Multi element doped type-II heterostructure assemblies (N, S- TiO2/ZnO) for electrochemical crystal violet dye degradation

Document Type : Reasearch Paper

Authors

• Dilip Kumar Behara
• Devangam Vishnu Priya Alugoti
• Palukuru Pooja Sree

Department of Chemical Engineering, JNTUACEA, Ananthapuramu-515002, India.

Abstract

Herein, we report multi-element doped Type-II heterostructure assembly consists of N, S doped TiO₂ and ZnO for electrochemical crystal violet dye degradation studies. Electrochemical measurements were performed on these synthesized N-S codoped TiO₂/ZnO compositeheterostructured assemblies which are fabricated on Titanium (Ti) substrate. It was observed that a composite electrode (N, S-TiO₂/ZnO@Ti) assembly has shown better efficiency metrics in comparison to all individual electrodes (bare Ti, TiO@Ti, ZnO@Ti) highlighting the importance of heterostructures. The findings of this article will help to design economic materials for complex dye molecule degradation studies as well as paves path towards better understanding of molecular mechanisms of dye degradation.

Keywords

• Crystal Violet Dye
• Synergistic Interaction
• Type-II heterostructure
• TiO2
• ZnO

20.1001.1.20088868.2020.11.4.1.5

---

Full Text

INTRODUCTION

Energy and environment are the two main important buzz words in today’s research world as both are prerequisites to economic and sustainable development of the country. In contemporary research, wastewater treatment has been given considerable importance due to the immense presence of dyes in various industry outlets creating not only a serious threat to human life but also effecting the global environment. These dyes containing water bodies not only create non-aesthetic pollution but also lead to dangerous by products via different reactions schemes such as oxidation, hydrolysis etc. [1, 2]. Further, some of these dyes are carcinogenic in nature and can cause serious damage to human life [3]. Therefore, it is essential to treat or remove the dyes from wastewater streams to minimize the impact of dyes in polluting the environment [4]. Numerous wastewater treatment methods such as conventional physical, chemical, and biological technologies are widely employed till date [5, 6]. However, they are not only consistent in terms of purity of separation but also expensive. In recent years, electrochemical oxidation processes emerged as best alternative methods for the treatment of wastewater, due to its ease of operation, fast reaction rate and being environmental friendly [7-13]. Further, there are no byproducts formed during this treatment and hence there is no requirement of sludge disposal techniques.

Semiconductors play an important role in electrochemical oxidation techniques and have been widely used for various applications such as water splitting, solar cell and pollutant degradation [14-16]. Among the various semiconductors, TiO₂ and ZnO have received much attention due to their low cost, chemical stability, and impressive photocatalytic performance [17, 18]. However, both TiO₂ and ZnO suffer from relatively large band gap which impedes thier applications. Therefore, it is essential to overcome this limitation for these materials for effective usage in various applications. However, the absorption coefficient of TiO₂ material will be improved by doping with non metals like nitrogen (N) and sulfur (S) which are iso-electronic species equivalent with TiO₂ and hence can be easily occupied into TiO₂ matrix [19-21]. Additionally, the addition of dopants to TiO₂ will not increase the the BET surface area by restraining the crystalline growth of TiO₂ but also increase the percentage of anatase phase [22-23]. Further, doping will fetch to form interstitial sites/oxygen vacancy sites of TiO₂ lattice and is shown to promote other interesting effects with regard to the electro-catalytic activity of the TiO₂, such as decreasing the electron-hole recombination rate and changingthe kinetics of electron (e^-) or hole (h⁺) transfer [24, 25]. It is known that forming composite/heterostructure with suitable band edge material like ZnO will improve the performance due to synergistic interaction as well as synchronized charge transport [26, 27]. Therefore, a composite assembly of TiO₂ and ZnO will improve the performance by forming a Type-II heterostructure [28, 29].

Herein, we propose electrochemical degradation of crystal violet dye using N-S co-doped TiO₂/ZnO composite heterostructured assemblies fabricated on titania (Ti) substrate. Titania substrate is chosen as good support due to its stability, and better conductivity. Since the conduction band (CB) edge of TiO₂ is -4.15eV and that of ZnO is -4.21eV w.r.t. Absolute Vacuum Scale [30], there is a net movement of charge carriers from TiO₂ to ZnO, which break the dye molecule at a faster rate. The fabricated electrodes were characterized using varous surface characterization techniques. Further, to monitor electrochemical activity, analytical techniques such as cyclic voltammetry and UV-Vis spectroscopy measurements were used. The faster rate of decolourization is achieved with N, S-TiO₂/ZnO electrode (~9hrs) in comparison to individual electrodes. This signifies the importance of iso-electronic non- metal (N, S) doped metal oxide Type-II Heterostructure in facilitating the fast charge carrier movements and thereby enhancing the electrochemical performance. The outcomes of the current article will not only give direction in understanding the molecular mechanism of dye degradation but also paves path towards exploration of low-cost catalytic heterostructure assemblies for different catalytic/electro-catalytic applications.

EXPERIMENTAL SECTION

Materials and Methods

Titanium tetraiso-propoxide (TTIP) [Ti{OCH(CH₃)₂}₄], zinc nitrate (Zn(NO₃)₂.6H₂O), potassium hydroxide(KOH), ethanol (C₂H₅OH), ammonia(NH₃), thiourea ((NH₂)₂CS), sodium sulphate (Na₂SO₄), and isopropyl alcohol (C₃H₈O) purchased from Sigma Aldrich Chemicals (India) Ltd. Nafion (10%) liquid solution is purchased DuPont Chemicals, USA. Crystal violet dye purchased from Sigma-Aldrich is used as a model pollutant in the present study. All the reagents were systematically graded and used as received without further purification. All solutions were prepared with distilled water.

Synthesis of doped/un-doped and co-doped nanoparticles

Proportionate amount fo ethanol (150 ml) and de-ionized water (3.75 ml) of were taken and stirred for half an hour to get a homogenous mixture. Controlled amount (~ 9 ml) of TTIP (titanium tetra iso-propoxide) was added drop by drop to this solution while stirring. Further, the stirring is continued for 4 hrs at 85 ºC. The obtained solution centrifuged at 2000 rpm for 30 min followed by drying and calcination at 400 ºC for 3 hours in a muffle furnace to get TiO₂nanoparticles. Ammonia, thiourea or both were added during the synthesis of TiO₂ to yield the formation of N-TiO₂, S-TiO₂ and N, S-TiO₂ nanoparticles respectively as shown in Fig.1 [24].

Synthesis of ZnOnanoparticles

Zinc Oxide (ZnO) nanoparticles were synthesized via direct precipitation method using zinc nitrate hexa hydrate (Zn (NO₃)₂.6H₂O) and potassium hydroxide (KOH) as precursors as shown in Fig.1[31]. An aqueous solution of 0.2 M zinc nitrate hexa hydrate and 0.4 M of potassiumhydroxide solutions were prepared separately. The addition of KOH solution to zinc nitrate solution at room temperature results in the white suspension. Finally, the white product was centrifuged at 5000 rpm for 20 minutes and washed several times times with distilled water and absolute ethanol. The final product was calcined at 500 ºC in an air atmosphere for 3 hours in a muffle furnace to get ZnO nanoparticles.

Synthesis of N, S -TiO₂/ZnOnanocomposite

The synthesis of N, S-TiO₂/ZnOnanocomposite is shown in Fig.1. Approximately 10 mg of nitrogen, sulphur co-doped titanium dioxide (N, S-TiO₂), is added to 0.2 M of zinc nitrate hexa hydrate (Zn (NO₃)₂.6H₂O) and 0.4M KOH. To the zinc nitrateand N, S-TiO₂ solutions, KOH solution was slowly added drop wise at room temperature under vigorous stirring and white suspension will be formed which was centrifuged at 5000 rpm for 20 min. The formed product was dried and calcined at 500 ºC for 3 hours to get N, S-TiO₂/ZnO nanocomposites.

Electrode Fabrication

Approximately 40 mg of synthesized TiO₂, N-TiO₂, S-TiO₂, N, S-TiO₂, N, S-TiO₂/ZnO nanoparticles were mixed with 2 ml of isopropanol and 20 µl of Nafion solution. The mixture was and sonicated for 1 hour and the obtained slurry was pasted on the titania (Ti) plate. Later it is dried at 60 ºC on a hot plate to form a uniform surface on Ti substrate (effective electrode surface ~1cm²). The electrode fabrication is shown in Fig.1* (supplementary information).

Electrochemical Measurements

Cyclic voltammetric(CV) studies were performed by using a potentiostat (K-Lyte 1.0) to monitor crystal violet dye degradation before and after performing electrochemical analysis. Three electrode system measurements were done using silver/slver chloride as reference electrode, platinum mesh as counter and fabricated catalyst assembly as working electrode. Further, crystal violet dye (55 mg/L) with 1M of Na₂SO₄ was used as supporting electrolyte solution for electrochemical measurements.

RESULTS and DISCUSSION

Characterization of Nanoparticles

X-Ray Diffraction Analysis

Crystallography characteristic of synthesized TiO₂ and ZnO was analyzed using X- ray diffractometer (XRD) with Cu K_α radiation (λ = 1.5406 Å). Obtained diffraction spectrum is then indexed and interpreted by JCPDS data (TiO₂:21-1272, ZnO: 36-1451) [25, 32]. XRD spectra of TiO₂, N-TiO₂, S-TiO_2, N, S-TiO₂, ZnO, and N, S-TiO₂/ZnOnanoparticles were shown in Fig. 2a and Fig 2* (supplementary information). The intense peaks at 2θ values of TiO₂, N-TiO₂, S-TiO_2, N,S-TiO₂ were observed at 25.48˚, 38.72˚, 48.26˚, 55.43˚, 63.22˚ which corresponds to (101), (004), (200), (211), (024) atomic planes respectively confirm the formation of tetragonal Anatase phase matching with JCPDS 21-1272 [24, 25]. The average crystallite size of TiO₂ and ZnO estimated from the Debye-Scherrer formula are 4 and 12 nm respectively. The widening and reduction in internsites of TiO₂ diffraction peaks were observed for doped (N, S) material matrices and implying that reduction in the crystal size and the degree of crystallinity for N-TiO₂, S-TiO₂, N, S-TiO₂ samples as shown in Fig. 2a. Similarly, ZnO exhibits peaks at 31.79º, 34.44º, 36.27º, 47.69º, 57.42º, 63.11º, 68.22º corresponds to 100, 002, 101, 012, 110, 013, 112 indices confirm hexagonal wurtzite structure. Additionally, the composite shows crystalline phase peaks of both N, S-TiO₂andZnO, which confirms the formation of composite (N, S-TiO₂/ZnO). Overall diffraction patterns from XRD patterns indicate the highly crystalline nature of Anatase phase TiO₂, ZnO and N, S-TiO₂/ZnOsamples.

FTIR analysis

The surface functional groups present in the sample can be known from Fourier Transform Infrared Spectroscopy (FTIR). The vibration mode observed at 3483.44 cm^-1 is attributed to hydroxyl groups (-OH) on the surface of the sample. Further, the transmission band observed at 2029.01 cm^-1 can be ascribed to the stretching vibration mode surface hydroxyl groups bonded to titanium atoms (Ti-OH). The fundamental crystal lattice vibrations of TiO₂ appear at 665.87 cm^-1 ascribed to the stretching vibrations of Ti-O bonds as depicted in Fig. 2(b) [33]. The vibrations at 3418.96 cm^-1 were assigned to N-H stretching, 1854.81 cm^-1 corresponds to N-H bending which come from N-TiO₂ as depicted in Fig. 2. (black colour) [34]. Similarly, broadband peaks observed at 2782.05 cm^-1 corresponds to

N=C=S stretching and the peak at 1256.18 cm^-1are assigned to the asymmetric stretching frequency of S=O bonds of S-TiO₂ [35]. For N, S-TiO₂both N-TiO₂, S-TiO₂ and TiO₂ peaks were obtained (red colour).A peak around 3534.65 cm^-1 is assigned to the stretching vibration and bending vibration of surface hydroxyl groups on ZnO. The peak at 1568.07 cm^-1was attributed to the O-C-O stretching vibrations. The peak observed at 1418.84 cm^-1 is known to be from adsorbed water (bending vibrations). Lastly, the peak at 660.89 cm^-1 is assigned to ZnO stretching vibrations [31].

SEM, TEM and EDX analysis

Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive X-ray Spectroscopy (EDS) were used to determine surface morphology and elemental composition of nanoparticles. From Fig. 3, it is apparent that the surface morphology of the TiO₂ changes with doping of N, S. Basically TiO₂ surface morphology was more or less spherical as depicted in the inset image of Fig. 3(a). Similarly, Zinc Oxide (ZnO) nanoparticles are rod like structure at many places with uneven distribution as shown in Fig 3*b(supplementary information). Further, the addition of dopants (N and S) to the crystalline matrix will rupture the surface of TiO₂ and lead to crystal distortion as shown in Fig 3*c, 3*d, and 3*e (supplementary information). The doped samples show the variation of shape that resultsfrom the agglomeration particles of TiO₂ with N, S dopants as shown in Fig. 3(b). EDX usually used to measure the chemical composition of the synthesized nanoparticles. The EDX spectrum of TiO₂ confirms the presence of Ti and O (inset image of Fig 3a). Further, elemental mapping confirms the presence of all elements including dopants (N, S) in all the samples from EDAX spectrum as shown in Fig 4* (supplementary information).Though EDX confirms the elemental mapping as well as the presence of composite sample elements but the surface material distribution is not uniform after doping. It may be probably due to surface segregation of the added dopants, and the lattice was completely distorted which was confirmed from the SEM image (Fig. 3c and Fig 3*f supplementary information). Further, to confirm the formation of composite or Heterostructure of materials, TEM images were taken. Fig 3(d) shows the TEM image of a composite sample with N, S doped TiO₂ and ZnO. It is to be noted that TiO₂nanoparticles are sandwiched between ZnOnanoparticles at many places and the adjacent location of these two nanoparticlesconfirms the formation of nanocomposite/Heterostructure as evident from inset image of Fig 3d.

Cyclic Voltammetry (CV) studies

A potential range of -1.5 V to +1.5 Vwasselected and controlled current density of 0.1 mA/cm² was maintained for all electrochemical measurements. Cyclic voltammograms exhibit oxidation peak during forward scan and reduction peak during the reverse scan of the electrolysis process. The oxidation peak potential varies for catalyst surface and different samples exhibit different oxidation potentials as shown in Fig 5* (supplementary information).The oxidation of reactive azo groups can be observed by keenly observing the change in currents produced from oxidation and reduction reactions during electrolysis [33]. Further, the absence of peak or reduction in peak will be observed after electrolysis or completion of dye degradation due to complete reduction of chromophoric groups. The electrolyzed solution at this moment was analyzed with HPLC to identify any functional groups present. It is observed that no amino group and no intermediate were detected which indicates the the reactive azo groups were reduced to amines. Fig 4a shows the CV of TiO₂ catalyst fabricated on Ti substrate shows oxidation and reduction peaks at potential of +1.05V and -0.4V respectively. It is understood that if more number of reactive groups are there in dye molecule then it leads to produce more current in CV. Similarly, the decrease in peak current after complete degradation indicates the reduction of reactive groups. The shift in oxidation peak to 0.8V in case of composite assembly N, S-TiO₂/ZnO indicates decolourization of dye and less or non-availability of azo groups in electrolyte solution as shown in Fig 4b. An oxidation peak at 0.8 V is observed for N, S-TiO₂/ZnO samples fabricated on Ti substrate as shown from the voltammogram curve (Fig 4b). Further, the reduction in anodic peaks in CV were observed during electrolysis which indicates the exisiting azo groups were reduced to amines. Additionally, the presence of Na₂SO₄ will favor oxidation of dye by forming persulfate ions. The progress of degradation can be monitored by performing chronoamperometry at a fixed voltage at different time intervals. Chrono-amperometric studies were performed at a fixed voltage (i.e. voltage where oxidation peak rises in CV) 1.2V vs. Ag/AgCl for N, S-TiO₂/ZnO@Ti at a scan rate of 0.05V/s.

Ultra Violet-Visible Spectra analysis

Ultra Violet-Visible Spectra analysis were perfomed to identify the extent of crystal violet dye absorption at different time intervals. The crystal violet dye spectra show an absorbance at 576 nm wavelength. The absorbance in the visible region is attributed to the presence of chromophores (azo) groups. During electrochemical degradation, the aromatic rings as well as –N=N–bonds will break down into small molecules, which results in the decrease of absorbance. The absorption becomes zero at λ_max=576 nm indicates the reduction of auxochrome groups. Different electrode assemblies such as Ti/TiO₂, Ti/N-TiO₂, Ti/S-TiO₂, Ti/ZnO, and Ti/N, S-TiO₂/ZnO composite assemblies were used to degrade the crystal violet dye and the decolourization vs. time plots were shown in Fig 6* (supplementary information).The absorbance vs wavelength plots for TiO₂ and N, S-TiO₂/ZnO samples were shown in Fig 5a and 5b respectively. A decrease in absorbance with time is observed for both samples and a faster rate of decrease is observed for the composite sample than pristine TiO₂. Further, the decolourization efficiency is calculated by using the following formula.

% decolourization = (1)

Where the absorption of dye solution before and after electrochemical treatment were indicated with at A₀ and A at λ_max as shown in Fig. 5c. Complete decolourization ( > 90% decolourization) is achieved for all samples at different times by electrochemical treatment. The degradation time is about 14 hr. and 8 hr. for Ti/TiO₂, and Ti/N, S-TiO₂/ZnO respectively as shown in Fig. 5c.The time in attaining max % decolourization for various electrode assemblies was shown in Table 1 for information. The degradation rate varies with varying of electrode assembliesdue to the change in an electrochemical active surface area (ECSA), which is responsible for adsorption of more dye molecules that interact with oxidizing agents easily [24]. It is observed that Ti/N, S-TiO₂/ZnOnanocomposite showed a faster rate of decolourization compared to Ti/TiO₂ electrode assembly due to the charge carrier flow path available at the interface of semiconductor materials having suitable band edge positions in forming Type-II Heterostructuresas shown in Fig. 5d. [24, 36]. Therefore, doped metal oxides Heterostructureachieve faster degradation compared to individual metal oxides and exemplify the importance of heterostructure assemblies in dye degradation studies. The degradation of crystal violet dye occurs through N- dealkylation process [37] and degradation pathways were well explained in our previously published articles [11, 12].

CONCLUSIONS

Present work aims to understand multi-element doped metal oxide Heterostructures (N, S-TiO₂/ZnO) for electrochemical degradation of crystal violet dye. The synthesized/fabricated electrode assemblies were confirmed using different characterization techniques such as XRD, SEM, TEM, EDS, UV-vis, and FTIR. Experimental results depict that doped heterostructure assembly (N, S-TiO₂/ZnO) degrade crystal violet dye much faster (8 hrs.) than pristine TiO₂ (14 hrs.). Electrochemical measurements show that Ti/N, S-TiO₂/ZnO heterostructured electrode shows better performance of dye degradation (%) in comparison to the all individual electrode assemblies.This can be ascribed may be due to 1) increase in electrochemical active surface area which accommodates more oxidants to interact with dye molecules; 2) increase in electronic conductivity due to appropriate band edges. The results of the current study will help to design low-cost catalytic assemblies in degrading complex dyes of industrial importance.

ACKNOWLEDGEMENTS

The authors wish to thank the Department of Chemical Engineering, JNTUACEA for providing laboratory and powder X-ray diffractometer facilities for XRD analysis. Authors, convey special thanks to D. HimaBindu, for assisting in taking SEM images at IIT BHU. Further, authors wish to acknowledge OTRI-JNTUA and RIPER for providing UV-vis and FTIR characterization of sample analysis.

RESEARCH FUNDING

This work was supported partly from IEI, RandD grant in aid (2016-17) and UGC XII plan funds of JNT University, Ananthapur (AP).

DISCLOSURE STATEMENT

No potential conflict of interest was reported by the authors

 

References

1. Prieto, O., Fermoso, J., Nunez, Y., Del Valle, J. L., andIrusta, R., (2005), Decolouration of textile dyes in wastewaters by photocatalysis with TiO₂. Sol Energy. 79: 376-383.

2. Hao, O. J., Kim, H., and Chiang, P. C., (2000),Decolorization of wastewater. Crit. Rev. Environ.SciTechnol, 30: 449-505.

3. Afkhami, A., andMoosavi, R., (2010), Adsorptive removal of Congo red, a carcinogenic textile dye, from aqueous solutions by maghemitenanoparticles. J Hazard Mater. 174:398-403.

4. Eggen, R. I., Hollender, J., Joss, A., Schärer, M., andStamm, C., (2014),Reducing the discharge of micropollutants in the aquatic environment: the benefits of upgrading wastewater treatment plants. Environ. Sci. Technol. 48: 7683-7689.

5. Ahn, D. H., Chang, W. S., and Yoon, T. I., (1999), Dyestuff wastewater treatment using chemical oxidation, physical adsorption and fixed bed biofilm process. Process Biochem. 34: 429-439.

6. El-Gohary, F. A., Abou-Elela, S. I., andAly, H. I., (1995), Evaluation of biological technologies for wastewater treatment in the pharmaceutical industry. Water Sci Technol. 32:13-20.

7. Comninellis, C., (1994),Electrocatalysis in the electrochemical conversion/combustion of organic pollutants for waste water treatment. ElectrochimActa.39: 1857-1862.

8. Martinez-Huitle, C. A., and Ferro, S., (2006),Electrochemical oxidation of organic pollutants for the wastewater treatment: direct and indirect processes. Chem Soc Rev. 35:1324-1340.

9. Panizza M., Bocca C., Cerisola G., (2000), Electrochemical treatment of wastewater containing polyaromatic organic pollutants. Water Res. 34: 2601-2605.

10. Chen G., (2004), Electrochemical technologies in wastewater treatment. Sep. Purif. Technol. 38: 11-41.

11. Behara D. K., Palukuru P. S., Devangam A. V. P., (2020), N, S-codoped TiO₂/Fe₂O₃ heterostructure assemblies for electrochemical degradation of crystal violet dye. Iran. J. Chem. Chem. Eng. 39: 171-180.

12. Behara D. K., Mukkara S. M., Jalajakshi T., (2019), TiO₂/Fe₂O₃: Type-I heterostructures for electrochemical dye degradation/water splitting studies. J. Inst. Eng. India Ser. E. 100: 189-198.

13. Zhou M., Yu Q., Lei L., Barton G., (2007), Electro-Fenton method for the removal of methyl red in an efficient electrochemical system. Sep. Purif. Technol. 57: 380-387.

14. Chen C., Ma W., Zhao J., (2010), Semiconductor-mediated photodegradation of pollutants under visible-light irradiation. Chem. Soc. Rev. 39: 4206-4219.

15. Behara D. K., Upadhyay A. P., Sharma G. P., Kiran B., Sivakumar S., Pala R. G. S., (2015), Heterostructures based on TiO₂ and silicon for solar hydrogen generation. Adv. Func. Mater. 219-281.

16. Kamat P. V., (2008), Quantum dot solar cells. Semiconductor nanocrystals as light harvesters. J. Phys. Chem. C. 112: 18737-18753.

17. Nagaveni K., Hegde M. S., Ravishankar N., Subbanna G. N., Madras G., (2004), Synthesis and structure of nanocrystalline TiO₂ with lower band gap showing high photocatalytic activity. Langmuir. 20: 2900-2907.

18. Lee K. M., Lai C. W., Ngai K. S., Juan J. C., (2016), Recent developments of zinc oxide based photocatalyst in water treatment technology: A review. Water Res. 88: 428-448.

19. Umebayashi T., Yamaki T., Itoh H., Asai K., (2002), Band gap narrowing of titanium dioxide by sulfur doping. Appl. Phys. Lett. 81: 454-456.

20. Darzi S. J., Mahjoub A. R., Bayat A., (2016), Synthesis and characterization of visible light active S-doped TiO₂ nanophotocatalyst. Int. J. Nano Dimens. 7: 33-38.

21. Liu G., Yang H. G., Wang X., Cheng L., Pan J., Lu G. Q., Cheng H. M., (2009), Visible light responsive nitrogen doped anatase TiO₂ sheets with dominant {001} facets derived from TiN. J. Am. Chem. Soc. 131: 12868-12869.

22. Devi L. G., Murthy B. N., Kumar S. G., (2010), Photocatalytic activity of TiO₂ doped with Zn²⁺ and V⁵⁺ transition metal ions: Influence of crystallite size and dopant electronic configuration on photocatalytic activity. Mater. Sci. Eng. B. 166: 1-6.

23. Horikawa T., Katoh M., Tomida T., (2008), Preparation and characterization of nitrogen-doped mesoporoustitania with high specific area. Microp. Mesop. Mater.110: 397-404.

24. Behara D. K., Ummireddi A. K., Aragonda V., Gupta P. K., Pala R. G. S., Sivakumar S., (2016),  Coupled optical absorption, charge carrier separation, and surface electrochemistry in surface disordered/hydrogenated TiO₂ for enhanced PEC water splitting reaction. Phys. Chem. Chem. Phys. 18: 8364-8377.

25. Behara D. K., Sharma G. P., Upadhyay A. P., Gyanprakash M., Pala R. G. S., Sivakumar S., (2016), Synchronization of charge carrier separation by tailoring the interface of Si–Au–TiO₂heterostructures via click chemistry for PEC water splitting. Chem. Eng. Sci. 154: 150-169.

26. Xiao F. X., (2012), Construction of highly ordered ZnO–TiO₂nanotube arrays (ZnO/TNTs) heterostructure for photocatalytic application. ACS Appl. Mater. Interfaces. 4: 7055-7063.

27.  Zhang P., Shao C., Li X., Zhang M., Zhang X., Sun Y., Liu Y., (2012), In situ assembly of well-dispersed Au nanoparticles on TiO₂/ZnOnanofibers: A three-way synergistic heterostructure with enhanced photocatalytic activity. J. Hazard Mater. 237: 331-338.

28. Guo S., Han S., Mao H., Dong S., Wu C., Jia L., Bo Ch., Li J., (2014), Structurally controlled ZnO/TiO₂ heterostructures as efficient photocatalysts for hydrogen generation from water without noble metals: The role of microporous amorphous/crystalline composite structure. J. Power Sourc. 245: 979-985.

29. Lei J. F., Li L. B., Shen X. H., Du K., Ni J., Liu C. J., Li W. S., (2013), Fabrication of ordered ZnO/TiO₂ heterostructures via a templating technique. Langmuir. 29: 13975-13981.

30. Wang R., Tan H., Zhao Z., Zhang G., Song L., Dong W., Sun Z., (2014), Stable ZnO@TiO₂ core/shell nanorod arrays with exposed high energy facets for self-cleaning coatings with anti-reflective properties. J. Mater. Chem. A. Mater. 2: 7313-7318.

31. Ghorbani, H. R., Parsa Mehr F., Pazoki H., Mosavar Rahmani B., (2015), Synthesis of ZnO nanoparticles by precipitation method. Orient. J. Chem. 31: 1219-1221.

32. Al Abdullah K., Awad S., Zaraket J., Salame C., (2017), Synthesis of ZnO nanopowders by using sol-gel and studying their structural and electrical properties at different temperature. Energy Procedia. 119: 565-570.

33. Wang C., Hu Q., Huang J., Wu L., Deng Z., Liu Z., Cao Y., (2013), Efficient hydrogen production by photocatalytic water splitting using N-doped TiO₂ film. Appl. Surf. Sci. 283: 188-192.

34. Yang G., Jiang Z., Shi H., Xiao T., Yan Z., (2010), Preparation of highly visible-light active N-doped TiO₂ photocatalyst. J. Mater. Chem. 20: 5301-5309.

35. Ivanov S., Barylyak A., Besaha K., Bund A., Bobitski Y., Wojnarowska-Nowak R., Yaremchuk I., Kus-Liśkiewicz M., (2016), Synthesis, characterization, and photocatalytic properties of sulfur-and carbon-codoped TiO₂ nanoparticles. Nanoscale Res. Lett. 11: 140-147.

36. Gholipour M. R., Dinh C. T., Béland F., Do T. O., (2015), Nanocompositeheterojunctions as sunlight-driven photocatalysts for hydrogen production from water splitting. Nanoscale. 7: 8187-8208.

37. Kim C., Kim J. T., Kim K. S., Jeong S., Kim H. Y., Han Y. S., (2009), Immobilization of TiO₂ on an ITO substrate to facilitate the photoelectrochemical degradation of an organic dye pollutant. Electrochim. Acta. 54: 5715-5720.