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Article

Electrostatic Self-Assembly of CdS Quantum Dots with Co9S8 Hollow Nanotubes for Enhanced Visible Light Photocatalytic H2 Production

by
Yuqing Yan
1,
Yonghui Wu
1,
Chenggen Lu
1,
Yu Wei
1,
Jun Wang
1,
Bo Weng
2,
Wei-Ya Huang
1,
Jia-Lin Zhang
1,
Kai Yang
1 and
Kangqiang Lu
1,*
1
Jiangxi Provincial Key Laboratory of Functional Crystalline Materials Chemistry, School of Chemistry and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou 341000, China
2
CAS Key Laboratory of Urban Pollutant Conversion, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China
*
Author to whom correspondence should be addressed.
Molecules 2024, 29(15), 3530; https://doi.org/10.3390/molecules29153530
Submission received: 21 June 2024 / Revised: 25 July 2024 / Accepted: 25 July 2024 / Published: 26 July 2024
(This article belongs to the Special Issue Green Catalysis Technology for Sustainable Energy Conversion)
Browse Figures
Versions Notes

Abstract

:
CdS quantum dots (CdS QDs) are regarded as a promising photocatalyst due to their remarkable response to visible light and suitable placement of conduction bands and valence bands. However, the problem of photocorrosion severely restricts their application. Herein, the CdS QDs-Co9S8 hollow nanotube composite photocatalyst has been successfully prepared by loading Co9S8 nanotubes onto CdS QDs through an electrostatic self-assembly method. The experimental results show that the introduction of Co9S8 cocatalyst can form a stable structure with CdS QDs, and can effectively avoid the photocorrosion of CdS QDs. Compared with blank CdS QDs, the CdS QDs-Co9S8 composite exhibits obviously better photocatalytic hydrogen evolution performance. In particular, CdS QDs loaded with 30% Co9S8 (CdS QDs-30%Co9S8) demonstrate the best photocatalytic performance, and the H2 production rate reaches 9642.7 μmol·g−1·h−1, which is 60.3 times that of the blank CdS QDs. A series of characterizations confirm that the growth of CdS QDs on Co9S8 nanotubes effectively facilitates the separation and migration of photogenerated carriers, thereby improving the photocatalytic hydrogen production properties of the composite. We expect that this work will facilitate the rational design of CdS-based photocatalysts, thereby enabling the development of more low-cost, high-efficiency and high-stability composites for photocatalysis.

1. Introduction

With the increasingly serious environmental pollution and the increasing demand for energy, the development and utilization of sustainable clean energy to achieve green development has become a hot topic [1,2,3,4]. In recent years, photocatalytic hydrogen evolution has attracted much attention due to its advantages of zero carbon emission, high efficiency and sustainability, and is considered a promising energy conversion method [5,6,7]. Therefore, the utilization of photocatalytic technology to produce hydrogen energy represents a feasible strategy for alleviating environmental pollution and energy crises [8,9,10]. The practical application of photocatalytic hydrogen production technology is contingent upon three key factors: low cost, high efficiency and high stability [11,12]. One of the most commonly employed modification strategies to improve the photocatalytic H2 evolution properties of semiconductors is the introduction of precious metals (such as Au, Ag, Pd and Pt) through doping. Nevertheless, precious metals are limited and expensive. Consequently, the development of cost-effective, environmentally friendly and highly active photocatalysts represents a significant and pressing challenge [13,14].
In recent years, metal sulfides have become a research focus in the field of photocatalytic hydrogen evolution on account of their exceptional light absorption properties and unique electronic structure. Among these, CdS has been extensively studied owing to its appropriate band gap and position of energy bands [15,16]. Moreover, CdS exhibits diverse morphologies and structures, involving zero-dimensional (0D) quantum dots, one-dimensional (1D) nanorods, two-dimensional (2D) nanosheets and three-dimensional (3D) cubes [17]. CdS QDs are considered to be a promising photocatalytic material due to their small size (<10 nm), high electron mobility and abundant recoverable light [18,19]. However, the issue of easy hole oxidation decomposition (photocorrosion) severely restricts the application of CdS [20]. Among various strategies to alleviate CdS photocorrosion, the rational incorporation of a cocatalyst is an effective approach [21]. Co9S8 serves as a widely used cocatalyst with advantages such as easy availability, abundant active sites and adjustable chemical composition [22]. In particular, the hollow-structured Co9S8 possesses a large specific surface area and enhances the absorption of light by multiple reflections, which is of significant importance in improving photocatalytic properties. Additionally, the electrostatic self-assembly method is an efficient and environmentally friendly preparation method for nanoparticles, which is expected to prepare highly active photocatalysts [23].
Herein, the CdS QDs-Co9S8 composite photocatalyst is successfully prepared through electrostatic self-assembly. Compared to blank CdS QDs, the CdS QDs-Co9S8 composite demonstrates enhanced photocatalytic H2 production performance. Notably, the optimal CdS QDs-30%Co9S8 exhibits a photocatalytic hydrogen production rate of 9642.7 μmol·g−1·h−1, approximately 60.3 times that of blank CdS QDs. Cyclic experiments indicate that the introduction of Co9S8 cocatalyst effectively prevents photocorrosion on the surface of CdS QDs. Moreover, subsequent characterizations confirm that loading Co9S8 cocatalyst effectively promotes the separation and migration of photogenerated carriers, thereby improving the photocatalytic properties of CdS QDs. This work illustrates the significant role of Co9S8 as a cocatalyst in the field of photocatalytic H2 production, and is expected to provide a useful reference for the development of effective and stable photocatalysts.

2. Results and Discussion

The synthesis process of the CdS QDs-Co9S8 composite photocatalyst is shown in Figure 1. Initially, the Co9S8 nanotubes are achieved through a two-step hydrothermal approach, followed by treatment with APTES to impart a positive charge. Subsequently, the treated Co9S8 nanotubes are subjected to an electrostatic assembly process with CdS QDs, resulting in the formation of the CdS QDs-Co9S8 composite photocatalyst. A diagram of the prepared samples diagram is depicted in Figure S1. As illustrated in Figure S1a,b, CdS QDs exhibit a yellow powder, while Co9S8 nanotubes display a black powder. Upon assembly of CdS QDs and Co9S8 nanotubes, the resulting CdS QDs-Co9S8 composite appears yellowish-green (Figure S1c).
Figure 2a,b display the Zeta potentials of APTES-modified Co9S8 and CdS QDs suspension dispersed in deionized water, respectively. It can be observed that the Zeta potentials of APTES-modified Co9S8 and CdS QDs are 13.8 mV and −30 mV, respectively, which means that APTES-modified Co9S8 is positively charged, while CdS QDs is negatively charged. This result provides a good basis for the assembly of the CdS QDs-Co9S8 composite [24].
Morphological and microstructural analyses of CdS QDs, Co9S8 and CdS QDs-30%Co9S8 were conducted using scanning electron microscopy (SEM) and transmission electron microscope (TEM). As shown in Figure 3a, the TEM image reveals that the diameter of CdS QDs is roughly 4 nm, consistent with previous literature [25]. Furthermore, as displayed in Figure 3b, the high-resolution TEM (HRTEM) image exhibits a lattice spacing of 0.35 nm corresponding to the (111) crystal face of CdS QDs, indicating its successful preparation [25]. Meanwhile, the TEM and HRTEM images of Co9S8 (Figure S2) demonstrate the successful synthesis of Co9S8 nanotubes. As depicted in Figure S2b, the 0.23 nm of lattice spacing corresponds to the (420) crystal face of Co9S8. Figure 3c illustrates a hollow nanotube structure with a diameter of approximately 200 nm for Co9S8. As exhibited in Figure 3d, CdS QDs-30%Co9S8 inherits the hollow nanotube structure of Co9S8. It is worth noting that the hollow structure exposes a large specific surface area and enhances the absorption of light by multiple reflections, which is of significant importance in improving the photocatalytic properties. Furthermore, it can be observed that CdS QDs are evenly decentralized on the Co9S8 nanotubes. As presented in Figure 3e, the EDS spectra illustrate the presence of Co, Cd and S elements in the CdS QDs-30%Co9S8 composite. Moreover, the composition of all the composite photocatalysts is quantitatively analyzed using inductively coupled plasma emission spectrometry (ICP-OES). As indicated in Table S1, as the Co9S8 load increases, the proportion of the Co element rises, while the proportion of the Cd element decreases, consistent with the anticipated results. In addition, the element mapping results of CdS QDs-30%Co9S8 indicate that CdS QDs are uniformly distributed on the surface of Co9S8 nanotubes (Figure 3f). These results demonstrate the successful synthesis of the CdS QDs-30%Co9S8 composite.
The crystal structure and phase composition of the prepared samples were investigated by X-ray diffraction (XRD). Figure 4a illustrates the XRD patterns of CdS QDs, Co9S8 and CdS QDs-30%Co9S8. It can be observed that the XRD peak of CdS exhibits a relatively strong intensity, indicating its robust crystal phase. In contrast, the XRD peak of Co9S8 displays a relatively weak intensity, suggesting its inferior crystal phase. For Co9S8, the diffraction peaks at 2θ = 29.9°, 31.4°, 37.4°, 39.5°, 47.5°, 52.3° and 54.6° correspond to the crystal planes (311), (222), (400), (331), (511), (400) and (531) of Co9S8, respectively (JCPDS: 65-1765) [26]. As for CdS QDs, the characteristic peaks at 26.2°, 43.6° and 51.7° can be related to the crystal faces (111), (220) and (311) of CdS (JCPDS: 75-1546), respectively [27]. In addition, the XRD diffraction curve of CdS QDs-30%Co9S8 is highly similar to that of CdS QDs, except that a weak peak at 39.5° belongs to the (331) crystal plane of Co9S8, demonstrating the successful assembly of the CdS QDs-Co9S8 composite. Furthermore, the (111) crystal face of CdS QDs exhibits a strong characteristic diffraction peak, resulting in a peak of Co9S8 at 29.9° masked by CdS QDs. The optical properties of a series of samples are determined by UV–vis diffuse reflectance spectroscopy (DRS). Figure 4b exhibits the light absorption curves of CdS QDs, Co9S8 and the CdS QDs-30%Co9S8 composite. The blank CdS presents a distinct absorption edge at near 570 nm. Moreover, Co9S8 illustrates strong absorption across the entire spectral range, suggesting excellent light collection ability from ultraviolet to visible light regions. Notably, the CdS QDs-30%Co9S8 composite displays superior light harvesting capability compared to CdS alone, which indicates the enhanced light absorption achieved through the introduction of the Co9S8 cocatalyst in the composite photocatalyst.
X-ray photoelectron spectroscopy (XPS) analysis (Figure 5) of the CdS QDs-30%Co9S8 composite is performed in order to further determine the chemical state and elemental composition of the prepared sample. As the survey spectra shown in Figure 5a, Co, Cd and S elements are present in the CdS QDs-30%Co9S8 composite, which further confirms the successful assembly of CdS QDs and Co9S8 cocatalyst. In the XPS spectra of Cd 3d (Figure 5b), the two characteristic peaks at 410.2 eV and 403.4 eV belong to Cd 3d3/2 and Cd 3d5/2, respectively, which demonstrates Cd exists in the form of +2 valence in the binary composite photocatalyst CdS QDs-30%Co9S8 [21]. As illustrated in Figure 5c, the distinct peaks at the binding energies of 160.1 eV and 161.9 eV belong to S 2p3/2 and S 2p1/2, respectively, confirming the existence of S2− [28]. In addition, the XPS spectra of Co 2p displayed in Figure 5d can be divided into two spin-orbital dual peaks and two satellite peaks (identified as “Sat.”). The first dual peaks at 780.3 eV and 776.6 eV and the second dual peaks at 796.8 eV and 794.5 eV can be attributed to Co 2p3/2 and Co 2p1/2, respectively, demonstrating the existence of Co2+ and Co3+ [29]. The XPS results confirm that the prepared composite contains CdS and Co9S8, which indicates the successful preparation of this hybrid.
In order to compare the photocatalytic performance of pure CdS QDs and CdS QDs-Co9S8 composites illuminated by visible light, a photocatalytic hydrogen evolution experiment is conducted using TEOA as a sacrificial agent. As displayed in Figure 6a, on account of the serious recombination of photogenerated carriers, the blank CdS QDs exhibit low photocatalytic activity, which demonstrates a hydrogen production rate of 159.8 μmol·g−1·h−1. When CdS QDs are combined with 5%, 10%, 30% and 50% Co9S8 nanotubes, the different proportions of the CdS QDs-Co9S8 composites show enhanced photocatalytic activity. As the loading capacity of Co9S8 is increased, the photocatalytic H2 production rate of the CdS QDs-Co9S8 composites exhibits a corresponding increase. In particular, the optimal CdS QDs-30%Co9S8 composite photocatalyst demonstrated a hydrogen production rate of 9642.7 μmol·g−1·h−1, which is 60.3 times that of pure CdS QDs. Nevertheless, when the Co9S8 cocatalyst content continually increased, the hydrogen production rate of the CdS QDs-Co9S8 composite decreased. This phenomenon may be attributed to the high proportion of cocatalysts, which results in the masking of the CdS QDs’ active sites during hydrogen evolution. As demonstrated in Table 1, the photocatalytic H2 evolution rate of the CdS QDs-30%Co9S8 composite is superior to that of similar photocatalysts documented in the literature. Furthermore, the photocatalytic stability of the CdS QDs-30%Co9S8 composite photocatalyst is evaluated by cyclic experiment. As illustrated in Figure 6b, the CdS QDs-30%Co9S8 composite photocatalyst demonstrates a stable photocatalytic activity following five cycles. These findings demonstrate that the CdS QDs-Co9S8 composite is an efficacious and stable photocatalyst. In addition, Figure 6c,d demonstrate that there is no obvious change in the SEM image and XRD pattern of the CdS QDs-30%Co9S8 composites following cycling, which further shows that the composites have excellent stability.
The separation efficiency of photogenerated carriers can be evaluated through the photoluminescence (PL) measurement. As displayed in Figure 7a, the CdS QDs-30%Co9S8 composite photocatalyst exhibits lower PL intensity than blank CdS QDs, indicating that the introduction of the Co9S8 cocatalyst has an effective inhibition effect on the photogenerated electron–hole pair recombination, which can enhance the photocatalytic hydrogen evolution performance [39,40,41]. Figure 7b shows the instantaneous photocurrent response of blank CdS QDs and CdS QDs-30%Co9S8 [42]. As demonstrated in Figure 7b, the photocurrent response of CdS QDs-30%Co9S8 composite is significantly higher than that of blank CdS QDs, which indicates the improved photogenerated carrier separation efficiency of the CdS QDs-30%Co9S8 composite [43,44,45]. Additionally, the charge migration behavior at the catalyst–electrolyte interface is investigated through electrochemical impedance spectroscopy (EIS). Generally, a smaller radius of curvature results in lower resistance for the catalyst during the charge transfer process. As we can see from the EIS Nyquist diagram (Figure 7c), the CdS QDs-30%Co9S8 composite exhibits a smaller curvature radius than that of the CdS QDs, which demonstrates that the CdS QDs-30%Co9S8 composite represents a faster-photogenerated carrier transfer rate and a lower charge transfer resistance [46,47,48,49]. As displayed in Figure 7d,e, both CdS QDs and Co9S8 exhibit type IV isotherms in their N2 adsorption–desorption isotherms, indicating the mesoporous nature of these materials. The pore size distribution of CdS QDs, Co9S8 and CdS QDs-30%Co9S8 is illustrated in Figure S3 and Table S2, further confirming their mesoporous characteristics. Furthermore, the BET surface areas of CdS QDs and Co9S8 are 1.50 m2/g and 8.23 m2/g, respectively, while that of CdS QDs-30%Co9S8 is 116.76 m2/g (Figure 7f). The larger BET surface area of the hybrid photocatalyst compared to that of Co9S8 and CdS QDs suggests that the structure of quantum dots on hollow nanotubes can expand the catalyst’s surface area, thereby enhancing the photocatalytic properties of the composite catalysts. Moreover, the N2 adsorption isotherm and corresponding BET-specific surface areas of all other ratios of the composites have been investigated to identify discernible trends. As depicted in Figure S4, all composites exhibit higher BET-specific surface area than blank CdS and Co9S8, and their BET-specific surface area roughly decreases with increasing Co9S8 load. This phenomenon may be attributed to the high loading amount of Co9S8, resulting in nanotube stacking and consequently reducing the composite’s BET-specific surface area.
The band structure information of CdS and Co9S8 can be acquired through UV–vis absorption spectra (Figure 4b) and Mott–Schottky plots (Figure 8). The band gap energy (Eg) of the synthesized samples is determined through the Tauc equation: (αhν)2 = A(hν − Eg), where α, ν, h and A are the absorption coefficient, frequency of light, Planck’s constant and proportionality constant, respectively. As depicted in Figure 8a,b, CdS QDs and Co9S8 exhibit Eg of 2.36 eV and 1.87 eV, respectively. The Mott–Schottky (M-S) method is employed to ascertain the semiconductor type and band potential. Figure 8c,d illustrates that both CdS and Co9S8 display a positive slope, indicating their n-type semiconductor nature [50]. From the M-S diagram, it is evident that the flat band potential (Vfb) for CdS QDs is −0.47 V, while that of Co9S8 is −0.29 V (vs. Ag/AgCl). Since the conduction potential (ECB) of n-type semiconductors is approximately −0.2 V negative than Vfb, it can be calculated that the ECB of CdS and Co9S8 are −0.67 V and −0.49 V (vs. Ag/AgCl), respectively [51]. According to the conversion relationship, we determine that the ECB of CdS is −0.47 V, while that of Co9S8 is −0.29 V (vs. NHE). Based on the Eg of CdS and Co9S8, their valence band potential (EVB) can be calculated as 1.89 V and 1.58 V using the formula EVB = ECB + Eg.
Based on the aforementioned characterizations, a potential mechanism for visible-light-driven photocatalytic hydrogen evolution by CdS QDs-Co9S8 has been put forward. The band positions and band gaps of CdS and Co9S8 have been determined through Mott–Schottky analysis and UV–vis DRS transformation plots. Since Co9S8 (−0.29 V) exhibits a lower conduction band potential (ECB) than CdS (−0.47 V), it suggests that the photogenerated electrons from CdS will be transferred to the CB of Co9S8. As exhibited in Figure 9, the irradiation of visible light results in the excited electrons in the valence band (VB) of CdS QDs jumping to the CB, accompanied by the generation of photogenerated holes in the VB. Due to the tight interfacial contact between CdS QDs and Co9S8, photogenerated electrons are transferred from the CB of CdS QDs to the CB of Co9S8 instead of being trapped by holes. Subsequently, the electrons that have accumulated on Co9S8 combine with H+ to form H2. Meanwhile, the remaining photogenerated holes in the VB of CdS rapidly oxidize the sacrificial agent triethanolamine, forming a complete reaction cycle. Furthermore, the nanotube structure of Co9S8 provides a multitude of active sites for photocatalytic hydrogen production reactions, and combined with the multiple reflections of light in the hollow structure of Co9S8, the photocatalytic H2 evolution performance of the CdS QDs-Co9S8 composite is significantly enhanced.

3. Experimental Section

3.1. Materials

Anhydrous chromium chloride (CdCl2), sodium hydroxide (NaOH), cobalt chloride hexahydrate (CoCl2·6H2O), 3-aminopropyl triethoxysilane (APTES) and 3-mercaptopropionic acid (C3H6O2S, MPA) were supplied by Shanghai Macklin Biochemical Co., Ltd. (Shanghai, China). Triethanolamine (C6H15NO3, TEOA) and anhydrous ethanol (C2H6O) were purchased from Xilong Scientific Co., Ltd. (Shantou, China). Sodium sulfide 9-hydrate (Na2S·9H2O) was supplied by Shanghai Aladdin Biochemical Technology Co., Ltd. (Shanghai, China). Urea (CH4N2O) was purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China).

3.2. Preparation of CdS QDs

In a typical experiment, 1.7 mmol MPA (3-mercaptopropionic acid) and 1 mmol CdCl2 were dissolved in 20 mL of deionized water. The pH was then modulated to about 10 through the addition of sodium hydroxide solution. The resulting solution was then diverted into a three-necked flask, which was sealed and the air outlet preserved. Subsequently, 5 mL of Na2S solution (0.2 mol/L) was added to the above solution in an atmosphere of argon gas and magnetically stirred. The solution was then heated to 373 K, after which the yellow solution was agitated for 0.5 h. Once the solution had cooled, 50 mL of ethanol was added to precipitate it. The resulting yellowish product was obtained after extraction, filtration, washing and drying.

3.3. Preparation of Co9S8 Nanotubes

The preparation process of Co9S8 nanotubes referred to the two-step hydrothermal method in previous work [52,53]. Firstly, Co(CO3)0.35Cl0.20(OH)1.10 nanorods were synthesized as a precursor for Co9S8 nanotubes. This was achieved by dissolving CoCl2·6H2O (5 mmol) and CH4N2O (5 mmol) in 40 mL deionized water and ultrasounding the solution for 30 min. Subsequently, the solution was diverted into a 50 mL Teflon autoclave and reacted in a 393 K oven for 10 h. The precipitate was then gathered through centrifugation and washed multiple times with anhydrous ethanol and deionized water. The pink precursor was obtained following drying at 333 K for several hours. Subsequently, the synthesized Co(CO3)0.35Cl0.20(OH)1.10 precursors (110 mg) were mixed to 40 mL of Na2S solution (5 mg/mL) in the Teflon liner and stirred for an hour. The liner was then diverted into a stainless-steel autoclave and heated to a temperature of 433 K for a period of 8 h. During the vulcanization process, the inner material of the rod-like precursor underwent a reaction and fell off, thereby obtaining the Co9S8 of the hollow nanotube structure. Subsequently, the product was isolated through suction filtration, washed with anhydrous ethanol and deionized water and dried at 333 K for 12 h, and the dried product (black powder) was collected for further processing.

3.4. Positive Electrochemical Treatment of Co9S8 Nanotubes

The prepared 100 mg Co9S8 nanotubes were dispersed in 50 mL C2H5OH and ultrasonic until the solution was uniform. Then, 2 mL of APTES (3-aminopropyl triethoxysilane) solution was added to the ultrasonic-treated Co9S8 nanotube ethanol solution and stirred for 20 min. Subsequently, the product was maintained in a water bath at 333 K for a period of four hours, centrifuged and washed with anhydrous ethanol and deionized water on several occasions. The obtained product was then dried in a 333 K oven and collected for use.

3.5. Electrostatic Assembly of CdS QDs-Co9S8

Typically, 50 mg CdS QDs was dispersed in 50 mL deionized water and ultrasounded for 5 min. A certain proportion of 5%/10%/30% (2.5 mg/5 mg/15 mg) electropositive Co9S8 nanotubes were dispersed in deionized water by the same method described above and ultrasonic. After ultrasound, the Co9S8 nanotube solution was injected into the CdS QDs solution and stirred for a period of 2.5 h. Subsequently, the mixed solution was subjected to centrifugation and multiple washes with deionized water, after which it was dried in an oven at 333 K to yield the dried yellowish-green product.

3.6. Activity Evaluation of Photocatalytic H2 Evolution

Photocatalytic H2 production was conducted within a 50 mL closed quartz reactor. Typically, 1 mL of triethanolamine (TEOA) and 5 mL of deionized water were added to a sealed quartz reactor containing 5 mg of CdS QDs-Co9S8 composite photocatalyst, followed by ultrasound until the solution was uniform. Subsequently, pure argon gas was implanted into the quartz reactor for half an hour to remove residuary air. A 300 W xenon lamp (PLS-SXE300D, Perfectlight, Beijing, China) with an ultraviolet cut-off filter (λ ≥ 420 nm) was used as the light source. Following a two-hour illumination period, 1 mL of mixed gas was injected into the gas chromatograph (GC7900, Techcomp, Shanghai, China) to detect the peak areas of hydrogen and argon, and the hydrogen production rate of the photocatalyst was then converted according to the hydrogen production coefficient given. Additionally, the stability of the CdS QDs-Co9S8 composite photocatalyst was evaluated by conducting tests for 5 cycles under the same conditions after centrifugation, washing and drying.

4. Conclusions

In summary, Co9S8 hollow nanotubes were prepared through a two-step hydrothermal approach as a cocatalyst, and the CdS QDs-Co9S8 composite photocatalysts were successfully prepared through a straightforward electrostatic self-assembly method. The electrostatic self-assembly strategy ensures a tight interfacial contact between CdS QDs and Co9S8 nanotubes. By adjusting the proportion of Co9S8 nanotubes in the composite, the photocatalytic hydrogen evolution rate of the optimal CdS QDs-30%Co9S8 nanotubes is 9642.7 μmol·g−1·h−1, approximately 60.3 times that of blank CdS QDs. The cyclic experiment demonstrates that the introduction of Co9S8 cocatalysts effectively prevents photocorrosion on the surface of CdS QDs. A series of characterization experiments illustrate that the introduction of Co9S8 hollow nanotubes resulted in a more uniform and dispersed growth of CdS QDs particles, as well as the promotion of the separation and migration of photogenerated carriers. As a result, the CdS QDs-Co9S8 composite exhibits excellent activity and stability in photocatalytic hydrogen production. This work provides new perspectives for the rational construction of stable, environmentally friendly and highly active composite photocatalysts to realize efficient photocatalytic H2 evolution.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/molecules29153530/s1. Figure S1. Schematic representation of the samples for (a) CdS QDs, (b) Co9S8 and (c) CdS QDs-Co9S8. Figure S2. (a) TEM image and (b) HRTEM image of Co9S8. Figure S3. Pore size distributions of (a) CdS QDs, (b) Co9S8 and (c) CdS QDs-30%Co9S8. Figure S4. Nitrogen adsorption–desorption isotherms of (a) CdS QDs-5%Co9S8, (b) CdS QDs-10%Co9S8 and (c) CdS QDs-50%Co9S8. Table S1. Summary of the ICP analysis results of the samples of CdS QDs-5%Co9S8, CdS QDs-10%Co9S8, CdS QDs-30%Co9S8 and CdS QDs-50%Co9S8. Table S2. The average pore size distributions of the prepared photocatalysts.

Author Contributions

Conceptualization, Y.Y. and K.L.; methodology, C.L.; software, Y.W. (Yonghui Wu); validation, Y.W. (Yu Wei), Y.Y. and J.W.; formal analysis, K.Y.; investigation, J.-L.Z.; resources, K.L. and B.W.; data curation, Y.W. (Yonghui Wu); writing—original draft preparation, Y.Y.; writing—review and editing, K.L.; visualization, W.-Y.H.; supervision, K.L.; project administration, B.W.; funding acquisition, K.L. All authors have read and agreed to the published version of the manuscript.

Funding

This work was supported by Jiangxi Provincial Natural Science Foundation (No. 20224BAB203018, 20224ACB213010, 20232BAB213050, 20232ACB203022), the Jiangxi Province “Double Thousand Plan” (No. jxsq2023102143, jxsq2023102142, jxsq2023201086, jxsq2023102141, jxsq2019102053), the National Natural Science Foundation of China (No. 22366018, 5236005), the Program of Qingjiang Excellent Young Talents, JXUST (No. JXUSTQJBJ2020005) and Jiangxi Provincial Key Laboratory of Functional Crystalline Materials Chemistry (2024SSY05161).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Acknowledgments

The authors would like to thank Chen Weiwei from Shiyanjia Lab (www.shiyanjia.com) for the XPS analysis on January 2024 and Jiangxi Qianvi New Materials Co., Ltd. for SEM analysis and TEM analysis provided by zkec (www.zkec.cc) on 1 May 2024.

Conflicts of Interest

The authors declare no conflicts of interest.

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Molecules 29 03530 g001
Figure 1. Synthesis diagram of CdS QDs-Co9S8 composite photocatalyst.
Figure 1. Synthesis diagram of CdS QDs-Co9S8 composite photocatalyst.
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Figure 2. Zeta potential of (a) APTES-modified Co9S8 and (b) CdS QDs suspension dispersed in deionized water.
Figure 2. Zeta potential of (a) APTES-modified Co9S8 and (b) CdS QDs suspension dispersed in deionized water.
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Figure 3. (a) TEM image and (b) HRTEM image of CdS QDs. SEM images of (c) Co9S8 and (d) CdS QDs-30%Co9S8. (e) The EDS spectrum of CdS QDs-30%Co9S8. (f) The element mapping results of CdS QDs-30%Co9S8.
Figure 3. (a) TEM image and (b) HRTEM image of CdS QDs. SEM images of (c) Co9S8 and (d) CdS QDs-30%Co9S8. (e) The EDS spectrum of CdS QDs-30%Co9S8. (f) The element mapping results of CdS QDs-30%Co9S8.
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Figure 4. (a) XRD pattern of blank CdS QDs, Co9S8 and CdS QDs-30%Co9S8 composite. (b) UV−vis diffuse reflection spectra of blank CdS QDs, Co9S8 and CdS QDs-30%Co9S8 composite.
Figure 4. (a) XRD pattern of blank CdS QDs, Co9S8 and CdS QDs-30%Co9S8 composite. (b) UV−vis diffuse reflection spectra of blank CdS QDs, Co9S8 and CdS QDs-30%Co9S8 composite.
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Figure 5. XPS spectra for (a) the survey spectra of the CdS QDs-30%Co9S8 composite, (b) Cd 3d, (c) S 2p and (d) Co 2p.
Figure 5. XPS spectra for (a) the survey spectra of the CdS QDs-30%Co9S8 composite, (b) Cd 3d, (c) S 2p and (d) Co 2p.
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Figure 6. (a) Photocatalytic hydrogen production rates of blank CdS QDs and CdS QDs-Co9S8 composite. (b) Cyclic stability test of CdS QDs-30%Co9S8 photocatalytic hydrogen production. (c) The SEM images of CdS QDs-30%Co9S8 composite after cyclic test. (d) XRD patterns of the CdS QDs-30%Co9S8 before and after cyclic test.
Figure 6. (a) Photocatalytic hydrogen production rates of blank CdS QDs and CdS QDs-Co9S8 composite. (b) Cyclic stability test of CdS QDs-30%Co9S8 photocatalytic hydrogen production. (c) The SEM images of CdS QDs-30%Co9S8 composite after cyclic test. (d) XRD patterns of the CdS QDs-30%Co9S8 before and after cyclic test.
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Figure 7. (a) Steady-state photoluminescence (PL) emission spectra with an excitation wavelength of 500 nm. (b) Transient photocurrent spectra. (c) EIS Nyquist plots. Nitrogen adsorption–desorption isotherms of (d) blank CdS QDs, (e) Co9S8 and (f) CdS QDs-30%Co9S8 composite.
Figure 7. (a) Steady-state photoluminescence (PL) emission spectra with an excitation wavelength of 500 nm. (b) Transient photocurrent spectra. (c) EIS Nyquist plots. Nitrogen adsorption–desorption isotherms of (d) blank CdS QDs, (e) Co9S8 and (f) CdS QDs-30%Co9S8 composite.
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Figure 8. The band gap energy of (a) CdS QDs and (b) Co9S8. Mott–Schottky plots of (c) CdS QDs and (d) Co9S8.
Figure 8. The band gap energy of (a) CdS QDs and (b) Co9S8. Mott–Schottky plots of (c) CdS QDs and (d) Co9S8.
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Figure 9. Mechanism diagram of CdS QDs-Co9S8 in photocatalytic hydrogen production driven by visible light.
Figure 9. Mechanism diagram of CdS QDs-Co9S8 in photocatalytic hydrogen production driven by visible light.
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Table 1. Contrast of the H2 production performance of the CdS-based photocatalysts.
Table 1. Contrast of the H2 production performance of the CdS-based photocatalysts.
PhotocatalystsLight SourcesSacrificial AgentsH2 (μmol·g−1·h−1)Reference
CdS QDs-30% Co9S8300 W Xe lamp
(λ ≥ 420 nm)		TEOA9642.7this work
CdS/TiO2@Ti3C2300 W Xe lamp
(λ ≥ 420 nm)		TEOA3115.0[30]
CdS QDs/Ni2P/B-TiO2300 W Xe arc lampNa2S/Na2SO33303.9[31]
CdS/Au/KTaO3Xe lamp
(λ ≥ 420 nm)		Na2S/Na2SO32892.0[32]
CdS QDs/CeO2300 W Xe lamp
(λ ≥ 300 nm)		Na2S/Na2SO3101.1[33]
Ni@NiO/CdS500 W Xe lampTEOA4380.0[34]
CuS/CdS300 W Xe lamp
(λ ≥ 420 nm)		lactic acid (10 vol%)5617.0[35]
UiO-66-NH2@CdS300 W Xe lamp
(λ ≥ 420 nm)		Na2S/Na2SO32028.5[36]
ZnO-Cu-CdS300 W Xe lamp
(λ ≥ 420 nm)		glycerol4655.0[37]
Ag2S-CdS300 W Xe lamp
(λ ≥ 420 nm)		lactic acids (2 vol%)777.3[38]
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Yan, Y.; Wu, Y.; Lu, C.; Wei, Y.; Wang, J.; Weng, B.; Huang, W.-Y.; Zhang, J.-L.; Yang, K.; Lu, K. Electrostatic Self-Assembly of CdS Quantum Dots with Co9S8 Hollow Nanotubes for Enhanced Visible Light Photocatalytic H2 Production. Molecules 2024, 29, 3530. https://doi.org/10.3390/molecules29153530

AMA Style

Yan Y, Wu Y, Lu C, Wei Y, Wang J, Weng B, Huang W-Y, Zhang J-L, Yang K, Lu K. Electrostatic Self-Assembly of CdS Quantum Dots with Co9S8 Hollow Nanotubes for Enhanced Visible Light Photocatalytic H2 Production. Molecules. 2024; 29(15):3530. https://doi.org/10.3390/molecules29153530

Chicago/Turabian Style

Yan, Yuqing, Yonghui Wu, Chenggen Lu, Yu Wei, Jun Wang, Bo Weng, Wei-Ya Huang, Jia-Lin Zhang, Kai Yang, and Kangqiang Lu. 2024. "Electrostatic Self-Assembly of CdS Quantum Dots with Co9S8 Hollow Nanotubes for Enhanced Visible Light Photocatalytic H2 Production" Molecules 29, no. 15: 3530. https://doi.org/10.3390/molecules29153530

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