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Low temperature synthesis and photocatalytic property of perovskite-type LaCoO3 hollow spheres
Accepted Manuscript Low temperature synthesis and photocatalytic property of perovskite-type La‐ CoO3 hollow spheres Shasha Fu, Helin Niu, Zhiyin Tao, Jiming Song, Changjie Mao, Shengyi Zhang, Changle Chen, Dong Wang PII: DOI: Reference:
S0925-8388(13)01004-9 http://dx.doi.org/10.1016/j.jallcom.2013.04.092 JALCOM 28386
To appear in: Received Date: Revised Date: Accepted Date:
29 October 2012 12 April 2013 13 April 2013
Please cite this article as: S. Fu, H. Niu, Z. Tao, J. Song, C. Mao, S. Zhang, C. Chen, D. Wang, Low temperature synthesis and photocatalytic property of perovskite-type LaCoO3 hollow spheres, (2013), doi: http://dx.doi.org/ 10.1016/j.jallcom.2013.04.092
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Low temperature synthesis and photocatalytic property of perovskite-type LaCoO3 hollow spheres Shasha Fua, Helin Niua,*, Zhiyin Taoa, Jiming Songa, Changjie Maoa, Shengyi Zhanga, Changle Chenb*, Dong Wang,c a
School of Chemistry and Chemical Engineering, Anhui University, Hefei 230039, P. R. China
b
CAS Key Laboratory of Soft Matter Chemistry and Department of Polymer Science &
Engineering, University of Science and Technology of China, Hefei, 230026, P. R. China c
Department of Bio-Health Technology, College of Biomedical Science, Kangwon National
University, Chuncheon, Gangwon-Do, 200-701, Korea Corresponding author: E-mail: [email protected] , [email protected]
Abstract Hollow perovskite-type LaCoO3 was successfully fabricated by surface-ion adsorption method utilizing the carbonaceous colloids as template under relatively low calcination temperature. Carbonaceous colloids not only acted as templates but also offered internal heat source during calcination process. The impact of calcined temperature and time on the structure and morphology of the product were studied and the possible formation process of perovskite-type LaCoO3 hollow spheres was illustrated. The obtained product was characterized by SEM, TEM, XRD, TG-DSC, ICP-OES, BET and UV-visible absorption spectra. The photocatalytic activities for
degradation of methylene blue, methyl orange and neutral red were tested. The good photocatalytic degradation activity of the three different dyes and the band gap of 2.07 eV make it a promising candidate material for photocatalytic applications.
Keywords: Perovskite; Hollow; Low temperature; Colloids template; Photodegradation. 1. Introduction Perovskite-type oxides have attracted considerable attentions because of their unique properties such as various types of oxygen vacancy order[1], intrinsic oxygen reduction reaction activity[2], high conductivity[3], magnetic properties [4], excellent performance as cathode or anode [5, 6] and high Seebeck coefficient [7, 8]. Perovskite-type lanthanum cobaltate (LaCoO3) is cheap, environmentally friendly and highly active in oxidation processes, making it a very promising material for many applications including highly active catalysts for energy storage [9], cathode for fuel cell [5, 10], catalytic reduction of NOx in automotive exhausts [11], catalytic oxidation of volatile organic compounds (VOCs) [12, 13], degradation of combustion soot [11, 14-17], catalytic oxidation of CO [18] and photocatalytic degradation reactions [19]. The catalytic performance of the LaCoO3 is significantly influenced by synthetic procedures, calcination temperature/time and so on. Many synthetic methods have been reported such as solid state reaction[4, 20, 21], sol-gel method[22], chemical co-precipitation method[23], combustion method[24], freeze-drying based method[25, 26] and microwave-assisted method[27]. However, most of these methods involve high-temperature and time-consuming solid-state reaction in order to mix the metal oxides on molecular level.
Meanwhile, the catalytic performance of LaCoO3 is not good enough for practical applications, mainly due to their low surface area (on the order of m2g−1) and strong tendency to sinter during high temperature calcination. It is well known that hollow and porous structures can greatly influence catalytic activities [28, 29]. Therefore, it is intriguing to explore the properties of hollow or porous LaCoO3 materials as well as new environmentally friendly and low power consuming synthetic routes. Recently, considerable studies have been dedicated to synthesize porous perovskites utilizing traditional templates [20] such as anodic aluminium oxide (AAO) [30-32], ZnO[33], silicon[34], corn starch[35] and polystyrene sphere[36]. In contrast, research on green colloids templates is relatively scarce. Carbonaceous colloid, a green colloid template prepared from saccharide solution, has integral and uniform surface with functional layers, which ensures homogeneity of the reagents and makes the surface modification unnecessary. Moreover, its size can be easily modified. It is reported that carbonaceous colloid template has been applied to produce monometallic oxides with hollow structures. However, this method has never been applied to prepare bimetallic perovskite-type oxide with hollow structures. Recently, our group reported the synthesis of porous La2O2CO3 hollow microspheres in saccharide solution using a one-pot hydrothermal method [37]. Herein, we report the fabrication of hollow perovskite-type LaCoO3 by surface-ion adsorption method utilizing the carbonaceous colloids as template at low calcination temperature. The influence of calcination time and temperature on the structures of LaCoO3 was investigated and its formation mechanism was discussed. The
photocatalytic degradation of the organic dyes including methylene blue, methyl orange and neutral red was employed to probe the photocatalytic properties of the hollow perovskite-type LaCoO3 under UV irradiation.
2. Experimental 2.1. Preparation methods All reagents were purchased from Sinopharm Chemical Reagent Co., Ltd, China. These reagents in analytic grade were used without further purification. 2.1.1. Synthesis of carbonaceous microspheres In a typical experiment, 6.0 g glucose was dissolved in 40 ml distilled water to form a clear solution. The solution was then sealed in a 40 ml autoclave with a Teflon seal and maintained at 160 °C for 12 h. Dark puce products were obtained after centrifugation at 8000 rpm for 10 min. A rinsing process involving three cycles of centrifugation/washing/redispersion was performed with water or ethanol, respectively. Carbonaceous microspheres using as the templates to prepare LaCoO3 hollow spheres were obtained after oven-drying at 70 °C for more than 6 h. 2.1.2. Synthesis of LaCoO3 hollow spheres 2.17 g La(NO3)3·6H2O and 1.46 g of Co(NO3)2·6H2O were dissolved in 50.0 ml distilled water. The ratio between the components was carefully controlled in [La3+]:[Co2+] = 1:1. In order to form core-shell structure precursor, 1.0 g carbonaceous colloids spheres with diameter of 100 nm [38]were added into the above solution and evenly dispersed with the assistance of ultrasonication. Then the reaction solution was placed at room temperature overnight to achieve
adsorption–desorption equilibrium between carbonaceous spheres, Co2+ and La3+ ions. A rinsing process involving 2 cycles of centrifugation/washing/re-dispersion was performed with water or ethanol, respectively. After dried in an oven in air with at 80 °C for 6 h, the obtained precursor was denoted as carbon@LaCo. In a typical procedure, carbon@LaCo was calcined in air at 550 °C for 4 h to remove the carbon core. Then it was allowed to cool to room temperature. The as-formed products were accumulated at the bottom of the crucible. In order to study the effect of calcination parameter on morphology, calcination time was set as 6 and 8 hours respectively by keeping temperature at 550 °C, and the temperature was set as 500 °C and 600 °C respectively by keeping calcination time for 4 h. Samples obtained for 4h, 6h and 8h at 550 °C were marked as 550-4, 550-6 and 550-8 respectively and samples obtained at 500 °C, 550 °C and 600 °C for 4 h were marked as 500-4, 550-4 and 600-4 respectively. 2.2. Structure and morphology characterization of products The structure and morphology of the product have been studied by X-ray diffractometry (XRD, X’pert PRO SUPER, Cu Kα radiation, λ=1.54056 Å), field emission scanning electron microscopy (FESEM, FEI Sirion 200) and transmission electron microscopy (TEM, Hitachi H-800). Thermo-gravimetric analysis and differential scanning calorimetry (TGA–DSC) were used to test the weight loss of the products. TGA–DSC data was recorded with a thermal analysis instrument (WCT-1D BOIF) in air flow at the heating rate of 10 °C/min. Elemental analysis for the carbon@LaCo precursor was determined with an
inductively coupled plasma (ICP-OES, IRIS Intrepid II, Thermo Electron, USA). Nitrogen adsorption and desorption isotherms were measured at 77 K using a Micromeritics ASAP2020M+ C system. Specific surface areas were determined by the BET method and the mesopore size distribution was determined by the Barrett–Joyner–Halenda (BJH) method. For the BJH analysis, the pore size distribution was obtained from the analysis of the adsorption branch of the isotherm. Ultraviolet–visible (UV–vis) spectroscopy of the products was measured by UV–vis diffuse reflectance spectroscopy (Hitachi U-4100). An American Beckman Coulter Delsa 440SX Zeta Potential Analyzer was used to perform the zeta potential determination. 2.3. Photocatalytic degradation experiment Photocatalytic activity of the LaCoO3 hollow spheres was assessed using the degradation reaction of methylene blue (MB) exposed to UV-vis light. The reaction suspensions containing methylene blue and LaCoO3 photo-catalyst were irradiated by the 30W UV light high-pressure mercury lamp with continuous magnetic stirring at room temperature. Absorbance measurements were recorded by 752PC UV-vis spectrophotometer. 10 mg of MB was dissolved in 1000 ml of deionized water to form 10 mg/L MB solution. The 0.2 g/L suspension was prepared by adding 10 mg LaCoO3 hollow spheres into 50 ml of MB solution. Prior to irradiation, the suspensions were magnetically stirred for 60 min in the dark to ensure adsorption/degradation equilibrium of methylene blue with the catalyst. After that, keeping the distance between liquid level and light source about 10 cm, the mixture
was subjected to UV irradiation. 2 ml of the supernatant was taken out by syringe at different time intervals and same volume of freshly as-prepared organic solution was added into the separation of the precipitate followed by ultrasonic. The dye obtained after separation was determined using 752PC UV-visible spectrophotometer. The degradation of methylene blue absorption peak intensity at 664 nm was monitored as a function of UV exposure time. Similar experiments were carried out by replacing methylene blue with methyl orange and neutral red, respectively.
3. Results and discussion 3.1. Characterization of carbonaceous microsphere and carbon@LaCo composite microspheres Carbonaceous spheres were prepared by a reported method[38]. As seen from Fig. 1a, the surface of the spheres is rough and porous, which not only provides high surface area, but also aids penetration of reactive species. In order to study the adsorption mechanism, the zeta potential of carbonaceous spheres was measured in aqueous solution by Delsa 440SX Zeta Potential Analyzer. The result of -36 mV indicates that carbonaceous spheres are negatively charged and stably dispersed in aqueous solution, thus allowing an easy manipulation of the evenly surface adsorption of carbonaceous spheres with Co2+ and La3+ ions. As shown in Fig. 1b, the carbon@LaCo precursor has a smooth ball shape, which indicates that the pore on the carbonaceous spheres surface might be filled with Co and La precursors. In order to quantify the ratio between La and Co ions adsorbed on carbonaceous spheres, inductively coupled plasma was used to determine carbon@LaCo precursors. The results display that the
mass proportions of loaded cobalt and lanthanum of the dissolved carbon@LaCo precursor in subboiled HNO3 are 1.7% and 4.1%, respectively, which illuminates that the ratio between La and Co ions adsorbed on carbonaceous spheres are almost 1:1. The TG curve in Fig. 1c shows that the burning of carbonaceous spheres is completed before 550 °C. DSC curve of the carbonaceous spheres demonstrats two exothermic peaks. According to the report by Tan's et al.[39], the first exothermic peak at 320 °C should be attributed to the dehydration and densification of carbonaceous sphere templates, and the second exothermic peak at 492 °C should be due to combustion of carbonaceous spheres. The result shows that the carbonaceous spheres not only acted as templates but also offered internal heat source in the formation process of the porous perovskite-type LaCoO3 hollow spheres. In Fig. 1d, the total weight loss of carbon@LaCo composite microspheres is 89.6%, indicating LaCoO3 product yield about 10.4 %. DSC curve of carbon@LaCo composite spheres shows only one sharp exothermic peak centered at 409 °C which indicates that the heat generated by burning of carbonaceous sphere templates and the crystallization process of the perovskite LaCoO3 at the same time might be the reason for synthesis LaCoO3 in relatively low calcining temperature 550 °C. 3.2. Characterization of LaCoO3 hollow spheres XRD was used to characterize the crystal structure of the sample which was obtained by calcining at 550 °C for 4 h (Fig. 2.). The major diffraction peaks of the products can be indexed to the perovskite-type cubic structure of LaCoO3 with Pm 3 m space group (JCPDS file no. 75-0279). This suggests that the main component
of products obtained by calcining at 550 °C for 4 h is perovskite-type cubic structure of LaCoO3. It is well documented that synthesis of perovskite-type LaCoO3 was generally under the condition of the existence of chelating agents [7, 8, 11, 18, 26]. So, a suitably designed method to synthesize hollow perovskite-type LaCoO3 under low temperature conditions without chelator is worthy of investigation. Speculated formation mechanism of perovskite-type LaCoO3 hollow spheres at relatively low temperature is illustrated in Scheme 1. What’s more, the template used here can be substituted for a series of carbonaceous spheres derivatives which possess core-shell nanostructures with carbonaceous polysaccharide shells, such as Ag@C [40], Te@C [41], Pt@C [42], SnO2@C[43], Fe3O4@C [44], and so on. Herein, this strategy can be extended to develop a general and facile method for preparation of core-shell nanostructures with perovskite-type LaCoO3 shells and functional cores. The morphology analyses of products were carried out using TEM and SEM techniques. The TEM image of hollow LaCoO3 spheres obtained by calcining at 550 °C for 4 h is presented in Fig.3a. The average particle size is about 50 nm. Overall, all particles have tight size distribution with connection between spheres. In comparison with the carbon@LaCo precursor (Fig. 1c), diameter of the hollow product shrink from 150 nm before calcination to 50 nm after calcination. Shrinking in the particle size can be attributed to disappearance of hard templates and the formation of the perovskite-type LaCoO3 in calcining process. The thickness of LaCoO3 shell is found to be around 10 nm. Fig. 3b is the SEM image of products obtained by calcining at
550 °C for 4 h, and uniform broken and unbroken hollow spheres in large scale are observed, which indicates that perovskite-type LaCoO3 hollow spheres are prepared successfully by carbon template method. Brunauer–Emmett–Teller (BET) gas sorptometry measurements have been conducted to examine the hollow perovskite-type LaCoO3. Fig. 4 shows the N2 adsorption/desorption isotherm and the pore-size distribution (inset) of the hollow perovskite-type LaCoO3. The isotherm can be categorized as type IV with a distinct hysteresis loop observed in the range of 0.6–1.0 p/p0, which is the characteristic of mesoporous metal oxides prepared by the hard-templating method. The specific surface area is determined to be 43.39 m²/g and the pores size distribution with the peak located at about 50 nm according to the BJH analysis. This result is consistent with the TEM image of hollow LaCoO3 spheres (Fig. 3a ). 3.3 The impact of calcination time and temperature on products To investigate the effect of the calcination time on the particle morphology and structure, samples were calcined at 550 °C ranging from 4 h to 8 h. Sample 550-8 shows stronger and more acute diffraction peaks of LaCoO3 than sample 550-4 with a weak impurity peak of La2O3 and Co3O4 (Fig. 5). As seen from Fig. 6a, the SEM image of sample 550-6 shows that LaCoO3 hollow spheres have all melted and collapsed into solid particles with increasing agglomeration. As shown in Fig. 5, all major XRD peaks of sample 550-8 are matched with the LaCoO3 with the standard XRD pattern (JCPDS Card, 75-0279) and no phase transformation from cubic LaCoO3 to rhombohedral is observed even after 8 h of calcinating at 550 °C. With
longer calcination time, the crystallinity of cubic LaCoO3 gets better, while LaCoO3 hollow spheres melt and collaps into solid particles when calcined for more than 4 h. To investigate the effect of the calcination temperature on morphology and structure of samples, samples were calcined at different temperatures for 4 h. XRD patterns of samples 500-4, 550-4, 600-4 are shown in Fig. 7. It is hard to distinguish rhombohedral LaCoO3 from cubic one due to their similarities in diffractions. However, According to the report by Tien-Thao’s et al.[45], the twin peaks (110) (104) at 2 θ =33.17, 33.54 are characteristic peaks of rhombohedral LaCoO3(JCPDS Card, 48-0123), which means the rhombohedral phase appears after 4 h of calcination at 600°C. As shown in Fig. 7 and Fig.8, higher calcination temperatures lead to better crystallinity. Meanwhile, the hollow structure completely disappeared and the structural transformation from cubic to rhombohedral was observed after calcined at 600 °C for 4 h. Moreover, sample 500-4 possesses stronger La2O3 and Co3O4 diffraction peaks than sample 550-4. TEM image of sample 500-4 (Fig. 8a) shows that the morphology of sample 500-4 is similar to that of sample 550-4 but with diameter of about 80 nm. Sample 600-4 presented in Fig. 8b shows agglomerated nanoparticles with the size of about 50 nm. It indicates that LaCoO3 hollow spheres might be melted and collapsed with the increasing of calcination temperature, ultimately forming agglomerated nanoparticles. 3.4 Formation mechanism The formation process of perovskite-type LaCoO3 hollow spheres is illustrated in
Scheme 1, which can be divided in two steps. The carbonaceous colloid template is hydrophilic, negatively charged (a ζ-potential of −36mV), which is prepared from saccharide solution, and functionalized with OH and C=O groups with large surface area and nano-scale pores distributed uniformly on its surface. Firstly, carbon@LaCo are formed through the surface-layer-adsorption process [44]: the functional groups and nano-scale pores in the surface layer are able to bind or capture La3+, Co2+ cations through coordination or electrostatic interactions, upon dispersal of the carbonaceous spheres in the solution with [La3+]:[Co2+] = 1:1. Sequential removal of carbon cores, densification, cross-linking, and phase transformation of La and Co in the layer via calcination process resulted in the formation of LaCoO3 hollow spheres. The TG-DSC data in Fig. 1c and Fig. 1d clearly demonstrated that the carbonaceous polysaccharide microspheres not only acted as templates but also served as internal heat source in the formation process. This might be the key reason as to why the perovskite-type structure can be formed at relatively low temperature (550 °C). 3.5 UV-vis spectroscopy of products To study the light response of our hollow perovskite-type LaCoO3 product, optical absorption of the sample 550-4 was studied. The strong absorption peak at 220-550 nm with maxim at 233 nm (Fig. 9), and the absorption edge at ca. 600 nm (corresponding to band gap 2.07 eV) indicate the possibility of utilizing UV-vis light for photocatalysis. The electronic structures of perovskite-type LaCoO3 are used to explain the reason for the formation of narrow band gap (~2.07 eV) and the broad absorption
peak at 220-550 nm. In the cubic perovskites, the octahedral ligand field produced by the O atoms that surrounding each Co atom split the tenfold degenerated levels of Co into sixfold t2g and fourfold eg levels. The theoretical calculations on the electronic structures of the octahedrally coordinated Co d6 ion showed that the low-spin state ( t 26g e g0 ) was more stable than the high-spin state ( t 24g eg2 ) when 10Dq≥2J, 10Dq was the cubic crystal field and J was the intra-atomic exchange interaction[46]. As reported by Ravindran et al.[47], the top of valence band is dominated by the Co 3d to O 2p hybridized band. The nearby bottom of the conduction band mainly included Co 3d states. More detailed explanation is given by Korotin et al.[48], in which the calculations on the Co low-spin configuration ( t 26g e g0 ) in LaCoO3 shows that the top of the valence band is formed by mixture the oxygen 2p states with Co t2g orbitals and the bottom of conduction band is predominanted by the eg orbitals. The calculated energy gap 2.06 eV is in good agreement with our experimental value 2.07 eV [48]. However, it should be noted that our experimental data is based on cubic modification, while the calculated value (2.06 eV) is based on rhombohedrally distorted phase modification.
3.6 Photocatalytic results To demonstrate the applications of the fabricated LaCoO3 hollow spheres, their photocatalytic activities on the degradation of methylene blue, methyl orange, neutral red were investigated, respectively. Fig. 10a shows the UV-vis spectra of methylene blue with LaCoO3 hollow spheres. The intensity of the characteristic absorption peak
at ~ 664 nm drops gradually with irradiation time and reaches a maximum of 87 % consumption after 100 min (as shown in Fig. 10b). The plot of the blank control shows that the photolysis of dyes without catalyst is negligible. The degradation rate by LaCoO3 hollow spheres is comparable with that of P25. No new absorption bands are observed in either the visible or ultraviolet region, indicating the complete and clean degradation of methylene blue. Fig. 10c and 10d show the absorption spectra of methyl orange degradation. The characteristic peak of methyl orange at 463 nm decreases dramatically with time. Similar to that of methylene blue, the degradation of methyl orange reaches maximum of 89% after 100 min. Fig. 10e and 10f show the absorption spectra of neutral red degradation. The characteristic peak of neutral red is at 525 nm. The maximum degradation ratio of neutral red is 88% after 40 min. The degradation rate for both methyl orange and neutral red is similar to that of P25. The degradation of neutral red is faster than the other two dyes under the same conditions. Clearly, LaCoO3 hollow sphere is highly active in photocatalytic degradation of the organic dyes such as methylene blue, methyl orange and neutral red. The possible catalytic mechanism is proposed as follows. The hollow structure of perovskite-type LaCoO3 provides not only large surface area for adsorption of the reactants but also more active sites for photocatalytic process [49, 50]. With UV irradiation, the valence band electron of LaCoO3 could be excited to the conduction band, generating valence band hole (h +). The hole (h +) is a strong oxidant on the surface of LaCoO3 hollow spheres and could oxidize OH- and H2O into OH· (free radical), which is also highly
oxidizing. Subsequently, the OH· radical can oxidize adjacent organic molecules through a series of reactions, ultimately transforming the organic dyes into CO2.
4. Conclusions We reported a novel and simple method to synthesize perovskite-type LaCoO3 hollow nanospheres in relatively low calcination temperature at 550 °C by utilizing the carbonaceous colloids as template and internal heat source. The influence of calcination temperature and time on the morphology and crystallinity of the product was investigated. The UV-vis analysis shows that the product has a broad and strong absorption at 220-550 nm region with narrow band gap of 2.07 eV. The LaCoO3 hollow nanospheres exhibit excellent photocatalytic activity for degradation of methylene blue, methyl orange and neutral red, making it a promising candidate for other environmentally friendly applications.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 21071002 and 21275006), Key Project of Anhui Provincial Education Department (KJ2013A029), Natural Science Foundation of Anhui Province (11040606Q02 and 11040606M34) and 211 project of Anhui University
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Captions of Fig.s Fig. 1. (a) SEM of carbonaceous microspheres; (b) SEM of carbon@LaCo precursor; (c) TG-DSC of carbonaceous microspheres; (d) TG-DSC of carbon@LaCo composite microspheres. Fig. 2. XRD pattern of products obtained using calcining at 550 °C for 4 h. Fig. 3. The carbonaceous cores removed via calcination at 550 °C for 4 hours (a) TEM of products obtained by calcining at 550 °C for 4 h; (b) SEM of products obtained by calcining at 550 °C for 4 h. Fig. 4. Nitrogen adsorption/desorption isotherm and Barrett–Joyner–Halenda (BJH) pore size distribution plot (inset) of the hollow perovskite-type LaCoO3. Fig. 5. XRD patterns of the products calcined at 550 °C for 4 h, 6 h and 8 h. Fig. 6. SEM images of the products calcined at 550 °C for 6 h (a) and 8 h (b) Fig. 7. XRD patterns of the products calcined at 500 °C, 550 °C, 600 °C for 4 h. Fig. 8. TEM images of the products calcined for 4 h at 500 °C (a) and 600 °C (b) Fig. 9. UV -visible absorption spectra of the hollow perovskite-type LaCoO3 Fig. 10. Time-dependent absorption spectra of three organic dyes with LaCoO3 hollow spheres catalyst: (a) methylene blue; (c) methyl orange (e) neutral red; Plot of dye concentration versus irradiation time for the photodegradation of organic dyes catalyzed LaCoO3 hollow spheres(■) and P25 (▲) and the blank without catalyst (●) under UV irradiation: (b) methylene blue; (d) methyl orange (f) neutral red. (1, 2, 3, 4, 5, 6 represented the degradation time of 0, 20, 40, 60, 80, 100 min.). Scheme 1. Illustration of the formation process for perovskite-type LaCoO3 hollow
spheres.
10 100 90
200nm
80 70 60 50 40 30 20 10
0 DSC exo
-10 -20
99.2% 320
492
-30 TG
-40
0 100
200
300
400
500
600
700
heat flow (mW/mg)
c
Weight loss (%)
a
800
ο
d
110 100 90 80 70 60 50 40 30 20 10 0
Weight loss (%)
b
DSC exo
89.6%
409
100
200
300
400
TG
500 ο
Temperature/ C
Fig. 1
600
700
10 0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100
800
heat flow (mW/mg)
Temperature/ C
120
110
100
220
310
211
221
111
40
210
60
200
80
100
Intensity(a.u.)
LaCoO3-750279
20 0 20
30
40
50
2θ (deg.)
Fig. 2
60
70
80
a
b
Fig. 3
140
100 80 60
Pore Volume (cm 3/g)
Quantity Adsorbed (cm3/g STP)
0.25
120
---
0.20 B
0.15 0.10 0.05 0.00 0
20
40
40
60
80 100 120 140
Pore Diameter (nm)
20 0 0.0
0.2
0.4
0.6
Relative Pressure (P/Po)
Fig. 4
0.8
1.0
104
LaCoO3-750279 #La2O3
*
#
134
306
208
122
202
012
Intensity(a.u.)
024
214
*Co3O4
550-8
#
550-6 550-4
20
30
40
50
2θ (deg.)
Fig. 5
60
70
80
Fig. 6
110
Intensity(a.u.)
104
600-4
*
550-4
#
500-4 20
30
40
50
2θ (deg.)
Fig. 7
60
70
80
Fig. 8
0.45 0.40
233nm
Abs.(a.u.)
0.35 0.30 0.25 0.20 0.15 0.10
UV Region
Visible Region
0.05 0.00 200
300
400
500
600
wavelength(nm)
Fig. 9
700
800
0
0.6
0.4
1
a
1-A0 2-20min 3-40min 4-60min 5-80min 6-100min
4
0.0 500
Blank LaCoO3
40
3
0.2
5 6
550
600
P25 60
80
650
100
700
0
20
40
Wavelength(nm)
60
80
100
Time/min
c
1
0
d
0.6 0.5
2
0.4
3
0.3
40
P25
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80
5 6
0.1
400
450
500
550
100
600
0
Wavelength(nm)
e
0.6
20
40
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20
Blank LaCoO3 P25
0.5 0
0.3 0.2
2
f
60
80
3
0.1
4 0.0 400
40
D%
1-A 2-20min 3-40min 4-60min
0.4
100
Time/min
0
1
0.7
Absorbance(a.u.)
Blank LaCoO3
4
0.2
0.0 350
20
1-A0 2-20min 3-40min 4-60min 5-80min 6-100min
D%
Absorbance(a.u.)
0.7
b
20
2
D%
Absorbance(a.u.)
0.8
450
500
100
550
600
0
650
10
20
30
40
Time/min
Wavelength(nm)
Fig. 10
50
60
Carbon sphere
Carbon@LaCo
LaCoO3 Hollow sphere Calcination
Co2+ La3+ LaCoO3
Scheme 1
Highlights ·Hollow perovskite-type LaCoO3 is fabricated by green colloidal template method. ·Carbonaceous colloids acted as templates and offered internal heat source. ·The calcination temperature to form perovskite-type LaCoO3 was dropped to 550 °C. ·The photocatalytic properties were studied upon UV irradiation. ·Hollow perovskite-type LaCoO3 shows excellent photocatalytic activity on dyes.
S0925-8388(13)01004-9 http://dx.doi.org/10.1016/j.jallcom.2013.04.092 JALCOM 28386
To appear in: Received Date: Revised Date: Accepted Date:
29 October 2012 12 April 2013 13 April 2013
Please cite this article as: S. Fu, H. Niu, Z. Tao, J. Song, C. Mao, S. Zhang, C. Chen, D. Wang, Low temperature synthesis and photocatalytic property of perovskite-type LaCoO3 hollow spheres, (2013), doi: http://dx.doi.org/ 10.1016/j.jallcom.2013.04.092
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.
Low temperature synthesis and photocatalytic property of perovskite-type LaCoO3 hollow spheres Shasha Fua, Helin Niua,*, Zhiyin Taoa, Jiming Songa, Changjie Maoa, Shengyi Zhanga, Changle Chenb*, Dong Wang,c a
School of Chemistry and Chemical Engineering, Anhui University, Hefei 230039, P. R. China
b
CAS Key Laboratory of Soft Matter Chemistry and Department of Polymer Science &
Engineering, University of Science and Technology of China, Hefei, 230026, P. R. China c
Department of Bio-Health Technology, College of Biomedical Science, Kangwon National
University, Chuncheon, Gangwon-Do, 200-701, Korea Corresponding author: E-mail: [email protected] , [email protected]
Abstract Hollow perovskite-type LaCoO3 was successfully fabricated by surface-ion adsorption method utilizing the carbonaceous colloids as template under relatively low calcination temperature. Carbonaceous colloids not only acted as templates but also offered internal heat source during calcination process. The impact of calcined temperature and time on the structure and morphology of the product were studied and the possible formation process of perovskite-type LaCoO3 hollow spheres was illustrated. The obtained product was characterized by SEM, TEM, XRD, TG-DSC, ICP-OES, BET and UV-visible absorption spectra. The photocatalytic activities for
degradation of methylene blue, methyl orange and neutral red were tested. The good photocatalytic degradation activity of the three different dyes and the band gap of 2.07 eV make it a promising candidate material for photocatalytic applications.
Keywords: Perovskite; Hollow; Low temperature; Colloids template; Photodegradation. 1. Introduction Perovskite-type oxides have attracted considerable attentions because of their unique properties such as various types of oxygen vacancy order[1], intrinsic oxygen reduction reaction activity[2], high conductivity[3], magnetic properties [4], excellent performance as cathode or anode [5, 6] and high Seebeck coefficient [7, 8]. Perovskite-type lanthanum cobaltate (LaCoO3) is cheap, environmentally friendly and highly active in oxidation processes, making it a very promising material for many applications including highly active catalysts for energy storage [9], cathode for fuel cell [5, 10], catalytic reduction of NOx in automotive exhausts [11], catalytic oxidation of volatile organic compounds (VOCs) [12, 13], degradation of combustion soot [11, 14-17], catalytic oxidation of CO [18] and photocatalytic degradation reactions [19]. The catalytic performance of the LaCoO3 is significantly influenced by synthetic procedures, calcination temperature/time and so on. Many synthetic methods have been reported such as solid state reaction[4, 20, 21], sol-gel method[22], chemical co-precipitation method[23], combustion method[24], freeze-drying based method[25, 26] and microwave-assisted method[27]. However, most of these methods involve high-temperature and time-consuming solid-state reaction in order to mix the metal oxides on molecular level.
Meanwhile, the catalytic performance of LaCoO3 is not good enough for practical applications, mainly due to their low surface area (on the order of m2g−1) and strong tendency to sinter during high temperature calcination. It is well known that hollow and porous structures can greatly influence catalytic activities [28, 29]. Therefore, it is intriguing to explore the properties of hollow or porous LaCoO3 materials as well as new environmentally friendly and low power consuming synthetic routes. Recently, considerable studies have been dedicated to synthesize porous perovskites utilizing traditional templates [20] such as anodic aluminium oxide (AAO) [30-32], ZnO[33], silicon[34], corn starch[35] and polystyrene sphere[36]. In contrast, research on green colloids templates is relatively scarce. Carbonaceous colloid, a green colloid template prepared from saccharide solution, has integral and uniform surface with functional layers, which ensures homogeneity of the reagents and makes the surface modification unnecessary. Moreover, its size can be easily modified. It is reported that carbonaceous colloid template has been applied to produce monometallic oxides with hollow structures. However, this method has never been applied to prepare bimetallic perovskite-type oxide with hollow structures. Recently, our group reported the synthesis of porous La2O2CO3 hollow microspheres in saccharide solution using a one-pot hydrothermal method [37]. Herein, we report the fabrication of hollow perovskite-type LaCoO3 by surface-ion adsorption method utilizing the carbonaceous colloids as template at low calcination temperature. The influence of calcination time and temperature on the structures of LaCoO3 was investigated and its formation mechanism was discussed. The
photocatalytic degradation of the organic dyes including methylene blue, methyl orange and neutral red was employed to probe the photocatalytic properties of the hollow perovskite-type LaCoO3 under UV irradiation.
2. Experimental 2.1. Preparation methods All reagents were purchased from Sinopharm Chemical Reagent Co., Ltd, China. These reagents in analytic grade were used without further purification. 2.1.1. Synthesis of carbonaceous microspheres In a typical experiment, 6.0 g glucose was dissolved in 40 ml distilled water to form a clear solution. The solution was then sealed in a 40 ml autoclave with a Teflon seal and maintained at 160 °C for 12 h. Dark puce products were obtained after centrifugation at 8000 rpm for 10 min. A rinsing process involving three cycles of centrifugation/washing/redispersion was performed with water or ethanol, respectively. Carbonaceous microspheres using as the templates to prepare LaCoO3 hollow spheres were obtained after oven-drying at 70 °C for more than 6 h. 2.1.2. Synthesis of LaCoO3 hollow spheres 2.17 g La(NO3)3·6H2O and 1.46 g of Co(NO3)2·6H2O were dissolved in 50.0 ml distilled water. The ratio between the components was carefully controlled in [La3+]:[Co2+] = 1:1. In order to form core-shell structure precursor, 1.0 g carbonaceous colloids spheres with diameter of 100 nm [38]were added into the above solution and evenly dispersed with the assistance of ultrasonication. Then the reaction solution was placed at room temperature overnight to achieve
adsorption–desorption equilibrium between carbonaceous spheres, Co2+ and La3+ ions. A rinsing process involving 2 cycles of centrifugation/washing/re-dispersion was performed with water or ethanol, respectively. After dried in an oven in air with at 80 °C for 6 h, the obtained precursor was denoted as carbon@LaCo. In a typical procedure, carbon@LaCo was calcined in air at 550 °C for 4 h to remove the carbon core. Then it was allowed to cool to room temperature. The as-formed products were accumulated at the bottom of the crucible. In order to study the effect of calcination parameter on morphology, calcination time was set as 6 and 8 hours respectively by keeping temperature at 550 °C, and the temperature was set as 500 °C and 600 °C respectively by keeping calcination time for 4 h. Samples obtained for 4h, 6h and 8h at 550 °C were marked as 550-4, 550-6 and 550-8 respectively and samples obtained at 500 °C, 550 °C and 600 °C for 4 h were marked as 500-4, 550-4 and 600-4 respectively. 2.2. Structure and morphology characterization of products The structure and morphology of the product have been studied by X-ray diffractometry (XRD, X’pert PRO SUPER, Cu Kα radiation, λ=1.54056 Å), field emission scanning electron microscopy (FESEM, FEI Sirion 200) and transmission electron microscopy (TEM, Hitachi H-800). Thermo-gravimetric analysis and differential scanning calorimetry (TGA–DSC) were used to test the weight loss of the products. TGA–DSC data was recorded with a thermal analysis instrument (WCT-1D BOIF) in air flow at the heating rate of 10 °C/min. Elemental analysis for the carbon@LaCo precursor was determined with an
inductively coupled plasma (ICP-OES, IRIS Intrepid II, Thermo Electron, USA). Nitrogen adsorption and desorption isotherms were measured at 77 K using a Micromeritics ASAP2020M+ C system. Specific surface areas were determined by the BET method and the mesopore size distribution was determined by the Barrett–Joyner–Halenda (BJH) method. For the BJH analysis, the pore size distribution was obtained from the analysis of the adsorption branch of the isotherm. Ultraviolet–visible (UV–vis) spectroscopy of the products was measured by UV–vis diffuse reflectance spectroscopy (Hitachi U-4100). An American Beckman Coulter Delsa 440SX Zeta Potential Analyzer was used to perform the zeta potential determination. 2.3. Photocatalytic degradation experiment Photocatalytic activity of the LaCoO3 hollow spheres was assessed using the degradation reaction of methylene blue (MB) exposed to UV-vis light. The reaction suspensions containing methylene blue and LaCoO3 photo-catalyst were irradiated by the 30W UV light high-pressure mercury lamp with continuous magnetic stirring at room temperature. Absorbance measurements were recorded by 752PC UV-vis spectrophotometer. 10 mg of MB was dissolved in 1000 ml of deionized water to form 10 mg/L MB solution. The 0.2 g/L suspension was prepared by adding 10 mg LaCoO3 hollow spheres into 50 ml of MB solution. Prior to irradiation, the suspensions were magnetically stirred for 60 min in the dark to ensure adsorption/degradation equilibrium of methylene blue with the catalyst. After that, keeping the distance between liquid level and light source about 10 cm, the mixture
was subjected to UV irradiation. 2 ml of the supernatant was taken out by syringe at different time intervals and same volume of freshly as-prepared organic solution was added into the separation of the precipitate followed by ultrasonic. The dye obtained after separation was determined using 752PC UV-visible spectrophotometer. The degradation of methylene blue absorption peak intensity at 664 nm was monitored as a function of UV exposure time. Similar experiments were carried out by replacing methylene blue with methyl orange and neutral red, respectively.
3. Results and discussion 3.1. Characterization of carbonaceous microsphere and carbon@LaCo composite microspheres Carbonaceous spheres were prepared by a reported method[38]. As seen from Fig. 1a, the surface of the spheres is rough and porous, which not only provides high surface area, but also aids penetration of reactive species. In order to study the adsorption mechanism, the zeta potential of carbonaceous spheres was measured in aqueous solution by Delsa 440SX Zeta Potential Analyzer. The result of -36 mV indicates that carbonaceous spheres are negatively charged and stably dispersed in aqueous solution, thus allowing an easy manipulation of the evenly surface adsorption of carbonaceous spheres with Co2+ and La3+ ions. As shown in Fig. 1b, the carbon@LaCo precursor has a smooth ball shape, which indicates that the pore on the carbonaceous spheres surface might be filled with Co and La precursors. In order to quantify the ratio between La and Co ions adsorbed on carbonaceous spheres, inductively coupled plasma was used to determine carbon@LaCo precursors. The results display that the
mass proportions of loaded cobalt and lanthanum of the dissolved carbon@LaCo precursor in subboiled HNO3 are 1.7% and 4.1%, respectively, which illuminates that the ratio between La and Co ions adsorbed on carbonaceous spheres are almost 1:1. The TG curve in Fig. 1c shows that the burning of carbonaceous spheres is completed before 550 °C. DSC curve of the carbonaceous spheres demonstrats two exothermic peaks. According to the report by Tan's et al.[39], the first exothermic peak at 320 °C should be attributed to the dehydration and densification of carbonaceous sphere templates, and the second exothermic peak at 492 °C should be due to combustion of carbonaceous spheres. The result shows that the carbonaceous spheres not only acted as templates but also offered internal heat source in the formation process of the porous perovskite-type LaCoO3 hollow spheres. In Fig. 1d, the total weight loss of carbon@LaCo composite microspheres is 89.6%, indicating LaCoO3 product yield about 10.4 %. DSC curve of carbon@LaCo composite spheres shows only one sharp exothermic peak centered at 409 °C which indicates that the heat generated by burning of carbonaceous sphere templates and the crystallization process of the perovskite LaCoO3 at the same time might be the reason for synthesis LaCoO3 in relatively low calcining temperature 550 °C. 3.2. Characterization of LaCoO3 hollow spheres XRD was used to characterize the crystal structure of the sample which was obtained by calcining at 550 °C for 4 h (Fig. 2.). The major diffraction peaks of the products can be indexed to the perovskite-type cubic structure of LaCoO3 with Pm 3 m space group (JCPDS file no. 75-0279). This suggests that the main component
of products obtained by calcining at 550 °C for 4 h is perovskite-type cubic structure of LaCoO3. It is well documented that synthesis of perovskite-type LaCoO3 was generally under the condition of the existence of chelating agents [7, 8, 11, 18, 26]. So, a suitably designed method to synthesize hollow perovskite-type LaCoO3 under low temperature conditions without chelator is worthy of investigation. Speculated formation mechanism of perovskite-type LaCoO3 hollow spheres at relatively low temperature is illustrated in Scheme 1. What’s more, the template used here can be substituted for a series of carbonaceous spheres derivatives which possess core-shell nanostructures with carbonaceous polysaccharide shells, such as Ag@C [40], Te@C [41], Pt@C [42], SnO2@C[43], Fe3O4@C [44], and so on. Herein, this strategy can be extended to develop a general and facile method for preparation of core-shell nanostructures with perovskite-type LaCoO3 shells and functional cores. The morphology analyses of products were carried out using TEM and SEM techniques. The TEM image of hollow LaCoO3 spheres obtained by calcining at 550 °C for 4 h is presented in Fig.3a. The average particle size is about 50 nm. Overall, all particles have tight size distribution with connection between spheres. In comparison with the carbon@LaCo precursor (Fig. 1c), diameter of the hollow product shrink from 150 nm before calcination to 50 nm after calcination. Shrinking in the particle size can be attributed to disappearance of hard templates and the formation of the perovskite-type LaCoO3 in calcining process. The thickness of LaCoO3 shell is found to be around 10 nm. Fig. 3b is the SEM image of products obtained by calcining at
550 °C for 4 h, and uniform broken and unbroken hollow spheres in large scale are observed, which indicates that perovskite-type LaCoO3 hollow spheres are prepared successfully by carbon template method. Brunauer–Emmett–Teller (BET) gas sorptometry measurements have been conducted to examine the hollow perovskite-type LaCoO3. Fig. 4 shows the N2 adsorption/desorption isotherm and the pore-size distribution (inset) of the hollow perovskite-type LaCoO3. The isotherm can be categorized as type IV with a distinct hysteresis loop observed in the range of 0.6–1.0 p/p0, which is the characteristic of mesoporous metal oxides prepared by the hard-templating method. The specific surface area is determined to be 43.39 m²/g and the pores size distribution with the peak located at about 50 nm according to the BJH analysis. This result is consistent with the TEM image of hollow LaCoO3 spheres (Fig. 3a ). 3.3 The impact of calcination time and temperature on products To investigate the effect of the calcination time on the particle morphology and structure, samples were calcined at 550 °C ranging from 4 h to 8 h. Sample 550-8 shows stronger and more acute diffraction peaks of LaCoO3 than sample 550-4 with a weak impurity peak of La2O3 and Co3O4 (Fig. 5). As seen from Fig. 6a, the SEM image of sample 550-6 shows that LaCoO3 hollow spheres have all melted and collapsed into solid particles with increasing agglomeration. As shown in Fig. 5, all major XRD peaks of sample 550-8 are matched with the LaCoO3 with the standard XRD pattern (JCPDS Card, 75-0279) and no phase transformation from cubic LaCoO3 to rhombohedral is observed even after 8 h of calcinating at 550 °C. With
longer calcination time, the crystallinity of cubic LaCoO3 gets better, while LaCoO3 hollow spheres melt and collaps into solid particles when calcined for more than 4 h. To investigate the effect of the calcination temperature on morphology and structure of samples, samples were calcined at different temperatures for 4 h. XRD patterns of samples 500-4, 550-4, 600-4 are shown in Fig. 7. It is hard to distinguish rhombohedral LaCoO3 from cubic one due to their similarities in diffractions. However, According to the report by Tien-Thao’s et al.[45], the twin peaks (110) (104) at 2 θ =33.17, 33.54 are characteristic peaks of rhombohedral LaCoO3(JCPDS Card, 48-0123), which means the rhombohedral phase appears after 4 h of calcination at 600°C. As shown in Fig. 7 and Fig.8, higher calcination temperatures lead to better crystallinity. Meanwhile, the hollow structure completely disappeared and the structural transformation from cubic to rhombohedral was observed after calcined at 600 °C for 4 h. Moreover, sample 500-4 possesses stronger La2O3 and Co3O4 diffraction peaks than sample 550-4. TEM image of sample 500-4 (Fig. 8a) shows that the morphology of sample 500-4 is similar to that of sample 550-4 but with diameter of about 80 nm. Sample 600-4 presented in Fig. 8b shows agglomerated nanoparticles with the size of about 50 nm. It indicates that LaCoO3 hollow spheres might be melted and collapsed with the increasing of calcination temperature, ultimately forming agglomerated nanoparticles. 3.4 Formation mechanism The formation process of perovskite-type LaCoO3 hollow spheres is illustrated in
Scheme 1, which can be divided in two steps. The carbonaceous colloid template is hydrophilic, negatively charged (a ζ-potential of −36mV), which is prepared from saccharide solution, and functionalized with OH and C=O groups with large surface area and nano-scale pores distributed uniformly on its surface. Firstly, carbon@LaCo are formed through the surface-layer-adsorption process [44]: the functional groups and nano-scale pores in the surface layer are able to bind or capture La3+, Co2+ cations through coordination or electrostatic interactions, upon dispersal of the carbonaceous spheres in the solution with [La3+]:[Co2+] = 1:1. Sequential removal of carbon cores, densification, cross-linking, and phase transformation of La and Co in the layer via calcination process resulted in the formation of LaCoO3 hollow spheres. The TG-DSC data in Fig. 1c and Fig. 1d clearly demonstrated that the carbonaceous polysaccharide microspheres not only acted as templates but also served as internal heat source in the formation process. This might be the key reason as to why the perovskite-type structure can be formed at relatively low temperature (550 °C). 3.5 UV-vis spectroscopy of products To study the light response of our hollow perovskite-type LaCoO3 product, optical absorption of the sample 550-4 was studied. The strong absorption peak at 220-550 nm with maxim at 233 nm (Fig. 9), and the absorption edge at ca. 600 nm (corresponding to band gap 2.07 eV) indicate the possibility of utilizing UV-vis light for photocatalysis. The electronic structures of perovskite-type LaCoO3 are used to explain the reason for the formation of narrow band gap (~2.07 eV) and the broad absorption
peak at 220-550 nm. In the cubic perovskites, the octahedral ligand field produced by the O atoms that surrounding each Co atom split the tenfold degenerated levels of Co into sixfold t2g and fourfold eg levels. The theoretical calculations on the electronic structures of the octahedrally coordinated Co d6 ion showed that the low-spin state ( t 26g e g0 ) was more stable than the high-spin state ( t 24g eg2 ) when 10Dq≥2J, 10Dq was the cubic crystal field and J was the intra-atomic exchange interaction[46]. As reported by Ravindran et al.[47], the top of valence band is dominated by the Co 3d to O 2p hybridized band. The nearby bottom of the conduction band mainly included Co 3d states. More detailed explanation is given by Korotin et al.[48], in which the calculations on the Co low-spin configuration ( t 26g e g0 ) in LaCoO3 shows that the top of the valence band is formed by mixture the oxygen 2p states with Co t2g orbitals and the bottom of conduction band is predominanted by the eg orbitals. The calculated energy gap 2.06 eV is in good agreement with our experimental value 2.07 eV [48]. However, it should be noted that our experimental data is based on cubic modification, while the calculated value (2.06 eV) is based on rhombohedrally distorted phase modification.
3.6 Photocatalytic results To demonstrate the applications of the fabricated LaCoO3 hollow spheres, their photocatalytic activities on the degradation of methylene blue, methyl orange, neutral red were investigated, respectively. Fig. 10a shows the UV-vis spectra of methylene blue with LaCoO3 hollow spheres. The intensity of the characteristic absorption peak
at ~ 664 nm drops gradually with irradiation time and reaches a maximum of 87 % consumption after 100 min (as shown in Fig. 10b). The plot of the blank control shows that the photolysis of dyes without catalyst is negligible. The degradation rate by LaCoO3 hollow spheres is comparable with that of P25. No new absorption bands are observed in either the visible or ultraviolet region, indicating the complete and clean degradation of methylene blue. Fig. 10c and 10d show the absorption spectra of methyl orange degradation. The characteristic peak of methyl orange at 463 nm decreases dramatically with time. Similar to that of methylene blue, the degradation of methyl orange reaches maximum of 89% after 100 min. Fig. 10e and 10f show the absorption spectra of neutral red degradation. The characteristic peak of neutral red is at 525 nm. The maximum degradation ratio of neutral red is 88% after 40 min. The degradation rate for both methyl orange and neutral red is similar to that of P25. The degradation of neutral red is faster than the other two dyes under the same conditions. Clearly, LaCoO3 hollow sphere is highly active in photocatalytic degradation of the organic dyes such as methylene blue, methyl orange and neutral red. The possible catalytic mechanism is proposed as follows. The hollow structure of perovskite-type LaCoO3 provides not only large surface area for adsorption of the reactants but also more active sites for photocatalytic process [49, 50]. With UV irradiation, the valence band electron of LaCoO3 could be excited to the conduction band, generating valence band hole (h +). The hole (h +) is a strong oxidant on the surface of LaCoO3 hollow spheres and could oxidize OH- and H2O into OH· (free radical), which is also highly
oxidizing. Subsequently, the OH· radical can oxidize adjacent organic molecules through a series of reactions, ultimately transforming the organic dyes into CO2.
4. Conclusions We reported a novel and simple method to synthesize perovskite-type LaCoO3 hollow nanospheres in relatively low calcination temperature at 550 °C by utilizing the carbonaceous colloids as template and internal heat source. The influence of calcination temperature and time on the morphology and crystallinity of the product was investigated. The UV-vis analysis shows that the product has a broad and strong absorption at 220-550 nm region with narrow band gap of 2.07 eV. The LaCoO3 hollow nanospheres exhibit excellent photocatalytic activity for degradation of methylene blue, methyl orange and neutral red, making it a promising candidate for other environmentally friendly applications.
Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant Nos. 21071002 and 21275006), Key Project of Anhui Provincial Education Department (KJ2013A029), Natural Science Foundation of Anhui Province (11040606Q02 and 11040606M34) and 211 project of Anhui University
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Captions of Fig.s Fig. 1. (a) SEM of carbonaceous microspheres; (b) SEM of carbon@LaCo precursor; (c) TG-DSC of carbonaceous microspheres; (d) TG-DSC of carbon@LaCo composite microspheres. Fig. 2. XRD pattern of products obtained using calcining at 550 °C for 4 h. Fig. 3. The carbonaceous cores removed via calcination at 550 °C for 4 hours (a) TEM of products obtained by calcining at 550 °C for 4 h; (b) SEM of products obtained by calcining at 550 °C for 4 h. Fig. 4. Nitrogen adsorption/desorption isotherm and Barrett–Joyner–Halenda (BJH) pore size distribution plot (inset) of the hollow perovskite-type LaCoO3. Fig. 5. XRD patterns of the products calcined at 550 °C for 4 h, 6 h and 8 h. Fig. 6. SEM images of the products calcined at 550 °C for 6 h (a) and 8 h (b) Fig. 7. XRD patterns of the products calcined at 500 °C, 550 °C, 600 °C for 4 h. Fig. 8. TEM images of the products calcined for 4 h at 500 °C (a) and 600 °C (b) Fig. 9. UV -visible absorption spectra of the hollow perovskite-type LaCoO3 Fig. 10. Time-dependent absorption spectra of three organic dyes with LaCoO3 hollow spheres catalyst: (a) methylene blue; (c) methyl orange (e) neutral red; Plot of dye concentration versus irradiation time for the photodegradation of organic dyes catalyzed LaCoO3 hollow spheres(■) and P25 (▲) and the blank without catalyst (●) under UV irradiation: (b) methylene blue; (d) methyl orange (f) neutral red. (1, 2, 3, 4, 5, 6 represented the degradation time of 0, 20, 40, 60, 80, 100 min.). Scheme 1. Illustration of the formation process for perovskite-type LaCoO3 hollow
spheres.
10 100 90
200nm
80 70 60 50 40 30 20 10
0 DSC exo
-10 -20
99.2% 320
492
-30 TG
-40
0 100
200
300
400
500
600
700
heat flow (mW/mg)
c
Weight loss (%)
a
800
ο
d
110 100 90 80 70 60 50 40 30 20 10 0
Weight loss (%)
b
DSC exo
89.6%
409
100
200
300
400
TG
500 ο
Temperature/ C
Fig. 1
600
700
10 0 -10 -20 -30 -40 -50 -60 -70 -80 -90 -100
800
heat flow (mW/mg)
Temperature/ C
120
110
100
220
310
211
221
111
40
210
60
200
80
100
Intensity(a.u.)
LaCoO3-750279
20 0 20
30
40
50
2θ (deg.)
Fig. 2
60
70
80
a
b
Fig. 3
140
100 80 60
Pore Volume (cm 3/g)
Quantity Adsorbed (cm3/g STP)
0.25
120
---
0.20 B
0.15 0.10 0.05 0.00 0
20
40
40
60
80 100 120 140
Pore Diameter (nm)
20 0 0.0
0.2
0.4
0.6
Relative Pressure (P/Po)
Fig. 4
0.8
1.0
104
LaCoO3-750279 #La2O3
*
#
134
306
208
122
202
012
Intensity(a.u.)
024
214
*Co3O4
550-8
#
550-6 550-4
20
30
40
50
2θ (deg.)
Fig. 5
60
70
80
Fig. 6
110
Intensity(a.u.)
104
600-4
*
550-4
#
500-4 20
30
40
50
2θ (deg.)
Fig. 7
60
70
80
Fig. 8
0.45 0.40
233nm
Abs.(a.u.)
0.35 0.30 0.25 0.20 0.15 0.10
UV Region
Visible Region
0.05 0.00 200
300
400
500
600
wavelength(nm)
Fig. 9
700
800
0
0.6
0.4
1
a
1-A0 2-20min 3-40min 4-60min 5-80min 6-100min
4
0.0 500
Blank LaCoO3
40
3
0.2
5 6
550
600
P25 60
80
650
100
700
0
20
40
Wavelength(nm)
60
80
100
Time/min
c
1
0
d
0.6 0.5
2
0.4
3
0.3
40
P25
60
80
5 6
0.1
400
450
500
550
100
600
0
Wavelength(nm)
e
0.6
20
40
60
80
20
Blank LaCoO3 P25
0.5 0
0.3 0.2
2
f
60
80
3
0.1
4 0.0 400
40
D%
1-A 2-20min 3-40min 4-60min
0.4
100
Time/min
0
1
0.7
Absorbance(a.u.)
Blank LaCoO3
4
0.2
0.0 350
20
1-A0 2-20min 3-40min 4-60min 5-80min 6-100min
D%
Absorbance(a.u.)
0.7
b
20
2
D%
Absorbance(a.u.)
0.8
450
500
100
550
600
0
650
10
20
30
40
Time/min
Wavelength(nm)
Fig. 10
50
60
Carbon sphere
Carbon@LaCo
LaCoO3 Hollow sphere Calcination
Co2+ La3+ LaCoO3
Scheme 1
Highlights ·Hollow perovskite-type LaCoO3 is fabricated by green colloidal template method. ·Carbonaceous colloids acted as templates and offered internal heat source. ·The calcination temperature to form perovskite-type LaCoO3 was dropped to 550 °C. ·The photocatalytic properties were studied upon UV irradiation. ·Hollow perovskite-type LaCoO3 shows excellent photocatalytic activity on dyes.