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.

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 %.

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.

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).

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.

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.

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].

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.

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.

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].

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.