F. Parrino et al. / Journal of Alloys and Compounds 682 (2016) 686e694
687
applications.
The physico-chemical features of LaFeO3 satisfy also the re-
quirements for its use as photocatalyst. LaFeO3 is a charge-transfer-
type semiconductor and exhibits an optical band-gap energy be-
tween 2.1 eV (for dense polycrystalline samples) and 2.6 eV (for
powdered samples) [9,10].
Under visible light irradiation, this compound shows stable
photocurrent [11] and photocatalytic activity, although the activity
has been reported only in aqueous solution for oxidative degra-
dation of dyes [12e16] and for water decomposition [17].
layer, using a FEI Quanta 200 SEM microscope operating in high
vacuum at 20 kV. Diffuse reflectance UVevis spectra were recorded
in the range 200e800 nm by using a Shimadzu UV-2401 PC in-
strument, with BaSO4 as the reference sample. Raman spectra were
obtained by means of a BWTek-i-Raman Plus System, equipped
with a 785 nm diode laser. The measurements were performed
focusing the sample by a 20ꢂ magnification lens; spot size was
around 50 m
m. The accuracy of Raman shift was around 3 cmꢀ1. The
power of the laser used was 10% of the maximum value, which is
around 300 mW. XPS analysis was performed with a Perkin-Elmer
The effect of partial substitution of La and/or Fe on the photo-
catalytic properties of the ferrite has been poorly described in the
recent literature. The partial substitution of La with divalent Ca or
monovalent Li ions causes an increase of the photocatalytic
degradation of methylene blue [18,19]. A similar beneficial effect is
obtained by Fe partial substitution with Zn or Mn [20,21]. In these
papers it is reported that the cation substitution gives rise to an
electronic unbalance compensated by an increment of the oxida-
tion state of a part of Fe(III) to Fe(IV), being oxygen vacancies
formed to preserve the electroneutrality. However, in case of
F
5600-ci spectrometer using a non-monochromatized Al/Mg dual
anode X-ray source (Al-K 1486.6 eV, Mg-K 1253.6 eV). The
samples were analysed as powders mounted on a double-sided
adhesive tape. The examined area was 800 m and the working
a
a
m
pressure lower than 10ꢀ9 mbar. The spectrometer was calibrated by
assuming the binding energy (BE) of the Au 4f7/2 line at 83.9 eV
with respect to the Fermi level. The standard deviation for the BEs
values was 0.2 eV. Survey scans were obtained in the 0e1200 eV
range. Detailed scans were recorded for the La3d, Fe2p, Cu2p, O1s
and C1s regions. The BEs shifts due to sample charging were cor-
rected by referencing all the energies to the C1s set at 284.8 eV,
arising from the adventitious carbon. The analysis involved Shirley-
type background subtraction, non-linear least-squares curve fitting,
adopting Gaussian Lorentzian peak shapes and peak area deter-
mination by integration. The atomic percentage content was eval-
uated from peak areas using sensitivity factors supplied by Perkin-
Elmer. Compositional data were averaged over three spots on each
sample. The experimental uncertainty on the reported atomic
composition values does not exceed 10%. Photocurrent measure-
ments were performed in a three electrodes electrochemical cell by
using a CH Instruments Electrochemical Analyzer. An aqueous Ag/
AgCl electrode (AMEL) was used as reference, a platinum foil
(surface equal to ca. 0.5 cm2) as the counter electrode and the
working electrode consisted of a porous monocrystalline film of the
perovskite deposited on a titanium foil. The working electrode was
prepared as follows: a titanium foil was first cut into 6.2 ꢂ 2.1 cm
pieces and then degreased by sonication in acetone, rinsed with
demineralized water and blown dry in an air stream. 200 mg of the
perovskite powder were suspended in 1 ml of absolute ethanol and
La0.8Sr0.2Fe1ꢀxCuxO3ꢀ [11,22] a complex effect on the photo-
d
electrochemical properties has been reported.
To the best of our knowledge, studies on the photoactivity of the
solid solution LaFe1ꢀxCuxO3ꢀd in gas-solid regime are not reported
in the current literature. In this work Cu-substituted LaFeO3 pe-
rovskites (x ¼ 0.05, 0.10, 0.20 and 0.40) have been prepared by
citrate auto-combustion synthesis and their photocatalytic activity
was tested by using 2-propanol oxidation in gas-solid regime as a
probe reaction, in analogy to what reported for other solids
[23e25]. Moreover, the investigated physico-chemical features
have been correlated to the photocatalytic activity.
2. Experimental
2.1. Perovskites preparation and characterization
Analytical grade La2O3, Fe(NO3)3$9H2O, Cu(NO3)2$2.5H2O, citric
acid, nitric acid and NH4OH were used as the starting materials. A
specific amount of dried La2O3 was dissolved in a nitric acid solu-
tion to prepare La(NO3)3$6H2O. The experimental details have been
sonicated for 20 min, hence 200 mL of the suspension was drop-
previously reported [5]. LaFeO3 (LF), LaFe0.95Cu0.05O3ꢀ (LFC05),
casted onto a fixed area (4.2 ꢂ 1.7 cm) of the titanium foil using a
scotch tape as frame and spacer. The obtained film was dried at
room temperature, covered with a glass plate and pressed for 3 min
at 200 kg/cm2 using an IR pressing tool according to a procedure
similar to that described in literature [26]. Such a procedure yields
an opaque and slightly translucent perovskite layer having an
excellent mechanical stability. The working electrode was then
placed in the photoelectrochemical cell so that the film was
completely immersed in the 0.1 M Na2SO4 aqueous solution used as
the electrolyte. The photocurrent was measured at the open circuit
potential previously recorded for each working electrode. The
electrochemical cell was irradiated from the front-side with a
700 W medium pressure Hg lamp, alternating light and dark pha-
ses. All the photoelectrochemical tests were carried out at room
temperature and under atmospheric air.
d
LaFe0.90Cu0.10O3ꢀ
(LFC10), LaFe0.80Cu0.20O3ꢀ
(LFC20) and
d
d
LaFe0.60Cu0.40O3ꢀd (LFC40) powders were prepared by citrate auto-
combustion of the dry gel obtained from a solution of the corre-
sponding nitrates in citric acid solution. The resulting lightweight
powders were calcined at 600 ꢁC for 3 h in air. For the sake of
comparison, a sample (LFC05imp) containing LF and CuO was
prepared by the wet impregnation method. In particular 0.025 g of
Cu(NO3)2$2.5H2O were dissolved in water and the solution was
poured onto 0.5 g of LF sample. After 24 h drying at room tem-
perature, the powder was calcined for 3 h at 400 ꢁC, a temperature
considerably lower compared to that reached during the citrate
autocombustion method. In this way CuO can be obtained, but solid
state interaction with LF and possible interdiffusion between the
two phases are not significant. The phase purity was determined for
each composition by powder X-ray diffraction. The patterns were
collected at room temperature (Bruker D8 Advance diffractometer)
2.2. Photoreactivity experiments
using Cu-K
a
1 radiation (40 kV, 40 mA,
l
¼ 0.15406 nm); the step
scan of 2
q
was 0.02ꢁ with a scan rate of 1.2ꢁ minꢀ1. The crystallite
The photoreactivity of the perovskites samples in gas-solid
regime was evaluated by using a cylindrical Pyrex batch photo-
reactor (V ¼ 130 ml, external diameter ¼ 93 mm, external
height ¼ 22 mm) provided with a silicon/teflon septum. The
powders were irradiated from the top by means of a SOLARBOX
apparatus (CO.FO.ME.GRA.) equipped with a 1500 W high pressure
Xenon lamp placed at a distance of 165 mm from the upper surface
domain size was calculated using the Scherrer equation. The spe-
cific surface areas (SSA) of the powders were determined in a Flow
Sorb 2300 apparatus (Micromeritics) by using the single point BET
method. Scanning electron microscopy (SEM) observations and
energy-dispersive X-ray (EDX) analyses were performed on the
powdered samples after deposition by sputtering of a thin gold