Introduction

With the rapid economic growth, the industrial waste water containing various synthetic dyes has attracted considerable attention, due to serious threat to human health and ecological environment (Fu et al. 2019; Cheng et al. 2016; Li et al. 2014; Peng et al. 2018). As a promising method to address this issue, semiconductor-based photocatalytic oxidation technology has a hopeful application prospect in wastewater treatment owing to its environmental friendly, energy saving, mild reaction conditions and without secondary contamination (Fu et al. 2019; Dong et al. 2019; Li et al. 2019a, b; Monti et al. 2016; Wu et al. 2016; Yadav and Purkait 2016; Yang et al. 2015; Yi et al. 2019). In the past decades, in order to take full use of solar light, numerous scientists have devoted to explore various semiconductor photocatalysts with visible light photocatalytic activity for degradation of various synthetic dyes, such as g-C3N4, Ag/AgCl, Bi2WO6 and V2O3 (Qin et al. 2019; Gashti et al. 2012; Montazer and Pakdel 2011; Shekofteh-Gohari et al. 2018; Zinatloo-Ajabshir and Salavati-Niasari 2016; Ouyang et al. 2018).

Among of all them, Ag/AgX (Cl, Br and I) have attracted considerable attention recently, due to prominent capability of harvesting solar energy in a very broad visible spectrum (Bi and Ye 2009). This outstanding visible light photocatalytic performance of Ag/AgX can be ascribed to, besides the surface plasmon resonance (SPR) effect of Ag nanoparticles, the highly dispersed AgX species. AgX can improve the separation efficiency of photo-generated charge carriers and generate electron–hole pairs when they absorb photos, resulting in enhanced photocatalytic activity of the substrates (Jiang and Zhang 2011; Liu et al. 2017). This is the reason that Ag/AgX exhibits better visible light photocatalytic activity than that of Ag nanoparticles. Nevertheless, the photocatalytic activity of Ag/AgX is limited by its photocorrosion during the continuous visible light photocatalytic process (Zhang et al. 2014), which seriously hinders their photocatalytic stability. Fortunately, numerous investigations showed that silver halides can maintain their stability and photocatalytic activity with the assistance of other semiconductor materials (TiO2, SiO2 and ZnO etc.) to promote the transfer of the photo-excited carriers (Shekofteh-Gohari et al. 2018; Sun et al. 2014). For example, Zang and Farnood synthesized a catalyst by doping AgBr on TiO2 that can efficiently degrade MO under visible light irradiation, following the significant improvement of stability of AgBr during photocatalytic process (Zang and Farnood 2008). Yu et al. prepared a durable AgI–TiO2/PAN catalyst, which showed a highly photocatalytic activity in degradation of MO. Moreover, the degradation efficiency of MO over AgI–TiO2/PAN was much higher as compared to that of pure PAN, AgI/PAN and TiO2/PAN, due to the synergistic effect between AgI and TiO2 (Yu et al. 2016). Noteworthily, preparation of heterostructures via incorporating two or more semiconductor materials has been demonstrated as one of the best choices to enhance photocatalytic activity resulting from the synergistic effects between different semiconductors. Recently, ceric dioxide (CeO2) as a relatively inexpensive rare earth semiconductor has been widely utilized to fabricate heterostructure photocatalysts (Lin et al. 2015; Kaviyarasu et al. 2016; Sangsefidi et al. 2017). Inspiration from these investigations, introduction of CeO2 into Ag/AgX to improve its stability and photocatalytic activity is highly desirable. More importantly, CeO2 can couple well with Ag/AgX due to their matched band structures (Wen et al. 2017). Based on this reason, Zeng et al. firstly prepared Ag/AgCl–CeO2 composite photocatalysts with 5.12 to 17.76 wt% Ag/AgCl for the photodegradation of norfloxacin (NOF) under visible light irradiation (Wen et al. 2017). However, the photocatalytic efficiencies of reported Ag/AgCl/CeO2 were still quite low. As mentioned in the literature, the AgX content had a strong effect in their photocatalytic activity (Yu et al. 2016). Inspiration from this result, we introduced a little amount of CeO2 into Ag/AgCl to prepared novel Ag/AgCl/CeO2. Herein, CeO2 plays a role in facilitating the transfer of photogenerated electrons and improving the stability of Ag/AgCl, resulting in the novel Ag/AgCl/CeO2 composite materials with higher photocatalytic activity and structural stability under visible light irradiation.

Apart from photocatalytic activity, photocatalysts with facile and durable reusability are also essential for practical application. It is well known that the photocatalyst powder may reduce its photocatalytic activity seriously after aggregation in practical applications. Moreover, the powdery catalyst is not conducive to recycling, which may cause secondary contamination and potential cytotoxic effects (Du et al. 2018). To address these drawbacks, such photocatalyst powders usually were anchored on the surfaces of substrates (Ran et al. 2019; Wang et al. 2017; Tissera et al. 2015). Among all of them, cotton fabric was frequently employed as the substrate to immobilize nanoparticles, due to its various characteristics including good mechanical properties, large specific surface area, porosity, controllable structure, and biodegradability (Guan et al. 2019; Li et al. 2008; Rafatullah et al. 2010; Ran et al. 2019; Zhan et al. 2019). Moreover, cotton fabric is easily modified owing to existence of abundant active moieties. In our previous work, the Ag/AgCl/ZIF-8/TiO2 coated cotton fabric was successfully prepared for highly efficient phtocatalytic degradation of dyes under visible light irradiation (Guan et al. 2019). This functional cotton fabric can be easily reused to photocatalyze dye degradation without centrifugation or filtration. However, the Ag/AgCl/ZIF-8/TiO2 easily detached from cotton fabric during photocatalytic process. Therefore, another objective of this work is to realize steady interfacial adhesion between Ag/AgX/CeO2 and cotton fabrics to form efficient and durable photocatalysts for dye degradation.

In the current work, an Ag/AgCl/CeO2 decorated cotton fabric with high visible light photocatalytic activity and easy reusability was successfully developed (Scheme 1). First, Ag/AgCl/CeO2 composites are prepared via a facile co-precipitation method, and then the surface was modified with amino groups via utilization of (3-aminopropyl) triethoxysilane (ATPMS) as a coupling agent. Subsequently, such amino-functionalized Ag/AgCl/CeO2 nanoparticles are grafted on the aldehyde functionalized cotton fabric via “amine–aldehyde” chemistry reaction. The surface microstructure and composition of resultant cotton fabric are characterized by FT-IR, SEM, EDS and XPS measurements. Secondly, the photocatalytic performance of Ag/AgCl/CeO2 decorated cotton fabric is evaluated via degradation of methylene blue (MB), rhodamine B (RhB) and methyl orange (MO) under visible light irradiation. Finally, the recyclability and the possible mechanism of Ag/AgCl/CeO2 decorated cotton fabric towards various dyes are also investigated in detail.

Scheme 1
scheme 1

Illustration of the fabricated route of Ag/AgCl/CeO2 grafted cotton fabrics as highly efficient photocatalysts

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Experimental

Materials

Cotton fabric (CF) with plain weave structure was purchased from a local fabric store. Methylene blue (MB, 98.5%), rhodamine B (RhB), methyl orange (MO), ceric dioxide (CeO2, 99%), sodium periodate (NaIO4, 99.5%), silver nitrate (AgNO3, 99.0%), zinc chloride (ZnCl2, 98%), ethanol (CH3CH2OH, 99.7%), ethylene glycol ((CH2OH)2, 99.5%), tert-butylacohol (C4H10O, 99.0%), ethylenediamine tetra acetic acid disodium salt (C10H14N2Na2O8, 98.0%), p-benzoquinone (C6H4O2, 98.0%), (3-aminopropyl) triethoxysilane (ATPMS) were bought from China Kelon Chemical Reagent Co., Ltd without further purification before used.

Preparation of photocatalysts

Preparation of Ag/AgCl/CeO2

0.374 g of AgNO3 was dissolved in 40 mL of deionized water, and then a certain proportion of CeO2 (3 wt%, 7 wt%, 10 wt%, 13 wt% and 16 wt%) was added to the solution under magnetic stirring for 30 min. Next, 0.13 g ZnCl2 was added into the suspending solution and the mixture was stirred for 30 min. Then, the suspension was irradiated by a xenon lamp for 30 min with stirring to achieve the photo-reduction. Subsequently, the suspension was filtered and washed three times with deionized water and dried at 60 °C for 2 h to obtain the Ag/AgCl/CeO2 nanoparticles with different CeO2 weight ratios of 3%, 5%, 7%, 10%, 13%, and 16%, denoted as Ag/AgCl/CeO2 (3 wt%), Ag/AgCl/CeO2 (5 wt%), Ag/AgCl/CeO2 (7 wt%), Ag/AgCl/CeO2 (10 wt%) and Ag/AgCl/CeO2 (16 wt%), respectively, for further procedure.

Amino functionalized Ag/AgCl/CeO2

Herein, amino functional Ag/AgCl/CeO2 was prepared (Scheme 1a). First, a certain amount of coupling agent (ATPMS) was dissolved into ethanol/DI water mixture (v:v = 85:15) to form 5 wt% solution. Then, 2 g of Ag/AgCl/CeO2 was added with magnetic stirring for 2 h at room temperature. Finally, the suspension was filtered and washed three times with excess deionized water to remove residual ATPMS and dried at 60 °C in vacuum oven to obtain the surface amino functionalized Ag/AgCl/CeO2 nanoparticles with varying CeO2 contents.

Preparation of aldehyde functionalized cotton fabric

Cotton fabric (4 cm × 4 cm) was immersed in 100 mL of sodium periodate solution with various concentrations (0.01 g mL− 1, 0.03 g mL− 1, 0.05 g mL− 1, 0.07 g mL− 1 and 0.10 g mL− 1) at 50 °C for 4 h under dark conditions, respectively (Scheme 1b). Subsequently, the aldehyde functionalized cotton fabric was soaked in 0.1% (w/w) ethylene glycol solution with stirring for 30 min at room temperature. Finally, the resultant cotton fabric was washed and dried. According to the increment of concentration of NaIO4, the aldehyde functionalized cotton fabrics were labeled as CF-1, CF-2, CF-3, CF-4 and CF-5, respectively.

Preparation of Ag/AgCl/CeO2 decorated cotton fabric

0.25 g amino functionalized Ag/AgCl/CeO2 nanoparticles were dispersed in 100 mL of deionized water at 50 °C, then aldehyde functionalized cotton fabrics were immersed into solution for 2 h to realize Ag/AgCl/CeO2 graft on the surface of cotton fabric via “amine–aldehyde” chemistry reaction (Scheme 1c). Subsequently, the resultant samples were washed by deionized water for three times, and then dried at 60 °C for 24 h for following experiments. According to the degree of aldehyde of cotton fabric, the Ag/AgCl/CeO2 grafted cotton fabrics were named as Ag/AgCl/CeO2–CF1, Ag/AgCl/CeO2–CF2, Ag/AgCl/CeO2–CF3, Ag/AgCl/CeO2–CF4 and Ag/AgCl/CeO2–CF5, respectively. Herein, Ag/AgCl/CeO2 coated untreated cotton fabric was also prepared for comparison according to the same procedure, and denoted as Ag/AgCl/CeO2–CF0.

Characterization

FT-IR spectra of the samples were measured with a Nicolet 560 Fourier transform infrared spectrophotometer (USA) in the wavenumber ranging from 600 to 4000 cm− 1, the fabric samples were measured by ATR accessory. X-ray photoelectron spectroscopy (XPS) measurements were performed on a Kratos XSAM800 system with an Al Ka (hv = 1486.6 eV) 150 W, the shift in binding energy caused by the relative surface charge is calibrated with reference to the peak of C 1s. The surface morphologies of the samples were recorded on a JEOL JSM-7500F Field Emission Scanning Electron Microscope (SEM). The UV–visible spectroscopy of the samples was carried out on a UV-2700 spectrophotometer (Shimadzu) in the range of 200–800 nm. The light absorption properties of the samples were recorded using a UV–Vis spectrophotometer. The electrochemical impedance spectroscopy measurement (EIS) was carried out on electrochemical workstation (Instrument Model: CHI660E) with the frequency range from 100 kHz to 0.01 Hz and the amplitude of 5 mV. The counter and the reference electrodes were platinum wire and saturated calomel electrode (SCE), respectively, and the electrolyte solution was phosphate buffer saline.

Photocatalytic activity

The photocatalytic activity of the prepared samples were carried out by photocatalytic degradation of dyes under visible light irradiation (λ > 420 nm) of a 500 W xenon lamp (GXZ500, Shanghai, China). The distance between the light source and the sample is 25 cm. The resultant coated sample was immersed in a 50 mL solution having a concentration of 20 mg L− 1. Subsequently, the solution was stored in the dark for 30 min to achieve an adsorption–desorption equilibrium. During the irradiation time, 3 mL of the degradation solution was taken out and the absorbance of the solution was measured at intervals of 15 min, and then poured back into the degradation solution after the test. The absorbance ratio of the solution (At/A0) was used to measure the photodegradation efficiency of the prepared sample. A0 is the initial absorbance of the degradation liquid, At is the absorbance of the degradation liquid in reaction time t after being irradiated.

In addition, the photocatalytic stability and recyclability of Ag/AgCl/CeO2–CF were also investigated. The photocatalytic mechanism was studied by different scavenger capture experiments in order to detect potential active species involved in the degradation process.

Results and discussion

Photocatalytic activity of Ag/AgCl/CeO2

First of all, a series of Ag/AgCl/CeO2 nanoparticles were prepared. Their structures were confirmed by SEM and XPS, respectively. The corresponding results were presented in Figs. S1 and S2. The results confirmed that the Ag/AgCl/CeO2 heterostructures were successfully prepared. Then, the photocatalytic performances of the as-prepared samples with various compositions were investigated via the photodegradation of MB experiment under visible light irradiation. As shown in the UV–Vis spectra of all powder samples (Fig. 1a), it is obvious that introduction of CeO2 into Ag/AgCl can effectively improve light absorption range and intensity from 400 to 800 nm. As expected, the photocatalytic activities of Ag/AgCl/CeO2 heterostructures were better than both of pure Ag/AgCl and CeO2 (Fig. 1b). However, the photocatalytic activity of Ag/AgCl/CeO2 decreased after the amount of CeO2 exceeded 10 wt%. This result may be ascribable to the aggregation of CeO2 particles, and thus results in a shading effect (Jiang et al. 2019). As shown in Fig. 1b, the Ag/AgCl/CeO2(10%) exhibited the best photocatalytic activity, which can be attributed to the good synergistic effect between Ag/AgCl and CeO2, resulting in enhanced separation efficiency of photogenerated electron–hole pairs. For this reason, the Ag/AgCl/CeO2(10%) was utilized in following experiments. Moreover, as shown in Fig. 1c, Ag/AgCl/CeO2(10%) could degrade almost MB, RhB and MO within 15 min, further suggesting that Ag/AgCl/CeO2(10%) could be considered as an effective photocatalyst.

Fig. 1
figure 1

a UV–Vis diffuse reflectance spectra of as-prepared samples, b photocatalytic degradation of MB by different samples under visible-light irradiation, c photocatalytic degradation of MB, RhB and MO by Ag/AgCl/CeO2(10%)

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Characterization of Ag/AgCl/CeO2 decorated cotton fabrics

Herein, the FT-IR, XPS and SEM were employed to confirm the structures of Ag/AgCl/CeO2 decorated cotton fabrics (Ag/AgCl/CeO2–CF). First of all, the successful fabrication of Ag/AgCl/CeO2–CF based on “amine–aldehyde” chemistry reaction was confirmed by FT-IR measurement. Taking Ag/AgCl/CeO2–CF5 as an example, the corresponding results were presented in Fig. 2. Compared to pristine cotton fabric, besides the characteristic absorption bands at 3300 cm− 1, at 2897 cm− 1 and at 1027–1060 cm− 1 from cellulose could be observed (Chen et al. 2019), it is obvious that the oxidization of cotton fabric by NaIO4 created aldehyde groups on its surface, which could be indicated by the appearance of characteristic peak at 1740 cm− 1 for the stretching vibration of C=O of the aldehyde group (Wan et al. 2017; Zhao et al. 2015) (Fig. 2b). Further, the aldehyde functionlized cotton fabrics reacted with amino functionalized Ag/AgCl/CeO2, resulting in the characteristic adsorption bands at 1740 cm− 1 disappeared and a new characteristic adsorption band peak of –Si–O–C– could be found at 1021 cm− 1 on cotton fabric (Wen et al. 2019), which belonged to the amino functionalized Ag/AgCl/CeO2 (Fig. S3). Based on above mentioned results, it demonstrated that amino functionalized Ag/AgCl/CeO2 nanoparticles have been successfully grafted on the surface of cotton fabric via formation of Schiff base structure.

Fig. 2
figure 2

FTIR spectra of a pristine cotton fabric, b aldehyde functionalized cotton fabric and c Ag/AgCl/CeO2 decorated cotton fabric

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In order to further confirm the composition of resultant cotton fabric, the XPS measurement was performed. The survey XPS spectrum of Ag/AgCl/CeO2–CF5 was displayed in Fig. 3a, the results showed that the presence of C, N, O, Si, Ag and Cl elements in the as-prepared sample, proved that amino functionalized Ag/AgCl/CeO2 nanoparticles have been successfully loaded onto the cotton fabric via “amino-aldehyde” chemistry reaction. Noteworthily, the peak of Ce was not observed in the Fig. 3a due to the weak signal of Ce. Additionally, the high-resolution narrow spectrum of the main characteristic elements C, O, Ag, Si and N were presented in Fig. 3b–f, respectively. The C1s was showed in Fig. 3b, deconvolution of the C1s spectrum could give several peaks, the peaks at 284.6 eV, 286.6 eV, 288.1 eV could be attributed to C–C, C–O, and C=O bond from cotton fabric, respectively. The composition C–Si (283.5 eV), C–N (285.5 eV), C–OH (288.1 eV) bond mainly resulted from ATPMS (Li et al. 2019a, b; Wan et al. 2017; Wen et al. 2019). And the C=N species suggested the chemical reaction between cotton fabric and Ag/AgCl/CeO2, which was consistent with the results from FT-IR. The XPS spectrum of O1s was also fitted, as shown in Fig. 3c, the three fitting peaks at 533.1 eV, 535.3 eV, 531.9 eV could be attributed to C–O, C=O or Si–OH and Si–O–Si, which could also verify that ATPMS existed on the fabric. Ag 3d was displayed in Fig. 3d, the peaks at 373.7 eV and 367.8 eV could be attributed to the Ag0 species, the peaks at 373.1 eV and 367.2 eV could be ascribed to the Ag+ species, proved the presence of Ag/AgCl (He et al. 2017; Yao and Liu 2014). Meanwhile, as shown in Fig. 3e, the peaks at 101.4 eV, 102.3 eV, 103.1 eV are the binding energy of Si–O–Si, Si–O–H and Si–O–C, respectively (Li et al. 2008). The N and Si element could both attribute in amino functionalized Ag/AgCl/CeO2, indicating that the Ag/AgCl/CeO2 nanoparticles were successfully grafted on the cotton fabric surface.

Fig. 3
figure 3

XPS spectrum of a Ag/AgCl/CeO2–CF and its high-resolution XPS spectra: b C 1s; c O 1s; d Ag 3d; e Si 2p; f N 1s

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After FT-IR and XPS measurements, the SEM test was employed to visualize the microstructure of prepared Ag/AgCl/CeO2–CF. The surface morphologies of samples were presented in Fig. 4. It was showed that the pristine cotton fabric was smooth with few pits and grooves (see Fig. 4a). However, the nanoparticles can be obviously observed on Ag/AgCl/CeO2–CF (Fig. 4b–f). Moreover, the amount of Ag/AgCl/CeO2 increased with increment of aldehyde degree of cotton fabric. This result further demonstrated that Ag/AgCl/CeO2 loaded on the surfaces of cotton fabrics based on “amine–aldehyde” chemistry reaction. It is good agreement with the results from FT-IR measurement. In addition, utilization of Ag/AgCl/CeO2–CF5 as an example, the EDS analysis was conducted. The results indicated that existence of Ag, O, Si, N, Cl and Ce elements in the resultant sample (Fig. 4g). Meanwhile, a detail chemical analysis was performed using element mapping, as shown in Fig. 4h–l, it can be clearly seen that the distribution of the Ag element is consistent with the Cl. Moreover, the Si and Ce elements were distributed on the entire surface, indicating that Ag/AgCl/CeO2 had been grafted on the cotton fabric. Until now, it can be concluded from the results of FT-IR, XPS and SEM measurements that the targeted Ag/AgCl/CeO2–CFs has been successfully prepared.

Fig. 4
figure 4

SEM images of a CF; b Ag/AgCl/CeO2–CF1; c Ag/AgCl/CeO2–CF2; d Ag/AgCl/CeO2–CF3; e Ag/AgCl/CeO2–CF4; f Ag/AgCl/CeO2–CF5; g EDS spectrum of Ag/AgCl/CeO2–CF5; h Ag element map; i Cl element map; j Ce element map; k Si element map; l O element map, respectively

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UV–Vis data analysis

In order to better understand photocatalytic behavior, the optical absorption property was also investigated by UV–Vis diffuse reflectance spectra in order to verify whether the functionalized cotton fabric can absorb visible light. As shown in Fig. 5a, no absorption peaks can be observed from the pristine cotton fabric in the range from 200 to 800 nm, indicating that it has no ultraviolet-visible absorption capacity. Nevertheless, after grafting Ag/AgCl/CeO2 on the surface of cotton fabric, its absorption capacity in the range from 200 to 400 nm was significantly improved. Based on the results from UV–Vis spectra, the band gap of different materials can be calculated by the formula (Tauc 1974): \(\left( {Ahv} \right)=K{(hv - Eg)^{\frac{n}{2}}}\). Where A, h, v, K and Eg represent the absorption coefficient, Planck’s constant, incident light frequency, a constant and the band-gap energy, respectively. Taking Ag/AgCl/CeO2–CF5 as an example, Fig. 4b shows the bandgaps of different composites by plotting (Ahv)2 versus hv. The Egs of Ag/AgCl and CeO2 from Ag/AgCl/CeO2–CF5 were 2.83 eV and 3.75 eV, respectively, these results are similar with the data from the previous report (Wen et al. 2017).

Fig. 5
figure 5

UV–Vis diffuse reflectance spectra of a Ag/AgCl/CeO2–CF5 and cotton fabric; b the curve between (Ahv)2 and photon energy (hv) of the as-prepared samples

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Photocatalytic activity of Ag/AgCl/CeO2 decorated cotton fabrics

After successful preparation of Ag/AgCl/CeO2–CFs, their photocatalytic activities were investigated in detail. First, the photocatalytic activity of cotton fabric with different amounts of Ag/AgCl/CeO2 was studied. As shown in Fig. 6, all samples exhibited satisfactory visible light photocatalytic performance. As expected, the Ag/AgCl/CeO2–CF5 presented the highest degradation efficiency among all of them (Fig. 6a). This result can be ascribed to the possession of most of the photocatalyst on the surface of the cotton fabric by Ag/AgCl/CeO2–CF5, resulting in excellent photocatalytic capacity. Herein, EIS was also utilized to evaluate the electrochemical performance of photocatalysts. The corresponding results were shown in Fig. 6b. The arc radius represents the electron transfer efficiency. It is well known that the smaller radius indicates a better the electron transfer rate. The arc radius of Ag/AgCl/CeO2–CF5 was smaller than other samples, implying that it has better photocatalytic performance due to the faster electron transfer rate than that of others (Qi et al. 2019), suggesting that more Ag/AgCl/CeO2 had loaded on the cotton fabric surface, which was consistent with the SEM results.

Based on above mentioned results, the Ag/AgCl/CeO2–CF5 was employed in further experiments due to its outstanding photocatalytic activity. For the purpose of evaluating the photocatalytic property of Ag/AgCl/CeO2–CF5 catalysts, MB, RhB and MO were selected as contaminants. As shown in Fig. 6c, the Ag/AgCl/CeO2–CF5 showed excellent degradation efficiency for all dyes. The degradation efficiencies of MB, RhB and MO were 99.5%, 95.2% and 92.6%, respectively, under visible light irradiation for 180 min. This result indicated that the Ag/AgCl/CeO2–CF5 has high photocatalytic activity for different dyes. Meanwhile, the comparison optical images of the colors of dye solutions before and after degradation were inserted in Fig. 6c. The color of solutions all changed to colorless after visible light irradiation. Additionally, the degradation efficiency of Ag/AgCl/CeO2–CF towards MB under nature sunlight irradiation was about 95.2% within 75 min irradiation (Fig. S4), indicating its bright prospect in practical application.

Fig. 6
figure 6

a Photocatalytic degradation of MB by different samples under visible-light irradiation, b EIS spectra of different samples, c photocatalytic degradation of different dyes by Ag/AgCl/CeO2–CF5 under visible-light irradiation

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Photocatalytic stability of Ag/AgCl/CeO2 decorated cotton fabrics

Apart from photocatalytic efficiency, reusability and stability of the prepared catalyst are also crucial parameters to assess its performance in practical application. The photocatalytic stability of Ag/AgCl/CeO2–CF5 was investigated by recycle photodegradation of MB solution under visible-light irradiation. Herein, the photocatalytic stability of Ag/AgCl/CeO2–CF0 was also investigated for comparison. As indicated in Fig. 7, the result reveals after five recycling runs for MB removal, the photocatalytic activity of Ag/AgCl/CeO2–CF5 was not significantly decreased during the recycle experiments; its photocatalytic degradation efficiency was maintained at approximately 91.8% and around 80.7% after three and five cycles, respectively. However, as for Ag/AgCl/CeO2–CF0, the photocatalytic degradation efficiency sharply decreased from 86.6 to 49.1% after five cycles. This decrease might be attributed large amount of Ag/AgCl/CeO2 detached from cotton fabric during the recycle processes. Therefore, the Ag/AgCl/CeO2–CF5 composites had better photocatalytic stability than Ag/AgCl/CeO2–CF0 in the photodegradation process. Importantly, the SEM image of Ag/AgCl/CeO2–CF5 after five cycles clearly showed that its surface morphology was similar to the freshly prepared Ag/AgCl/CeO2–CF5 (Fig. S5). It also confirmed that the Ag/AgCl/CeO2 did not easily to fall off of the composite during the photocatalytic process. On the basis of above results, it indicates that Ag/AgCl/CeO2–CF5 will be an effective photocatalyst in environmental remediation.

Fig. 7
figure 7

Cycle of photodegradation of MB (MB = 20 mg L− 1; 50 mL) by a Ag/AgCl/CeO2–CF5 and b Ag/AgCl/CeO2–CF0, respectively

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To be noted, the initial pH of the degradation solution is one of the influential parameters on the catalyst surface and oxidation power of free radicals; it might have a great effect on photocatalytic reactions. As shown in Fig. 8, a series of pH values were controlled by NaOH or CH3COOH to explore the effect of the pH value of solution on the photocatalytic process. The photodegradation efficiency increased with increasing of pH from 4 to 8 and then decreased with further increase with pH value of solution. Therefore, the Ag/AgCl/CeO2–CF5 displayed the best phtodegradation efficiency towards MB under neutral environment. The reasons can be explained as follows. Under the acidic solutions, the Ag/AgCl/CeO2 surface presents positively charged in acidic medium, electrostatic interactions between the photocatalyst surface and MB (cationic dye) are weak, resulting in decrease adsorption (Li et al. 2015; Liu et al. 2015). Meanwhile, it is not conducive to the production of hydroxyl radicals under alkaline conditions, resulting in the decease of photocatalytic efficiency (Mohamed et al. 2017; Fan et al. 2018). Generally, Ag/AgCl/CeO2–CF5 exhibited acceptable photocatalytic efficiency under broad pH range.

Fig. 8
figure 8

Photocatalytic degradation of MB by Ag/AgCl/CeO2–CF5 with pH values under visible-light irradiation

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Mechanism of photocatalytic activity enhancement

In order to explore the predominant active species generated in the reaction system, three typical chemicals, t-BuOH, EDTA-2Na and p-benzoquinone (BQ) were employed as the scavengers of superoxide radical (·OH), hole (h+) and hydroxyl radical (·O2−), respectively (Chen et al. 2016; Serpone et al. 2000; Zhao et al. 2012). As depicted in Fig. 9, the degradation efficiency of Ag/AgCl/CeO2–CF without any scavengers can reach 99.5% within 120 min. When 1 mmol of t-BuOH and BQ as the scavenger for ·OH and ·O2− radical species was added into the photocatalytic procedure separately, the degradation efficiency of Ag/AgCl/CeO2–CF decreased from 99.5 to 86.2% and 82.8%, and the degradation efficiency of Ag/AgCl/CeO2–CF decreased to 59.2% in the presence of 1 mmol EDTA-2Na. Thus, it can be inferred that for MB photocatalytic degradation in the Ag/AgCl/CeO2–CF composite system, ·OH, h+ and ·O2− all play a role in the whole photocatalytic reaction, but holes played important roles compared to ·OH and ·O2−.

Fig. 9
figure 9

Effects of various scavengers on the photocatalytic degradation of MB

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On the basis of the above results, a reasonable photocatalytic mechanism was proposed (Scheme 2). First of all, the valence band (Wen et al. 2017) (VB) of CeO2 (+ 2.37 eV) is more negative than oxidation potential of OH−/·OH (+ 2.40 eV), meaning that the H2O cannot be oxidized by photo excited holes in theory. Meanwhile, ·O2− is also cannot be generated in the AgCl because the conduction band (CB) of CeO2 (− 0.353 eV) is more negative than AgCl (+ 0.09 eV) (Zhu et al. 2016; Li et al. 2018). Secondly, Ag nanoparticles were good electron acceptors with a lower Fermi level than the CB of AgCl, which promotes the transfer of photoelectrons from AgCl to Ag nanoparticles under visible light radiation, and then recombine with holes that transferred from the VB of CeO2. As a result, the photoelectrons were effectively separated. Meanwhile, the electrons in the CB of CeO2 can react with the O2 to form reactive superoxide radicals. The holes at VB of AgCl can react with H2O molecules to form ·OH or oxidize Cl− to Cl0. Subsequently, the formed h+, ·OH, Cl0 and ·O2− were taken part in the MB photodegradation system. This analysis was identical with the results of trapping experiment of reactive species.

Scheme 2
scheme 2

Possible photocatalytic mechanism of MB with Ag/AgCl/CeO2–CF

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Conclusion

To sum up, novel Ag/AgCl/CeO2 decorated cotton fabrics were successfully fabricated via “amine–aldehyde” chemistry. Interestingly, the amount of Ag/AgCl/CeO2 on the cotton fabric surface can be adjusted by the degree of aldehyde of cotton fabrics. As results, the Ag/AgCl/CeO2–CFs presented excellent photocatalytic activity towards synthetic dyes. The degradation efficiencies of Ag/AgCl/CeO2–CFs for MB, RhB and MO can all reach approximately 95% within 180 min under visible light irradiation. Meanwhile, the Ag/AgCl/CeO2–CFs presented good stability under various acidic and basic solutions, the degradation efficiency could maintain above 80.0% after five degradation cycles. Most importantly, the Ag/AgCl/CeO2–CFs can be easily reused without centrifugation and filtration. Therefore, it is proposed that Ag/AgCl/CeO2–CFs could be a promising candidate for the removal of various dyes from industrial waste for environmental remediation.