Cu photocatalyst: Preparation and high performance for degradation of organic dye

Accepted Manuscript Title: Sepiolite/Cu2 O/Cu photocatalyst: preparation and high performance for degradation of organic dye Authors: Peisan Wang, Chunxia Qi, Luyuan Hao, Pengchao Wen, Xin Xu PII: DOI: Reference:

S1005-0302(18)30231-7 https://doi.org/10.1016/j.jmst.2018.03.023 JMST 1341

To appear in: Received date: Revised date: Accepted date:

10-2-2018 9-3-2018 22-3-2018

Please cite this article as: Wang P, Qi C, Hao L, Wen P, Xu X, Sepiolite/Cu2 O/Cu photocatalyst: preparation and high performance for degradation of organic dye, Journal of Materials Science and amp; Technology (2018), https://doi.org/10.1016/j.jmst.2018.03.023 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.

Sepiolite/Cu2O/Cu photocatalyst: preparation and high performance for degradation of organic dye

a

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Peisan Wang a,b, Chunxia Qi c, Luyuan Hao a, Pengchao Wen a, Xin Xu a,*

Chinese Academy of Sciences Key Laboratory of Materials for Energy Conversion,

Department of Materials Science and Engineering, University of Science and

b c

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Technology of China, Hefei 230026, China.

Department of Chemistry, Anhui Medical University, Hefei 230032, China

Department of Chemical Engineering, Hefei Normal University, Hefei 230601,

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China

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[Received 10 February 2018; revised 9 March 2018; accepted 22 May 2018]

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*Corresponding author.

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E-mail address: [email protected] (Xin Xu).

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A novel ternary sepiolite/Cu2O/Cu (SCC) nanocomposite was successfully

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synthesized by a facile one-pot method. The Cu2O/Cu nanoparticles in the SCC nanocomposite are well dispersed on the sepiolite surface. It exhibited enhanced photocatalytic performance in the degradation of congo red (CR), remarkably superior

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to that of Cu2O or Cu2O/Cu nanoparticles. Elemental copper in the SCC serves as a good electron acceptor to promote the transfer of photo-generated electrons in Cu2O and suppress the recombination of the photo-generated electrons and holes of the composite. The enhanced photocatalytic efficiency is attributed to the synergistic 1

effect of sepiolite and Cu2O/Cu. This type of SCC nanocomposites is a promising candidate as photocatalytic material for environmental protection.

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Keywords: Sepiolite/Cu2O/Cu, Nanocomposite, Photocatalytic, Organic dye

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1. Introduction

Cuprous oxide (Cu2O), a p-type semiconductor, is an interesting photocatalyst.

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The visible light can activate a narrow band gap (2.2 eV), compere with ZnO or TiO2,

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which is showed by the Cu2O [1-5]. Till now, many studies have been reported about

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Cu2O as a photocatalyst to degradation organic pollutants [6-9]. However, Cu2O,

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particularly those with nanoscale structure, has no activity for easy photocorrosion if

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not undering pretreatment [10]. For instance, Ho prepared submicrometer-sized Cu2O with systematic morphological evolution from cube to short hexapod, whose tests of

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photodegradation revealed unfortunately no effect on the methyl blue [11]. Polyhedral

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Cu2O microcrystals were found that the degradation ratio were even less than 30% for the photodegradation of dye molecules with visible-light irradiation [12, 13]. The rhodamine B degradationrates are 51.83% by Cuprous Oxide [14]. The above studies

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show that the photocatalytic effect of bare Cu2O microstructure materials is unsatisfactory. For the sake of enhancing the Cu2O photocatalytic activity nano/micromaterials, some of Cu2O composite structures doped with different metal nanoparticles were synthesized, the maximum photodegradation ratio of organic 2

pollutants is up to 88% under visible-light irradiation [15-18]. However, the ultrafine Cu2O/Cu particles quite possibly agglomerate into larger particles, which can weakened the photocatalytic activity of sample. It is a good method to avoid the agglomeration of Cu2O/Cu powders by disperse Cu2O/Cu particles onto porous

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mineral. The sepiolite is a monolayered hydrated magnesium silicate clay mineral. The formula

of

half-unit

cell

of

sepiolite

is

assigned

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theoretical

as

Si12O30Mg8(OH,F)4(OH2)4·8H2O [19, 20]. The structure of sepiolite is built up by two

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Si-O tetrahedral layers pack and a Mg-O or Mg-OH octahedral sheet which is resulted

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as zeolite-like channels. Generally, the morphology of sepiolite shows micro-fibrous.

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The micro-fibrous structure can offer high porous volume as 0.4 cm3/g and over 300

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m2/g specific surface area. And the high porous volume and high specific surface area

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can show a strong adsorption ability of organics [21, 22]. For instance, the photocatalysts like TiO2 and ZnO can be dispersed on the sepiolite to improve their

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photocatalytic performance [23-28].

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In the present work, we developed a new nanocomposite based on sepiolite immobilized Cu2O/Cu nanoparticles by a facile one-pot method that has not been previously reported. Porous sepiolite can help the composite to overcome the

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agglomeration of Cu2O/Cu nanoparticles and enhance the photocatalytic activity of Cu2O/Cu. Nano-sized Cu particles can prevent Cu2O from photocorrosion and improve the stability of the nanocomposite. Furthermore, prepared ternary sepiolite/Cu2O/Cu nanocomposite show high specific surface area, which is a key 3

prerequisite for dye adsorption, and exhibits good photocatalytic ability for degradation organics like Congo red (CR).

2. Experimental

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2.1. Materials The natural mineral containing >95% of sepiolite was bought from the

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Community of Madrid in Spain and provided by University of Science and

Technology of China. Other reagents including Cu(NO3)2·3H2O, HCl, C2H5OH and

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(CH2OH)2 (EG) were all analytical grade. These reagents were used without further

with deionized water.

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2.2. SCC photocatalyst preparation

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purification. Glasswares were immersed by chromic acid lotion for 12 h and washed

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SCC photocatalysts were prepared via a solvothermal method. Typically, the raw sepiolite powder was firstly added into hydrochloric acid (1.2 mol/L) (the ratio of HCl

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and sepiolite was 10:1) and stirred severely for 60 min. The mixture suspension

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solution was retrieved by centrifuging and dried at 120 °C. Secondly, As shown in Scheme 1, 0.1g of Cu(NO3)2·2H2O and 0.025 g sepiolite powder were added in a 17 mL of ethanol and 8.5 mL of EG mixed solution and stirred for 6 h. The resulted

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solution was airproofed in a teflon-lined stainless-steel autoclave and heated at 170 °C for 8 h. Then the precipitate was centrifuged, washed with deionized water for 7 times and dried at 60 °C for 24 h. The preparation process is shown in Scheme 1. 2.3. SCC nanocomposite characterization 4

The SCC nanocomposite was characterized through a series of techniques. The crystalline phases of samples were identified by X-ray diffraction (XRD) analysis under a Japan MapAHF X-ray diffractometer (Cu Kα1 radiation; scanning rate of 2o/min). The X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250, Al Kα

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radiation) and Fourier transform infrared spectroscopy (FTIR, NEXUS-870, KBr pellet, 4000 - 500 cm−1) were also used.

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The morphology and the microstructure of the samples were observed by field-emission scanning electron microscopy (FESEM, JEOL-6390LA) and

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high-resolution transmission electron microscopy (HRTEM, Hitachi H-800). The

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energy dispersive X-ray spectra (EDX) were measure under a detector equipped in

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SEM system.

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The UV-visible diffuse reflectance spectra of the samples were measured under a

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TU-1901 model UV-visible spectrophotometer. The measurements of specific surface area, specific pore volume, and pore size were carried out by BET and BJH using a

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Micromeritics Tristar II 3020M equipment.

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2.4. Photocatalytic degradation of CR The photocatalytic performance of SCC was explored by degradation of CR

aqueous solution. 10 mg SCC was dispersed in a 50 mL CR solution with a

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concentration of 10 mg/L. The soliquoid was stirred in darkness for 30 min to attain adsorption equilibrium, then continuously stirred under 200 W xenon lamp irradiation. At an interval of 5 min, a fixed amount of soliquoid was taken out and centrifuged for further testing on a Hitach UV-4100 spectrophotometer. The photocatalytic 5

degradation rate of Congo red was counted based on the following equation. D  (

A0  A1 )  100% A0

where D is the photocatalytic degradation ratio of CR, Ao and A1 are the absorbance

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values of CR before and after reaction.

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3. Results and discussion 3.1. SCC nanocomposite

The XRD patterns of the pretreated sepiolite and SCC nanocomposite are shown

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in Fig. 1. A typical diffraction diagram of pure sepiolite (JCPDS No. 13-0595) can be

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seen (Fig. 1(a)) with the characteristic reflection at d110 =12.0 Å which corresponds to

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the interlayer distance of sepiolite structure [29]. In Fig. 1(b), the diffraction peaks of sepiolite still exist, and six new peaks appear at 2 values of 29.5°, 36.4°, 42.4°, 61.6°,

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73.8°and 77.3° corresponding to (110), (111), (200), (220), (311) and (222) of cuprous

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oxide (JCPDS No. 65-3288), respectively. In addition, three values of 43.3°, 50.5° and 74.2° according to (111), (200) and (220) of crystalline copper (JCPDS No.04-0836)

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are also observed. Therefore, a great deal of cuprous oxide and copper were immobilized into/on sepiolite powder as evidenced by XRD. For one thing,

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comparing Fig. 1(a) with (b), some peaks of SCC weaken or vanish; this is because of the imperfect crystalloid for discontinuing the fiber unit [30]. FTIR spectra further confirm the formation of SCC as shown in Fig. 2. The bands at 3690–3565 cm−1 are attributed to the Mg-OH in the Mg-O octahedron [15] stretching vibration. The peak at 3424 cm−1 is assigned to water and the obscure -OH groups stretching vibrations 6

located in the Mg–O octahedron emerges at about 1635 cm−1. For another, the -OH groups bending vibration on the sepiolite surface at 1398 cm−1 are weak but strengthen in SCC composites. It can be caused by the regain of the sepiolite structure when divalent copper ion exchanges with sepiolite. Furthermore, as shown in Fig.

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2(b), the strong band at 628 cm−1 originates from the Cu–O stretching vibrations in Cu2O [15], and the stretching vibration of Si-O at 1094 cm−1 shown in Fig. 2(a) shifts

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to 1019 cm−1 in Fig. 2(b), which may be caused by the interaction of sepiolite with Cu/Cu2O.

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SEM and TEM images of the morphological textures of sepiolite powder before

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and after modification are shown in Figs. 3 and 4. It can be found that the

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unpretreated sepiolite has a fibrous morphology with clean and smooth surface (Figs.

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3(a) and 4(a)). After being treated with hydrochloric acid, the surface of sepiolite

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particles become rough (Fig. 3(b)), this contributes the loading of nanophotocatalyt on sepiolite. Fig. 3(c) exhibits the nanorod like morphology of nanocomposites. The

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TEM image in Fig. 4(b) confirms that the Cu2O/Cu nanoparticles with the average

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size of ca. 10 nm anchor closely to sepiolite powder surface. The EDX spectrum (Fig. 3(d)) demonstrates that the rod-like structure in Fig. 3(c) contains O, Si, Mg, Cu, and C elements, which indicates the formation of SCC nanocomposites. And the atom

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percentage of O, Mg, Al, Si, and Cu, are 39.32%, 7.46%, 0.45%, 47.26% and 5.51%, respectively. The specific surface area of sepiolite and SCC were measured by N 2 adsorption-desorption method in Fig. 5. Total pore volume and the BET surface area 7

are 0.47 cm3/g and 256.5 m2/g for sepiolite, and 0.27 cm3/g and 112.1 m2/g for SCC nanocomposites, separately. The decrease of surface area of SCC composite comparing with sepiolite could be attributed to the formation of Cu2O/Cu nanoparticles on the sepiolite surface. Both sepiolite and SCC show an IV type N 2

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adsorption isotherm with an evident hysteresis loop, indicating the presence of mesopores in both samples [31]. The pore size distributions of the sepiolite (Fig. 5(b))

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and SCC (Fig. 5(d)) are within the range of 2-50 nm.

X-Ray photoelectron spectroscopy can probe the eletron informatinon of the

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surface SCC (Fig. 6). Fig. 6(a) depicts the binding energies of Cu (2p), C (1s), O (1s),

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Si, and Mg KLL of SCC nanocomposite. The other peaks at 285 eV, 1096 eV、1000

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eV, 979 eV, 103 eV, 154 eV, 307 eV correspond to C1s, OKLL, Si and Mg KLL in the

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sepiolite. Fig. 6(b) shows two bands of binding energies of 952.1 eV and 932.2 eV

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that can be assigned to Cu 2p1/2 and Cu 2p3/2. For comparison, the oxidation states between Cu+ and Cu0, a Cu LM2 Auger spectrum (Fig. 6(c)) was further taken on the

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product; the existing of Cu LM2 kinetic energy of 916.75 eV and Cu 2p3/2 binding

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energy of 932.2 eV suggests that Cu0 and Cu+ coexist in the composite [32]. Fig. 6(d) depicts that the O (1s) peak binding energy at 532 eV was labeled as an internal reference. Consequently, the XPS results further prove that the product is mainly

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composed of sepiolite/Cu2O/Cu. The UV–Vis DRS (ultraviolet–visible diffuse reflectance absorptive spectra) were further executed to reveal the optical absorbance of the SCC composite shown in Fig. 7. The absorption peak at about 651 nm（Fig. 7(a)）corresponds to the band gap 8

energies of 1.733 eV (Fig. 7(b)). In addition, the absorption edge expands to near infrared region. Compared with the direct band gaps of Cu2O/Cu (2.02 eV) [15], SCC had a lower band gap energy and its sensitivity to visible light-near infrared region is enhanced.

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3.2. Photocatalytic performance of SCC The photocatalysis by SCC, which is one of the advanced oxidation processes

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(AOPs) [33-35]. The photocatalytic performance of SCC is assessed by monitoring

the degradation of CR in an aqueous solution under simulated sunlight irradiation, the

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results as shown in Fig. 8(a). Apperently, CR molecules adsorbed on SCC after the

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suspension was placed in the dark. With extending the illumination time, the

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absorption peak intensity reduced sequentially, reaching zero after 50 min

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illumination, and the color of CR solution vanished completely (Fig. 8(a) inset),

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indicating that CR molecules degraded entirely. However, as shown in Fig. 8(b) CR molecules adsorped on sepiolite reached equilibrium in the dark, the decreased extent

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of absorption peak does not decrease with illumination, which indicates that the

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removing of CR resulted mainly from sepiolite particles adsorption. As shown in Fig. 8(c), the degradation ratio of CR is about 36.1% and 95.1% in the presence of sepiolite and SCC, respectively. In addition, according to Fig. 8(d), the degradation

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ratio of CR is about 54.3%, 71.2% and 95.1% using Cu2O, Cu2O/Cu and SCC sample under the same irradiation conditions, respectively. These results confirm the higher photocatalytic performance of the Cu2O/Cu loaded sepiolite for this reason a synergistic effect of the both components. 9

3.3. Mechanism of photocatalysis Based on the above results, the degradation mechanism of CR on SCC nanocomposite can be illustrated in Fig. 9. Cu2O semiconductor with band gap of 2.17 eV absorbs the visible light to produce electron-hole pairs with high activity, which

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may react with OH− or O2 to form oxidative radicals including •OH or •O2−. The oxidative radicals like • OH or • O2− have strong oxidation property for

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decomposition the benzene and organic branches of CR as CO2, H2O and some soluble salt. The presence of Cu improved the visible-light absorbance and promoted

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the photocatalic performance [36]. In addition, the photopic energy can be transferred

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to semiconductor Cu2O in order to further enhance the generation of the hole and

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electron [37]. Besides, the anchor of Cu nanoparticles on the surface of sepiolite as

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well as Cu2O also promote the generation of photo holes and electrons on the surface

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of sepiolite. So, the adsorptive action of sepiolite and light-response ability of

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Cu2O/Cu have a synergistic effect in the photocatalytic degradation of CR molecules.

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4. Conclusion

The SCC composites were successfully prepared via a simple one-pot method.

The acid-treated sepiolite fibers display an admirable loading for Cu2O/Cu

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nanoparticles. Porous sepiolite enhances the adsorption of SCC for CR, and Cu nanoparticles improve the photo-catalysis properties of Cu2O and therefore increase the visible light utilization, enhance the photocatalytic performance degradation of CR. The degradation ratio of CR reaches about 95.1% in aqueous solution. The SCC 10

composites display a promising photocatalysis performance in treating wastewater containing dyes.

Acknowledgements

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This research was supported by the National Natural Science Foundation of China (Grant No. 51372238), the CNPC-CAS Strategic Cooperation Research

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Program (2015A-4812), and the Provincial Teaching Research Project of Anhui

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Province (2014jyxm010).

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Figures captions Scheme 1. Schematic presentation of SCC preparation procedure. Figure 1. XRD patterns of (a) sepiolite and (b) SCC. Figure 2. FT-IR spectra of (a) acidized sepiolite and (b) SCC.

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Figure 3. (a-c) SEM images of raw sepiolite, acid treated sepiolite and SCC, respectively, (d) EDX spectrum of the SCC.

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Figure 4. TEM images of (a) sepiolite and (b) SCC.

Figure 5. (a, c) N2 adsorption/desorption isotherms and (b, d) BJH pore diameter

XPS of SCC nanocomposites: (a) survey spectrum, (b) Cu 2p, (c) Cu

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Figure 6.

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distributions of sepiolite and SCC.

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LM2 Auger spectrum; (d) O1s.

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Figure 7. (a) UV-vis DRS spectrum and (b) (αhγ)2 vs. hγ plot of SCC.

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Figure 8. Absorption spectra of CR solution with irradiation time in the presence of (a) SCC nanocomposite and (b) sepiolite. (c) Photodegradation ratios of CR in the

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presence of sepiolite and SCC, (d) Absorption spectra of CR in the presence of Cu2O,

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Cu2O/Cu and SCC

Figure 9. Schematic illustration of the proposed photocatalytic degradation

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mechanism.

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Figure 1

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Scheme 1

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