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tructural, optical and photocatalytic properties of NiO–SiO2 nanocompositesrepared by sol–gel technique
tif Mossad Alia,b,∗, Rasha Najmyc
Department of Physics, Faculty of Science, King Khalid University, Abha, Saudi ArabiaDepartment of Physics, Faculty of Science, Assiut University, Assiut, EgyptGirls’ College of Education, Science Departments, King Khalid University, Saudi Arabia
r t i c l e i n f o
rticle history:eceived 2 August 2012eceived in revised form 4 September 2012ccepted 5 September 2012vailable online 30 October 2012
a b s t r a c t
NiO–SiO2 nanocomposite thin films were prepared by sol–gel technique. The formed films gels werecalcined at 200, 400, 600, 800 and 1100 ◦C for 4 h leading to NiO–SiO2 nanocomposites. For these films,the X-ray diffraction, the Fourier transform infrared absorption and the field emission scanning electronmicroscopy were investigated. Our results show that the NiO(2 0 0) diffraction peak dominates in thepreset work. NiO phase appeared in the XRD patterns for all samples. The newly prepared photocatalysts
eywords:ol–geliOiO2
anocomposites
films have been evaluated by the determination of their photonic efficiencies for degradation of methy-lene. The results reveal that the photonic efficiency increases with increasing calcination temperatureup to 600 ◦C with the maximum photonic efficiency being 0.62%. In addition, porous structures wereobserved and improve the sensitivity of these nanocomposites for gas sensing.
Recently, several research efforts have been devoted to thereparation of metal oxide nanostructures, because of their uniqueptical and electronic properties and their potential applicationsn various fields, such as biotechnology, optoelectronics, and catal-sis [1–4]. In addition, one of the very attractive research fieldss synthesis of semiconductor materials nanoparticles of differentizes and shapes. Among them, nickel oxide (NiO) appears to bespecially efficient in numerous applications from sensing deviceso solar energy conversion, photovoltaic devices, electrochromiclms, and as catalyst material for wastewater depollution, dueo the presence of excess oxygen [4–7]. Moreover, NiO–SiO2anocomposites have received much attention in the area of metalxide composites. NiO–SiO2 nanocomposites have been used ineactions as catalysts [8,9], gas or optical gas sensors [10,11].
Several techniques have been used to process nanocompos-te thin films such as ion implantation [12], sputtering [13],o-evaporation [14] thermal decomposition, hydrothermal syn-
hesis, chemical precipitation, molten-salt synthesis, anodic arclasma, ball milling, solvothermal and the sol–gel route [15–24].he sol–gel method, which is used in the present work, has several
∗ Corresponding author at: Department of Physics, Faculty of Science, King Khalidniversity, Abha, Saudi Arabia. Tel.: +966 7241 7099; fax: +966 7241 8319.
advantages for the preparation of single or composite oxide films,useful for the preparation of the nanocomposites, i.e. nanoparticlesin an amorphous silica, low cost, simple, quick and low temperatureprocessing, good control of the film stoichiometry and large thick-nesses attainable by using successive coatings [24–26]. In addition,it involves the synthesis from hydrolysis and condensation pro-cesses of both matrix and metal oxides, followed by a thermaltreatment in a reducing atmosphere to induce the nucleation andgrowth of metallic clusters. The sol–gel method represents a way ofsynthesizing mixed oxides such as ZnO, V2O5 and TiO2/SiO2 whichdisplay excellent properties suitable for many applications. There-fore, numerous catalyst supports or catalysts have been preparedby this procedure which leads to high purity, homogeneous mate-rials with large surface areas and these properties are key factorsfor the photocatalysis applications [27–30].
The purpose of this work is to synthesize novel NiO–SiO2 nano-structure using sol gel method. Moreover, the effects of annealingtemperature on the microstructure, optical and photocatalyticproperties of the films are illustrated by details.
2. Experimental details
2.1. Film preparation
SiO2 sol was created from the addition of a tetraethoxysi-lane (TEOS), H2O, C2H5OH, and HCl. The equivalent molar ratiowas 1:4:5:0.25. The mixture was stirred for 30 min at room
ascribed to stretching vibration of C H bond. Finally, the stretch-ing absorption bands due to H OH and Si OH bonds are observedat approximately 3500 and 3750 cm−1, respectively [34], mergedwith noise from the spectrometer.
A.M. Ali, R. Najmy / Cat
emperature to complete hydrolysis then forming the sol. Torepare the NiO:SiO2 nanocomposite films, Ni(NO3)·6H2O wasdded to SiO2 sol under vigorous stirring for 3 h to obtain 5 wt%iO:SiO2 composites. The films were deposited on a silicon (Si)
ubstrate (P(1 0 0)), by spin coating technique. Si substrates wereished with acetone and then submerged in a beaker containing
queous solution of HF (5%) for surface treatment, then rinsed withater and dried in air. Spin speed was fixed at 2000 rpm with a
pinning time of 60 s for each spin speed. After aging in air for 24 h,he samples were annealed at different temperatures ranged from00 to 1100 ◦C for 4 h.
.2. Films characterization
Field emission scanning electron microscopy (FESEM; JEOL-SM-7600F) was used to investigate the general morphologies ofhe nanocomposite NiO–SiO2 thin films. The structural proper-ies of the overall films were studied by X-ray diffraction (XRD)
easurements using a SHIMADZU XD-D1 with Cu K� radiation� = 1.5406 A). The vibrational spectra were measured by a FTIRpectrometer (JASCO FT/IR-610) at a normal light incident andnder a vacuum condition, using the same Si wafer with the Si sub-trates, as a reference. The chemical composition was examined bysing energy dispersive spectroscopy (EDS), attached with FESEM.
.3. Photocatalytic testing
The photocatalytic tests were performed in an aqueous solu-ion using methylene blue (Aldrich, �max = 661 nm) as the probe
olecule. The samples were irradiated with 1 mW/cm2 UV(A) light20 W UV tube, Eurolite). The sample was pre-irradiated with UV(A)ight (1 mW/cm2) for 24 h to decompose the remaining organicontamination by photocatalytic reaction. The NiO–SiO2 thin films1 cm × 1 cm) were horizontally fixed in the upper part of a cylin-rically shaped glass cell with an inside diameter of 5.5 cm and
height of 6.5 cm and then immersed into the cell. The concen-ration of dye adsorption and test solutions was 0.01 mmol/l. Oneundred milliliters of the methylene blue (MB) solution was put inontact with the thin films allowing the dye to adsorb in the darkor 12 h. After the adsorption of the dye was complete, the sampleas put in contact with 0.01 mmol/l MB solution and irradiatedith 1 mW/cm2 UV light. The photodegradation of the dye was
ollowed by measuring the absorption spectra at regular intervalsing a UV–VIS spectrophotometer Cary 100Bio (Varian, Australia).he photonic efficiency � being defined as the ratio of the degrada-ion rate and the incident photon flux was calculated for all testedlms from these results. Total organic carbon (TOC) analysis ofhe samples was measured using Phoenix 8000 UV-persulfate TOCnalyzer.
. Results and discussion
The XRD patterns of the nanocomposite NiO–SiO2 thin filmsrepared by sol–gel technique on Si substrates and annealed atifferent temperatures have been shown in Fig. 1. From this fig-re we observed a broad signal at about 22.5◦ for all samples,hich is due to the SiO2 vitreous matrix or NiO(0 0 6) diffractioneak. In addition, a strong Si(1 1 1) peak at 28.1◦ was observed. The
ntensity of this peak decreases with annealing temperatures. Onhe other hand, the patterns of NiO–SiO2 nanocomposites displayepresentative NiO peaks centered at 2� = 38.5◦, 44.9◦, 59◦, due to1 1 1), (2 0 0), and (2 2 0) planes, respectively [31]. Since the rela-
ive intensity of the NiO(2 0 0) diffraction peaks, compared to the1 1 1) and (2 2 0) peaks, increase gradually by increasing annealingemperatures. So, we suggested that the NiO(2 0 0) diffraction peakominates in the preset work. Kamyabi-Gol et al. [32] observed that
Fig. 1. X-ray diffraction spectra for nanocomposite NiO–SiO2 thin films annealed atdifferent temperatures.
the NiO phase did not appear in the XRD patterns for the samplesbefore drying. This could be related to the fact that the samples havetrapped components such as water, alcohol and other volatile con-stituents which could alter the X-ray analysis and, in simple terms,hide the NiO phase. In addition, they also observed that both SiO2and NiO are present in different heat treated samples, in agreementwith the results that were reported by Takeuchi et al. [33].
The difference in the chemical bonding states was detected byFTIR absorption spectra shown in Fig. 2. As shown in Fig. 2, thereis a very pronounced band appearing at 1060–1080 cm−1, togetherwith less pronounced band at 460 cm−1, and two weak bands at 555and 800 cm−1, corresponding to the vibration absorption of Si–O–Sigroups [33,34], indicating that all the samples are mainly com-posed of silica network. In addition, band at 470 cm−1 is assignedto the stretching vibration of Ni–O bond. Ni–O–Ni stretching modeis observed at 665 cm−1. The absorption bands observed at 1230and 1300–1520 cm−1 accompanied with a noise are assigned tonitrate group. Water molecules are identified by the bending modeat around 1650 cm−1. Two peaks located 2330 and 2360 cm−1 arerelated to stretching of Si CH3. Peaks at 2850 and 2920 cm−1 are
Fig. 2. IR transmittance spectra for nanocomposite NiO–SiO2 thin films with differ-ent annealing values.
4 A.M. Ali, R. Najmy / Catalysis
(sdaNaop
tive oxygen species (i.e. H O and OH•). Both, holes and OH• are
Fig. 3. FESEM images of nanocomposite NiO–SiO2 thin films.
Fig. 3 presents the field emission scanning electron microscopyFESEM) micrograph of nanocomposites NiO–SiO2 thin films. Aseen in Fig. 3 porous structure was observed. In addition, an energyispersive X-ray (EDX) spectra from the spectra 1 and 2 regions arelso shown in Fig. 4. From EDX analysis, the atomic percentage ofi, O and Si in spectrum 1 region are 3.3, 27.7 and 69, respectively,
nd the atomic percentage of Si in spectrum 2 region is 99.9. Basedn these results (FESEM micrographs and EDX analysis), it was pro-osed that the porous structure of the silica matrix could improve
Fig. 4. Typical FESEM images (a and b) and its corresponding ED
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the sensitivity of these nanocomposites for gas sensing becausemore paths would be available for the gas molecules diffusing inthe film and allowing reach the NiO reactive particle.
NiO is a p-type semiconductor with a large band gap energy(Eg = 3.5 eV), valence band (3.1 eV vs. SCE) and conduction band(−0.5 V vs. SCE) that makes it suitable for dye-sensitized solar cells,water splitting and photocatalytical processes [35–38]. In addition,it is well known that, heterogeneous photocatalysis is a process bywhich the irradiation of a metal-semiconductor produces an neg-atively charged electrons (e−) and a positively charged holes (h).The photo-excitation of semiconductor particles by means of lightwith a higher energy than its band gap generates an excess elec-trons in the conduction band and an electron vacancies (holes) inthe valence band, as shown in Fig. 5. The problem is these electronsand holes migrate to surfaces and recombine with release of pho-tons. This is a major drawback as it reduces the quantum efficiencyof photocatalysis [39,40]. To solve this problem, the immobilizationof NiO nanocatalyst onto silica support results into the inhibitionof the electron–hole recombination on the metal oxides nanopar-ticles. As shown in Fig. 5, an excitation by light of appropriatewavelength (� < 385 nm), each of the absorbed photons can gen-erate an electron–hole pair within the bulk or at the NiO surface.The hole in the valence band can react with H2O or hydroxideions adsorbed at the particle surface to produce hydroxyl radicals(OH•), while the electron in the conduction band can reduce O2 toproduce superoxide radicals (O2
•−) and subsequently other reac-
2 2
extremely reactive towards organic compounds with which theyare in contact. The photocatalytic efficiencies of the newly syn-thesized NiO–SiO2 films was assessed for the photodegradation
S spectra (c and d) of nanocomposites NiO–SiO2 thin films.
ig. 5. Schematic illustration of the proposed mechanism to explain the enhancedctivity for photodegradation of methylene blue (MB) over mesoporous NiO–SiO2
anocomposites as photocatalysts.
f aqueous solutions of methylene blue (MB) [0.01 mmol/l] by theetermination of the respective photonic efficiencies. The decom-osition of the dye under UV(A) light irradiation was determined byeasuring absorption spectra using a UV–VIS spectrophotometer.
he experimental results are shown in Fig. 6. The strong absorp-ion bands of MB located at � = 291 and 661 nm steadily decreasepon increasing irradiation times. This experiment clearly showshat the decoloration of MB can be achieved under UV-light irra-iation when the solution is put in contact with the thin films. MBan decolorize either by the oxidative degradation of the dye or byhe two-electron reduction to its colorless form [41,42]. We couldetect a small peak of the characteristic absorption band of leuco-B at 256 nm. Hence, the decoloration of MB is attributed to the
xidative degradation of the dye. For this film, the absorbance at64 nm is decreases from 0.86 to 0.13 after nearly 6 h of irradiation.he finding results were confirmed by total organic carbon (TOC)easurements. A correlation between the photonic efficiencies andiO–SiO2 films annealed at different temperatures has been clearlybserved as shown in Fig. 7. They reveal that the photonic efficiency
ncreases with increasing annealing temperature up to 600 ◦C withhe maximum photonic efficiency being 0.62%. Subsequently, thehotonic efficiency gradually decreases with increasing calcinationemperature at 800 ◦C reaching a value of 0.38%. This difference can-
ig. 6. Absorbance vs. wavelength as a function of illumination time for the pho-ocatalytic degradation of methylene blue (MB) on NiO–SiO2 films calcined at00 ◦C (I = 1 mW/cm2), MB concentration [0.01 mmol/l], volume of MB (test solu-ion) = 100 ml, illuminated NiO–SiO2 films area = 1 cm × 1 cm.
Fig. 7. Dependence of photonic efficiency of NiO–SiO2 films calcined at 200, 400,600 and 800 ◦C (I = 1 mW/cm2), MB concentration [0.01 mmol/l], volume of MB (testsolution) = 100 ml, illuminated NiO–SiO2 films area = 1 cm × 1 cm.
not be explained by different surface areas or crystallinity both ofwhich are even. The high photonic efficiencies of the NiO–SiO2 filmscalcined at 600 ◦C can be attributed to a fast transport of the tar-get molecule MB to the active sites. The latter can be expected dueto its facile diffusion through the ordered porous network. There-fore, we conclude that the NiO–SiO2 films supports the transportproperties of all reactants involved in the photocatalytic processand, thus, enhances the overall photocatalytic activity. Hence, thephotocatalytic
•OH production is expected to occur mainly on an
internal surface. Furthermore, MB adsorption onto NiO–SiO2 filmsshould take place mainly within the pores of this high surface areamaterial.
4. Conclusion
Effect of calcinations temperature on the surface morphological,structural and optical properties of NiO–SiO2 nanocomposite thinfilms was examined by X-ray diffraction, Fourier transform infraredabsorption and field emission scanning electron microscopy. Theintensity of the NiO(2 0 0) diffraction peaks, compared to the (1 1 1)and (2 2 0) peaks, increase by increasing annealing temperatures.Porous structure was also observed. A correlation between thephotonic efficiencies and NiO–SiO2 films calcined at different tem-perature has been investigated. The high photonic efficiencies ofthe NiO–SiO2 films calcined at 600 ◦C can be attributed to a fasttransport of the target molecule methylene blue (MB) to the activesites. The latter can be expected due to its facile diffusion throughthe ordered porous network.
Acknowledgments
Financial support by King Abdulaziz City for Science and Tech-nology under grant number: 08-NAN153-7, and also the financialsupport of Deanship of Scientific Research are gratefully acknowl-edged.
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