An efficient sol-gel auto-combustion assisted visible light responsive ...

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Fabrication of nanocrystalline LaFeO3: An efficient solegel auto-combustion assisted visible light responsive photocatalyst for water decomposition K.M. Parida*, K.H. Reddy, S. Martha, D.P. Das, N. Biswal Colloids and Materials Chemistry Department, CSIR-Institute of Minerals and Materials Technology, Bhubaneswar 751013, Orissa, India

article info

abstract

Article history:

Synthesis of nano photocatalysts, LaFeO3 with orthorhombic perovskite structure by

Received 10 June 2010

solegel auto-combustion method was demonstrated. The samples were characterized by

Received in revised form

PXRD, SEM, HRTEM, XPS and optical absorption studies. Photocatalytic water decompo-

30 July 2010

sition over LaFeO3 nanoparticles activated at various temperatures without any co-cata-

Accepted 5 August 2010

lyst were investigated under visible light irradiation (l >> 420 nm). Highest amount of H2

Available online 9 September 2010

and O2 evolved in 180 min over the LaFeO3 activated at 500 C was recorded to be

Keywords:

The pronounced activity of nano LaFeO3 samples towards water decomposition was

Solegel auto-combustion method

consistent with BET-surface area and particle size analyses.

1290 mmol and 640 mmol, respectively having apparent quantum efficiency (AQE) 8.07%.

Perovskite type

ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

Orthorhombic LaFeO3 Nanoparticles Water decomposition

1.

Introduction

The photocatalytic production of hydrogen and oxygen is potentially one of the most promising ways for the conversion and storage of solar energy. Photocleavage of water over photocatalysts has been intensively studied since the discovery of HondaeFujishima effect [1]. It has been reported that some metal oxide photocatalysts i.e. Sr2Nb2O7 [2], La2Ti2O7 [3], La4CaTi5O17 [4], K2La2Ti3xMxO10þd [5] and NiO/ NaTaO3:La photocatalyst with a high quantum efficiency; z50% [6] are active for evolution of H2 and O2 from water under UV irradiation. A few oxides are also active under visible light such as RbPb2Nb3O10 [7], MgWOx [8] and NixIn1xTaO4 [9], yet their efficiencies are very low. Metal chalconides such as CdS, ZnS etc. have been extensively studied, but they are vulnerable to fatal photo-corrosion. Recently, some metal oxides [10e14], (oxy)sulfides [10,11,15e18], and (oxy)nitrides [19e22] function as active photocatalysts for H2

and O2 evolution from aqueous solutions containing suitable sacrificial electron acceptors and donors under visible light irradiation. Oxygen evolution over visible-light-driven photocatalysts such as WO3 [10,11], RbPb2Nb3O10 [7,10], BiVO4 [10,11], Bi2WO6 [11], AgNbO3 [13], and Ag3VO4 [14] using a sacrificial reagent was also reported by esteem researchers. On the other hand, there are few active metal oxides e.g. Pt/ HPb2Nb3O10 [10] and metal-doped SrTiO3 [12] were tested for H2 evolution under visible light. However, it is a matter of temptation in the current age to develop a visible light responsive photocatalyst. There are meagre investigations on hydrogen production over a semiconducting material in absence of any co-catalyst [23,24]. Perovskite type photocatalysts are quite encouraging materials for water splitting owing to their stability in water. As an important functional material, LaFeO3 with a typical ABO3-type perovskite structure has many applications such as catalytic oxidation, surface electronic states and gas-sensitive

* Corresponding author. Tel.: þ91 674 2581636425; fax: þ91 674 2581637. E-mail address: [email protected] (K.M. Parida). 0360-3199/$ e see front matter ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2010.08.029

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Table 1 e The textural, optical properties and water decomposition results of nano LaFeO3.

Fig. 1 e Schematic representation of LaFeO3 preparation.

characters [25e27]. To the best of our knowledge, few papers were published on their photocatalytic activity. The excellent catalytic activity of LaFeO3 is well known, because of its various exceeding properties like high stability, non-toxicity and small band gap energy [28e30]. We report herein, the synthesis of nano LaFeO3 with an orthorhombic structure by solegel auto-combustion method using citric acid (C6H8O7) as a complexing reagent, La (NO3)3.6H2O and Fe (NO3)3.9H2O as metal precursors and the photoactivity towards water decomposition using AgNO3 and CH3OH as sacrificial reagent under visible light illumination.

Catalysts

BE (eV)

LeFeO3-500 LaFeO3-600 LeFeO3-700 LeFeO3-800 LeFeO3-900

2.11 2.10 2.09 2.08 2.07

Material and methods

2.1.

Material synthesis

25.8 22.55 20.04 8.5 5.8

24 28 65.7 79.1 104.1

1290 1180 1040 910 820

640 580 500 460 380

According to the stoichiometric composition, specified amount of Fe (NO3)3.9H2O (purity 98%), La (NO3)3.6H2O (purity 99.9%) and citric acid (purity 99.5%) were dissolved in distilled water. The molar amount of citric acid was equal to total molar amount of metal nitrates in solution. Ammonium hydroxide was slowly added to adjust the pH 7.0 and also to stabilize the nitrateecitrate sol. Then the solution was kept stirred continuously at 60 C. Afterwards, the stabilized nitrateecitrate sol was poured into a tray and heated slowly to 130 C. The change in viscosity and colour was observed as the sol turned into a brown, puffy, porous dry gel, which on further heating automatically gets ignited as a result of thermally induced oxidationereduction reaction. Finally, the solid dry gel is formed by auto-ignition. The solid dry gel is then activated at different temperatures from 500 to 900 C for 2 h. The catalysts are designated as LaFeO3-500, LaFeO3-600 and so on. The whole process was schematically represented in Fig. 1.

2.2.

2.

Particle Amount Amount BETsize of H2 of O2 Surface area (m2/g) (nm) evolution evolution (mmol) (mmol)

Methods of characterization

Analytical grade Fe (NO3)3.9H2O, La (NO3)3.6H2O, C6H8O7.H2O and NH4OH were used as raw materials to prepare LaFeO3.

The samples were characterized by PXRD, DRUV-vis, XPS, SEM and HRTEM. The PXRD pattern of all the samples was recorded on a X’pert PRO PANanalytica diffractometer with automatic control. The patterns were run with monochromatic Mo radiation from 2q ¼ 10e40 with a scan rate of 2 /min. Diffused

Fig. 2 e PXRD pattern of LaFeO3 (a) dry gel, (b) 500, (c) 600, (d) 700, (e) 800 and (f) 900 C. The solid squares indicate the peaks corresponding to NH4NO3.

Fig. 3 e DRUV-vis spectra of (a) dry gel, (b) 500, (c) 600, (d) 700, (e) 800 and (f) 900 C activated LaFeO3.


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Fig. 4 e SEM micrographs of LaFeO3 activated at (a) 500, (b) 600, (c) 700, (d) 800 and (e) 900 C.

reflectance UVevis (DRUV-vis) spectra of the samples were taken with a Varian Cary 100 spectrophotometer equipped with a diffuse reflectance accessory in the region 200e800 nm with boric acid as reference. The reflectance spectra were converted into KubelkaeMunk function (F (R)) which is proportional to the absorption co-efficient for low values of F (R). X-Ray photoelectron spectroscopy (XPS) measurements were performed on a VG Microtech Multilab ESCA 3000 spectrometer with a non-monochromatised MgKa X-ray source. Energy resolution of the spectrometer was set at 0.8 eV with MgKa radiation at pass energy of 50 eV. The binding energy correction was performed using the C1s peak of carbon at 284.9 eV as a reference. Scanning electron microscopic (SEM) images were taken on a Hitachi S-3400N. Prior to the analyses, the samples were sputtered with a thin film of gold. High resolution Transmission electron microscopic (HRTEM) images were obtained on Philips TECHNAI G2 operated at

200 kV, in which samples were prepared by dispersing the powdered samples in ethanol by sonication for 3 min and then drop-drying on a copper grid coated with carbon film.

2.3.

Water decomposition

The catalytic activity and deactivation were studied in a batch reactor. About 0.05 g of catalyst was suspended in 20 ml of an aqueous solution containing 10 vol.% of CH3OH and 0.05 M of AgNO3 solution for H2 and O2 evolution, respectively. The solution was kept under stirring with a magnetic stirrer, prohibiting particles to settle at the bottom of the reactor. Prior to irradiation, the reaction mixture was purged with N2 in order to remove dissolved gases. A 125 W medium pressure Hg visible lamp (l >> 420 nm) (Scientific Aids and Instruments Corporation (SAIC), Chennai) was used as light source. The evolved gas was collected by water displacement technique

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˚ molecular sieve column and analyzed by GC-17A using 5 A with a thermal conductivity detector (TCD). The light intensity was measured by Digital Illuminance Meter (TES-1332A, Taiwan) with inbuilt Si-diode. A comparison of the retention

Fig. 5 e XPS spectra of (a) La3d, (b) Fe2p and (c) O1s of LaFeO3 activated at 500 C.

time of the peaks that appeared on the chromatogram with standards confirmed that the gases were hydrogen and oxygen. The apparent quantum efficiencies (AQE) for H2 and O2 were calculated by using the following equation. AQEH2 ¼

2 number of H2 molecule evolved 100% Numberof incident photonson reacting surface

AQEO2 ¼

4numberof O2 molecule evolved 100% Number of incident photonson reacting surface

3.

Results and discussion

3.1.

Characterization

Fig. 2 shows the characteristic PXRD patterns of various LaFeO3 samples synthesized in the present investigation. It is observed that some NH4NO3 species (solid squares) exists in LaFeO3 dry gel and converted into orthorhombic LaFeO3 at 500 C and above with an ABO3- type perovskite (JCPDS card number 15148 (a ¼ 5.556, b ¼ 5.565 and c ¼ 7.862)). The diffraction peaks of all the samples are comparatively broad, a typical characterstic of a nanoparticle. The peaks of LaFeO3 nanoparticles become narrower and stronger with increasing activation temperature indicating the well crystallization of the samples. The diffraction peak intensity increases with enhancement of the activation temperature. Some minor peaks of impurities were observed at 2q of 36.59, 38.42 and 39.602 . According to standard JCPDS card, they are the diffraction peaks of La2O3, Fe2O3 and Fe3O4, respectively. As was observed by Kato and Kudo [31], the orthorhombic phase accelerates the excited energy transfer of photogenerated-electron/hole pairs in the crystal. This could be the possible reason of the photocatalytic activity of LaFeO3 towards water decomposition. Table 1 represents the BET-surface area and particle size results of all the samples activated from 500 to 900 C. With increase in activation temperature, surface area was observed to decrease from 25.8 to 5.8 m2/g and increase in particle size from 24 to 104 nm (Table 1). It is understood that the particle size and surface area are inversely related to each other [32]. Our results are in agreement with this concept. From the optical absorption spectra (Fig. 3), we can see the red shift of absorption edge i.e. blue shift of band gap energy which indicates that LaFeO3 nanoparticles are active under the visible light illumination. In general, the valence band of oxide photocatalysts consists of O2p orbital and the electronic transition takes place from O2p/Fe3d orbital [27]. The band gap energy of the prepared nanoparticles was determined to be 2.45 eV and in the range of 2.11e2.07 eV for LaFeO3 dried gel and LaFeO3 samples at different activation temperature (500e900 C), respectively. A minor change in band gap energy values was observed for LaFeO3 activated at various temperatures. Fig. 4 shows the SEM micrographs of LaFeO3 nanoparticles activated at 500e900 C. The product was observed to be of low density, loose and porous material at 500 C which is believed to be favorable for catalytic activity. After 500 C, the nanoparticles exhibit an irregular porous morphology. The


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Fig. 6 e HRTEM micrographs of LaFeO3 activated at (a) 500, (b) 600, (c) 700, (d) 800 and (e) 900 C.

particles connect with each other to form a large network system with irregular sizes and shapes due to the large number of escaping gases from the rapid decomposition of organic compounds and the strong redox reaction during solegel auto-combustion. The particle morphology is observed to be changing with increasing the activation temperature of the LaFeO3 from 500 to 900 C. At the highest activation temperature, the irregular porous morphology began to appear and the morphology transformed slowly to nano hexagonal shape. Fig. 5(a), (b), and (c) reveals the XPS spectra of La3d, Fe2p and O1s, respectively. The binding energies 834.88 and 710.5 eV are assigned to La 3d5/2 (Fig. 5(a)) and Fe 2p3/2 (Fig. 5

(b)), respectively for the LaFeO3 sample activated at 500 C corresponding to La3þ and Fe3þ ions in oxide form [33,34]. The broad and asymmetric O1s XPS spectra (Fig. 5(c)) correspond to two kinds of O chemical states according to the binding energy range (526.0e534.0 eV), including crystal lattice oxygen (OL) and hydroxyl oxygen (OH) with increasing binding energy [35,36]. The OL XPS signal is attributed to the contribution of LaeO and FeeO in LaFeO3 crystal lattice and its peak position is at about 529.2 eV. The OH XPS is closely related to the hydroxyl groups resulting mainly from the chemisorbed water and its peak position is at about 531.3 eV (Fig. 5(c)). The HRTEM micrographs were presented in the Fig. 6. Although, few reports emphatically suggest no correlation can

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Fig. 7 e (A) Evolution of H2 in presence of CH3OH (10 vol.%) and (B) Evolution of O2 in presence of AgNO3 (0.05 M) as sacrificial agents during photodecomposition of water under visible light (125 W) illumination (C) Reusability study over the LaFeO3 activated at 500 C. (D) The amount of H2 and O2 gas evolution over LaFeO3 nanoparticles as a function of activation temperature.

be drawn among water splitting activity, surface area, particle size but in our case the above surface properties are responsible for catalytic activity towards water decomposition reaction. The particle morphology is observed to be changing with increasing the activation temperature of the LaFeO3 from 500 to 900 C. From the HRTEM micrograph, it is clearly observed that the particles with irregular morphology were transformed slowly to nano hexagonal shape. Again the particle size is also increased from 24 to 104 nm with increasing the activation temperature from 500 to 900 C.

3.2. Photocatalytic activities of LaFeO3 towards water decomposition Photocatalytic H2 and O2 production over various photocatalysts was evaluated. Blank experiments showed no appreciable H2 and O2 evolution in the absence of either irradiation or photocatalyst. The amount of hydrogen and oxygen

evolution over LaFeO3 nanoparticles activated at different temperatures under visible light irradiation using methanol and silver nitrate are shown in Fig. 7(A) and (B), respectively. Visible light activity of all the synthesized samples is well observed which is consistent with the surface area and particle size. Maximum H2 and O2 gas was evolved over LaFeO3 nanoparticles activated at 500 C. In order to study the reusability and stability of the LaFeO3 activated at 500 C towards prolonged light illumination, after 180 min, N2 gas was purged for removal of gases and reactivation of the catalysts. It is clearly observed from the Fig. 7(C), the activity of the catalysts remain almost unchanged from one cycle to other. The amount of gas evolution over native LaFeO3 nanoparticles are plotted as a function of the activation temperature as shown in Fig. 7(D). The amount of gas evolution markedly decreased with the increase of the activation temperature from 500 to 900 C. Moreover, the activity order of LaFeO3 nanoparticles is in good agreement with the


4.

Fig. 8 e Schematic representation of the mechanism of water splitting.

characterization results. The higher hydrogen and oxygen evolution in the present case can be ascribed to the lower particle size and high surface area of these materials [37]. In other words, the smaller the particle size, higher is the surface area and also photocatalytic activity. This can be explained on the basis of the following three aspects; first, the smaller is the particle size, the wider is the band gap, the stronger is the oxidizing and reducing ability of photoinduced holes and electrons, respectively [38]. Second, if the particle size is small the migrating time of photoinduced charge carriers to surfaces is short. Thus, photoinduced charge carriers can have much chance to reach to surfaces in advance of recombination and get captured to initiate photochemical reactions resulting in the increase in the activity. Thirdly, if the particle size is small, the surface area is large. The large surface area is favorable for photocatalytic reactions. In the present case, BET-surface area is decreasing from 25.8 to 5.8 m2/g. It can be clearly visible that the H2 evolution activity of 500 C activated LaFeO3 might be due to the highest BET-surface area and lowest particle size (24e104 nm). When light with energy larger than the band gap is incident on the catalyst, electron and holes are generated in the conduction band and valence band, respectively. Water molecules are reduced by electrons to form hydrogen and oxidized by holes to form oxygen leading to overall water splitting (Fig. 8). As observed in Fig. 7(A) and (B), the highest amount of H2 and O2 was evolved over 500 C activated LaFeO3 which is consistent with the evidences obtained from particle size and surface area. Here, we have used CH3OH and AgNO3 as sacrificial agents to evaluate the liberated amount of H2 and O2, respectively. When the reaction was carried out in aqueous solutions including easily oxidisable reducing agents i.e. methanol, the photogenerated holes irreversibly oxidizes methanol instead of water. This makes the photocatalyst electron rich which enhances the hydrogen evolution process [11]. On the other hand, in presence of electron acceptors such as Agþ, the photogenerated electrons in the conduction band are consumed by them and an O2 evolution reaction is enhanced [11]. These reactions using sacrificial reagents are regarded as half reactions and often employed for test reactions of photocatalytic H2 or O2 evolution.

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Conclusions

The prepared nano semiconductor particles; LaFeO3 was characterized by PXRD, DRUV-vis, BET-surface area, SEM, HRTEM etc. The PXRD pattern confirms the orthorhombic structure of LaFeO3. The blue shift in band gap energy was evidenced from the DRUV-vis spectra. SEM and HRTEM micrographs showed the transformation of irregular structure to nanosized hexagonal shape with increase in activation temperature from 500 to 900 C. Restrain of crystal growth and increase in surface area were supported by HRTEM and BET studies. The photoactivity of nano semiconductors follows the order; LaFeO3-500 > LaFeO3-600 > LaFeO3-700 > LaFeO3800 > LaFeO3-900 which is supported by the results obtained from particle size analysis and BET-surface area. Highest result was obtained over 500 C activated LaFeO3 producing 1290 and 640 mmol of H2 and O2 in 180 min with apparent quantum efficiency ca. 8.07% under visible light illumination.

Acknowledgements The authors are very much thankful to Prof. B.K. Mishra, Director, IMMT for giving permission to publish the work, Mr. G.K. Pradhan for doing SEM and Dr. K.R. Patil, NCL, Pune for XPS analyses. The financial assistance by CSIR for funding Networking Project (NWP 22) is greatly acknowledged.

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