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BiOBr composites: The upconversion effect of CQDs
Journal of Alloys and Compounds 685 (2016) 34e41
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Enhanced visible light photocatalytic activity and stability of CQDs/BiOBr composites: The upconversion effect of CQDs Fangfang Duo, Yawen Wang**, Caimei Fan*, Xiaochao Zhang, Yunfang Wang College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 March 2016 Received in revised form 19 May 2016 Accepted 23 May 2016 Available online 25 May 2016
CQDs/BiOBr composites with oxygen vacancies were synthesized by a simple hydrolysis method at room temperature. The obtained composites exhibited much higher photocatalytic activity and stability than pure BiOBr under visible light. The upconversion effect of CQDs on BiOBr was studied by photoluminescence (PL) spectra, transient photocurrent (PC) responses measurements, electron spin resonance (ESR) analysis, reactive species scavenger experiments, and RhB and BPA degradation under monochromatic light irradiation. The results demonstrated that the upconversion effect of CQDs could improve O 2 formation on BiOBr during photocatalytic process via more electron transfer in oxygen vacancies owing to the desirable long wavelength visible light and near infrared (NIR) light absorption property of CQDs. Moreover, the upconversion effect of CQDs could refresh the oxygen vacancies on BiOBr and therefore maintain the photocatalytic stability of BiOBr. © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Upconversion CQDs/BiOBr composites Oxygen vacancies Photocatalytic activity
1. Introduction Development of highly efficient photocatalysts for the solar energy conversion and the organic pollutants elimination has been considered to be one of the pivotal investigative tasks [1e4]. TiO2 is the most used and studied photocatalyst during the last decades because of its high activity, excellent stability, and low cost. However, the wide band gap nature of TiO2 (band gap of 3.2 eV) limits its practical applications due to its poor response to visible light accounting for about 43% of solar light [5]. Thus, many research groups have been focusing on developing novel semiconductors with improved visible light photocatalytic efficiency. Fortunately, bismuth oxyhalide BiOX (X ¼ F, Cl, Br, or I) compounds have received much attention due to their efficient photocatalytic performances in the degradation of aqueous organic pollution, arising from their unique tetragonal layered structure and indirect transition band gap [6e8]. Among these BiOX photocatalysts, BiOBr has attracted great interest owing to its relatively superior catalytic activity under visible light irradiation [9]. To further improve the photacatalytic activity of BiOBr, many strategies have been developed, such as noble metals decoration [10], ion
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Wang), [email protected] (C. Fan).
doping [11e13], coupling with other semiconductor photocatalysts [14e17], and so on. In addition, coupling of BiOBr with metal-free carbonaceous materials, such as carbon fibers [18,19], graphene [20], carbon nitride [21], and carbon quantum dots (CQDs) [22,23] can receive an expected improvement in the photocatalytic activity by reducing the recombination of the electron-hole pairs. However, CQDs, a novel class of nanocarbons with sizes below 10 nm, have been of greatly interest because of its robust chemical inertness, superior stability against photobleaching, and low cytotoxicity [24], also possess the unique photoluminescence (PL) upconversion property [25]. The PL upconversion effect can convert the long wavelength visible light and near infrared (NIR) light radiation into shorter wavelength UV and visible light emission [26], which made CQDs a kind of very promising material for bioimaging, solar cells, photocatalysis etc. applications [27,28]. Many photocatalysts, like Ag3PO4 [29], Fe2O3 [30], and Cu2O [31] applied the upconversion effect of CQDs for harvesting light to improve the photocatalytic activity. For example, a CQDs/Ag3PO4 composite exhibits enhanced photocatalytic activity, since the CQDs have an ability to harness visible light and NIR light (700e1000 nm), while the corresponding PL upconversion spectra are in the range of 300e650 nm, from which the Ag3PO4 (2.4 eV with light adsorption edge about 516 nm) would be partly excited [29]. However, the insight mechanism of the upconversion effect of CQDs on semiconductors photocatalysts still need to pursue in order to open up a new vista for efficient photocatalysis using more low energy light spectra.
http://dx.doi.org/10.1016/j.jallcom.2016.05.259 0925-8388/© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
F. Duo et al. / Journal of Alloys and Compounds 685 (2016) 34e41
It is known that the oxygen vacancies formed on the materials surface can contribute to the efficient photocatalytic activity under visible light [32e34]. First, surface OVs with their typical defect states can trap electrons or holes to inhibit the charge carriers’ recombination, and may also promote the transfer of these trapped charge carriers to the adsorbates. Second, OVs with abundant localized electrons are of particular interests for the enhanced adsorption and activation of adsorbate gas molecules. For example, OVs of BiOCl can directly activate O2 to active oxygen species such 2 2 as *O 2 , O4 and O2 [35]. As well known that OVs can be produced depending on the following formation conditions of nonaqueous system, inert gas atmosphere, or ultraviolet light (l < 400 nm) [36]. However, OVs of oxide semiconductors would gradually be healed during their interaction with molecular oxygen under visible light, which greatly inhibited the stability of the semiconductors photocatalysts. So the stability and reusability of OVs are crucial for the practical environment application. Herein, we present a simple hydrolysis method at room temperature for the fabrication of CQDs/BiOBr composites with oxygen vacancies. The CQDs/BiOBr composites showed excellent photocatalytic activity under visible light and even NIR light irradiation. More interestingly, the composites exhibited great photocatalytic stability under visible light irradiation. Experimental results suggested the upconversion effect of CQDs played important roles on the excellent photocatalytic performance for CQDs/BiOBr composites. 2. Experimental 2.1. Synthesis All of the chemicals are analytical reagents without further purification. To prepare CQDs solid, D-Glucose (3.96 g) was dissolved in 40 mL deionized water and 2.67 mL ethylenediamine was then added. The above solution was transferred into a 100 mL sealed Teflon-lined stainless-steel autoclave and heated at 180 C for 11 h. After cooled to room temperature, the resulting solution was filtered with a 0.22 mm polyethersulfone membrane to remove the larger carbon nanoparticles. The CQDs were further dialyzed in a dialysis bag (cutoff molecular weight: 1000 Da) for 2 days. At last, the powder CQDs were obtained by freeze drying. The CQDs/BiOBr composites were synthesized by a facile room temperature hydrolysis method. Typically, 2.425 g of Bi(NO3)3$5H2O was added into 20 mL ethylene glycol (EG) solution, stirring until the homogeneous solution then a certain amount of CQDs solution (15 g/L, dispersed in distilled water) was added into the above solution. After that, the mixture solution was ultrasound for 20 min. Subsequently, the 50 mL aqueous solution containing of KBr (0.595 g) and NaOH (0.200 g) was added into the above solution and stirred for 0.5 h. The resulting samples were collected and washed with deionized water and ethanol and then dried at 60 C in the oven for 4 h. The CQDs/BiOBr composites prepared by changing the amount of CQDs solution of 1, 3, and 6 mL were labeled as 1-CQDs/BiOBr, 3-CQDs/BiOBr, and 6-CQDs/BiOBr, respectively. 2.2. Characterization X-ray powder diffraction (XRD) was carried out on a D/max2500 diffractometer with Cu Ka radiation and the scanning range was 10 e80 . Fourier transform infrared spectroscopy (FT-IR) analysis was carried out on a Shimadzu 8400 spectrophotometer. The scanning electron microscope (SEM) images were recorded on a Nanosem 430 Field Emission Scanning Electron Microscope. Transmission electron microscopy (TEM) and high-resolution
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transmission electron microscopy (HRTEM) were recorded on a JEOL-2011 microscope (Japan, 200 kV). UVevis diffused reflectance spectra (DRS) of the samples were obtained for the dry-pressed film samples using a UVevis spectrophotometer (UV-3600, Shimadzu, Japan). X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo ESCALAB 250 Xi XPS system with an Al Ka (hv ¼ 1486.6 eV) 150 W, 500 mm beam spot source. The excitation, emission photoluminescence (PL) spectra of CQDs and the photoluminescence (PL) emission spectra for pure BiOBr and 3CQDs/BiOBr sample were collected on a Horiba Fluoromax-4 (France) luminescence spectrometer using a Xe lamp as the excitation source. In situ electron spin resonance (ESR) spectra were conducted on a Bruker model ESR JES-FA200 spectrometer. The g factor was obtained by taking the signal of manganese as standard. Magnetic parameters of the radicals detected were obtained from direct measurements of the magnetic field and microwave frequency. The ESR signals of radicals spin-trapped were examined by spin-trap reagent DMPO in water and methanol. The UVevis adsorption spectra of the centrifugated solutions were analyzed using a Varian Cary 50 prode UVevis spectrometer. The fluorescence (PL) quantum yield was measured by Edinburgh FLs 980 fully functional steady/transient fluorescence spectrometer.
2.3. Photocatalytic activity measurements The photocatalytic activities of the samples were evaluated by the degradation of rhodamine B (RhB) and bisphenol A (BPA), under visible light irradiation. A 500 W Xe lamp with a 420 nm filter plate (AC/250 V, LanSheng electronics co., LTD, Shanghai) was used as visible light source. The 3 W monochromatic light lamps with the wavelength of 365, 590, 630, and 980 nm are made in Shenzhen lamplic co., LTD. In a typical experiment, the photocatalyst (30 mg) was added into RhB solution (100 mL, 10 mg/L), BPA and SMX solution (50 mL, 10 mg/L) to produce a suspension for the degradation reaction at room temperature with the magnetic stirring. Before light irradiation, the suspension was stirred for 1 h in the dark to ensure an adsorption-desorption equilibrium of the degradation material on the surface of the photocatalyst. Then the suspension was exposed to visible light irradiation under magnetic stirring. At given time intervals, about 3 mL suspension was sampled and centrifuged to remove the photocatalyst particles. The degradation efficiency (DE) was calculated according to the following equation, DE (%) ¼ (1C/C0) 100%
(1)
where C0 and C represent the adsorption equilibrium concentration of solution prior to irradiation and the concentration after certain interval irradiation time, respectively.
2.4. Photoelectrochemical measurements The photocurrents (PC) and electrochemical impedance spectroscopy (EIS) measurements were performed on an electrochemical analyzer (Solartron SI 1287 Instruments). The system is in a standard three-electrode configuration with a Pt wire as the counter electrode, and calomel electrode as a reference electrode. A 500 W Xenon lamp served as a UVevis light source. 0.1 M Na2SO4 aqueous solution was used as the electrolyte. The working electrodes were prepared as follows: the 0.03 g ground sample was mixed with 1 ml ethanol to make slurry and then ultrasound for 20 min. Afterwards, the turbid liquid was coated onto a 0.5 5 cm F-doped stannic oxide glass electrode and these electrolytes were dried at 60 C for 3 h.
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3. Results and discussion 3.1. Chemical and optical characterization The CQDs were prepared by a facile hydrothermal method and CQDs/BiOBr composites were synthesized by a simple room temperature hydrolysis method (see experimental details in the Supporting Information). A simple constitution process of the CQDs/BiOBr composites was shown in Scheme 1. The XRD patterns for as-prepared composites were shown in Fig. S1. The characteristic diffraction peaks were readily indexed to BiOBr tetragonal phase (JCPDS card #73-2061) without interference peaks. A broad peak at around 25 was clearly seen in the XRD pattern of CQDs. This is consistent with the (002) lattice plane of carbon-based materials [37,38]. The spectra for all the CQDs/BiOBr composites were similar to BiOBr nanocrystals, indicating that the lattice structure of BiOBr did not change upon the introduction of CQDs. There was also no observable shift of the characteristic diffraction peaks in the spectra for the CQDs/BiOBr composites, suggesting that the deposited CQDs were not incorporated into the BiOBr lattice [39]. No obvious peaks attributed to CQDs were detected in the CQDs/BiOBr possibly due to the small quantity and particle size of CQDs dopant, as well as their high dispersion [40,41]. We further acquired the transmission electron microscopy (TEM) image of the as-synthesized CQDs, as shown in Fig. 1a. The obtained results demonstrated that CQDs were monodisperse with a spherical shape diameter of 4e9 nm and the insert HRTEM image revealed that the CQDs have an interplanar spacing of 0.32 nm, corresponding to the crystallographic (002) plane of the XRD pattern (Fig. S1). Fig. 1b showed the normal photoluminescence (PL) spectra of CQDs. The solution of CQDs was blue under 365 nm UV lamp irradiation (insert in Fig. 1b), indicating the photoluminescence presence property of CQDs. It could be seen that the PL spectra were dependent on the excitation wavelength, and the strongest peak excited at 390 nm, consistented with previous fluorescence analyses [42]. The tunable emissions of the CQDs could be a result of varied fluorescence characteristics of particles of different sizes of the CQDs and the distribution of different emissive sites on the surface of the CQDs [43]. In addition to conventional fluorescence emissions, up-conversion fluorescence emission is an optical phenomenon wherein the fluorescence emission wavelength is shorter than the used excitation wavelength. The up-conversion florescence emissions likely originated from a multi-photon excitation process e an essential feature of upconversion fluorescence emission [44]. The unique PL upconversion property of CQDs was shown in Fig. 1c. It was found that the long excitation wavelength changed from 500 to 1100 nm, and the upconverted shorter emissions peaks shifted from 350 to 600 nm, respectively. Remarkably, CQDs had an obvious PL upconversion behavior. To investigate the morphology of the CQDs modified BiOBr
samples, the scanning electron microscopy (SEM) was recorded and shown in Fig. 1d. As shown, the surface morphological feature of 3CQDs/BiOBr consisted of plentiful nanosheets constructing cluster like microspheres. Additional energy-dispersive X-ray (EDX) mappings for carbon, bromine, oxygen, and bismuth were given in Fig. S2. Fig. 1e and f displayed the TEM and HRTEM image of 3CQDs/BiOBr composite. It was clear that the CQDs were dispersed on the surface of BiOBr nanosheets, which was benefit for the transfer of the photoinduced electrons between CQDs and BiOBr. The compositions of CQDs/BiOBr were further confirmed by FT-IR and XPS spectra as shown in Fig. S3 and Fig. S4, respectively. To further explore the optical property impacts of CQDs on BiOBr, we investigated the UVevis diffuse reflectance spectra (DRS) of CQDs/BiOBr composites, as shown in Fig. 2a. Apparently, pure BiOBr had an absorption edge at the wavelength of 430 nm with an absorption tail to 650 nm. This absorption tail indicated there was OVs existence [45]. With the addition of CQDs, there was an obvious red shift from 430 to 492 nm in the absorption edge and an improved absorbance in the visible light region. Interestingly, such a red shift seems to depend linearly on the CQDs content. It is well known that the optical absorption near the band edge follows the formula (ahn)1/2 ¼ A (hn Eg), where a, h, n, Eg, and A are absorption coefficient, Planck constant, light frequency, band gap, and a constant, respectively [46]. The value of exponent, n determines the nature of electronic transition. For BiOBr, the value of n is 2 for the indirect transition and the linear extrapolation of (ahn)1/2 to zero gave the band gap energies of the samples [47]. The Eg values of BiOBr, 1-CQDs/BiOBr, 3-CQDs/BiOBr, and 6-CQDs/BiOBr can be estimated to be 2.64, 2.55, 2.25, and 1.71 eV, respectively (Table S1). We considered the red shift of absorption band edges corresponding to the reduced band gaps and improved absorbance in visible region may due to the CQDs, which can make BiOBr to excite under long wavelength light illumination (larger than 430 nm) by the upconversion ability, and utilize more visible light. The existence of OVs could also be proved by low temperature electron spin resonance (ESR) spectra as shown in Fig. 2b. It can be found that the intensity of ESR signal at g factor ~2.0060 for 3-CQDs/BiOBr is much higher than that of pure BiOBr at g factor ~2.0067 under the quantitative comparison condition. In any case, the amount of oxygen vacancies increased after the CQDs introduction. 3.2. Photocatalytic activity and mechanism In view of the effect of CQDs on the photocatalytic performances of BiOBr, we employed CQDs/BiOBr composites for rhodamine B (RhB) and bisphenol A (BPA) removal under visible light irradiation (Fig. 3). Fig. 3a showed the photocatalytic decomposition of RhB over CQDs/BiOBr composites under visible light irradiation. For pure BiOBr, 67% of RhB was photocatalytically decomposed after 20 min, while all the CQDs/BiOBr composites exhibited much higher photocatalytic activities than BiOBr. Especially, the sample
Scheme 1. Preparation route for CQDs/BiOBr composites.
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Fig. 1. (a) TEM image (insert: HRTEM image), (b) PL spectra (insert: the photograph of CQDs solution under 365 nm UV lamp irradiation), and (c) upconversion PL spectra of CQDs, (d) SEM, (e) TEM, and (f) HRTEM images of sample 3-CQDs/BiOBr.
3-CQDs/BiOBr had the highest degradation rate of 92%. To further explore the photocatalytic ability of CQDs/BiOBr composites, the photodegradation of BPA a typical colorless and harmful organic pollutants, was investigated under visible light irradiation (Fig. 3b). Results revealed that 96.8% of BPA was decomposed for 3-CQDs/ BiOBr composite after 4 h, which was much higher than pure BiOBr.
To gain insight into the active species effects on the photocatalysis process in detail, we detected the oxidative species in the catalytic systems of BiOBr and 3-CQDs/BiOBr under visible light by adding different scavengers (p-benzoquinone (PBQ) for O 2, ammonium oxalate (AO) for hþ, and tertiary butanol (TBA) for OH). As shown in Fig. 4a,b. TBA had weak inhibition efficiency of 3.5%
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a 1.4
b
1.2
BiOBr 3-CQDsBiOBr
0.8 0.6 0.4 0.2 0.0 200
BiOBr 1-CQDs/BiOBr 3-CQDs/BiOBr 6-CQDs/BiOBr CQDs 300
400
g=2.0060
Intensity (a.u.)
Absorbance
1.0
g=2.0067
500
600
700
Wavelength (nm)
800
3420 3440 3460 3480 3500 3520 3540 3560 3580 3600
Magnetic field
Fig. 2. (a) UVevis diffuse reflectance spectra (DRS) and (b) EPR spectra (at 77 K) of CQDs/BiOBr composites.
Fig. 3. Photocatalytic performances of (a) RhB and (b) BPA over CQDs/BiOBr composites under visible light irradiation.
and 3.7% of RhB degradation for BiOBr and 3-CQDs/BiOBr, respectively, indicating that OH had no effect on the photocatalytic degradation of RhB process. There were 68.9% and 47.9% depressions for AO addition on BiOBr and 3-CQDs/BiOBr respectively. For PBQ addition, 22.5% and 29.3% depressions were shown on BiOBr and 3-CQDs/BiOBr respectively. It can be seen that hole played a crucial role during degradation., while O 2 radicals also played an important role. Obviously, the generated O 2 in 3-CQDs/ BiOBr was much effective than that of BiOBr. Meanwhile, the ESR spin-trap tests with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) were performed (Fig. 4c). 3-CQDs/BiOBr showed the stronger signals of O 2 than that of BiOBr under visible light irradiation, indicating more O 2 was generated during the irradiation of 3-CQDs/ BiOBr. Considering the band gap position of BiOBr (calculated in the supporting information), it was not difficult to found that the conduction band energy (ECB) for BiOBr is 0.64 eV, which was more positive than that of E(O2/O 2 ) (0.046 eV) and not in favor of O2 generation. It was widely accepted that molecular oxygen can be easily charged by the localized electrons at OVs on the surface [35]. Therefore, the generation of O 2 was due to the OVs on BiOBr surface. If the O 2 was generated by OVs, how to explain the increased O2 on CQDs/BiOBr, which had similar intensity of OVs to BiOBr. To
find the reason, we monitored the transient photocurrent (PC) responses measurements as shown in Fig. 4d. There was a significantly higher photocurrent for CQDs/BiOBr in comparison with BiOBr. The results may have two reasons. One, the separation efficiency of electrons-hole pairs was improved by CQDs, which was proved by the results of electrochemical impedance spectra (EIS) and steady-state PL spectra (Fig. S5). Another, the numbers of generated electrons were increased after CQDs composited. According to the PL upconversion effect of CQDs, which could absorb long wavelength light of 500e1100 nm and emit shorter wavelength light of 350e600 nm, CQDs/BiOBr could utilize longer wavelength light to generate more electron-hole pairs. To prove this, we then carefully studied the photocatalytic activity of BiOBr, CQDs, and 3-CQDs/BiOBr composite under different monochromatic light (590, 630, and 980 nm). As shown in Fig. 5a, the photolysis of RhB could be ignored, and there was scarcely any decomposition of RhB by CQDs under light irradiation. When exposed to the 590 nm light irradiation, BiOBr showed only 4.5% RhB degradation, which may be attributed to the OVs defect states in BiOBr with a weak absorption tail to 650 nm as the statement of DRS analyses. However, 3-CQDs/BiOBr showed much higher superior photocatalytic degradation efficiency of RhB for 64% under 590 nm light irradiation than BiOBr. Even under 630 and 980 nm
F. Duo et al. / Journal of Alloys and Compounds 685 (2016) 34e41
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Fig. 4. Scavenger experiments of reactive species during the photocatalytic degradation of RhB over BiOBr (a) and 3-CQDs/BiOBr sample (b) under visible light irradiation. ESR spectra of radical adducts trapped by DMPO in methanol dispersion (c), and transient photocurrent response (d) of BiOBr and 3-CQDs/BiOBr under visible light, respectively.
Fig. 5. (a) RhB degradation of BiOBr and 3-CQDs/BiOBr under the monochromatic light irradiation at wavelength 365, 590, 630, and 980 nm for 4 h, and (b) BPA degradation of BiOBr and 3-CQDs/BiOBr under the monochromatic light irradiation at wavelength of 365 and 590 nm for 3 h and 7 h, respectively.
light irradiation, 3-CQDs/BiOBr composite showed efficient RhB degradation of 58% and 5.4%, respectively. The similar phenomenon can be seen in the degradation of BPA in Fig. 5b. Under 590 nm light irradiation, 77% BPA was degraded for 3-CQDs/BiOBr, while BiOBr had almost no degradation effect. This result well demonstrated that, under the whole visible light region irradiation, more electrons-hole pairs could be generated on CQDs/BiOBr composite
owing to the upconversion effect of CQDs. Accordingly, the electrons localized on OVs may also increase, which subsequently promoted the formation of O 2 by the molecular oxygen activation with OVs. Therefore, the upconversion effect of CQDs could efficiently improve the generation number of O 2 on BiOBr with OVs. As known OVs of oxide semiconductors would gradually be healed during their interaction with molecular oxygen [48], so we
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1.0 0.8
1-C/C 0
0.6 0.4 0.2 0.0
3-CQDs/BiOBr BiOBr
7
6
5
4
3
2
1
s cle Cy
Fig. 6. Cycling runs for the photodegradation of RhB of BiOBr and 3-CQDs/BiOBr under visible light.
Table 1 The quantum yield of sample CQDs and 3-CQDs/BiOBr powder. Sample
CQDs
3-CQDs/BiOBr
Quantum yield (QY)
28.89%
18.12%
monitored the reusability of BiOBr and 3-CQDs/BiOBr, as shown in Fig. 6. As might have been expected, BiOBr only maintained 53.6% of its initial degradation efficiency of RhB corresponding to 46.4% depression after 7 circles. But interestingly, the photocatalytic degradation of RhB over 3-CQDs/BiOBr had only 9.5% decline after 7 circles. It was reported that under ambient conditions, OVs on the (001) facet of BiOCl can only be generated under UV light [45]. Just in our case, the CQDs could convert the visible light into 350e600 nm light, which cover part of UV light region. Therefore, even though the dissociative healing of OVs could not be avoided,
the converted UV light could in situ refresh OVs on BiOBr for the sustainable generation of O 2 under visible light, and then kept the stable photocatalytic activity. Meanwhile, Fig. S6 displayed the comparison photographs of CQDs and 3-CQDs/BiOBr immersed into the distilled water for 7 days. Results demonstrated that the aqueous solution remained colorless, confirming that the CQDs modified on the surface of BiOBr were highly stable. The fluorescence (PL) quantum yield of CQDs and CQDs/BiOBr powder are 28.89% and 18.12%, respectively (Table 1). It is known that the higher quantum yield of CQDs corresponding to better fluorescence properties. As photoluminescence originates from surface energy traps, when a certain excitation wavelength illuminates the CQDs, a surface energy trap dominates the emission. The CQDs/BiOBr composite has lower quantum yield value than CQDs after covered on the surface of BiOBr, which may be due to the reducing surface energy traps of CQDs after combined with BiOBr. On the basis of the above experiments results, a possible explanation for the enhanced photocatalytic activity and stability of the CQDs/BiOBr composites under visible light was illustrated in Scheme 2. The photogenerated electrons on the conduction band of BiOBr could not be able to$reduce oxygen to O 2 , the O2 was only arisen from the activation of chemisorbed O2 through one-electron transfer via the OVs states. After modified with CQDs, the CQDs on the surface of BiOBr can greatly improve the generation numbers of electrons-hole pairs by upconverting the long wavelength light of 500e1100 nm to 350e600 nm for exciting BiOBr. The electrons localized on OVs would also be increased to promote more O 2 formation. As a result, the increased hþ and O 2 were responsible for the photocatalytic performances acting more efficiently. In addition, the OVs on the BiOBr surface could be sustainably supplemented by the converted UV light, which could maintain the stability of photocatalytic activity to a great extent. 4. Conclusions In summary, we have synthesized CQDs/BiOBr composites with oxygen vacancies by a simple hydrolysis method at room temperature. CQDs/BiOBr composites exhibited high photocatalytic
Scheme 2. Schematic diagram of the upconversion of CQDs on enhancing the photocatalytic activity of BiOBr under visible light irradiation.
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efficiency and long term stability under visible light. It is demonstrated the upconversion effect of CQDs played important roles on the excellent photocatalytic performance for CQDs/BiOBr composites. One side, under the whole visible light region irradiation, more O2 could be generated on CQDs/BiOBr composites via more electrons transfer in oxygen vacancies owing to the upconversion effect of CQDs. On the other side, the OVs on the BiOBr surface could be sustainably supplemented by the converted UV light, which could maintain the stability of photocatalytic activity to a great extent. These findings gain deep insight into the interaction modes of CQDs with photocatalysts, and also provide a new approach to improve the stability of photocatalysts. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21176168, and 21303116), Shanxi Province Science Foundation (2013021011-2). Natural Science Foundation for Young Scientists of Shanxi Province (201402-10193). Young academic leader in Shanxi Province (154010141-s). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.05.259. References [1] H.X. Shi, G.Y. Li, H.W. Sun, T.C. An, H.J. Zhao, P.K. Wong, Appl. Catal. B 158 (2014) 301e307. [2] L.Q. Ye, J.Y. Liu, C.Q. Gong, L.H. Tian, T.Y. Peng, L. Zan, ACS Catal. 2 (2012) 1677e1683. [3] S.A. Ansari, M.M. Khan, M.O. Ansari, M.H. Cho, Sol. Energy Mater. Sol. Cells 141 (2015) 162e170. [4] F. Niu, D. Chen, L.S. Qin, T. Gao, N. Zhang, S. Wang, Z. Chen, J.Y. Wang, X.G. Sun, Y.X. Huang, Sol. Energy Mater. Sol. Cells 143 (2015) 386e396. [5] Z.H. Ai, W.K. Ho, S.C. Lee, L.Z. Zhang, Environ. Sci. Technol. 434 (2009), 143e4150. [6] W.Y. Su, J. Wang, Y.X. Huang, W.J. Wang, L. Wu, X.X. Wang, P. Liu, Scr. Mater 62 (2010) 345e348. [7] J. Jiang, K. Zhao, X.Y. Xiao, L.Z. Zhang, J. Am. Chem. Soc. 134 (2012) 4473e4476. [8] L.Q. Jing, W. Zhou, G.H. Tian, H.G. Fu, Soc. Rev. 42 (2013) 9509e9549. [9] J. Zhang, F.J. Shi, J. Lin, D.F. Chen, J.M. Gao, Z.X. Huang, X.X. Ding, C.C. Tang, Chem. Mater 20 (2008) 2937e2941. [10] X.M. Zhang, G.B. Ji, Y.S. Liu, X.G. Zhou, Y. Zhu, D.N. Shi, P. Zhang, X.Z. Cao, B.Y. Wang, Phys. Chem. Chem. Phys. 17 (2015) 8078e8086. [11] G.H. Jiang, X.H. Wang, Z. Wei, X. Li, X.G. Xi, R.B. Hu, B.L. Tang, R.J. Wang, S.T. Wang, J. Mater. Chem. A 1 (2013) 2406e2410. [12] H.L. Lin, X. Li, J. Cao, S.F. Chen, Y. Chen, Catal. Commun. 49 (2014) 87e91. [13] M.Q. He, W.B. Li, J.X. Xia, L. Xu, J. Di, H. Xu, S. Yin, H.M. Li, M.N. Li, Appl. Surf. Sci. 331 (2015) 170e178.
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Enhanced visible light photocatalytic activity and stability of CQDs/BiOBr composites: The upconversion effect of CQDs Fangfang Duo, Yawen Wang**, Caimei Fan*, Xiaochao Zhang, Yunfang Wang College of Chemistry and Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 14 March 2016 Received in revised form 19 May 2016 Accepted 23 May 2016 Available online 25 May 2016
CQDs/BiOBr composites with oxygen vacancies were synthesized by a simple hydrolysis method at room temperature. The obtained composites exhibited much higher photocatalytic activity and stability than pure BiOBr under visible light. The upconversion effect of CQDs on BiOBr was studied by photoluminescence (PL) spectra, transient photocurrent (PC) responses measurements, electron spin resonance (ESR) analysis, reactive species scavenger experiments, and RhB and BPA degradation under monochromatic light irradiation. The results demonstrated that the upconversion effect of CQDs could improve O 2 formation on BiOBr during photocatalytic process via more electron transfer in oxygen vacancies owing to the desirable long wavelength visible light and near infrared (NIR) light absorption property of CQDs. Moreover, the upconversion effect of CQDs could refresh the oxygen vacancies on BiOBr and therefore maintain the photocatalytic stability of BiOBr. © 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords: Upconversion CQDs/BiOBr composites Oxygen vacancies Photocatalytic activity
1. Introduction Development of highly efficient photocatalysts for the solar energy conversion and the organic pollutants elimination has been considered to be one of the pivotal investigative tasks [1e4]. TiO2 is the most used and studied photocatalyst during the last decades because of its high activity, excellent stability, and low cost. However, the wide band gap nature of TiO2 (band gap of 3.2 eV) limits its practical applications due to its poor response to visible light accounting for about 43% of solar light [5]. Thus, many research groups have been focusing on developing novel semiconductors with improved visible light photocatalytic efficiency. Fortunately, bismuth oxyhalide BiOX (X ¼ F, Cl, Br, or I) compounds have received much attention due to their efficient photocatalytic performances in the degradation of aqueous organic pollution, arising from their unique tetragonal layered structure and indirect transition band gap [6e8]. Among these BiOX photocatalysts, BiOBr has attracted great interest owing to its relatively superior catalytic activity under visible light irradiation [9]. To further improve the photacatalytic activity of BiOBr, many strategies have been developed, such as noble metals decoration [10], ion
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (Y. Wang), [email protected] (C. Fan).
doping [11e13], coupling with other semiconductor photocatalysts [14e17], and so on. In addition, coupling of BiOBr with metal-free carbonaceous materials, such as carbon fibers [18,19], graphene [20], carbon nitride [21], and carbon quantum dots (CQDs) [22,23] can receive an expected improvement in the photocatalytic activity by reducing the recombination of the electron-hole pairs. However, CQDs, a novel class of nanocarbons with sizes below 10 nm, have been of greatly interest because of its robust chemical inertness, superior stability against photobleaching, and low cytotoxicity [24], also possess the unique photoluminescence (PL) upconversion property [25]. The PL upconversion effect can convert the long wavelength visible light and near infrared (NIR) light radiation into shorter wavelength UV and visible light emission [26], which made CQDs a kind of very promising material for bioimaging, solar cells, photocatalysis etc. applications [27,28]. Many photocatalysts, like Ag3PO4 [29], Fe2O3 [30], and Cu2O [31] applied the upconversion effect of CQDs for harvesting light to improve the photocatalytic activity. For example, a CQDs/Ag3PO4 composite exhibits enhanced photocatalytic activity, since the CQDs have an ability to harness visible light and NIR light (700e1000 nm), while the corresponding PL upconversion spectra are in the range of 300e650 nm, from which the Ag3PO4 (2.4 eV with light adsorption edge about 516 nm) would be partly excited [29]. However, the insight mechanism of the upconversion effect of CQDs on semiconductors photocatalysts still need to pursue in order to open up a new vista for efficient photocatalysis using more low energy light spectra.
http://dx.doi.org/10.1016/j.jallcom.2016.05.259 0925-8388/© 2016 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
F. Duo et al. / Journal of Alloys and Compounds 685 (2016) 34e41
It is known that the oxygen vacancies formed on the materials surface can contribute to the efficient photocatalytic activity under visible light [32e34]. First, surface OVs with their typical defect states can trap electrons or holes to inhibit the charge carriers’ recombination, and may also promote the transfer of these trapped charge carriers to the adsorbates. Second, OVs with abundant localized electrons are of particular interests for the enhanced adsorption and activation of adsorbate gas molecules. For example, OVs of BiOCl can directly activate O2 to active oxygen species such 2 2 as *O 2 , O4 and O2 [35]. As well known that OVs can be produced depending on the following formation conditions of nonaqueous system, inert gas atmosphere, or ultraviolet light (l < 400 nm) [36]. However, OVs of oxide semiconductors would gradually be healed during their interaction with molecular oxygen under visible light, which greatly inhibited the stability of the semiconductors photocatalysts. So the stability and reusability of OVs are crucial for the practical environment application. Herein, we present a simple hydrolysis method at room temperature for the fabrication of CQDs/BiOBr composites with oxygen vacancies. The CQDs/BiOBr composites showed excellent photocatalytic activity under visible light and even NIR light irradiation. More interestingly, the composites exhibited great photocatalytic stability under visible light irradiation. Experimental results suggested the upconversion effect of CQDs played important roles on the excellent photocatalytic performance for CQDs/BiOBr composites. 2. Experimental 2.1. Synthesis All of the chemicals are analytical reagents without further purification. To prepare CQDs solid, D-Glucose (3.96 g) was dissolved in 40 mL deionized water and 2.67 mL ethylenediamine was then added. The above solution was transferred into a 100 mL sealed Teflon-lined stainless-steel autoclave and heated at 180 C for 11 h. After cooled to room temperature, the resulting solution was filtered with a 0.22 mm polyethersulfone membrane to remove the larger carbon nanoparticles. The CQDs were further dialyzed in a dialysis bag (cutoff molecular weight: 1000 Da) for 2 days. At last, the powder CQDs were obtained by freeze drying. The CQDs/BiOBr composites were synthesized by a facile room temperature hydrolysis method. Typically, 2.425 g of Bi(NO3)3$5H2O was added into 20 mL ethylene glycol (EG) solution, stirring until the homogeneous solution then a certain amount of CQDs solution (15 g/L, dispersed in distilled water) was added into the above solution. After that, the mixture solution was ultrasound for 20 min. Subsequently, the 50 mL aqueous solution containing of KBr (0.595 g) and NaOH (0.200 g) was added into the above solution and stirred for 0.5 h. The resulting samples were collected and washed with deionized water and ethanol and then dried at 60 C in the oven for 4 h. The CQDs/BiOBr composites prepared by changing the amount of CQDs solution of 1, 3, and 6 mL were labeled as 1-CQDs/BiOBr, 3-CQDs/BiOBr, and 6-CQDs/BiOBr, respectively. 2.2. Characterization X-ray powder diffraction (XRD) was carried out on a D/max2500 diffractometer with Cu Ka radiation and the scanning range was 10 e80 . Fourier transform infrared spectroscopy (FT-IR) analysis was carried out on a Shimadzu 8400 spectrophotometer. The scanning electron microscope (SEM) images were recorded on a Nanosem 430 Field Emission Scanning Electron Microscope. Transmission electron microscopy (TEM) and high-resolution
35
transmission electron microscopy (HRTEM) were recorded on a JEOL-2011 microscope (Japan, 200 kV). UVevis diffused reflectance spectra (DRS) of the samples were obtained for the dry-pressed film samples using a UVevis spectrophotometer (UV-3600, Shimadzu, Japan). X-ray photoelectron spectroscopy (XPS) measurements were performed on a Thermo ESCALAB 250 Xi XPS system with an Al Ka (hv ¼ 1486.6 eV) 150 W, 500 mm beam spot source. The excitation, emission photoluminescence (PL) spectra of CQDs and the photoluminescence (PL) emission spectra for pure BiOBr and 3CQDs/BiOBr sample were collected on a Horiba Fluoromax-4 (France) luminescence spectrometer using a Xe lamp as the excitation source. In situ electron spin resonance (ESR) spectra were conducted on a Bruker model ESR JES-FA200 spectrometer. The g factor was obtained by taking the signal of manganese as standard. Magnetic parameters of the radicals detected were obtained from direct measurements of the magnetic field and microwave frequency. The ESR signals of radicals spin-trapped were examined by spin-trap reagent DMPO in water and methanol. The UVevis adsorption spectra of the centrifugated solutions were analyzed using a Varian Cary 50 prode UVevis spectrometer. The fluorescence (PL) quantum yield was measured by Edinburgh FLs 980 fully functional steady/transient fluorescence spectrometer.
2.3. Photocatalytic activity measurements The photocatalytic activities of the samples were evaluated by the degradation of rhodamine B (RhB) and bisphenol A (BPA), under visible light irradiation. A 500 W Xe lamp with a 420 nm filter plate (AC/250 V, LanSheng electronics co., LTD, Shanghai) was used as visible light source. The 3 W monochromatic light lamps with the wavelength of 365, 590, 630, and 980 nm are made in Shenzhen lamplic co., LTD. In a typical experiment, the photocatalyst (30 mg) was added into RhB solution (100 mL, 10 mg/L), BPA and SMX solution (50 mL, 10 mg/L) to produce a suspension for the degradation reaction at room temperature with the magnetic stirring. Before light irradiation, the suspension was stirred for 1 h in the dark to ensure an adsorption-desorption equilibrium of the degradation material on the surface of the photocatalyst. Then the suspension was exposed to visible light irradiation under magnetic stirring. At given time intervals, about 3 mL suspension was sampled and centrifuged to remove the photocatalyst particles. The degradation efficiency (DE) was calculated according to the following equation, DE (%) ¼ (1C/C0) 100%
(1)
where C0 and C represent the adsorption equilibrium concentration of solution prior to irradiation and the concentration after certain interval irradiation time, respectively.
2.4. Photoelectrochemical measurements The photocurrents (PC) and electrochemical impedance spectroscopy (EIS) measurements were performed on an electrochemical analyzer (Solartron SI 1287 Instruments). The system is in a standard three-electrode configuration with a Pt wire as the counter electrode, and calomel electrode as a reference electrode. A 500 W Xenon lamp served as a UVevis light source. 0.1 M Na2SO4 aqueous solution was used as the electrolyte. The working electrodes were prepared as follows: the 0.03 g ground sample was mixed with 1 ml ethanol to make slurry and then ultrasound for 20 min. Afterwards, the turbid liquid was coated onto a 0.5 5 cm F-doped stannic oxide glass electrode and these electrolytes were dried at 60 C for 3 h.
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3. Results and discussion 3.1. Chemical and optical characterization The CQDs were prepared by a facile hydrothermal method and CQDs/BiOBr composites were synthesized by a simple room temperature hydrolysis method (see experimental details in the Supporting Information). A simple constitution process of the CQDs/BiOBr composites was shown in Scheme 1. The XRD patterns for as-prepared composites were shown in Fig. S1. The characteristic diffraction peaks were readily indexed to BiOBr tetragonal phase (JCPDS card #73-2061) without interference peaks. A broad peak at around 25 was clearly seen in the XRD pattern of CQDs. This is consistent with the (002) lattice plane of carbon-based materials [37,38]. The spectra for all the CQDs/BiOBr composites were similar to BiOBr nanocrystals, indicating that the lattice structure of BiOBr did not change upon the introduction of CQDs. There was also no observable shift of the characteristic diffraction peaks in the spectra for the CQDs/BiOBr composites, suggesting that the deposited CQDs were not incorporated into the BiOBr lattice [39]. No obvious peaks attributed to CQDs were detected in the CQDs/BiOBr possibly due to the small quantity and particle size of CQDs dopant, as well as their high dispersion [40,41]. We further acquired the transmission electron microscopy (TEM) image of the as-synthesized CQDs, as shown in Fig. 1a. The obtained results demonstrated that CQDs were monodisperse with a spherical shape diameter of 4e9 nm and the insert HRTEM image revealed that the CQDs have an interplanar spacing of 0.32 nm, corresponding to the crystallographic (002) plane of the XRD pattern (Fig. S1). Fig. 1b showed the normal photoluminescence (PL) spectra of CQDs. The solution of CQDs was blue under 365 nm UV lamp irradiation (insert in Fig. 1b), indicating the photoluminescence presence property of CQDs. It could be seen that the PL spectra were dependent on the excitation wavelength, and the strongest peak excited at 390 nm, consistented with previous fluorescence analyses [42]. The tunable emissions of the CQDs could be a result of varied fluorescence characteristics of particles of different sizes of the CQDs and the distribution of different emissive sites on the surface of the CQDs [43]. In addition to conventional fluorescence emissions, up-conversion fluorescence emission is an optical phenomenon wherein the fluorescence emission wavelength is shorter than the used excitation wavelength. The up-conversion florescence emissions likely originated from a multi-photon excitation process e an essential feature of upconversion fluorescence emission [44]. The unique PL upconversion property of CQDs was shown in Fig. 1c. It was found that the long excitation wavelength changed from 500 to 1100 nm, and the upconverted shorter emissions peaks shifted from 350 to 600 nm, respectively. Remarkably, CQDs had an obvious PL upconversion behavior. To investigate the morphology of the CQDs modified BiOBr
samples, the scanning electron microscopy (SEM) was recorded and shown in Fig. 1d. As shown, the surface morphological feature of 3CQDs/BiOBr consisted of plentiful nanosheets constructing cluster like microspheres. Additional energy-dispersive X-ray (EDX) mappings for carbon, bromine, oxygen, and bismuth were given in Fig. S2. Fig. 1e and f displayed the TEM and HRTEM image of 3CQDs/BiOBr composite. It was clear that the CQDs were dispersed on the surface of BiOBr nanosheets, which was benefit for the transfer of the photoinduced electrons between CQDs and BiOBr. The compositions of CQDs/BiOBr were further confirmed by FT-IR and XPS spectra as shown in Fig. S3 and Fig. S4, respectively. To further explore the optical property impacts of CQDs on BiOBr, we investigated the UVevis diffuse reflectance spectra (DRS) of CQDs/BiOBr composites, as shown in Fig. 2a. Apparently, pure BiOBr had an absorption edge at the wavelength of 430 nm with an absorption tail to 650 nm. This absorption tail indicated there was OVs existence [45]. With the addition of CQDs, there was an obvious red shift from 430 to 492 nm in the absorption edge and an improved absorbance in the visible light region. Interestingly, such a red shift seems to depend linearly on the CQDs content. It is well known that the optical absorption near the band edge follows the formula (ahn)1/2 ¼ A (hn Eg), where a, h, n, Eg, and A are absorption coefficient, Planck constant, light frequency, band gap, and a constant, respectively [46]. The value of exponent, n determines the nature of electronic transition. For BiOBr, the value of n is 2 for the indirect transition and the linear extrapolation of (ahn)1/2 to zero gave the band gap energies of the samples [47]. The Eg values of BiOBr, 1-CQDs/BiOBr, 3-CQDs/BiOBr, and 6-CQDs/BiOBr can be estimated to be 2.64, 2.55, 2.25, and 1.71 eV, respectively (Table S1). We considered the red shift of absorption band edges corresponding to the reduced band gaps and improved absorbance in visible region may due to the CQDs, which can make BiOBr to excite under long wavelength light illumination (larger than 430 nm) by the upconversion ability, and utilize more visible light. The existence of OVs could also be proved by low temperature electron spin resonance (ESR) spectra as shown in Fig. 2b. It can be found that the intensity of ESR signal at g factor ~2.0060 for 3-CQDs/BiOBr is much higher than that of pure BiOBr at g factor ~2.0067 under the quantitative comparison condition. In any case, the amount of oxygen vacancies increased after the CQDs introduction. 3.2. Photocatalytic activity and mechanism In view of the effect of CQDs on the photocatalytic performances of BiOBr, we employed CQDs/BiOBr composites for rhodamine B (RhB) and bisphenol A (BPA) removal under visible light irradiation (Fig. 3). Fig. 3a showed the photocatalytic decomposition of RhB over CQDs/BiOBr composites under visible light irradiation. For pure BiOBr, 67% of RhB was photocatalytically decomposed after 20 min, while all the CQDs/BiOBr composites exhibited much higher photocatalytic activities than BiOBr. Especially, the sample
Scheme 1. Preparation route for CQDs/BiOBr composites.
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Fig. 1. (a) TEM image (insert: HRTEM image), (b) PL spectra (insert: the photograph of CQDs solution under 365 nm UV lamp irradiation), and (c) upconversion PL spectra of CQDs, (d) SEM, (e) TEM, and (f) HRTEM images of sample 3-CQDs/BiOBr.
3-CQDs/BiOBr had the highest degradation rate of 92%. To further explore the photocatalytic ability of CQDs/BiOBr composites, the photodegradation of BPA a typical colorless and harmful organic pollutants, was investigated under visible light irradiation (Fig. 3b). Results revealed that 96.8% of BPA was decomposed for 3-CQDs/ BiOBr composite after 4 h, which was much higher than pure BiOBr.
To gain insight into the active species effects on the photocatalysis process in detail, we detected the oxidative species in the catalytic systems of BiOBr and 3-CQDs/BiOBr under visible light by adding different scavengers (p-benzoquinone (PBQ) for O 2, ammonium oxalate (AO) for hþ, and tertiary butanol (TBA) for OH). As shown in Fig. 4a,b. TBA had weak inhibition efficiency of 3.5%
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F. Duo et al. / Journal of Alloys and Compounds 685 (2016) 34e41
a 1.4
b
1.2
BiOBr 3-CQDsBiOBr
0.8 0.6 0.4 0.2 0.0 200
BiOBr 1-CQDs/BiOBr 3-CQDs/BiOBr 6-CQDs/BiOBr CQDs 300
400
g=2.0060
Intensity (a.u.)
Absorbance
1.0
g=2.0067
500
600
700
Wavelength (nm)
800
3420 3440 3460 3480 3500 3520 3540 3560 3580 3600
Magnetic field
Fig. 2. (a) UVevis diffuse reflectance spectra (DRS) and (b) EPR spectra (at 77 K) of CQDs/BiOBr composites.
Fig. 3. Photocatalytic performances of (a) RhB and (b) BPA over CQDs/BiOBr composites under visible light irradiation.
and 3.7% of RhB degradation for BiOBr and 3-CQDs/BiOBr, respectively, indicating that OH had no effect on the photocatalytic degradation of RhB process. There were 68.9% and 47.9% depressions for AO addition on BiOBr and 3-CQDs/BiOBr respectively. For PBQ addition, 22.5% and 29.3% depressions were shown on BiOBr and 3-CQDs/BiOBr respectively. It can be seen that hole played a crucial role during degradation., while O 2 radicals also played an important role. Obviously, the generated O 2 in 3-CQDs/ BiOBr was much effective than that of BiOBr. Meanwhile, the ESR spin-trap tests with 5,5-dimethyl-1-pyrroline N-oxide (DMPO) were performed (Fig. 4c). 3-CQDs/BiOBr showed the stronger signals of O 2 than that of BiOBr under visible light irradiation, indicating more O 2 was generated during the irradiation of 3-CQDs/ BiOBr. Considering the band gap position of BiOBr (calculated in the supporting information), it was not difficult to found that the conduction band energy (ECB) for BiOBr is 0.64 eV, which was more positive than that of E(O2/O 2 ) (0.046 eV) and not in favor of O2 generation. It was widely accepted that molecular oxygen can be easily charged by the localized electrons at OVs on the surface [35]. Therefore, the generation of O 2 was due to the OVs on BiOBr surface. If the O 2 was generated by OVs, how to explain the increased O2 on CQDs/BiOBr, which had similar intensity of OVs to BiOBr. To
find the reason, we monitored the transient photocurrent (PC) responses measurements as shown in Fig. 4d. There was a significantly higher photocurrent for CQDs/BiOBr in comparison with BiOBr. The results may have two reasons. One, the separation efficiency of electrons-hole pairs was improved by CQDs, which was proved by the results of electrochemical impedance spectra (EIS) and steady-state PL spectra (Fig. S5). Another, the numbers of generated electrons were increased after CQDs composited. According to the PL upconversion effect of CQDs, which could absorb long wavelength light of 500e1100 nm and emit shorter wavelength light of 350e600 nm, CQDs/BiOBr could utilize longer wavelength light to generate more electron-hole pairs. To prove this, we then carefully studied the photocatalytic activity of BiOBr, CQDs, and 3-CQDs/BiOBr composite under different monochromatic light (590, 630, and 980 nm). As shown in Fig. 5a, the photolysis of RhB could be ignored, and there was scarcely any decomposition of RhB by CQDs under light irradiation. When exposed to the 590 nm light irradiation, BiOBr showed only 4.5% RhB degradation, which may be attributed to the OVs defect states in BiOBr with a weak absorption tail to 650 nm as the statement of DRS analyses. However, 3-CQDs/BiOBr showed much higher superior photocatalytic degradation efficiency of RhB for 64% under 590 nm light irradiation than BiOBr. Even under 630 and 980 nm
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39
Fig. 4. Scavenger experiments of reactive species during the photocatalytic degradation of RhB over BiOBr (a) and 3-CQDs/BiOBr sample (b) under visible light irradiation. ESR spectra of radical adducts trapped by DMPO in methanol dispersion (c), and transient photocurrent response (d) of BiOBr and 3-CQDs/BiOBr under visible light, respectively.
Fig. 5. (a) RhB degradation of BiOBr and 3-CQDs/BiOBr under the monochromatic light irradiation at wavelength 365, 590, 630, and 980 nm for 4 h, and (b) BPA degradation of BiOBr and 3-CQDs/BiOBr under the monochromatic light irradiation at wavelength of 365 and 590 nm for 3 h and 7 h, respectively.
light irradiation, 3-CQDs/BiOBr composite showed efficient RhB degradation of 58% and 5.4%, respectively. The similar phenomenon can be seen in the degradation of BPA in Fig. 5b. Under 590 nm light irradiation, 77% BPA was degraded for 3-CQDs/BiOBr, while BiOBr had almost no degradation effect. This result well demonstrated that, under the whole visible light region irradiation, more electrons-hole pairs could be generated on CQDs/BiOBr composite
owing to the upconversion effect of CQDs. Accordingly, the electrons localized on OVs may also increase, which subsequently promoted the formation of O 2 by the molecular oxygen activation with OVs. Therefore, the upconversion effect of CQDs could efficiently improve the generation number of O 2 on BiOBr with OVs. As known OVs of oxide semiconductors would gradually be healed during their interaction with molecular oxygen [48], so we
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F. Duo et al. / Journal of Alloys and Compounds 685 (2016) 34e41
1.0 0.8
1-C/C 0
0.6 0.4 0.2 0.0
3-CQDs/BiOBr BiOBr
7
6
5
4
3
2
1
s cle Cy
Fig. 6. Cycling runs for the photodegradation of RhB of BiOBr and 3-CQDs/BiOBr under visible light.
Table 1 The quantum yield of sample CQDs and 3-CQDs/BiOBr powder. Sample
CQDs
3-CQDs/BiOBr
Quantum yield (QY)
28.89%
18.12%
monitored the reusability of BiOBr and 3-CQDs/BiOBr, as shown in Fig. 6. As might have been expected, BiOBr only maintained 53.6% of its initial degradation efficiency of RhB corresponding to 46.4% depression after 7 circles. But interestingly, the photocatalytic degradation of RhB over 3-CQDs/BiOBr had only 9.5% decline after 7 circles. It was reported that under ambient conditions, OVs on the (001) facet of BiOCl can only be generated under UV light [45]. Just in our case, the CQDs could convert the visible light into 350e600 nm light, which cover part of UV light region. Therefore, even though the dissociative healing of OVs could not be avoided,
the converted UV light could in situ refresh OVs on BiOBr for the sustainable generation of O 2 under visible light, and then kept the stable photocatalytic activity. Meanwhile, Fig. S6 displayed the comparison photographs of CQDs and 3-CQDs/BiOBr immersed into the distilled water for 7 days. Results demonstrated that the aqueous solution remained colorless, confirming that the CQDs modified on the surface of BiOBr were highly stable. The fluorescence (PL) quantum yield of CQDs and CQDs/BiOBr powder are 28.89% and 18.12%, respectively (Table 1). It is known that the higher quantum yield of CQDs corresponding to better fluorescence properties. As photoluminescence originates from surface energy traps, when a certain excitation wavelength illuminates the CQDs, a surface energy trap dominates the emission. The CQDs/BiOBr composite has lower quantum yield value than CQDs after covered on the surface of BiOBr, which may be due to the reducing surface energy traps of CQDs after combined with BiOBr. On the basis of the above experiments results, a possible explanation for the enhanced photocatalytic activity and stability of the CQDs/BiOBr composites under visible light was illustrated in Scheme 2. The photogenerated electrons on the conduction band of BiOBr could not be able to$reduce oxygen to O 2 , the O2 was only arisen from the activation of chemisorbed O2 through one-electron transfer via the OVs states. After modified with CQDs, the CQDs on the surface of BiOBr can greatly improve the generation numbers of electrons-hole pairs by upconverting the long wavelength light of 500e1100 nm to 350e600 nm for exciting BiOBr. The electrons localized on OVs would also be increased to promote more O 2 formation. As a result, the increased hþ and O 2 were responsible for the photocatalytic performances acting more efficiently. In addition, the OVs on the BiOBr surface could be sustainably supplemented by the converted UV light, which could maintain the stability of photocatalytic activity to a great extent. 4. Conclusions In summary, we have synthesized CQDs/BiOBr composites with oxygen vacancies by a simple hydrolysis method at room temperature. CQDs/BiOBr composites exhibited high photocatalytic
Scheme 2. Schematic diagram of the upconversion of CQDs on enhancing the photocatalytic activity of BiOBr under visible light irradiation.
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efficiency and long term stability under visible light. It is demonstrated the upconversion effect of CQDs played important roles on the excellent photocatalytic performance for CQDs/BiOBr composites. One side, under the whole visible light region irradiation, more O2 could be generated on CQDs/BiOBr composites via more electrons transfer in oxygen vacancies owing to the upconversion effect of CQDs. On the other side, the OVs on the BiOBr surface could be sustainably supplemented by the converted UV light, which could maintain the stability of photocatalytic activity to a great extent. These findings gain deep insight into the interaction modes of CQDs with photocatalysts, and also provide a new approach to improve the stability of photocatalysts. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (21176168, and 21303116), Shanxi Province Science Foundation (2013021011-2). Natural Science Foundation for Young Scientists of Shanxi Province (201402-10193). Young academic leader in Shanxi Province (154010141-s). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.05.259. References [1] H.X. Shi, G.Y. Li, H.W. Sun, T.C. An, H.J. Zhao, P.K. Wong, Appl. Catal. B 158 (2014) 301e307. [2] L.Q. Ye, J.Y. Liu, C.Q. Gong, L.H. Tian, T.Y. Peng, L. Zan, ACS Catal. 2 (2012) 1677e1683. [3] S.A. Ansari, M.M. Khan, M.O. Ansari, M.H. Cho, Sol. Energy Mater. Sol. Cells 141 (2015) 162e170. [4] F. Niu, D. Chen, L.S. Qin, T. Gao, N. Zhang, S. Wang, Z. Chen, J.Y. Wang, X.G. Sun, Y.X. Huang, Sol. Energy Mater. Sol. Cells 143 (2015) 386e396. [5] Z.H. Ai, W.K. Ho, S.C. Lee, L.Z. Zhang, Environ. Sci. Technol. 434 (2009), 143e4150. [6] W.Y. Su, J. Wang, Y.X. Huang, W.J. Wang, L. Wu, X.X. Wang, P. Liu, Scr. Mater 62 (2010) 345e348. [7] J. Jiang, K. Zhao, X.Y. Xiao, L.Z. Zhang, J. Am. Chem. Soc. 134 (2012) 4473e4476. [8] L.Q. Jing, W. Zhou, G.H. Tian, H.G. Fu, Soc. Rev. 42 (2013) 9509e9549. [9] J. Zhang, F.J. Shi, J. Lin, D.F. Chen, J.M. Gao, Z.X. Huang, X.X. Ding, C.C. Tang, Chem. Mater 20 (2008) 2937e2941. [10] X.M. Zhang, G.B. Ji, Y.S. Liu, X.G. Zhou, Y. Zhu, D.N. Shi, P. Zhang, X.Z. Cao, B.Y. Wang, Phys. Chem. Chem. Phys. 17 (2015) 8078e8086. [11] G.H. Jiang, X.H. Wang, Z. Wei, X. Li, X.G. Xi, R.B. Hu, B.L. Tang, R.J. Wang, S.T. Wang, J. Mater. Chem. A 1 (2013) 2406e2410. [12] H.L. Lin, X. Li, J. Cao, S.F. Chen, Y. Chen, Catal. Commun. 49 (2014) 87e91. [13] M.Q. He, W.B. Li, J.X. Xia, L. Xu, J. Di, H. Xu, S. Yin, H.M. Li, M.N. Li, Appl. Surf. Sci. 331 (2015) 170e178.
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