Synthesis and Photocatalytic Activity of TiO2/CdS Nanocomposites with Co-Exposed Anatase Highly Reactive Facets

In this work, TiO2/CdS nanocomposites with co-exposed {101}/[111]-facets (NH4F-TiO2/CdS), {101}/{010} facets (FMA-TiO2/CdS), and {101}/{010}/[111]-facets (HF-TiO2/CdS and Urea-TiO2/CdS) were successfully synthesized through a one-pot solvothermal method by using [Ti4O9]2− colloidal solution containing CdS crystals as the precursor. The crystal structure, morphology, specific surface area, pore size distribution, separation, and recombination of photogenerated electrons/holes of the TiO2/CdS nanocomposites were characterized. The photocatalytic activity and cycling performance of the TiO2/CdS nanocomposites were also investigated. The results showed that as-prepared FMA-TiO2/CdS with co-exposed {101}/{010} facets exhibited the highest photocatalytic activity in the process of photocatalytic degradation of methyl orange (MO), and its degradation efficiency was 88.4%. The rate constants of FMA-TiO2/CdS was 0.0167 min−1, which was 55.7, 4.0, 3.7, 3.5, 3.3, and 1.9 times of No catalyst, CdS, HF-TiO2/CdS, NH4F-TiO2/CdS, CM-TiO2, Urea-TiO2/CdS, respectively. The highest photocatalytic activity of FMA-TiO2/CdS could be attributed to the synergistic effects of the largest surface energy, co-exposed {101}/{010} facets, the lowest photoluminescence intensity, lower charge-transfer resistance, and a higher charge-transfer efficiency.


Introduction
Since Fujishima and Honda discovered that titanium dioxide (TiO 2 ) could photodecompose water to produce hydrogen in the 1970s [1], TiO 2 , as a traditional semiconductor material, has been widely utilized in the fields of photocatalytic degradation of pollutants, dye-sensitized solar cells, lithium-ion batteries, gas sensors, etc., due to its relatively good chemical stability, low cost, non-toxicity, and environmentally friendly nature [2][3][4]. However, TiO 2 is subject to many limitations in industrial application due to its relatively large bandgap (anatase:~3.2 eV, rutile:~3.0 eV) and the relatively rapid recombination rate of photogenerated electrons and holes [5,6]. Recent studies have shown that changing the crystal phase, grain size, morphology, specific surface area, heterogeneous structure, and exposed facets of TiO 2 is an effective way to increase the photocatalytic activity. Especially, the configuration of heterogeneous structure with exposed highly reactive facets plays an important role in improving the charge separation efficiency and photocatalytic activity of TiO 2 [7]. Therefore, it is necessary to design and synthesize TiO 2 -based semiconductor composite photocatalyst with exposed highly reactive facets to broaden its optical light absorption range and accelerate the separation of photogenerated electrons and holes, thus improving the photocatalytic efficiency [8,9]. To extend the light absorption range of TiO 2 to the visible region, numerous efforts have been made to combine TiO 2 with other semiconductors, such as Fe 2 O 3 [10], CdS [11], Cu 2 O [12], Ag 3 PO 4 [13], WO 3 [14], ZnS [15], and Bi 2 S 3 [16]. Among various inorganic semiconductors, cadmium sulfide (CdS) is considered to be one of the best cocatalyst candidates because of its appropriate bandgap (2.4 eV), low price and appropriate band edge position for photocatalytic redox reaction [4,17]. Therefore, TiO 2 /CdS composites have been widely applied in the fields of photocatalysis and photovoltaics due to their simple composition, easily controlled microstructure and high extinction coefficient [18,19]. For example, Gao et al. reported the synthesis, characterization, and photocatalytic activity of the CdS-loaded TiO 2 microspheres with exposed {001} facets [20]. Wang et al. demonstrated that coating CdS nanoparticles on anatase TiO 2 films with exposed {001} facets can greatly improve the photoelectrochemical water splitting ability [21]. Dai et al. prepared surface-fluorinated anatase TiO 2 nanosheets with exposed {001} facets/CdS-diethylenetriamine nanobelts composites, which improved the ability of water to split into H 2 under visible light [22]. Ma [24,25]. Using appropriate morphology-controlling agents can selectively adsorb on the surface of TiO 2 crystal with more active sites, which can not only reduce its surface free energy, but also inhibit the crystal growth along the corresponding direction, so as to expose specific crystal facets [26]. Among the various types of capping agents, fluorine morphology-controlling agents, such as hydrofluoric acid (HF), ammonium fluoride (NH 4 F), and non-fluorine morphology-controlling agents, such as urea, and formic acid (FA) are often employed. Generally speaking, the F − ionized by HF and NH 4 F are easy to adsorb on the {001}-faceted surfaces of anatase TiO 2 to reduce its surface energy and facilitate its growth [27,28]. However, it is undeniable that HF and NH 4 F will generate toxic and corrosive substances at high temperature [29], and it is difficult to remove the passivation of F − on the crystal surfaces of anatase TiO 2 [30]. Owing to these limitations, a novel methodology for synthesizing anatase TiO 2 crystals with exposed specific surfaces using morphology-controlling agents without fluorine was developed. Carbon dioxide decomposed by urea at high temperature is converted into carbonate in alkaline solution, which can adsorb on the {001} crystal planes of anatase TiO 2 and reduce its surface energy, thus leading to the formation of {001} crystal planes [29]. As an organic weak acid, formic acid is preferentially attached onto the specific {101} surfaces to reduce its surface energy and facilitate the oriented growth of crystal along [1] direction during the growth of anatase TiO 2 crystal, resulting in the reduction of {001} crystal plane [31].
In this study, TiO 2 /CdS nanocomposites with co-exposed anatase colloidal solution containing CdS crystals. The photocatalytic degradation of MO performance of the asobtained TiO 2 /CdS nanocomposites was investigated under ultraviolet irradiation. To our knowledge, this is the first report that TiO 2 /CdS nanocomposites with co-exposed anatase highly reactive facets exhibit excellent photocatalytic activity for MO degradation.

Synthesis of CdS Nanoparticles
The synthesis method of CdS nanoparticles was as follows: 7.1190 g of Cd(NO 3 ) 2 ·4H 2 O and 1.9036 g of thiourea were added to a hydrothermal vessel. Then 30 mL of deionized water and 30 mL of absolute ethyl alcohol were added into the above vessel and magnetically stirred for 30 min. Then, 1.0080 g of ammonium fluoride (NH 4 F) was added to the above-mixed solution and continue stirring for 30 min. After that, the above autoclave with a white solution was sealed and put into a constant temperature blast drying oven (GZX-GF101-1-BS, Hefei Kejing Material Technology Co. Ltd., Hefei, Anhui, China) and maintained at 180 • C for 24 h. Finally, the orange-yellow precipitates were collected by centrifugation and washed several times with deionized water.

Preparation of [Ti 4 O 9 ] 2− Colloidal Solution from H 2 Ti 4 O 9
Layered potassium tetratitanate fiber K 2 Ti 4 O 9 was synthesized via conventional solidstate calcination using potassium carbonate (K 2 CO 3 ) and titanium dioxide (TiO 2 ) as raw materials [32]. Briefly, 14.5115 g K 2 CO 3 (0.105 mol) and 31.9681 g TiO 2 (0.400 mol) were ground evenly in an agate mortar and then heated in a high-temperature box resistance furnace at 900 • C for 24 h using an alumina crucible. The resultant K 2 Ti 4 O 9 (30.00 g) was immersed in 1.0 mol/L HNO 3  The 0.05 g CdS nanoparticles prepared above were transferred to four hydrothermal vessels, which containing 5.0 mL of hydrofluoric acid (HF), 2.0009 g NH 4 F, 2.0074 g urea, and 5.0 mL of formic acid (FMA), respectively. Then, 70 mL of [Ti 4 O 9 ] 2− colloidal solution was added to the four hydrothermal vessels and stirred magnetically for 30 min. Subsequently, the four hydrothermal vessels were sealed and maintained at 180 • C for 24 h to yield a yellow solid consisting of TiO 2 and CdS. Finally, the yellow TiO 2 /CdS composites were centrifugated and repeatedly washed with deionized water, and the corresponding TiO 2 /CdS composites were named as HF-TiO 2 /CdS, NH 4 F-TiO 2 /CdS, Urea-TiO 2 /CdS, and FMA-TiO 2 /CdS, respectively. The transformation of the [Ti 4 O 9 ] 2− colloidal solution containing CdS and various morphology-controlling agents to TiO 2 /CdS nanocomposites in the acidic (HF and formic acid) and alkaline conditions (NH 4 F, urea), can be described as follows [33,34]: The schematic of the possible growth mechanism of TiO 2 /CdS nanocomposites is depicted in Figure 1.

Measurement of Photocatalytic Activity
The photocatalytic performances of the as-synthesized TiO 2 /CdS nanocomposites were determined by detecting degradation of methyl orange (MO) under a 175 W lowpressure mercury lamp irradiation (Shanghai Mingyao Glass Hardware Tool Factory, Shanghai, China). Typically, 0.15 g of TiO 2 /CdS nanocomposite was suspended in 150 mL MO aqueous solution (10 ppm), and a 2 h of dark-reaction was carried out under stirring conditions to achieve the adsorption-desorption. Following, 5 mL of suspension together with the catalyst was drawn out every 15 min and centrifuged at 2500 rpm for 10 min to wipe off the TiO 2 /CdS nanocomposites. The concentrations of the MO solution were determined spectrophotometrically using a TU-1901 ultraviolet-visible spectrophotometer at a maximum absorption wavelength of 465 nm. For comparison, the photocatalytic activities of the commercial TiO 2 (CM-TiO 2 , 96.8% anatase and 3.2% rutile) and as-synthesized CdS samples were also investigated. The stability and recyclability of the TiO 2 /CdS nanocomposites were investigated by degrading 10 ppm MO solution (150 mL). The photocatalytic degradation efficiency (η) can be calculated according to the formula: η=(1-c t /c 0 ) × 100%, where c 0 and c t are the initial concentration of MO solution and the final concentration after irradiation for a certain time, respectively.  215), respectively, indicating that TiO 2 /CdS nanocomposites were successfully synthesized. The crystallite size of CdS, HF-TiO 2 /CdS, NH 4 F-TiO 2 /CdS, FMA-TiO 2 /CdS, and Urea-TiO 2 /CdS was estimated using Scherrer's formula [35], and the average grain size was 37.6 nm, 48.1 nm, 79.2 nm, 74.5 nm, 77.8 nm. The sharp (004) diffraction peak in the TiO 2 /CdS nanocomposites represents the preferential growth of anatase {001} facets [36]. Compare with HF-TiO 2 /CdS nanocomposites, the (100) diffraction peak of CdS in the NH 4 F-TiO 2 /CdS, FMA-TiO 2 /CdS, and Urea-TiO 2 /CdS nanocomposites is not obvious, because it overlaps greatly with the (101) diffraction peak of anatase TiO 2 in the TiO 2 /CdS nanocomposites [21].

FESEM and FESEM-EDS Analysis
The morphology of the K 2 Ti 4 O 9 , H 2 Ti 4 O 9 , CdS, and TiO 2 /CdS nanocomposites synthesized in the presence of different morphology control agents were characterized by FESEM. Figure 3a shows the FESEM image of K 2 Ti 4 O 9 , which composed of square-rods with the length of 0.60-2.06 µm, width of 0.13-0.26 µm, and thickness of 0.05-0.10 µm. After ion exchange, the resulting H 2 Ti 4 O 9 remains the square-rod morphology of the precursor K 2 Ti 4 O 9 , and extends along the [10]-direction (Figure 3b) [37]. Figure 3c and d are the FESEM images of CdS, which are composed of dendritic-like dimers formed by agglomeration of nanobipyramids, nanospindles, nanocuboids, and irregular nanoparticles.  Energy-dispersive X-ray spectroscopy (EDX) analysis of prepared K 2 Ti 4 O 9 , H 2 Ti 4 O 9 and CdS samples is shown in Table 1. It can be seen that the prepared K 2 Ti 4 O 9 sample contains not only potassium, titanium, and oxygen elements, but also a small amount of carbon from the reaction raw material K 2 CO 3 . The obtained H 2 Ti 4 O 9 sample after ion-exchange with HNO 3 solution contains not only titanium and oxygen elements, but also a small amount of potassium and carbon from the reaction material K 2 Ti 4 O 9 and HNO 3 , respectively, indicating that K + ions have not completely replaced by H + ions after three times of ion exchanges. The obtained CdS sample contains sulfur, cadmium, carbon, nitrogen, and oxygen elements, among which a small amount of carbon, nitrogen, and oxygen came from thiourea. The results of EDX analysis of TiO 2 /CdS composites prepared by hydrothermal treatment of tetra titanate colloidal suspension under the action of different morphology control agents are listed in Table 2. It can be seen that the obtained TiO 2 /CdS composites contain not only titanium, oxygen, cadmium, and sulfur elements, but also a small amount of carbon (came from CdS and N(CH 3 ) 4 OH), nitrogen (came from N(CH 3 ) 4 OH), and fluorine (came from HF and NH 4 F) elements. And the content of CdS particles in the prepared TiO 2 /CdS composites is different, the mass fraction (or atom fraction) of CdS in the TiO 2 /CdS composites increases in an order of HF-TiO 2 /CdS (71.583%) > FMA-TiO 2 /CdS (23.042%) > Urea-TiO 2 /CdS (22.512%) > NH 4 F-TiO 2 /CdS (21.690%). The content of CdS particles in the prepared HF-TiO 2 /CdS composite is much higher than those of TiO 2 /CdS composites, which may be due to the strong corrosivity and solubility of HF, resulting in the reduction of TiO 2 content in the HF-TiO 2 /CdS composite [26].

TEM and HRTEM Analysis
The morphological and structural characteristics of the as-obtained CdS and TiO 2 /CdS samples were further investigated by TEM and HRTEM images, as shown in Figures 4 and 5. A low-resolution TEM image of the as-obtained CdS sample is displayed in Figure 4a (101) and (011) planes, respectively, of the tetragonal anatase TiO 2 , as shown in Figure 5f. Additionally, the (101) and (011) planes parallel to the lateral edges of the nanocuboids with an interfacial angle of 82 • , therefore, we can confirm that the crystal planes exposed on the top (or bottom) and lateral planes are [111]-facets and {101} facets, respectively.

Photoluminescence Analysis
Photoluminescence (PL) spectroscopy is an efficient technique to characterize the separation and recombination of photogenerated electron-hole pairs on the photocatalyst surface [40]. As can be seen from Figure 7, CdS, CM-TiO 2 , and TiO 2 /CdS all exhibit obvious emission peak near 560 nm, which may be due to the rapid recombination of photogenerated electron-hole pairs [23]. In addition, compared with CdS sample, some other emission peaks were observed at 394, 437, 466, 480, and 616 nm in the CM-TiO 2 and TiO 2 /CdS nanocomposites. The first emission peak at 394 nm can be attributed the emission of the band-band PL process of anatase TiO 2 , the other emission peaks can be attributed the excitonic PL process at the band edge of anatase TiO 2 [38,41]. As the PL spectra are produced by the recombination of charge carriers, the lower emission intensity meanings lower recombination and higher separation efficiency of photogenerated electrons and holes [42,43]. In other words, the lower emission intensities indicate the existence of oxygen defects in the prepared TiO 2 /CdS nanocomposites [44,45] The PL emission peak intensities of FMA-TiO 2 /CdS and Urea-TiO 2 /CdS are much weaker than pure CdS, CM-TiO 2 , NH 4 F-TiO 2 /CdS, and HF-TiO 2 /CdS samples, indicating the better separation of photogenerated electrons/holes. Remarkably, FMA-TiO 2 /CdS nanocomposite exhibits the lowest PL intensity in all samples, indicating that the synergistic effects of the maximum specific surface and the TiO 2 /CdS heterostructure are beneficial to impede the recombination of photogenerated carriers and enhance the photocatalytic performance [46].

Electrochemical Impedance Spectroscopy Analysis
The interfacial charge-carrier separation and transfer efficiency of the HF-TiO 2 /CdS, NH 4 F-TiO 2 /CdS, FMA-TiO 2 /CdS, and Urea-TiO 2 /CdS nanocomposites was investigated by Electrochemical impedance spectroscopy (EIS) measurements at a frequency range of 1.0 MHz to 0.1 Hz with a signal amplitude of 0.01 V. Nyquist plots of TiO 2 /CdS nanocomposites obtained under different morphology control agents were shown in Figure 8. It can be seen from the Nyquist plots that the order of the impedance arc radius of the nanocomposites is FMA-TiO 2 /CdS < Urea-TiO 2 /CdS < NH 4 F-TiO 2 /CdS < HF-TiO 2 /CdS, indicating that FMA-TiO 2 /CdS nanocomposite has a lower charge-transfer resistance and a higher charge-transfer efficiency, thus improving the photocatalytic efficiency [4,47].

Photocatalytic Activity
The ultraviolet-light-driven photocatalytic activities of TiO 2 /CdS composites prepared by adding different morphology capping agents in the mixed solution of H 2 Ti 4 O 9 and CdS were measured by the degradation of the MO solution. As shown in Figure 9a  It is widely accepted that the crystal structure, specific surface area, separation efficiency of photogenerated electron-hole pairs and exposed facets play an important role in the photocatalytic reaction [48][49][50]: the suitable heterostructure, the larger specific surface area, the higher separation efficiency of photogenerated electron-hole pairs, and the higher reactive exposed facet are conductive to the improvement of the photocatalytic activity. A possible photocatalytic mechanism of TiO 2 /CdS heterostructure for the photodegradation of MO under ultraviolet light irradiation is shown in Figure 10. Under the ultraviolet light irradiation, the photogenerated electrons (e − ) on the conduction band (CB) of CdS can easily transfer to the CB of TiO 2 through the heterostructure interface owing to the CB potential of CdS (−0.89 eV) is more negative than that of TiO 2 (−0.30 eV), in contrast, the photogenerated holes (h + ) on the valence band (VB) of TiO 2 can migrate to the VB of CdS through the heterostructure interface owing to the VB potential of TiO 2 (+2.90 eV) more positive than that of CdS (+1.45 eV) [5,40]. The separation of the photogenerated e − (in TiO 2 ) and photogenerated h + (in CdS) decreases the recombination of photogenerated e − /h + pairs, thus enhancing the photocatalytic activity of TiO 2 /CdS nanocomposites. Consequently, the photogenerated h + on the VB of CdS can directly oxidize the water molecules absorbed on its surface to generate ·OH radicals, meanwhile, the photogenerated e − on the CB of TiO 2 can directly reduce the oxygen molecules absorbed on its surface to form ·O 2 − radicals. The generated ·OH and ·O 2 − radicals have strong oxidability, which can oxidize MO into water, carbon dioxide, and mineral acid. The possible photocatalytic degradation mechanisms are described as follows [40]:  The photocatalytic performance of the FMA-TiO 2 /CdS nanocomposite is significantly improved due to the suitable heterostructure between TiO 2 and CdS, the larger specific area, and the higher separation efficiency of photogenerated electron-hole pairs. By comparing the TiO 2 /CdS nanocomposites, we can find that the MO degradation efficiencies for FMA-TiO 2 /CdS reached 88.4%, which was much higher than other TiO 2 /CdS nanocomposites under the same test environment conditions. The difference in photocatalytic activity among the four TiO 2 /CdS nanocomposites can be ascribed to the different specific surface areas and separation efficiency of photogenerated electron-hole pairs. The specific surface area of FMA-TiO 2 /CdS nanocomposite is much higher than those of other TiO 2 /CdS nanocomposites, which is conducive to adsorbing more MO molecules on the surface of TiO 2 /CdS nanocomposite, thereby improving the photocatalytic performance [51]. The PL intensity and the impedance arc radius of FMA-TiO 2 /CdS nanocomposite are much lower than those of other TiO 2 /CdS nanocomposites, which are beneficial to decrease the recombination of the photogenerated charge carriers and improve the charge transfer efficiency. Additionally, a large proportion of exposed {010} facets in FMA-TiO 2 /CdS nanocomposite have superior surface atom structure and surface electronic structure, which also contributes to the improvement of photocatalytic performance [52].

Conclusions
In summary, TiO 2 /CdS nanocomposites with co-exposed highly reactive anatase The crystal structure, morphology, specific surface area, pore size distribution, separation, and recombination of photogenerated electrons/holes of the TiO 2 /CdS nanocomposites were characterized by XRD, FESEM, specific surface area, and porosity analyzer, PL and EIS. Photocatalytic degradation of MO performance of the as-obtained TiO 2 /CdS nanocomposites was investigated under ultravi-olet irradiation. It is worth mentioning that the FMA-TiO 2 /CdS nanocomposite exhibits better photocatalytic activity due to the synergistic effects of its suitable heterojunction structure, the largest specific surface area and the exposed highly reactive anatase {010} facets in comparison with the pure CdS, CM-TiO 2 and other TiO 2 /CdS nanocomposites.