Introduction

Effluents from paper, textile and dyeing industries and petroleum refineries are contaminated with organic pollutants such as dyes, phenols and hydrocarbons. These chemicals and their metabolites being toxic and non-biodegradable in nature cause serious environmental pollution impacting human health as well as eco-system [1]. For example, nitro aromatic compounds exhibit high toxicity and/or mutagenicity for living organisms either directly or through some of their catabolic metabolites [2]. The usual physical, chemical. and biological methods adopted for the removal of organic pollutants from contaminated water are inefficient and generate a large amount of sludge as a solid waste that requires secondary treatment. Advanced oxidative photo-catalytic degradation of organic pollutants in water and air may provide an excellent solution for environmental cleanup without generating toxic byproducts as this technique enables complete oxidation of the pollutants and their conversion to benign species. Absorption of a photon of wavelength adequate to band gap energy (E_g) of the semiconductor photo-catalyst results in the promotion of an electron from its Valence Band (VB) to Conduction Band (CB) creating a hole (h+) at the VB. The photo-generated electron/ hole pair can either recombine releasing energy as heat or these charge carriers migrate to the photo-catalyst’s surface. Whereas, the photo-excited electrons at the CB reacts with surface adsorbed oxygen, the holes at the VB interact with water molecule forming highly reactive hydroxyl radicals that enable degradation of organic pollutant molecules forming CO₂, H₂O, and simple non-toxic end products.

The researchers in this area have mainly focused on understanding the fundamentals of photo-catalytic processes and to explore the possible means of improving efficiencies of high band gap photo-catalysts in the visible region of radiation in order to harvest the freely available solar energy thus making the process cost effective [3-6]. Nano-size (1-100 nm) semiconductor particles due to their large specific surface areas have great potential as water and air purifier as these can provide redox active media. Besides it, the nanoparticles can also exhibit their size and shape-dependent optical, electronic and catalytic properties [7].

Though a wide range of semiconductor material may be used in photo-catalytic degradation reactions, yet an ideal photo-catalyst should be chemically inert, stable toward photo-corrosion, non-toxic, photoactive under visible light and of low-cost [8]. Titanium dioxide (TiO₂) has been used in glass manufacturing, electro ceramics, self-cleaning surfaces and as a photo-catalyst, under UV irradiation, for photo degradation of numerous organic pollutants in water and air [9-11]. It may be due to its high ultraviolet absorption, biological and chemical inertness, cost effectiveness, non-toxicity, and long-term stability against chemical corrosion.

Reza, et al. [12] has reviewed the parameters affecting photo-catalytic degradation of dyes using TiO₂. Due to the large band gap of TiO₂, its photo-excitation requires high energy UV radiation and its catalytic efficiency is low in solar radiation. A higher rate of electron-hole recombination in photo-excited TiO₂ also reduces its catalytic efficiency. In addition to these drawbacks, high cost of UV source, required for the photo-excitation, the use of TO₂, as a catalyst for a large scale treatment of water may not be economical. Therefore, this has prompted the researchers for further studies to improve photo-catalytic efficiency of high band gap photo-catalysts under visible radiation. Such studies mostly involve modification of the photo-catalysts by doping metals/non-metals or by adopting surface-complex-mediated path [13-19].

Another approach for improving the photo-catalytic efficiency of high band gap energy (Eg) semiconductors could be by mixing these with a low band gap semiconductor. According to the inter-particle electron-transfer theory, visible light can photo-excite a low band gap semiconductor which in turn can photosensitize another semiconductor with high electrical potential and large band gap resulting in the enhanced photo-catalytic reaction. Sant & Kamat [20] investigated inter-particle electron transfer between size-quantized CdS and TiO₂ semiconductor nanoclusters. They have found that electron transfer from low band gap photo excited CdS (Eg = 2.4 eV) to high band gap TiO₂ (Eg = 3.2 eV) depends on the particle size of TiO₂ charge and the transfer was observed only when TiO₂ particles were sufficiently large (>12 Å). However, according to Tian, et al. [21], coupling of TiO₂ with another semiconductor with lower band gap energy such as WO₃ (Eg =2,7 eV) to serve as an electron accepter, can improve the photo-catalytic properties of TiO₂. As a basic function, WO₃ has a suitable conduction band potentially to allow the transfer of photo-generated electrons from TiO₂ facilitating effective charge separation. In addition to this, formation of a monolayer of WO_x species on TiO₂ can significantly increase the surface acidity [22,23]. Owing to such enhanced acidity, TiO₂/WO₃ can absorb more hydroxyl groups and simultaneously more organic reactants on its surface resulting in an improvement of the photocatalytic activity vis-à-vis the pure TiO₂ photo-catalyst. Boga et al. [24] from spectroscopic and structural analysis of TiO₂/WO₂ have reported that due to higher charge separation efficiency and the surface hydrophilicity, the above composite photo-catalyst can enhance the photo-catalytic degradation of oxalic acid under UV light in aqueous media.

However, no reports have appeared on the photo-catalytic activity of TiO₂/WO₃ or N-doped TiO₂/WO₃ composite photo-catalysts for photo-degradation of phenol red-an organic pollutant in contaminated water. Phenol red exists as a red crystal that is stable in air and has solubility 0.77 g L^-1 in water. It exhibits a gradual transition in color from yellow (λ_max = 443 nm) to red (λ_max = 570 nm) over the pH range 6.8 to 8.2. Phenol Red (PR) also known as phenol sulfonaphthalein is a pH indicator and frequently used in cell biological laboratories for determining kidney function, is toxic in nature. On contact with eye and skin, it causes irritation and if inhaled, it is harmful for respiratory tract and digestive system. This paper reports a comparative photo-catalytic degradation of phenol red in aqueous medium using sol-gel and hydrothermally synthesized and duly characterized Nano size TiO₂, N-doped-TiO₂, TiO₂/WO₃ and N-doped TiO₂/WO₃ composites as photo-catalysts.

Materials and Methods

Chemicals

Titanium tetrachloride TiCl₄ (MW 189.87 g.mol^-1, 99.9%; SD chemicals); TiO₂ (Degussa p-25, MW: 79.9 g.mol^-1, 99%, Germany); WO₃ (MW 231.84 g.mol^-1, 99%, SD fine chemicals,); Ethanol, C₂H₃OH (MW 58.03 g.mol^-1, 99.9%, Park scientific); Ammonium hydroxide, NH₄OH (MW 35.04 g.mol^-1, Abron chemicals); Urea (MW 60.06 g.mol^-1, extra pure BLULUX)) and Phenol Red (C₁₉H₁₄O₅S, MW: 354.38 g·mol^-1, BLULUX) were of analytical grade. Molecular structure of Phenol Red is shown in Figure 1.

Methods

Preparation of TiO₂ Nanoparticles

Nano Size Titanium Dioxide (TiO₂) was prepared by following sol-gel route [25,26]. The precursor titanium tetrachloride (TiCl₄) was added to distilled water taken in a glass conical flask, kept in an ice bath. To this, ethanol and ammonium hydroxide solution were mixed with vigorous stirring for 30 min. The precipitate thus obtained was thoroughly washed with distilled water for the removal of chloride ions, dried at 110 ˚C. The product was calcinated at 400 ˚C. for four hours, cooled and ground to get a fine powder.

Preparation of N-doped TiO₂

Nitrogen doped TiO₂ nanoparticles were prepared by solid phase reaction method, described elsewhere [26]. Typically, urea and as-synthesized TiO₂ with molar ratio = 1:3, were mixed in a ceramic crucible and calcinated at 400 ˚C for 4 hrs. The product was cooled to room temperature, crushed to get fine powder and then stored in a moisture-free atmosphere before further use.

Preparation of WO₃ coupled TiO₂

As-synthesized TiO₂ and WO₃ (molar ratio 95: 5) were thoroughly mixed with equimolar mixture of ethanol and water [27], dried at 110 ˚C and calcinated at 400 ˚C for four hrs. The product was then cooled and crushed to a fine powder.

Preparation of N-doped TiO₂/WO₃

As-synthesized N-doped TiO₂ and WO₃ (95:5, w/w) were thoroughly mixed in an equimolar mixture of ethanol and water. The mixture was dried up at 110 ˚C for 2 h and then calcinated at 400 ˚C for 4h. The product was crushed to fine powder at room temperature before further use.

XRD Analysis

XRD patterns of as-prepared photo-catalysts powders were obtained over 2θ range: 20-60˚ using an X-Ray Diffractometer (Bruker D8 Advance XRD) and Cu Kα radiation (λ = 0.154056 nm). The X-ray diffractometer was operated at 40 kV and 40 mA under scan rate 0.02˚ min^-1

SEM Analysis

A drop of photo-catalyst powder dispersion in acetone was released over the carbon SEM stub. The stub was then heated in a vacuum oven for 10 min at 200 ˚C and subsequently cooled to room temperature. The SEM image of each sample was obtained using accelerating voltage 15 kV at different magnifications.

UV/Visible Analysis

For determining photo absorption edge and band gap energy of photo-catalyst powders, optical absorbance of their aqueous suspension (0.01%, w/v) were recorded over 200-800 nm using a UV/Visible spectrophotometer (JASCOV 550).

Photocatalytic Degradation Study

Photo-catalytic degradation of Phenol-Red (PR) was carried out using a batch type reactor, described, elsewhere [28]. It comprised of a quartz tube with an effective volume of 250 mL and provided with an inlet for air-purging and an outlet for the collection of reaction mixture. Each time, 100 mL PR aqueous solution of specified initial concentration (pH =6.7) mixed with a known amount of photo-catalyst powder was taken in the reactor tube and stirred in dark for an hour to achieve sorption-desorption equilibrium between photo-catalyst powder and PR solution. A UV lamp (12W, Philips) emitting radiation at 254 nm and operated at 230 Volts and 50 Hz, was used as a UV source. The lamp was kept at 12 cm above the top of the reactor tube. Exposure of PR solution to the solar radiation was done on clear days, during 10 AM to 4 PM. at Haramaya University, Ethiopia (9025’30” N & 4202’0’’E). During irradiation, the reaction mixture was purged with air at flow rate 100 ml L^-1. A five mL each of reaction mixture was collected at a regular interval, centrifuged at 4000 rpm for 5 minutes and filtered through a 0.5 μm pore size membrane. Absorbance of the filtrate was recorded at 432 nm on a spectrophotometer (JASCOV 550) and the percent degradation of the substrate (PR) was calculated using the relation-

% Degradation = [(A_o-A_t)/A_o] x 100 (1)

Where, A_o and A_t are absorbance values at initial stage of reaction and at time ‘t’, respectively.

Results and Discussion

XRD Analysis

XRD patterns of as-synthesized photo-catalysts: TiO₂, N-doped TiO₂, TiO₂, /WO₃ and N-doped TiO₂, /WO₃ are presented in Figure 2. The XRD of TiO₂ powder calcinated at 400 ˚C showing diffraction peaks at 2θ = 25.5, 38.7, 48.2, 54.9 and 64.0° corresponding to the crystal planes (101), (004), (200), (105) and (204), respectively, revealed the existence of tetragonal anatase phase of TiO₂ in agreement with the standard spectrum (JCPDS Card Nos. 21-1272). However, the reflections at 27.0, 56.6 and 69.4° from the crystal planes (110), (220) and (301) also indicated the presence of some rutile phase [JCPDS 84-1286)] as well in the synthesized TiO₂ powder. The observed XRD pattern for N-doped TiO₂ was similar to that of calcinated TiO₂ suggesting that crystal structure of the later is unaltered by the doped nitrogen. Since no additional diffraction peak is observed in the XRD pattern of N-doped TiO₂ vis-à-vis TIO₂, it suggests that the anatase and rutile phases of TiO₂ remain intact on doping N. It also implies that the guest N atoms in the crystal lattice are distributed substituting equal number of oxygen atoms [29]. Further, in the XRD patterns of TiO₂/WO₃ and N-TiO₂/WO₃s composites, the observed low intensity additional peaks at 25.0, 28.1, 35.2, 37.9 and 52.0 are from (110), (200), (111), (201) and (220), respectively, planes of WO₃ hexagonal phase (JCPDS Card No. 033-1387) and the peaks pattern suggests the compositing WO₃ with TiO₂ results in shifting of anatase ⇌ rutile equilibrium in favor of rutile phase.

The average crystallite size of as-synthesized photo-catalysts powders were obtained using Dubye Scherrer equation-

D = K.λ / (β.cosθ) (2)

Where, D is crystallite size in nanometer, K is shape factor constant equal to 0.9, β is the full width at half maximum (FWHM) in radians of the most intense diffraction peak, λ is the X-ray wave length (0.15406 nm) and θ is Bragg’s angle. Average crystallite size, diffraction angle (2θ) of most intense diffraction peak and β values are given in Table 1. Average crystallite size of synthesized photo-catalyst powders were in the range 9-12 nm

SEM Analysis

Scanning Electron Microscopic (SEM) images for photo-catalysts powders using accelerating voltage 15 kV at similar magnification, are presented in Figure 3. Particles consisting of coherently scattered domain in the size range 5-50 nm are found to be agglomerated and of polycrystalline nature. The observed SEM images distinctly display the irregular shapes as well as size of the individual particles and the fine detail of surface topography. The observed diverse shaped SEM images included Nano platelet structure, with non-uniform size with thickness 60 to 300 nm and width/diameter 0.5 to 1.5 μm. Also in the SEM figures are nanorods-nanobelts 40-70 nm thick 10-15 μm long. In the SEM images, some urchin-like structures, with dimensions in the nanorange, were also observed. At identical magnification of SEM, average particle size of synthesized TiO₂/WO₃ or N- TiO₂/WO₃ composites is smaller compared to pure TiO₂ and N-doped TiO₂ photo-catalysts. This suggests that compositing WO₃ with TiO₂ not only promotes transforming anatase to rutile phase, described above, but can also prevents the growth of the catalyst crystallites.

Further, in SEM images of TiO₂/WO₃ or N-TiO₂/WO₃ composite photo-catalysts, WO₃ particles appear smaller in size and brighter in appearance compared to the TiO₂ particles. These features may be attributed to the larger atomic number of W (compared to Ti). As a basic principle, element with larger atomic number tends to agglomerate less resulting in its smaller size, but it appears brighter owing to more intense back-scattering of surface electrons.

EDS Analysis

The elemental distribution of synthesized photo-catalyst material was investigated using energy dispersive X-ray system (EDS). The EDS spectra of TiO₂, N-doped TiO₂, TiO₂/WO₃ and N-doped TiO₂/WO₃ are presented in Figure 4. In case of TiO₂ and N-doped TiO₂ photo-catalysts, only two peaks, one for oxygen and the other for titanium, are observed. However, the absence of nitrogen peak in case of N-doped TiO₂ EDS spectra may be due to the overlap of nitrogen (K) and Titanium (L) energy peaks. Whereas, in the TiO₂/WO₃ composite sample, Ti and W percent concentrations (w/w) were: 97.9 and 2.1, respectively, while for N-TiO₂/WO₃ these were 96.2 and 3.8%, respectively. Since no peak representing any other foreign element, within the detection limit of the EDS spectrometer, were detected, it also established the purity of as synthesized photo-catalyst samples.

UV/Visible Spectroscopic Analysis

UV–Visible diffuse reflectance spectra of synthesized photo-catalyst powders dispersed in aqueous medium are presented in Figure 5. Absorption edges of TiO₂, N-doped TiO₂, TiO₂/WO₃ and N-TiO₂/WO₃ composites were 397, 492, 485 and 520 nm, respectively. The observed red shift of absorption edge from 397 nm to 495 nm on doping N in TiO₂ may be due to the incorporation of 2p energy levels of N within the valence and conduction bands of TiO₂ causing band gap narrowing. The observed absorption edge shifting to higher wavelength in case of N-doped TiO₂ also ensures successful doping of N into TiO₂. Absorption wavelength of TiO₂/WO₂ composite is also well extended to higher wave length (485 nm) vis-à-vis TiO₂ (397 nm). This may be attributed to the intermixing of energy levels of the two photo-catalysts leading to the lowering of band gaps. Highest observed absorption wave length (520 nm) in case of N-doped TiO₂/WO₂ composite, among the studied photo-catalysts, may be due to cumulative synergetic effect of band gap narrowing caused due to N-doping as well as compositing the two photo-catalysts.

Band gap energy of photo-catalysts were calculated using the relation-

Eg = 1240/λ (3)

Where, E_g is band gap energy in electron volts, and λ is the absorption edge wavelength (nm).

The band gap energy thus calculated for TiO₂, N-doped TiO₂, TiO₂/WO₃ and N-TiO₂/WO₃, are: 3.12, 2.52, 2.56 and 2.38 eV, respectively.

Photo-Catalytic Degradation Study

Plots of percent photo-catalytic degradation of Phenol Red (PR) over different photo-catalysts as a function of time, using substrate initial concentration, 50 mgL^-1 and photo-catalyst load 150 mgL^-1, under UV and solar radiations are presented in Figure 6 and Figure 7, respectively. Phenol red degradation at three hrs. Over uncalcined TiO₂, calcined TiO₂, Degussa p-25, N-doped TiO₂, TiO₂/WO₃ and N-doped TiO₂/WO₃ under UV radiation were: 1.39, 28.6, 37.9, 40.3, 68.7 and 74.7 %, respectively and under solar radiation were: 1.4, 54.3, 56.4, 83.3, 85.2 and 92.6, respectively. Percent degradation values of PR at 3 hrs. using its initial concentration 50 mgL^-1 and photo-catalyst load 150 mgL^-1 are presented in Table 2. Photo-catalytic activity of Degussa P-25, was found to be higher compared to uncacined TiO₂ or calcined TiO₂ under UV as well as visible radiations. It may be because the Degussa P-25 mainly consists of photo-active anatase phase (60-70%) (along with less active minor amounts of rutile and other amorphous forms), whereas the uncalcined TiO₂ is a photo-inactive amorphous substance and calcinated TiO₂ may be having lesser proportion of anatage in the mixed phases than in Degussa P-25 [29]. Degradation of phenol red over N-doped TiO₂ was higher compared to Degussa P-25 and pure TiO₂ under solar radiation than under UV radiation. This may be attributed to the red-shift in photo-absorption enhancing photo response of N-doped TiO₂ in the visible region and a lowered rate of electron-hole recombination. The improved photo-catalytic performance under UV as well as solar radiations in case of coupled semiconductor system (TiO₂/WO₃) compared to pure TiO₂ can be attributed to synergetic effects of enhanced charge separation efficiency as well the surface hydrophilicity in case of the composite semiconductors [30].

Since among the studied photo-catalysts, N-doped TiO₂/WO₃ composite could exhibit as high as 92.6% degradation of PR at 3 hrs using dye initial concentration 50 mgL^-1 and photo-catalyst load 150 mgL^-1 under solar radiation, this composite photo-catalyst can be recommended for a low cost and efficient treatment of bulk water or industrial effluent contaminated with phenol red.

Conclusion

Nano size nitrogen doped TiO₂ and TiO₂/WO₃ composite semiconductor powders were synthesized using sol-gel and hydrothermal techniques. Nitrogen doped TiO₂ exhibited higher degradation of phenol red dye under visible/solar radiation than under UV light due to lowering of band gap and thus enhanced photo absorption in the visible region of radiation. Nitrogen doping as well as compositing of WO3 with TiO₂ showed synergetic effect towards improving the photo-catalytic degradation of phenol red in aqueous solution. With the dye initial concentration: 50 mg L^-1 and N-doped TiO₂/WO₃ photo-catalyst load: 150 mgL^-1 its degradation, as high as 92.6%, could be achieved within three hrs. Therefore, the above composite photo-catalyst can be recommended for a low cost and efficient treatment of bulk water or effluents contaminated with phenol red and the purified water thus obtained may be used for domestic and agriculture sectors.

Acknowledgement

The authors thank the Research and Extension Office of Haramaya University for providing the financial grant for the research (HURG-2014-03-03 and HURG-2016-03-02) and Project MAT2016-77496-R (AEI/FEDER, UE) from Spain for materials characterization.

Figure 1: Molecular structure of Phenol- red (phenolsulfonphthalein).

Figure 2: XRD Patterns of as-synthesized TiO₂, N-doped TiO₂, TiO₂/WO₃ and N-doped TiO₂/WO₃ photo-catalysts powders.

Figure 3: Scanning Electron Microscopic (SEM) images of synthesized photo-catalysts powders at accelerating voltage 15 kV. (A): Calcined TiO₂, (B): N-doped TiO₂, (C): TiO₂/WO₃, (D): N-TiO₂/WO_3.

Figure 4: Energy Dispersive X-Ray Spectra (EDS) and elemental percent composition (w/w) (excluding N and O) (A):  TiO₂ [Ti =100 %]; (B): N-doped TiO₂, [Ti =100 %]; (C): TiO₂/WO₃ [Ti=97.9% and W=2.1%]; (D): N-doped TiO₂/WO₃[Ti=96.2% and W=3.8%].

Figure 5: UV-Visible diffuse absorption spectra of synthesized photo-catalysts: A = TiO₂, B = N-doped TiO₂; C= TiO₂/WO₃ composite and D= N-doped TiO₂/WO₃ composite.

Figure 6: Percent degradation of Phenol Red (PR) under UV radiations as a function of time (initial concentration of PR = 50 mg L^-1; Photo-catalyst load= 150 mgL^-1).

Figure 7: Percent degradation of Phenol Red (PR) under Solar radiations as a function of time (initial concentration of PR = 50 mg L^-1; Photo-catalyst load= 150 mgL^-1).

References

1. Roldán MD, Blasco R, Caballero FJ, Castillo F (1997) Degradation of p-Nitrophenol by the Phototrophic Bacterium Rhodobacter capsulatus. Arch Microbiol 169: 36-42.
2. Anvari FH, Amirkolaie AK, Jalali AM, Miandare HK, Sayed AH, et al. (2018) Environmental Pollution and Toxic Substances; Cellular Apoposis as a Key Parameter in a Sensible Model Like Fish. Aquat Toxicol 204: 144-159.
3. Guesh K, Taddesse AM, Yadav OP (2014) Effect of Ag-N co-doping in Nanosize TiO₂ on Photo-catalytic Degradation of Methyl Orange Dye. J Surf. Sci Tech 29: 1-14.
4. Yadav M, Ali S, Yadav OP (2015) Photo-catalytic Degradation of Victoria Blue-B Dye Using ZnS and ZnS/Fe₂O₃ Composite Nanoparticles Under Visible Radiation. Der Chemica Sinica 6: 28-37.
5. Elizondo GP, Hernandez-Ramirez A (2017) Photo-catalytic Degradation of Ibuprofen Using TiO₂ Sensitized by Ru(II) Polyaza Complexes. Photochem. Photobiol Sci 16: 31-37.
6. Ani IJ, Akpan UG, Olutoye MA Hameed BH (2018) Photo-catalytic Degradation of  Pollutants in Petroleum Refinery Wastewater by TiO2-and ZnO-based Photo catalysts: Recent Development. J Cleaner Production 205: 930-954.
7. Kakhki RM, Hedayat S, Mohammadzadeh K (2019) Novel, Green and Low Cost Synthesis of Ag Nanoparticles with Superior Adsorption and Solar Based Photo- catalytic activity. J Mater Sci Materials in Electronics 30: 8788-8795.
8. Bharkhnade DS, Pangarkar VG, Beenackers AACM (2002) Photo-catalytic Degradation for Environmental Applications-A Review. J Chem Technol Biotechnol 77: 102-116.
9. Pawar M, Sendoğdular ST, Gouma P (2018) A Brief Overview of TiO₂ Photocatalyst for Organic Dye Remediation: Case Study of Reaction Mechanisms Involved in Ce-TiO₂ Photo-catalysts System. J Nanomaterials Article ID 5953609.
10. In S, Orlov A, Berg R, García F, Jimenez SP, et al. (2007) Effective Visible Light-activated B-doped and B, N-co-doped TiO₂ Photo-catalysts.J Amer Chem Soc 129: 13790-13791.
11. Xie J, Jiang D, Chen M, Li D, Zhu J, et al. (2010) Preparation and Characterization of Monodisperse Ce-doped TiO₂ Microspheres with Visible Light photocatalytic Activity. Colloids and Surfaces A: Physicochem. Eng. Aspects 372: 107-114.
12. Reza KM, Kurny ASW, Gulshan F (2017) Parameters Affecting Photo-catalytic Degradation of Dyes Using TiO₂-A Review. Appl Water Sci 7: 1569-1578.
13. Kim S, Choi W (2005) Visible-Light-Induced Photo-catalytic 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 109: 5143-5149.
14. Ihara T, Miyoshi M, Iriyama Y, Matsumoto O, Sugihara S (2003) Visible –Light- Active Titanium Oxide Photo-catalyst Realized by an Oxygen-deficient Structure and by Nitrogen Doping. Applied Catalysis B 42: 403-409.
15. Welderfael T, Yadav OP, Taddesse AM, Kaushal J (2013) Synthesis, Characterization and Photo-catalytic Activities of Ag-N-codoped ZnO nanoparticles for Degradation of Methyl Red. Bull Chem Soc Ethiop 27: 221-232.
16. Nibret A, Yadav OP, Diaz I, Taddesse AM (2015) Cr-N co-doped ZnO Nanoparticles: Synthesis, Characterization and Photo-catalytic Activity for degradation of Thymol Blue. Bull Chem Soc Ethiop 29: 247-258.
17. Balcha A, Yadav OP, Dey T (2016) Photo-catalytic Degradation of Methylene Blue Dye by ZnO Nanoparticles obtained from Precipitation and Sol-gel Methods. Environ Sci Pollut Res 23:  25485-25493.
18. Dagme Z, Yadav OP, Taddesse AM (2017) Photo-catalytic Degradation of Methylene Blue Dye in Water Over Mn-N co-doped ZnS Nanoparticles. Current Phys Chem 7: 1-9.
19. Bargole S, George S, Saharan VK, Pandit, AB (2019) Synthesis and Characterization of Samarium and Nitrogen doped TiO2 Photo-catalysts for Photo-degradation of 4-acetamidophenol in Combination with Hydrodynamic and Acoustic Cavitation. Separat Purif Technol 209: 254-269.
20. Sant PA, Kamat PV (2002) Interparticle Electron Transfer Between Size-quantized CdS and TiO₂ Semiconductor Nanoclusters. Phys Chem Chem Phys 4: 198-2003.
21. Tian J, Sang Y, Zhao Z, Zhou W, Wange D, et al. (2013) Enhanced Photo-catalytic Performances of CeO₂/TiO₂Nanobelt Heterostructures Small 9: 3864-3872.
22. Kwon YT, Song KY, Lee WI, Choi GJ, Do YR (2000) Photo-catalytic Behavior of WO3-Loaded TiO₂ in an Oxidation Reaction. J Catal 191: 192-199.
23. Song KY, Park MK, Kwon YT, Lee HW, Chung WJ, et al. (2001) Preparation of Transparent Particulate MoO₃/TiO₂ and WO₃/TiO₂ and Their Photo-catalytic Properties. Chem Mater 13: 2349-2355.
24. Boga B, Szekely I, Pap Z, Baia L, Baia M (2018) Detailed Spectroscopic and Structural Analysis of TiO₂/WO₃ Semiconductors. J Spectr Article ID 6260458.
25. Bessekhouad Y, Robert D, Weber JV (2003) Preparation of TiO₂ Nanoparticles by Sol-Gel route. Intern J Photoenergy 5: 153-158.
26. Wawrzyniak B, Morawski AW, Tryba B (2006) Preparation of TiO₂-Nitrogen-Doped Photocatalyst Active under Visible Light. Inter J of Photoenergy 23: 1-8.
27. Ren G, Gao Y, Yin J, Xing A, Liu H (2011) Synthesis of High-Activity TiO₂/WO₃ Photo-catalyst via Environmentally Friendly and Microwave Assisted Hydrothermal Process. J Chem Soc Pak 33: 666-670.
28. Achamo T, Yadav OP (2016) Removal of 4-Nitrophenol from Water Using Ag-N-P Tridoped TiO₂ by Photo-catalytic Oxidation Technique. Anal Chem Insight 11: 29:34.
29. Huang BS, Chang FY, Wey MY (2010) An Efficient Composite Growing N-doped TiO₂ on Multi-Walled Carbon Nanotubes Through Sol–gel Process. J Nanoparticle Res 12: 2503-2510.
30. Grigioni I, Stamplecoskie KG, Jara DH, Dozzi MV, Oriana A, et al. (2017) Wavelength-dependent Ultrafast Charge Carrier Separation in the WO₃/BiVO₄ Coupled system. ACS Energy Letters 2: 1362-1367.