The role of crystalline TiO2 nanoparticle in enhancing the photocatalytic and photoelectrocatalytic properties of CdS nanorods

Journal of Alloys and Compounds 695 (2017) 1080e1087

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The role of crystalline TiO2 nanoparticle in enhancing the photocatalytic and photoelectrocatalytic properties of CdS nanorods Qianqian Shen a, b, Jinbo Xue a, b, c, *, Haocheng Zhao d, Mingzhe Shao a, b, Xuguang Liu a, b, e, Husheng Jia a, b, f, ** a Key Laboratory of Interface Science and Engineering in Advanced Materials (Taiyuan University of Technology), Ministry of Education, Taiyuan, 030024, PR China b Research Centre of Advanced Materials Science and Technology, Taiyuan University of Technology, Taiyuan, 030024, PR China c Frederick Seitz Materials Research Laboratory, University of Illinois at Urbana-Champaign, Urbana, IL, 61801, USA d Department of Electrical Engineering, Shanxi Institute of Energy, Taiyuan, 030600, PR China e College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan, 030024, PR China f College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan, 030024, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 July 2016 Received in revised form 15 October 2016 Accepted 25 October 2016 Available online 26 October 2016

A series of heterojunctions, based on CdS nanorods and TiO2 nanocrystals, were prepared by solvothermal process. Various techniques such as X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy and UVevis diffuse reflectance spectroscopy were used to characterize as-prepared composites. The results demonstrate that well crystalline TiO2 nanocrystals play an important role in photocatalytic and photoelectrochemical properties. The well crystalline TiO2 nanocrystals are beneficial to the formation of good chemical bonding between TiO2 nanocrystal and CdS nanorod, which gives positive effect on the optical property and photoelectrochemical performance of this heterostructure. Further, the good bonding at the interface of TiO2 nanocrystals and CdS nanorod is beneficial to the formation of larger width of space charge region, which gives rise to more band bending in CdS to improve the separation and transition efficiency of photo-induced electrons and holes in CdSeTiO2 heterojunctions. © 2016 Elsevier B.V. All rights reserved.

Keywords: Heterostructures TiO2 nanocrystals Energy band bending Photocatalyst Photoelectrocatalyst

1. Introduction The eternal pursuit for sustainable energy and pollutant-free environment keeps moving. In this context, photocatalysis is a promising and significant process for energy storage and conversion to chemical energy [1]. Cadmium sulfide (CdS), one of the most important semiconductor materials, has applications in a wide range of fields including photocatalysis [2e4], photovoltaics [5], and chemical sensors [6]. With a bandgap of around 2.4 eV, which matches well with the visible spectral range of solar irradiation, CdS exhibits excellent photocatalytic activity because of its highly effective absorption of solar energy. As a visible-light-driven

* Corresponding author. Key Laboratory of Interface Science and Engineering in Advanced Materials (Taiyuan University of Technology), Ministry of Education, Taiyuan, 030024, PR China. ** Corresponding author. College of Materials Science and Engineering, Taiyuan University of Technology, Taiyuan, 030024, PR China. E-mail address: [email protected] (J. Xue). http://dx.doi.org/10.1016/j.jallcom.2016.10.233 0925-8388/© 2016 Elsevier B.V. All rights reserved.

photocatalyst, it has been extensively investigated and its photocatalytic activity has been found to be influenced by a variety of factors including preparation conditions, particle size, morphology, and crystallinity [7]. For example, features such as good crystallinity, large surface area, and hexagonal crystal phase have been found to favor higher photocatalytic activity [8]. In contrast to nanoparticles or bulk materials, one dimensional (1D) structures have unique advantages as potential photocatalysts [9e12]. First, the 1D geometry leads to a fast and long-distance electron transport. Second, the light absorption and scattering can be obviously enhanced because of the high aspect ratio of the 1D structure. Third, 1D structures are expected to have larger specific surface area and pore volume, compared with their particle counterparts. On the other hand, there is an inherent drawback for CdS-based photocatalysts: the photocorrosion [13,14], because the sulfide ion is highly prone to oxidation by photogenerated holes. The photocorrosion effect makes CdS very unstable as a photocatalyst and greatly obstructs its practical application. Another main concern of

Q. Shen et al. / Journal of Alloys and Compounds 695 (2017) 1080e1087

CdS-based photocatalysts is the toxicity of leached Cd2þ ions. It is therefore absolutely vital to develop suitable surface engineering methods to inhibit photocorrosive damage to CdS nanoparticles [15]. If the surface of CdS nanostructure is modified by carbon [16,17] and oxides (TiO2, ZnO, SiO2) [18e20], the good properties of CdS and carbon or oxides will be integrated into the hybrids, which is advantageous to overcoming some intrinsic defects of CdS. As two of the mostly studied semiconductor photocatalysts in practical applications, CdS and TiO2 have attracted great attention, and to make better use of them, these two semiconductors are often coupled each other, because of their matched band structures and complementary optical and photocatalytic properties [21e26]. Further, TiO2 is a more inert material than CdS and is less affected in physiological conditions. Given that CdS easily suffers from the corrosion of acid and base, TiO2 can help it to increase the resistance. Liu and co-workers obtained CdS@TiO2 core-shell nanowires by a two-step solvothermal method [27]. Compared with bare CdS nanowires, CdS@TiO2 nanocomposites exhibited enhanced conversion and yield in selective oxidation of alcohols to aldehydes. Chen and co-workers [28] coated an ultrathin-layer TiO2 shell on CdS nanospheres by electrostatic assembly, giving a composite highly active for selective photoreduction of heavy metal ions. However, most researches are focused on coupling CdS with amorphous TiO2 to increase the photocatalytic properties and alleviate the photocorrosion of CdS. The studies on combining with crystalline TiO2 are infrequent though, compared with amorphous TiO2, crystalline TiO2 coated onto CdS might result in efficient photoinduced charge carrier transfer, which is distinctly different from amorphous TiO2 and carbon coating onto CdS [29]. Herein, we modify CdS nanorod surface with crystalline TiO2 nanoparticles by facile solvothermal method, during which the crystalline nanoparticles are well knit to CdS nanorod surface. The coupling of TiO2 and CdS enhances the physicochemical property of each. Dressing the CdS core with such cystalline TiO2 nanoparticles can efficiently improve the lifespan and transfer of photoinduced electrons from CdS core to surface TiO2, thereby leading to high photocatalytic efficiency for photoredox reactions over CdS. So, this heterojunction exhibits excellent photocatalytic and photoelectrocatatlytic performance. Especially, this heterojunction structure can improve the light corrosion resistance of CdS. 2. Experimental section 2.1. Materials Cadmium nitrate(Cd(NO3)2$4H2O), thiourea(CN2H4S), ethylenediamine(C2H8N2), titanium isopropylate(Ti{OCH(CH3)2}4), isopropanol(C3H8O) were purchased from Alfa Aesar. Deionized water was obtained from local sources. All reagents were of analytical grade and used without further purification. 2.2. Synthesis The 1D CdSeTiO2 heterojunctions were fabricated by a facile and template-free two-step solvothermal method, as illustrated in Fig. 1. 2.2.1. Synthesis of CdS nanorods In a typical synthesis, 0.1 M of Cd(NO3)2$4H2O dissolved in 20 mL of ethylenediamine was add to 0.76 g of thiourea to form a mixture. Afterward the clear solution was continuously stirred for about 20 min. The reaction mixture was then transferred into a 30 mL Teflon-lined stainless steel autoclave. The autoclave was heated to 180  C and kept at this temperature for 24 h. After synthesis, the reaction solution was cooled to room temperature

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naturally. Then the sample was rinsed with ethanol for three times and deionized water for twice. Finally, the sample was dried at 60  C for 12 h in vacuum for further use. 2.2.2. Preparation of CdSeTiO2 heterojunction To fabricate the CdSeTiO2 heterojunction photocatalyst, different amount of titanium isopropylate (TTIP) was mixed with 20 mL of isopropyl alcohol, then 0.2 g of the obtained CdS nanorod was added to the mixture and dissolved by stirring for 20 min. After that, 1.5 mL of deionized water was add to the mixture dropwise. Then the mixture was transferred into a 30 mL teflon-lined stainless steel autoclave and heated to 150  C and kept at 150  C for 24 h. After cooling, the sample was rinsed with ethanol for three times and deionized water for twice, Finally, the sample was dried at 60  C for 12 h in vacuum. The obtained samples were denoted as CdSeTiO2-X, in which“X” stands for the added amount of titanium isopropylate. 2.3. Characterization The crystalline phase of CdSeTiO2-X heterojunction smaples was identified by X-ray diffraction (XRD) on a Bruker D8 diffractometer. The morphologies of CdSeTiO2-X heterojunction samples were characterized on a JEOL JSM-6700F field emission scanning electron microscope (FESEM). High resolution transmission electron microscopy (HRTEM) observation was carried out with a JEOL JEM-2010 electron microscope. A UVevis spectrometer (Lambda750S Perkin-Elmer) was used to record the UVevis absorption spectra of the samples. X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo Scientific ESCA Lab250 spectrometer which consists of a monochromatic Al Ka as the X-ray source. 2.4. Photocatalytic and photoelectrochemical evaluation 2.4.1. Photocatalytic activity The photocatalytic activity of as-prepared CdSeTiO2-X heterojunction samples was assessed by degrading model contaminant dye methylene blue (MB) in aqueous solution under visible light irradiation. An incandescent lamp (100 W, irradiation distance: 20 cm) was used as light source. In typical photocatalytic test, 0.01 g of samples was added in 30 mL of 10 mg/L methylene blue solution. The suspension was maintained in the dark under stirring for 1 h to reach adsorption-desorption equilibrium before irradiation. During the reaction, sampling was done at a regular interval of 20 min for analysis. UVevis absorption spectra of irradiated samples were measured with a UVevis spectrophotometer. 2.4.2. PEC performance Photoelectrochemical analysis was conducted in a threeelectrode cell in which the CdSeTiO2-X samples, Pt foil, and SCE served as working, counter and reference electrodes, respectively. The electrolyte was 0.1 M Na2S aqueous solution, which was purged with N2 gas for 30 min. A 500 W xenon lamp (TrustTech, Beijing, China) with 420 nm filter was used as light source. The photocurrent was recorded by the CHI660D electrochemical workstation. 3. Results and discussion The crystallographic structure and phase composition of CdSeTiO2-X heterojunction samples were examined by XRD with the diffraction angle 2q ranging from 20 to 80 , as shown in Fig. 2. The CdS/TiO2-0 g sample shows well-developed crystalline phase, which is indexed to the hexagonal structure of CdS with lattice constants a ¼ b ¼ 4.132 Å and c ¼ 6.734 Å (JCPDS No. 41e1049). The

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Fig. 1. Schematic flowchart for two-step synthesis of CdSeTiO2-X heterojunctions.

Fig. 2. XRD pattern of pure CdS nanorods (a) and CdSeTiO2-X heterojunctions: (b) X ¼ 0.3 g; (c) X ¼ 0.6 g; (d) X ¼ 0.9 g; (e) X ¼ 1.2 g; (f) X ¼ 1.5 g.

characteristic peaks at 2q values of 24.81, 26.51,28.21, 36.61, 43.71, 47.91 and 51.81 can be attributed to the (100), (002), (101), (102), (110), (103) and (112) crystal planes of CdS, respectively, as shown in Fig. 2a, which are attributed to the high purity of asprepared CdS nanorods. In CdSeTiO2 heterojunction samples, the characteristic diffraction peaks for TiO2 are not obviously observed from the pattern owing to their relatively low diffraction intensity. However, XRD pattern of CdSeTiO2 heterojunction shows broader shoulder at some characteristic peaks, especially at 24.81, as shown in Fig. 2bef. This is because that the (100) peak of hexagonal CdS might overlap with the (101) peak of anatase TiO2. In addition, the broadened XRD peak reflects the dispersion of very small TiO2 nanocrystallites on CdS nanorods. This indicates a well crystalline nature of the TiO2 nanoparticles [30]. It is worth noting that a wellcrystalline material is beneficial to photovoltaic application owing to its high charge transport properties and low recombination losses.

Fig. 3aeg shows the SEM images of CdS nanorods, CdSeTiO2-X heterojunctions and TiO2 nanoparticles. CdS nanorods show the morphology of nanorods with a diameter of ~50 nm and a length of ~800 nm (Fig. 3a). As seen in Fig. 3g, as-prepared TiO2 exhibits uniform nanoparticles with a grain size of ~10 nm on a larger scale. From Fig. 3 bef, it can be seen that small granular particles are attached on the CdS nanorod surfaces, making the surfaces of nanorod much rougher with the amount of titanium isopropylate changing from 0.3 g to 1.5 g. Furthermore, the aggregation of TiO2 nanoparticles becomes more and more serious with increment of amount of titanium isopropylate. Typical TEM images of CdS nanorods and CdSeTiO2-X heterojunctions are shown in Fig. 4. As shown in Fig. 4a, CdS nanorod has a uniform morphology of smooth rod with a diameter of about 50 nm. As can be seen from Fig. 4bef, after the deposition of TiO2 onto the CdS nanorods, TiO2 nanocrystals are loaded onto the wall of CdS nanorods, and the morphology of TiO2 clusters varies greatly with the change of the amount of titanium isopropylate from 0.3 g to 1.5 g. For lower amount, i.e. 0.3 g, a small number of TiO2 nanocrystals are found around the CdS nanorods. As the amount of titanium isopropylate increases to 0.9 g, the TiO2 nanocrystals are uniformly dispersed along the CdS nanorods. When the amount of titanium isopropylate is up to 1.5 g, the TiO2 nanocrystals become aggregated to form nanoclusters. High-resolution TEM images further reveal the microscopic phase of CdSeTiO2-0.9 nanohybrids, as shown in Fig. 5aee. In Fig. 5b, two sets of lattice fringes are distinctly observed; the interplanar distance of 0.35 nm agrees well with the (101) planes of anatase TiO2. Meanwhile, the fast Fourier transform pattern (Fig. 5c) originating from the HRTEM image (Fig. 5b) shows two directions of diffraction spots, which correspond to (011) plane and (101) plane of anatase TiO2. From Fig. 5d and e, the interplanar distance of 0.34 nm can be indexed to the (002) plane of hexagonal CdS. The HRTEM images also suggest abundant defects in CdS nanorods. Furthermore, cross-sectional HRTEM images show intimate contact between CdS nanorod and TiO2 nanocrystal. The observed lattice distance of ~0.34 nm corresponds to the (002) plane of hexagonal CdS. As for the TiO2 nanocrystals, the measured lattice distance is ~0.35 nm, consistent with the (101) planes of

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Fig. 3. FESEM images of pure CdS nanorods(a), pure TiO2 nanoparticles(g) and CdSeTiO2-X heterojunctions: (b) X ¼ 0.3 g; (c) X ¼ 0.6 g; (d) X ¼ 0.9 g; (e) X ¼ 1.2 g; (f) X ¼ 1.5 g.

Fig. 4. TEM images of pure CdS nanorods(a) and CdSeTiO2-X heterojunctions: (b) X ¼ 0.3 g; (c) X ¼ 0.6 g; (d) X ¼ 0.9 g; (e) X ¼ 1.2 g; (f) X ¼ 1.5 g.

anatase TiO2. The epitaxial relationship in the CdSeTiO2 system originates from the fact that in-situ formed TiO2 nanocrystals are loaded on CdS nanorod. All these results clearly demonstrate that the CdSeTiO2 nanohybrids have good bonding between TiO2 nanocrystal and CdS nanorod, which would give positive effect on the optical property and photoelectrochemical performance of the heterostructure, as will be discussed in later section. XPS measurement was performed to investigate the change of the surface bonding of CdSeTiO2 heterojunction(CdSeTiO2-0.9 g). Cd 3d core levels are shown in Fig. 6a; two peaks located at 411.9 and 405.2 eV correspond to the Cd 3d3/2 and Cd 3d5/2 peaks, respectively, and are assigned to Cd2þ of CdS with a spin-orbit separation of 6.7 eV, consistent with the reported values [24]. The result indicates that the Cd 3d core levels of the TiO2eCdS heterojunction (CdSeTiO2-0.9 g) are similar with those of pure CdS [31]. The asymmetric band S 2p spectrum of CdSeTiO2-0.9 g in Fig. 6b includes three individual peaks with respective binding energies 161.8, 162.3 and 162.9 eV; The peaks at 161.8 and 162.9 eV can be

assigned to the S atoms bonded to Cd atoms, which correspond to S 2p3/2 and S 2p1/2 of CdS, respectively [32]. Meanwhile, the peak at about 162.3 eV can be assigned to the S atoms bonded to Ti atoms [33]. The Ti 2p3/2 and 2p1/2 XPS peaks in the CdSeTiO2-0.9 g heterojunction are centered at binding energies of 458.3 and 464.0 eV, as shown in Fig. 6c, which are typical for the Ti4þ-O bonds in TiO2 [34]. Moreover, the asymmetric band of O 1s of CdSeTiO20.9 g heterojunction can be fitted into three peaks in Fig. 6d. The peak at 529.6 eV is assigned to the crystal lattice oxygen of TieO, and the peak at 531.8 eV can be assigned to hydroxyl oxygen [35,36]. Meanwhile, the peak at 529.2 eV can be assigned to OeCd bonding [37]. These results demonstrate the chemical bonding between TiO2 nanocrystal and CdS nanorod, not just physical attachment. The results are identified with HRTEM and XRD results. So, in-situ formation of TiO2 nanocrystals on CdS nanorod is beneficial to the formation of chemical bonding between TiO2 nanocrystal and CdS nanorod. The UVeVis absorption spectra of CdS, TiO2 and CdSeTiO2-X are

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Fig. 5. HRTEM images of CdSeTiO2-0.9 g heterojunction (a), (b and c) TiO2 nanocrystal and corresponding FFT, (d and e) CdS nanorod indicated by green arrow in (a). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

Fig. 6. XPS survey spectrum for CdSeTiO2-0.9 g heterojunction: Cd 3d (a), S 2p (b), Ti 2p (c) and O 1s (d).

shown in Fig. 7. In Fig. 7a, CdS exhibits obvious visible light absorption, with the absorption edge of about 580 nm. However, the absorption edge of TiO2 is about 380 nm (UV region), corresponding to the band gap of 3.2 eV of anatase TiO2, as shown in Fig. 7g. From Fig. 7bef, it is clear that the CdSeTiO2-X exhibits light absorption capacity in the UV and visible light region, but the adsorption edge of CdSeTiO2-X is red-shifted with respect to that of TiO2 at 350 nm (① and ②inset in Fig. 7) and blue-shifted with respect to that of CdS at 500 nm (③ and ④inset in Fig. 7). This is understandable because

the coupling effect between CdS and TiO2 contributes to this phenomenon. On the other hand, it is also indicated that the CdSeTiO2X heterostructures have good bonding between TiO2 nanocrystal and CdS nanorod. In order to evaluate the photocatalytic performance of asprepared samples, the photoactivities of the samples towards aqueous phase degradation of methylene blue, a well-known organic-based dye pollutant in waste water produced from textile and other industrial processes, was studied. The results are shown

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Fig. 7. UVevis absorptions of pure CdS nanorods(a), pure TiO2 nanoparticles(g) and CdSeTiO2-X heterojunctions: (b) X ¼ 0.3 g; (c) X ¼ 0.6 g; (d) X ¼ 0.9 g; (e) X ¼ 1.2 g; (f) X ¼ 1.5 g.

in Fig. 8. Fig. 8 shows the comparison of photocatalytic degradation of MB over bare CdS and CdSeTiO2-X heterojunction under visible light irradiation. C0 and C1 are the concentrations of initial MB solution and MB solution remaining after each time interval of irradiation, respectively. The photo-degradation rate increases with the amount of titanium isopropylate ranging from 0 g to 0.9 g; however, a decrease is observed at the amount of titanium isopropylate ranging from 1.2 g to 1.5 g. The heterojunction fabricated with 0.9 g of titanium isopropoxide shows the best photocatalytic performance. The removal efficiency of MB is 90.4% by using CdSeTiO2-0.9 g heterojunction, much higher than that by using pure CdS nanorod (23.9%). The heterojunction can maintain a better photocatalytic efficiency with the extension of time.

Fig. 8. Photocatalytic activities of pure CdS nanorods(a), pure TiO2 nanoparticles(g) and CdSeTiO2-X heterojunctions on photodegradation of MB on irradiation with visible-light (L > 420 nm): (b) X ¼ 0.3 g; (c) X ¼ 0.6 g; (d) X ¼ 0.9 g; (e) X ¼ 1.2 g; (f) X ¼ 1.5 g.

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Compared with pure CdS, CdSeTiO2-X photocatalysts show improved photocatalytic activity for degrading MB under visible light, which may result from the modification effect of TiO2 on CdS. We propose that an effective interfacial charge-transfer process occurs at the heterojunction interface, primarily involving electron transfer from the conduction band (CB) of CdS to that of TiO2 [21]. The CB and valence band (VB) potentials of CdS are at 1 eV and 1.42 eV, respectively [38]; the CB edge potential of CdS (1 eV) is more negative than that of TiO2(0.5 eV) [38]; since electrons on the surface of CdS particles can transfer easily from the CB of CdS to that of TiO2 via the well-developed interface, the photo-generated holes and electrons can stay on the surface of CdS and TiO2, respectively; the recombination rate of electron and hole pairs is reduced. Thus, the separation of photo-induced electronhole pairs is improved and the lifetime of the carriers is prolonged, indicating that the recombination of photogenerated electron-hole pairs is hampered in the CdSeTiO2 -X, compared with that in CdS. A schematic illustration of the proposed reaction pathway for MB photo-degradation over the CdSeTiO2-X nanocomposites under visible light irradiation is shown in Fig. 9. Photo-excited electrons in CB of CdS cores transfer to the CB of TiO2 shells under visible light irradiation. The electrons reduce surface-absorbed O2 over TiO2 active sites to form superoxide radical ($O 2 ) [39]. On the other hand, the holes remaining in CdS react with surface-absorbed H2O to generate $OH owing to the presence of gaps between TiO2 nanocrystals, which has strong oxidation ability to decompose MB even to CO2 and H2O [40]. The degradation process can be expressed as follows [41]: CdS þ hn/CdS(e)þCdS(hþ)

(1)

CdS(e)þTiO2 / CdS þ TiO2 (e)

(2)

TiO2 (e)þO2 / TiO2þ$O 2

(3)

CdS(hþ)þOH / $OH þ CdS

(4)

Dyeþ$OH / degradation products

(5)

Dye þ hþ / oxidation products

(6)

Dye þ e / reduction products

(7)

Fig. 9. Schematic diagram of proposed reaction pathway for MB photodegradation over the CdSeTiO2-X heterojunction under visible light irradiation.

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Photoelectrochemical performance of bare CdS and CdSeTiO2-X heterojunction electrodes was studied. As shown in Fig. 10, all electrodes show good reproducibility and stability of transient anodic photocurrent as the UVevisible light illumination is switched-on and switched-off. It is interesting to note that CdSeTiO2-0.9 g composite electrode shows a photocurrent density of 0.5 mA cm2, which is about 4 times that of bare CdS electrode, with a photocurrent density of 0.12 mA cm2. This remarkably increased photocurrent may be the result of faster electron transfer and more efficient separation of charge carriers due to the integrating of CdS with nanocrystal TiO2. However, CdSeTiO2-0.3 g and CdSeTiO2-0.6 g composite electrodes exhibite lower photocurrent density than bare CdS. The reason may be that the conductivity of these composite electrodes is inferior than that of bare CdS, which would hamper the transition of photo-induced electrotron and hole. With the increase of TiO2 nanocrystals on the surface of CdS nanorods, the enhanced space charge region in the heterostructure will accelerate the transition of photo-induced electrons and holes, which is most contributive to the photocurrent. More TiO2 nanocrystals cause larger band bending according to Eq. (8) [42,43], and the scheme of the space charge region and band bending in CdS is shown in Fig. 11.

D¼

1  2εr ε0 VBB 2 eNd

(8)

where D is the width of space charge region; εr and ε0 are the relative dielectric constant of the semiconductor and the vacuum permittivity, respectively; e is the elementary charge; Nd is dopant concentration; VBB (band bending value) in the space charge region is schematically plotted in Fig. 11. From Eq. (8), D is proportional to the squared root of VBB. As is well known, the width of space charge region in heterostructures increases with the increase in the number and size of semiconductor and in the crystallization [44e47]. The CdSeTiO2-X system is also like this case. Thus, as the amount of TiO2 nanocrystals increases, the larger width of space charge region would give rise to more band bending in CdS, which would facilitate the separation efficiency of photo-induced electrons and holes. Further, the good bonding between TiO2 nanocrystals and CdS nanorods is

Fig. 11. Schematic diagrams of band scheme, space charge region and electric potential of the depletion layer (space charge layer) in CdS semiconductor.

beneficial to the transition efficiency of photo-induced electrons and holes. However, the excessive TiO2 nanocrystals would be unfavorable to the transition efficiency of photo-induced holes in electrolyte. So, the photocurrent density of CdSeTiO2-X increases at first to a maximum value and then decreases with increasing amount of TiO2 nanocrystals. On the other hand, from the FESEM image (Fig. 3), it is shown that the length of CdS nanorods keep their length at X  0.9, but become shorter at X > 0.9. In CdSeTiO2X heterostructure, the CdS nanorod plays the role of transportation tunnel for the photo-induced electrons. So, the shorter CdS nanorod decreases the transportation rate of the photo-induced electrons, which degenerates the photocatalytic and photoelectrochemical performance of the CdSeTiO2-X heterostructure samples. As a result, CdSeTiO2-0.9 sample shows the best photocatalytic and photoelectrochemical performance. 4. Conclusion In summary, CdSeTiO2-X hetejunctions were fabricated by twostep solvothermal method. Compared with bare CdS nanorods, asprepared CdSeTiO2-X hetejunctions exhibit enhanced photocatalytic activities for MB photodecomposition under visible light irradiation. The improved photoactivity is ascribed to the efficient separation of photogenerated electron and hole charge carriers between CdS and TiO2 nanocrystals. The remarkable photoresponse should be ascribed to the hetero-epitaxial relationship of highquality crystalline TiO2 nanocrystals and CdS nanorod which originates from the in-situ formation of TiO2 nanocrystals on CdS nanorod. The good chemical bonding between TiO2 nanocrystal and CdS nanorod gives positive effect on the optical property and photoelectrochemical performance of the heterostructure, and is beneficial to the formation of larger width of space charge region, which gives rise to more band bending in CdS to improve the separation and transition efficiency of photo-induced electrons and holes in CdSeTiO2 heterojunctions. Acknowledgement

Fig. 10. Transient photocurrent curves of pure CdS nanorods electrode(a) and CdSeTiO2-X heterojunctions electrodes: (b) X ¼ 0.3 g; (c) X ¼ 0.6 g; (d) X ¼ 0.9 g; (e) X ¼ 1.2 g; (f) X ¼ 1.5 g.

This study was funded by National Natural Science Foundation of China [51402209]; Natural Science Foundation of Shanxi Province [201603D121017, 201601D102020, 2015021075]; Scientific and Technologial Innovation Programs of Higher Education Institutions in Shanxi [2016124]; Program for Science and Technology Development of Shanxi [20140321012-01]; Shanxi Provincial Key Innovative Research Team in Science and Technology [201513002-10];

Q. Shen et al. / Journal of Alloys and Compounds 695 (2017) 1080e1087

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