Recent Progress on Green-Derived Tin Oxide (SnO₂) for the Degradation of Textile Dyes: A Review

Abstract

Water contamination from textile dyes is a major environmental hazard. This is due to the textile industry serving among the biggest manufacturers, thus the extensive usage of these dyes. Several methods for the treatment of these pollutants have been used; however, they have limitations in terms of cost, forming secondary pollution, and effectiveness. Metal oxides such as tin oxide (SnO₂) have been identified as potential photocatalysts for the degradation of these dyes. The potential of SnO₂-based photocatalysts, especially those made using green techniques, has been at the forefront of current research. The physical and optical properties, green synthesis techniques, and photocatalytic uses of SnO₂ NPs are examined. Furthermore, methods to improve photocatalytic effectiveness through the formation of heterostructures are also explored. Lastly, the conclusion and future perspectives of these materials as suitable candidates for water treatment are highlighted.

1. Introduction

Water pollution poses a major health hazard to both the environment and humanity globally, resulting in water scarcity in many regions [1]. This is because the current economy’s rapid growth is a factor that contributes to water scarcity by causing natural resources to be continuously depleted while industrial pollution is generated [2]. It has been discovered that numerous industries, including the textile industry, examples of which are paints, cosmetics, paper, plastic, leather, and rubber, have been reported as responsible contributors to water pollution, in addition to household effluents [3,4,5].

Dating back to ancient times, natural dyes derived from natural materials such as wood, minerals, flowers, vegetables, and insect secretions have been used in the textile industry. However, due to the over-reliance on this industry, a huge strain has been added to global water [6,7]. High water demands coupled with extreme discharge and pollution caused by dying, bleaching, and printing use water for the application of chemicals and dyes, and in clothing and fabric [8]. The textile industry is the 2nd biggest polluter of water, coming after agriculture. On average, 280 × 10³ tons of textile dyes (Figure 1) are leached and released annually [9].

The textile dyes in water streams cause bioaccumulation, thus several countries have added restrictions on their usage [10,11]. For example, the US and European governments, through their ecology and toxicology associations, put limitations on the usage of heavy metals as colorants in the textile industry [12,13,14]. In Asian countries, namely Japan, India, and China, azo dyes have been banned since they contain compounds such as 3,3-dimethylbenzidine [15]. When these dyes (Figure 2) enter water streams, such as lakes and oceans, the food pyramid is affected, algal bloom is produced, and mutagenic effects are also noted on aquatic plants and other microorganisms [11].

A variety of conventional techniques, such as chemical, biological, and physical processes, have been used for the removal of these dyes [15,16]. The physical techniques consist of adsorption, membrane filtration, coagulation–flocculation, and reverse osmosis [17,18,19]. The chemical methods include advanced oxidation processes (AOPs), such as Fenton reactions, photochemical reactions, ultraviolet irradiation, oxidation, ozonation, and electrochemical destruction [4,9]. For the biological techniques, enzymatic degradation and biosorption are the most preferred. Also, other green methods, such as Fenton-based Advanced Oxidation Processes (AOPs), have been used in oxidative/reductive degradation against Mordant Blue Dye [1]. None of the methods can fully remove or degrade these synthetic dyes from wastewater, despite their extensive use as remedies. This is because every strategy has a unique set of drawbacks, and in this case, these traditional treatment methods have been reported to show limitations of high cost, being time-consuming, poor efficiency, recycling challenges, and the production of secondary pollution [9,20,21,22]. Furthermore, chemical approaches have shown an additional constraint of accumulating concentrated sludge, which consequently causes a disposal issue, in addition to these general limitations. [10,23,24]. Thus, researchers have moved towards photocatalysis in the last two decades as it has been shown to be one of the most successful methods of treatment of wastewaters containing dyes because of the low cost, quick oxidation process, and non-toxicity [9,24,25].

For the photocatalysis process to be efficient, a photocatalyst is required to have improved optical and surface characteristics, which include a low band gap, higher surface-volume ratio, and higher surface energy [26]. For the degradation of organic dyes from wastewater, numerous researchers have employed a variety of photocatalytic materials, including metal–organic frameworks (MOFs), carbon-based materials, polymers, and metal oxides [27,28]. However, semiconducting photocatalysts were preferred over these materials [29]. Moreover, these semi-conductive materials, which include metals, metal oxides, and their composites, have emerged as a fascinating field in nanoscience and technology because of their crucial optical, physicochemical, and electrical characteristics [30,31]. These metal oxide semiconductor (MOS) nanoparticles (NPs) include ZnO, TiO₂, CuO, CdO, NiO, CeO₂, ZrO₂, and many others [9,15]. When they break down the organic dyes in wastewater, their intrinsic qualities—such as having a heterogeneous photocatalyst with a broad range of light absorption, redox capabilities, charge dissociation lifespan, sensitivity to light, and stability—also impact their photodegradation efficiency [9]. As a result, tin oxide (SnO₂) from amongst the MOSs has drawn a lot of interest lately from researchers due to its exceptional ability to absorb UV radiation, which produces reactive oxygen species (ROS) and increases dye degradation efficiency [32]. Furthermore, SnO₂ is an n-type semiconductor with additional features of a rutile crystal structure, a tetragonal form, and a bandgap of roughly 3.6 eV [5,16,33,34,35].

Though this material has been used widely in catalysis, the synthesis of it using conventional techniques such as hydrothermal, solvothermal, chemical vapor deposition, etc., for synthesizing SnO₂ has also demonstrated drawbacks such as high operating costs, time consumption, and the use of hazardous chemicals [36,37,38,39,40]. Generating safe and efficient low-cost materials has been at the forefront in circumventing some of these limitations [32,41,42]. This involves the usage of natural reductants from plant extracts with phytochemicals that can serve as reducing and capping agents from natural sources such as agricultural waste (peels, pith, corbs, stalks, and roots), microbes (bacteria, fungi, algae, and enzymes), animals (proteins, vitamins, nutrients (iron, iodine, etc.), as well as plants (leaves, roots, fruits, barks, latex, seeds, and stems) [43,44,45]. Furthermore, plant extracts have drawn a lot of interest in green synthesis since they help create desired nanomaterials in terms of size, shape, and functionality [38].

Thus, in this review, the synthesis of tin oxide via the green route and its composites are investigated for the degradation of textile dyes. Synthesis using plant extracts is highlighted, including the modification and application of these materials as photocatalysts.

2. SnO₂ Nanoparticles

Tin is an n-type semiconductor with a bandgap of 3.6 eV and vacant oxygen defects. It has a tetragonal crystal structure with two oxidation states (Sn²⁺, Sn⁴⁺). The tetragonal structure of SnO₂ results from Sn ions being in a hexacoordinated state, whereby the O atoms are tri-coordinated. Al-Hamdi et al. [1] reported that SnO₂ forms a single cubic structure with a rutile phase. This material has a high carrier density, chemical stability, and oxygen vacancies. It has the potential to possess optical transparency in the visible region, recombination resistance, and chemical stability. It has previously been applied in optoelectronic devices such as gas sensing, dye sensing, and as a photocatalyst [3]. This is caused by its faster charge transport and charge separation size distribution, which leads to its higher electron mobility (100–200 cm²V⁻¹s⁻¹) [4].

The optoelectronic properties this material possesses are mostly influenced by its synthesis conditions, its stoichiometry in relation to O₂, and the presence and absence of impurities. Several authors have synthesized SnO₂ materials from 0D to 3D in an effort to obtain various optical properties [4]. These included nanowires, nanosheets, nanobelts, etc. From photoluminescence studies, authors were able to obtain information about the defects, structure, and impurities of a material. Lee et al. [13] formed 1D materials of SnO₂ via thermal evaporation, and a peak at 595 nm was noted. At room temperature, low SnO₂ emissions had been noted. The PL of SnO₂ was mainly caused by the energy states found in the bandgap because of defects such as oxygen vacancies and interstitial and dangling bonds. Other authors also noted that upon synthesizing nanoribons and nanorods, it was observed at room temperature that a red light luminescence at 605 nm was observed. This was caused by oxygen vacancies. Nanoblades, via the hydrothermal process at low temperature, were also formed, and from their analysis, a blue shift emission at 445 nm was noted [21]. This could be caused by the triple to ground state transition. From these studies, it can be noted that the growth mechanism can influence the photoluminescence efficiency of these materials.

3. Synthesis of SnO₂ Nanoparticles

The bottom-up and top-down methods are the two basic techniques used to produce metallic nanostructures (Figure 3). The objective of top-down techniques, which encompass lithography, ball milling, sputtering, and electrospinning as listed in Table 1, is to generate nanoparticles by progressively segmenting or reducing bulk materials [37]. However, these methods have several drawbacks, as highlighted in Table 1, including high costs, elevated impurity levels, structural defects, and a wide distribution of particle sizes and shapes [38]. In contrast, bottom-up methods are used to synthesize nanomaterials cluster by cluster, molecule by molecule, or atom by atom. Notable techniques in this category include precipitation, atomic layer deposition, chemical vapor deposition, hydrothermal/solvothermal methods, and microwave-assisted heating, as also indicated in Table 1 [24,39]. The bottom-up method is extensively utilized in the synthesis of nanostructures due to its cost-effectiveness, ability to produce a diverse array of morphologies, and narrow particle size distribution (ranging from 1 to 20 nm), which can be modified by varying the starting precursors [40,41,42,43,44].

Table 1. Conventional methods used for the removal of textile pollutants.

Remediation Methods	Functions	Advantages	Disadvantages	Ref.
Lithography	Techniques that employed concentrated electron or light beam to synthesize or fabricate structures or patterns at the nanoscale.	Commercial fabrication on nano science devices Capacity to create a cluster of the required shape and size from a single nanoparticle.	Require high equipment maintenance Costly equipment Fluids produce bubbles during synthesis	[37,46,47,48]
Ball milling	It uses a mechanical process to break larger particles into smaller particle sizes	Produce ultra-fine nanoparticles Provides higher yield Greater densification Homogeneity	Requires high energy Irregular morphologies Potential of contamination.	[49]
Sputtering	Process of depositing nanoparticles on a surface using a metal-based material after ions have clashed with the surface.	Strong adherence Homogeneity Capacity to deposit complex compounds	Low deposition rates High maintenance Risk of film stress and contamination Limited scalability for large-area coatings	[37,49,50]
Electrospinning	It is an electrostatic method that fabricates fiber materials by pulling a polymer solution under a high electric field, resulting in nanofiber production.	Creates fibers with a high surface-to-volume ratio Adjustable porosity Uniform structure	High maintenance Low biocompatibility Long biodegradation	[51,52]
Precipitation	Used for producing nanoparticles by combining chemical components in a solution, resulting in the formation of solid precipitates.	High purity Large surface area materials Simple	Use of toxic reagents and chemicals High sludge production Difficulty in separation	[49,53]
Hydrothermal	A process that creates nanoparticles by carrying out reactions at high temperatures and pressures in an autoclave reactor in a sealed vessel	Controlled particle size and structure Surface functionality Optical properties of the resultant nanomaterials	Uses high temperature Lengthy response periods High energy consumption	[40,54]
Microwave	Uses microwave irradiation to heat reaction mixtures that form nanoparticles.	Excellent purity Small particle size Produce porous nanoparticles Quick reaction times Scalable Efficient	Non-stable particles Particle aggregation	[55]
Chemical vapor deposition	Used to decompose gaseous reactants using light, heat, and plasma to form solid nanoparticles	No need for capping/reducing agents High purity nanoparticles	Use of higher temperature and pressure	[56]
Spray pyrolysis	Entails atomizing metal solutions while maintaining a high temperature and gas flow rate in order to break down the metal and create nanoparticles	Regulated crystallinity, composition, chemical uniformity Adaptability for large-area films	Difficulty in real-time observation and modelling of nanoparticle formation High-temperature requirements Relatively slow deposition rates	[38]
Biological	A process that uses microbes (bacteria, fungi, plants, and algae) to eliminate pollutants	Simple Eco-friendly Cost effective	Slow process Microbes require extra attention. Non-selective Low degradation efficiency	[57,58]

Table 1. Conventional methods used for the removal of textile pollutants.

Remediation Methods	Functions	Advantages	Disadvantages	Ref.
Lithography	Techniques that employed concentrated electron or light beam to synthesize or fabricate structures or patterns at the nanoscale.	Commercial fabrication on nano science devices Capacity to create a cluster of the required shape and size from a single nanoparticle.	Require high equipment maintenance Costly equipment Fluids produce bubbles during synthesis	[37,46,47,48]
Ball milling	It uses a mechanical process to break larger particles into smaller particle sizes	Produce ultra-fine nanoparticles Provides higher yield Greater densification Homogeneity	Requires high energy Irregular morphologies Potential of contamination.	[49]
Sputtering	Process of depositing nanoparticles on a surface using a metal-based material after ions have clashed with the surface.	Strong adherence Homogeneity Capacity to deposit complex compounds	Low deposition rates High maintenance Risk of film stress and contamination Limited scalability for large-area coatings	[37,49,50]
Electrospinning	It is an electrostatic method that fabricates fiber materials by pulling a polymer solution under a high electric field, resulting in nanofiber production.	Creates fibers with a high surface-to-volume ratio Adjustable porosity Uniform structure	High maintenance Low biocompatibility Long biodegradation	[51,52]
Precipitation	Used for producing nanoparticles by combining chemical components in a solution, resulting in the formation of solid precipitates.	High purity Large surface area materials Simple	Use of toxic reagents and chemicals High sludge production Difficulty in separation	[49,53]
Hydrothermal	A process that creates nanoparticles by carrying out reactions at high temperatures and pressures in an autoclave reactor in a sealed vessel	Controlled particle size and structure Surface functionality Optical properties of the resultant nanomaterials	Uses high temperature Lengthy response periods High energy consumption	[40,54]
Microwave	Uses microwave irradiation to heat reaction mixtures that form nanoparticles.	Excellent purity Small particle size Produce porous nanoparticles Quick reaction times Scalable Efficient	Non-stable particles Particle aggregation	[55]
Chemical vapor deposition	Used to decompose gaseous reactants using light, heat, and plasma to form solid nanoparticles	No need for capping/reducing agents High purity nanoparticles	Use of higher temperature and pressure	[56]
Spray pyrolysis	Entails atomizing metal solutions while maintaining a high temperature and gas flow rate in order to break down the metal and create nanoparticles	Regulated crystallinity, composition, chemical uniformity Adaptability for large-area films	Difficulty in real-time observation and modelling of nanoparticle formation High-temperature requirements Relatively slow deposition rates	[38]
Biological	A process that uses microbes (bacteria, fungi, plants, and algae) to eliminate pollutants	Simple Eco-friendly Cost effective	Slow process Microbes require extra attention. Non-selective Low degradation efficiency	[57,58]

In the hydrothermal method, high temperatures are used, above the boiling point of water, by conducting reactions in a sealed vessel [59]. Due to the significant control they offer over the composition, size, and structure of the resultant nanomaterials, the method is gaining increasing popularity. Although the usage is high, it has certain limitations, such as lengthy response periods that can range from a few minutes to 24 h, which make it difficult to use and lead to considerable energy usage [60]. A notable disadvantage of these reactors is that the temperature is not uniform. Ren et al. [40] synthesized SnO₂ NPs using the hydrothermal method (Figure 3). Different materials of nanorods, flowerlike microspheres, and hydrangea microspheres were formed.

In the microwave method, excellent purity, small particle size, and quick reaction times were noted. However, the stability of the nanostructures and particle aggregation were some of the limiting factors reducing the high usage of this synthesis method. Manimaran et al. [31] used the microwave-assisted method to form highly porous tetragonal materials with a particle size of 18 nm. From the PL and zeta potential analysis, improved photoluminescence properties were noted, and these could assist in photodegradation applications. For the chemical vapor deposition method, reactive gas-phase species are used to coat materials in the CVD process. The species in the vapor phase undergo chemical reactions at high temperatures. The process of CVD involves multiple phases [34]. First, the gases are adjusted into various chambers. Thereafter, gaseous species are moved to various locations, which ultimately results in their adsorption. The interaction between the reactants and the substrate leads to the formation of nanostructured materials. Finally, the materials exit the compartment after going through desorption. Unfortunately, the limited growth controllability of this method has restricted its broader application in the production of high-quality nanostructures. Lastly, another popular method, solvothermal synthesis, has been shown to have numerous benefits, including inexpensive precursors, environmental safety, low temperature processing (which improves stoichiometric control), and the capacity to regulate parameters and to control the final product [17,61]. It can be used with other techniques such as hot pressing, microwave, electrochemistry, ultrasound, and optical radiation. Another advantage of solvothermal synthesis is its rapid growth rates, which are made possible by rapid diffusion processes. However, there are several disadvantages to the technique, such as the need for expensive autoclaves, issues about safety during the reaction, and the inability to see the reaction process [23]. Though several methods have been used to produce materials of distinct shapes and properties, they still suffer from certain limitations. For example, the hydrothermal approach tends to have lengthy response periods of synthesis, stability, and particle aggregation, and lastly, elevated growth temperature is still a major limitation [57,62].

Among the bottom-up methods, green synthesis has been on the rise. The are methods which include the use of plant, fungi, and bacterial materials. The phytochemicals contained in these extracts, which include quercetin, polyphenols, and tannins, can be used as reducing and capping agents for these materials; thus, research has mostly moved toward materials formed through this route [28].

4. Green Synthesis of SnO₂ Nanoparticles

The synthesis of SnO₂ NPs using green materials has been on the rise in the last decade (

Table 2. Green synthesis of SnO₂ NPs and their characteristics.

Reductant	Product Calcination Temperature and Duration	Extract Dosage	Extract Temperature and Duration	Morphology and Phase Identification	Particle Size (nm)	Refs.
Sambucus Canadensis leaf extract	350 °C for 30 min	20 g in 250 mL	60 °C for 12 h	Plant like	-	[21]
Actinidia deliosa (Kiwi) peel extract	300 °C for 3 h	1:10 ratio	60 °C for 2 h	Sphere	5–10	[22]
Catunaregam spinosa (barks roots)	450 °C for 2 h	300 g in 500 mL	60 °C for 2 h	Sphere	47 ± 2	[23]
Jujube fruit extract	500 °C for 1 h	200 g in 200 mL	80 °C for 30 min	Sphere	18	[24]
Psidium Guajava leaf extract	400 °C for 4 h	-	60 °C for 4 h	Sphere Tetragonal rutile	8	[25]
Camelia sinesis	400 °C for 1 h	0.5 g in 50 mL 1 g in 50 mL 2 g in 50 mL	60 °C for 1 h	Quazi-spherical Hexagonal rutile	-	[26]
Cotton bool waste Peel extract	200 °C for 3 h 500 °C for 3 h 700 °C for 3 h	3 g in 100 mL	80 °C for 30 min	Sphere Tetragonal	4 8 13	[27]
Aspalathus linearis	200 °C for 3 h 700 °C for 3 h 900 °C for 3 h	-	-	Quasi-spherical	1–5	[28]
Persia Americana	300 °C for 3 h	-	-	Sphere Tetragonal rutile	4	[5]
Daphne mucronata (D. mucronata) leaf extract	Oven dried 100 °C for 6 h	20 g in 500 mL	100 °C for 1 h	Different shapes	64	[6]
Garcinia Cambogia	500 °C for 2 h	10 g in 100 mL	100 °C for 5 min.	Sphere Tetragonal	14 11 10	[29]
Tnospora crispa (TCSE) stem extract	550 °C for 4 h	4 g in 150 mL	100 °C	Rods and spheres	-	[30]
Solanum nigrum leaf extract	600 °C for 6 h	10 g in 250 mL	85 °C in 20 min	Sphere	5 & 18	[31]
Galaxaura elongata (red algae).	500 °C for 3 h	2 g in 200 mL	70 °C for 48 h	Sphere Tetragonal	35	[32]
Croton macrostachyus leaf extract	500 °C for 3 h	10 g in 200 mL	60 °C for 4 h	Sphere	32	[17]
marine alga, Ulva sp.	400 °C for 3 h	2 g in 100 mL	70 °C for 20 min	Sphere and cuboidal	5	[33]
Aquilaria malaccensis leaf extract	800 °C for 2 h	80 g in 400 mL	80 °C for 20 min	Sphere tetragonal	27	[20]
Indigofera tinctoria leaf extract	No calcination, oven dried for 8 h at 60 °C	5 g in 100 mL	60 °C for 30 min	Sphere and nanosheets Tetragonal	-	[34]
Murraya paniculate (orange jasmine) leave extract	Oven dried at 60 °C	10 g in 100 mL	80 °C for 180 min	Sphere and sheets	3–5	[35]

). In particular, the usage of plant extracts, as these are readily available and do not require tedious preservation steps and optimization to suit specific conditions, unlike fungi and bacteria [46]. Microorganisms such as fungi generally experience delayed growth rates, resulting in a prolonged synthesis process, and lastly, detaching the formed NPs from the fungi can be difficult. In the usage of plant extracts, phytochemicals consisting of enol groups, for example, phenols and flavonoids, are utilized as reducing agents [47]. In the presence of a methyl group, the extracts can also serve as a capping agent; thus, plants often play a dual role of reducing/capping depending on the phytochemicals present. Moreover, phytochemicals consisting of an ortho-substituted (OH) group can influence the particle size, optical properties, and morphology of the materials [48].

Jeyaraj et al. [13] using Brassica Juncea seed extract, managed to form spherically shaped SnO₂ materials with a zeta potential of −40.1 mV and a particle size of 6.6 nm [13]. Suresh et al. [14], in the presence of Delonix elata leaf extract, used three synthesis routes, sonication, wet chemical, and the microwave-assisted method for the green synthesis of SnO₂ NPs. From the plant extract, polyhydroxy compounds and quercetin were the phytochemicals of choice, as these consisted of OH groups and could be used as ligation agents. From the analysis, all the materials exhibited O-Sn-O stretches in the 500–650 cm⁻¹ range. Highly crystalline materials were also shown via XRD. From Figure 4a–f, it can also be noted that these materials had a cluster-like foam morphology with limited agglomeration in all the samples, and moreover, EDS also confirmed the formation of these materials with the major elements of Sn and O recorded. From the three materials, the MW-SnO₂ recorded the highest surface area of 196 m²g, likely influenced by the lowest particle size of 13 nm. Lastly, the MW-SnO₂ was the most thermally stable of the formed materials.

Gonzalez et al. [20] studied the effect of plant concentration (1, 2, 4%) on the synthesis of SnO₂ in understanding the optical and structural properties. From FTIR, a characteristic band in the fingerprint region of Sn-O at 601 cm⁻¹ was observed. XRD, through phase identification, noted a tetragonal rutile crystalline phase. Moreover, as the plant concentration increased, the crystallite size also increased to 15, 16, and 21 nm, respectively. This suggests that the Tillia cordata plant assisted in SnO₂ crystal growth. From Figure 5, cluster-like quasi-spherically shaped materials were observed. A high agglomeration on the particles was noted. This formed morphology may be caused by low nucleation due to the higher organic content of the matter.

Ahmad et al. [21] used Sambacus Canadensis to form agglomerated clusters of plate-like NPs. A maximum wavelength of 285 nm was obtained, and at 586 cm⁻¹, an asymmetrical vibration of the Sn-O-Sn bond was obtained.

Gomathi et al. [22], using a one-step method, used a kiwi peel extract of Actinidia deliciosa to form spherically shaped small NPs with a particle size of 10 nm or less. Meanwhile, Haritha et al., in the presence of root barks of C. sniposa, synthesized spherically shaped NPs with a particle size of 47 nm. Through GC-MS, metabolites such as 7-hydroxy-6-methoxy-2H-benzopyran were identified to be in abundance, in the range of 61% [23]. Honarmand et al. managed to form tetragonal rutile SnO₂ NPs. Through GC-MS, quercetic, a flavonoid derivative, was able to reduce metal ions into metal NPs [24]. Kumar et al. [25], in the presence of guava leaf extract, confirmed the formation of SnO₂ through an absorbance peak at 314 nm. The small particle sizes in the quantum confinement range, less than 10 nm, could greatly assist the materials as photocatalysts.

5. Green-Derived SnO₂ as Photocatalysts for Dye Degradation

Studies on the photocatalytic degradation of SnO₂ (

Table 3. Photocatalytic degradation of textile dyes using green SnO₂.

Photocatalyst	Dye	Bandgap	Light Source (Watts)	Parameters (Dosage, Conc and pH)	Contact Time	Degradation Removal	References
SnO2	CBB MR CBB MR	-	Solar Solar UV lamp UV lamp	30 mg	90 min	2.87% 1.94% 2.87% 0.27%	[21]
SnO2	MB MO RhB	-		5 mg 10−5 M	8 min 8 min 6 min	89% 86% 97%	[22]
SnO2	CR	-	UV Reactor	2.5 mg 10−4 M	45 min	90%	[23]
SnO2	MB EBT	-	Solar	8 mg 100 ppm	300 min	90% 83%	[24]
SnO2	RY186	-	Solar (637 W)	10 mg 40 ppm	180 min	90%	[25]
SnO2	MO MB RhB	4.02 eV 3.95 eV 3.79 eV	Solar and UV lamp	50 mg 15 ppm	180 min	81% 100% 100%	[26]
SnO2	MB MO	2.68 eV 2.98 eV 3.14 eV	UV lamp (125 Watts)	20 mg 10 ppm	30 min 40 min	99% 98%	[27]
SnO2	CR MB EY	-	UV Lamp	2.5 × 10−5 2.7 × 10−6 M	20 min	2.2 nm 2.2 nm 19 nm	[28]
SnO2	R6G	-	Solar	20 mg 15 ppm	390 min	99%	[6]
SnO2	MB	-	Solar	20 mg 15 ppm	240 min	90%	[29]
SnO2	MB RhB	3.45 eV	Solar	50 mg 10 ppm	200 min	93% 47%	[30]
SnO2	EB	3.3 eV	Solar	2 mg 1.5 × 10−4 M	90 min	96%	[31]
SnO2	RhB MB	3.03 eV, 2.71 eV, 2.61 eV, and 2.41 eV	UV lamp	50 mg 0.1 g/L pH = 6	80 min	91% 96%	[17]
SnO2 QDs	MB MO RhB	3.80 eV	Solar	15 mg 1 × 10−4 M	16 min 26 min 44 min	94% 87% 93%	[32]
SnO2	MB	4.17 eV 3.78 eV 4.0 eV	Solar	10 mg 10−4 M	240 min	96%	[37]
Psidium Guajava	RY	3.64 eV	Sunlight		180 min	90	[40]
SnO2	MB	3.12 eV			70 min	94%	[41]
SnO2	V4BSN	3.6 eV	UV lamp (125 Watts)	10 mg 25 ppm	40 min	100%	[43]
SnO2	RhB	-	Solar UV lamp (125 Watts)	50 mg 20 ppm	30 min	97% 46%	[44]
SnO2	RhB	-	Solar	100 mg 1 × 10−5 M pH	90 min	86%	[45]
SnO2	EBT	-	UV lamp (500 W)	(2.87 × 10−5 M, 0.211 g L−1 pH = 9	120 min	36%	[18]

MB = Methylene Blue, MO = Methyl Orange, EBT = Eriochrome Black T, MR = Methylene Red, RhB = Rhodamine Blue, RY186 = Reactive yellow 186, CBB = Coomasie brilliant Blue, CR = Congo Red, EY = Eosin Y, R6G = Rhodamine 6 G, V4BSN = Acid Violet 3.

) or modified SnO₂ as a nano or composite catalyst for its effectiveness against different dyes are included in

Table 4. Photocatalytic degradation of SnO₂ composites for dye degradation.

Photocatalyst	Dye	Bandgap	Light Source (Watts)	Parameters (Dosage, Conc and pH)	Contact Time	Degradation Removal	References
SnO2/NGO	MB MO	-	Solar	20 mg 20 ppm	150 min	89% 94%	[34]
SnO2:Zn@g-C3N4	MG	3.22 eV	Solar	30 mg pH = 9 10 ppm	100 min	94%	[35]
Ag/SnO2-bento	CV		visible	1 g/L−1	30 min	97	[37]
PANI-SnO2	RhB MO MB MR	2.68 eV		100 mg 20 ppm		84% 92% 60% 90%	[42]
SnO2/gCN	MB	2.44 eV	UV lamp (50 W)	50 mg 10 ppm	120 min	99%	[59]
Sr:SnO2	MB CV	2.43 eV	Solar	25 mg 10 ppm	150 min	83% 68%	[60]
Fe3O4/SnO2/Pr+	MB RhB		50 W LED		150 min	89% 83%
SnO2/ZnO	MB		UV		150 min	88%	[57]
ZnO/SnO2	MB MO RhB		Solar sunlight		55 min 55 min 65 min	100	[62]

. The bandgap energies of these SnO₂ materials have been reported to range from 2.4 to 4.17 eV, making it an n-type (oxygen-deficient) semiconductor [35,39]. Given that it is an amphoteric white substance with optical transparency and UV photoactivation within the UV range [37,44], it is anticipated to function effectively during the photocalysis process. As a result, it has been utilized against organic dyes in a number of studies gathered in Table 3.

Begum et al. [11] used a Parkia speciosa Hassk plant extract to synthesise SnO₂ nanoparticles for the degradation of Acid yellow 23 under sunlight. The material was completely degraded after 24 min at optimum conditions of 20 mg, 5 ppm, and pH = 3. In another study, amino acids were used as a reductant for the synthesis of SnO₂, which was used to remove 96% of the MB dye [4].

Honarmand et al. [24] synthesized SnO₂ using Jujube fruit extract to degrade MB, and 88% degradation efficiency was recorded after 5 h. For the photocatalytic process, the light used in these reports was either sunlight or lamp light, and others have used the photocatalytic reactor to degrade various organic pollutants. Most of these results indicate that a higher degradation and removal of dyes in longer irradiation times was achieved.

Luque et al. [26] synthesized a green-derived catalyst with three separate bandgaps of 4.02 eV, 3.95 eV, and 3.79 eV. Degradation of 81%, 100% and 100% for MO, MB, and RhB, respectively, was noted [26]. These findings demonstrate that the larger bandgap of 4.02 eV provided a lower percentage removal of MO as compared to MB and RhB, which both had 100% removal. This suggests that the best removal is produced by narrower bandgaps, while wider bandgaps result in poorer removal percentages. In another study, Borah et al. [33] synthesized a green catalyst with a bandgap of 3.80 eV and obtained removal percentages of 94%, 87%, and 93% against MB, MO, and RhB [33]. The results of the two studies were similar because the SnO₂ nano catalyst for the photodegradation against three dyes (MB, MO, and RhB) was produced using a natural reductant/stabilizer. The bandgaps in both of these studies were higher than the bulk bandgap of SnO₂, which is 3.60 eV, which could be due to the addition of plant extracts during the synthesis process. Therefore, these results demonstrate that MB and RhB are more easily removed by the nano catalyst than the MO dye. In another study, Selvam et al. [45] synthesized SnO₂ (CP) using a carica papaya extract and produced a narrower bandgap of 3.09 eV, and a degradation of 86% of RhB using sunlight within 90 min was recorded. Tammina et al. [43] also had the same bandgap of 3.60 eV, and in their study, they removed 96% of the V4BSN dye within 100 min while using a UV lamp (125 Watts) [61]. Kaur et al. [30] degraded 93% of MB and 47% of RhB in 200 min using a 125 watt UV lamp and a 3.45 eV catalyst [30]. Kumar et al. [25], in the presence of sunlight, with a bandgap of 3.64 eV, had a degradation efficiency of 90% against Reactive Yellow dyes in 180 min at a rate of 0.00476 min⁻¹. High reusability of the catalyst was also noted. Singh et al., using a catalyst derived from Punica granatum with a particle size of 20 nm, managed to degrade 92% of MB dye in 240 min in the presence of sunlight. A reaction rate of 6.86 × 10⁻³ in the degradation of this textile pollutant was recorded.

Ahmad et al. [21] also generated green SnO₂ at 350 °C, producing plant-like flower morphological structures and used their catalyst to degrade various dyes. A maximum degradation removal of 2.87% was recorded after 90 min. This suggests that several factors, such as bandgap size, morphology of the catalyst, and synthesis conditions leading to the formation of these materials, may have influenced degradation by preventing the pollutant from passing through the active areas on the surface. Even though in this particular study the activity was limited, it is very rare to obtain degradation rates of less than 5% even for bare green SnO₂ materials; hence, several factors may have influenced this activity.

In many studies, spherical morphology materials, irrespective of the particle size, light source, and bandgap, were noted to have high degradation rates [49,50,63]. For example, three experiments that synthesized spherical morphologies at low temperatures were able to deteriorate textile dyes by more than 99%. This was demonstrated by Luque et al. [26], who removed 99% of MB and RhB, Haq et al. [6], who degraded 99% of R6G, and Narasaiah et al. [27], who degraded 98% and 99% of MO and MB, respectively. This is consistent with the findings that morphological structure influences degradation efficiency. In these circumstances, the spherical shape allows light to penetrate the surface equally because there are no structural constraints or corners. This facilitated light absorption on the surface, causing radicals to attack the trapped dye molecules.

Apart from morphology, the majority of the investigations in Table 3 demonstrated greater degradation rates of over 90% with lower bandgap energies (<3 eV) at shorter time frames (<45 min), and higher band energies (above 3) had efficient rates in extended time frames (2–4 h, etc.). Narasaiah et al. [27] managed to degrade 99% of MB in 30 min at 2.68 eV and 98% of MO in 40 min at 2.98 eV under a UV lamp (125 Watts)

Though there are several patterns that have been noted in terms of relating morphology, bandgap, and degradation rates, with these green materials, there are still many factors contributing to the high efficiency or lack thereof. This could be owing to a variety of variables, including the use of optimum degradation conditions and the contribution of a light source. Thus, a demonstration of other contributing elements influencing the efficacy of the degradation of organic dyes with a modified catalyst is noted.

6. Limitations of Green SnO₂ and Their Modification for Dye Degradation

Although SnO₂ has exhibited a lot of potential, unfortunately, single-component materials have been discovered to possess weaker photocatalytic activity due to their large bandgap, high electron–hole pair recombination rate, and requirement for UV light for activation [51,52].

Furthermore, SnO₂ has little to no light absorption in the visible portion of the solar spectrum due to its wide bandgap semiconductor nature [17,36]. Moreover, its photoexcitation is only triggered by UV radiation [9,29]. It has also been reported that SnO₂ becomes agglomerated in suspension at high loading and after a few photodegradation cycles, making it rather challenging to extract and recycle from the solution [1]. Therefore, in an attempt to overcome it, researchers often change the surface material or add a lower bandgap MOS to minimize the bandgap [25,27] (Table 4). As noted in Figure 5, outside of the normal limitations faced by these materials, morphology, structural changes, surface area, and particle size play a role. Thus, the literature has indicated that modifying SnO₂ characteristics with various doping species or in combination with other semiconductors is a promising approach to overcome this constraint of a wide bandgap and can increase its photocatalytic degradation efficiency [53,54]. The materials that are considered for doping purposes can be from a variety of alkaline earth metals, rare earth elements, and transition metals, in addition to the lower bandgap of other MOSs [26,55].

In a study by Kumar et al. [34], using SnO₂ modified with graphene oxide, they managed an 89% degradation of MB and 94% of MO under sunlight for 150 min [38]. Snigdha et al. [35] formed a multicomponent sheet of SnO₂/Zn@g-C₃N₄ using Murraya Paniculata leaves. A 94% degradation of MG dye was achieved with a rate constant of 0.299 min⁻¹. The authors noted that the photoelectrons layered in gC₃N₄ assisted in the reduction of the recombination rate. Further, h+ and O₂⁻ were the main cause of degradation of the MG dye. Heyradi showed that upon loading Ag on SnO₂ supported on bentonite in the presence of perovskia abrotanoides plant extract, at optimum conditions of pH 4, enhanced degradation rates of 97% of 100 ppm CV dye were recorded in under 30 min (Figure 6). In this study, on top of depositing the low bandgap metal, the effect of variables such as pH, dosage, concentration, and time had to be investigated to obtain these high degradation rates. This high efficiency may have also been caused by the quick charge separation in the presence of Ag, while plasmonic decay caused by the transfer of electrons and holes also assisted [56,58]. Moreover, the •OH and O²⁻ radicals were noted to be the species responsible for the degradation of this textile dye [37].

Kanwal et al. [42] also used 100 mg of PANI-SnO₂ catalyst with a bandgap of 2.68 to degrade 20 ppm of various textile dyes. A 60% or above degradation of all the pollutants was noted. In another study, Minh et al. [59] modified SnO₂ with graphitic carbon nitride (gCN) to form a nanocomposite with a reduced bandgap of 2.44 eV, and managed to remove 99% of MB within 120 min using a 50 watt UV lamp [59]. Kaur et al. [60] reported on strontium-doped SnO₂ with a bandgap of 2.43 eV for the degradation of MB and CV dyes. In the presence of solar light, a degradation of 83% and 68% was recorded in 150 min.

These studies (Table 4), though they did not report a significant improvement from green bare SnO₂ efficiency, due to the extended time, it is known that the presence of carbon in the formation of composites assists in the improvement of the surface area, thus increasing the surface for active sites.

Despite the efficiency of the nanocomposite catalyst against dye degradation and bandgap reduction, the modification of SnO₂ with other metals has been reported to show limitations of agglomeration, so other researchers have opted to use carbon-based or polymers for stability and fast recombination rate, as in the previous study [19].

In addition to the intrinsic photocatalytic limitations of SnO₂, practical considerations related to scalability, cost, and recyclability must also be acknowledged. Although green synthesis offers a sustainable and environmentally friendly approach for the synthesis of SnO₂ nanoparticles, there is still the challenge of scaling up that is encountered in industrial applications. The seasonal availability of raw materials, the varying compositions of phytochemical concentrations, and variability in plant extract composition may result in inconsistent nanoparticle properties, leading to difficulties in large-scale production. Furthermore, while green synthesis reduces the need for hazardous chemicals, the cost of extraction, purification, and potential low yields may offset its economic advantages if not optimized. Another important factor is the recyclability and reusability of green-derived SnO₂ photocatalysts.

In a real wastewater setting, agglomeration is an issue since it often leads to a reduced surface area after multiple photocatalytic cycles, resulting in reusability and recovery challenges [64,65]. This poses limitations for continuous flow systems commonly used in industry. Thus, future work should focus on improving the stability of the photocatalyst through forming heterostructures, developing standardized and scalable synthesis processes, and conducting life cycle and cost–benefit assessments to determine the true feasibility of green SnO₂ for industrial wastewater remediation.

7. Conclusions and Future Perspectives

This review has outlined the progress made in the application of green-synthesized SnO₂ nanoparticles for the photocatalytic degradation of textile dyes. While SnO₂ might be promising due to its stability, non-toxicity, and availability, its large bandgap and rapid electron–hole recombination remain significant limitations, thereby restricting its efficiency under visible light. Green synthesis offers an eco-friendly alternative to conventional chemical methods, yet challenges such as agglomeration, inconsistent extract compositions, and limited catalyst recyclability persist. Researchers have attempted to address these issues through doping with low-bandgap metals, forming heterojunctions with other semiconductors, and incorporating carbon-based materials or polymers to enhance stability, surface area, and light absorption. However, further advancements are required to optimize these modifications for consistent and large-scale use.

In the future, more attention should be given to better understanding and controlling processes that are plant-mediated by identifying exactly the specific phytochemicals that participate in the nanoparticle synthesis. This will assist in the scalability as well as the reproducibility in industrial applications. Moreover, efforts should be based on the development of SnO₂-based photocatalysts that can utilize the entire solar spectrum, unlike just UV light, and that can be performed through advanced strategies, such as plasmonic enhancement, hybrid composite formation, and defect engineering. Amongst other issues, reusability and stability remain a concern; thus, there is also a need for the development of a recyclable catalyst that can undergo multiple cycles without losing its efficiency. Most importantly, future studies should conduct environmental assessments, ensuring that there are no unknown or unintended potential ecological risks associated with green-derived SnO₂.

Furthermore, whilst recent research mainly uses single dye solutions that are controlled in laboratory conditions, real textile wastewater has major challenges. Thus, it is imperative that real effluent testing and pilot-scale studies are conducted to evaluate practical applications. It is also crucial to consider whether green-synthesized SnO₂ can surpass or match conventionally synthesized materials in terms of cost, efficiency, and durability. Comparative performance studies will be key to determining their industrial viability. Although green-derived SnO₂ poses a significant potential in sustainable wastewater technologies, an extensive multidisciplinary research approach would be beneficial to overcome the outstanding limitation and realize its full application potential.