Raman spectroscopy as a tool for the elucidation of nanoparticles with core–shell structure of TiO2 and MoO3

doi:10.1016/j.vibspec.2010.03.010

Vibrational Spectroscopy

Volume 54, Issue 2, 18 November 2010, Pages 89-92

https://doi.org/10.1016/j.vibspec.2010.03.010 Get rights and content

Abstract

In this work, we report the synthesis of TiO₂ nanoparticles and core–shell TiO₂–MoO₃ and MoO₃–TiO₂ nanoparticles dispersed into porous Vycor glass by the metallo-organic decomposition process (MOD). The nanoparticles were analyzed using Raman spectroscopy by monitoring the E_2g band (phonons region for anatase TiO₂) and the bands located at 820 and 996 cm⁻¹ (vibrational modes of α-MoO₃) when Mo is present. The diameters of the TiO₂ nanoparticles dispersed in the glass matrix were calculated by means of the phonon confinement model and the values obtained are 3.9, 4.6, and 5.7 nm depending on the number of impregnation–decomposition cycles used in the nanoparticle syntheses.

Introduction

In recent years, much of the research in nanomaterials has focused on semiconductor nanoparticles, since the nanoparticles display a variety of fundamentally interesting physical and chemical properties resulting directly from their size and dimensionality. Semiconductor based core–shell nanoparticles (CSN) represent an important type of advanced material, since their properties can be tailored to suit applications such as modulation of optical properties [1], [2], magnetism [3], [4], heterogeneous catalysts [5], [6], and solar energy conversion [7], [8].

Nanocrystalline anatase TiO₂ is a potential material for photocatalytic applications [9]. However, its large band gap of 3.2 eV (outside the most intense region of the solar spectrum, centered at 2.6 eV) is a problem that researchers must overcome. To make TiO₂ suitable for practical solar energy conversion, materials on the nanometer scale with core–shell structure have been synthesized [10], [11]. Another approach is the combination of anatase TiO₂ and orthorhombic α-MoO₃ (band gap of 2.9 eV), that could result in heterostructured materials with band gaps of ca. 2.7 eV, due to the arrangements of the valence and conduction bands.

Among the various techniques that can be used for the characterization of nanostructures, Raman spectroscopy is presented as a potential tool, since it provides important information about connectivities in those materials [12], [13], [14], [15]. Theoretically, the fundamental q ∼ 0 Raman selection rule is relaxed for a finite-size domain, allowing participation of phonons away from the Brillouin zone center [16]. The phenomenon of spatial confinement of optical phonons was studied by Richter et al. [17]. It was shown that the Raman spectra of nanocrystalline semiconductors are red-shifted and broadened due to the relaxation of the q-vector selection rule in finite-size nanocrystals. The phonon uncertainty is Δq ∼ 1/d, where d is the diameter of a nanocrystal or quantum dot. This spatial confinement inside nanocrystals gives rise to a shift and to an asymmetric broadening of the Raman band in nanostructures compared to bulk crystals [18], [19]. The phonon confinement effects in the Raman scattering of TiO₂ nanocrystals have been studied [20], [21], but due to the large number of polymorphs of molybdenum oxide, its study by the phonon confinement model becomes difficult.

In this work, we report the synthesis of core–shell nanoparticles of titanium and molybdenum oxides produced by metallo-organic decomposition process in a porous matrix. The samples were analyzed by Raman spectroscopy.

Section snippets

Synthesis of core–shell nanoparticles of TiO₂ and MoO₃

The TiO₂–MoO₃ and MoO₃–TiO₂ core–shell nanoparticles dispersed in porous Vycor glass (PVG, Corning Glass^®) were prepared by the metallo-organic decomposition process, which consists in impregnation followed by decomposition of the titanium (IV) and molybdenum (VI) precursors. PVG was impregnated with titanium (IV) di-(propoxy)-di-(2-ethylhexanoate), Ti(Prop)₂(Hex)₂, or molybdenum (VI) 2-ethylhexanoate, Mo(Hex)₆. These metallo-organic compounds were thermally decomposed under air at 1023 K for 8 h

Results and discussion

PVG/xTiO₂ with x = 3, 5 and 7 were analyzed using Raman spectroscopy under the same analysis conditions. The bulk anatase TiO₂ spectrum is presented for comparison (Fig. 1) and shows two intense bands located at ca. 144 and 638 cm⁻¹ [21], [22]. The spectrum of pure PVG, characterized by broad bands typical of vitreous materials [21], is also presented. The presence of anatase TiO₂ related Raman bands in PVG/xTiO₂ samples is indicative of the titania phase present inside PVG. The most intense band

Conclusions

In this work, we report the synthesis of a series of TiO₂–MoO₃ and MoO₃–TiO₂ nanoparticles inside porous Vycor glass. Raman spectroscopy shows that this synthesis method allows to control the preparation of core–shell nanoparticles in the Vycor^® glass pores. The size-dependent Raman spectra band shifts are explained based on the phonon confinement model. A detailed analysis of the behavior of the E_2g band of the anatase TiO₂ and of the bands attributed to α-MoO₃ supplies important information

Acknowledgements

The authors are grateful to FAPESP and CNPq for financial support, to Prof. D.L.A. Faria and Prof. M.L.A. Temperini (IQ-USP, São Paulo, Brazil) and Prof. A.G. Souza Filho (UFC, Brazil) for assistance with the Raman measurements, and to Prof. C.H. Collins (IQ-UNICAMP, Brazil) for English revision. This is a contribution of National Institute of Science and Technology in Complex Functional Materials—Inomat (CNPq-MCT/FAPESP).

References (26)

M. Fan et al.
J. Magn. Magn. Mater.
(2009)
X.L. Yang et al.
J. Catal.
(2005)
H. Richter et al.
Solid State Commun.
(1981)
I.O. Mazali et al.
J. Phys. Chem. Solids
(2005)
T. Siciliano et al.
Mater. Chem. Phys.
(2009)
S.A. Ivanov et al.
J. Phys. Chem. B
(2004)
V.I. Klimov et al.
Nature
(2008)
D. Cangussu et al.
J. Appl. Phys.
(2009)
D.P. Wang et al.
Chem. Mater.
(2009)
N.G. Park et al.
Langmuir
(2004)

S. Chappel et al.

Langmuir

(2002)

G.K. Zhang et al.

Langmuir

(2008)

S.H. Elder et al.

J. Am. Chem. Soc.

(2000)

Cited by (24)

Adaptive tribological performance of porous ticn-mos<inf>2</inf> composite coatings in response to fluctuating humidity conditions
2024, Surface and Coatings Technology
To improve the friction and wear performance of coatings in aerospace engineering under wet and dry alternating conditions, the porous hard TiCN coating was prepared by a combination of multi-arc ion plating and oblique angle deposition. The study found that when the titanium target current was reduced from 80 A to 60 A, the pore size of the porous TiCN coating increased from 0.30 μm to 0.93 μm. In comparison to the low hardness of stainless steel substrate of 3.4 GPa, the hardness of TiCN coatings with pore sizes of 0.93 and 0.30 μm rose to 10 and 22 GPa, respectively. The existence of pores can effectively improve the hydrophilicity of the coatings. However, as the pore size increased, the anti-wear lifetime of the coatings were also decrease. This process was followed by ultrasonication and hydrogen reduction of (NH₄)₂MoS₄ to infuse MoS₂ directly into the pores to fabricate the TiCN-MoS₂ composite coatings. The findings revealed MoS₂ atomic sheets are randomly embedded in the pores of the TiCN coating. In a stable humidity environment, as the relative humidity increased, the friction coefficient of the TiCN-MoS₂ porous composite coating increased and the anti-wear lifetime decreased. In addition, the anti-wear lifetime of the TiCN-MoS₂ porous composite coating at 20 % relative humidity environment is 128,322 laps, which exceeded the 38,099 laps of the dense TiCN-MoS₂ composite coating. This proved that the porous composite coating had better anti-wear lifetime due to the continuous release of MoS₂ in the pores under low humidity conditions. The tribological behavior of the porous composite coatings was systematically investigated under alternating humidity conditions, with a comparison against the dense TiCN-MoS₂ composite coatings. The porous composite coatings possessed better self-adaptation under alternating humidity conditions, with coefficients of friction of 0.05 and 0.15 at 10 % and 70 % RH air, respectively. A detailed discussion was conducted on the relationship between tribofilm composition and tribological properties under different humidity environments.
Effect of Cr addition on oxidation and tribological properties of MoB<inf>2-z</inf> films by magnetron sputtering
2024, Surface and Coatings Technology
Molybdenum diboride films are anticipated to be used in cutting applications as a potential replacement for nitride coatings due to their high hardness and good high-temperature stability. However, the films undergo severe oxidation at 600 °C, which does not guarantee good wear resistance. The addition of Cr element is chosen to improve the film's oxidation resistance, which in turn improves the film's friction and wear performance at higher temperatures. The Mo-Cr-B films were prepared by magnetron sputtering by changing Cr target current. The oxidation properties of the films at 600 °C and the tribological performance and the wear mechanism at 25 °C, 400 °C and 600 °C were investigated. The results reveal that the MoB_2-z film and Cr-alloyed MoB_2-z films have higher hardness (>30 Gpa). And the Mo_0.67Cr_0.33B_2-z film and Mo_0.51Cr_0.49B_2-z film exhibit much greater oxidation resistance at 600 °C than do MoB_2-z film and Mo_0.79Cr_0.21B_2-z film. The Mo_0.67Cr_0.33B_2-z film demonstrates good wear resistance at 25 °C, 400 °C and 600 °C. Especially at 600 °C, the Mo_0.67Cr_0.33B_2-z film clearly outperforms the MoB_2-z film in terms of wear resistance.
Influence of crystallite size on color properties and NIR reflectance of TiO<inf>2</inf>@NiTiO<inf>3</inf> inorganic pigments
2021, Ceramics International
Citation Excerpt :
Moreover, it should be noted that the peaks of NiTiO3 were stronger than ones of TiO2, which was in reverse to phase composite of XRD above, and the band of TiO2 at 235 cm−1 almost disappeared for all pigments. This corroborated the core-shell of pigments [26,27]. And the FT-IR spectra of as-prepared pigments also presented more characteristics of NiTiO3 in Fig. S1.
Crystallite size may significantly impact optical properties of inorganic pigments. However, limited studies have so far been reported in this area. In this work, TiO₂@NiTiO₃ complex pigments were synthesized by precipitation-calcination method. Results showed that the increase in calcination temperature led to the formation of TiO₂@NiTiO₃ pigments with crystallite sizes from 29 to 82.6 nm but similar morphologies and aggregate particle sizes (around 1 μm). The yellow component (b*) significantly increased by 7 with crystallite size, but the integrated NIR reflectance decreased slightly (4.02%). Also, the characteristic absorption of NiTiO₃ pigments in NIR region declined the integrated NIR reflectance values.
MoO<inf>3</inf> nanoparticle formation on zeolitic imidazolate framework-8 by rotary chemical vapor deposition
2018, Microporous and Mesoporous Materials
Citation Excerpt :
A monolayer-dispersed MoO3 on TiO2 has been synthesized by impregnation technique [21], reaching pour performances. To further advance the photochromic properties, numerous strategies such as metalorganic decomposition [22], hydrothermal method [23], surfactant micelle nucleation [24] and sol-gel process [25] have been researched achieving MoO3-TiO2 nanostructures with a significant improvement. However, with these techniques, the use of surfactants and high temperature second stage thermal treatments are required.
For the first time, MoO₃ nanoparticles (NPs) with a size ranging from 1.5 to 60 nm were deposited on spray dried zeolitic imidazolate framework-8 (ZIF-8) by rotary chemical vapor deposition (RCVD) in order to improve its photocatalytic performance. A direct effect of the deposition time on the metal oxide loading was observed. In a time frame between 0.9 and 2.7 ks the metal oxide loading can be increased from 1 to 3 wt%. All the MoO₃-NPs/ZIF-8 catalysts were tested towards the methylene blue photodegradation using sunlight. MoO₃-NPs/ZIF-8 3 wt% RCVD reached a conversion of 82% and 95% after 180 and 300 min, respectively.
Study of structure of the TiO<inf>2</inf>-MoO<inf>3</inf> bilayer films by Raman spectroscopy
2014, Materials Research Bulletin
In this work, TiO₂–MoO₃ films were easily prepared by dip-coating technique and metallo-organic decomposition process (MOD). Raman analyses indicate the formation of TiO₂ in anatase phase and orthorhombic phase of α-MoO₃. It was observed that the Raman bands intensities attributed to TiO₂ and MoO₃ oxides were dependent on the number of decomposition–deposition cycles (DDC). The different number of DDC generates films with different thicknesses and the Raman signal was sensitive to this variation. Raman analyses provided qualitative information about the bilayer structure of the bi-component TiO₂–MoO₃ films, which was confirmed by scanning electron microscopy. In this direction, the dip-coating technique and MOD process can be an efficient strategy to facile preparation of many samples to be used in applications.
Photocatalytic degradation of methylene blue by MoO<inf>3</inf> modified TiO<inf>2</inf> under visible light
2014, Cuihua Xuebao/Chinese Journal of Catalysis
Citation Excerpt :
Figure 2 illustrates the Raman spectra of MoO3 and MoO3/P25(x). In the spectrum of MoO3, the bands at 242 and 343 cm−1 are due to the Mo–O–Mo deformation [9,10], and the band at 287 cm−1 is the result of deformation of the Mo=O bond [11]. The other three bands at 671, 820, and 996 cm−1 are assignable to the triply coordinated oxygen (Mo3–O) stretching mode, which results from edge-shared oxygens in common to three MoO6 octahedra [12], the Mo–O–Mo stretching of corner sharing MoO6 octahedra [11], and the symmetric Mo=O stretching mode [12], respectively, in the orthorhombic α-MoO3 phase.
MoO₃/P25 catalysts were prepared by an impregnation method. The catalysts were characterized by X-ray diffraction, ultraviolet-visible spectrophotometry, Fourier transform infrared spectroscopy, and laser Raman spectroscopy, and their photocatalytic activty was evaluated by the degradation of methylene blue dye under visible light. The monolayer dispersion threshold of MoO₃ on P25 was around 0.1 g/g. The strong interaction between the monolayer-dispersed tetrahedral-coordinated molybdenum oxide species and P25 led to a decrease in the band gap of P25, thus increasing the visible light absorption of the catalyst. Crystalline MoO₃ was formed on catalysts with a MoO₃/P25 mass ratio above 0.1. In these cases, the visible light absorption of the catalysts decreased with increasing MoO₃ content. The band gap of the catalyst was not the only factor affecting its photocatalytic activity for the degradation of methylene blue under visible light. MoO₃/P25 with the MoO₃ to P25 mass ratio of 0.25, which possessed not only suitable band gap but also a certain amount of crystalline MoO₃, showed the best catalytic performance.

View all citing articles on Scopus

View full text

Raman spectroscopy as a tool for the elucidation of nanoparticles with core–shell structure of TiO2 and MoO3

Abstract

Introduction

Section snippets

Synthesis of core–shell nanoparticles of TiO2 and MoO3

Results and discussion

Conclusions

Acknowledgements

J. Magn. Magn. Mater.

J. Catal.

Solid State Commun.

J. Phys. Chem. Solids

Mater. Chem. Phys.

J. Phys. Chem. B

Nature

J. Appl. Phys.

Chem. Mater.

Langmuir

Langmuir

Langmuir

J. Am. Chem. Soc.

Raman spectroscopy as a tool for the elucidation of nanoparticles with core–shell structure of TiO₂ and MoO₃

Synthesis of core–shell nanoparticles of TiO₂ and MoO₃