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Article

Multi-Walled Carbon Nanotubes Modified NiCo2S4 for the Efficient Photocatalytic Reduction of Hexavalent Chromium

by
Qiu Jin
,
Ziye Zheng
,
Yuxiao Feng
,
Shuang Tian
and
Zuoli He
*
Shandong Key Laboratory of Water Pollution Control and Resource Reuse, School of Environmental Science and Engineering, Shandong University, Qingdao 266237, China
*
Author to whom correspondence should be addressed.
C 2023, 9(4), 99; https://doi.org/10.3390/c9040099
Submission received: 15 July 2023 / Revised: 15 October 2023 / Accepted: 17 October 2023 / Published: 20 October 2023
(This article belongs to the Special Issue Carbon-Based Catalyst (2nd Edition))
Browse Figures
Versions Notes

Abstract

:
Hexavalent chromium (Cr(VI)) compounds are considered to be occupational carcinogens, which can be transferred from the environment to the human body and pose a significant threat to human health. It is particularly urgent to explore a more efficient catalyst for removing Cr(VI) to comply with discharge standards. The addition of CNTs enables the separation and transfer of photogenerated charges. Thus, we synthesized a range of NiCo2S4 hybrid materials with different multi-walled carbon nanotube (MWCNTs) contents using a two-step hydrothermal method. The composites had significant advantages compared to pure NiCo2S4, such as an enhanced visible light absorption, increased specific surface area, high electron–hole pair separation, and fast electron transport. Thus, MWCNT addition enabled efficient photocatalytic performances in terms of reducing hexavalent chromium (Cr(VI)). Among all the composite samples, the MWCNT/NiCo2S4 with 0.050 g of MWCNTs achieved the highest efficiency in reducing Cr(VI) under light irradiation, which showed a removal rate close to 100% within 40 min. Such CNT-based composite photocatalysts could be used to reduce the highly toxic Cr(VI) in environmental applications.

1. Introduction

In the past decades, water resource pollution has become more and more severe with industrialization development [1]. Heavy metal pollution due to the massive application of heavy metals in industry is gaining widespread attention [2,3]. Among them, hexavalent chromium (Cr(VI)) is often used as an essential raw material in electroplating, pigment processing, leather making, photographic, textile, and other applications. Chromium is usually present in two different oxidation states, hexavalent chromium (Cr(VI)) and trivalent chromium (Cr(III)) [4,5]. Cr(III) is less toxic in the environment and plays a vital role in metabolic processes as a trace metal essential to human nutrition [6,7]. In addition, it can be readily precipitated out of wastewater as Cr(OH)3 in an alkaline or neutral solution and removed as solid waste. Unfortunately, Cr(VI) is 100 times more toxic than Cr(III), which the US has classified as a top-rated toxic pollutant. This was by the Environmental Protection Agency (EPA) due to its dangerous persistence and extremely low degradability in water and soil. Thus, the reduction of Cr(VI) to Cr(III) is considered to be a critical step in the treatment of wastewater containing Cr(VI) [8,9,10]. In addition, Cr(VI) can quickly enter food chains and destroy human enzyme systems through ingestion, inhalation, or skin contact, which poses a considerable threat to human health [11,12]. Studies have shown that releasing potentially carcinogenic, highly toxic, and non-degradable Cr(VI) into the environment can cause health problems such as pulmonary gland obstruction, epidermal irritation, hepatitis, and cancer [11,13,14]. Therefore, the conversion of Cr(VI) into Cr(III) is a desirable method for strictly controlling the concentration of Cr(VI) in industrial wastewater so that the wastewater can be safely discharged.
Various conventional methods have been adopted to remove Cr(VI) from water environments, including ion exchange, adsorption, chemical reduction, bioremediation, physical reverse osmosis, and membrane separation [15,16,17,18,19,20]. However, traditional methods often result in ancillary contaminants, a high energy intake, and a low efficacy, which are relatively expensive and environmentally unfriendly [21,22]. Actual industrial wastewater is often treated using the chemical reduction method, which reduces Cr(VI) to Cr(III) by adding reducing agents (such as NaHSO3, Na2SO3, and iron powder, etc.) to the wastewater, and then adding lime or sodium hydroxide for precipitation and separation [23,24]. The reducing agent needs to be added in an acidic environment, and the amount of reducing agent added must be more significant than the theoretical amount, but a large amount would cause the wastewater chemical oxygen demand to rise [25]. The main advantages of this method are its relatively mature technology, simple operation, low investment, and good results in engineering applications. However, it has a large amount of sludge and will produce secondary pollution, which is not in line with the development concept of green environmental protection.
Photocatalysis is a promising technology for achieving the one-step removal of Cr(VI) using sunlight due to its efficient, simple, and environmentally friendly features [26,27,28,29]. It has been extensively studied in environmental and energy fields [30,31,32,33]. More and more semiconductor photocatalysts [34] are being investigated and applied for Cr(VI) reduction, hydrogen production, carbon dioxide reduction, and the degradation of organic pollutants such as TiO2 [35,36,37,38], g-C3N4 [39,40,41], ZnIn2S4 [42,43], NiCo2S4 [43], CuS [44], and BiVO4 [39,45]. Unmodified photocatalysts tend to have a short photogenerated carrier lifetime and low specific surface area, leading to inefficient photocatalysis, which limits their development and practical application [46]. As an ideal additive phase for composite materials, carbon nanotubes (CNTs) have excellent electrical and mechanical properties, and their suitable pore size distribution allows for a better dispersion of metal components [43,47,48,49,50]. Moreover, their unique and stable structure and morphology, especially their surface properties, can be modified in different ways according to the needs of people, making them an ideal new catalyst carrier material [51]. The research on CNTs as catalyst carrier materials has mainly focused on loading active components onto CNTs [52,53,54]. In the photocatalysis of a single semiconductor, the photogenerated electrons migrate to the conduction band (CB) and then tend to return to the valence band (VB) again, which significantly reduces the photocatalytic efficiency. The addition of CNTs enables the separation and transfer of photogenerated charges [25,55]. In this study, CNTs were hybridized with NiCo2S4. CNTs as a carrier not only inhibited the aggregation of NiCo2S4 and increased the specific surface area of the catalysts, but also effectively inhibited the recombination of photogenerated carriers through charge transfer. The research content of this paper is instructive for the preparation and application of CNTs in photocatalysts.
Herein, a series of MWCNT-NiCo2S4 (C-NCS) composites with different MWCNT contents were prepared using a relatively facile two-step hydrothermal process. The synthesized composites were utilized for the photocatalytic reduction of Cr(VI) under UV-Visible radiation to evaluate their photocatalytic activity. The materials were characterized in detail using XRD, SEM, TEM, XPS, and BET, etc. The selective removal performance of the composites for Cr(VI) was also explored. Compared to the pure NiCo2S4, the composites with added MWCNTs showed a better photocatalytic activity. Among them, 0.050MWCNT-NiCo2S4 had the best performance in the photocatalytic reduction of Cr(VI).

2. Materials and Methods

2.1. Synthesis of Composite Photocatalysts

Firstly, 0.4 g of PVP (K30) was added to 50 mL of ultrapure water and ultrasonically dispersed well. In total, 0.050 g of multi-walled carbon nanotubes (MWCNTs) was added to the above solution, followed by 30 min of treatment using an ultrasonic reactor (SONICS VCX1500, SONICS & MATERIALS INC, 53 CHURCH HILL RD. NEWTOWN, CT, USA). The dispersion was stirred until it was cooled, then 0.1783 g of nickel chloride hexahydrate, 0.3568 g of cobalt chloride hexahydrate, 0.45 g of urea, and 10 mL of ultrapure water were added and stirred for 30 min. It was added to the hydrothermal kettle and reacted at 130 °C for 12 h. The obtained sample (precursor of 0.050C-NCS) was washed with ultrapure water and 75% alcohol and finally dried in an oven at 70 °C. In the second hydrothermal reaction step, 0.2 g of as-obtained product, 0.6 g of sodium sulfide nonahydrate, and 60 mL of ultrapure water were added to the hydrothermal kettle, sonicated for 2 min, stirred for 30 min, and then reacted hydrothermally at a temperature of 180 °C for 12 h. Similarly, various amounts of MWCNTs (0 g, 0.005 g, 0.020 g, 0.035 g, 0.050 g, 0.075 g, and 0.100 g) were added in the first step of the hydrothermal process to synthesize the MWNCT-NiCo2S4 composite samples. The obtained composite samples were named NCS, 0.005C-NCS, 0.020C-NCS, 0.035C-NCS, 0.050C-NCS, 0.075C-NCS, and 0.100C-NCS, respectively.

2.2. Photocatalytic Performance Measurements

The photocatalytic activity of the samples was evaluated by reducing Cr(VI) (K2Cr2O7 solution). In this reaction, 15 mg catalysts were dispersed into 30 mL of 50 ppm Cr(VI) solution and then stirred in the dark for 30 min to reach the absorption–desorption equilibrium. Next, the suspension was irradiated with a 300 W Xenon lamp (CEL-HXF300-T3, Beijing China Education Au-Light Co., Ltd, Beijing, China) with a VisREF filter (350~780 nm). A total of 1 mL of sample was, intermittently and in specific time intervals, filtered through a 0.22 µm pore-sized PTFE syringe filter to determine the removal rate of Cr(VI). The concentration of Cr(VI) was determined with a UV-Vis spectrophotometer (Shimadzu UV2600, Shimadzu corporation, Nishinokyo Kuwabara-cho, Nakagyo-ku, Kyoto, Japan) using a diphenylcarbonyldihydrazide colorimetric method.

3. Results

3.1. Photocatalytic Performances

We synthesized NiCo2S4 nanoparticles with different MWCNT contents using a two-step hydrothermal method, adding 0 g, 0.005 g, 0.020 g, 0.035 g, 0.050 g, 0.075 g, and 0.100 g. The products obtained were defined as NCS, 0.005C-NCS, 0.020C-NCS, 0.035C-NCS, 0.050C-NCS, 0.075C-NCS, and 0.100C-NCS, respectively. The photocatalytic performances of the samples were evaluated using a 60 min reduction experiment of Cr(VI) with a controlled amount of photocatalyst under visible light irradiation, as shown in Figure 1a. A concentration of 30 mL of 50 ppm Cr(VI) was chosen for the photoreduction experiments. A schematic diagram of this reactor is demonstrated in Figure S1. The results showed that, when using pure NiCo2S4 as the photocatalyst, Cr(VI) could only be reduced by 64.4% in 60 min. However, all composites containing MWCNTs exhibited an excellent photocatalytic activity. In particular, 0.050C-NCS could reduce Cr(VI) entirely within 40 min, which exhibited the best photocatalytic performance. Figure 1b shows the results of the control experiment for the reduction of Cr(VI) by 0.050C-NCS, and it can be seen that the concentration of Cr(VI) was almost constant under light conditions only. At the same time, only 17.5% of Cr(VI) was adsorbed under dark conditions, and the MWCNT had almost no photocatalytic effect on Cr(VI), which further demonstrates the essential role of this composite in the photocatalytic reduction process of Cr(VI). Furthermore, the electron spin resonance (ESR) detection results (Figure S2) also indicated that a distinct ·O2- the characteristic signal peak was detected at 0.050C-NCS. In contrast, the detection of pristine NiCo2S4 indicated that the intensity of the O2− signal was significantly lower than that of 0.050C-NCS. This result also demonstrated an improvement in the photocatalytic performance when a certain amount of MWCNT was added. By verifying the effect of scavengers on photocatalytic efficiency, the active species with contributing roles in the photocatalytic process can be elucidated [56]. As shown in Figure S3, potassium bromate (10 mM) hindered the photocatalytic reduction process, which demonstrates that the photoelectrons were mainly responsible for the Cr(VI) photocatalytic reduction enhancement of the 0.050C-NCS. Furthermore, we compared the reaction conditions and reduction activity of different photocatalysts reported in the recent literature with the present work, and the results are listed in Table S1. Compared to previous studies, 0.050C-NCS exhibited an excellent photocatalytic reduction of Cr(VI).
The separation rate of photogenerated electron–hole pairs is an essential factor affecting the photocatalytic activity of a photocatalyst. A higher separation efficiency of the photogenerated electron–hole pairs will result in a higher photocatalytic activity of the photocatalyst. To evaluate photocatalysts’ charge carrier separation and transport efficiency, photocurrent and Electrochemical Impedance Spectroscopy (EIS) analyses were conducted. The photocurrent response in Figure 2a indicates that 0.050 CNT-NiCo2S4 showed the highest photocurrent density because of a much lower charge carrier recombination in 0.050C-NCS. The improved separation efficiency and lowest recombination rate of the photogenerated electron–hole pairs facilitated efficient photocatalytic reactions [57,58]. The photoreaction and separation of carriers are effective when the arc-radius of the EIS plot is less [59,60]. The EIS of the fabricated photocatalysts was analyzed to investigate the charge carriers’ interface transfer/recombination efficiency. As can be seen from Figure 2b, 0.050C-NCS presented a much smaller arc size than the pure NiCo2S4, demonstrating that 0.050C-NCS had a lower electron transfer resistance and a positive effect on photo-induced carrier transfer. The Nyquist data of 0.050C-NCS were fitted with an equivalent circuit via Zview software (2.1.0.0) to determine the exact values of the RESR and Rct resistances [61,62]. Figure S4 presents the fitted model of 0.050C-NCS with the most suitable modified Randle circuit model in the inset image. The value of the RESR and Rct of the 0.050C-NCS sample electrode came to be 36.77 and 66.31 Ω, respectively. All of the above results demonstrate the excellent photocatalytic performance of 0.050C-NCS, consistent with the trend of the photocatalytic reduction results.
The effect of inorganic ions on photocatalytic activity has been reported in many studies. Among them, inorganic anions, such as NO3−, Cl−, SO42−, and PO43−, may have two inhibitory effects: the first is the competitive adsorption of ions and pollutant molecules to the active sites on the TiO2 surface, leading to a decrease in the photocatalytic efficiency. The second is that anions can react with the photogenerated holes, thus reducing the amount of oxides that degrade the pollutant molecules [12,63]. We investigated the effect of coexisting ions in the photocatalytic reduction process of Cr(VI). In Figure 3a, it is seen that the inhibition of the Cr(VI) reduction process was enhanced when the concentration of NaCl was gradually increased, which might have been due to the competition between chloride ions and Cr(VI) adsorption. Similarly, the addition of 10 mM of HCO3− and SO42− to the solution had a relatively strong inhibitory effect on the reduction process of Cr(VI) (Figure 3b). The effect of cations on the reduction process of Cr(VI) was investigated in Figure 3c, from which it can be seen that the addition of Cu2+ significantly improved the photocatalytic reduction rate of Cr(VI). This is because the positively charged Cu2+ was more likely to capture the electrons generated by photoexcitation on the catalyst surface than the negatively charged Cr2O72−, and Cu2+ was preferentially reduced to Cu+. At the same time, Cu+ could rapidly undergo redox reactions with Cr2O72−, with Cr2O72− being reduced to Cr3+ and Cu+ being oxidized to Cu2+. Since Cu2+ could capture the electrons generated on the catalyst surface faster than Cr2O72−, the recombination rate of the electron–hole pairs on the catalyst surface was significantly reduced, which resulted in a higher utilization of electrons on the catalyst surface. Therefore, a significant enhancement in the photocatalytic reduction rate occurred when a small amount of Cu2+ was added to the reaction system. The above process shows that Cu2+ plays a catalytic role in the photocatalytic reduction process of Cr(VI). As shown in the figure, the presence of cations such as Mg2+, Ca2+, K+, and Na+ in the solution will reduce the reduction efficiency of Cr(VI). This inhibitory effect is mainly because these ions can quickly get the photoexcited electrons on the semiconductor conduction band, thus competing with Cr(VI) for electrons, inevitably reducing the reduction efficiency.

3.2. Structural and Morphological Analysis

The X-ray diffraction (XRD) results of the NiCo2S4 with different MWCNT contents are shown in Figure 4. For the samples, eleven diffraction spikes were recorded at 26.8°, 31.6°, 33.0°, 38.3°, 47.4°, 50.5°, 55.3°, 58.1°, 62.5°, 65.1°, and 69.3°, respectively, indexed as 022, 031, 222, 040, 242, 511, 044, 253, 620, 053, and 244 using JCPDS No. 20-0782 [64]. It can be clearly seen from the image that the diffraction peaks of these as-prepared composites were relatively narrow and independent, indicating that the composites exhibited a better crystalline state. However, no prominent diffraction peaks of the MWCNTs were observed, indicating that they were highly dispersed in the composites. With the introduction of MWCNTs, the NiCo2S4 was relatively dispersed, and the peak intensity decreased. The change in grain size can be reflected in the peak width of the diffraction peak [65]. It can be seen from the graph that the grain size of the pure NiCo2S4 was slightly larger. Furthermore, the grain sizes of the NiCo2S4, 0.020C-NCS, 0.050C-NCS, and 0,100C-NCS photocatalysts were further calculated to be 321, 262, 136, and 148 nm, respectively, by using the Debye–Scherrer method [66,67]:
D = k λ / β cos θ
The photocatalytic activity generally increased with a decrease in the grain size [68]. The larger grain size of NiCo2S4 indicated that the photocatalytic reduction reaction of Cr(VI) with the pure NiCo2S4 had poor performance. The previous experimental results of photocatalytic reduction also confirmed this conclusion.
The structure and morphology of 0.050C-NCS were further characterized using SEM and TEM, as shown in Figure 5. From the SEM of 0.050C-NCS (Figure 5a,b), there were a large number of NiCo2S4 nanoparticles clustered on the surface of the carbon nanotubes. The appearance of the carrier MWCNTs and nanoparticle shape of NiCo2S4 proved that the NiCo2S4 nanoparticles were successfully loaded on the MWCNTs via hydrothermal synthesis. The nanostructure of 0.050C-NCS was further characterized using transmission electron microscopy (TEM), as shown in Figure 5c. The results of the TEM further observed that the morphology of 0.050C-NCS showed an apparent hollow tubular structure. In the high-resolution TEM (HR-TEM) image of 0.050C-NCS, it exhibits an ordered lattice structure (Figure 5d). The measured lattice fringe spacing between two adjacent planes was 0.27 nm, which matched well with the (222) lattice plane of NiCo2S4, indicating the successful synthesis of NiCo2S4 materials. In addition, the element distribution of 0.050C-NCS (Figure 5e) was tested using SEM-EDS. The result showed that 0.050C-NCS comprised Ni in a light blue color, Co in a green color, S in a yellow color, C in a blue color, and N in red color, indicating the successful synthesis of 0.050C-NCS. Additionally, the Energy dispersive spectroscopy of 0.050 CNT-NiCo2S4 is presented in Figure S5.

3.3. Nitrogen Adsorption–Desorption Experiment

The specific surface area of photocatalysts is one of the critical factors affecting the performance of catalysts. The higher the specific surface area, the more reaction sites of the catalyst, which is beneficial to enhancing the photocatalytic performance of the catalyst. We can obtain the type of BET isotherm from the four informative sections of the BET plot: initial steepness, rounded knee, plateau, and final steepness. The prepared photocatalyst’s BET isotherm (Figure 6 or Figures S6–S8) was of type II, as it had a low saturation pressure, an extended plateaus, and an almost vertical rise at a higher relative pressure, which expresses the presence of a macro-sized pore. The higher the relative pressure, the more adsorption occurs, so the interaction between the adsorbent and the adsorbate is relatively weak. The photocatalyst’s hysteresis was H3 type (slit-type pores) that almost diffused or overlapped with the adsorption curve, expressing the simple adsorption on and desorption from the surface [69]. The H3 hysteresis loops and III-type isotherms proved the mesoporous nature of the catalysts, and the pore size ranges counted in Table 1 were all within the mesoporous range. The specific surface area (SBET), pore volume, and pore size of the prepared samples are presented in Table 1. The SBET of the samples gradually increased with an increase in the MWCNT addition. Among them, the SBET and pore volume of 0.050C-NCS increased to some extent compared to the pure NiCo2S4, which might have been due to the doping of MWCNTs inhibiting the crystallinity of NiCo2S4, resulting in a decrease in the grain size, thus increasing the SBET and pore volume. Increasing the pore volume and SBET of photocatalysts could provide more reaction sites for reducing Cr(VI). However, too much MWCNT addition will lead to the encapsulation of NiCo2S4 by the MWCNTs, reducing the photocatalytic performance.

3.4. Composition and Band Structure Analysis

To further determine the chemical compositions of the elements in the samples, 0.050C-NCS was characterized using X-ray photoelectron spectroscopy (XPS), and the results are shown in Figure 7. The peak positions of C, Ni, Co, S, N, and O can be observed in the survey spectra of Figure 7a, and the distribution of each element was relatively uniform. Specifically, the element N was present at a binding energy of 400 eV, and its content was relatively low. The absorption of some oxygen-containing substances in the air by the material might have caused the presence of the O 1s characteristic peak. As shown in Figure 7b, 0.050C-NCS showed two satellite peaks and two spin-orbit double peaks. The two spin-orbit double peaks were about 853.1 eV and 870 eV, corresponding to Ni2+, and the peaks were about 855.9 eV and 873.8 eV, corresponding to Ni3+, which proved the presence of Ni2+ and Ni3+ in 0.050C-NCS [58]. As shown in Figure 7c, in 0.050C-NCS, there were two spin-orbit double peaks at about 778.5 eV and 793.3 eV, corresponding to Co3+, and at about 797.3 eV and 781.08 eV, corresponding to Co2+. As shown in Figure 7d, the S 2p spectrum showed two peaks at 162.5 eV and 161.5 eV, attributed to S 2p1/2 and S 2p3/2, which showed the valence state of S2− [58]. The fractional peak at 163.5 eV was attributed to the metallic sulfur (M-S) in 0.050C-NCS, demonstrating the presence of the Ni-Co-S phase [63]. The other peak with a binding energy of 168.6 eV represented the oxidized S bond, meaning that some surface sulfide ions were oxidized during the XPS test. The C 1s spectrum was decomposed into three characteristic peaks at 288, 285.8, and 284.8 eV (Figure 7e), which could be attributed to the C=O/C=N, C-O/C-N, and C=C/C-C bonds, respectively [70,71]. From the N 1s spectra, the nitrogen atoms contained different oxidation states, including pyridine-N (398.4 eV), pyrrole-N (400.1 eV), and pyridine-N-oxide (402.2 eV) (Figure 7f) [72].
To investigate the band structure alignment of 0.050C-NCS, UV-Vis Diffuse Reflectance Absorption Spectra (DRS) were obtained, as shown in Figure 8. As shown in Figure 8a, 0.050C-NCS presented a clear absorption edge at around 485.2 nm. The optical energy gap was calculated by using the given equation: αhυ = A(hυ − Eg)n, where h, α, υ, A, and Eg are the equation parameters [73]. The bandgap energy of 0.050C-NCS was 2.56 eV (Figure 8b). Figure S9 shows that the absorption edge of the pure NiCo2S4 was at 417.2 nm. The light absorption of 0.050C-NCS showed a light red shift compared to the pure NiCo2S4, indicating that introducing MWCNTs enhanced the light absorption performance. As shown in Figure 8c, the slope of the Mott–Schottky plot is positive, so 0.050C-NCS belonged to the n-type semiconductor. Generally, the flat band potential was close to the bottom of the conduction band of the n-type semiconductor. The flat band potential calculated from the x-intercept of the linear regions of this plot was −0.63 eV (vs. Ag/AgCl) for 0.050C-NCS (−0.43 eV, vs. NHE). The conduction band was smaller than the flat band by 0.2 eV. Thus, the conduction band of 0.050C-NCS was −0.63 eV (vs. NHE). According to the equation EVB = ECB + Eg [74], the valence band position (EVB) of 0.050C-NCS could be calculated as 1.93 eV. The relative band structure alignment of 0.050C-NCS is illustrated in Figure 8d. Also, the potential of ECr6+(Cr2O72−/Cr3+) could be calculated as 1.33 eV when the solution was acidic. Therefore, Cr(VI) can be easily reduced by the 0.050C-NCS photocatalyst system. Adding MWCNTs accelerated the charge transfer, which was more conducive to the reaction.
The results indicated that MWCNTs incorporated into NiCo2S4 nanoparticles improved the efficiency of visible light absorption and excitation separation. The high activity of 0.050C-NCS was attributed to the high charge transfer and low photoexcited electron–hole recombination properties. The good chemical and thermal stability, large surface area, and excellent mechanical properties of MWCNTs make them ideal as support materials for hybridization with photocatalysts for applications in modifying photocatalysts. When MWCNTs were hybridized with NiCo2S4, they not only exerted the advantage of inhibiting photocatalyst aggregation. Moreover, they also acted as a sink in the transfer of photogenerated electrons. In this photocatalyst system, MWCNTs acted as “electron sinks” to receive the photogenerated electrons from NiCo2S4, effectively separating the electron–hole pairs. Thus, the recombination of photogenerated carriers was effectively suppressed. Moreover, the photogenerated electrons on MWCNT could participate in the photocatalytic reaction and effectively photocatalytically reduce Cr(VI) to Cr(III). In addition, the reaction equation for the photocatalytic reduction of Cr(VI) to Cr(III) is as follows [45]:
Cr2O72− + 14 H+ + 6e− → 2Cr3+ + 7 H2O

3.5. Stability of the Catalyst

In addition, as a photocatalyst with an excellent performance, it should remain stable under repeated cycles. Cyclic photocatalytic experiments confirmed the stability of 0.050C-NCS. As shown in Figure 9, the results showed that 0.050C-NCS maintained a high reduction rate after being used five times. It maintained a 91.4% reduction rate in the fourth photocatalytic Cr(VI) reduction, while a sharp decrease occurred in the fifth cycle. In general, 0.050CNT-NiCo2S4 had a relatively good application for reducing Cr(VI) in the first four cycles.

4. Conclusions

In summary, a facile two-step hydrothermal method was used to prepare CNT-NiCo2S4 composites with different MWCNT contents. The composite samples showed excellent photocatalytic activities with a good reduced Cr(VI) stability. The 0.050C-NCS sample could reduce Cr(VI) within 40 min, attributed to the large specific surface area, enhanced visible light absorption, and efficient separation and transport of photogenerated electron–hole pairs resulting from the MWCNT introduction. MWCNTs could receive photogenerated electrons from NiCo2S4 and effectively separate the photogenerated electron–hole pairs. Therefore, the recombination of photogenerated carriers was effectively suppressed. After the addition of MWCNTs, an improvement in the visible light absorption and carrier separation efficiency of NiCo2S4 was realized, and the photogenerated electrons on the MWCNTs could effectively photocatalytic reduce Cr(VI) to Cr(III). Therefore, the MWCNT-based composite photocatalyst could effectively reduce Cr(VI). This work’s strategy for building a CNT-based photocatalyst provides a promising method for completely removing high-valence heavy metal ions.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/c9040099/s1. Figure S1: The schematic diagram of the reactor; Figure S2: ESR spectrum of the •O2− produced by NiCo2S4 and 0.050 CNT-NiCo2S4 under visible-light irradiation; Figure S3: Effect of scavengers on photocatalytic degradation of Cr(VI) during the trapping experiments of the 0.050 CNT-NiCo2S4; Figure S4: Circuit fitted Nyquist plot of the 0.050C-NCS with a suitable modified Randle circuit (inset); Figure S5: Energy dispersive spectroscopy of 0.050 CNT-NiCo2S4; Figure S6: N2 adsorption and desorption isotherms and corresponding pore-size distribution curves (inset) for 0.005 CNT-NiCo2S4; Figure S7: N2 adsorption and desorption isotherms and corresponding pore-size distribution curves (inset) for 0.035 CNT-NiCo2S4; Figure S8: N2 adsorption and desorption isotherms and corresponding pore-size distribution curves (inset) for 0.075 CNT-NiCo2S4; Figure S9: UV-Vis Diffuse Reflectance Absorption Spectra (DRS) of as-prepared samples with different amounts of CNT; Table S1: Comparison of photocatalytic efficiency for photocatalytic reduction of Cr(VI) between previously reported literature and the present work.

Author Contributions

Methodology, validation, formal analysis, writing—original draft, Q.J.; investigation, data curation, visualization, Y.F., S.T. and Z.Z.; writing—review and editing, conceptualization, supervision, funding acquisition, Z.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Young Taishan Scholars Program of Shandong Province (no. tsqn.201909026), the Youth Interdisciplinary Science and Innovative Research Groups of Shandong University (no. 2020QNQT014), the Shandong University Future Youth Grant Program (no. 61440089964189).

Data Availability Statement

Data will be made available on request.

Acknowledgments

The authors thank Sen Wang from State Key Laboratory of Microbial Technology of Shandong University for assisting with SEM analysis.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

References

  1. Li, J.; You, J.; Wang, Z.; Zhao, Y.; Xu, J.; Li, X.; Zhang, H. Application of alpha-Fe2O3-based heterogeneous photo-fenton catalyst in wastewater treatment: A review of recent advances. J. Environ. Chem. Eng. 2022, 10, 108329. [Google Scholar] [CrossRef]
  2. Li, S.; Cai, M.; Wang, C.; Liu, Y.; Li, N.; Zhang, P.; Li, X. Rationally designed Ta3N5/BiOCl S-scheme heterojunction with oxygen vacancies for elimination of tetracycline antibiotic and Cr(VI): Performance, toxicity evaluation and mechanism insight. J. Mater. Sci. Technol. 2022, 123, 177–190. [Google Scholar] [CrossRef]
  3. Kholisa, B.; Matsena, M.; Chirwa, E.M.N. Evaluation of Cr(VI) reduction using indigenous bacterial consortium isolated from a municipal wastewater sludge: Batch and kinetic studies. Catalysts 2021, 11, 1100. [Google Scholar] [CrossRef]
  4. Cheng, L.; He, R.L.; Min, D.; Li, W.W.; Liu, D.F.; Yu, H.Q. Engineering a rhamnose-inducible system to enhance the extracellular electron transfer ability of Shewanella genus for improved Cr(VI) reduction. ACS EST Eng. 2021, 1, 842–850. [Google Scholar] [CrossRef]
  5. Jin, Q.; Dai, M.; Zhan, X.; Wang, S.; He, Z. Carbon nanotubes and graphene composites used in Cr(VI) detection techniques: A review. J. Alloys Compd. 2022, 922, 166268. [Google Scholar] [CrossRef]
  6. Djouider, F. Radiolytic formation of non-toxic Cr(III) from toxic Cr(VI) in formate containing aqueous solutions: A system for water treatment. J. Hazard. Mater. 2012, 223, 104–109. [Google Scholar] [CrossRef] [PubMed]
  7. Meinhold, V.; Hoehlich, D.; Mehner, T.; Lampke, T. Electrodeposition of thick and crack-free Fe-Cr-Ni coatings from a Cr (III) electrolyte. Coatings 2022, 12, 56. [Google Scholar] [CrossRef]
  8. Qi, Y.; Jiang, M.; Cui, Y.-L.; Zhao, L.; Liu, S. Novel reduction of Cr(VI) from wastewater using a naturally derived microcapsule loaded with rutin-Cr(III) complex. J. Hazard. Mater. 2015, 285, 336–345. [Google Scholar] [CrossRef]
  9. Xu, J.-J.; Gu, H.-Y.; Chen, M.-D.; Li, X.-P.; Zhao, H.-W.; Yang, H.-B. Dual Z-scheme Bi3TaO7/Bi2S3/SnS2 photocatalyst with high performance for Cr(VI) reduction and TC degradation under visible light irradiation. Rare Metals 2022, 41, 2417–2428. [Google Scholar] [CrossRef]
  10. Qin, C.; Pan, G.; Zhang, Y.; Ding, F.; Qu, J.; Xu, X.; Su, X. Efficient reduction of Cr (VI) to Cr (III) over a TiO2-supported palladium catalyst using formic acid as a reductant. Catalysts 2022, 12, 179. [Google Scholar] [CrossRef]
  11. Zhang, T.; Pan, Z.L.; Song, D.G.; Huang, H.; Yu, J.; Wen, Y.C.; Lu, J.L.; Bian, Z.F.; Qian, X.F. Interstitial compound Fe3C-soped Fe(0) nanoparticles embedded in mesoporous carbon boosting Cr(VI) removal. ACS EST Eng. 2022, 3, 131–137. [Google Scholar] [CrossRef]
  12. Yang, D.Z.; Chu, Z.T.; Feng, X.Z.; Ge, Q.Y.; Wang, R.H.; Zhang, J.; Li, S.Y.; Zheng, R.J.; Wei, W.F.; Yi, S.P.; et al. Dual ions neutralized and stabilized red mud for chromium(VI) polluted soil remediation. ACS EST Eng. 2022, 2, 913–923. [Google Scholar] [CrossRef]
  13. Zarras, P.; Miller, C.E.; Webber, C.; Anderson, N.; Stenger-Smith, J.D. Laboratory and field studies of Poly(2,5-bis(N-methyl-N-hexylamino)phenylene vinylene) (BAM-PPV): A potential wash primer replacement for army military vehicles. Coatings 2014, 4, 687–700. [Google Scholar] [CrossRef]
  14. Gao, Y.; Yang, X.; Lu, X.; Li, M.; Wang, L.; Wang, Y. Kinetics and mechanisms of Cr(VI) removal by nZVI: Influencing parameters and modification. Catalysts 2022, 12, 999. [Google Scholar] [CrossRef]
  15. Gao, Y.; Sun, W.; Yang, W.; Li, Q. Palladium nanoparticles supported on amine-functionalized glass fiber mat for fixed-bed reactors on the effective removal of hexavalent chromium by catalytic reduction. J. Mater. Sci. Technol. 2018, 34, 961–968. [Google Scholar] [CrossRef]
  16. Shang, H.; Zhang, S.; Zhu, X. Biomass cellulose component and Fe mineral catalysis help Cr(VI) to realize almost 100% pyrolysis reduction efficiency. ACS EST Eng. 2021, 1, 1441–1448. [Google Scholar] [CrossRef]
  17. Vaddi, D.R.; Gurugubelli, T.R.; Koutavarapu, R.; Lee, D.-Y.; Shim, J. Bio-stimulated adsorption of Cr(VI) from aqueous solution by groundnut shell activated carbon@Al embedded material. Catalysts 2022, 12, 290. [Google Scholar] [CrossRef]
  18. Rahmat, S.T.; Alias, N.; Kumar, R.; Tan, W.K.; Kawamura, G.; Matsuda, A.; Lockman, Z. Electrophoretic deposition of graphene oxide and reduced graphene oxide on the rutile phase of TiO2 nanowires for rapid reduction of Cr (VI) under simulated sunlight irradiation. Catalysts 2022, 12, 1282. [Google Scholar] [CrossRef]
  19. Wan, Z.; Mao, Q.; Xiang, J.; Ma, D.; Tang, H. Greatly increased visible-light photocatalytic activity of SnS2/carbon nanotube composite for Cr(VI) reduction: Insights into effects of solid acid structure. J. Mater. Sci. Technol. 2023, 161, 233–244. [Google Scholar] [CrossRef]
  20. Yu, M.D.; Mao, X.H.; He, X.S.; Zheng, M.X.; Meng, Y.; He, F.; Xi, B.D. Enhanced sequestration of chromium by mechanochemically silicified microscale zerovalent iron: Role of the silicate-modified surface. ACS EST Eng. 2023, 3, 1604–1613. [Google Scholar] [CrossRef]
  21. Asimakopoulos, G.; Karakassides, A.; Baikousi, M.; Gioti, C.; Moschovas, D.; Avgeropoulos, A.; Bourlinos, A.B.; Douvalis, A.P.; Salmas, C.E.; Karakassides, M.A. Nanoporous carbon magnetic hybrid derived from waterlock polymers and its application for hexavalent chromium removal from aqueous solution. C-J. Carbon Res. 2021, 7, 69. [Google Scholar] [CrossRef]
  22. Smith, V.A.; Rivera, J.F.A.; Bello, R.; Rodriguez-Aguado, E.; Elshaer, M.R.; Wodzinski, R.L.; Bashkova, S. The role of surface chemistry and polyethylenimine grafting in the removal of Cr (VI) by activated carbons from cashew nut shells. C-J. Carbon Res. 2021, 7, 27. [Google Scholar] [CrossRef]
  23. Feng, H.-F.; Yu, Y.-X.; Jiang, S.-Q.; Shang, J.; Cheng, Y.; Wang, L.; Hao, W.-C.; Wang, T.-M. Synthesis of magnetic core-shell iron nanochains for potential applications in Cr(VI) ion pollution treatment. Rare Metals 2021, 40, 176–179. [Google Scholar] [CrossRef]
  24. Zhou, H.; Chen, Y. Effect of acidic surface functional groups on Cr(VI) removal by activated carbon from aqueous solution. Rare Metals 2010, 29, 333–338. [Google Scholar] [CrossRef]
  25. Yu, J.X.; Fu, Q.; Yang, H.S.; Ouyang, G.F.; Wang, J.H.; Hao, Z.P. Synergistic catalytic organic pollutants degradation and Cr(VI) reduction by carbon nanotubes through an electron-transfer mechanism without external energy or chemical input. ACS EST Eng. 2022, 2, 1221–1228. [Google Scholar] [CrossRef]
  26. Mu, F.H.; Dai, B.L.; Zhao, W.; Zhou, S.J.; Huang, H.B.; Yang, G.; Xia, D.H.; Kong, Y.; Leung, D.Y.C. Construction of a novel Ag/Ag3PO4 /MIL-68(In)-NH2 plasmonic heterojunction photocatalyst for high-efficiency photocatalysis. J. Mater. Sci. Technol. 2022, 101, 37–48. [Google Scholar] [CrossRef]
  27. Xiong, J.; Zeng, H.Y.; Xu, S.; Peng, J.F.; Liu, F.Y.; Wang, L.H. Enhancing the intrinsic properties of flower-like BiOI by S-doping toward excellent photocatalytic performances. J. Mater. Sci. Technol. 2022, 118, 181–189. [Google Scholar] [CrossRef]
  28. Zhang, M.; Zhao, X.; Dong, Y.; Hu, C.; Xiang, X.; Zeng, X.; Jia, J.; Jin, C.; Ding, L.; Chen, X. In-situ synthesis of 0D/1D CeO2/Zn0.4Cd0.6S S-scheme heterostructures for boosting photocatalytic remove of antibiotic and chromium. Ceram. Int. 2023, 49, 5842–5853. [Google Scholar] [CrossRef]
  29. Zhang, M.; Dong, Y.; Peng, M.; Chen, X.; Zheng, Q.; Liao, J.; Chen, W. Band gap-controllable N, P co-doped carbon@ZnxCd1−xIn2S4 for photocatalytic reduction of chromium(VI) and degradation of tetracycline hydrochloride. Sep. Purif. Technol. 2023, 322, 124294. [Google Scholar] [CrossRef]
  30. Shearer, C.J.; Alvino, J.F.; Batmunkh, M.; Metha, G.E. Pt nanocluster co-catalysts for photocatalytic water splitting. C-J. Carbon Res. 2018, 4, 64. [Google Scholar] [CrossRef]
  31. Ratova, M.; Sawtell, D.; Kelly, P.J. Micro-patterning of magnetron sputtered titanium dioxide coatings and their efficiency for photocatalytic applications. Coatings 2020, 10, 68. [Google Scholar] [CrossRef]
  32. Roveri, M.; Goidanich, S.; Toniolo, L. Artificial ageing of photocatalytic nanocomposites for the protection of natural stones. Coatings 2020, 10, 729. [Google Scholar] [CrossRef]
  33. Shen, R.; Hao, L.; Chen, Q.; Zheng, Q.; Zhang, P.; Li, X. P-Doped g-C3N4 nanosheets with highly dispersed Co0.2Ni1.6Fe0.2P cocatalyst for efficient photocatalytic hydrogen evolution. Acta Phys.-Chim. Sin. 2022, 38, 2110014. [Google Scholar]
  34. Saleh, T.S.; Badawi, A.K.; Salama, R.S.; Mostafa, M.M.M. Design and development of novel composites containing nickel ferrites supported on activated carbon derived from agricultural wastes and its application in water remediation. Materials 2023, 16, 2170. [Google Scholar] [CrossRef]
  35. Raditoiu, V.; Raditoiu, A.; Raduly, M.F.; Amariutei, V.; Gifu, I.C.; Anastasescu, M. Photocatalytic behavior of water-based styrene-acrylic coatings containing TiO2 sensitized with metal-phthalocyanine tetracarboxylic acids. Coatings 2017, 7, 229. [Google Scholar] [CrossRef]
  36. Huang, G.Q.; Ye, W.N.; Lv, C.X.; Butenko, D.S.; Yang, C.; Zhang, G.L.; Lu, P.; Xu, Y.; Zhang, S.C.; Wang, H.W.; et al. Hierarchical red phosphorus incorporated TiO2 hollow sphere heterojunctions toward superior photocatalytic hydrogen production. J. Mater. Sci. Technol. 2022, 108, 18–25. [Google Scholar] [CrossRef]
  37. Zhu, H.; Tan, J.; Qiu, J.; Wang, D.; Zhao, Z.; Lu, Z.; Huang, G.; Liu, X.; Mei, Y. Gold nanoparticles decorated titanium oxide nanotubes with enhanced antibacterial activity driven by photocatalytic memory effect. Coatings 2022, 12, 1351. [Google Scholar] [CrossRef]
  38. Alasri, T.M.; Ali, S.L.; Salama, R.S.; Alshorifi, F.T. Band-structure engineering of TiO2 photocatalyst by AuSe quantum dots for efficient degradation of malachite green and phenol. J. Inorg. Organomet. Polym. Mater. 2023, 33, 1729–1740. [Google Scholar] [CrossRef]
  39. Zhang, S.J.; He, Z.L.; Xu, S.S.; Li, X.; Zhang, J.; Zhan, X.P.; Dai, M.; Wang, S.G. In situ liquid-phase growth strategies of g-C3N4 solar-driven heterogeneous catalysts for environmental applications. Sol. RRL 2021, 5, 2100233. [Google Scholar] [CrossRef]
  40. Jin, Z.; Li, Y.; Hao, X. Ni, Co-based selenide anchored g-C3N4 for boosting photocatalytic hydrogen evolution. Acta Phys.-Chim. Sin. 2021, 37, 1912033. [Google Scholar]
  41. Fernandez-Catala, J.; Greco, R.; Navlani-Garcia, M.; Cao, W.; Berenguer-Murcia, A.; Cazorla-Amoros, D. g-C3N4-based direct Z-scheme photocatalysts for environmental applications. Catalysts 2022, 12, 1137. [Google Scholar] [CrossRef]
  42. Dai, M.; Yu, H.; Chen, W.; Qu, K.-A.; Zhai, D.; Liu, C.; Zhao, S.; Wang, S.; He, Z. Boosting photocatalytic activity of CdLa2S4/ZnIn2S4 S-scheme heterojunctions with spatial separation of photoexcited carries. Chem. Eng. J. 2023, 470, 144240. [Google Scholar] [CrossRef]
  43. Xiong, Z.; Hou, Y.; Yuan, R.; Ding, Z.; Ong, W.-J.; Wang, S. Hollow NiCo2S4 Nanospheres as a cocatalyst to support ZnIn2S4 nanosheets for visible-light-driven hydrogen production. Acta Phys.-Chim. Sin. 2022, 38, 2111021. [Google Scholar]
  44. Zeng, J.-Y.; Wang, X.-S.; Xie, B.-R.; Li, Q.-R.; Zhang, X.-Z. Large pi-conjugated metal-organic frameworks for infrared-light-driven CO2 reduction. J. Am. Chem. Soc. 2022, 144, 1218–1231. [Google Scholar] [CrossRef]
  45. Lan, C.; Meng, L.; Xu, N. One-pot synthesis of the direct Z-scheme AgInS2/AgIn5S8 QDs heterojunction for efficient photocatalytic reduction of Cr6+ in neutral condition. Colloids Surf. A Physicochem. Eng. Asp. 2022, 632, 127762. [Google Scholar] [CrossRef]
  46. Dai, M.; He, Z.; Zhang, P.; Li, X.; Wang, S. ZnWO4-ZnIn2S4 S-scheme heterojunction for enhanced photocatalytic H2 evolution. J. Mater. Sci. Technol. 2022, 122, 231–242. [Google Scholar] [CrossRef]
  47. He, Z.; Byun, J.-H.; Zhou, G.; Park, B.-J.; Kim, T.-H.; Lee, S.-B.; Yi, J.-W.; Um, M.-K.; Chou, T.-W. Effect of MWCNT content on the mechanical and strain-sensing performance of thermoplastic polyurethane composite fibers. Carbon 2019, 146, 701–708. [Google Scholar] [CrossRef]
  48. He, Z.; Zhou, G.; Byun, J.H.; Lee, S.K.; Um, M.K.; Park, B.; Kim, T.; Lee, S.B.; Chou, T.W. Highly stretchable multi-walled carbon nanotube/thermoplastic polyurethane composite fibers for ultrasensitive, wearable strain sensors. Nanoscale 2019, 11, 5884–5890. [Google Scholar] [CrossRef]
  49. Zhang, S.; Zhang, N.; Zhang, J. Controlled synthesis of carbon nanotubes: Past, present and future. Acta Phys.-Chim. Sin. 2020, 36, 1907021. [Google Scholar] [CrossRef]
  50. Adusei, P.K.; Johnson, K.; Kanakaraj, S.N.; Zhang, G.; Fang, Y.; Hsieh, Y.-Y.; Khosravifar, M.; Gbordzoe, S.; Nichols, M.; Shanov, V. Asymmetric fiber supercapacitors based on a FeC2O4/FeOOH-CNT hybrid material. C-J. Carbon Res. 2021, 7, 62. [Google Scholar] [CrossRef]
  51. Zhang, J.; Dai, M.; Zhang, S.; Dai, M.; Zhang, P.; Wang, S.; He, Z. Recent progress on carbon-nanotube-based materials for photocatalytic applications: A review. Sol. RRL 2022, 6, 2200243. [Google Scholar] [CrossRef]
  52. Liu, S.-Q.; Wang, S.; Dai, G.-P.; Lu, J.; Liu, K. Enhanced visible-light photocatalytic activity and stability of nano-sized Ag2CO3 combined with carbon nanotubes. Acta Phys.-Chim. Sin. 2014, 30, 2121–2126. [Google Scholar]
  53. Toloman, D.; Stefan, M.; Macavei, S.; Barbu-Tudoran, L.; Popa, A. Photocatalytic self-cleaning PVDF membrane blended with MWCNT-ZnO nanocomposites for RhB removal. Coatings 2023, 13, 594. [Google Scholar] [CrossRef]
  54. Miklec, K.; Grcic, I.; Radetic, L.; Cingesar, I.K.; Vrsaljko, D. Photocatalytic oxidation of amoxicillin in CPC reactor over 3D printed TiO2-CNT@PETG static mixers. Coatings 2023, 13, 386. [Google Scholar] [CrossRef]
  55. Gomis-Berenguer, A.; Iniesta, J.; Fermin, D.J.; Ania, C.O. Photoelectrochemical response of WO3/nanoporous carbon anodes for photocatalytic water oxidation. C-J. Carbon Res. 2018, 4, 45. [Google Scholar] [CrossRef]
  56. Falara, P.P.; Ibrahim, I.; Zourou, A.; Sygellou, L.; Sanchez, D.E.; Romanos, G.E.; Givalou, L.; Antoniadou, M.; Arfanis, M.K.; Han, C.; et al. Bi-functional photocatalytic heterostructures combining titania thin films with carbon quantum dots (C-QDs/TiO2) for effective elimination of water pollutants. Environ. Sci. Pollut. Res. 2023. [Google Scholar] [CrossRef]
  57. Bai, S.; Li, X.; Kong, Q.; Long, R.; Wang, C.; Jiang, J.; Xiong, Y. Toward enhanced photocatalytic oxygen evolution: Synergetic utilization of plasmonic effect and schottky junction via interfacing facet selection. Adv. Mater. 2015, 27, 3444–3452. [Google Scholar] [CrossRef]
  58. Yuan, K.; Gao, T.-J.; Yang, Y.; Luo, W.; Li, S.; Zhang, C.-Y.; Xu, J.-X.; Li, N.; Zhu, Y.-R. Template sacrificial controlled synthesis of hierarchical nanoporous carbon@NiCo2S4 microspheres for highperformance hybrid supercapacitors. Rare Metals 2023, 42, 2643–2657. [Google Scholar] [CrossRef]
  59. Rashid, M.; Hassan, W.; Aadil, M.; Somaily, H.H.; Mahdi, N.M.; Lataef, R.; Taki, A.G.; Srithilat, K.; Baamer, D.F.; Albukhari, S.M.; et al. Solar-light-driven and magnetically recoverable doped nano-ferrite: An ideal photocatalyst for water purification applications. Opt. Mater. 2023, 135, 113192. [Google Scholar] [CrossRef]
  60. Nazik, G.; Aadil, M.; Zulfiqar, S.; Hassan, W.; Rahman, A.; Ibrahim, S.M.; Naseem, K.; Sheikh, T.A.; Akhtar, M.N. Synthesis of doped metal sulfide nanoparticles and their graphene reinforced nanohybrid for Pb(II) detection. Z. Für Phys. Chem. 2023, 237, 1257–1285. [Google Scholar] [CrossRef]
  61. Aadil, M.; Zulfiqar, S.; Agboola, P.O.; Aboud, M.F.A.; Shakir, I.; Warsi, M.F. Fabrication of graphene supported binary nanohybrid with multiple approaches for electrochemical energy storage applications. Synth. Met. 2021, 272, 116645. [Google Scholar] [CrossRef]
  62. Tariq, R.; Zulfiqar, S.; Somaily, H.H.; Warsi, M.F.; Ayman, I.; Hanif, F.; Akhtar, M.; Aadil, M. Synthesis of carbon supported iron oxide nanochips and their composite with glutathione: A novel electrochemical sensitive material. Surf. Interfaces 2022, 34, 102350. [Google Scholar] [CrossRef]
  63. Dai, M.; He, Z.; Cao, W.; Zhang, J.; Chen, W.; Jin, Q.; Que, W.; Wang, S. Rational construction of S-scheme BN/MXene/ZnIn2S4 heterojunction with interface engineering for efficient photocatalytic hydrogen production and chlorophenols degradation. Sep. Purif. Technol. 2023, 309, 123004. [Google Scholar] [CrossRef]
  64. Guo, M.-L.; Wu, Z.-Y.; Zhang, M.-M.; Huang, Z.-J.; Zhang, K.-X.; Wang, B.-R.; Tu, J.-C. Coupling interface constructions of FeOOH/NiCo2S4 by microwave-assisted method for efficient oxygen evolution reaction. Rare Metals 2023, 42, 1847–1857. [Google Scholar] [CrossRef]
  65. Aadil, M.; Hassan, W.; Somaily, H.H.; Ejaz, S.R.; Abass, R.R.; Jasem, H.; Hachim, S.K.; Adhab, A.H.; Abood, E.S.; Alsafari, I.A. Synergistic effect of doping and nanotechnology to fabricate highly efficient photocatalyst for environmental remediation. J. Alloys Compd. 2022, 920, 165876. [Google Scholar] [CrossRef]
  66. Tamam, N.; Aadil, M.; Hassan, W.; Ejaz, S.R.; Najm, Z.M.; Alsafari, I.A.; Aman, S.; Trukhanov, A.V.; Al-Buriahi, M.S.; Boukhris, I. Surfactant assisted synthesis of nanostructured Mn-doped CuO: An efficient photocatalyst for environmental remediation. Ceram. Int. 2022, 48, 29589–29600. [Google Scholar] [CrossRef]
  67. Ishfaq, M.; Aadil, M.; Ejaz, S.R.; Hassan, W.; Panduro-Tenazoa, N.M.; El Sayed, M.E.; Murshed, M.N.; El-Bahy, Z.M. Synthesis of binary metal doped CeO2 via the subcritical hydrothermal method for photo-mineralizing methyl orange dye. J. Alloys Compd. 2023, 960, 170661. [Google Scholar] [CrossRef]
  68. Badovinac, I.J.; Peter, R.; Omerzu, A.; Salamon, K.; Saric, I.; Samarzija, A.; Percic, M.; Piltaver, I.K.; Ambrozic, G.; Petravic, M. Grain size effect on photocatalytic activity of TiO2 thin films grown by atomic layer deposition. Thin Solid Films 2020, 709, 138215. [Google Scholar] [CrossRef]
  69. Li, T.; Aadil, M.; Zulfiqar, S.; Anwar, A.; Yakout, S.M.; Panduro-Tenazoa, N.M.; Mubeen, S. Synthesis of doped and porous CuO with boosted light-harvesting features for the photocatalytic mineralization of azo dyes. Ceram. Int. 2023, 49, 27827–27836. [Google Scholar] [CrossRef]
  70. Bai, Y.; Xu, S.; Chen, J.; Sun, X.; Zhao, S.; Chang, J.; He, Z. Ti3C2@g-C3N4/TiO2 ternary heterogeneous photocatalyst for promoted photocatalytic degradation activities. Coatings 2023, 13, 655. [Google Scholar] [CrossRef]
  71. He, Z.; Kim, C.; Lin, L.H.; Jeon, T.H.; Lin, S.; Wang, X.C.; Choi, W. Formation of heterostructures via direct growth CN on h-BN porous nanosheets for metal-free photocatalysis. Nano Energy 2017, 42, 58–68. [Google Scholar] [CrossRef]
  72. Yu, H.; Xu, S.; Zhang, S.; Wang, S.; He, Z. In-situ construction of core–shell structured TiB2-TiO2@g-C3N4 for efficient photocatalytic degradation. Appl. Surf. Sci. 2022, 579, 152201. [Google Scholar] [CrossRef]
  73. He, Z.; Que, W.; Chen, J.; Yin, X.; He, Y.; Ren, J. Photocatalytic degradation of methyl orange over nitrogen-fluorine codoped TiO2 nanobelts prepared by solvothermal synthesis. ACS Appl. Mater. Interfaces 2012, 4, 6816–6826. [Google Scholar] [CrossRef] [PubMed]
  74. Wang, L.; Karuturi, S.; Zan, L. Bi2S3-In2S3 heterostructures for efficient photoreduction of highly toxic Cr6+ enabled by facet-coupling and Z-scheme structure. Small 2021, 17, 2101833. [Google Scholar] [CrossRef] [PubMed]
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Figure 1. (a) Photocatalytic reduction curves of Cr(VI) in the presence of as-prepared samples with different amounts of MWCNTs (15 mg photocatalyst, 30 mL 50 ppm Cr(VI)); and (b) control experiment of photocatalytic reduction of Cr(VI) for 0.050C-NCS.
Figure 1. (a) Photocatalytic reduction curves of Cr(VI) in the presence of as-prepared samples with different amounts of MWCNTs (15 mg photocatalyst, 30 mL 50 ppm Cr(VI)); and (b) control experiment of photocatalytic reduction of Cr(VI) for 0.050C-NCS.
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Figure 2. (a) Transient photocurrent responses of the as-prepared samples; and (b) electrochemical impedance spectra (EIS) of the as-prepared samples.
Figure 2. (a) Transient photocurrent responses of the as-prepared samples; and (b) electrochemical impedance spectra (EIS) of the as-prepared samples.
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Figure 3. Photocatalytic reduction curves of Cr(VI) solution containing different concentrations of NaCl (a); and (b) sodium salts (10 mM) and (c) inorganic salts (10 mM).
Figure 3. Photocatalytic reduction curves of Cr(VI) solution containing different concentrations of NaCl (a); and (b) sodium salts (10 mM) and (c) inorganic salts (10 mM).
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Figure 4. XRD pattern of as-prepared samples.
Figure 4. XRD pattern of as-prepared samples.
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Figure 5. (a,b) SEM images, (c) TEM images, and (d) HRTEM images of 0.050C-NCS. (e) Elemental mapping of 0.050C-NCS.
Figure 5. (a,b) SEM images, (c) TEM images, and (d) HRTEM images of 0.050C-NCS. (e) Elemental mapping of 0.050C-NCS.
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Figure 6. N2 adsorption and desorption isotherms and corresponding pore-size distribution curves (inset) for (a) C-NCS; (b) 0.020C-NCS; (c) 0.050C-NCS; and (d) 0.100C-NCS.
Figure 6. N2 adsorption and desorption isotherms and corresponding pore-size distribution curves (inset) for (a) C-NCS; (b) 0.020C-NCS; (c) 0.050C-NCS; and (d) 0.100C-NCS.
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Figure 7. XPS survey spectra (a) and high-resolution XPS spectrum of 0.050C-NCS ((b) Ni 2p; (c) Co 2p; (d) S 2p; (e) C 1s; and (f) N 1s).
Figure 7. XPS survey spectra (a) and high-resolution XPS spectrum of 0.050C-NCS ((b) Ni 2p; (c) Co 2p; (d) S 2p; (e) C 1s; and (f) N 1s).
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Figure 8. (a) UV-Vis Diffuse Reflectance Absorption Spectra (DRS); (b) direct band gap energy; (c) Mott–Schottky plots, and (d) schematic illustration of 0.050C-NCS.
Figure 8. (a) UV-Vis Diffuse Reflectance Absorption Spectra (DRS); (b) direct band gap energy; (c) Mott–Schottky plots, and (d) schematic illustration of 0.050C-NCS.
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Figure 9. Stability of 0.050C-NCS within five consecutive cycles.
Figure 9. Stability of 0.050C-NCS within five consecutive cycles.
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Table 1. The BET parameters of the samples.
Table 1. The BET parameters of the samples.
SampleSBET (m2/g)Pore Size (nm)Pore Volume (cm3/g)
NCS16340.133
0.005C-NCS22270.145
0.020C-NCS51220.290
0.035C-NCS71180.328
0.050C-NCS91200.460
0.075C-NCS100180.439
0.100C-NCS125150.467
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Jin, Q.; Zheng, Z.; Feng, Y.; Tian, S.; He, Z. Multi-Walled Carbon Nanotubes Modified NiCo2S4 for the Efficient Photocatalytic Reduction of Hexavalent Chromium. C 2023, 9, 99. https://doi.org/10.3390/c9040099

AMA Style

Jin Q, Zheng Z, Feng Y, Tian S, He Z. Multi-Walled Carbon Nanotubes Modified NiCo2S4 for the Efficient Photocatalytic Reduction of Hexavalent Chromium. C. 2023; 9(4):99. https://doi.org/10.3390/c9040099

Chicago/Turabian Style

Jin, Qiu, Ziye Zheng, Yuxiao Feng, Shuang Tian, and Zuoli He. 2023. "Multi-Walled Carbon Nanotubes Modified NiCo2S4 for the Efficient Photocatalytic Reduction of Hexavalent Chromium" C 9, no. 4: 99. https://doi.org/10.3390/c9040099

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