Synthesis and investigation of optical properties and enhancement photocatalytic activity of TiO₂–SnO₂ semiconductor for degradation of organic compounds

Abstract

Industrial wastewater treatment using UV irradiation in combination with oxidants or catalysts (TiO₂) has attracted attention as a promising substitute for conventional methods. Studying the preparation and characterization of TiO₂–SnO₂ nanocomposites with different ratios, as well as their use in the enhanced photocatalytic degradation of acid red 37 dye in aqueous solution under UV irradiation as a model pollutant, are the goals of the research. The crystalline structures of the prepared nanomaterials were confirmed by XRD and the surface morphology of the samples was studied by TEM. The elemental compositions of the catalysts were confirmed by EDAX. The optical properties of the powder samples were analyzed with UV–Vis spectroscopy and their band gaps were estimated. The photocatalytic degradation was investigated using several advanced oxidation techniques using a batch photoreactor. The TiO₂–SnO₂ (90:10) nanocomposite showed the best degradation efficiency.

Introduction

However, as we learn more about how finite the earth’s supply of fossil raw materials has sparked a flurry of initiatives to discover environmentally friendly and long-term replacements. Aside from attempting to employ more renewable as a fundamental chemical feedstock, one of the key difficulties for science and engineering is meeting the world’s expanding energy demand¹. As a result, it is critical to create new efficient technologies for converting energy from renewable sources. Hydrogen is one of the most interesting energy resources since it allows for the effective use of a fuel cell to convert chemical energy into electrical energy with the only byproduct being water². As a result, an obvious benefit of the enormous reduction in pollutants is a hydrogen economy.

Due to its many benefits, such as its simplicity of use, high efficiency, and absence of secondary pollutants, a lot of focus has lately been placed on photocatalysis as one of the emerging advanced oxidation methods. It is widely recognized as method of treating wastewater that shows the greatest promise as being eco-friendly to the environment^3,4. Catalysts play a crucial part in the photocatalytic process, as we all know. Titanium dioxide, among the many photocatalysts, is of particular interest because of its chemical stability, peculiar physical characteristics, and lack of toxicity. It is frequently used in energy conversion, hydrogen production, air purification, and wastewater treatment⁵. Titanium dioxide, a semiconductor, helps electrons to go from the valence band to the conduction band when exposed to light.

Compared to more traditional chemical oxidation techniques, photocatalytic applications, and semiconductor-based photocatalysis in particular, stand out as the most attractive means for decomposing harmful substances to non-hazardous output. This is due to the fact that semiconductors are (i) cheap, (ii) are non-toxic, (iii) have a high surface area, (iv) have broad absorption spectra with high absorption coefficients, (v) exhibit tunable properties that can be modified through size reduction, doping, sensitizers, etc., (vi) afford the facility for multielectrons transfer process, and (vii) can be used for long periods of time without significantly losing its photocatalytic activity.

In order for a semiconductor to be photochemically active, its redox potential must be positive enough for the photogenerated valence band hole to produce adsorbed OH radicals that can oxidise organic pollutants and negative enough for the conduction band electron to reduce adsorbed O₂ to superoxide.

The category of photocatalysts known as metal-oxide semiconductor photocatalysts includes TiO₂. TiO₂ is a good n-type semiconductor photocatalyst because of its high activity in Fig. 1, sustained resistance to photo- and chemical-corrosion during reaction circumstances, strong oxidizing power, stable substance, and nontoxic. TiO₂nanostructures, on the other hand, are a potential semiconductor for a variety of applications, including batteries, solar cells, dye pollution catalysis, hydrogen generation, and water splitting^6,7,8.

The three crystal phases of TiO₂are rutile, brookite and anatase⁹. Although antase and rutile are the most commonly investigated phases, anatase has a greater photocatalytic degradation activity with organic pollutants than rutile due to its crystal structural arrangement¹⁰. TiO₂ can only absorb ultraviolet light due to its high band gap energy 3.2 eV for anatase and 3.0 eV for rutile, respectively. This prevents TiO₂from being used for visible light absorption¹¹.

Fig. 1

Schematic of the chemical reaction pathway of an n-type semiconductor such as TiO₂.

Tin oxide (SnO₂) has sparked a lot of scientific interest. SnO₂ is an outstanding optical and electrical material that, at room temperature, is an of n-type semiconductor that has a wide band gap (3.6 eV), unusual optical transparency, low resistivity, and a high theoretical specific capacity. The electron mobility in SnO₂ is quite high (between 100 and 200 cm² V¹ s¹), which suggests that photoexcited electron transport happens more quickly. SnO₂ nanoparticles (SnO₂ NPs) have high band gap energies, high stability, and unusual structural and optical properties. Another distinctive property of SnO₂ NPs is the blue shift of the band edge transition energy. Furthermore, as the wide surface area serves to increase photocatalytic reaction sites, the small size of SnO₂ NPs may be a contributing factor in the considerable degradation rate of the organic dye on as-produced SnO₂NPs with diameters smaller than 10 nm and increases electron-hole separation efficiency¹².

Results and discussion

HR-TEM of TiO2–SnO2 wt% nanocomposites

TiO₂-SnO₂ nanocomposite samples were examined with HR-TEM to investigate the homogenety and the morphology of the samples. Prior to the investigation, the samples were coated with gold using sputtering coater (model: S 150 B, Edwards High Vacuum Ltd., England).

Figure 2 shows the HR-TEM picture of the synthesized TiO₂–SnO₂ (97:3) wt% nanocomposite. The particles had tetragonal shape with a diameter of 26.3–88 nm, according to a detailed analysis of the micrograph.

Fig. 2

HR-TEM of TiO₂–SnO₂ (97:3) wt% nanocomposite.

Figure 3 shows the HR-TEM picture of the synthesized TiO₂–SnO₂ (90:10) wt% nanocomposite. The particles had a roughly tetragonal shape and semi round shape with a diameter of 27.6–54.5 nm, according to a detailed analysis of the micrograph.

Fig. 3

HR-TEM of TiO₂–SnO₂ (90:10) wt% nanocomposite.

Figure 4 shows the HR-TEM picture of the prepared TiO₂–SnO₂ (80:20) wt% nanocomposite. The particles had tetragonal and semi round shaped with a diameter of 30.2–72.2 nm, according to a detailed analysis of the micrograph.

Fig. 4

HR-TEM of TiO₂–SnO₂ (80:20) wt% nanocomposite.

It can be concluded from the previous Figs. 2, 3 and 4 that increasing of SnO₂ (3 wt%) in TiO₂–SnO₂nanocomposite lead to an increase in the particle size due to nucleation and growth happen¹³. Further increase in SnO₂ to 10 wt% results in increasing the particle size, whereas further increase in SnO₂ up to 20 wt% results in decreasing the particle size of the nanocomposite due to agglomeration.

XRD of the prepared TiO2–SnO2 nanocomposites

The purity, crystallite size, and crystal structure of the generated nanoparticles are all examined using XRD analysis. The width of a certain XRD peak in a diffraction pattern connected with a particular planar reflection inside the crystal unit cell is used to calculate crystallite size. The size of the crystallites is therefore reflected in the peak broadening.

XRD of TiO2–SnO2 (97:3) wt% nanocomposite

Figure 5 illustrates the XRD pattern of TiO₂–SnO₂ (97:3) wt% nanocomposite sample with diffraction peaks at 2θ = 25.30, 26.56, 27.42, 33.92, 36.09, 37.81, 38.65, 41.29, 47.99, 51.85, 54.03, 55.0, and 68.92. The crystal tetragonal structure’s allocated pattern, which indicates how it was formed, corresponds to TiO₂–SnO₂ (97:3) wt%. The measured diffraction peaks are consistent having a standard card issued by the Joint Committee of Powder Diffraction Standards (JCPDS card No 041-1445 and 001-0562) which showed a slight peak shift and an increase in the intensity of peak, with an average crystal size of 42.3 nm.

XRD of TiO2–SnO2 (90:10) wt% nanocomposite

Figure 5 depicts the XRD pattern of TiO₂–SnO₂ (90:10) wt% nanocomposite sample with diffraction peaks at 2θ = 25.33, 26.66, 27.43, 33.82, 36.08, 37.92, 41.24, 48.08, 51.75, 54.33, 56.68, 62.72, and 68.92. The tetragonal crystal structure’s allocated pattern, which indicates how it was formed corresponds to TiO₂–SnO₂ (90:10) wt%. The measured diffraction peaks are consistent having a standard card issued by the Joint Committee of Powder Diffraction Standard (JCPDS card No 041-1445 and 001-0562) which showed a slight peak shift and an increase in the intensity of peak, with an average crystal size of 37.09 nm.

XRD of TiO2–SnO2 (80:20) wt% nanocomposite

Figure 5 depicts the XRD pattern of TiO₂–SnO₂ (80:20) wt% nanocomposite sample with diffraction peaks at 2θ = 25.28, 26.57, 27.39, 33.87, 36.92, 35.92, 37.78, 41.29, 48.09, 51.70, 54.14, 62.77, and 68.86. The tetragonal crystal structure’s allocated pattern, which indicates how it was formed corresponds to TiO₂–SnO₂ (80:20) wt%. The measured diffraction peaks are consistent with the Joint Committee of Powder Diffraction standard (JCPDS card No 041-1445 and 001-0562) which showed a slight peak shift and an increase in the intensity of peak, with an average crystal size of 38.5 nm.

Fig. 5

XRD of the prepared TiO₂–SnO₂ nanocomposites.

According to the results as shown Fig. 5 which obtained from XRD of TiO₂-SnO₂ nanocomposites, it could be concluded that the average crystal size by Scherrer Eq. (1) of TiO₂–SnO₂ (97:3) wt% was 42.3 nm and was found to decrease to 37.05 with increasing the w % of SnO₂. Further increase of SnO₂ wt% to 20 resulted in decreasing the average crystal size to 38.5 nm.

$$B(2\theta )=\frac{{K\lambda }}{{L\cos \theta }}$$

(1)

Peak width (B) is inversely proportional to crystallite size (L).

K (the Scherrer constant)—λ wavelength

According to the results as shown Figs. 5, 6 and 7 which obtained from XRD of TiO₂-SnO₂ nanocomposites, it could be concluded that the average crystal size of TiO₂–SnO₂ (97:3) wt% was 42.3 nm and was found to decrease to 37.05 with increasing the wt% of SnO₂. Further increase of SnO₂ wt% to 20 resulted in decreasing the average crystal size to 38.5 nm. respectively according to Table 1.

Table 1 Average crystal size (nm) of the prepared TiO₂-SnO₂ nanocomposites.

EDAX of the prepared TiO2–SnO2 nanocomposites

The obtained results from the EDAX of TiO₂–SnO₂ nanocomposites as shown in Figs. 6, 7 and 8 confirmed the elemental ratios of the prepared nanocomposites.

Fig. 6

EDAX of TiO₂–SnO₂ (97:3) wt% nanocomposite.

Fig. 7

EDAX of TiO₂–SnO₂ (90:10) wt% nanocomposite.

Fig. 8

EDAX of TiO₂–SnO₂ (80:20) wt% nanocomposite.

FTIR of the prepared TiO2–SnO2 nanocomposites

As shown from Figs. 9, 10 and 11 the FTIR of TiO₂–SnO₂ nanocomposites shows that the peaks of TiO₂ & SnO₂ were slightly shifted due to the change in SnO₂ loading percentage.

Fig. 9

FTIR of TiO₂–SnO₂ (97:3) wt% nanocomposite.

Fig. 10

FTIR of TiO₂–SnO₂ (90:10) wt% nanocomposite.

Fig. 11

FTIR of TiO₂–SnO₂ (80:20) wt% nanocomposite.

XPS of the prepared TiO2–SnO2 nanocomposites

The analytical technique (XPS), commonly known as electron spectroscopy for chemical analysis (ESCA), that uses X-rays to bombard a specimen and then analyses the energy of the released electrons (Figs. 12 and 13).

Fig. 12

XPS of TiO₂–SnO₂ (90:10) wt% nanocomposite calcined at 500 °C for 3 h (Survey spectra).

Fig. 13

XPS of TiO₂–SnO₂ (90:10) wt% nanocomposite calcined at 500 °C for 3 h (O 1s region).

XPS of TiO2–SnO2 (90:10) wt% nanocomposite

Figures 14, 15, 16 and 17 shows the results of XPS analysis of the elements’ chemical make up and content. Peaks of O 1s, Ti 2p, Sn 3d, and C1s were seen in the survey spectra. Figure 14 in the region of 1200-0 eV, showing that Ti, O made up the majority of the samples. Figure 15 displays the spectrum of asymmetric TiO₂-SnO₂ (90:10) wt% with an O 1s core level. The spectrum was fitted by two Gaussian peaks at around 531.44 eV (Ti-O-Sn) and 528.75 eV. (Sn–O). Although there were still some Tin ions dispersed on the surface of TiO₂, it was hypothesized that Ti-O-Sn bonds connecting the composite sample were generated it in the majority. This increased the oxygen concentration on the surface. It could be fitted into 6 peaks in the spectrum in Ti 2p region (Fig. 16) with energies of 464.15, 457.94, 471.54, 459.98, 462.91, and 464.69 eV. 464.15 and 457.94 eV peaks were attributed to Ti⁴⁺2p_1/2 and Ti⁴⁺2p_3/2, respectively. Figure 17 displays the doublet Sn 3d spectrum, with binding energies of 495.03 and 486.49 eV, respectively, for Sn³⁺3d_3/2 and Sn³⁺3d_5/2lines¹⁴.

Fig. 14

XPS of TiO₂–SnO₂ (90:10) wt% nanocomposite calcined at 500 °C for 3 h (Ti 2p region).

Fig. 15

XPS of TiO₂–SnO₂ (90:10) wt% nanocomposite calcined at 500 °C for 3 h (Sn 3d region).

Fig. 16

The band gap energy of TiO₂–SnO₂ (97:3) wt% nanocomposite.

Fig. 17

The band gap energy of TiO₂–SnO₂ (90:10) wt% nanocomposite.

Finally, from the previous Figs. 12, 13, 14 and 15 it was found that the catalysts might have more lattice defects. The presence of Sn³⁺, could make it easier for electrons to be captured. As a result, the likelihood of photoinduced electrons and holes recombining was decreased¹⁴.

Calculations of band gap energy of TiO₂–SnO₂nanocomposites

Calculation of band gap energy of TiO₂–SnO₂(97:3) wt% nanocomposite

It was found that, increasing the amount of SnO₂ in TiO₂–SnO₂ nanocomposite causes a red-shift to longer wavelengths. This has to do with the development of new energy levels in TiO₂band gap^6,15. Oxygen vacancies are the principal source of the tiny absorption edges in the visible area^16,17.

The UV-Vis analysis is applied to investigate and calculate the band gap energy by using Tauc Eq. (1).

$$\alpha {\text{h}}\upsilon \,=\,{\text{A}}{({\text{h}}\upsilon - {\text{Eg}})^{\text{n}}},$$

(1)

where n = 1/2 for direct band gap materials and n = 2 for indirect band gap materials, α is absorption coefficient, h is the Planck constant, υ is the wavenumber, A is a constant, and E_gis the band gap energy¹⁸. The band gap energy (E_g) value of TiO₂–SnO₂ (97:3) wt% is determined from the plot of (αhυ)² against photon energy in electron volts. The derived band gap energy from Figs. 16, 17 and 18 was obtained from the slope of the line. The calculated band gap energy of TiO₂–SnO₂ (97:3) wt% nanccomposite was found to be 2.91 eV which was a less value compared with TiO₂ (3.2 eV) and SnO₂(3.6 eV)¹⁹.

Figure 17 depicts the band gap energy (E_g) of TiO₂–SnO₂ (90:10) wt% nanocomposite. The band gap energy (E_g) of TiO₂–SnO₂ (93:7) wt% is determined from the plot of (αhυ)² against photon energy in electron volts which was found to be 2.8 eV when compared with TiO₂ (3.2 eV) and SnO₂ (3.6 eV). So, it is decreased with increasing the wt% of SnO₂in the nanocomposite¹⁹.

Figure 18 depicts the band gap energy (E_g) of TiO₂–SnO₂ (80:20) wt% nanocomposite. The calculated band gap energy was found to be 2.86 eV. So, it is increased again with increasing the wt% of SnO₂to 20 in the nanocomposite¹⁹.

Fig. 18

The band gap energy of TiO₂–SnO₂ (80:20) wt% nanocomposite.

From the previous results on band gap calculation, it was found according to band gap values as shown in Table 2 that the prepared nanocomposites have lower band gap energies than native TiO₂ and SnO₂.

When SnO₂ wt% increases from (3–10), the band gap decreases from 2.91 to 2.8 eV and then return to increase when SnO₂ wt% increases from (15–20). This may be due to band gap energy of the coupled oxides should results from band gap overlap with the individual oxides. The band gap was found to shrink slightly as Sn concentration increased. Reportedly, SnO₂ exhibits a narrow indirect band gap (3.4 eV) and a wide direct band gap (< 3.4 eV) (1.6 to 2.9 eV). The shift in the band gap energy with increasing the wt% of SnO₂ is likely due to changes in crystal shifting on the plane, as indicated by XRD.

Also, doping of TiO₂ with SnO₂ creates Fermi level below the conduction band of the nanocomposite, so the band gap of TiO₂–SnO₂ nanocomposite decreases compared with native TiO₂ and SnO₂alone²⁰.

Also, it was found that increasing the percentage of SnO₂ from 3 to 10 wt% resulted in decreasing the Fermi level position and hence decreasing the band gap energy of TiO₂-SnO₂ nanocomposite from 2.91 to 2.8 eV. Further increasing in the wt% of SnO₂ from 15 to 20 resulted in increasing the band gap from 2.82 to 2.86 eV respectively. This increase in the band gap energy is probably due to the formation of homojunction of SnO₂–SnO₂ on the expense of TiO₂ -SnO₂ nanocomposite. So, the prepared nanocomposites need lower energy for electron excitation from VB to CB than native TiO₂ and SnO₂.

It was concluded also that TiO₂-SnO₂ (90:10) wt% nanocomposite has the lowest band gap value (2.8 eV), so (90:10) wt% is the optimum ratio which means it needs lower energy for excitation as shown in Table 2.

Table 2 Effect of SnO₂ percentage on TiO₂-SnO₂ nanocomposite band gap energy.

Photocatalysis of acid red 37 dye

The photocatalytic degradation of aqueous acid Red 37 dye solution was studied with different AOPs using UV/TiO₂, UV/SnO₂, UV/TiO₂–SnO₂ (97:3), UV/TiO₂–SnO₂ (90:10), UV/TiO₂–SnO₂ (80:20), UV/TiO₂–SnO₂ (90:10)/H₂O₂, UV/TiO₂–SnO₂ (90:10)/S₂O₈ and UV/TiO₂–SnO₂ (90:10)/IO₄⁻. In every experiment, the batch photoreactor was employed as the photoreactor.

Effect of native TiO2, SnO2 and TiO2–SnO2 nanocomposites as photocatalysts

Photodegradation was caused when UV light with a wavelength of 254 nm was used to irradiate slurry solutions of TiO₂ and SnO₂ (0.4 g L⁻¹) containing acid red 37 dye. This can be represented as shown in Fig. 1as mentioned in the introduction^1,21,22,23. A first order model for the degradation of dye was used to visualize the data. The slopes of the straight lines on plots of ln (C/C_o) over time, where C_o represents the starting concentration of acid red 37 dye and C is the dye concentration at time t, were used to calculate the values of the first-order rate constant (k_app). The time necessary for the reactants to deteriorate to half of their starting concentrations is known as the half-life time (t_1/2) of the pseudo first order reaction. t_1/2 and k_app are inversely proportional as shown by Eq. (2).

$${{\text{t}}_{{\text{1}}/{\text{2}}}}={\text{ }}0.{\text{693}}/{{\text{k}}_{{\text{app}}}}.$$

(2)

It was discovered that increasing the apparent rate constant (k_app) causes the t_1/2 to decrease, as indicated in Table 3.

Table 3 The apparent rate constants and half life times for photocatalytic degradation of acid red 37 dye using UV/TiO₂, UV/SnO₂, UV/TiO₂–SnO₂ (97:3), UV/TiO₂–SnO₂ (90:10) and UV/TiO₂–SnO₂ (80:20).

Irradiation of SnO₂ (0.4 g L⁻¹) with UV light of wavelength 254 nm resulted in a lower photodegradation of acid red 37 dye compared with UV/TiO₂. But SnO₂ exhibited good catalytic performance and stability. The superior photocatalytic activity of TiO₂ over SnO₂ may be attributed to smaller TiO₂ particle size, quick e⁻/h⁺ pair recombination rate and poor quantum yield of SnO₂in aqueous solutions²⁴.

According to Fig. 19, the synthesized TiO₂–SnO₂ nanocomposite exhibits a higher photocatalytic activity compared with native TiO₂ or SnO₂. TiO₂–SnO₂ composite’s high photocatalytic activity can be attributed to the more effective separation of photoinduced electron–hole (e⁻/h⁺) pairs; specifically, the n-p heterojunction of TiO₂–SnO₂ causes the photogenerated holes to migrate toward the interface and the electrons to migrate toward the bulk.

Because the conduction band of SnO₂ CB_SnO2 is more positive than that of the conduction band of TiO₂ CB_TiO2, the excited electrons on TiO₂ can also move to SnO₂ whereas the photogenerated holes can also move to the valance band of SnO₂ loading VB_SnO2 to the conduction band of TiO₂ VB_TiO2 (e⁻/h⁺) pairs separation as shown in Fig. 20. In comparison with native TiO₂ or SnO₂, TiO₂–SnO₂ nanocomposite demonstrated much better photocatalytic activity. Degradation of acid red 37 dye via photocatalysis depends significantly on the content of SnO₂ present in TiO₂–SnO₂ nanocomposite. It was discovered that increasing the quantity of SnO₂ from 3 to 10 wt% led to a stronger photocatalytic degradation of the dye. This may be due to decreasing in the band gap as shown in band gap calculation to a certain limit at TiO₂–SnO₂ (90:10) wt% nanocomposite, which is the lowest band gap that avoid the recombination and decreases the time needed to transfer the electrons and holes between CB and VB which gives the highest photocatalytic activity. The decreasing in photocatalytic degradation of the dye with increasing SnO₂ content from 10 to 20 wt% may be due to the formation of SnO₂–SnO₂ homojunctions which decrease the number of TiO₂–SnO₂ heterojunctions and also increase the number of free SnO₂particles, which possess low photocatalytic activity²⁵.

Fig. 19

Change of ln (C/C_o) with time for photocatalytic degradation of acid red 37 dye using UV/TiO₂, UV/SnO₂, UV/TiO₂–SnO₂ (97:3), UV/TiO₂–SnO₂ (93:7), UV/TiO₂–SnO₂ (90:10), UV/TiO₂–SnO₂ (85:15) and UV/TiO₂–SnO₂ (80:20).

Fig. 20

Relative energy position of the bands and the process of charge separation in theTiO₂/SnO₂ system.

Effect of addition of various inorganic oxidants to TiO2–SnO2 (90:10) nanocomposite

Effect of addition of H2O2 to TiO2–SnO2 (90:10) wt% nanocomposite

TiO₂–SnO₂ (90: 10) wt% nanocomposite with SnO₂ (10 wt%) was utilized to explore the influence of several oxidants (H₂O₂, Na₂S₂O₈ and NaIO₄) on the photocatalytic breakdown of acid red 37 dye based on earlier findings. Figure 21 depicts the effect of adding different concentrations of hydrogen peroxide to TiO₂–SnO₂ (90:10) (0.4 g L⁻¹) for photodegradation of acid red 37 dye under UV irradiation²¹. The data was plotted using a first-order dye destruction model. The addition low concentration of hydrogen peroxide (2 × 10⁻² M) to UV/TiO₂-SnO₂ (90:10) wt% (0.4 g L⁻¹) increases the rate of photocatalytic degradation of acid red 37 dye when compared with UV/TiO₂-SnO₂ (90:10) wt% alone. The rate of photodegradation increased when the concentration of hydrogen peroxide was increased to 6 × 10⁻² M with k_app ranged from 789 × 10⁻⁴− 997 × 10⁻⁴ min⁻¹, then decreased with further increasing of hydrogen peroxide concentration to 8 × 10⁻² M with k_app of 698 × 10⁻² M (Table 1).

A power law relationship describes the effect of rate-determining species:

$${{\text{k}}_{{\text{app}}}}={\text{ K }}{\left[ {{{\text{H}}_{\text{2}}}{{\text{O}}_{\text{2}}}} \right]^{\text{n}}},$$

(3)

where k_app and K are the apparent and true rate constants, respectively. Plotting the logarithm of k_app against the logarithm of the concentration of hydrogen peroxide in Fig. 22 produced a value of 2.3 for the exponent, n, the order of reaction with regard to the oxidant species (Table 1). According to the following reaction, the photogenerated conduction band electrons of TiO₂–SnO₂ are more efficiently trapped by H₂O₂ than they are by O₂, which may explain why the rate of dye degradation is increased when using UV/TiO₂–SnO₂ (90: 10) wt%/H₂O₂ system as compared with UV/TiO₂–SnO₂(90:10) wt% alone^22,23:

$${{\text{H}}_{\text{2}}}{{\text{O}}_{\text{2}}}\,+\,{{\text{e}}^ - }_{{({\text{cb}})}}{ \to ^ - }{\text{OH }}{+^ \cdot }{\text{OH}}{\text{.}}$$

(4)

Fig. 21

Change of ln (C/C_o) with time for photocatalytic degradation of acid red 37 dye using 0.4 g/L of TiO₂–SnO₂ (90: 10) wt% with different concentrations of H₂O₂ (M).

Fig. 22

Change of ln k_app (min⁻¹) with ln [H₂O₂] (M) for photocatalytic degradation of acid red 37 dye.

Additional oxidizing species (·OH), can aid in the oxidative decay process as a whole, besides the scavenging action which slowing the recombination interaction between photogenerated carriers, (electrons and holes). Since the employed UV source’s main output was at 254 nm, significant photolysis of the H₂O₂would have taken place, creating additional hydroxyl radicals^25,26. Thus,

$${{\text{H}}_{\text{2}}}{{\text{O}}_{\text{2}}}\,+\,{\text{hn}} \to {{\text{2}}^ \cdot }{\text{OH}}.$$

(5)

By raising the H₂O₂ concentrations (2 × 10⁻²–6 × 10⁻² M), dye oxidation and degradation are accelerated as well as the electron scavenging. Further increasing in H₂O₂ concentration to 8 × 10⁻² M decreases the photocatalytic degradation rate due to the recombination of the resulting of OH radicals which occurs at high concentration of H₂O₂²⁶.

Effect of addition of S2O82− to TiO2–SnO2 (90:10) wt% nanocomposite

The oxygen on the surface of irradiated TiO₂–SnO₂ (90: 10) wt% suspension acts as a natural sink for the photogenerated conduction band electrons. The sorbed H₂O or hydroxyl ions on TiO₂–SnO₂surface are subsequently oxidized by the remaining holes to produce the OH radicals. It could be advantageous to use an electron acceptor other than oxygen that is more effective^27,28,29.

The impact of persulfate addition to TiO₂–SnO₂ (90:10) wt% (0.4 g L⁻¹) on the photocatalytic degradation of acid red 37 dye is shown in Fig. 23. The rate of deterioration using TiO₂–SnO₂ (90:10) (0.4 g L⁻¹) was found to be accelerated by addition of a little amount of persulfate (1.0 × 10⁻³ M). The dye degraded completely and quickly when the persulfate concentration was increased to 10 × 10⁻³ M (2.9 min). Figure 24 shows the reaction rate order with regard to persulfate, and it was determined to be 0.5 (Table 4). The persulfate anions may capture the photogenerated conduction band electrons of TiO₂–SnO₂ more than O₂ or H₂O₂ and produce the powerful oxidizer SO₄^⋅−30.

$${{\text{S}}_{\text{2}}}{{\text{O}}_{\text{8}}}^{{{\text{2}} - }}+{\text{ }}{{\text{e}}^ - }_{{({\text{cb}})}} \to {\text{ S}}{{\text{O}}_{\text{4}}}^{{{\text{2}} - }}+{\text{ S}}{{\text{O}}_{\text{4}}}^{{ \cdot - }}$$

(6)

Additionally at a wavelength of 254 nm, the sulphate radical anion can interact with the solvent and produces ^·OH in the following processes^6,31,32:

$${{\text{S}}_{\text{2}}}{{\text{O}}_{\text{8}}}^{{{\text{2}} - }}+{\text{ hn}} \to {\text{ 2 S}}{{\text{O}}_{\text{4}}}^{{ \cdot - }}$$

(7)

$${\text{S}}{{\text{O}}_{\text{4}}}^{{ \cdot - }}+{\text{ }}{{\text{H}}_{\text{2}}}{\text{O }}{ \to ^ \cdot }{\text{OH}}\,+\,{\text{S}}{{\text{O}}_{\text{4}}}^{{{\text{2}} - }}+{\text{ }}{{\text{H}}^+}$$

(8)

As a result, increasing the persulfate concentrations (1 × 10⁻³–10 × 10⁻³ M) improved both the trapping of photogenerated conduction band electrons and the production of SO₄^·− and ^⋅OH which led to higher rate of photocatalytic degradation.

Fig. 23

Change of ln (C/C_o) with time for photocatalytic degradation of acid red 37 dye using 0.4 g/L of TiO₂–SnO₂ (90: 10) wt% with different concentrations of S₂O₈²⁻ (M).

Fig. 24

Change of ln k_app (min⁻¹) with ln [S₂O₈²⁻] (M) for photocatalytic degradation of acid red 37 dye.

Effect of addition of IO4− to TiO2–SnO2 (90:10) wt% nanocomposite

The effect of addition of periodate to TiO₂–SnO₂ (90:10) wt% (0.4 g L⁻¹) for photocatalytic degradation of the studied acid red 37 dye is shown in Fig. 25. It can be seen that the addition of very low concentration of periodate (1 × 10⁻⁴ M) to UV/TiO₂–SnO₂ (90:10) wt% (0.4 g L⁻¹) resulted in higher photocatalytic degradation rate compared with UV/TiO₂–SnO₂ (90–10) wt% (0.4 g L⁻¹) system alone. Also, increasing the periodate concentration to 1 × 10⁻³ resulted in higher photocatalytic degradation of the dye compared with UV/TiO₂–SnO₂ (90:10) wt% (0.4 g L⁻¹)/H₂O₂ or UV/TiO₂–SnO₂ (90:10) wt% (0.4 g L⁻¹)/S₂O₈²⁻ systems, indicating the effectiveness of periodate over peroxide or persulfate.

The reaction rate order with respect to periodate was found to be 2.9 (Fig. 26; Table 4). The enhancement of the dye degradation may be due to the scavenging of the photogenerated conduction band electrons of the excited TiO₂–SnO₂ by periodate which is more efficient than trapping with O₂, H₂O₂ or S₂O₈²⁻as follows⁷:

$${\text{I}}{{\text{O}}_{\text{4}}}^{ - }+{\text{8}}{{\text{e}}^ - }_{{({\text{cb}})}}\,+\,{\text{8}}{{\text{H}}^+} \to {\text{ 4}}{{\text{H}}_{\text{2}}}{\text{O}}\,+\,{{\text{I}}^ - }$$

(9)

In addition, under UV irradiation (254 nm), a variety of highly reactive radical- and non-radical intermediates (IO₃⋅, ^⋅OH, and IO₄^⋅) are produced during the photolytic degradation of periodate which enhanced the degradation rate⁸:

$${\text{I}}{{\text{O}}_{\text{4}}}^{ - }+{\text{ hn}} \to {\text{ I}}{{\text{O}}_{\text{3}}}^{ \cdot }+{\text{ }}{{\text{O}}^{ \cdot - }},$$

(10)

$${{\text{O}}^{ \cdot - }}+{\text{ }}{{\text{H}}^+}{ \leftrightarrow ^ \cdot }{\text{OH}}$$

(11)

$$^{ \cdot }{\text{OH}}\,+\,{\text{I}}{{\text{O}}_{\text{4}}}^{ - } \to {\text{O}}{{\text{H}}^ - }+{\text{I}}{{\text{O}}_{\text{4}}}^{ \cdot }$$

(12)

Fig. 25

Change of ln (C/C_o) with time for photocatalytic degradation of acid red 37 dye using 0.4 g/L of TiO₂–SnO₂ (90:10) wt% with different concentrations of IO₄⁻ (M).

Fig. 26

Change of ln k_app (min⁻¹) with ln [IO₄⁻] (M) for photocatalytic degradation of acid red 37 dye.

The formation of IO₃^⋅, O^⋅−, ^⋅OH, and IO₄^⋅ as well as the trapping of the photogenerated conduction band electrons of TiO₂ and SnO₂ (90:10) wt% are both enhanced by an increase in periodate concentration.

Qamar et al.³³ have shown that the degradation of chrysoidine R and Acid Red 29 dyes was hastened when peroxide or periodate was added to UV light containing TiO₂. It was discovered that the response rate followed the sequence:

• UV/TiO₂/S₂O₈²⁻ > UV/TiO₂/H₂O₂ > UV/TiO₂.

Similarly, Qamar et al.³⁴ observed higher rates of chromotrope 2B and amido black 10 B photocatalytic degradation in the presence of H₂O₂, KBrO₃, and (NH₄)₂S₂O₈. The pace of deterioration was about:

• UV/TiO₂/BrO₃⁻ > UV/TiO₂/S₂O₈²⁻ > UV/TiO₂/H₂O₂ > UV/TiO₂.

Finally, we can showed the total results that for three addition of inorganic antioxidants H₂O₂,SO₄⁻ and IO₄⁻ in shown Table 4 and compare rate of reaction with TiO₂ and SnO₂ alone then after addition inorganic antioxidant.

Table 4 Collective data of apparent rate constants, half life times and reaction orders for photocatalytic degradation of acid red 37 dye.

Materials and methods

Artificial polluted wastewater preparation

An aqueous solution of Acid Red 37 dye (1.04 × 10^–4 M) were prepared in double distilled water as a model for wastewater pollutant. The pH of TiO₂–SnO₂/dye suspension was 6.1. Addition of H₂O₂, NaIO₄ or Na₂S₂O₈ to TiO₂–SnO₂/dye suspension decreases the pH below 6.1. A few drops of an alkali were added to adjust the pH to its original value 6.1.

Preparation of TiO2–SnO2 nanocomposites

TiO₂–SnO₂ nanocomposites were prepared through a solid-state reaction route. The starting material was TiO₂ (99.58%). 3, 7, 10, 15 and 20 wt% of SnO₂ powder have been added to TiO₂. The powders have been mixed uniformly by grinding in mortar with pestle for 3 h to get fine powders. The resultant powders have been annealed in air at 500 °C for 3 h in an electric muffle furnace³⁵.

Schematic diagram of batch photoreactor

The reactor used for studying the photodegradation of the dye in all experiments was a batch photoreactor as shown Fig. 27. To increase solution fluidization and access oxygen for Eq. (13)

$${\text{Adsorbed oxygen}}:{\text{ }}\left( {{{\text{O}}_{\text{2}}}} \right){\text{ads}}\,+\,{\text{e}} - \to {{\text{O}}_{\text{2}}}^{{ - \cdot }},$$

(13)

air was blown into the reaction solution using an air pump at a flow rate of 10 m³/h. Blowing cooled air into the solution eliminated the lamp’s heat effect and kept the temperature at around 25 °C³⁶. It is consists of a glass container (250 ml). The contents (TiO₂/dye, SnO₂/dye, TiO₂–SnO₂/dye, or TiO₂–SnO₂/dye/oxidant) of the container were agitated by a magnetic stirrer and kept purged with air at a rate of 3000 ml min⁻¹. The dye solution was stirred with TiO₂–SnO₂ nanocomposite in the dark for 30 min before irradiation with UV to obtain equilibrium adsorption. Irradiation was carried out with a tubular low pressure mercury lamp (total rating 43 W, total UV output at 513 nm 13.4 W, and length 120 cm, Voltarc Tubes Inc., USA) was located 10 cm from the surface of the dye solution. The total intensity reaching the slurry solution was measured using a UVX radiometer (UV Products Ltd., Cambridge) equipped with a sensor with peak sensitivity at 254 nm was 4 mW cm⁻². Samples of dye solution (1.0 × 10⁻⁴ mol/L) which prepared in distilled water as a model for wastewater pollutants were removed from the container via a sample port periodically and measured after filtration through 0.2 μm polyether sulfone membrane. All photodegradation experiments were done in a temperature of 22 ± 2 °C.

Fig. 27

Schematic diagram of the batch photoreactor.

Measurement of photocatalytic activity

The photocatalytic performance of native TiO₂, SnO₂ nanoparticles and TiO₂–SnO₂nanocomposites was investigated by the decomposition of acid red 37 dye in an aqueous solution under UV light (254 nm). Absorbance was readily described by Beer-Lambert law³⁷

$${\text{A}}\,=\,\varepsilon {\text{LC}}$$

(14)

Where A is absorbance, ε is the molar extinction coefficient, L is the light path length (1.00 cm) and C is the concentration. The absorbance which given at each irradiation time was related to a specific dye concentration using a calibration curve for acid red 37 dye solutions with different concentrations. The percent of degradation of the dye could be calculated according to equation (15) :

$${\text{Degradation }}\% {\text{ }}={\text{ }}{{\text{C}}_{\text{o}}} - {\text{ C}}/{{\text{C}}_{\text{o}}} \times {\text{ 1}}00,$$

(15)

Where C_o = initial concentration of dye solution, C = concentration of dye solution after photoirradiation at time t.

Conclusions

The photocatalytic degradation of the investigated acid red 37 dye using TiO₂ nanoparticles was found to be higher than that of SnO₂. The photocatalytic activity results showed that the couple oxide TiO₂–SnO₂ exhibited much higher photocatalytic activity than the native SnO₂ and native TiO₂.Heterogeneous photocatalytic degradation of the investigated acid red 37 dye using TiO₂–SnO₂ nanocomposite with different wt% (3–20) indicated that: UV/TiO₂–SnO₂ (90:10) wt% performing the higher photocatalytic degradation of the dye than other ratios. Heterogeneous photocatalytic degradation using UV/TiO₂–SnO₂ (90:10)/Oxidant (H₂O₂, S₂O₈²⁻, or IO₄⁻) systems was found to be more effective than UV/SnO₂ or UV/TiO₂ and follows the following order:

• UV/SnO₂ < UV/TiO₂ < TiO₂–SnO₂ (90:10) < UV/TiO₂–SnO₂ (90:10)/H₂O₂ < UV/TiO₂–SnO₂ (90:10)/Na₂S₂O₈ < UV/TiO₂–SnO₂ (90:10)/NaIO₄.