The Syntheses, Characterizations, and Photocatalytic Activities of Silver, Platinum, and Gold Doped TiO2 Nanoparticles

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  • ABSTRACT

    Different weight percentages of Ag, Pt, and Au doped nano TiO2 were synthesized using the acetic acid hydrolyzed sol-gel method.The crystallite phase, surface morphology combined with elemental composition and light absorption properties of the doped nano TiO2 were comprehensively examined using X-ray diffraction (XRD), N2 sorption analysis, transmission electron microscopic (TEM),energy dispersive X-ray, and DRS UV-vis analysis. The doping of noble metals stabilized the anatase phase, without conversion to rutile phase. The formation of gold nano particles in Au doped nano TiO2 was confirmed from the XRD patterns for gold. The specific surface area was found to be in the range 50 to 85 m2/g. TEM images confirmed the formation a hexagonal plate like morphology of nano TiO2.The photocatalytic activity of doped nano TiO2 was evaluated using 4-chlorophenol as the model pollutant. Au doped (0.5 wt %) nano TiO2 was found to exhibit higher photocatalytic activity than the other noble metal doped nano TiO2, pure nano TiO2 and commercial TiO2 (Degussa P-25). This enhanced photocatalytic activity was due to the cathodic influence of gold in suppressing the electron-hole recombination during the reaction.


  • KEYWORD

    4-chlorophenol , Hexagonal plate , Noble metal doped TiO2 , Photocatalytic activity , Sol-gel

  • 1. Introduction

    Semiconductor photocatalysis has become a promising technology for environmental remediation. Of the various types of photocatalyst, titanium dioxide is now under intensive investigation for practical application to antimicrobial, deodorization,air and water purification and wastewater treatment because of its favorable physico-chemical properties, low cost, ease of availability and high stability [1, 2]. However, it is active only under UV irradiation (λ = 388 nm), which accounts for less than 5%of solar light. There have been various attempts to utilize visible light with TiO2 photocatalysis, including dye sensitization, semiconductor coupling and impurity doping [3-6].

    The doping of TiO2 with different metals and metal ions has frequently been attempted to not only retard the fast recombination of electron-hole pair, but to also enhance the absorption of visible light by providing defective states in the band gap [7]. In these cases, modifications of the TiO2 surface with a noble metal and its ions have been successful used to improve the photocatalytic activity of TiO2 and; thus, increase the quantum yield due to an increase in the rate of electron transfer to the oxidant.However, the electronic transitions from the valence band to defective states or from defective states to the conduction band may be allowed under sub-band gap illumination to show the activity of visible light. Several studies have focused on the optical properties of TiO2 films and the dosage of Pt, Pd and Ag into the films to absorb visible light [8]. As the Fermi levels of these noble metals are lower than that of TiO2, photoexcited electrons can be transferred from the conduction band to the metal particles deposited on the surface of TiO2, while photogenerated valence band holes remain in the TiO2. These activities greatly reduce the possibility of electron-hole recombination, resulting in efficient separation and stronger photocatalytic reactions [9].The band gap of TiO2 can be decreased and visible light absorption increased due to the introduction of a noble metal into the lattice of TiO2[10].

    Anpo and Takeuchi [11] employed electron spin resonance(ESR) signals to investigate the electron transfer from TiO2 to Pt particles. It was found that Ti3+ signals increased with increasing irradiation time; whereas, the loading of Pt reduced the amount of Ti3+. They concluded that the occurrence of electron transfer might take place from the TiO2 to Pt particles. As electrons accumulated on the noble metal particles, their Fermi levels shifted closer to the conduction band of TiO2, resulting in more negative energy levels [12]. Furthermore, small metal particles deposited on the TiO2 surface exhibit a more negative Fermi level shift. The electrons accumulated on the metal particles can be transferred to adsorb dissolved oxygen by the photogenerated electrons to produce superoxide anion radicals (O2-) on the surface. These superoxide radicals are the critical species for the decomposition of pollutant molecules [13]. Therefore, noble metals with a suitable work function can help electron transfer, leading to enhanced photocatalytic activity.

    Chao et al. [14] studied the effect of Ag doping on TiO2 using the sol-gel method and found that Ag doping promoted anatase to rutile transformation and increased the specific surface area.As a result of electron?hole pair separation, the photocatalytic activity improved the efficiently. Moreover, Ag nanoparticles could facilitate electron excitation by creating a local electric field in the ultraviolet region, and the plasmon resonance effect of Ag nanoparticles in the visible light region also showed reasonable enhancement in this electric field [15, 16]. Dobosz and Sobczy?ski [17] reported that Ag doped TiO2 caused 3.6 times higher mineralization of phenol than pure TiO2. Sclafani and Herrmann [18] investigated the influence of deposited silver on the photocatalytic activity of rutile and anatase allotropic forms of TiO2 and found that silver deposition was beneficial for the activity of the rutile and detrimental for the anatase phase in the oxidation of 2-propanol. Similarly, Moonsiri et al. [19] reported that Pt doped TiO2 showed a slight increase in the degradation of 4-chlorophenol (4-CP) in the absence of oxygen, but this was decrease in the presence of oxygen. Moreover, the deposited Pt particles were demonstrated to serve as trapping centers for electrons excited from TiO2 during irradiation, which led to improved quantum efficiency for photochemical reactions [20].Gold is capable of producing the highest Schottky barrier of all the metals; thus, facilitating electron capture. Zanella et al. [21]prepared Au/TiO2 photocatalyst using a deposition-precipitation method and showed improved activity in the oxidation of CO. Li et al. [22] reported the synthesis of a highly active mesoporous titania photocatalyst, with homogenously embedded gold nanoparticles within the framework, to enhance light absorption property and improve the quantum efficiency.

    4-CP, a known endocrine disruptor, is a toxic and non-biodegradable organic compound present in the wastewater of pulp and paper, dyestuff, pharmaceutical and agrochemical industries [23]. This compound is currently removed from the wastewater by conventional treatment methods. Semiconductor mediated photocatalytic oxidation has been accepted as a promising alternative to conventional methods because most pollutants can be completely mineralized to CO2 with a suitable catalyst in the presence of UV light illumination. In this article,the preparation of different weight percentages of Ag, Pt, and Au doped TiO2 nanoparticles as well as exhaustive characterizations of these materials are presented. In addition, the photocatalytic activity of these materials in the mineralization of 4-CP in aqueous medium was also attempted in order to evaluate the efficiency of these materials as photocatalysts.

    2. Materials and Methods

       2.1. Materials

    Titanium(IV) tetraisopropoxide (Ti(OiPr)4), silver nitrate (Ag NO3), hexachloroplatinic(IV) acid hexahydrate (H2Pt Cl6.6H2O)and gold(III) chloride (AuCl3) were purchased from E. Merck India, and usedfor the preparation of noble metal doped nano TiO2. Glacial acetic acid (SRL Ltd.) and 4-CP (SRL Ltd.) were used as such. Double distilled water was used for the preparation of all solutions.

    The synthesis procedure adopted by Venkatachalam et al.was used for the preparation of nano TiO2. Titanium(IV) isopropoxide[16]; the glacial acetic acid and water were maintained in the molar ratio 1:10:350. Titanium(IV) isopropoxide (18.6 mL)was hydrolyzed using glacial acetic acid (35.8 mL) at 0˚C. Water(395 mL) was drop wise added to this solution under vigorous stirring for 1 hr, followed by ultrasonication for 15 min. The stirring was continued for a further 1 hr. The prepared solution was kept in the dark for 24 hr to allow the nucleation process. After this period, the solution was placed in an oven at 70˚C for 12 hr to allow the gelation and aging processes. The gel was then dried at 100˚C and subsequently crushed into fine powder and calcinated in a muffle furnace at 500˚C for 5 hr. The calcination temperature was attained at a heating rate of about 5˚C per min. The same procedure was adopted for the preparation of Ag, Pt and Au doped nano TiO2 by the addition of stoichiometric amounts of silver nitrate, hexachloroplatinic(IV) acid and gold(III) chloride,respectively. The doping concentrations of the noble metals are expressed in wt %.

       2.2. Catalyst Characterization

    The X-ray diffraction (XRD) patterns of pure and noble metal doped nano TiO2 were recorded on a PANalytical X’Pert Pro Xray diffractometer, using CuKα radiation as the X- ray source.The diffractograms were recorded in the 2θ range 10-80, in steps of 0.02 with a count time of 20 sec at each point. The particle size was determined from the broadening of the diffraction peak using the Scherrer formula, D = Kλ / β cosθ, where D is the crystallite size (nm), K the Scherrer constant, λ the wavelength of the X-ray source, β the full width at half-maximum and θ Bragg’s angle. Transmission electron microscopic (TEM) images were recorded using a JEOL TEM-3010 (JEOL, Tokyo, Japan) electron microscope, operating at an accelerating voltage of 300 KeV.Scanning electron microscopic (SEM) images were recorded using a scanning electron microscope (HITACHI COM-S-4200;Hitachi, Tokyo, Japan). The surface area of the nano TiO2 was determined using a Belsorp mini II sorption analyzer (BEL JAPAN Inc., Osaka, Japan), with nitrogen as the sorbent, at 77 K. Prior to analyses, the samples were degassed for 3 hr at 250℃, under vacuum (10-5 mbar) in the degas port of the adsorption analyzer.FT-IR spectra were recorded using a FT-IR spectrometer (Nicolet Avator 360). DRS UV-vis spectra of the synthesized materials were recorded in the scan range 210-900 nm, using a UV-Visible spectrophotometer (Shimadzu model 2450; Shimadzu, Kyoto,Japan), equipped with an integrating sphere, with BaSO4 used as the reference.

       2.3. Photocatalytic Degradation Studies and Analysis

    The photocatalytic degradation of 4-CP was performed in aqueous medium in a slurry batch reactor. A cylindrical photochemical reactor, 30 x 2 cm (height x diameter), provided with a water circulation arrangement to maintain the temperature in the range 25-30℃, was used in all the experiments. The UV

    irradiation was carried out using 8 x 8 W low pressure mercury lamps built into a lamp housing, with polished anodized aluminium reflectors placed 12 cm away from the reactor. These lamps emit predominantly UV radiation at a wavelength of 254 nm, with another 8 lamps arranged alternately to emit UV radiation at a wavelength of 365 nm. The reactor set-up was covered with aluminium foil followed by a black cloth to prevent UV light leakage. A 1,000 mg L-1 4-CP stock solution was prepared in double distilled water and diluted to the required concentration.In a typical procedure, 100 mg of the catalyst was added to 100 mL 4-CP solution of 100 mg L-1 and the resultant slurry stirred for 30 min to attain equilibrium. The slurry was then irradiated with UV light of either 254 or 365 nm, with continuous purging of air using CO2.

    Aliquots were withdrawn at specific time intervals and analyzed after centrifugation and filtration through a 0.2 ㎛ membrane to remove the catalyst particles. The extent of mineralization of 4-CP was monitored using a total organic carbon analyzer (TOC) (Shimadzu TOC-V CPN; Shimadzu).

    3. Results and Discussion

       3.1. XRD

    The XRD patterns of pure and different wt % (0.05, 0.1, 0.3,0.5, 0.8, and 1.0) Ag doped nano TiO2 catalysts are shown in Fig.1. The patterns exclusively corresponded to the anatase phase,without any evidence of the rutile structure. There was no pattern corresponding to doped Ag and; hence, Ag particles were not adequately present to produce their characteristic patterns.The intensities of the patterns for 0.05 and 0.1 wt % Ag doped nano TiO2 were less than that of pure nano TiO2. In addition,the patterns were also broadened compared to the others; thus,clearly indicating suppression of the growth of TiO2 crystals by the doping with Ag [24]. Hence, Ag at the same loading rates may be dispersed uniformly on the active sites of the crystals of TiO2 and prevent its dimensional growth. However, with high rates of Ag doping, the suppression of growth was not clearly seen. Silver may not incorporate into the lattice of TiO2 due to its higher ionic size (1.26 ?) than titanium (0.605 ?) [24]. It is also evident from Fig. 1 that the 2θ values of all the Ag doped nano TiO2 catalysts exhibited similar patterns. The intensities of the patterns increased with increasing dopant concentration above 0.1 wt %. Hence, the formation of large crystallites of TiO2 may be favoured by increasing the amount of Ag. Therefore, an increase in the size of Ag crystallites can enhance the TiO2 crystallite size. The crystallite sizes of pure and Ag doped nano TiO2 were calculated using the Scherrer equation, the values of which are given in Table 1.

    The XRD patterns of pure and different wt % (0.1, 0.3, 0.5, and 1.0) Pt doped nano TiO2 catalysts are shown in Fig. 2. Irrespective of the wt % of Pt doping, all the materials exhibit similar patterns, without any patterns corresponding to platinum. This revealed they may possess small particle sizes, below the detectability limit of XRD. The intensities of the patterns were similar for all Pt doped nano TiO2, indicating similarTiO2 particle sizes. It was concluded that Pt may not interfere with the growth of TiO2 crystallites during the synthesis, as no Pt(OH)4 particles were adsorbed on the active sites of TiO2 crystallites.

    The XRD patterns of pure and different wt % (0.1, 0.3, 0.5, and 1.0) Au doped nano TiO2 catalysts are shown in Fig. 3. The intenadsities of the patterns were nearly equal for all Au doped nano TiO2. The initial product, Au(OH)3, formed during the synthesis was not adsorbed on the active phase of the TiO2 crystallites like Pt doped nano TiO2. The weak diffraction patterns shown in Fig.3d for 1.0 wt % gold doped nano TiO2 at 2θ = 38.2˚, 44.5˚, 64.6˚,and 77.6˚, corresponding to (111), (200), (220), and (311) planes,respectively, are for the standard face centered cubic phase of Au[25]. The formation of Au crystallites in 1.0 wt % Au doped nano TiO2 was also verified in the DRS UV-vis analysis, as discussed below.

       3.2. Nitrogen Sorption Studies

    The nitrogen adsorption?desorption isotherms of pure and Ag doped (0.1, 0.3, 0.5, and 1.0 wt %) nano TiO2 catalysts are shown in Fig. 4. Hysteresis loops were observed in all the isotherms,which is clear evidence of the formation of interparticle

    voids or pores in all the Ag doped nano TiO2. The specific surface area, pore volume and pore diameter of pure and Ag doped nano TiO2 derived from the nitrogen sorption analyses are presented in Table 1. The surface areas of 0.1 and 0.3 wt % Ag doped nano TiO2 were higher than that of nano TiO2 by about 22 to 25 m2/g;whereas, the increase was only 8 m2/g in the case of 0.5 wt % Ag doped nano TiO2. However, 1.0 wt % Ag doped nano TiO2 showed a surface area of 5 m2/g less than nano TiO2. This observation was in accordance with the intensity of the XRD patterns in Fig.1. The pore volume of Ag doped nano TiO2 was higher than pure nano TiO2. Since the surface area of 1.0 wt % Ag doped nano TiO2 was less than pure nano TiO2, the pore volume may contribute significantly to the surface area. Therefore, only a fraction of particles may be used for the formation of interparticle voids, with the rest of the particles possibly remaining free. Moreover, the pore diameters of the catalysts also exhibited similar results. For example, 0.1 wt % Ag doped nano TiO2 showed a smaller pore diameter and higher surface area than 1.0 wt % Ag doped nano TiO2. Therefore 0.1 wt % Ag doped nano TiO2 may use more particles for the formation of interparticle voids than 1.0 wt % Ag doped nano TiO2.

    The nitrogen adsorption?desorption isotherms of pure and Pt doped (0.1, 0.3, 0.5, and 1.0 wt %) nano TiO2 are shown in Fig.5. All the materials exhibit similar features, like the Ag doped nano TiO2. The formation of interparticle voids was evident from the hysteresis loop. The surface areas of the catalysts derived from the isotherms are presented in Table 1, and were found to vary in the range 67-72 m2/g. The surface area of pure nano TiO2 was higher than the Pt doped nano TiO2. However, an increase in the amount of Pt doping did not significantly influence the surface area, which was also clearly seen from the enhanced adsorption of N2 in the nano TiO2 compared to other doped nano TO2. The interparticle pore volume of pure TiO2 was higher than for the doped TiO2, which was evident from the large height of the capillary condensation in the case of pure TiO2. Hence, it was concluded that there may be higher interparticle interactions in pure nano TiO2 than Pt doped nano TiO2. The nitrogen adsities

    sorption?desorption isotherms of pure and Au doped (0.1, 0.3,0.5, and 1.0 wt %) nano TiO2 are shown in Fig. 6. The isotherms showed similar characteristics to those of the Ag and Pt doped nano TiO2. Hence, Au also suppressed interparticle interactions and prevented the formation of interparticle voids, similar to Pt.As the result of the formation of tiny dopant particles on the active site of TiO2, interparticle interactions may be suppressed by isolation.

       3.3. TEM

    The TEM images of 0.5 wt % Ag doped nano TiO2 are shown in Figs. 7a and b. The particles appeared to have different shapes and sizes in the TEM image. Agglomeration was clearly evident and the lattice fringes for a single particle were clearly seen in the TEM images (Fig. 7b). The products consisted of well crystallized nanosized TiO2, which was nearly uniform, especially in diameter. The particle size was close to 5 to 10 nm, with the observed sizes of the crystallites comparable to those calculated from the XRD patterns. Thus, the presence of nano sized particles was confirmed from the TEM images. The selected area electron diffraction (SAED, inset Fig. 7a) pattern showed distinct

    and good diffraction rings corresponding to the anatase phase[26]. The intensity of the diffraction rings obtained from the particles indicated the crystalline nature of the particles, with a narrow particle size distribution.

    The TEM images of 0.5 wt % Pt and 0.5 wt % Au doped nano TiO2 are shown in Figs. 8 and 9, respectively. The particles had different sizes, some big particles were found to have a hexagonal plate like morphology; whereas, small particles possessed a different morphology, with a slight concentration of aggregates.Platinum and gold could be spread over the surface of TO2 particle at the atomic level. This was verified from the XRD results,as discussed above. A single hexagonal particle is presented in Figs. 8b and 9b. A clear hexagonal shaped morphology with lattice fringes was clearly evident. The uniform fringes, with a lattice spacing of 0.35 nm, corresponding to the (101) anatase phase, observed over the particle clearly endorsed that each nanoparticle consisted of a single anatase grain. The fringes of the doped anatase lattice were expanded and showed considerable waviness (indicated by the arrow in Fig. 8). These defects were possibly due to electronic stress that may exist from metal doping [27].

       3.4. EDX

    The energy dispersive X-ray (EDX) spectra of 0.5 wt % Ag, Pt and Au doped TiO2 are shown in Fig. 10a to c, respectively. The peaks corresponding to titanium, oxygen and the respective doped metal can be clearly seen in these spectra. In addition to these peaks, the presence of Cu (source) and Si (grid) were also observed. The results of an elemental analysis confirmed the homogenous distribution of metal nano particles in the TiO2 lattice.

       3.5. DRS UV-vis Spectra

    The DRS UV-vis spectra of nano TiO2 and Ag doped (0.1, 0.3,0.5, and 1.0 wt %) nano TiO2 are shown in Fig. 11. An absorption peak, with a centre at 560 nm, was observed for Ag doped nano TiO2 the intensity of which increased with increasing Ag content.This was assigned to the surface plasmon resonance peak of spatially confined electrons in Ag nanoparticles [28]. Band gap excitation occurred near 400 nm in all the spectra. However, the band gap excitation was slightly shifted to longer wavelengths

    with increasing amount of Ag dopant. This was due to the high refractive index of TiO2 and the interaction between Ag nanoparticles and the TiO2 matrix [29]. Based on the origin of excitation,the band gap values of nano TiO2 and 1.0 wt % Ag doped nano TiO2 were calculated, and the values found to be 3.28 and 3.1 eV,respectively. The band gap values of all the other Ag doped nano TiO2 were between these two values. In addition to the plasmon resonance peak, two absorbance maxima were observed between 200 and 400 nm, one close to 250 and the other at 325 nm.Therefore, this analysis illustrates the formation of two groups of nanoparticles with difference sizes. Based on the similar appearance of all the spectra, it was established that varying the Ag content during the synthesis may not influence the crystallization of nano TiO2. In other words, Ag may not enter into the lattice of TiO2 during crystallization.

    The DRS UV-vis spectra of pure and Pt doped nano TiO2 are presented in Fig. 12. For pure nano TiO2, the absorbance was close to zero over the entire visible region, covering the range 400-700 nm. However, the absorbance was slightly increased above zero for all Pt doped nano TiO2. This illustrates the origin of the plasmon band for Pt particles. However, the characteristic

    patterns of Pt metal crystallite were not observed in the XRD.Hence, the size of Pt particles may be very diminutive beside TiO2 particles; therefore, could not be detected by XRD. Again,two absorbance maxima were observed for all Pt doped nano TiO2, similar those for Ag doped nano TiO2. Hence, the Pt precursor may not influence the growth of TiO2 nano crystallites. The band gap excitation was shifted to a significantly longer wavelength on the addition of Pt to TiO2, as predicted by Maxwell-Garnett theory [30]. This can be attributed to overlapping of the origin of the band gap excitation of TiO2 with the plasmon band of Pt, since Pt metal particles do not interact with TiO2 particle.In addition, a confinement effect in metal nano particles will reduce the mean free path of electrons, resulting in variation of the optical constants from that of bulk particles [31].

    The DRS UV-vis spectra of pure and Au doped nano TiO2 are presented in Fig. 13. The spectra displayed similar characteristics to those of Ag and Pt doped nano TiO2. The absorbance in the visible region was more pronounced with higher Au doping.Moreover, the absorption peak appearing at 600 nm was ascribed to the surface plasmon resonance of Au nanoparticles[32]. Due to the high refractive index of TiO2, the surface plasmon resonance was shifted to a longer wavelength (red shift)from its value of 520 nm in water [33]. The band gap excitation was also shifted to longer wavelengths with increasing amounts of Au. This red shift may have been caused by the interaction of gold with TiO2 nanoparticles [34]. The adsorption of Au on TiO2 surfaces creates a metal-semiconductor Schottky junction,which possibly facilitates charge separation [35].

    From the DRS UV-vis analysis, it was concluded that the dopant metals; namely, Ag, Au and Pt, did not influence the crystallization of TiO2. The red shift in the band gap excitation revealed by the DRS UV-vis spectra can be attributed to the introduction of metal atom energy levels into the band gap of TiO2. The presence of energy levels below the conduction band and above the valence band may influence the photocatalytic activity and;thus, the dopant metal atom can act as electron or hole scavengers.Thus, the high level of dispersion and interaction between the dopant metal atom and TiO2 particles would be expected to enhance the catalytic activity of metal doped nano TiO2.

       3.6. Evaluation of Photocatalytic Activity of Noble Metal Doped Nano TiO2

    The degradation of 4-CP in aqueous medium using nano TiO2 catalysts was studied under UV irradiation at a wavelength of 254 nm. The progress of degradation was measured using the TOC. The plots of TOC vs. irradiation time for all the catalysts are shown in Fig. 14. The gradual decrease in the TOC with respect to irradiation time was evident for all the catalysts. The appearance of nearly similar trends in the decrease of the TOC illustrates the same mechanism of degradation prevailed with all the catalysts. Ag doped (0.5 wt %) nano TiO2 exhibited a higher 4-CP adsorption capacity than the others and; thus, showed a high rate of TOC reduction. This catalyst required about 180 minutes to bring the TOC value to zero.

    The degradation of 4-CP with 0.5 wt % Ag doped nano TiO2 was studied for 5 hr, the results of which are illustrated in Fig.15. As the chlorine in 4-CP was removed as chloride ions during degradation, the chloride ion concentration in the reaction mixture was monitored at regular intervals during the irradiation to predict the progress of degradation. The concentration of chloride ions showed a rapid increase up to 2 hr, but after 2 hr the concentration attained a steady state, illustrating completion of the degradation of 4-CP. Hence, the irradiation time for the maximum degradation of 4-CP was found to be close to 2 hr. The chloride ion concentration attained a steady value between 20 and 25 ppm after 2.5 hr. The calculated chlorine content of 4-CP was 28 ppm. As these two values were close, it was established that 4-CP was completely mineralized without any intermediate.It was also confirmed that there was no adsorption due to 4-CP without undergoing mineralization at the end of the process.The determination of the chloride ion concentration was the ultimate aim in this study for monitoring mineralization.

    The photocatalytic degradation of 4-CP was tested over 0.5 wt % Ag, Pt and Au doped nano TiO2 catalysts. The progress of the reaction was monitored by determining the TOC of aliquots withdrawn at regular time intervals. The plots of TOC vs. irradiation time for all nano TiO2 and metal doped TiO2 are depicted in Fig. 16. The rates of decreasing TOC were in the order: Degussa TiO2 < pure nano TiO2 < Ag doped nano TiO2 < Pt doped nano TiO2 < Au doped nano TiO2. Hence, the influence of metal doping on the degradation rate was clearly evident. Therefore, the band gap excitation electrons may be rapidly trapped by the doped metal before they were annihilated by falling into the holes. Such an observation has already been reported in the literature [36,37]. Another important observation in the rates of degradation by the metal doped catalysts was that the rates of decrease of TOC for the doped metals were very similar, although there were slight differences in their electro-negativities. The electro-negativities of the doped metal atoms varied in the order: Ag (1.93) 2 required 170, 150, and 120 min, respectively,for complete mineralization of 4-CP. Hence, the metal doped nano TiO2 catalysts were better than pure nano TiO2 for the photocatalytic mineralization of 4-CP. In order to verify the reproducibility of the results, the reaction was carried out three times, with similar results obtained. This revealed that the dopant metals did not escape from the surface of TiO2. Although there is no covalent bonding between TiO2 and the doped metal,the latter might be sufficiently buried inside the surface of nano TiO2 for reproducibility of the degradation rate. Such sub-surface doping could be achieved during synthesis, as the doped metal ions were in the forms of their hydroxides; the appropritiate

    precursors for getting into the surface of growing TiO2 crystals.When one or more extra TiO2 layers were formed around the metal hydroxides, the newly formed metal dots could very well buried within the TiO2 during the calcination process. At the structural level, a dopant may introduce stress into the lattice of TiO2; thus, inducing the formation of defects. Conversely, the lattice of TiO2 can impose non-typical coordination modes on the dopant metal [38].

    4. Conclusions

    Nano TiO2, and Ag, Pt and Au doped nano TiO2 were prepared using the sol-gel method. The characterization of the prepared catalysts revealed the formation of nano particles, with sizes ranging from 5 to 15 nm for all the catalysts. The light absorbance properties of all the catalysts were found to be similar,irrespective of the nature of the doped metal. Thus, the precursors of the doped metals were established as having a negligible influence on the crystallization of nano TiO2. The dopant metal particles did not show their characteristic XRD patterns in the case of Ag and Pt doped nano TiO2. This confirmed that the sizes of the dopant metal particles were below the detectability limit of this technique. However, patterns were observed for gold nanoparticles in the Au doped nano TiO2. The photocatalytic mineralization of 4-CP over the catalysts established the activity order as: Au doped nano TiO2 > Pt doped nano TiO2 > Ag doped nano TiO2 > pure nano TiO2, and established the cathodic influence of metals in suppressing electron-hole recombination during the reaction. The reactivity order followed the electronegativity and electron affinity order of the dopant metal atom.From this study, it is concluded that Ag, Pt and Au doped nano TiO2 catalysts could be better materials than bulk commercial TiO2 for the mineralization of organic pollutants.

  • 1. Fujishima A, Rao TN, Tryk DA 2000 Titanium dioxide photocatalysis [J. Photochem. Photobiol. C: Photochem. Rev] Vol.1 P.1-21 google
  • 2. Rehman S, Ullah R, Butt AM, Gohar ND 2009 Strategies of making TiO2 and ZnO visible light active [J. Hazard. Mater] Vol.170 P.560-569 google
  • 3. Moon J, Yun CY, Chung KW, Kang MS, Yi J 2003 Photocatalytic activation of TiO2 under visible light using Acid Red 44 [Catal.Today] Vol.87 P.77-86 google
  • 4. Kim S, Choi W 2005 Visible-light-induced photocatalytic degradation of 4-chlorophenol and phenolic compounds in aqueous suspension of pure titania: demonstrating the existence of a surface-complex-mediated path [J. Phys. Chem. B] Vol.109 P.5143-5149 google
  • 5. Li XZ, Li FB 2001 Study of Au/Au3+-TiO2 photocatalysts toward visible photooxidation for water and wastewater treatment [Environ. Sci. Technol] Vol.35 P.2381-2387 google
  • 6. Wu CG, Chao CC, Kuo FT 2004 Enhancement of the photo catalytic performance of TiO2 catalysts via transition metal modification. [Catal Today] Vol.97 P.103-112 google
  • 7. Mitsuhara K, Kitsudo Y, Matsumoto H 2010 Electronic charge transfer between Au nano-particles and TiO2-terminated SrTiO3(0 0 1) substrate [Surf. Sci] Vol.604 P.548-554 google
  • 8. Subramanian V, Wolf E, Kamat PV 2001 Semiconductor-metal composite nanostructures. To what extent do metal nanoparticles improve the photocatalytic activity of TiO2 films? [J. Phys. Chem. B] Vol.105 P.11439-11446 google
  • 9. Haick H, Paz Y 2003 Long-range effects of noble metals on the photocatalytic properties of titanium dioxide [J. Phys. Chem.B] Vol.107 P.2319-2326 google
  • 10. Ni M, Leung MK, Leung DY, Sumathy K 2007 A review and recent developments in photocatalytic water-splitting using TiO2 for hydrogen production [Renew. Sustain. Energ. Rev] Vol.11 P.401-425 google
  • 11. Anpo M, Takeuchi M 2003 The design and development of highly reactive titanium oxide photocatalysts operating under visible light irradiation [J. Catal] Vol.216 P.505-516 google
  • 12. Subramanian V, Wolf EE, Kamat PV 2004 Catalysis with TiO2/gold nanocomposites. effect of metal particle size on the fermi level equilibration [J. Am. Chem. Soc.] Vol.126 P.4943-4950 google
  • 13. Liu SX, Qu ZP, Han XW, Sun CL 2004 A mechanism for enhanced photocatalytic activity of silver-loaded titanium dioxide [Catal. Today] Vol.93 P.877-884 google
  • 14. Chao HE, Yun YU, Xingfang HU, Larbot A 2003 Effect of silver doping on the phase transformation and grain growth of solgel titania powder [J. Eur. Ceram. Soc] Vol.23 P.1457-1464 google
  • 15. Herrmann JM, Tahiri H, Ait-Ichou Y, Lassaletta G, Gonzalez-Elipe AR, Fernandez A 1997 Characterization and photocatalytic activity in aqueous medium of TiO2 and Ag-TiO2 coatings on quartz [Appl. Catal. B Environ.] Vol.13 P.219-228 google
  • 16. Zhao G, Kozuka H, Yoko T 1996 Sol-gel preparation and photoelectrochemical properties of TiO2 films containing Au and Ag metal particles [Thin Solid Films] Vol.277 P.147-154 google
  • 17. Dobosz A, Sobczy?ski A 2003 The influence of silver additives on titania photoactivity in the photooxidation of phenol [Water Res.] Vol.37 P.1489-1496 google
  • 18. Sclafani A, Herrmann JM 1998 Influence of metallic silver and of platinum-silver bimetallic deposits on the photocatalytic activity of titania (anatase and rutile) in organic and aqueous media [J. Photochem. Photobiol. A Chem] Vol.113 P.181-188 google
  • 19. Moonsiri M, Rangsunvigit P, Chavadej S, Gulari E 2004 Effects of Pt and Ag on the photocatalytic degradation of 4-chlorophenol and its by-products [Chem. Eng. J] Vol.97 P.241-248 google
  • 20. Wang X, Yu JC, Yip HY, Wu L, Wong PK, Lai SY 2005 A mesoporous Pt/TiO2 nanoarchitecture with catalytic and photocatalytic functions [Chem. Eur. J] Vol.11 P.2997-3004 google
  • 21. Zanella R, Giorgio S, Henry CR, Louis C 2002 Alternative methods for the preparation of gold nanoparticles supported on TiO2 [J. Phys. Chem. B] Vol.106 P.7634-7642 google
  • 22. Li H, Bian Z, Zhu J, Huo Y, Lu Y 2007 Mesoporous Au/TiO2 nanocomposites with enhanced photocatalytic activity [J. Am.Chem. Soc] Vol.129 P.4538-4539 google
  • 23. Johnson SK, Houk SK, Feng J, Houk RS, Johnson DC 1999 Electrochemical incineration of 4-chlorophenol and the identification of products and intermediates by mass spectrometry [Environ. Sci. Technol] Vol.33 P.2638-2644 google
  • 24. Cao Y, Tan H, Shi T, Tang T, Li J 2008 Preparation of Ag-doped TiO2 nanoparticles for photocatalytic degradation of acetamiprid in water [J. Chem. Technol. Biotechnol] Vol.83 P.546-552 google
  • 25. Liu Y, Chen L, Hu J, Li J 2010 TiO2 nanoflakes modified with gold nanoparticles as photocatalysts with high activity and durability under near UV irradiation [J. Phys. Chem. C] Vol.114 P.1641-1645 google
  • 26. Ma R, Sasaki T, Bando Y 2004 Layer-by-layer assembled multilayer films of titanate nanotubes Ag- or Au-loaded nanotubesand nanotubes/nanosheets with polycations [J. Am. Chem.Soc.] Vol.126 P.10382-10388 google
  • 27. Liu ZL, Cui ZL, Zhang ZK 2005 The structural defects and UV-VIS spectral characterization of TiO2 particles doped in the lattice with Cr3+ cations [Mater. Charact] Vol.54 P.123-129 google
  • 28. Stathatos E, Lianos P, Falaras P, Siokou A 2000 Photocatalytically deposited silver nanoparticles on mesoporous TiO2 films [Langmuir] Vol.16 P.2398-2400 google
  • 29. He J, Ichinose I, Kunitake T, Nakao A 2002 In situ synthesis of noble metal nanoparticles in ultrathin TiO2-gel films by a combination of ion-exchange and reduction processes [Langmuir] Vol.18 P.10005-10010 google
  • 30. Driessen MD, Grassian VH 1998 Photooxidation of trichloroethylene on Pt/TiO2 [J. Phys. Chem. B] Vol.102 P.1418-1423 google
  • 31. Bohren CF 1983 Huffman DR. Absorption and scattering of light by small particles P.329 google
  • 32. Li X, Fan T, Zhou H, Zhu B, Ding J, Zhang D 2008 A facile way to synthesize biomorphic N-TiO2 incorporated with Au nanoparticles with narrow size distribution and high stability [Microporous Mesoporous Mater] Vol.116 P.478-484 google
  • 33. Chen S, Liu Y, Wu G 2005 Stabilized and size-tunable gold nanoparticles formed in a quaternary ammonium-based room-temperature ionic liquid under gamma-irradiation [Nanotechnology] Vol.16 P.2360-2364 google
  • 34. Yang L, Li GH, Zhang LD 2000 Effects of surface resonance state on the plasmon resonance absorption of Ag nanoparticles embedded in partially oxidized amorphous Si matrix [Appl.Phys. Lett.] Vol.76 P.1537-1539 google
  • 35. Jakob M, Levanon H, Kamat PV 2003 Charge Distribution between UV-irradiated TiO2 and gold nanoparticles: Determination of shift in the Fermi level [Nano Lett] Vol.353 P.358 google
  • 36. Egerton TA, Mattinson JA 2008 The influence of platinum on UV and ‘visible’ photocatalysis by rutile and Degussa P25. J.Photochem. Photobiol [A Chem.] Vol.194 P.283-289 google
  • 37. Smirnova N, Vorobets V, Linnik O, Manuilov E, Kolbasov G 2010 Photoelectrochemical and photocatalytic properties of mesoporous TiO2 films modified with silver and gold nanoparticles [Surf. Interface Anal.] Vol.42 P.1205-1208 google
  • 38. Park JB, Graciani J, Evans J 2009 High catalytic activity of Au/CeOx/TiO2(110) controlled by the nature of the mixedmetal oxide at the nanometer level [Proc. Natl. Acad. Sci. U S A] Vol.106 P.4975-4980 google
  • [Fig. 1.] XRD patterns of pure nano TiO2 and Ag doped nano TiO2. (a)Nano TiO2 (b) 0.05 wt % (c) 0.1 wt % (d) 0.3 wt % (e) 0.5 wt % (f) 0.8wt % and (g) 1.0 wt %.
    XRD patterns of pure nano TiO2 and Ag doped nano TiO2. (a)Nano TiO2 (b) 0.05 wt % (c) 0.1 wt % (d) 0.3 wt % (e) 0.5 wt % (f) 0.8wt % and (g) 1.0 wt %.
  • [Fig. 2.] XRD patterns of pure nano TiO2 and Pt doped nano TiO2 withdifferent wt %. (a) pure (b) 0.1 wt % (c) 0.3 wt % (d) 0.5 wt % and (e)1.0 wt %.
    XRD patterns of pure nano TiO2 and Pt doped nano TiO2 withdifferent wt %. (a) pure (b) 0.1 wt % (c) 0.3 wt % (d) 0.5 wt % and (e)1.0 wt %.
  • [Fig. 3.] XRD patterns of Au doped nano TiO2 with different wt %. (a)0.1 wt % (b) 0.3 wt % (c) 0.5 wt % and (d) 1.0 wt %.
    XRD patterns of Au doped nano TiO2 with different wt %. (a)0.1 wt % (b) 0.3 wt % (c) 0.5 wt % and (d) 1.0 wt %.
  • [Fig. 4.] N2 adsorption isotherms of pure and Ag doped nano TiO2 withdifferent wt %. (a) pure (b) 0.1 wt % (c) 0.3 wt % and (d) 0.5 wt %.
    N2 adsorption isotherms of pure and Ag doped nano TiO2 withdifferent wt %. (a) pure (b) 0.1 wt % (c) 0.3 wt % and (d) 0.5 wt %.
  • [Table 1.] Physico-chemical characteristics of Ag Pt and Au doped nano TiO2
    Physico-chemical characteristics of Ag Pt and Au doped nano TiO2
  • [Fig. 5.] N2 adsorption isotherms of pure and Pt doped nano TiO2 withdifferent wt %. (a) pure (b) 0.1 wt % (c) 0.3 wt % and (d) 0.5 wt %.
    N2 adsorption isotherms of pure and Pt doped nano TiO2 withdifferent wt %. (a) pure (b) 0.1 wt % (c) 0.3 wt % and (d) 0.5 wt %.
  • [Fig. 6.] N2 adsorption isotherms of pure and Au doped nano TiO2 withdifferent wt %. (a) pure (b) 0.1 wt % (c) 0.3 wt % and (d) 0.5 wt %.
    N2 adsorption isotherms of pure and Au doped nano TiO2 withdifferent wt %. (a) pure (b) 0.1 wt % (c) 0.3 wt % and (d) 0.5 wt %.
  • [Fig. 7.] TEM images of (a) 0.5 wt % Ag doped nano TiO2 and (b)magnified image of a single nano particle with clear lattice fringes.
    TEM images of (a) 0.5 wt % Ag doped nano TiO2 and (b)magnified image of a single nano particle with clear lattice fringes.
  • [Fig. 8.] TEM images of (a) 0.5 wt % Pt doped nano TiO2 and (b)magnified image of a single nano particle with clear lattice fringes.
    TEM images of (a) 0.5 wt % Pt doped nano TiO2 and (b)magnified image of a single nano particle with clear lattice fringes.
  • [Fig. 9.] TEM images of (a) 0.5 wt % Pt doped nano TiO2 and (b)magnified image of a single nano particle with clear lattice fringes.
    TEM images of (a) 0.5 wt % Pt doped nano TiO2 and (b)magnified image of a single nano particle with clear lattice fringes.
  • [Fig. 10.] EDX spectra of (a) 0.5 wt % Ag doped nano TiO2 and (b) 0.5 wt % Pt doped nano TiO2 and (c) 0.5 wt % Au doped nano TiO2.
    EDX spectra of (a) 0.5 wt % Ag doped nano TiO2 and (b) 0.5 wt % Pt doped nano TiO2 and (c) 0.5 wt % Au doped nano TiO2.
  • [Fig. 11.] DRS UV-vis spectra of pure and Ag doped nano TiO2 (a) pure(b) 0.1 wt % (c) 0.3 wt % (d) 0.5 wt % and (e) 1.0 wt %.
    DRS UV-vis spectra of pure and Ag doped nano TiO2 (a) pure(b) 0.1 wt % (c) 0.3 wt % (d) 0.5 wt % and (e) 1.0 wt %.
  • [Fig. 12.] DRS UV-vis spectra of pure and Pt doped nano TiO2 (a) pure(b) 0.1 wt % (c) 0.3 wt % (d) 0.5 wt % and (e) 1.0 wt %.
    DRS UV-vis spectra of pure and Pt doped nano TiO2 (a) pure(b) 0.1 wt % (c) 0.3 wt % (d) 0.5 wt % and (e) 1.0 wt %.
  • [Fig. 13.] DRS UV-vis spectra of pure and Au doped nano TiO2. (a) pure(b) 0.1 wt % (c) 0.3 wt % (d) 0.5 wt % and (e) 1.0 wt %.
    DRS UV-vis spectra of pure and Au doped nano TiO2. (a) pure(b) 0.1 wt % (c) 0.3 wt % (d) 0.5 wt % and (e) 1.0 wt %.
  • [Fig. 14.] Photocatalytic mineralization of 4-CP over nano TiO2 Agdoped TiO2 and Degussa (P-25) TiO2 (●) Degussa (○) pure (▼) 0.3wt % (Δ) 0.5 wt % and (■) 1.0 wt %.
    Photocatalytic mineralization of 4-CP over nano TiO2 Agdoped TiO2 and Degussa (P-25) TiO2 (●) Degussa (○) pure (▼) 0.3wt % (Δ) 0.5 wt % and (■) 1.0 wt %.
  • [Fig. 15.] Concentration of chloride ions formed during the degradationof 4-CP over 0.5 wt % Ag doped TiO2.
    Concentration of chloride ions formed during the degradationof 4-CP over 0.5 wt % Ag doped TiO2.
  • [Fig. 16.] Photocatalytic mineralization of 4-CP over different noblemetal doped nano TiO2 nano TiO2 and Degussa (P-25) TiO2. (●)Degussa (○) pure (Δ) 0.5 wt % Ag doped TiO2 (▼) 0.5 wt % Pt dopedTiO2 and (■) 0.5 wt % Au doped TiO2.
    Photocatalytic mineralization of 4-CP over different noblemetal doped nano TiO2 nano TiO2 and Degussa (P-25) TiO2. (●)Degussa (○) pure (Δ) 0.5 wt % Ag doped TiO2 (▼) 0.5 wt % Pt dopedTiO2 and (■) 0.5 wt % Au doped TiO2.