Facile Preparation of CNT/Ag2S Nanocomposites with Improved Visible and NIR Light Photocatalytic Degradation Activity and Their Catalytic Mechanism

In this work, a series of carbon nanotubes (CNT)/Ag2S hybrid nanocomposites were successfully prepared by a facile precipitation method. Transmission electron microscope (TEM) observation indicates that Ag2S nanoparticles with an average particle size of ~25 nm are uniformly anchored on the surface of CNT. The photocatalytic activities of the CNT/Ag2S nanocomposites were investigated toward the degradation of rhodamine B (RhB) under visible and near-infrared (NIR) light irradiation. It is shown that the nanocomposites exhibit obviously enhanced visible and NIR light photocatalytic activities compared with bare Ag2S nanoparticles. Moreover, the recycling photocatalytic experiment demonstrates that the CNT/Ag2S nanocomposites possess excellent photocatalytic stability. The photoelectrochemical and photoluminescence measurements reveal the efficient separation of photogenerated charges in the CNT/Ag2S nanocomposites. This is the dominant reason behind the improvement of the photocatalytic activity. Based on active species trapping experiments, the possible photocatalytic mechanism of CNT/Ag2S nanocomposites for dye degradation under visible and NIR light irradiation was proposed.


Introduction
Recently, photocatalysis has come to be regarded as a promising technology to solve environmental pollution and energy problems [1][2][3]. As one of the typical photocatalysts, TiO 2 can only respond to ultraviolet (UV) light (less than 5% solar energy) owing to its wide bandgap (~3.2 eV), which seriously limits its application in the photocatalytic field. It is well known that visible light and near-infrared light (NIR) accounts for about 48% and 46% of solar energy, respectively [4]. In view of the effective utilization of sunlight energy, much work has focused on the development of novel photocatalysts that can respond to visible light or NIR light [5][6][7][8].
Silver sulfide (Ag 2 S), as an important chalcogenide, has been widely studied due to its outstanding optical limiting properties, good chemical stability and potential applications in photoconductive cells, photovoltaic cells and superionic conductors [9,10]. More importantly, Ag 2 S exhibits a narrow bandgap (~1.0 eV) and a large absorption coefficient, which is very suitable for the effective absorption of visible light and NIR light. These prominent properties make it a promising candidate for the photodegradation of organic pollutants and water splitting under visible and NIR light irradiation [11][12][13][14][15]. On the other hand, because of its appropriate energy-band potentials, Ag 2 S has been widely employed as an ideal cocatalyst to combine with other photocatalysts, thus creating efficient composite

Fabrication of Ag 2 S Nanoparticles
The Ag 2 S nanoparticles were prepared by a precipitation method. AgNO 3 (2 mmol) was added into distilled water (30 mL) under magnetic stirring (pH~6.1). Na 2 S (1 mmol) was introduced into distilled water (20 mL) to form a homogeneous solution (pH~11.7). After that, Na 2 S solution was added drop by drop into AgNO 3 solution under vigorous magnetic stirring for 5 h, during which a black suspension was obtained (pH~5.8). During the preparation process, the pH value of the solution was maintained at a natural condition. The obtained black product was separated by centrifugation, washed with distilled water several times, and then dried in an oven at 60 • C for 6 h.

Fabrication of Carbon Nanotubes (CNT)/Ag 2 S Nanocomposites
For the preparation of CNT/Ag 2 S nanocomposites (Scheme 1), a certain amount of multi-walled CNT, purchased from Nanjing XFNano Materials Tech Co. Ltd. (Nanjing, China), was introduced into distilled water (30 mL) with ultrasonic treatment and magnetic stirring to obtain a homogeneous suspension. Subsequently, AgNO 3 (2 mmol) was dissolved into the CNT suspension under magnetic stirring (pH~4.1). On the other hand, Na 2 S (1 mmol) was added into distilled water (20 mL) to form a uniform solution (pH~11.8). During the above process, the pH value of the solution was maintained at a natural condition. The following precipitate process and washing/drying procedure was similar to that for Ag 2 S preparation. To investigate the effect of CNT content on the photocatalytic activity of CNT/Ag 2 S nanocomposites, a series of CNT/Ag 2 S nanocomposites with different mass contents of CNT (0.8%, 1.6%, 2.4% and 4.8%) were prepared. These composite samples were correspondingly termed as 0.8% CNT/Ag 2 S, 1.6% CNT/Ag 2 S, 2.4% CNT/Ag 2 S and 4.8% CNT/Ag 2 S. These composite samples were correspondingly termed as 0.8% CNT/Ag2S, 1.6% CNT/Ag2S, 2.4% CNT/Ag2S and 4.8% CNT/Ag2S. Scheme 1. The schematic illustration of preparation process for carbon nanotubes (CNT)/Ag2S nanocomposite.

Photocatalytic Activity Test
The photocatalytic activities of the photocatalyst were investigated toward the degradation of RhB separately under illumination of visible light (300 W xenon lamp with a 420 nm cut-off filter) and NIR light (300 W xenon lamp with an 800 nm cut-off filter). Typically, 0.1 g sample was added into 200 mL RhB solution (concentration: 5 mg L −1 ). Before photocatalytic reaction, the above mixture was magnetically stirred in the dark for 0.5 h to achieve adsorption-desorption equilibrium between the photocatalyst and RhB molecules. During the photocatalytic process, a small volume of the reaction solution was sampled at a given time interval and centrifuged to separate the photocatalyst. The concentration of RhB solution was measured by detecting the absorbance of the obtained supernate at a wavelength of ~554 nm using an ultraviolet-visible (UV-vis) spectrophotometer. The photocatalytic stability of the photocatalyst was evaluated by recycling degradation experiment. After each photocatalytic reaction, the photocatalyst was collected and recovered by washing and drying. The recovered photocatalyst was introduced into the new RhB solution for the next photocatalytic experiment under the same condition. To detect the active species responsible for dye degradation, the active species trapping experiment was carried out. Ethanol (10% by volume) and ethylene diamine tetraacetic acid (EDTA, 2 mM) were employed as the scavengers of hydroxyl (•OH) and photogenerated holes (h + ), respectively. The trapping experimental process was performed under the same conditions as described for the above photocatalytic procedure. To examine the role of superoxide (•O2 − ) and hydrogen peroxide (H2O2), N2 gas was bubbled into the reaction solution to expel the dissolved O2 and thus prevent the generation of •O2 − and H2O2.

Characterization
The phase purity of the photocatalyst was detected by X-ray diffractometer (XRD) (Bruker AXS, Karlsruhe, Germany). The morphology observation of the photocatalyst was performed by a field-emission scanning electron microscope (SEM) (JEOL Ltd., Tokyo, Japan) and field emission transmission electron microscope (TEM) (JEOL Ltd., Tokyo, Japan). The composition and surface chemical states of the photocatalyst were investigated though X-ray photoelectron spectroscopy (XPS) (Physical Electronics, Chanhassen, MN, USA).The UV-vis diffuse reflectance spectra of the photocatalyst were measured using a UV-vis spectrophotometer with BaSO4 as a reference (Beijing Purkinje General Instrument Co. Ltd., Beijing, China). The photoluminescence (PL) spectra of the photocatalyst were obtained on a fluorescence spectrophotometer with the excitation wavelength of Scheme 1.
The schematic illustration of preparation process for carbon nanotubes (CNT)/Ag 2 S nanocomposite.

Photocatalytic Activity Test
The photocatalytic activities of the photocatalyst were investigated toward the degradation of RhB separately under illumination of visible light (300 W xenon lamp with a 420 nm cut-off filter) and NIR light (300 W xenon lamp with an 800 nm cut-off filter). Typically, 0.1 g sample was added into 200 mL RhB solution (concentration: 5 mg L −1 ). Before photocatalytic reaction, the above mixture was magnetically stirred in the dark for 0.5 h to achieve adsorption-desorption equilibrium between the photocatalyst and RhB molecules. During the photocatalytic process, a small volume of the reaction solution was sampled at a given time interval and centrifuged to separate the photocatalyst. The concentration of RhB solution was measured by detecting the absorbance of the obtained supernate at a wavelength of 554 nm using an ultraviolet-visible (UV-vis) spectrophotometer. The photocatalytic stability of the photocatalyst was evaluated by recycling degradation experiment. After each photocatalytic reaction, the photocatalyst was collected and recovered by washing and drying. The recovered photocatalyst was introduced into the new RhB solution for the next photocatalytic experiment under the same condition. To detect the active species responsible for dye degradation, the active species trapping experiment was carried out. Ethanol (10% by volume) and ethylene diamine tetraacetic acid (EDTA, 2 mM) were employed as the scavengers of hydroxyl (•OH) and photogenerated holes (h + ), respectively. The trapping experimental process was performed under the same conditions as described for the above photocatalytic procedure.

Characterization
The phase purity of the photocatalyst was detected by X-ray diffractometer (XRD) (Bruker AXS, Karlsruhe, Germany). The morphology observation of the photocatalyst was performed by a field-emission scanning electron microscope (SEM) (JEOL Ltd., Tokyo, Japan) and field emission transmission electron microscope (TEM) (JEOL Ltd., Tokyo, Japan). The composition and surface chemical states of the photocatalyst were investigated though X-ray photoelectron spectroscopy (XPS) (Physical Electronics, Chanhassen, MN, USA). The UV-vis diffuse reflectance spectra of the photocatalyst were measured using a UV-vis spectrophotometer with BaSO 4 as a reference (Beijing Purkinje General Instrument Co. Ltd., Beijing, China). The photoluminescence (PL) spectra of the photocatalyst were obtained on a fluorescence spectrophotometer with the excitation wavelength of~265 nm (Shimadzu RF-6000, Kyoto, Japan). Photoelectrochemical measurements were performed on a CHI 660C electrochemical workstation (Shanghai Chenhua Instrument Co. Ltd, Shanghai, China) equipped with a three-electrode cell configuration. The working electrode preparation and measurement procedures were the same as those reported in the literature [52]. The used electrolyte was 1 mol L −1 Na 2 SO 4 aqueous solution, and the light source was a 300 W xenon lamp with a 420 nm cut-off filter.

X-Ray Diffractometer (XRD) Analysis
The XRD patterns of bare Ag 2 S nanoparticles and the 4.8% CNT/Ag 2 S nanocomposite are shown in Figure 1. For the Ag 2 S nanoparticles, all of the diffraction peaks can be indexed to the monoclinic structure of Ag 2 S (JCPDS Card No. 14-0072). Notably, the XRD pattern of 4.8% CNT/Ag 2 S is very similar to that of the Ag 2 S nanoparticles and no trace of other impurities is detected, suggesting that the phase structure of Ag 2 S does not undergo obvious change when coupled with CNT. In addition, no characteristic diffraction peaks of CNT are observed in the XRD pattern of the nanocomposite, which is mainly due to its low content and weak diffraction intensity [48]. ~265 nm (Shimadzu RF-6000, Kyoto, Japan). Photoelectrochemical measurements were performed on a CHI 660C electrochemical workstation (Shanghai Chenhua Instrument Co. Ltd, Shanghai, China) equipped with a three-electrode cell configuration. The working electrode preparation and measurement procedures were the same as those reported in the literature [52]. The used electrolyte was 1 mol L −1 Na2SO4 aqueous solution, and the light source was a 300 W xenon lamp with a 420 nm cut-off filter.

X-Ray Diffractometer (XRD) Analysis
The XRD patterns of bare Ag2S nanoparticles and the 4.8% CNT/Ag2S nanocomposite are shown in Figure 1. For the Ag2S nanoparticles, all of the diffraction peaks can be indexed to the monoclinic structure of Ag2S (JCPDS Card No. 14-0072). Notably, the XRD pattern of 4.8% CNT/Ag2S is very similar to that of the Ag2S nanoparticles and no trace of other impurities is detected, suggesting that the phase structure of Ag2S does not undergo obvious change when coupled with CNT. In addition, no characteristic diffraction peaks of CNT are observed in the XRD pattern of the nanocomposite, which is mainly due to its low content and weak diffraction intensity [48].

Optical Absorption Properties
It is noted that the optical absorption properties of semiconductors have an important effect on their performances, which can be determined by UV-vis diffuse reflectance spectra (DRS) measurements [53,54]. The DRS spectra of Ag2S nanoparticles and CNT/Ag2S nanocomposites with different CNT contents are displayed in Figure 2. It can be seen that bare Ag2S nanoparticles exhibit strong light absorption in the entire range of the UV-vis light region. The combination of CNT with Ag2S nanoparticles leads to an enhanced light absorption in the whole wavelength range. Moreover, the absorption intensity of the CNT/Ag2S photocatalyst increases with increasing CNT content.

Optical Absorption Properties
It is noted that the optical absorption properties of semiconductors have an important effect on their performances, which can be determined by UV-vis diffuse reflectance spectra (DRS) measurements [53,54]. The DRS spectra of Ag 2 S nanoparticles and CNT/Ag 2 S nanocomposites with different CNT contents are displayed in Figure 2. It can be seen that bare Ag 2 S nanoparticles exhibit strong light absorption in the entire range of the UV-vis light region. The combination of CNT with Ag 2 S nanoparticles leads to an enhanced light absorption in the whole wavelength range. Moreover, the absorption intensity of the CNT/Ag 2 S photocatalyst increases with increasing CNT content.        Elemental mapping is an important method to observe the elemental distribution and microstructure of materials [55]. Figure 4a presents the dark field scanning TEM (DF-STEM) image of 4.8% CNT/Ag 2 S, and the corresponding elemental maps are shown in Figure 4b-d. It is seen that the CNT presents the elemental distribution of C, and the anchored nanoparticles consist of Ag and S elements. This indicates the formation of a CNT/Ag 2 S hybrid structure. Elemental mapping is an important method to observe the elemental distribution and microstructure of materials [55]. Figure 4a presents the dark field scanning TEM (DF-STEM) image of 4.8% CNT/Ag2S, and the corresponding elemental maps are shown in Figure 4b-d. It is seen that the CNT presents the elemental distribution of C, and the anchored nanoparticles consist of Ag and S elements. This indicates the formation of a CNT/Ag2S hybrid structure.

X-Ray Photoelectron Spectroscopy (XPS) Analysis
The surface chemical states and elemental composition of 4.8% CNT/Ag2S were detected by XPS, as depicted in Figure 5. In the spectrum of Ag 3d (Figure 5a), the core-electron binding energies of Ag 3d3/2 and Ag 3d5/2 are found at 374.5 and 368.5 eV, respectively, which correspond to the characteristic peaks of Ag + [56]. The S 2p spectrum (Figure 5b) displays two obvious fitted peaks at 162.8 and 161.5 eV, which are attributed to S 2p1/2 and S 2p3/2 of S 2− , respectively. In addition, the weak peak located at ~164.5 eV belongs to the satellite peak of S 2p1/2, which is consistent with previous report [57]. The deconvoluted spectrum of C 1s is presented in Figure 5c, where the peak at 284.7 eV is assigned to the sp 2 hybridized carbon (C-C) and the peak at 285.5 eV is ascribed to the defect of hybridized carbon [58].

X-ray Photoelectron Spectroscopy (XPS) Analysis
The surface chemical states and elemental composition of 4.8% CNT/Ag 2 S were detected by XPS, as depicted in Figure 5. In the spectrum of Ag 3d (Figure 5a), the core-electron binding energies of Ag 3d 3/2 and Ag 3d 5/2 are found at 374.5 and 368.5 eV, respectively, which correspond to the characteristic peaks of Ag + [56]. The S 2p spectrum (Figure 5b) displays two obvious fitted peaks at 162.8 and 161.5 eV, which are attributed to S 2p 1/2 and S 2p 3/2 of S 2− , respectively. In addition, the weak peak located at~164.5 eV belongs to the satellite peak of S 2p 1/2 , which is consistent with previous report [57]. The deconvoluted spectrum of C 1s is presented in Figure 5c, where the peak at 284.7 eV is assigned to the sp 2 hybridized carbon (C-C) and the peak at 285.5 eV is ascribed to the defect of hybridized carbon [58]. Elemental mapping is an important method to observe the elemental distribution and microstructure of materials [55]. Figure 4a presents the dark field scanning TEM (DF-STEM) image of 4.8% CNT/Ag2S, and the corresponding elemental maps are shown in Figure 4b-d. It is seen that the CNT presents the elemental distribution of C, and the anchored nanoparticles consist of Ag and S elements. This indicates the formation of a CNT/Ag2S hybrid structure.

X-Ray Photoelectron Spectroscopy (XPS) Analysis
The surface chemical states and elemental composition of 4.8% CNT/Ag2S were detected by XPS, as depicted in Figure 5. In the spectrum of Ag 3d (Figure 5a), the core-electron binding energies of Ag 3d3/2 and Ag 3d5/2 are found at 374.5 and 368.5 eV, respectively, which correspond to the characteristic peaks of Ag + [56]. The S 2p spectrum (Figure 5b) displays two obvious fitted peaks at 162.8 and 161.5 eV, which are attributed to S 2p1/2 and S 2p3/2 of S 2− , respectively. In addition, the weak peak located at ~164.5 eV belongs to the satellite peak of S 2p1/2, which is consistent with previous report [57]. The deconvoluted spectrum of C 1s is presented in Figure 5c, where the peak at 284.7 eV is assigned to the sp 2 hybridized carbon (C-C) and the peak at 285.5 eV is ascribed to the defect of hybridized carbon [58].

Photocatalytic Activities
The visible and NIR light photocatalytic activities of the CNT/Ag2S photocatalyst were assessed by the degradation of RhB. Figure 6a presents the concentration change of RhB photocatalyzed by the samples with visible light illumination. For comparison, the blank experiment was carried out without the addition of photocatalysts. Only a slight decrease in RhB concentration is observed after 5 h illumination, suggesting that the self-degradation of RhB under visible light irradiation can be neglected. When Ag2S nanoparticles are used as the photocatalyst, about 28.9% of RhB is degraded after visible light irradiation for 5 h, indicating that Ag2S nanoparticles exhibit weak photocatalytic activity. This phenomenon is probably attributed to the high recombination rate of photogenerated charges in pure Ag2S [11][12][13]. The combination of CNT with Ag2S nanoparticles leads to an obvious enhancement in the photocatalytic activity of Ag2S nanoparticles. After 5 h exposure, the degradation percentage of the dye over 0.8% CNT/Ag2S, 1.6% CNT/Ag2S, 2.4% CNT/Ag2S and 4.8% CNT/Ag2S is observed as ~53.2%, ~72.4%, ~88.5% and ~77.5%, respectively. It is found that when the content of CNT is increased from 0.8% to 2.4%, the photodegradation percentage of the dye is gradually raised. The 2.4% CNT/Ag2S composite exhibits the optimal degradation percentage. However, further increasing the content of CNT leads to the decreased photoactivity of the resultant composites. This could be attributed to the fact that the excessive CNT may shield the light, thus decreasing the photon absorption of the photocatalysts [48]. The photocatalytic reaction process can be fitted by the first-order kinetic equation Ln (C0/Ct) = kappt [59], as shown in Figure 6b. It is found that the 2.4% CNT/Ag2S composite exhibits the highest apparent first-order reaction rate constant, kapp, which is about 5.9 times higher than that for bare Ag2S nanoparticles. Compared with graphene-modified Ag2S, the visible-light-driven photocatalytic activity Ag2S can be much improved by the decoration of CNT [32]. Figure 6c displays the variation of RhB concentration as a function of NIR light irradiation over the samples. It is seen that the samples also manifest an important NIR photodegradation activity. Photocatalyzed by the optimal 2.4% CNT/Ag2S composite with 5 h of NIR light exposure, ~59.8% of RhB is observed to be degraded. Photodegradation kinetics analysis, as shown in Figure 6d, implies that the 2.4% CNT/Ag2S composite has a NIR light photocatalytic activity about 5.4 times higher than that of bare Ag2S nanoparticles. Figure 6e

Photocatalytic Activities
The visible and NIR light photocatalytic activities of the CNT/Ag 2 S photocatalyst were assessed by the degradation of RhB. Figure 6a presents the concentration change of RhB photocatalyzed by the samples with visible light illumination. For comparison, the blank experiment was carried out without the addition of photocatalysts. Only a slight decrease in RhB concentration is observed after 5 h illumination, suggesting that the self-degradation of RhB under visible light irradiation can be neglected. When Ag 2 S nanoparticles are used as the photocatalyst, about 28.9% of RhB is degraded after visible light irradiation for 5 h, indicating that Ag 2 S nanoparticles exhibit weak photocatalytic activity. This phenomenon is probably attributed to the high recombination rate of photogenerated charges in pure Ag 2 S [11][12][13]. The combination of CNT with Ag 2 S nanoparticles leads to an obvious enhancement in the photocatalytic activity of Ag 2 S nanoparticles. After 5 h exposure, the degradation percentage of the dye over 0.8% CNT/Ag 2 S, 1.6% CNT/Ag 2 S, 2.4% CNT/Ag 2 S and 4.8% CNT/Ag 2 S is observed as 53.2%,~72.4%,~88.5% and~77.5%, respectively. It is found that when the content of CNT is increased from 0.8% to 2.4%, the photodegradation percentage of the dye is gradually raised. The 2.4% CNT/Ag 2 S composite exhibits the optimal degradation percentage. However, further increasing the content of CNT leads to the decreased photoactivity of the resultant composites. This could be attributed to the fact that the excessive CNT may shield the light, thus decreasing the photon absorption of the photocatalysts [48]. The photocatalytic reaction process can be fitted by the first-order kinetic equation Ln (C 0 /C t ) = k app t [59], as shown in Figure 6b. It is found that the 2.4% CNT/Ag 2 S composite exhibits the highest apparent first-order reaction rate constant, k app , which is about 5.9 times higher than that for bare Ag 2 S nanoparticles. Compared with graphene-modified Ag 2 S, the visible-light-driven photocatalytic activity Ag 2 S can be much improved by the decoration of CNT [32]. Figure 6c displays the variation of RhB concentration as a function of NIR light irradiation over the samples. It is seen that the samples also manifest an important NIR photodegradation activity. Photocatalyzed by the optimal 2.4% CNT/Ag 2 S composite with 5 h of NIR light exposure,~59.8% of RhB is observed to be degraded. Photodegradation kinetics analysis, as shown in Figure 6d, implies that the 2.4% CNT/Ag 2 S composite has a NIR light photocatalytic activity about 5.4 times higher than that of bare Ag 2 S nanoparticles. Figure 6e In addition to the photocatalytic activity, recyclability of photocatalysts is considered to be another important criterion for their practical application. The reusability of 2.4% CNT/Ag2S was examined by the recycling photocatalytic degradation of RhB separately under visible and NIR light irradiation. As shown in Figure 7a, after three consecutive cycles of photocatalytic reaction, no obvious decrease of the RhB degradation percentage is detected. To further evaluate the structural and morphological stability of 2.4% CNT/Ag2S after recycling photocatalytic reaction, the TEM observation, XPS detection and XRD characterization were carried out. The XRD pattern in Figure 7b demonstrates that Ag2S undergoes no detectable structural change and remains the monoclinic structure. The XPS spectrum of Ag 3d for the recovered 2.4% CNT/Ag2S photocatalyst (Figure 7c) suggests that the Ag2S in the composite is stable without being reduced after consecutive In addition to the photocatalytic activity, recyclability of photocatalysts is considered to be another important criterion for their practical application. The reusability of 2.4% CNT/Ag 2 S was examined by the recycling photocatalytic degradation of RhB separately under visible and NIR light irradiation. As shown in Figure 7a, after three consecutive cycles of photocatalytic reaction, no obvious decrease of the RhB degradation percentage is detected. To further evaluate the structural and morphological stability of 2.4% CNT/Ag 2 S after recycling photocatalytic reaction, the TEM observation, XPS detection and XRD characterization were carried out. The XRD pattern in Figure 7b demonstrates that Ag 2 S undergoes no detectable structural change and remains the monoclinic structure. The XPS spectrum of Ag 3d for the recovered 2.4% CNT/Ag 2 S photocatalyst (Figure 7c) suggests that the Ag 2 S in the composite is stable without being reduced after consecutive photocatalytic reaction, and the similar report can be found in other literature [60]. The TEM image (Figure 7d) shows that Ag 2 S nanoparticles are still well anchored onto CNT, and no obvious exfoliate phenomenon is observed. The above results reveal that the CNT/Ag 2 S photocatalyst exhibits good photocatalytic and structural stability.
Micromachines 2019, 10, x 9 of 15 photocatalytic reaction, and the similar report can be found in other literature [60]. The TEM image (Figure 7d) shows that Ag2S nanoparticles are still well anchored onto CNT, and no obvious exfoliate phenomenon is observed. The above results reveal that the CNT/Ag2S photocatalyst exhibits good photocatalytic and structural stability.

Photogenerated Charge Behavior
It is well known that the photocatalytic activity of photocatalysts is highly related to their photogenerated charges behavior. Transient photocurrent response, electrochemical impedance spectroscopy (EIS) and PL spectroscopy are useful methods to investigate the separation behavior of photoinduced charges [61,62]. Figure 8a presents the photocurrent response plots of Ag2S and 2.4% CNT/Ag2S under intermittent visible light irradiation. It can be seen that both the samples exhibit fast photocurrent responses with on-off cycles. The photocurrent value of 2.4% CNT/Ag2S is much higher than that of bare Ag2S, indicating that the introduction of CNT promotes the separation of photogenerated electrons and holes in Ag2S. Figure 8b shows the EIS spectra of the samples, indicating that 2.4% CNT/Ag2S exhibits a smaller impedance arc radius than that of bare Ag2S. This reveals a higher efficiency of charge transfer in the CNT/Ag2S photocatalyst. The PL spectra of Ag2S and 2.4% CNT/Ag2S are shown in Figure 8c. An obvious emission peak is observed at ~363 nm for both the samples, which is probably attributed to the recombination of photogenerated charges. The 2.4% CNT/Ag2S photocatalyst exhibits a relatively weak PL emission peak compared with bare Ag2S, which further confirms the efficient separation of photogenerated charges in the composite.

Photogenerated Charge Behavior
It is well known that the photocatalytic activity of photocatalysts is highly related to their photogenerated charges behavior. Transient photocurrent response, electrochemical impedance spectroscopy (EIS) and PL spectroscopy are useful methods to investigate the separation behavior of photoinduced charges [61,62]. Figure 8a presents the photocurrent response plots of Ag 2 S and 2.4% CNT/Ag 2 S under intermittent visible light irradiation. It can be seen that both the samples exhibit fast photocurrent responses with on-off cycles. The photocurrent value of 2.4% CNT/Ag 2 S is much higher than that of bare Ag 2 S, indicating that the introduction of CNT promotes the separation of photogenerated electrons and holes in Ag 2 S. Figure 8b shows the EIS spectra of the samples, indicating that 2.4% CNT/Ag 2 S exhibits a smaller impedance arc radius than that of bare Ag 2 S. This reveals a higher efficiency of charge transfer in the CNT/Ag 2 S photocatalyst. The PL spectra of Ag 2 S and 2.4% CNT/Ag 2 S are shown in Figure 8c. An obvious emission peak is observed at~363 nm for both the samples, which is probably attributed to the recombination of photogenerated charges. The 2.4% CNT/Ag 2 S photocatalyst exhibits a relatively weak PL emission peak compared with bare Ag 2 S, which further confirms the efficient separation of photogenerated charges in the composite.

Photocatalytic Mechanisms
To clarify the photocatalytic mechanism of the CNT/Ag2S photocatalyst, active species trapping experiments were performed to determine the main active species involved in the photocatalytic reaction, as shown in Figure 9. Under visible light irradiation, the introduction of ethanol results in a slight decrease of degradation percentage, suggesting that •OH plays a minor role in the photocatalytic degradation of RhB. In contrast, the photocatalytic degradation of RhB obviously decreases with the addition of EDTA, which indicates that h + is the major active species responsible for the degradation of the dye. •O2 − and H2O2, generally generated from the reaction between photogenerated electrons and O2, could also be the active species in the photocatalytic reaction. The N2 purging can expel O2 dissolved in the solution and inhibit the generation of •O2 − or H2O2. It is found that the degradation of RhB is remarkably suppressed after N2 purging, revealing that •O2 − and/or H2O2 play an important role in the photocatalytic reaction. Under NIR light irradiation, a similar active species trapping behavior is observed.

Photocatalytic Mechanisms
To clarify the photocatalytic mechanism of the CNT/Ag 2 S photocatalyst, active species trapping experiments were performed to determine the main active species involved in the photocatalytic reaction, as shown in Figure 9. Under visible light irradiation, the introduction of ethanol results in a slight decrease of degradation percentage, suggesting that •OH plays a minor role in the photocatalytic degradation of RhB. In contrast, the photocatalytic degradation of RhB obviously decreases with the addition of EDTA, which indicates that h + is the major active species responsible for the degradation of the dye.

Photocatalytic Mechanisms
To clarify the photocatalytic mechanism of the CNT/Ag2S photocatalyst, active species trapping experiments were performed to determine the main active species involved in the photocatalytic reaction, as shown in Figure 9. Under visible light irradiation, the introduction of ethanol results in a slight decrease of degradation percentage, suggesting that •OH plays a minor role in the photocatalytic degradation of RhB. In contrast, the photocatalytic degradation of RhB obviously decreases with the addition of EDTA, which indicates that h + is the major active species responsible for the degradation of the dye. •O2 − and H2O2, generally generated from the reaction between photogenerated electrons and O2, could also be the active species in the photocatalytic reaction. The N2 purging can expel O2 dissolved in the solution and inhibit the generation of •O2 − or H2O2. It is found that the degradation of RhB is remarkably suppressed after N2 purging, revealing that •O2 − and/or H2O2 play an important role in the photocatalytic reaction. Under NIR light irradiation, a similar active species trapping behavior is observed. The possible photocatalytic mechanism of the CNT/Ag 2 S photocatalyst is presented in Figure 10. When the reaction system is irradiated by visible light (Figure 10a), the Ag 2 S nanoparticles are excited to generate electrons and holes. It is noted that the RhB molecule can also absorb visible light in the wavelength range from 500 to 600 nm (Figure 6e,f), and therefore its photosensitization effect during the visible-light photocatalytic process should be considered. In this photosensitization process, the photogenerated electrons from excited RhB molecule will transfer to the conduction band (CB) of Ag 2 S because the redox potential of RhB (−1.42 V vs. normal hydrogen electrode (NHE)) is negative to the CB potential of Ag 2 S (−0.3 V vs. NHE) [63]. On the other hand, Under NIR light irradiation (Figure 10b), only Ag 2 S nanoparticles can be excited, leading to the generation of electrons and holes. Whether irradiated by visible or NIR light, the recombination rate of photogenerated charges in Ag 2 S is high, and thus only a small fraction of them participate in the photocatalytic reaction. It is demonstrated that CNT can act as an excellent electron acceptor due to its efficient electron conductivity [37]. Therefore, after the formation of hybrid structures between Ag 2 S nanoparticles and CNT, the photogenerated electrons in Ag 2 S can easily migrate to CNT, which inhibits the recombination of photogenerated charges and leads to the enhancement of photocatalytic activity. This charge migration process is feasible from a thermodynamic point of view because the Fermi level of CNT (+0. 44   The possible photocatalytic mechanism of the CNT/Ag2S photocatalyst is presented in Figure 10. When the reaction system is irradiated by visible light (Figure 10a), the Ag2S nanoparticles are excited to generate electrons and holes. It is noted that the RhB molecule can also absorb visible light in the wavelength range from 500 to 600 nm (Figure 6e,f), and therefore its photosensitization effect during the visible-light photocatalytic process should be considered. In this photosensitization process, the photogenerated electrons from excited RhB molecule will transfer to the conduction band (CB) of Ag2S because the redox potential of RhB (−1.42 V vs. normal hydrogen electrode (NHE)) is negative to the CB potential of Ag2S (−0.3 V vs. NHE) [63]. On the other hand, Under NIR light irradiation (Figure 10b), only Ag2S nanoparticles can be excited, leading to the generation of electrons and holes. Whether irradiated by visible or NIR light, the recombination rate of photogenerated charges in Ag2S is high, and thus only a small fraction of them participate in the photocatalytic reaction. It is demonstrated that CNT can act as an excellent electron acceptor due to its efficient electron conductivity [37]. Therefore, after the formation of hybrid structures between Ag2S nanoparticles and CNT, the photogenerated electrons in Ag2S can easily migrate to CNT, which inhibits the recombination of photogenerated charges and leads to the enhancement of photocatalytic activity. This charge migration process is feasible from a thermodynamic point of view because the Fermi level of CNT (+0.44 V vs. NHE) is positive to the CB potential of Ag2S (−0.3 V vs. NHE) [64,65]. The photogenerated electrons in the Fermi level of CNT cannot reduce O2 to •O2 − (E 0 (O2/•O2 − ) = −0.13 V vs. NHE) [66], but they can reduce O2 to produce H2O2 (E 0 (O2/H2O2) = +0.695 vs. NHE) [67]. This confirms that H2O2 is one of the active species responsible for the degradation of the dye, which explains the inhibition phenomenon of the dye degradation after N2 purging. Furthermore, a portion of H2O2 could transform into •OH through a series of reactions [65], which is considered to be the major route for the generation of •OH in this reaction. On the other hand, compared with the redox potential of OH − /•OH (+1.89 V vs. NHE), the photogenerated holes in the VB of Ag2S (+0.7 V vs. NHE) is not positive enough to oxidize OH − to •OH [65,68]. However, the photogenerated holes in Ag2S can directly oxide the dye, as confirmed by the active species trapping experiment.

Conclusions
A series of CNT/Ag2S photocatalyst with different contents of CNT have been successfully synthesized through a simple precipitation method. It is found that the CNT/Ag2S photocatalyst exhibit obviously enhanced visible and NIR light photocatalytic activity for the degradation of RhB when compared with bare Ag2S nanoparticles. Moreover, the CNT/Ag2S photocatalysts are

Conclusions
A series of CNT/Ag 2 S photocatalyst with different contents of CNT have been successfully synthesized through a simple precipitation method. It is found that the CNT/Ag 2 S photocatalyst exhibit obviously enhanced visible and NIR light photocatalytic activity for the degradation of RhB when compared with bare Ag 2 S nanoparticles. Moreover, the CNT/Ag 2 S photocatalysts are demonstrated to be stable visible and NIR light photocatalysts. The enhanced photocatalytic activity of the CNT/Ag 2 S photocatalyst is mainly attributed to the excellent electron-accepting ability of CNT, which can serve as an electron trap to promote the separation of photogenerated charges in Ag 2 S nanoparticles. The results of this study provide an efficient modification route for the enhancement of visible and NIR light photocatalytic activity of Ag 2 S nanoparticles, which is beneficial for their practical applications in the photocatalytic field.