Introduction

Rhodamine B (RhB) is a highly water soluble, basic red dye of xanthene class. It is found as a reddish violet powder and comes under the trade name of D and C red no. 19 (molecular formula = C28H31ClN2O3; molecular weight = 479.02). It is widely used as a colorant in textiles and food stuffs. It is also a well-known water tracer fluorescent. It causes irritation to skin, eyes and respiratory tract. The carcinogenicity, reproductive and developmental toxicity, neurotoxicity and chronic toxicity towards humans and animals have been experimentally proven (Jain et al. 2007; Kelesoglu et al. 2012; Dlamini et al. 2011).

The traditional ways of processing wastewater containing dyes through coagulation–flocculation or adsorption by activated carbon are inefficient since they just transfer the dye molecules from liquid to solid. In the past few years, photocatalysis using semiconductor has attracted more as a new and efficient method to treat wastewater containing colorants. It is well documented that TiO2 and ZnO can only absorb a small fraction of sunlight (ultraviolet and near ultraviolet) due to their broad band gap (3.0–3.2 eV) and therefore, it is necessary to modify photocatalysts which can effectively be utilized the solar energy to a greater extent (Kou et al. 2012; Beltran-Heredia et al. 2011).

Cyclodextrins (CDs) are cyclic oligosaccharides composed of 6, 7 and 8 d-glucopyranose rings termed α-, β- and γ-CD, respectively. CDs are shaped like truncated cones, with a hydrophobic cavity and a hydrophilic exterior. The precise number of hydroxyl groups according to the number of glucose units makes it possible to establish strong interactions with other molecules and can form inclusion complexes with guest molecules (Serra-Gomez et al. 2012; Xin et al. 2012). As for the application of CD in environmental chemistry is concerned, it is more interesting and important to know if CD inclusion will change the environmental behavior and the fate of organic pollutants and if there could be any possibility to make use of CD in pollutants control (Zhang et al. 2007). Attarchi et al. prepared Ag/TiO2/β-CD nanocomposite and tested its photocatalytic activity for degradation of methylene blue dye (Attarchi et al. 2013), Yang et al. (2014) reported the self-assembly of bioinspired catecholic cyclodextrin TiO2 heterosupramolecule with high adsorption capacity and efficient visible-light photoactivity, photodegradation of 4,4-bis(4-hydroxyphenyl)valeric acid and its inclusion complex with β-cyclodextrin in aqueous solution (Salomatova et al. 2014) has been reported recently.

We have reported photocatalytic decoloration of dyes with TiO2/β-CD and ZnO/β-CD using visible, UV and solar light and the results were compared with bare TiO2 and ZnO (Velusamy et al. 2011, 2013; Rajalakshmi et al. 2013; Pitchaimuthu et al. 2013; Velusamy et al. 2014). The present study is aimed to investigate the feasibility of solar light for photocatalytic decoloration of RhB dye in different conditions. The effect of various parameters such as initial dye concentration, irradiation time, amount of catalysts and pH have been investigated on the photocatalytic decoloration of RhB in the presence of TiO2/β-CD and ZnO/β-CD as a photocatalyst and the results were compared with bare TiO2 and ZnO. The mineralization of RhB dye was further confirmed by Chemical Oxygen Demand (COD). A decoloration pathway based on GC–MS was studied to confirm the photocatalytic decoloration of RhB dye.

Experimental

Materials

RhB dye obtained from Loba Chemie (India) was used as such. The catalysts viz, TiO2 and ZnO were received from SD fine chemicals, India. β-CD was purchased from Hi Media Chemicals (P) Ltd. All the other chemicals and reagents were used as received.

Samples preparation

Reported procedures were employed for the preparation of RhB/β-CD complex (Pitchumani et al. 1994; Velusamy et al. 2011, 2014). To a saturated solution of β-CD in distilled water, equimolar amount of RhB dye was added and stirred continuously for 24 h. The precipitated inclusion complex was filtered and washed with diethyl ether to remove uncomplexed RhB dye and dried in an air oven at 60 °C for 2–3 h. The precipitate thus obtained is used for the characterization of RhB/β-CD complex by FT-IR and 1H NMR analyses.

Metal oxides (MOs) such as TiO2 and ZnO were added into 0.01 mol/L β-CD solution and stirred for 20 min. The suspended solution was centrifuged; the solids were separated and washed three times with water. The solids were dried with an infrared lamp for UV-DRS, PXRD and FE-SEM analyses (Lu et al. 2004).

Exactly 10 × 10−6 M of RhB dye was prepared using deionised water and various concentrations of β-CD like 1.25, 2.50, 3.75, 5.00, 6.25 and 7.50 × 10−5 M were prepared in a 25 mL SMF. These two solutions were mixed thoroughly with magnetic stirrer for 24 h. Then these samples were analyzed with UV–visible spectrophotometer (Velusamy 1998).

For GC–MS analysis, the photocatalytic decoloration experiments were carried out under different irradiation times and irradiated samples were collected. The dye solution was extracted using CHCl3 as a solvent. The solvent was evaporated. The residue was then used for the analyses.

Characterization

X-ray diffraction patterns of powder samples were recorded with a high-resolution powder X-ray diffractometer model RICH SIERT & Co with CuKα radiation as the X-ray source (λ = 1.5406 × 10−10 m). FE-SEM images were obtained on a Carl ZEISS (ΣIGMA Series, Germany) microscope taken at an accelerated voltage of 2 kV. UV–visible diffuse reflectance spectra were recorded on a Shimadzu 2550 UV–visible spectrophotometer with BaSO4 as the background between 200 and 700 nm. NMR spectra were recorded at 400 MHz Bruker NMR spectrometer (in DMSO–d6). UV–visible spectra were recorded using UV–visible spectrophotometer (Shimadzu UV-1700) and the scan range was from 400 to 800 nm. FT-IR spectra were recorded using FT-IR spectrometer (Shimadzu model 8400S) in the region 4,000–400 cm−1 using KBr pellets. Absorbance of the dye was determined with visible spectrophotometer (ELICO-207). The pH of the dye solution was measured using digital pen pH meter (HANNA instrument, Portugal). The GC–MS spectra was recorded by GC (Perkin-Elmer Auto System) equipped with an HP5-MS capillary column (30 × 250 micron) and 1-μm film thickness (Perkin-Elmer elite series) and interfaced directly to the MS (Perkin-Elmer turbo mass spectrophotometer). The GC column was operated at a temperature of 50 °C for 2 min then increased to 280 °C at the rate of 10 °C/min in split mode. Injection volume 0.1 μL with helium as carrier gas was used. The other analytical conditions are: EI impact ionization 70 eV, injection temperature 280 °C, source temperature 180 °C.

Photocatalytic experiments

All experiments were carried out under identical conditions using Borosilicate beakers as the sample holders. Exactly 50 mL of RhB dye solution with required amount of catalysts and at definite concentration was irradiated under solar light. Prior to irradiation, the reaction mixture was stirred in the dark for 20 min to achieve adsorption saturation. At regular intervals, 5 mL of the sample was taken out and centrifuged to remove the catalysts. Absorbance at 544 nm was measured to determine the concentration of RhB after the photocatalytic decoloration process. The photocatalytic reaction was carried out during the daytime. The light intensity during the photocatalytic reaction was measured using lux meter (between 11.30 AM to 1.30 PM) and found that it was in the range of 944–1,090 lux. COD analyses were done by open reflux method (Alhakimi et al. 2003).

Results and discussion

Characterization of photocatalysts

Optical properties

UV–visible diffuse reflectance spectra of TiO2, ZnO, TiO2/β-CD and ZnO/β-CD give information about the optical properties of the catalysts. The reflectance data reported as (αhν)2(eV)2 values have been obtained by the application of the Kubelka–Munk algorithm. The band gaps of the catalysts have been deduced from the Tauc plot. Figure 1 indicates the plot of (αhν)2(eV)2 vs energy. The extrapolation of the rising segment of (αhν)2(eV)2 to the abscissa at zero F® provides the band gap energy as 3.35 eV for TiO2, 3.27 eV for ZnO, 3.31 eV for TiO2/β-CD and 3.23 eV for ZnO/β-CD nanocomposites. The results showed that the band gap energy of MO/β-CD nanocomposites is lower than the corresponding metal oxides. Therefore, MO/β-CD nanocomposites can be excited to produce more electron–hole pairs under same light illumination which could result in higher photocatalytic activity (Karunakaran et al. 2010).

Fig. 1
figure 1

Diffuse reflectance spectra of a TiO2, b ZnO, c TiO2/β-CD, d ZnO/β-CD

Full size image

Phase structure

The XRD patterns of TiO2/β-CD and ZnO/β-CD nanocomposites are similar to that of TiO2 and ZnO particles (Rajalakshmi et al. 2013). The XRD analysis of TiO2 reveals that the sample exhibits single phase belongs to anatase-type TiO2 which is identified by comparing the spectra with the JCPDS file # 21-1272 (Velusamy et al. 2014; Pitchaimuthu et al. 2013). Diffraction peaks at 25.38º, 37.90º, 48.07º, 53.94º and 55.18º correspond to (101), (004), (200), (105) and (211) planes of TiO2, respectively. The XRD spectra of ZnO and ZnO/β-CD composites provide all of the characteristic peaks which are well matched with that of hexagonal wurtzite ZnO (JCPDS file # 36-1451) (Velmurugan and Swaminathan 2011). The relatively high intensity of the peak for (101) plane is an indicative of anisotropic growth and implies a preferred orientation of the crystallites. The peaks corresponding to the MOs exhibit appreciable differences in MO/β-CD nanocomposites. In particular, the peaks related to β-CD of 2θ values from 12.38º to 12.70º are hidden and the peaks of 2θ values are getting reduced when it is blended with TiO2. In the same way, the peaks correspond to ZnO of 2θ values at 31.74º, 31.76º, 31.78º and 36.18º to 36.36º are getting reduced when ZnO is blended with β-CD. All the above evidences imply that there is a strong binding between β-CD and MOs. This indicates that the incorporation of β-CD on MOs provide no change in the crystalline structure of the bare TiO2 and ZnO. But the main peak intensity of the both TiO2/β-CD and ZnO/β-CD nanocomposites are slightly lower than bare TiO2 and ZnO.

Morphology

The morphology of the samples was investigated by FE-SEM analyses. Figure 2a–e depicts FE-SEM of β-CD, TiO2, ZnO, TiO2/β-CD and ZnO/β-CD nanocomposites, respectively. The surface morphology of pristine β-CD (Fig. 2a) shows its amorphous nature. TiO2 crystals have spherical morphology with uniform particle size (Fig. 2b). ZnO has prismatic morphology with uniform particles containing wurtzite lattice structure (Fig. 2c). Whereas in TiO2/β-CD nanocomposites (Fig. 2d) and ZnO/β-CD (Fig. 2e), two different types of particles are observed. The smaller size particles correspond to β-CD and larger particles correspond to MOs such as TiO2 and ZnO. This indicates that the β-CD molecules are adsorbed on the surface of the metal oxides. It is also observed that the surface of MO nanocomposites (TiO2/β-CD and ZnO/β-CD) is very loosely backed in nature. This kind of surface can provide a better adsorption environment and more active sites for the photocatalytic decoloration reactions.

Fig. 2
figure 2

FE-SEM images of a β-CD, b TiO2, c ZnO, d TiO2/β-CD, e ZnO/β-CD

Full size image

Evidence for complex formation

Dissociation constant measurement

Based on UV-absorption spectral studies (using the Benesi and Hildebrand equation) (Pitchumani et al. 1994; Velusamy 1998), the dissociation constant (K D) is evaluated as 0.378 × 10−5 M (Figs. 3, 4). The value of KD indicated that the dye molecules form strong inclusion complex with β-CD.

Fig. 3
figure 3

UV-visible spectra for complexation of RhB/β-CD a β-CD, b RhB, c 1:1 RhB/β-CD, d 1:2 RhB/β-CD, e 1:3 RhB/β-CD, f 1:4 RhB/β-CD, g 1:5 RhB/β-CD

Full size image
Fig. 4
figure 4

Plot of [C] [S]/Δ OD vs {[C] + [S]} for RhB/β-CD complex

Full size image

FT-IR spectra

FT-IR techniques are not generally utilized for the detection of inclusion compounds because the resultant spectra have super position of host and guest bands. However, in this study RhB dye has some characteristics FT-IR absorption in regions where β-CD does not show absorption in the corresponding regions is used to identify the host–guest interaction (Velusamy et al. 1996). FT-IR spectra of complexation between RhB dye and β-CD (1:1) and its physical mixture (1:1) are presented in Fig. 5. The aromatic –C–H stretching of RhB is observed at 2,977.89 cm−1 is shifted with reduced intensity to 2,972.74 cm−1 in the RhB/β-CD complex. Similarly, the peaks at the frequencies 1,640.17, 1,577.66, 1,640 and 894.91 cm−1 corresponding to the functional groups such as –COOH, –C=C–, –C–N= and –C–O–C– are shifted to 1,706.88, 1,589.23, 1,647.10 and 1,182.28 cm−1, respectively, in the RhB/β-CD complex. It indicates that there is a strong inclusion complex between RhB and β-CD.

Fig. 5
figure 5

FT-IR spectra of a β-CD, b RhB, c 1:1 physical mixture of RhB/β-CD, d 1:1 complex of RhB/β-CD (KBr pellets)

Full size image

1H NMR analysis

Formation of an inclusion complex between RhB and β-CD was also inferred from 1H NMR studies of RhB/β-CD complex. The chemical shifts (in ppm) given for β-CD protons are shifted in RhB/β-CD complex (Fig. 6; Table 1). The upfield shift was occurred for H1 and H3 and H5, which are located in the inner side of β-CD. The changes of chemical shift (Δδ) of H3 and H5 suggested that the RhB dye molecules are encapsulated into the hydrophobic cavity of β-CD. On the contrary, the chemical shifts of H2 and H4 which are located on the outer surface of β-CD and the narrow opening of β-CD, undergo downfield shift by the RhB dye molecules. H6 proton does not undergo any changes. Similarly, for RhB protons there is no change in Hc protons and Ha and Hb protons are shifted to upfield because of the interaction between RhB and β-CD (Table 2). These observations are consistent with the notation that the phenyl ring of RhB dye molecule is suitably encapsulated into the β-CD cavity.

Fig. 6
figure 6

1H NMR spectra of RhB/β-CD inclusion complex (DMSO-d6)

Full size image
Table 1 1H NMR chemical shifts of β-CD protons in RhB/β-CD complex
Full size table
Table 2 1H NMR chemical shifts of RhB protons in RhB/β-CD complex
Full size table

Assessment of photocatalytic activity

Effect of initial concentration

The photocatalytic decoloration of RhB dye was carried out by varying the initial concentration of the dye and fixing all other processing parameters constant. Figure 7 shows that the percentage of decoloration decreases with increase in initial concentration of RhB from 95.0 to 60.0 % for TiO2, 97.0–66.0 % for ZnO, 98.8–76.0 % for TiO2/β-CD and 99.0–84.0 % for ZnO/β-CD nanocomposite. The possible explanation for this behavior is that as the initial concentration of the dye increases (1) the path length of the photons entering into the solution decreases and in low concentration the reverse effect is observed, thereby increasing the number of photon absorption by the catalyst in lower concentration and hence percentage of decoloration decreases. (2) Due to the lack of active sites with the increase in the initial concentration of RhB, the amount of dye adsorbed on the catalyst surface gets increased. Due to this, only fewer photons reach at the surface of photocatalyst. This results decrease in concentration of •OH and O •−2 radicals thereby decrease in photocatalytic decoloration activity (Wang et al. 2008; Aliabadi and Sagharigar 2011; Dong et al. 2010). From the results it was also inferred that the 167 × 10−7 M concentration of RhB dye was decolorized up to 95.0 % using ZnO/β-CD nanocomposite at the optimum reaction conditions such as irradiation time = 120 min; dose = 2 g/L and pH = 3.6.

Fig. 7
figure 7

Effect of initial concentration on photocatalytic decoloration of RhB dye under solar light irradiation time = 120 min; dose = 2 g/L; pH = 3.6

Full size image

Effect of irradiation time

The effect of irradiation time for the removal of RhB dye by semiconductor mediated photocatalysis is shown in Fig. 8. It has been found that with the increase in irradiation time, percentage of decoloration increases from 72.5 to 98.4 % for TiO2, 83.3–99.8 % for ZnO, 87.0–99.9 % for TiO2/β-CD and 92.1–99.9 % for ZnO/β-CD nanocomposite. This is attributed to the fact that, as the irradiation time is increased, the dye molecules have enough time to react with the catalysts and form hydroxyl radicals which accelerate photocatalytic decoloration reaction and also as time proceeds, this concentration gradient decreases due to accumulation of dye molecules on vacant sites and thus saturation stage was almost perceived and hence percentage of decoloration increases (Daneshvar et al. 2003a, b).

Fig. 8
figure 8

Effect of irradiation time on photocatalytic decoloration of RhB dye under solar light irradiation. [RhB] = 4.1 × 10−7 M for TiO2, 16.7 × 10−7 M for ZnO, 41.7 × 10−7 M for TiO2/β-CD, 167 × 10−7 M for ZnO/β-CD; dose = 2 g/L; pH = 3.6

Full size image

Kinetics of photocatalytic decoloration

Reaction kinetics gives enough information about the reaction rates and the mechanisms by which the reactants are converted to the products. Kinetics is determined for all the catalysts as ln C 0/C t vs irradiation time after the photocatalytic decoloration process (Fig. 9). The results showed that the photocatalytic decoloration of RhB obeys apparently pseudo-first-order kinetics and the rate expression is given by

$$\ln \, C_{0} /C_{t} = \, kt$$
(1)

where C t is the concentration of RhB at time t, C 0 is the initial concentration of RhB and k is the pseudo-first-order rate constant (Wang et al. 2009a, b).

Fig. 9
figure 9

Kinetics of photocatalytic decoloration of RhB dye under solar light irradiation

Full size image

Effect of amount of catalysts

Effect of amount of catalysts is an important factor that affects the photocatalytic decoloration reactions. Experiments were performed by varying the amount of catalysts from 0.6 to 7.2 g/L. The results (Fig. 10) showed that the percentage of photocatalytic decoloration increased by increasing the amount of catalysts from 76.5 to 89.5 % for TiO2, 85.0–96.8 % for ZnO, 88.0–98.8 % for TiO2/β-CD and 90.5–99.5 % for ZnO/β-CD nanocomposite. Irrespective of the nature of the dye and photocatalysts, the percentage of removal was found to enhance linearly (certain level) with increase in the dose of the catalyst indicating the heterogeneous regime. This may probably be due to: (1) increase in the extent of dye adsorption molecules on the catalyst surface; (2) increase in the number of surface active sites; (3) enhanced generation of hydroxyl radicals due to increase in the concentration of charge carriers. However, at higher amount of catalyst, the reaction rate tends to decrease which may be attributed to; (1) the deactivation of activated molecules by collision with ground state molecules; (2) the agglomeration of the catalyst particles at higher amount of catalysts, which covers the part of photosensitive area retarding the photon absorption and also the dye adsorption; (3) turbidity at higher amount of catalysts results in the shadowing effect thus decreasing the penetration depth of light irradiation; (4) high degree of scattering by the catalyst particles and increase in the opacity. Hence above a certain level, the additional catalyst amount does not get involved in catalytic activity and further increment in the reaction rate was not observed. Hence, the optimum level of dose is 2 g/L (Daneshvar et al. 2003a, b; Grzechulska and Morawski 2002; Wang et al. 2009a, b; Rauf and Salman Ashraf 2009; Damardji et al. 2009; Chen et al. 2004).

Fig. 10
figure 10

Effect of amount of catalysts on photocatalytic decoloration of RhB dye under solar light irradiation. [RhB] = 4.1 × 10−7 M for TiO2, 16.7 × 10−7 M for ZnO, 41.7 × 10−7 M for TiO2/β-CD, 167 × 10−7 M for ZnO/β-CD; irradiation time = 120 min; pH = 3.6

Full size image

Effect of pH

The pH of the aqueous solution is an important variable in the evaluation of aqueous phase-mediated photocatalytic reactions. The effect of pH on the photocatalytic decoloration of RhB dye was studied under constant experimental conditions such as amount of catalysts (2 g/L), irradiation time (120 min) and concentration of the dye [4.1 × 10−7 M (TiO2); 16.7 × 10−7 M (ZnO); 41.7 × 10−7 M (TiO2/β-CD); 167 × 10−7 M (ZnO/β-CD)]. The experiments were carried out by changing in the initial pH value of the dye from 1 to 12. It has been observed from Fig. 11 that the percentage of decoloration increased considerably with an increase in pH from 78.0 to 92.8 % for TiO2, 86.0–95.8 % for ZnO, 86.9–98.6 % for TiO2/β-CD and 90.0–99.8 % for ZnO/β-CD nanocomposite and it showed percentage of decoloration is higher at strong basic condition than the neutral pH. The reason is under alkaline conditions, the concentrations of perhydroxyl radicals and hydroxide ions are getting increased. During catalyst-based photochemical reactions, both perhydroxyl as well as hydroxyl ions produce large number of hydroxyl radicals in the aqueous medium, which effectively take part in the decoloration of organic pollutants present in RhB dye (Wang et al. 2009a, b; Kansal et al. 2008; Santhi et al. 2011).

$${\text{pH}}\, > \,\,{\text{pH}}_{\text{ZPC}} \quad {\text{M}} - {\text{OH }} + {\text{ OH}}^{ - } \to {\text{MO}}^{ - } + {\text{ H}}_{2} {\text{O}}$$
(2)
$${\text{pH}}\, < \,\,{\text{pH}}_{\text{ZPC}} \quad {\text{M}} - {\text{OH }} + {\text{ H}}^{ + } \to {\text{MOH}}_{2}^{ + }$$
(3)
Fig. 11
figure 11

Effect of pH on photocatalytic decoloration of RhB dye under solar light irradiation. [RhB] = 4.1 × 10−7 M for TiO2, 16.7 × 10−7 M for ZnO, 41.7 × 10−7 M for TiO2/β-CD, 167 × 10−7 M for ZnO/β-CD; irradiation time = 120 min; dose = 2 g/L

Full size image

The pH of the reaction has a significant effect on the surface properties of the metal oxide photocatalysts, which include the surface charge of the particles, the extent of aggregation of the catalyst particles, and the band edge position of metal oxides. The zero charge point (pHZPC) for ZnO is 8.8 and TiO2 is at 6.2. The extent of positive charges on the metal oxide surface decreases with increasing pH and reaches zero at pHZPC. Therefore, pH significantly affects the adsorption–desorption properties of the compounds on the surface of the catalysts. At pH > pHZPC, MO− is the predominant species, whereas at pH < pHZPC, MOH2 + is the predominant species according to the Eqs. (2) and (3).

Determination of COD

As the reduction of COD reflects the extent of photocatalytic decoloration of an organic species, the reduction of COD was studied as a function of pH using solar light irradiation.

Figure 12 presents the comparison between removal of COD of RhB with all the catalysts and shown that the COD decreases with increase in pH and initial COD values (460.6 mg O2/L for TiO2; 458.4 mg O2/L for ZnO; 401.06 mg O2/L for TiO2/β-CD and 346.07 mg O2/L for ZnO/β-CD) were reduced after photocatalytic decoloration process. COD is significantly reduced with ZnO/β-CD nanocomposites up to 18.09 % than other catalysts (Dukkanci et al. 2010).

Fig. 12
figure 12

Determination of COD for photocatalytic decoloration of RhB dye under solar light irradiation. [RhB] = 4.1 × 10−7 M for TiO2, 16.7 × 10−7 M for ZnO, 41.7 × 10−7 M for TiO2/β-CD, 167 × 10−7 M for ZnO/β-CD; irradiation time = 120 min; dose = 2 g/L

Full size image

Role of β-CD for the enhancement of photocatalytic activity of catalysts

A suitable mechanism for the photocatalytic decoloration of RhB with MO/β-CD is proposed as given below (Eqs. 4–12) and also schematically represented in Scheme 1. The RhB dye enters into the cavity of β-CD, which is linked to the MO surface in the equilibrium stage. Since CDs have higher affinity for the MO surface than RhB, β-CD molecules could adsorb on the surface of the MO and occupy the reaction sites β-CD would capture holes on the active surface of MO resulting in the formation of stable MO/β-CD complex (Eq. 4). Hence, the reaction between the reactants such as RhB and MO/β-CD complex should be the key step in the photocatalytic degradation of RhB in MO suspension containing β-CD (Eq. 5) (Zhang et al. 2011). An electron is rapidly injected from the excited RhB to the conduction band (Hoffmann et al. 1995; Macwan et al. 2011; Zhang et al. 2009; Wang et al. 2006a). The enhancement of photocatalytic decoloration of RhB mainly resulted in the enhanced adsorption of RhB on MO surface and moderate inclusion depth of RhB in the β-CD cavity. Since β-CD can include the RhB into its cavity and adsorbed onto the surface of MO, it could play a role as bridge or channel for RhB to get onto the MO surface and accumulate the higher concentrations, which makes the decoloration of RhB more easy and effective in the presence of hydroxyl radicals produced during photocatalytic decoloration processes. MO/β-CD system apparently exhibited declined photoluminescence intensity compared with MO system which indicates that the electron/hole recombination has been suppressed in the MO/β-CD nanocomposite systems. Therefore, the quantum efficiency of MO has been enhanced as the recombination of e −CB /h +VB is getting lowered, which resulted in the generation of more hydroxyl and superoxide radicals (Eqs. 6–9). Thus, the decoloration of the RhB and RhB+• could be accelerated which leads to the formation of mineralized products (Eqs. 10–12) (Wang et al. 2006b; Lu et al. 2004).

$$\text{MO} \; + \;\upbeta\; - \;\text{CD} \; \to \;\text{MO} /\upbeta\; - \;\text{CD}$$
(4)
$$\text{RhB} + \text{MO} /\upbeta - \text{CD} \; \to \;\text{MO} /\upbeta - \text{CD}^{{ \bullet }} \text{RhB}$$
(5)
$$\text{MO} /\upbeta - \text{CD}^{{ \bullet }} \text{RhB} + \text{h} \nu \; \to \;\text{MO} /\upbeta - \text{CD}^{{{ \bullet }1}} \text{RhB} * \, + \text{MO} /\upbeta - \text{CD}^{{{ \bullet }3}} \text{RhB} *$$
(6)
$$\text{MO} /\upbeta - \text{CD}^{{{ \bullet }3}} \text{RhB} * \to \left( \text{e} \right)\text{MO} /\upbeta - \text{CD} + \text{RhB}^{{{ \bullet } + }}$$
(7)
$$\text{MO} /\upbeta - \text{CD}^{{{ \bullet }1}} \text{RhB} * \, + \text{O}_{2 } \to \text{MO} /\upbeta - \text{CD}^{{ \bullet }} \text{RhB} +^{1} \text{O}_{2}$$
(8)
$$\left( \text{e} \right)\text{MO} /\upbeta - \text{CD} + \text{O}_{2 } \,\, \to \,\text{MO} /\upbeta - \text{CD} + \text{O}_{2}^{ - }$$
(9)
$$\text{RhB}^{{ + { \bullet }}} \,\, \to \,\text{Products}$$
(10)
$$\text{RhB} \, +^{1} \text{O}_{2} \, \to \text{Products}$$
(11)
$$\text{RhB} +^{{ \bullet }} \text{O}_{2}^{ - } \,\, \to \,\text{Products}$$
(12)
Scheme 1
scheme 1

Mechanism for the enhancement of photocatalytic decoloration of RhB dye with MO/β-CD systems

Full size image

Photocatalytic pathway for the photodecoloration of RhB dye

Identification of possible intermediate products during the photocatalytic reaction is the best way to understand the photocatalytic decoloration reaction mechanism. To study the photocatalytic decoloration pathway of RhB dye under solar light irradiation as well as to identify the possible intermediate products during the photocatalytic decoloration, experiments were conducted with ZnO/β-CD as a photocatalyst. GC–MS spectra of the irradiated aqueous solutions of RhB at different time intervals were observed with the time interval of 60 min. Major intermediates during the photocatalytic decoloration process were proposed using m/z (M − 1) values of the GC–MS spectra (Fig. 13). Normally, RhB dye was found to be very stable under solar light irradiation in the absence of catalysts. Based on the results of GC–MS spectra, the fragmentation pathway and intermediates for the solar light-induced photocatalytic decoloration of RhB dye have been proposed and shown in Scheme 2. In photocatalytic decoloration of RhB dye three main steps are involved. They are (1) N-deethylation, (2) cleavage of chromophore, (3) mineralization of dye. N-deethylation followed by carboxylation lead to the formation of possible isomerized intermediates corresponding to the m/z value of 281. After carboxylation, the formed intermediate degraded into possible intermediate with m/z values of 267 and 253 and m/z values 121, 165 and 91 indicates that the RhB dye molecule is effectively oxidized by the catalysts into smaller molecules such as benzoic acid, phthalic acid and propane 1,2,3-triol. The formed oxidized products were then mineralized into CO2, H2O, NO3 − and NH4 + as reported in literatures (Kansal et al. 2007; Yu et al. 2009; Sun et al. 2009; He et al. 2009; Luan et al. 2011; Silva et al. 2011; Mehrdad et al. 2011; Li et al. 2007).

Fig. 13
figure 13

GC-MS spectra for the photocatalytic decoloration of RhB dye under solar light irradiation

Full size image
Scheme 2
scheme 2

Photocatalytic decoloration pathway for the photocatalytic decoloration of RhB dye under solar light irradiation

Full size image

Conclusions

In summary, ZnO/β-CD nanocomposite shows superior photocatalytic activities towards photocatalytic decoloration of RhB dye under solar light irradiation, which is due to the lower band gap energy of ZnO in ZnO/β-CD system. Moreover, concentration of RhB, irradiation time, dose of the catalysts used and pH of the reaction medium have significant effects on photocatalytic decoloration of RhB dye. β-CD can include the RhB into its cavity and is adsorbed onto the surface of MO, it could play a role as bridge or channel for RhB to get onto the MO surface and accumulate the higher concentrations, which makes the decoloration of RhB more easily and effectively in the presence of hydroxyl radicals produced during photocatalytic decoloration processes. The GC–MS spectral studies confirm the photocatalytic decoloration pathway of RhB dye which follows the steps such as N-deethylation, cleavage of chromophore and mineralization of dye. Based on the findings, ZnO/β-CD nanocomposite can be used as an efficient photocatalyst for the photocatalytic decoloration of RhB dye.