Synthesis, characterization and photocatalytic activity of cubic-like CuCr2O4 for dye degradation under visible light irradiation

Applied Surface Science 319 (2014) 350–357

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Synthesis, characterization and photocatalytic activity of cubic-like CuCr2 O4 for dye degradation under visible light irradiation Wenhui Yuan a,∗ , Xiaoxia Liu a , Li Li b a b

School of Chemistry and Chemical Engineering, South China University of Technology 510640, Guangdong, Guangzhou, PR China College of Environmental Science and Engineering, South China University of Technology 510006, Guangdong, Guangzhou, PR China

a r t i c l e

i n f o

Article history: Received 25 May 2014 Received in revised form 17 July 2014 Accepted 25 July 2014 Available online 1 August 2014 Keywords: Hydrothermal synthesis CuCr2 O4 Photocatalytic activity Organic dyes

a b s t r a c t CuCr2 O4 nanoparticles with cubic-like morphology were prepared via hydrothermal synthesis method without template. The CuCr2 O4 samples were characterized by thermogravimetry and differential scanning calorimetry (TG–DSC), X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), UV–vis diffuse reflectance spectra (DRS) and Zeta potentials, respectively. The results indicated that cubic-like CuCr2 O4 could be successfully synthesized by calcining the precursor at 600 ◦ C, and the calcination temperature greatly influenced the morphology and optical performance of CuCr2 O4 . The pH at the point of zero charge (pHpzc ) of the CuCr2 O4 calcined at 600 ◦ C was about 4.52. The photocatalytic activity of CuCr2 O4 was evaluated for degradation of rhodamine B (RhB), methylene blue (MB), and methyl orange (MO) in the presence of H2 O2 under visible light irradiation and the effects of the calcination temperature, dosage of photocatalyst, etc., on photocatalytic activity were studied in detail. The photocatalytic results revealed that the CuCr2 O4 photocatalyst was of high activity for degradation of RhB (96.8%) and MB (99.5%), but very low activity for degradation of MO (14%). The CuCr2 O4 sample calcined at 600 ◦ C possesses the best photocatalytic activity, and the optimal dosage of the CuCr2 O4 photocatalyst is 0.4 g/L. © 2014 Elsevier B.V. All rights reserved.

1. Introduction The environmental problem, especially water pollution, has drawn great attention in recent years [1]. Dyes as one of the main types of pollutants in wastewater are emphasized [2–4]. The wastewater containing dyes has a great influence on humans, animals, and the ecosystem because many dyes are not readily biodegradable and very toxic to aquatic life and the human body [5,6]. There are many techniques applied in wastewater treatment, such as ultrafiltration, adsorption, coagulation and heterogeneous photocatalytic oxidation [7]. Among these techniques, heterogeneous photocatalytic technology has been extensively considered as a “green” and energy-saving means to degrade the soluble dyes in wastewater [8–10]. At the current stage, many kinds of photocatalysts have been widely applied in the field of photodegradation of dyes. However, several widely used photocatalysts such as TiO2 and ZnO have a wide band gap which makes them sensitive only

∗ Corresponding author. Present address: School of Chemistry and Chemical Engineering, South China University of Technology, Wushan, Tianhe, Guangzhou 510640, PR China. Tel.: +86 2087111887; fax: +86 2087111887. E-mail address: [email protected] (W. Yuan). http://dx.doi.org/10.1016/j.apsusc.2014.07.158 0169-4332/© 2014 Elsevier B.V. All rights reserved.

in UV region [11,12]. It is unfortunate that only about 4% of the solar spectrum falls in the UV range [9], so the development of visible-light photocatalyst has become a research focus. CuCr2 O4 with a spinel structure is an important p-type semiconductor photocatalyst that is highly effective in visible light [13,14]. With a bandgap energy of 1.4 eV, spinel CuCr2 O4 is a versatile catalyst due to its stable structure [1,15]. It has been widely applied in hydrogenation, dehydrogenation, oxidation, hydrogen production, propellant combustion, alkylation, and decomposition of organic compounds [16–22]. CuCr2 O4 can be synthesized by solid-state thermal method, coprecipitation, sol–gel method and hydrothermal synthesis method [23–26]. Most of these methods had shortcomings of high reaction temperature, long reaction time and complex reaction equipment, however, hydrothermal synthesis method is a cost-effective and easy route for preparing homogeneous, fine crystalline oxide materials with controlled particle size and morphology [27]. Recently, several studies reported that the CuCr2 O4 nanoparticles with different morphologies were synthesized. Hirunsit and Faungnawakij [28] prepared Cu–Cr spinel-oxide-type (CuCr2 O4 ) grains with irregular shape by a citric-acid complexation method, and it displayed excellent catalytic activity for hydrogen production via steam reforming of the oxygenated hydrocarbons. Geng et al. [29] reported that the

W. Yuan et al. / Applied Surface Science 319 (2014) 350–357

2. Experimental 2.1. Materials Copper nitrate trihydrate (Cu(NO3 )3 ·3H2 O) and chromium nitrate nonahydrate (Cr(NO3 )3 ·9H2 O) were purchased from Aladin (Shanghai, China). Sodium hydroxide pellets (NaOH), rhodamine B (RhB), methylene blue (MB), methyl orange (MO), hydrogen peroxide (H2 O2 , 30 wt% in water) and ethanol were supplied by Sinopharm Chemical Reagent Co., Led (Beijing, China). All chemicals were of analytical reagent grade and used without further purification. 2.2. Preparation of CuCr2 O4 The CuCr2 O4 samples were synthesized by hydrothermal synthesis method. In a typical synthesis procedure, 5 mmol Cu(NO3 )3 ·3H2 O and 10 mmol Cr(NO3 )3 ·9H2 O were dissolved in 50 mL deionized water and stirred for 20 min to form a homogeneous solution. The pH value of solution obtained above was adjusted and maintained at 10.3 with an attached pH regulator through dropwise adding 5 mol/L NaOH aqueous solution, and then the solution was heated to 70 ◦ C with vigorous stirring and kept for 4 h until a homogeneous suspension was obtained. Subsequently, the suspension was transferred into a 100 mL Teflon-lined stainless steel autoclave and heated up to 180 ◦ C and maintained for 11 h, and then cooled to room temperature naturally. The precipitates in the solution were centrifuged and respectively washed with 0.1 mol/L HCl solution, deionized water and ethanol several times until a neutral pH value was obtained. The obtained samples were dried at 90 ◦ C for 6 h, and then calcined at different temperatures (500–800 ◦ C) for 4 h in air. 2.3. Characterization The thermogravimetry (TG) and differential scanning calorimetry (DSC) of the precursor of CuCr2 O4 sample was carried on a Netzsch STA449C analyzer (Germany) in the temperature range from room temperature to 1000 ◦ C in air with a heating rate of 10 ◦ C/min. The crystallinity and phase analyses of as-prepared samples were conducted by Bruker D8 Advance (Germany) X˚ at ray diffractometer (XRD) with Cu-K␣ irradiation ( = 1.54 A) 40 kV and 40 mA. The morphologies of as-prepared samples were analyzed by JEOL JEM 2010 EX (Japan) transmission electron microscopy (TEM) with accelerating voltage of 200 kV. The X-ray photoelectron spectroscopy (XPS) measurement was detected on a Kratos Axis Ultra DLD (Britain) photoelectron spectrometer using Al K␣ (1486.6 eV) radiation. The UV–vis diffuse reflectance spectra

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octahedral-like CuCr2 O4 particles could be synthesized by calcining as-burt power at 600–1000 ◦ C prepared by a sol–gel combustion process, and it could be served as solar-absorbing pigment to fabricate thickness sensitive spectrally selective paint coatings which displayed good spectrally selective properties and excellent long term stability. Hosseini et al. [30] synthesized sphere-like CuCr2 O4 nanospinels via a sol–gel combustion method and found that it exhibited good catalytic activity toward 2-propanol combustion. But there is very little research about the synthesis of cubic-like CuCr2 O4 nanoparticles via a hydrothermal synthesis method. In this work, we report the effect of calcination temperature on the structure, morphology, and photocatalytic activity of CuCr2 O4 prepared by a hydrothermal synthesis method. Moreover, the photocatalytic activities for degradation of organic dyes such as Rhodamine B (RhB), methylene blue (MB), and methyl orange (MO) on CuCr2 O4 as-prepared under visible light irradiation without and with the assistance of H2 O2 were investigated.

351

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Fig. 1. TG and DSC curves of the precursor of CuCr2 O4 sample.

(DRS) were recorded with a Hitachi U-3010 (Japan) UV–vis spectrophotometer with BaSO4 as the reference. The Zeta potentials were measured by Malvern Zetasiser Nano ZS (Britain). 2.4. Photocatalytic activity test The photocatalytic activities of as-prepared CuCr2 O4 samples were evaluated by degradation of organic dye pollutants such as RhB, MB and MO under simulative visible light at room temperature. A 250 W tungsten–halogen lamp equipped with a 420 nm cut-off filter was used as a visible light source. Typically, 20 mg of catalyst was dispersed in 50 mL, 20 mg/L aqueous solution of dye. Before starting the irradiation, the suspension was stirred vigorously for 30 min in dark to achieve adsorption/desorption equilibrium. After that, 50 ␮L of 30% H2 O2 was added into the above suspension which was exposed to the light after 3 min. At certain time intervals, 3 mL of suspension was sampled and centrifuged. The absorbance of the supernatant was analyzed by UV–vis spectrophotometer (UV-2450). 3. Results and discussion 3.1. TG and DSC analysis Fig. 1 shows the TG–DSC thermograms of the precursor of CuCr2 O4 sample. From the TG curve, the weight loss from room temperature to about 1000 ◦ C was 29.2%, which may be divided into three major weight loss stages. The first weight loss below 180 ◦ C is owing to the dehydration of adsorbed water molecules, the second located at 180–350 ◦ C is attributed to the decomposition of partial hydroxyl and loosely bound nitrates, and the third weight loss situated at 380–485 ◦ C is very little, which may be due to the crystallization process of CuCr2 O4 samples. From the DSC curve, the sharp endothermic peak located at 395 ◦ C was observed, which is ascribed to the formation of the bond Cu–O–Cr in CuCr2 O4 [31]. 3.2. XRD analysis The XRD patterns of the precursor and CuCr2 O4 samples calcined at different temperatures are presented in Fig. 2. As can be seen, there is no diffraction peak of the precursor, and the samples calcined at 500 ◦ C are composed of Cr2 O3 and CuO phases [29]. For the samples calcined at 600, 700 and 800 ◦ C, the diffraction peaks are in agreement with the JCPDS No.34-0424, which can be indexed to a

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3.3. TEM analysis Fig. 3 displays the TEM images of CuCr2 O4 samples calcined at different temperatures and the spent photocatalyst. As shown in Fig. 3(a), the CuCr2 O4 sample calcined at 500 ◦ C is composed of irregular shape nanoparticles with slight agglomeration. However, from Fig. 3(b)–(d), the samples prepared at 600, 700, and 800 ◦ C are made up of almost homogeneously distributed uniform cubic-like nanoparticles. It can be clearly found that the size of the CuCr2 O4 nanoparticles becomes bigger with the calcination temperature increased from 600 to 800 ◦ C [33], which is consistent with the results of the XRD analysis. Fig. 3(e) shows the TEM image of CuCr2 O4 photocatalyst calcined at 600 ◦ C after five cycles of photodegradation experiments, which revealed that the particle shape and size of the CuCr2 O4 sample were kept unchanged after photocatalytic degradation of organic dyes. 3.4. XPS spectra analysis The sample calcined at 600 ◦ C was characterized by XPS to elucidate the elemental composition and chemical states of CuCr2 O4 photocatalyst, as shown in Fig. 4. The binding energy of 284.6 eV for carbon 1s peak was used to calibrate the XPS spectra. Fig. 4(a) shows the XPS survey spectrum of CuCr2 O4 sample, and the three elements of Cu, Cr and O are identified in the sample. The Cu 2p,

Cr O 2 3 CuO 800 °C

Intensity / (a.u.)

tetragonal phase of spinel CuCr2 O4 . The mean crystallite size calculated according to Debye–Scherrer equation [32] are around 18 nm, 37 nm and 42 nm for CuCr2 O4 samples calcined at corresponding temperature, respectively. Moreover, the peaks intensity becomes stronger and shaper with increasing temperature, which indicates that the crystallite size of CuCr2 O4 samples increased with increasing calcination temperature. Therefore, the monophase (tetragonal phase) of CuCr2 O4 crystallites can form when the precursor is calcined at the temperature higher than 500 ◦ C.

700 °C 600 °C 500 °C precursor

CuCr2O4 JCPDS No. 34-0424 10

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2 / (°) Fig. 2. XRD patterns of CuCr2 O4 samples.

Cr 2p and O1s XPS spectra are represented in Fig. 4(b)–(d), respectively. The peaks at 932.9 and 952.8 eV are assigned to the Cu 2p3/2 and Cu 2p1/2 (Fig. 4(b)), which demonstrates that Cu is in the form of Cu2+ . And the intermediate peaks between 2p3/2 and 2p1/2 energy levels are the satellite peaks, which are generally observed for the copper ion with an oxidation state of 2+ [20]. The peaks located at 574.5 and 583.9 eV are corresponding to Cr 2p3/2 and Cr 2p1/2 (Fig. 4(c)), which is consistent with the value of Cr3+ . From Fig. 4(d), the O 1s spectrum is obviously asymmetric, so the spectrum is fitted with two peaks. The stronger peak at 529.2 eV is attributed to the lattice oxygen of metal oxides, and the smaller peak with the binding energy of 527.8 eV is due to the absorbed oxygen or

Fig. 3. TEM images of (a)–(d) CuCr2 O4 samples calcined at 500, 600, 700, 800 ◦ C, and (e) the spent catalyst calcined at 600 ◦ C.

W. Yuan et al. / Applied Surface Science 319 (2014) 350–357

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Fig. 4. XPS spectra of CuCr2 O4 sample calcined at 600 ◦ C (a) survey XPS spectrum; (b) Cu 2p; (c) Cr 2p; (d) O 1s.

the surface hydroxyl species [34]. Quantitative XPS measurements showed that the atomic surface molar ratio Cu:Cr:O = 1:2.09:4.13 which is very close to 1:2:4. The obtained results confirm that the CuCr2 O4 photocatalyst is synthesized successfully.

Fig. 5 illustrates the UV–vis diffuse reflectance spectra of the precursor and the CuCr2 O4 samples calcined at different temperatures. As can be seen, the CuCr2 O4 samples calcined at different temperatures all have strong absorption in the visible range and the absorption intensity of the samples is slightly decreasing with the calcination temperature increased from 500 to 800 ◦ C. However, the spectrum of the precursor of CuCr2 O4 sample consists of a weak absorption in the visible range. It is concluded that the calcination temperature and calcination process have great influence on the absorption properties of CuCr2 O4 .

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The photocatalytic activities of CuCr2 O4 photocatalysts were assessed by degradation of different dyes in water under visible light irradiation at room temperature. Fig. 6 shows the photocatalytic degradation of RhB on CuCr2 O4 samples calcined at 500, 600, 700, and 800 ◦ C in the presence of H2 O2 (50 ␮L) under visiblelight irradiation. As can be seen, the degradation rate of RhB increases with increasing calcination temperature from 500 ◦ C to

600 ◦ C, which may be due to containing impurity phases of CuCr2 O4 samples calcined at 500 ◦ C. However, it decreases with increasing calcination temperature from 600 ◦ C to 800 ◦ C. The reason for this phenomenon is that the photocatalyst loses the surface area at higher calcination temperature which results from aggregation and grain size growth [7].

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Fig. 7. Photocatalytic degradation of RhB in the presence of different catalysts and dosage of CuCr2 O4 sample calcined at 600 ◦ C.

The photocatalytic degradation of RhB in the presence of different catalysts and dosage of CuCr2 O4 sample calcined at 600 ◦ C is shown in Fig. 7. Obviously, control experiments show that there is no appreciable RhB degradation using H2 O2 (50 ␮L) or

pure CuCr2 O4 (20 mg) as photocatalyst after irradiation 60 min respectively. Interestingly, CuCr2 O4 showed relatively low photocatalytic activity for the degradation of RhB although it has a strong absorption in the visible range, which is attributed to the

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Fig. 8. Absorption spectra of (a) RhB, (b) MB and (c) MO aqueous solutions taken at different times and (d) photocatalytic degradation of RhB, MB and MO using CuCr2 O4 (20 mg) calcined at 600 ◦ C in the presence of H2 O2 under visible light irradiation.

W. Yuan et al. / Applied Surface Science 319 (2014) 350–357

CuCr2 O4 + h → CuCr2 O4 (h+ + e− )

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H2 O2 + CuCr2 O4 (e− ) → OH• + OH− + CuCr2 O4

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OH + dye → Degradation products

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easy recombination of photogenerated electrons and holes. However, the photocatalytic activity of CuCr2 O4 catalyst for degradation of RhB is significantly enhanced in the presence of H2 O2 (50 ␮L). And the degradation rate of RhB increases with increasing the catalyst dosage from 10 to 20 mg, and then almost unchanged with the catalyst dosage from 20 to 50 mg. Moreover, more than 95% of RhB molecules are degraded by 20 mg CuCr2 O4 with the assistance of H2 O2 . These results demonstrate that the optimal dosage of the CuCr2 O4 catalyst is 20 mg and H2 O2 played a significant role on degradation of RhB. Recently, several studies have illustrated that H2 O2 can efficiently enhance the photocatalytic degradation of organic dyes [35,36]. The possible pathway of the photocatalytic degradation of dyes has been proposed as follows:

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In order to further study the photocatalytic activity of CuCr2 O4 catalyst calcined at 600 ◦ C, the photocatalytic degradation of different dyes experiments were carried out in the presence of H2 O2 under visible light irradiation. The temporal evolution of absorption spectra and photocatalytic efficiencies of RhB, MB and MO under the conditions stated above are displayed in Fig. 8. RhB and MB both belong to the cationic dye, while the MO is the anionic azo dye [35]. From Fig. 8(a) and (b), the characteristic absorption peaks of RhB and MB are 554 and 664 nm, respectively, which decrease

2

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pH Fig. 9. Zeta potential of CuCr2 O4 catalyst calcined at 600 ◦ C as a function of pH.

rapidly with the increase of irradiation time and almost disappear after irradiation for 60 min. Moreover, there is no other absorption peak in the whole spectrum under further irradiation. However, the characteristic absorption peak of MO which is at 464 nm only decreases a little with extension of the exposure time, as shown

Fig. 10. Schematic interaction model between RhB, MB, MO and CuCr2 O4 .

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Fig. 11. (a) Recyclability tests of CuCr2 O4 (20 mg) photocatalyst prepared at 600 ◦ C for degradation of RhB in the presence of H2 O2 under visible light irradiation for 60 min; (b) XRD pattern of fresh and spent photocatalysts.

in Fig. 8(c). The above results show that the CuCr2 O4 photocatalyst has an excellent visible light photocatalytic activity for degradation of the cationic dyes, but it is opposite for the anionic dyes, which is attributed to the charge on the surface of CuCr2 O4 photocatalyst. Fig. 9 presents the surface zeta potential versus pH value for the CuCr2 O4 photocatalyst calcined at 600 ◦ C. Clearly, the pH at the point of zero charge (pHpzc ) of CuCr2 O4 catalyst is about 4.52 and the CuCr2 O4 catalyst is negatively charged (−31.9 mV) on its surface in the neutral solution. Therefore, it repulses the anionic dye in the solution and the MO molecules could not be largely adsorbed by the catalyst, while the RhB and MB (the cationic dyes) molecules are adsorbed by electrostatic attraction [37], as shown in Fig. 10, which could also be confirmed by the adsorption of dyes in the dark, as shown in Fig. 8(d). The degradation rate of RhB and MB on CuCr2 O4 photocatalyst is up to 96.8% and 99.5%, respectively, in the presence of H2 O2 under visible light irradiation in 60 min. Nevertheless, only 14% of MO is degraded under the same conditions. For the cationic dye, the photocatalytic degradation activity of MB is better than that of the RhB, which could be attributed to the more complicated and larger structures of the RhB molecules. To understand the photostability of the CuCr2 O4 photocatalyst calcined at 600 ◦ C during the photocatalytic degradation reaction, the photocatalyst which was collected from the degradation solution was reused to degrade the RhB dye under the same conditions. The results of recyclability tests were shown in Fig. 11(a). The CuCr2 O4 photocatalyst exhibits an excellent photocatalytic degradation activity, and the degradation rate is up to 91.6% even after five successive cycles. Moreover, the crystal structure of the CuCr2 O4 stays unchanged and the crystallinity is also well even after five successive runs, as shown in Fig. 11(b), which demonstrates that the CuCr2 O4 photocatalyst is of excellent stability under visible light irradiation. 4. Conclusions The cubic-like CuCr2 O4 spinel nanoparticles were successfully synthesized by the hydrothermal synthesis method. It is found that the calcination temperature has a great effect on the morphology and the photocatalytic activity of CuCr2 O4 catalyst. The particle size of the CuCr2 O4 photocatalysts increases with the calcinations temperature increased from 600 to 800 ◦ C but the photocatalytic activity decreases. The CuCr2 O4 photocatalyst calcined at 600 ◦ C shows excellent photocatalytic activity for degradation of RhB and MB cationic dyes but not for MO anionic dye, which is due to the negative charges on the surface of CuCr2 O4 photocatalyst

(−31.9 mV) in the neutral solution. Moreover, the photocatalyst is very stable and can be reused several times without obvious loss of photocatalytic activity. Therefore, the CuCr2 O4 photocatalyst can be considered as a promising material for degradation of organic dyes in the wastewater.

Acknowledgments The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (No. 20976057) and research fund of The Guangdong Provincial Engineering Research Center of Green Fine Chemicals, China.

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