Developments in visible-light active TiO2/SnX (X = S and Se) and their environmental photocatalytic applications – A mini-review
Graphical abstract
Introduction
In the past few decades, the energy shortage and environmental pollution related issues emerging from the discharge of various industries organic effluents have been a matter of great concern for human society [1], [2], [3]. These organic pollutants are hazardous for humans as well as for animals, which rationalize their poor health. To overcome these environmental pollution issues, researchers across the globe have focused on developing innovative methods that are capable to remove completely these contaminants from the ecosystem [4], [5]. Photocatalysis, a green and environmentally friendly technique having wide range of applications (Fig. 1) emerges out as the best alternative over the conventional techniques to resolve the environmental contaminants problems [6], [7]. Although photocatalysis has a wide range of applications but its utility in water splitting and degradation of aqueous organic contaminants has gained more interest among the scientific community [8], [9], [10].
In the photocatalytic process, the semiconductor materials upon irradiation with a photon having energy equal or higher energy than its optical energy gap, produce charge species that decompose the toxic organic pollutants into non-toxic compounds [11]. For the practical industrial applications of the semiconductor photocatalysts, their strong absorption in the wide range of spectrum and long life span of charge carriers are indispensable requirements [12], [13]. Metal oxide such as zinc oxide, titanium dioxide, bismuth oxide, tin oxide, and copper oxide etc. are promising materials having a wide range of potential applications in the field of photocatalysis and photoelectrochemical [14], [15], [16], [17], [18]. Among the different semiconductor materials, TiO2 is the most broadly used photocatalyst due to its chemical stability, low toxicity, environmentally friendly nature, better absorption, and reusability [19], [20], [21], [22].
Due to its wide optical band gap of 3.2 eV, it activates only in the ultraviolet (UV) region and possesses little efficiency to harvest solar energy. To overcome such disadvantage of TiO2, some strategies like modification in geometry (e.g. nanosheet, nanoribbon, nanotube, nanoparticles etc.), surface area and porosity increase, and introduction of metal, non-metal, noble metal, or formation of heterostructure with other semiconductor materials as shown in Fig. 2 have been demonstrated successfully [23], [24], [25], [26], [27], [28], [29]. Among the above-mentioned modification strategy, the mixing of TiO2 with economical, earth-abundant and narrow band gap MXSY or MXOY based materials such as CdS, SnS, Fe2O3, CuO, PbS, CdTe, SnS2, ZnS, CdSe, Bi2O3 etc. have attracted the massive scientific interest for their exceptional and valuable optical, photoelectrochemical and photocatalytic applications [30], [31], [32], [33], [34], [35].
Among them, chalcogenides of Cd are most widely used as photocatalyst but due to their toxic nature, they are dangerous for the environment and human health [36]. The chalcogenides of tin has in recent times emerged as an alternative solution compared to Cd based due to its several advantages such as less toxicity, earth abundance, chemically stability, cost-effective, possessing unique structural properties and strong absorption in the visible region [37]. Tin sulfides occur in phases such as SnS (tin sulfide) and SnS2 (tin disulfide) with variable coordination numbers and oxidation states of + 2 and + 4 with associated respective p and n-type conductivity. In the Sn2S3 phase, Sn is present in both + 2 and + 4 oxidation states with n-type conductivity [38]. SnS, SnS2 have been broadly used in tin chalcogenides due to their novel optical and structural properties. Yang et al. [39] found that the nanoflakes of SnS2 prepared using SnCl2 and excess sulfur powder give better photocatalytic activity for the decomposition of methyl orange under the illumination of solar light. The coupling of SnS2 with wide band gap SnO2 semiconductor gives superior catalytic performance than that of individual SnS2 and SnO2 photocatalyst [40]. Due to various advantages of SnX (X = S, Se), the various preparation methods in different synthesis conditions have been adopted to obtain the multipurpose TiO2 composite with SnX (X = S, Se) for their interesting novel properties. In this review, we discuss the various design and synthesis strategies of the TiO2/ SnX (X = S, Se) composites with the special emphasis on variation in the structural, morphological, optical and photocatalytic activity of the TiO2/SnX composites. The effect of synthesis conditions and the addition of different precursors are highlighted in designing such TiO2/Sn based composites. We envisage that the availability of such comprehensive focussed literature insights will be beneficial as the basis for further development strategies of designing highly efficient semiconductor materials for water purification.
Section snippets
TiO2/SnS and TiO2/SnS2 heterostructure materials and properties
Tin and its compounds with sulfur such as SnS, Sn4S5, Sn3S4, Sn2S3, SnS2, etc. are non-toxic, environment-friendly, chemically stable and possessing better catalytic properties are widely used in the different fields of research [41], [42]. Among the different forms of tin sulfides, SnS and SnS2 are cost-effective, more earth-abundant and stable in both acidic and alkaline mediums. Due to the narrow energy gap and their layered structural features, the sulfur containing Sn compounds have many
TiO2/SnSe and TiO2/SnSe2 heterostructure materials and properties
Tin monoselenide (SnSe), diselenide (SnSe2) and Sn2Se3 are three major stoichiometric forms of tin selenide system which belong to the IV-VI group [86]. Several defects in the SnSe structure can be created by the Se-interstitial (Sei), Se-vacancy (VSe), Sn-interstitial (Sni) and Sn -vacancy (VSn). Such defects create the different energy levels which absorb radiation from near infrared and visible region of the electromagnetic spectrum and strongly influenced the electronic and optical
Photocatalytic activities
Intrinsic parameters like surface defect, crystal phase, band potentials and doping, etc. and extrinsic such as temperature, charge on the catalyst surface, catalyst dosage, concentration and pH of the solution significantly influence the photocatalytic efficiency of the nano-composites. Catalyst loading is an important factor for application on commercial scale and at high catalyst loading activity decreases due to sedimentation of particles and scattering of light while at low catalyst
Conclusions
The progress of the UV visible light-driven composites of TiO2/SnX (X = S and Se) are summarized in this review for the photocatalytic oxidation of organic molecules. Some traditional and novel cost-effective typical routes for the designing of TiO2/SnX (X = S and Se) possessing high photoconversion efficiency are also discussed. It is found that both surfactant and solvent affect the shape, size and morphology of nanocomposite. The phase formation of composite is also affected by the pH of the
CRediT authorship contribution statement
Shankar Sharma: Writing – original draft. Anuj Mittal: Writing – review & editing. Nar Singh Chauhan: Investigation, Writing – review & editing. Peter R. Makgwane: Writing – review & editing. Kavitha Kumari: Writing – review & editing. Sanjeev Maken: Visualization. Naveen Kumar: Project administration, Supervision, Visualization, Writing – review & editing.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
One of the author SS is thankful to University Grant Commission, New Delhi, India for financial help in the form of SRF.
Shankar Sharma is the doctorate student under the supervision of Dr. Naveen Kumar in the Department of Chemistry, Maharshi Dayanand University, Rohtak, India. He has completed his master degree from the same department and is currently working in the field of material chemistry and published many articles in the reputed journals.
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Cited by (0)
Shankar Sharma is the doctorate student under the supervision of Dr. Naveen Kumar in the Department of Chemistry, Maharshi Dayanand University, Rohtak, India. He has completed his master degree from the same department and is currently working in the field of material chemistry and published many articles in the reputed journals.
Dr. Anuj Mittal done his Ph.D under the supervision of Dr. Naveen Kumar from Department of Chemistry, Maharshi Dayanand University, Rohtak, India in July, 2020. His area of interest is application of semiconductor materials for environmental applications and published more than 20 articles in the reputed journals.
Dr. N.S. Chauhan is an Assistant Professor in the Department of Biochemistry, Maharshi Dayanand University Rohtak, India. For the past 15 years, he is pursuing research in the area of material science and system biology. He has published dozens of scientific papers on the synthesis and characterization of nanomaterials and microbial system biology.
Dr. Peter R. Makgwane is currently a principal scientist at the Council for Scientific and Industrial Research (CSIR) of South Africa and associate professor at the chemistry department of the University of the Western Cape (UWC). He previously worked for Mintek and Sasol petroleum as a scientist. His research interests include mainly heterogeneous catalysis and photocatalysis with respect to nanomaterials design for their applications in renewable chemicals conversion, environmental remediation and gas chemical sensing.
Kavitha Kumari is a doctorate student in the Department of Chemistry, Deenbandhu Chottu Ram University of Science and Technology, Murthal, Sonipat. Her area of interest is the thermodynamic characterization and materials chemistry.
Prof. Sanjeev Maken worked as Professor in the Department of Chemistry, Deenbandhu Chottu Ram University of Science and Technology, Murthal, Sonipat. He worked in Korean Institute of Chem. Eng. Seoul under Brain Pool Programme, South Korea. She also worked as an invited researcher/scientist in University of Minho, Portugal. He research interest is CO2 Absorption, Solution thermodynamics and modeling, thin oxide films, Waste-to-Energy. He has published more than 130 articles in the journal of high repute.
Dr. Naveen Kumar is working as an Assistant Professor in the Department of Chemistry, Maharshi Dayanand University, Rohtak, India for last 11 years. He is actively engaged in the researh for last 17 years and his keen area of research interest materials chemistry and mainly working in area of synthesis and application of nano-composite materials. He also worked in Department of applied Physics, University of Politechnica, Valencia, Spain in an international research project entitled as “Development of a new generation CIGS based Solar cells”. He has published more than 40 research articles in the journals of high repute.