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BiVO4 nanoheterostructures with enhanced visible-light photocatalytic activity
Journal of Alloys and Compounds 688 (2016) 891e898
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Graphitic carbon nitride quantum dots loaded on leaf-like InVO4/ BiVO4 nanoheterostructures with enhanced visible-light photocatalytic activity Xue Lin a, Da Xu a, Jia Zheng a, Minshan Song b, Guangbo Che a, *, Yushuang Wang a, Yang Yang a, Chang Liu a, Lina Zhao a, Limin Chang c, ** a Key Laboratory of Preparation and Applications of Environmental Friendly Materials (Jilin Normal University), Ministry of Education, Changchun, 130103, PR China b School of Mathematics and Physics, Jiangsu University of Science and Technology, Zhenjiang, PR China c College of Chemistry, Jilin Normal University, Siping, 136000, PR China
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a b s t r a c t
Article history: Received 12 April 2016 Received in revised form 27 May 2016 Accepted 26 July 2016 Available online 27 July 2016
The visible-light photocatalytic performance of g-C3N4 quantum dots (CNQDs)/InVO4/BiVO4 nanoheterostructures was investigated. The as-prepared nanoheterostructures included CNQDs assembling on the surface of leaf-like InVO4/BiVO4 crystals. The nanocomposite displayed much higher visible-light photocatalytic efficiency than that of pure CNQDs, BiVO4, InVO4, CNQDs/InVO4, CNQDs/BiVO4 or InVO4/ BiVO4. It was elucidated that the excellent photocatalytic activity of CNQDs/InVO4/BiVO4 can be ascribed to the extended absorption in the visible light region resulting from CNQDs loading, and the efficient separation of photogenerated electrons and holes through CNQDs/InVO4/BiVO4 heterostructure. The quenching effects of different scavengers demonstrated that O 2 played the major role in the Rh B degradation. © 2016 Elsevier B.V. All rights reserved.
Keywords: g-C3N4 quantum dots InVO4/BiVO4 Heterostructure Photocatalysis Visible light
1. Introduction The development of semiconductor photocatalysts with excellent catalytic performance and good stability under visible light is an important issue in photocatalysis broad, and is also significant in solving present energy and environment problems [1e4]. Recently, a great deal of research focused on the synthesis of quantum dots (QDs)-based composites has been reported [5]. The QDs can be employed to utilize hot electrons or to generate multiple charge carriers with a single high-energy photon. The design of QDs-based composites is an effective strategy to improve the photodegradation efficiency of photocatalysts under visible light irradiation. At the same time, the QDs based materials additionally render a rare opportunity to rationalize the mechanistic pathways of the catalytic reaction by resorting to theoretical models with fewer atoms and thus higher accuracies than those of the bulk
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (G. Che). http://dx.doi.org/10.1016/j.jallcom.2016.07.275 0925-8388/© 2016 Elsevier B.V. All rights reserved.
materials, eventually leading to an atomic-level insight into the process of the catalysis reaction to study the mutual relationship between the structure of the materials and their corresponding properties [5]. For example, as an analogue, graphene has been fabricated into narrow nanoribbons (GNRs) and quantum dots (GQDs) with sizes less than 10 nm. The strong quantum confinement and edge effects when the sizes are down to 10 nm render graphene excellent optical properties, which have been used to design GQD-based photocatalysts, such as GQD/TiO2, GQD/g-C3N4, GQD/CdSe, and GQD/InVO4/BiVO4 [6e9]. Graphitic carbon nitride (CN) as a newly developed metal free semiconductor has been broadly applied in various fields, such as photocatalytic hydrogen evolution, pollutants degradation, oxygen reduction reaction, and bioimaging [10e12]. Recent years, many works presented that g-C3N4 as an efficient visible-light photocatalyst, due to its particular superiority for environment application [13,14]. However, there are still some limitations in the bulk g-C3N4 photocatalytic system, including its low specific surface area and poor quantum yield [15]. In this context, it would be interesting to develop g-C3N4 quantum dots (CNQDs) based materials for photocatalysis application. Recently, there are a few
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reports concentrated on CNQDs based heterostructures, for example CNQDs/g-C3N4, CNQDs/graphene, CNQDs/BiPO4, and CNQDs/TiO2 [15e18]. Thanks to the heterostructures formed between the semiconductors, the recombination of photo-generated electrons and holes can be effectively decreased, thus increasing the quantum efficiency of catalytic system. However, until now, there is no report focusing on the assembly of CNQDs with leaf-like InVO4/BiVO4 nanostructures. Furthermore, no attention has been paid to the photocatalysis mechanism of this material, which has remained unclear to date. Herein, we report a successful attempt at the fabrication of CNQDs/InVO4/BiVO4 hierarchical nanostructures via a facile in situ precipitation method, and the photocatalytic activity of the nanostructures was investigated by measuring the degradation of rhodamine B (Rh B) under visible light (l > 420 nm). Furthermore, the photocatalytic mechanism of CNQDs/InVO4/BiVO4 was investigated through reactive species trapping experiments. Finally, the stability of the CNQDs/InVO4/BiVO4 photocatalyst was also examined. 2. Experimental
wrapping CNQDs on InVO4/BiVO4 particles, 6 mL of the CNQDs solution (1 mg mL1) was added and the reaction temperature was kept at 70 C for 60 min. The resulting suspension was filtered, washed with deionized water three times and dried at 60 C for 24 h in a vacuum oven. The theoretical wrapping amount of CNQDs was 2 wt%. For comparison, CNQDs/InVO4 and CNQDs/BiVO4 were prepared under the same conditions. 2.2. Characterization of photocatalysts The crystal structures of the samples were characterized by Xray diffraction (XRD) on a Rigaku (Japan) D/max 2500 X-ray diffractometer (Cu Ka radiation, l ¼ 0.154 18 nm). The morphologies and structure details of the as-synthesized samples were detected by using field emission scanning microscopy (SEM, JSM-6510) and transmission electron microscopy (TEM, JEM-2100F). X-ray photoelectron spectroscopy (XPS) analysis was performed with an ESCALa-b220i-XL electron spectrometer (VGScientific, England) using 300 W Al Ka radiation. The photoluminescence (PL) spectra of the photocatalysts were obtained by a F4500 (Hitachi, Japan) photoluminescence detector with an excitation wavelength of
2.1. Preparation of photocatalysts 2.1.1. Preparation of CNQDs Bulk g-C3N4 was prepared by heating melamine for 4 h to 550 C and kept at this temperature for another 4 h in air [15]. CNQDs was prepared by using the Yu’ method [15]: First, 1 g of bulk g-C3N4 was treated in the mixture of concentrated sulfuric acid (H2SO4) (20 mL) and nitric acid (HNO3) (20 mL) for about 2 h at room temperature. The mixture was then diluted with deionized water (1 L) and washed for several times. Second, 50 mg of the obtained solid was dispersed in 30 mL concentrated NH3$H2O, and then the mixed suspension was transferred into a 20 mL Teflon-lined stainless steel autoclave and heated at 200 C for 24 h. Upon cooling to room temperature, the precipitate was washed with water for several times to remove the adsorbed NH3 molecules. Third, 10 mg of the synthesized solid was dispersed in 100 mL water, and then ultrasound for about 6 h. The as-obtained aqueous suspension was then centrifuged at about 7000 rpm, and dialyzed in a dialysis bag to remove large-sized nanoparticles. 2.1.2. Preparation of InVO4/BiVO4 nanostructures Leaf-like InVO4/BiVO4 crystals were synthesized through a simple hydrothermal method [19]. In a typical procedure, In(NO3)3$4.5H2O (0.0764 g) and Bi(NO)3$5H2O (0.388 g) were dissolved in 3 mL of HNO3 (2 mol L1) followed by vigorous stirring for 3 h. At the same time, NH4VO3 solid (0.117 g) was added in 12 mL of deionized water under vigorous stirring for a uniform suspension. Then, the solution was added to the suspension and afterwards stirred for additional 3 h at room temperature. After carefully adjusting the pH value of 3 using 25 wt% NH3$H2O solution, the mixed suspension was transferred into a 20 mL Teflon-lined steel autoclave, which was heated in an oven at 150 C for 24 h. At last, the obtained InVO4/BiVO4 composite was collected and washed with ethanol and distilled water several times, and dried at 100 C for 2 h. The theoretical value of InVO4 loading amount was 20 wt%. The pure InVO4 sample and pure BiVO4 sample were fabricated under the same conditions. 2.1.3. Preparation of CNQDs/InVO4/BiVO4 photocatalyst The as-prepared InVO4/BiVO4 particles (300 mg) were mixed with 200 mL of deionized water by ultrasonication for 30 min. Then, 1.0 mL of 5% polyethylene glycol (PEG) 2000 solution was added and the dispersion was stirred for another 10 min. For
Fig. 1. XRD patterns of as-prepared samples.
Fig. 2. FTIR spectra of the as-synthesized CNQDs and CNQDs/InVO4/BiVO4.
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325 nm. The UVevis diffuse reflectance spectra (DRS) were recorded using a scan UVevis spectrophotometer (UV-2550).
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collected and analyzed after centrifugation. The Rh B concentration was analyzed by a UV-2550 spectrometer to record intensity of the maximum band at 552 nm in the UVevis absorption spectra.
2.3. Photocatalytic activities studies 2.4. Active species trapping experiments The photocatalytic properties of the as-prepared samples were evaluated using Rh B as a model compound. The Rh B is a very stable compound, which has been used widely as a representative reaction for examining the performance of numerous visible light active catalysts [15,16]. In experiments, the Rh B solution (0.01 mmol L1, 100 mL) containing 0.05 g of photocatalyst was mixed in a pyrex reaction glass. A 300 W Xe lamp (l > 420 nm) was employed to provide visible light irradiation. A 420 nm cut-off filter was inserted between the lamp and the sample to filter out UV light (l < 420 nm). Prior to visible light illumination, the suspension was strongly stirred in the dark for 40 min. Then the solution was exposed to visible light irradiation under magnetic stirring. At given time intervals, 4 mL of the suspension was periodically
For detecting the active species during photocatalytic reactivity, some sacrificial agents, such as 2-propanol (IPA), ammonium oxalate (AO) and 1,4-benzoquinone (BQ) were employed as the hydroxyl radical (OH) scavenger, hole (hþ) scavenger and superoxide radical (O 2 ) scavenger, respectively [20e22]. The method was similar to the former photocatalytic activity test with the addition of 1 mmol of quencher in the presence of Rh B. 3. Results and discussion Fig. 1 shows the XRD patterns of as-prepared samples. All the diffraction peaks in the XRD pattern of BiVO4 can be indexed to
Fig. 3. XPS spectra of the as-obtained CNQDs/InVO4/BiVO4 sample: (a) C 1s spectrum, (b) N 1s spectrum, (c) Bi 4f spectrum, (d) In 3d spectrum, (e) V 2p spectrum.
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BiVO4 phase (JCPDS No. 14e0688). For InVO4, the diffraction peaks can be readily indexed into specific crystal planes of InVO4 phase, which is in good agreement with the standard data (JCPDS No. 48e0898). When coupling the two compounds, the main characteristic diffraction peaks of BiVO4 and InVO4 did not change obviously. The CNQDs/InVO4/BiVO4 sample displays a coexistence of BiVO4 phase (JCPDS No. 14e0688) and InVO4 phase (JCPDS No. 48e0898), revealing that the mixture of BiVO4 and InVO4 is the main existing form of as-synthesized CNQDs/InVO4/BiVO4 sample. In addition, there is no any diffraction peaks of CNQDs can be detected for CNQDs/InVO4/BiVO4, CNQDs/BiVO4, and CNQDs/InVO4 samples, which may be resulted from small crystal size or low percentage of CNQDs. Herein, the successful loading of CNQDs was illustrated by FT-IR and XPS. The FT-IR spectra of the as-synthesized samples are displayed in Fig. 2. In the spectrum of CNQDs, several absorption bands at 1800 - 890 cm 1 are ascribed to either C]N or CeN stretching vibrations [23]. The broad adsorption band centered at 3200 cm1
is attributed to the NeeH stretching vibration [23]. The main characteristic peaks of CNQDs are observed in the spectrum of CNQDs/InVO4/BiVO4 sample, showing the successful loading of CNQDs. Fig. 3 reveals the XPS spectra of the as-prepared CNQDs/ InVO4/BiVO4 composite. C 1s XPS spectrum (Fig. 3a) clearly displays two characteristic peaks located at 284.8 eV and 288.6 eV, respectively. The former is assigned to the adventitious hydrocarbon from the XPS instrument itself and defect-containing sp2-hybridized carbon atoms present in graphitic domains, whereas the latter one is associated with CeNeC coordination [24]. Fig. 3b reveals the N 1s XPS spectrum, it can be seen that two binding energies could be separated including triazine rings (CeNeC, 397.4 eV) and tertiary nitrogen (N(C)3, 399.8 eV) [23]. XPS signals of Bi 4f are detected at binding energies at 162.9 eV (Bi 4f7/2) and 157.7 eV (Bi 4f5/2) (Fig. 3c), corresponding to Bi3þ [24]. The In 3d peaks are observed at about 444.4 eV and 452.3 eV (Fig. 3d), ascribed to In3þ [25]. The V 2p peak is centered at around 530.4 eV (Fig. 3e), which is attributed to V5þ [25].
Scheme 1. Schematic illustration for the preparation of CNQDs/InVO4/BiVO4 nanoheterostructure.
Fig. 4. SEM images of as-synthesized BiVO4 (a), BiVO4/InVO4 (b), CNQDs/InVO4/BiVO4 (c), and the corresponding EDS elemental mapping images of CNQDs/InVO4/BiVO4 (deg).
X. Lin et al. / Journal of Alloys and Compounds 688 (2016) 891e898
The CNQDs/InVO4/BiVO4 nanoheterostructure was synthesized via two main processes, as illustrated in Scheme 1. The first step was taken to prepare CNQDs. On this basis, the second step was taken to wrap CNQDs on the surface of leaf-like InVO4/BiVO4 crystals. Fig. 4 displays the SEM images of the as-fabricated BiVO4, InVO4/BiVO4, and CNQDs/InVO4/BiVO4 samples. All the samples show a leaf-like morphology, revealing that low amount InVO4 or CNQDs loading didn't have significant influence on the morphology of BiVO4 crystals. The elemental mapping of the CNQDs/InVO4/ BiVO4 sample is displayed in Fig. 4deg. Maps of BieM, In-L, NeK, and CeK have the same shape and location, showing the existence of BiVO4, InVO4, and CNQDs in the as-fabricated CNQDs/InVO4/ BiVO4 composite. This gives solid evidence of the formation of CNQDs/InVO4/BiVO4 heterostructure. In order to further ascertain the nanoheterostructure of CNQDs, InVO4, and BiVO4, the assynthesized CNQDs/InVO4/BiVO4 sample was tested by transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM), as shown in Fig. 5. Fig. 5a reveals the leaf-like structure of CNQDs/InVO4/BiVO4 sample. From Fig. 5b, it can be observed that g-C3N4 quantum dots were loaded on the surface of leaf-like InVO4/BiVO4 crystals. From Fig. 5c, by measuring the lattice fringes, the resolved interplanar distances are determined to be around 0.309 and 0.337 nm, corresponding to the (121) plane of BiVO4 and the (220) plane of InVO4, respectively. The absorbance properties of the as-prepared samples were studied through UVevis diffuse reflectance spectroscopy (DRS), as shown in Fig. 6a. The basal absorption edges of pure InVO4 and
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BiVO4 occur at wavelength 520 nm, whereas pure g-C3N4 has an absorption edge at around 450 nm. For the InVO4/BiVO4, CNQDs/ BiVO4, CNQDs/InVO4, and CNQDs/InVO4/BiVO4 samples, the curves show significant red-shift of the absorption edges. According to the plot of ahv ¼ A(hv - Eg)n/2, the band gaps (Eg) of BiVO4, InVO4, and CNQDs were estimated to be 2.40, 2.40, and 2.70 eV, respectively (Fig. 6b). Meanwhile, the Eg of CNQDs/InVO4/BiVO4 is determined to be 2.00 eV, showing that the heterostructuring of these materials lead to band gap variation. The band structure of CNQDs/InVO4/BiVO4 can be estimated according to the following empirical equations:
EVB ¼ X Ee þ 0:5Eg
(1)
ECB ¼ EVB Eg
(2)
where EVB is the valence band edge potential, ECB is the conduction band edge potential, X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms, Ee is the energy of free electrons on the hydrogen scale (about 4.5 eV). Thus, the EVB of BiVO4, InVO4, and CNQDs are calculated to be 2.858, 1.70 and 1.58 eV vs NHE and their corresponding ECB are 0.458, 0.70, and 1.12 eV vs NHE, respectively. The energy band structure diagram of BiVO4, InVO4, and CNQDs is thus schematically illustrated, as shown in Scheme 2. Under visible light irradiation, CNQDs, BiVO4, as well as InVO4 can be excited to produced hþ and e. Under normal case, most of
Fig. 5. TEM image (a) and HRTEM images (b,c) of the as-fabricated CNQDs/InVO4/BiVO4 sample.
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Fig. 6. (a) UVevis DRS of as-synthesized samples. (b) Plots of (ahv)2 versus photon energy (hn) for the band gap energies of BiVO4, InVO4, CNQDs, and CNQDs/InVO4/ BiVO4.
electrons-holes pairs recombine rapidly, thus pure CNQDs, BiVO4, or InVO4 has a respectively low photocatalytic activity. For the heterostructured photocatalyst, several possible pathways might be considered for the generation of photogenerated charge carriers. First, when BiVO4 and InVO4 are simultaneously excited under visible light irradiation, the photogenerated electrons in the CB of InVO4 flow into the CB of BiVO4 (electron transfer I: InVO4/BiVO4), since the CB of InVO4 is more negative than that of BiVO4 [25]. Simultaneously, the photogenerated holes in the VB of BiVO4 flow into the VB of InVO4 (hole transfer I: BiVO4/InVO4), because the VB of InVO4 is more negative than that of BiVO4. Second, the photogenerated electrons in the CB of CNQDs can transfer to the CB of InVO4 (electron transfer II: CNQDs/InVO4) or directly move to the CB of BiVO4 (electron transfer III: CNQDs/BiVO4). At the same time, there is the transfers of holes in the VB of InVO4 into the VB of CNQDs (hole transfer II: InVO4/CNQDs) and that of BiVO4 into the VB of CNQDs (hole transfer III: BiVO4/CNQDs). It is envisaged that the processes of these transfers are faster than the electron-hole recombination process between the VB and CB of BiVO4, InVO4 or CNQDs. These charge transfers would reduce the electron-hole pair recombination and prolong the life-time of charges, thus improving the photocatalytic performance. The photocatalytic activities of the as-prepared materials were investigated by comparing the photodecomposition rate of Rh B under visible light irradiation (Fig. 7a). From the catalytic experiments, the blank test demonstrates that the degradation of Rh B was extremely slow without any photocatalyst under visible light illumination. CNQDs/InVO4/BiVO4 sample was detected to be more photoactive towards Rh B solution than the pure CNQDs, BiVO4, InVO4, CNQDs/BiVO4, CNQDs/InVO4, and InVO4/BiVO4 samples. In the presence of CNQDs/InVO4/BiVO4, almost 100% of the Rh B molecules were decomposed within 40 min visible light irradiation. This is because the efficient heterostructure interface between two or three components can restrain the recombination of photoinduced charges effectively [26,27]. The as-prepared CNQDs/InVO4/ BiVO4 composite photocatalyst displayed much higher phtocatalytic activity than InVO4/BiVO4 photocatalysts in our reported works [9,19], showing that CNQDs loading is beneficial to the improvement of photocatalytic performance of catalysts. Fig. 7b
Scheme 2. Schematic diagram of the separation and transfer of photogenerated charges in the ternary heterostructure under visible light irradiation.
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Fig. 7. (a) Photodegradation efficiencies of Rh B as a function of irradiation time for different photocatalysts. (b) ln(C0/C) versus time curves of Rh B degradation over BiVO4, InVO4, CNQDs, InVO4/BiVO4, and CNQDs/InVO4/BiVO4 samples. (c) Room temperature PL spectra of as-synthesized samples under the excitation wavelength of 325 nm. (d) Cycling runs for the photocatalytic degradation of Rh B over CNQDs/InVO4/BiVO4 sample under visible light irradiation. (e) Trapping experiment of active species during the photocatalytic degradation of Rh B over CNQDs/InVO4/BiVO4 sample under visible light irradiation.
reveals that the photocatalytic degradation of Rh B on different catalysts fits pseudo-first-order kinetics, ln(C0/C) ¼ kt, where C is the concentration of the Rh B at time t, C0 is the initial concentration of the Rh B solution, and the slope k is the apparent reaction rate constant. The results imply that k value for CNQDs/InVO4/BiVO4 sample (0.120 min1) is higher than those of pure BiVO4 (0.056 min1), InVO4 (0.026 min1), CNQDs (0.061 min1), and InVO4/ BiVO4 composite (0.074 min1). These results indicate that the formation of the heterojunction structure of CNQDs/InVO4/BiVO4 could enhance the photocatalytic efficiency of single CNQDs, BiVO4, InVO4, and InVO4/BiVO4 composite. A comparison of the PL spectra of the as-synthesized photocatalysts under an excitation wavelength of 325 nm is shown in Fig. 7c. It can be seen that the PL peak intensity of CNQDs/InVO4/ BiVO4 decreases obviously. This result shows that the heterostructure effect of CNQDs, InVO4, and BiVO4 contributes to the
effective electronehole pair separation, which may be a reason for the CNQDs/InVO4/BiVO4 sample showing enhanced photocatalytic efficiency under visible light irradiation. In addition, the CNQDs coating can improve the visible light absorption efficiency (Fig. 6a), which is also beneficial for the nanoheterostructure to be able to photolyze Rh B, thus enhancing the photocatalytic performance of the CNQDs/InVO4/BiVO4 composite. The stability of photocatalysts is another important issue for their practical application. Therefore, the cycling runs for the degradation of Rh B on the CNQDs/InVO4/BiVO4 sample were further performed to evaluate the photocatalytic stability, as displayed in Fig. 7d. The stability testing results revealed that the degradation rate of the CNQDs/InVO4/BiVO4 photocatalyst showed slightly decrease after the continuous five-run repeated irradiation within 280 min. The above mentioned results demonstrate that the as-obtained CNQDs/InVO4/BiVO4 nanoheterostructure can be
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employed as a promising photocatalyst with excellent activity and desirable stability under visible light irradiation. In order to investigate the photocatalytic mechanism of the CNQDs/InVO4/BiVO4 composite, the effect of scavengers on the degradation of Rh B was tested in the photocatalytic oxidation process, as revealed in Fig. 7e. As can be seen in Fig. 7e, under visible-light irradiation of the CNQDs/InVO4/BiVO4 photocatalyst, the photodegradation rate of Rh B had slight decrease after the addition of hydroxyl radical scavenger IPA and hole scavenger AO, indicating that hydroxyl radicals and holes were involved but not main radical species. In contrast, the photocatalytic degradation of Rh B was significantly repressed in presence of superoxide radical scavenger BQ. According to the above results, it can be concluded that active species O 2 radicals were the main oxygen active species for CNQDs/InVO4/BiVO4 photocatalyst in the Rh B solution under visible light illumination. 4. Conclusions In conclusion, for the first time we demonstrated the synthesis of CNQDs/InVO4/BiVO4 composite photocatalyst via an easily accessible route. The as-prepared g-C3N4 QDs were well assembled on the surface of leaf-like InVO4/BiVO4 crystals. Due to the favorable heterostructures, the CNQDs/InVO4/BiVO4 composite exhibited significantly enhanced photocatalytic performance under visible light irradiation. The decolorization rate of Rh B could reach 100% for the CNQDs/InVO4/BiVO4 within 40 min visible light illumination. The present findings revealed that the CNQDs/InVO4/ BiVO4 composite architecture could be employed as one of promising visible light-induced photocatalysts. Acknowledgment This work was supported by National Natural Science Fundation of China (21407059, 21576112, 51404108), the Science Development Project of Jilin Province (20130521019JH), the Science and Technology Research Project of the Department of Education of Jilin Province (2015220) and the Science Development Project of Jiangsu Province (BK20140527).
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Graphitic carbon nitride quantum dots loaded on leaf-like InVO4/ BiVO4 nanoheterostructures with enhanced visible-light photocatalytic activity Xue Lin a, Da Xu a, Jia Zheng a, Minshan Song b, Guangbo Che a, *, Yushuang Wang a, Yang Yang a, Chang Liu a, Lina Zhao a, Limin Chang c, ** a Key Laboratory of Preparation and Applications of Environmental Friendly Materials (Jilin Normal University), Ministry of Education, Changchun, 130103, PR China b School of Mathematics and Physics, Jiangsu University of Science and Technology, Zhenjiang, PR China c College of Chemistry, Jilin Normal University, Siping, 136000, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 12 April 2016 Received in revised form 27 May 2016 Accepted 26 July 2016 Available online 27 July 2016
The visible-light photocatalytic performance of g-C3N4 quantum dots (CNQDs)/InVO4/BiVO4 nanoheterostructures was investigated. The as-prepared nanoheterostructures included CNQDs assembling on the surface of leaf-like InVO4/BiVO4 crystals. The nanocomposite displayed much higher visible-light photocatalytic efficiency than that of pure CNQDs, BiVO4, InVO4, CNQDs/InVO4, CNQDs/BiVO4 or InVO4/ BiVO4. It was elucidated that the excellent photocatalytic activity of CNQDs/InVO4/BiVO4 can be ascribed to the extended absorption in the visible light region resulting from CNQDs loading, and the efficient separation of photogenerated electrons and holes through CNQDs/InVO4/BiVO4 heterostructure. The quenching effects of different scavengers demonstrated that O 2 played the major role in the Rh B degradation. © 2016 Elsevier B.V. All rights reserved.
Keywords: g-C3N4 quantum dots InVO4/BiVO4 Heterostructure Photocatalysis Visible light
1. Introduction The development of semiconductor photocatalysts with excellent catalytic performance and good stability under visible light is an important issue in photocatalysis broad, and is also significant in solving present energy and environment problems [1e4]. Recently, a great deal of research focused on the synthesis of quantum dots (QDs)-based composites has been reported [5]. The QDs can be employed to utilize hot electrons or to generate multiple charge carriers with a single high-energy photon. The design of QDs-based composites is an effective strategy to improve the photodegradation efficiency of photocatalysts under visible light irradiation. At the same time, the QDs based materials additionally render a rare opportunity to rationalize the mechanistic pathways of the catalytic reaction by resorting to theoretical models with fewer atoms and thus higher accuracies than those of the bulk
* Corresponding author. ** Corresponding author. E-mail address: [email protected] (G. Che). http://dx.doi.org/10.1016/j.jallcom.2016.07.275 0925-8388/© 2016 Elsevier B.V. All rights reserved.
materials, eventually leading to an atomic-level insight into the process of the catalysis reaction to study the mutual relationship between the structure of the materials and their corresponding properties [5]. For example, as an analogue, graphene has been fabricated into narrow nanoribbons (GNRs) and quantum dots (GQDs) with sizes less than 10 nm. The strong quantum confinement and edge effects when the sizes are down to 10 nm render graphene excellent optical properties, which have been used to design GQD-based photocatalysts, such as GQD/TiO2, GQD/g-C3N4, GQD/CdSe, and GQD/InVO4/BiVO4 [6e9]. Graphitic carbon nitride (CN) as a newly developed metal free semiconductor has been broadly applied in various fields, such as photocatalytic hydrogen evolution, pollutants degradation, oxygen reduction reaction, and bioimaging [10e12]. Recent years, many works presented that g-C3N4 as an efficient visible-light photocatalyst, due to its particular superiority for environment application [13,14]. However, there are still some limitations in the bulk g-C3N4 photocatalytic system, including its low specific surface area and poor quantum yield [15]. In this context, it would be interesting to develop g-C3N4 quantum dots (CNQDs) based materials for photocatalysis application. Recently, there are a few
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reports concentrated on CNQDs based heterostructures, for example CNQDs/g-C3N4, CNQDs/graphene, CNQDs/BiPO4, and CNQDs/TiO2 [15e18]. Thanks to the heterostructures formed between the semiconductors, the recombination of photo-generated electrons and holes can be effectively decreased, thus increasing the quantum efficiency of catalytic system. However, until now, there is no report focusing on the assembly of CNQDs with leaf-like InVO4/BiVO4 nanostructures. Furthermore, no attention has been paid to the photocatalysis mechanism of this material, which has remained unclear to date. Herein, we report a successful attempt at the fabrication of CNQDs/InVO4/BiVO4 hierarchical nanostructures via a facile in situ precipitation method, and the photocatalytic activity of the nanostructures was investigated by measuring the degradation of rhodamine B (Rh B) under visible light (l > 420 nm). Furthermore, the photocatalytic mechanism of CNQDs/InVO4/BiVO4 was investigated through reactive species trapping experiments. Finally, the stability of the CNQDs/InVO4/BiVO4 photocatalyst was also examined. 2. Experimental
wrapping CNQDs on InVO4/BiVO4 particles, 6 mL of the CNQDs solution (1 mg mL1) was added and the reaction temperature was kept at 70 C for 60 min. The resulting suspension was filtered, washed with deionized water three times and dried at 60 C for 24 h in a vacuum oven. The theoretical wrapping amount of CNQDs was 2 wt%. For comparison, CNQDs/InVO4 and CNQDs/BiVO4 were prepared under the same conditions. 2.2. Characterization of photocatalysts The crystal structures of the samples were characterized by Xray diffraction (XRD) on a Rigaku (Japan) D/max 2500 X-ray diffractometer (Cu Ka radiation, l ¼ 0.154 18 nm). The morphologies and structure details of the as-synthesized samples were detected by using field emission scanning microscopy (SEM, JSM-6510) and transmission electron microscopy (TEM, JEM-2100F). X-ray photoelectron spectroscopy (XPS) analysis was performed with an ESCALa-b220i-XL electron spectrometer (VGScientific, England) using 300 W Al Ka radiation. The photoluminescence (PL) spectra of the photocatalysts were obtained by a F4500 (Hitachi, Japan) photoluminescence detector with an excitation wavelength of
2.1. Preparation of photocatalysts 2.1.1. Preparation of CNQDs Bulk g-C3N4 was prepared by heating melamine for 4 h to 550 C and kept at this temperature for another 4 h in air [15]. CNQDs was prepared by using the Yu’ method [15]: First, 1 g of bulk g-C3N4 was treated in the mixture of concentrated sulfuric acid (H2SO4) (20 mL) and nitric acid (HNO3) (20 mL) for about 2 h at room temperature. The mixture was then diluted with deionized water (1 L) and washed for several times. Second, 50 mg of the obtained solid was dispersed in 30 mL concentrated NH3$H2O, and then the mixed suspension was transferred into a 20 mL Teflon-lined stainless steel autoclave and heated at 200 C for 24 h. Upon cooling to room temperature, the precipitate was washed with water for several times to remove the adsorbed NH3 molecules. Third, 10 mg of the synthesized solid was dispersed in 100 mL water, and then ultrasound for about 6 h. The as-obtained aqueous suspension was then centrifuged at about 7000 rpm, and dialyzed in a dialysis bag to remove large-sized nanoparticles. 2.1.2. Preparation of InVO4/BiVO4 nanostructures Leaf-like InVO4/BiVO4 crystals were synthesized through a simple hydrothermal method [19]. In a typical procedure, In(NO3)3$4.5H2O (0.0764 g) and Bi(NO)3$5H2O (0.388 g) were dissolved in 3 mL of HNO3 (2 mol L1) followed by vigorous stirring for 3 h. At the same time, NH4VO3 solid (0.117 g) was added in 12 mL of deionized water under vigorous stirring for a uniform suspension. Then, the solution was added to the suspension and afterwards stirred for additional 3 h at room temperature. After carefully adjusting the pH value of 3 using 25 wt% NH3$H2O solution, the mixed suspension was transferred into a 20 mL Teflon-lined steel autoclave, which was heated in an oven at 150 C for 24 h. At last, the obtained InVO4/BiVO4 composite was collected and washed with ethanol and distilled water several times, and dried at 100 C for 2 h. The theoretical value of InVO4 loading amount was 20 wt%. The pure InVO4 sample and pure BiVO4 sample were fabricated under the same conditions. 2.1.3. Preparation of CNQDs/InVO4/BiVO4 photocatalyst The as-prepared InVO4/BiVO4 particles (300 mg) were mixed with 200 mL of deionized water by ultrasonication for 30 min. Then, 1.0 mL of 5% polyethylene glycol (PEG) 2000 solution was added and the dispersion was stirred for another 10 min. For
Fig. 1. XRD patterns of as-prepared samples.
Fig. 2. FTIR spectra of the as-synthesized CNQDs and CNQDs/InVO4/BiVO4.
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325 nm. The UVevis diffuse reflectance spectra (DRS) were recorded using a scan UVevis spectrophotometer (UV-2550).
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collected and analyzed after centrifugation. The Rh B concentration was analyzed by a UV-2550 spectrometer to record intensity of the maximum band at 552 nm in the UVevis absorption spectra.
2.3. Photocatalytic activities studies 2.4. Active species trapping experiments The photocatalytic properties of the as-prepared samples were evaluated using Rh B as a model compound. The Rh B is a very stable compound, which has been used widely as a representative reaction for examining the performance of numerous visible light active catalysts [15,16]. In experiments, the Rh B solution (0.01 mmol L1, 100 mL) containing 0.05 g of photocatalyst was mixed in a pyrex reaction glass. A 300 W Xe lamp (l > 420 nm) was employed to provide visible light irradiation. A 420 nm cut-off filter was inserted between the lamp and the sample to filter out UV light (l < 420 nm). Prior to visible light illumination, the suspension was strongly stirred in the dark for 40 min. Then the solution was exposed to visible light irradiation under magnetic stirring. At given time intervals, 4 mL of the suspension was periodically
For detecting the active species during photocatalytic reactivity, some sacrificial agents, such as 2-propanol (IPA), ammonium oxalate (AO) and 1,4-benzoquinone (BQ) were employed as the hydroxyl radical (OH) scavenger, hole (hþ) scavenger and superoxide radical (O 2 ) scavenger, respectively [20e22]. The method was similar to the former photocatalytic activity test with the addition of 1 mmol of quencher in the presence of Rh B. 3. Results and discussion Fig. 1 shows the XRD patterns of as-prepared samples. All the diffraction peaks in the XRD pattern of BiVO4 can be indexed to
Fig. 3. XPS spectra of the as-obtained CNQDs/InVO4/BiVO4 sample: (a) C 1s spectrum, (b) N 1s spectrum, (c) Bi 4f spectrum, (d) In 3d spectrum, (e) V 2p spectrum.
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BiVO4 phase (JCPDS No. 14e0688). For InVO4, the diffraction peaks can be readily indexed into specific crystal planes of InVO4 phase, which is in good agreement with the standard data (JCPDS No. 48e0898). When coupling the two compounds, the main characteristic diffraction peaks of BiVO4 and InVO4 did not change obviously. The CNQDs/InVO4/BiVO4 sample displays a coexistence of BiVO4 phase (JCPDS No. 14e0688) and InVO4 phase (JCPDS No. 48e0898), revealing that the mixture of BiVO4 and InVO4 is the main existing form of as-synthesized CNQDs/InVO4/BiVO4 sample. In addition, there is no any diffraction peaks of CNQDs can be detected for CNQDs/InVO4/BiVO4, CNQDs/BiVO4, and CNQDs/InVO4 samples, which may be resulted from small crystal size or low percentage of CNQDs. Herein, the successful loading of CNQDs was illustrated by FT-IR and XPS. The FT-IR spectra of the as-synthesized samples are displayed in Fig. 2. In the spectrum of CNQDs, several absorption bands at 1800 - 890 cm 1 are ascribed to either C]N or CeN stretching vibrations [23]. The broad adsorption band centered at 3200 cm1
is attributed to the NeeH stretching vibration [23]. The main characteristic peaks of CNQDs are observed in the spectrum of CNQDs/InVO4/BiVO4 sample, showing the successful loading of CNQDs. Fig. 3 reveals the XPS spectra of the as-prepared CNQDs/ InVO4/BiVO4 composite. C 1s XPS spectrum (Fig. 3a) clearly displays two characteristic peaks located at 284.8 eV and 288.6 eV, respectively. The former is assigned to the adventitious hydrocarbon from the XPS instrument itself and defect-containing sp2-hybridized carbon atoms present in graphitic domains, whereas the latter one is associated with CeNeC coordination [24]. Fig. 3b reveals the N 1s XPS spectrum, it can be seen that two binding energies could be separated including triazine rings (CeNeC, 397.4 eV) and tertiary nitrogen (N(C)3, 399.8 eV) [23]. XPS signals of Bi 4f are detected at binding energies at 162.9 eV (Bi 4f7/2) and 157.7 eV (Bi 4f5/2) (Fig. 3c), corresponding to Bi3þ [24]. The In 3d peaks are observed at about 444.4 eV and 452.3 eV (Fig. 3d), ascribed to In3þ [25]. The V 2p peak is centered at around 530.4 eV (Fig. 3e), which is attributed to V5þ [25].
Scheme 1. Schematic illustration for the preparation of CNQDs/InVO4/BiVO4 nanoheterostructure.
Fig. 4. SEM images of as-synthesized BiVO4 (a), BiVO4/InVO4 (b), CNQDs/InVO4/BiVO4 (c), and the corresponding EDS elemental mapping images of CNQDs/InVO4/BiVO4 (deg).
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The CNQDs/InVO4/BiVO4 nanoheterostructure was synthesized via two main processes, as illustrated in Scheme 1. The first step was taken to prepare CNQDs. On this basis, the second step was taken to wrap CNQDs on the surface of leaf-like InVO4/BiVO4 crystals. Fig. 4 displays the SEM images of the as-fabricated BiVO4, InVO4/BiVO4, and CNQDs/InVO4/BiVO4 samples. All the samples show a leaf-like morphology, revealing that low amount InVO4 or CNQDs loading didn't have significant influence on the morphology of BiVO4 crystals. The elemental mapping of the CNQDs/InVO4/ BiVO4 sample is displayed in Fig. 4deg. Maps of BieM, In-L, NeK, and CeK have the same shape and location, showing the existence of BiVO4, InVO4, and CNQDs in the as-fabricated CNQDs/InVO4/ BiVO4 composite. This gives solid evidence of the formation of CNQDs/InVO4/BiVO4 heterostructure. In order to further ascertain the nanoheterostructure of CNQDs, InVO4, and BiVO4, the assynthesized CNQDs/InVO4/BiVO4 sample was tested by transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM), as shown in Fig. 5. Fig. 5a reveals the leaf-like structure of CNQDs/InVO4/BiVO4 sample. From Fig. 5b, it can be observed that g-C3N4 quantum dots were loaded on the surface of leaf-like InVO4/BiVO4 crystals. From Fig. 5c, by measuring the lattice fringes, the resolved interplanar distances are determined to be around 0.309 and 0.337 nm, corresponding to the (121) plane of BiVO4 and the (220) plane of InVO4, respectively. The absorbance properties of the as-prepared samples were studied through UVevis diffuse reflectance spectroscopy (DRS), as shown in Fig. 6a. The basal absorption edges of pure InVO4 and
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BiVO4 occur at wavelength 520 nm, whereas pure g-C3N4 has an absorption edge at around 450 nm. For the InVO4/BiVO4, CNQDs/ BiVO4, CNQDs/InVO4, and CNQDs/InVO4/BiVO4 samples, the curves show significant red-shift of the absorption edges. According to the plot of ahv ¼ A(hv - Eg)n/2, the band gaps (Eg) of BiVO4, InVO4, and CNQDs were estimated to be 2.40, 2.40, and 2.70 eV, respectively (Fig. 6b). Meanwhile, the Eg of CNQDs/InVO4/BiVO4 is determined to be 2.00 eV, showing that the heterostructuring of these materials lead to band gap variation. The band structure of CNQDs/InVO4/BiVO4 can be estimated according to the following empirical equations:
EVB ¼ X Ee þ 0:5Eg
(1)
ECB ¼ EVB Eg
(2)
where EVB is the valence band edge potential, ECB is the conduction band edge potential, X is the electronegativity of the semiconductor, which is the geometric mean of the electronegativity of the constituent atoms, Ee is the energy of free electrons on the hydrogen scale (about 4.5 eV). Thus, the EVB of BiVO4, InVO4, and CNQDs are calculated to be 2.858, 1.70 and 1.58 eV vs NHE and their corresponding ECB are 0.458, 0.70, and 1.12 eV vs NHE, respectively. The energy band structure diagram of BiVO4, InVO4, and CNQDs is thus schematically illustrated, as shown in Scheme 2. Under visible light irradiation, CNQDs, BiVO4, as well as InVO4 can be excited to produced hþ and e. Under normal case, most of
Fig. 5. TEM image (a) and HRTEM images (b,c) of the as-fabricated CNQDs/InVO4/BiVO4 sample.
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Fig. 6. (a) UVevis DRS of as-synthesized samples. (b) Plots of (ahv)2 versus photon energy (hn) for the band gap energies of BiVO4, InVO4, CNQDs, and CNQDs/InVO4/ BiVO4.
electrons-holes pairs recombine rapidly, thus pure CNQDs, BiVO4, or InVO4 has a respectively low photocatalytic activity. For the heterostructured photocatalyst, several possible pathways might be considered for the generation of photogenerated charge carriers. First, when BiVO4 and InVO4 are simultaneously excited under visible light irradiation, the photogenerated electrons in the CB of InVO4 flow into the CB of BiVO4 (electron transfer I: InVO4/BiVO4), since the CB of InVO4 is more negative than that of BiVO4 [25]. Simultaneously, the photogenerated holes in the VB of BiVO4 flow into the VB of InVO4 (hole transfer I: BiVO4/InVO4), because the VB of InVO4 is more negative than that of BiVO4. Second, the photogenerated electrons in the CB of CNQDs can transfer to the CB of InVO4 (electron transfer II: CNQDs/InVO4) or directly move to the CB of BiVO4 (electron transfer III: CNQDs/BiVO4). At the same time, there is the transfers of holes in the VB of InVO4 into the VB of CNQDs (hole transfer II: InVO4/CNQDs) and that of BiVO4 into the VB of CNQDs (hole transfer III: BiVO4/CNQDs). It is envisaged that the processes of these transfers are faster than the electron-hole recombination process between the VB and CB of BiVO4, InVO4 or CNQDs. These charge transfers would reduce the electron-hole pair recombination and prolong the life-time of charges, thus improving the photocatalytic performance. The photocatalytic activities of the as-prepared materials were investigated by comparing the photodecomposition rate of Rh B under visible light irradiation (Fig. 7a). From the catalytic experiments, the blank test demonstrates that the degradation of Rh B was extremely slow without any photocatalyst under visible light illumination. CNQDs/InVO4/BiVO4 sample was detected to be more photoactive towards Rh B solution than the pure CNQDs, BiVO4, InVO4, CNQDs/BiVO4, CNQDs/InVO4, and InVO4/BiVO4 samples. In the presence of CNQDs/InVO4/BiVO4, almost 100% of the Rh B molecules were decomposed within 40 min visible light irradiation. This is because the efficient heterostructure interface between two or three components can restrain the recombination of photoinduced charges effectively [26,27]. The as-prepared CNQDs/InVO4/ BiVO4 composite photocatalyst displayed much higher phtocatalytic activity than InVO4/BiVO4 photocatalysts in our reported works [9,19], showing that CNQDs loading is beneficial to the improvement of photocatalytic performance of catalysts. Fig. 7b
Scheme 2. Schematic diagram of the separation and transfer of photogenerated charges in the ternary heterostructure under visible light irradiation.
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Fig. 7. (a) Photodegradation efficiencies of Rh B as a function of irradiation time for different photocatalysts. (b) ln(C0/C) versus time curves of Rh B degradation over BiVO4, InVO4, CNQDs, InVO4/BiVO4, and CNQDs/InVO4/BiVO4 samples. (c) Room temperature PL spectra of as-synthesized samples under the excitation wavelength of 325 nm. (d) Cycling runs for the photocatalytic degradation of Rh B over CNQDs/InVO4/BiVO4 sample under visible light irradiation. (e) Trapping experiment of active species during the photocatalytic degradation of Rh B over CNQDs/InVO4/BiVO4 sample under visible light irradiation.
reveals that the photocatalytic degradation of Rh B on different catalysts fits pseudo-first-order kinetics, ln(C0/C) ¼ kt, where C is the concentration of the Rh B at time t, C0 is the initial concentration of the Rh B solution, and the slope k is the apparent reaction rate constant. The results imply that k value for CNQDs/InVO4/BiVO4 sample (0.120 min1) is higher than those of pure BiVO4 (0.056 min1), InVO4 (0.026 min1), CNQDs (0.061 min1), and InVO4/ BiVO4 composite (0.074 min1). These results indicate that the formation of the heterojunction structure of CNQDs/InVO4/BiVO4 could enhance the photocatalytic efficiency of single CNQDs, BiVO4, InVO4, and InVO4/BiVO4 composite. A comparison of the PL spectra of the as-synthesized photocatalysts under an excitation wavelength of 325 nm is shown in Fig. 7c. It can be seen that the PL peak intensity of CNQDs/InVO4/ BiVO4 decreases obviously. This result shows that the heterostructure effect of CNQDs, InVO4, and BiVO4 contributes to the
effective electronehole pair separation, which may be a reason for the CNQDs/InVO4/BiVO4 sample showing enhanced photocatalytic efficiency under visible light irradiation. In addition, the CNQDs coating can improve the visible light absorption efficiency (Fig. 6a), which is also beneficial for the nanoheterostructure to be able to photolyze Rh B, thus enhancing the photocatalytic performance of the CNQDs/InVO4/BiVO4 composite. The stability of photocatalysts is another important issue for their practical application. Therefore, the cycling runs for the degradation of Rh B on the CNQDs/InVO4/BiVO4 sample were further performed to evaluate the photocatalytic stability, as displayed in Fig. 7d. The stability testing results revealed that the degradation rate of the CNQDs/InVO4/BiVO4 photocatalyst showed slightly decrease after the continuous five-run repeated irradiation within 280 min. The above mentioned results demonstrate that the as-obtained CNQDs/InVO4/BiVO4 nanoheterostructure can be
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employed as a promising photocatalyst with excellent activity and desirable stability under visible light irradiation. In order to investigate the photocatalytic mechanism of the CNQDs/InVO4/BiVO4 composite, the effect of scavengers on the degradation of Rh B was tested in the photocatalytic oxidation process, as revealed in Fig. 7e. As can be seen in Fig. 7e, under visible-light irradiation of the CNQDs/InVO4/BiVO4 photocatalyst, the photodegradation rate of Rh B had slight decrease after the addition of hydroxyl radical scavenger IPA and hole scavenger AO, indicating that hydroxyl radicals and holes were involved but not main radical species. In contrast, the photocatalytic degradation of Rh B was significantly repressed in presence of superoxide radical scavenger BQ. According to the above results, it can be concluded that active species O 2 radicals were the main oxygen active species for CNQDs/InVO4/BiVO4 photocatalyst in the Rh B solution under visible light illumination. 4. Conclusions In conclusion, for the first time we demonstrated the synthesis of CNQDs/InVO4/BiVO4 composite photocatalyst via an easily accessible route. The as-prepared g-C3N4 QDs were well assembled on the surface of leaf-like InVO4/BiVO4 crystals. Due to the favorable heterostructures, the CNQDs/InVO4/BiVO4 composite exhibited significantly enhanced photocatalytic performance under visible light irradiation. The decolorization rate of Rh B could reach 100% for the CNQDs/InVO4/BiVO4 within 40 min visible light illumination. The present findings revealed that the CNQDs/InVO4/ BiVO4 composite architecture could be employed as one of promising visible light-induced photocatalysts. Acknowledgment This work was supported by National Natural Science Fundation of China (21407059, 21576112, 51404108), the Science Development Project of Jilin Province (20130521019JH), the Science and Technology Research Project of the Department of Education of Jilin Province (2015220) and the Science Development Project of Jiangsu Province (BK20140527).
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