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Highly Efficient Photocatalytic Degradation Performance of CsPb(Br1-xClx)3-Au Nanoheterostructures Xianbin Feng, Hongmei Ju, Tinghui Song, Tingsen Fang, Wenchao Liu, and Wei Huang ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b06023 • Publication Date (Web): 04 Feb 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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ACS Sustainable Chemistry & Engineering

Highly Efficient Photocatalytic Degradation Performance of CsPb(Br1-xClx)3-Au Nanoheterostructures Xianbin Feng†, Hongmei Ju†, Tinghui Song, Tingsen Fang, Wenchao Liu*, Wei Huang* Key Laboratory of Flexible Electronics (KLOFE) & Institute of Advanced Materials (IAM), Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University (NanjingTech), 30 South Puzhu Road, Nanjing 211816, China *Corresponding author: [email protected] (Dr. Liu), [email protected] (Prof. Huang) †These authors contributed equally to this work.

Abstract All-inorganic CsPbX3 (X = I, Br, Cl) perovskite nanocrystals (NCs) have attracted much attention in clean energy field such as solar cell and light-emitting diode due to the excellent opto-electric properties. Herein, we extended the application of perovskite NCs to photocatalytic degradation filed. We demonstrated a facile and strategy for highly efficient and feasible synthesis of pure CsPb(Br1-xClx)3 NCs and CsPb(Br1-xClx)3-Au

nanoheterostructures.

The

photocatalytic

performance

of

CsPb(Br1-xClx)3-Au for degrading water-insoluble carcinogenic Sudan red III under visible light irradiation was characterized by UV-vis absorption spectra. CsPb(Br1-xClx)3-Au nanoheterostructures showed excellent photocatalytic activities, which can degrade about 71% of Sudan red III within 6 hours. This study provides a new

way

to

use

semiconductor

perovskite-metal

nanoheterostructures

in

photocatalytic applications. Keywords: perovskite, photocatalytic degradation, nanoheterostructures, nanocrystals, Sudan Red

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Introduction All-inorganic CsPbX3 (X = I, Br, Cl) perovskite nanocrystals (NCs) have attracted increasing attention as one of the most promising materials for the next generation opto-electronic applications due to their advantages of

high

photoluminescence quantum yields, tunable band gap, low defect density, long carrier life-time, broad emission spectra tunability, short radiative life-times and better environmental stability than organic-inorganic hybrid perovskite1-5. Since 2015, various forms of CsPbX3 NCs such as nanocubes1, nanowires6, nanoplatelets7, 8, quantum dots9-11, and nanorods12 have been synthesized by varying surfactant ligands, reaction temperature and duration time. Up to now, the optoelectronic applications of CsPbX3 perovskite NCs focus on solar cells13, 14，light-emitting devices15, 16, lasers17, and photodetectors18 and so on. Other applications such as photocatalysis were rarely reported. Xu YF et.al19 reported the application of CsPbBr3 NCs as photocatalysts to convert CO2 into solar fuels. Kong ZC et.al20 synthesized CsPbBr3@Zeolitic nanocomposites for photocatalytic CO2 reduction. To our best knowledge, up to now, there are no reports about the photocatalytic degradation of pollutants of CsPbX3 and CsPbX3-Au nanoheterostructures. In this paper, we explored the potential application of all-inorganic perovskite NCs and CsPb(Br1-xClx)3-Au semiconductor-metal nanoheterostructures in photo-degradation of Sudan red III pollutants under visible light. Sudan red III is a lipophilic azo dye with a compound of naphthalene, as shown in figure S1 in Supporting Information. Apart from uses of dyeing of paints, engine oils and cloth polishes, it was also used as food additives illegally. Since 1995, Sudan


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red as food additives are prohibited since it is carcinogenic and has significant toxic effects on human liver and kidney organs due to the nature of their chemical structure. More importantly, Sudan red dyes are normally water-insoluble, and difficult to be removed once it enters into the digestive system. Therefore, it is meaningful to develop a feasible and efficient method to degrade Sudan red pollutants. Herein, pure CsPbX3 NCs and CsPb(Br1-xClx)3-Au nanoheterostructures were synthesized as a new type of

photocatalyst to degrade Sudan red III. The Pure CsPbX3 NCs were

synthesized using hot-inject method and the CsPb(Br1-xClx)3-Au nanoheterostructures were obtained by reducing HAuCl4.3H2O on the surface of CsPbBr3 NCs. CsPb(Br1-xClx)3-Au nanoheterostructures exhibited superior photoactivities compared to pure perovskite NCs under visible light irradiation, suggesting a promising future in photocatalysis degradation field. Experimental Materials Cs2CO3 (Aladdin, 99.9%), PbBr2 (Aladdin, 99.0%), octadecene (ODE, Aladdin, 90%), oleic acid (OA), oleylamine (OLA, Aladdin), ethyl acetate, toluene (Shanghai Lingfeng Chemical Reagent Co. Ltd., ≥ 99.5%), HAuCl4.3H2O (Sigma Aldrich), Sudan red III (Aladdin) were purchased and used without further purification. Synthesis of CsPb(Br1-xClx)3 NCs The synthesis of perovskite NCs follows the previous report by Protesescu20. Cs2CO3 (814.0 mg) was added into octadecene (ODE, 40 mL) and oleic acid (OA, 2.5 mL) solution in 100 mL 3-neck flask. After being dried for 1 h at 120 ºC, it was



heated to 150 ºC under the protection of N2. In another 100 mL 3-neck flask, PbBr2 (53.9 mg) and PbCl2 (23.0 mg) in ODE (5 mL) were dried under N2 at 120 ºC for 1 h, then OA (0.5 mL) and OLA (0.5 mL) were injected into the mixture. After 30 min, the solution was heated to 180 ºC, and then the hot Cs-oleate solution (0.4 mL) (preheated to 100 ºC) was injected quickly. After 5 s, the mixture solution was immediately cooled in the ice-water. The obtained CsPb(Br1-xClx)3 NCs were purified by adding ethyl acetate (20 mL) and centrifugated at high speed of 12000 rpm, and then redispersed in toluene. Synthesis of CsPb(Br1-xClx)3-Au nanoheterostructures Au NCs decorated CsPb(Br1-xClx)3 were synthesized as the following steps: 9.8 mg of HAuCl4.3H2O was added into the CsPbBr3 NCs dispersed toluene (20 mL), after stirring 5 min, the mixture was stored in the 20 mL vial. HAuCl4.3H2O can be reduced by oleylamine connecting with the surface of the CsPbBr3, resulting in the Au NCs attachment on the surface of perovskite NCs. Figure S2 in Supporting Information shows the synthesis process for CsPb(Br1-xClx)3-Au nanoheterostructures. Characterization X-ray diffraction (XRD) spectra was measured in a diffractometer (D8 Advance) with Cu Kα radiation (λ=1.5406 Å). The size, structural phase and morphology of perovskite and Au NCs were obtained by a transmission electron microscopy (TEM, JEOL JEM200CX). Energy-dispersive X-ray spectroscopy (EDS, HORIBA, EX-250) was used to confirm the chemical composition. The absorption spectra were obtained by an ultraviolet spectrophotometer (SHIMADZU UV1750). Fluorescence spectra


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were measured by a fluorescence spectrometer (F-4600). Photocatalytic activity Prior to the illumination, pure perovskite NCs and CsPb(Br1-xClx)3-Au nanoheterostructures dispersed in toluene (2 mL) were mixed with 18 mL of 2.7 × 10−6 M Sudan red III toluene solution. The mixture was stirred in the dark for 30 min to ensure the adsorption and desorption equilibrium between the perovskite photocatalyst and Sudan red III. A 300 W Xe lamp was used to illuminate the solution. The concentration (C) change of Sudan red III was characterized through measuring the UV–vis absorption of the solution. The photocatalytic performance was further investigated through the profile of C0/C vs time. Results and discussion XRD was used to confirm the crystal phase of CsPb(Br1-xClx)3 NCs and CsPb(Br1-xClx)3-Au nanoheterostructures, as shown in figure 1. The peaks of 2θ= 15.21°, 21.49°, 30.69°, 34.19°, 37.60° and 43.69° nicely match the diffractions from (100), (110), (200), (210), (211) and (202) planes of cubic perovskite CsPbBr3 (JCPDS 18-0364). When the HAuCl4.3H2O was added into CsPbBr3 solution, a narrow, sharp and strong peak at 38.2° appears, which is assigned to cubic (111) plane of Au. It means the successful formation of Au and CsPb(Br1-xClx)3 composite phase. Figure 2 shows TEM, HRTEM, and SAED images of pure CsPb(Br1-xClx)3 and CsPb(Br1-xClx)3-Au nanoheterostructures. Figure 2a and b show that pure perovskite NCs present a typical cubic morphology with an average size of 12 nm and a narrow size distribution. Figure 2c shows that CsPb(Br1-xClx)3 NCs are nearly monodispersed



and Au NCs randomly attach on their surface. The CsPb(Br1-xClx)3 NCs have a uniform size with an average diameter of 9.0 nm, and Au NCs have a uniform size with an average diameter of 2.9 nm. HRTEM (figure 2d) image shows that the d-spacing of 0.22 nm corresponds to (111) lattice plane of cubic Au and d-spacing of 0.40 nm corresponds to (110) lattice plane of CsPb(Br1-xClx)3. The Au NCs are decorated on the corners of CsPb(Br1-xClx)3 cubic NCs. The size of perovskite NCs in nanoheterostructures is relatively smaller than that of pure perovskite NCs. In our synthesis process, the formation process of CsPb(Br1-xClx)3-Au nanoheterostructures includes two steps: perovskite NCs are synthesized at higher temperature of 180 oC and then Au is grown on the surface of perovskite NCs through an in-situ reduction mechanism at lower room temperature, as shown in figure 2e. The oleylamine surrounding the perovskite NCs acts as the reductant to reduce Au(III) ions into Au(0) on the surface of perovskite NCs while the cubic perovskite NCs act as the crystal seed for the growth of Au NCs. Finally, Au NCs are randomly deposited on the surface of the cubic perovskite NCs seed. The formation of Au NCs is accompanied with a partial dissolution of perovskite lattice, resulting in the slightly reduction of the size of perovskite NCs. The quantized energy dispersive spectroscopy (EDS) (Figure S3 and Table S1 in Supporting Information) indicates that CsPb(Br1-xClx)3 NCs are nearly stoichiometric and the molar ratio of Au/CsPb(Br1-xClx)3 is around 10%. Figure S4 in Supporting Information shows the fluorescence and absorption spectra of CsPbBr3 NCs before and after addition of HAuCl4·3H2O. The absorption edge of CsPb(Br1-xClx)3-Au


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nanoheterostructures shows a slight blue-shift due to the introduction of Cl element. The emission spectrum shows a peak position at 520 nm, exhibiting a blue shift following the introduction of Au. Moreover, a decrease in the ﬂuorescence intensity is also observed when Au NCs are attached on CsPbBr3 NCs. This reduction is due to the transfer of photoexcited electrons from semiconductor NCs to Au NCs, resulting in a charge-separated state21, 22, which will give help to photocatalytic degradation activity. Photocatalytic activity of the as-synthesized bare CsPb(Br1-xClx)3 NCs and CsPb(Br1-xClx)3-Au nanoheterostructures is evaluated through the degradation of Sudan red III under visible light irradiation. Figure 3a and 3b are the absorption spectra of Sudan red III degradation using CsPb(Br1-xClx)3-Au nanoheterostructures (10 mg) and CsPb(Br1-xClx)3 NCs (10 mg). The absorption intensity of Sudan red III degraded by CsPb(Br1-xClx)3-Au nanoheterostructures decreases rapidly with the irradiation time compared to that by pure CsPb(Br1-xClx)3. Figure 3c is the concentration (C/C0) versus irradiation time for photo-degradation of Sudan red III by CsPb(Br1-xClx)3-Au nanoheterostructures, where C0 is the original concentration and C is the concentration of Sudan red III after irradiation for time t. After exposure to visible light for 6 h, nearly 71 % of Sudan red III was degraded by the CsPb(Br1-xClx)3-Au nanoheterostructures, while only 20 % of Sudan red III was degraded by pure perovskite NCs. For comparison, Sudan red III is rarely degraded while there are no catalysts. The CsPb(Br1-xClx)3-Au nanoheterostructures show good photocatalytic performance in degrading Sudan red III pollution with a



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stable molecular structure. The good photocatalytic efficiency can be attributed to the efficient

separation

CsPb(Br1-xClx)3-Au

and

transfer

of


the The

photogenerated effect

of

carriers

in

the

CsPb(Br1-xClx)3-Au

nanoheterostructures concentration on the photocatalytic performance is also evaluated, as shown in figure S5 in Supporting Information. Figure 3d suggests that the

photocatalytic

performance

increases

as

the

CsPb(Br1-xClx)3-Au

nanoheterostructures catalyst increases from 5 mg to 10 mg, and then decreases as the catalyst increases further to 20 mg. Now we discuss the possible mechanism of Sudan red III degradation by CsPb(Br1-xClx)3-Au


As

shown

in

figure

4,

in

the

nanoheterostructures, under light irradiation, the electrons are excited from the valence band to the conduction band of the semiconductor perovskite CsPb(Br1-xClx)3 NCs, leaving holes at the valence. After contact, the energy band bending at the Au-CsPb(Br1-xClx)3 occurs once the interface is formed since Au has a lower Fermi level, as shown in figure 4b. As a result, an equilibrium Femi level is created. At the interface of perovskite and Au, a ‘built in’ electric field directing from perovskite to Au forms. The electric field quickly sweeps free carriers out, hence the region is depleted of free carriers. The built of depletion region can enhance the separation and inhibit the recombination of photoinduced holes and electrons and further increase the photocatalytic properties23,

24.

The holes are thus transferred from semiconductor

CsPb(Br1-xClx)3 to metal Au while electrons moves towards the perovskite. The dissolved O2 can easily trap photoelectrons to produce superoxide anion radicals (O2-),


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and the photoholes will react with OH− to form hydroxy radicals (·OH) on the surface of Au NCs. Then, Sudan red III can be strongly degraded to nontoxic product by O2or ·OH25, 26. It should be noted that the decoration of Au NCs here acts like a hole reservoir and the capability of Au NCs reservoir to receive holes from perovskite NCs is size-dependent. It would show high efficiency only when the size of Au NCs is below 5 nm27. The size of Au NCs in our nanoheterostructure is 2.9 nm, resulting in superior photocatalytic properties. Also, the high volume to surface ratio of perovskite-Au nanoheterostructures will help to increase the photo-degradation rates since the reaction rate is directly proportional to the surface area where the Sudan red III is degraded. Conclusions In conclusion, bare and Au decorated CsPb(Br1-xClx)3 NCs have been synthesized for the photocatalytic degradation of Sudan red III pollutant. TEM confirmed that 2.9 nm Au NCs have been decorated on the surface of 9.0 nm CsPb(Br1-xClx)3 NCs. The degradation performance study suggested that 71% Sudan red III was degraded by CsPb(Br1-xClx)3-Au nanoheterostructures after visible light irradiation for 6 h, which is superior to bare CsPb(Br1-xClx)3 NCs. The ‘built-in’ electric

field

at

semiconductor-metal

interface

in

the

CsPb(Br1-xClx)3-Au

nanoheterostructures, small size of Au NCs and high volume to surface ratio promotes higher light absorption and higher carrier separation, leading to better photocatalytic performance. Our study paves a new way to take the advantage of halide perovskite materials for promising applications in photocatalytic degradation of pollutants.



Acknowledgements This work was supported by the Natural Science Foundation of the Jiangsu Higher Education Institutions of China (Grant No. 17KJA430009) and the National Natural Science Foundation of China (51202108).


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Figure

1.

XRD

pattern

of

the

CsPb(Br1-xClx)3-Au

nanoheterostructures,

CsPb(Br1-xClx)3 NCs and CsPbBr3 NCs. The XRD patterns of CsPbBr3 (JCPDS 18-0364) and Au (JCPDS04-0784) are shown below for comparison.



Figure 2. (a, b) TEM and HRTEM images of bare CsPbBr3 NCs, (c, d) TEM and HRTEM images of CsPb(Br1-xClx)3-Au nanoheterostructures, insets are size distribution statistics of CsPb(Br1-xClx)3 NCs and Au NCs. (e) Reduction of Au(III) at the surface of CsPbBr3 NCs to form CsPb(Br1-xClx)3-Au nanoheterostructures.


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Figure 3. The absorption spectra of Sudan red III degraded using (a) CsPb(Br1-xClx)3-Au

nanoheterostructures

and

(b)

CsPb(Br1-xClx)3

NCs

with

illumination time, (c) concentration (Ct/C0) changes vs time of Sudan red III using different catalysts and (d) different catalyst concentration.



Figure 4. Proposed mechanisms for the photocatalytic degradation of Sudan red III using CsPb(Br1-xClx)3-Au nanoheterostructures. FFM is fermi level of Au, FFS is Fermi level of perovskite, F is Equilibrium Fermi level, VB is valance band, CB is conduction band, Eg is band gap.


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For Table of Contents Use Only

The introduce of gold nanoparticles significantly enhances the photocatalytic degradation of Sudan Red III performance of CsPb(Br1-xClx)3 nanocrystals.


3-Au Nanoheterostructures - ACS Publications - American Chemical

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