An efficient removal of methyl violet from aqueous solution by an AC-Bi/ZnO nanocomposite material

Abstract

An activated charcoal (AC) supported bismuth (Bi)-doped zinc oxide (ZnO) nanocomposite material was synthesized by a precipitation method. The obtained material was characterized by high resolution-scanning electron microscopy (HR-SEM) with energy dispersive X-ray analysis (EDX), X-ray diffraction (XRD), Fourier transform-Raman spectroscopy (FT-RAMAN), photoluminescence spectroscopy (PL), UV-visible diffuse reflectance spectroscopy (UV-Vis-DRS), X-ray photoelectron spectroscopy (XPS) and Brunauer–Emmett–Teller (BET) analysis. The BET surface area, pore radius and pore volume of the materials were calculated by applying the BET equation to the sorption isotherms. The cyclic voltammetry (CV) analysis suggested an electrochemical redox reaction. The production of hydroxyl radicals (˙OH) on the surface of the UV-irradiated photocatalysts was detected by a photoluminescence technique using coumarin as a probe molecule. The photocatalytic activity of the AC-Bi/ZnO material is demonstrated through the photodegradation of methyl violet (MV) under UV-light irradiation. AC-Bi/ZnO has an increased absorption in the UV region; moreover, it shows excellent UV-light driven photocatalytic performance. The photocatalyst of AC-Bi/ZnO reveals enhanced photocatalytic activities as compared to ZnO and Bi/ZnO for the degradation of harmful MV dyes. The enhanced photocatalytic activity of AC-Bi/ZnO is attributed to the low recombination rates of photoinduced electron hole pairs, caused by the transfer of electrons and holes between ZnO and AC supported Bi³⁺ ions. The mechanism of the photocatalytic effect of the AC-Bi/ZnO nanocomposite material has been discussed. The proposed use of AC-Bi/ZnO in water purification technique is promising.

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1. Introduction

Nowadays, water sources are facing serious threats from hazardous pollutants due to various activities. These hazardous compounds include heavy metals, anions, organic compounds and dyes. These contaminants could threaten public health and atmosphere when they exceed the tolerance limits in water.^1–4 Everyday a large amount of unconsumed dye produced by the textile and printing industries is discharged into the environment. The presence of dyes and pigments in water causes considerable damage to the marine environment.^5–7 Basic dyes have high intensity of colour and are extremely toxic even in a very low concentration.^8–10 Methyl violet (MV) may cause severe skin and eye irritation. The inhalation of MV may also cause irritation to the respiratory tract, whereas ingestion causes irritation to the gastrointestinal tract.¹¹ Hence, dye removal is of enormous value.

Metal oxides are the most excellent choice for heterogeneous photocatalysis to remove various pollutants, dyes and phenolic compounds.^12–15 Among the metal oxides, ZnO is an alternative for TiO₂ due to the similar band gap energy (3.37 eV) as well as similar photocatalytic mechanism and capacity.^16–18 ZnO is a semiconductor with high photosensitivity, non-toxic nature, low cost, and environmentally friendly features for photocatalytic applications.¹⁹ ZnO has a competitive photocatalytic activity greater in some cases than TiO₂, for example, in the decolourization of Reactive Blue19, textile dyes and azo dyes in aqueous suspensions.^20–22 Furthermore, semiconductor thin films have been found to photodegrade dyes and insecticides.^23,24 ZnO nanoparticles can be prepared using various methods, including hydrothermal, electro deposition (ED), chemical vapor deposition (CVD), electrochemical, solution combustion, sol–gel and precipitation methods.^25–31 Among all these methods, the precipitation technique provides a suitable control of the nucleation, ageing and growth of the particles in solution. Direct precipitation is also one of the most simple and cost effective methods for the bulk production of materials.³² Moreover, both the size and the morphology have an influence on the properties of photocatalysts.³³

In this method, particle growth is due to the interactions between different aqueous solutions; therefore, very small particles are formed. These small size particles with lower solubility dissolve and re-precipitate on the surface of larger particles in the solution; thus, agglomeration takes place in the solution as the particles clog together to decrease surface energy.³⁴ Doping is one of the most widely used strategies to increase the photocatalytic activity of semiconductors.³⁵ Many dopants, such as transition metals (Mn, Cu and Co) and rare earth elements (La, Ce and Er), could enhance the photocatalytic properties of ZnO.^36–39 The effect of Bismuth-doped ZnO nanocomposite materials for the photocatalytic degradation of Congo Red was discussed very recently.³¹ The doping of Bi into ZnO is expected to shift the absorption edge of ZnO and influence the separation rate of the photoinduced charge carriers of ZnO because of the different structures of the electronic shells and sizes of Bi and Zn.⁴⁰ Bismuth is a type of p-block metal with a d10 configuration, and the hybridized valence band by O 2p and Bi 6s can narrow the band gap, as well as favor the mobility of photo-generated holes in the valance band.^41,42 It is worth noting that the studies of bismuth-based photocatalysts focus on the degradation of organic pollutants.^43,40

Photocatalytic oxidation is an economical process owing to the fact that it involves only a photocatalyst and light source.⁴⁴ An alternative approach for decolourization is the addition of charcoal due to its valuable features in the chemical, physical and biological processes.^45–47 The function of charcoal is versatile: (i) it acts as a dye adsorbent, not only in straightforward adsorption processes but also in charcoal enhanced coagulation and membrane filtration processes; (ii) it catalyses ˙OH production in advanced oxidation processes; (iii) it generates strong oxidizing agents (mostly, hydroxyl (˙OH) radicals) in electrochemical dye oxidations; (iv) it catalyses anaerobic (azo) dye reduction and supports biofilm growth in microbial dye removal.⁴⁸ Although charcoal has higher adsorption properties, it usually does not degrade the dye. However, charcoal loaded on a semiconductor oxide enhances the degradation efficiency of the semiconductor oxide by its synergetic effects and effectively degrades the dye. The increase in photocatalytic activity of semiconductor oxides by the addition of charcoal has been well established.^49,50

In this study, we report the synthesis, characterization and photocatalytic activity of an AC-supported Bi-doped ZnO nanocomposite material. The AC-Bi/ZnO material was synthesized by a simple and cost effective precipitation method. The obtained material was characterized by HR-SEM with EDX, XRD, FT-RAMAN, PL, DRS, XPS, BET and CV analysis. In the application section, we have studied its photocatalytic activity towards MV in aqueous solution under UV-light irradiation. Herein, we highlight and evaluate the recent progress in the development of photocatalytic activity of AC-Bi/ZnO nanocomposite materials. The enhanced photocatalytic activity of AC-Bi/ZnO, which is caused due to the reduction in the recombination of electron–hole pairs, decreased band gap energy, increased surface area, prevented aggregation, and increased ˙OH formation, improves the electron transfer and increases the absorption of light intensity; thus, they are desired elaborately. The AC-Bi/ZnO reveals enhanced photocatalytic activities as compared to ZnO and Bi/ZnO for the degradation and decolourization of MV under UV-light irradiation for 0–60 minutes.

2. Experimental section

2.1. Materials

Zinc acetate dihydrate (Zn(CH₃COO)₂·2H₂O), bismuth nitrate pentahydrate (Bi(NO₃)₃·5H₂O), activated charcoal, oxalic acid (C₂H₆O₆), anhydrous ethanol (C₂H₅OH), coumarin (1 mM of 4-hydroxy coumarin) and Methyl Violet (C₂₅H₃₀ClN₃) were the guaranteed reagents of Sigma Aldrich and used as supplied. The aqueous solutions were prepared by using double distilled water.

2.2. Synthesis of AC supported Bi-doped ZnO nanocomposite material

The AC supported Bi-doped ZnO nanocomposite material was synthesized by a precipitation method (Scheme 1). 0.24 g of Bi(NO₃)₃·5H₂O (0.1 M) and 0.003 g of activated charcoal (0.05 M) together with 0.4 M of oxalic acid were dissolved in 20 mL of anhydrous ethanol. The resulting solution was added dropwise into a 20 mL solution of 2.7 g of Zn(CH₃COO)₂·2H₂O (0.8 M) in anhydrous ethanol at room temperature under vigorous stirring until a precipitate formed. The obtained precipitate was washed with water and ethanol. Then, the precipitate was collected and dried in an oven at 100 °C for 12 h in air. The resulting powder was finally calcined at 600 °C for 3 h in a muffle furnace to get AC-Bi/ZnO. It is found that this catalyst contained 8.5 wt% of AC, 21.5 wt% of Bi and 70 wt% of ZnO. The bare ZnO and Bi-doped ZnO nanocomposites are also synthesized by the same precipitation method.

Scheme 1 Schematic for the synthesis of AC-Bi/ZnO nanocomposite material.

2.3. Analysis of hydroxyl radical (˙OH)

The formation of hydroxyl radicals (˙OH) are similar to the photocatalytic activity experiments.⁵¹ A photoluminescence (PL) technique with coumarin (1 mM) as a probe molecule was used to investigate the formation of hydroxyl radicals on the surface of AC-Bi/ZnO illuminated by UV irradiation for 0 to 60 min.

2.4. Adsorption and oxidation kinetics on the surface of AC-Bi/ZnO material

Activated charcoal (AC) is an excellent adsorbent of countless pollutants. Its industrial applications include the adsorptive removal of odour, colour, taste and other unwanted organics and inorganics from waste water and drinking water.⁴⁸ The organic substrates from the dye effluents diluted in aqueous medium are considered to be first adsorbed on the surface of the AC (Fig. 1), wherein they migrate to the Bi/ZnO particles and they are oxidized in the vicinity of Bi/ZnO by radical species, such as hydroxyl radicals (˙OH) and superoxide radical anions (O₂˙⁻), which are formed by the reaction with photogenerated holes (h⁺) and electrons (e⁻), respectively. Hence, a good understanding of the physicochemical properties of the support material is very important to achieve efficient photodegradation.

Fig. 1 Dye effluents on the AC-Bi/ZnO surface are desirable for photodegradation kinetics.

2.5. Photocatalytic activity

The photocatalytic activity of the photocatalyst was evaluated by the photodegradation of MV. The light source used was a UV lamp. The reaction was maintained at ambient temperature (303 K). In a typical experiment, aqueous suspensions of dye (40 mL, 2 × 10⁻⁴ M) and 0.08 g of the photocatalyst powder were placed in reaction tubes. Prior to irradiation, the suspension was magnetically stirred in dark to ensure the establishment of an adsorption–desorption equilibrium. The suspension was kept under constant air-equilibrated conditions. At the given intervals of irradiation times, the suspension was measured spectrophotometrically (573 nm) by diluting it four times to keep the absorptions within the Beer–Lambert law limit.

2.6. Characterization methods

High-resolution scanning electron microscopy and elementary dispersive X-ray analysis experiments were carried out on a FEI Quanta FEG 200 instrument with EDX analyzer facility at 25 °C. The sample was prepared by placing a small quantity of the prepared nanocomposites on a carbon coated copper grid and allowing the solvent to evaporate. X-ray diffraction spectra was recorded on the X'PERT PRO model X-ray diffractometer from Pan Analytical instruments operated at a voltage of 40 kV and a current of 30 mA with Cu Kα radiation. Fourier transform-Raman spectra were recorded with an integral microscope Raman system RFS27 spectrometer equipped with a 1024 × 256 pixels liquefied nitrogen-cooled germanium detector. The 1064 nm line of the Nd:YAG laser (red laser) was used to excite the sample. To avoid intensive heating of the sample, the laser power at the sample was not higher than 15 mW. Each spectrum was recorded with an acquisition time of 18 s. Photoluminescence spectra at room temperature were recorded using a Perkin-Elmer LS 55 fluorescence spectrometer. Nanoparticles were dispersed in chloroform and excited using light of wavelength 300 nm. The UV-visible diffuse reflectance spectra of nanomaterials were recorded with a UV-3600 SHIMADZU (Japan) spectrometer in the range of 800–200 nm. To determine the textural properties of the synthesized materials, N₂ adsorption–desorption measurements were carried out at 77 K using a Micromeritics ASAP 2020 V3.00 H instrument. X-ray photoelectron spectroscopy was performed on a monochromatic Al Kα source instrument (Omicron Nanotechnology, GMBH, Germany) operating at 14 kV and 20 mA for an X-ray power of 280 W. Spectra were collected with a photoelectron take-off angle of 90° from the sample surface plane with the energy steps of 0.10 eV and pass energy of 20 eV. The spectra were referenced to the binding energy of C (1s) (285 eV). The BET method was utilized to calculate the specific surface area of the materials. By using the Barrett–Joyner–Halenda (BJH) model, the pore size distribution was derived from the desorption branches of the isotherms at a relative pressure (P/P₀) of 0.98. Cyclic voltammograms (CVs) were performed by using a CHI 604C electrochemical analyzer (CHI Instruments Inc., Austin, TX). A conventional three-electrode cell was used, including an Ag/AgCl (saturated KCl) electrode as the reference electrode, a Pt wire served as a counter electrode, and a glassy carbon electrode (GCE) coated with synthesized AC-Bi/ZnO as a working electrode. UV-Vis (ultraviolet and visible light) absorbance spectra were measured over a range of 800–200 nm with a Shimadzu UV-1650PC recording spectrometer using a quartz cell with a 10 mm optical path length.

3. Results and discussion

3.1. Surface morphology and elemental analysis

The HR-SEM images of the Bi/ZnO and AC-Bi/ZnO nanocomposites are shown in Fig. 2a and b respectively. The HR-SEM images revealed that individual particles comprised a collection of elongated particles of various sizes and shapes. The HR-SEM images of the Bi/ZnO shows an average particle size of 80 nm and AC-Bi/ZnO shows an average particle size of 30 nm. The abovementioned observation that the particle size of Bi/ZnO was greater than that of AC-Bi/ZnO suggested that in the absence of AC, Bi/ZnO aggregates more easily during the calcination process. AC acted as a barrier, which controlled the growth of Bi/ZnO particles and prevented their aggregation. The aggregation of particles would inevitably reduce their total surface area exposed to the outer environment, leading to a lowered photocatalytic activity as observed in many reaction systems.¹⁸ Moreover, the decrease in particle size can be correlated with the observed increase of the surface area. This can be further discussed in BET surface area analysis. The surface properties, undoubtedly, have a great influence on the photocatalytic activity.⁵² AC-Bi/ZnO is characterized by larger surface area, increased oxygen vacancy, increased absorption of light intensity and enhanced photocatalytic activity. EDX analysis confirms Zn, Bi and O are present in Bi/ZnO (Fig. 2c), whereas Zn, Bi, C and O are present in AC-Bi/ZnO composite material (Fig. 2d). EDX analysis clearly displays intense peaks between 2.0 keV and 2.5 keV corresponding to bismuth L1 (2.3 keV), which is similar to the major constituents of Bi/ZnO and AC-Bi/ZnO.

Fig. 2 HR-SEM images of (a) Bi/ZnO, (b) AC-Bi/ZnO nanocomposite material, EDX analysis of (c) Bi/ZnO, (d) AC-Bi/ZnO nanocomposite material.

3.2. XRD analysis

The obtained XRD of the ZnO, Bi/ZnO and AC-Bi/ZnO nanocomposites are shown in Fig. 3a–c, respectively. The peaks at 31.7°, 34.3°, 36.2°, 47.4°, 56.5°, 62.8°, and 67.8° are from the diffractions for ZnO (100), (002), (101), (102), (110), (103) and (112) crystal planes, respectively (JCPDS no. 36-1451). The relatively high intensity of the (101, 002 and 100) peaks are an indication of the anisotropic growth and implies a preferred orientation of the crystallites.^53,54 The peaks at 22.5°, 24.8°, 30.1°, 33.6°, 36.2°, 40.4°, 47.2°, and 56.1° are the diffractions of the Bi (003), (101), ZnO (100), (002), (101), Bi (110) and ZnO (102), (110) crystal planes, respectively. It confirms that Bi³⁺ was present in ZnO lattice⁴⁰ (JCPDS no. 36-1451, 44-1246 and 79-0206).

Fig. 3 XRD patterns of (a) ZnO nanomaterial, (b) Bi/ZnO nanocomposite material and (c) AC-Bi/ZnO nanocomposite material.

AC-Bi/ZnO peaks at 26.5°, 27.1°, 31.9°, 34.6°, 36.1°, 44.8°, 50.2°, 56.1° and 60.2° are the diffractions of the C (300), Bi (012), ZnO (100), (002), (101), Bi (015), C (511), ZnO (110), C (161) crystal planes, respectively. It shows Bi and C are present in ZnO material (JCPDS no. 36-1451, 44-1246 and 50-0927). As shown in Fig. 3, all the strong peaks can be indexed as the pure hexagonal phase of wurtzite-type ZnO, which agrees well with the reported data (JCPDS no. 36-1451 and 79-0206). These results indicate that AC-supported Bi-doped ZnO have a hexagonal phase of wurtzite structure. The sharp peaks of ZnO indicate the high degree of crystallinity (Fig. 3a). AC-Bi/ZnO decreases the peak intensity of the (101), (001) and (100) diffraction planes, which indicate AC act as barrier and Bi³⁺ in the lattice of ZnO. Fig. 3b and c show that the diffraction peaks (002) are slightly left-shifted, which is mainly due to the larger radius of Bi³⁺ versus that of Zn²⁺. This observation suggests that a portion of Bi³⁺ ions was incorporated into the ZnO lattice⁵⁵ by replacing Zn²⁺ with Bi³⁺.

The average crystalline size (L) of the ZnO, Bi/ZnO and AC-Bi/ZnO particles are calculated by the Debye–Scherrer formula,⁵⁶ L = 0.89λ/β cos θ, where L is the crystalline size (in nm), λ is the wavelength (in nm), β is the full width at half maximum intensity (FWHM-in radian), and θ is Bragg diffraction angle (°). The average crystalline size of ZnO and Bi/ZnO composite was figured out to be about 92.5 and 78.5 nm. The average crystalline sizes of AC-Bi/ZnO composite are almost 29.5 nm.

3.3. FT-RAMAN analysis

The Raman spectra of ZnO, Bi/ZnO and AC-Bi/ZnO are shown in Fig. 4. The characteristic peaks at 212, 380, 438, 569, and 1150 cm⁻¹, which correspond to the 2TA; 2E₂ (low), A₁ (TO), E₂ (high), A₁ (LO) and 2A₁ (LO), 2E₁ (LO); 2LO fundamental phonon modes of ZnO, respectively. The Raman peak at 438 cm⁻¹ is attributed to the ZnO nonpolar optical phonons of high-E₂ mode,⁵⁷ which is one of the characteristic peaks of wurtzite ZnO (Fig. 4a).

Fig. 4 FT-RAMAN spectra of (a) ZnO nanomaterial, (b) Bi/ZnO nanocomposite material and (c) AC-Bi/ZnO nanocomposite material.

Bi/ZnO showed peaks at 229, 549, 592, 912, 1080, 1472 cm⁻¹ (Fig. 4b). The characteristic peaks are at 229, 549, and 592 cm⁻¹, which correspond to the shifted peaks of 2 TA; 2E₂ (low), A₁ (LO) and E₁ (LO), respectively. The peak at 229 cm⁻¹ attributed to the 2TA; 2E₂ (low) mode is one of the characteristic peaks of the Bi–O bonds. The band at 549 cm⁻¹ corresponds to the A₁ symmetry with LO modes. The A₁ (LO) became a sharp peak and shifted about 20 cm⁻¹ towards lower energy. It is generally accepted that the A₁ (LO) peak caused by polar branches appears at about 549 cm⁻¹. Bi was present in the ZnO nanomaterial is confirmed from the abovementioned observation. AC-supported Bi/ZnO shows peaks at 232, 323, 441, and 1580 cm⁻¹ (Fig. 4c). The shifted peaks at 232 and 323 cm⁻¹ are attributed to the 2 TA; 2E₂ (low) and E₂ (high)–E₂ (low) mode, which is one of the characteristic peaks of Bi–O bonds. The shifted peak at 441 cm⁻¹ is attributed to the high-E₂ mode, which is one of the characteristic peaks of wurtzite ZnO. Above 400 cm⁻¹, the intensities of the peaks are increased, and several small peaks also observed. The high intensity of the 1580 cm⁻¹ was assigned to G-band, which corresponds to carbon present in the Bi/ZnO composite.

3.4. PL analysis

The photoluminescence spectra of ZnO, Bi/ZnO and AC-Bi/ZnO are shown in Fig. 5a–c, respectively. As photoluminescence occurs due to electron–hole recombination, its intensity is directly proportional to the rate of electron–hole recombination.⁵⁸ ZnO showed four emissions at 400, 436, 480 and 530 nm. Bi/ZnO showed four emissions at 401, 438, 482 and 531 nm. AC-Bi/ZnO also showed four emissions at 401, 438, 487 and 531 nm. AC-supported Bi-doping of ZnO slightly shifts the emission of ZnO. AC-Bi/ZnO shows a near UV emission band at 401 nm and a blue-green band at 487 nm. However, the PL intensity at 401 nm is lower than that of the synthesized ZnO. This near UV emission corresponds to the exciting recombination that is related to the near-band edge emission of the ZnO nanomaterial.^59–62 The blue-green emission is due to surface defects in the ZnO.⁶² A reduction of PL intensity at 401 nm by AC-Bi/ZnO compared to synthesized ZnO and Bi/ZnO indicates the suppression of the recombination of the photogenerated electron–hole pairs by AC-Bi/ZnO. This leads to a higher photocatalytic activity of AC-Bi/ZnO nanocomposite materials.

Fig. 5 PL spectra of (a) ZnO nanomaterial, (b) Bi/ZnO nanocomposite material and (c) AC-Bi/ZnO nanocomposite material.

3.5. UV-Vis-DRS analysis

The UV-Vis-DRS spectra of ZnO, Bi/ZnO and AC-Bi/ZnO are shown in Fig. 6. The direct band gap of the synthesized materials has been determined from the Tauc plots. The plots of [F(R)hν]² versus the photon energy (hν) provide the direct band gap of the synthesized ZnO, Bi/ZnO and AC-Bi/ZnO as 3.19, 3.15 and 3.12 eV, respectively. UV-Vis-DRS results demonstrated a decrease in the direct band gap of AC-Bi/ZnO compared to ZnO and Bi/ZnO. These results reveal AC-Bi/ZnO will be useful as an effective photocatalyst. In addition, the UV-Vis spectrum in the diffuse reflectance mode (R) was transformed to the Kubelka–Munk (KM) function, F(R), to separate the extent of light absorption from scattering.⁶³ F(R) values have been calculated from the reflectance (R) by the application of the Kubelka–Munk algorithm: [F(R) = (1 − R)²/2R].^64,65 The KM plot (Fig. 6d) shows that AC-Bi/ZnO has a strong absorption in the UV region of 200–400 nm, indicating more photocatalytic activity in UV region. This reveals that AC-Bi/ZnO can be used as an UV light active photocatalytic material. Strong UV light absorption may lead to an improved generation of electron–hole pairs, which enhances the photocatalytic activity of AC-Bi/ZnO.

Fig. 6 Direct band gap of (a) AC-Bi/ZnO, (b) Bi/ZnO, (c) ZnO and (d) plot of Kubelka–Munk function versus the energy of light absorbed by the catalysts.

3.6. XPS analysis

The composition and chemical states of the elements in AC-Bi/ZnO were examined by X-ray photoelectron spectroscopy. The typical X-ray photoelectron survey spectrum of the AC-Bi/ZnO indicates that the material consists of Zn, Bi, O and C (Fig. 7). The O 1s profile is asymmetric and can be fitted to two symmetrical peaks α and β located at 530.5 and 533.5 eV, respectively, indicating two different types of O species in the sample. These peaks should be associated with the lattice oxygen of ZnO and chemisorbed oxygen caused by the surface hydroxyls (OH).⁶⁶ The C 1s peak is attributed to the activated charcoal present in the synthesized material.⁶⁷ The XPS spectra of Zn 2p, and the peak positions of Zn 2p_1/2 and Zn 2p_3/2 are located at 1045.5 eV and 1019.9 eV. Compared to the peak positions to those in the Handbook of X-ray Photoelectron Spectroscopy,⁶⁸ we can conclude that Zn is in the state of Zn²⁺. The peaks of Bi 4f_5/2 and Bi 4f_7/2 at 158.9 and 164.2 eV, respectively, reveal that bismuth is in the Bi³⁺ state.^69,70

Fig. 7 XPS survey spectrum of AC-Bi/ZnO nanocomposite material.

3.7. BET surface area analysis

The corresponding N₂ adsorption–desorption isotherms and BJH desorption pore distribution of all the investigated catalysts are shown in Fig. 8. The surface area of ZnO, Bi/ZnO and AC-Bi/ZnO was determined using the nitrogen gas adsorption method. The N₂ adsorption–desorption isotherms of the catalysts exhibited a hysteresis loop, which is typical of a type II pattern representing the predominant nonporous structure according to the international union of pure and applied chemistry (IUPAC) classification.⁷¹ A sharp increase in the adsorption volume of N₂ was observed and located in the P/P₀ at 0.98. This sharp increase can be attributed to the capillary condensation, indicating the good homogeneity of the sample and a macro pore size since the P/P₀ position of the inflection point is related to the pore size.⁷² The pore size distribution of ZnO, Bi/ZnO and AC-Bi/ZnO are given in inset Fig. 8a–c, respectively. The BET surface and total pore volume of the catalysts was given in Table 1. The BET surface area of AC-Bi/ZnO (19.4 m² g⁻¹) is higher than Bi/ZnO (15.3 m² g⁻¹) and bare ZnO (8.3 m² g⁻¹). It is clear that Bi³⁺ doping is effective for changing the morphology of ZnO, and big particle size leads to decrease in the BET surface area, this result agrees well with the result of BET surface area. The higher surface area of the AC-Bi/ZnO enhances the photocatalytic activity.

Fig. 8 N₂ adsorption–desorption isotherms of (a) ZnO (b) Bi/ZnO and (c) AC-Bi/ZnO (inset: BJH desorption pore distribution).

Table 1 Surface properties of the materials

Properties	ZnO	Bi/ZnO	AC-Bi/ZnO
BET surface area	8.3 m^2 g^−1	15.3 m^2 g^−1	19.4 m^2 g^−1
Total pore volume (single point)	0.026 cm^3 g^−1	0.12 cm^3 g^−1	0.16 cm^3 g^−1

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3.8. Electrochemical enhancement study

Photocatalysis is a surface phenomenon as the photogenerated charge carriers diffuse to the surface to initiate redox reactions.⁷³ A modified electrode was constructed by ZnO and AC-Bi/ZnO via the mechanical attachment. Fig. 9 shows the effect of using unmodified/modified GCE on the electrochemical oxidation/reduction potential of potassium chloride (KCl) based on the data from cyclic voltammetry. When the GCE was not modified with any photocatalyst, there is hardly any enhancement or decrement in the cycles. With ZnO modified GCE, it was observed that the electrochemical oxidation of KCl is an irreversible process due to the peak (anodic current and the (E_pa) peak potential of 0.931 V). However, with AC-Bi/ZnO modified GCE, the absence of a well-defined reduction GCE was observed, showing that the electrochemical redox reaction of KCl is a reversible process (enhanced anodic and cathodic current and the (E_pa) peak potential of 0.547 V and 0.245 V). These results indicated that AC-Bi/ZnO modified GCE had larger adsorption–desorption and higher electrochemical response than that of ZnO. Several literature reports have indicated that GCEs modified with metal ion and carbon materials were found to perform better in comparison to bare GCE.^63,74,75 This suggests that the presence of AC and Bi in the ZnO could increases current and improves the relative electron transfer.

Fig. 9 CVs of (a) uncoated GCE with 0.1 M KCl (green curve), (b) ZnO coated GCE in 0.1 M KCl (blue curve) and (c) AC-Bi/ZnO coated GCE in 0.1 M KCl (red curve).

3.9. Hydroxyl radical analysis

The photocatalytic activity of the as-synthesized materials was further confirmed by the detection of ˙OH. In order to confirm the existence of hydroxyl radicals, the formed hydroxyl radicals on the surface of AC-Bi/ZnO illuminated by UV light were detected by PL techniques. The PL emission spectra excited at 330 nm in a coumarin solution suspended with AC-Bi/ZnO were measured at different time intervals of 0, 30 and 60 min. Fig. 10a displays that PL signals were observed at 391 to 522 nm. The main peak is located at about 391, 398, 404, 417, 447 and 522 nm. The occurrence of the emission peaks observed in the UV-visible region is attributed to the electronic transitions mediated by the defect levels such as oxygen vacancies in the band gap.^76,77 Maximum PL intensity was found for AC-Bi/ZnO at 60 min (Fig. 10b). This suggests that fluorescence is caused by the chemical reactions of coumarin with hydroxyl radicals formed in photocatalytic reactions.⁵¹ Hence, hydroxyl radicals are the reactive oxidation species in the AC-Bi/ZnO sample and finally induce the degradation of MV. Moreover, the AC-Bi/ZnO with maximal photocatalytic activity produced much more reactive hydroxyl radicals.⁷⁸ The increase of ˙OH formation on AC-Bi/ZnO can be caused by the better separation of free carriers retarding the recombination reactions in the semiconductor through the capture of electrons by AC-Bi/ZnO and enhanced yields of reaction between photogenerated holes and adsorbed water molecules on the photocatalyst surface.

Fig. 10 PL spectra of AC-Bi/ZnO under excitation at 330 nm in coumarin solution (a) UV-light irradiation of AC-Bi/ZnO at 60 min and (b) UV-light irradiation of AC-Bi/ZnO obtained using different time intervals (0, 30 and 60 min).

3.9.1. Photocatalytic performance of the AC-Bi/ZnO nanocomposite material. Dye effluents from textile industries are becoming a severe environmental trouble because of their high chemical oxygen demand content, resistance to chemicals, and unacceptable colour, photochemical and biological degradation.⁷⁹ We have chosen the photodegradation of MV as model dye with UV irradiation to evaluate the photocatalytic activity of the proposed photocatalyst. Fig. 11 shows the time course of decrease in the absorbance of MV under UV light irradiation. The photodegradation process of MV, the UV-Vis spectra and the colours of MV aqueous solution as a function of UV-light irradiation time in the presence of the photocatalysts (ZnO, Bi/ZnO and AC-Bi/ZnO) are illustrated in Fig. 11a–c. It can be seen from the UV-visible spectra changes that the strong adsorption peak of MV solution at 573 nm steadily decreased and degraded with increasing light irradiation time, and the initial violet colour of the solution gradually became lighter. The comparative studies of the photocatalytic activity of ZnO, Bi/ZnO and AC-Bi/ZnO photocatalysts for photodegradation as well as decolourization of MV is shown in Fig. 11d. AC-Bi/ZnO exhibited excellent photocatalytic activity for MV under UV-light irradiation compared to that of ZnO and Bi/ZnO.

Fig. 11 Absorption spectra changes of methyl violet solution (2 × 10⁻⁴ M, 40 mL) in the presence of photocatalysts (a) ZnO, (b) Bi/ZnO and (c) AC-Bi/ZnO under UV-light (365 nm) irradiation at 10 minute interval. (d) Comparative studies of the photocatalytic activity (absorbance vs. time (min)) of ZnO, Bi/ZnO and AC-Bi/ZnO photocatalysts for the photodegradation and decolourization of MV under UV-light (365 nm) irradiation at 10 minute interval.

Fig. 12a shows the time course of the percentage decolourization of MV under UV-light irradiation. After 60 min of irradiation time, AC-Bi/ZnO can decolourize the MV in aqueous solution up to 94.5% compared to that of ZnO and Bi/ZnO (66.5% and 82.5%). Under dark conditions, initially there is a decrease in dye concentration with AC-Bi/ZnO and this is due to the adsorption of the dye on the catalyst. Under UV light irradiation, the degradation of MV occurred with all the catalysts. However, AC-Bi/ZnO exhibited higher photocatalytic activity than ZnO and Bi/ZnO photocatalysts. Fig. 12b shows the time course of the decrease in the concentration of MV under UV-light irradiation. Initially the MV dye concentration was 2 × 10⁻⁴ M after photodegradation takes place with respect to time. The blank run (without UV light) was 30 minutes 1.4 × 10⁻⁴ M than UV-irradiation takes at 0 to 60 minutes. After 60 minutes of irradiation time AC-Bi/ZnO can photodegrade MV in aqueous solution up to 5.4 × 10⁻⁵ M compared to that of ZnO and Bi/ZnO (7.4 × 10⁻⁵ M and 6.4 × 10⁻⁵ M). An increase in the irradiation time (0 to 60 minutes) with respect to a decrease the dye concentration can be seen; therefore, AC-Bi/ZnO shows higher photocatalytic activity. As shown in Fig. 12, no significant photodegradation of MV was observed in the absence of AC and Bi, which indicates that the photosensitizing effects of MV could be ignored in this case. It was also found that AC-Bi/ZnO showed a much higher photocatalytic efficiency than ZnO, and exhibited the best photocatalytic performance for MV photodegradation.

Fig. 12 (a) Comparison of the photocatalytic activity (dye decolouration (%) vs. time (min)) of decolouration of MV dye by ZnO, Bi/ZnO and AC-Bi/ZnO under UV-light (365 nm) irradiation at 10 minute interval. (b) Comparisons of the photocatalytic activity (concentration (M) vs. time (min)) of decolouration of MV dye by ZnO, Bi/ZnO and AC-Bi/ZnO under UV-light (365 nm) irradiation at 10 minute intervals.

Fig. 13a was evaluated by measuring the time dependent degradation efficiency of MV in an aqueous catalyst suspension under UV light exposure. Because MV itself is a dye which is active to UV-light, its photocatalytic activity is in the order of AC-Bi/ZnO > Bi/ZnO > ZnO. The photodegradation kinetics of MV on ZnO, Bi/ZnO and AC-Bi/ZnO were evaluated using the pseudo-first-order model:

ln(C₀/C_t) = K_appt (1)

where K_appt is the rate constant [min], C₀ is the initial concentration of MV, and C_t is the concentration of MV at reaction time (t).⁸⁰ From the results (Fig. 13b), one can see that AC-Bi/ZnO showed the highest degradation rate constant, approximately 1.098 × 10⁻⁴ s⁻¹, which was greater than that of ZnO (∼0.822 × 10⁻⁴ s⁻¹) and Bi/ZnO (∼0.958 × 10⁻⁴ s⁻¹).

Fig. 13 (a) Kinetic of methyl violet degradation upon irradiation and (b) photodegraded efficiencies for the ZnO, Bi/ZnO and AC-Bi/ZnO, variations in ln(C₀/C) as function of irradiation time and linear fits of catalysts.

For AC-Bi/ZnO, it is generally accepted that the photocatalytic activity is initiated by the photogeneration of electron–hole pairs. The ˙OH radicals, produced by an electron and the holes, are the primary oxidative species in a photocatalytic degradation and decoloration reaction, and they attack azo groups causing the decomposition of chromophores.^81–83 The photocatalytic process is influenced by various factors, in which specific surface area and the transport properties of photoinduced charge carriers are two key factors.⁸⁴ AC-Bi/ZnO is quite active for photocatalytic MV degradation, because it has a larger BET surface area (19.4 m² g⁻¹) than the prepared ZnO (8.3 m² g⁻¹) and Bi/ZnO (15.3 m² g⁻¹) to facilitate a more efficient reaction of the AC-Bi/ZnO with organic dye contaminants; thus, resulting in enhanced photocatalytic activity. UV-Vis-DRS (Fig. 6d) shows AC-Bi/ZnO has strong absorption in the UV region of 200–400 nm; moreover, there is a slight increase in absorbance in visible region. Hence, AC-Bi/ZnO can be used as an UV light active semiconductor photocatalytic material. Therefore, an experiment was conducted to test its photocatalytic activity under solar light (sunny days between 11 am and 12 pm). The irradiation of MV solution (2 × 10⁻⁴ M) with a borosilicate glass tube of 50 mL capacity (40 mL of MV solution with 0.08 g of catalyst) was performed for 1 hour, under the same condition as was discussed for UV-light. Solar light produced 70% decolourization (degradation at 7.0 × 10⁻⁵ M). 94.5% decolourization (degradation at 5.4 × 10⁻⁵ M) was obtained with UV light. AC-Bi/ZnO shows higher photocatalytic activity due to the reduced band gap energy, smaller particle size, electron hole recombination and enhanced surface area, which are the determining factors of enhanced photocatalytic efficiency.

3.9.2. The mechanism of photocatalytic effect of the AC-Bi/ZnO nanocomposite material. The AC-supported Bi-doped ZnO nanocomposite material absorbs UV light and an electron from its valence band (vb), gets excited to the conduction band (cb) and generates a positively charged hole in the valence band (h_vb⁺) and negative charge in the conduction band (e_cb⁻), according to the eqn (2).

The chemisorbed H₂O molecules interact with the valence band holes forming ˙OH radicals (eqn (3)), which attack the dye molecules successively to obtain degradation. Furthermore, the conduction band electrons e_cb⁻ interact with dissolved O₂, producing superoxide radical anions (O₂˙⁻) as shown in eqn (4). On the other hand, h_vb⁺ could interact with donor ⁻OH and HO₂˙ forming ˙OH radicals that attack the dye as in eqn (5)

AC-Bi/ZnO + hv → h_vb⁺ + e_cb⁻ (2)

(H₂O → H⁺ + OH⁻) + h_vb⁺ → H⁺ + ˙OH (3)

O₂ + e_cb⁻ → (O₂˙⁻) + (H⁺ + OH⁻) → HO₂˙ + ⁻OH (4)

HO₂˙ + ⁻OH + h_vb⁺ → ˙OH (5)

Thus, the photocatalytic cycle comprises three steps (i) illumination induces a transition of electrons from the valance band (VB) to the conduction band (CB), leaving an equal number of vacant sites (holes), (ii) the excited electrons and holes on Bi ion migrate to ZnO surface, facilitating the charge separation and higher photocatalytic efficiency. This charge separation is also revealed by the reduction of the PL intensity of the proposed catalyst compared to ZnO. This process not only facilitates the charge separation, but also accumulates electrons and holes in Bi and ZnO, respectively. If large proportions of electron hole pairs recombine, the input energy is dissipated in the form of heat or emitted light. To prevent the recombination of electron–hole pairs, the AC-supported Bismuth on the ZnO surface acts as a barrier (Fig. 14). Effective electron–hole pair separation at the heterojunction interfaces can significantly improve the photocatalytic performance of the composite materials.^73,85 The hetero-junctions that are formed between the AC-supported ZnO and Bi provide an internal electric field that facilitates the separation of the electron–hole pairs and induces faster carrier migration. (iii) The ˙OH radical was commonly recognized as the main reactive species responsible for the degradation of organic dyes.^86,87 The photogenerated electrons could react with the oxygen molecules adsorbed on the surface of AC-Bi/ZnO to yield O₂˙⁻. On the other hand, the photogenerated holes react with H₂O molecules to produce ˙OH. O₂˙⁻, ˙OH and photogenerated holes to degrade the reactive dyes absorbed on the surface of AC-Bi/ZnO. Thus, they often enhance the photocatalytic activity.

Fig. 14 Schematic illustration of AC supported Bi-doped ZnO mechanism for the photocatalytic process.

4. Conclusion

An AC-supported Bi-doped ZnO nanocomposite material was synthesized by a precipitation method, and characterized by HR-SEM with EDX, XRD, FT-RAMAN, PL, DRS, XPS, BET and CV analysis. The results confirmed the formation of the AC-Bi/ZnO nanocomposite material. HR-SEM and XRD analysis of ZnO and Bi/ZnO showed an average particle size of 78.4 nm. However, AC-Bi/ZnO showed an average particle size of 29.5 nm. The decrease in particle size can be correlated with the observed increase of the surface area. XRD, EDX and XPS reveal the presence of Zn, O, Bi and C and their oxidation states in the AC-Bi/ZnO. In the XRD spectra, all strong peaks are indexed as the pure hexagonal phase of wurtzite structure. FT-Raman spectra reveal the optical phonons of high-E₂ mode, which corresponds to the wurtzite ZnO structure. PL spectra confirm the suppression of the recombination of the photogenerated electron–hole pairs by the AC-Bi/ZnO nanomaterial. UV-Vis-DRS demonstrated a decrease in the direct band gap of AC-Bi/ZnO compared to ZnO and Bi/ZnO. The BET surface area of AC-Bi/ZnO is higher than Bi/ZnO and ZnO. The higher surface area of the AC-Bi/ZnO enhances the photocatalytic activity. AC-Bi/ZnO has potential to be used as a mediator for electrochemical analysis due to the increases in current and improvement of relative electron transfer. AC-Bi/ZnO caused the retarding of the recombination reactions, which occurs after the excitation of semiconductors with UV light. An increase of the OH radicals formation during UV irradiation in AC-Bi/ZnO was observed due to the better separation of free carriers; however, there was no linear relation of the photocatalyst ability to OH radical formation on AC and Bi in ZnO surfaces. AC-Bi/ZnO reveals enhanced photocatalytic activities as compared to ZnO and Bi/ZnO for the photodegradation and decolourization of MV under UV-light irradiation for 0 to 60 minutes. The mechanism of dye degradation is proposed for the higher photocatalytic activity of AC-Bi/ZnO.

Conflict of interest

The authors declare no competing financial interest.

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