Down-conversion phosphors as noble-metal-free co-catalyst in ZnO for efficient visible light photocatalysis

Accepted Manuscript Title: Down-conversion phosphors as noble-metal-free co-catalyst in ZnO for efficient visible light photocatalysis Author: Haipeng Chu Xinjuan Liu Jiaqing Liu Wenyan Lei Jinliang Li Tianyang Wu Ping Li Huili Li Likun Pan PII: DOI: Reference:

S0169-4332(16)31458-1 http://dx.doi.org/doi:10.1016/j.apsusc.2016.07.031 APSUSC 33599

To appear in:

APSUSC

Received date: Revised date: Accepted date:

27-3-2016 1-7-2016 3-7-2016

Please cite this article as: Haipeng Chu, Xinjuan Liu, Jiaqing Liu, Wenyan Lei, Jinliang Li, Tianyang Wu, Ping Li, Huili Li, Likun Pan, Down-conversion phosphors as noblemetal-free co-catalyst in ZnO for efficient visible light photocatalysis, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.07.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Down-conversion phosphors as noble-metal-free co-catalyst in ZnO for efficient visible light photocatalysis Haipeng Chua, Xinjuan Liua,, Jiaqing Liub, Wenyan Leia, Jinliang Lib, Tianyang Wua, Ping Lic, Huili Lib, and Likun Panb

a

Institute of Coordination Bond Metrology and Engineering, College of Materials

Science and Engineering, China Jiliang University, Hangzhou 310018, China b

Engineering Research Center for Nanophotonics & Advanced Instrument, Ministry

of Education, Department of Physics, East China Normal University, Shanghai 200062 China c



Shanghai Nanotechnology Promotion Center, Shanghai 200237, China

Corresponding author. Tel: 86 571 86872475; Fax: 86 571 86872475; E-mail: [email protected] 1

Graphical Abstract

ZnO-Lu3(Al,Si)5(O,N)12:Ce3+ hybrid photocatalysts were synthesized via a fast microwave-assisted approach for visible light photocatalytic degradation of organic pollutions with a high degradation rate of 91%.

2

Research Highlights 

ZnO-Lu3(Al,Si)5(O,N)12:Ce3+ were synthesized via a facile microwave-assisted method.



Lu3(Al,Si)5(O,N)12:Ce3+ acts as co-catalyst to facilitates the self-sensitized degradation of MB.



ZnO-Lu3(Al,Si)5(O,N)12:Ce3+ exhibited enhanced visible light photocatalytic activity.



A high MB degradation rate of 91% was achieved under visible light irradiation.

3

Abstract Exploring novel visible light responsive photocatalysts is one of greatly significant issues from the viewpoint of using solar energy. Here we report the yellow-orange emitting α-Si3N4-doped Lu3Al5O12:Ce3+ (Lu3Al5-xSixO12-xNx:Ce3+) phosphors as a noble-metal-free co-catalyst for enhanced visible light photocatalytic activity of ZnO. The results show that ZnO-Lu3Al5-xSixO12-xNx:Ce3+ hybrid photocatalysts using a fast microwave-assisted approach exhibits a 91% methylene blue (MB) degradation under visible light irradiation at 240 min, which evidence the synergistic effect of ZnO and Lu3Al5-xSixO12-xNx:Ce3+ that suppress the rate of charge recombination and increase the self-sensitized degradation of MB. ZnO-down conversion phosphors can be envisaged as potential candidate in environmental engineering and solar energy applications.

Keywords: Down conversion phosphors; ZnO; photocatalysts; microwave-assisted approach

4

1. Introduction With industrialization and population growth, the environmental contaminations such as heavy metal ions and organic chemical compounds are becoming the overwhelming problems all over the world. Conventional treatment methods, such as adsorption, ultrafiltration, coagulation, reverse osmosis, etc. have been carried out to solve the problem, while they have some drawbacks due to the increasing number of refractory materials found in waste-water effluents, difficulties in the complete removal of color, and expensiveness [1]. Hence, it is critical to develop novel techniques that are able to remove the pollutants from water efficiently. Semiconductor photocatalysis as a novel and environmentally-friendly technology has attracted wide attention in recent years due to its potential applications in oxidation of organic pollutants [2-5], reduction of heavy metal ions and carbon dioxide [5-9], evolution of hydrogen or oxygen gas [10, 11]. One of the major factors in photocatalysis is the limited light absorption of photocatalysts in the incident solar spectrum [12-18]. For example, ZnO as a promising photocatalyst can respond only ultraviolet light due to wide band gap. To make full use of solar energy, intensive investigation has been carried out to expand the light absorption range of wide band gap semiconductors that respond to visible light, such as coupling with narrow band gap semiconductors, and doping with metals or nonmetals [19, 20]. Recently, the light-converting materials have attracted wide interest in photocatalysis and solar energy cells [21]. Up conversion material-ZnO hybrid photocatalysts have been demonstrated with excellent visible light photocatalytic activity due to good match 5

between the photon emission of phosphors and band gap of ZnO [22]. In our previous works [23, 24], ZnO-down converting phosphors such as Y3Al5O12:Ce3+ (YAG:Ce3+), Y2O2S:Eu3+ etc. was found to exhibit better photocatalytic activity under visible light irradiation compared with pure ZnO by utilizing the light down-converting characteristics of the phosphors to facilitate the self-degradation of dye. It is known that different phosphors can emit different visible light with different wavelengths. Up to now, the amount of exploration of such a kind of novel composites has been not nearly enough so far. Lu3Al5O12:Ce3+ is iso-structure of Y3Al5O12:Ce3+, but shows a higher luminescent intensity, quantum yield and thermal stability than Y3Al5O12:Ce3+ [25]. Therefore, the incorporation of Lu3Al5O12:Ce3+ into ZnO to form hybrid materials should be a promising method to enhance the visible light photocatalytic activity of ZnO. Currently, research indicated that the incorporation of α-Si3N4 into Lu3Al5O12:Ce3+ to obtain Lu3Al5-xSixO12-xNx:Ce3+ (x is the atom ratio of α-Si3N4 and Lu3Al5O12:Ce3+) leads to an obvious red shift and broadens the emission spectrum, which is responsible for the enhancement of the photocatalytic activity. Unfortunately, so far the influence of light down converting property for Lu3Al5-xSixO12-xNx:Ce3+ with different α-Si3N4 doped content on the photocatalytic activity of ZnO has not yet been reported. In

this

work,

we

report

a

fast

strategy

for

synthesizing

ZnO-

Lu3Al5-xSixO12-xNx:Ce3+ (ZLS) hybrid photocatalysts by a microwave-assisted reaction of ZnO precursor with a Lu3Al5-xSixO12-xNx:Ce3+ suspension, and their photocatalytic 6

activity was investigated. Microwave irradiation can heat the reactant in closed vessels under high pressure and stirring to a high temperature in a short time by transferring energy selectively to microwave absorbing polar solvents. More energy input at the same temperature and enormous acceleration in reaction can be achieved, which means that a reaction in conventional hydrothermal or solvothermal method that takes several hours can be completed over the course of minutes or a reaction that would not proceed previously will now proceed, typically, with higher yields [26]. What is more, it is possible to program and to control the different synthesis steps [27]. Therefore, microwave synthesis has been accepted as a promising method for rapid heating, higher reaction rate and selectivity, lower reaction temperature, less reaction time, homogeneous thermal transmission, and the phase purity with better yield [28]. ZLS hybrid photocatalysts exhibit enhanced photocatalytic activity in the degradation of methylene blue (MB) under visible light irradiation compared with pure ZnO. The photocatalytic mechanism was also studied in terms of a series of characterization and controlled experiments using radical scavengers.

2. Experimental 2.1 Synthesis of ZnO-Lu3Al5-xSixO12-xNx:Ce3+ composites Lu3Al5-xSixO12-xNx:Ce3+ with x=0-0.21 phosphors were fabricated by a simple solid-state reaction, which has been reported in the literature [25]. A certain amount of Lu3Al5-xSixO12-xNx:Ce3+ with x=0-0.21 were added into 20 ml 0.1 M ZnSO4 solution, which was placed in a 35 ml microwave tube, and then the solution was 7

sonicated for 30 min to produce uniform dispersion. A dilute NaOH solution was added into the solution until the pH value reached 12. The mixture was then placed into an automated focused microwave oven (Explorer-48, CEM Co.) and treated at 150 °C with a microwave irradiation power of 150 W for 10 min. The as-synthesized ZLS samples with 1 wt.% Lu3Al4.91Si0.09O11.91N0.09:Ce3+,Lu3Al4.85Si0.15O11.85N0.15:Ce3+, and Lu3Al4.79Si0.21O11.79N0.21:Ce3+, named as ZLS-1-3, ZLS-1 and ZLS-1-6, were isolated by filtration, washed for three times with distilled water, and finally dried in a vacuum oven at 60 °C for 24 h. The ZLS samples with 0.5 and 2 wt.% Lu3Al4.85Si0.15O11.85N0.15:Ce3+, named as ZLS-0.5 and ZLS-2, respectively, was synthesized for comparison. Pure ZnO was synthesized by a direct microwave assisted reaction for comparison. The detailed synthesis mechanism is as follows [14]. Under alkaline condition of pH > 9, a conversion from Zn+ and ZnOH+ ions to Zn(OH)2 nucleation happens. Microwave irradiation can be used to synthesize the hydrophilic nanoparticles or to improve the surface wettability by increasing surface free

energy

[29].

Therefore,

Zn(OH)2

is

attached

easily

around

Lu3Al5-xSixO12-xNx:Ce3+ due to the improvement of surface hydrophilicity of Lu3Al5-xSixO12-xNx:Ce3+. The Zn(OH)2 is transformed to ZnO in the surrounding of Lu3Al5-xSixO12-xNx:Ce3+ under microwave heating. Therefore, a good contact between ZnO and Lu3Al5-xSixO12-xNx:Ce3+ is formed in the ZLS composites [30, 31]. For the electrochemical impedance spectra (EIS) testing, 90 mg sample with 0.2 ml 2.5 wt.% polyvinyl alcohol binder was homogenously mixed in water to form slurry. Then, the resultant slurries were coated on the graphite flake (2 cm × 2 cm). Finally, these 8

prepared electrodes were dried in a vacuum oven at 60 °C for 24 h. 2.2 Characterization The surface morphology and structure of the samples were characterized by field-emission scanning electron microscopy (FESEM, Hitachi S-4800), and X-ray diffraction (XRD, Holland Panalytical PRO PW3040/60) with Cu Kα radiation (V=30 kV, I=25 mA). X-ray photoelectron spectroscopy (XPS) measurement was carried out using a Thermo ESCALAB 250 Xi spectrometer with a monochromatic Al Kα X-ray source. The Brunauer-Emmett-Teller (BET) specific surface areas of the samples were evaluated on the basis of nitrogen adsorption isotherms measured at 77 K using a BELSORP-max nitrogen adsorption apparatus (Micrometitics, Norcross, GA). The diffuses absorption spectra of the samples were recorded using a PerkinElmer Lambda750S UV-Vis-NIR spectrophotometer equipped with an integrated sphere attachment by using BaSO4 as a reference. Photoluminescence (PL) spectra at room temperature were examined by fluorescence spectrophotometer (HORIBA Jobin Yvon fluoromax-4). EIS measurements were carried out on an electrochemical workstation (AUTOLAB PGSTAT302N) under dark conditions using a three electrode configuration with the as-prepared films as working electrode, a Pt foil as counter electrode and a standard calomel electrode as reference electrode. The electrolyte was 5 mg l-1 MB aqueous solution. EIS were recorded in the frequency range of 0.1 Hz-1 MHz, and the applied bias voltage and ac amplitude were set at open-circuit voltage and 10 mV. 2.3 Photocatalytic experiments 9

The photocatalytic performance of the as-prepared samples was evaluated through the experiments of photocatalytic degradation of MB under visible light irradiation. The samples (2 g l-1) were dispersed in 80 ml MB aqueous solutions (5 mg l-1). The suspensions were magnetically stirred in the dark for 30 min to reach the adsorption-desorption equilibrium. Under ambient conditions and stirring, the mixed suspensions were exposed to visible light irradiation produced by a 400 W metal halogen lamp (λ > 400 nm) with a cut off filter. At certain time intervals, 2 ml of the mixed suspensions were extracted and centrifuged to remove the photocatalyst. The filtrates were analysed by recording the UV-vis spectra of MB using a Hitachi U-3900 UV-vis spectrophotometer. A recycled photocatalytic activity test was carried out according to the above-mentioned procedure. After each run of photocatalytic reaction, the fresh MB aqueous solution was injected, and the separated photocatalyst was washed with deionized water carefully and used again. To investigate the photocatalytic mechanism, trapping experiments were carried out to determine the main reactive species in the photocatalytic process. The experimental procedure was similar to the photocatalytic activity measurement except that the radical scavengers were added into the reaction system. 2.4 Analysis of hydroxyl radicals (·OH) The formation of hydroxyl radicals at the photocatalyst/water interface under visible light irradiation was detected by a PL technique using coumarin as probe molecule, which reacted with ˙OH radicals to produce the highly fluorescent product, 7-hydroxycoumarin. This method relies on the PL signal at 450 nm corresponding to 10

the hydroxylation of coumarin with ˙OH generated at the photocatalyst/water interface. The experimental procedures used are similar to the measurement of photocatalytic activity except that MB aqueous solution is replaced by 0.5 mmol l-1 coumarin aqueous solution. After irradiation every 15 min, the reaction solution was filtered and analyzed on a HORIBA Jobin Yvon fluoromax-4 fluorescence spectrophotometer under excitation at 332 nm.

3. Results and discussion 3.1 Characterizations Fig.

1(a)-(c)

shows

the

FESEM

images

of

as-synthesized

ZnO,

Lu3Al4.85Si0.15O11.85N0.15:Ce3+ and ZLS-1, respectively. Lu3Al4.85Si0.15O11.85N0.15:Ce3+ shows the particle morphology with diameters in the range of 0.5-2 μm. Some Lu3Al4.85Si0.15O11.85N0.15:Ce3+ particles are aggregated randomly into large particles. ZnO displays the particles nanostructure with diameter in the range of 50-150 nm. The morphologies of ZLS-0.5, ZLS-2, ZLS-1-3 and ZLS-1-6 (not shown here) are similar to that of ZLS-1. It is clearly observed from Fig. 1(c) that the morphology of ZnO in the ZLS composite is similar to that of pure ZnO, which indicates that the introduction of Lu3Al4.85Si0.15O11.85N0.15:Ce3+ in ZnO does not affect the formation of ZnO obviously. The Lu3Al4.85Si0.15O11.85N0.15:Ce3+ particles are distributed into ZnO. Fig. 1(d) shows the XRD patterns of Lu3Al4.85Si0.15O11.85N0.15:Ce3+, ZnO and ZLS-1. All peaks in the Lu3Al4.85Si0.15O11.85N0.15:Ce3+ pattern can be indexed as the cubic garnet structured Lu3Al5O12 (JCPDS 18-0761) [25]. The peaks at 31.5º, 34.1º, 36º, 11

47.2º, 56.4º, 62.6º, and 67.7º are indexed to (100), (002), (101), (102), (110), (103), and (112) planes of wurtzite structured ZnO (JPCDS 36-1451), respectively. Furthermore, it can be observed that the main diffraction peaks of ZLS-1 are similar to those of pure ZnO, suggesting that the presence of Lu3Al4.85Si0.15O11.85N0.15:Ce3+ does not result in the development of new crystal orientations of ZnO. Compared with pure ZnO, new peaks corresponding to Lu3Al4.85Si0.15O11.85N0.15:Ce3+ appear in the XRD

pattern

of

ZLS-1,

which

further

confirms

the

existence

of

Lu3Al4.85Si0.15O11.85N0.15:Ce3+ in the composite. However, the intensities of the peaks corresponding to Lu3Al4.85Si0.15O11.85N0.15:Ce3+ in the XRD pattern of ZLS-1 are very weak, which may be due to the low amount of in the ZLS-1 composites [14]. In order to investigate the chemical composition of ZLS hybrid photocatalysts (ZLS-1), XPS measurements were carried out. Fig. 2(a) and (b) show the high resolution XPS spectra of Zn 2p and O 1s. The peak at 1021.7 eV is attributed to Zn 2p3/2, which is similar to those reported in the literature [32]. The O 1s spectrum of ZLS-1 is fitted to two peaks. The peak of O 1s at 530 eV corresponds to lattice oxygen of ZnO, and a higher binding energy of 531.7 eV is assigned to mixed contributions from surface hydroxides. Fig. 2(c-f) shows the high resolution XPS spectra of Lu 4d, Al 2p, Ce 3d and N 1s. Two characteristic peaks located at 205.9 eV and 196.3 eV can be assigned to Lu 4d5/2 and Lu 4d3/2, respectively, which is agreement with the values for Lu(III) compounds. The Al 2p spectrum shows a peak centered at 74.7 eV. The Ce 3d spectrum displays the spin-orbit split lines of Ce 3d5/2 and Ce 3d3/2 at 885 eV and 904 eV, respectively. The binding energy located at 397.4 12

eV for N 1s spectrum could be ascribed to the Si-N bond, which indicates that the existence of Si and N in the ZLS hybrid photocatalysts. Fig. 3 shows the PL excitation and emission spectra of Lu3Al5O12:Ce3+ and Lu3Al4.85Si0.15O11.85N0.15:Ce3+ phosphors. Upon the excitation of 445 nm, the main emission peak for Lu3Al5O12:Ce3+ is observed at 515 nm. When the α-Si3N4 is doped into Lu3Al5O12:Ce3+, the emission peaks shift to longer wavelength. Our previous work reported that the emission peak shifts from 515 nm to 558 nm with the increasing

x

from

0

to

0.21

[25].

Therefore,

the

emission

band

of

Lu3Al5-xSixO12-xNx:Ce3+ phosphors matches the absorption range of the MB (about 550-700 nm), which is beneficial to the self-sensitized destruction of MB. Fig. 4 shows the nitrogen adsorption-desorption isotherms of ZnO and ZLS-1. It can be observed that all of them show type IV isotherms with H3 hysteresis loop. The specific surface areas of ZnO and ZLS-1 are 6 and 17 m2 g-1, respectively. The result means that the incorporation of Lu3Al5-xSixO12-xNx:Ce3+ phosphors increases the specific surface areas of hybrid photocatalyst, which is beneficial to the adsorption of MB [14, 23, 24, 33]. Fig. 5 shows the UV-Vis absorption spectra of ZnO, ZLS-1-3, ZLS-1 and ZLS-1-6. The characteristic absorption peak at 360 nm is dominated by ZnO. It can be observed that ZLS hybrid photocatalysts exhibit higher absorbance in the visible light and the intensity increases with the increase of α-Si3N4 content, which is ascribed to the contribution from Lu3Al5-xSixO12-xNx:Ce3+ phosphors. The result is similar to those reported in the case of composite materials [34, 35]. Such an enhancement of 13

light absorption should increase the number of photo-generated electrons and holes to participate in the photocatalytic reaction. The charge transfer and recombination behaviour of the as-prepared samples was studied by analysing the EIS spectra at open-circuit voltage in dark conditions. Fig. 6 shows the typical Nyquist plots of ZnO, ZLS-1-3, ZLS-1 and ZLS-1-6. The semicircle in the EIS spectra is due to the contribution from the charge transfer resistance (Rct) and constant phase element (CPE) at the photocatalyst/electrolyte interface. The inclined line, resulting from the Warburg impedance ZW, corresponds to the ion-diffusion process in the electrolyte. The corresponding equivalent circuit is shown in the inset of Fig. 6. It can be observed that the Rct decreases with the increase of α-Si3N4 content, which is beneficial to the separation and transportation of photo-induced carriers [24]. The explanation may be due to the good contact between ZnO and Lu3Al5-xSixO12-xNx:Ce3+ [23, 24, 33]. However, when the α-Si3N4 content is further increased (ZLS-1-6), the Rct increases. The result indicates that excessive α-Si3N4 doping into Lu3Al5O12:Ce3+ can act as recombination centre instead of providing an electron pathway and promote the recombination of electron-hole pairs [23, 24].

3.2 Photocatalytic activity The effect of α-Si3N4 content on the photocatalytic activity of ZLS hybrid photocatalysts was observed. Photocatalytic degradation of MB by ZnO, ZLS-1-3, ZLS-1 and ZLS-1-6 was performed under visible light irradiation. Fig. 7(a) shows the 14

UV-vis absorption spectra of MB with irradiation time under visible light irradiation using ZLS-1. It is observed that the UV-vis absorption of MB, related to its concentration in the solution, becomes weak with the increase in the irradiation time, while its shape does not vary during the photocatalytic process. Fig. 7(b) displays the time-dependent reduction rates of MB by ZnO, ZLS-1-3, ZLS-1 and ZLS-1-6 under visible light irradiation. The normalized temporal concentration changes (C/C0) of MB during the photocatalytic process are proportional to the normalized maximum absorbance (A/A0), which can be derived from the change in the MB absorption profile during the photocatalysis process. It is observed that the concentration of MB is hardly reduced under visible light irradiation in the absence of the photocatalyst, which is in good agreement with the reported results [36-40]. The degradation rate of MB for pure ZnO under visible light irradiation at 240 min is 13% due to the self-sensitized degradation of MB [36]. Because the photocatalytic reaction is carried out on the photocatalysts surface, the illumination and photocatalysts are necessary in the degradation of MB process [36]. It can be also found that ZLS hybrid photocatalysts exhibit better photocatalytic activity than pure ZnO. The photocatalytic activity of ZLS depends on the proportion of α-Si3N4 in the hybrid photocatalysts. When Lu3Al5-xSixO12-xNx:Ce3+ is introduced into ZnO, with the increase of α-Si3N4 content, the degradation rate increases to 41% for ZLS-1-3 and reaches a maximum value of 91% for ZLS-1 at 240 min. However, when the Lu3Al5-xSixO12-xNx:Ce3+ content is further increased, the degradation rate decreases to 30% for ZLS-1-6 at 240 min. Furthermore, the influence of Lu3Al5-xSixO12-xNx:Ce3+ content on the 15

photocatalytic activity of ZLS hybrid photocatalysts was also studied under visible light irradiation (Fig. S1). It can be observed that the ZLS hybrid photocatalysts with 1 wt.% Lu3Al4.85Si0.15O11.85N0.15:Ce3+ (ZLS-1) exhibits the best photocatalytic activity under visible light irradiation. It is well known that the stability and reusability of photocatalysts are very important for practical application. The reusability of ZLS hybrid photocatalysts (ZLS-1) was investigated under visible light irradiation with three times of cycling uses, as shown in Fig. 8(a). It is noteworthy that only insignificant decrease for photocatalytic activity is found, which may be due to the loss of photocatalyst during collection process [41]. The crystal structure of ZLS-1 after the photocatalytic reaction (labeled as irr-ZLS-1) was characterized by XRD. It can be seen that the crystal structure of irr-ZLS-1 did not show obvious changes before and after the photocatalytic reaction (Fig. 8(b)). All results confirm that the ZLS hybrid photocatalysts can retain excellent photocatalytic stability and reusability under the studied conditions.

3.3 Mechanism of Photocatalytic activity Visible light photocatalytic degradation of dyes using ZnO photocatalyst is a typical self-sensitized degradation process [23, 24]. In photocatalytic degradation of MB process using ZnO (Fig. 9), the MB is excited under visible light irradiation to MB*, and thus generates the electron-hole pairs. The CB and VB of ZnO are -0.45 and 2.75 V (vs. NHE), respectively [42]. The redox potentials of E0 (MB*/MB) (-0.71 V, 16

vs. NHE) is negative than the CB level of ZnO [43]. Therefore, the photo-generated electrons can migrate from MB* species to the conduction band of ZnO. Because the CB level of ZnO is negative than the O2/·O2- (-0.28 V, vs. NHE) [44] and O2/H2O2 (0.695 V, vs. NHE) [45] potential, the photo-generated electrons of ZnO can reduce the adsorbed O2 to produce the superoxide radical anions (·O2-) and hydrogen peroxide (H2O2). These new forms of intermediates will interact to produce ·OH radials [36, 37]. Moreover, the ·OH/H2O potential (2.3 V, vs. NHE) is negative than the VB of ZnO [44], thus the photo-generated holes of ZnO can oxide H2O to form ·

OH, which is confirmed by the PL method using coumarin as a probe molecule (Fig.

7) [15, 46]. Fig. 10(a) shows the changes of the PL spectra for coumarin solution with the irradiation time in the presence of ZLS-1. It is observed that the spectra have an identical shape with maximum wavelength at about 450 nm, which indicates that the 7-hydroxycoumarin generated by the reaction of ·OH with coumarin was formed on the surface of ZLS hybrid photocatalysts. The PL intensity increases with the irradiation time, which suggests that the amount of ·OH radicals produced in the photo-irradiated process increases. These results are in good agreement with the previous study [47]. Fig. 10(b) shows a comparison of the photo-induced PL intensity at 450 nm for ZnO, ZLS-1-3, ZLS-1 and ZLS-1-6 with irradiation time. It is found that ZLS-1 exhibits a maximal PL intensity, which is also beneficial to the enhancement of photocatalytic activity. The PL intensity of ZnO is higher than that of ZLS-1-3 and ZLS-1-6, which is similar to the reported result [48] The produced radicals are responsible for the photocatalytic degradation of MB. In addition, the 17

holes also contribute to the oxidative pathways for the degradation of organic pollutants. However, the photocatalytic degradation rate of MB using ZnO under visible light irradiation is limited because of low self-sensitized degradation and slow interfacial electron transfer [49]. When Lu3Al5-xSixO12-xNx:Ce3+ phosphors are introduced into ZnO, the Lu3Al5-xSixO12-xNx:Ce3+ phosphors can emit the visible light at a longer wavelength (515-558 nm), which can be easily absorbed by MB and thus effectively excite the MB to generate more electron-hole pairs, resulting in the improvement of the self-sensitized destruction of MB, which enhances the visible light photocatalytic activity of ZnO. During visible light photocatalysis, besides self-sensitized degradation of dye, the adsorption, light absorption as well as the charge separation and transportation are crucial factors [33, 50]. Therefore, besides the enhancement of self-sensitized degradation of MB, the improvement of photocatalytic activity of ZLS-1 should also be mainly ascribed to the lower electron-hole pair recombination, which has been confirmed by EIS measurement. However, when the α-Si3N4 content is further increased (ZLS-1-6), the Rct increases. The result indicates that excessive α-Si3N4 doping into Lu3Al5O12:Ce3+ can act as recombination centre instead of providing an electron pathway and promote the recombination of electron-hole pairs [23, 24]. To understand the possible photocatalytic mechanism, the main oxidative species in the photocatalytic process were detected through the controlled experiments. Tert-butanol (t-BuOH, 1 mmol), benzoquinone (C6H4O2, 1 mmol) and edetate 18

disodium (EDTA-Na, 1 mmol) were employed as scavengers for hydroxyl radicals (·OH), superoxide anion radicals (·O2-) and holes (h+), respectively. As shown in Fig. 11, the photocatalytic activity of ZLS-1 decreases slightly with the addition of ·O2-, and reduces largely with the addition of ·OH and h+. The results indicates that the photo-induced ·OH and h+ mainly govern the photocatalytic process, which is consistent with the literature [51]. The major reaction steps are summarized as follows: MB  hν  MB*  h   e e   O 2  O2

2e   O 2  2H   H 2 O 2 H 2 O 2  e  . OH  OH  h   H 2 O. OH  H 

h  / . OH  MB  H 2 O  CO 2

4. Conclusions ZLS

hybrid

photocatalysts

were

successfully

synthesized

via

a

microwave-assisted reaction of the ZnO precursor with a Lu3Al5-xSixO12-xNx:Ce3+ suspension using a microwave system and their photocatalytic activity were investigated. The results indicate that (i) ZLS hybrid photocatalysts exhibit a better photocatalytic activity than pure ZnO; (ii) the photocatalytic activity of ZLS is dependent on the proportion of α-Si3N4 in the hybrid photocatalysts and ZLS-1 achieves the highest MB degradation rate of 91% under visible light irradiation at 240 19

min; (iii) the enhanced photocatalytic activity is mainly ascribed to the reduction of photoelectron-hole

pair

Lu3Al5-xSixO12-xNx:Ce3+,

recombination and

the

in light

ZnO

with

the

introduction

down-converting

effect

of of

Lu3Al5-xSixO12-xNx:Ce3+, which facilitates the self-sensitized degradation of MB.

Acknowledgments Financial support from the National Natural Science Foundation of China (No. 21401180) is gratefully acknowledged.

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Figure captions Fig. 1 FESEM images of (a) LASG:Ce3+ with x=0.15, (b) ZnO particles and (c) ZLS-1; (d) XRD patterns of (A) LASG:Ce3+ with x=0.15, (B) ZnO and (C) ZLS-1. Fig. 2 XPS spectra of (a) Zn 2p, (b) O 1s, (c) Lu 4d, (d) Al 2p, (e) Ce 3d and (f) N 1s. Fig. 3 PL excitation and emission spectra of LAG:Ce3+ and LASG:Ce3+ with x=0.15. Fig. 4 Nitrogen adsorption-desorption isotherms of ZnO and ZLS-1. Fig. 5 UV-vis absorption spectra of ZnO, ZLS-1-3, ZLS-1 and ZLS-1-6. Fig. 6 Nyquist plots of ZnO, ZLS-1-3, ZLS-1 and ZLS-1-6. Inset is the corresponding equivalent circuit model. Fig. 7 (a) UV-vis absorbance of MB with the variation of visible light irradiation time using ZLS-1; (b) Photocatalytic degradation of MB by ZnO, ZLS-1-3, ZLS-1 and ZLS-1-6 under visible light irradiation. Fig. 8 (a) Photo-stability of ZLS-1 by investigating its photocatalytic activity with three times of cycling uses; (b) XRD patterns of irr-ZLS-1 and ZLS-1. Fig. 9 Proposed photocatalytic mechanism for ZLS hybrid photocatalysts under visible light irradiation. Fig. 10 (a) PL spectral changes for coumarin solution during the illumination using ZLS-1; (b) comparison of PL intensity at 450 nm against irradiation time. Fig. 11 Photocatalytic degradation of MB by ZLS-1 with the addition of hole and hydroxyl radical scavengers under visible light irradiation.

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Accepted Manuscript Title: Down-conversion phosphors as noble-metal-free co-catalyst in ZnO for efficient visible light photocatalysis Author: Haipeng Chu Xinjuan Liu Jiaqing Liu Wenyan Lei Jinliang Li Tianyang Wu Ping Li Huili Li Likun Pan PII: DOI: Reference:

S0169-4332(16)31458-1 http://dx.doi.org/doi:10.1016/j.apsusc.2016.07.031 APSUSC 33599

To appear in:

APSUSC

Received date: Revised date: Accepted date:

27-3-2016 1-7-2016 3-7-2016

Please cite this article as: Haipeng Chu, Xinjuan Liu, Jiaqing Liu, Wenyan Lei, Jinliang Li, Tianyang Wu, Ping Li, Huili Li, Likun Pan, Down-conversion phosphors as noblemetal-free co-catalyst in ZnO for efficient visible light photocatalysis, Applied Surface Science http://dx.doi.org/10.1016/j.apsusc.2016.07.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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