ABSTRACT In this research, a series of perovskite-type photocatalysts La1-xSrxCoO3 x = 0, 0.2, 0.4, 0.6, 0.8 and nanometric photocatalyst La0.6Sr0.4CoO3 have been fabricated by sol-gel m
OVERVIEW
Photocatalysis
A semiconductor was a substance, usually a solid chemical element or compound that could conduct electricity under some conditions but not others, making it a good medium for the control of electrical current Its conductance varies depending on the current or voltage applied to a control electrode, or on the intensity of irradiation by infrared (IR), visible light, ultraviolet (UV), or X-rays [20]
The specific properties of a semiconductor depended on the impurities, or dopants, added to it An N-type semiconductor carried current mainly in the form of negatively-charged electrons, in a manner similar to the conduction of current in a wire A P-type semiconductor carried current predominantly as electron deficiencies called holes A hole had a positive electric charge, equal and opposite to the charge on an electron In a semiconductor material, the flow of holes occurred in a direction opposite to the flow of electrons
Semiconductors were particularly useful as photocatalysts because of a favorable combination of electronic structure, light absorption properties, charge transport characteristics and excited-state lifetimes A semiconductor, by definition, was nonconductive in its undoped ground state because an energy gap, the band gap, exists between the top of the filled valence band and the bottom of the vacant conduction band Thus, electron transport between these bands must occur only with appreciable energy change In semiconductor photocatalysis, excitation of an electron from the valence band to the conduction band was accomplished by absorption of a photon of energy equal to or higher than the bandgap energy of the semiconductor This light- induced generation of an electron-hole pair is a prerequisite step in all semiconductor- mediated photocatalytic processes Photogenerated species tended to recombine and dissipate energy as heat of photons because the kinetic barrier for the electron-hole recombination process is low In order to have a photocatalyzed reaction, the e - -h + recombination, subsequent to the initial charge separation, must be prevented as much as possible However, conduction band electrons and valence band holes could be
5 separated efficiently in the presence of an electric field, such as the one formed spontaneously in the space charge layer of a semiconductor-fluid or a semiconductor- metal interface Therefore, the lifetimes of photogenerated carriers increased and the possibility was offered to these species to exchange charge with substrates adsorbed on the photocatalyst surface and initiate chemical reactions [20]
Several semiconductors had energies of their bandgap sufficient for catalyzing a wide range of redox reactions An ideal photocatalyst should be inexpensive, non- toxic, highly photoactive and stable in the conditions in which the pollutant is present
Some well-known photocatalyst materials were TiO2, WO3, SrTiO3 and Fe2O3 and LaCoO3 [21] The advantage of using semiconductor based materials as photoactive catalysts in the detoxification of pollutants was the complete mineralization into environment friendly products, without generation of waste, which was not possible in the case of any other treatment method [22] The other advantages included regeneration, reusability and active under easily available UV-visible photo-light
Photocatalysis was a process where a catalyst participates in modifying the rate of a chemical transformation of the reactants without being consumed in the end under light irradiation [23] In other words, light was a main factor that activates catalyst activity and elevates reaction rate
Chlorophyll of plants is a typical natural photocatalyst The difference between chlorophyll photocatayst and synthesized one was, usually chlorophyll captures sunlight to turn water and carbon dioxide into oxygen and glucose, but on the contrary photocatalyst created strong oxidation agent and electronic holes to break down the organic matter to carbon dioxide and water in the presence of photocatalyst, light and water [24]
6 Figure 1 Schematic diagram comparing the actions of a man-made photocatalyst
(TiO2) with a natural one (chlorophyll) [24]
The use of semiconductors as photocatalysts in environmental treatment studying were attracting more attention than other conventional methods [23] In this method, catalyst itself was not modified during the process and need not to provide other raw materials for the reaction system In addition, the advantages of this method included possible performance under ambient temperature and pressure, using artificial UV or nature light source, inexpensive and non-toxic materials
Heterogeneous photocatalysts such as titanium oxide (TiO2) were a promising material for water purification It could degrade water pollutants such as aromatic chemicals, various dyes and domestic wastewaters In addition, it was able to degrade microorganisms completely into carbon dioxide, water and mineral acid [23]
1.1.3 Mechanism of photocatalytic degradation of MB:
Figure 2 showed the mechanism of MB photocatalytic degradation
7 Figure 2 Schematic diagram of photocatalytic process initiated by photon acting on the semiconductor [25]
When the electron in the valence band of the semiconductor absorbed a photon with energy equal to or greater than the band gap (∆E) of the semiconductor, the electron became excited and jumped to the conduction band, leaving a positively charged hole , hVB +, in the valence band [26] Besides the recombination with the electron, the positively charged hole could oxidize water molecules to form hyper- reactive hydroxyl free radicals ( OH) The resulting hydroxyl radicals were the main agent that attack the chemical pollutant molecules or microorganisms cells to purify water [25] The excited electron could react with dissolved oxygen to form the oxygen radical, O2- , which was also active toward organic pollutants According to this, the relevant reactions at the semiconductor surface causing the degradation of dyes could be expressed as follow [25]:
TiO2 + hυ (UV) → TiO2 (eCB - + hVB +) (1) TiO2 (hVB +) + H2O → TiO2 + H + + OH (2) TiO2 (hVB +) + OH - → TiO2 + OH (3) TiO2 (eCB -) + O2 → TiO2 + O2- (4)
Dye + eCB - → reduction products (8) where hυ is photon energy require to excite the semiconductor electron from the valence band (VB) region to conduction band (CB) region
1.1.4 Mechanism of photocatalytic degradation of CO 2 :
The photocatalytic conversion of CO2 into useful products, mainly methane CH4 and other inorganic and organic substances such as CO [27], HCOOH [12], HCHO [12], CH3OH [28], has been studied for a long time Even though knowledge was still limited, the reaction mechanisms proposed until now are based on the observed charge transfer and reaction orders that have been acquired from macroscopic electrochemical testing According to the literature, CO2 photochemical reduction follows different reaction pathways, depending on the used photocatalyst and on the experimental conditions [27] The mechanism represented for the photocatalytic conversion of CO2 in water as the reductant using TiO2 was shown below [29]:
Figure 3 Schematic representation of the photocatalytic reduction of CO2 with H2O on the anchored titanium oxide [29]
When UV light with sufficient photonic energy (hυ) and associated appropriate wavelength were used, photon-generated electrons and holes were formed on the catalyst surface The hole first reacted with the H2O vapor adsorbed on the catalyst,
9 hydroxyl radicals ( OH) and hydrogen ion (H + ) were produced The water was then oxidized by the OH radicals, and oxygen and H + are formed In the meantime, CO2 molecules were reduced to carbon radicals ( C), with CO as a byproduct At the end, the carbon radicals reacted with H + to yield CH4 The eventual presence of CO in the gaseous product mixture was due to an incomplete CO2 reduction, which in turn could be due to different reasons such as insufficient H + amount, or the recombination of C radicals and oxygen on the catalyst surface [29].
Perovskite compounds
The ideal perovskite-type structure is cubic with space groupPm3m-O l h In the unit formula of perovskite-type oxides ABO3, A is the larger cation and B is the smaller cation In this structure, the B cation is 6-fold coordinated and the A cation is 12-fold coordinated with the oxygen anions Figure 4 depicts the corner sharing octahedra that form the skeleton of the perovskite structure, in which the center position is occupied by the A cation Alternatively, this structure could be viewed with the B cation placed in the center of the octahedron and the A cation is in the center of the cube The perovskite structure was thus a superstructure with a ReO3-type framework built up by the incorporation of A cations into the BO6 octahedra The significance and role of the ReO3-type framework as a host structure for deriving numerous structures of metal oxides has been emphasized by Raveau [30]
In the ideal structure, where the atoms were touching one another, the B-O distance was equal to a/2 (a is the cubic unit cell parameter) while the A-O distance was (a/ 2) and the following relationship between the ionic radii holds: rA + rO = 2 (rB + rO) However, it was found that the cubic structure was still retained in ABO3 compounds, even though this equation is not exactly obeyed As a measure of the deviation from the ideal situation, Goldschmidt introduced a tolerance factor (t), defined by the equation [30]:
10 which was applicable at room temperature to the empirical ionic radii Although for an ideal perovskite t is unity, this structure was also found for lower t-values (0.75< t
99 %
Guangzhou Jinhuada Chemical Reagent Co
Cobalt (II) nitrate hexahydrate Co(NO3)2.6H2O > 99 %
Guangzhou Jinhuada Chemical Reagent Co
Guangzhou Jinhuada Chemical Reagent Co
Oven (170 o C, MMM, Medcenter Einrichtungen GmbH, Germany )
UV-vis spectroscopymeter (250 V, Model 4001/4, Thermo electron corporation,
Furnace (30-3000 o C, Nabertherm, Nabertherm GmbH, Germany)
Centrifuge (6000 min -1 , 230 V, Hochstzul Drehzahl, Hermle Labortechnik,
Ultrasonicator (500 W, Power sonic 410, Hwashin Technology Co, Korea)
Analytical balance (AR2140, Ohaus Corp Pine Brook, NJ USA)
China Gas Chromatography 2000 equipped with a flame ionization detector (FID)
Synthesis processing of La1-xSrxCoO3 nanophotocatalysts could be divided in two stages:
Synthesis of La1-xSrxCoO3 powders using sol-gel method and find the proper x value that gives the highest photocatalytic activity
Synthesis of nano perovskite La1-xSrxCoO3 with the above x value using carbonaceous microspheres template
3.2.2.1.1 Synthesis of perovskite-type La 1-x Sr x CoO 3 using the sol-gel method:
A series of perovskite photocatalysts La1-xSrxCoO3 with variable Sr content (x = 0, 0.2, 0.4, 0.6, 0.8) were fabricated by the sol-gel citrate method The metal nitrates were weighed to the nominal compositions and dissolved in 60 mL deionized water with Co concentration of 0.25 mol/L Citric acid monohydrate, CA.1H2O, was also added to this solution as chelating agent with molar ratio of citric acid/metals to be 1.5/1 The resulting mixture was then heated in water bath at 70 o C under continuous stirring After 4h heating, the clear pink solution transformed into gel This dark pink gel was dried in an oven in air at 140 o C overnight and following calcined at 850 o C for 4h with heating rate of 5 o C/min In order to obtained perovskite powders as photocatalytic materials, the 850 o C calcined powders were pulverized in ethanol media using 5mm dia zirconia balls in 12h and following drying in electric oven in
27 overnight The obtained perovskite powders were labeled as LSC followed by two numbers where the first two indicate the molar ratio between lanthanum and strontium, the next “S” letter presents for sol-gel method and the last three numbers indicate the calcined temperature For instance, the La0.6Sr0.4CoO3 sample prepared at 850 o C was labeled as LSC64-S850 However, when x is equal to 0, the product was marked simply as LC-S850
Here, the amount of strontium was changed from x = 0 to x = 0.8 to investigate the Sr-doping effect on the crystal structure of LaCoO3 catalyst while the temperature and time calcination were fixing at 850 o C, 4h according to L.F Liotta [18]
The flowchart of the experimental process was shown in Figure 14
La(NO 3 ) 3 6H 2 O Sr(NO 3 ) 2 Co(NO 3 ) 2 6H 2 O CA 1H 2 O DI water
Figure 14 Flowchart of La1-xSrxCoO3 synthesis process by the sol-gel method
3.2.2.1.2 Synthesis of La 1-x Sr x CoO 3 nanoparticles using carbonaceous microspheres template:
In a typical experiment, 4.7562 g glucose was dissolved in 100 ml distilled water to form a colorless solution by magnetic stirring at 60 o C for 30 min and then ultrasonicating at 30 o C for 30 min Subsequently, the solution was transferred into a 125 ml autoclave with a Teflon seal and heated at 170 o C for 11 h Afterwards the autoclave was cooled to room temperature naturally The dark puce products were separated by centrifugation at 4000 rpm for 30 min Then a rinsing process involving three cycles of centrifugation/washing/ultrasonication was performed with water, respectively Finally, carbonaceous microspheres using as the templates to prepare La0.6Sr0.4CoO3 hollow spheres were obtained after vacuum drying at 80 o C for more than 6 h
Carbonaceous microspheres ã 60 o C ã 30 min ã 30 o C ã 30 min ã 170 o C ã 11 h ã 4000 rpm ã 30 min ã 30 o C ã 30 min ã 1 h ã 30 o C ã 30 min DI water
Figure 15 Flowchart of carbonaceous microspheres synthesis process Synthesis of La 1-x Sr x CoO 3 nanophotocatalysts:
Here, the nano perovskite La0.6Sr0.4CoO3 was taken as an example and prepared as follows: 1.3 g La(NO3)3 6H2O, 0.424 g Sr(NO3)2 and 0.556 g of Co(NO3)2 6H2O
29 were dissolved in 50.0 ml distilled water Subsequently, 1.0 g carbonaceous microspheres were dispersed in the above solution with the assistance of ultrasonication for 10 min at 30 o C Then the resulting suspension was magnetically stirred at room temperature for 24 h to achieve the adsorption–desorption equilibrium between the carbonaceous spheres, La 3+ , Sr 2+ , Co 2+ ions After dried at 80 o C in air for 1 day, the obtained precursor was calcined at 550 o C (or 500 o C) for 4 h with a heating rate of 2.0 o C/min to remove the carbonaceous template Then it was cooled to room temperature The nano perovskite La0.6Sr0.4CoO3 was accumulated at the bottom of the crucible The resulting products fabricated at 500 o C and 550 o C were defined as LSC64-M500 and LSC64-M550, respectively
In this experiment, the calcined temperature (500 o C and 550 o C), which based on the results of Helin Niu [13], was chosen to synthesize the nanophotocatalysts
Nano perovskite La 0.6 Sr 0.4 CoO 3 ã 30 o C ã 10 min ã 30 o C ã 24 h ã 30 o C ã 20 min ã 80 o C ã 1 day ã 550 o C or 500 o C ã 4 h
Figure 16 Flowchart of nano perovskite La0.6Sr0.4CoO3 synthesis process
30 3.2.2.2 Photocatalytic degradation of methylene blue experiment:
In this study, the photocatalytic degradation experiment was performed to:
Choose the proper x coefficient that gives the highest photocatalytic activity
Evaluation the variation of photocatalytic activity between the different MB concentrations ranging from 10 ppm to 30 ppm
Compare the MB degradation efficiency of photocatalyst between two methods: sol-gel method and surface-ion adsorption method
To determine the amount of MB remaining in solution after treatment, a calibrate curve described the relationship between the concentration of MB and the coefficient A (absorption coefficient of MB solution measured by UV-Vis) was built Analytical method UV-vis was used to examine the change in the concentration of methylene blue in photocatalytic reaction In this study, the calibrate curve was constructed based on MB solution concentration was premeditated by weighing the exact amount of MB dissolved in solution The concentrations of MB were 0, 2, 4, 6, 8, 10 ppm, respectively The MB solution was measured with a wavelength of 664 nm, the wavelength of maximum adsorption of MB
The photocatalytic degradation experiment can be described below:
In order to evaluate influence of Sr content on MB photodegradation, the suspensions containing methylene blue and La1-xSrxCoO3 photocatalyst were irradiated by the Pen-Ray@ Light Source lamp with continuous magnetic stirring at room temperature The 0.5 g/L suspension was prepared by adding 0.5 g La1-xSrxCoO3 photocatalysts into 1.0 L of 30 ppm MB solution Prior to UV irradiation, the suspensions were magnetically stirred for 65 min in the dark to ensure the adsorption/desorption equilibrium of methylene blue with the catalyst After that, the mixture was subjected to UV irradiation Each 6 ml of the supernatant was taken out by syringe at different time intervals and centifugated to separate the catalyst and MB solution The MB concentration remaining after photocatalytic treatment was determined using 4001/4 UV–visible spectrophotometer with a wavelength of 664 nm
The sample would be measured three times at a definite time interval and the average
31 value was then recorded as the final result The degree of methylene blue degradation was also calculated according to the following equation:
Co: the initial concentration of MB solution C: the concentration of MB solution after photocatalysis Ao: the intensity of MB solution before photocatalytic reaction A: the intensity of MB solution after photocatalytic reaction
Table 2 Experimental conditions for MB adsorption stage
No M catalyst :V MB 30 ppm mg/l t, min , nm
Table 3 Experimental conditions of MB degradation stage under UV light
No M catalyst :V MB 30 ppm mg/l t, min nm
3.2.2.3 Photocatalytic reduction of CO 2 experiment:
Figure 17 Experimental setup model for the photocatalytic reduction of CO2
The experimental setup for the photocatalytic reduction of CO2 was shown in Figure 17 In the liquid phase reaction, the catalyst loading was 0.1 g of cat in 5.0 ml distilled water Before each test, the solution was saturated with CO2 by flowing CO2 gas (99.999%) for 30 min The experiments were then carried out in a batch reactor with the temperature controlled at 60 o C and UV-irradiated using a 9W UV lamp with a wavelength of 254 nm for 24h The main product of CO2 reduction reaction, CH4, were analyzed by a gas chromatography (China Chromatography 2000) equipped with a flame ionization detector (FID)
A blank test was carried out similar to CO2 reduction experiment but without photocatalysts present
X-ray diffraction (XRD) patterns of the specimens were recorded using an X-ray diffractometer (D2 Phaser, Bruker) equipped with a Cu K radiation source (1.5406 Å) and nickel filter Microstructure analysis was performed with a Scanning Electron Microscope (SEM, JSM-6500F, JEOL) Ultraviolet–visible Diffuse Reflection Spectroscopy (UV-vis DRS) of the photocatalyst was investigated with a UV–vis spectrophotometer (Varian Cary-100) in the wavelength between 200 nm and 800 nm using BaSO4 as a reference In addition, the specific surface area of LSC64-S850, LSC64-M500, and LSC64-M550 powders was measured by the Brumauer – Emmett – Teller (BET) method using Quantochrome NOVA 1000e, Nitrogen
X-ray Diffraction (XRD) was a method used for identifying the atomic and molecular structure of a crystal, in which the crystalline atoms cause a beam of incident X-rays to diffract into many specific directions [40] By measuring the angles and intensities of these diffracted beams, a crystallographer could produce a three- dimensional picture of the density of electrons within the crystal From this electron density, the mean positions of the atoms in the crystal could be determined, as well as their chemical bonds, their disorder and particle diameter [40]
The crystal structures of the prepared perovskite-type oxides were examined by powder X-ray diffractometer (D2 Phaser, Bruker) with Cu Kα radiation (2θ = 15 o – 80 o , step = 0.03 o , λ = 1.5406 Å) The sample for analysis were ground using a mortar and pestle, followed by loading onto glass slides The accelerating voltage was set to be 45 kV and the operating beam current was 45 mA The amount of sample for analysis was about 0.2 g
The Scanning Electron Microscope (SEM) could give us a lot of information about the catalysts such as external morphology, chemical composition, and crystalline structure It permitted the observation and characterization of inorganic materials on a nanometer (nm) to micrometer (àm) scale [44] SEM was advantageous to
34 characterize powder morphology and size because of its ease of operation, simple sample preparation, large depth of field, and nanoscale resolution
The morphologies of powders were investigated using a field emission scanning electron microscope (SEM, JSM-6500F, JEOL) at accelerating voltages of 10 kV
Specially, the powders for analysis were dispersed on carbon tape attached SEM tub
The excess loose powder was tapped off the stub The amount of sample for analysis was about 0.1 g
The surface area of particles could be evaluated by several methods Among them, Brunauer – Emmett – Teller (BET) theory was an important analysis method for the measurement of the specific surface of a material, originally published by Stephen Brunauer, Paul Hugh Emmett and Edward Teller in 1938 [45] The specific surface area of a powder was determined by physical adsorption of a gas on the surface of the solid and by calculation the amount of adsorbate gas corresponding to a monomolecular layer on the surface The determination was usually carried out at the temperature of liquid nitrogen The amount of gas adsorbed could be measured by a volumetric or continuous flow procedure [45] This method was an accurate and direct expression for specific surface area measurements for particles
In this experiment, the surface area was determined using nitrogen adsorption- desorption measurements, which were analyzed with the Brunauer – Emmett – Teller (BET) method These measurements were carried out at 77K using a multi-point method after the sample was vacuum-degassed for 3 hours at 300 o C to remove moisture The BET surface area measurement is based on the isothermal curve of gas adsorption and desorption at low relative pressure (0.05