Yuming
Jin‡
a,
Xiaofeng
Shen‡
a,
Zixiao
Liu
b,
Zhaojie
Wang
b,
Bo
Zhu
b,
Pengfei
Xu
a,
Li
Luo
a and
Lisha
Zhang
*a
aState Key Laboratory for Modification of Chemical Fibers and Polymer Materials, State Environmental Protection Engineering Center for Pollution Treatment and Control in Textile Industry, College of Environmental Science and Engineering, Donghua University, Shanghai, 201620, China. E-mail: lszhang@dhu.edu.cn; Fax: +86-21-67792522; Tel: +86-21-67792548
bCollege of Materials Science and Engineering, Donghua University, Shanghai 201620, China
First published on 24th November 2017
Development of efficient and recyclable photocatalysts has drawn more and more attention. Herein, we have prepared a kind of core–shell fiber-shaped NiTiO3–Bi2MoO6 heterojunction as an efficient and easily recyclable visible-light-driven photocatalyst. NiTiO3 nanofibers have been obtained by an electrospinning–calcination method; and in situ growth of Bi2MoO6 is conducted on the surface of NiTiO3 by a simple solvothermal method. NiTiO3–Bi2MoO6 heterojunctions are composed of NiTiO3 nanofibers with a diameter of about 150 nm whose surfaces are decorated by Bi2MoO6 nanoneedles with a diameter of ∼8 nm and length of ∼35 nm. Under visible light irradiation, NiTiO3–Bi2MoO6 heterojunctions exhibit significantly high photocatalytic activities compared with NiTiO3, Bi2MoO6 and a mechanical mixture (22 wt% NiTiO3 + 78 wt% Bi2MoO6) when rhodamine B (RhB) or 4-chlorophenol (4-CP) are selected as model pollutants. For example, NiTiO3–Bi2MoO6 heterojunctions achieve the highest photodegradation efficiency of RhB (97.2%) compared to 21.9% by NiTiO3, 40.3% by Bi2MoO6 and 45.0% by the mechanical mixture. The enhanced photocatalytic activities can be mainly attributed to the efficient separation of photogenerated electron–hole pairs. During the photocatalytic degradation of RhB, superoxide radical ions (˙O2−) and photogenerated holes (h+) are detected as the main active species. Moreover, NiTiO3–Bi2MoO6 heterojunctions can be recycled by sedimentation and still possess excellent stability. Therefore, NiTiO3–Bi2MoO6 heterojunctions have great potential to act as efficient and recyclable photocatalysts in practical applications of environmental purification.
It is well known that the construction of heterojunctions between different materials (TiO2 and C3N4)20,21 is an efficient way to improve the photocatalytic performance. To improve the photocatalytic activity of NiTiO3, some NiTiO3-based heterojunctions have been developed, such as NiTiO3–Ag3VO4 nanoparticles,22 NiTiO3–CdS nanoparticles,23 mesoporous NiTiO3–C3N4,24 and NiTiO3–Fe2O3 nanofibers.25 These NiTiO3-based heterojunctions exhibit enhanced photocatalytic activity compared to pure NiTiO3 photocatalysts. For example, Yang Qu et al. have reported that NiTiO3–CdS heterojunctions can degrade 90% of Cr(VI) after 2 h under visible light irradiation, which is higher than that (70%) by pure NiTiO3 nanorods.23 However, these powder-shaped photocatalysts also suffer from several drawbacks, such as complex preparation process, recycling difficulty and unsatisfactory photocatalytic activity.
Recently, bismuth-based photocatalysts have been reported to have excellent photocatalytic activities under visible light irradiation.26–28 Among the bismuth-based semiconductors, Bi2MoO6 with a band gap of about 2.66 eV has been prepared as a promising VLD photocatalyst to degrade organic pollutants due to its broad visible-light range and long photogenerated electron-pair lifetime.11,28–30 Meanwhile, Bi2MoO6 has been widely used to form heterojunctions with other materials to enhance the photocatalytic efficiency, such as Bi2MoO6–sulfide (Bi2MoO6–CdS14), Bi2MoO6–metal oxide (Bi2MoO6–TiO231) and Bi2MoO6–carbon (Bi2MoO6–graphene26). These heterojunctions show a significant increase in photocatalytic activity for degrading organic pollutants. Bi2MoO6 may be a potential candidate as a co-catalyst for NiTiO3 due to its well-matched band gaps, which can suppress the recombination of photoinduced charge carries. To our knowledge, there is no report on the synthesis of NiTiO3–Bi2MoO6 heterojunctions.
The electrospinning method has been recognized as an efficient technique to prepare nonwoven cloth and nanofibers which are easily recyclable. By using electrospinning-assisted methods, our group has prepared several efficient and easily recyclable photocatalysts, including Ta3N5–Pt nonwoven cloth,32 Fe2O3–AgBr nonwoven cloth33 and Ta3N5/Bi2MoO6 core–shell nanofibers.34 However, these are just preliminary works to fabricate composite photocatalysts by the electrospinning method. It is of great importance to further develop other inexpensive, efficient and recyclable photocatalysts. Herein, we developed novel NiTiO3–Bi2MoO6 core–shell fiber-shaped heterojunctions as an efficient and easily reusable photocatalyst. NiTiO3–Bi2MoO6 core–shell fiber-shaped heterojunctions were obtained through an electrospinning–calcination–solvothermal method. Importantly, NiTiO3–Bi2MoO6 heterojunctions exhibited higher photocatalytic activity in degrading organic pollutants (such as RhB and 4-CP) under visible light irradiation than pure NiTiO3 or Bi2MoO6. Furthermore, the mechanism of the advantageous photocatalytic activity was also proposed.
The second step was to grow Bi2MoO6 on the surface of NiTiO3 nanofibers by a solvothermal method to form NiTiO3–Bi2MoO6 heterojunctions. The SEM images (Fig. 3a and b) reveal that NiTiO3–Bi2MoO6 heterojunctions are also composed of nanofibers. Obviously, one can find that there are many small nanoneedles on the surface of the NiTiO3 nanofibers. The diameter of the nanofibers increases to ∼185 nm, due to the coating of the nanoneedles. Further information about the NiTiO3–Bi2MoO6 heterojunctions can be obtained from TEM images (Fig. 3c and d). The TEM images confirm that the sample is composed of nanofibers whose surface is decorated with plenty of nanoneedles. These nanoneedles have a diameter of ∼8 nm and length of ∼35 nm. The high-resolution TEM image (Fig. 3d) taken from the nanofiber demonstrates that the lattice spacings are 0.25 nm and 0.315 nm, which are in accordance with the values of the NiTiO3 (110) plane and Bi2MoO6 (113) plane, respectively. The above results confirm the formation of NiTiO3–Bi2MoO6 core–shell fibers.
The XRD patterns of the NiTiO3–Bi2MoO6 heterojunctions, NiTiO3 nanofibers and Bi2MoO6 nanoparticles were further studied for investigating the phase of the as-prepared photocatalysts (Fig. 4). The diffraction peaks of NiTiO3 at 24.1°, 33.1°, 35.7°, 40.9°, 49.4° and 54.0° are assigned to the (012), (104), (110), (113), (024), (116) and (018) crystal planes (JCPDS Card No. 33-0960) respectively. In addition, the diffraction peaks of Bi2MoO6 at 28.3°, 32.6°, 46.7° and 55.5° are assigned to the (131), (002), (202) and (331) crystal planes (JCPDS Card No. 76-2388) respectively. It is obvious that the XRD patterns of the NiTiO3–Bi2MoO6 heterojunctions can be indexed to a mixture of NiTiO3 and Bi2MoO6, and no trace of any impurity peak was observed in the XRD pattern of NiTiO3–Bi2MoO6 which indicated the high purity of these heterojunctions.
The nitrogen adsorption–desorption isotherms of NiTiO3 nanofibers, Bi2MoO6 nanoparticles and NiTiO3–Bi2MoO6 heterojunction samples were also investigated (Fig. 5). The Brunauer–Emmett–Teller (BET) surface area of the NiTiO3 nanofibers is calculated to be 3.6 m2 g−1, while that of the Bi2MoO6 nanoparticles is 34.6 m2 g−1. Interestingly, after the decoration of Bi2MoO6 nanoneedles on the surface of the NiTiO3 nanofibers, the resulting NiTiO3–Bi2MoO6 heterojunction exhibits a greatly enhanced BET surface area (∼37.0 m2 g−1) compared with the NiTiO3 nanofibers, which should result from the introduction of Bi2MoO6 nanoneedles with large surface area. Furthermore, we also calculated the pore size distribution from the desorption branches, which reveals the existence of nanopores. The NiTiO3 nanofibers, Bi2MoO6 nanoparticles and NiTiO3–Bi2MoO6 heterojunction samples have a broad pore size distribution centered at about 31 nm, 22 nm and 22 nm respectively. The presence of nanopores may serve as transport paths for organic pollutant molecules.
Fig. 5 Nitrogen adsorption–desorption isotherms of NiTiO3, Bi2MoO6 and NiTiO3–Bi2MoO6 heterojunctions. |
The optical absorption of NiTiO3–Bi2MoO6 heterojunctions as well as NiTiO3 fibers and Bi2MoO6 was measured by a UV-vis-NIR spectrometer (Fig. 6). The pure NiTiO3 nanofibers exhibit an intense absorption in the visible light region with an absorption edge around 545 nm, which is similar to a previous report.18 Two absorption bands at about 450 nm and 510 nm are observed due to the crystal field splitting of NiTiO3 with the 3d8 band associated with Ni2+ splitting into two sub-bands, which can be called Ni2+ → Ti4+ charge-transfer bands.35 In addition, the pure Bi2MoO6 exhibits an absorption edge around 460 nm. After the decoration of Bi2MoO6 on the surface of NiTiO3, NiTiO3–Bi2MoO6 heterojunctions display a broad phtotoabsorption from ultraviolet to visible light with an edge around 540 nm. This fact reveals that NiTiO3–Bi2MoO6 heterojunctions have a broad photoabsorption region, resulting in great potential as an excellent VLD photocatalyst.
To further demonstrate that the photocatalytic performance actually results from the excitation of the NiTiO3–Bi2MoO6 photocatalyst instead of a dye sensitization mechanism, colorless organic pollutant 4-CP was chosen as the example pollutant. Degradation of 4-CP was measured by high-performance liquid chromatography (Fig. 8a). NiTiO3, Bi2MoO6, and the mechanical mixture can adsorb 1.9%, 4.8% and 6% 4-CP respectively after 30 min with magnetic stirring in darkness. NiTiO3–Bi2MoO6 heterojunctions adsorb 27% 4-CP under the same conditions, resulting from their high surface area (37.0 m2 g−1) and hierarchical pores. In the subsequent photocatalytic process, the blank control indicates that no 4-CP is degraded without photocatalysts. With pure NiTiO3 or Bi2MoO6 as photocatalysts, the photocatalytic degradation efficiency of 4-CP is 13% or 28% after 120 min, respectively. The degradation efficiency of the mechanical mixture (22 wt% NiTiO3 + 78 wt% Bi2MoO6) is only 30% after 120 min. Importantly, when the NiTiO3–Bi2MoO6 heterojunction is used as the photocatalyst, 91% 4-CP can be degraded under visible light irradiation after 120 min, which is better than NiTiO3, Bi2MoO6 and even the mechanical mixture. The kinetic curves are shown in Fig. 8b. The kinetic rate constant of the NiTiO3–Bi2MoO6 heterojunctions (0.01957 min−1) is much higher than those of the NiTiO3 fibers (0.00117 min−1), Bi2MoO6 (0.00231 min−1), mechanical mixture (0.00179 min−1). Obviously, NiTiO3–Bi2MoO6 heterojunctions exhibit the best photocatalytic activities. In addition, GC/MS analysis reveals that by using NiTiO3–Bi2MoO6 photocatalysts, there are six predominant intermediate products at 30 min of the photodegradation of 4-CP (Fig. S2 and Table S1, ESI†).
Mineralization of organic pollutants is an important aspect in pollutant treatment. A typical way to determine the mineralization degree of organic pollutants is to measure the total organic carbon (TOC). The mineralization degree of RhB was studied by adding 250 mg NiTiO3–Bi2MoO6 photocatalyst in 250 mL RhB aqueous solution (50 mg L−1) with magnetic stirring under visible light irradiation. The TOC value continuously decreases with the increase of irradiation time, which indicates that RhB is steadily mineralized (Fig. 9). After 7 h of reaction, the TOC value decreases from 43.48 mg L−1 to 23.95 mg L−1, revealing that RhB mineralization reaches a high ratio of 45%. This fact shows that NiTiO3–Bi2MoO6 photocatalysts can efficiently mineralize organic pollutants under visible light irradiation.
Fig. 9 TOC removal during RhB (50 mg L−1, 250 mL) degradation in the presence of NiTiO3–Bi2MoO6 heterojunctions (250 mg) under visible light irradiation. |
To further explore the mechanism of photocatalytic degradation, it is crucial to find out the major active species responsible for the degradation of organic pollutants with NiTiO3–Bi2MoO6 heterojunctions during the photocatalytic process. Isopropyl alcohol (IPA), benzoquinone (BQ), ammonium oxalate (AO) and AgNO3 are commonly used to capture hydroxyl free radicals (˙OH), superoxide radicals (˙O2−), photogenerated holes (h+) and electrons (e−), respectively.38 By adding four different scavengers in RhB solution with NiTiO3–Bi2MoO6 heterojunctions, trapping experiments were conducted to find out the most effective species (Fig. 10a). When IPA or AgNO3 is added, the degradation efficiency of RhB remains 92.4% or 93.6%, which is close to the result (∼97.2%) without scavengers. This fact indicates that ˙OH and e− are not the major active species responsible for RhB degradation. On the contrary, with the addition of BQ or AO, the degradation efficiency of RhB decreases to 37.1% or 47.1%, indicating that the photocatalytic activities can be significantly suppressed. The kinetic rate constant decreases dramatically from 0.02592 min−1 to 0.00142 min−1 and 0.00392 min−1 respectively (Fig. 10b). These facts verify that O2− and h+ are the major active species in the photocatalytic degradation of RhB with NiTiO3–Bi2MoO6 heterojunctions.
The stability of the NiTiO3–Bi2MoO6 heterojunctions is a critical factor for practical application. Four consecutive runs of photocatalytic tests were conducted to measure the stability (Fig. 11). After each cycle, NiTiO3–Bi2MoO6 heterojunctions were separated by centrifugation from solution, washed thoroughly with deionized water and then dried for the next use. The recycling efficiency of NiTiO3–Bi2MoO6 heterojunctions is determined to be 95.1% in each cycle. After four cycles, the degradation efficiency of RhB has a very slight decrease from 97.2% at the first cycle to 93.3% at the fourth cycle (Fig. 11). This slight decrease in degradation efficiency should result from the imperfect centrifugation efficiency (95.1%) instead of the loss of photocatalytic activity. Furthermore, we investigated the chemical stability of NiTiO3–Bi2MoO6 heterojunctions by analyzing the Ni, Ti, Bi, and Mo concentration in the clear solution after the centrifugation. Inductively coupled plasma results (Table S2, ESI†) indicate that the concentrations of all metal elements (Ni, Ti, Bi, Mo) are below 0.01 μg mL−1, which suggests that NiTiO3–Bi2MoO6 heterojunctions have very low solubility in water. Therefore, these facts indicate that NiTiO3–Bi2MoO6 heterojunctions can be easily recyclable and have high stability.
The electronic structures and energy band of NiTiO3 and Bi2MoO6 were studied. On the basis of their energy band diagram, the photocatalytic process of the NiTiO3–Bi2MoO6 heterojunction system can be proposed (Fig. 12). Both NiTiO3 and Bi2MoO6 can generate not only electrons in their conduction band but also holes in their valence band under visible light irradiation. Based on the fact that the CB and VB of NiTiO3 are lower than those of Bi2MoO6, the Fermi levels of NiTiO3 and Bi2MoO6 tend to move up and down respectively when the heterojunction structure forms between NiTiO3 and Bi2MoO6.17,39 As a result, the photogenerated electrons in the CB of Bi2MoO6 tend to flow into that of NiTiO3. Meanwhile, photogenerated holes are available for the transfer from VB of NiTiO3 to that of Bi2MoO6. Thus, an equilibrium is formed (Fig. 12). Under these circumstances, electrons gathering on the CB of NiTiO3 are successfully forced to produce more O2− that can be effectively utilized to decompose organic pollutants (such as RhB and 4-CP). The holes stored in the VB of Bi2MoO6 can also oxidize organic pollutants directly. Therefore, the formation of heterojunctions between NiTiO3 and Bi2MoO6 can promote the separation of photoelectron–hole pairs, which further leads to high photocatalytic activity.
Fig. 12 Schematic diagram of electron–hole pair separation and possible reaction mechanism of NiTiO3–Bi2MoO6 heterojunctions under visible light irradiation. |
Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7nj03367b |
‡ These authors contributed equally to this work. |
This journal is © The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2018 |