- Email: [email protected]
TiO2 arrays on FTO with enhanced photocatalytic activity and reusability
Accepted Manuscript Title: A facile preparation of immobilized BiOCl nanosheets/TiO2 arrays on FTO with enhanced photocatalytic activity and reusability Author: Yinghua Shen Xiang Yu Weitian Lin Yi Zhu Yuanming Zhang PII: DOI: Reference:
S0169-4332(16)32793-3 http://dx.doi.org/doi:10.1016/j.apsusc.2016.12.076 APSUSC 34623
To appear in:
APSUSC
Received date: Revised date: Accepted date:
12-10-2016 7-12-2016 10-12-2016
Please cite this article as: Yinghua Shen, Xiang Yu, Weitian Lin, Yi Zhu, Yuanming Zhang, A facile preparation of immobilized BiOCl nanosheets/TiO2 arrays on FTO with enhanced photocatalytic activity and reusability, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.12.076 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A facile preparation of immobilized BiOCl nanosheets /TiO2 arrays on FTO with enhanced photocatalytic activity and reusability Yinghua Shena, Xiang Yu a,b, Weitian Lina, Yi Zhua,
Yuanming Zhang a*
a
Department of Chemistry, Jinan University, Guangzhou, 510632, P.R. China
b
Analytical & Testing Center, Jinan University, Guangzhou, 510632, P.R. China
These authors contributed equally to this work.
*
Corresponding Authors: Tel: +86 2085222756
Email: [email protected](Y.Zhang)
Graphical abstract
Highlights:
Immobilized BiOCl nanosheets/TiO2 arrays hybrid photocatalyst were fabricated.
The degradation efficiency of BCTO-3 can still reach 91.7% after eight cycles.
The immobilized BCTO-3 can be recycled for removal of organic pollutants in water.
Abstract：Forming a hybrid structure is considered as an efficient strategy toward improving the photocatalytic activity of TiO2-based photocatalyst. In this work, we report a facile impregnation method to prepare BiOCl nanosheets on rutile TiO2 nanorod arrays on transparent conductive fluorine-doped tin oxide (FTO) substrate. According to RhB photocatalytic degradation experiments, the degradation efficiency of the immobilized BiOCl/TiO2 (denoted as BCTO-3) hybrid photocatalyst can reach 99.1% after visible light irradiation for 3 h, and its efficiency is higher than that of pure BiOCl (42.7%) and TiO2 (44.8%), respectively. The enhancement is demonstrated to be the match of energy level between BiOCl and TiO2. Hence, the separation and transfer of photogenerated electron-hole pairs are obviously improved, which have been illustrated by the result of the photoluminescence spectra analysis and photoelectrochemical performance. Moreover, the degradation efficiency of BCTO-3 can still reach 91.7% after eight times photodegradation cycle experiments. Due to the easy recycling and excellent durability, the immobilized BCTO-3 photocatalyst is considered as a promising photocatalytic material for the removal of organic pollutants in aqueous eco-environments.
Keywords: immobilized photocatalyst; impregnation method; BiOCl nanosheets; TiO2 nanorod arrays.
1. Introduction Recently, there has been significant attention on the developments of semiconductor photocatalysts owing to their potential applications in renewable energy from water splitting and environmental cleaning by the decomposition of organic pollutants [1-3]. In traditional photocatalytic processes, the semiconductor photocatalysts utilized in the treatment of organic pollutants in water are mostly in the form of unsupported suspensions, which may easily diffuse into the solution as secondary pollutants and increase the cost of water purification process [4-6]. Compared with universal powder photocatalysts, the immobilized photocatalyst appears to be more promising and valuable for practical application owing to its easy reusability and non-secondary effect. And this consequently makes the immobilized semiconductor photocatalysts become one of the hot research subjects of materials science. Since the original report by Fujishima and Honda in 1972 [7, 8], TiO2 has drawn stupendous attention and become one of the most popular semiconductor photocatalysts as a result of its good chemical stability, oxidizability, low cost and non-toxicity [9, 10]. According to the pioneer researches, oriented single-crystalline TiO2 can uniformly grow on the FTO substrate to form the film by the hydrothermal method [11, 12]. Compared with the bulk TiO2, the single-crystalline 1D TiO2 nanorod arrays can significantly prolong the life time of photogenerated charge carriers and hold vast potential for the photocatalytic application in virtue of its high aspect ratio and effective electrical pathway for electrons [13-15]. However, TiO2 nanorod arrays suffer from the rapid recombination of photogenerated electron-hole pairs, which results in the suppressed photocatalytic activity and limited practical application of TiO2 arrays. To address this problem, considerable energies have been devoted by researchers, including coupling with other semiconductors, carbon materials and noble metals [16]. In this work, the strategy is mainly centred on modifying the TiO2 nanorod arrays with another semiconductor, whose redox energy levels for its corresponding conduction and valence band differ from that of TiO2 [17]. Among numerous semiconductors, bismuth oxychloride (BiOCl) has emerged as one of most promising candidates in virtue of its high photocorrosion stability and good biocompatibility. As an important V-VI-VII ternary compound, BiOCl has the open layered structure consisting of the [Bi2O2] slabs intercalated by double slabs of Cl atoms [18-20], which can provide a large enough
space to polarize the related atoms and orbitals, and engender an internal static electric field to help separate the photogenerated electron-hole pairs [21, 22]. This is particularly beneficial in semiconductor hybrid composites referring to the fabrication of hierarchical or grafted structures. On one hand, since the maximum potential of valence band (VB) and the minimum potential of conduction band (CB) of BiOCl are both more positive than the corresponding bands of TiO2 [23], electrons can migrate from the CB of TiO2 to the CB of BiOCl, meanwhile holes could reversely transfer from the VB of BiOCl to the VB of TiO2, which could significantly facilitate the separation of photoinduced electron-hole pairs in the structure consist of TiO2 and BiOCl. On the other hand, in virtue of the different property of n-type TiO2 and p-type BiOCl, there will form the depletion region at the interface between layers of the two components, which results in the efficient separation of the charge carries [24-27]. To our knowledge, there were a few previous reports about the immobilized BiOCl/TiO2 photocatalyst for photodegradation of organic pollutants. Cai et al. [28] grafted BiOCl nanosheets onto the TiO2 nanotubes by the hydrothermal method. Accordingly, a deep exploitation of immobilized BiOCl/TiO2 hybrid photocatalyst with improved photocatalytic activity and reusability by a facile preparation still holds considerable significance for the applied prospect in the treatment of water contamination. In our work, a facile impregnation method has been developed to graft the sheet-like BiOCl onto the upright position of as-prepared rutile TiO2 nanorod arrays by utilizing the BiCl3 as Bi3+ source at room temperature. The BiOCl/TiO2 (denoted as BCTO) hybrid photocatalyst exhibits enhanced photocatalytic activity and good reusability by the degradation of RhB with the visible light illumination compared with BiOCl nanosheets and TiO2 nanorods. Meanwhile, the degradation efficiency of BCTO-3 can still reach 91.7% after eight times photodegradation cycle experiments. In addition, a series of characterization were employed to provide a reasonable explanation about the mechanism for the improved photocatalytic activity of the as-fabricated immobilized BCTO composites.
2. Experimental details 2.1 Preparation of rutile TiO2 nanorod arrays on FTO The rutile TiO2 nanorod arrays were experimentally prepared on an FTO substrate (3 cm*7 cm) by the hydrothermal process. Typically, the mixture containing 40 mL concentrated hydrochloric
acid (36.5%~38% by weight) and the same volume of deionized water, was stirred under room temperature for 5 min to form the homogeneous solution before the addition of 1.33 mL titanium butoxide (97%). Keep stirring the solution, until the whole mixture became clear and transparent. Two pieces of FTO substrates, ultrasonically cleaned for 60 min in ethanol, were placed at an angle against the wall of the Teflon-liner with the conducting side facing down. Finally, the sealed autoclave with the mixture and FTO substrate was placed in the oven at 170 °C for 4 h. After synthesis, the ethanol and deionized water were alternatively utilized to rinse the samples for several times before dried in air. 2.2 Synthesis of BiOCl nanosheet on rutile TiO2 nanorod arrays 0.45 g BiCl3 was added to 10 mL absolute ethanol containing 0.05 mL concentrated hydrochloric acid (36.5%~38% by weight). Keep stirring the mixture for 0.5 h to make it homogenous. Then, 0.4 mL of the mixing solution was sucked by flnnpipette and sprayed on the as-prepared TiO2 arrays. When the ethanol was nearly volatilized out under the atmosphere condition, the as-prepared sample was baked in the oven at 80 °C for 0.5 h. Finally, the substrate was soaked in deionized water for 0.5 h to fabricate the BiOCl nanosheets on rutile TiO2 nanorod arrays, and then dried at 80 °C for 0.5 h. In order to investigate the effects of the amount of BiOCl nansheets grafting on TiO2 nanorod arrays toward the photocatalytic activity, the mass of BiCl3 was experimentally adjusted to prepare different immobilized BiOCl/TiO2 (BCTO) composites by repeating the foregoing procedure. Furthermore, 0.4 mL BiCl3 solution with the same mass as the one for BCTO composite of the best photocatalytic performance was used to prepare BiOCl nanosheets on bare FTO substrate for comparison. The BCTO composites using 0.15 g, 0.30 g, o.45 g and 0.60 g BiCl3 as the Bi3+ source were denoted as BCTO-1, BCTO-2, BCTO-3 and BCTO-4, respectively. The preparation of immobilized BiOCl/TiO2 on FTO was summarized as follows in Scheme 1. 2.3 Characterization The structure and phase characterization of the as-synthesized samples were conducted by the X-ray diffractometer (XD-2) with 0.154 nm Cu Kα radiation source. The morphology and crystal structure were observed by field emission scanning electron microscopy (FE-SEM, Zeiss ULTRA 55) and transmission electron microscopy (TEM, JEOL-2100F). To investigate the composition and chemical state of the samples, the energy dispersive spectroscopy (EDS, Bruker/Quanta 200)
and X-ray photoelectron spectroscopy (XPS, ESCALab-250) were experimentally utilized. The light absorption abilities of products were analyzed by the UV-Vis diffuse reflectance spectroscopy (DRS, Cary 5000) with BaSO4 as the reference. And the photoluminescence (PL) spectra of samples were characterized by the fluorescence spectrometer (RF-5301 PC). To study the photoelectrochemical properties of the as-prepared samples, electrochemical impedance spectroscopy (EIS) and transient photocurrent response were recorded by the electrochemical analyzer (SP-150, France) with a standard three-electrode configuration. The Pt wire, saturated Ag/AgCl electrode and as-prepared immobilized samples were separately utilized as the counter electrode, reference electrode and working electrode. The process was conducted in 0.1 M Na2SO4 aqueous solution with the 150 W Xenon lamp cutting off ultraviolet (λ > 420 nm) as the visible light source. 2.4 Evaluation of photocatalytic activity and determination of reactive species In order to evaluate the photocatalytic activity of the as-prepared samples, the RhB (a common cationic dye) was selected as the model organic pollutant, and the photodegradation reaction was conducted under ambient conditions using the 150 W Xenon lamp with a cut-off glass filter (λ > 420 nm) as the visible light source. In a typical photodegradation process, one piece of FTO substrate (3cm * 7cm) loading as-synthesized sample was placed in a quartz beaker containing 100 mL RhB with the concentration of 2.5 mg/L. To establish the absorption-desorption equilibrium, the mixture was stirred magnetically for o.5 h in dark before the light on. The running water was utilized to ensure the system temperature retaining at room temperature throughout the whole process. At a given time intervals, about 2.5 mL suspensions was sucked out and measured the absorbance with a UV-Vis spectrophotometer. The removal efficiency of dye was calculated by the following equation: η=
Ct 100% C0
(1)
where η means the degradation efficiency of RhB solution, C0 and Ct mean the initial concentration and the concentration at time t, respectively. Additionally, the trapping experiments with various scavengers, including p-benzoquinone (BQ), isopropyl alcohol (IPA) and triethanolamine (TEOA), were performed to analyze the effects of different radicals in photocatalytic process. The adding dosages of scavengers were according
to the previous studies.
3. Results and discussion 3.1 Structure, morphology and chemical composition characterization Fig. 1 exhibits the XRD patterns of the FTO, TiO2, BiOCl and BCTO-1 to BCTO-4. As it’s shown, curves a displays the diffraction pattern of bare FTO substrate, while curve b presents its pattern after the hydrothermal process. Apart from the diffraction peaks of FTO substrate, others appearing in curve b are consonant with the tetragonal rutile phase of TiO2 (JCPDS NO. 21-1276, a = b = 0.4593 nm, and c = 0.2959 nm), indicating the successful synthesis of TiO2 arrays. Meanwhile, the distinct peaks emerged at 36.0°, 54.3°, and 62.7°correspond to the (101), (211), and (002) crystalline planes of the rutile TiO2 structures, confirming the pure rutile phase of obtained TiO2 by the hydrothermal method. According to the diffraction pattern of pure BiOCl nanosheets by curve c, the peaks situated at 12.0°, 25.9°, and 33.5° can be separately attributed to the (001), (101), and (102) planes of its tetragonal phase (JCPDS NO. 06-0249, a = b = 0.3891 nm, c = 0.7369 nm). Moreover, the diffraction peak of (001) plane is obviously stronger than those of others, which possibly resulted from the favored growth orientation of BiOCl nanosheets. All the reflections of BiOCl nanosheets are narrow and sharp, suggesting their good crystallinity. Curves d to g belong to the BCTO-1 to BCTO-4, respectively. According to curves d-g, no extra characteristic peaks can be observed, implying the high purity of as-prepared products. The morphology of rutile TiO2 nanorods, BiOCl nanosheets, and BCTO-3 were characterized by the FESEM. As it’s shown in Fig. 2a, the TiO2 nanorod arrays can regularly grow on FTO substrate via the hydrothermal process. According to the top view of inset, the nanorods appear to contain many step edges and their diameters range from 100 nm to 200 nm. From the cross-section view of the TiO2 nanorods (Fig. 2b), one can see that the nanorods with the height of 1±0.1 µm are nearly perpendicular to the FTO substrate. The inset of Fig. 2b has shown the cross-section of BCTO-3. Fig. 2c-2f have shown the typical FESEM images of BCTO-1 to BCTO-4, respectively. As the images show, some BiOCl nanosheets are freely growing on the upper surface of rutile TiO2 nanorod arrays, while others randomly embed into the interval between TiO2 nanorods. Meanwhile, the distribution of BiOCl nanosheets becomes much denser with the increasing mass of the BiCl3, so as the average size and thickness of nanosheets.
The further demonstration of the microstructural feature about BCTO composite was analyzed via the TEM and HETEM. Fig. 3a is a typical TEM image of immobilized BCTO-3 composite, which consisted of the TiO2 nanorod and irregular BiOCl nanosheet. Fig. 3b is the HETEM image of the corresponding area signed by the dotted line in Fig. 3a. As it’s shown, the lattice spacing of 0.205 nm and 0.247 nm corresponds to (210) plane of rutile TiO2 and (003) plane of tetragonal BiOCl, respectively. To clarify the elemental distribution of the product, the EDS mapping was performed. Fig. 3c-3f show the distributions of Ti, O, Bi and Cl, confirming the composition and structure of BCTO composite. XPS was employed to analyze the surface properties and the oxidation states of the BCTO-3 composite, and the whole results are illustrated in Fig. 4. As it’s shown, the survey spectrum (Fig. 4a) of the BCTO-3 reveals the dominant presence of Ti, Bi, O and Cl. According to Fig. 4b, the peaks located at 164.5 eV and 159.2 eV could be separately ascribed to the spin orbital splitting photoelectrons of Bi 4f5/2 and Bi 4f7/2, indicating the presence of Bi in the trivalent oxidation state [29]. In Fig. 4c, the peaks of Ti 2p1/2 and Bi 4d3/2 are partially overlapped, resulting in a broad bump in the vicinity of 466.3 eV. The splitting peaks of Ti 2p centred at 464.4 eV and 458.5 eV are respectively related to Ti 2p1/2 and Ti 2p3/2 for the normal state of Ti4+ in TiO2, and the peak around 466.2 eV belongs to Bi 4d3/2 [30]. In the light of the high resolution spectra of Cl 2p (Fig. 4d), the peaks situated at 199.6 eV and 198.0 eV can be separately assigned to Cl 2p1/2 and Cl 2p3/2, confirming the Cl- in BiOCl [31]. As observed in Fig. 4e, the O1s peak have been fitted into three peaks: the peak at 529.8 eV belongs to the Bi-O bond from [Bi2O2] slabs of BiOCl layered structure [32], the peak at 530.1 eV is associated with the Ti-O bond inTiO2, while the last peak located at 532.1 eV is owing to the O-H bond from the surface absorbed water [33, 34]. The results of XPS further demonstrate the coexistence of BiOCl and TiO2 in BCTO composite. Furthermore, no signals of other impurities can be detected in BCTO-3 composite, indicating the good consonance with XRD studies. 3.2 Optical absorption behavior DRS was employed to detect the photoabsorption ability of the as-prepared samples. Fig. 5a depicts the diffuse reflectance absorption spectra of BiOCl nanosheets, TiO2 nanorods and all the immobilized BCTO films. According to Fig. 5a, the absorption spectra of rutile TiO2 nanorods and all the immobilized BCTO composites exhibit the stepped curves with a slight shift to the region
of visible light. Moreover, the plots of immobilized BCTO composites appear to show the similar tendency with that of TiO2 nanorods. Owing to the reflection from the ordered tubular or grooved array structure, there was still a slight absorption at the region of visible light [35]. Meanwhile, the absorption of BiOCl nanosheets mainly exhibits at the UV region. Fig. 5b and the inset separately represent the plots of (αhυ)1/n versus hυ for rutile TiO2 nanorods and BiOCl nanosheets. According to the results of optical absorption property, the band gap energy of the samples could be figured out by the following equation [36]:
h A(h Eg ) n / 2
(2)
where a, h, υ, A, Eg, and n are the absorption coefficient, Planck's constant, incident light frequency, constant, the band-gap energy and an integer, respectively. Among them, n depends on the characteristic of the optical transition in a semiconductor (n =1 for direct transition and n =4 for indirect transition). On the basis of the optical transition, the value of n is 1 to TiO2, while it is 4 to BiOCl. According to the results above, the band gap of rutile TiO2 nanorods and BiOCl nanosheets are separately estimated to be 3.04 eV and 3.22 eV. Although the Eg value of the BCTO composites is difficult to determine directly by the equation above, the degradation results would indicate that the composites composed of BiOCl nanosheets and TiO2 nanorods hold the hybrid band gap conformation with the synergistic photocatalysis effect in the following section. 3.3 Photocatalytic activity and stability The photocatalytic activities of as-fabricated immobilized BCTO composites were evaluated by the photodegradation efficiency of RhB under ambient condition with the illumination of visible light (λ > 420 nm). Fig. 6a shows the temporal spectrum evolution of RhB solution when using the immobilized BCTO-3 composite as photocatalyst. From Fig. 6a, it could be seen that the maximal absorption peak of RhB at 553 nm gradually decreased with the illumination time increasing, indicating the decomposition of RhB. This could also be evidenced by the colour change of RhB solution from the inset. To study the photocatalytic activity of various as-fabricated samples, comparative experiments were performed under identical condition. As presented in Fig. 6b, the immobilized BCTO composites performed the significantly improved photocatalytic activity when compared with the pure rutile TiO2 nanorods and BiOCl nanosheets. Additionally, the BCTO-3 exhibits the optimal performance, which could decompose 99.1% of RhB after 3 h of
irradiation. According to the photocatalytic performance of various immobilized BCTO composites, it reveals that the photocatalytic activities are related to the amount of BiOCl nanosheets grating on TiO2 nanorod arrays. Without BiOCl nanosheets, the photocatalytic activity of rutile TiO2 nanorod arrays is lowest. Within a certain range, the activity of BCTO films increases with the increasing amount of BiOCl nanosheets, which probably results from the closer contacted interface between the two components. Nevertheless, excessive distribution of BiOCl nanosheets may enshroud the activated sites of the photocatalyst and result in the declining photocatalytic activity [34]. According to Fig. 6c, the photocatalytic degradation of RhB on pure rutile TiO2 nanorods, BiOCl nanosheets and BCTO-3 can match with pseudo-first-order kinetics model: -Ln (C/C0) = kt
(3)
where the slope k means the pseudo-first-order rate constant. The results reveal that k value of BCTO-3 (2.59 h-1) is much higher than those of TiO2 nanorods (0.20 h-1) and BiOCl nanosheets (0.19 h-1), indicating the faster reaction rate of immobilized BCTO-3 composite. As presented in Fig. 6d, the photocatalytic activity of BCTO-3 can still degrade 91.7% of RhB after eight times cycle experiments, demonstrating the high stability and good reusability of immobilized BCTO-3. The photocatalytic degradations of RhB on various photocatalytic films have been widely studied, and the results were summarized in Table 1. As it’s shown, the immobilized BCTO-3 composite in our work holds a higher photocatalytic activity than those previously reported photocatalytic films [37-40], indicating its great prospect for practical application in the environment cleaning. 3.4 Effects of active species To our best knowledge, there contains various reactive species in the photocatalytic process, including hydroxyl radicals (·OH), active holes (h+) and superoxide radicals (·O2-), which can directly determine the photocatalytic performance. In general, the roles of·OH, h+ and·O2- were investigated via the introduction of IPA (a quencher of ·OH) [41], TEOA (a quencher of h+) [42] and BQ (a quencher of ·O2-) [43], respectively. Fig. 7 shows the trapping experiment results of immobilized BCTO-3 composite in the presence of various scavengers under Xenon lamp irradiation (λ > 420 nm). Generally, the significance of reactive species depends upon the impact on the photodegradation efficiency of RhB with its capture. Compared with the result without
quencher, the more considerable impact implies the more significant role of reactive species in the reaction. As it’s shown in Fig. 7, the photodegradation efficiency of RhB is hardly affected by the addition of IPA, indicating an unimportant role of ·OH. Conversely, the degradation efficiency of RhB evidently decreased by the adoption of BQ or TEOA, which consequently indicates that ·O2and h+, especially the h+, play the major roles in the photocatalytic oxidation process. 3.5 Possible photocatalytic mechanism of immobilized BCTO composite To provide a reasonable explanation on the photocatalytic mechanism of immobilized BCTO composite, the corresponding band positions of TiO2 nanorods and BiOCl nanosheets are figured out by the following formula [44]: 1 EVB E e E g 2
(4)
where EVB and Eg mean the VB edge potential and the band gap energy of the semiconductor, respectively. Ee is the energy of free electrons on the hydrogen scale (ca. 4.5 eV), and χ is the electronegativity of the semiconductor, which is defined as the arithmetic mean of the atomic electron affinity and the first ionization energy. Based on the value of EVB and Eg, the potential of the conduction band edge (ECB) can be calculated by ECB = EVB - Eg [45]. Since the Eg of pure BiOCl nanosheets and the rutile TiO2 nanorod arrays have been calculated to be 3.22 and 3.04 eV, their relative band positions (ECB/EVB) could be estimated to be 0.22/3.44 eV and −0.12/2.92 eV (vs. NHE) via the above formula , respectively. Based on the band gap structure of immobilized BCTO composite and the analysis of reactive species above, the photocatalytic mechanism of immobilized BCTO composite under the illumination of visible light is proposed and summarized in Scheme 2. Owing to the potentials for VB and CB of BiOCl more positive than the corresponding bands of TiO2, there will form a staggered band gap structure in the immobilized BCTO composite, which is favorable for the transfer of photogenerated charge carriers [46]. Under the irradiation, the photoinduced electrons of TiO2 nanorods can be excited and transfer from the VB to CB, then injected into the CB of BiOCl nanosheets, while the photoinduced holes will simultaneously migrate from the VB of BiOCl nanosheets and reversely move to the VB of TiO2 nanorods, which can inhibit the recombination of electron-hole pairs and make the charge separation more efficient. Additionally, according to reactive species analysis, the h+ , as well as the·O2- generated from the capture of
electron by molecular oxygen in water is the main active species, what means that if photoinduced electron-hole pairs can be separated more efficiently, it will make a contribution to increasing the amount of h+ and ·O2-and resulting in the improved photocatalytic activity. In order to prove the speculation above, the photoluminescence spectra, transient photocurrent response and electrochemical impedance spectroscopy were experimentally performed. Fig. 8 displays the PL emission spectrum of TiO2 nanorods, BiOCl nanosheets and BCTO-3 with an excitation wavelength of 360 nm. As present in Fig. 8, the peak of BiOCl nanosheets with high intensity ranges from 417 nm to 443 nm, and the main peak of TiO2 nanorods centred at 468 nm (2.63 eV) can be assigned to the oxygen vacancies. As presented in the inset, the PL spectrum of BCTO-3 composite shows a similar trend to that of pure TiO2 nanorods, while the intensity of the former is obviously much lower. As it’s well known, the intensity of PL emission spectrum correlates with the recombination rate of the charge carriers, and a lower intensity implies a lower electron-hole pairs recombination rate. According to the PL result, the immobilized BCTO-3 composite has the lowest recombination of photogenerated electron-hole pairs due to its lowest emission intensity, indicating the efficient migration of charge carries between TiO2 nanorods and BiOCl nanosheets. Fig. 9a exhibits the transient photocurrent response of rutile TiO2 nanorods, BiOCl nanosheets and BCTO-3 with several 10s on-off cycles under intermittent visible light illumination. As present in Fig. 9a, the photocurrent density will significantly increase with the light on and straightway fall to the primary value as long as the light off, indicating the key factor of illumination to generate photocurrent on photoanode in the system. In comparison with pure TiO2 nanorods and BiOCl nanosheets, the immobilized BCTO-3 composite exhibits a significantly increasing photocurrent density, which is about five times higher than that of rutile TiO2 arrays. Generally, the higher photocurrent the sample holds, the more effective separation of electron-hole pairs it will gain, which evidences that the immobilized BCTO-3 composite holds the most efficient separation due to its strongest photocurrent. The EIS measurements of rutile TiO2 nanorods, BiOCl nanosheets and immobilized BCTO-3 composite were experimentally conducted under visible light irradiation in 0.10 M Na2SO4 solution with the frequency from 100 kHz to 0.01 Hz, and the results were presented in Fig. 9b in
the form of Nyquist plot. According to Fig. 9b, BCTO-3 holds the smallest diameter of the Nyquist circle in contrast with pure TiO2 nanorods and BiOCl nanosheets, indicating not only its significant decrease of resistance and capacitance but also its fast interfacial charge transfer process and efficient separation of photogenerated charge carries [47]. Based on the analysis of PL, transient photocurrent response and EIS, the immobilized BCTO-3 composite exhibits the best performance, indicating its optimal photocatalytic activity. And this is obviously in good consonance with the photodegradation result of RhB solution and significantly proves the speculation above.
4. Conclusion In summary, we developed a facile impregnation method to graft the sheet-like BiOCl at the upright position of as-prepared rutile TiO2 nanorod arrays by using the BiCl3 as Bi3+ source, and form an immobilized photocatalyst. The as-prepared immobilized BCTO composites, especially the BCTO-3 exhibited optimized photocatalytic activity toward degradation of RhB in contrast with pure BiOCl nanosheets and rutile TiO2 arrays under visible light irradiation, which could be mainly ascribed to the match of energy level between BiOCl nanosheets and TiO2 nanorods. More attractively, the degradation efficiency of BCTO-3 can still reach 91.7% after eight times photodegradation cycle experiments, which shows the high stability and excellent durability of BCTO-3. In summary, this work could provide a new insight into broadening the application of TiO2 nanorods and BiOCl nanosheets, and developing new preparation route of the immobilized photocatalysts with enhanced photocatalytic activity.
Acknowledgements This work was supported by National Key Project in Control and Management of Polluted Water Bodies (No. 2013ZX07105-005).
References [1] F. Raziq, C. Li, M. Humayun, Y. Qu, A. Zada, H. Yu, L. Jing, Synthesis of TiO2/g-C3N4 nanocomposites as efficient photocatalysts dependent on the enhanced photogenerated charge separation, Materials Research Bulletin, 70 (2015) 494-499. [2] Y. Ma, Z. Chen, D. Qu, J. Shi, Synthesis of chemically bonded BiOCl@Bi2WO6 microspheres with
exposed (020) Bi2WO6 facets and their enhanced photocatalytic activities under visible light irradiation, Applied Surface Science, 361 (2016) 63-71. [3] Z. Ni, Y. Sun, Y. Zhang, F. Dong, Fabrication, modification and application of (BiO)2CO3-based photocatalysts: A review, Applied Surface Science, 365 (2016) 314-335. [4] Y. Chen, D.D. Dionysiou, TiO2 photocatalytic films on stainless steel: The role of Degussa P-25 in modified sol–gel methods, Applied Catalysis B: Environmental, 62 (2006) 255-264. [5] B. Erjavec, P. Hudoklin, K. Perc, T. Tišler, M.S. Dolenc, A. Pintar, Glass fiber-supported TiO2 photocatalyst: Efficient mineralization and removal of toxicity/estrogenicity of bisphenol A and its analogs, Applied Catalysis B: Environmental, 183 (2016) 149-158. [6] A.Y. Shan, T.I.M. Ghazi, S.A. Rashid, Immobilisation of titanium dioxide onto supporting materials in heterogeneous photocatalysis: A review, Applied Catalysis A: General, 389 (2010) 1-8. [7] X. Li, J. Yu, M. Jaroniec, Hierarchical photocatalysts, Chemical Society reviews, 45 (2016) 2603-2636. [8] J. Wen, X. Li, W. Liu, Y. Fang, J. Xie, Y. Xu, Photocatalysis fundamentals and surface modification of TiO2 nanomaterials, Chin. J. Catal., 36 (2015) 2049-2070. [9] Y. Zhang, S. Liu, Z. Xiu, Q. Lu, H. Sun, G. Liu, TiO2/BiOI heterostructured nanofibers: electrospinning–solvothermal two-step synthesis and visible-light photocatalytic performance investigation, Journal of Nanoparticle Research, 16 (2014). [10] J. Low, B. Cheng, J. Yu, Surface modification and enhanced photocatalytic CO2 reduction performance of TiO2: a review, Applied Surface Science, 392 (2017) 658-686. [11] H. Zhang, X. Liu, Y. Li, Q. Sun, Y. Wang, B.J. Wood, P. Liu, D. Yang, H. Zhao, Vertically aligned nanorod-like rutile TiO2 single crystal nanowire bundles with superior electron transport and photoelectrocatalytic properties, J. Mater. Chem., 22 (2012) 2465-2472. [12] Z. Chai, J. Gu, P. Qiang, X. Yu, W. Mai, Facile conversion of rutile titanium dioxide nanowires to nanotubes for enhancing the performance of dye-sensitized solar cells, CrystEngComm, 17 (2015) 1115-1120. [13] J. Wang, W.-D. Zhang, Modification of TiO2 nanorod arrays by graphite-like C3N4 with high visible light photoelectrochemical activity, Electrochimica Acta, 71 (2012) 10-16. [14] Z.-Q. Liu, P.-Y. Kuang, R.-B. Wei, N. Li, Y.-B. Chen, Y.-Z. Su, BiOBr nanoplate-wrapped ZnO nanorod arrays for high performance photoelectrocatalytic application, RSC Adv., 6 (2016) 16122-16130. [15] Y. Zhao, X. Huang, X. Tan, T. Yu, X. Li, L. Yang, S. Wang, Fabrication of BiOBr nanosheets@TiO2 nanobelts p–n junction photocatalysts for enhanced visible-light activity, Applied Surface Science, 365 (2016) 209-217. [16] L. Gomathi Devi, R. Kavitha, A review on plasmonic metal–TiO2 composite for generation, trapping, storing and dynamic vectorial transfer of photogenerated electrons across the Schottky junction in a photocatalytic system, Applied Surface Science, 360 (2016) 601-622. [17] F. Boxberg, N. Sondergaard, H.Q. Xu, Elastic and piezoelectric properties of zincblende and wurtzite crystalline nanowire heterostructures, Adv Mater, 24 (2012) 4692-4706. [18] J. Jiang, K. Zhao, X. Xiao, L. Zhang, Synthesis and facet-dependent photoreactivity of BiOCl single-crystalline nanosheets, Journal of the American Chemical Society, 134 (2012) 4473-4476. [19] C. Cao, L. Xiao, C. Chen, Q. Cao, Synthesis of novel Cu2O/BiOCl heterojunction nanocomposites and their enhanced photocatalytic activity under visible light, Applied Surface Science, 357 (2015) 1171-1179.
[20] R. He, J. Zhang, J. Yu, S. Cao, Room-temperature synthesis of BiOI with tailorable (001) facets and enhanced photocatalytic activity, Journal of colloid and interface science, 478 (2016) 201-208. [21] L. Ye, Y. Su, X. Jin, H. Xie, C. Zhang, Recent advances in BiOX (X = Cl, Br and I) photocatalysts: synthesis, modification, facet effects and mechanisms, Environmental Science: Nano, 1 (2014) 90. [22] R.a. He, S. Cao, P. Zhou, J. Yu, Recent advances in visible light Bi-based photocatalysts, Chin. J. Catal., 35 (2014) 989-1007. [23] L. Ye, J. Liu, C. Gong, L. Tian, T. Peng, L. Zan, Two Different Roles of Metallic Ag on Ag/AgX/BiOX (X = Cl, Br) Visible Light Photocatalysts: Surface Plasmon Resonance and Z-Scheme Bridge, ACS Catalysis, 2 (2012) 1677-1683. [24] B.N. Pal, I. Robel, A. Mohite, R. Laocharoensuk, D.J. Werder, V.I. Klimov, High-Sensitivity p-n Junction Photodiodes Based on PbS Nanocrystal Quantum Dots, Advanced Functional Materials, 22 (2012) 1741-1748. [25] C. Cao, C. Hu, W. Shen, S. Wang, J. Wang, Y. Tian, Fabrication of a novel heterostructure of Co3O4-modified TiO2 nanorod arrays and its enhanced photoelectrochemical property, Journal Of Alloys And Compounds, 550 (2013) 137-143. [26] H. Tong, S. Ouyang, Y. Bi, N. Umezawa, M. Oshikiri, J. Ye, Nano-photocatalytic Materials: Possibilities and Challenges, Advanced Materials, 24 (2012) 229-251. [27] J. Cao, J. Xing, Y. Zhang, H. Tong, Y. Bi, T. Kako, M. Takeguchi, J. Ye, Photoelectrochemical Properties of Nanomultiple CaFe2O4/ZnFe2O4 pn Junction Photoelectrodes, Langmuir: the ACS journal of surfaces and colloids, 29 (2013) 3116-3124. [28] Y. Cai, P. Wang, Y. Ye, J. Liu, Z. Tian, Y. Liu, C. Liang, Grafting BiOCl nanosheets onto TiO2 tubular arrays to form a hierarchical structure with improved photocatalytic performance, RSC Advances, 3 (2013) 19064. [29] G. Dai, J. Yu, G. Liu, Synthesis and Enhanced Visible-Light Photoelectrocatalytic Activity of p−n Junction BiOI/TiO2 Nanotube Arrays, The Journal of Physical Chemistry C, 115 (2011) 7339-7346. [30] C. Liao, Z. Ma, G. Dong, J. Qiu, BiOI nanosheets decorated TiO2 nanofiber: Tailoring water purification performance of photocatalyst in structural and photo-responsivity aspects, Applied Surface Science, 314 (2014) 481-489. [31] Y. Li, Q. Wang, B. Liu, J. Zhang, The {001} facets-dependent superior photocatalytic activities of BiOCl nanosheets under visible light irradiation, Applied Surface Science, 349 (2015) 957-969. [32] K. Zhang, C. Liu, F. Huang, C. Zheng, W. Wang, Study of the electronic structure and photocatalytic activity of the BiOCl photocatalyst, Applied Catalysis B: Environmental, 68 (2006) 125-129. [33] Z. Ai, W. Ho, S. Lee, L. Zhang, Efficient Photocatalytic Removal of NO in Indoor Air with Hierarchical Bismuth Oxybromide Nanoplate Microspheres under Visible Light, Environmental Science & Technology, 43 (2009) 4143-4150. [34] X.J. Wang, W.Y. Yang, F.T. Li, J. Zhao, R.H. Liu, S.J. Liu, B. Li, Construction of amorphous TiO2/BiOBr heterojunctions via facets coupling for enhanced photocatalytic activity, Journal of hazardous materials, 292 (2015) 126-136. [35] C. Zhang, S. Liu, T. Shi, Z. Tang, Improved model-based infrared reflectrometry for measuring deep trench structures, Journal of the Optical Society of America a-Optics Image Science And Vision, 26 (2009) 2327-2335. [36] H.-P. Jiao, X. Yu, Z.-Q. Liu, P.-Y. Kuang, Y.-M. Zhang, One-pot synthesis of heterostructured Bi2S3/BiOBr microspheres with highly efficient visible light photocatalytic performance, RSC Adv., 5
(2015) 16239-16249. [37] C. Wang, X. Zhang, Y. Zhang, Y. Jia, B. Yuan, J. Yang, P. Sun, Y. Liu, Morphologically-tunable TiO2 nanorod film with high energy facets: green synthesis, growth mechanism and photocatalytic activity, Nanoscale, 4 (2012) 5023. [38] Y. Liang, C. Guo, S. Cao, Y. Tian, Q. Lui, A High Quality BiOCl Film with Petal-Like Hierarchical Structures and Its Visible-Light Photocatalytic Property, Journal of Nanoscience and Nanotechnology, 13 (2013) 919-923. [39] Y. Wang, Y.Z. Zheng, S. Lu, X. Tao, Y. Che, J.F. Chen, Visible-light-responsive TiO2-coated ZnO:I nanorod array films with enhanced photoelectrochemical and photocatalytic performance, ACS applied materials & interfaces, 7 (2015) 6093-6101. [40] X. Zhao, Y. Wu, W. Yao, Y. Zhu, Photoelectrochemical properties of thin Bi2WO6 films, Thin Solid Films, 515 (2007) 4753-4757. [41] P. Ji, J. Zhang, F. Chen, M. Anpo, Study of adsorption and degradation of acid orange 7 on the surface of CeO2 under visible light irradiation, Applied Catalysis B: Environmental, 85 (2009) 148-154. [42] L. Ye, K. Deng, F. Xu, L. Tian, T. Peng, L. Zan, Increasing visible-light absorption for photocatalysis with black BiOCl, Physical chemistry chemical physics : PCCP, 14 (2012) 82-85. [43] S.A. Khan, N. Singh, K. Saleem, Synthesis, characterization and in vitro antibacterial activity of thiourea and urea derivatives of steroids, European journal of medicinal chemistry, 43 (2008) 2272-2277. [44] D. Sun, J. Li, L. He, B. Zhao, T. Wang, R. Li, S. Yin, Z. Feng, T. Sato, Facile solvothermal synthesis of BiOCl–TiO2heterostructures with enhanced photocatalytic activity, CrystEngComm, 16 (2014) 7564. [45] F. Duo, Y. Wang, X. Mao, X. Zhang, Y. Wang, C. Fan, A BiPO4/BiOCl heterojunction photocatalyst with enhanced electron-hole separation and excellent photocatalytic performance, Applied Surface Science, 340 (2015) 35-42. [46] H. Cheng, B. Huang, Y. Dai, Engineering BiOX (X = Cl, Br, I) nanostructures for highly efficient photocatalytic applications, Nanoscale, 6 (2014) 2009-2026. [47] J. Di, J. Xia, Y. Ge, L. Xu, H. Xu, M. He, Q. Zhang, H. Li, Reactable ionic liquid-assisted rapid synthesis of BiOI hollow microspheres at room temperature with enhanced photocatalytic activity, J. Mater. Chem. A, 2 (2014) 15864-15874.
Fig. 1. XRD patterns of a: the FTO; b: TiO2; c: BiOCl; d: BCTO-1; e: BCTO-2; f: BCTO-3; g: BCTO-4.
Fig. 2. FE-SEM images of (a) TiO2 nanorod arrays; (b) cross-section view of the TiO2 nanorod arrays; (c) BCTO-1; (d) BCTO-2; (e) BCTO-3; (f) BCTO-4, and the inset in (a) is the top view of TiO2 nanorods at high magnifications.
Fig. 3. (a-b) TEM and HRTEM images of the BCTO-3; (c-f) EDS mapping distribution of Ti, O, Bi and Cl taken on a detailed region of the BCTO-3.
Fig. 4. XPS spectra of the immobilized BCTO-3 composite: (a) survey spectra; (b) Bi 4f; (c) Ti 2p; (d) Cl 2p; (e) O 1s.
Fig. 5. (a) UV-vis diffused reflectance spectra of the as-prepared samples; (b) the plot of (αhυ)2 versus hυ for rutile TiO2 ,and the inset in (b) is the plot of (αhυ)1/2 versus hυ for BiOCl.
Fig. 6. (a) absorption changes of RhB solution in the presence of the immobilized BCTO-3composite; (b) comparative experiments for the degradation of RhB with all the as-prepared samples under visible light; (c) kinetic fit for the degradation of RhB with TiO2, BiOCl and BCTO-3, and reaction rate constant k obtained from linear fitting; (d) recycling tests on immobilized BCTO-3composite for the degradation of RhB under visible light.
Fig. 7. Trapping experiment of active species during the photocatalytic oxidation of RhB.
Fig. 8. The PL spectrum of pure TiO2, BiOCl and BCTO-3, and the inset is the amplification of PL spectrum of TiO2 and BCTO-3.
Fig. 9. (a) Transient photocurrent responses of pure TiO2 nanorods, BiOCl nanosheets and immobilized BCTO-3 composite in 0.10 mol/L Na2SO4 aqueous solution under visible light irradiation (λ > 420 nm); and (b) the corresponding of electrochemical impedance spectroscopy with the frequency from 100 kHz to 0.01 Hz.
Scheme 1. Illustration of the preparation of immobilized BCTO composites on FTO.
Scheme 2. Schematic diagram of the electron-hole pairs separation and the proposed reaction mechanism over the immobilized BCTO composite under visible light.
Table 1 The photodegradation of RhB on various immobilized photocatalysts. Immobilized photocatalyst
RhB concentration (mg/L)
Volume (mL)
Degradation efficiency
Time (h)
Light source
Reference
TiO2 BiOCl ZnO:I/TiO2 Bi2WO6 BCTO-3
10.0 1.0 2.4 5.0 2.5
10 100 100 100 100
50% 52.5% 97% 20% 99.1%
5 8 6 8 3
UV Visible Visible Visible Visible
37 38 39 40 This work
S0169-4332(16)32793-3 http://dx.doi.org/doi:10.1016/j.apsusc.2016.12.076 APSUSC 34623
To appear in:
APSUSC
Received date: Revised date: Accepted date:
12-10-2016 7-12-2016 10-12-2016
Please cite this article as: Yinghua Shen, Xiang Yu, Weitian Lin, Yi Zhu, Yuanming Zhang, A facile preparation of immobilized BiOCl nanosheets/TiO2 arrays on FTO with enhanced photocatalytic activity and reusability, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.12.076 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A facile preparation of immobilized BiOCl nanosheets /TiO2 arrays on FTO with enhanced photocatalytic activity and reusability Yinghua Shena, Xiang Yu a,b, Weitian Lina, Yi Zhua,
Yuanming Zhang a*
a
Department of Chemistry, Jinan University, Guangzhou, 510632, P.R. China
b
Analytical & Testing Center, Jinan University, Guangzhou, 510632, P.R. China
These authors contributed equally to this work.
*
Corresponding Authors: Tel: +86 2085222756
Email: [email protected](Y.Zhang)
Graphical abstract
Highlights:
Immobilized BiOCl nanosheets/TiO2 arrays hybrid photocatalyst were fabricated.
The degradation efficiency of BCTO-3 can still reach 91.7% after eight cycles.
The immobilized BCTO-3 can be recycled for removal of organic pollutants in water.
Abstract：Forming a hybrid structure is considered as an efficient strategy toward improving the photocatalytic activity of TiO2-based photocatalyst. In this work, we report a facile impregnation method to prepare BiOCl nanosheets on rutile TiO2 nanorod arrays on transparent conductive fluorine-doped tin oxide (FTO) substrate. According to RhB photocatalytic degradation experiments, the degradation efficiency of the immobilized BiOCl/TiO2 (denoted as BCTO-3) hybrid photocatalyst can reach 99.1% after visible light irradiation for 3 h, and its efficiency is higher than that of pure BiOCl (42.7%) and TiO2 (44.8%), respectively. The enhancement is demonstrated to be the match of energy level between BiOCl and TiO2. Hence, the separation and transfer of photogenerated electron-hole pairs are obviously improved, which have been illustrated by the result of the photoluminescence spectra analysis and photoelectrochemical performance. Moreover, the degradation efficiency of BCTO-3 can still reach 91.7% after eight times photodegradation cycle experiments. Due to the easy recycling and excellent durability, the immobilized BCTO-3 photocatalyst is considered as a promising photocatalytic material for the removal of organic pollutants in aqueous eco-environments.
Keywords: immobilized photocatalyst; impregnation method; BiOCl nanosheets; TiO2 nanorod arrays.
1. Introduction Recently, there has been significant attention on the developments of semiconductor photocatalysts owing to their potential applications in renewable energy from water splitting and environmental cleaning by the decomposition of organic pollutants [1-3]. In traditional photocatalytic processes, the semiconductor photocatalysts utilized in the treatment of organic pollutants in water are mostly in the form of unsupported suspensions, which may easily diffuse into the solution as secondary pollutants and increase the cost of water purification process [4-6]. Compared with universal powder photocatalysts, the immobilized photocatalyst appears to be more promising and valuable for practical application owing to its easy reusability and non-secondary effect. And this consequently makes the immobilized semiconductor photocatalysts become one of the hot research subjects of materials science. Since the original report by Fujishima and Honda in 1972 [7, 8], TiO2 has drawn stupendous attention and become one of the most popular semiconductor photocatalysts as a result of its good chemical stability, oxidizability, low cost and non-toxicity [9, 10]. According to the pioneer researches, oriented single-crystalline TiO2 can uniformly grow on the FTO substrate to form the film by the hydrothermal method [11, 12]. Compared with the bulk TiO2, the single-crystalline 1D TiO2 nanorod arrays can significantly prolong the life time of photogenerated charge carriers and hold vast potential for the photocatalytic application in virtue of its high aspect ratio and effective electrical pathway for electrons [13-15]. However, TiO2 nanorod arrays suffer from the rapid recombination of photogenerated electron-hole pairs, which results in the suppressed photocatalytic activity and limited practical application of TiO2 arrays. To address this problem, considerable energies have been devoted by researchers, including coupling with other semiconductors, carbon materials and noble metals [16]. In this work, the strategy is mainly centred on modifying the TiO2 nanorod arrays with another semiconductor, whose redox energy levels for its corresponding conduction and valence band differ from that of TiO2 [17]. Among numerous semiconductors, bismuth oxychloride (BiOCl) has emerged as one of most promising candidates in virtue of its high photocorrosion stability and good biocompatibility. As an important V-VI-VII ternary compound, BiOCl has the open layered structure consisting of the [Bi2O2] slabs intercalated by double slabs of Cl atoms [18-20], which can provide a large enough
space to polarize the related atoms and orbitals, and engender an internal static electric field to help separate the photogenerated electron-hole pairs [21, 22]. This is particularly beneficial in semiconductor hybrid composites referring to the fabrication of hierarchical or grafted structures. On one hand, since the maximum potential of valence band (VB) and the minimum potential of conduction band (CB) of BiOCl are both more positive than the corresponding bands of TiO2 [23], electrons can migrate from the CB of TiO2 to the CB of BiOCl, meanwhile holes could reversely transfer from the VB of BiOCl to the VB of TiO2, which could significantly facilitate the separation of photoinduced electron-hole pairs in the structure consist of TiO2 and BiOCl. On the other hand, in virtue of the different property of n-type TiO2 and p-type BiOCl, there will form the depletion region at the interface between layers of the two components, which results in the efficient separation of the charge carries [24-27]. To our knowledge, there were a few previous reports about the immobilized BiOCl/TiO2 photocatalyst for photodegradation of organic pollutants. Cai et al. [28] grafted BiOCl nanosheets onto the TiO2 nanotubes by the hydrothermal method. Accordingly, a deep exploitation of immobilized BiOCl/TiO2 hybrid photocatalyst with improved photocatalytic activity and reusability by a facile preparation still holds considerable significance for the applied prospect in the treatment of water contamination. In our work, a facile impregnation method has been developed to graft the sheet-like BiOCl onto the upright position of as-prepared rutile TiO2 nanorod arrays by utilizing the BiCl3 as Bi3+ source at room temperature. The BiOCl/TiO2 (denoted as BCTO) hybrid photocatalyst exhibits enhanced photocatalytic activity and good reusability by the degradation of RhB with the visible light illumination compared with BiOCl nanosheets and TiO2 nanorods. Meanwhile, the degradation efficiency of BCTO-3 can still reach 91.7% after eight times photodegradation cycle experiments. In addition, a series of characterization were employed to provide a reasonable explanation about the mechanism for the improved photocatalytic activity of the as-fabricated immobilized BCTO composites.
2. Experimental details 2.1 Preparation of rutile TiO2 nanorod arrays on FTO The rutile TiO2 nanorod arrays were experimentally prepared on an FTO substrate (3 cm*7 cm) by the hydrothermal process. Typically, the mixture containing 40 mL concentrated hydrochloric
acid (36.5%~38% by weight) and the same volume of deionized water, was stirred under room temperature for 5 min to form the homogeneous solution before the addition of 1.33 mL titanium butoxide (97%). Keep stirring the solution, until the whole mixture became clear and transparent. Two pieces of FTO substrates, ultrasonically cleaned for 60 min in ethanol, were placed at an angle against the wall of the Teflon-liner with the conducting side facing down. Finally, the sealed autoclave with the mixture and FTO substrate was placed in the oven at 170 °C for 4 h. After synthesis, the ethanol and deionized water were alternatively utilized to rinse the samples for several times before dried in air. 2.2 Synthesis of BiOCl nanosheet on rutile TiO2 nanorod arrays 0.45 g BiCl3 was added to 10 mL absolute ethanol containing 0.05 mL concentrated hydrochloric acid (36.5%~38% by weight). Keep stirring the mixture for 0.5 h to make it homogenous. Then, 0.4 mL of the mixing solution was sucked by flnnpipette and sprayed on the as-prepared TiO2 arrays. When the ethanol was nearly volatilized out under the atmosphere condition, the as-prepared sample was baked in the oven at 80 °C for 0.5 h. Finally, the substrate was soaked in deionized water for 0.5 h to fabricate the BiOCl nanosheets on rutile TiO2 nanorod arrays, and then dried at 80 °C for 0.5 h. In order to investigate the effects of the amount of BiOCl nansheets grafting on TiO2 nanorod arrays toward the photocatalytic activity, the mass of BiCl3 was experimentally adjusted to prepare different immobilized BiOCl/TiO2 (BCTO) composites by repeating the foregoing procedure. Furthermore, 0.4 mL BiCl3 solution with the same mass as the one for BCTO composite of the best photocatalytic performance was used to prepare BiOCl nanosheets on bare FTO substrate for comparison. The BCTO composites using 0.15 g, 0.30 g, o.45 g and 0.60 g BiCl3 as the Bi3+ source were denoted as BCTO-1, BCTO-2, BCTO-3 and BCTO-4, respectively. The preparation of immobilized BiOCl/TiO2 on FTO was summarized as follows in Scheme 1. 2.3 Characterization The structure and phase characterization of the as-synthesized samples were conducted by the X-ray diffractometer (XD-2) with 0.154 nm Cu Kα radiation source. The morphology and crystal structure were observed by field emission scanning electron microscopy (FE-SEM, Zeiss ULTRA 55) and transmission electron microscopy (TEM, JEOL-2100F). To investigate the composition and chemical state of the samples, the energy dispersive spectroscopy (EDS, Bruker/Quanta 200)
and X-ray photoelectron spectroscopy (XPS, ESCALab-250) were experimentally utilized. The light absorption abilities of products were analyzed by the UV-Vis diffuse reflectance spectroscopy (DRS, Cary 5000) with BaSO4 as the reference. And the photoluminescence (PL) spectra of samples were characterized by the fluorescence spectrometer (RF-5301 PC). To study the photoelectrochemical properties of the as-prepared samples, electrochemical impedance spectroscopy (EIS) and transient photocurrent response were recorded by the electrochemical analyzer (SP-150, France) with a standard three-electrode configuration. The Pt wire, saturated Ag/AgCl electrode and as-prepared immobilized samples were separately utilized as the counter electrode, reference electrode and working electrode. The process was conducted in 0.1 M Na2SO4 aqueous solution with the 150 W Xenon lamp cutting off ultraviolet (λ > 420 nm) as the visible light source. 2.4 Evaluation of photocatalytic activity and determination of reactive species In order to evaluate the photocatalytic activity of the as-prepared samples, the RhB (a common cationic dye) was selected as the model organic pollutant, and the photodegradation reaction was conducted under ambient conditions using the 150 W Xenon lamp with a cut-off glass filter (λ > 420 nm) as the visible light source. In a typical photodegradation process, one piece of FTO substrate (3cm * 7cm) loading as-synthesized sample was placed in a quartz beaker containing 100 mL RhB with the concentration of 2.5 mg/L. To establish the absorption-desorption equilibrium, the mixture was stirred magnetically for o.5 h in dark before the light on. The running water was utilized to ensure the system temperature retaining at room temperature throughout the whole process. At a given time intervals, about 2.5 mL suspensions was sucked out and measured the absorbance with a UV-Vis spectrophotometer. The removal efficiency of dye was calculated by the following equation: η=
Ct 100% C0
(1)
where η means the degradation efficiency of RhB solution, C0 and Ct mean the initial concentration and the concentration at time t, respectively. Additionally, the trapping experiments with various scavengers, including p-benzoquinone (BQ), isopropyl alcohol (IPA) and triethanolamine (TEOA), were performed to analyze the effects of different radicals in photocatalytic process. The adding dosages of scavengers were according
to the previous studies.
3. Results and discussion 3.1 Structure, morphology and chemical composition characterization Fig. 1 exhibits the XRD patterns of the FTO, TiO2, BiOCl and BCTO-1 to BCTO-4. As it’s shown, curves a displays the diffraction pattern of bare FTO substrate, while curve b presents its pattern after the hydrothermal process. Apart from the diffraction peaks of FTO substrate, others appearing in curve b are consonant with the tetragonal rutile phase of TiO2 (JCPDS NO. 21-1276, a = b = 0.4593 nm, and c = 0.2959 nm), indicating the successful synthesis of TiO2 arrays. Meanwhile, the distinct peaks emerged at 36.0°, 54.3°, and 62.7°correspond to the (101), (211), and (002) crystalline planes of the rutile TiO2 structures, confirming the pure rutile phase of obtained TiO2 by the hydrothermal method. According to the diffraction pattern of pure BiOCl nanosheets by curve c, the peaks situated at 12.0°, 25.9°, and 33.5° can be separately attributed to the (001), (101), and (102) planes of its tetragonal phase (JCPDS NO. 06-0249, a = b = 0.3891 nm, c = 0.7369 nm). Moreover, the diffraction peak of (001) plane is obviously stronger than those of others, which possibly resulted from the favored growth orientation of BiOCl nanosheets. All the reflections of BiOCl nanosheets are narrow and sharp, suggesting their good crystallinity. Curves d to g belong to the BCTO-1 to BCTO-4, respectively. According to curves d-g, no extra characteristic peaks can be observed, implying the high purity of as-prepared products. The morphology of rutile TiO2 nanorods, BiOCl nanosheets, and BCTO-3 were characterized by the FESEM. As it’s shown in Fig. 2a, the TiO2 nanorod arrays can regularly grow on FTO substrate via the hydrothermal process. According to the top view of inset, the nanorods appear to contain many step edges and their diameters range from 100 nm to 200 nm. From the cross-section view of the TiO2 nanorods (Fig. 2b), one can see that the nanorods with the height of 1±0.1 µm are nearly perpendicular to the FTO substrate. The inset of Fig. 2b has shown the cross-section of BCTO-3. Fig. 2c-2f have shown the typical FESEM images of BCTO-1 to BCTO-4, respectively. As the images show, some BiOCl nanosheets are freely growing on the upper surface of rutile TiO2 nanorod arrays, while others randomly embed into the interval between TiO2 nanorods. Meanwhile, the distribution of BiOCl nanosheets becomes much denser with the increasing mass of the BiCl3, so as the average size and thickness of nanosheets.
The further demonstration of the microstructural feature about BCTO composite was analyzed via the TEM and HETEM. Fig. 3a is a typical TEM image of immobilized BCTO-3 composite, which consisted of the TiO2 nanorod and irregular BiOCl nanosheet. Fig. 3b is the HETEM image of the corresponding area signed by the dotted line in Fig. 3a. As it’s shown, the lattice spacing of 0.205 nm and 0.247 nm corresponds to (210) plane of rutile TiO2 and (003) plane of tetragonal BiOCl, respectively. To clarify the elemental distribution of the product, the EDS mapping was performed. Fig. 3c-3f show the distributions of Ti, O, Bi and Cl, confirming the composition and structure of BCTO composite. XPS was employed to analyze the surface properties and the oxidation states of the BCTO-3 composite, and the whole results are illustrated in Fig. 4. As it’s shown, the survey spectrum (Fig. 4a) of the BCTO-3 reveals the dominant presence of Ti, Bi, O and Cl. According to Fig. 4b, the peaks located at 164.5 eV and 159.2 eV could be separately ascribed to the spin orbital splitting photoelectrons of Bi 4f5/2 and Bi 4f7/2, indicating the presence of Bi in the trivalent oxidation state [29]. In Fig. 4c, the peaks of Ti 2p1/2 and Bi 4d3/2 are partially overlapped, resulting in a broad bump in the vicinity of 466.3 eV. The splitting peaks of Ti 2p centred at 464.4 eV and 458.5 eV are respectively related to Ti 2p1/2 and Ti 2p3/2 for the normal state of Ti4+ in TiO2, and the peak around 466.2 eV belongs to Bi 4d3/2 [30]. In the light of the high resolution spectra of Cl 2p (Fig. 4d), the peaks situated at 199.6 eV and 198.0 eV can be separately assigned to Cl 2p1/2 and Cl 2p3/2, confirming the Cl- in BiOCl [31]. As observed in Fig. 4e, the O1s peak have been fitted into three peaks: the peak at 529.8 eV belongs to the Bi-O bond from [Bi2O2] slabs of BiOCl layered structure [32], the peak at 530.1 eV is associated with the Ti-O bond inTiO2, while the last peak located at 532.1 eV is owing to the O-H bond from the surface absorbed water [33, 34]. The results of XPS further demonstrate the coexistence of BiOCl and TiO2 in BCTO composite. Furthermore, no signals of other impurities can be detected in BCTO-3 composite, indicating the good consonance with XRD studies. 3.2 Optical absorption behavior DRS was employed to detect the photoabsorption ability of the as-prepared samples. Fig. 5a depicts the diffuse reflectance absorption spectra of BiOCl nanosheets, TiO2 nanorods and all the immobilized BCTO films. According to Fig. 5a, the absorption spectra of rutile TiO2 nanorods and all the immobilized BCTO composites exhibit the stepped curves with a slight shift to the region
of visible light. Moreover, the plots of immobilized BCTO composites appear to show the similar tendency with that of TiO2 nanorods. Owing to the reflection from the ordered tubular or grooved array structure, there was still a slight absorption at the region of visible light [35]. Meanwhile, the absorption of BiOCl nanosheets mainly exhibits at the UV region. Fig. 5b and the inset separately represent the plots of (αhυ)1/n versus hυ for rutile TiO2 nanorods and BiOCl nanosheets. According to the results of optical absorption property, the band gap energy of the samples could be figured out by the following equation [36]:
h A(h Eg ) n / 2
(2)
where a, h, υ, A, Eg, and n are the absorption coefficient, Planck's constant, incident light frequency, constant, the band-gap energy and an integer, respectively. Among them, n depends on the characteristic of the optical transition in a semiconductor (n =1 for direct transition and n =4 for indirect transition). On the basis of the optical transition, the value of n is 1 to TiO2, while it is 4 to BiOCl. According to the results above, the band gap of rutile TiO2 nanorods and BiOCl nanosheets are separately estimated to be 3.04 eV and 3.22 eV. Although the Eg value of the BCTO composites is difficult to determine directly by the equation above, the degradation results would indicate that the composites composed of BiOCl nanosheets and TiO2 nanorods hold the hybrid band gap conformation with the synergistic photocatalysis effect in the following section. 3.3 Photocatalytic activity and stability The photocatalytic activities of as-fabricated immobilized BCTO composites were evaluated by the photodegradation efficiency of RhB under ambient condition with the illumination of visible light (λ > 420 nm). Fig. 6a shows the temporal spectrum evolution of RhB solution when using the immobilized BCTO-3 composite as photocatalyst. From Fig. 6a, it could be seen that the maximal absorption peak of RhB at 553 nm gradually decreased with the illumination time increasing, indicating the decomposition of RhB. This could also be evidenced by the colour change of RhB solution from the inset. To study the photocatalytic activity of various as-fabricated samples, comparative experiments were performed under identical condition. As presented in Fig. 6b, the immobilized BCTO composites performed the significantly improved photocatalytic activity when compared with the pure rutile TiO2 nanorods and BiOCl nanosheets. Additionally, the BCTO-3 exhibits the optimal performance, which could decompose 99.1% of RhB after 3 h of
irradiation. According to the photocatalytic performance of various immobilized BCTO composites, it reveals that the photocatalytic activities are related to the amount of BiOCl nanosheets grating on TiO2 nanorod arrays. Without BiOCl nanosheets, the photocatalytic activity of rutile TiO2 nanorod arrays is lowest. Within a certain range, the activity of BCTO films increases with the increasing amount of BiOCl nanosheets, which probably results from the closer contacted interface between the two components. Nevertheless, excessive distribution of BiOCl nanosheets may enshroud the activated sites of the photocatalyst and result in the declining photocatalytic activity [34]. According to Fig. 6c, the photocatalytic degradation of RhB on pure rutile TiO2 nanorods, BiOCl nanosheets and BCTO-3 can match with pseudo-first-order kinetics model: -Ln (C/C0) = kt
(3)
where the slope k means the pseudo-first-order rate constant. The results reveal that k value of BCTO-3 (2.59 h-1) is much higher than those of TiO2 nanorods (0.20 h-1) and BiOCl nanosheets (0.19 h-1), indicating the faster reaction rate of immobilized BCTO-3 composite. As presented in Fig. 6d, the photocatalytic activity of BCTO-3 can still degrade 91.7% of RhB after eight times cycle experiments, demonstrating the high stability and good reusability of immobilized BCTO-3. The photocatalytic degradations of RhB on various photocatalytic films have been widely studied, and the results were summarized in Table 1. As it’s shown, the immobilized BCTO-3 composite in our work holds a higher photocatalytic activity than those previously reported photocatalytic films [37-40], indicating its great prospect for practical application in the environment cleaning. 3.4 Effects of active species To our best knowledge, there contains various reactive species in the photocatalytic process, including hydroxyl radicals (·OH), active holes (h+) and superoxide radicals (·O2-), which can directly determine the photocatalytic performance. In general, the roles of·OH, h+ and·O2- were investigated via the introduction of IPA (a quencher of ·OH) [41], TEOA (a quencher of h+) [42] and BQ (a quencher of ·O2-) [43], respectively. Fig. 7 shows the trapping experiment results of immobilized BCTO-3 composite in the presence of various scavengers under Xenon lamp irradiation (λ > 420 nm). Generally, the significance of reactive species depends upon the impact on the photodegradation efficiency of RhB with its capture. Compared with the result without
quencher, the more considerable impact implies the more significant role of reactive species in the reaction. As it’s shown in Fig. 7, the photodegradation efficiency of RhB is hardly affected by the addition of IPA, indicating an unimportant role of ·OH. Conversely, the degradation efficiency of RhB evidently decreased by the adoption of BQ or TEOA, which consequently indicates that ·O2and h+, especially the h+, play the major roles in the photocatalytic oxidation process. 3.5 Possible photocatalytic mechanism of immobilized BCTO composite To provide a reasonable explanation on the photocatalytic mechanism of immobilized BCTO composite, the corresponding band positions of TiO2 nanorods and BiOCl nanosheets are figured out by the following formula [44]: 1 EVB E e E g 2
(4)
where EVB and Eg mean the VB edge potential and the band gap energy of the semiconductor, respectively. Ee is the energy of free electrons on the hydrogen scale (ca. 4.5 eV), and χ is the electronegativity of the semiconductor, which is defined as the arithmetic mean of the atomic electron affinity and the first ionization energy. Based on the value of EVB and Eg, the potential of the conduction band edge (ECB) can be calculated by ECB = EVB - Eg [45]. Since the Eg of pure BiOCl nanosheets and the rutile TiO2 nanorod arrays have been calculated to be 3.22 and 3.04 eV, their relative band positions (ECB/EVB) could be estimated to be 0.22/3.44 eV and −0.12/2.92 eV (vs. NHE) via the above formula , respectively. Based on the band gap structure of immobilized BCTO composite and the analysis of reactive species above, the photocatalytic mechanism of immobilized BCTO composite under the illumination of visible light is proposed and summarized in Scheme 2. Owing to the potentials for VB and CB of BiOCl more positive than the corresponding bands of TiO2, there will form a staggered band gap structure in the immobilized BCTO composite, which is favorable for the transfer of photogenerated charge carriers [46]. Under the irradiation, the photoinduced electrons of TiO2 nanorods can be excited and transfer from the VB to CB, then injected into the CB of BiOCl nanosheets, while the photoinduced holes will simultaneously migrate from the VB of BiOCl nanosheets and reversely move to the VB of TiO2 nanorods, which can inhibit the recombination of electron-hole pairs and make the charge separation more efficient. Additionally, according to reactive species analysis, the h+ , as well as the·O2- generated from the capture of
electron by molecular oxygen in water is the main active species, what means that if photoinduced electron-hole pairs can be separated more efficiently, it will make a contribution to increasing the amount of h+ and ·O2-and resulting in the improved photocatalytic activity. In order to prove the speculation above, the photoluminescence spectra, transient photocurrent response and electrochemical impedance spectroscopy were experimentally performed. Fig. 8 displays the PL emission spectrum of TiO2 nanorods, BiOCl nanosheets and BCTO-3 with an excitation wavelength of 360 nm. As present in Fig. 8, the peak of BiOCl nanosheets with high intensity ranges from 417 nm to 443 nm, and the main peak of TiO2 nanorods centred at 468 nm (2.63 eV) can be assigned to the oxygen vacancies. As presented in the inset, the PL spectrum of BCTO-3 composite shows a similar trend to that of pure TiO2 nanorods, while the intensity of the former is obviously much lower. As it’s well known, the intensity of PL emission spectrum correlates with the recombination rate of the charge carriers, and a lower intensity implies a lower electron-hole pairs recombination rate. According to the PL result, the immobilized BCTO-3 composite has the lowest recombination of photogenerated electron-hole pairs due to its lowest emission intensity, indicating the efficient migration of charge carries between TiO2 nanorods and BiOCl nanosheets. Fig. 9a exhibits the transient photocurrent response of rutile TiO2 nanorods, BiOCl nanosheets and BCTO-3 with several 10s on-off cycles under intermittent visible light illumination. As present in Fig. 9a, the photocurrent density will significantly increase with the light on and straightway fall to the primary value as long as the light off, indicating the key factor of illumination to generate photocurrent on photoanode in the system. In comparison with pure TiO2 nanorods and BiOCl nanosheets, the immobilized BCTO-3 composite exhibits a significantly increasing photocurrent density, which is about five times higher than that of rutile TiO2 arrays. Generally, the higher photocurrent the sample holds, the more effective separation of electron-hole pairs it will gain, which evidences that the immobilized BCTO-3 composite holds the most efficient separation due to its strongest photocurrent. The EIS measurements of rutile TiO2 nanorods, BiOCl nanosheets and immobilized BCTO-3 composite were experimentally conducted under visible light irradiation in 0.10 M Na2SO4 solution with the frequency from 100 kHz to 0.01 Hz, and the results were presented in Fig. 9b in
the form of Nyquist plot. According to Fig. 9b, BCTO-3 holds the smallest diameter of the Nyquist circle in contrast with pure TiO2 nanorods and BiOCl nanosheets, indicating not only its significant decrease of resistance and capacitance but also its fast interfacial charge transfer process and efficient separation of photogenerated charge carries [47]. Based on the analysis of PL, transient photocurrent response and EIS, the immobilized BCTO-3 composite exhibits the best performance, indicating its optimal photocatalytic activity. And this is obviously in good consonance with the photodegradation result of RhB solution and significantly proves the speculation above.
4. Conclusion In summary, we developed a facile impregnation method to graft the sheet-like BiOCl at the upright position of as-prepared rutile TiO2 nanorod arrays by using the BiCl3 as Bi3+ source, and form an immobilized photocatalyst. The as-prepared immobilized BCTO composites, especially the BCTO-3 exhibited optimized photocatalytic activity toward degradation of RhB in contrast with pure BiOCl nanosheets and rutile TiO2 arrays under visible light irradiation, which could be mainly ascribed to the match of energy level between BiOCl nanosheets and TiO2 nanorods. More attractively, the degradation efficiency of BCTO-3 can still reach 91.7% after eight times photodegradation cycle experiments, which shows the high stability and excellent durability of BCTO-3. In summary, this work could provide a new insight into broadening the application of TiO2 nanorods and BiOCl nanosheets, and developing new preparation route of the immobilized photocatalysts with enhanced photocatalytic activity.
Acknowledgements This work was supported by National Key Project in Control and Management of Polluted Water Bodies (No. 2013ZX07105-005).
References [1] F. Raziq, C. Li, M. Humayun, Y. Qu, A. Zada, H. Yu, L. Jing, Synthesis of TiO2/g-C3N4 nanocomposites as efficient photocatalysts dependent on the enhanced photogenerated charge separation, Materials Research Bulletin, 70 (2015) 494-499. [2] Y. Ma, Z. Chen, D. Qu, J. Shi, Synthesis of chemically bonded BiOCl@Bi2WO6 microspheres with
exposed (020) Bi2WO6 facets and their enhanced photocatalytic activities under visible light irradiation, Applied Surface Science, 361 (2016) 63-71. [3] Z. Ni, Y. Sun, Y. Zhang, F. Dong, Fabrication, modification and application of (BiO)2CO3-based photocatalysts: A review, Applied Surface Science, 365 (2016) 314-335. [4] Y. Chen, D.D. Dionysiou, TiO2 photocatalytic films on stainless steel: The role of Degussa P-25 in modified sol–gel methods, Applied Catalysis B: Environmental, 62 (2006) 255-264. [5] B. Erjavec, P. Hudoklin, K. Perc, T. Tišler, M.S. Dolenc, A. Pintar, Glass fiber-supported TiO2 photocatalyst: Efficient mineralization and removal of toxicity/estrogenicity of bisphenol A and its analogs, Applied Catalysis B: Environmental, 183 (2016) 149-158. [6] A.Y. Shan, T.I.M. Ghazi, S.A. Rashid, Immobilisation of titanium dioxide onto supporting materials in heterogeneous photocatalysis: A review, Applied Catalysis A: General, 389 (2010) 1-8. [7] X. Li, J. Yu, M. Jaroniec, Hierarchical photocatalysts, Chemical Society reviews, 45 (2016) 2603-2636. [8] J. Wen, X. Li, W. Liu, Y. Fang, J. Xie, Y. Xu, Photocatalysis fundamentals and surface modification of TiO2 nanomaterials, Chin. J. Catal., 36 (2015) 2049-2070. [9] Y. Zhang, S. Liu, Z. Xiu, Q. Lu, H. Sun, G. Liu, TiO2/BiOI heterostructured nanofibers: electrospinning–solvothermal two-step synthesis and visible-light photocatalytic performance investigation, Journal of Nanoparticle Research, 16 (2014). [10] J. Low, B. Cheng, J. Yu, Surface modification and enhanced photocatalytic CO2 reduction performance of TiO2: a review, Applied Surface Science, 392 (2017) 658-686. [11] H. Zhang, X. Liu, Y. Li, Q. Sun, Y. Wang, B.J. Wood, P. Liu, D. Yang, H. Zhao, Vertically aligned nanorod-like rutile TiO2 single crystal nanowire bundles with superior electron transport and photoelectrocatalytic properties, J. Mater. Chem., 22 (2012) 2465-2472. [12] Z. Chai, J. Gu, P. Qiang, X. Yu, W. Mai, Facile conversion of rutile titanium dioxide nanowires to nanotubes for enhancing the performance of dye-sensitized solar cells, CrystEngComm, 17 (2015) 1115-1120. [13] J. Wang, W.-D. Zhang, Modification of TiO2 nanorod arrays by graphite-like C3N4 with high visible light photoelectrochemical activity, Electrochimica Acta, 71 (2012) 10-16. [14] Z.-Q. Liu, P.-Y. Kuang, R.-B. Wei, N. Li, Y.-B. Chen, Y.-Z. Su, BiOBr nanoplate-wrapped ZnO nanorod arrays for high performance photoelectrocatalytic application, RSC Adv., 6 (2016) 16122-16130. [15] Y. Zhao, X. Huang, X. Tan, T. Yu, X. Li, L. Yang, S. Wang, Fabrication of BiOBr nanosheets@TiO2 nanobelts p–n junction photocatalysts for enhanced visible-light activity, Applied Surface Science, 365 (2016) 209-217. [16] L. Gomathi Devi, R. Kavitha, A review on plasmonic metal–TiO2 composite for generation, trapping, storing and dynamic vectorial transfer of photogenerated electrons across the Schottky junction in a photocatalytic system, Applied Surface Science, 360 (2016) 601-622. [17] F. Boxberg, N. Sondergaard, H.Q. Xu, Elastic and piezoelectric properties of zincblende and wurtzite crystalline nanowire heterostructures, Adv Mater, 24 (2012) 4692-4706. [18] J. Jiang, K. Zhao, X. Xiao, L. Zhang, Synthesis and facet-dependent photoreactivity of BiOCl single-crystalline nanosheets, Journal of the American Chemical Society, 134 (2012) 4473-4476. [19] C. Cao, L. Xiao, C. Chen, Q. Cao, Synthesis of novel Cu2O/BiOCl heterojunction nanocomposites and their enhanced photocatalytic activity under visible light, Applied Surface Science, 357 (2015) 1171-1179.
[20] R. He, J. Zhang, J. Yu, S. Cao, Room-temperature synthesis of BiOI with tailorable (001) facets and enhanced photocatalytic activity, Journal of colloid and interface science, 478 (2016) 201-208. [21] L. Ye, Y. Su, X. Jin, H. Xie, C. Zhang, Recent advances in BiOX (X = Cl, Br and I) photocatalysts: synthesis, modification, facet effects and mechanisms, Environmental Science: Nano, 1 (2014) 90. [22] R.a. He, S. Cao, P. Zhou, J. Yu, Recent advances in visible light Bi-based photocatalysts, Chin. J. Catal., 35 (2014) 989-1007. [23] L. Ye, J. Liu, C. Gong, L. Tian, T. Peng, L. Zan, Two Different Roles of Metallic Ag on Ag/AgX/BiOX (X = Cl, Br) Visible Light Photocatalysts: Surface Plasmon Resonance and Z-Scheme Bridge, ACS Catalysis, 2 (2012) 1677-1683. [24] B.N. Pal, I. Robel, A. Mohite, R. Laocharoensuk, D.J. Werder, V.I. Klimov, High-Sensitivity p-n Junction Photodiodes Based on PbS Nanocrystal Quantum Dots, Advanced Functional Materials, 22 (2012) 1741-1748. [25] C. Cao, C. Hu, W. Shen, S. Wang, J. Wang, Y. Tian, Fabrication of a novel heterostructure of Co3O4-modified TiO2 nanorod arrays and its enhanced photoelectrochemical property, Journal Of Alloys And Compounds, 550 (2013) 137-143. [26] H. Tong, S. Ouyang, Y. Bi, N. Umezawa, M. Oshikiri, J. Ye, Nano-photocatalytic Materials: Possibilities and Challenges, Advanced Materials, 24 (2012) 229-251. [27] J. Cao, J. Xing, Y. Zhang, H. Tong, Y. Bi, T. Kako, M. Takeguchi, J. Ye, Photoelectrochemical Properties of Nanomultiple CaFe2O4/ZnFe2O4 pn Junction Photoelectrodes, Langmuir: the ACS journal of surfaces and colloids, 29 (2013) 3116-3124. [28] Y. Cai, P. Wang, Y. Ye, J. Liu, Z. Tian, Y. Liu, C. Liang, Grafting BiOCl nanosheets onto TiO2 tubular arrays to form a hierarchical structure with improved photocatalytic performance, RSC Advances, 3 (2013) 19064. [29] G. Dai, J. Yu, G. Liu, Synthesis and Enhanced Visible-Light Photoelectrocatalytic Activity of p−n Junction BiOI/TiO2 Nanotube Arrays, The Journal of Physical Chemistry C, 115 (2011) 7339-7346. [30] C. Liao, Z. Ma, G. Dong, J. Qiu, BiOI nanosheets decorated TiO2 nanofiber: Tailoring water purification performance of photocatalyst in structural and photo-responsivity aspects, Applied Surface Science, 314 (2014) 481-489. [31] Y. Li, Q. Wang, B. Liu, J. Zhang, The {001} facets-dependent superior photocatalytic activities of BiOCl nanosheets under visible light irradiation, Applied Surface Science, 349 (2015) 957-969. [32] K. Zhang, C. Liu, F. Huang, C. Zheng, W. Wang, Study of the electronic structure and photocatalytic activity of the BiOCl photocatalyst, Applied Catalysis B: Environmental, 68 (2006) 125-129. [33] Z. Ai, W. Ho, S. Lee, L. Zhang, Efficient Photocatalytic Removal of NO in Indoor Air with Hierarchical Bismuth Oxybromide Nanoplate Microspheres under Visible Light, Environmental Science & Technology, 43 (2009) 4143-4150. [34] X.J. Wang, W.Y. Yang, F.T. Li, J. Zhao, R.H. Liu, S.J. Liu, B. Li, Construction of amorphous TiO2/BiOBr heterojunctions via facets coupling for enhanced photocatalytic activity, Journal of hazardous materials, 292 (2015) 126-136. [35] C. Zhang, S. Liu, T. Shi, Z. Tang, Improved model-based infrared reflectrometry for measuring deep trench structures, Journal of the Optical Society of America a-Optics Image Science And Vision, 26 (2009) 2327-2335. [36] H.-P. Jiao, X. Yu, Z.-Q. Liu, P.-Y. Kuang, Y.-M. Zhang, One-pot synthesis of heterostructured Bi2S3/BiOBr microspheres with highly efficient visible light photocatalytic performance, RSC Adv., 5
(2015) 16239-16249. [37] C. Wang, X. Zhang, Y. Zhang, Y. Jia, B. Yuan, J. Yang, P. Sun, Y. Liu, Morphologically-tunable TiO2 nanorod film with high energy facets: green synthesis, growth mechanism and photocatalytic activity, Nanoscale, 4 (2012) 5023. [38] Y. Liang, C. Guo, S. Cao, Y. Tian, Q. Lui, A High Quality BiOCl Film with Petal-Like Hierarchical Structures and Its Visible-Light Photocatalytic Property, Journal of Nanoscience and Nanotechnology, 13 (2013) 919-923. [39] Y. Wang, Y.Z. Zheng, S. Lu, X. Tao, Y. Che, J.F. Chen, Visible-light-responsive TiO2-coated ZnO:I nanorod array films with enhanced photoelectrochemical and photocatalytic performance, ACS applied materials & interfaces, 7 (2015) 6093-6101. [40] X. Zhao, Y. Wu, W. Yao, Y. Zhu, Photoelectrochemical properties of thin Bi2WO6 films, Thin Solid Films, 515 (2007) 4753-4757. [41] P. Ji, J. Zhang, F. Chen, M. Anpo, Study of adsorption and degradation of acid orange 7 on the surface of CeO2 under visible light irradiation, Applied Catalysis B: Environmental, 85 (2009) 148-154. [42] L. Ye, K. Deng, F. Xu, L. Tian, T. Peng, L. Zan, Increasing visible-light absorption for photocatalysis with black BiOCl, Physical chemistry chemical physics : PCCP, 14 (2012) 82-85. [43] S.A. Khan, N. Singh, K. Saleem, Synthesis, characterization and in vitro antibacterial activity of thiourea and urea derivatives of steroids, European journal of medicinal chemistry, 43 (2008) 2272-2277. [44] D. Sun, J. Li, L. He, B. Zhao, T. Wang, R. Li, S. Yin, Z. Feng, T. Sato, Facile solvothermal synthesis of BiOCl–TiO2heterostructures with enhanced photocatalytic activity, CrystEngComm, 16 (2014) 7564. [45] F. Duo, Y. Wang, X. Mao, X. Zhang, Y. Wang, C. Fan, A BiPO4/BiOCl heterojunction photocatalyst with enhanced electron-hole separation and excellent photocatalytic performance, Applied Surface Science, 340 (2015) 35-42. [46] H. Cheng, B. Huang, Y. Dai, Engineering BiOX (X = Cl, Br, I) nanostructures for highly efficient photocatalytic applications, Nanoscale, 6 (2014) 2009-2026. [47] J. Di, J. Xia, Y. Ge, L. Xu, H. Xu, M. He, Q. Zhang, H. Li, Reactable ionic liquid-assisted rapid synthesis of BiOI hollow microspheres at room temperature with enhanced photocatalytic activity, J. Mater. Chem. A, 2 (2014) 15864-15874.
Fig. 1. XRD patterns of a: the FTO; b: TiO2; c: BiOCl; d: BCTO-1; e: BCTO-2; f: BCTO-3; g: BCTO-4.
Fig. 2. FE-SEM images of (a) TiO2 nanorod arrays; (b) cross-section view of the TiO2 nanorod arrays; (c) BCTO-1; (d) BCTO-2; (e) BCTO-3; (f) BCTO-4, and the inset in (a) is the top view of TiO2 nanorods at high magnifications.
Fig. 3. (a-b) TEM and HRTEM images of the BCTO-3; (c-f) EDS mapping distribution of Ti, O, Bi and Cl taken on a detailed region of the BCTO-3.
Fig. 4. XPS spectra of the immobilized BCTO-3 composite: (a) survey spectra; (b) Bi 4f; (c) Ti 2p; (d) Cl 2p; (e) O 1s.
Fig. 5. (a) UV-vis diffused reflectance spectra of the as-prepared samples; (b) the plot of (αhυ)2 versus hυ for rutile TiO2 ,and the inset in (b) is the plot of (αhυ)1/2 versus hυ for BiOCl.
Fig. 6. (a) absorption changes of RhB solution in the presence of the immobilized BCTO-3composite; (b) comparative experiments for the degradation of RhB with all the as-prepared samples under visible light; (c) kinetic fit for the degradation of RhB with TiO2, BiOCl and BCTO-3, and reaction rate constant k obtained from linear fitting; (d) recycling tests on immobilized BCTO-3composite for the degradation of RhB under visible light.
Fig. 7. Trapping experiment of active species during the photocatalytic oxidation of RhB.
Fig. 8. The PL spectrum of pure TiO2, BiOCl and BCTO-3, and the inset is the amplification of PL spectrum of TiO2 and BCTO-3.
Fig. 9. (a) Transient photocurrent responses of pure TiO2 nanorods, BiOCl nanosheets and immobilized BCTO-3 composite in 0.10 mol/L Na2SO4 aqueous solution under visible light irradiation (λ > 420 nm); and (b) the corresponding of electrochemical impedance spectroscopy with the frequency from 100 kHz to 0.01 Hz.
Scheme 1. Illustration of the preparation of immobilized BCTO composites on FTO.
Scheme 2. Schematic diagram of the electron-hole pairs separation and the proposed reaction mechanism over the immobilized BCTO composite under visible light.
Table 1 The photodegradation of RhB on various immobilized photocatalysts. Immobilized photocatalyst
RhB concentration (mg/L)
Volume (mL)
Degradation efficiency
Time (h)
Light source
Reference
TiO2 BiOCl ZnO:I/TiO2 Bi2WO6 BCTO-3
10.0 1.0 2.4 5.0 2.5
10 100 100 100 100
50% 52.5% 97% 20% 99.1%
5 8 6 8 3
UV Visible Visible Visible Visible
37 38 39 40 This work