Photocatalytic Removal of Toluene with CdIn2S4/CNFs as Catalyst: Effect of Ozone Addition

CdIn 2 S 4 (CIS) has attracted much attention in the photocatalysis research field due to its structural stability and photoelectric properties. However, it's difficult to recycle when after usage, so application of CIS photocatalysis in removing volatile organic compounds (VOCs) is still limited and the literature on applying carbon nanofibers (CNFs) as electron acceptor is scarce. In this study, a novel CIS/CNFs composite was synthesized via a simple hydrothermal method. X-ray diffraction (XRD), Scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) were applied to characterize the structure, microtopography and composition of CIS/CNFs photocatalyst prepared. The results showed that CNFs with the size of about 300 nm were favorable connected with CIS to form 3D conductive network and CIS particles with the average size of 100 nm grew onto the surface of CNFs uniformly. Results of photocatalytic degradation tests indicate that under visible light irradiation, degradation of toluene reached the optimal level of 86% as the CIS doped with 3% CNFs. It proved that the composite material prepared had an excellent photocatalytic recycle efficiency via repeated experiments. Furthermore, 95% removal efficiency was achieved as 200 ppm ozone was added into the system and mineralization rate are also improved. Derived from the intermediates detected, possible pathways of toluene degradation were proposed. Hence, this study presents a new method to synthesize photocatalyst with visible-light driven ozone-enhanced photocatalysis process toward indoor air pollutants removal VOCs. the is to 95%. Ozone catalytic oxidation has significantly improved toluene removal efficiency and mineralization rate. Meanwhile, after ten times of repeated use, the photocatalytic activity decreased by only 3.42%, proving that the CIS/CNFs composite material has eximious repeated use performance. Through photocurrent and PL spectrum analysis, it is found that the CIS/CNFs composite material has a better photoelectron and hole separation rate than CIS nanoparticles. The results show that the CIS/CNFs composite material has higher quantum efficiency and improved its photocatalytic activity. Based on the intermediate by-products measured by GC-MS, the mechanism of ozone catalytic oxidation of toluene was proposed. The results show that ozone can generate more hydroxyl and oxygen radicals, thereby further reduced the recombination of electron-hole pairs. Increasing the ozone consumption rate and completely degrade the remaining ozone needs further investigation in subsequence. This study provides a new strategy to prepare the CIS/CNFs composites with high photocatalytic activity and excellent recyclable performance.


1.Introduction
Along with the continuous progress of industrialization and enhancement standard of living, excessive amounts of volatile organic compounds (VOCs) have been used and released from indoor decoration and other aspects [1]. Long-term exposure to VOCs may lead to influence of human body respiratory system and urinary system [2].
Indoor toluene has the characteristics of wide source, low concentration and long duration [3]. Traditional methods for removing VOCs include adsorption [4] and condensation [5], but secondary pollution may occur. Thermal catalytic oxidation [6], non-thermal plasma [7] and photocatalysis [1] have been proved to be effective for VOCs removal. However, low mineralization rate, high energy consumption and deactivation of catalyst are the disadvantages. To solve the bottleneck, combination of above techniques to treat VOCs containing gas streams has recently gained much attention [8][9][10].Ozone-enhanced photocatalytic oxidation(O3-PCO) is one of the promising VOCs control technologies [11,12]. The recent studies found that introduction of ozone not only improves the performance of photocatalytic oxidation, but also facilitates catalyst regeneration [13]. Therefore, in order to improve the efficiency of ozone-enhanced photocatalytic oxidation，which is essential to develop stable structure and excellent properties photocatalyst.
CdIn2S4 (CIS) is a ternary n-type chalcogenide with superior thermal stability and unique photoelectric properties; for instance, narrow band gap (2.0 eV) and large specific surface area which benefit the rapid excitation of charge carriers due to effective absorption of visible light [14][15][16]. CIS is more stable than CdS which suffer from serious photo-corrosion under illumination due to the presence of In 3+ along with Cd 2+ ions [17,18]. However, powdered materials are difficult to recycle after dispersion, which limits their application in the real environment [19].
Carbon nanofibers (CNFs) as a suitable electron acceptor can inhibit the secondary recombination of electron holes due to its superior electron transport behavior, stable physicochemical properties and high adsorption capacity [20][21][22][23]. Zhang et al. [24] adopted nano-carbon fiber to support metal gold ions and compounded them with In2S3. The results indicate that the photocatalytic activity was enhanced due to the network structure of composite nanofiber, and the degradation efficiency of Rhodamine B was greatly improved. At the same time, combination of CNFs with catalyst effectively solves the defects such as photo-corrosion. Wang et al. [25] and Zhang et al. [26] synthesized Bi2WO6/CNFs composite material by paring Bi2WO6 with CNFs. The results showed that heterogeneous structure was formed between the composite materials and there was a strong interaction, leading to enhanced photocatalytic activity.
Moreover, due to the three-dimensional (3D) network structure, adding CNFs enhances the electron transmission as well as facilitates the recovery and reuse of the catalyst [24,27]. Therefore, it could expect that coupling CIS with electrospun CNFs to prepare CIS/CNFs would inhibit the electron-hole pair recombination to bring about enhanced photoactivity and regeneration of catalyst.
In this study, the CIS/CNFs photocatalytic material was synthesized by hydrothermal method. The physical and chemical structures of photocatalysts prepared were investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Photoluminescence (PL) was used to investigate the photoelectric properties of the samples. The catalytic ability of the CIS/CNFs materials for the degradation of toluene was evaluated under visible light, and ozone-enhanced photocatalysis oxidation was applied to improve the removal efficiency. Through the analysis of the experimental data, a viable catalytic pathway for the degradation of pollutants by the materials prepared was explored and discussed. This study provides a new highperformance catalyst for degrading toluene.

Chemicals and materials.
In this study, all reagents were of analytical-grade and used as received. Reagents BET surface area and pore volume of catalysts were determined by nitrogen adsorption (Micromeritics ASAP 2020, USA). The crystal structure of the assynthesized CIS/CNFs by hydrothermal method was determined by Bruker X-ray diffractometer (D8-Advance, Germany) using Cu-Kα radiation. Data was collected at scan rate of 0.02° s −1 and from 10° to 80° (2θ, diffraction angle). High-resolution transmission electron microscopy (HRTEM) micrographs were obtained with a Tecnai G2 F20 S-TWIN microscope and operated at 200 kV. Scanning electron microscope (SEM) images of the catalysts were obtained on a Nova NanoSEM 450 microscope operated at 200 kV. X-ray phototoelectron spectroscopy (XPS) analyses were performed on a Thermo Fisher Scientific ESCALAB 250 photoelectron spectrometers.
Photoluminescence (PL) spectra was carried out on a spectrometer with an excitation wavelength of 325 nm (Varian Cary Eclipse, America). Photocurrent responses was observed by electrochemical workstation (CHI 660B).

Photocatalytic tests
Catalytic degradation of toluene was tested in a self-designed reactor at room temperature. The experimental setup is shown in Fig. 1. The volume of dark organicglass reactor is 0.5 L. Prior to each experiment, 50 mg photocatalyst powder was uniformly dispersed in 20 mL ethanol solution and then coated on a round glass plate with a diameter of 5 cm, placing the sample-coated dishes in the bottom of reactor with a glass slide cover. After that, the standard gas (60 ppm toluene in air) was passed into the reactor. The reactor was kept in the dark condition for 1 h to achieve an equilibrium of adsorption and desorption. The initial toluene concentration was remained at 60 ppm after adsorption equilibrium. The glass slide cover on the petri dish was then removed to begin the catalytic oxidation of toluene. Xenon lamp (MAX-350, Nmerry Technology.Co) occluded by a fixed wavelength filter produces a visible light source. where Tolueneinitial and Toluenefinal are the initial and final concentrations of toluene (ppmv), respectively.
The ozone utilization rate was calculated by the following equation: where Ozoneinitial and Ozonefinal are the initial and final concentrations of ozone (ppmv), respectively.
The toluene mineralization rate was calculated by the following equation: Toluene mineralization rate =  under the same conditions. We speculate that when too much CNFs is doped, some pores might be blocked due to the high dispersability of CNFs and the active sites of CIS would also be reduced as a result of reduced specific surface area. Therefore, 4% is chosen as the maximum doping rate of CNFs.
All degradation tests were conducted for three times and low deviations (less than 10%) were observed. As can be seen from Fig. 2(b), when the doping rate of CNFs is 3%, the photocatalytic degradation of toluene reached the optimal (86.1%) and as the doping rate increases to 4%, photocatalytic efficiency is reduced to 72%. As too much carbon nanofibers are added, carbon nanofiber may cover part of the CIS particles, to reduce light absorption and utilization. Results of toluene removal obtained in this study are summarized and compared with other studies under the same experimental protocol, as shown in Table   1. It can be observed that the performance of CIS/CNFs-3 prepared via simple hydrothermal method is comparable to those reported in literature over various catalysts.

Toluene degradation performance and mineralization rate of different processes
Ozone is a powerful oxidizing agent which has been applied in a wide range of photocatalytic oxidation (PCO) to improve the performance. In this study, 200 ppm of ozone was introduced into the system to conduct ozone-enhanced photocatalytic oxidation (O3-PCO) tests of toluene with CIS, CNFs, and CIS/CNFs, respectively, as shown in Fig. 3. The experiment is divided into three processes, i.e., CIS/CNFs+Vis, CIS/CNFs +O3 and CIS/CNFs +O3+Vis. Toluene degradation efficiencies achieved with CIS +Vis and CIS/CNFs +O3 reach 86.1% and 75.6%, respectively. After ozone is injected, the efficiency of PCO system increases significantly to 95%.
In the meantime, the introduction of ozone significantly improves the mineralization rate of toluene, indicating that ozone and PCO system had excellent synergistic sequence. For photocatalyst tests, the photocatalyst gradually became deactivated after being operated for specific sometimes, which may be caused by the adhesion of intermediates produced in the degradation process of toluene to the surface of the catalyst, resulting in the blocking of the active sites on the catalyst surface and the oxygen vacancy. However, the situation is greatly improved for O3-PCO, showing a slow upward trend throughout the process. Therefore, the introduction of ozone can improve the durability of catalysts, because catalyst deactivation is inhibited to some extent. As shown in Fig. S1, toluene mineralization rates of 87.3% and 76.8% and ozone utilization rates of 84.9% and 69.5% are achieved with CIS/CNFs+O3+Vis and CIS/CNFs+O3, respectively. It indicates that O3-PCO has the optimal toluene mineralization rate and ozone consumption rate in two methods, and toluene mineralization rate is positively correlated with ozone consumption rate. The mineralization rate of toluene is affected to some extent by the ozone consumption rate, but it is not completely dependent on the latter. Meanwhile, in CIS/CNFs+O3+Vis, the ozone utilization rate is over 80%, indicating that ozone is well consumed and utilized.

Influence of relative humidity on ozone-enhanced photocatalysis
In order to explore the influence of relative humidity on the performance of O3-PCO system, the gas streams with various relative humidity including 20%, 40% and 80%, respectively, were introduced into the system with CIS/CNFs-3 as catalyst for tests (Fig. S2). The results show that when the relative humidity is increased from 20% to 80%, removal rate of toluene increases from 60% to 95%, and there exists a positive correlation. As the relative humidity is increased, more water vapor can participate in the photocatalytic reaction, thus generating more hydroxyl radicals with strong oxidability, thereby the removal efficiency of toluene is improved.
In brief, the above experimental result confirmed the photocatalytic activity of CIS/CNFs and showed 95% degradation activity for toluene. At the same time, introduction of ozone into the system not only improved the photocatalytic efficiency, but also increased toluene mineralization rate. In the preliminary experiment, investigated the influence of different concentrations of ozone on the photocatalytic oxidation of toluene. Unfortunately, with the increase of ozone concentration, the ozone utilization rate was decreased, so 200ppm was chosen as the experimental ozone concentration. The N2 sorption isotherms of CIS and CIS/CNFs-3 showed type III character with a well-developed and distinct hysteresis loop (Fig. S4). The BET analysis of CIS material and CIS/CNFs-3 composite material indicates that specific surface areas of CIS and CIS/CNFs-3 composite material are 38.6 m 2 g -1 and 43.3 m 2 g -1 , respectively.

Kinetic analysis
As CNFs is doped, the specific surface area of CIS material is enhanced, a larger specific surface area can expose more surface active sites, which is favorable for the adsorption of toluene and ozone, so the photocatalytic degradation of toluene is also improved.

Crystalline structures
The crystal structure and phase properties of CNFs, CIS, and CIS/CNFs samples with various doping contents were investigated by XRD pattern and the results as in Fig. 4. respectively. As for CNFs, the first wide peak at 2θ=25.0° is corresponding to (002) crystal plane. The sharp and strong diffraction peak of pure CIS indicates that the sample has good crystallinity. As CIS nanoparticles were grown on the carbon fiber surface, the XRD patterns of all CIS/CNFs composites were similar to those of pure CIS materials. The peak in the (311) plane of CIS/CNFs composite material was compared with that of pure CIS as shown in Fig. 2(b), and an obvious shift to lower 2θ values was observed, which may be caused by the strong interaction between CIS and CNFs. Meanwhile, with the increase of CNFs doping content, the deviation of the peak become more intensive. In addition, it was observed that CIS diffraction peaks was broadened and weakened in intensity as 3% carbon fiber was doped. This indicates that doping CNFs is not conducive to CIS crystallinity, the catalyst with poor crystallinity generates more surface defects in the catalyst structure, which is favorable to the adsorption and decomposition of toluene and ozone molecules [28]. A high defect density is conducive to the adsorption of ozone molecules on the surface of the catalyst [29]. The average grain size (D) of the catalysts can be calculated from the line broadening of XRD peaks using Scherrer's formula [30].
where K is a constant (0.89); λ is the wavelength of the X-ray radiation (Cu Kɑ= 0.1541 nm); β is the full width at half-maximum and θ is the angle at position of peak maximum [31]. The as-calculated grain size of monomeric CIS is 27.5 nm, while the grain sizes of series composites CIS/CNFs are 25.6 nm. In general, particle of smaller grain size has a larger specific surface area [29]. What's more, the grain sizes also imply that CIS/CNFs are favorable to the adsorption of toluene and ozone at defect sites. This result was also supported by the data obtained by BET analysis. The above results indicate that CIS/CNFs composite material was successfully synthesized and CIS was not destroyed during the process of CNFs growth.

Morphology of CIS/CNFs photocatalyst
The morphology of the sample is analyzed by SEM and TEM, as shown in Fig. 5.
The diameter of carbon nanofibers is about 300 nm and the lengths are up to several microns. It has a high aspect ratio, and the surface is smooth and closely connected to form a 3D conductive network, which is beneficial to the transmission of electrons generated during light irradiation. The SEM image of CIS is shown in Fig. 5(b), indicating that the CIS particles have 3D octahedral shape, and the illustration clearly shows that CIS octahedral particles have good crystal faces. In addition, as shown in

Structure and composition of CIS/CNFs photocatalyst
The surface element states of the constituent elements in the CIS and CIS/CNFs nanocomposite samples were investigated by XPS spectra as shown in Fig. 6. The spectra reveal all of nanocomposite contain Cd, In, S, and C elements. The highresolution XPS spectra of C 1s as shown in Fig. 6(d) and could be deconvoluted into three peaks centered at 284.6 eV, 285.7 eV and 286.6 eV, respectively. The peak at 284.6 eV is attributed to C-C bond derived from amorphous carbon phase or indefinite carbon [32]. Peaks at 285.7 eV are characteristic of C-O groups. In addition, the weak peak at 286.6 eV was ascribed to carboxyl carbon (OC = O) [33,34]. Fig. 6(a) showed the Cd 3d spectra could be deconvoluted into two peaks, the peaks at 405.5 and 412.26 eV were attributed to Cd 3d5/2 and 3d3/2 states, respectively, showing a normal state of Cd 2+ in sulfide environment [35]. As shown in Fig. 6(b), in the In 3d spectra, the peaks at 444.6 eV and 452.14 eV were respectively assigned to the 3d5/2 and 3d3/2 states when the In 3+ ion is coordination with the sulfide ion [16]. In Fig. 6(c), S 2p spectra were deconvoluted into two peaks at 161.31 eV and 162.49 eV, which were assigned to 2p3/2 and 2p1/2 levels of S 2− , respectively [36]. In addition, the spin orbital splits of Cd 3d, In (d) C 1s.

Photochemical analysis
The photocurrent had been considered as an efficient method to estimate recombination rate of electron-hole pairs [37]. As shown in Fig. 7, the photocurrent spectrum was obtained by using xenon lamp as light source and Na2SO4 as electrolyte.

PL spectra of CIS and CIS/CNFs-3 composites
Photoinduced electron-hole recombination causes negative impact on photocatalyst photodegradation activity and reduce the degradation efficiency of toluene, and the recombination rate of electron-hole is directly proportional to the photoluminescence (PL) intensity [38]. Therefore, PL spectra are used to study the degree of electron hole separation of these photocatalysts. The quenching of PL emission spectra of the CIS/CNFs can be attributed to efficient electron transfer between CIS and CNFs. Despite of the fact that CIS has a narrow band gap and a high spectral response range, visible light can be absorbed and utilized.
However, due to the narrow band gap of CIS, electron-hole pairs are easy to recombine.
On the other hand, the CIS/CNFs composite material can effectively inhibit recombination of the electron-hole pairs and promote the efficient separation of photogenerated carriers due to rapid transfer of interfacial charge. As a result, the photocurrent density of CIS/CNFs composite material is significantly higher than that of CIS. It can be concluded that the addition of CNFs was beneficial to light absorption, effective photoelectron transfer between CIS and CNFs promotes the electron-hole pairs separation, thus improving the corresponding photocatalytic activity.

Photocatalytic recycling and stability of CIS/ CNFs
The stability of catalysts is of great importance to the successful application. In order to test the stability of CIS/CNFs-3 composite materials for ozone-enhanced photocatalysis, the catalyst was reused for 10 times, and each experiment was carried out under the same condition for three tests, and observed the XRD patterns before and after the use of the catalyst, as shown in Fig. 9.

2-Theta (degree)
Intensity (a.u.) Within ten cycles of the experiment, CIS/CNFs-3 composite material reveals low level of photocatalytic activity reduction, degradation efficiency decreases slightly from 95% to 90.42% during 3 h tests, indicating that used sample still has good photocatalytic activity. The composite of carbon nanofibers can effectively inhibit the photo-corrosion and ozone corrosion. Fig. 9(b) shows the XRD pattern of CIS/CNFs-3 composite before and after XRD pattern of before and after 10 times repeated use. After 10 times reactions, the peak position and ratios were almost unchanged compared with the fresh photocatalyst. The results proved that the micro-morphology of the photocatalyst did not change after the 10 times repeated tests, indicating that CIS/CNFs-3 has significant stability for ozone-enhanced photocatalysis.

Before After
3.6 Possible degradation mechanisms 3.6.1. Photocatalytic degradation mechanism Based on the intermediates detected, possible pathways of toluene degradation were proposed as shown in Fig. 10. With the irradiation of visible light, CIS is excited to produce photogenic electrons and holes. Due to phenomenal conductivity of carbon nanofibers, the conduction band of CIS is excited to generate photogenerated electrons and transfer to carbon nanofibers. Moreover, the close combination of CIS and CNFS provides a good transmission platform for photogenerated carriers. Therefore, the photogenerated electron-hole pairs can be separated effectively. In the process of toluene degradation, the holes in the CIS valency band oxidize H2O(g) molecules to O2 and H + , then photogenerated electrons on carbon nanofibers combine with O2 to produces O2 -• (superoxide radical) with strong redox and further react with water molecules to generate OH• (hydroxyl radical). According to the above results, the transport and degradation mechanism of the photo-generated carrier of CIS/CNFs composite photocatalyst can be described by the following equation: According to the stoichiometry, one oxygen molecule reacts with three electron holes and two water molecules to produce four OH• [9,39]. Toluene is then oxidized to carbon dioxide and water [40][41][42]. The above experimental results proved that during ozone into the PCO system not only improves the removal efficiency of toluene, but also contributes significantly to the mineralization of toluene. At present, the mainstream view believes that its mechanism is associated the synergistic effect of mutual influence [13]. It is believed that O• and OH• played major roles in O3-PCO system and occur in CIS/CNFs+O3 and CIS/CNFs +Vis+O3.
In the CIS/CNFs+O3 process, oxygen molecules react with active sites of the photocatalyst, O• reacts further with water vapor in the gas phase, and one ozone produces two OH•. At the same time, as a high electron affinity gas, ozone is easier to capture the electrons generated by light, thus reducing the electron-hole pair recombination rate and accelerating the formation of OH• [43].
With the irradiation of visible light, CIS is excited to produce photogenic electrons and holes in the CIS/CNFs +Vis+O3 process. At the same time, O2 -•, OH -, and H + are generated. The photogenic electrons react with ozone to generate O3 -, which combines with H + to generate HO3• and further decomposes to OH•. Subsequently, ozone reacts with the OHgenerated in photocatalysis to form O3 -•and hydrogen peroxyl radical (HO2•), which further produces the highly oxidizing superoxide radical (O2 -•), and the above process is repeated.
The effect of humidity on toluene removal achieved with O3-PCO indicate that water molecules play an important role in the initial stage of O3-PCO chain reaction. However, in the case of high humidity, ozone molecules and water molecules compete for adsorption sites on the catalyst surface, which is not conducive for toluene removal [44].
In summary, two powerful oxidants which OH• and O•, are involved two ways in the degradation of toluene.
(1) When OH• is the main oxidant oxidizing for toluene, OH• was H-abstraction from methyl resulting in the production of benzyl alcohol and benzaldehyde, which is further attacked by OH• oxidant and then the aromatic ring opens, gradually forming CO2 and H2O. In this pathway ( Fig. 11(a)), byproducts such as formic acid and benzaldehyde are abundant [45,46].
(2) The primary pathway of toluene oxidation by O• is via abstraction of two H atoms from methyl to directly produce benzaldehyde which is then opened after being continuously attacked by O•, and then reacts similar to OH• [13,47]. In this path, the intermediate product is only benzaldehyde (Fig. 11(b)). Compared with OH•, the reaction steps with O• as oxidant are shorter, the intermediates are fewer, and the reaction is faster. As will be readily seen, compared with PCO reaction, O3-PCO process produces more oxidants and less byproducts.
The above analysis is based on the previous studies and analyses of the by-products attached to the photocatalyst surface. The above processes are all based on the degradation of toluene under the ideal state. We observed some intermediates using GC-MS, as shown in Fig. S5 respectively. Meanwhile, the undecomposed toluene characteristic peak appears at 6 min [48]. Therefore, according to ideal state and the actual observation, we summarized reactions leading to toluene degradation in O3-PCO system (Fig. 11).
(A) OH• degradation process (B) O• degradation process Fig. 11 Possible mechanism for ozone-enhanced photocatalysis of toluene Therefore, when the above multiple reactions are combined on the surface of photocatalyst as a more powerful oxidant, ozone greatly increases the amount and rate of OH• and O• formation. Moreover, due to the high electron affinity of ozone, it is easier to capture the electrons that have transited to the conduction band after being exposed to visible light, which means that it is easier to decrease the recombination rate of the electron-hole pairs by scavenging photo-induced electrons. As a result, the efficiency of photocatalytic oxidation toward toluene was improved.

Conclusions
In this study, CdIn2S4/CNFs were synthesized by a simple hydrothermal method, and the composite materials were irradiation by visible light to degrade toluene, besides take advantage of ozone to enhanced degradation efficiency. The successful preparation of CIS/ CNFS composites by hydrothermal method was confirmed by XRD and XPS.
Compared with CIS nanoparticles, CIS/CNFs composite materials have a higher specific surface area, leading to CIS/CNFs have significant adsorption capacity. The 3D conductive network formed by CIS nanoparticles and carbon nanofibers was closely combined as shown in SEM and TEM. The results showed that CNFs (about 300 nm in diameter) were well connected with CIS to form 3D conductive network and the CIS with 100 nm in average particle size were uniformly grown onto the surface of CNFs.
Under visible light irradiation, the degradation efficiency of toluene achieved with CIS doped with 3% CNFs is 86%. For purpose of further improve the efficiency, with the introduction of 200 ppm ozone, the toluene removal efficiency is increased to 95%.
Ozone catalytic oxidation has significantly improved toluene removal efficiency and mineralization rate. Meanwhile, after ten times of repeated use, the photocatalytic activity decreased by only 3.42%, proving that the CIS/CNFs composite material has eximious repeated use performance. Through photocurrent and PL spectrum analysis, it is found that the CIS/CNFs composite material has a better photoelectron and hole separation rate than CIS nanoparticles. The results show that the CIS/CNFs composite material has higher quantum efficiency and improved its photocatalytic activity.