Due to the rapid growth of population size and water consumption, wastewater treatment is fundamental process in recent decades. For this reason, semiconductor photocatalysis has been considered as an alternative method for the degradation/oxidation or reduction of different organic pollutants and demonstrated to be a technically viable cleanup process. Among various photocatalysts, metal oxide semiconductors have been intensively investigated because of photo-stability, non-toxicity and low cost. There are many reports on metal oxide semiconductors for wastewater treatment applications. However, there are limitations on semiconductor materials for efficient catalytic redox reaction applications. In this particular work, silica supported TiO2/Ag2O catalyst with TiO2 inside and Ag2O outside with efficient electron and hole separation for the purpose of oxidation reaction; p-types Ag2O and CuxO inside and n-type TiO2 outside on SiO2 spherical particle for the purpose of reduction reactions were synthesized. Furthermore, NiO/NiS composite catalyst with simple ion-exchange method was also synthesized for the purpose of reduction reaction. The resulting catalysts were characterized with different techniques and the catalytic performances of the catalysts were also evaluated with selected standard method of different organic pollutants degradation or reduction. Specifically, this work demonstrates four parts.
In the first part, we develop the n-type TiO2 coated on SiO2 support abbreviated as SiO2/TiO2 (ST) followed by deposition of p-type Ag2O nanoparticles outside for the purpose of photocatalytic degradation of organic pollutants. Different composite catalysts were prepared with changing the amount AgNO3 (such as 0%, 5%, 10%, 20%, and 30 %) and the composites were abbreviated as ST, STA-5, STA-10, STA-20, and STA-30, respectively. The composite catalysts were characterized with different techniques and tested for Rhodamine (RhB) dye degradation under UV and visible light. Among the composite catalysts, the degradation efficiency of STA-20 was the highest and it degraded about 99% within 40 min under UV light-irradiation. However, the ST, STA-5, STA-10, and STA-30 composite catalysts could degrade about 21%, 47%, 58%, and 75% of the dye, respectively. Furthermore, the STA-5, STA-10, STA-20, and STA-30 composites were also tested and about 39%, 47%, 57%, and 42% of the dye, respectively, was degraded under visible light source. Hence, the formation of p-n junction heterostructure between n-type TiO2 and p-type Ag2O could enhance the degradation of RhB in both UV and visible light irradiation. It could be also potentially applicable photocatalyst for solving many environmental problems.
In the second part, the n-type TiO2 semiconductor nanoparticles were coated on the p-type Ag2O nanoparticles-deposited at SiO2 spherical particles through facile method for catalytic reduction of 4-nitrophenol. The as-prepared spherical composite abbreviated as SiO2/Ag2O@TiO2 was also characterized by different techniques and tested as a catalyst towards 4-nitrophenol (4-NP) reduction into 4-aminophenol (4-AP) with NaBH4 as a reducing agent at room temperature. This work combines an interesting design with the n-type TiO2 with the rich in electron outward and the p-type Ag2O with the rich in electronic hole inward to form the p/n junction for the purpose of efficiently separating the charge carrier to have longer lifetime of outside electrons for catalytic reduction reactions. The SiO2/Ag2O@TiO2 composite catalyst showed the best performance in the reduction of 4-NP to 4-AP within 30 seconds. Our results reveals that the p-n junction combined composite sphere was superior and efficient for reduction of 4-nitrophenol without using the light source. Over all, the SiO2/Ag2O@TiO2 composite can be used as a cost-effective reduction catalyst for converting the toxic 4-NP into the useful 4-AP, an industrial organic intermediate compound.
In the third part, the p-type CuxO (x=1 or 2) nanoparticles deposited on SiO2 spherical particle inside and coated with thin layered n-type TiO2 semiconductors outside was also designed for reduction purpose. The composite material, abbreviated as SiO2@CuxO@TiO2, was also characterized and the catalytic performance of the composite was tested for the reductions of 4-Nitrophenol (4-NP) and 2-Nitroaniline (2-NA). Complete reductions of 4-NP and 2-NA took, 210 and 150 sec, respectively. The catalytic efficiency of the composite material may be also associated with electrons and holes separation resulted from the p-n junction formation between p-type CuxO and n-type TiO2 and the built-in electric field. Moreover, the hydride ion and electrons released from NaBH4 together with outward electrons from n-type TiO2, synergistically, are also responsible for the reduction of nitro aromatic compounds. Our design of composite material from low-priced metal oxides was successful towards reduction of nitro-aromatic compounds.
In the fourth part of this work, the NiO/NiS composite catalyst synthesis was designed with a simple and facile method. The composite materials with different sulfur sources were characterized and applied for catalytic reduction of 4-nitrophenol (4-NP) in aqueous solution. The catalyst universalities towards reduction/ or decolorization of other organic dyes such as methylene blue (MB), methyl orange (MO), and rhodamine B (RhB) were also evaluated. The NiO/NiS (Ni-5) composite catalyst prepared with 10 mmol of Ni(Ac)2•4H2O and 5 mmol of thioacetamide had better catalytic activity and the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) took only 6 min. The composite catalyst was also excellent for reductions of MB, MO, RhB, and mixture of organic pollutants including 4-NP, MB, MO, and RhB and the reductions were completed in 60, 30, 90, and 210 sec, respectively. The results showed that the Ni-5 composite catalyst was highly active and efficient for complete reductions of organic pollutants at room temperature. The catalytic efficiency of the composite material is originated from NiS as a catalyst and facilitates electron transfer together with the NiO hierarchical structure. The pollutants and the hydrogen adsorbed on the surface of the catalyst can be connected with each other so that reduction occurs easily. Therefore, the NiO/NiS composite material formed from low-cost nickel-based metal oxide and sulfide can be successful towards the reduction of organic pollutants at room temperature.
Key words: p-n junction, photocatalyst, degradation, oxidation, reduction, metal oxides, TiO2, Ag2O, CuxO, composite, amino-aromatics, dye.

Due to the rapid growth of population size and water consumption, wastewater treatment is fundamental process in recent decades. For this reason, semiconductor photocatalysis has been considered as an alternative method for the degradation/oxidation or reduction of different organic pollutants and demonstrated to be a technically viable cleanup process. Among various photocatalysts, metal oxide semiconductors have been intensively investigated because of photo-stability, non-toxicity and low cost. There are many reports on metal oxide semiconductors for wastewater treatment applications. However, there are limitations on semiconductor materials for efficient catalytic redox reaction applications. In this particular work, silica supported TiO2/Ag2O catalyst with TiO2 inside and Ag2O outside with efficient electron and hole separation for the purpose of oxidation reaction; p-types Ag2O and CuxO inside and n-type TiO2 outside on SiO2 spherical particle for the purpose of reduction reactions were synthesized. Furthermore, NiO/NiS composite catalyst with simple ion-exchange method was also synthesized for the purpose of reduction reaction. The resulting catalysts were characterized with different techniques and the catalytic performances of the catalysts were also evaluated with selected standard method of different organic pollutants degradation or reduction. Specifically, this work demonstrates four parts.
In the first part, we develop the n-type TiO2 coated on SiO2 support abbreviated as SiO2/TiO2 (ST) followed by deposition of p-type Ag2O nanoparticles outside for the purpose of photocatalytic degradation of organic pollutants. Different composite catalysts were prepared with changing the amount AgNO3 (such as 0%, 5%, 10%, 20%, and 30 %) and the composites were abbreviated as ST, STA-5, STA-10, STA-20, and STA-30, respectively. The composite catalysts were characterized with different techniques and tested for Rhodamine (RhB) dye degradation under UV and visible light. Among the composite catalysts, the degradation efficiency of STA-20 was the highest and it degraded about 99% within 40 min under UV light-irradiation. However, the ST, STA-5, STA-10, and STA-30 composite catalysts could degrade about 21%, 47%, 58%, and 75% of the dye, respectively. Furthermore, the STA-5, STA-10, STA-20, and STA-30 composites were also tested and about 39%, 47%, 57%, and 42% of the dye, respectively, was degraded under visible light source. Hence, the formation of p-n junction heterostructure between n-type TiO2 and p-type Ag2O could enhance the degradation of RhB in both UV and visible light irradiation. It could be also potentially applicable photocatalyst for solving many environmental problems.
In the second part, the n-type TiO2 semiconductor nanoparticles were coated on the p-type Ag2O nanoparticles-deposited at SiO2 spherical particles through facile method for catalytic reduction of 4-nitrophenol. The as-prepared spherical composite abbreviated as SiO2/Ag2O@TiO2 was also characterized by different techniques and tested as a catalyst towards 4-nitrophenol (4-NP) reduction into 4-aminophenol (4-AP) with NaBH4 as a reducing agent at room temperature. This work combines an interesting design with the n-type TiO2 with the rich in electron outward and the p-type Ag2O with the rich in electronic hole inward to form the p/n junction for the purpose of efficiently separating the charge carrier to have longer lifetime of outside electrons for catalytic reduction reactions. The SiO2/Ag2O@TiO2 composite catalyst showed the best performance in the reduction of 4-NP to 4-AP within 30 seconds. Our results reveals that the p-n junction combined composite sphere was superior and efficient for reduction of 4-nitrophenol without using the light source. Over all, the SiO2/Ag2O@TiO2 composite can be used as a cost-effective reduction catalyst for converting the toxic 4-NP into the useful 4-AP, an industrial organic intermediate compound.
In the third part, the p-type CuxO (x=1 or 2) nanoparticles deposited on SiO2 spherical particle inside and coated with thin layered n-type TiO2 semiconductors outside was also designed for reduction purpose. The composite material, abbreviated as SiO2@CuxO@TiO2, was also characterized and the catalytic performance of the composite was tested for the reductions of 4-Nitrophenol (4-NP) and 2-Nitroaniline (2-NA). Complete reductions of 4-NP and 2-NA took, 210 and 150 sec, respectively. The catalytic efficiency of the composite material may be also associated with electrons and holes separation resulted from the p-n junction formation between p-type CuxO and n-type TiO2 and the built-in electric field. Moreover, the hydride ion and electrons released from NaBH4 together with outward electrons from n-type TiO2, synergistically, are also responsible for the reduction of nitro aromatic compounds. Our design of composite material from low-priced metal oxides was successful towards reduction of nitro-aromatic compounds.
In the fourth part of this work, the NiO/NiS composite catalyst synthesis was designed with a simple and facile method. The composite materials with different sulfur sources were characterized and applied for catalytic reduction of 4-nitrophenol (4-NP) in aqueous solution. The catalyst universalities towards reduction/ or decolorization of other organic dyes such as methylene blue (MB), methyl orange (MO), and rhodamine B (RhB) were also evaluated. The NiO/NiS (Ni-5) composite catalyst prepared with 10 mmol of Ni(Ac)2•4H2O and 5 mmol of thioacetamide had better catalytic activity and the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) took only 6 min. The composite catalyst was also excellent for reductions of MB, MO, RhB, and mixture of organic pollutants including 4-NP, MB, MO, and RhB and the reductions were completed in 60, 30, 90, and 210 sec, respectively. The results showed that the Ni-5 composite catalyst was highly active and efficient for complete reductions of organic pollutants at room temperature. The catalytic efficiency of the composite material is originated from NiS as a catalyst and facilitates electron transfer together with the NiO hierarchical structure. The pollutants and the hydrogen adsorbed on the surface of the catalyst can be connected with each other so that reduction occurs easily. Therefore, the NiO/NiS composite material formed from low-cost nickel-based metal oxide and sulfide can be successful towards the reduction of organic pollutants at room temperature.
Key words: p-n junction, photocatalyst, degradation, oxidation, reduction, metal oxides, TiO2, Ag2O, CuxO, composite, amino-aromatics, dye.

References
[1] M. Pera-Titus, V. Garcı́a-Molina, M.A. Baños, J. Giménez, S. Esplugas, Degradation of chlorophenols by means of advanced oxidation processes: a general review, Appl. Catal., B, 47 (2004) 219-256.
[2] M. Shanmugam, A. Alsalme, A. Alghamdi, R. Jayavel, Enhanced photocatalytic performance of the graphene-V2O5 nanocomposite in the degradation of methylene blue dye under direct sunlight, ACS Appl. Mater. Interfaces, 7 (2015) 14905-14911.
[3] A. Leudjo Taka, K. Pillay, X. Yangkou Mbianda, Nanosponge cyclodextrin polyurethanes and their modification with nanomaterials for the removal of pollutants from waste water: A review, Carbohydr. Polym., 159 (2017) 94-107.
[4] H. Hu, J.H. Xin, H. Hu, X. Wang, D. Miao, Y. Liu, Synthesis and stabilization of metal nanocatalysts for reduction reactions – a review, J. Mater. Chem. A, 3 (2015) 11157-11182.
[5] C.C. Nguyen, N.N. Vu, T.-O. Do, Recent advances in the development of sunlight-driven hollow structure photocatalysts and their applications, J. Mater. Chem. A, 3 (2015) 18345-18359.
[6] B. Subash, B. Krishnakumar, M. Swaminathan, M. Shanthi, Highly efficient, solar active, and reusable photocatalyst: Zr-loaded Ag-ZnO for Reactive Red 120 dye degradation with synergistic effect and dye-sensitized mechanism, Langmuir, 29 (2013) 939-949.
[7] O.A. Zelekew, D.-H. Kuo, Synthesis of a hierarchical structured NiO/NiS composite catalyst for reduction of 4-nitrophenol and organic dyes, RSC Adv., 7 (2017) 4353-4362.
[8] J. Wang, T. Tsuzuki, B. Tang, X. Hou, L. Sun, X. Wang, Reduced graphene oxide/ZnO composite: reusable adsorbent for pollutant management, ACS Appl. Mater. Interfaces, 4 (2012) 3084-3090.
[9] T. Jiao, H. Zhao, J. Zhou, Q. Zhang, X. Luo, J. Hu, Q. Peng, X. Yan, Self-assembly reduced graphene oxide nanosheet hydrogel fabrication by anchorage of chitosan/silver and its potential efficient application toward dye degradation for wastewater treatments, ACS Sustain. Chem. & Eng., 3 (2015) 3130-3139.
[10] M.J. Ahmed, M. Ahmaruzzaman, A facile synthesis of Fe3O4-charcoal composite for the sorption of a hazardous dye from aquatic environment, J. Environ. Manage., 163 (2015) 163-173.
[11] H.-T. Ren, S.-Y. Jia, Y. Wu, S.-H. Wu, T.-H. Zhang, X. Han, Improved photochemical reactivities of Ag2O/g-C3N4 in phenol degradation under uv and visible light, Ind. Eng. Chem. Res., 53 (2014) 17645-17653.
[12] F. Shen, W. Que, Y. He, Y. Yuan, X. Yin, G. Wang, Enhanced photocatalytic activity of ZnO microspheres via hybridization with CuInSe2 and CuInS2 nanocrystals, ACS Appl. Mater. Interfaces, 4 (2012) 4087-4092.
[13] J. Wen, X. Li, W. Liu, Y. Fang, J. Xie, Y. Xu, Photocatalysis fundamentals and surface modification of TiO2 nanomaterials, Chin. J. Catal., 36 (2015) 2049-2070.
[14] I. Zinicovscaia, Conventional methods of wastewater treatment, in: I. Zinicovscaia, L. Cepoi (Eds.) cyanobacteria for bioremediation of wastewaters, Springer International Publishing, Cham, 2016, pp. 17-25.
[15] C.A. Martínez-Huitle, E. Brillas, Decontamination of wastewaters containing synthetic organic dyes by electrochemical methods, Appl. Catal., B, 87 (2009) 105-145.
[16] D.P. Zagklis, P.G. Koutsoukos, C.A. Paraskeva, A combined coagulation/flocculation and membrane filtration process for the treatment of paint industry wastewaters, Ind. Eng. Chem. Res., 51 (2012) 15456-15462.
[17] K. Pelendridou, M.K. Michailides, D.P. Zagklis, A.G. Tekerlekopoulou, C.A. Paraskeva, D.V. Vayenas, Treatment of olive mill wastewater using a coagulation-flocculation process either as a single step or as post-treatment after aerobic biological treatment, J. Chem. Technol. Biotechnol., 89 (2014) 1866-1874.
[18] G. Lian, X. Zhang, H. Si, J. Wang, D. Cui, Q. Wang, Boron nitride ultrathin fibrous nanonets: one-step synthesis and applications for ultrafast adsorption for water treatment and selective filtration of nanoparticles, ACS Appl. Mater. Interfaces, 5 (2013) 12773-12778.
[19] Y. Tian, L. Chen, T.-L. Jiang, Simulation of a membrane bioreactor system for wastewater organic removal: biological treatment and cake layer−membrane filtration, Ind. Eng. Chem. Res., 50 (2011) 1127-1137.
[20] J. Wang, P. Zhang, B. Liang, Y. Liu, T. Xu, L. Wang, B. Cao, K. Pan, Graphene oxide as an effective barrier on a porous nanofibrous membrane for water treatment, ACS Appl. Mater. Interfaces, 8 (2016) 6211-6218.
[21] J.-D. Lee, S.-H. Lee, M.-H. Jo, P.-K. Park, C.-H. Lee, J.-W. Kwak, Effect of coagulation conditions on membrane filtration characteristics in coagulation−microfiltration process for water treatment, Environ. Sci. Technol., 34 (2000) 3780-3788.
[22] P. Liu, L. Zhang, Adsorption of dyes from aqueous solutions or suspensions with clay nano-adsorbents, Sep. Purif. Technol., 58 (2007) 32-39.
[23] A. Ajmal, I. Majeed, R.N. Malik, M. Iqbal, M.A. Nadeem, I. Hussain, S. Yousaf, Zeshan, G. Mustafa, M.I. Zafar, M.A. Nadeem, Photocatalytic degradation of textile dyes on Cu2O-CuO/TiO2 anatase powders, Journal of Environmental Chemical Engineering, 4 (2016) 2138-2146.
[24] S. Chowdhury, R. Balasubramanian, Graphene/semiconductor nanocomposites (GSNs) for heterogeneous photocatalytic decolorization of wastewaters contaminated with synthetic dyes: A review, Appl. Catal., B, 160-161 (2014) 307-324.
[25] W. Lu, T. Xu, Y. Wang, H. Hu, N. Li, X. Jiang, W. Chen, Synergistic photocatalytic properties and mechanism of g-C3N4 coupled with zinc phthalocyanine catalyst under visible light irradiation, Appl. Catal., B, 180 (2016) 20-28.
[26] S. Bai, J. Jiang, Q. Zhang, Y. Xiong, Steering charge kinetics in photocatalysis: intersection of materials syntheses, characterization techniques and theoretical simulations, Chem. Soc. Rev., 44 (2015) 2893-2939.
[27] W.-K. Jo, R.J. Tayade, Recent developments in photocatalytic dye degradation upon irradiation with energy-efficient light emitting diodes, Chinese Journal of Catalysis, 35 (2014) 1781-1792.
[28] T.S. Natarajan, M. Thomas, K. Natarajan, H.C. Bajaj, R.J. Tayade, Study on UV-LED/TiO2 process for degradation of Rhodamine B dye, Chem. Eng. J., 169 (2011) 126-134.
[29] K. Kabra, R. Chaudhary, R.L. Sawhney, Treatment of hazardous organic and inorganic compounds through aqueous-phase photocatalysis:  A review, Ind. Eng. Chem. Res., 43 (2004) 7683-7696.
[30] R.a. He, S. Cao, P. Zhou, J. Yu, Recent advances in visible light Bi-based photocatalysts, Chinese Journal of Catalysis, 35 (2014) 989-1007.
[31] N. Liang, M. Wang, L. Jin, S. Huang, W. Chen, M. Xu, Q. He, J. Zai, N. Fang, X. Qian, Highly efficient Ag2O/Bi2O2CO3 p-n heterojunction photocatalysts with improved visible-light responsive activity, ACS Appl. Mater. Interfaces, 6 (2014) 11698-11705.
[32] M. Xu, L. Han, S. Dong, Facile fabrication of highly efficient g-C3N4/Ag2O heterostructured photocatalysts with enhanced visible-light photocatalytic activity, ACS Appl. Mater. Interfaces, 5 (2013) 12533-12540.
[33] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Environmental applications of semiconductor photocatalysis, Chem. Rev., 95 (1995) 69-96.
[34] T. Lin, J. Wang, L. Guo, F. Fu, Fe3O4@MoS2 Core–shell composites: preparation, characterization, and catalytic application, J. Phys. Chem. C, 119 (2015) 13658-13664.
[35] H.S. Wu, L.D. Sun, H.P. Zhou, C.H. Yan, Novel TiO2-Pt@SiO2 nanocomposites with high photocatalytic activity, Nanoscale, 4 (2012) 3242-3247.
[36] T. Kamegawa, H. Seto, S. Matsuura, H. Yamashita, Preparation of hydroxynaphthalene-modified TiO2 via formation of surface complexes and their applications in the photocatalytic reduction of nitrobenzene under visible-light irradiation, ACS Appl. Mater. Interfaces, 4 (2012) 6635-6639.
[37] S. Shuang, R. Lv, Z. Xie, Z. Zhang, Surface plasmon enhanced photocatalysis of Au/Pt-decorated TiO2 nanopillar arrays, Sci Rep, 6 (2016) 26670.
[38] A.L. Luna, M.A. Valenzuela, C. Colbeau-Justin, P. Vázquez, J.L. Rodriguez, J.R. Avendaño, S. Alfaro, S. Tirado, A. Garduño, J.M. De la Rosa, Photocatalytic degradation of gallic acid over CuO–TiO2 composites under UV/Vis LEDs irradiation, Appl. Catal., A, 521 (2016) 140-148.
[39] Y. Xie, B. Yan, H. Xu, J. Chen, Q. Liu, Y. Deng, H. Zeng, Highly regenerable mussel-inspired Fe3O4 @polydopamine-Ag core-shell microspheres as catalyst and adsorbent for methylene blue removal, ACS. Appl. Mater. Interfaces, 6 (2014) 8845-8852.
[40] M.T. Yagub, T.K. Sen, S. Afroze, H.M. Ang, Dye and its removal from aqueous solution by adsorption: A review, Adv. Colloid Interface Sci., 209 (2014) 172-184.
[41] H. Lachheb, E. Puzenat, A. Houas, M. Ksibi, E. Elaloui, C. Guillard, J.-M. Herrmann, Photocatalytic degradation of various types of dyes (Alizarin S, Crocein Orange G, Methyl Red, Congo Red, Methylene Blue) in water by UV-irradiated titania, Appl. Catal., B, 39 (2002) 75-90.
[42] C.Y. Teh, P.M. Budiman, K.P.Y. Shak, T.Y. Wu, Recent advancement of coagulation–flocculation and its application in wastewater treatment, Ind. Eng. Chem. Res., 55 (2016) 4363-4389.
[43] B.R. Ganapuram, M. Alle, R. Dadigala, A. Dasari, V. Maragoni, V. Guttena, Catalytic reduction of methylene blue and Congo red dyes using green synthesized gold nanoparticles capped by salmalia malabarica gum, International Nano Letters, 5 (2015) 215-222.
[44] R. Begum, R. Rehan, Z.H. Farooqi, Z. Butt, S. Ashraf, Physical chemistry of catalytic reduction of nitroarenes using various nanocatalytic systems: past, present, and future, J. Nanopart. Res., 18 (2016).
[45] S.T. Tan, A. Ali Umar, M.M. Salleh, (001)-Faceted hexagonal ZnO nanoplate thin film synthesis and the heterogeneous catalytic reduction of 4-nitrophenol characterization, J. Alloys Compd., 650 (2015) 299-304.
[46] P. Guo, L. Tang, J. Tang, G. Zeng, B. Huang, H. Dong, Y. Zhang, Y. Zhou, Y. Deng, L. Ma, S. Tan, Catalytic reduction–adsorption for removal of p-nitrophenol and its conversion p-aminophenol from water by gold nanoparticles supported on oxidized mesoporous carbon, J. Colloid Interface Sci., 469 (2016) 78-85.
[47] B. Lai, Z. Chen, Y. Zhou, P. Yang, J. Wang, Z. Chen, Removal of high concentration p-nitrophenol in aqueous solution by zero valent iron with ultrasonic irradiation (US–ZVI), J. Hazard. Mater., 250-251 (2013) 220-228.
[48] S. Gazi, R. Ananthakrishnan, Metal-free-photocatalytic reduction of 4-nitrophenol by resin-supported dye under the visible irradiation, Appl. Catal., B, 105 (2011) 317-325.
[49] L. Li, Y. Feng, Y. Liu, B. Wei, J. Guo, W. Jiao, Z. Zhang, Q. Zhang, Titanium dioxide nanoparticles modified by salicylic acid and arginine: Structure, surface properties and photocatalytic decomposition of p-nitrophenol, Appl. Surf. Sci., 363 (2016) 627-635.
[50] S.M. El-Sheikh, A.A. Ismail, J.F. Al-Sharab, Catalytic reduction of p-nitrophenol over precious metals/highly ordered mesoporous silica, New J. Chem., 37 (2013) 2399-2407.
[51] S.M. Ansar, C.L. Kitchens, Impact of gold nanoparticle stabilizing ligands on the colloidal catalytic reduction of 4-Nitrophenol, ACS Catal., 6 (2016) 5553-5560.
[52] B.K. Ghosh, S. Hazra, B. Naik, N.N. Ghosh, Preparation of Cu nanoparticle loaded SBA-15 and their excellent catalytic activity in reduction of variety of dyes, Powder Technology, 269 (2015) 371-378.
[53] S. Du, Z. Liao, Z. Qin, F. Zuo, X. Li, Polydopamine microparticles as redox mediators for catalytic reduction of methylene blue and rhodamine B, Catal. Comm., 72 (2015) 86-90.
[54] F. Liu, Y.H. Leung, A.B. Djurišić, A.M.C. Ng, W.K. Chan, Native Defects in ZnO: effect on dye adsorption and photocatalytic degradation, J. Phys. Chem. C, 117 (2013) 12218-12228.
[55] W. Jiang, X. Wang, Z. Wu, X. Yue, S. Yuan, H. Lu, B. Liang, Silver oxide as superb and stable photocatalyst under visible and near-infrared light irradiation and its photocatalytic mechanism, Ind. Eng. Chem. Res., 54 (2015) 832-841.
[56] R. Patel, S. Suresh, Decolourization of azo dyes using magnesium–palladium system, J. Hazard. Mater., 137 (2006) 1729-1741.
[57] K. Yu, H. Zhang, J.-h. Lv, L.-h. Gong, C.-m. Wang, L. Wang, C.-x. Wang, B.-b. Zhou, High-efficiency photo- and electro-catalytic material based on a basket-like {Sr⊂P6Mo18O73} cage, RSC Adv., 5 (2015) 59630-59637.
[58] R.J. Tayade, T.S. Natarajan, H.C. Bajaj, Photocatalytic degradation of methylene blue dye using ultraviolet light emitting diodes, Ind. Eng. Chem. Res., 48 (2009) 10262-10267.
[59] J.A. Laszlo, Regeneration of dye-saturated quaternized cellulose by bisulfite-mediated borohydride reduction of dye azo groups:  an improved process for decolorization of textile wastewaters, Environ. Sci. Technol., 31 (1997) 3647-3653.
[60] Z. Gan, A. Zhao, M. Zhang, W. Tao, H. Guo, Q. Gao, R. Mao, E. Liu, Controlled synthesis of Au-loaded Fe3O4@C composite microspheres with superior SERS detection and catalytic degradation abilities for organic dyes, Dalton Trans, 42 (2013) 8597-8605.
[61] F. Liu, S. Chung, G. Oh, T.S. Seo, Three-dimensional graphene oxide nanostructure for fast and efficient water-soluble dye removal, ACS Appl. Mater. Interfaces, 4 (2012) 922-927.
[62] S. Banerjee, S.C. Pillai, P. Falaras, K.E. O’Shea, J.A. Byrne, D.D. Dionysiou, New Insights into the mechanism of visible light photocatalysis, J. Phys. Chem. Lett., 5 (2014) 2543-2554.
[63] X. Li, J. Yu, J. Low, Y. Fang, J. Xiao, X. Chen, Engineering heterogeneous semiconductors for solar water splitting, J. Mater. Chem. A, 3 (2015) 2485-2534.
[64] D. Xu, P. Diao, T. Jin, Q. Wu, X. Liu, X. Guo, H. Gong, F. Li, M. Xiang, Y. Ronghai, Iridium Oxide Nanoparticles and Iridium/Iridium Oxide Nanocomposites: Photochemical Fabrication and Application in Catalytic Reduction of 4-Nitrophenol, ACS Appl. Mater. Interfaces, 7 (2015) 16738-16749.
[65] X. Li, P. Liu, Y. Mao, M. Xing, J. Zhang, Preparation of homogeneous nitrogen-doped mesoporous TiO2 spheres with enhanced visible-light photocatalysis, Appl. Catal., B, 164 (2015) 352-359.
[66] C. Liu, C. Cao, X. Luo, S. Luo, Ag-bridged Ag2O nanowire network/TiO2 nanotube array p-n heterojunction as a highly efficient and stable visible light photocatalyst, J Hazard Mater, 285 (2015) 319-324.
[67] M.D. Susman, Y. Feldman, A. Vaskevich, I. Rubinstein, Chemical deposition of Cu2O nanocrystals with precise morphology control, ACS Nano, 8 (2014) 162-174.
[68] L. Liu, W. Yang, W. Sun, Q. Li, J.K. Shang, Creation of Cu2O@TiO2 composite photocatalysts with p–n heterojunctions formed on exposed Cu2O facets, their energy band alignment study, and their enhanced photocatalytic activity under illumination with visible light, ACS Appl. Mater. Interfaces, 7 (2015) 1465-1476.
[69] B. Jiang, L. Jiang, X. Shi, W. Wang, G. Li, F. Zhu, D. Zhang, Ag2O/TiO2 nanorods heterojunctions as a strong visible-light photocatalyst for phenol treatment, J Sol-Gel Sci Technol, 73 (2014) 314-321.
[70] M. Tobajas, C. Belver, J.J. Rodriguez, Degradation of emerging pollutants in water under solar irradiation using novel TiO2-ZnO/clay nanoarchitectures, Chem. Eng. J., 309 (2017) 596-606.
[71] M. Wang, Y. Hu, J. Han, R. Guo, H. Xiong, Y. Yin, TiO2/NiO hybrid shells: p-n junction photocatalysts with enhanced activity under visible light, J. Mater. Chem. A, 3 (2015) 20727-20735.
[72] C. Siriwong, N. Wetchakun, B. Inceesungvorn, D. Channei, T. Samerjai, S. Phanichphant, Doped-metal oxide nanoparticles for use as photocatalysts, Prog. Cryst. Growth Charact. Mater., 58 (2012) 145-163.
[73] J.F. Lei, L.B. Li, X.H. Shen, K. Du, J. Ni, C.J. Liu, W.S. Li, Fabrication of ordered ZnO/TiO2 heterostructures via a templating technique, Langmuir, 29 (2013) 13975-13981.
[74] Y. Li, B. Wang, S. Liu, X. Duan, Z. Hu, Synthesis and characterization of Cu2O/TiO2 photocatalysts for H2 evolution from aqueous solution with different scavengers, Appl. Surf. Sci., 324 (2015) 736-744.
[75] D.S. Kim, S.-Y. Kwak, The hydrothermal synthesis of mesoporous TiO2 with high crystallinity, thermal stability, large surface area, and enhanced photocatalytic activity, Appl. Catal., A, 323 (2007) 110-118.
[76] H. He, Y. Miao, Y. Du, J. Zhao, Y. Liu, P. Yang, Ag2O nanoparticle-decorated TiO2 nanobelts for improved photocatalytic performance, Ceram. Intern., 42 (2016) 97-102.
[77] S. Khanchandani, S. Kumar, A.K. Ganguli, Comparative study of TiO2/CuS sore/shell and composite nanostructures for efficient visible light lhotocatalysis, ACS Sustainable Chemistry & Engineering, 4 (2016) 1487-1499.
[78] Y. Yu, Y. Tang, J. Yuan, Q. Wu, W. Zheng, Y. Cao, Fabrication of N-TiO2/InBO3 heterostructures with enhanced visible photocatalytic performance, J. Phys. Chem. C, 118 (2014) 13545-13551.
[79] S. Sun, L. Wu, C.E. Png, P. Bai, Nanoparticle loading effects on the broadband absorption for plasmonic-metal@semiconductor-microsphere photocatalyst, Catal. Today, 278 (2016) 312-318.
[80] W. Qian, P.A. Greaney, S. Fowler, S.-K. Chiu, A.M. Goforth, J. Jiao, Low-temperature nitrogen doping in ammonia solution for production of N-Doped TiO2-hybridized graphene as a highly efficient photocatalyst for water treatment, ACS Sustain. Chem. Eng., 2 (2014) 1802-1810.
[81] J. Low, B. Cheng, J. Yu, Surface modification and enhanced photocatalytic CO2 reduction performance of TiO2: a review, Appl. Surf. Sci., 392 (2017) 658-686.
[82] A. Pearson, S.K. Bhargava, V. Bansal, UV-switchable polyoxometalate sandwiched between TiO2 and metal nanoparticles for enhanced visible and solar light photococatalysis, Langmuir, 27 (2011) 9245-9252.
[83] Y. Wang, F. Pan, W. Dong, L. Xu, K. Wu, G. Xu, W. Chen, Recyclable silver-decorated magnetic titania nanocomposite with enhanced visible-light photocatalytic activity, Appl. Catal., B, 189 (2016) 192-198.
[84] Z. Chen, L. Fang, W. Dong, F. Zheng, M. Shen, J. Wang, Inverse opal structured Ag/TiO2 plasmonic photocatalyst prepared by pulsed current deposition and its enhanced visible light photocatalytic activity, J. Mater. Chem. A, 2 (2014) 824-832.
[85] W. Zhou, H. Liu, J. Wang, D. Liu, G. Du, J. Cui, Ag2O/TiO2 nanobelts heterostructure with enhanced ultraviolet and visible photocatalytic activity, ACS Appl. Mater. Interfaces, 2 (2010) 2385-2392.
[86] Y. Wang, L. Liu, L. Xu, X. Cao, X. Li, Y. Huang, C. Meng, Z. Wang, W. Zhu, Ag2O/TiO2/V2O5 one-dimensional nanoheterostructures for superior solar light photocatalytic activity, Nanoscale, 6 (2014) 6790-6797.
[87] B. Sun, G. Zhou, T. Gao, H. Zhang, H. Yu, NiO nanosheet/TiO2 nanorod-constructed p–n heterostructures for improved photocatalytic activity, Appl. Surf. Sci., 364 (2016) 322-331.
[88] O. Mehraj, B.M. Pirzada, N.A. Mir, M.Z. Khan, S. Sabir, A highly efficient visible-light-driven novel p-n junction Fe2O3/BiOI photocatalyst: Surface decoration of BiOI nanosheets with Fe2O3 nanoparticles, Appl. Surf. Sci., 387 (2016) 642-651.
[89] C. Han, Q. Shao, J. Lei, Y. Zhu, S. Ge, Preparation of NiO/TiO2 p-n heterojunction composites and its photocathodic protection properties for 304 stainless steel under simulated solar light, J. Alloys Compd., 703 (2017) 530-537.
[90] K. Kalpana, V. Selvaraj, A novel approach for the synthesis of highly active ZnO/TiO2/Ag2O nanocomposite and its photocatalytic applications, Ceram. Int., 41 (2015) 9671-9679.
[91] L. Yang, S. Luo, Y. Li, Y. Xiao, Q. Kang, Q. Cai, High Efficient Photocatalytic Degradation of p-Nitrophenol on a Unique Cu2O/TiO2 p-n Heterojunction Network Catalyst, Environ. Sci. Technol., 44 (2010) 7641-7646.
[92] D. Sarkar, C.K. Ghosh, S. Mukherjee, K.K. Chattopadhyay, Three dimensional Ag2O/TiO2 type-II (p-n) nanoheterojunctions for superior photocatalytic activity, ACS Appl. Mater. Interfaces, 5 (2013) 331-337.
[93] J. Bandara, C.P.K. Udawatta, C.S.K. Rajapakse, Highly stable CuO incorporated TiO2 catalyst for photocatalytic hydrogen production from H2O, Photochem. Photobiol. Sci., 4 (2005) 857-861.
[94] M. Wang, J. Han, Y. Hu, R. Guo, Y. Yin, Carbon-incorporated NiO/TiO2 Mesoporous shells with p–n heterojunctions for efficient visible light photocatalysis, ACS Appl. Mater. Interfaces, 8 (2016) 29511-29521.
[95] D. Praveen Kumar, N. Lakshmana Reddy, B. Srinivas, V. Durgakumari, V. Roddatis, O. Bondarchuk, M. Karthik, Y. Ikuma, M.V. Shankar, Stable and active CuxO/TiO2 nanostructured catalyst for proficient hydrogen production under solar light irradiation, Sol. Energy Mater. Sol. Cells, 146 (2016) 63-71.
[96] S. Yang, D. Xu, B. Chen, B. Luo, X. Yan, L. Xiao, W. Shi, Synthesis and visible-light-driven photocatalytic activity of p–n heterojunction Ag2O/NaTaO3 nanocubes, Appl. Surf. Sci., 383 (2016) 214-221.
[97] Y. Cao, Q. Li, Y. Xing, L. Zong, J. Yang, In situ anion-exchange synthesis and photocatalytic activity of AgBr/Ag2O heterostructure, Appl. Surf. Sci., 341 (2015) 190-195.
[98] L. Chen, H. Hua, Q. Yang, C. Hu, Visible-light photocatalytic activity of Ag2O coated Bi2WO6 hierarchical microspheres assembled by nanosheets, Appl. Surf. Sci., 327 (2015) 62-67.
[99] X. Liu, J. Liu, H. Chu, J. Li, W. Yu, G. Zhu, L. Niu, Z. Sun, L. Pan, C.Q. Sun, Enhanced photocatalytic activity of Bi2O3–Ag2O hybrid photocatalysts, Appl. Surf. Sci., 347 (2015) 269-274.
[100] H. Yu, R. Liu, X. Wang, P. Wang, J. Yu, Enhanced visible-light photocatalytic activity of Bi2WO6 nanoparticles by Ag2O cocatalyst, Appl. Catal., B, 111-112 (2012) 326-333.
[101] H.-T. Ren, S.-Y. Jia, J.-J. Zou, S.-H. Wu, X. Han, A facile preparation of Ag2O/P25 photocatalyst for selective reduction of nitrate, Appl. Catal., B, 176-177 (2015) 53-61.
[102] N. Wei, H. Cui, Q. Song, L. Zhang, X. Song, K. Wang, Y. Zhang, J. Li, J. Wen, J. Tian, Ag2O nanoparticle/TiO2 nanobelt heterostructures with remarkable photo-response and photocatalytic properties under UV, visible and near-infrared irradiation, Appl. Catal., B, 198 (2016) 83-90.
[103] F. Niu, Y. Jiang, W. Song, In situ loading of Cu2O nanoparticles on a hydroxyl group rich TiO2 precursor as an excellent catalyst for the Ullmann reaction, Nano Research, 3 (2010) 757-763.
[104] X. An, K. Li, J. Tang, Cu2O/reduced graphene oxide composites for the photocatalytic conversion of CO2, ChemSusChem, 7 (2014) 1086-1093.
[105] S. Sun, Recent advances in hybrid Cu2O-based heterogeneous nanostructures, Nanoscale, 7 (2015) 10850-10882.
[106] Z.Z. Chen, E.W. Shi, Y.Q. Zheng, W.J. Li, B. Xiao, J.Y. Zhuang, Growth of hex-pod-like Cu2O whisker under hydrothermal conditions, J. Cryst. Growth, 249 (2003).
[107] D. Liu, D. Li, D. Yang, Synthesis of colloidal NiO nanocrystals by a hot-injection approach with a protecting ligand, Cryst. Res. Technol., 51 (2016) 313-317.
[108] L. Tian, L.Y. Yep, T.T. Ong, J. Yi, J. Ding, J.J. Vittal, Synthesis of NiS and MnS Nanocrystals from the Molecular Precursors (TMEDA)M(SC{O}C6H5)2 (M = Ni, Mn), Crystal Growth & Design, 9 (2009) 352-357.
[109] S. Zhang, J. Li, T. Wen, J. Xu, X. Wang, Magnetic Fe3O4@NiO hierarchical structures: preparation and their excellent As(v) and Cr(vi) removal capabilities, RSC Adv., 3 (2013) 2754-2764.
[110] J.H. Pan, Q. Huang, Z.Y. Koh, D. Neo, X.Z. Wang, Q. Wang, Scalable synthesis of urchin- and flowerlike hierarchical NiO microspheres and their electrochemical property for lithium storage, ACS Appl. Mater. Interfaces, 5 (2013) 6292-6299.
[111] U.M. Patil, R.R. Salunkhe, K.V. Gurav, C.D. Lokhande, Chemically deposited nanocrystalline NiO thin films for supercapacitor application, Appl. Surf. Sci., 255 (2008) 2603-2607.
[112] J. Ma, J. Yang, L. Jiao, Y. Mao, T. Wang, X. Duan, J. Lian, W. Zheng, NiO nanomaterials: controlled fabrication, formation mechanism and the application in lithium-ion battery, Cryst.Eng.Comm., 14 (2012) 453-459.
[113] C. Luo, D. Li, W. Wu, C. Yu, W. Li, C. Pan, Preparation of 3D reticulated ZnO/CNF/NiO heteroarchitecture for high-performance photocatalysis, Appl. Catal., B, 166–167 (2015) 217-223.
[114] X. Rong, F. Qiu, J. Qin, H. Zhao, J. Yan, D. Yang, A facile hydrothermal synthesis, adsorption kinetics and isotherms to Congo Red azo-dye from aqueous solution of NiO/graphene nanosheets adsorbent, J. Ind. Eng. Chem., 26 (2015) 354-363.
[115] S. Ci, Z. Wen, Y. Qian, S. Mao, S. Cui, J. Chen, NiO-microflower formed by nanowire-weaving nanosheets with interconnected ni-network decoration as supercapacitor electrode, Sci Rep, 5 (2015) 11919.
[116] Y. Zhu, C. Cao, S. Tao, W. Chu, Z. Wu, Y. Li, Ultrathin nickel hydroxide and oxide nanosheets: synthesis, characterizations and excellent supercapacitor performances, Sci Rep, 4 (2014) 5787.
[117] C. Shifu, Z. Sujuan, L. Wei, Z. Wei, Preparation and activity evaluation of p–n junction photocatalyst NiO/TiO2, J. Hazard. Mater., 155 (2008) 320-326.
[118] C.-J. Chen, C.-H. Liao, K.-C. Hsu, Y.-T. Wu, J.C.S. Wu, P–N junction mechanism on improved NiO/TiO2 photocatalyst, Catal. Comm., 12 (2011) 1307-1310.
[119] H.K. Kadam, S.G. Tilve, Advancement in methodologies for reduction of nitroarenes, RSC Adv., 5 (2015) 83391-83407.
[120] X. Yang, H. Zhong, Y. Zhu, H. Jiang, J. Shen, J. Huang, C. Li, Highly efficient reusable catalyst based on silicon nanowire arrays decorated with copper nanoparticles, J. Mater. Chem. A, 2 (2014) 9040.
[121] A. Hernández-Gordillo, A.G. Romero, F. Tzompantzi, R. Gómez, Kinetic study of the 4-Nitrophenol photooxidation and photoreduction reactions using CdS, Appl. Catal., B, 144 (2014) 507-513.
[122] Z.H. Farooqi, K. Naseem, R. Begum, A. Ijaz, Catalytic reduction of 2-Nitroaniline in aqueous medium using silver nanoparticles functionalized polymer microgels, J. Inorg. Organomet. Polym., 25 (2015) 1554-1568.
[123] K. Li, Z. Zheng, X. Huang, G. Zhao, J. Feng, J. Zhang, Equilibrium, kinetic and thermodynamic studies on the adsorption of 2-nitroaniline onto activated carbon prepared from cotton stalk fibre, J. Hazard. Mater., 166 (2009) 213-220.
[124] Z. Dong, X. Le, X. Li, W. Zhang, C. Dong, J. Ma, Silver nanoparticles immobilized on fibrous nano-silica as highly efficient and recyclable heterogeneous catalyst for reduction of 4-nitrophenol and 2-nitroaniline, Appl. Catal., B, 158-159 (2014) 129-135.
[125] P. Yang, A.-D. Xu, J. Xia, J. He, H.-L. Xing, X.-M. Zhang, S.-Y. Wei, N.-N. Wang, Facile synthesis of highly catalytic activity Ni–Co–Pd–P composite for reduction of the p-Nitrophenol, Appl. Catal., A, 470 (2014) 89-96.
[126] M. Tian, C. Dong, X. Cui, Z. Dong, Nickel and cobalt nanoparticles modified hollow mesoporous carbon microsphere catalysts for efficient catalytic reduction of widely used dyes, RSC Adv., 6 (2016) 99114-99119.
[127] J. Liu, Q. Zhang, J. Yang, H. Ma, M.O. Tade, S. Wang, J. Liu, Facile synthesis of carbon-doped mesoporous anatase TiO(2) for the enhanced visible-light driven photocatalysis, Chem Commun (Camb), 50 (2014) 13971-13974.
[128] J. Liu, L. Han, H. Ma, H. Tian, J. Yang, Q. Zhang, B.J. Seligmann, S. Wang, J. Liu, Template-free synthesis of carbon doped TiO2 mesoporous microplates for enhanced visible light photodegradation, Science Bulletin, 61 (2016) 1543-1550.
[129] Y.-C. Chang, D.-H. Chen, Catalytic reduction of 4-nitrophenol by magnetically recoverable Au nanocatalyst, J. Hazard. Mater., 165 (2009) 664-669.
[130] A. Hernández-Gordillo, M. Arroyo, R. Zanella, V. Rodríguez-González, Photoconversion of 4-nitrophenol in the presence of hydrazine with AgNPs-TiO2 nanoparticles prepared by the sol–gel method, J. Hazard. Mater., 268 (2014) 84-91.
[131] M.M. Mohamed, M.S. Al-Sharif, Visible light assisted reduction of 4-nitrophenol to 4-aminophenol on Ag/TiO2 photocatalysts synthesized by hybrid templates, Appl. Catal., B, 142-143 (2013) 432-441.
[132] X. Fang, Z. Liu, M.-F. Hsieh, M. Chen, P. Liu, C. Chen, N. Zheng, Hollow mesoporous aluminosilica spheres with perpendicular pore channels as catalytic nanoreactors, ACS Nano, 6 (2012) 4434-4444.
[133] Z.D. Pozun, S.E. Rodenbusch, E. Keller, K. Tran, W. Tang, K.J. Stevenson, G. Henkelman, A Systematic Investigation ofp-Nitrophenol reduction by bimetallic dendrimer encapsulated nanoparticles, J. Phys. Chem. C, 117 (2013) 7598-7604.
[134] J.-H. Noh, R. Meijboom, Synthesis and catalytic evaluation of dendrimer-templated and reverse microemulsion Pd and Pt nanoparticles in the reduction of 4-nitrophenol: The effect of size and synthetic methodologies, Appl. Catal., A, 497 (2015) 107-120.
[135] G. Zhou, Y. Liu, M. Luo, Q. Xu, X. Ji, Z. He, Peptide-capped gold nanoparticle for colorimetric immunoassay of conjugated abscisic acid, ACS Appl. Mater. Interfaces, 4 (2012) 5010-5015.
[136] R.K. Narayanan, S.J. Devaki, Brawny silver-hydrogel based nanocatalyst for reduction of nitrophenols: studies on kinetics and mechanism, Ind. Eng. Chem. Res., 54 (2015) 1197-1203.
[137] C. Chu, Z. Su, Facile synthesis of AuPt alloy nanoparticles in polyelectrolyte multilayers with enhanced catalytic activity for reduction of 4-nitrophenol, Langmuir, 30 (2014) 15345-15350.
[138] P. Veerakumar, S.M. Chen, R. Madhu, V. Veeramani, C.T. Hung, S.B. Liu, Nickel nanoparticle-decorated porous carbons for highly active catalytic reduction of organic dyes and sensitive detection of Hg(II) ions, ACS. Appl. Mater. Interfaces, 7 (2015) 24810-24821.
[139] T.R. Mandlimath, B. Gopal, Catalytic activity of first row transition metal oxides in the conversion of p-nitrophenol to p-aminophenol, J. Mol. Catal. A: Chem., 350 (2011) 9-15.
[140] C. Sarkar, S.K. Dolui, Synthesis of copper oxide/reduced graphene oxide nanocomposite and its enhanced catalytic activity towards reduction of 4-nitrophenol, RSC Adv., 5 (2015) 60763-60769.
[141] W. Che, Y. Ni, Y. Zhang, Y. Ma, Morphology-controllable synthesis of CuO nanostructures and their catalytic activity for the reduction of 4-nitrophenol, J. Phys. Chem. Solids, 77 (2015) 1-7.
[142] C. Huang, W. Ye, Q. Liu, X. Qiu, Dispersed Cu2O octahedrons on h-BN nanosheets for p-nitrophenol reduction, ACS Appl. Mater. Interfaces, 6 (2014) 14469-14476.
[143] Z. Jin, C. Liu, K. Qi, X. Cui, Photo-reduced Cu/CuO nanoclusters on TiO2 nanotube arrays as highly efficient and reusable catalyst, Sci Rep, 7 (2017) 39695.
[144] A.K. Sasmal, S. Dutta, T. Pal, A ternary Cu2O-Cu-CuO nanocomposite: a catalyst with intriguing activity, Dalton Trans., 45 (2016) 3139-3150.
[145] A. Goyal, S. Kapoor, P. Samuel, V. Kumar, S. Singhal, Facile protocol for reduction of nitroarenes using magnetically recoverable CoM0.2Fe1.8O4 (M = Co, Ni, Cu and Zn) ferrite nanocatalysts, RSC Adv., 5 (2015) 51347-51363.
[146] M. Bassetto, S. Ferla, F. Pertusati, S. Kandil, A.D. Westwell, A. Brancale, C. McGuigan, Design and synthesis of novel bicalutamide and enzalutamide derivatives as antiproliferative agents for the treatment of prostate cancer, Eur. J. Med. Chem., 118 (2016) 230-243.
[147] Y. Zuo, J.M. Song, H.L. Niu, C.J. Mao, S.Y. Zhang, Y.H. Shen, Synthesis of TiO2-loaded Co0.85Se thin films with heterostructure and their enhanced catalytic activity for p-nitrophenol reduction and hydrazine hydrate decomposition, Nanotechnology, 27 (2016) 145701.
[148] Characterization of solids, in: S.E. Dann (Ed.) Reactions and Characterization of Solids, The Royal Society of Chemistry, 2000, pp. 48-80.
[149] J.D. Andrade, X-ray Photoelectron Spectroscopy (XPS), in: J.D. Andrade (Ed.) Surface and interfacial aspects of biomedical polymers: Volume 1 Surface Chemistry and Physics, Springer US, Boston, MA, 1985, pp. 105-195.
[150] J.I. Goldstein, D.E. Newbury, P. Echlin, D.C. Joy, C.E. Lyman, E. Lifshin, L. Sawyer, J.R. Michael, The SEM and Its Modes of Operation, in: Scanning Electron Microscopy and X-ray Microanalysis: Third Edition, Springer US, Boston, MA, 2003, pp. 21-60.
[151] D.J. Smith, Chapter 1 Characterization of nanomaterials using transmission electron microscopy, in: Nanocharacterisation (2), The Royal Society of Chemistry, 2015, pp. 1-29.
[152] A. E. Morales, E. S Mora, U. Pal, Use of diffuse reflectance spectroscopy for optical characterization of un-supported nanostructures, Rev. Mex. Fis., S 53 (2007) 18–22.
[153] L. Wu, Y. Zhou, W. Nie, L. Song, P. Chen, Synthesis of highly monodispersed teardrop-shaped core–shell SiO2/TiO2 nanoparticles and their photocatalytic activities, Appl. Surf. Sci., 351 (2015) 320-326.
[154] Y. Zhao, X. Huang, X. Tan, T. Yu, X. Li, L. Yang, S. Wang, Fabrication of BiOBr nanosheets@TiO2 nanobelts p–n junction photocatalysts for enhanced visible-light activity, Appl. Surf. Sci., 365 (2016) 209-217.
[155] K.K. Paul, R. Ghosh, P.K. Giri, Mechanism of strong visible light photocatalysis by Ag2O-nanoparticle-decorated monoclinic TiO2(B) porous nanorods, Nanotechnology, 27 (2016) 315703.
[156] W. Zhou, H. Liu, J. Wang, D. Liu, G. Du, S. Han, J. Lin, R. Wang, Interface dominated high photocatalytic properties of electrostatic self-assembled Ag2O/TiO2 heterostructure, Phys Chem Chem Phys, 12 (2010) 15119-15123.
[157] X. Hu, Q. Zhu, X. Wang, N. Kawazoe, Y. Yang, Nonmetal-metal-semiconductor-promoted P/Ag/Ag2O/Ag3PO4/TiO2 photocatalyst with superior photocatalytic activity and stability, J. Mater. Chem. A, 3 (2015) 17858-17865.
[158] G. Tian, Y. Chen, H.-L. Bao, X. Meng, K. Pan, W. Zhou, C. Tian, J.-Q. Wang, H. Fu, Controlled synthesis of thorny anatase TiO2 tubes for construction of Ag-AgBr/TiO2 composites as highly efficient simulated solar-light photocatalyst, J. Mater. Chem, 22 (2012) 2081-2088.
[159] A.P. Dementjev, O.P. Ivanova, L.A. Vasilyev, A.V. Naumkin, D.M. Nemirovsky, D.Y. Shalaev, Altered layer as sensitive initial chemical state indicator, Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 12 (1994) 423-427.
[160] O.A. Zelekew, D.-H. Kuo, Facile synthesis of SiO2@CuxO@TiO2 heterostructures for catalytic reductions of 4-nitrophenol and 2-nitroaniline organic pollutants, Appl. Surf. Sci., 393 (2017) 110-118.
[161] D. Wang, H. Shen, L. Guo, F. Fu, Y. Liang, Design and construction of the sandwich-like Z-scheme multicomponent CdS/Ag/Bi2MoO6 heterostructure with enhanced photocatalytic performance in RhB photodegradation, New J. Chem., 40 (2016) 8614-8624.
[162] J. Zhang, H. Liu, Z. Ma, Flower-like Ag2O/Bi2MoO6 p-n heterojunction with enhanced photocatalytic activity under visible light irradiation, J. Mol. Catal. A: Chem., 424 (2016) 37-44.
[163] S. Ma, J. Xue, Y. Zhou, Z. Zhang, Photochemical synthesis of ZnO/Ag2O heterostructures with enhanced ultraviolet and visible photocatalytic activity, J. Mater. Chem. A, 2 (2014) 7272-7280.
[164] T. Shi, X. Hao, J. Ma, H. Liu, G. Gai, Y. Zhang, Preparation of Ag2O/TiO2/fly-ash cenospheres composite photocatalyst, Materials Letters, 183 (2016) 444-447.
[165] A.T. Kuvarega, R.W.M. Krause, B.B. Mamba, Nitrogen/palladium-codoped TiO2 for efficient visible light photocatalytic dye degradation, J. Phys. Chem. C, 115 (2011) 22110-22120.
[166] O.A. Zelekew, D.-H. Kuo, J.M. Yassin, K.E. Ahmed, H. Abdullah, Synthesis of efficient silica supported TiO2/Ag2O heterostructured catalyst with enhanced photocatalytic performance, Appl. Surf. Sci., 410 (2017) 454-463.
[167] W. Zhou, G. Du, P. Hu, Y. Yin, J. Li, J. Yu, G. Wang, J. Wang, H. Liu, J. Wang, H. Zhang, Nanopaper based on Ag/TiO2 nanobelts heterostructure for continuous-flow photocatalytic treatment of liquid and gas phase pollutants, J Hazard. Mater, 197 (2011) 19-25.
[168] J. Tian-hao, L. Yang, D. Li-ya, H. Peng, S. Jia-yue, Facile preparation and visible-light photocatalysis enhancement of N-Doped β-TiO2 nanobelts, Chem. Res. Chin. Univ., 28 (2012) 721-726.
[169] J.S. Hammond, S.W. Gaarenstroom, N. Winograd, X-ray photoelectron spectroscopic studies of cadmium- and silver-oxygen surfaces, Anal. Chem., 47 (1975) 2193-2199.
[170] B.A. Zaccheo, R.M. Crooks, Stabilization of alkaline phosphatase with Au@Ag2O nanoparticles, Langmuir, 27 (2011) 11591-11596.
[171] Y.V. Zubavichus, Y.L. Slovokhotov, M.K. Nazeeruddin, S.M. Zakeeruddin, M. Grätzel, V. Shklover, Structural characterization of solar cell prototypes based on nanocrystalline TiO2 anatase sensitized with ru complexes. X-ray Diffraction, XPS, and XAFS Spectroscopy Study, Chem. Mater., 14 (2002) 3556-3563.
[172] K.P.O. Mahesh, D.-H. Kuo, B.-R. Huang, Facile synthesis of heterostructured Ag-deposited SiO2@TiO2 composite spheres with enhanced catalytic activity towards the photodegradation of AB 1 dye, J. Mol. Catal. A: Chem., 396 (2015) 290-296.
[173] S. Praharaj, S. Nath, S.K. Ghosh, S. Kundu, T. Pal, Immobilization and recovery of Au nanoparticles from anion exchange resin:  resin-bound nanoparticle matrix as a catalyst for the reduction of 4-nitrophenol, Langmuir, 20 (2004) 9889-9892.
[174] P. Zhang, R. Li, Y. Huang, Q. Chen, A Novel approach for the in situ synthesis of Pt–Pd nanoalloys supported on Fe3O4@C core–shell nanoparticles with enhanced catalytic activity for reduction reactions, ACS Appl. Mater. Interfaces, 6 (2014) 2671-2678.
[175] M. Li, G. Chen, Revisiting catalytic model reaction p-nitrophenol/NaBH4 using metallic nanoparticles coated on polymeric spheres, Nanoscale, 5 (2013) 11919-11927.
[176] Z. Zhang, T. Sun, C. Chen, F. Xiao, Z. Gong, S. Wang, Bifunctional nanocatalyst based on three-dimensional carbon nanotube-graphene hydrogel supported Pd nanoparticles: one-pot synthesis and its catalytic properties, ACS. Appl. Mater. Interfaces, 6 (2014) 21035-21040.
[177] J. Huang, S. Vongehr, S. Tang, H. Lu, X. Meng, Highly catalytic Pd−Ag bimetallic dendrites, J. Phys. Chem. C, 114 (2010) 15005-15010.
[178] A. Boateng, A. Brajter-Toth, Nanomolar detection of p-nitrophenol via in situ generation of p-aminophenol at nanostructured microelectrodes, Analyst, 137 (2012) 4531-4538.
[179] Y. Shi, X.-L. Zhang, G. Feng, X. Chen, Z.-H. Lu, Ag–SiO2 nanocomposites with plum-pudding structure as catalyst for hydrogenation of 4-nitrophenol, Ceram. Intern., 41 (2015) 14660-14667.
[180] N. Muthuchamy, A. Gopalan, K.-P. Lee, A new facile strategy for higher loading of silver nanoparticles onto silica for efficient catalytic reduction of 4-nitrophenol, RSC Adv., 5 (2015) 76170-76181.
[181] X. Wang, S. Li, H. Yu, J. Yu, S. Liu, Ag2O as a new visible-light photocatalyst: self-stability and high photocatalytic activity, Chem. Eur. J., 17 (2011) 7777-7780.
[182] X. Hu, C. Hu, R. Wang, Enhanced solar photodegradation of toxic pollutants by long-lived electrons in Ag–Ag2O nanocomposites, Appl. Catal., B, 176–177 (2015) 637-645.
[183] Z. Jiang, D. Jiang, A.M. Showkot Hossain, K. Qian, J. Xie, In situ synthesis of silver supported nanoporous iron oxide microbox hybrids from metal-organic frameworks and their catalytic application in p-nitrophenol reduction, PCCP, 17 (2015) 2550-2559.
[184] O. Ahmed Zelekew, D.-H. Kuo, A two-oxide nanodiode system made of double-layered p-type Ag2O@n-type TiO2 for rapid reduction of 4-nitrophenol, PCCP, 18 (2016) 4405-4414.
[185] X.H. Guo, J.Q. Ma, H.G. Ge, Synthesis and characterization of Cu2O/Au and its application in catalytic reduction of 4-nitrophenol, Russ. J. Phys. Chem. A, 89 (2015) 1374-1380.
[186] S. Sathasivam, D.S. Bhachu, Y. Lu, N. Chadwick, S.A. Althabaiti, A.O. Alyoubi, S.N. Basahel, C.J. Carmalt, I.P. Parkin, Tungsten doped TiO2 with enhanced photocatalytic and optoelectrical properties via aerosol assisted chemical vapor deposition, Sci. Rep., 5 (2015) 10952.
[187] G. Deroubaix, P. Marcus, X-ray photoelectron spectroscopy analysis of copper and zinc oxides and sulphides, Surf. Interface Anal., 18 (1992) 39-46.
[188] X. Liu, Y. Sun, C. Feng, C. Jin, F. Xiao, Ni/amorphous CuO core–shell nanocapsules with enhanced electrochemical performances, J. Power Sources, 245 (2014) 256-261.
[189] B. Ulgut, S. Suzer, XPS studies of SiO2/Si system under external bias, J. Phys. Chem. B, 107 (2003) 2939-2943.
[190] F. Xia, X. Xu, X. Li, L. Zhang, L. Zhang, H. Qiu, W. Wang, Y. Liu, J. Gao, Preparation of bismuth nanoparticles in aqueous solution and its catalytic performance for the reduction of 4-nitrophenol, Ind. Eng. Chem. Res., 53 (2014) 10576-10582.
[191] Y. Zhang, X. Yuan, Y. Wang, Y. Chen, One-pot photochemical synthesis of graphene composites uniformly deposited with silver nanoparticles and their high catalytic activity towards the reduction of 2-nitroaniline, J. Mater. Chem, 22 (2012) 7245.
[192] T. Aditya, A. Pal, T. Pal, Nitroarene reduction: a trusted model reaction to test nanoparticle catalysts, Chem. Commun (Camb), 51 (2015) 9410-9431.
[193] W. Zhang, Y. Sun, L. Zhang, In situ synthesis of monodisperse silver nanoparticles on sulfhydryl-functionalized poly(glycidyl methacrylate) microspheres for catalytic reduction of 4-nitrophenol, Ind. Eng. Chem. Res., 54 (2015) 6480-6488.
[194] Y. Tian, Y. Liu, F. Pang, F. Wang, X. Zhang, Green synthesis of nanostructed Ni-reduced graphene oxide hybrids and their application for catalytic reduction of 4-nitrophenol, Colloids and Surfaces A: Physicochem. Eng. Aspects, 464 (2015) 96-103.
[195] A.A. Ismail, A. Hakki, D.W. Bahnemann, Mesostructure Au/TiO2 nanocomposites for highly efficient catalytic reduction of p-nitrophenol, J. Mol. Catal. A: Chem., 358 (2012) 145-151.
[196] M.M. Mohamed, M.S. Al-Sharif, Visible light assisted reduction of 4-nitrophenol to 4-aminophenol on Ag/TiO2 photocatalysts synthesized by hybrid templates, Appl. Catal., B, 142–143 (2013) 432-441.
[197] S. Tang, S. Vongehr, X. Meng, Carbon spheres with controllable silver nanoparticle doping, J. Phys. Chem.C, 114 (2010) 977-982.
[198] M.H. Rashid, T.K. Mandal, Synthesis and catalytic application of nanostructured silver dendrites, J. Phys. Chem.C, 111 (2007) 16750-16760.
[199] S. Kandula, P. Jeevanandam, Synthesis of Cu2O@Ag polyhedral core-shell nanoparticles by a thermal decomposition approach for catalytic applications, Eur. J. Inorg. Chem., 2016 (2016) 1548-1557.
[200] J. Zhang, G. Chen, M. Chaker, F. Rosei, D. Ma, Gold nanoparticle decorated ceria nanotubes with significantly high catalytic activity for the reduction of nitrophenol and mechanism study, Appl. Catal., B, 132-133 (2013) 107-115.
[201] Y. Fu, T. Huang, L. Zhang, J. Zhu, X. Wang, Ag/g-C3N4 catalyst with superior catalytic performance for the degradation of dyes: a borohydride-generated superoxide radical approach, Nanoscale, 7 (2015) 13723-13733.
[202] T. Bhowmik, M.K. Kundu, S. Barman, Ultra small gold nanoparticles-graphitic carbon nitride composite: an efficient catalyst for ultrafast reduction of 4-nitrophenol and removal of organic dyes from water, RSC Adv., 5 (2015) 38760-38773.
[203] C. Wei, C. Cheng, Y. Cheng, Y. Wang, Y. Xu, W. Du, H. Pang, Comparison of NiS2 and alpha-NiS hollow spheres for supercapacitors, non-enzymatic glucose sensors and water treatment, Dalton Trans, 44 (2015) 17278-17285.
[204] W. Zhang, Y. Wang, Z. Wang, Z. Zhong, R. Xu, Highly efficient and noble metal-free NiS/CdS photocatalysts for H2 evolution from lactic acid sacrificial solution under visible light, Chem. Commun. (Camb), 46 (2010) 7631-7633.
[205] L.P. Zhu, G.H. Liao, Y. Yang, H.M. Xiao, J.F. Wang, S.Y. Fu, Self-assembled 3D flower-like hierarchical beta-Ni(OH)2 hollow architectures and their in situ thermal conversion to NiO, Nanoscale Res Lett, 4 (2009) 550-557.
[206] L.S. Zhong, J.S. Hu, H.P. Liang, A.M. Cao, W.G. Song, L.J. Wan, Self-assembled 3D flowerlike iron oxide nanostructures and their application in water treatment, Adv. Mater., 18 (2006) 2426-2431.
[207] X. Liu, J. Zhao, Y. Cao, W. Li, Y. Sun, J. Lu, Y. Men, J. Hu, Facile synthesis of 3D flower-like porous NiO architectures with an excellent capacitance performance, RSC Adv., 5 (2015) 47506-47510.
[208] X. Yan, X. Tong, L. Ma, Y. Tian, Y. Cai, C. Gong, M. Zhang, L. Liang, Synthesis of porous NiS nanoflake arrays by ion exchange reaction from NiO and their high performance supercapacitor properties, Materials Letters, 124 (2014) 133-136.
[209] Y. Wang, Q. Zhu, L. Tao, X. Su, Controlled-synthesis of NiS hierarchical hollow microspheres with different building blocks and their application in lithium batteries, J. Mater. Chem, 21 (2011) 9248-9254.
[210] S. Saha, A. Pal, S. Kundu, S. Basu, T. Pal, Photochemical green synthesis of calcium-alginate-stabilized Ag and Au nanoparticles and their catalytic application to 4-nitrophenol reduction, Langmuir, 26 (2010) 2885-2893.
[211] S. Farhadi, M. Kazem, F. Siadatnasab, NiO nanoparticles prepared via thermal decomposition of the bis(dimethylglyoximato)nickel(II) complex: A novel reusable heterogeneous catalyst for fast and efficient microwave-assisted reduction of nitroarenes with ethanol, Polyhedron, 30 (2011) 606-613.
[212] L. Gao, R. Li, X. Sui, R. Li, C. Chen, Q. Chen, Conversion of chicken feather waste to N-doped carbon nanotubes for the catalytic reduction of 4-nitrophenol, Environ. Sci. Technol, 48 (2014) 10191-10197.
[213] P. Herves, M. Perez-Lorenzo, L.M. Liz-Marzan, J. Dzubiella, Y. Lu, M. Ballauff, Catalysis by metallic nanoparticles in aqueous solution: model reactions, Chem. Soc. Rev., 41 (2012) 5577-5587.
[214] M. Hajfathalian, K.D. Gilroy, A. Yaghoubzade, A. Sundar, T. Tan, R.A. Hughes, S. Neretina, Photocatalytic enhancements to the reduction of 4-nitrophenol by resonantly excited triangular gold–copper nanostructures, J. Phys. Chem. C, 119 (2015) 17308-17315.
[215] H. Shang, K. Pan, L. Zhang, B. Zhang, X. Xiang, Enhanced activity of supported Ni catalysts promoted by Pt for rapid reduction of aromatic nitro compounds, Nanomaterials, 6 (2016) 103.
[216] P.K. Sahoo, B. Panigrahy, D. Bahadur, Facile synthesis of reduced graphene oxide/Pt-Ni nanocatalysts: their magnetic and catalytic properties, RSC Adv., 4 (2014) 48563-48571.
[217] S. Zhang, S. Gai, F. He, Y. Dai, P. Gao, L. Li, Y. Chen, P. Yang, Uniform Ni/SiO2@Au magnetic hollow microspheres: rational design and excellent catalytic performance in 4-nitrophenol reduction, Nanoscale, 6 (2014) 7025-7032.
[218] S. Oros-Ruiz, R. Gómez, R. López, A. Hernández-Gordillo, J.A. Pedraza-Avella, E. Moctezuma, E. Pérez, Photocatalytic reduction of methyl orange on Au/TiO2 semiconductors, Catal. Comm., 21 (2012) 72-76.
[219] M. Tian, X. Cui, C. Dong, Z. Dong, Palladium nanoparticles dispersed on the hollow aluminosilicate microsphere@hierarchical γ-AlOOH as an excellent catalyst for the hydrogenation of nitroarenes under ambient conditions, Appl. Surf. Sci., 390 (2016) 100-106.