Synthesis of Durian-like TiO 2 @CdS Core-Shell Structure and Study on H 2 Generation Properties

: Novel durian-like TiO 2 @CdS core-shell particles were synthesized through a solvothermal method in ethylenediamine solution and the obtained nanocomposites were characterized by scanning electron microscopy (SEM), powder X-ray diffraction (XRD), and transmission electron microscopic (TEM) techniques. It can be seen from the characterization that the synthesized core-shell structured particles show uniform size. The possible formation mechanism of TiO 2 @CdS core-shell particles is also presented schematically. CdS grows on the TiO 2 surface in the form of nanorods, turning the TiO 2 @CdS composite particles into durian-like structures. The durian-like TiO 2 @CdS core-shell particles prepared in the experiment can overcome the disadvantages of TiO 2 and CdS, respectively. They not only produce a higher yield of H 2 than pure TiO 2 ; the durian-like TiO 2 @CdS nanostructures formed at 180 ◦ C for 16 h produced 2.5 times as much H 2 as did TiO 2 , also showing enhanced stability as compared with pure CdS. durian like TiO 2 photocatalytic 2 yield of H 2 of pure CdS durian like TiO 2 stable, change of the hybrid materials TiO 2 @CdS is not changed during photocatalytic reaction. tion like 2


Introduction
In recent years, photocatalysis via titanium dioxide (TiO 2 ) has been widely investigated as a promising method for environmental and energy applications [1][2][3], and functional properties have been extensively used in various fields including photocatalytic degradation of pollutants [4][5][6], photocatalytic CO 2 reduction into energy fuels, water splitting, supercapacitors and lithium-ion batteries [7][8][9][10]. However, due to its broad band gap of 3.2 eV, TiO 2 is only sensitive to the light of wavelengths below 380 nm belonging to the UV range, which covers only 5% of the entire solar spectrum [11]. This draw back dramatically limits its conversion efficiency in solar applications. Recent research has focused on improving the photocatalytic efficiency of TiO 2 through a variety of material engineering approaches, such as improving charge electroactive surface area [12], utilization of co-catalyst immobilization, controlled faceting, and enhancing the visible-light activity of TiO 2 through doping [13][14][15][16][17]. Irfan et al., successfully utilized CoSe as a cocatalyst and ZnSe as a visible-light active semiconductor to significantly enhance the photocatalytic hydrogen production on ZnSe/CdS under visible light irradiation for inexpensive conversion of solar energy to H 2 [18]. Pan et al., have successfully developed a hybrid photocatalytic system consisting of CdS NSs as PS and free-standing Fe 2 P NPs as cocatalysts for photocatalytic hydrogen production from water under visible light [19]. According to previous reports, the charge can be separated efficiently at the heterointerface, thus suitable design of the heterointerface between the other semiconductor catalysts and TiO 2 will be an effective strategy for enhancing the performance of photocatalysis [20,21]. 2 of 9 Cadmium sulfide (CdS) which is an important II-VI semiconductor (Eĝ = 2.42 eV (515 nm) at room temperature) with many excellent physical and chemical properties [22], has been used as a light-harvesting sensitizer to improve the photoelectric properties of TiO 2 because of its appropriate direct energy band gap. Recently, TiO 2 /CdS composite materials have attracted great interest. They could compensate the disadvantages of the individual components based on some synergistic effects, such as efficient charge separation and migration, expanded visible light response, and improved photostability [23][24][25]. Cao et al., prepared TiO 2 /CdS core-shell nanorod arrays with visible light activity by a two-step method. Thish increased the probability of electron-hole separation and extended the range of the TiO 2 photoresponse from ultraviolet to visible regions [26].
Many fundamental properties of semiconductor materials are dependent on the size and shape of their particles. Controlling the relevant size and shape would provide opportunities for tailoring properties of materials and offer possibilities for observing attractive and useful physical phenomena [27]. Therefore, materials engineering in nanometer-scale has attracted much attention in optics, electronics, magnetics, catalysis, and ceramics [28][29][30][31][32]. In this paper, TiO 2 @CdS core-shell particles, with CdS nanorods as shell layers and TiO 2 nanoparticles as the corresponding core units, were successfully fabricated, in order to overcome the disadvantages of both TiO 2 and CdS, thus gaining better optical and electric performances. To the best of our knowledge, this is the first report on TiO 2 @CdS core-shell particles with a durian-like surface structure.

SEM and TEM Characterizations
SEM and TEM techniques were employed to characterize the morphology of the TiO 2 @CdS particles obtained with different reaction times. As shown in Figure 1a, the commercially available TiO 2 nanoparticles with diameters of about 100 nm have a smooth surface. After adding a CdCl 2 solution, SC(NH 2 ) 2 , H 2 NCH 2 CH 2 NH 2 and conducting continuous heating for 8 h, we find the surface of TiO 2 is no longer smooth due to the formation of a TiO 2 @CdS composite ( Figure 1b). With an extended heating time (16 h), we observe the formation of durian-like TiO 2 @CdS particles ( Figure 1c) with various nanorods on the TiO 2 surface, with a diameter of 50 nm. We further used TEM to characterize the TiO 2 @CdS particles. As shown in Figure 1d, the clear contrast between the core and the shell indeed proves that the CdS nanorods coat the surface of the TiO 2 nanoparticles. Furthermore, as shown in Figure 2 and Table 1, the ratio of S:Cd is ca. 1:1. The results of EDX basically indicate that the surface rods are CdS. Based on the above observations, we can conclude that the reaction time played a crucial role in controlling the durian-like morphology of the TiO 2 @CdS. With the help of ethylenediamine, the CdS grows into a rod-like structure on the surface of the TiO 2 particles forming durian-like TiO 2 @CdS particles (Figure 1c), as further confirmed by TEM (Figure 1d).

XRD Characterization
The powder XRD patterns of the TiO 2 , TiO 2 @CdS and CdS samples prepared with different reaction times are shown in Figure 3. A typical diffraction peak located at 2θ = 25.3 • is attributed to the (101) crystal planes of the anatase-phase TiO 2 (JCPDS no. . The peaks at 26.5 • and 28.2 • observed in the XRD of TiO 2 @CdS and CdS particles are attributed to (002) and (101) crystal planes of hexagonal CdS. If the reaction time lasts longer, the (002) diffraction peak of the TiO 2 @CdS samples becomes both stronger and narrower, as shown in Figure 3 (TiO 2 @CdS, 16 h). The unusual (002) diffraction peak indicates that the sample (TiO 2 @CdS, 16 h) is preferentially orientated along the c axis. The rod shape is confirmed by SEM and TEM photographs. In addition, the typical diffraction peak located at 2θ = 25.3 • of anatase-phase TiO 2 is weak which can be explained by the thick coating of CdS on the surface of TiO 2 . This result is also verified by the TEM photograph.

XRD Characterization
The powder XRD patterns of the TiO2, TiO2@CdS and CdS samples prepared with different reaction times are shown in Figure 3. A typical diffraction peak located at 2θ = 25.3° is attributed to the (101) crystal planes of the anatase-phase TiO2 (JCPDS no. 21-1272). The peaks at 26.5° and 28.2°observed in the XRD of TiO2@CdS and CdS particles are attributed to (002) and (101) crystal planes of hexagonal CdS. If the reaction time lasts

XRD Characterization
The powder XRD patterns of the TiO2, TiO2@CdS and CdS samples prepared with different reaction times are shown in Figure 3. A typical diffraction peak located at 2θ = 25.3° is attributed to the (101) crystal planes of the anatase-phase TiO2 (JCPDS no. . The peaks at 26.5° and 28.2°observed in the XRD of TiO2@CdS and CdS particles are attributed to (002) and (101) crystal planes of hexagonal CdS. If the reaction time lasts

The Formation Mechanism of TiO 2 @CdS Core-Shell Particles
The possible formation mechanism of TiO 2 @CdS core-shell particles is depicted schematically in Figure 4. The first step is to link the Cd 2+ to the TiO 2 surface through inorganic grafting which is achieved by impregnating TiO 2 nanoparticles in a CdCl 2 solution. In the second step, the addition of SC(NH 2 ) 2 promotes generation of CdS on the surface of the TiO 2 nanoparticle, which is used as the seed. In the third step, CdS grows in the form of nanorods on the surface of TiO 2 with the help of ethylenediamine, which turns the composite particle into a durian-like structure.
longer, the (002) diffraction peak of the TiO2@CdS samples becomes both stronger and narrower, as shown in Figure 3 (TiO2@CdS, 16 h). The unusual (002) diffraction peak indicates that the sample (TiO2@CdS, 16 h) is preferentially orientated along the c axis. The rod shape is confirmed by SEM and TEM photographs. In addition, the typical diffraction peak located at 2θ = 25.3° of anatase-phase TiO2 is weak which can be explained by the thick coating of CdS on the surface of TiO2. This result is also verified by the TEM photograph.

The Formation Mechanism of TiO2@CdS Core-Shell Particles
The possible formation mechanism of TiO2@CdS core-shell particles is depicted schematically in Figure 4. The first step is to link the Cd 2+ to the TiO2 surface through inorganic grafting which is achieved by impregnating TiO2 nanoparticles in a CdCl2 solution. In the second step, the addition of SC(NH2)2 promotes generation of CdS on the surface of the TiO2 nanoparticle, which is used as the seed. In the third step, CdS grows in the form of nanorods on the surface of TiO2 with the help of ethylenediamine, which turns the composite particle into a durian-like structure.

Photocatalytic Reaction for H2 Generation
In order to verify the photocatalyst activity of the TiO2@CdS particles, the photocatalytic hydrogen evolution ability under light irradiation was studied, and the results are shown in Table 2. We find that the efficiency of H2 generation in the produced hybrid

Photocatalytic Reaction for H 2 Generation
In order to verify the photocatalyst activity of the TiO 2 @CdS particles, the photocatalytic hydrogen evolution ability under light irradiation was studied, and the results are shown in Table 2. We find that the efficiency of H 2 generation in the produced hybrid materials increases in the order TiO 2 < TiO 2 @CdS (8 h) < durian-like TiO 2 @CdS (16 h) with a maximum H 2 generation rate of 1065 µmol·h −1 ·g −1 for the durian-like TiO 2 @CdS core/shell particles (16 h). This is two times larger than the activity of bare TiO 2 particles. The yield of H 2 of durian-like TiO 2 @CdS (16 h) is higher than that of TiO 2 @CdS (8 h).
The results indicate that the durian-like TiO 2 @CdS particles have better photocatalytic activity than TiO 2 particles since the CdS nanorod can expand the visible light response and improve charge separation and migration. The yield of H 2 of pure CdS is higher than that of durian-like TiO 2 @CdS (16 h), but pure CdS is less stable, as suggested by a color change from yellow to black. The color of the hybrid materials TiO 2 @CdS is not changed during the photocatalytic reaction. Additionally, the reproducibility experiments have been performed. The H 2 generation rate of durian-like TiO 2 @CdS (16 h) decreased to 800 µmol·h −1 ·g −1 after four cycle reaction (see the Figure 5). In contrast, the H 2 generation rate of pure CdS dropped to 400 µmol·h −1 ·g −1 in the second cycle reaction. Meanwhile, the SEM experiments of both CdS and durian-like TiO 2 @CdS after photocatalytic reaction have also been performed ( Figure 6). The results indicated that the particle size of durian-like TiO 2 @CdS slightly changed as compared with the pure CdS. In addition, no obvious change was observed before and after catalysis in the XRD of the TiO 2 @CdS (Figure 6e). Thus, the reproducibility of H 2 generation rate of durian-like TiO 2 @CdS is basically good. Compared with the state-of-the-art catalysts, the activity and stability of TiO 2 @CdS is comparable.
Catalysts 2022, 12, x FOR PEER REVIEW 6 of 9 over the CdS can be reduced, and the photooxidation reaction may occur on the surface of TiO2 rather than the CdS. Therefore, the carriers can be separated and photocorrosion can be inhibited. In addition, the photo-current responses of CdS, TiO2@CdS and TiO2 were tested to gain a deeper understanding of the efficacy of photoexcited charge separation. As shown in Figure 6g, we can observe that the photocurrent intensity of TiO2@CdS (16) was higher than that of TiO2, indicating more effective charge separation in TiO2@CdS (16). Although the CdS exhibited the highest photocurrent, the H2 generation rate of pure CdS dropped seriously.   From the above results, we can observe that the activity of H 2 generation over TiO 2 @CdS was higher than that of the TiO 2 , and the stability of TiO 2 @CdS was better than that of CdS. Thus, we can assume that the charged species can be separated over the composites, and photocorrosion is severely inhibited. Hence, we can assume that a typical Z-scheme mechanism was occurring over the TiO 2 @CdS. Under simulated sunlight illumination, both TiO 2 and CdS can be excited to produce photoinduced electrons and holes. The photoinduced electrons in CdS tend to keep in the CB of CdS, and the photoinduced holes in TiO 2 remain in the VB of TiO 2 . Meanwhile, the photoinduced electrons in the CB of TiO 2 combine with the photoinduced holes in VB of CdS (Figure 6f). Thus, the holes over the CdS can be reduced, and the photooxidation reaction may occur on the surface of TiO 2 rather than the CdS. Therefore, the carriers can be separated and photocorrosion can be inhibited. In addition, the photo-current responses of CdS, TiO 2 @CdS and TiO 2 were tested to gain a deeper understanding of the efficacy of photoexcited charge separation. As shown in Figure 6g, we can observe that the photocurrent intensity of TiO 2 @CdS (16) was higher than that of TiO 2 , indicating more effective charge separation in TiO 2 @CdS (16). Although the CdS exhibited the highest photocurrent, the H 2 generation rate of pure CdS dropped seriously.

Preparation of TiO 2 @CdS Core-Shell Particles
These particles were synthesized by a solvothermal method [33,34]. The commercial anatase-phase TiO 2 nanoparticles (1 g, 0.013 mol) were added to dilute nitric acid (10 mL, 1 mol·L −1 ) with ultrasound for 4 h, and were collected and washed with distilled water to neutral. The final products were dried at 80 • C for 4 h in a vacuum box. The obtained TiO 2 (0.1 g) nanoparticles and 5 mL of an aqueous solution of CdCl 2 (1 mol·L −1 ) were added to a beaker with a capacity of 100 mL and were stirred magnetically for 4 h at room temperature. 5 mL of a thiourea solution (1.5 mol·L −1 ) and the mixture described above were added to a teflon-lined stainless steel autoclave with a 25 mL capacity. The autoclave was filled with 10 mL ethylenediamine solution up to about 80% of the total volume. The solution was stirred and treated with ultra-sound. The autoclave was maintained at 180 • C for 8-16 h and then allowed to cool to room temperature. Yellow precipitates were collected and washed with ethanol and distilled water to remove residues of organic material. The final products were dried at 80 • C for 4 h in a vacuum box. The pure CdS was prepared by the same method without TiO 2 .

Characterization of TiO 2 @CdS Core-Shell Particles
The morphologies of the TiO 2 nanoparticles and durian-like TiO 2 @CdS core-shell particles were characterized by scanning electron microscopy (SEM, SU8020) and transmission electron microscopy (TEM, JEM-2100). To facilitate the investigation of the detailed surface morphology of TiO 2 @CdS, no conductive metal layer such as Au, Pd/Au, Cr, or carbon layer was coated on the sample surface. The samples were further analyzed with a Bruker D8 Avance X-ray diffractometer (XRD) using Ni-filtered Cu Kα radiation at 40 kV and 40 mA in the 2θ range of 20-80 • , with a scan rate of 0.02 • per second.

Photocatalytic Experiment
Photocatalytic reactions of hydrogen production by water splitting were conducted in a gas-closed system with a side irradiation Pyrex cell. An aluminum alloy shell was employed outside the Pyrex cell to reflect and gather the visible light originating from Xe lamp. A water cycling system was used to maintain the reaction temperature.
Photocatalyst powder (10 mg) was dispersed in a solution (2 mL) containing 0.35 mol·L −1 Na 2 S and 0.25 mol·L −1 Na 2 SO 3 . After being evacuated and flushed by N 2 gas for over 10 min, the photocatalysts were irradiated by visible light (λ ≥ 300 nm) from a 300 W Xe lamp for 4 h. The amount of H 2 gas was determined using a gas chromatograph (Bruker GC-4890, using column type TDX01).

Conclusions
In summary, novel durian-like TiO 2 @CdS core-shell particles were synthesized through a solvothermal method in ethylenediamine solution. As demonstrated, CdS coated the surface of the TiO 2 material. The reaction time plays a crucial role in controlling the nucleation and growth of crystallites; with a prolonged reaction time, CdS gradually forms a rod shape, which effectively enhances visible light absorption and possesses a high electron affinity. The durian-shaped TiO 2 @CdS composite not only produced a higher yield of H 2 than pure TiO 2 , but also possessed an enhanced stability as compared with the pure CdS. Therefore, the durian-like TiO 2 @CdS core-shell particles might be a good catalyst for application in photocatalysis.