INTRODUCTION
In this materialistic world, nothing is free. The life we are enjoying today has come to us at the cost of our environment. Globally, almost every country across the world has been facing detrimental threats to the environment in the form of pollution and this burning issue has become a matter of paramount concern for environmentalists, ecologists, and scientists. Intensive use of agricultural and industrial practices, as well as excessive use of energy resources, have brought the contamination toxicity level in the environment to an alarming height. The level of water and aerial pollution continuously increases the level of harmful contaminants through the emission of toxic gases, dyes from cosmetic and textile industries as well as heavy metals from agriculture, chemical industries, pharmaceutical, and domestic waste [1-4]. The elevated level of harmful chemicals not only damages the ecosystem but also causes serious diseases in living beings [5]. To curb the growth and removal of these hazardous pollutants, various conventional and modern methods are available such as chemical methods, and physical and physio-chemical methods [6]. Advanced oxidation phenomena involving UV radiation, ozonation, and Fenton oxidation [7-8] are helpful in dyes synthetic dyes degradation but all the above-mentioned processes require lots of chemical substances, which again make them uneconomical. Unfortunately, all these available methods are associated with one or more practical limitations with them despite their efficacy. In recent years, to solve these nerve-wracking environmental challenges, persistent efforts are shown by the researchers to explore and develop innovative techniques by using low-cost and eco-friendly materials for environment purification. In this series of efforts, the Photocatalysis technique has emerged as a promising candidate for a “green and eco-friendly” method to eliminate toxins present in the environment as well as for clean fuel production [9-10]. It’s been more than a century since the term “Photocatalysis” first came into the limelight of scientific literature. In 1911, various research communications were published incorporating the concept of Photocatalysis. The semiconducting materials which show photocatalytic activity upon conversion of irradiating light energy into chemical energy of electron-hole pairs are known as photocatalysts. Hence, while opting for a suitable photocatalyst for particular photocatalytic activity, bandgap of catalyst, level of toxicity, cost and availability are some of the important parameters which must be taken into consideration [11]. Fig. 1  shows the application of photocatalysis for the removal of different types of pollutants generally present in the water environment.

Properties and uses of SnO2 
Among various semiconducting metal oxides based photocatalysts (TiO2, Fe2O3,  ZnO, etc), Tin Oxide (SnO2) gained tremendous attention due to its wide range of applications in different fields such as photodegradation of pollutants, electrodes for lithium-ion batteries, gas sensing, dye-based solar cells, and optoelectronic devices, etc. as shown in Fig.2. Its low-cost availability, non-toxic nature, optical transparency, long-term stability, and high thermal stability consider SnO2 an excellent photocatalyst [12]. Fig. 2 shows the versatile applications of SnO2  in various fields.
Tin oxide is mainly an inorganic compound with the chemical formula SnO2. It usually appears as a colorless solid or powder that is insoluble in water. Stannic oxide is the other name for SnO2 [13]. Tin oxide is an n-type semiconductor that shows a wide bandgap of about 3.6 eV in bulk. SnO2 crystal phase structure is analogous to the rutile structure of TiO2 and belongs to P42/mnm space group having lattice parameters as a=b=4.738 A˚ and c= 3.187 A˚. The different physicochemical properties of SnO2 semiconductor material are represented in Table 1 [14].                        
Doping is considered a successful proven tool for tailoring the morphological, electrical, and optical properties of metal oxides. The reported literature is in agreement with the fact that oxygen vacancies offer themselves as highly occurring recombination centers in the SnO2 emission process [15]. SnO2 is considered a good host for doping with transition metals. The catalytic efficiency of SnO2 nanoparticles significantly enhances doping with different transition metals (TMs). Transition metal-doped SnO2 nanoparticles exhibit tunable bandgap and a high active surface area which further helps in improving the degradation response in the photocatalytic mechanism [16].
Although ample work on SnO2 nanoparticles has been reported by the research community, still SnO2 nanoparticles are hot area of research. A large number of research publications including some comprehensive review articles covering various aspects of SnO2 have already been published. For instance, Al Hamdi et al. [17] published a comprehensive review of SnO2 as a photocatalyst for the water remediation process. R.Rajput .et.al reviewed the development of Hydro/solvothermal synthesized visible light-responsive modified SnO2 nanoparticles for water treatment [14]. Y.Tadesse and co-workers reviewed green synthesis methodologies, mechanisms, and applications of Tin oxide nanoparticles [18].
A very limited number of reviews have been published within the last five years on the use of tin oxide as the photocatalyst for water remediation applications by the increasing interest of the scientific community in tin oxide as the potential photocatalyst. However, most of the review papers reported only selective transition metals doped SnO2-based photocatalytic materials. To the best of our knowledge, this is the first review report that encompassed the doping of complete first and second transition series elements with SnO2 towards its applications for the removal of various water pollutants. This review focuses primarily on the purification of synthetic dyes wastewater by spotlighting the role of first and second-series transition metal doping in SnO2 nanoparticles.
Hence, the present review intends to assess a detailed analysis of the first and second series of transition metals doped SnO2 nanoparticles photocatalyst covering the most commonly used synthesis methods, doping effects, different parameters affecting the photocatalytic activity, and also review the progress of removal of water-based toxic contaminations through SnO2 nanoparticles. Fig. 3 shows the roadmap of this review paper focusing on photodegradation mechanism, synthesis strategies, transition metal doping, influencing parameters, and applications of SnO2 Photocatalyst.

SnO2 semiconductor as a photocatalyst
Among different metal oxide semiconductors, SnO2 has gained the widespread attention of researchers due to its multifaceted applications. As stated above, SnO2 is an n-type semiconductor material with a bandgap, i.e. 3.6 eV, which corresponds to activation with photons of the wavelength of about 350 nm (UV-A range). R. Saravanan and co-workers [19] reported that in the photocatalytic process, a redox reaction i.e. successive photo-oxidation and reduction of catalyst takes place upon irradiation of light energy. The photocatalysis mechanism initiates when light energy of suitable wavelength (E≥ Eg), where Eg stands for bandgap energy, falls on the surface of semiconducting material in terms of photons. The valence shell electrons absorb energy from photons and jump to the conduction band of material which results in the formation of e−/h+ pairs. The h+ in the valence band oxidized and react with the H2O molecules to generate hydroxyl radicals (OH•). The e− present in the conduction band reacts with dissolved oxygen and triggers the formation of superoxide-free radical (O2−•) anion or hydroperoxyl (•O2H) radicals. After that these radicals react with the intermediate and convert the toxic pollutants into CO2 and H2O. The overall reaction steps are shown below [20]. Fig. 4 shows the schematic photodegradation mechanism of SnO2 nanoparticles as a photocatalyst.
              

Due to the wide bandgap energy value, SnO2 can only be activated in the UV range of the electromagnetic spectrum [21]. However, doping or semiconductor coupling can modify the catalytic activity of SnO2 by tailoring the absorption spectrum from the UV range to the visible range. As compared to pure SnO2, doped SnO2 has shown magnificent photocatalytic activity attributed to the high surface area with effective separation of photogenerated EHP and centralized electric field enhancement effect [22-23].

Synthesis techniques for SnO2 nanoparticles
For the synthesis of SnO2 nanoparticles, various strategies such as sol-gel, hydrothermal, co-precipitation, solvothermal, and solution-based methods, etc have been adopted. Some most commonly used techniques for the synthesis of SnO2 are discussed as follows:

Sol-Gel Method 
The Sol-Gel process has been considered a key technology that has gained widespread popularity in recent years, due to its simplicity and flexible nature. It allows using different materials to synthesize metal oxide and their nanocomposites at an affordable price. The choice of drying conditions also plays a crucial role in the formation of aerogels and xerogels [24]. Fig. 5  shows a schematic flowchart of the Sol-Gel method.
Azam. et. al prepared Mn-doped SnO2, SnCl4·5H2O, and MnCl2·4H2O as the precursors via a sol-gel approach. They studied the effect of Mn doping on the structural and optical properties of SnO2 NPs. It was observed that with the increase in Mn concentration, the crystallite size tends to reduce as the incorporation of Mn ions into the host lattice prevents the growth of crystal grains [25].
Kumar et.al [26] synthesized SnO2 spherical nanoparticles by using  Psidium Guajava Leave Extract and degraded reactive yellow 186 dye under direct sunlight. The biosynthesized SnO2 nanoparticles photodegraded 90% of the dye with in 3hrs. This shows that the green synthesis method is an emerging technique that can be explored further to obtain promising results for solar-driven water remediation.
The sol-gel method is a simple and cost-effective technique that provides good control over the size and shape of the nanoparticles and also produces better homogeneity results [27]. 

Hydrothermal Method 
Hydrothermal synthesis has been considered a solution reaction-based approach. The process of formation of nanomaterials can occur in a wide range of temperatures i.e. from room temperature to very high temperature. The synthesis process takes place in a closed Teflon-coated stainless-steel autoclave. This method is widely used to synthesize different types of nanomaterials [28]. Fig. 6 shows a pictorial diagram of the hydrothermal technique.
Huang et al [29] reported the fabrication of 3D SnO2 hierarchical superstructures (SOHS) using template-free hydrothermal synthesis. They selected methylene blue dye to examine the photodegradation activity of the sample and observed that the sample SOHS-1 has achieved the highest degradation efficiency among all the other samples. The enhanced degradation performance is ascribed due to the high surface-to-volume ratio and abundant catalytic activity sites. 
K. Bhuvaneswari and her group utilized the hydrothermal technique for the preparation of SnO2 nanoparticles and a plate-like structure using cetyl trimethyl ammonium bromide (CTAB) surfactant. It was reported that the pure SnO2 samples were poor absorbers as compared to CTAB synthesized samples. Under UV-vis light irradiation, they analyzed the prepared sample for the degradation of RhB, MB, and MO dye and found that the CTAB- SnO2-24 h sample shows higher photocatalytic efficiency as the refined morphology provides a more active surface for the reaction and hence enhanced the efficiency [30].

Co-precipitation Method 
Co-precipitation is a simple, fast, and cost-effective process to synthesize pure and doped nanoparticles. It involves the atomic mixing of particles which further yields the products with ideal stoichiometry at a low-temperature range. Due to its simplicity, it is widely used for industrial applications and also provides good morphological controls. Ahmad and his group [31] prepared pure and Cd-doped SnO2 using SnCl2.2H2O and anhydrous CdCl2 as precursor materials. They found that the particle size decreased initially on 1% of doping and further increased with increasing dopant concentration due to the expansion of the lattice attributed to the swapping of cations of different radii.
L. Nejati-Moghadam et al [32] successfully synthesized SnO2 nanoparticles using bis (acetylacetone) ethylenediamine as a capping agent and ammonia as a precipitation agent. They opted for Methyl orange and Eriochromschwarz-T as model pollutants to check the photodegradation performance of the prepared material and observed the complete degradation of dyes within 120 min. With the increasing irradiation time, the concentration of dyes tends to decrease as more dye is absorbed on the catalyst surface.

Solvothermal Method
Solvothermal synthesis is a facile technique that can produce a variety of organized structures relatively at high temperatures. The characteristics of the material can be tailored by altering some parameters such as reaction time, temperature, solvent type, precursor type, etc. 
Tikkun Jia et al [33] used this methodology for the synthesis of Zn doped SnO2 hierarchical architectures of different morphologies and the degradation of RhB dye was evaluated under UV lamp exposure. The alkaline quantity (NaOH) of the solution had a noticeable effect on the morphology and formed nanoflowers and nanourchin structures. The samples with urchin morphology reflected better photocatalytic activity due to the intrinsic oxygen vacancies created by the Zn2+ ions into the host lattice. 
Bhuvaneswari et al [34] successfully synthesized EDA (ethylenediamine) assisted SnO2 nanorods and they reported that the addition of EDA significantly altered the morphology and optical absorption spectra. The degradation of methylene blue dye was monitored and the EDA-SnO2 nanorods have shown excellent degradation of dye in 90 min. The enhanced performance was attributed due to the more intrinsic oxygen vacancies which provide high surface activity.

Transition metal doping in SnO2
Doping is the modification of photocatalyst by introducing impurities in it, which helps in reducing the bandgap. Doping is a part of bandgap engineering, which helps in avoiding the recombination process by enhancing the trapping of electrons [35]. 
Doping not only alters the morphology and surface area but also helps in improving photocatalytic activity. The introduction of dopants in the photocatalyst exhibits excellent performance due to the following reasons-
● It helps in avoiding the electron-hole recombination process
● Providing enhanced surface area
● Helps in increasing the pore size of the sample [16]
Though, control doping with different materials provides novel possibilities to optimize the properties of semiconducting nanomaterials and is a beneficial method to achieve enhanced efficiency and photoluminescence in the visible range. However, as compared to bare systems the longer emission lifetime of doped semiconductor nanomaterial is still facing challenges in their utilization for many practical devices. Hence, whether “To dope or not to dope” is still debatable [36].  
According to IUPAC, a transition metal is defined as an element whose atom has an incomplete d subshell. Various researchers have doped SnO2 with different transition metals such as Zn, Ni, Co, and Mn. Doping of SnO2 with transition elements not only optimizes the electronic structure and conductivity but also increases the catalytic activity of the material [37].
This section incorporates the doping of SnO2 semiconductors with the first and second series of transition metals for their photocatalytic applications toward water purification.

First transition series elements doped SnO2 nanoparticles:
Scandium (Sc)-doped SnO2
Scandium (Sc) is classified as a 3d transition metal and also a rare earth element rather than an earth-abundant element. Due to poor availability (rare earth metal) and wide bandgap energy of about 6.0 eV, which remains only active in the UV region hence Scandium as a photocatalyst or as a dopant for photocatalytic applications is still quite challenging. The effect of Sc doping on photocatalytic properties has not been much reported but still, this material has the potential to be explored further yet [38].

Titanium (Ti)-doped SnO2
Titanium is considered a transition element that belongs to the d block and group 4 of the periodic table. It has high reactivity with oxygen and doping of Ti into SnO2 lattice results in a decrease in lattice constants.  Lei Ran et al [39] synthesized the hollow structured Ti-doped SnO2 via an improved Stober method. The doping of Ti into SnO2 prevents the recombination process and results in enhancing photocatalytic activity. The photocatalytic activity of the obtained sample was investigated by the decomposition of methylene blue (MB) under UV and visible-light illumination.
Hanen Letif et al. [40] successfully prepared the Ti-doped SnO2 by facile and low-cost co-precipitation route at different concentrations. The pure and doped nanoparticles were crystallized in the tetragonal structure. Precisely, the highest concentration of Ti improved the photocatalytic performance. The improvement in efficiency was due to the extended absorption edge from the UV light to the visible light region. 

Vanadium (V)-doped SnO2
Vanadium is the 20th most abundant element which lies left of chromium and right of titanium in the first series of transition metals. The incorporation of vanadium into the SnO2 lattice reduced the cell volume due to the small radii of vanadium ions.
 J. Mazloom et al [41] synthesized V-doped SnO2 using the sol-gel route. As compared to the pure sample the quenching in green luminescence intensity was observed in the doped sample. V doping decreased the intensity and possesses high photocatalytic performance as a result of the reduced bandgap. To degrade the methylene blue and rhodamine B, the obtained sample exhibited excellent photocatalytic activity.
Ch. Venkata Reddy et al [42] prepared V doped SnO2 at different concentrations of vanadium via combustion synthesis technique. X-ray photoelectron spectroscopy confirmed the existence of V4+ species in the SnO2 lattice. With increasing dopant concentration bandgap energies decreases and also enhance the photocatalytic activity, as doping in SnO2 shift the absorption edge to the visible region.
R. Shyamala et al [43] synthesized V doped SnO2 using ammonium metavanadate and stannous chloride by sol-gel approach. They concluded that with an increase in the dopant amount the absorption edge shows a redshift and hence the value of bandgap energies decreases from 3.77 to 2.9 eV. They studied the photodegradation of MO and concluded that the V/SnO2 sample exhibited higher photocatalytic performance due to a lower bandgap value. 
H. Letif et al [44] incorporated the co-precipitation method to prepare the V-doped SnO2 NPs and their results displayed the photocatalytic degradation of rhodamine B under UV light illumination. The absorption edge of the doped sample exhibited a red shift due to increasing dopant concentration hence, the doped samples achieved higher photocatalytic activity as compared to the bare sample.

Chromium (Cr)-doped SnO2
Chromium with atomic number 24 belongs to group 6 of the periodic table. The highly polished chromium can reflect about 70% of visible and 90% of visible light. As the ionic radius of Cr3+ (63 °A) is close to that of Sn4+ (74 °A), which means that Cr3+ ions can easily incorporate into the SnO2 lattice or substitute the position of Sn4+  in the crystal without altering its rutile structure [45].
Ch. Venkata Reddy and group [46] successfully prepared the Chromium (Cr)-doped SnO2 quantum dots (QDs) with different doping concentrations via a simple combustion technique. The XPS spectra confirm the existence of Sn4+, Cr3+, and O ions respectively in the host lattice and the Cr-doped SnO2 QDs exhibit higher photocatalytic activity as the introduction of Cr ions into the lattice decreased the intensity and hence reduces the recombination rate of photogenerated electron and hole.                    
Taybeh Karimi et al [47] reported the preparation of pure and doped (Cr)-doped tin dioxide (SnO2) nanoparticles via a chemical precipitation route. Their studies reveal that the particle size decreases due to the incorporation of Cr ions into the host lattice as dopants affect the growth mechanism of the particles. Also, the small size of the particles offers the remarkable photocatalytic degradation of methylene blue dye.

Manganese (Mn)-doped SnO2
Mn is classified as the third most abundant transition element. The substitution of Mn+4 (0.53◦ A) ion in place of Sn+4 (0.69◦A) ions into the host lattice results in the contraction of the cell parameters.  K. Anandan et al [48] described the synthesis of bare and doped Mn-doped SnO2 nanoparticles by precipitation method. Their optical studies revealed that the bandgap of the Mn-doped SnO2 nanoparticles increased on increasing the Mn concentration due to the small particle size. 
L. Sakwises and co-workers [49] investigated SnO2 and Mn-doped SnO2 particles prepared via chemical synthetic technique. The group used this material in the photocatalytic degradation of methylene blue. They have reported that the pure SnO2 exhibits higher efficiency rather than the doped sample due to the same oxidation state of Mn and Sn, hence no difference was notified on partial substitution of Mn ions into SnO2 lattice.
M. Ramamoorthy et al [50] used a chemical precipitation route and synthesized Mn-doped SnO2 loaded with (0.5 g) corn cob activated carbon. The photocatalytic activity of obtained samples was studied by photodegradation of methylene blue dye under sunlight illumination. The doped samples loaded with corn cob exhibited higher degradation efficiency as the loading of corn cob could be the carrier for generated electrons, hence improving the efficiency. 
Pritam Borker et al [51] used the co-precipitation technique to synthesize Mn-doped SnO2 nanowires and the photocatalytic activity of nanocomposites was studied by the degradation of naphthol blue-black dye under UV light. They reported that the dye was not able to degrade in the dark, therefore the dye solution was bubbled with O2 to enhance the photocatalytic performance. Aeration provides better results as it prevents the recombination process and hence enhanced the photocatalytic efficiency of Mn-doped SnO2 nanowires.

Iron (Fe)-doped SnO2
Iron is a metal that belongs to group 8 and the first transition series of the periodic table. Fe atoms are incorporated into SnO2 lattice at substitutional or at interstitial sites. The cell volume and lattice parameters gradually decrease with increasing dopant amount [52]. Marauo Davis et al. [53] successfully synthesized Fe-doped SnO2 nano architectures via a sol-gel route using inorganic salts as starting materials. They concluded that only 5% of dopant concentration degrades about 55% of dye under and this can be only achieved due to the small crystallite size, high-internal surface area, and porous aerogel network.
Zhang et al. [54] successfully used a simple solvothermal technique to fabricate Fe-doped SnO2 echinus-like particles. They investigated the synthesized material for degradation of RhB and Cr (VI) under UV light illumination and Fe-doped SnO2 samples displayed better degradation performance due to the high active surface area and high porosity.
Othmen et al. [55] prepared Fe-doped tin dioxide nanoparticles using a hydrothermal process with different concentrations of Fe and the presence of Fe4+ ions in the host lattice was detected by Mössbauer spectroscopy. Under UV exposure the addition of iron diminishes the photocatalytic efficiency but is only enhanced under visible light due to the wide bandgap values of SnO2 samples. 
R. Mani et al [56] utilized a chemical precipitation technique to synthesize (Fe) doped SnO2 nanoparticles. TEM revealed that the samples are spherical in shape and the average size was about 24-42nm. Further, the photocatalytic degradation of phenol and benzoic acid was studied and doped samples have a high-efficiency rate due to narrow bandgap value and high active sites.
Amna Afzaal et al [57] incorporated sol-gel and hydrothermal routes to synthesized SnO2-SiO2 and Fe doped SnO2-SiO2 nanocomposites respectively, using a zwitterionic surfactant. The incorporation of iron into nanocomposite observed the redshift due to the small bandgap and transfer of electrons thus refining the optical properties hence, the doped nanocomposite exhibits enhanced degradation efficiency of methylene blue.
Othmen et al [58] used three steps elaboration method and successfully reported the synthesis of Fe-doped SnO2 NPs, which were further loaded on rGO sheets. They studied the photodegradation of rhodamine B dye under visible light irradiation and due to the presence of oxygen functional groups in graphene oxide, the electrons were entrapped by dissolved oxygen on the surface of the semiconductor, hence increasing the photocatalytic performance of the doped sample. 
Qing Wang et al [59] synthesized Fe doped SnO2 at different concentrations with decorated layer g-C3N4 via chemical precipitation technique. The photocatalytic performance of prepared samples was evaluated under visible light illumination by degradation of Rhodamine B (RhB) and Methylene blue (MB). The dopant helps in reducing the bandgap value and improved the photocatalytic activity.

Cobalt (Co)-doped SnO2
Cobalt with atomic number 27 belongs to group VIII of the periodic table. The presence of Co ions in the SnO2 lattice results in decreasing the grain size and increasing oxygen deficiency of the SnO2 lattice. These properties can influence the photocatalytic performance of pure SnO2. Entradas et al [60] reported the Co-doped SnO2 nanopowders via a chemical route and their optical study demonstrated a redshift due to band-to-tail and tail-to-tail transitions. They also studied the photocatalytic behavior of prepared nanocomposites in the degradation of 4-hydroxybenzoic acid (4-HBA) under UV light and complete photodegradation of dye was achieved within 60 min.
R. Mani et al [61] successfully synthesized pure and Co-doped SnO2 nanoparticles via chemical precipitation. The effect of doping on the structural, optical, and photocatalytic activity was studied by using different characterization techniques. The doped material showed highly promising photodegradation properties when checked for the degradation of phenol and benzoic acid and was found superior to bare SnO2 nanoparticles. The enhanced catalytic performance was attributed due to small bandgap values and high specific surface area.
Z. Nasir et al [62] utilized the co-precipitation method to prepare Co-doped tin oxide nanoparticles and further their photocatalytic and antimicrobial properties were investigated. The photocatalytic performance of Co-doped SnO2 NPs was examined against MB and the increasing level of dopant concentration results in enhancing the photocatalytic activity due to the formation of more trapping sites and lower recombination rate.
D. Toloman et al [63] prepared Co-doped SnO2 nanoparticles via chemical precipitation. The structure of the samples was in the tetragonal rutile phase and the presence of Co ions in the host lattice results in declining the oxygen valencies. The obtained doped samples showed high photocatalytic efficiency against RhB solution under visible light illumination due to small recombination rates, visible light absorption, and high amounts of •OH and •O− 2 radicals.

Nickel (Ni)-doped SnO2
Nickel is the first-row transition element in the periodic table and belongs to a group (VIIIb) of the periodic table. It is a naturally occurring metallic element with a shiny appearance. H. Chen et al [64] used the hydrothermal process to synthesize Nickel-doped tin dioxide microspheres with various doping amounts and further characterized by using different techniques. The prepared samples show excellent photocatalytic efficiency as compared to pure SnO2 under visible light irradiation. The dopant plays a vital role in reducing bandgap and recombination rate, further boosting the activity and stability of the catalyst. 
M. Kandasamy et al [65] prepared Ni-doped SnO2 nanoparticles (NPs) via a co-precipitation route and then investigated the properties for sensing and photocatalytic applications. The photocatalytic degradation of Rhodamine B (RhB), Congo red (CR), and Direct red (DR) dyes were monitored under Visible light irradiation. The doped NPs showed enhanced degradation efficiency due to the termination of the recombination process at higher doping concentrations.
Ateeq Ahmed et al [66] synthesized Ni-doped SnO2 NPs via sol-gel technique with different amounts of dopant. The degradation rate of RhB was studied under UV light irradiation and reported that SnO2 with 6% of Ni doping exhibited higher photocatalytic activity due to better adsorption of dye on the surface. 
Chen and group [67] used a hydrothermal technique to synthesize Ni-doped SnO2 quantum dots and SnCl4.5H2O, NiCl2.6H2O is the main precursor, vitamin C as a stabilizer, and Na2CO3 as a precipitator. The effect of the doping concentration on the degradation efficiency was investigated and revealed that the doping reduces the bandgap and recombination rate. Hence, enhanced the photocatalytic activity of the doped catalyst. 
S. Asaithambi et al [68] prepared rutile structured Ni-doped tin oxide NPs using a simple co-precipitation technique. It was observed that on increasing the level of doping, the average size of particles decreased from about 27 nm to 22 nm as a result of defects produced by the Ni doping. The photodegradation of methylene blue was examined for the pure and doped sample under a visible light source and the doped sample exhibited higher photodegradation efficiency due to its smaller size and high active surface area.

Copper (Cu)-doped SnO2
Copper with atomic no. 29 classified as a transition element belongs to group 11 of the periodic table having a face-centered cubic structure. S. Vadivel et al [69] reported the synthesis of bare and copper (Cu) doped SnO2 nanocrystalline thin films via the chemical bath deposition method. The redshift in spectrum displayed the decrease in band gap value due to charge-transfer transitions, hence promoting the photocatalytic behavior of Cu doped samples.
M. Sathishkumar et al [70] demonstrated the photocatalytic and antibacterial activity of pure and Cu-doped SnO2 NPs synthesized by a microwave-assisted method. They studied the degradation properties of dyes were examined under UV light illumination and higher efficiency was obtained at about 9% of Cu doping. Due to the smaller size of particles and small surface roughness, the highest wavelength value was only observed for 9% of Cu doping.

Zinc (Zn)-doped SnO2
Zinc with atomic no. 30 belongs to group 12 of the periodic table. It is the 24th most abundant element in the earth’s crust.  X. Jia et al [71] fabricated Zn doped SnO2 nanoparticles through the precipitation technique. The photocatalytic activities of samples were evaluated using rhodamine B dye and the obtained decomposition rate of RhB for doped SnO2 nanoparticles was about 99% due to smaller bandgap value and high surface activity.
In another study, the photodegradation of brilliant green (organic dye) under UV light was studied by N. Shanmugam et al [72]. The obtained results revealed that 0.75 M of Zn-doped SnO2 decolorized brilliant green faster than other doping concentrations due to the narrow bandgap value which promotes the production of exciton and thus favors the photodegradation rate. 
W. Soltan et al [73] prepared nanocrystalline and nanoporous Zn-doped SnO2 materials via a simple polyol method. Fine-tuning of textural and optical properties was done by varying the zinc concentration. They revealed that the higher dosage of a catalyst lowers the degradation rate due to the turbidity of the solution and agglomeration of NPs. Hence, the optimum dosage of catalyst improves the performance and the complete discoloration of MB solution was achieved after 120 min. 
M. Yurddaskal and group [74] compared the catalytic performance of pure and Zn-doped SnO2 nanoparticles at different concentrations. The photocatalytic activity of prepared samples was evaluated by studying the decomposition of MB dye solution under a UV light source. The 1% doped sample exhibited higher efficiency but further doping reduced the degradation performance due to the increasing recombination process.
Chu et al [75] fabricated the Zn-doped SnO2 flower-like nanostructures via hydrothermal technique and further, the photodegradation of RhB dye was studied under visible light illumination. The prepared Zn-doped SnO2 nanostructures achieved higher photocatalytic activity in the decomposition of rhodamine B dye than pure SnO2 as doping results in higher charge separation and lower electron-hole recombination.
Lu et al [76] synthesized Zn-doped SnO2 pompon-like hierarchical structure using the hydro-thermal route and zinc nitrate, sodium oxalate and zinc nitrate hexahydrate are the major precursors. Further, MB, MO, RhB, and CR dyes were selected as model pollutants to examine the degradation efficiency of synthesized materials. Among them, the Zn-doped SnO2 catalyst showed excellent photodegradation behavior due to the oxygen vacancies and more doping sites.
Suthakaran et al [77] introduced the surfactant-assisted hydrothermal method to synthesize undoped and Zn doped SnO2 NPs using sodium hexametaphosphate (SHMP) as a surfactant. Further, they examine the synthesized NPs for the photodegradation of methyl violet dye. The incorporation of surfactant reduces the intensity of the strong absorption band with increasing time which indicates the decolorization of MV and hence enhances the photocatalytic activity. Table 2. shows a summary of the Photocatalytic behavior of the first series of Transition metals doped SnO2 semiconductors. 

Second transition series elements doped SnO2 nanoparticles:
Yttrium (Y) doped SnO2
Yttrium with atomic no. 39 is considered the rare-earth element which belongs to group 3 of the periodic table. The surface separation of Y3+ ions creates oxygen vacancies that possess great optical conductivity. The effect of yttrium (Y3+) doping obstructs the recombination process and enhances photocatalytic performance.  A. Baig et al [97] reported the photodegradation of Y-doped SnO2 NPs, which were prepared by a hydrothermal process with various doping amounts. They examined the structural, optical, and photocatalytic properties of the obtained sample. The doping results in decreasing the band gap values and thus provides more active surface sites. Hence, doping enhances the photodegradation of methylene blue dye in the visible region.

Zirconium (Zr) doped SnO2
Zr with atomic no. 40 is a strong transition metal that lies in group 4 of the periodic table. Suthakaran et al [98] used the surfactant-assisted hydrothermal method to synthesize pure and Zr doped SnO2 NPs. Tin (IV) chloride pentahydrate and zirconyl chloride octahydrates are the main precursors and sodium hexametaphosphate is used as a surfactant. An increment in the doping amount results in lower recombination rates thus promoting the efficiency rate. Further, the synthesized NPs for the photodecomposition of methyl violet (MV) dye was investigated under sunlight illumination for 120 min. 
Similarly, A. Baig et al [99] synthesized the Zr doped SnO2 nanostructures via a low-cost co-precipitation technique. The obtained nanoparticles were spherical and consisted of agglomerated particles and 4% Zr doping showed enhanced photodegradation of methyl orange dye which occurs due to the defects produced by the NPs.
Niobium (Nb) doped SnO2
Niobium is a light grey transition metal with atomic no. 41 and belongs to group 5 of the periodic table.  A. Sadeghzadeh-Attar [100] fabricated the Nb-doped SnO2/V2O5 hetero- structured nanocomposites by using hydrothermal and liquid-phase deposition-based processes. SnO2 nanotubes were doped with different concentrations and the synthesized nanocomposite exhibited higher degradation due to the efficient charge separation.

Molybdenum (Mo) doped SnO2
Molybdenum is a chemical element in group 6 of the periodic table with atomic no. 42. It exists in two different oxidation states such as Mo6+ and Mo4+ having ionic radii. Therefore, it is expected that more Mo ions could be incorporated into the SnO2 lattice by replacing more Sn4+ ions. N. Manjula et al [101] successfully investigated the photocatalytic behavior of Mo-doped SnO2 (SnO2: Mo) nanopowders synthesized by a cost-effective chemical method. Doping shifts the absorption edge towards the visible region therefore, the doped sample exhibit a higher degradation rate as compared to the bare sample.

Palladium (Pd) doped SnO2
Palladium with atomic no. 46 belongs to group 10 in the periodic table. It is considered the most precious and rarest earth metal. R. Janmanee et al [102] utilized the thermal decomposition method to synthesize Palladium doped SnO2 NPs using Vent Pulp as the dispersant and tin tetrachloride pentahydrate and ammonium hydroxide as a precursor. The photocatalytic activity of the obtained samples for the degradation of sucrose and glucose under UVA-light irradiation was examined and it was concluded that due to their smaller size, the Pd doped SnO2 NPs demonstrated a good efficiency rate.

Silver (Ag) doped SnO2
Silver with chemical symbol Ag and atomic no. 47 is a white lustrous metal located in period 5 and group 11 of the periodic table. It is classified as a soft, white, and lustrous transition element. K. Vignesh et al [103] reported the photocatalytic behavior of Ag-doped SnO2 modified with curcumin. The NPs were synthesized via combined precipitation and chemical impregnation techniques. The modified photocatalysts revealed a redshift in the visible region and the enhanced activity of the Cu-Ag-SnO2 sample was ascribed due to the existence of more reactive oxygen species.
S. Ansari et al [104] used silver in the synthesis of enhanced SnO2 nanocomposites using an electrochemically active biofilm. The prepared material was then analyzed in the degradation of various organic dyes and toxins, such as methyl orange, methylene blue, 4-nitrophenol, and 2-chlorophenol. They showed higher photocatalytic activity as compared to pure SnO2 nanostructures upon exposure to light in the visible region due to a lower recombination process.
M. Ahmed et al [105] successfully prepared mesoporous Ag- SnO2 NPs via sol-gel process using PVP as the pore and structure-directing agent. Due to the deposition of Ag ions on the wall of pores, super micropores were created and the hydroxyl radicals and holes are responsible for the degradation of methylene blue dye.
B. Babu et al [106] successfully synthesized Ag doped SnO2 QDS via one-pot synthesis using hydrazine. The photocatalytic activity of prepared material was synthesized at different amounts of Ag. Doping of Ag ions results in increasing the high active surface area thus the doped sample degrades about 98% of Rhodamine B (RhB) dye. Table 3. shows a summary of the Photocatalytic behavior of Second series Transition metals doped SnO2 semiconductors.
Some second series transition elements such as Technetium (Tc), Ruthenium (Ru), Rhodium (Rh), and Cadmium (Cd) are not widely reported in the literature as a dopant with SnO2 for photocatalytic applications. Technetium-99 (Tc) is a problematic fission product and due to its long half-life, it complicates the long-term disposal of nuclear waste [120]. K. R. Arangayagam et al [121] reported the synthesis Ru doped ZnO as photocatalyst but no literature reported for Ru doped SnO2 for photocatalytic applications. Rh and Cd doped semiconductors are reported in the literature for different applications such as in the field of hydrogen production, dye-sensitized solar cells, and sensors with other metal oxide semiconductors [112-124].

Parameters altering the photocatalytic activity of SnO2 Nanoparticles 
Different operating parameters can affect the photocatalytic degradation of various pollutants present in the wastewater such as catalyst concentration, pH value, light intensity, temperature, surface area, and crystallinity. This section concisely reviews some of the following parameters [125].

Catalyst Concentration
Various studies reveal that the photocatalytic efficiency first increases along with the catalyst loading and then reduces at a high dosage. An extreme concentration of catalyst particles would block the path of light, which further results in the scattering of light and hence decrease the photocatalytic activity [126-127]. 
S. Chakraborty et al [128] studied the effect of SnO2 NPs loading on photocatalytic degradation of 4-aminopyridine under solar light exposure for 120 min. They reported the effect of two parameters such as catalyst concentration and pH of the solution on photocatalytic activity of SnO2 nanoparticles and the catalyst with various concentrations were added to the 4-AP solution. On increasing the catalyst dosage, the absorption of photons also increases followed by the increment in active sites on the surface of the catalyst. Hence, the photocatalytic efficiency increase with increasing catalyst loading. Barkha Rani et al [129] successfully explained the significant influence of catalyst amount on the photocatalytic degradation of methylene blue dye. The synthesized samples revealed that the increase in catalyst amount provides higher active sites for the adsorption of dye. Hence, the degradation rate was found to increase with an increased dosage amount.

Effect of pH
pH is one of the most significant factors which not only affects the oxidation potential but also influences the charge of the potential [130]. A slightly acidic pH range enhances the attraction of the pollutant to the photocatalyst surface, which results in increasing the degradation efficiency. Also, the degradation rate declines if the range of pH drops below a certain value. Hence it is an important factor that can modify the photocatalytic activity of the particles [131]. Z. Fzhu [132] et al analyzed the efficacy of pH on the photocatalytic activity of SnO2 microspheres, synthesized via microwave solvothermal technique. The RhB dye degradation efficiency was studied at different pH ranges. They found that at pH 2.91 and 6.18, the absorbance of dye molecules was 2-3 times higher due to the availability of large binding sites for dye molecules.
Similarly, M. Najjar et al [133] studied the effect of pH on the photocatalytic activity and reported that the photodegradation of EBT dye is enhanced when the pH of the reaction mixture is higher due to the attraction between cationic dye molecules and OH- ions.

Light intensity
When the incident energy is equal to or more than the bandgap energy then only the semiconductor catalyst absorbs it. On increasing the intensity of incident light, the feasibility of catalyst excitation was also raised. Light of lower intensity reduces the generation of free radicals, hence reducing the degradation rate. Therefore, photocatalytic activity increases with increasing light intensity [134]. M. Dhanlakshmii et al [135] fabricated the visible-light-driven Ir doped SnO2 NPs. They concluded that the enhancement in the degradation efficiency rate results in an increment of light intensity as the light of higher intensity is capable of generating more reactive radicals hence improving the photodegradation rate. 

Temperature
The photodegradation rate also depends on the temperature of the reaction. The photocatalytic activity increases at elevated temperature range due to improvement in the mobility of charge carriers. However, further, an increase in temperature beyond a certain range results in increasing the recombination process and hence reducing the degradation rate [136]. K. Prakash et al [137] reported the formation of SnO2 photocatalyst for mineralization of methylene blue dye solution at different temperature ranges. They found that increase in the annealing temperature range produces remarkable enhancement in the degradation efficiency of dye as the movement of photoelectron-hole pairs generates more OH radicals. But the further increase in the temperature results in increasing the recombination rate thus better results is acquired when the temperature range is retained between 20 ◦C and 70 ◦C.
Similarly, Hao Yuon [138] et al analyzed the impact of calcination temperature on the methyl orange dye degradation and the results revealed that the sample prepared at a higher temperature reported enhanced degradation efficiency and then decreases due to variation in particle size and phase composition of the particles.

Surface area and Crystallinity
Size and surface area play a very crucial role in the photocatalytic efficiency of photocatalysts. Small size and high active surface area offer enhanced degradation qualities towards the removal of pollutants from the existing environment. Therefore, modification by doping not only reduces the bandgap value but also enlarges the surface of the catalyst [139]. Soumia Haya and co-workers [140] compared the photocatalytic efficiency of pure and Sr doped SnO2 NPs for the degradation of methylene blue dye under UV light exposure. The effect of doping was examined by investigating the crystal morphology. The average size was calculated by the Debye Scherer equation which revealed the decrement in crystal size hence providing a large surface area that further offers the more active sites thus promoting the photocatalytic efficiency.
Ameer Azam et al [25] studied the efficacy of Mn ions doping on the structural and optical properties of SnO2 NPs prepared by the sol-gel approach. The crystal structure and morphological studies were carried out and it was observed that the crystal size reduced from 16.2 nm to 7.1 nm as the doping of Mn ions prevents the growth of the crystal. The effect of various influencing factors on the particle size and crystallinity of SnO2 nanoparticles is summarized in Table 4.

Applications of Bare/Doped SnO2 Photocatalyst for Degradation of wastewater pollutants
Globally, the uncontrolled increasing level of water pollution has turned into a major threat. The major pollutants are broadly classified into organic, inorganic, and biological contaminants. Among them, organic pollutants are of major concern due to their mutagenic effects even after exposure to a little amount [141]. The majority of organic pollutants are emitted with large-scale industrial and agricultural practices i.e. reckless use of chemicals and fertilizers [142]. Fig. 7 shows the various applications of SnO2 toward water decontamination.
Various researchers reported the mineralization of wastewater pollutants using SnO2 has been summarized below:

Degradation of pharmaceutical pollutants 
Pharmaceuticals products (PP) like sulfamono-methoxine (SMM), naproxen (NPX), ciprofloxacin (CIP), amoxicillin (AMX), tetracycline, and many more are significantly used for healthcare systems. These products are difficult to degrade as they produce secondary pollution [143]. Hojamberdiev et al [144] examined the degradation process for different pharmaceuticals and personal care products (PPCPs): metoprolol, carbamazepine, acetaminophen, and triclosan. Under visible light exposure, the obtained sample degraded about 70% of acetaminophen, 67% of metoprolol, 40% of carbamazepine, and 40% of triclosan within 120 min. The difference in the removal efficiency of PPCPs is due to the variation in physicochemical properties of the composite, chemical structures of PPCPs, and the interactions between PPCP molecules and the photo-catalyst surface.
Begum and group [145] utilized a chemical precipitation technique to synthesize SnO2 nanoparticles using anhydrous aspartic acid and surfactants at different annealing temperatures. They evaluated the photocatalytic activity of the synthesized sample for detoxification of carbamazepine (CBZ), an antiseptic drug. The obtained SC1 and SS1 NPs samples can degrade about 97% and 92% respectively, beneath UV-C light within 1 h.
Chu et al [146] successfully prepared bismuth-doped SnO2 quantum dots using a one-step hydrothermal process. The photocatalytic behavior of the prepared sample was analyzed for the mineralization of ciprofloxacin hydrochloride (CIP) and RhB dye solution under-stimulated sunlight illumination and the degradation efficiencies were 1.75 and 1.53 times higher than that of the pure sample. The outstanding performance of the composites was due to the enhanced absorbance of light and lower recombination rate.
Begum et al [147] introduced the hydrothermal synthesis process to produce SnO2/activated carbon nanocomposites tin chloride pentahydrate, sugarcane juice, and activated carbon. The prepared sample was used further for the mineralization of naproxen beneath sunlight irradiation and showed that the obtained nanocomposite degraded 94% of the naproxen due to the availability of larger active sites available on the surface.

Photocatalytic Degradation of Dyes
Nowadays, synthetic dyes are the major abundant pollutant detected in water bodies. These highly pigmented dyes cause eutrophication and agitations in marine life [148]. The degradation mechanism of dyes is shown below in Fig. 8.
Jyoti Bala Kaundal et al [149] synthesized SnO2 decorated Polystyrene (SnO2-PS) polymer nanocomposites using thermocol packing waste via a sol-gel chemical route and their photocatalytic studies revealed that 99% of indigo dye was degraded within 30 minutes due to the smaller size of NPs hence, makes it a preferable photocatalyst. 
Morvarid Najjar et al [133] synthesized SnO2 nanoparticles with the help of the sol-gel synthesis route and the gel obtained was calcined at different temperatures. They concluded that the increase in irradiation time results in decreasing the absorbance of dye solution and the obtained degradation rate of Eriochrome Black T (EBT) dye was about 35.9% 
Taehee Kim et al [150] synthesized SnO2 aerogel/reduced graphene oxide (rGO) nanocomposites via the sol-gel technique. The obtained nanocomposites exhibit enhanced photocatalytic activity for the degradation of methyl orange due to the high surface area of graphene flakes. Hence, the prepared nanocomposites are termed a suitable candidate for the photodegradation of pollutants in industrial wastewater. 
Vijay Kumar et al [151] utilized a two-step sol-gel approach to synthesize SnO2/CdS heterostructures and they concluded that prepared SnO2/CdS heterostructures exhibit improved photocatalytic performance due to the excessive separation of photogenerated electrons and holes in the photocatalytic region.
Al-Hamdi et al [139] prepared Iodine doped tin oxide (SnO2:I) nanoparticles using the sol-gel approach. In this work, the photocatalytic activities of synthesized NPs for phenol degradation were studied and the iodine doped SnO2 under UV irradiation degrades phenol very quickly within 30 minutes due to the high optical absorption of doped particles. 
Photocatalytic degradation of other organic pollutants 
Aromatic compounds are considered another organic pollutant that is mainly discharged into water bodies by different industries[152-153]. These are colorless or pale yellow solids with one or more hydroxyl groups attached to the ring. The discharge of these pollutants into the environment causes a significant threat to the ecosystem [154].
Al-Hamdi et al [155] reported the mineralization of phenol with rare earth metals doped SnO2 NPs. Lanthanum, cerium, and neodymium were used as dopants and they showed that the lanthanum doped SnO2 was tremendously effective for the degradation of phenol as they are most photoactive. Under UV- light exposure more than 95% of phenol mineralized from the sample. The results revealed that the obtained doped sample was much better than the bare sample.
K. Saravanakumar et al [113] introduced a simple and fast one-step hydrothermal route to produce spheres like Ag/SnO2 nanocomposite. Different spectroscopic and microscopic techniques were used to evaluate their light absorption and morphological properties. The incorporation of Ag into the SnO2 lattice improved the photocatalytic performance due to the reduced recombination mechanism, which was examined by the PL spectrum. Further, beneath visible light illumination, 91% of phenol was degraded within 50 min.
Liu et al [156] successfully fabricated the core−shell structural CdS@SnO2 nanorods. The integration of Cds having a small bang value i.e. 204 eV with SnO2 having a wide bandgap of about 3.6 eV was found to be favorable. Under visible light exposure, the photocatalytic performance for the oxidation of benzyl alcohol to benzaldehyde was higher than that of neet semiconductors as a result of an extended lifetime and improved e−–h+ pairs separation.
J.Ebrahimian et al [157] reported the production of SnO2 nanoparticles via the green synthesis route using the extract of chaste tree (Vitex agnus-castus) with casticin, quercetin, and kaempferol as reducing and stabilizing agents. 91.7% of dye degraded within 190 min due to a higher adsorption rate of Co2+ ions on the surface.

CONCLUSION
This review paper significantly highlights the modification of SnO2 by doping of first and second transition series metals. The key issues that are addressed in this review are as follows: 
1. SnO2 is considered the most preferable and attractive photocatalyst for the removal of pollutants present in wastewater. Also, the addition of transition metal dopants further improves photocatalytic performance.
2. Different synthesis methodologies such as sol-gel, hydrothermal, and co-precipitation are the most approachable routes for preparing doped SnO2 nanoparticles, owing to their simplicity. The sol-gel method is considered the most preferable method due to its better homogeneity results.
3. Various studies reveal that TM doping helps in reducing the recombination process which further improves the photocatalytic behavior of nanoparticles. Hence, it is possible to acquire the visible light active photocatalyst by altering the different operating parameters such as pH, dopant concentration, time, and temperature which significantly affect the photocatalytic performance.
4. TM doping is found to increase the photodegradation behavior of SnO2-based photocatalysts. The above-summarized results exhibited the excellent photodegradation of dyes, pharmaceutical waste, and other toxic organic pollutants. Various researchers have successfully synthesized transition metal-doped SnO2 nanoparticles, but the first and second series transition metal-doped SnO2 for their photocatalytic applications have yet not been fully explored.
5. With a thorough analysis, it has been observed that the majority of the work with SnO2 nanoparticles as photocatalysts for water purification is reported under UV and Visible light radiation only. But recent developments in the field show that it is possible to use solar-driven SnO2 nanoparticles for the removal of synthetic dyes/toxins.
6. We have compared the effect of different transition metal dopants used in tin oxide nanoparticles based on their synthesis technique, source of irradiation used, types of contaminations removed, and obtained photodegradation efficiency.
The ability of SnO2 nanoparticles to completely deal with different toxic pollutants in water bodies without producing harmful by-products has unrolled the novel research approach to be pursued. SnO2 is still a hot topic for new research findings across the world. Various researchers are working on it by varying the different doping elements and concentrations to get desired outcomes. As it is a well-known fact that transition elements are quite popular elements for doping, we hope that this review will help the researcher community.

FUNDING
This research did not receive any specific grant from any funding agencies.

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.