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Journal of the Chilean Chemical Society

On-line version ISSN 0717-9707

J. Chil. Chem. Soc. vol.61 no.1 Concepción Mar. 2016

http://dx.doi.org/10.4067/S0717-97072016000100014 

 

MOLYBDENUM TRIOXIDE THIN FILMS DOPED WITH GOLD NANOPARTICLES GROWN BY A SEQUENTIAL METHODOLOGY: PHOTOCHEMICAL METAL-ORGANIC DEPOSITION (PMOD) AND DC-MAGNETRON SPUTTERING

 

C. CASTILLO1, G. BUONO-CORE*1, C. MANZUR1 N. YUTRONIC2, R. SIERPE2, G. CABELLO3, B. CHORNIK4.

1Instituto de Química, Pontificia Universidad Católica de Valparaíso, Valparaíso, Chile.

2Departamento de Química, Facultad de Ciencias, Universidad de Chile.

3Departamento de Ciencias Básicas, Facultad de Ciencias, Universidad del Bío-Bío, Chillán, Chile

4 Departamento de Física, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile.

e-mail: gbuonoco@ucv.cl


ABSTRACT

Gold nanoparticles (AuNPs) were deposited by DC-magnetron sputtering onto molybdenum trioxide (MoO3) thin films grown by Photochemical Metal-Organic Deposition (PMOD) on Si(100) and borosilicate glass substrates. The chemical, optical and morphology properties of the films were studied by UV/ Vis Spectroscopy, Scanning Electron Microscopy (SEM), X-Ray Photoelectron Spectroscopy (XPS), and X-Ray Diffraction (XRD). SEM revealed that AuNPs formed after 5 s of sputtering. AuNPs are spherical and have both an average diameter of 18 nm and a relatively narrow size distribution. As the deposition time increases, larger structures are formed by an aggregation of AuNPs. XPS studies of the AuNP/MoO3 films on Si(100) showed the presence of Mo(VI) and Mo(V), which indicated that the films were primarily non-stoichiometric molybdenum oxides. The occurrence of oxygen vacancies in the substrate play an important role to stabilize the AuNPs.

Keywords A. oxides, A. thin films, A. nanostructures, B. sputtering; C. x-ray diffraction, C. photoelectron spectroscopy


 

1. INTRODUCTION

Unique properties can be achieved by reducing a material to the nanometer scale. Metal nanoparticles (MNPs) present attractive optical, electronic, magnetic and chemical properties, among others, which are not displayed in the bulk state 12. Gold nanoparticles (AuNPs) are the most stable MNP; they can be synthesized with different sizes and shapes; and they are easily functionalized with diverse ligands 3. AuNPs have numerous applications in nanomedicine 45 biotechnology 6, microelectronics 7, optics 8-10, gas sensing 11-13 and catalysis14-20

AuNPs have been prepared using various approaches that can be divided into physical and chemical methods. Included among the chemical methods are reduction 21, photochemistry using UV irradiation 22,23, photochemistry using X-ray irradiation 24, sonochemistry 25,26, and sonoelectrochemistry 27. A physical method that can be used is cathodic deposition using magnetron sputtering. This physical method has several advantages over chemical syntheses, such as a lack of surface contamination from solvent or precursor molecules, which may be important for subsequent assays and/or applications. Furthermore, physical deposition is cheaper and faster and does not damage the environment 28-30.

Gold catalysts with selected support materials can efficiently promote many reactions that normally occur at much higher temperatures or with lower selectivity when catalyzed by other metals. It has been shown that AuNPs supported on metal oxides exhibit unique activity at room temperature because of a meta-stable interatomic bonding between gold atoms 31. It has been shown that substrates such as ceria 32, titania 33,34 and titanates 35 can stabilize AuNPs in which oxygen vacancies play an important role in the stabilizing effect. It is generally accepted that interfaces provide adequate sites for chemical reactions, and metallic Au surfaces are required as reservoirs for a determined reactant 36,37. Gold nanoparticles also play an important role in the development of chemical and biological sensors. A sensor usually has two functional components: one component selectively binds with the analyte, and the other provides transduction of this interaction into a detectable signal 38. Considering the chromogenic properties of molybdenum oxide on the one hand and the catalytic properties of gold nanoparticles on the other, a binary system that is formed by molybdenum oxide coated with gold nanoparticles could potentially be of use in the field of catalysis and/or in the fabrication of new specific sensors. For example, it has been found that the UV-light coloration performance of MoO3 thin films can be enhanced by modification of the MoO3 surface with gold nanoparticles 39. In this work, the synthesis and characterization of a AuNPs/MoO3 system is examined. To synthesize the support material, i.e., molybdenum oxide, a pure photochemical deposition method such as PMOD is used, followed by the synthesis of AuNPs using a DC-magnetron sputtering method. The synthesis of MoO3 thin films deposited onto Si(100) by PMOD, has been described at length in a previous publication from our group 40.

2. Materials and methods

2.1    General procedure

X-ray photoelectron spectra (XPS) were recorded on an XPS-Auger Perkin Elmer electron spectrometer Model PHI 1257 in an ultra-high vacuum chamber, with a hemispherical electron energy analyzer and an Al Ka X-ray source (hv = 1486.6 eV). The pressure of the spectrometer during data acquisition was maintained at 10-9 mbar. The binding energy (BE) scale was calibrated using adventitious carbon set to 284.8 eV. The accuracy of the BE scale was ± 0.1 eV. High resolution spectra were fitted using Gaussian-Lorentzian curves and a Shirley background 41. The approximate composition of the surface was determined for each element by dividing the individual peak area, after appropriate background subtraction, by their respective atomic sensitivity factors (ASF). XRD patterns were obtained using a Bruker D8 Advance diffractometer. The X-ray source was Cu 40 kV/40 mA. The surface plasmon resonance (SPR) effect of AuNPs on Mo oxide films was examined by solid-state UV-Vis spectrophotometry from 200 to 800 nm. References of barium sulfate (BaSO4) and a Mo oxide thin film deposited onto quartz were used. The equipment used was a Shimadzu UV-2450 spectrophotometer with an integrating sphere ISR-2200 and controlled by the UVProbe software version 1.10. First, the diffuse reflectance of the samples was measured; then, via the Kubelka-Munk mathematical transformation, all absorbances were obtained. Scanning electron microscopy (SEM) images were obtained on a LEO 1420VP at an accelerating voltage of 25 kV and coupled to an energy dispersive analysis instrument, Oxford 7424 model. For FE-SEM images a Leo Zeiss Supra 35-VP model was used with accelerating voltages of 15 kV and 2 kV. Samples were prepared by coating the molybdenum trioxide thin films with Au nanoparticles through the sputtering method with deposition times of 1, 2, 3, 4, 5 and 20 s. The total count of Au nanoparticles on the oxide was evaluated using SEM images. A histogram was built with the Origin 8 software with a Gaussian fit analysis. Mathematical treatments of the average sizes with standard deviations were calculated by this program.

2.2    Materials

A gold target (Quorum Technologies Ltd.) was cleaned with ethanol, acetone, and deionized water and dried at 80 °C prior to use. Molybdenum thin films were deposited onto Si(100) and borosilicate glass.

2.3    Synthesis of MoO3 thin films.

Post-annealing of the MoO3 films was carried out at 400 °C for 2 h under a continuous flow of synthetic air in a programmable Lindberg tube furnace and then allowed to slowly return to room temperature.

2.4    Synthesis of AuNPs on Mo^3 thin films.

AuNPs were deposited onto MoO3 thin films by Magnetron Sputtering with a DC diode sputtering single source. The equipment used was a Magnetron Sputter Coater Pelco SC-6 with a gold target (99.99%). Pre-deposited MoOthin films were introduced into the vacuum chamber at a pressure of 0.05 mbar. Argon was used for sputtering with an ionization current of 25 mA and the gold foil as the cathode. The deposition times were 1, 2, 3, 4, 5 and 20 s, and the MoO3 film was maintained at constant pressure.

 

3.    RESULTS    AND    DISCUSSION

3.1 Synthesis and characterization of Au NPs onto MoO3 thin films

The formation of AuNPs on the MoO3 surface was confirmed by monitoring the UV/Vis absorption band associated with the surface plasmon resonance (SPR). SPR absorption band measurements provide information about the presence and estimated size of the AuNPs on the oxide film. This method was used to evaluate the effects of different sputtering times on the deposition of gold NPs and to determine the optimum deposition time 128. Surface plasmon absorption spectra of spherical gold nanoparticles prepared by DC-sputtering for different deposition times are shown in Fig 1. As observed from the figure, the absorption spectrum maximum is shifted from 560 nm for NPs produced by 3 s of sputter depositing to 585 nm for NPs produced during a 5-s deposition.


Fig. 1 UV/Vis surface plasmon resonance absorption of gold nanoparticles deposited onto MoO3
thin films by DC-magnetron sputtering with different deposition times.

The bathochromic shift is also accompanied by a broadening of the band and an increase in the intensity, which is associated with an increase in the diameter and number of the gold particles. This could also result from NP aggregation or an interparticle coupling phenomenon produced by the proximity of metal NPs 42. As a consequence, NPs that are arranged side by side behave as a single, larger structure with a collective effect on the plasmon resonance. Longer sputtering times were evaluated; however, the surface plasmon effect was lost, likely because of an excess of AuNPs.

SEM studies showed that nanoparticles deposited under these conditions have an apparent spherical shape and were homogeneously dispersed on the surface layer of oxide (Fig. 2a). Increasing the time of sputtering produces an increase in the size of the NPs. The average size of the Au particles that formed using a 5-s deposition, as determined by a size distribution study of the SEM image, was measured to be 18 nm, with a relatively narrow size distribution (histogram Fig. 2c). SEM images and their respective histograms show dispersity of nanostructure sizes. Therefore, 5 s corresponds to the optimum deposition time. As previously discussed, the increase in bandwidth and the absorption band shift towards higher wavelengths are the result of a dipolar coupling between closely spaced particles 28,43,44. NP aggregations on the oxide surface are observed for samples that were subjected to 20 s of sputter depositing, as shown in the SEM image (Fig. 2b), and have an average particle size of 460 nm, as determined by the particle size distribution (histogram, Fig. 2d).


Fig. 2 SEM images of MoO3 films doped with AuNPs (a) deposition time = 5 s / Mag. = 100.000x (b) depos3tion time = 20 s / Mag. = 20,000x.
Histograms of particle size distribution for the AuNP/MoO3 films (c) deposition time = 5 s and (d) deposition time = 20 s

To obtain information concerning the chemical state of the AuNPs, the samples were characterized by XPS. The low-resolution XPS spectrum of a sample that contained 18-nm AuNPs (Fig. 3) shows the characteristic signals of O and Mo for the support and the signal of Au for the AuNPs, thus confirming the presence of metal in the film. The signal intensity was significantly lower for Mo and O than for Au, which was the predominant species in the test area. The presence of the C 1s signal is associated with the adsorbed carbon impurities on the surface of the film.


Fig.3 Wide scan XPS spectrum of an Au/moO3 thin film (sputtering time = 5 sec).

The high-resolution spectra of the Au 4f lines (Fig. 4a) show one doublet located at 83.9 and 87.5 eV, which correspond to Au 4f7/2 and Au 4f5/2, respectively. These values are in agreement with the data reported by other authors 45 and indicate that the chemical species present in the film corresponds to Au (0), without other satellite signals 45-49.

 

The crystal structure of the AuNPs was studied by XRD. The diffractogram of an Au/MoO3 film with 470 nm NPs (Fig. 5a) clearly shows the presence of orthorhombic a-MoO3 preferentially oriented along the [020] axis 55-57, but no signals attributed to the gold NPs can be observed. This is likely due to a combination of factors, including the small particle size, the small grain size, and the relatively low concentrations of Au relative to MoO3. However, an XRD diffractogram using a grazing incidence angle (Fig. 6b) shows diffraction peaks at 20 = 38.2° for the flat Au (111), 20 = 44.2° for Au (200) and 20 = 64.9° for Au (220), in accordance with the literature (JCPDS No. 04-0784). These results suggest that 470 nm AuNPs would exhibit a face centered cubic (fcc) structure 24,58.


Fig. 4 High-resolution XPS spectra of Au/MoO3 films (a) Au 4f and (b)

For the case of Mo 3d (Fig. 4b, Table 1) the predominant species (> 90%) corresponds to Mo (VI), with binding energies of 232.7 eV for Mo 3d5/2 and 235.7 eV for Mo 3d3/2 50-54. The presence of signals assigned to Mo(V) species at 231.6 eV for Mo 3d5^ and 234.4 eV for Mo 3d3^, could be explained by the over-irradiation of the photodeposited oxide or the catalytic action of the noble metal on the MoO3 surface. Finally, the O 1s peak shows the same composition as that determined for MoO3 films without Au doping, with an oxygen contribution from the oxide in the film and from adsorbed OH 41.


Table 1 Standard binding energies for Mo 3d photoelectron peaks associated with different oxidation states 50.

 


Fig. 5 XRD spectra of a MoO3 film (annealed at 400°C for 2 h) doped with 470 nm AuNPs deposited by magnetron sputtering, (a)
measured with the Bragg-Brentano instrumental function and (b) with grazing incidence.

3.2 Optical Properties

The variation in absorbance spectra of the MoO3 thin films doped with AuNPs is indicative of changes in the optical properties of the films. The optical band gap of the films can be estimated from the Tauc equation using their absorption spectra measured at room temperature. From solid-state band theory, the relation between the absorption coefficient a and the energy of the incident light hv is given by

where A is the probability parameter for the transition and Eg is the optical band gap energy. For allowed direct transitions, the coefficient n = 2, and for allowed indirect transitions, n = 1/2. The values of E are estimated from the intersection of the extrapolated linear part of the (ahv)2 curves with the energy axis (Fig. 6). The optical band gaps computed from the above relation are estimated to be 3.60 and 3.21 eV (when n = 2) for undoped and AuNP doped MoO3 films, respectively. The value of the optical band-gap energy for the undoped MoO3 film is in good agreement with the reported values for this non-stoichiometric oxide 53,54,57. Once MoO3 films are doped with AuNPs, the band gap decreases to 3.21 eV. A possible explanation is that the deposition of catalytic metal nanoparticles forms additional intermediate electronic states in the metal oxide, which reside between the top of the valence band and the bottom of the conduction band 13,59.


Fig. 6 Optical Band-Gap of MoO3 thin films undoped and doped with AuNPs.

4. CONCLUSIONS

The results of our studies demonstrate that the deposition of gold nanoparticles onto MoO3 films can be successfully achieved by a combination of PMOD and DC-magnetron sputtering methods. AuNPs with an average size of 18 nm formed after 5-s sputter depositing, and their size was directly related to the time used in the sputtering process. Increased deposition time caused significant particle growth, which reached 460 nm after a 15-s deposition. XRD using grazing incidence angle indicated that the Au in the Au/MoO3 films exhibited a monocrystalline face-centered cubic structure. The gold possessed a single oxidation state, Au (0), as determined by high-resolution XPS analysis. XRD analysis also showed the presence of orthorhombic a-MoO3 with a (020) preferential orientation. XPS surface analysis demonstrated the different contributions of Mo(VI) and Mo(V) oxidation states present in the thin film. Finally, gold doping caused a decrease in the MoO3 optical bandgap, which was probably due to the formation of intermediate electronic states. The gasochromic performance of the AuNP doped MoO3 films towards H2 gas is currently being investigated by our group. We expect that the deposition of Au nanoparticles on the MoO3 thin films will significantly improve the gasochromic properties of such films by increasing the sensitivity to H2, improving the dynamic optical performance, and lowering the operating temperature.

ACKNOWLEDGMENTS

This research was supported by FONDECYT, Chile (Project No.1110439) and Pontificia Universidad Católica de Valparaíso (Project D.I. No. 125.756/11). C. Castillo thanks CONICYT, Chile for a doctoral fellowship and doctoral thesis support (Project No 24110020). We also thank Dr. Michael Noyong of Institut für Anorganische Chemie at Aachen, for preparing the SEM images.

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