Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Fabrication of Nanosized Ag3PO4 and Self-Assembly on Cotton Fabrics to Enhance Visible Light Photocatalytic Activities Yanyan Wang,†,‡,§ Xian Zhang,*,†,§ Xin Ding,†,§ Ping Zhang,†,‡,§ Mengting Shu,†,‡,§ Jianjun Ding,†,§ Kang Zheng,†,§ and Xingyou Tian*,†,§
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†
Institute of Applied Technology, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230088, People’s Republic of China ‡ University of Science and Technology of China, Hefei 230026, People’s Republic of China § Key Laboratory of Photovolatic and Energy Conservation Materials, Chinese Academy of Sciences, Hefei 230088, People’s Republic of China S Supporting Information *
ABSTRACT: A facile route has been developed for the preparation of stable and recyclable Ag3PO4 nanoparticles to upgrade the photocatalytic capacity and practicality. Ag3PO4 nanoparticles are first in situ deposited on a negatively charged potassium alginate/CNT colloidal solution, and then Ag3PO4− CNT and TiO2 assemble alternately on cotton fabric with the attraction of positive and negative charges. Taking advantage of the excellent absorptive capacity and recyclable nature of cotton fabric, the Ag3PO4−CNT/TiO2 photocatalytic coating presents great photocatalytic performance and recyclability for both liquid rhodamine B and gas toluene decomposition. The 7 bilayer (BL) form is the best for RhB degradation with a minimum residual of 3.94% within 5 min under visible light, and the 5BL form presents the highest toluene degradation rate (95.96%) within 60 min under simulated sunlight. Furthermore, the stability of Ag3PO4 has been greatly improved. The photocatalytic mechanism of this coating system is analyzed by ESR measurement. This work provides a nice strategy to realize the practical application of nanoscale powder photocatalysts in both liquid and gas pollutant removal. method.15 Compared to Ag3PO4, CNT/Ag3PO4 had better photocatalytic performance which benefited from the electrical conductivity of CNT facilitating the transfer of photogenerated e− from Ag3PO4 to CNT. Although the introduction of carbon materials can upgrade the photocatalytic activity and durability of Ag3PO4, the obtained Ag3PO4 composites are usually on the order of a micrometer. It has been recognized that reducing the particle size of photocatalysts can effectively increase the specific surface area, leading to the exposure of more active sites and shortening the migrating distance of photogenerated carriers from the interior to the surface.16−18 Hence, Ag3PO4 nanoparticles are expected to upgrade the photocatalytic property. Inspired by Ag3PO4 growth controlled by the interaction between Ag+ and the oxygen group on graphene oxide,19−21 potassium alginate (PA) as a negative polyelectrolyte that is rich in the oxygen group can be anticipated to be a well-suited support for preparing Ag3PO4 nanoparticles. In addition, it has been confirmed that CNT can be well dispersed by PA in our previous work.22 Therefore, it is
1. INTRODUCTION Semiconductor photocatalytic technology has been widely investigated as an environmentally friendly approach to the removal of environmental pollutants since the discovery of water splitting on a TiO2 electrode with ultraviolet light irradiation by Fujishima and Honda in 1972.1−4 Recently, silver orthophosphate (Ag3PO4) as a visible-light-driven photocatalyst has successfully attracted a great deal of attention from researchers because of its extraordinary photooxidative capability.5−9 However, the practical application of Ag3PO4 is restricted by its intrinsic defects, including the instability of the photooxidative process and the rapid recombination of photogenerated h+−e− pairs. It has been confirmed that combining Ag3PO4 with carbon materials can efficiently improve the photocatalytic activity10−14 in which carbon materials act as electron acceptors not only to enhance the stability of Ag3PO4 but also to hinder the recombination of photon-generated carriers. For instance, Cui et al. assembled Ag3PO4 in 3D graphene aerogels via an in situ ion filtration− precipitation method in which graphene aerogels promoted photogenerated h+−e− pairs separation because of its excellent electrical conductivity.12 Therefore, the 3D Ag3PO4/graphene aerogels exhibited an enhancement in activity and stability. Xu et al. prepared CNT/Ag3PO4 composites by a two-step © XXXX American Chemical Society
Received: Revised: Accepted: Published: A
May 21, 2019 July 4, 2019 July 9, 2019 July 9, 2019 DOI: 10.1021/acs.iecr.9b02766 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Figure 1. Diagram of Ag3PO4−CNT synthesis (a) and LBL self-assembly process (b).
Co., Ltd. Both AgNO3 and Na3PO4·12H2O were acquired from Sinopharm Chemical Reagent Co., Ltd., and HCl was obtained from Shanghai Chemical Co., Ltd. 2.2. Preparation of Ag3PO4−CNT Nanoparticles. As shown in Figure 1a, Ag3PO4−CNT is synthesized by an in situ precipitation method. In detail, AgNO3 (0.51 g) was added to 200 mL of PA/CNT solution (PA = 0.1 wt %, CNT = wt %) with stirring and held for 1 h to ensure an sufficient combination between Ag+ and PA. Then a certain Na3PO4· 12H2O aqueous solution was slowly added to the mixture. The Ag3PO4−CNT colloidal solution was obtained after stirring the mixture for 1 h. The corresponding powder photocatalysts were acquired by centrifugation and freeze-drying. For Ag3PO4 nanoparticles, the preparation method was similar to the synthesis of Ag3PO4−CNT, while the PA/CNT solution was replaced by PA. Bulk Ag3PO4 was also prepared with the above synthesis process in the absence of PA and CNT conditions. 2.3. LBL Self-Assembly Process. The positively charged colloidal solution was obtained through dispersing TiO2 nanoparticles (0.2 g) in aqueous solution at pH 4 (200 mL). Meanwhile, the pH value of the above prepared Ag3PO4−CNT colloidal solution was also adjusted to 4. The assembly process of the Ag3PO4−CNT/TiO2 photocatalytic coating is shown in Figure 1b. Before the beginning of assembly, the fabric was rinsed with deionized water and dried. Then the cotton fabric was alternately dipped into TiO2 and Ag3PO4−CNT solutions for 2 min. Rinsing and drying were imperative procedures after each immersion. The same steps were implemented to achieve specified bilayer numbers (2BL, 5BL, and 7BL). 2.4. Measurements and Characterization. The surface morphologies and element distribution of coated fabrics are explored by a field emission scanning electron microscope (SEM). Thermogravimetric analysis (TGA, PerkinElmer, USA) and inductively coupled plasma−atomic emission spectrometry (ICP−AES, PlasmaQuad 3) are utilized to characterize the coating growth. Chemical compositions of
workable to combine Ag3PO4 nanoparticles with PA/CNT. For powder photocatalysts, it usually tends to agglomerate in the bottom of the container, making them difficult to recollect and utilize from the pollution system especially in sewage treatment. Hence, constructing Ag3PO4−CNT powder photocatalysts on a stable substrate is also expected for easy collection and reuse. In recent years, the layer-by-layer (LBL) self-assembly technique has become a potential strategy for constructing a functional coating on a substrate,23,24 especially on a nonplanar substrate. The deposition order of functional materials can be adjusted during the deposition process. Cotton fabric, as an environmentally friendly renewable material with a loose structure, may be a well-suited substrate for supporting powder photocatalysts via an LBL self-assembly technique. It not only will prevent photocatalysts from depositing on the bottom of reactor but also will easily achieve the collection and reuse of photocatalysts from the pollution system. Furthermore, cotton fabric exhibits superior adsorption capacity for organic pollutants in our previous work,25 which is beneficial to the improvement of photocatalytic performance. Herein, we report a facile route to the preparation of stable and recyclable Ag3PO4 nanoparticles through negatively charged Ag3PO4−CNT and positively charged TiO2 alternating deposition on fabrics. All of the Ag3PO4−CNT/TiO2 photocatalytic coatings exhibit nice photocatalytic activity for the decomposition of both liquid RhB and gas toluene pollutants. In addition to photocatalytic performance, the coated fabrics present durability and reusability.
2. EXPERIMENTAL SECTION 2.1. Materials. Cotton fabric, as a substrate, was purchased from an online store. Titanium dioxide (TiO2) was obtained from Degussa. Carbon nanotubes (CNTs, length 0.5−2 μm) were provided by Nanjing XFNANO Materials Tech Co., Ltd. PA was obtained from Qingdao Bright Moon Seaweed Group B
DOI: 10.1021/acs.iecr.9b02766 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
prepared bulk Ag3PO4 and Ag3PO4 nanoparticles. It can be found that bulk Ag3PO4 present irregular shapes with large sizes on the order of a micrometer, while the size of Ag3PO4 nanoparticles is very small, indicating that the growth of Ag3PO4 can be availably controlled by the oxygen group on PA. From Figure 2c, we clearly see that Ag3PO4 nanoparticles are spherelike with a uniform size of about 10 nm; moreover, all of the Ag3PO4 nanoparticles are observed on the surface of PA, which further confirms the growth process of Ag3PO4. A high-resolution TEM image of Ag3PO4 nanoparticles is shown in Figure 2d. The lattice fringes can be observed to have a spacing of 0.212 nm corresponding to the (220) crystal plane. The crystal structure of Ag3PO4 nanoparticles is further verified by selected-area electron diffraction. Four diffraction rings (Figure 2e) can be distinctly discovered, which correspond to the (210), (220), (400), and (420) crystal planes, respectively. The elemental compositions of Ag3PO4 nanoparticles are also examined by EDS (Figure 2f), which proves that Ag, P, and O are the constituent elements with an atomic mole ratio of Ag/P close to 3:1. The morphology of the Ag3PO4−CNT composite was also investigated by SEM (Figure 2g,h). It can see that there exists intimate contact between CNT and Ag3PO4 nanoparticles, which is helpful for the transfer of photon-generated e−. The structures and surface elemental compositions of asprepared Ag3PO4 samples are examined by XRD, FTIR, Raman spectra, and XPS, respectively. As shown in Figure 3a, bulk Ag3PO4, Ag3PO4 nanoparticles, and Ag3PO4−CNT exhibit similar diffraction peaks which can be indexed to the body-centered cubic Ag3PO4 (JCPDF no. 06-0505).26,27 These characteristic peak intensities weaken with the introduction of PA, implying the formation of smaller Ag3PO4. As shown in Figure 3b, the FTIR characteristic peaks of Ag3PO4 are detected in all as-prepared Ag3PO4 samples, in which the absorption peaks at 551, 859, 1015, and 1400 cm−1 correspond to the OP−O bending vibration, symmetric stretching vibration of P−O−P rings, asymmetric stretching of P−O−P groups, and stretching vibration of PO, respectively.28,29 Interestingly, for Ag3PO4 nanoparticles and Ag3PO4−CNT, the peak at 1015 cm−1 shift to a higher wavenumber (1072 cm−1) compared to that of bulk Ag3PO4, indicating that Ag3PO4 has been substantially attached to PA. To verify the existence of CNT in Ag3PO4−CNT, the Raman spectra are presented in Figure 3c. For Ag3PO4 nanoparticles, the peak at 344 cm−1 arises from the bending vibration of the tetrahedral PO43− group; the peaks at 556 and 707 cm−1 originate from the symmetric stretch of P−O−P bonds; and the peaks at 910 and 1003 cm−1 are ascribed to oxygen bond motion vibrations in phosphate chains.10,30 Compared to Ag3PO4 nanoparticles, new absorption peaks are observed at 1350 and 1605 cm−1 in Ag3PO4−CNT, corresponding to the D and G bands, respectively.15 The D band is associated with sp3 carbon signifying the tube end, and the G band corresponds to sp2 carbon in a hexagonal lattice. This result strongly confirms the existence of CNT in the Ag3PO4−CNT composite. The Ag, P, O, and C elements are observed in the XPS survey spectrum of Ag3PO4−CNT (Figure 3d). The C 1s fine spectrum (Figure 3e) is fitted into two peaks at 284.4 and 285.1 eV, which correspond to the sp2 graphitic CC/C−C structure and the sp3 irregular C atoms in the nanotube structure or the C−H structure in PA.31,32 In Ag 3d (Figure 3f), the fine spectrum is divided into two main peaks at 368.0 and 374.0 eV, corresponding to binding energies of Ag 3d5/2 and Ag 3d3/2
the coating are explored via XPS analysis. Ultraviolet−visible spectrophotometry is employed to record the light absorption performance of samples. The photoluminescence (PL) spectra of samples are monitored with a fluorometer (FluoroMax-4) with a 220 nm excitation wavelength. Electrochemical impedance spectroscopy (EIS, CHI660D, Chenhua) is presented to analyze the charge recombination of the coated fabrics. The morphologies of the as-prepared Ag3PO4 nanoparticles are viewed with TEM. FTIR and XRD are employed to survey the composition of powder photocatalysts. Raman spectra (Renishaw inVia Reflex) were recorded with a 532 nm laser as an excitation source to confirm the existence of CNT. Electron spin resonance is employed to speculate about the possible photocatalytic mechanism. The photocatalytic activities of coated fabrics are evaluated via both liquid pollutant (RhB) and gas pollutant (toluene) degradation. In detail, 1 g of coated fabrics was placed into 35 mL of RhB solution (1.25 × 10−5 mol/L) and then stirred in the dark to realize adsorption saturation. Then a small amount reaction solution was taken out to detect the concentration change in RhB 5 min after light irradiation. A xenon lamp (300 W) acted as the light source. Toluene degradation was also carried out in a closed quartz reactor (350 mL). First, 0.58 g of cotton fabric was placed in the reactor, followed by toluene (0.2 μL) being injected into it and hot air being employed to vaporize the toluene. Then the reactor was placed in the dark for 1 h to ensure the adsorption−desorption balance. Gas chromatography was employed to monitor the toluene concentration.
3. RESULTS AND DISCUSSION 3.1. Characterization of As-Prepared Ag3PO4 Samples. Figure 2a,b shows low-magnification TEM images of as-
Figure 2. TEM images of bulk Ag3PO4 (a) and Ag3PO4 nanoparticles (b), HRTEM images of Ag3PO4 nanoparticles (c and d), the SAED pattern (e) and EDS image (f) of Ag3PO4 nanoparticles, and SEM images of Ag3PO4−CNT (g and h). C
DOI: 10.1021/acs.iecr.9b02766 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 3. (a) XRD pattern and (b) FTIR spectra of bulk Ag3PO4, Ag3PO4 nanoparticles, and Ag3PO4−CNT. (c) Raman spectra of Ag3PO4 nanoparticles and Ag3PO4−CNT. (d) XPS survey spectrum, (e) C 1s XPS, and (f) Ag 3d XPS of Ag3PO4−CNT.
Figure 4. SEM images and Ti, Ag, and P mapping of uncoated and coated fabrics.
of Ag+, respectively.26 The XPS result verifies the existence of CNT and Ag+ in Ag3PO4−CNT. 3.2. Characterization of the Ag3PO4−CNT/TiO2 coating. Ag3PO4−CNT/TiO2 is assembled on fabric by alternately depositing TiO2 and as-prepared Ag3PO4−CNT colloidal solutions. Because electrostatic attraction is the interaction for forming the photocatalytic coating, the zeta potential values of TiO2 and Ag3PO4−CNT colloidal solutions are surveyed under different pH values before beginning coating construction. In Figure 1S, there is a reverse potential at pH 2−6 for TiO2 and Ag3PO4−CNT colloidal solutions. Because mild experimental conditions and a relatively large difference in the
reverse potential are need in this LBL self-assembly process, pH 4 is chosen to construct the photocatalytic coating. Surface morphologies of coating samples are examined. In Figure 4, it can be found that the untreated fabric possesses a smooth interface, while all of the treated fabrics have a rough interface with the incorporation of Ag3PO4−CNT/TiO2. Moreover, the roughness gradually increases with the increasing bilayer number, indicating a progressive increase in the loading amount. In addition, the typical fabric structure, composed of fiber bundles, is maintained in coated fabrics, which indicates that the photocatalytic coating obviously cannot change the fabric structure and the coated fabrics will maintain a strong adsorption capacity for contaminants. The D
DOI: 10.1021/acs.iecr.9b02766 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 5. (a) Ti 2p, (b) C 1s, and (c) Ag 3d XPS of 7BL coated fabric. (d) TGA curves of uncoated and coated fabrics. (e) Ag and (f) P element contents of 7BL from ICP−AES.
pollutants to accelerate the degradation reaction. Hence, the adsorption abilities of samples are investigated, and RhB is selected as a typical organic pollutant. As shown in Figure 6,
strengthening mapping signals of Ti, Ag, and P also verify that the content of the coating increases as the bilayer number increases. Meanwhile, other information given by mapping signals is that the distribution of Ag3PO4−CNT/TiO2 is uniform on the fiber surface. The surface elemental compositions of coated fabrics are explored through XPS characterization (Figure 5a−c) of 7BL. In the Ti 2p fine spectrum, the spectrum can be fitted into two typical peaks at 458.5 and 464.1 eV, corresponding to the binding energies of Ti 2p3/2 and Ti 2p1/2 of Ti4+ in TiO2, respectively.33,34 After the Gaussian curve fitting, the C 1s curve is deconvolved into three peaks at 284.4, 286.0, and 287.5 eV. The peak at 284.4 eV corresponds to the sp2 graphitic CC/C−C structure of CNT, while the peaks at 286.0 and 287.5 eV are attributed to carbon atoms attached to different oxygen-containing groups in cotton fiber.31 For the Ag 3d fine spectrum, it is divided into two main peaks at 368.0 and 374.0 eV coupling with binding energies of Ag 3d5/2 and Ag 3d3/2, corresponding to Ag+ in Ag3PO4.26 The XPS results strongly confirm that the photocatalytic coating of Ag3PO4− CNT/TiO2 is successfully constructed on cotton fabric. As shown in Figure 5d, TGA is employed to investigate Ag3PO4−CNT/TiO2 coating growth, and the loading amount on cotton fabric is calculated. Then the loading amount acts as a function of 2BL, 5BL, and 7BL to record the coating growth, and the corresponding curve is shown in the inset. The coating amount increases gradually with increasing bilayer number. Furthermore, ICP−AES analysis is also used to explore the change in the coating content. From Figure 5e,f, it can be found that Ag and P element contents increase with the bilayer number from 2 to 7, indicating that the content of Ag3PO4 increases gradually in the self-assembly process. This result is consistent with the TGA analysis. 3.3. Photocatalytic Degradation of Pollutants. The excellent adsorption capacity of photocatalysts is beneficial to achieving sufficient contact between photocatalysts and
Figure 6. Adsorption curve of samples for RhB.
the adsorption capacities of cotton fabrics are all much stronger than those of powder photocatalysts, which can be attributed to the loose structure of the fabric. In addition, it is noteworthy that the absorptive capacity of the fabric is strengthened gradually with the increasing bilayer number, indicating that the roughness of the surface increases the adsorption capacity of the coated fabric. This result demonstrates that it is a good strategy to construct powder photocatalysts on cotton fabric to improve the absorptive capacity. The optical performances of the coated fabrics are examined by UV−vis absorption spectra, PL spectra, and electrochemical impedance spectroscopy (EIS), respectively. Compared to the pure cotton fabric, the light absorbance capacity of coated fabrics shows obvious improvement in the test region (Figure 7a), and the absorption edge of coated samples is around 400 nm, which originates from the band gap of TiO2.35,36 The absorption intensities gradually increase with the increasing number of bilayers, indicating that more light will participate in E
DOI: 10.1021/acs.iecr.9b02766 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research
Figure 7. (a) UV−vis absorption spectra, (b) photoluminescence spectra, and (c) Nyquist plots of coated samples.
Figure 8. Degradation curves of (a) bulk Ag3PO4, Ag3PO4 nanoparticles, and Ag3PO4−CNT and of (b) coated fabrics for RhB. (c) Degradation curves of coated fabrics for toluene.
Figure 9. (a) Recycling test, (b) Ag 3d XPS of 7BL before and after the recycling test, and morphology (c) before and (d) after the degradation reaction of 7BL.
the photocatalytic degradation reaction. For PL spectra of coated fabrics (Figure 7b), the emission peak intensities decrease with the increase in bilayer number, signifying that the recombination of h+ and e− is gradually suppressed, benefiting an improved quantum efficiency. EIS plots of coated fabrics are also given (Figure 7c) in which the semicircles representing charge-transfer resistance decrease gradually with the increase in the bilayer number. It implies that the charge transfer has increased sequentially, which is conducive to promoting photocatalytic activity.
To verify the wide applicability of this photocatalytic coating, different types of pollutants are utilized to evaluate the degradation activity. In terms of liquid pollutant degradation, RhB being widely used in the textile industry is selected as a typical organic pollutant. As shown in Figure 8a,b, all powder photocatalysts (bulk Ag3PO4, Ag3PO4 nanoparticles, and Ag3PO4−CNT) present low saturated adsorption capacities. Their photocatalytic degradation rates are 33.15, 72.28, and 45.97% within 25 min under the participation of visible light, respectively. Interestingly, with the incorporation F
DOI: 10.1021/acs.iecr.9b02766 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research of CNT, the photocatalytic activity of Ag3PO4−CNT decreases by 26.31% compared to that of Ag3PO4 nanoparticles. This might be attributed to the competitive light absorption between CNT and Ag3PO4. Interestingly, with the introduction of the Ag3PO4−CNT/TiO2 coating (Figure 8b), it can be found that all of the coated fabrics show a great increase in RhB degradation in which only 23.32, 15.60, and 3.94% RhB are left within 5 min, corresponding to 2BL, 5BL, and 7BL, respectively. Although pure cotton fabric does not present any photocatalytic activity for RhB, >90% of the RhB can be removed from all of the coated fabrics within 25 min under visible light irradiation. This result indicates that the introduction of the Ag3PO4−CNT/TiO2 coating on cotton fabric can optimize the photocatalytic activity of powder photocatalysts. For gas pollutant degradation, toluene acting as an indoor VOC is selected to evaluate the degradation performance of coating samples, and simulated sunlight is employed as the light source. In Figure 8c, all fabrics display similar low adsorption capacities for toluene. For pure cotton fabric, the toluene content does not exhibit an obvious reduction within 60 min under illumination, which suggests that uncoated fabric has no photocatalytic activity for toluene. Oppositely, all of the coated fabrics display brilliant photocatalytic capacities. The photocatalytic decomposition rates of 2BL, 5BL, and 7BL are 91.57, 95.96, and 86.08%, respectively. Although 5BL is the best, there is not much difference among the photocatalytic capacities of all of the coated fabrics. This may be ascribed to the fact that gas pollutant molecules do not easily enter the interior of the coating, which are inclined to directly contact and react with the surface photocatalytic coating, resulting in all of the coated fabrics showing similar photocatalytic activity. Furthermore, the similar and small adsorption capacities also imply that interaction between coated fabrics and toluene is restricted to the surface layer of the coating. Undeniably, the coated fabrics exhibit excellent photocatalytic activity for gas pollutant decomposition. From the results of the degradation for RhB and toluene, it is concluded that the coating fabrics possess a great potential for practical application in the removal of environmental pollutants. Recyclability is an essential characteristic for photocatalysts to realize practical applications. Here, 7BL and RhB are selected as a typical photocatalyst and organic pollutant for evaluating the durability and recycling of coating samples. After four cycles, although the degradation capacity of 7BL shows a slight decrease (Figure 9a), the coated fabric still exhibits high degradation activity, suggesting excellent stability of the selfassembled coating which can be ascribed to the strong interaction between the Ag3PO4−CNT/TiO2 coating and fabrics, with the photocorrosion of Ag3PO4 suppressed by CNT. Significantly, the coated fabrics are conveniently recycled after filtration from the reaction solution for the next use. To further explore the stability of Ag3PO4 after the photocatalytic degradation experiment, the XPS spectrum of Ag 3d was provided in Figure 9b. Compared to the Ag 3d fine spectrum before the degradation reaction, the spectrum of Ag 3d after the reaction is deconvolved into three peaks located at 368.0, 368.2, and 374.0 eV, which correspond to the binding energies of Ag 3d5/2 (Ag+), Ag 3d5/2 (Ag0), and Ag 3d3/2 (Ag+), respectively.26,37 The result indicated that a part of the lattice Ag+ is converted to Ag0 during the degradation reaction, while the peak area of Ag0 is only 22.72%, implying that lattice Ag+ is still the main state of existence in the photocatalytic coating
and that the photocorrosion of Ag3PO4 is efficiently suppressed by the introduction of CNT. Figure 9c,d showed the morphology of 7BL before and after the degradation reaction. It could be seen that both of them exhibited rough fiber surface, and there did not appear to be an obvious shedding phenomenon after the degradation reaction. Thus, it was concluded that the coating material is very suitable in practical applications in the polluted environment. 3.4. Photocatalytic Mechanism. The electron spin resonance (ESR) is utilized to detect possible active species in the photocatalytic degradation reaction. As shown in Figure 10a, the peaks of DMPO−•OH are discovered with the
Figure 10. ESR spectra of (a) DMPO−•OH adducts in aqueous dispersions and (b) DMPO−O2−• adducts in methanol dispersions.
participation of visible light,38 and the peak intensities increased significantly when the light source changed to simulated sunlight. However, no signals of DMPO−•OH are observed in the dark. In Figure 10b, the peaks of DMPO-O2−• are detected under simulated sunlight irritation,39 while there are not any corresponding signals appearing in the dark or with the participation of visible light. The ESR result indicated that light irritation is indispensable to generating active species of • OH and O2−• in which •OH can be formed with the participation of visible light, while simulated sunlight irritation is required to generate O2−• on the surface of coated fabrics. On the basis of the ESR analysis, we give the probable photocatalytic mechanism for the Ag3PO4−CNT/TiO2 photocatalytic coating under different illumination conditions, and the corresponding schematic diagram is shown in Figure 11. In terms of the photocatalytic degradation of pollutants, there exist three major steps: adsorption for pollutants, photon
Figure 11. Degradation reaction diagram of the coating fabric. G
DOI: 10.1021/acs.iecr.9b02766 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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photocatalysts to construct powder photocatalyst on the fabric via a layer-by-layer self-assembly technique.
absorption and photogenerated carriers transfer, and catalytic surface reactions. With the advantage of cotton fabric, the coated fabric exhibits great adsorption capacity for pollution (RhB) which is conducive to realizing the sufficient contact between the Ag3PO4−CNT/TiO2 coating and pollution. As for Ag3PO4, photogenerated electrons and holes not only tend to recombine but also would react with lattice Ag+, resulting in the photocorrosion of Ag3PO4. When the Ag3PO4−CNT/ TiO2 photocatalytic coating is exposed to visible light, only Ag3PO4 (CB = 0.45 eV, VB = 2.9 eV)28,40 is motivated to generate photogenerated h+ and e− in which e− will jump to its CB and the corresponding h+ is left on VB. Then h+ will transfer to the VB of TiO2 owing to the higher valence band position compared to that of Ag3PO4 through the heterojunction between them.17,41 Meanwhile, the photogenerated e− will transfer into CNT through the intimate contact interface with Ag3PO4 and CNT because of the excellent electrical conductivity of CNT. It would greatly inhibit the photocorrosion of Ag3PO4 to improve the stability. The recombination of photogenerated holes and electrons has been effectively suppressed. From the signals of ESR, there also exist •OH radicals which may be generated via holes oxidizing H2O molecules adsorbing on the coating surface. Then the • OH radicals and holes will participate in the degradation reaction of the organic pollutant (RhB). When the light source is simulated sunlight, TiO2 and Ag3PO4 are both stimulated. Through the interface of Ag3PO4−CNT/TiO2 composites, the h+ on the VB of Ag3PO4 will move to the VB of TiO2 and e− from TiO2 will transfer to the CB of Ag3PO4. Understandably, • OH radicals are still formed by combination h+ with H2O. In addition, DMPO-O2−• signals are also detected under simulated sunlight irritation, which suggests that some of the photogenerated e− on the CB of TiO2 react with O2 molecules adsorbing on the coating surface. Hence, both •OH and O2−• radicals can participate in the degradation reaction of toluene under simulated sunlight irritation. Furthermore, holes with strong oxidizability may directly degrade toluene. According to the mechanism discussion, the excellent photocatalytic activity of this coating system can be explained via two aspects. In terms of the structure of the coating system, fabric not only acts as a support to prevent agglomeration of the photocatalysts and to endow its recyclable characteristic but also plays the role of adsorbent, which greatly enhances the adsorption capacity of photocatalysts for pollution. For the photocatalytic composites of Ag3PO4−CNT/TiO2, heterojunction formation is beneficial to facilitating charges separation and transfer and effectively suppresses the recombination of photogenerated holes and electrons. Moreover, the stability of Ag3PO4 has been greatly improved via photogenerated electrons transferring into the conductive CNT in time.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b02766. Zeta potential value of Ag3PO4/CNT and TiO2 at different pH values (PDF)
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (X.Z.). *E-mail:
[email protected] (X.T.). ORCID
Xian Zhang: 0000-0002-7910-1562 Jianjun Ding: 0000-0002-7597-2141 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are grateful for the support of the Anhui Provincial Natural Science Foundation (1708085MB46) and the CASHIPS Director’s Fund (YZJJ201523).
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REFERENCES
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4. CONCLUSIONS In this work, the Ag3PO4−CNT/TiO2 coating was successfully assembled on fabric via a facile layer-by-layer self-assembly technique. This photocatalytic coating exhibited obvious advantages in collection and reuse. Meanwhile, the coated fabrics presented excellent photocatalytic activity for RhB and toluene. The photocatalytic mechanism of this coating system was analyzed by ESR measurement. We demonstrated that it was an effectively strategy for the practical application of H
DOI: 10.1021/acs.iecr.9b02766 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
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DOI: 10.1021/acs.iecr.9b02766 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
