Coprecipitates Synthesis of CaIn2O4 and Its Photocatalytic

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Coprecipitates synthesis of CaIn2O4 and its photocatalytic degradation of methylene blue by visible light irradiation Rengaraj Selvaraj, Yunfan Zhang, Younghun Kim, Mika Sillanpää, and Cheuk-Wai Tai Ind. Eng. Chem. Res., Just Accepted Manuscript • Publication Date (Web): 18 Jun 2014 Downloaded from http://pubs.acs.org on July 3, 2014

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Industrial & Engineering Chemistry Research

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Coprecipitates synthesis of CaIn2O4 and its photocatalytic

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degradation of methylene blue by visible light irradiation

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Yunfan Zhang1, Rengaraj Selvaraj2*, Mika Sillanpää1*, Younghun Kim3, Cheuk-Wai Tai4

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1

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Technology, Mikkeli FI-50100, Finland

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2

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123, Al-Khoudh, Muscat, Sultanate of Oman

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3

Department of Chemical Engineering, Kwangwoon University, Seoul 139-701, Korea

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4

Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm

Laboratory of Green Chemistry, Faculty of Technology, Lappeenranta University of

Department of Chemistry, College of Science, Sultan Qaboos University, P.O. Box. 36., P.C.

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University, S-106 91 Stockholm, Sweden

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* Corresponding author: Tel: +968-9337 6294; e-mail: [email protected] (R. S)

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+358 400 205215; e-mail: [email protected] (M.S)

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ACS Paragon Plus Environment


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Abstract

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Visible light-activated photocatalyst calcium indate (CaIn2O4) has been successfully prepared

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by a new approach with lower temperature calcinations. The sub-micron sized samples were

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characterized by X-ray diffraction (XRD), field-emission scanning electron microscopy

5

(FE-SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy

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(XPS) and UV-Vis diffused reflectance spectroscopy (DRS). High purity CaIn2O4 particles

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were synthesized by controlling the sintering temperature and their photocatalytic

8

performance were evaluated. Under visible light irradiation, 10 ppm of Methylene Blue (MB)

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solution was effectively degraded in 3.0 hours. The excellent photocatalytic activity of

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CaIn2O4 is attributed to the high purity, small particle size and large surface area because of

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lower sintering temperature by thermal decomposition of metal oxalate precursor.

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Keywords: CaIn2O4, coprecipitation, low temperature, methylene blue, photocatalysis.

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1. Introduction

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Since the semiconductor TiO2 electrode was reported to be able to electrochemical

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water splitting, heterogeneous photocatalysis has drawn considerable attention for

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environmental remediation applications1-4. Photocatalytic degradation of organic dyes, in

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particular, have been studied extensively5 because dye removal from industrial effluent is one

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major environmental issue that urgently needs to be solved. However, the traditional

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photocatalysts such as TiO2 and ZnO only work under UV light due to their wide band gaps.

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This drawback seriously limits their applications under solar light, because solar spectrum

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contains only 4% of UV light. Therefore, visible-light active photocatalysts, such as CdS6,

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In2S37, 8, Bi2WO69 and CaIn2O410 have been developed.

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Calcium indate (CaIn2O4), spinel compound, is a semiconductor with orthorhombic

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structure10. As a photocatalyst, CaIn2O4 was first investigated for methylene blue (MB) dye

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photocatalytic degradation under visible light irradiation in 200311. It is reported that 100 ml

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MB solution (the concentration was about 47.8 µmol/L and the initial pH value of the

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solution was nearly 7), could be completely degraded over 0.3 g CaIn2O4 under visible light

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irradiation for 120 min. The photocatalytic efficiency is still as high as nearly 50% MB

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degradation while the incident light wavelength is longer than 580 nm. Hereafter, a series of

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MIn2O4 (M = Ca, Sr and Ba) semiconductors were synthesized and the substitution effects of

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M2+ ions on the structural and photocatalytic properties for MB dye degradation were

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systematically studied in 200412. It was found that CaIn2O4 has outstanding photocatalytic

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performance. The high sintering temperature (>1050 °C), however, has a negative effect on 3



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its photocatalytic degradation of MB, because the larger crystalline synthesized under high

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temperature has smaller surface area. For instance, the surface area of CaIn2O4 prepared at

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1000 and 1050 °C were 1.27 and 0.86 m2/g. The remarkable photocatalytic performance

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could be attributed to crystal structure of CaIn2O413. The structure contains significantly

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distorted InO6 polyhedral network with great dipole moment promoting the charge separation

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that is beneficial for the photoelectrons transfer.

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Conventionally, CaIn2O4 has been prepared by the solid-state reaction (SSR) method.

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The starting materials are mixed and calcined at elevated temperature for a particular long

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reaction time14. However, the SSR method does not mix the precursors at the molecular level

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so that the agglomeration may occur; and therefore, the final products have low surface area

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and high impurity15. In order to improve the preparation process, some other approaches have

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been suggested. Ge firstly developed a sol-gel method with citric acid and polyethylene

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glycol (PEG)16. After annealing at 1000 °C, CaIn2O4 with low impurity was obtained. In

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2009, a solution-combustion method was also invented to synthesize CaIn2O4 rods at 200 °C

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by using glycine as fuel with post-annealing at 1100 °C for 12 hours in air15. The prepared

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sample was ordinary CaIn2O4 rods with the diameter of 300 nm and length of 2 µm with

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significantly high photocatalytic activity for MB degradation.

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In this paper, we synthesized CaIn2O4 by thermal decomposition of metal oxalate

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precursors at 800, 900 and 1000 °C. The obtained samples were characterized by XRD,

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FE-SEM, EDX, XPS and UV-Vis diffuse reflectance spectroscopy; and their photocatalytic

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properties were examined via degradation of MB under visible light irradiation. 4


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2 Experimental section

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2.1 Raw materials

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Indium

nitrate

(In(NO3)2),

calcium

nitrate

(Ca(NO3)2•4H2O),

oxalic

acid

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((H2C2O4)•2H2O), diethylamine ((CH3-CH2)2NH), ammonia (25%), calcium carbonate

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(CaCO3), indium oxide (In2O3), TiO2 (P25, 21nm) and ZnO were all analytical grade

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materials (Sigma-Aldrich, Finland). The nitrate salts, In(NO3)2 and Ca(NO3)2•4H2O, were

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used for introducing metal elements. The precipitating agent was oxalic acid. The

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diethylamine (CH3-CH2)2NH and ammonia solution were used to control pH. The de-ionized

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water was used during the entire synthesis procedure.

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2.2 Sample preparations

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The preparation of calcium indate was achieved by a modified coprecipitation

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method17. The In(NO3)2 and Ca(NO3)2•4H2O in stoichiometric ratio (2:1) dissolved into 30

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ml nitric acid aqueous solution (4.3%). Meanwhile, oxalic acid (20% excess) was also

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dissolved into water solution and diethylamine was added to adjust the pH at 11. Then, the

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aqueous solution of nitrates was dropwisely added to oxalic acid solution with vigorous

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magnetic stirring.

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(25%) during the whole coprecipitate process. Finally, the oxalate precipitates precursors

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were obtained by filtration, washing with deionized water with pH = 11 and dried in the air at

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80 °C for 18 hours. In order to obtain powder CaIn2O4, the precursors were calcined in air at

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800, 900 and 1000 °C for 6 hours and named as CIO-800, CIO-900 and CIO-1000.

The pH was constantly maintained at 11 by adding ammonia solution

5



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1

In order to compare the difference between calcium indate compounds synthesized

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from different routes, CaIn2O4 prepared by solid-state reaction (SSR) method was also

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conducted12. The stoichiometric amounts (1:1) of CaCO3 and In2O3 were mixed with 20 ml of

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an ethanol solution. After fully blending, the mixture was dried in air at 250 °C for 5 hours

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and calcined at 900 °C for 12 hours. Eventually, the calcined samples were vigorously

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grounded and eventually sintered at 1050 °C for 12 hours in air. The final product is

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designated as CIO-SSR.

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2.3 Characterization of CaIn2O4

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The structural analysis of the final products were performed using a (RINT 2000)X-ray

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diffractometer equipped with graphite monochromatized radiation Cu Kα1 (λ = 1.5406 Å). An

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accelerating voltage of 40 kV and emission current of 1000 mA were adopted for the

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measurements. For each sample, the scanning range was from 20 to 80 ° and the scanning

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speed was 0.02 ° per second. The morphology and microstructure were characterized by

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field-emission scanning electron microscopy (Hitachi S-4800). This microscope was

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equipped with Energy-dispersive X-ray spectrometer (EDAX Genesis 2), which was used for

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qualitative and quantitative analysis of chemical composition. X-ray photoelectron

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spectroscopy (XPS) was conducted with a Sigma Probe (ThermoVG, U.K.) X-ray

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photoelectron spectrometer, of which the source is Al Kα radiation (1.486 eV). The

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photoemitted electrons from the sample were analysed in a hemispherical energy analyzer at

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a pass energy of Ep = 20 eV. All spectra were obtained with an energy step of 0.1 eV and a

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dwell time of 50 ms. A software package (Avantage Thermo VG) was used to analyze the 6


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XPS data. For survey spectrum, the binding energy scanned from 0.00 to 1000.00 in 1.00 eV

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steps. In slow scanning mode, the scanning step was 0.10 eV. The absorption spectrum of the

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samples in the diffused reflectance spectrum mode was recorded in the wavelength range

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between 200 and 1000 nm using a spectrophotometer (V-670, JASCO), with BaSO4 as a

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reference. From the absorption edge, the band gap values were calculated by the

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extrapolation method.

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2.4 Evaluation of photocatalytic behavior

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The photocatalytic activities of samples were evaluated by photocatalytic degradation

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of MB under a 150 watt Xenon lamp with visible light filter (PLS-SXE300C, Beijing

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Perfectlight Co. Ltd., China). For each experiment, 0.3 g catalyst was mixed with 100 mL of

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MB aqueous solution (10 ppm) in photocatalytic Pyrex glass reactor with a cooling water

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jacket. The light source placed about 10 cm apart from the MB solution surface. After

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adjusting the pH to 7 by 0.1M NaOH and HCl, the suspension was treated under magnetic

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stirring for 30 min at room temperature in the dark to ensure the homogenous dispersion of

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photocatalyst and adsorption/desorption equilibrium. When the photocatalytic reaction

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started, the suspensions solution were taken at certain time intervals, and centrifuged with

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4000 revolutions per minute (rpm) for 5 mins to separate photocatalyst from suspension

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solution. The reaction temperature was maintained at ~20 °C. The supernatants were

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collected to evaluate the concentration of MB by UV-Vis spectrometer (Perkin-Elmer

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Lambda 45 UV-vis spectrometer). The absorbance of MB at 664 nm was used to calculate

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the degradation rate according to the following equation: 7



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% =

− − = (1)

1

where C is the concentration of MB and A represents the measured absorbance of MB at 664

2

nm. The subscript of 0 and t denote the initial state and the reaction time, respectively. Total

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Organic Carbon Analyzer (Shimadzu TOC-V CPH) was also used to study the mineralization

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of prepared sample during degradation. NPOC (Non-Purgeable Organic carbon) method was

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adopted for all samples.

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3. Results and discussion

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3.1 Crystal structure characterization

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Powder X-ray diffraction was used to study the crystallization behavior of samples.

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The XRD patterns of CaIn2O4 samples calcined at different temperatures and approaches are

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shown in Fig. 1. By indexing the diffraction peaks with references, the samples only contain

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CaIn2O4 (PDF NO. 00-017-0643) and In2O3 (PDF NO. 01-073-6440) phases. The CIO-800

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sample includes mainly CaIn2O4, but the peaks at 2θ = 30.57, 35.45 and 51.07 ° denote that

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certain amount of indium oxide existed. The impurity of CIO-800 attributed to inadequate

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annealing temperature or time. The quantities and intensities of the reflections of In2O3 were

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reduced after the precursor was annealed at 900 °C.

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CIO-900 was 99.79%, which indicates that the majority of sample was CaIn2O4 phases and

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only minor proportion of In2O3 was present. The purity of CaIn2O4 sintered at 1000 °C is

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further improved to 99.86%. Hence, the purity of CaIn2O4 phase was increased with elevated

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sintering temperatures. The XRD pattern of the sample prepared by conventional solid-state

After calculation, the purity of

8


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reaction method is also represented in Fig. 1 for comparison. Even though the samples

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CIO-SSR were sintered at 900 and 1050 °C for 12 hours, respectively, the impurity was still

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fairly high (about 50%). The poor purity of CaIn2O4 in CIO-SSR sample was the result of

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agglomeration during calcinations15. Therefore, CaIn2O4 with high purity is able to be

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produced via thermal decomposition of metal oxalate precursor at a lower temperature and in

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a shorter reaction time.

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3.2 Microstructure and compositional analysis

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The FE-SEM images in Fig. 2 illustrate the morphology and microstructure of

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CaIn2O4 samples prepared by thermal decomposition of coprecipitate precursor under

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different temperatures. It is clearly seen that most of the particles had irregular shapes and

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diverse sizes, few crystals with facets also were observed. The rod-like particles were ranging

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from 100 to few hundreds nm. Most of particles were connected with each others,

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corresponding to the diffusion effect during sintering. Moreover, the particle size had

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obviously enlarged with increasing calcination temperature.

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The quantitative elemental analysis of the CaIn2O4 samples was performed by EDX.

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As a typical spectrum shows in Fig. 3, only calcium, indium, oxygen and carbon exist. The

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small amount of carbon was detected because of the carbon conductive tapes on the sample

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holder. The atomic ratio of Ca, In and O in the samples were calculated and are listed at

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Table 1. Since the In Lb peak was overlapped with Ca K peak, the atomic ratio of In and Ca

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may have a tiny difference with their theoretical values. The small variation between the

9



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Page 10 of 25

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measured and theoretical atomic ratio could be caused by various reasons, such as the

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thickness variation, particle size and the existence of pores.

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Table 1. Atomic ratio of samples determined by quantitative analysis via EDX

Sample name Atomic ratio CIO-800

CIO-900

CIO-1000

O

60.415

59.43

58.48

In

25.12

25.18

24.50

Ca

13.88

15.39

17.02

4

5

3.3 XPS analysis

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To further investigate the chemistry of samples, XPS analysis was conducted. The

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XPS spectra are able to show the quality and composition of the samples. The Fig. 4 shows a

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typical spectrum of CIO-900 recorded by XPS. In Fig.4 A), the survey spectrum of the

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CIO-900 contains no peak other than In 3d, O 1s, Ca 2p and C 1s. The existence of carbon

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peak was attributed to the adsorption of the residue of organic chemical (chelating agent or

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oxalic acid) during synthetic process. The XPS spectra for O 1s, In 3d and Ca 2p were also

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recorded in slow scanning mode and are illustrated in Figs. 4 B) and C). The O 1s contains

13

two peaks at 529 and 531.5 eV. This asymmetric XPS spectrum indicates that different

10


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oxygen species exist in the near surface region. It is known that metallic oxide (O2-)

2

contribute to the peak at 529 eV

3

work

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peaks at 443.5 and 451.1 eV are corresponding to the binding energy of In 3d5/2 and In 3d3/2,

5

respectively18, which can be assigned to In3+ state. The Ca 2p spectrum observed on Fig. 4 D)

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are Ca 2p3/2 and Ca 2p1/2 at 347.0 eV and 350.5 eV, respectively, which are consistent with

7

the reported literature 19.

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3.4 UV-Vis diffuse reflectance spectroscopic (DRS) study

18

and similar spectrum also has been observed in previous

16

. Another peak at 531.5 eV may arise from oxidized hydrocarbon. The two strong

9

In order to inspect the optical properties of the CaIn2O4 compounds prepared under

10

different sintering temperatures, the UV-Vis diffuse reflectance spectroscopy (DRS) was

11

used to record the spectra of samples in the wavelength range between 200 and 800 nm,

12

shown in Fig. 5. The determined adsorption edges and band gaps of prepared CaIn2O4 were

13

also listed in the graph. It confirmed that MB solution was directly decomposed by CaIn2O4

14

under visible light irradiation. It is also worth to notice that CIO-900 and CIO-1000 have

15

similar adsorption spectrum, which is consistent with the previous study of pure CaIn2O4 11.

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In contrast, the CIO-800 and CIO-SSR have different adsorption spectrum, which may

17

related to the impurity In2O3 exist in these two samples. In2O3 is an n-type semiconductor

18

with a bad gap about 2.5 eV20. The coexistence of In2O3 and CaIn2O4 as composite may

19

change its light adsorption16.

20

11



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1

3.5 Photocatalytic degradation performance

2

The MB degradation under visible light irradiation by a Xenon lamp at room

3

temperature was investigated. The time-dependent degradation rates of MB over

4

photocatalysts were shown in Fig. 6A. The absorbance of MB at 664 nm was used to

5

determine the concentration of MB solution. After three hours irradiation time, it is obvious

6

that CIO-900 have the best photocatalytic performance on MB degradation among tested

7

photocatalysts. More than 95 percent of MB has been degraded over CIO-900 under visible

8

light. For CIO-800 and CIO-1000, only 81 and 78 % of MB were degraded after three hour

9

reaction, respectively. In order to compare the photocatalytic activity among different

10

photocatalysts, TiO2 and ZnO were used as references to test their efficiency on MB

11

degradation. The results showed in Fig.6A indicate that both TiO2 and ZnO adsorbed lots of

12

MB molecule before the photocatalysis reaction started. This is because TiO2 (35-65 m2/g)

13

and ZnO particles used in this case have large surface area. However, the concentrations of

14

MB over TiO2 and ZnO were reducing much slower than that of over CaIn2O4 under light

15

irradiation when the adsorption equilibrium reached. Therefore, it proofs that CaIn2O4 is

16

more efficient than TiO2 and ZnO on MB degradation under visible light irradiation. As a

17

comparison, MB degradation by CIO-SSR was also conducted with the same procedure. It is

18

found that CaIn2O4 prepared by solid-state method has also good photocatalytic performance.

19

More than 87% of MB was degraded, which is higher than CIO-800 and CIO-1000 but still

20

lower than CIO-900. Therefore, comparing photocatalytic performance of calcium indate

12


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1

obtained via solid-state approach, the thermal decomposition of coprecipitate precursor could

2

obtain high crystalline phase of CaIn2O4 with high photocatalytic degradation rate of MB.

3

The photocatalytic performances of CaIn2O4 samples obtained from different

4

sintering temperatures and methods were also studied. The results exhibit that after three

5

hours reaction, the degradation rates of CIO-800, CIO-900 and CIO-1000 were 81%, 95%

6

and 78% respectively. It indicates that calcining temperature have obviously influence on the

7

photocatalytic performances of CaIn2O4 obtained from metallic oxalate precursor. Few

8

reasons may leads to this difference: First of all, as we confirmed from XRD pattern, the 800

9

°C calcined temperature could only prepare samples with certain amount of impurity, which

10

may have negative impact on its photocatalytic behavior. However, the growth of crystal is

11

accelerating if the sintering temperature is too high. In this case, the particle size of CaIn2O4

12

calcined at 1000 °C may be too big to provide enough reaction sites on the surface of

13

particles. Moreover, the adsorption edges of prepared samples were also varied with sintering

14

temperature. The UV-Vis spectrum illustrated that CIO-900 have the highest adsorption

15

edges among prepared CaIn2O4 samples. To sum up all the factors, the CaIn2O4 prepared at

16

900 °C provide the best photocatalytic performance on MB degradation due to its high purity,

17

moderate particle size and high adsorption edge. However, the TOC results (Fig.6 B)

18

illustrate that the total organic carbon concentration didn’t change as much as MB

19

concentration. After six hours reaction, the TOC only reduced 10%. This finding indicates

20

that CaIn2O4 could efficiently degrade MB molecule, but it will took very long time to

21

mineralize the organic dye to CO2 and H2O. 13



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1

3.6 Stability of CaIn2O4

2

The stability of photocatalyst is a very important property in practice. In order to

3

evaluate the stability of CaIn2O4, CIO-900 was used to conduct recycle photocatalytic

4

experiment and its results are shown in Fig. 7. The reaction time of 3.5 hours was taken for

5

each experiment for thoroughly degrading MB. It is found that the degradation rate decreased

6

from 98 to 92% after first recycle, but it was still more than 90% after four times recycle. As

7

the result, CaIn2O4 possess excellent photo-stability under visible light irradiation.

8

3.7 Mechanism discussion

9

A possible mechanism can be deduced based on the characterization and

10

photocatalytic experiments. For CaIn2O4 prepared by thermal decomposition method, the

11

purity of samples is very high but the band gap is varying with sintering temperature.

12

Therefore, CIO-900, which has small band gap value, provide the best photocatalytic

13

efficiency. However, for CaIn2O4 synthesized by solid-state reaction method, high amount of

14

impurity In2O3 exist in the samples. The high photocatalytic activity may be related to the

15

In2O3-CaIn2O4 composite21. In this system, the photo-excited electrons in the conduction

16

band of CaIn2O4 inject into the lower conduction band of In2O3 and the photo-excited holes

17

in the valence band of In2O3 also jump to the valence band of CaIn2O4. Moreover, In2O3 is

18

able to trap the photo-excited electrons efficiently21.

19

20 14


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1

4. Conclusions

2

The visible light activated photocatalyst CaIn2O4 with high purity was successfully

3

synthesized by coprecipitation method at pH = 11. The final products were obtained after

4

thermal decomposition of oxalate precursor at elevated temperature for six hours. The

5

solid-state reaction was also adopted to prepare CaIn2O4 for comparison. By powder-XRD

6

assessment, the CaIn2O4 with high purity were obtained from coprecipitation method when

7

the sintering temperature is at or above 900 °C. However, the sample prepared from

8

solid-state reaction had poor purity, even though it undergone calcinations at 900 and 1050

9

°C for 12 hours. Hence, the coprecipitation method has several advantages over conventional

10

solid-state method: (1) lower sintering temperature, (2) shorter processing time, (3) high

11

purity and (4) smaller particle size, i.e. larger surface area. The photocatalytic degradation

12

experiments proved that synthesized samples have excellent activity on decomposition of MB

13

under sodium lamp irradiation. In the 3.0 hours reaction, almost 95% of methylene blue is

14

degraded over CaIn2O4 sintered at 900 °C. Also, the efficiency of CaIn2O4 is higher than

15

TiO2 (P25).

16

Acknowledgments

17 18

We would like to acknowledge the support provided by the Research Foundation at Lappeenranta University of Technology and MVTT foundation.

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mesoporous hierarchical-like In2S3 hollow microspheres composed of nanofibers and

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Indates with Octahedrally Coordinated d10 Configuration. II. Roles of Geometric and

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Electronic Structures. J. Phys. Chem. B 2003, 107, 7970-7975.

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Chen,

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A

Study

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Luminescent

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Properties

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Crist, pp. 548. ISBN 0-471-49265-5. Wiley-VCH, October 2000. 2000, 1

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core–shell like composite In2O3@CaIn2O4 for efficient degradation of Methylene Blue by

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visible light. Appl. Catal., A 2007, 321, 1-6.

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2 3

Figure 1. The XRD patterns of the samples prepared by different methods and sintered at

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temperatures. The (▼) denote the peaks of In2O3.

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1 2

Figure 2. SEM images of samples - A) CIO-800, B) CIO-900 and C) CIO-1000.

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2

Figure 3. EDX spectrum of a CaIn2O4 sample (CIO-900).

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1

2

Figure 4. XPS analysis of the CaIn2O4 (CIO-900): A) survey spectrum; B) oxygen spectrum;

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C) indium 3d spectrum and D) calcium spectrum.

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1

2

Figure 5. The UV-Vis DRS spectrum of CaIn2O4 photocatalysts from different synthetic

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routes; (Inset) Adsorption edges and Band gaps of CaIn2O4 prepared from different methods

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and sintered at different temperatures.

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Figure 6. A): Time-dependent degradation rate of methylene blue (10 mg/L) over different

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photocatalysts; B): (Inset) TOC measurement of MB solution over CIO-900 for different

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reaction times.

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2 3

Figure 7. Cycling degradation performance of MB done by CIO-900.

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Coprecipitates Synthesis of CaIn2O4 and Its Photocatalytic

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