M A T E R I A L S C H A R A C T E R I Z A T I O N 6 4 ( 2 0 1 2 ) 3 6 – 4 2
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Optical properties of La2CuO4 and La2−xCaxCuO4 crystallites inUV–vis–NIR region synthesized by sol–gel process
Yifeng Li, Jianfeng Huang⁎, Liyun Cao, Jianpeng Wu, Jie FeiKey Laboratory of Auxiliary Chemistry and Technology for Chemical Industry, Ministry of Education,Shaanxi University of Science and Technology, Xi'an 710021, China
Article history:Received 24 August 2011Received in revised form22 November 2011Accepted 24 November 2011
La2CuO4 and La2−xCaxCuO4 crystallites were prepared via a simple sol–gel process. The as-prepared La2CuO4 and La2−xCaxCuO4 crystallites were characterized by X-ray diffraction,transmission electron microscope and UV–vis–NIR spectra. Results show that the grainsize of La2CuO4 crystallites increases with the increase of heat treatment temperaturefrom 600 °C to 800 °C. Optical properties show that La2CuO4 crystallites have broad absorp-tion both in the UV–vis region and in the NIR region. The band gap of the as-prepared crys-tallites decreases from 1.367 eV to 1.284 eV with the increase of calcination temperaturefrom 600 °C to 800 °C. In the series of La2−xCaxCuO4 compounds (x=0.05, 0.08, 0.10, 0.12,0.15 and 0.20), all of the samples exhibit an orthogonal crystal structure and the solubilitylimit of Ca2+ in La2CuO4 is within the range of x=0.12–0.15. In the whole UV–vis–NIR region,La2−xCaxCuO4 crystallites exhibit a broad absorption and the corresponding band gap firstincreases and then decreases with increasing of Ca2+ content.
La2CuO4 is a perovskite-like type mixed oxide [1], which iscomposed of alternating layers of (LaO)2 rock salt and the c-axis CuO2 sheet. It is a p-type anti-ferromagnetic semiconduc-tor [2] and becomes a superconductor by either replacingsome of the La3+ ions with rare earth cationic such as Ca2+,Sr2+ and Ba2+, or by intercalating excess oxygen into the inter-stitial sites. The catalytic properties of La2CuO4 also make ituseful in various oxidation and reduction reactions. In addi-tion, it has great potential to be used as electrode materials.
Today, sustainable energy gains considerable interest, stim-ulated by oil price instabilities and negative environmental ef-fects by fossil fuel combustion. Also, expected energyshortages and the exponentially growing energy demand urgethe need for alternative and new energy resources. Currently,however, sustainable energy is too expensive to meet thisneed for large-scale applications. One of the promising poten-tial energy sources is hydrogen, since it generates water after
combustion and is thus a clean fuel. To date, the hydrogen pro-duction processes are based on the polluting steam reformingreaction of hydrocarbons with steam. One of the means to pro-duce hydrogen is to decompose water, where it is decomposedinto hydrogen and oxygen under light excitation. For effectivephoto-catalysts, the band gap should be large enough to sup-port the 1.23 eV dissociation energy of water. On the otherhand, the band gap should be smaller than 2.1 eV [3], whichwould allow the materials to capture and harvest most of thesunlight energy. Both ion doped and un-doped La2CuO4 can beused as a photocathode for the photo-electrochemical decom-posing of water, in which the Cu2+ ion offers the small bandgap of this material while the incorporated lanthanum ion pro-vides the energy level adjustment. According to Spijker's re-search [4], the La2CuO4 electrode acts as a photocathode forthe photo-electrochemical decomposing of water and shows a0.5 mA/cm2 photocurrent.
Up to now, the majority of the research on La2CuO4 andLa2−xMxCuO4 (M=Ca2+, Sr2+ and Ba2+) are focused on
Fig. 1 – XRD patterns of the gel precursor and La2CuO4 crystallites calcined at (a) precursor, (b) 500 °C, (c) 600 °C, (d) 650 °C,(e) 700 °C and (f) 800 °C for 2 h.
37M A T E R I A L S C H A R A C T E R I Z A T I O N 6 4 ( 2 0 1 2 ) 3 6 – 4 2
superconductivity [5,6] and catalytic performance [7,8], fewstudies in literature about the preparation and optical propertiesof La2CuO4 and La2−xCaxCuO4 crystallites in the UV–vis–NIR re-gion are reported. In the present work, a simple sol–gel pro-cess was employed to prepare La2CuO4 and La2−xCaxCuO4
crystallites, and the influence of heat treatment temperatureand the doping level x on their optical properties were partic-ularly investigated.
Fig. 2 – TEM images of the La2CuO4 crystallites calcined a
2. Experimental
2.1. Synthesis of La2CuO4 and La2−xCaxCuO4 Crystallites
Firstly, according to the stoichiometric composition of La2CuO4
Fig. 3 – UV–vis–NIR absorption spectra of the La2CuO4 crystallites prepared at (a) 600 °C, (b) 650 °C, (c) 700 °C and (d) 800 °C for 2 h.
38 M A T E R I A L S C H A R A C T E R I Z A T I O N 6 4 ( 2 0 1 2 ) 3 6 – 4 2
(Ca(NO3)2·4H2O) and cupric nitrate (Cu(NO3)2·3H2O) were dis-solved in distilled water to form a homogeneous solution.Then, citric acid (C6H8O7·H2O, CA) in a calculated molar ratioCA:(La3++Ca2++Cu2+)=1:1 was added to obtain a complex pre-cursor under magnetic stirring. And this ratio was maintainedthroughout this work. The pH value was controlled at about2.0 by 5 vol.% ammonia water and the solution was stirred for1 h at room temperature. After that, the obtained sol was driedcompletely at 90 °C to achieve a gel precursor. The as-preparedgel was sintering at 500–800 °C for 2 h to obtain the La2CuO4
and La2−xCaxCuO4 crystallites.
2.2. Characterization
The phase composition of the as-prepared powders was char-acterized by X-ray diffraction (XRD, Rigaku D/Max-2200PC) inthe 2θ range of 15–70° with Cu Kα radiation. The morphologyand grain size of the samples were observed using JEM-3010transmission electron microscope (TEM) at an acceleration
Fig. 4 – Relationship between (αhν)2 and hν of the La2CuO4 crystafor 2 h.
voltage of 100 kV. TEM samples were prepared by dispersingthe powders in ethanol, placing the drop of the solution on acarbon coated nickel grid and subsequent drying. Optical ab-sorption measurements were carried out using Lambda 950UV/Vis/NIR spectrophotometer in the wavelength rangingfrom 200 nm to 2500 nm.
3. Results and Discussion
3.1. XRD, TEM and Optical Property Analyses of La2CuO4
Crystallites
In order to make clear the influence of calcination tempera-ture on the composition and morphology of the final prod-ucts, sol–gel reactions were carried out at room temperaturewith La3+:Cu2+ ratio of 2:1, and sintering at 500 °C, 600 °C,650 °C, 700 °C, and 800 °C for 2 h, respectively. Fig. 1 showsthe XRD patterns of the precursor and La2CuO4 crystallites
llites prepared at (a) 600 °C, (b) 650 °C, (c) 700 °C and (d) 800 °C
Fig. 5 – XRD patterns of the La2−xCaxCuO4 crystallites calcined at 700 °C for 2 h, (a) x=0, (b) x=0.05, (c) x=0.08, (d) x=0.10,(e) x=0.12, (f) x=0.15 and (g) x=0.20.
39M A T E R I A L S C H A R A C T E R I Z A T I O N 6 4 ( 2 0 1 2 ) 3 6 – 4 2
calcined at different temperatures for 2 h. For the gel precur-sor, no diffraction peak is detected, demonstrating its amor-phous state, as shown in Fig. 1(a). After sintered at 500 °C(Fig. 1(b)), weak diffraction peaks of La2O3, CuO and LaCuO2
are obtained although small peaks of La2CuO4 are observed.When the sintering temperature reaches 600 °C, monophaseof La2CuO4 (JCPDS file 38-0709) is achieved. This synthesistemperature is lower than that prepared by using an amor-phous hetero-nuclear complex as a precursor (650 °C) [9].When the calcination temperature increases from 650 °C(Fig. 2(d)) to 800 °C (Fig. 2(f)), no phase transition or changehappened although the increase in peak intensity is detected.
In order to investigate the relationship between the grain sizeof La2CuO4 and the calcination temperature, the samples wereobserved by TEM (Fig. 2). Fig. 2(a) exhibits the precursor heat trea-ted at low temperature (600 °C), the grain size is in the range of50–80 nm. With the increase of calcination temperature, the in-crease in grain size is obvious. The crystallite size is about60–80 nm (Fig. 2(b)), 70–90 nm (Fig. 2(c)) and 150–200 nm(Fig. 2(d)) with the sintering temperature of 650 °C, 700 °C and800 °C, respectively.
To study the effect of the grain size on the optical property,the UV–vis–NIR absorption spectra (Fig. 3) of the La2CuO4 crys-tallites prepared at different calcination temperatures(Fig. 3(a) 600 °C, Fig. 3(b) 650 °C, Fig. 3(c) 700 °C and Fig. 3(d)800 °C) were measured. It is shown that La2CuO4 has a broad
Table 1 – Lattice parameters and cell volume of differentdoping levels (x).
absorption both in the UV–vis region (200–800 nm) and in theNIR region (1000–2500 nm). The shapes of curves are similarto each other though some minor differences in absorbencyare observed. In the UV–vis region, the absorption intensityof the as-prepared La2CuO4 crystallites increases with increas-ing the calcination temperature, and shows a reverse tenden-cy in the NIR region. The difference in calcinationtemperature also leads to a blue shift in the absorption edgeof UV–vis region, which may be a result from the decrease ingrain size of La2CuO4 crystallites.
According to the light absorption theory of crystal mate-rials [10], the movement of electrons between the band gapof semiconductor materials can be represented by Eq. (1) asshown below.
αhνð Þ2 ¼ A hν−Eg� � ð1Þ
where α is the photo absorption coefficient, h is the Plankconstant (6.626×10−34 J/s), ν is the light frequency, A is a con-stant, and Eg is the energy band gap. According to this linearrelationship, the energy band gap Eg of La2CuO4 crystallitescan be calculated by setting (αhν)2=0.
Fig. 4 displays the relationship between (αhν)2 and hν ofLa2CuO4 crystallites sintered at different temperatures. Theenergy band gap is equal to the tangent line of the linear seg-ment on the (αhν)2~hν curve. It can be concluded that theband gap of La2CuO4 crystallites decreases with the increaseof calcination temperature, which may result from the quan-tum size effect corresponding with the above mentionedblue shift. Thus, the optical band gap can easily be controlledby adjusting the grain size, which can be achieved by varyingthe heat treatment temperature. In addition, La2CuO4 has acharge-transfer (CT) gap of about 1 eV–2 eV between the va-lence band of the occupied O 2p orbital and the conductiveband of the empty Cu 3d orbital [11,12]. The obtained bandgap is 1.367 eV (Fig. 4(a)), 1.346 eV (Fig. 4(b)), 1.307 eV(Fig. 4(c)) and 1.284 eV (Fig. 4(d)) for the sintering temperatureof 600 °C, 650 °C, 700 °C and 800 °C respectively, which are allin the range of 1–2 eV.
Fig. 6 – UV–vis–NIR absorption spectra of the La2−xCaxCuO4 crystallites calcined at 700 °C for 2 h, (a) x=0.05, (b) x=0.08,(c) x=0.10, (d) x=0.12, (e) x=0.15 and (f ) x=0.20.
40 M A T E R I A L S C H A R A C T E R I Z A T I O N 6 4 ( 2 0 1 2 ) 3 6 – 4 2
3.2. XRD and Optical Property Analyses of La2−xCaxCuO4
Crystallites
Ion doping is one of the crucial factors that affect the opticalband gap. To study the effects of calcium ion concentrationon the UV–vis–NIR absorption, the sol–gel process were car-ried out with various La:Ca:Cu ratios of (2−x):x:1 (x=0, 0.05,0.08, 0.10, 0.12, 0.15 and 0.20) and sintering at 700 °C for 2 h.
Fig. 7 – Schematic diagram of the La2CuO4 cryst
Clearly, all of the La2−xCaxCuO4 oxides display a singlephase of orthorhombic structure, and the peak intensity ofLa2−xCaxCuO4 decreases with the increase of Ca2+ content(Fig. 5). It may be attributed to the distortions in the ion latticedue to the calcium doping, which may lead to negative effectson the crystallization. In the range of x=0–0.12, with the in-crease of the calcium amount, the diffraction peaks of (004)and (006) gradually shift to lower diffraction angles, while the
Fig. 8 – Relationship between (αhν)2 and hν of the La2−xCaxCuO4 crystallites calcined at 700 °C for 2 h, (a) x=0.05, (b) x=0.08,(c) x=0.10, (d) x=0.12, (e) x=0.15 and (f ) x=0.20.
41M A T E R I A L S C H A R A C T E R I Z A T I O N 6 4 ( 2 0 1 2 ) 3 6 – 4 2
diffraction peaks of (020) and (200) gradually shift to higher dif-fraction angles. When the x value reaches 0.15 and 0.20, thediffraction peaks display no significant change comparedwith x=0.12, which infers that the solubility limit of Ca2+ inLa2CuO4 is within x=0.12–0.15 using this synthesis method.This result is a little higher than that reported by Li et al., i.e.within x=0.10–0.12. In addition, the diffraction peaks of theLa2CaCu2O6 phase are generated for x≥0.15, which is in accordwith the result reported by Moodenbaugh et al. [13].
The lattice parameters and cell volume data of theLa2−xCaxCuO4 crystallites acquired from the Rietveldrefinement of XRD patterns are listed in Table 1. When thedoping level is within x=0.12, the lattice parameters a and bdecrease gradually with the increase in concentration ofcalcium ion, but parameter c shows a reverse tendency.Despite the increasing of c, the overall effect of substitutingLa ions by smaller Ca ions will lead to a decrease in the cellvolume. Beyond x=0.12, the lattice parameters and cellvolume changes a little, which is well in agreement with theresults reported by Li et al. [14].
Fig. 9 – Relationship between band gap and x o
Fig. 6 is the absorption spectra in the whole UV–vis–NIR re-gion for the La2−xCaxCuO4 crystallites. Clearly, all of the sam-ples exhibit a broad absorption. The curves are similar to eachother within x=0.05–0.12, and change when x=0.15–0.12. Itmay be due to the existence of La2CaCu2O6 impurity phasein the La2−xCaxCuO4 samples, which leads to the differencein optical property.
The substitution of Ca2+ for La3+ can be represented by(using the Kroeger–Vink notation) Eqs. (2) and (3), in whichthe compensating defects are oxygen vacancies (VO
·· ) andholes (h·), respectively [15].
CaO ¼ Ca′La þ 1=2V ::O þ LaxLa þ 3=2Ox
O ð2Þ
CaO ¼ 1=4O2ðgÞ þ Ca′La þ h: þ LaxLa þ 3=2OxO ð3Þ
Fig. 7 indicates schematic diagram of the La2CuO4 structurealong with the doping of Ca2+. It is shown that un-dopedLa2CuO4 has an orthorhombic K2NiF4 type lattice consisting ofCuO2 planes perpendicular to the c-axis, separated by layers of(LaO)2. In the process of Ca2+ doping, the substitution of Ca2+
42 M A T E R I A L S C H A R A C T E R I Z A T I O N 6 4 ( 2 0 1 2 ) 3 6 – 4 2
for La3+ occurred in the LaO layer and the corresponding oxygenvacancies or holes were generated in the sheet of CuO2.
As shown in Fig. 8, a series of band gaps of La2−xCaxCuO4
are calculated according to the light absorption theory men-tioned above. The band gap is 1.353 eV (Fig. 8(a)), 1.380 eV(Fig. 8(b)), 1.431 eV (Fig. 8(c)), 1.463 eV (Fig. 8(d)), 1.132 eV(Fig. 8(e)) and 1.102 eV (Fig. 8(f)) when the x value is 0.05,0.08, 0.10, 0.12, 0.15 and 0.20, respectively. Compared withLa2CuO4 crystallites prepared in the same condition(Eg=1.307 eV, 700 °C), the band gap of La2−xCaxCuO4 changesgreatly and increases with the increase of Ca2+ content inthe range of x=0.5–0.12. It may be due to the increase inbond-mismatch, which is in line with the variation of latticeparameters a, b and c. It is known that, in the un-dopedLa2CuO4, there is a bond-length mismatch between theLa\O bonds in the (LaO)2 layers and the Cu\O bonds in theCuO2 sheets. When doped with Ca2+, the radio of Ca2+ is smal-ler than the La3+ host, which means substitution will enhancethe bond-mismatch by contracting the (LaO)2 layers. This willlead to the expanding of the CuO2 sheets, and result in the in-crease in band gap of La2−xCaxCuO4 crystallites.
Fig. 9 shows the relationship between the Eg (band gap) andx level. There is a linear correlation with a slope of about 1.29when 0≤x≤0.12. Above x=0.12, the band gap of La2−xCaxCuO4
crystallites drops off quickly, this abnormal phenomenonmay be attributed to the appearance of La2CaCu2O6 phasewhen the Ca2+ content is over the solubility limit(x=0.12–0.15).
Based on the above research, it can be concluded that theband gap of un-doped and doped La2CuO4 can be modifiedand controlled by the calcination temperature and the dopinglevel of Ca2+ ion, which will enable the crystallites to be po-tentially used as effective photo-catalysts.
4. Conclusions
(1) La2CuO4 crystallites could be prepared at low tempera-ture (600 °C) via a sol–gel process. The grain size ofLa2CuO4 crystallites increased from 50–80 nm to150–200 nm with the increase of calcination tempera-ture from 600 °C to 800 °C. It exhibited a broad absorp-tion band both in the UV–vis region (200–800 nm) andin the NIR region (1000–2500 nm). The band gap of theas-prepared La2CuO4 crystallites decreased from1.367 eV to 1.284 eV with the increase of sintering tem-perature from 600 °C to 800 °C.
(2) In the series of La2 − xCaxCuO4 oxides (x=0.05, 0.08,0.10, 0.12, 0.15, 0.2), all of the samples exhibited apure La2 −xCaxCuO4 phase within the solubility limitof x=0.12–0.15, and La2CaCu2O6 impurity phase gener-ated when x≥0.15 within the solubility limit, with theincrease of x level, the increase in crystallite lattice pa-rameters a, b and decrease in c are detected, whichleads to the enhance in bond-mismatch. In the wholeUV–vis–NIR region, the La2− xCaxCuO4 samples dis-played a broad absorption band and the band gap in-creased linearly with doping level when 0≤x≤0.12,which will meet the demand of effective photo-catalyst applications.
Acknowledgments
This work was supported by the National Natural ScienceFoundation of China (grant no. 50942047), the National Natu-ral Science Foundation of China (grant no. 51102196), the Nat-ural Science Foundation of Shaanxi Province (grant no.2010JM6001) and the Graduate Innovation Found of ShaanxiUniversity of Science and Technology.
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