R. Reduced graphene oxide-TiO2 nanocomposite as a promising visible-light-active photocatalyst for the conversion of carbon dioxide.,” Nanoscale Res. Lett. 8(1) (2013).
[16] Xiang, Q. and Jaroniec, M. Nanoscale enhanced photocatalytic H2 -production activity of graphene-modified titania nanosheets. Material Science. (2011), 3670–3678.
[17] Tan, L. L., Ong, W. J., Chai, S. P. and Mohamed, A. R. Noble metal modified reduced graphene oxide/TiO2 ternary nanostructures for efficient visible-light-driven photoreduction of carbon dioxide into methane. Appl.
Catal. B Environ., 166–167 (2015), pp. 251–259.
[18] Haggerty, J. E. S., Schelhas, L.T., Kitchaev, D.A., Mangum, J.S., Garten, L. M., Sun, W., Stone, K.H., Perkins, J.D., Toney, M.F., Ceder, G., Ginley, D.F., Gorman, B.P., and Tate, J.. High-fraction brookite films from amorphous precursors. Sci. Rep., 7(1) (2017), 15232.
[19] Mukifza, A., Yusof, S., Awang, H. B., and Farid, E. M. Synthesis and characterization of titanium dioxide using a caustic hydrothermal with moderate molarity and ratio from synthetic rutile waste . 1(2) (2016), 12–15.
[20] Gupta, S. M. and Tripathi, M. A review of TiO2 nanoparticles. Chinese Sci. Bull., 56(16) (2011), 1639–1657.
[21] Anatase Titanium XRD pattern.pdf. . [22] Rutile Titanium XRD pattern.pdf. .
[23] Fan, W., Lai, Q., Zhang, Q. and Wang, Y. Nanocomposites of TiO2 and reduced graphene oxide as efficient photocatalysts for hydrogen evolution. (2011), 10694–10701.
[24] Shen, J. , Shi, M., Yan, B., Ma, H, Li, N. and Ye, M. Ionic liquid-assisted one-step hydrothermal synthesis of TiO2-reduced graphene oxide composites. Nano Res., 4(8) (2011), 795–806.
[25] Pelaez, M., Nolan, N. T., Pillai, S. C., Seery, M. K., Falaras, P. , Kontos, A. G., Dunlop, P.S.M., Hamilton, J.W.J., Byrne, J.A., O'Shea, K., Entezari, M.H. and Dionysiou, D.D. A review on the visible light active titanium dioxide photocatalysts for environmental applications. Appl. Catal. B Environ. 125 (2012), 331–349, 2012.
[26] Hao, X., Lü, L., Liang, B., Li, C., Wu, P. and Wang, J. Solvent extraction of titanium from the simulated ilmenite sulfuric acid leachate by trialkylphosphine oxide. Hydrometallurg. 113–114 (2012), 185–191.
[27] Suwarnkar, M.B., Dhabbe, R. S., Kadam, a. N. and Garadkar, K. M. Enhanced photocatalytic activity of Ag doped TiO2 nanoparticles synthesized by a microwave assisted method. Ceram. Int. 40(4) (2014), 5489–5496.
[28] Mills, A. and Le Hunte, S. An overview of semiconductor photocatalysis. Photochem. & Photobio. 108(1) (1997), 1–35.
[29] Shavanova, K., Bakakina, Y., Burkova, I., Shtepliuk, I., Viter, R., Ubelis, A., Beni, V., Starodub, N., Yakimova, R., Khranovskyy, V., Application of 2D non-graphene materials and 2D oxide nanostructures for biosensing technology oxide nanostructures for biosensing technology. Sensor (Base). 16(3) (2016), 1–23.
[30] Chen, Y., Huang, W., He, D., Situ, Y. and Huang, H. Construction of heterostructured g-C3N4/Ag/TiO2
microspheres with enhanced photocatalysis performance under visible-light irradiation. Appl. Mater. Interfaces.
6(16) (2014), 14405-14414.
[31] Liu, J & Hui, X.,Yuanguo, X., Song, Y, Jiabiao, L., Yan, Z., Wang, L., Huang, L., Ji, H. and Li, H. Graphene quantum dots modified mesoporous graphite carbon nitride with significant enhancement of photocatalytic activity. Applied Catal. B, Environ. (2017).
67