Preparation and characterization of novel CuBi₂O₄/SnO₂ p–n heterojunction with enhanced photocatalytic performance under UVA light irradiation

2.1 Chemical reagents

The starting materials used in the synthesis: α-Bi₂O₃ (99.99%), CuO (99.99%) and SnO₂ (99.99%) were all obtained from Aldrich chemical company Ltd. Congo red (C.I. 22020, MW = 696.67 g mol⁻¹, C₃₂H₂₄N₆O₆S₂·2Na, λ_max = 497 nm and pKa = 4) and other chemicals used in the experiments (NH₄OH and H₂SO₄) were purchased from C.I.S.A. Espagne. Distilled water was used for the preparation of various solutions.

2.2 Preparation of p-CuBi₂O₄

The p-CuBi₂O₄ powder was prepared according to the previously reported procedure (Arai et al., 2007; Liu et al., 2010a,b; Elaziouti et al., 2012). α-Bi₂O₃ and CuO were used as starting materials. The stoichiometric proportion mixture of Bi₂O₃ and CuO was previously ground for a period of time in an agate mortar, and then heated at the rate of 5 °C/min in a muffle oven (Linn High Therm) and thermally treated at 750 °C for 72 h in air. After the muffle oven was naturally cooled to room temperature, the black CuBi₂O₄ powder was ground in the agate mortar and then was collected as the precursor to prepare the composite photocatalyst p-CuBi₂O₄/n-SnO₂.

2.3 Preparation of semiconductor p-CuBi₂O₄/n-SnO₂

Combined semiconductors CuBi₂O₄/SnO₂ were prepared by a physical milling technique. Different CuBi₂O₄/SnO₂ powder samples were prepared in a ratio (mass concentration) of 5%, 10%, 20%, 30%, 40% and 50% respectively, by varying the amount of CuBi₂O₄ and subsequently milled in the agate mortar for 30 min. The final samples were used for the determination of characterization and photocatalytic activity.

2.4 Characterization

Phase evolution and crystalline of the resulting powders were characterized by X-ray diffraction (XRD) in an automatic D8 Bruker AXS diffractometer using CuKα radiation (λ = 1.5406 Å). XRD diffractogrammes were collected in 10–70° intervals with a scan speed of 10°/min. The crystallite average size (d_DRX) calculation using the Scherrer (Pullar et al., 1988) equation is as follows (Eq. 1):

Scanning electron microscopy (SEM) (Hitachi S-4800 N) is used to characterize the morphology of the particles.

UV–Vis diffuse reflectance spectroscopy measurements were carried out using a Perkin Elmer Lambda 650 spectrophotometer equipped with an integrating sphere attachment. The analysis range was from 200 to 900 nm, and polytetrafluoroethylene (PTFE, Teflon) was used as a reflectance standard.

The residual RC concentrations during the course of degradation were monitored with UV mini-1240 Spectrophotometer (Shimadzu UV mini-1240) in the range 200–800 nm, using 1 cm optical pathway cells.

2.5 Photocatalytic study measurements

The photodegradation of CR catalyzed by the p-CuBi₂O₄/n-SnO₂ samples was investigated under UVA light irradiation. 100 mg of catalyst was suspended in a CR solution (200 mL, 20 mg/L) in quartz cell tube. The suspension pH value was previously adjusted at 8 using NaOH/H₂SO₄ solutions using a (Hanna HI 210) pH meter. Prior to UV light irradiation, the suspension was stirred with magnetic stirrer (Speedsafe™ Hanna) for 30 min under dark conditions at 298 K to ensure the establishment of adsorption/desorption equilibrium between the catalyst and CR. The sample was then irradiated at 298 K using 6 W ultraviolet (λ = 365 nm, BLX-E365) photoreactor under continuous stirring. As the reaction proceeded, a 5 ml suspension was taken at 20 min intervals during the catalytic reaction and was centrifuged using centrifuge (EBA-Hetlich) at 3500 rpm for 15 min to completely remove photocatalyst particles. The residual RC concentrations during the course of degradation were monitored with a UV mini-1240 Spectrophotometer (Shimadzu UV mini-1240) in the range 200–800 nm, using 1 cm optical pathway cells.

The effect of initial pH on the photocatalytic degradation of CR only was conducted from pH 6–12 for avoiding dye aggregation. Data obtained from the photocatalytic degradation experiments were then used to calculate the degradation efficiency η′ (%) of the substrate (Eq. 2):

According to Planck’s Law and some further calculation, we can find that the absorption wavelength of the photoreactor can be calculated by determining its band gap value (Eq. 3):

From the calculation, in order to absorb a UVA-light wavelength, the band gap value of the photoreactor has to be below 3.49 eV and above 3.30 eV.

The photocatalytic degradation efficiency of catalyst for the degradation CR was quantified by the measurement of dye apparent first order rate constants under operating parameters. Surface catalyzed reactions can often be adequately described by a monomolecular Langmuir–Hinshelwood mechanism, in which an adsorbed substrate with fractional surface coverage θ is consumed at an initial rate given as follows (Eq. 4) (Vasanth Kumar et al., 2008):

The half-life of dye degradation at various process parameters was raised from Eq. (8):

3.1 XRD analysis of (x wt%) CuBi₂O₄/SnO₂ Nanocomposites

Fig. 1, shows the XRD patterns of the as-synthesized (5 wt%) CuBi₂O₄/SnO₂ nanocomposites in comparison with those of CuBi₂O₄ precursor and pure SnO₂. Diffraction peaks of pure SnO₂ (Fig. 1a) at 2θ of 26.63°, 33.73°,37.94°, and 51.79° can be indexed as the (1 1 0), (1 0 1), (2 0 0), and (2 1 1) planes of tetragonal rutile structure of stannic oxide (lattice constant a = 4.7373(2) Å et b = 3.1865(3) Å), which is in good agreement with standard value (JCPDS file No. 41–1445). The crystallite sizes of pure SnO₂, d_XRD, deduced from the XRD patterns by calculation of the Scherrer equation, was found to be 50 nm. The diffraction peaks of the Cubi₂O₄ precursor (Fig. 1b) at 2θ of 28.03°, 29.73°, 30.73°, 32.54°, 33.36° and 46.71° were respectively indexed as (2 1 1), (2 2 0), (0 0 2), (1 0 2), (3 1 0), and (4 1 1) planes of pure tetragonal phase of crystalline Cubi₂O₄, according to the Joint Committee Powder Diffraction Standards (P4₂/mnm, JCPDS file No. 42–0334) with lattice constants (a = 8.5004 Å, c = 5.819 Å) calculated from their corresponding XRD pattern data obtained by Fullprof program. Both precursors CuBi₂O₄ and pure SnO₂ show preferred (0 0 2) crystallographic orientation owing to the preparation route of the sample during the XRD analysis. On the other hand, the XRD patterns of (5 wt%) CuBi₂O₄/SnO₂ nanocomposite exhibited characteristic diffraction peaks of both CuBi₂O₄ and SnO₂ crystalline phases, suggesting that CuBi₂O₄ and SnO₂ are coexistent in the composites as separate phases. It can be seen from Fig. 1c, that at 5% mass concentration of Cubi₂O₄, the diffraction pattern of the nanocomposite materials was approximately similar to that of pure SnO₂. This is probably due to the high crystallinity of the SnO₂ phase and the lowest amount of CuBi₂O₄ present in the composites (5 wt%), thus appearing as the dominant peaks in the XRD spectra of the nanocomposite sample. Compared with SnO₂, the XRD patterns (Fig. 2) in the 2θ range from 20° to 37° of (5 wt%) CuBi₂O₄/SnO₂ exhibits the same broader peaks, indicating a high solubility of CuBi₂O₄ in SnO₂ matrix. Thus the presence of (5 wt%) Cubi₂O₄ has obviously no effect on the particle size of the (5 wt%) CuBi₂O₄/SnO₂ nanocomposite.

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Figure 1

XRD patterns of pure SnO₂ (a) precursor CuBi₂O₄ (b) and the synthesized (5 wt%) CuBi₂O₄/SnO₂ (c).

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Figure 2

XRD patterns of pure SnO₂ (a) precursor CuBi₂O₄ (b) and the synthesized (5 wt%) CuBi₂O₄/SnO₂ (c) in the 2θ range from 20 to 37.

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3.2 SEM analysis

Fig. 3a, illustrates typical SEM image of CuBi₂O₄ powder synthesized by solid-state reaction of CuO and α-Bi₂O₃ at 750 °C for 24 h, pure SnO₂ and (5 wt%) CuBi₂O₄/SnO₂ nanocomposites. It can be seen that, for the CuBi₂O₄, the appearance is a shapeless sheet, and the particle size of the CuBi₂O₄ is about 10–20 μm. Fig. 3b shows typical high-resolution SEM image of CuBi₂O₄ powder to further show the details of the nanoparticles. As shown in Fig. 3b, it clearly shows two different crystal shapes on the CuBi₂O₄ surface, corresponding to two different particle sizes of CuBi₂O₄. The appearance of CuBi₂O₄ is a shape sheet and a well-defined tetragonal phase with the crystallite diameter of the CuBi₂O₄ about 5 μm, whereas groups of smaller particles do not have any specific shape with size up to 500 nm tend to cover the bigger particles. However, pure SnO₂ from SEM analysis (Fig. 3c) clearly shows two different tetragonal-shaped nanoparticle structures on the SnO₂ surface, which can be assigned to SnO₂ (ion radius Sn⁴⁺: rSn⁴⁺ = 0.071 nm) with a particle size in the range of 0.1 μm and SnO (ion radius Sn²⁺: rSn²⁺ = 0.112 nm) with approximately 0.2 μm dimensions, which agrees with the UV–Vis diffuse reflectance spectrum of SnO₂ in Fig. 4. Both nanoparticles are close to each other in the form of chains.

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Figure 3

SEM images of (a) low-resolution of CuBi₂O₄ (b) high-resolution of precursor CuBi₂O₄ (c) pure SnO₂ (d) (5 wt%) CuBi₂O₄/SnO₂ nanocomposites.

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Figure 4

UV–visible absorbance spectra of SnO₂ and CuBi₂O₄ synthesized by solid-state reaction at 750 °C for 24 h.

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The as synthesized (5 wt%) CuBi₂O₄/SnO₂ nanocomposite via the physical milling synthesis method (Fig. 3d) clearly shows the presence of SnO₂ nanoparticles deposited onto the CuBi₂O₄ surface, displaying a particle size of 0.1–0.2 μm and strong assembly of the nanoparticles measuring from 0.2 μm to 1 μm. Such aggregation can be explained by the solid-state synthesis route, which generally requires repeated mechanical mixing process and a high temperature process.

3.3 UV–Vis diffuse reflectance spectra and band gap energy

Fig. 4, shows the UV–Vis absorbance spectra of CuBi₂O₄ synthesized by solid-state reaction at 750 °C for 24 h and pure SnO₂. It is clear from the recorded UV–visible spectrum of SnO₂ that two absorption bands are observed in the UV region at 210–245 nm and at 270 nm. Generally, the absorption band at 210–245 nm of SnO₂ in the UV region originates from the charge-transfer transition between the O 2p and Sn 4d states in O²⁻ and Sn⁴⁺. The band at 270 nm is attributed, either to the inter-valence transition of Sn⁴⁺/Sn²⁺ (Shen et al., 1994), or to the charge-transfer transition s → p of Sn²⁺ ions (Teegarden, 1966).

The UV–visible spectrum of CuBi₂O₄ nanostructures synthesized by solid-state reaction at 750 °C for 24 h is presented in Fig. 4. It can be seen that it has strong and broad absorption in the range of 200–900 nm. This suggests that the prepared sample absorbs both UV and visible light. Obviously, for CuBi₂O₄ nanostructures, the broad absorption band observed in the UV–visible region was attributed to the charge-transfer transition between the O 2p and Cu 3dx² − y² states in O²⁻ and Cu²⁺, respectively (Nathan et al., 2012). Fig. 5 shows UV–Vis diffuse reflectance spectra of a series of photocatalysts (x wt%) CuBi₂O₄/SnO₂ for x = 0–20 wt%. A progressive red shift in the band gap absorption is observed with an increase in the amount of (x wt%) CuBi₂O₄ in the SnO₂ matrix.

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Figure 5

UV–visible absorbance spectra of a series of (x wt%) CuBi₂O₄/SnO₂ (with x = 0–20 wt%) nanocomposites.

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Pure SnO₂ has a negligible absorption in the range of UVA light (355 nm ⩽ λ ⩽ 375 nm) because the catalyst is only effective under ultraviolet irradiation (λ ⩽ 345 nm). With an increase in the amount of x wt% of CuBi₂O₄, the absorption edge of the sample has some red shift. The red-shift observed in the nanocrystalline SnO₂ would explain the formation of localized states within the band gap owing to oxygen vacancies and increase in Sn²⁺ ion concentration. This phenomenon is due to the shift of absorbance band toward the longer wavelength (Lu et al., 2009; Charitidis et al., 2005). The red shift is presumably ascribed to the homogeneous dispersion of CuBi₂O₄ particles within SnO₂ matrix. The onset absorption edges and band gap energies of the as-synthesized CuBi₂O₄ particle, pure SnO₂ nanoparticle and series of (x wt%) CuBi₂O₄/SnO₂ nanocomposites are shown in Figs. 4 and 5, respectively.

The absorption onsets of crystalline semiconductor were determined by linear extrapolation from the infection point of the curve (Absorbance versus λ_{Absorp. Edge}) to the baseline and Eg = 1240/λ_{Absorp. Edge} as shown in Figs. 4 and 5. The as-synthesized CuBi₂O₄ exhibits an absorption onset at 900 nm, which corresponds to the band gap energy of 1.38 eV. This value is lower than that reported in the literature; 1.5 eV (Arai et al., 2007; Liu et al., 2010a,b). It is clear from the recorded spectrum (Fig. 4) that the pure nanocrystalline SnO₂ has an absorption onset at 345 nm, which matches the band gap energy of 3.59 eV, attributing to stannic oxide (SnO₂). These results are well in agreement with values reported in the literature (Casey and Stephenson, 1990). The optical properties of the as-synthesized CuBi₂O₄ and pure SnO₂ nanoparticles are reported in Table 1.

It is widely accepted that electronic transport properties depend on the physical and structural characteristics of photocatalyst, such as crystallite size, morphology, phase structure and amount of CuBi₂O₄ loaded (Yu et al., 2008). As reported from the UV–visible results in Fig. 5 and Table 1, for the series of (x wt%) CuBi₂O₄/SnO₂ nanocomposite materials, the band gap energy slightly decreased from 3.59 to 3.49 eV as the amount of CuBi₂O₄ was increased up to 20 wt% on the SnO₂ matrix, suggesting that a number of micro p–n heterostructures CuBi₂O₄/SnO₂ will be formed after doping CuBi₂O₄ powder into SnO₂ particles. Hence it was concluded that CuBi₂O₄ played a role in electron separation. The increase in photocatalytic activity of CuBi₂O₄/SnO₂ would be attributed to the suppression of electron–hole recombination by the effective electron separation (Masami et al., 2013). So, the decrease in the band gap energy with an enhanced absorption intensity of the (5wt%) CuBi₂O₄/SnO₂ nanocomposites upon loading the amount of CuBi₂O₄ could be ascribed to the homogeneous dispersion of CuBi₂O₄ within the SnO₂ matrix in the bulk of the catalyst and the formation of conducting network at very low temperature.

Chun-Ming et al. (2007) have already reported band gap narrowing effect for doped SnO₂ nanoparticles. However there is no clear understanding of this phenomenon. A direct–indirect transition has been proposed by Rakhshani et al. (1998). In order to explain the band gap narrowing effect, many groups have suggested that alloying effect of parent compound with some impurity phases may be responsible for the band gap narrowing (Park and Kim, 2003; Barreau et al., 2002). So we think that for the samples containing up to 5 wt% CuBi₂O₄ concentration, SnO₂–SnO_2-x alloying effect may be responsible for the band gap narrowing effect. For the SnO₂ nanoparticles above 5 wt% CuBi₂O₄ concentration there is a huge drop in the band gap. This may be due to the formation of sub-bands in between the band gap and the conduction band and sub-bands are merging with the conduction band to form a continuous band. This is perfectly in agreement with the XRD analytical results and consistent with the previously reported work (Chun-Ming et al., 2007).

3.4 Photocatalytic activity tests

3.4.1

3.4.1 Effect of pH solution on the photocatalytic activity of (5 wt%) CuBi₂O₄/SnO₂ nanocomposite

In order to study the effect of initial pH on the degradation efficiency of (5 wt%) CuBi₂O₄/SnO₂ nanocomposite catalyst on photodecomposition of CR, experiments were carried out at various pH, ranging from 6 to12 for avoiding dye aggregation. Results showed that the pH significantly affected the photocatalytic degradation efficiency of both CR. As shown in Fig. 6 and Table 2, for CR, the degradation rate increased from 12.03 to 58.06% as the pH value was increased from 6 to 8, and then decreased to 12.27 at pH 12. The maximum degradation rate of CR (58.06%) was achieved at pH 8. For this reason, the pH 8 was selected for subsequent experiments.

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Figure 6

Effect of the pH solution on the photocatalytic redox of CR under UVA light irradiation ([(5 wt% CuBi₂O₄/SnO₂]=0.5 g/L, [CR]= 20 mg/L, T = 298 K, λ_max = 365 nm, I = 90 J/cm² and irradiation time = 100 min).

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Table 2 Results of the effect of the pH solution on the photocatalytic redox of CR under UVA light irradiation ([(5 wt% CuBi₂O₄/SnO₂] = 0.5 g/L, [CR] = 20 mg/L, T = 298 K, λ_max = 365 nm, I = 90 J/cm² and irradiation time = 100 min).

pH initial	Adsorption activity η (%)	Photocatalytic activity η′ (%)
2	Dye aggregation
4
6	0.93	12.03
7	33.69	42.16
8	31.38	58.06
10	8.64	13.77
12	0.46	12.27

It is commonly accepted that in photocatalyst/aqueous systems, the potential of the surface charge is determined by the activity of ions (e.g. H⁺ or pH). A convenient index of the tendency of a surface to become either positively or negatively charged as a function of pH is the value of the pH required to give zero net charge (pH _PZC) (Zhang et al., 1998; Yath, 1974). pH _PZC is a critical value for determining the sign and magnitude of the net charge carried on the photocatalyst surface during adsorption and the photocatalytic degradation process. Most of the semiconductor oxides are amphoteric in nature, can associate Eq. (15) or dissociate Eq. (17) proton. To explain the relationship between the layer charge density and the adsorption, so-called Model of Surface Complexation (SCM) was developed (Fernandez et al., 2002), which consequently affects the sorption–desorption processes as well as the separation and transfer of the photogenerated electron–hole pairs at the surface of the semiconductor particles. In the 2-pK approach we assume two reactions for surface protonation.

The zero point charge pH _PZC for SnO₂ (about 5) is approximately identical to that of (5 wt%) CuBi₂O₄/SnO₂ which is mainly composed of SnO₂ nanopowders (shown in XRD patterns), since there is no adsorption of CR ions than the potential determining H⁺/OH⁻ at the surface of CuBi₂O₄ particles. This is often the case for pure (“pristine surface”) oxides in water.

When the pH is lower than the pH _PZC value, the system is said to be “below the PZC”. Below the PZC, the acidic water donates more protons than hydroxide groups, and so the adsorbent surface is positively charged (attracting anions/repelling cations), according to the following reaction Eqs. (9), (10):

(9)

$$\text{pH}<{\text{pH}}_{\text{PZC}}(5\text{wt}\%){\text{CuBi}}_2{\text{O}}_4{\text{SnO}}_2+{\text{H}}^{+}\to (5\text{wt}\%){\text{CuBi}}_2{\text{O}}_4{\text{SnO}}_2{\text{H}}^{+}$$

(10)

$$(5\text{wt}\%){\text{CuBi}}_2\text{O}4/{\text{SnO}}_2{\text{H}}^{+}+{\text{CR}}^{-}\to [(5\text{wt}\%){\text{CuBi}}_2{\text{O}}_4/{\text{SnO}}_2{\text{H}}^{+}\text{,}{\text{CR}}^{-}](\text{electrostatic interaction})$$ Conversely, above pH _PZC the surface is negatively charged (attracting cations/repelling anions), given by the following reaction Eqs. (11) and (12):

(11)

$$\text{pH}>{\text{pH}}_{\text{PZC}}(5\text{wt}\%){\text{CuBi}}_2{\text{O}}_4/{\text{SnO}}_2+{\text{OH}}^{-}\to (5\text{wt}\%){\text{CuBi}}_2{\text{O}}_4/{\text{SnO}}^{-}+{\text{H}}_2\text{O}$$

(12)

$$(5\text{wt}\%){\text{CuBi}}_2{\text{O}}_4/{\text{SnO}}^{-}+{\text{CR}}^{-}\to [(5\text{wt}\%){\text{CuBi}}_2{\text{O}}_4/{\text{SnO}}^{-}\text{,}{\text{CR}}^{-}](\text{electrostatic repulsion})$$ The experimental data revealed that higher adsorption and degradation activities of CR were obtained in alkali medium (i.e. pH = 8). Even though (5 wt%) CuBi₂O₄/SnO₂ nanocomposite showed good photocatalytic efficiency among the CuBi₂O₄ loadings, but it seems to be that this activity may not be up to our expectation. The poor adsorption affinity of the composite toward organic pollutant may be the primary cause of the observed efficiency. As the adsorption reaction between anionic dye/composite takes place on the solid surface, this means that the main dye-composite interaction is a coulomb-type interaction. However, other interaction, such as hydrophobic effect between guest molecules on the composite surface (Xie et al., 2005) and/or H-bonding can contribute to the adsorption of organic dye on composite surface, favoring the dye agglomeration. Thus, the enhancement of adsorption activity was not exclusively caused by the attractive interaction among dye-composite because there are no obvious electrostatic interactions between the negatively charged (5wt%) CuBi₂O₄/SnO₂ surface (pH > pH _PZC) and CR anionic dye (pH > pK_a) at pH 8. As was reported previously, we believe that it is mainly due to the hydrophobic effect between guest molecules on the composite surface and/or H-bonding. Thus, a strong adsorption can lead to a drastic decrease in the active centers on the catalyst surface, which results in a decrease in the absorption of the light quanta by the catalyst and consequently reducing the kinetic reaction. As a result, the moderate photodegradation activity can be ascribed to the (5 wt%) CuBi₂O₄/SnO₂-mediated photocatalytic oxidation of CR dye under alkali medium. It is noteworthy that heterogeneous photocatalytic processes substantially depend on a variety of environment conditions as mentioned in Introduction section. Thus, the presence of the tightly physically bonded or close contact interfaces between the two materials, by which the photoinduced charge transfer from one particle to the other through interfaces spatially is available, can lead to a strong photocatalytic redox of CR over the combined catalysts.

At neutral and acidic media (pH < 8), lower photocatalytic efficiency of catalyst can be explained by the following possible reasons. First, the electrostatic repulsion forces between the negatively charged (5 wt%) CuBi₂O₄/SnO₂ surface and CR anionic dye, mainly sulfonated groups (-SO⁻₃), affecting strongly the accessibility of the surface reducing species to the CR photocatalytic oxidation/reduction kinetics. Second, the excess of H₃O⁺ ions, especially at pH = 6, enters into electrostatic interactions with both the negatively charged (5 wt%) CuBi₂O₄/SnO₂ surface and RC dye anions, leading to a minimum adsorption extent at pH 6. Furthermore we found that, where the adsorption of dye was strong, photodegradation remarkably occurred.

At pH higher than pH _PZC value (i.e. pH > 8), excess of hydroxyl anions facilitates photogeneration of ^•OH radicals which are accepted as primary oxidizing species responsible for photocatalytic degradation, resulting in the enhancement of the efficiency of the process. However, a dramatic decrease in the degradation efficiency could be explained on the basis of amphoteric behaviors of (5 wt%) CuBi₂O₄/SnO₂ catalyst. The negatively charged surface of (5 wt%) CuBi₂O₄/SnO₂ catalyst (highly concentration of hydroxide ions) and the great negatively charged RC dye anions resulted in electrostatic repulsion, leading to the reduction in the efficiency of the photodegradation process. There were similar results in the previous reports (Liu et al., 2010a,b).

3.4.2

3.4.2 Effect of the amount of CuBi₂O₄ on the photocatalytic activity of (x wt%) CuBi₂O₄/SnO₂

The effect of the amount of CuBi₂O₄ on photocatalytic degradation of CR was conducted over a range of catalyst amounts from x = 0 to x = 100 wt%. As observed in Fig. 7 and Table 3, it is evident that the photocatalytic redox of CR greatly depends on the amount of doped CuBi₂O₄. The photocatalytic activity increased drastically from 25.62 to 58.06% as the catalyst amount was raised from x = 0 to x = 5 wt%. Upon further increase in the CuBi₂O₄ amount beyond x = 5 wt%, the photocatalytic activity decreased gradually, almost reaching 3.13% at x = 100 wt%. The maximum photocatalytic activity of (x wt%) CuBi₂O₄/SnO₂ (58.06%) under UVA light irradiation was achieved within 100 min of light illumination time when the amount of doped CuBi₂O₄ x was 5 wt%, which is obviously about 2.3 times higher than the value of 25.62% over pure SnO₂. So there is an optimum CuBi₂O₄ contents for high dispersion morphology of nanoparticles CuBi₂O₄ on the SnO₂ surface with high activity. The effective electron–hole separation both at the physically bonded interfaces and in the two semiconductors as well as charge defect during the physical mixing method was believed to be mainly responsible for the remarkably enhanced photocatalytic activity of (5 wt%) CuBi₂O₄/SnO₂ in the course of the photocatalytic redox conversion of CR. But until now, there are no reports about synergistic effect between SnO₂ and CuBi₂O₄ in the (5 wt%) CuBi₂O₄/SnO₂ nanocomposite under visible light excitation. From Fig. 7, it is clear that the photocatalytic activity of SnO₂ is significantly increased under the presence of an amount of CuBi₂O₄ (5 wt%) compared to pure SnO₂ and the CuBi₂O₄ samples. These results strongly suggest the existence of a synergistic effect between SnO₂ and the CuBi₂O₄ in the (5 wt%) CuBi₂O₄/SnO₂ nanocomposite under UVA light excitation.

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Figure 7

Effect of the amount of CuBi₂O₄ on the photocatalytic redox of CR under UVA light irradiation ([(x wt%) CuBi₂O₄/SnO₂]=0.5 g/L, [CR]= 20 mg/L, pH = 8, T = 298 K, λ_max = 365 nm, I = 90 J/cm² and irradiation time = 100 min).

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Table 3 Results of the effect of the amount of CuBi₂O₄ on the photocatalytic redox of CR under UVA light irradiation ([(x wt%) CuBi₂O₄/SnO₂,0.5 g/L, [CR] = 20 mg/L, pH = 8, T = 298 K, λ_max = 365 nm, I = 90 J/cm² and irradiation time = 100 min).

Amount of CuBi2O4 x (%)	Adsorption activity η (%)	Photocatalytic activity η′ (%)
0	2.92	25.62
5	31.38	58.06
10	22.24	31.48
20	16.21	19.69
30	12.52	12.95
40	11.01	8.01
50	5.18	3.64
100	4.71	3.13

However, at higher amount of doped CuBi₂O₄ than 5 wt%, the photocatalytic redox activity of (x wt%) CuBi₂O₄/SnO₂ photocatalyst was obviously decreased on further increase in the amount of CuBi₂O₄. Thus, such an above occurrence in the present experiment is primarily attributed to overlapping of adsorption sites of SnO₂ particles as a result of overcrowding of the CuBi₂O₄ granule owing to the decrease in screening effect and interfering of light. Similar trends were reported in a series of p–n heterojunction photocatalysts p-CuBi₂O₄/n-TiO₂ (Lin et al., 2008) and CuBi₂O₄/BiVO₄ (Masami et al., 2013) with high photocatalytic activity under visible and UV light irradiation.

3.4.3

3.4.3 Effect of UV light and catalyst

The photocatalytic activities of all three CuBi₂O₄, SnO₂ and (5 wt%) CuBi₂O₄/SnO₂ photocatalysts were assessed by the photocatalytic redox reaction of Congo red (CR) aqueous solution under UVA light irradiation. Variations of CR reduced concentration (C/C₀) versus visible-light irradiation time over different catalysts under different experimental conditions through alone (CR self-photolysis), CuBi₂O₄/UVA, SnO₂/UVA and (5 wt%) CuBi₂O₄/SnO₂/UVA are presented in Fig. 8. The synergistic effect between SnO₂ and CuBi₂O₄ in the (5 wt%) CuBi₂O₄/SnO₂ nanocomposite under UVA light excitation showed that (5 wt%) CuBi₂O₄/SnO₂ exhibited higher photocatalytic efficiency, as compared to the single phases CuBi₂O₄ and SnO₂. The highest efficiency was obtained, under UVA light irradiation over (5 wt%) CuBi₂O₄/SnO₂, as a result of 58.06% degradation of CR for 100 min of irradiation time. However, the photocatalytic degradation of CR over single phases CuBi₂O₄ and SnO₂ was only 3.13 and 25.62% respectively. With 20 mg/L of CR in the direct photolysis for the same optimum irradiation time, disappearance of dye was negligible (0.49%). On the basis of these results, the high decomposition of CR dye in the presence of (5 wt%) CuBi₂O₄/SnO₂ catalyst is exclusively attributed to the photocatalytic reaction of the combined semiconductor particles under UVA light irradiation. As known, one of the basic requirements for the combined photocatalysts with a higher activity is the presence of the tightly physically bonded or close contact interfaces between the two materials, by which the photoinduced charge transfer from one particle to the other via interfaces spatially is available. Thus, such an above occurrence in the present experiment is primarily assigned to the charge defect during the physical mixing method, which is advantageous for the effective electron–hole separation and the suppression of the recombination rate of the photogenerated charge carriers; hence result in an improvement of the probability of light-generated carriers transfer via interfaces spatially available. A similar result was reported in the heterojunction semiconductor SnO₂/SrNb₂O₆ with an enhanced photocatalytic activity (Gurlo, 2006).

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Figure 8

Photocatalytic degradation kinetics of CR at different experimental conditions ([Catalyst]=0.5 g/L, [CR]= 20 mg/L, pH = 8, T = 298 K, λ_max = 365 nm, I = 90 J/cm² and irradiation time = 100 min).

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3.4.3.1

3.4.3.1 Kinetic modeling

The photocatalytic degradation of CR over different experimental conditions is displayed in Table 4. As it can be seen, the straight lines for the entire as-prepared samples of the plots of ln C/C₀ versus t with high regression coefficients (R² = 0.945–0.991), for the pseudo-first-order kinetic model strongly suggest that all the photodegradation systems were a pseudo-first-order model. Exception was observed in the cases of direct photolysis and photocatalysis reaction in the presence of the single phase CuBi₂O₄ respectively.

Table 4 Kinetic parameters of photocatalytic degradation of CR on (5 wt%) CuBi₂O₄/SnO₂, compared to the pure and combined catalyst systems ([Catalyst]=0.5 g/L, [CR] = 20 mg/L, pH = 8, T = 298 K, λ_max = 365 nm, I = 90 J/cm² and irradiation time = 100 min).

Systems	Adsorption activity η (%)	Photocatalytic activity η′ (%)	K1 (min−1)	t1/2 (min)	R2 (%)
CR/UV-A	–	0.49	–	–	–
CR/SnO2/UVA	2.92	25.62	0.0045	154.032	0.945
CR/CuBi2O4/UVA	0	3.130	0.0002	3465.736	0.203
CR/(5 wt.%)CuBi2O4-SnO2/UVA	31.38	58.06	0.052	13.329	0.991

3.5 Discussion of mechanism

The above analysis shows that the migration direction of the photogenerated charge carrier depends on the band edge positions of the two semiconductors. There are three methods to determine the band edge positions: experiments based on photoelectrochemical techniques, calculation according to the first principle, and predicting theoretically from the absolute (or Mulliken) electronegativity (Kim et al., 1993; Butler and Ginley, 1978; Xu and Schoonen, 2000). The first one is not always easy to handle, and the second one cannot obtain the absolute energy of band edges with respect to vacuum and always has large discrepancies between calculated and measured values. The third one is a simple approach with reasonable results for many oxide photocatalysts (Xu and Schoonen, 2000). The conduction band edge of a semiconductor at the point of zero charge (pHzpc) can be predicted by Eq. (13):

Fig. 9 depicts reaction schemes of CuBi₂O₄ (a) and SnO₂ (b) as the p and n type respectively for charge separation for the reductivity/oxidizability improvement model. CuBi₂O₄ is a p-type semiconductor, which always exhibits good stability under UVA visible illumination, and SnO₂ is determined as an n-type material. It was reported previously that that the band gap of CuBi₂O₄ is 1.38 eV, which can be excited by photons with wavelengths below 900 nm, whereas SnO₂ with band gap of about 3.59 eV can be excited by photons with wavelengths of 345 nm. Under UVA (λ_UVA = 355–375 nm → Eg_ = 3.3–3.49 eV) light irradiation, the energy of the excitation light is sufficient to directly excite the CuBi₂O₄ (λ = 900 nm → Eg = 1.38 eV) semiconductor and it is large enough to yield a higher energy level of SnO₂ (λ = 345 nm → Eg = 3.59 eV) portion of the photocatalyst.

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Figure 9

Reaction schemes of CuBi₂O₄ (a) and SnO₂ (b) as the p- and n-type semi-conductor respectively for charge separation for the reductivity/oxidizability improvement model (electron and hole ).

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According to the band edge position (Table 5), for the p-CuBi₂O₄ alone (Fig. 9 a), the electronic potential of the conduction band of p-CuBi₂O₄ is around −0.44 eV/NHE at pH 7 which is more negative than that of superoxide radical (E₀ (O₂/O₂^•−) = −0.28 V/NHE at pH 7). This indicated that the electron photoproduced at the conduction band directly reduced O₂ into O₂^•−. These reduced O₂^•− can subsequently transfer the charge to the species present in the reaction medium that is preferentially adsorbed onto the p-CuBi₂O₄ particles. Hence, the superoxide radical (O₂^•−) reduces the recombination of the charge carriers enhancing the activity in the UVA light. However, the p-CuBi₂O₄ valence band of +0.94 eV/NHE at pH 7, is too negative than that of hydroxyl radical (E₀ (H₂O/^•OH)) = + 2.8 V/NHE at pH 7). The holes photogenerated in the p-CuBi₂O₄ are not able to oxidize H₂O to ^•OH.

p-CuBi₂O₄ powder formed in our laboratory by the solid-state reaction of CuO and α-Bi₂O₃ at 750 °C for 24 h, exhibits a black color. The presence of non stoichiometric regions of the nominally p-CuBi2O4 particles or small domains of binary oxide phases of Cu_xO or Bi_xO, undetected by XRD data, as unstable impurity phases which could be originated from a number of processes such as reduction of the p-CuBi₂O₄, could be responsible for higher recombination rates. Thus, the result is consistent with the previous study in the electrochemical synthesis and characterization of p-CuBi₂O₄ thin film photocathodes (Magesh et al., 2009). Therefore, CuBi₂O₄ alone shows negligible photocatalytic activity under UVA light. As a result, less efficient charge-carrier separation, and thus the increment of photocatalytic activity were restricted (Elaziouti et al., 2012).

For pure n-SnO₂ (Fig. 9b) which is only effective under ultraviolet irradiation (λ = 345 nm), shows little photocatalytic activity under UVA light. These observations can be explained as follows: The reduction of Sn⁺⁴ to Sn⁺² requires a potential of +0.15 V/NHE at pH 7 and the oxidation of Sn⁺² to Sn⁺⁴ needs −0.15 V/NHE at pH 7. The SnO₂ valence band of SnO₂ is around +3.55 eV (vs. NHE at pH 7) which is more positive than Sn⁺² to Sn⁺⁴ oxidation potential. The valence electron of SnO₂ can hence oxidize Sn⁺² to Sn⁺⁴. But the conduction band of SnO₂ has a potential of −0.05 eV/NHE at pH 7, which is more negative than Sn⁺⁴ to Sn⁺² reduction potential. Hence, the conduction band electrons of SnO₂ may be able to reduce Sn⁺⁴ to Sn⁺². These reduced Sn⁺² and oxidized Sn⁺⁴ species can subsequently transfer the charge to the species present in the reaction medium. Hence, the Sn ions can suppress the recombination of the charge carriers, enhancing the activity in the UVA light.

In contrast, (5 wt%) p-CuBi₂O₄/n-SnO₂ composite exhibits a higher photocatalytic activity compared to that of single photocatalyst (p-CuBi₂O₄ and n-SnO₂). To overcome the recombination problem of composite photocatalyst while maintaining its high efficiency, an artificial z-scheme photocatalytic system, which mimics the z-scheme in the natural photosynthesis of green plants (Sasaki et al., 2008) has been suggested for p-CuBi₂O₄/n-SnO₂ composite mediated photocatalytic degradation of Congo red as probe pollutant. The solid state z-scheme mechanism, in which two UVA light sensitive photocatalysts, p-CuBi₂O₄ and n-SnO₂ in our case, with different levels of band gap energy Eg, is utilized without any intermediates. This system, however, cannot be considered to be a genuine z-scheme under irradiation by visible light because bare n-SnO₂, can only generate electrons when irradiated with UV light (Hyeong et al., 2011). The z-scheme photocatalysis system diagram is depicted in Fig. 10.

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Figure 10

Mechanism of CR degradation in a z-scheme photocatalysis system consisting of CuBi₂O₄/SnO₂ heterojunction under UVA-light irradiation. (electron and hole ).

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So, when p-type semiconductor CuBi₂O₄ and n-type semiconductor SnO₂ were connected to each other, a number of micro p–n heterojunction CuBi₂O₄/SnO₂ will be formed between p-CuBi₂O₄ powder and n-SnO₂ granule at the interfaces of p-CuBi₂O₄ loaded n-SnO₂ composite. According to the band edge position (Table 5), the electronic potential of the conduction band of n-SnO₂ is slightly more anodic than that of p-CuBi₂O₄, whereas, the hole potential of the valence band top of n- SnO₂, is more positive than that of p-CuBi₂O₄. The electron–hole pairs will be created under UVA light illumination. The electrons that are photogenerated by n-SnO₂ are transferred to the valence band of p-CuBi₂O₄. These electrons then recombine with holes photogenerated at p-CuBi₂O₄. The electrons that are excited at p-CuBi₂O₄ then have a sufficiently high potential to participate in the reduction step of the reaction. By taking advantage of a combination of p-CuBi₂O₄ and n-SnO₂, the probabilities of electron–hole recombination would be decreased significantly, thus resulting in more electrons available on CuBi₂O₄ surface and a larger amount of holes on SnO₂ surface acting as powerful oxidants respectively (Eqs. 14, 15). The stepwise photocatalytic mechanism is illustrated below: