Preparation and characterization of TiO2/GaN nanostructure composite for a degradation reaction of methylene blue

162 JOURNAL OF SCIENCE OF HNUE DOI: 10.18173/2354-1059.2017-0045 Mathematical and Physical Sci. 2017, Vol. 62, Iss. 8, pp. 162-169 This paper is available online at PREPARATION AND CHARACTERIZATION OF TiO2/GaN NANOSTRUCTURE COMPOSITE FOR A DEGRADATION REACTION OF METHYLENE BLUE Nguyen Cao Khang Center for Nano Science and Technology, Hanoi National University of Education Abstract. In an attempt to extend light absorption of a TiO2-based photocatalyst toward the visible light range, a new type of photocatalyst TiO2/GaN powder was prepared using a simple process. The crystal phase composition, structure and light absorption of the new photocatalysts were comprehensively examined by X-ray diffraction (XRD) and UV-visible absorption spectra. X-ray diffraction results indicate that the effect of temperature and of the mole ratio of Ti/Ga concentration is dependent upon the structure of the materials. A significant shift of the absorption edge to lower energy was observed in the UV-visible absorption spectra. The results of analyses in this study indicate that TiO2/GaN can increase light absorption in the visible range. The experiment demonstrated that the photooxidation efficiency of methylene blue using TiO2/GaN powder is significantly higher than that when using pure TiO2. The development of such photocatalysts may be considered a breakthrough in large-scale utilization of solar energy to address environmental needs. Keywords: TiO2, composite, GaN, photocatalyst. 1. Introduction Since the discovery of the photocatalytic splitting of water on a TiO2 electrode by Fujishima and Honda in 1972 [1], enormous effort has been spent on the study of TiO2 under light illumination due to its various potential applications, such as photovoltaics and photocatalysis [2, 3]. However, with its high band gap value of 3.2 eV for anatase phase, it only utilizes the UV portion of the solar spectrum as an energy source (less than 5% of total sunlight energy). There was an exponential growth in research activities occured in nanoscience and nanotechnology during the 1990s. New physical and chemical properties emerge when the size of the material becomes smaller and smaller, down to nanometer scale. In particular, TiO2 nano showed an enhanced photocatalytic performance but not considerable than bulk structure [4]. The modification of light harvesting properties of TiO2 nanostructure material using many different methods have been become important research topics in an attempt to achieve an efficient operating range under UV and visible light [5]. Received September 13, 2017. Accepted September 30, 2017. Contact Nguyen Cao Khang, email: khangdhsp@gmail.com Preparation and characterization of TiO2/GaN nanostructure composite for a degradation reaction 163 Doping TiO2 nanomaterials with other elements can narrow the electronic properties and, thus, alter the optical properties of TiO2 nanomaterials. Different elements including both metals and non-metals have been successfully doped into TiO2 nanomaterials and a significant influence of dopants on photoreactivity was observed [6, 7]. Specially, coupled semiconductor photocatalysts increase the efficiency of a photocatalytic process by increasing the charge separation and extending the energy range of photoexcitation for the system [8, 9]. Besides advantages in photoactivity, the doping materials also show many weak points in environmental techniques such as metasable structure, with a clotting of dopants. On the other hand, composite semiconductors have shown many strong points in application. The photocatalytic activity of composite materials has been researched widely with interesting initial achievements, in particular with the composites TiO2 and GaN. The main goal of this research is to synthesize a TiO2/GaN nanostructure composite, characterize these particles using various techniques, and estimate the photoactivity of the samples. 2. Content 2.1. Experiment First, a TTiP solution was dropped into distilled water to form a colloidal solution. Nitric acid was added under continuous stirring to obtain a transparent homogeneous sol solution. A calculated amount of Ga(NO3)3 powder (corresponding to [Ti]/[Ga] = 3/1 and 6/1, respectively) was added to the stirring solution and the mixture was stirred magnetically for 30 minutes. Next, ammonia solution (NH4OH) was added in an amount sufficient to made a white precipitate. The precipitate was filtered and dried in the air at 100 ºC before being crushed. 75 mL of MB solution at 10 ppm and a 75 mg photocatalyst sample were added inside the reactor vessel under constant stirring for 30 minutes in the dark before irradiation to reach equilibrium. After the equilibrium absorption process, the concentration of MB solution was considered as being 100%. During irradiation the reactor vessel was kept under magnetic stirring to maintain a homogeneous suspension to promote absorption on the surface of photocatalytic particles. The discolored solution was taken for spectral measurement with an UV–vis spectrophotometer at various times intervals after centrifugation. 2.2. Results The crystalline phases present in the photocatalysts were identified by X-ray powder diffraction. In Fig. 1 and Fig. 2, the XRPD pattern of two photocatalyst systems are shown as a function of the calcination temperatures of 500, 600, 700, 800 and 900 ºC. This result indicates that the crystalline phase of TiO2 in all photocatalyst samples is anatase at 500 ºC. At a calcination temperature of 600 ºC and higher, the characteristic peak of the anatase phase in the TiO2/GaN samples was observed to be sharper and clearer, particularly at 2θ of 25.4º. The anatase phase was identified at 2θ of 25.4º (); 38.1º (); 48.2º (); 53.9º (); 55.1º () and 62.8º () respectively, corresponding with the standard XRD. In both systems, when the calcination temperature was 700 °C, rutile phase with diffraction peaks located at the 27.5º () started to appear. For the 800 °C and 900 °C annealed samples, the peak at the 27.5º was sharper and peaks of the rutile phase were also observed clearly at 2θ of 36.1º (); 41.2º () and 56.7º () respectively, corresponding with the standard XRD pattern. However, an anatase phase was still present in the composite samples. Interestingly, the prominent presence of the peaks at 2θ of 32.5; 34.5 and 58.1º (marked with * in XRD patterns) indicate that a wurtzite phase of GaN formed when the calcination temperature was higher than 800 ºC. The peaks at 2θ of 32.5; 34.5 and 58.1º were obtained by reflection of X-ray at families of Nguyen Cao Khang 164 planes , and , respectively, and they were matched with the standard XRPD pattern of GaN. The wurtzite type structure of GaN was obtained at 350 ºC when GaCl3 was used as a precursor of Ga [10]. However, normally, GaN was obtained only when annealed samples were calcinated at 850 °C and a wurtzite type structure was formed clearly at higher temperatures [11]. These results explain that diffracted peaks of the wurtzite phase only start appearing in X-ray pattern of samples calcined at 800 °C and only present sharper when the calcination temperature is 900 °C. 20 30 40 50 60 70 Wurtzite* * * ** S31-800 o C S31-500 o C S31-600 o C S31-700 o C S31-900 o C In te n si ty ( a. u .) 2  (deg.) Fig.1. XRD patterns of TiO2/GaN samples with [Ti]/[Ga] = 3/1 as a function of calcinations temperatures 20 30 40 50 60 70 * Wurtzite ** S61-600 o C S61-500 o C S61-800 o C S61-700 o C S61-900 o C In te n si ty ( a. u .) 2  (deg.) Fig.2. XRD patterns of TiO2/GaN samples with [Ti]/[Ga] = 6 as a function of calcinations temperatures Fig. 3 and Fig. 4 show the remarkable effect of the presence of GaN crystals on the formation process of crystalline phases of TiO2 at 500, 700, 800, and 900 °C, respectively. In TiO2 nanoparticle preparation from Ti(OH)4, the produced amorphous TiO2 transformed into anatase when heated at a temperature of 350 °C and the anatase phase was stable between 400 °C and 700 °C before changing into a rutile phase at temperatures higher than 750 °C, as reported by Reyes- Coronado D. et al. [12]. Fig. 3 shows the effect of GaN cystals on the crystalline process of the anatase phase. This was seen clearly by comparing the anatase peaks in the X-ray pattern of the composites with a sample of TiO2 nano that was synthesised from Ti(OH)4 in the same calcination conditions (named TiO2/NH3-500 °C and TiO2/NH3-700 °C). A comparison of two XRPD patterns shows that the Preparation and characterization of TiO2/GaN nanostructure composite for a degradation reaction 165 crystalline of the anatase phase became more hardly with an increased ratio [Ga]/[Ti]. As seen in the patterns, the peaks corresponding to the anatase phase have a strong reduction of intensity with increasing gallium content. 20 30 40 50 60 70 Anatase Rutile TiO 2 /NH 3 -500 o C S61-500 o C S31-500 o C In te n si ty ( a. u .) 2 (deg.) 20 30 40 50 60 70 Anatase Rutile S31-700 o C TiO 2 /NH 3 -700 o C S61-700 o C In te n si ty ( a .u .) 2 (deg.) Fig.3. XRD patterns of samples at calcination temperature of 500 ºC (a) and 700 ºC (b) Fig. 4 shows the effect of the GaN crystals on the transformation of anatase to the rutile phase. A sample TiO2 nano was also synthesised in the same calcination conditions to comparing (named TiO2/NH3 - 800 ºC and TiO2/NH3 - 900 ºC). The transformation of anatase to rutile was reduced by the presence of Ga in the samples. For 900 °C annealed samples, the TiO2 nanoparticles crystalise absolutely in the rutile phase. However, in all TiO2/GaN samples, TiO2 consists of mixed phases of anatase and rutile. 20 30 40 50 60 70 * Anatase Rutile TiO 2 /NH 3 -800 o C S61-800 o C S31-800 o C In te n si ty ( a. u .) 2 (deg.) 20 30 40 50 60 70 * * * * * * Wurtzite Anatase Rutile S31-900 o C S61-900 o C TiO 2 /NH 3 -900 o C In te n si ty ( a. u .) 2 (deg.) Fig.4. XRD patterns of samples at calcination temperature of 800 ºC (a) and 900 ºC (b) From all the XRPD results, apart from characteristic diffraction peaks of TiO2 and wurtzite crystals phase, no strange diffraction peaks was observed. This indicates that there is only surfaced contact between TiO2 and wurtzite particles and no formation of strange crystals phase in all of the samples. This result shows that the production of GaN from the precursors occurred without the formation of intermediate Ga2O3 crystals. However, the possibility that amorphous Ga2O3 is formed prior to the production of GaN cannot be excluded. This differs from previously (a) (b) (a) (b) Nguyen Cao Khang 166 reported findings in other studies which showed that the conversion of GaN from Ga(NO3)3 precursors usually proceeds through the formation of intermediate GaxOy [13]. TEM photographs of composite nanoparticles prepared at different ratios of [Ti] and [Ga] are shown in Fig. 5. An analysis of TEM photographs shows clearly that the sample contains irregular forms and aggregated particles with an average size of 10 nm. Fig.5. TEM photographs of TiO2/GaN powder obtained at (a) 500 ºC and (b) 900 ºC Fig. 6 shows the corresponding UV-vis diffuse reflectance spectra of pure TiO2 nanostructure anatase and TiO2/GaN composite prepared with [Ti]/[Ga] equal to 3/1 (a) and 6/1 (b) as a function of calcination temperature. The composite samples have an apparent absorption in the visible region between 400 and 600 nm (the samples are bright yellow in color). At lower calcination temperatures (below 700 o C), the optical absorption edge of synthesised samples shifted negligibly, whereas when the calcination temperature was increased to higher than 800 o C, a red- shift in the optical absorption edge of samples toward the visible light region, at 600 nm, was observed. The negligible increase of absorption corresponding to the below 700 o C annealed samples could be explained by the influence of the N doping level on the band gap structure of the TiO2. A similar result was reported for the N-doped TiO2 catalyst prepared using similar methods [14]. 300 400 500 600 TiO 2 /GaN (S31) [Ti]/[Ga] = 3/1 800 o C 900 o C 700 o C Anatase A b s. ( a. u .) Wavelength (nm) 300 400 500 600 TiO 2 /GaN (S61) [Ti]/[Ga] = 6/1 900 o C 800 o C 700 o C Anatase A b s. ( a. u .) Wavelength (nm) Fig.6. UV-vis diffusive reflectance spectra of pure Anatase and TiO2/GaN obtained with [Ti]/[Ga] equal to 3/1 (a) and 6/1 (b) Fig. 7 shows an insignificant difference between the UV-vis diffuse reflectance spectra of S31-700 o C, S61- 700 o C and TiO2/NH3-700 o C. This is easy to understand because there was no formation of any strange phase in the S31 and S61 samples. When calcination temperatures were higher than 700 o C, a rutile phase of TiO2 in the composite samples started to appear and it (a) (b) Preparation and characterization of TiO2/GaN nanostructure composite for a degradation reaction 167 predominated over the other phases in the 800 and 900 o C annealed samples (as seen in the XRD pattern results). The energy band gap of the rutile phase is 3.0 eV, corresponding to excited light with λ equal to 420 nm as seen in the UV-vis diffuse reflectance spectra of S31 - 900 oC, S61 - 900 o C. However, noticeably, in the spectra of these samples, there were two individual absorption regions. The first absorption region corresponding to wavelengths from 380 to 420 nm is well known due to the optical transition of electrons from VB to CB in a pure rutile phase. The second absorption region corresponds to wavelengths from 420 to 550 nm. The presence of this region could be explained by the concurrent influence of the N doping level and the formation of the wurtzite phase on the band gap structure of the TiO2. It is possible that the formation of the wurtzite phase effected the absorption ability of the composite synthesized at 800 and 900 o C as seen when comparing the UV-vis diffuse reflectance spectra of S31 - 900 o C and S61 - 900 o C with the TiO2/NH3-900 o C sample. 300 350 400 450 500 550 Anatase TiO 2 /NH 3 S61S31 700 o C A b s. ( a .u .) Wavelength (nm) 350 400 450 500 550 600 800 o C TiO 2 /NH 3 S61 S31 A b s. ( a. u .) Wavelength (nm) 350 400 450 500 550 600 TiO 2 /NH 3 S61 900 o C S31 A b s. ( a .u .) Wavelength (nm) Fig.7. UV-vis diffusive reflectance spectra of photocatalysts at different calcination temperatures: 700 ºC (a), 800 ºC (b), and 900 ºC (c) For the photocatalytic tests, methylene blue (MB) was used as the model organic pollutant to evaluate the photocatalytic activity of the TiO2/GaN composite nanomaterials. In Fig. 8, we show the visible light-induced photocatalytic activity for the degradation of MB over a selective photocatalyst synthesized under different conditions. The P25 samples are essentially inactive Nguyen Cao Khang 168 since they could not be activated by visible light due to their large energy band gap (3.2 eV). However, with the similar band gap, nano materials indicates the surpassingly in photocatalytic activity, as shown in Fig. 8. The photocatalytic activity of selected composites was enhanced greatly under visible light irradiation. This can be explained basing on following: First, the formation of the wurtzite phase aside from the phase types of TiO2 at calcination temperatures of more than 800 ºC as indicated in the XRPD pattern results. Second, the absorption edge corresponding to synthesized samples shifted into the visible region as shown in UV-vis diffusive reflectance spectra. The N doping level had an obvious influence on the photocatalytic activity of the TiO2 samples when comparing the catalytic performance of S31 - 700 ºC, TiO2/NH3 - 700 ºC with TiO2 nano anatase. Although the N doping level was effected positively, the inactive photocatalytic result of TiO2/NH3 - 900 ºC could be explained by the formation of the catalytically less active rutile phase. Differing from the TiO2/NH3 - 900 ºC sample, the photocatalytic performance of both S31 - 900 ºC and S61 - 900 ºC samples was fairly active. Possible reasons include the concurrent influences of the N doping level, the evident formation of wurtzite and the obstinate presence of the anatase phase. The N doping level and the evident formation of wurtzite possibly effected on absorptive ability of materials whereas anatase is the catalytically active phase of TiO2. 0 1 2 3 50 60 70 80 90 100 C /C 0 Time (h) P25 Anatase S31-700 o C S61-900 o C S31-900 o C TiO 2 /NH 3 -700 o C TiO 2 /NH 3 -900 o C Fig.8. Photocatalytic degradation of MB over synthesised samples visible light irradiation (λ > 430 nm) 3. Conclusion TiO2/GaN nanosized powders were successfully prepared using simple means with urea as the nitrogen precursor. The TEM image shows that the TiO2/GaN sample had an average size of 10 nm. The XRD results of analyses in this study indicated that the TiO2/GaN is in the anatase phase when heated to a temperature lower than 700 o C and a rutile phase when heated to 800 o C or higher. From the absorption spectra, the band gap of the TiO2 samples was estimated to be 2.6 to 2.9 eV due to temperature or the mole ratio of Ti/Ga, while the band gap of TiO2 was 3.2 eV. For the decomposition of MB, the best TiO2/GaN sample is about 2 times higher than pure TiO2 with 40% of MB decomposed after 4 hours. Acknowledgment: This work was supported by the Ministry of Education and Training Grant No. B2017-SPH-30. Preparation and characterization of TiO2/GaN nanostructure composite for a degradation reaction 169 REFERENCES [1] A. Fujishima and K. Honda, 1972. Electrochemical Photolysis of Water at a Semiconductor Electrode. Nature, 238, pp. 37-38. [2] M. Grätzel, 2001. Photoelectrochemical celle. Nature, 414, pp. 338-344. [3] A. Hagfeldt and M. Gräetzel, 1995. Light-Induced Redox Reactions in Nanocrystalline Systems. Chem. Rev., 95, pp. 49-68. [4] A L. Linsebigler, G Lu and J.T. Yates, 1995. Photocatalysis on TiOn Surfaces: Principles, Mechanisms and Selected Result. Chem. 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