Litchi-like CdS/CdTiO₃–TiO₂ composite: synthesis and enhanced photocatalytic performance for crystal violet degradation and hydrogen production

Abstract

Using the programmed temperature hydrothermal or microwave-assisted hydrothermal method combined with the ion exchange method, a series of CdS/CdTiO₃–TiO₂ composites were prepared by adjusting the Cd : Ti element molar ratio and microwave synthesis conditions. The crystal structure, morphology, and surface physicochemical properties of the as-prepared composites were well characterized by X-ray diffraction (XRD), UV-visible diffuse reflectance spectroscopy (UV-vis/DRS), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and N₂ adsorption–desorption measurements. The results showed that the composites prepared under different synthesis conditions contained mixed crystal phases of CdS, CdTiO₃ and TiO₂; moreover, compared to samples prepared by the programmed temperature hydrothermal method, the conditions of the microwave-assisted hydrothermal synthesis showed greater impact on the as-prepared CdTiO₃ phase. The as-prepared composites basically showed a litchi-like structure, in which CdS/CdTiO₃–TiO₂ synthesized by the programmed temperature hydrothermal method had a smoother surface, but the dispersibility of the particles was not favorable, while compared with the composites just mentioned above, the CdS/CdTiO₃–TiO₂ composites prepared by the microwave-assisted hydrothermal method possessed a coarser surface, more irregular particles and better separation conditions. Simultaneously, compared with the monomer CdS, the Brunauer–Emmett–Teller surface area of these composites increased remarkably, and the optical absorption extended to the visible region. Moreover, a series of CdS/CdTiO₃–TiO₂ composites were tested under multiple conditions including ultraviolet light, visible light, simulated solar light and microwave-assisted irradiation to study the photocatalytic properties towards crystal violet (CV) photodegradation. The results showed that the composites presented the best photocatalytic activity when the microwave synthesis conditions were 160 °C for 6 h with 0.4 : 1 (40%) Cd : Ti molar ratio. Meanwhile, the composite also had an excellent photocatalytic hydrogen production capacity. According to trapping experimental results, we proposed the possible mechanism of the photodegradation and photocatalytic hydrogen production in aqueous systems.

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In recent years, the human environment have been increasingly seriously damaged by more and more environmental problems. The traditional methods of water treatment, such as carbon adsorption, coagulation, and activated sludge have some deficiencies in the actual treatment process, making it difficult to decompose pollutants completely, and sometimes also causing secondary pollution. Therefore, there is an urgent need to find an economical and effective way to completely degrade water contaminants. Photocatalytic oxidation technology, due to its green and environment-friendly features, is becoming a promising method for pollutant treatment. Since the 1970s, researchers have found that thousands of refractory organic compounds can be degraded under sunlight or artificial light, via TiO₂, ZnO or other photocatalysts.¹ As a common photocatalytic material, TiO₂ can absorb ultraviolet radiation to arouse electronic transitions, and then the formed photogenerated electrons and holes can react with dissolved oxygen adsorbed on the surface of the catalyst and water molecules to form strongly oxidizing OH˙ and ˙O₂⁻; thus, high photocatalytic activity can be obtained, thereby mineralizing organic contaminant compounds.

TiO₂ is cheap and easily obtained, but it has a wide band gap (3.2 eV); thus, it can be excited only under ultraviolet wavelengths less than 380 nm, which will effect the practical application of TiO₂ to a degree. Recently, in order to improve the light absorption property of TiO₂, some researchers have begun to modify TiO₂ by recombination and doping methods. Hamed Eskandarloo et al. combined CeO₂ and TiO₂ to obtain optimal performance from CeO₂/TiO₂ composites.² Jiang synthesized CdS -modified mesoporous TiO₂ core/shell microspheres (CdS/CS-TiO₂) with a strong visible light activity by an in situ synthesis method, and the sample showed excellent photocatalytic properties for the degradation of RhB and 4-CP under visible light irradiation.³

CdS is a type of IV semiconductor with a band gap of 2.4 eV at room temperature. CdS has good photoelectric properties when the wavelength is more than 463 nm. Taking cadmium acetate as the cadmium source and H₂S as the sulfur source, D. W. Jing et al. obtained a CdS photocatalyst with high photocatalytic activity towards hydrogen evolution from water splitting.⁴ However, as one kind of photocatalyst, CdS also has the problem of lower quantum efficiency and light corrosion in practical applications. Therefore, combining CdS and TiO₂ can not only expand the spectral response range of the composite, but also improve the stability of the material. Dipankar Barpuzary et al. successfully synthesized a one-dimensional CdS@TiO₂ core–shell nanocomposite material by two-step hydrothermal methods,⁵ and the lifetime of the carrier could be prolonged when the sample was irradiated with visible light, and the photocatalytic activity was improved.

In addition, compared with the traditional hydrothermal synthesis method, the microwave-assisted hydrothermal method has some characteristics including fast heating speed, uniform heating and high utilization rate of the heat energy. In recent years, microwave heating has been widely used in chemical reactions and the synthesis of materials, because microwave synthesis can optimize the product morphology and improve a variety of material properties.

Therefore, in this paper, a series of CdS/CdTiO₃–TiO₂ composite materials were fabricated by the programmed temperature hydrothermal method or microwave-assisted hydrothermal combined with the ion exchange method, and the main research aims were: (1) modify the optical absorption properties of TiO₂, by combining CdS and CdTiO₃ to enhance the optical absorption of the TiO₂-based composites, then further improve the photocatalytic properties of the composites and reduce the photocorrosion of CdS. (2) Explore the effect of Cd : Ti ratio on the photocatalytic performance. (3) Investigate the changing of experimental conditions to optimize the performance of the as-prepared composites. According to the comparison of the samples synthesized by the programmed temperature hydrothermal method and microwave-assisted hydrothermal method, by changing the synthesis temperature and microwave irradiation time, some effects of the microwave properties on the samples were researched. We expect that these effects of the microwave irradiation time and microwave temperature on the crystal size, band gap energy, specific surface area and surface morphology of samples can be revealed.

1. Experimental

1.1. Materials and instruments

Titanium(IV) isopropoxide (TTIP, 98%) was purchased from New Jersey Co. Ltd., USA. Cadmium chloride and thioacetamide (TAA) were purchased from Kaitong Chemical Reagent Co. Ltd., Tianjin, China. Analytical grade crystal violet (CV), methyl orange (MO), rhodamine B (RhB), and acid fuchsin (AF) were used as received without further purification. Double-distilled water was used in the experiments. The MDS-8G microwave reactor was purchased from Xin Yi Co., Shanghai, China.

1.2. Preparation of CdS/CdTiO₃–TiO₂ composite

1.2.1. Step 1: synthesis of CdS. First, a certain amount of HCl solution was added to a CdCl₂ solution, and the obtained solution was denoted as A. Then, TAA was dissolved in distilled water (10 mL) and added to solution A. The mixture was stirred for 40 min before it was heated to 70 °C, and then the resulting light yellow solution was heated until an orange solid was obtained. The product was washed with ethanol and distilled water and dried at 60 °C, and then the target product was obtained.

1.2.2. Step 2: synthesis of CdS/CdTiO₃–TiO₂. The precursor was prepared by the programmed temperature hydrothermal method and microwave-assisted hydrothermal method, respectively. The programmed temperature hydrothermal method is as described here: certain amounts of CdCl₂, glycerin, ethyl alcohol, diethyl ether and TTIP were mixed and stirred for 30 min, and finally heated at a rate of 2 °C min⁻¹ in a air blast dryer, before being held at 160 °C for 6 h in a Teflon-lined stainless steel autoclave. The reactant ratio in the microwave-assisted hydrothermal progress was the same as in the programmed temperature hydrothermal method, while the difference between two methods was that the reaction vessel was placed in the microwave reactor during the microwave-assisted hydrothermal process. Then, the as-obtained white precipitate was washed with ethyl alcohol, dried at 60 °C and calcined at 550 °C for 4 h. After that, both samples and TAA were dispersed in anhydrous ethanol to react in a chemical bath at less than 90 °C for 8 h. After drying, the yellow powder obtained was the final product, the CdS/CdTiO₃–TiO₂ composite. When changing the ratio of Cd : Ti in the two synthesis methods, the samples synthesised by the programmed temperature hydrothermal method were marked as X% HCT 160-6, and the samples synthesised by the microwave-assisted hydrothermal method were marked as X% WCT (X% indicates the molar ratio of Cd : Ti, for example, 0.4 : 1 = 40%); a series of CdS/CdTiO₃–TiO₂ composites with different microwave times and temperatures were also prepared, marked as WCT temperature–time (for example, WCT 160-6).

1.2.3. Characterization of photocatalysts. The X-ray diffraction (XRD) patterns of the as-prepared samples were recorded using an X-ray diffractometer (Bruker-AXS (D8)) equipped with Cu Kα X-ray radiation at 60 kV and 80 mA and with 2θ ranging from 20° to 80°. The morphologies of the as-prepared samples were examined by scanning electron microscopy (SEM) (HitachiS-4300) and high-resolution transmission electron microscopy (HRTEM) (JEM-2100F). The Brunauer–Emmett–Teller (BET) surface areas, pore volumes, and pore size distributions were determined from nitrogen adsorption isotherms at 77 K using a Quantachrome Nova Win II instrument. X-ray photo-electron spectroscopy (XPS) was performed using an ESCALAB 250Xi spectrometer equipped with Al Kα radiation at 300 W. The optical absorbance spectra of the as-prepared samples were recorded using a UV-visible spectrophotometer (TU-1901) equipped with a diffuse reflectance accessory, and BaSO₄ was used as the reflectance standard.

1.3. Photocatalytic activity test

In order to investigate the photocatalytic activity of the composite materials, crystal violet (CV) was chosen as a model molecule. The UV photocatalytic reaction apparatus and light source apparatus with a built-in light source consist of a cylindrical quartz outer tube and quartz sleeve surrounding. The catalyst and the suspension of the reaction solution were surrounded by the light source. The reaction temperature was controlled by cooling water at 20 ± 5 °C; an Hg lamp was used as the UV light source (125 W Hg lamp with the maximum emission at 313.2 nm). The visible light was obtained from a 400 W Xe lamp with the main emission lines greater than 410 nm. A microwave-assisted photocatalytic reaction device and an H-shaped 15 W microwave discharge electrodeless lamp (MDEL) with the maximum emission at 280 nm were used. A simulated sunlight source was made by using a 1000 W Xe-lamp with emission from UV to near IR spectra (ca. 300–1050 nm), and the distance between the lamp and the reaction liquid was 8.5 cm. The reaction system was kept at a constant temperature by ethanol cycling.

The concentration of the reaction solution (CV) was 50 mg L⁻¹. The amount of catalyst used for the UV and simulated sunlight photodegradation was identical at 0.15 g, and the volume of solvent was 90 mL. The amount of photocatalyst for the microwave-assisted photodegradation was 0.5 g, and the volume of solvent was 500 mL. The photocatalyst was dispersed into the solution by ultrasonication for 10 min and stirred in the dark for 30 min to obtain the adsorption–desorption equilibrium between the photocatalyst and organic pollutant. At given time intervals, 3 mL suspension was collected and centrifuged to remove the photocatalyst particles. All samples were analyzed using a UV-visible spectrophotometer (TU-1901).

1.4. Photocatalytic hydrogen evolution test

Photocatalytic hydrogen evolution experiments were carried out in a reactor with a closed and connected vacuum circulatory system (LabSolar-IIIAG, purchased from Beijing Perfect Light, China). 100 mg photocatalyst was dispersed in 50 mL distilled water, and 12.5 mmol Na₂S·9H₂O and 17.5 mmol Na₂SO₃ were added as sacrificial agents. After vacuum degassing, the photocatalytic hydrogen evolution test started with lighting and stirring. A 300 W Xe lamp (PLS-SXE 300, purchased from Beijing Perfect Light, China), fixed at a distance of 10 cm from the reaction solution, was used as the light source; high purity nitrogen as a carrier gas was at a flow rate of 0.5 mL s⁻¹; the output pressure was 0.4–0.5 MPa, and the working voltage and working current were about 20 mV and 50 mA, respectively. In the reaction process, the temperature of the reactor was kept at 5 °C by the circulating cooling water. Gas was collected after a certain irradiation time by using online gas chromatography (GC-7900, purchased from Shanghai Techcomp, China) to analyze the hydrogen production. The column was a 5 Å molecular sieve column, and the detector was a thermal conductivity detector (TCD). According to the peak area of the different reaction times, the production of hydrogen was calculated, and the catalytic activity of the photocatalyst was measured from the total hydrogen production over 8 h.

2. Results and discussion

2.1. XRD

In order to investigate the crystal structure of the as-prepared composites and the effect of the microwave synthesis conditions on the crystal size of CdS/CdTiO₃–TiO₂, X-ray diffraction tests were employed, and the results are shown in Fig. 1 and 2.

Fig. 1 XRD patterns of TiO₂, CdS and different CdS/CdTiO₃–TiO₂ composites prepared by programmed temperature hydrothermal synthesis.

Fig. 2 XRD patterns of TiO₂, CdS and a series of different samples prepared by microwave hydrothermal synthesis. (a) Hydrothermal synthesis of 40% CdS/CdTiO₃–TiO₂ composites with different synthesis times; (b) TiO₂, CdS and a series of 40% CdS/CdTiO₃–TiO₂ composites prepared by microwave hydrothermal synthesis with different synthesis temperatures.

The XRD patterns of TiO₂, CdS and X% HCT 160-6 composites synthesized by the programmed temperature hydrothermal method in different proportions are as shown in Fig. 1. The peaks at 2θ values of 24.9°, 43.8°, 47.9° and 51.9° correspond to the (100), (110), (103) and (112) planes of the CdS hexagonal phase (JCPDS41-1049).⁶ Simultaneously, those diffraction peaks at 2θ values of 25.31° (101) and 48.02° (200) indicate that the main phase of TiO₂ is anatase (JCPDS 21-1272).⁷ The peaks at 2θ values of 27.5°, 36.1°, 41.3° and 54.3° correspond to the (110), (101), (111) and (211) planes of rutile TiO₂ (JCPDS84-1283). As shown in Fig. 1, different proportions of CdS/CdTiO₃–TiO₂ have characteristic diffraction peaks of CdS and TiO₂ at the same time, the peaks at 2θ values of 31.1°, 34.2° and 41.3° belonging to the CdTiO₃ ilmenite phase (JCPDS 85-0452). Currently, there is only little study according to relevant reports,^8,9 CdTiO₃ often has higher stability, which can make up for the influence of CdS light corrosion during the reaction, and CdTiO₃ is also a kind of photocatalyst, whose presence can improve the photocatalytic activity of the composite to a certain extent. What we can observe from Fig. 1 is that the characteristic peaks of CdTiO₃ in the 40% HCT sample were more evident and sharper, indicating that the CdTiO₃ in the HCT sample was more perfect than that in the other proportion samples prepared by the programmed temperature hydrothermal method. Moreover, the average crystallite sizes of these CdS/CdTiO₃–TiO₂ composites, estimated using the Scherrer equation, are listed in Table S1 (ESI†). From Table S1,† with the increase in Cd molar ratio, the crystallite sizes of the samples prepared by the temperature programmed hydrothermal method show some trends: small → big → small; among them, the crystallite size of the sample of 40% HCT 160-6 is the largest. According to the relevant literature,¹⁰ the increase in crystallite sizes can effectively improve the photocatalytic activity, and these reports are consistent with our photocatalytic test results. Accordingly, the sample ratio was fixed at 40% in follow-up experiments using the microwave-assisted hydrothermal method.

The XRD patterns of TiO₂, CdS and composites synthesized by the microwave-assisted hydrothermal method under different synthesis conditions are presented in Fig. 2(a) and (b). As shown in Fig. 2, the peak at a 2θ value of ca. 25.3° (101) is a typical characteristic peak of anatase TiO₂;¹¹ at the same time, the peaks at 27.5° (110), 36.1° (101), 41.3° (111) and 54.3° (211) are some typical characteristic peaks of rutile TiO₂, and the peaks at 2θ values of 36.7° and 43.8° correspond to the (102) and (110) planes of CdS (JCPDS41-1049), respectively.^12,13 Compared with the as-mentioned monomers, the characteristic diffraction peaks of TiO₂ and CdS are found in the composites, simultaneously. The CdS/CdTiO₃–TiO₂ composites prepared under different microwave-assisted synthesis conditions possess the characteristic peaks of CdTiO₃, appearing at 31.1°, 41.3° and 34.2°. This indicates that composites treated with microwaves also exist in three mixed crystal phases of TiO₂, CdTiO₃ and CdS. By analysing Fig. 2, we can observe that with the increase in microwave radiation time, the characteristic peak of CdTiO₃ gradually becomes higher and sharper, attributed to the polarization during microwave synthesis, which can lead to a greater degree of perfect crystallinity. Of course, it is also one of the reasons that the photocatalytic activity is increased. According to Fig. 2(b), the intensity of the characteristic CdTiO₃ peak shows some change with the increase in temperature, indicating that the microwave temperature can influence the formation of CdTiO₃ to a certain extent.

In summary, from the comparison of the two synthetic methods, it can be found that the diffraction peaks of the samples prepared by microwave hydrothermal method are higher and sharper, attributed to the microwave polarization effect on the crystal growth. The average crystallite sizes of CdS/CdTiO₃–TiO₂ calculated using the Scherrer formula¹⁴ are listed in Table S1 (ESI†). Compared with the monomer TiO₂, with the CdS doping amount increasing, the crystallite size of the composite was changed from small to large and then to small again. The crystallite sizes of CdS/CdTiO₃–TiO₂ synthesized by the microwave-assisted hydrothermal method were generally larger than those of samples prepared by the hydrothermal synthetic method, indicating that the microwave hydrothermal treatment can promote the growth of the crystals. Simultaneously, we found that the crystallite size of the sample increased in varying degrees accompanied with the increase in the microwave synthesis temperature, which further showed that the microwave synthesis temperature had a certain effect on the crystallinity of the sample. According to the relevant reports,^15–17 the reduction in crystallite size leads to the quantization effect, altering the electric field gradients and reducing the separation of electron–hole pairs.¹⁸ Almquist and her co-workers reported that the photocatalytic performance of the nanoparticles improved upon an increase in the crystallite size due to the optimization of the optical properties and charge carrier dynamics.¹⁰ Moreover, the calculated lattice parameters of CdS/CdTiO₃–TiO₂ can be seen in Table S1 (ESI†). From Table S1 (ESI†), it can be observed, compared with pure TiO₂, that the lattice parameters of the composites had obviously changed with the increase in CdS doping amount, indicating that some of the Cd ions had doped into the TiO₂ lattices and generated CdTiO₃, resulting in some changes in the lattice parameters.

2.2. UV-visible diffuse reflectance spectroscopy

The effect of the two kinds of synthetic method and the microwave-assisted synthesis conditions (temperature and time) on the optical absorption properties of the CdS/CdTiO₃–TiO₂ samples were explored by UV-visible diffuse reflectance, as shown in Fig. 3 and S1 (ESI†). Fig. S1a (ESI†) shows the UV-vis/DRS results of CdS, TiO₂ and different proportion of CdS/CdTiO₃–TiO₂ composites prepared by the programmed temperature hydrothermal method. The absorption edge of TiO₂ is located only in the UV region < 320 nm, whereas pure CdS shows a strong absorbance in the visible region.^19,20 However, all composite samples produce a certain absorption intensity in the visible region. According to the absorption edge of CdS and TiO₂, it’s not difficult to find that the optical absorbance properties of the CdS/CdTiO₃–TiO₂ composites show greater difference. The optical absorption of the sample of 40% HCT 160-6 is located between 430 and 500 nm, which is significantly higher than that of the other composites, indicating that it will show higher visible light photocatalytic activity.

Fig. 3 UV-vis/DRS spectra (a) and the plot of transformed Kubelka–Munk function vs. absorption energy of light (b) of CdS/CdTiO₃–TiO₂ prepared by two methods.

The energy band gaps of all the samples were calculated from their optical absorption edges in the UV-vis/DRS spectra using the following equation:²¹

(αhν)ⁿ = K(hν − E_g)

where α is the absorption coefficient, K is related to the effective masses associated with the valence and conduction bands, n is 1/2 for a direct transition, hν is the absorption energy, and E_g is the band gap energy. By plotting a graph between (αhν)ⁿ and hν based on the spectral responses, shown in Fig. S1b (ESI†) and 3(b), the E_g values of the CdS/CdTiO₃–TiO₂ composites were calculated and listed in Table S2 (ESI†). As shown in Table S2,† the E_g values of the CdS/CdTiO₃–TiO₂ composites prepared by the two methods were between those of CdS (2.05 eV) and TiO₂ (3.2 eV), indicating that the composite materials can make up for defects in the short range of TiO₂ absorption, and can also improve the optical stability of CdS to a degree. Comparing the two synthetic methods, the E_g values of CdS/CdTiO₃–TiO₂ synthesized by the microwave-assisted hydrothermal method are smaller than those from the programmed temperature hydrothermal synthesis. Moreover, at the same microwave temperature (160 °C), the E_g value of the composite increased slightly when the microwave radiation time was lengthened, which may be related to the increase in CdTiO₃ concentration in the synthetic process.²² However, the increasing temperature of the microwave synthesis will make E_g values get smaller and then get larger. Based on the above results, we find that the sample of 40% WCT 160-6 possesses the best optical absorption properties (these results are consistent with subsequent photocatalytic experiments, and the photocatalytic activity of 40% WCT 160-6 is the best).

2.3. X-ray photoelectron spectroscopy

The surface elemental composition and the chemical state of the CdS/CdTiO₃–TiO₂ composite have been analyzed by XPS, and the obtained results are shown in Fig. 4. The typical spectrum of the sample clearly shows the presence of O, Ti, Cd, S, and C elements, where the observed carbon peak was due to absorbed gas molecules as an impurity.²³

Fig. 4 XPS full spectrum of the 40% WCT 160-6 composite (a) and patterns of O_1s (b), S_2p (c), Ti_2p (d) and Cd_3d (e).

The O_1s peak observed at a binding energy of 530.7 eV is attributed to O_1s originating from bulk lattice oxygen, as shown in Fig. 4(b).^24,25 Characteristic peaks of S_2p3/2 (161.7 eV) and S_2p1/2 (162.5 eV) confirm the presence of S in the −2 valence state, as shown in Fig. 4(c). The XPS spectrum of Ti is shown in Fig. 4(d), and the binding energy positions located at 459.5 and 465.3 eV are ascribed to Ti_2p3/2 and Ti_2p1/2 in the CdS/CdTiO₃–TiO₂ (40% WCT 160-6) sample, which are consistent with the binding energies of Ti⁴⁺.²⁶ Fig. 4(e) shows that Cd_3d mainly consists of 3d_5/2 and 3d_3/2 spin–orbit components at 405.8 and 412.5 eV, respectively, indicating the presence of Cd in the +2 valence state.^27–29 According to the relative peak intensities of Cd_3d, Ti_2p, O_1s and S_2p in the spectra, the relative contents of Cd, Ti, O and S are about 8.7 : 20 : 62 : 8.9, which were calculated by the sensitivity factor method.³⁰ The relative content ratio of Ti : O = 10 : 31 can prove that the photocatalyst contains TiO₂ and TiO₃²⁻, which is consistent with the XRD results.

2.4. Morphology

In order to investigate the surface morphologies of the composite materials, 5% HCT 160-6, 40% HCT 160-6, 40% WCT 160-6, WCT 100-6 and WCT 160-1 were analyzed by scanning electron microscopy.

Fig. 5 shows the SEM images of the 5% HCT 160-6, 40% HCT 160-6 and 40% WCT 160-6 composite materials at different typical scales. Fig. 5(a)–(f) show that the surface morphologies of the CdS/CdTiO₃–TiO₂ composites prepared by the temperature programmed hydrothermal treatment consist of spherical, irregular nanoparticles accumulating on the sphere outer surface, and both the uniformity of the sphere size and the surface smoothness decrease with the increase in CdS doping amount. Fig. 5(e) shows that most of the 40% HCT 160-6 spheres coincide with each other, without favorable separation. Fig. 5(g)–(i) show that the composites present a lichi-like structure with a relatively uniform size and rough surface, and the irregular particle accumulation can be observed on the relatively independent sphere outer surfaces; all these features are conducive to increasing the contact area between catalysts and reactants, which is then expected to improve the photocatalytic activities of the composites. Fig. 5(j)–(l) show SEM images of 40% WCT 100-6 at different typical scales. Compared with 40% WCT 160-6, although 40% WCT 100-6 exhibits a lichi-like structure, the sphere is irregular and doesn’t have good separation. Fig. 5(m)–(o) show SEM images of 40% WCT 160-1 at different typical scales. Compared with 40% WCT 160-6, the synthesized 40% WCT 160-1 consists of irregular spheres with poor separation and more irregular particles existing among the spheres under these synthetic conditions, which is probably due to the microwave radiation time being too short to accumulate particles to form spheres.

Fig. 5 SEM images of composites (a–c) 5% HCT; (d–f) 40% HCT; (g–i) 40% WCT 160-6; (j–l) 40% WCT 100-6 and (m–o) 40% WCT 160-1.

The morphological characteristics and the crystal structures of CdS/CdTiO₃–TiO₂ (40% WCT 160-6) are further investigated by HR-TEM and the results are illustrated as Fig. 6. Fig. 6(a)–(c) are the images of the edge and middle part of the spheres. Through use of software (Gatan Digital Micro group), we estimate the fringe spacing (as is shown in Fig. 6(d)–(j)). Several sets of fringe spacings are ca. 0.287, 0.385, 0.324, 0.352, 0.334, 0.207 and 0.176 nm, corresponding to CdTiO₃ (211), CdTiO₃ (110), rutile TiO₂ (110), anatase TiO₂ (101), CdS (002), CdS (110) and CdS (112), respectively. As is shown in Fig. 6(d), a hexagon can be observed from the FFT image, according to the FFT line plot. We find an obvious hexagonal CdS lattice fringe, which is taken as proof that CdS is indeed in the hexagonal phase, consistent with the XRD results. From the enlarged drawing of Fig. 6(c) (directional arrow in Fig. 6(c)), there are obvious dividing lines among the two sets of lattice fringes, which thus proves the as-prepared composites have rutile TiO₂ and anatase TiO₂ heterojunctions.

Fig. 6 HR-TEM images of the edge and middle part of a sphere (a–c), FFT line plot and FFT (d–j) of 40% WCT sample.

2.5. N₂ adsorption–desorption isotherms

In order to investigate the physicochemical properties of the different samples, the adsorption–desorption isotherm type, pore size distribution, surface area, total pore volume and average pore diameter of CdS/CdTiO₃–TiO₂, CdS, TiO₂ and 40% WCT with different microwave synthesis conditions were investigated by N₂ adsorption–desorption measurements, and the related results are shown in Fig. 7 and Table S3 (ESI†). As shown in Fig. 7, the N₂ adsorption–desorption isotherms of CdS, TiO₂, 40% HCT 160-6 and 40% WCT 160-6 reveal a type IV isotherm with H3 hysteresis loop according to the IUPAC classification, indicating the presence of mesopores.²⁴ The type of isotherm and the hysteresis loop are caused by the condensation of the capillary and the aggregation of particles.

Fig. 7 Nitrogen adsorption–desorption isotherms and Barrett–Joyner–Halenda (BJH) pore size distributions (insets) of different samples.

According to Table S3 (ESI†), the BET specific surface area of 40% WCT 160-6 is less than that of 40% 160-6 HCT, which is probably due to the 40% WCT 160-6 spheres being relatively independent, while the 40% HCT 160-6 spheres are not completely separated from each other. These results are also consistent with the SEM images. In Fig. 7, the illustrations of the BJH pore size distribution curves can be seen. Compared with the pore size distribution of 40% 160-6 WCT, that of 40% 160-6 HCT is more uniform, which should be attributed to the programmed temperature hydrothermal method being relatively mild in the synthesis process, leading to the internal structure of the photocatalyst being more uniform. However, the microwave-assisted hydrothermal synthesis reaction is very vigorous and quickly forms the catalyst, which could cause the non-uniform pore size distribution. The BET specific surface area, pore size distribution and total pore volume of the CdS/CdTiO₃–TiO₂ composites are larger than those of CdS, meaning that the composite materials based on the CdS structure have been optimized.

2.6. Photocatalytic activity of CdS/CdTiO₃–TiO₂

In order to investigate the photocatalytic activities of the different proportion CdS/CdTiO₃–TiO₂ composites prepared by programmed temperature hydrothermal synthesis, a series of experiments were carried out, including direct photolysis (without photocatalyst), CdS, P25 and CdS/CdTiO₃–TiO₂ composites (X% HCT) degrading crystal violet under UV light; the results of the relevant photocatalytic experiments are shown in Fig. 8. As can be seen from Fig. 8, the degradation efficiencies of the different photocatalysts were diverse. The activities of X% HCT were higher than those of the monomer and direct photocatalysis, due to TiO₂ having strong absorption in the ultraviolet region; after forming a composite with CdS, the synergy between TiO₂ and CdS can further improve the efficiency of the photocatalytic degradation. Of course, the portion of CdTiO₃ generated during synthesis also had a significant impact on catalysis. In addition, according to the experimental results of the different proportion photocatalysts, the UV photocatalytic degradation efficiency of 40% HCT was the best, and the photocatalytic activity of 50% HCT 160-6 decreased because of the reduction of the E_g value, resulting in the increase of the electron–hole pair recombination rate and the decrease of the photocatalytic quantum efficiency. Accordingly, the optimal ratio of the composite materials was 40%, which can be further confirmed.

Fig. 8 (a) Photocatalytic degradation profiles of CV for different catalysts under UV irradiation; (b) photocatalytic degradation histogram of CV for different catalysts under UV irradiation for 90 min.

A series of profiles of composites degrading crystal violet under UV light, wherein the composites were synthesized under different microwave synthesis conditions, are shown in Fig. 9. From Fig. 9(a), it can be observed that the photocatalytic activity is best when the microwave conditions are 160 °C for 6 h. The photocatalytic activities of the optimum catalyst prepared by both the temperature programmed hydrothermal method and microwave-assisted hydrothermal method to degrade CV under UV light are shown in Fig. 9(b). It can be clearly observed that the activity of the sample synthesized by the microwave-assisted hydrothermal method is much better under the same experimental conditions. Fig. 9(c) shows that all photocatalytic degradation of CV follows second order reaction kinetics.

Fig. 9 Results of UV degradation of CV with a series of photocatalysts synthesised by changing the conditions (a); comparison of the photocatalytic activity of 40% HCT and 40% WCT under UV light (b); kinetics profiles of CV degradation with different photocatalysts (c).

Fig. 10 is the comparison of the results of different dyes degraded under UV irradiation. According to the comparison, 40% CdS/CdTiO₃–TiO₂ composites prepared by the two methods have a certain universality for the degradation of different types of dyes under UV irradiation. As shown in Fig. 11, under visible light, simulated sunlight and microwave irradiation, the degradation effects of CV based on direct photocatalysis, CdS, P25, 40% HCT 160-6 and 40% WCT 160-6 are enhanced in sequence. In addition, it can be observed from Fig. S2 (ESI†) that, after several cycles of experiments, the degradation rates decreased to a certain degree. That is owing to the catalyst CdS/CdTiO₃–TiO₂ being a kind of sulfide-containing compound, and it is easily broken down under the high temperature treatment, leading to the decomposition of sulfide, which then results in the destruction of its composition and structure. Therefore, a purge of water and ethanol around CdS/CdTiO₃–TiO₂ in the recycling experiment was carried out, which meant that some dye molecules present in the pores of CdS/CdTiO₃–TiO₂ could not be removed completely. Because some of the pores of CdS/CdTiO₃–TiO₂ were blocked, this will affect its activity. Fig. S2c (ESI†) is the XRD patterns of the recycled product, and it can be observed that the composition of catalyst have no obvious changes, illustrating the catalyst can maintain the structure stability.

Fig. 10 Degradation results of CdS/CdTiO₃–TiO₂ for different organic pollutants under UV light.

Fig. 11 Degradation of CV by different photocatalysts under visible light, simulated sunlight and microwave irradiation conditions.

In order to investigate the photocatalytic hydrogen evolution ability of the as-prepared 40% WCT 160-6 photocatalyst, a series of experiments were carried out, as shown in Fig. 12. From Fig. 12, it can be clearly observed that the photocatalytic hydrogen evolution ability of TiO₂ is very weak before CdS and CdTiO₃ were combined with it, and this illustrates that the presence of CdS and CdTiO₃ can make up for the shortcoming of TiO₂ (E_CB = −0.29 eV vs. NHE) that the conduction band is too close to the potential of H₂/H₂O (0.0 eV vs. NHE), and owing to CdS (E_CB = −0.52 eV vs. NHE) being more negative, the presence of CdS and CdTiO₃ in the as-prepared composite can improve its ability for hydrogen production.³³

Fig. 12 Amounts of H₂ generated from TiO₂ and 40% WCT 160-6 photocatalyst.

According to above activity tests, we find that the sample of 40% WCT 160-6 has the best activity, which can be attributed to: (1) the sample 40% WCT 160-6 has the best optical absorption properties according to the UV-vis/DRS results and can be excited to generate electrons and holes under the simulated sunlight irradiation; therefore, the photocatalytic reaction can occur; (2) in the microwave reactor vessel, due to the role of the high temperature, pressure and microwaves, they can make the aggregation of the catalyst particles decrease, and the particle dispersion is more uniform, thereby improving the photocatalytic activity; (3) the band gap of TiO₂ is 3.2 eV, leading to its lower activity; after combining it with CdS and CdTiO₃, the band gap of the composite is dramatically narrowed, and thus the activity is greatly improved.

2.7. The formation mechanism of the CdS/CdTiO₃–TiO₂ composite

According to the relevant experimental data, the possible growth mechanism of the CdS/CdTiO₃–TiO₂ composite was discussed, with the specific content shown in Scheme S1 (ESI†). CdCl₂ and TTIP were mixed into a reaction kettle, in which the TiO₂ nanoparticles were hydrolyzed by water to afford Ti(IV) species. The surface energy was raised along with the rising temperature during the synthesis process, leading to these particles aggregating to rapidly form the precursor. During the growth of the CdS/CdTiO₃–TiO₂ composite, TAA functioned as a sulfur source and a bidentate ligand to form a relatively stable Cd–TAA complex. The S–C bond of thiourea can be cleaved during the chemical bath process. The resulting sulfur ions exchanged oxygen ions to react with Cd ions until the CdS was generated.

2.8. Photocatalytic mechanism

In order to explore the reaction mechanism of the as-prepared catalyst, trapping experiments with 40% WCT 160-6 were carried out, wherein the trapping agents were isopropanol and benzoquinone.^31,32 As shown in Fig. S3 (ESI†), benzoquinone and isopropanol successfully captured active groups in the UV photocatalytic experiments, that is, the superoxide radical (˙O₂⁻) and the hydroxyl radical (˙OH) played a major role during the process of photocatalytic degradation of the dyes.^25,33

Based on the above trapping experiments, a probable reaction mechanism was proposed (Scheme 1).

Scheme 1 The possible photocatalytic mechanism of CdS/CdTiO₃–TiO₂.

Scheme 1 is the possible photocatalytic reaction mechanism of the CdS/CdTiO₃–TiO₂ composite. According to the formula E_CB = χ − 4.5 − 0.5E_g, the conduction band and valence band locations of CdS, CdTiO₃ and TiO₂ can be calculated,³⁴ which are listed in Table 1. Under the light irradiation, photoelectrons can be excited from the valence band (VB) of CdS in CdS/CdTiO₃–TiO₂ to the conduction band (CB) of CdS, thus forming electron–hole pairs. Due to the formation of CdTiO₃ in the synthesis process, charge transfer paths are increased, which can promote charge transfer and the separation of photogenerated electron–holes, thereby increasing the overall photoelectric conversion efficiency and photocatalytic activity. Simultaneously, because of the interfacial contact between TiO₂ and CdS, photogenerated electrons can transfer to TiO₂, thus enhancing the lifetimes of the charge carriers. However, accumulated holes on the VB of CdS cannot produce ˙OH radicals by the oxidization of surface hydroxyl groups or adsorbed water molecules. Similarly, the electron on the CB of TiO₂ cannot reduce oxygen molecules (O₂) into superoxide radicals (O₂˙⁻). This is because the CB potential of TiO₂ (−0.29 eV vs. NHE) is more positive than the redox potential of O₂˙⁻ formation (O₂/O₂˙⁻ = −0.33 eV vs. NHE) and the VB potential of the CdS (+1.88 eV vs. NHE) is more negative than the potential required to oxidize H₂O or –OH to ˙OH radicals (+2.4 eV vs. NHE).^35,36 Therefore, in the reaction system, the molecular oxygen can be activated to form superoxide radicals on the CB of CdS, and the holes that are generated from the VB will react with H₂O to form OH˙ radicals on the VB of TiO₂. Thus, these reactive species can efficiently inhibit recombination of electron–hole pairs and prolong their lifetimes. The generation of OH˙ and O₂˙⁻ can be proved by trapping experiments, and they degrade and even mineralize organic pollutants to CO₂ and H₂O. At the same time, the conduction band position of the CdS/CdTiO₃–TiO₂ is more negative than the hydrogen electrode reaction potential (0.0 eV vs. NHE), which can satisfy the requirements of the hydrogen evolution reaction so that conduction band electrons may reduce water into hydrogen. Since the as-synthesized composite CdS/CdTiO₃–TiO₂ can simultaneously satisfy the above conditions of degradation and hydrogen production, it may have activities of both photocatalytic degradation of dye and photocatalytic hydrogen evolution.

Table 1 Conduction bands (CBs) and valence bands (VBs) of CdS, TiO₂ and CdTiO₃

Potential (eV)	CdS	TiO2	CdTiO3
ECB	−0.52	−0.29	−0.68
EVB	1.88	2.91	3.22

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3. Conclusions

A series of CdS/CdTiO₃–TiO₂ composites were prepared by programmed temperature hydrothermal and microwave-assisted hydrothermal methods. The results showed that the CdS/CdTiO₃–TiO₂ composites existed in three mixed crystal phases of TiO₂, CdTiO₃ and CdS. With the increase in the Cd doping amount, the crystallite size showed this trend: small → big → small. The crystallite sizes of the composites prepared by the microwave-assisted hydrothermal method were generally larger than those of the programmed temperature hydrothermal synthesis samples, and the crystallite size of the sample also increased to a degree along with the increasing microwave synthesis temperature, which indicated that the microwave hydrothermal treatment may promote the growth of crystals and had some influence on the sample crystallinity. The surface morphology of the CdS/CdTiO₃–TiO₂ composite prepared by the temperature programmed hydrothermal treatment was spherical with the accumulation of irregular nanoparticles on the outer surface of the spheres. Both the uniformity of the sphere size and the surface smoothness decreased with the increase in CdS doping amount. The surface morphology of the CdS/CdTiO₃–TiO₂ composite prepared by the microwave-assisted hydrothermal treatment showed a lichi-like structure owing to these features, like independent spherical structure, relatively uniform size, surface roughness, and the irregular particle accumulation on the sphere outer surface. Based on the results of photocatalytic degradation of CV by the CdS/CdTiO₃–TiO₂ composites, the best microwave synthesis conditions of the photocatalyst with the highest ultraviolet light catalytic activity were determined as 160 °C for 6 h with the molar ratio as 0.4 : 1, that is, 40% WCT 160-6. The as-prepared photocatalyst not only had the ability to degrade organic pollutants, but also exhibited excellent photocatalytic hydrogen production capacity, showing that the synergistic effect of CdS, CdTiO₃ and TiO₂ can help enhance its photocatalytic activity.

Acknowledgements

This study is supported by the National Natural Science Foundation of China (21376126), Natural Science Foundation of Heilongjiang Province, China (B201106), Scientific Research of Heilongjiang Province Education Department (12511592), Government of Heilongjiang Province Postdoctoral Grants, China (LBH-Z11108), Open Project of Green Chemical Technology Key Laboratory of Heilongjiang Province College, China (year 2013), Postdoctoral Researchers in Heilongjiang Province of China Research Initiation Grant Project (LBH-Q13172), Innovation Project of Qiqihar University Graduate Education (YJSCX2014-009X), and Qiqihar University in 2016 College Students Academic Innovation Team Funded Projects.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra05631h

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