Correlation of Surface Properties and Photocatalytic Activity of Nanocrystalline TiO2 on the Synthesis Route

Present work describes the synthesis of nanocrystalline TiO2 photocatalyst using sol-gel and solution combustion methods and their characterisation by powder X-ray Diffraction, Diffuse Reflectance Spectroscopy, surface area measurement, FT-IR, FT-Raman, Thermo Gravimetric Analysis, and Transmission Electron Microscopy. Their photocatalytic activity was evaluated by the degradation of two nitro aromatic pollutants, viz. para-nitroaniline (PNA) & meta-dinitrobenzoic acid (DNBA) commonly observed in nitroaromatic plants. Performance of the synthesized catalysts was compared with commercial Degussa P25 sample. The photocatalytic degradation and total mineralization were monitored using UV/VIS spectrophotometer and total organic carbon content analysis respectively. The materials properties such as crystallinity and surface hydroxyl group on the nanocrystalline TiO2 played crucial role for the total mineralization of the nitroaromatics.


Introduction
Subsequent to reporting of photocatalytic splitting of water on a TiO 2 electrode under ultraviolet (UV) light [1], a vast area of semiconductor mediated photocatalytic processes has opened. Among the various semiconductors, TiO 2 is widely studied for photocatalytic and other applications such as photovoltaics, photo-/ electrochromics and sensors [2]. Photocatalytic decomposition of pollutants is one of the prime applications of TiO 2 and is potentially used as the most efficient and eco-friendly process. Semiconductor sensitized oxidative mineralization of organic pollutant was first reported by Ollis and coworkers [3,4] for the mineralization of halogenated hydrocarbons, including trichloroethylene, dichloromethane, chloroform and carbon tetrachloride. The photocatalytic activity of a semiconductor is mainly determined [5] by (i) the light absorption properties, (ii) oxidation and reduction rates on the surface by the hole and electron, and (iii) the electron-hole recombination rate.
Higher photocatalytic reaction rates can be achieved on the surfaces with large surface area and constant surface density of adsorbents. However, with surface being defect site, the rate of recombination of the electron-hole pair will increases with increase in surface area [6]. Surface with minimum bulk defects (higher crystallinity) is obtainable by high temperature treatment. However, this in turn, can induce the aggregation of small nanoparticles thereby decreasing the surface area.
Hence, the relation between the structural & textural properties of the semiconductor and the photocatalytic activities is to be understood for determining an optimum photocatalyst. The physical and photochemical characteristics such as morphology, crystal phase, specific surface area, particle aggregate size and surface density of OH groups in the TiO 2 appear to be highly dependent on the exact experimental conditions used during synthesis. Among the various methods [2] used for preparation nanocrystalline TiO 2 , sol-gel route is a simple and versatile process for the synthesis of nanocrystalline materials. The solution combustion method is a single-step process that results into fine particles and large surface area oxide materials.

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Nano Hybrids Vol. 1 Present work describes synthesis of nanocrystalline TiO 2 using sol-gel [7] as well as solution combustion methods [8]. In a typical sol-gel process, the hydrolysis and polymerization reactions of the precursors, generally inorganic metal salts or metal organic compounds, such as metal alkoxide, leads to a formation of a suspension or a sol and complete polymerization and loss of solvent leads to the transition from the liquid sol into a solid gel phase [9][10][11]. Low content of water, low hydrolysis rates, and excess titanium alkoxide in the reaction mixture will lead to the formation of three dimensional polymeric skeletons with close packing that result from the development of Ti-O-Ti chains. With high hydrolysis rates for an average amount of water the formation of Ti(OH) 4 is favored [12]. In solution combustion method, an aqueous redox mixture containing stoichiometric amounts of metal salts and water-soluble fuel is heated rapidly. Nano-TiO 2 with higher dispersing ability, in water, with easy separation of the catalyst by centrifugation has been reported using this method [13,14].
Different characterization techniques were used to study the surface properties of the nanocrystalline TiO 2 obtained by these two methods. Their photocatalytic activity was evaluated for the degradation of nitro aromatic compounds. We have tried to correlate the surface properties of the two synthesized catalysts with their obtained photocatalytic activities. A comparative study of the synthesized TiO 2 catalysts with standard Degussa P25 sample, for their photocatalytic activity, has also been carried out. The selected nitro aromatic compounds for the degradation studies are pnitroaniline (PNA) and m-dinitrobenzoic acid (DNBA) which are commonly employed in the production of dyes and explosives. Nitro-aromatic compounds, in general, are toxic in nature, resistant to degradation and may act as an inhibitor for the biodegradation of other compounds in the waste materials [15]. DNBA is commonly found in the vicinity of ammunition plants and it's presence in ground-and drinking water is of great environmental concern. Identification of DNBA, from polar metabolites of explosives, and its fragmentation behaviour has been reported [16,17].
Toxicity of PNA in human body and its metabolites have identified and have reported from a death Nano Hybrids Vol. 1 case that happened after oral p-nitroaniline intake which occurred in Berlin in 2005 [18].
Photocatalytic as well as photolytic degradation of PNA has been reported by different research groups [19,20]. corresponds to the diffraction from the (101) crystal plane of anatase phase, (110) crystal plane of rutile phase and the (121) crystal plane of brookite phase respectively [21,22]. The phase content of a sample has been calculated from the integrated intensities of anatase (101), rutile (110), and brookite (121) peaks using the following equations:

Experimental
Where, W A and W B represent the mass fractions of anatase and brookite, respectively. A B is the integrated intensity of the brookite (121) peak, and K A and K B are two coefficients with values of 0.886 and 2.721 respectively. The anatase crystallite size of the catalysts was determined from the characteristic peak at 2θ = 25.3 for the (101) plane of the anatase phase using the Scherer formula [23], with a shape factor (K) of 0.9 as follows: Where, W = W b -W s , W b is the broadened profile width of experimental sample. Ws is the standard profile width of reference silicon sample, λ is the wavelength of X-ray radiation. The crystallinity of the catalysts was calculated with reference to P25 Degussa by taking the average of the five major anatase peaks of the catalyst (2θ = 25.3, 37.7, 47.9, 53.9 and 55.2 ).

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The Brunauer-Emmett-Teller (BET) surface areas [24] of the powders were determined from the nitrogen adsorption isotherm measured at 77.4 K using a volumetric adsorption set-up (Micromeritics ASAP 2010). Catalyst samples were degassed at 350 °C for 2 h under a vacuum of 5 x 10 -5 mm Hg prior to N 2 adsorption measurement. FT-IR analysis was carried out on a Perkin-Elmer GX spectrophotometer. The samples were incorporated in KBr pellets for the measurements.
The spectra were recorded in the range of 400-4000 cm -1 with a resolution of 4 cm -1 . FT-Raman spectra were recorded with Thermo Electron Corporation (Nicolet 6700) spectrophotometer. The Diffuse Reflectance Spectra were taken at room temperature in the range of 250-650 nm using Shimadzu UV-3101PC spectrophotometer equipped with an integrity sphere and BaSO 4 was used as the reference material. The bandgap energies of the catalysts were calculated according to the equation, where h is the plank's constant, c is the velocity of the light and λ is the wavelength of the adsorbed radiation.
The morphology of the synthesized catalysts was determined using Transmission Electron  Photocatalytic Activity. Photocatalytic degradation of p-nitroaniline and m-dinitrobenzoic acid was carried out using a locally fabricated reactor consisting of two parts [25]. The first part is a double wall quartz vessel having an empty chamber at the centre to immerse the irradiation source.
This quartz jacket has an inlet and an outlet meant for water circulation by which the temperature can be maintained throughout the reaction. The UV irradiation source used [26] was a 125W mercury Acetonitrile -Water (80% and 20%) mixture was used as a mobile phase with a flow rate of 0.2 ml min -1 in every analysis.

Results and Discussion
Structural Properties. X-ray diffraction patterns of the synthesized and P25 catalysts are shown in The full width at half-maximum (FWHM) of the diffraction peak is related to the size of the nanomaterials [27] as narrower the peak larger will be the crystallite size. The reinforcement of diffraction of the X-ray beam because of the periodicity of the individual crystallite domains leads to a tall narrow peak. If the crystals are randomly arranged or have low degrees of periodicity, it results into a broader peak. The broadened peak of SC shows the lowest crystallinity among the catalysts studied (Fig. 1).

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Nitrogen sorption isotherms (Fig. 2.) obtained for the two synthesized catalysts are of type IV (BDDT classification) [28] and have hysteresis loops. Type H2 hysteresis loops were obtained at low relative pressure between 0.4 and 0.7 which normally is observed in the case of pores with narrow necks wider bodies (ink bottle pores). Type H3 hysteresis loops were observed at high relative pressure between 0.8 and 1.0, often found with aggregates of plate like particles giving rise to slit like pores [21]. Pore size distribution curves of SG and SC are shown in the inset of Broad IR band at 3400-3437 cm -1 range is characterized by stretching -OH groups. IR band at 1020 cm -1 which was observed in P25 and SG is attributed to the Ti-OH deformation vibrations [29]. Literature contains a large amount of data on IR-spectral regions of deformation vibrations of adsorbed water molecules (~ 1620 cm -1 ) and was observed in all catalysts at 1622-1630 cm -1 range [29]. Intense band at 1020 cm -1 in P25 indicates the presence of higher amount of surface hydroxyl groups than in other synthesized TiO 2 catalysts.  Fig. 3. FT-IR spectra of synthesized and standard P-25 Degussa nanocrystalline titania catalysts.
FT-Raman spectra's of the two synthesized catalysts is shown in Fig. 4. All the six Ramanactive fundamentals [30], three E g modes centered around 144, 197, and 637 cm -1 , two B 1g modes at 397 and 517 cm -1 and one A 1g mode at 513 cm -1 , in the vibrational spectrum of the anatase TiO 2 were observed. Besides, a peak at around 444 cm -1 was present in the spectrum of SC corresponding to rutile phase of TiO 2 . It has been reported [30] that the most intense Eg (1) mode shows the blue shift and significant broadening with decrease in both crystallite size and crystallinity [31]. The observation is in agreement with the XRD results that SC has lowest crystallinity which leads toward in the broadening of Eg (1) mode with slight blue shift as compared to SG.   However there wasn't any reduction in the TOC value of these samples even after 4 hours emphasizing the efficiency of TiO 2 as a photocatalyst.
The results demonstrated that the catalytic activity of the synthesized photocatalysts was different due the difference in their surface area, crystallinity, phase composition, band gap and surface hydroxyl content present in it. Among the three TiO 2 photocatalyst samples studied P25 was dominated over SG and SC for the mineralization of both PNA and DNBA. This could be attributed to its higher crystallinity (from XRD results Fig. 1), and slightly lower band gap (from DRS analysis) than SG and SC. It has been reported that the photocatalytic efficiency of amorphous TiO 2 is negligible indicating that crystallinity is one of the important requirements for TiO 2 to be an active photocatalyst [33]. Lower band gap could help P25 to absorb more radiations from the incident light 74 Nano Hybrids Vol. 1 which might be able to enhance the electron-hole pair generation. It is recognized that a composite of anatase and rutile phases is beneficial for suppressing the recombination of photogenerated electrons and holes and by this means enhancing photocatalytic activity [34][35][36].
A large surface area can be determining factor in certain photocatalytic degradation reactions, as large amount of adsorbed organic molecules promotes the reaction rate [37,38]. However, powders with large surface area are usually associated with large amounts of crystal defects, which favor the recombination of electrons and holes leading to a lower mineralizing activity [6]. That could be the reason why SG and SC with higher surface area (80 and 95 m 2 /g respectively) than P25 (54 m 2 /g) showed lower mineralizing activity.

Conclusion
TiO 2 photocatalyst syntheses by Sol-gel and solution combustion methods produces nanocrystalline TiO 2 with difference in phase contents, crystallinity, crystallite size, surface area, and band gap.
Total mineralization of both PNA and DNBA was higher when P25 Degussa TiO 2 was used as the photocatalyst followed by SG and SC. Higher crystallinity and combination of anatase rutile phase of P25 over the synthesised catalysts are accountable for the greater mineralising power of P25 Degussa TiO 2 photocatalyst. Hence, the methods which can produce nanocrystalline TiO 2 with high crystallanity, low bandgap and higher surface area, along with other advantages, are preferable.