Introduction

Under the advocacy of green chemistry principles, biobased materials are increasingly employed in synthetic and catalytic fields. In addition, the design of chemical reaction must take into account two important aspects: enhancing the efficiency of single reaction to reduce energy consumption, as well as fabricating new green materials to meet the environmentally-friendly needs (Jung et al. 2015).

Photocatalytic degradation of pollutants is an efficient, attractive, and clean technology for environmental pollution abatement in both aqueous media and gas phase. There has been extensive research done in this area since the discovery of water splitting on semiconductor catalysts in 1972 (Fujishima and Honda 1972; Yang et al. 2013). As a typical n-type semiconductor nanocrystal of group II–VI, crystalline CdS is regarded as one of the most attractive visible light-driven photocatalysts due to its relatively narrow band gap (Eg = 2.42 eV, corresponding well to the spectrum of sunlight) (Dukovic et al. 2010; Hernández-Gordillo et al. 2013; Meng et al. 2017; Zhu et al. 2015). The relevant researches demonstrate that the morphology of CdS has a significant influence on the optical band gap which determines the performance of photocatalytic degradation (Bera et al. 2015; Liu et al. 2017). More recently, organic–inorganic hybrid materials with excellent properties have emerged as a new type of composite and attracted interests from many researchers around the world. For this reason, various CdS based materials with different morphologies and nanostructures (e.g., nanosheets, nanocrystals, nanowires and nanoparticles) were fabricated using inorganic or organic substrates as templates, creating potential applications in many fields including but not limited to photocatalysis (Cao et al. 2010; Chen et al. 2018; Liang et al. 2004; Shi et al. 2004; Zhou et al. 2006). For facilitating the reuse of the photocatalyst, a few studies on immobilization of CdS on inorganic or organic substrates (such as PVP, PEI-PEG, nafion membrane, ZnO layer) have been carried out (Qi et al. 2000; Venkatareddy et al. 2018; Wang et al. 2005, 2018; Zeng et al. 2002).

Under the enlightenment of bio-inspired fabrication (Maghsoudi-Ganjeh et al. 2019), many bio-based materials (e.g., cotton, silk, chitosan) were used as bio-templates for nano-fabrication (Drachuk et al. 2017; Lai et al. 2009; Li et al. 2016). Among these various bio-based materials, BC produced by fermentation of Acetobacter xylinum is a nanofibrous biomaterial and has been modified to fabricate nanocomposite materials for many applications including plant biomimicking, as well as biomedical, electrically conductive, optical, luminescent, proton conductive, antimicrobial, thermo-responsive applications (Qiu and Netravali 2014; Wang et al. 2019). BC has also been widely used as a matrix to fabricate nanocomposite for catalytic application, because it has unique properties such as high Young’s modulus (138 GPa), good tensile strength (> 2 GPa), high purity (free of lignin and hemicellulose), good water holding capacity, high crystallinity and excellent biocompatibility (Kashcheyeva et al. 2019). For example, Zhang et al. fabricated BC@TiO2 nanocomposite by immersing BC blocks into preceramic polymer solution of titanium source for photocatalytic degradation of Rhodamine B (Zhang et al. 2018). In addition, Wei et al. synthesized CoSe2 nanoparticles growing on carbon nanofibers (derived from BC fibers) as an efficient electrocatalyst for hydrogen evolution reaction (Wei et al. 2018). The unique porous 3D nanofibrous structure of BC matrix provided very high specific surface area containing plenty of active hydroxyl groups, which significantly enhanced the metal ion adsorption (Chaiyasat et al. 2019; Chen et al. 2009). Thus, it is feasible for BC to serve as an excellent matrix in the in-situ synthesis of CdS nanoparticles, and the application of BC will simplify the process of nano-fabrication and make the photocatalyst recycling easier. BC@CdS nanocomposite will be an excellent photocatalytic material because it can combine the unique properties of BC with the strong photocatalytic performance of CdS. Due to the existence of BC matrix, it can be expected that the novel adsorbent photocatalytic material will achieve enhanced photocatalytic performance and exhibit better cyclic utilization.

In this study, CdS nanoparticles were synthesized on different BC matrices by a highly efficient microwave-assisted solvothermal in-situ method (quick-heating and reacting, low-energy consumption, Bharti et al. 2018). A possible mechanism was proposed for the growth of CdS nanoparticles on BC and verified by Zeta potential and FT-IR analysis. The morphologies and crystal forms of CdS nanoparticles were also characterized using SEM, XRD and XPS. This study provided a facile, highly efficient wet chemical strategy to synthesize visible light responsive BC@CdS nanocomposite, with its adsorption and photocatalytic performance demonstrated by the degradation of MB. The overall formation mechanism and photocatalytic degradation process of BC@CdS nanocomposite are shown in Fig. 1.

Fig. 1
figure 1

Schematic representation of the formation mechanism and photocatalytic degradation process of BC@CdS nanocomposite

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Experimental

Materials and reagents

The fermentative hydrogel-like BC membranes were kindly provided by Tropical Fruits Processing Co. Ltd, Hainan, China. The BC membranes with average thickness of 3 cm, briefly boiled and treated three times with 0.1 M aqueous NaOH at 90 °C for 20 min to remove the bacteria and their residues, were cut into block-shaped pieces. The as-prepared BC membranes were thoroughly washed by deionized water to neutral. Other reagents were all purchased from Kemiou Chemical Reagent Co. Ltd, Tianjin, China, and used without further purification.

The preparation of different BC matrices

Different BC matrices were employed for the synthesis of CdS nanoparticles, including BC-hydrogel, BC-alcogel, and BC-aerogel.

BC-hydrogel The obtained hydrogel-like BC membranes (purified by the method mentioned above in Sect. 2.1) were cut into small blocks for experimental use.

BC-alcogel The BC-hydrogel was subjected to a solvent exchange in absolute ethanol stepwise to obtain BC-alcogel. Firstly, the BC-hydrogel was immersed in the absolute alcohol (50:50, v/v) for 12 h to obtain 50% BC-alcogel. Subsequently, 50% BC-alcogel was transferred to another fresh absolute ethanol bath for 12 h, then the 75% BC-alcogel was obtained. Lastly, the above step was repeated until the ethanol content rising up to 99%, and the BC-alcogel was prepared.

BC-aerogel The BC-hydrogel was subjected to freeze drying process to obtain BC-aerogel for experimental use.

Synthesis of BC@CdS nanocomposite

CdS nanoparticles were synthesized on different BC matrices (BC-hydrogel, BC-alcogel and BC-aerogel) using a microwave-assisted solvothermal in-situ method, and each BC block contained almost 7 mg dry BC fibers. Firstly, the squeezed BC-hydrogel, BC-alcogel (0.4 MPa, 5 min, Kumagai Rikii Kogyo Co. Ltd, Japan) and BC-aerogel were immersed into beakers containing 10 ml of 5 mmol/L CdCl2 aqueous alcohol solution (3:2, v/v) with continuous stirring (< 100 rpm) until reaching an adsorption equilibrium. Subsequently, the BC matrices were taken out and rinsed with deionized water several times to remove dissociated Cd2+ prior to transferring to another beaker containing 10 mL of 30 mmol/L thiourea aqueous alcohol solution (3:2, v/v). Thirdly, the matrices with thiourea solution were transferred to microwave synthesis reactor (Monowave 100, Anton Paar, Austria) and irradiated for 3 h at 120 °C after adjusting pH value to 9 with 0.1 mol L−1 NaOH aqueous solution. In the process, it was observed that the BC matrices changed to a yellowish color as shown in Fig. 1. Finally, the hybrid nanocomposite was further washed with deionized water and ethanol three times separately and dried overnight by freeze-drying system (alpha 1–4 LD plus, Christ, Germany) for characterization. The mass percentage of CdS was calculated in BC matrix based on the following Eq. 1:

$${w}_{t}\mathrm{\%}(CdS)=\frac{{\mathrm{W}}_{n}-{\mathrm{W}}_{0}}{{\mathrm{W}}_{n}}\times 100\%$$
(1)

In Eq. 1, where Wn is the weight of dry BC@CdS nanocomposite, W0 is the weight of dry BC gels.

Characterization of BC@CdS nanocomposite

The morphology of the BC@CdS nanocomposite was characterized with a Vega-3-SBH (Tescan, Czech) SEM apparatus at 10 kV. Before observation, the surface of the nanocomposite was coated with gold under vacuum. EDS analysis was further performed on this scanning electronic microscopy without gold spraying.

FT-IR spectra of the specimens were collected on a Vertex 70 FT-IR spectrometer (Bruker, Germany) in the range of 4000–400 cm−1 with a scan number of 32 times by using the method of KBr pellets.

The surface charge variation was characterized for BC fibers during synthetic process by a MÜTEK SZP-06 Zeta potential system, and the concentration of BC slurry was 0.5% (w/w) for test after homogeneous processing.

XRD pattern of the nanocomposite was obtained on an Advanced D8 XRD apparatus (Bruker, Germany) equipped with a Cu Kα radiation (k = 1.54 A°) as an X-ray resource, which operated (40 kV, 30 mA) at a scan rate of 0.1 per second in a 2θ range of 10°–60°. Before testing, the specimens were vacuum-dried at 60 °C for 12 h.

X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Thermo Fisher, America) measurements were performed on a PHI-5000 X-ray photoelectron spectrometer with an Al Kα excitation source (100.00 eV).

Thermal stability of the BC@CdS nanocomposite obtained was evaluated using thermogravimetric analyses (TGA). The thermograms were obtained using STA 449 F thermal analyzer (Netzsch, Germany) at a temperature range from 30 to 800 °C with a heating rate of 10 °C/min in N2 flow.

Adsorption performance of BC@CdS nanocomposite

Adsorption capacity

The adsorption measurement was performed with a Lambda 25 UV–Vis spectrophotometer (PerkinElmer, America) at λmax = 664 nm for the color intensity of MB. The concentration of MB was calculated on the basis of the standard curve for MB solution (S-Fig. 1).

The calculations of the adsorption capacity (q) were performed by the following Eq. 2:

$$q=\frac{\left({C}_{0}-{C}_{e}\right)V}{m}$$
(2)

where m is the weight of dry BC, C0 is the initial concentration of the MB solution, Ce is the equilibrium concentration of MB, and V is the volume of the MB solution.

Adsorption kinetics

BC@CdS nanocomposite (1.1 g) was immersed in 20 mL MB solution of 20 mg L−1 at room temperature until the adsorption equilibrium. At regular time intervals, the absorbance value of residual solution was measured and the adsorption capacity was calculated. The adsorption kinetics were calculated and determined in accordance with pseudo-first-order (Eq. 3) and pseudo-second-order kinetic models (Eq. 4) as described below (Duan et al. 2019).

$$\mathrm{ln}\left({q}_{e}-{q}_{t}\right)=\mathrm{ln}\left({q}_{e}\right)-{K}_{1}t$$
(3)
$$\frac{t}{{q}_{t}}=\frac{1}{{K}_{2}{q}_{e}^{2}}+\frac{t}{{q}_{e}}$$
(4)

where K1 is the pseudo-first-order rate constant (mg·g−1·min−1), K2 is the pseudo-second-order rate constant (mg·g−1·min−1), qe (mg·g−1) is the sorption capacity at equilibrium, qt (mg·g−1) is the sorption at the time of t.

Effect of pH value

Initial pH value of MB solution (20 mg L−1) was adjusted by 0.1 mol L−1 HCl or NaOH aqueous solution to change between 1.0 and 13.0 (1.0, 3.0, 5.0, 7.0, 9.0, 11.0,13.0). BC@CdS nanocomposite (1.0 g) was immersed in 20 mL of MB solution at room temperature for equilibrium time.

Adsorption isotherms

BC@CdS nanocomposite (1.1 g) was immersed into 20 mL MB solution with concentrations of 5 mg·L−1, 10 mg·L−1, 15 mg·L−1, 20 mg·L−1 and 25 mg·L−1 at 293 K for equilibrium time. The same procedures were applied at 303 K and 313 K. The adsorption isotherms were modeled by using the Langmuir and Freundlich isotherm models (Cheng et al. 2016).

Photocatalytic degradation activity of BC@CdS nanocomposite

Photocatalytic degradation tests were performed using MB (a representative organic dye) as a model contaminant under visible light irradiation with a Xe lamp. The degradation performance of the specimens was determined by measuring absorbance value (λ = 664 nm) of MB aqueous solution on a Lambda 25 UV–Vis spectrophotometer (PerkinElmer, America), the concentration of MB was calculated on the basis of absorbance of the standard curve (adsorbance to concentration, S-Fig. 1) of MB solution.

The experiments were performed following the steps below: BC and BC@CdS were firstly transferred into test tubes containing MB aqueous solution (20 mL, 20 mg/L). Prior to visible light irradiation, the suspensions were magnetically stirred for 30 min in the dark to reach adsorption/desorption equilibrium. Typically, the total concentration of MB was determined from the maximum adsorption (λ = 664 nm) measurements by UV–Vis spectroscopy. Ct/C0 (Ct and C0 are the concentration of MB at time t and 0 h) was used to describe the degree of MB degradation.

Results and discussion

Analysis of the formation process of CdS nanoparticles on BC matrices

According to Fig. 2a, a possible “anchoring-reacting-forming” pathway is proposed to reveal the formation mechanism of the CdS nanoparticles on BC matrix. In the synthetic process, Cadmium cations (Cd2+) were anchored on BC fibers by ion–dipole interaction (Shim et al. 2002) when BC matrix was immersed into CdCl2 solution. After being rinsed sufficiently to remove unanchored Cd2+, the BC matrix with immobilized Cd2+ was transferred to thiourea solution, and CdS nanoparticles were formed on BC 3D networks when thiourea released S2− with the effect of microwave. It was clear that the semitransparent BC matrix changed to a bright yellowish color (as shown in Fig. 1).

Fig. 2
figure 2

The analysis of forming process of CdS nanoparticles on BC matrix. a Assumptive forming process of CdS nanoparticles; b FT-IR spectra of BC, BC/Cd2+ and BC@CdS nanocomposite; c Zeta potential curve of BC fibers in the process of adsorbing Cd2+

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Figure 2b shows the results of FT-IR spectra for BC, BC/Cd2+ and BC@CdS nanocomposite. The results support the assumptive mechanism of the forming process. It can be seen that the peak at 3390 cm−1 corresponding to –OH stretching vibration declines to lower wavenumber (3343 cm−1) after transferring BC into CdCl2 solution and reaching the adsorption equilibrium, and the area of the peak decreases sharply. When CdS nanoparticles form on BC matrices (curve BC@CdS), the peak corresponding to –OH vibration obviously moves to higher wavenumber (3361 cm−1) with the increasing of the peak area (larger than curve BC/Cd2+ and smaller than curve BC). This indicates that the hydroxyl groups are the main active sites during the in-situ synthesis process. In the process, the hydroxyl groups of BC form strong coordination effect with CdS nanoparticlesMeanwhile, the peaks around 2976 cm−1, 1651 cm−1, 1049 cm−1 corresponding to stretching vibration of C–H, C=O, and C–O–C groups exhibit the same trends with the peak of 3390 cm−1, which reconfirms the coordination effect is the main interaction between BC matrix and CdS nanoparticles.

Secondly, Zeta potential is frequently employed to characterize fibers surface charge, which consists of plentiful surface hydroxyl groups of fibers. Figure 2c shows the Zeta potential variation after adding reagent CdCl2 into BC slurry. Zeta potential of BC fibers rises from − 29.3 mV (containing no Cd2+) to -5.1 mV and finally stabilizes at − 6.3 mV (containing Cd2+ with the concentration of 5 mmol/L). This indicates that Cd2+ ions are adsorbed by hydroxyl groups and anchored by ion–dipole interaction.

The characterization of BC@CdS nanocomposite

The analyses were carried out for characterization of BC@CdS nanocomposite with SEM, EDS, XRD and XPS. The results are shown in Figs. 3, 4, and 5.

Fig. 3
figure 3

SEM images of BC (a), BC@CdS-hydrogel (b), BC@CdS-alcogel (c), BC@CdS-aerogel (d); EDS analysis of BC@CdS nanocomposite (e)

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Fig. 4
figure 4

XRD patterns of BC and BC@CdS-alcogel nanocomposite

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Fig. 5
figure 5

XPS analysis of BC@CdS-alcogel nanocomposite (a–d)

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As is shown in Fig. 3a, pure BC matrix exhibits 3D self-supported structure containing porous and interconnected tunnels, which forms in the self-assemble process of bacterial cultivation. Figure 3b–d show SEM images of different BC@CdS nanocomposite. CdS particles on BC-hydrogel (Fig. 3b) agglomerate together and display inhomogeneous sizes because of the impact of bound-water in BC matrix. CdS particles on BC-alcogel (Fig. 3c) exhibit nanoscale sizes (approximately 100 nm) and distribute homogeneously. Besides, CdS particles on BC-aerogel exhibit much wider and bigger size (about 300–700 nm).

To identify the existence of the element Cd and S in BC@CdS nanocomposite, EDS analysis was proceeded. In Fig. 3e, Cd and S can be observed clearly, demonstrating that CdS nanoparticles are synthesized on BC matrices by in-situ method. Meanwhile, the loading percentage of CdS on different BC matrices is calculated and shown in Table 1. BC-aerogel is loaded with the most quantity of CdS (13.2% ± 0.3%) while BC-alcogel supports less mass of CdS (12.4% ± 0.3%), and BC-hydrogel bears the minimum weight of CdS (10.5% ± 0.3%).

Table 1 The loading percentage of CdS on BC-hydrogel, BC-alcogel and BC-aerogel
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The reasons for this phenomenon are as follows: the bound-water molecule layer on BC fibers has stronger polarity than ethanol molecules layer, which influences the homogeneity and quantity of adsorption of Cd2+ on BC fiber. The stronger polarity causes heterogeneous dispersion of CdS particles and minimizes CdS loading percentage. Meanwhile, the lack of ethanol results in uncontrolled growth of CdS particles on BC-aerogel, leading to much bigger size and increased loading percentage of CdS at the end. It can be concluded that BC-alcogel is the best matrix for synthesis of CdS nanoparticles.

Figure 4 shows three intensive diffraction peaks appearing at 14.6°, 16.9° and 22.8° corresponding to the crystallographic planes of (100), (010) and (110) of BC fibers (French 2014). Due to the presence of electron-rich Cd and sulfur, the peak intensities from the BC are diminished. To check the crystalline phase of CdS on the BC matrix, characteristic peaks from 23° to 60° which contains most of the peaks of CdS are also shown in Fig. 4. BC@CdS-alcogel nanocomposite presents prominent diffraction peaks at 2θ values of 24.8°, 26.5°, 28.2°, 36.6°, 43.7° and 51.8° which is corresponding with the (100), (002), (101), (102), (110), (112) planes of pure CdS standard card (JCPDS Card 41-1049) respectively. All of the diffraction peaks match well with those of the perfect hexagonal wurtzite-structured CdS (Zhou et al. 2019). Furthermore, BC@CdS-alcogel nanocomposite has other diffraction peaks at 26.4°, 43.8° and 51.9° corresponding with the (111), (220), (311) planes of pure CdS standard cards (JCPDS 65–2887). This confirms that the cubic structure of CdS has been synthesized on BC fibers (Li et al. 2009). The results clearly indicate that cubic and hexagonal crystalline phases of CdS nanoparticles coexist in BC-alcogel.

Further evidence was obtained for the high crystal quality by using XPS. As is shown in Fig. 5a and b, the binding energies of both O 1s and C 1s are identified respectively at 532.87 eV and 286.57 eV, which correspond to the glucose chain of BC fibers. From Fig. 5c and d, the binding energies of S 2p3/2, Cd 3d3/2 and Cd 3d5/2 are identified at 161.57 eV, 411.97 eV and 405.27 eV, which are consistent with the reported values in the literature (Xiong et al. 2009; Zhai et al. 2009). These results further confirm that the nanoparticles are pure CdS, which has been proven in the XRD results. According to XPS results, the percentage of Cd atom is 0.85%, the percentage of S atom is 0.61%, and the atomic ratio of Cd to S is calculated to be 1.37, indicating that almost Cd2+ ions are transformed to CdS nanoparticles.

Thermostability of BC@CdS nanocomposite

Thermostability is an important property of nanocomposite for future applications. Figure 6a and b shows TGA and DTG curves of BC and BC@CdS-alcogel nanocomposite. The TGA curves (Fig. 6a) of these two specimens exhibit two stages of weight loss in the temperature range from 30 to 780 °C. The weight loss below 140 °C is attributed to water evaporation. In the temperature range from 220 to 370 °C, a huge weight loss can be observed due to the degradation of cellulose. The decomposition temperature of BC is about 330 °C, whereas the decomposition temperature of BC@CdS-alcogel nanocomposite shifts to a higher temperature due to insertion of CdS. According to DTG curves (Fig. 6b), the BC@CdS-alcogel nanocomposite requires much higher temperature than natural BC when reaching the highest degradation rate. This indicates that CdS nanoparticles are efficiently inserted in BC-alcogel.

Fig. 6
figure 6

TG and DTG curves of BC and BC@CdS-alcogel nanocomposite

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Adsorption performance of BC@CdS nanocomposite

Figure 7a shows a real process and a possible mechanism of photocatalytic degradation of BC@CdS nanocomposite. A new photocatalytic degradation model “adsorption–accumulation–degradation” is proposed. As is shown in Fig. 7a, pure BC just possesses ability of adsorption for MB molecules without degradation after 180 min visible light irradiation. On the contrast, BC@CdS nanocomposite exhibits clear photocatalytic degradation effect on MB. The comparison on maximum equilibrium sorption capacity of different adsorbents for MB is shown in Table 2.

Fig. 7
figure 7

The possible photocatalytic degradation mechanism of BC@CdS nanocomposite (a); the adsorption performance of BC@CdS nanocomposite for MB (b–e)

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Table 2 The comparison on maximum equilibrium sorption capacity of different adsorbents for MB
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The adsorption performance of BC@CdS nanocomposite for MB was also investigated. Figure 7b shows the changes of adsorption capacity of BC@CdS nanocomposite for MB over time. It can be seen that the adsorption capacity increases dramatically and quickly reaches adsorption equilibrium within 30 min, which indicates that there is a strong interaction between adsorbent (BC@CdS nanocomposite) and adsorbate (MB).

Figure 7c shows the adsorption kinetic curve of the nanocomposite. The constants of the kinetic process are calculated and shown in Table 3. It is concluded that the linear correlation coefficient (R2 = 0.99996) simulated by pseudo-second-order kinetics is much better than that (R2 = 0.82899) simulated by pseudo-first-order kinetics. The R2 of the fitted curves for the two models suggests that the pseudo-second-orders model is more suitable to describe the adsorption kinetic behavior of the BC@CdS nanocomposite for MB, which demonstrates that the adsorption behavior is controlled by chemical adsorption process (Chen et al. 2019).

Table 3 The adsorption kinetic constants of BC@CdS nanocomposite for MB
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Figure 7d shows that the pH value plays a significant role during the adsorption process. With pH value increasing, the adsorption capacity of BC@CdS nanocomposite for MB increases at first and then decreases, the nanocomposite reaches the highest adsorption capacity at neutral pH. The adsorption ability of BC@CdS nanocomposite for MB is attributed to the electrostatic attraction between the negative charges of the adsorbent surface and the positive charge of the MB cation. The negatively charged sorption sites (C–O−), existing on the surface of BC, can adsorb the cationic dyes such as MB. At low pH value, the negatively charged sorption sites are protonated to form C–OH2+ by H+, which decreases surface negative charge of BC and reduces the adsorption capacity (Auta and Hameed 2014). When the pH value increases, OH− will influence the positive charge of MB cation, the sorption sites on the surface of BC fibers are harder to attract MB cation. So neutral pH value is beneficial for adsorption of MB.

Adsorption isotherms of BC@CdS nanocomposite for MB at different temperature (293 K, 303 K, 313 K) are presented in Fig. 7e. The adsorption capacity increases with the increase in MB concentration and surrounding temperature. This indicates that higher initial concentration of MB and surrounding temperature is advantageous for adsorption. The parameters of Langmuir model and Freundlich model are calculated and listed in Table 4. The best fit isotherm model for the system is compared by judging the correlation coefficients and R2 values. The Langmuir model shows the best fit with the highest R2 values at all temperatures compared with Freundlich model, suggesting that monolayer coverage of MB adsorbing onto the adsorbate (Liu et al. 2010).

Table 4 Adsorption isotherms parameters of BC@CdS nanocomposite for MB
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The photocatalytic performance of BC@CdS nanocompsite

The photocatalytic performance of BC@CdS-alcogel nanocomposite was studied. It can be seen in Fig. 8a, BC and BC@CdS-alcogel nanocomposite exhibits high adsorption ability within the first 30 min in darkness. The concentration of MB decreases to 60% because large quantities of MB molecules are adsorbed on BC surface. During the next 180 min, the reaction turns into accumulation and degradation stage, the nanocomposite exhibits highly degradation activity in comparison with pure BC. Finally, BC@CdS-alcogel nanocomposite shows high efficiency with 77.39% MB degradation after 180 min visible light irradiation. The strong adsorption of BC matrix for MB improves local concentration and promotes activity of photocatalytic degradation.

Fig. 8
figure 8

Photocatalytic degradation performances of BC@CdS-alcogel nanocomposite

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Figure 8b shows the results from the cyclic test of BC@CdS-alcogel nanocomposite for MB. The degradation activity of BC@CdS nanocomposite for MB exhibits a slight attenuation after five circles. This proves that the BC@CdS nanocomposite has good cyclic utilization. Table 5 shows the comparison of photocatalytic activity among BC@CdS-alcogel, BC@TiO2-air-3 and P25. The BC@CdS-alcogel also possesses the highest efficiency (28.3%·mg−1 h−1) of photocatalytic degradation for MB.

Table 5 Comparison on photocatalytic activity of different photocatalysts
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Conclusions

Highly efficient photocatalytic CdS nanoparticles were successfully synthesized on BC matrix by a facile microwave-assisted solvothermal in-situ method. A possible forming mechanism of CdS nanoparticles on BC matrix was proposed and verified. CdS nanoparticles on different BC matrices exhibited different morphology and different loading percentage. BC-alcogel was found to be the best matrix to prepare CdS nanoparticles, on which cubic and hexagonal CdS nanocrystals (approximately 100 nm) were distributed uniformly. This new kind of organic–inorganic hybrid nanocomposite followed an “adsorption–accumulation–degradation” pathway of photocatalytic degradation. The BC matrix had strong adsorption for MB, and the testing results showed that the adsorption kinetics fitted well to the pseudo-second-orders model and adsorption isotherms fitted better to the Langmuir model. Because of the synergistic effect of BC and CdS, this novel adsorbent nanocomposite (contained 12.4% CdS, about 0.91 mg for degradation experiment) was highly efficient for photocatalytic degradation of MB under visible light irradiation (77.39% removal, the degradation rate was 28.3%·mg−1·h−1). It was also convenient for cyclic utilization with slight attenuation.