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Abstract
In this work, composite thin films based on titanium dioxide and noble metal nanoparticles (Ag, Au and bimetallic Ag/Au alloys) (Me-TiO2) were synthesized using the RF magnetron sputtering technique. The obtained thin films were characterized by TEM, SEM, XRD analysis, and Raman spectroscopy. It was observed that annealing in an argon atmosphere led to the crystallization of the initially amorphous as-deposited TiO2 matrix. The analysis of transmission spectra revealed that the composite thin films exhibited two light absorption regions: the first is local minima in the visible range associated with localized surface plasmon resonance (LSPR) phenomena; the second is light absorption due to the energy band gap. The study demonstrates the possibility of tuning these parameters in the composite films by changing the composition of the metal NPs. The LSPR minima for Ag-TiO2 and Au-TiO2 films were located at about 485 nm and 606 nm, respectively. In the composite thin films with bimetallic ∼Ag0.54/Au0.46 alloy nanoparticles, the position of the absorption peak was found to be at 555 nm. The energy band gap of these films also varies almost linearly, decreasing with an increase in the Au content, so that the largest value among the annealed Me-TiO2 composites was observed for the Ag-TiO2.
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1. Introduction
Oxide semiconductors play a vital role in the functionality of photo/electro-catalytic and optoelectronic devices. Titanium dioxide (TiO2) stands out among the various oxides due to its exceptional optical and chemical properties, as well as its cost-effectiveness. Titanium dioxide is a polymorphic compound and it exists in three crystalline modifications: anatase, rutile, and brookite. Rutile and anatase are extensively empoyed as additives in polymers, sunscreens, dyes, as well as water treatment and photocatalysis applications [1–3]. Moreover, TiO2 has gained significant attention in environmental solutions, solar energy devices, biomedicine, and other fields [4].
Despite its advantages, TiO2 has limitations, leading to recent research efforts focused on enhancing its photocatalytic characteristics. One approach involves doping TiO2 with impurities of different chemical natures. Plasmonic composites, which combine titanium dioxide with noble metal nanoparticles, particularly silver (Ag) and gold (Au), have attracted considerable interest [3]. These metal nanoparticles can modify the absorption properties of TiO2-based composites through localized surface plasmon resonance (LSPR) phenomena. Numerous studies have already demonstrated the enhanced photoactivity of such composites in both the UV and visible ranges [3,4]. Furthermore, such compositions can possess excellent antibacterial properties. Several works have shown that Ag NPs deposited on TiO2 nanotubes, thin films, or powders exhibit enhanced activity against a wide variety of bacteria. Additionally, Me-TiO2 composites are utilized as functional coatings for surface-enhanced Raman spectroscopy (SERS) [5,6].
It is well known that the plasmon resonance wavelength of nanoparticles is mainly determined by their size, shape, composition and local environment. Therefore, by controlling these factors, the plasmon resonance wavelength of nanoparticles can be tailored for specific applications. Considering the composition factor, bimetallic nanoparticles have attracted considerable attention in recent years due to their advantages over monometallic nanoparticles, such as increased stability and enhanced plasmon resonance with a tunable frequency that can be optimized, for example, according to the excitation wavelength of the laser used in SERS measurements.
Among bimetallic nanoparticles, the Ag/Au alloy is the subject of active theoretical studies and various practical applications. This combination of metals could provide both antibacterial properties (due to the Ag ions) and chemical stability (due to the Au). In addition, the composition of the Ag/Au alloy can be adjusted to tune the position of the LSPR absorption peak. There are many research works investigating the optical properties, especially the dielectric function, of bimetallic colloidal NPs by changing the Ag/Au ratio [7,8]. However, the synthesis and properties of these NPs in solid-state matrices, which are commonly prepared by methods such as PVD [9,10], laser ablation and ion implantation, are reported in only a few works [6,11]. Therefore, here we focused on the optical properties and structure of Ag/Au-TiO2 composite thin films containing NPs of noble metals (Ag, Au) and their bimetallic (Ag/Au) alloy synthesized by RF magnetron sputtering.
2. Thin film preparation
Deposition of thin films was performed using a 3-inch circular magnetron (Angstrom Science ONYX) in non-reactive RF (13.56 MHz) mode. A rutile target of 4N purity was used as the source of titanium dioxide (TiO2) thin films. To obtain metal-doped composite (Me-TiO2) thin films, the flat silver and gold pellets were placed on the erosion zone of the sputtered target. Sputtering was carried out in the atmosphere of high purity (99.999%) argon gas. The thin films were grown on the surface of pre-cleaned quartz glass, c-Si and KBr crystals. The geometrical and technological parameters of the deposition processes can be found in Fig. 1 and Table 1, respectively. All deposited samples were subsequently annealed at 450°C for 20 min in an argon atmosphere using CVD equipment. This process resulted in the formation of isolated metal NPs embedded in crystalline TiO2 matrices. Heating was carried out for 40 minutes, starting from room temperature and reaching 450°C, followed by natural cooling.
Table 1. Deposition parameters of the Me-TiO2 thin films
3. Characterization of thin films
The composition of the as-deposited pure TiO2 and composite Me-TiO2 thin films was analyzed using energy dispersive spectroscopy (EDS) with the EDAX attachment on the Quanta 3D 200i scanning electron microscope (SEM, FEI). The thickness of the films was determined by scanning the cross-section of the as-deposited thin films on the surface of silicon substrate (c-Si, (100), 5-10 Ohm).
The formation of metal nanoparticles (NPs) was confirmed using bright field transmission electron microscopy (TEM, JEM 100 JEOL) with an accelerating voltage of 80 keV. For TEM analysis, thin films were deposited on KBr salt plates. Next, the salt was dissolved in warm water, and the films were transferred onto TEM grids.
The structure of the Me-TiO2 composites was studied using Raman spectroscopy on a Solver Spectrum (NT-MDT) instrument with a 473 nm excitation laser source, covering the range from 50 to 1450 сm-1, with an exposure time of 100s. Additionally, X-ray diffraction (XRD) analysis was conducted using Rigaku X-ray analytical system equipped with a CuKα monochromator.
The transmittance spectra of the films deposited on transparent quartz substrates were measured using a Shimadzu UV-3600 spectrophotometer in the wavelength range from 240 to 1200 nm, with a 1 nm slit.
4. Results and discussion
Characterization of the thin films structure
The results of the EDS analysis and thickness measurements for pure TiO2 and composite Me-TiO2 films are presented in Table 2. The as-deposited pure TiO2 thin film on the Si substrate shows a slight oxygen deficiency. Scanning the cross-section of the TiO2/Si structure reveals a thickness of approximately 122 nm. Composite thin films, obtained through sputtering of a combined “Ag/Au pellets – on – rutile wafer” target, contain about 5-7 at.% of noble metals. Among all the synthesized films, the Ag-TiO2 composite exhibits the lowest thickness (∼105 nm) for the identical deposition conditions. The addition of Au pellets increases the deposition rate of the films. This is related to the non-reactive sputtering of the oxide target. In the process of sputtering in argon atmosphere, the erosion zone of the target may slightly change its composition due to the oxygen depletion, which can increase the sputtering rate of the target. The thicknesses of composite Ag/Au-TiO2 and Au-TiO2 thin films are approximately 117 nm and 128 nm, respectively. Figure 2 presents SEM images of the cross-section of the synthesized films.
Fig. 2. SEM images of the cross-section of TiO2 (a) and Me-TiO2 thin films deposited on the c-Si substrate.
Table 2. Thicknesses and elemental composition of the as-deposited and annealed films
TEM analysis revealed the presence of metal nanoparticles in the as-deposited Me-TiO2 composite thin films, as illustrated in Fig. 3. Interestingly that composites with pure Ag and Au metals exhibit larger NPs sizes than composite thin films with Ag/Au alloy NPs. According to the distribution analysis using the ImageJ program [12], the mean size of Ag and Au nanoparticles was approximately ∼5 nm, while the mean size of bimetallic Ag/Au nanoparticles in the composite films was smaller, approximately 3-4 nm. The TEM and SEM images used for NP size determination are shown in Supplementary Information in Fig.1(S) and Fig.2(S). Figure 4 displays the obtained histograms representing the size distributions, with the lines in the histograms corresponding to a lognormal distribution.
The structure of the TiO2 matrix was determined by Raman spectroscopy. The Raman spectra of TiO2 and Me-TiO2 as-deposited thin films recorded at 473 nm excitation are shown in Fig. 5(a). The spectrum with broad peaks at about 400 cm-1 and 600 cm-1 corresponds to the amorphous phase of TiO2 thin films [13–15]. It is interesting to note that the addition of small amounts of Ag/Au to the TiO2 matrix leads to the disappearance of the typical amorphous TiO2 peaks, apparently indicating an even more disordered structure (Fig. 5(a)). In addition, in the case of composite films, there is a slight trend below 200 cm-1, which is often observed in the systems containing noble metals, especially Ag/Au NPs in metal oxides. There is no certain explanation for a broad peak at about 750 cm-1 in the case of Ag-TiO2 film, however, some researchers attribute it to the coupled LO-phonon-plasmon modes of TiO2 thus indicating the presence of free charge carriers that activate low-intensity or even silent Raman modes in the resonant regime [16–18]. However, this peak may also correspond to bending vibrations of the O-Ag-O bond in Ag2O [19].
It is well known, that annealing promotes the crystallization of amorphous TiO2 films [20,21]. In this work, we performed post-synthesis thermal annealing (450°C, Ar atmosphere, 20 min) to make TiO2 matrix crystalline and to promote further coalescence of Ag and Au atoms into NPs. As a result, the amorphous structure of titania was transformed into anatase phase. The Raman spectrum of annealed TiO2 thin film has peaks at about 143 cm-1, 394 cm-1 (B1g), 514 cm-1 (A1g) and 633 cm-1 (Eg), corresponding to anatase vibrational modes [22,23]. The Raman spectra of the composite Me-TiO2 thin films also show an anatase structure but with slightly shifted peaks apparently due to the distortions in cell parameters induced by the presence of the metal NPs and/or metal ions. The largest discrepancy is observed for Ag-TiO2 film, where the most prominent peak corresponding to Eg mode is shifted from 143 to 148 cm-1. In addition, several studies have shown that the presence of silver may inhibit the crystallization of TiO2 films and change lattice parameters [20,23].
The results of XRD studies of as-deposited and annealed thin films are shown in Fig. 6. Analysis of the XRD patterns also revealed a transition from the amorphous structure of the as-deposited films to the anatase phase after annealing. While the XRD patterns of the as-deposited thin films show no peaks (see Fig. 6(a)), the annealed thin films exhibit a peak at about 25 degrees (2 theta), which corresponds to X-ray reflection from (101) plane of anatase according to JCPDS Card No. 21-1272. However, a detailed analysis of this peak (Fig. 6(c)) revealed slight differences in its position and FWHM depending on noble metal content. The largest discrepancy is again observed for the Ag-TiO2 film, indicating the most significant changes in cell parameters, which was also shown by Raman studies. Furthermore, the crystallite size of thin films determined by the Scherer equation indicates that Ag-TiO2 film has the smallest crystallite size of ∼46 nm. For comparison, the remaining films have crystallite sizes of about ∼63-65 nm. The results of the detailed analysis of the XRD peak ((101) plane of anatase) are shown in Table 3. The peaks of Ag and Au were not detected because their concentrations were rather low and the thickness of the films was small.
Fig. 6. XRD patterns of (a) as-deposited and (b) annealed TiO2 and Me-TiO2 thin films; (c) high resolution XRD (101) peak of anatase.
Table 3. Analysis of XRD data ((101) plane) of the synthesized TiO2 and Me-TiO2 films
Optical properties of thin films
The optical properties of the prepared composites were analyzed using the transmission spectra of as-deposited and annealed films shown in Fig. 7(a) and (b), respectively. As can be seen in Fig. 7(a), the as-deposited Ag-TiO2 composite film exhibits a minimum at approximately 471 nm due to LSPR, while the Au-TiO2 composite film shows resonance absorption near 560 nm. The composite thin film with Ag/Au alloy NPs does not exhibit a distinct LSPR minimum, despite the presence of isolated NPs in the films after deposition. This possibly means that in as-deposited thin films, there are bimetallic Agx/Au1-x NPs with different ratios ranging from x = 0 to x = 1. On the other hand, there is a well-known dependence of the efficiency of LSPR absorption on the size of the plasmon nanoparticles, which in our case is the lowest for Ag/Au alloy nanoparticles. Thus, a simpler explanation can be found in the overlap of the interference curve of the composite film with the LSPR minimum of Ag/Au NPs. In addition, the Ag-TiO2 thin film demonstrates the lowest transmittance in the visible range which can be attributed to the presence of larger nanoparticles which contribute to the light scattering. It should be noted that interference minimum can overlap with the LSPR minima, however, the transmittance spectra of AgNP-TiO2 thin films with thickness <100 nm unambiguously confirm the plasmonic absorption of the as-deposited films (see Supplementary Information, Fig.3(S)). The reflectance spectra of as-deposited and annealed films can be found in Supplementary Information, Fig. 4(S).
As mentioned earlier, annealing leads to the crystallization of TiO2 matrix. Figure 7(b) further demonstrates that annealing induces changes in the transmission spectra of the films, partly due to the coalescence of the metal nanoparticles [24]. The temperature increases the diffusion to the surface which promotes the formation of larger metal crystals. As shown in Fig. 8, on the surface of annealed thin films large NPs have appeared. The mean size of these NPs according to the inserted histograms is approximately ∼20 nm for all composite thin films. It should be noted that the formation of NPs on the surface of TiO2 thin films enhances the functionality of these coatings in various applications, such as SERS, antibacterial protection, and electrochemical processes.
The annealed composite thin films exhibit distinct LSPR minima. For Ag-TiO2 the minimum is located at 485 nm. The redshift with respect to the as-deposited film is attributed to the increase in nanoparticle sizes. The LSPR minima for bimetallic Ag/Au-TiO2 and Au-TiO2 annealed thin films are observed at 555 nm and 606 nm, respectively. In the case of Ag/Au-TiO2 composite thin film, the observation of a single LSPR minimum suggests the formation of NPs of alloy with almost the same Ag/Au composition ratio [25]. The reason for the red shift of the bands is the same as for Ag-TiO2. Figure 9(a) demonstrates the positions of the LSPR minima for as-deposited and annealed Me-TiO2 composite thin films along with the results of the theoretical evaluation of the LSPR minima positions using the Mie theory [26,27]. The calculations were performed using the determined NPs size distribution (see Fig. 8), while the refractive index (RI) of anatase matrix was taken from Ref. [28] and dielectric permittivity of metals was given by the following expression [29,30]:
Fig. 9. Results of analysis of optical transmittance spectra: (a) - calculated and experimentally observed LSPR minimum positions; (b) - determined band gap energies for indirect allowed electron transition of as-deposited and annealed composite Ag1-xAux-TiO2 thin films with different values of x.
Here, ${\varepsilon _{bulk}}(\omega )$ denotes the dielectric function of bulk metals, which is obtained from the RI of the corresponding metal using data from Johnson and Christy in Ref. [31], ${\gamma _{bulk}}$ and ${\omega _p}\; $ are damping and plasmon frequencies, respectively. The size effect in Eq. (1) is taken into account by adding the modified damping frequency $\Gamma=\gamma_{\text {bulk }}+\eta v_F / R$ in the complex permittivity of metals, where ${\upsilon _F}$ – Fermi velocity, R – radius of NPs, and $\eta $ is a geometric factor equal to 1. The following Drude parameters were used for the calculation ${\upsilon _F}\sim 1.4 \times {10^6}$ m/s, $\hbar {\omega _{p({Au} )}} = 8.89$ eV, $\hbar {\Gamma _{\left( {Au} \right)}} = 0.07$ eV, $\hbar {\omega _{p({Ag} )}} = 9.04\; $ eV, $\hbar {\Gamma _{Ag}} = 0.02$ eV. Dielectric permittivity of bimetallic NPs was determined as: ${\mathrm{\varepsilon }_{bimet}}(x )= x{\varepsilon _{Au}} + ({1 - x} ){\varepsilon _{Ag}}$, where x is molar fraction of Au content.
As can be seen in Fig. 9(a), calculations using the above parameters yield the LSPR minima centered at the larger wavelengths. The discrepancy in the resonance position for Ag/Au-TiO2 and Au-TiO2 is apparently caused by the lower value of the real refractive index of the media due to the presence of NPs on the surface of the films. However, a significant blueshift in the case of Ag-TiO2 is attributed to the lower RI of the matrix due to porosity, rather large surface NPs and/or possible presence of silver oxide phase.
In addition, all annealed thin films demonstrate plasmonic coloration due to resonance absorption. The visible colors of the annealed films in transmitted light are shown on the right side of Fig. 7(b). While the annealed undoped TiO2 thin film has good transparency and no color, the composite thin films exhibit red-brown (Ag-TiO2), violet (Ag/Au-TiO2) and blue (Au-TiO2) coloration. In reflected light the colors are almost the same.
As can be seen from Fig. 7, both the as-deposited and annealed thin films exhibit an absorption edge in the transmittance spectra, which is typical for semiconducting materials. Therefore, using the optical transmittance spectra it is possible to determine the energy band gap of the pure TiO2 and Me-TiO2 composite thin films. Here, the absorption spectra were derived as $\alpha = \textrm{log}({1/T} )/d$, without taking into account the reflection of the films.
Linear interpolation of the Tauc plots for the indirect allowed transition shows that the energy band gap is about ∼3.3 and ∼3.2 eV for as-deposited and annealed TiO2 thin films, respectively. For composite thin films thermal annealing leads to an increase in the energy band gap. The energy band gap values determined for Me-TiO2 composite thin films are shown in Fig. 9. The largest value among the annealed composites is observed for Ag-TiO2 and it decreases with increasing Au content. In general, the obtained values of energy band gap are typical for TiO2 in anatase phase.
5. Conclusion
This work presents study of the composite thin films based on TiO2 matrix and noble metal NPs, including Ag, Au and their alloys Ag/Au synthesized using RF magnetron sputtering. The presence of metal NPs was confirmed by TEM analysis. The particle size distribution shows that the as-deposited Ag-TiO2 and Au-TiO2 films contain NPs with predominately larger size compared to Ag/Au-TiO2 films. The TiO2 matrices of all as-deposited films showed amorphous structure according to XRD and Raman analysis. Thermal annealing at 450°C in an argon atmosphere resulted in the appearance of larger NPs (mean size ∼20 nm) on the surface of Me-TiO2 composite thin films and the transformation of the TiO2 matrix from amorphous phase to anatase. Annealed composite thin films exhibit tunable LSPR absorption. The LSPR minima for Ag-TiO2, Ag/Au-TiO2 and Au-TiO2 films are located at about 485, 555 and 606 nm, respectively. In addition, the comprehensive study of Me-TiO2 composite thin films showed that among the composite thin films, annealed Ag-TiO2 had significantly smaller crystallite size and higher energy band gap. The composite thin films obtained in this study are believed to have potential applications such as optical filters, decorative coatings as well as antibacterial agents and SERS substrates.
Funding
Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan (АР09258922).
Acknowledgment
The authors acknowledge the Science Committee of the Ministry of Science and Higher Education of the Republic of Kazakhstan for the financial support under Research Grant No. АР09258922.
Disclosures
The authors declare no conflicts of interest.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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