Morphological regulation of sulfur-vacancy-rich CdS for tunable CO₂ photoreduction under visible light irradiation

Abstract

In this work, morphological control with a series of sulfur-vacancy-rich CdS photocatalysts has been achieved toward the optimization of their performances in CO₂ photoreduction. Results show that sulfur-vacancy-rich CdS nano-platelets (p-CdS-Vs) exhibit the highest CO₂ photoreduction activity with a CO yield of 4058.5 μmol h⁻¹ g⁻¹, which is 10 and 6 times those of sulfur-vacancy-rich CdS nanowires (w-CdS-Vs, 372.8 μmol h⁻¹ g⁻¹) and nanorods (r-CdS-Vs, 638.7 μmol h⁻¹ g⁻¹), respectively, amongst the highest numbers for CdS-based photocatalysts reported hitherto. The superior CO₂ photoreduction performance of p-CdS-Vs is attributable to its high efficiency of electron transport and suppressed recombination of photogenerated charge carriers. A mechanistic study indicates the critical role of surface sulfur vacancies that provide a microenvironment to trap unpaired electrons for the separation of photogenerated carriers so that the photocatalytic efficiency of CO₂-to-CO reduction is largely improved in this current system.

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1. Introduction

Photocatalytic carbon dioxide (CO₂) reduction into hydrocarbon fuel has been recognized as one of the promising means to alleviate environmental issues related to global warming and the energy crisis.^1,2 In a typical process, photogenerated charge carriers (photoelectrons and holes) are first induced based on efficient light absorption of semiconductor photocatalysts.^2,3 Once photocarriers are separated and transported to the surface of photocatalysts, photoelectrons initiate reduction reactions of CO₂ and H₂O into various carbonaceous products including carbon monoxide (CO), methane (CH₄), and other hydrocarbons. However, the practical application of CO₂ photoreduction is limited due to either the low selectivity of products or the poor stability of photocatalysts. Therefore, delicate design and controlled synthesis of viable semiconductor photocatalysts are vital for the development of a more sustainable system that converts solar energy into chemical energy.

As an efficient semiconductor photocatalyst, CdS has shown great potential in the field of CO₂ reduction. However, the photocatalytic activity of CdS is somehow unsatisfactory due to the possible catalyst deactivation by photocorrosion, particle aggregation, and rapid self-recombination of photogenerated carriers (photoelectrons and holes) during photocatalysis, which limit its further practical applications.^4,5 Therefore, improvement in the stability of CdS, coupled to the promotion of photocarrier separation, is crucial for the optimization of the efficiency of CO₂ photoconversion. Due to the different morphologies and sizes of photocatalysts, their photocatalytic activities are very different.^6,7 The macroscopic quantum tunneling effect and quantum size effect of photocatalysts with different morphologies will be different,^8,9 and these effects are closely related to the properties of electrons and holes in the photocatalysts, and thus changes in the morphology of semiconductor photocatalysts will affect the migration efficiency of photogenerated carriers, as well as the photocatalytic CO₂ reduction performance.

Sulfur vacancies refer to positions or vacancies within the chemical structure where sulfur atoms are not filled or absent. Near these vacancies, there is a local distribution of electrons to maintain charge balance.¹⁰ Sulfur vacancies can serve as active centers for catalytic reactions, providing adsorption and reaction sites.¹¹ They can also enhance the light absorption capacity of photocatalysts, thereby increasing the efficiency of photocatalytic reactions. Additionally, sulfur vacancies can regulate the migration and transfer of electrons and thus favor the separation efficiency of photogenerated charge carriers for further increasing the local electron density. To this end, in situ creation of sulfur vacancies may regulate the catalytic realms at defective sites and facilitate efficient carrier transportation, thereby increasing the lifetime and local density of photocarriers.

Therefore, a series of sulfur-vacancy-rich CdS (CdS-Vs) with various morphologies were synthesized using a solvothermal method. It is worth noting that different synthesis methods yield CdS with varying morphologies and sulfur vacancy contents. Among them, p-CdS-Vs exhibits the best performance in photocatalytic CO₂ reduction, attributed to its highest sulfur vacancy content, which effectively separates photogenerated charge carriers and enhances the ability for CO₂ reduction. This work provides a feasible strategy for defect engineering to enhance the photocatalytic activity of CO₂ reduction.

2. Experimental

2.1. Chemicals and characterization

Cadmium nitrate tetrahydrate (Cd(NO₃)₂·4H₂O), cadmium chloride (CdCl₂), sodium sulfide nonahydrate (Na₂S·9H₂O), ethylenediamine (C₂H₈N₂), acetonitrile (CH₃CN), and triethylamine (C₆H₁₅N) were purchased from Aladdin Industrial Corporation (Shanghai, China). Cadmium acetate dihydrate (Cd(CH₃COO)₂·2H₂O), sulfur (S), and thiourea (CS(NH₂)₂) were purchased from Macklin Biochemical Technology Co., Ltd (Shanghai, China). Ethylene glycol ((CH₂OH)₂) and sodium hydroxide (NaOH) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China).

The structural properties of catalysts were tested by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution microscopy (HRTEM), X-ray diffraction (PXRD), X-ray energy spectrum analysis (XPS), UV-vis diffuse reflectance spectroscopy (DRS), photoluminescence spectroscopy (PL), electron paramagnetic resonance (EPR) analysis, etc.

2.2. Preparation of sulfur-vacancy-rich CdS with different morphologies

2.2.1. Preparation of sulfur-vacancy-rich CdS nanorods (r-CdS-Vs). For the synthesis of r-CdS-Vs, 3.0 mmol Cd(NO₃)₂·4H₂O, 9.0 mmol CS(NH₂)₂ and 20 mL C₂H₈N₂ were added to 10 mL deionized water, and then stirred for 30 min to form a clarified dispersion. Afterwards, the above solution was transferred into a Teflon-lined stainless autoclave and heated at 180 °C for 12 h, and the resulting precipitate was washed several times with deionized water and anhydrous ethanol and dried under vacuum at 60 °C.

2.2.2. Preparation of sulfur-vacancy-rich CdS platelets (p-CdS-Vs). For the synthesis of p-CdS-Vs, 15 mmol NaOH was dissolved in 25 mL deionized water and transferred to a three-neck flask containing 4.0 mmol CdCl₂ and 20 mL ethylene glycol, and then ultrasonication was performed. The above solution was heated to 160 °C in an oil bath and then 4 mmol Na₂S·9H₂O was added to the three-neck flask and heated to 180 °C for 1 h. The resulting precipitate was washed several times with deionized water and anhydrous ethanol and dried under vacuum at 60 °C.

2.2.3. Preparation of sulfur-vacancy-rich CdS nanowires (w-CdS-Vs). For the synthesis of w-CdS-Vs, 2.0 mmol Cd(CH₃COO)₂·2H₂O and 30 mL C₂H₈N₂ were placed in a 100 mL beaker. 6.0 mmol S and 50 mL C₂H₈N₂ were added dropwise and then stirred for 30 min to form a clarified dispersion; the above solution was transferred into a Teflon-lined stainless autoclave and heated at 200 °C for 12 h. The resulting precipitate was washed several times and dried under vacuum at 60 °C.

2.3. Photoelectrochemical measurements

The CO₂ photoreduction experiment was carried out at room temperature and pressure with a 300 W xenon lamp (λ ≥ 400 nm, light intensity = 400 mW cm⁻²) as the light source. Specifically, 10.0 mg photocatalyst was added to a reactor containing 8 mL MeCN and TEA (v/v = 3 : 1). Then, the reaction system was degassed under vacuum and injected with CO₂ and this was repeated three times to ensure that the reactor was filled with CO₂. During the photocatalytic reaction, 1.0 mL gas was extracted hourly, and the product was analyzed using gas chromatography.

3. Results and discussion

3.1 Characterization

Surface morphologies and microstructures of p-CdS-Vs, w-CdS-Vs and r-CdS-Vs were studied using SEM, TEM, and HRTEM. As shown in Fig. 1a–f, w-CdS-Vs are 60 nm nanowires in diameter, r-CdS-Vs are uniform and smooth nanorods with a length in the range of 200–300 nm, and p-CdS-Vs are layered stacked nanowires in the thickness range of 5–20 nm. As indicated in the HR-TEM image depicted in Fig. 1g–i, CdS samples exhibited exceptional crystallinity with an interplanar spacing of approximately 0.34 nm, corresponding to the (0 0 2) crystallographic plane of CdS.¹² Moreover, the discontinuous lattice stripes (red circles) are attributed to sulfur vacancies.^13,14 It is preliminarily seen that there are more discontinuous regions in p-CdS-Vs than in w-CdS-Vs and r-CdS-Vs, and it can be inferred that p-CdS-Vs contains more sulfur vacancies.

Fig. 1 SEM images of w-CdS-Vs (a), r-CdS-Vs (b) and p-CdS-Vs (c); TEM images of w-CdS-Vs (d), r-CdS-Vs (e) and p-CdS-Vs (f); HR-TEM images of w-CdS-Vs (g), r-CdS-Vs (h) and p-CdS-Vs (i).

XRD patterns of the synthesized CdS with different morphologies were analyzed (Fig. 2a). The diffraction peaks at 2θ of 25.36°, 27.00°, 28.78°, 44.48°, 48.48° and 52.80°, which correspond to the crystal planes of (1 0 0), (0 0 2), (1 0 1), (1 1 0), (1 0 3) and (1 1 2) of CdS,⁵ and were attributed to the characteristics of CdS (JCPDS: 41-1049). The diffraction peak intensity of each crystal plane is high, and there are no other diffraction peaks, indicating the excellent crystallinity of the as-synthesized CdS.¹⁵ Among them, the diffraction peak intensity of r-CdS-Vs and w-CdS-Vs is relatively higher than that of p-CdS-Vs, likely due to the presence of more sulfur vacancies in the latter.

Fig. 2 (a) XRD of CdS with different morphologies; (b) high-resolution XPS full spectrum of CdS with different morphologies; and XPS spectra of Cd 3d (c) and S 2p (d).

X-ray photoelectron spectroscopy (XPS) was employed to determine the elemental composition and chemical valence of CdS with different morphologies. The XPS survey spectrum unambiguously indicated the presence of S and Cd (Fig. 2b). The C 1s peak is generated by the exogenous carbon of the XPS instrument. In the Cd 3d spectra (Fig. 2c), characteristics peaks near 404.9 eV and 411.7 eV corresponded to Cd 3d_5/2 and Cd 3d_3/2 respectively, indicating the predominant presence of Cd²⁺.^16,17 In the S 2p spectra (Fig. 2d), p-CdS-Vs exhibited two characteristics at 163.1 eV and 162.5 eV, corresponding to S 2p_3/2 and S 2p_1/2, indicating the predominant presence of S²⁻. Moreover, the binding energy of r-CdS-Vs and w-CdS-Vs shifts to a lower direction, indicating that different synthesis methods affect the chemical state of non-bonded S.

3.2 Photocatalytic reduction of CO₂ on CdS with different morphologies

To explore the difference in performances among CdS catalysts with different morphologies, photocatalytic CO₂ reduction experiments were conducted separately for three CdS morphologies. As shown in Fig. 3, the primary CO₂ reduction product for all CdS catalysts was CO, with p-CdS-Vs exhibiting the highest activity in CO₂ reduction. The CO yield reached 4058.5 μmol h⁻¹ g⁻¹ for p-CdS-Vs, which was roughly 10 and 6 times higher, compared to w-CdS-Vs (372.8 μmol h⁻¹ g⁻¹) and r-CdS-Vs (638.7 μmol h⁻¹ g⁻¹), respectively, amongst the highest numbers for CdS-based photocatalysts reported hitherto. The superior performance of p-CdS-Vs in CO₂ reduction can be attributed to its smaller bandgap, broader absorption range, faster charge carrier migration, and reduced charge carrier recombination rate.

Fig. 3 Photocatalytic performances: (a) yield of H₂; (b) yield of CO; (c) CH₄ isotope labeling; and (d) cyclic experiments.

The cyclic stability of photocatalysts is crucial for their practical applications. Therefore, the cyclic stability of p-CdS-Vs photocatalytic CO₂ reduction was evaluated (Fig. 3d). p-CdS-Vs exhibited linear growth in gas production over 30 h of continuous reaction under light irradiation, indicating its stability in photocatalytic CO₂ reduction. Furthermore, photocatalytic experiments were conducted using Ar as a substitute for CO₂. The results indicated that H₂ was the only gas product, confirming that CO₂ was the exclusive source of carbonaceous products.

3.3 Photochemical analysis

The photoelectrochemical properties of photocatalysts were investigated through the comprehensive analysis of optical and electrochemical measurements. UV-Vis diffuse reflectance spectroscopy (DRS) was used to test CdS with different morphologies. All CdS samples exhibited excellent visible light response, with p-CdS-Vs demonstrating a broader absorption range compared to r-CdS-Vs and w-CdS-Vs. By using the Kubelka–Munk function, the bandgap widths of p-CdS-Vs, r-CdS-Vs, and w-CdS-Vs were calculated to be 2.16 eV, 2.32 eV, and 2.40 eV, respectively, all of which are smaller than the bandgap width of pure CdS (2.40 eV).¹⁸ This indicates that the synthesized CdS is more readily excited by visible light.

Furthermore, the p-CdS-Vs exhibited a significantly improved photocurrent response in comparison with r-CdS-Vs and w-CdS-Vs (Fig. 4c), indicating noteworthy charge separation efficiency for p-CdS-Vs.^19,20 Additionally, electrochemical impedance spectroscopy (EIS) of different CdS was conducted (Fig. 4d); p-CdS-Vs exhibited a smaller arc radius, indicating faster internal charge transfer within the photocatalyst. In the photoluminescence spectrum (PL), the fluorescence intensity of p-CdS-Vs generated under visible light excitation is the weakest (Fig. 4f), indicating that p-CdS-Vs can effectively transfer photogenerated charge carriers with the lowest recombination rate of photogenerated electron–hole pairs. This may be attributed to the presence of more sulfur vacancies in p-CdS-Vs, which could effectively separate the electrons and holes generated in the system.

Fig. 4 (a) UV-vis DRS; (b) band gaps; (c) transient photocurrent response under visible irradiation; (d) EIS Nyquist plots; (e) PL and (f) EPR spectra of CdS samples.

3.4 Mechanism analysis

Through electron paramagnetic resonance (EPR), defects in CdS with different morphologies were examined, as shown in Fig. 4f. All catalysts exhibited distinct EPR signals at a g value of 2.002, indicating the presence of significant sulfur vacancies within the photocatalysts.²¹ Moreover, the sulfur vacancy density of p-CdS-Vs was observed to be much higher than that of r-CdS-Vs and w-CdS-Vs. Combining the photoelectrochemical test results, it is evident that p-CdS-Vs demonstrates efficient photogenerated electron transfer, which is attributed to its lower crystallinity that induces a higher lattice defect density such as disorder/dislocations and grain boundaries involving abundant sulfur vacancies. S vacancies can serve as the center of electron capture and promote photogenerated electron–hole separation for improved photocatalytic efficiency, thereby suppressing the recombination of photogenerated charge carriers and enhancing the performance of photocatalytic CO₂ reduction.^22,23

The above experimental results indicated that CdS (0 0 2) with surface S vacancies significantly enhanced the photocatalytic activity for CO₂ reduction. By means of theoretical calculations of charge density, the catalysts were analysed to study the charge distribution, and the role of sulfur vacancies in these catalysts towards CO₂ reduction was analyzed. The optimized structure diagram and charge distribution of model catalysts CdS (0 0 2) without surface S vacancies and CdS (0 0 2) with surface S vacancies are shown in Fig. S1 and S2.† The charge density distribution (2D surface slice) provided a more intuitive understanding of the changes in surface charge density due to the presence of S vacancies. Upon the introduction of S vacancies, the electron density around the sulfur atoms decreased, while the charge density around the cadmium atoms (indicated by red circles) showed both increases and decreases. This indicated that the surface charge distribution became uneven in the presence of S vacancies in CdS. As shown in Fig. 5, S vacancies introduced two states in the bandgap: (i) bonding states and anti-bonding states that acted as trapping sites for photogenerated charge carriers; and (ii) a reduction in electron accumulation caused by the S vacancies which served as active centers for the reaction and enhanced the ability of catalysts for photocatalytic CO₂ reduction.

Fig. 5 (a) Charge density distribution diagram (2D surface slice) of CdS(002) without surface S vacancies and (b) CdS(002) with surface S vacancies; (c) the charge density difference maps of CdS(002) without surface S vacancies and (d) CdS(002) with surface S vacancies (the yellow region represents the electron accumulation, the blue region represents the electron deletion, isosurface = 0.005 e Bohr⁻³).

Based on the above analyses and characterization, the mechanism of photocatalytic reduction of CO₂ with CdS-Vs is proposed as follows: under visible light irradiation, electrons (e⁻) on the valence band (VB) of the CdS photocatalyst absorb photon energy, undergoing an inter-band transition and inducing h⁺ on the VB (Scheme 1). The sulfur vacancies presented in the CdS photocatalyst trap electrons, thereby enhancing the efficiency of separation of photogenerated electron–hole pairs. The h⁺ on the VB oxidizes triethanolamine (TEA) to generate TEA⁺, while the e⁻ on the conduction band (CB) reduces CO₂ and H⁺ to produce CO, CH₄, and H₂.

Scheme 1 Mechanism of photocatalytic reduction of CO₂ by p-CdS-Vs.

4. Conclusion

In summary, three CdS photocatalysts with various morphologies were synthesized using a solvothermal method, and their performances and mechanism in photocatalytic CO₂ reduction were systematically investigated. The results demonstrated that p-CdS-Vs exhibited the highest activity in photocatalytic CO₂ reduction, with a CO production rate reaching 4058.5 μmol h⁻¹ g⁻¹, which was amongst the highest numbers for CdS-based photocatalysts reported hitherto. Cycling experiments further confirmed the stable performance of p-CdS-Vs in photocatalytic CO₂ reduction, with gas production rates increasing nearly linearly over 30 h. Electron paramagnetic resonance and photoelectrochemical tests revealed that p-CdS-Vs possessed higher photocurrent response and more sulfur vacancies, meanwhile significantly suppressing the recombination of photogenerated charge carriers. This study thus provides a deeper understanding of defect engineering in semiconductors with various morphologies to enhance the performance of CO₂ reduction.

Data availability

We declare that data included in this article are genuine and available upon request.

Conflicts of interest

The authors declare no conflicts of interest.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5qi00290g

‡ Equal contributions.

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