Structure and Composition of Titanium Nanocluster Films Prepared by

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Structure and Composition of Titanium Nanocluster Films Prepared by a Gas Aggregation Cluster Source M. Drabik,*,† A. Choukourov,† A. Artemenko,† O. Polonskyi,† O. Kylian,† J. Kousal,† L. Nichtova,† V. Cimrova,‡ D. Slavinska,† and H. Biederman† † ‡

Charles University in Prague, Faculty of Mathematics and Physics, V Holesovickach 2, 180 00 Prague 8, Czech Republic Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsky Sq. 2, 162 06 Prague 6, Czech Republic

bS Supporting Information ABSTRACT: Structure and composition of titanium nanocluster films were studied on samples prepared by a gas aggregation cluster source based on a planar magnetron following the Haberland concept. Elemental composition of the nanoclusters was studied by X-ray photoelectron spectroscopy (XPS), their crystal structure was determined by X-ray diffraction, and their optical properties were characterized by UV
vis spectroscopy. Both as-prepared (without any additional postdeposition treatment) and annealed films were studied. Annealing of selected nanocluster films was done in air at 420 °C for 1 h followed by a natural cool-down. Titanium nanocluster films were found to oxidize immediately when exposed to ambient atmosphere. The as-prepared nanoclusters were composed of both Ti and TiO2. The nanoclusters undergo further oxidation in time and their structure and properties change. The transformation from a metal to an oxide structure of the nanoclusters upon annealing ends up in a semistable anatase crystal form of TiO2 with traces of a thermodynamically stable rutile crystal form.

1. INTRODUCTION Nanostructured powders and films composed of ultrafine crystals with dimensions of several to tens of nanometers have received an increased attention in the past decade due to their unique chemical, physical, mechanical, electrical, and optical properties compared to bulk materials, which result from their dimensions. Titanium dioxide (TiO2) is a good example.1 Titania nanoparticles have been studied for prospective novel applications, for example, in biomedicine,2 environmental purification,3,4 or photovoltaics.5 Apart from the dimensions of the particles and the surface morphology, chemical composition and crystal structure also are important for the effectiveness of the above applications. TiO2 naturally occurs in three crystal structures: tetragonal anatase, rutile, and orthorhombic brookite. Irrespective of the crystalline structure, the Ti4+ ions are surrounded by six O2
ions that create a TiO6 octahedral basic block. The symmetry of anatase is lower than that of rutile. These differences in the lattice structures cause differences in mass densities and electronic band structures between the two forms of TiO2.6 Depending on the particular deposition process, preparation conditions, impurities, crystal dimensions, etc., the amorphous-anatase transition occurs at about 400 °C. The anatase form is stable up to around 800 °C, and above this temperature it transforms into the rutile phase.7,8 Also it can be assumed that the transformation of crystal structure of TiO2 nanoparticles is dependent on the temperature and the length of the annealing process.9 r 2011 American Chemical Society

Different gas phase methods are utilized for production of metal nanoclusters (e.g., review articles10,11). Milani and coworkers showed that TiO2 nanoparticles can be produced by annealing of titanium nanoparticles prepared by supersonic cluster beam deposition.12
14 Deposition of titanium nanoclusters and theoretical description of cluster formation was studied by Shyjumon and co-workers.15 Our previous study was focused on the morphology of thin films composed of titanium nanoclusters prepared by means of a gas aggregation cluster source under various experimental conditions. The cluster source utilized was based on a planar magnetron following the Haberland concept.16 We showed that the size of the nanoclusters and its distribution and overall morphology of the films is very stable, and no changes were observed even 7 months after their deposition.17 However, this might not be true for the chemical composition and crystal structure of the titanium nanocluster films as our preliminary research into the aging of the films suggested.18 The long-term stability of materials is crucial for any industrial application and among others also determines handling precautions. A detailed description of the composition and structure of titanium nanoparticle films and their changes upon aging and annealing are therefore the focus of this study.

Received: June 24, 2011 Revised: September 19, 2011 Published: September 22, 2011 20937

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2. EXPERIMENTAL SECTION The titanium nanocluster films were prepared using a gas aggregation cluster source based on a planar magnetron following the Haberland concept.16 DC power source was connected to a magnetron with a Ti target (purity 99.3%). The gas aggregation chamber attached to the housing of the magnetron was cooled by water. Argon (purity 99.99%) was used as a working gas at a pressure that was controlled by changing the input flow rate of the gas. Ti nanoclusters formed in the aggregation chamber were collected on a substrate positioned in front of the chamber orifice in a vacuum deposition system pumped by diffusion and rotary pumps. Titanium nanoclusters were prepared dependent on either the magnetron current or the argon pressure inside the cluster source. A more detailed description of the experimental setup can be found in ref 17. Film characteristics were studied at different time intervals after the preparation procedure during which the samples were stored in ambient air at room temperatures to describe any changes in their structure, composition, and optical properties. All metal nanocluster films were introduced to ambient atmosphere after deposition during their transfer to various characterization systems. Selected nanocluster films were annealed in air at 420 °C for 1 h followed by a natural cool-down. Different methods were utilized to fully characterize the titanium nanocluster films dependence on particular experimental conditions, namely, the current delivered to the magnetron and the aggregation chamber pressure and overall aging of the films. A transmission electron microscope (TEM, Jeol 2000FX) working at 200 kV was utilized for characterization of the size of nanoclusters. For the TEM characterization, a sub-monolayer of nanoclusters was deposited on copper grids with carbon foils. Scanning electron microscope (SEM, Tescan Mira II) working at 30 kV equipped with detectors of secondary electrons (SE) and backscattered electrons (BSE) was used to observe the morphology of the resulting films deposited on polished silicon substrates including the estimation of the size of nanoclusters. X-ray photoelectron spectroscopy (XPS, Specs Phobios 100) was utilized to determine the elemental composition of the film surface using Al Kα X-ray source (1486.60 eV) at the incidence angle of 45° to the sample surface plane. Pass energy of 40 eV for survey scans and 10 eV for high resolution (core level) scans was used for all measurements. For the purpose of elemental analysis, the films were prepared on polished silicon substrates. The XPS measurement was performed without any postdeposition modification of the sample surfaces, and no correction of the XPS data for surface roughness was done. The obtained XPS spectra were charge referenced to adventitious aliphatic carbon at 285.0 eV. Atomic concentrations were determined from peak areas corrected for photoelectron cross sections. The error bars of elemental ratios were derived from the relative error of determination of atomic concentration, which is estimated to be 2%. A 30:70 Gaussian-to-Lorentz curve fitting was used for deconvolutions of the high resolution spectra of elements according to data from X-ray Photoelectron Spectroscopy database of National Institute of Standards and Technology (NIST).19 The error bars of bonding ratios were derived from the relative error of determination of relative bonding state concentration from the core spectra, which is estimated to be 10%. The crystal structure of Ti nanoclusters deposited on polished silicon substrates was studied by X-ray diffraction (XRD). Panalytical X’Pert MRD diffractometer in the parallel beam geometry with an X-ray mirror was utilized. Cu Kα radiation (1.54056 Å) was used at an angle of incidence 1°. Crystal structure

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of the samples was matched to the database files provided by the International Centre for Diffraction Data (ICDD).20 Optical properties of the nanocluster films deposited on quartz glass substrates were characterized using an ultraviolet
visible spectrophotometer (UV
vis) Perkin-Elmer Lambda 35. Specular transmittance of the films was measured over a wavelength range of 190
900 nm. Reflectance of the films was measured using integrating sphere Labsphere RSA-PE-20 with samples positioned at an angle of incidence 8° behind the integrating sphere. Also transmittance of the films was measured using the same integrating sphere with samples positioned at an angle of incidence 0° in front of the integrating sphere. Certified reflectance standard Spectralon USRS-99-010 was used as a light spectral reference in all measurements using the integrating sphere. Transmittance and reflectance spectra of the samples studied using the integrating sphere were measured over a wavelength range of 250
900 nm. The absolute measurement error for the measurement of specular transmittance was estimated to be 0.5%, while the measurement error for the characterization using integrating sphere was estimated to be 2%.

3. RESULTS AND DISCUSSION Titanium nanocluster films were prepared with dependence on either the magnetron current (0.1
0.5 A) or the argon pressure inside the cluster source (50
200 Pa). However, most of the characterization was focused on the clusters prepared at 100 Pa and either 0.1 or 0.5 A. These experimental conditions were chosen because they lead to formation of considerably different nanoclusters as was shown in our previous study.17 Typical TEM micrographs of such nanocluster films can be seen in Figure 1. As can be seen, larger clusters (agglomerates) with a modal diameter of about 30 nm are formed at the lower magnetron current (Figure 1a), while smaller clusters with modal diameters of about 11 nm are formed at the higher current (Figure 1b). The size of clusters also affects their structure as we will discuss further in section 3.1. 3.1. Elemental Composition of the Films. Elemental composition and chemical structure of the surface of titanium nanocluster films was studied by XPS. Typical high resolution spectra are displayed in Figure 2 for a case study of as-prepared titanium nanocluster film deposited at a cluster source pressure of 100 Pa and magnetron current of 0.5 A as measured right after the deposition process and transfer to the analysis chamber through ambient air. The as-prepared titanium nanoclusters are partly oxidized and partly contaminated by carbon/hydrocarbon contamination which can be partly oxidized as well. The most complex is the doublet spectrum of Ti 2p photoelectron peak region (Figure 2a). The spectrum consists of two major peaks Ti 2p3/2 and Ti 2p1/2, which is caused by spin
orbit splitting.21 The deconvolution of the Ti 2p photoelectron peak showed that (at least) the top clusters on the film surface are formed by a metallic core (Ti, 454.1 eV) encapsulated by a shell of titanium oxides and titanium carbides (Ti
C, 455.5 eV). The presence of various titanium oxides points out that the nanoclusters in films are in various stages of the oxidation process (aging) which started right after the extraction of the sample to the ambient air and is still not completed. The most intense peak (greatest amount) holds the stoichiometric titanium dioxide (TiO2, at 458.7 eV), but we also can detect substoichiometric titanium oxides (TiOx, 457.0 eV), namely, titanium monoxide (TiO) and other titanium oxides (Ti2O3 and Ti3O5), which yields 1 e x < 2.22 It has already 20938

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Figure 1. TEM micrographs of Ti nanoclusters (magnification 100 000) prepared at a cluster source pressure of 100 Pa and magnetron currents of either 0.1 A (a) or 0.5 A (b).

been shown before that the fraction of metallic titanium in the clusters depends on the magnetron current, that is, the size of the clusters.17 The analysis of high resolution spectra of C 1s photoelectron peak reveals the nature of the carbon contamination. Generally, this contamination can originate either in the deposition process as a result of chemical reactions of hydrocarbon vapors from the oil pumping system with titanium in plasma or be acquired after the extraction of the nanoclusters to ambient air. In this case, the contamination is expected to be adsorbed without any chemical bonding to titanium due to the preferred reaction of titanium with oxygen. In Figure 2b, we can see that carbon is partly chemically bonded to titanium clusters (C
Ti bond at about 281.9 eV). It is highly probable that this represents the hydrocarbon contamination from the deposition process. However, most of the carbon is bonded in C
C/C
H backbone chains (C
C, 285.0 eV) which is most probably the contamination adsorbed from air after the deposition (dust, bacteria, etc.). The hydrocarbon contamination layer is also partly oxidized (C
O at about 286.4 eV or CdO at about 288.9 eV). However, most of the oxygen is primarily bonded to titanium (Figure 2c), forming titanium dioxide (TiO2, 530.4 eV) with a minor contribution from titanium monoxide (TiO) and other substoichiometric oxides which are together denoted as TiOx (532.8 eV). The general analysis of chemical composition of the titanium nanocluster films described above can be applied to all of the studied samples. However, the actual ratios of the respective elements (C/Ti and O/Ti) slightly change according to the deposition conditions (cluster source pressure and magnetron current) and

Figure 2. High resolution XPS of as-prepared titanium nanocluster film: Ti 2p (a), C 1s (b), and O 1s (c) photoelectron peaks. Spectra with corresponding fits and peak positions [eV] are displayed.

also over time (Figure S1, Supporting Information). The time evolution of the C/Ti and O/Ti elemental ratios can be viewed in Figure 3. Titanium nanoclusters were prepared at a cluster source pressure of 100 Pa and two different magnetron currents: 0.1 or 0.5 A. The greatest changes in the chemical structure of the films occur soon after the deposition process and extraction of the films to ambient atmosphere. Both C/Ti and O/Ti elemental ratios increase reflecting the oxidation of titanium and adsorption of hydrocarbon contamination from air (and its oxidation). After this early stage, 20939

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Figure 3. Time evolution of elemental ratio (C/Ti and O/Ti) of titanium nanocluster films prepared at a magnetron current of 0.1 or 0.5 A and cluster source pressure of 100 Pa as measured on as-prepared and annealed samples. Points are interconnected for a better readability of the graphs.

saturation is reached with minimal changes in the elemental ratios for a relatively long time of 5 weeks. Another dramatic change in the structure occurs after the annealing process. The C/Ti elemental ratio is decreased about five times in the case of bigger clusters (0.1 A) and about three times in the case of smaller clusters (0.5 A). This decrease is connected with thermal decomposition and removal of hydrocarbon contamination at high temperature. Nevertheless, some carbon contamination still remains on the surface (or is newly adsorbed) after the annealing process. The O/Ti elemental ratio also slightly decreases which is again connected with the removal of the oxidized hydrocarbons. At this stage after the annealing, the titanium nanoclusters are expected to be fully oxidized to stoichiometric titanium dioxide in a thermodynamically stable (or semistable) crystal structure. This will be discussed in detail in section 3.2. Further aging of the nanoclusters in ambient air at room temperature is thus connected mainly with adsorption of more hydrocarbon contamination from air and its oxidation, as suggested by an increase in C/Ti and O/Ti elemental ratios as measured two weeks after the annealing. These presumptions will be further discussed below based on an analysis of high resolution XPS. Despite the similarities in the evolution of elemental ratios for the clusters prepared under different magnetron currents, the actual values of the ratios differ for the respective samples. This is most probably connected with the differences in the size of the clusters (surface area of each of the clusters) and roughness of the films (total surface area of the films). The larger the surface areas of clusters and films, the higher the cluster oxidation and higher adsorption probability of the contamination can be expected. The nanoclusters prepared at current 0.1 A are bigger with a large amount of particle agglomerates (average size ∼ 30 nm), while clusters prepared at 0.5 A are smaller with a narrower diameter distribution (average size ∼ 11 nm).17 In addition to the elemental ratios, also the time evolution of the bonding ratios obtained from fitting respective high resolution XPS of the same samples was studied for the clusters prepared at 100 Pa and 0.5 A (Figure 4). As can be seen from the analysis of Ti 2p photoelectron peak, the relative amount of all bondings of titanium gradually decreases with respect to the amount of titanium dioxide, that is, the amount of oxygen in the films. It can be seen that all the clusters are almost completely oxidized to TiO2 and other titanium bondings are almost zero after the annealing and do not increase in time. The analysis of O

Figure 4. Time evolution of high resolution XPS fits of photoelectron peaks Ti 2p (top), O 1s (middle), and C 1s (bottom) as measured on asprepared and annealed titanium nanocluster films prepared at a magnetron current of 0.5 A and cluster source pressure of 100 Pa. Points are interconnected for a better readability of the graphs.

1s photoelectron peak is not that straightforward. Generally, it can be seen that most oxygen is bonded to titanium in stoichiometric titanium dioxide. The relative amount of oxidized hydrocarbon contamination strongly varies and might be caused by a random presence of the surface contamination. The amount of substoichiometric oxides decreases as the aging and oxidation proceeds over time. The relative amounts of all bonds other than TiO2 decrease below 10% after the annealing. Detailed analysis of C 1s region shows that a certain amount of carbon is bonded to titanium. This carbon is expected to be atomic or a fragment of small organic molecules that originate in the plasma deposition process. The highest relative amount of C
C suggests that most carbon contamination is composed of larger organic adsorbents on the surface of the films in various states of oxidation. Almost no chemical bonding of carbon to titanium can be observed after the annealing process, and thus the hydrocarbon contamination 20940

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Table 1. List of Diffraction Angles (2θ) with the associated Miller Indices (h k l) titanium

anatase

2θ [deg] (h k l) 2θ [deg]

Figure 5. Diffractograms of titanium nanocluster films prepared at a magnetron current of 0.5 A and cluster source pressure of 100 Pa measured at different times after the deposition. The measured diffraction patterns are compared to diffractograms of metallic titanium (T), anatase (A), and rutile (R) crystals.

remains present only via surface adsorption, as was anticipated based on the elemental analysis of the survey spectra above. We can thus conclude that despite almost no changes observed either in the size or morphology of the titanium nanoclusters during aging,17 the chemical structure of the clusters changes substantially when the samples are stored under standard room conditions. This is due to the nature of titanium which is very reactive and readily forms oxides and carbides.23 Similar results were obtained also for bigger clusters prepared at 100 Pa and 0.1 A (Figure S2, Supporting Information). 3.2. Crystal Structure. The crystal structure of the nanocluster films was determined by XRD. The obtained diffractograms of Ti nanoclusters prepared at 0.5 A and 100 Pa measured at different times after the deposition is displayed in Figure 5. Both as-prepared and annealed films were studied. By comparing the diffraction patterns to database files,20 metallic titanium and two crystal forms of titanium dioxide (anatase and rutile) were identified. The observed diffraction angles (2θ) with the corresponding reflection planes are listed in Table 1. On the basis of the observation of appearance and disappearance of particular diffraction peaks characteristic for respective crystals, we can conclude that the asprepared nanoclusters are mostly metallic titanium nanoparticles in hexagonal crystal structure. This observation corresponds to the TEM-SAD (transmission electron microscope selected-area diffraction) analysis performed previously.17 The metallic titanium nanoclusters spontaneously start to oxidize in air (as witnessed by XPS) and slowly transform into titania. The full transformation from Ti into crystalline TiO2 is accelerated and completed after the annealing process.24 The final structure is a mixture of both anatase and rutile crystal forms (Figure 5). Also, a weak but wide hump observed in the diffractogram of annealed nanocluster at about 31° could possibly be attributed to brookite (211) diffraction plane. However, the presence of brookite structure could not be confirmed by observation of some other diffraction planes at 25.4° (210) and at 25.7° (111) due to their overlapping by the intensive anatase (101) diffraction. These results are slightly different from the observations by Kholmanov et al.,13 who reported a mixture of anatase and rutile crystals prepared by supersonic cluster beam deposition only after annealing at 600 °C with a weight fraction of rutile 5%. However, the size of their crystals was about 40 nm which might be the cause of the difference.

rutile

(h k l)

2θ [deg]

(h k l)

35.1

(1 0 0)

25.4

(1 0 1)

27.5

(1 1 0)

40.7

(1 0 1)

37.9

(0 0 4)

36.2

(1 0 1)

63.0 70.3

(1 1 0) (1 0 3)

48.0 54.6

(2 0 0) (1 0 5)/(2 1 1)

41.5 54.6

(1 1 1) (2 1 1)

62.9

(2 0 4)

56.5

(2 2 0)

75.2

(2 1 5)

69.4

(3 0 1)/(1 1 2)

Table 2. Size of Nanoclusters Prepared at a Magnetron Current of 0.5 A and Cluster Source Pressure of 100 Pa as Estimated from Measurements by Different Characterization Techniques Ti nanocluster film as-prepared

(11 ( 2) nm

TEM

SEM

(14 ( 4) nm

(11 ( 3) nm

XRD

(Ti) annealed

anatase

(16 ( 4) nm

(13 ( 3) nm

(TiO2) rutile

(9 ( 3) nm

Average sizes of nanocrystallite grains D in the films prepared at a magnetron current of 0.5 A and cluster source pressure of 100 Pa were estimated from the diffractograms using Scherrer equation:25 D¼

0:9λ B cos θ

ð1Þ

where λ is the wavelength of the X-ray radiation, B is the broadening of the diffraction peak measured at half-maximum intensity (fwhm), and θ is the Bragg diffraction angle. Diffraction of the titanium (100) plane at 2θ = 35.1° was used for calculating the size of the as-prepared nanoclusters, while diffractions of anatase (101) at 2θ = 25.4° and rutile (110) at 2θ = 27.5° were used for calculating the size of the annealed nanoclusters. The size of titanium nanocrystallites was estimated to be about 11 nm, anatase crystallites about 13 nm and rutile crystallites about 9 nm after the correction for the instrument-related broadening in the parallel beam geometry (2θ = 0.3°) (see Table 2). All these values (except for rutile particles) correspond within the measurement error to the average size of nanoclusters estimated from electron microscopic measurements. The size of as-prepared titanium nanoclusters estimated from TEM measurement was 11 nm and from SEM measurement was 14 nm. The size of annealed Ti nanoclusters (TiO2) was estimated from SEM measurement at 16 nm.17 In the mentioned TEM characterization, it was concluded that the as-prepared nanoclusters (prepared at 0.5 A and 100 Pa) are mostly monocrystals, and thus the crystallite size estimated from the XRD measurement should correspond to the size of the nanoclusters estimated from TEM measurement. This assumption was confirmed. Nevertheless, it has to be noted that the size of the nanoclusters determined from the XRD measurement represents a lower bound on the nanoparticle size. The main source of error lies in the determination of the diffraction peak width (fwhm). It is (apart from the instrument-related 20941

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which are almost transparent (%TA ≈ 85
90). Total (diffuse and specular) transmittance of the films measured using integrating sphere corresponded (within a measurement error) to specular transmittance over the conjunct spectral ranges and is therefore not shown here. On the other hand, only a very limited difference of the order of a measurement error (∼2%) was observed in the reflectance spectra of both as-prepared and annealed films. A very low reflectance (%R ≈ 5
10) was observed on all the nanocluster films. This can be seen in Figure 6b where changes in the UV
vis transmittance and reflectance spectra as measured at different times after the deposition and after the annealing are displayed. The values in the graphs were taken at 450 nm of the respective transmittance and reflectance spectra at a given time. In order to illustrate the structure of the deposited titanium nanocluster films, the relation of the transmittance, reflectance, and absorbance of light interacting with a matter can be written as T þ R þ A¼1 where T stands for transmittance, R is reflectance, and A is absorbance. Both transmittance and reflectance can be divided into specular (S) and diffuse (D): TS þ TD þ RS þ RD þ A ¼ 1

Figure 6. Transmittance and reflectance spectra of Ti nanocluster film deposited at a magnetron current of 0.5 A and cluster source pressure of 100 Pa on a silica glass substrate. Thickness of the films in the measurement spot was 100 nm. Actual UV
vis spectra of as-prepared and annealed film compared to transmittance of a quartz substrate (a) and time evolution of values of transmittance and reflectance at 450 nm (b) are displayed.

broadening) affected also by other factors such as crystallite size or defects in the films. The relative amount of the anatase phase in the mixture, that is, weight fraction WA of the anatase phase for the annealed samples consisting of a mixture of both anatase and rutile phases, was estimated from the relative diffraction intensities based on the empirical equation derived by Spurr and Myers:26 WA ¼

1 1 þ 1:265

IR IA

ð2Þ

where IA and IR represent the integrated intensities of the strongest reflections of anatase (101) and rutile (110) crystal planes, respectively. The relative amount of anatase phase in the mixture was estimated to 60%. 3.3. Optical Properties. Optical properties of the titanium nanocluster films prepared at a magnetron current of 0.5 A and cluster source pressure of 100 Pa were characterized by measuring UV
vis transmittance and reflectance spectra. The UV
vis spectra collected right after the deposition (as-prepared) and after the annealing processes are presented in Figure 6a without substrate subtraction. The evolution of transmittance of the samples follows the trends observed previously.18 The as-prepared films absorb strongly in the visible part of the spectra (%TAP ≈ 45
60) as compared to the annealed films of the same thickness

The diffuse transmittance TD of all the films can be approximated to zero according to the outcomes of the transmittance measurements mentioned above. Reflectance of all the films can be considered almost constant within the visible spectral range (Figure 6a). These observations reflect the structure of the nanocluster films.27,28 Surface of the films (nanocluster film
air interface) is very rough (rms roughness ∼ 70 nm),17 and thus the specular reflection can be expected to be low. This corresponds to the observation that all the samples are visually matte: the as-prepared films are black, while the annealed ones are milky. On the other hand, the interface between the silica substrate and the nanocluster film can be considered homogeneously planar according to the size of the nanoclusters (∼10 nm),17 which limits light scattering at this interface, and the diffuse transmission is minimal as can be observed from equal transmission spectra measured in a standard setup and using integrating sphere. We can thus conclude that all changes observed in the transmittance spectra (Figure 6) are primarily caused by changes in the absorbance of the titanium nanocluster films. This can be stated under the assumption that there is no change in the absorbance of the quartz substrates which was proven by the measurement of transmittance and reflectance spectra. These spectra did not change over time and therefore are not shown here. According to the elemental analysis performed by XPS, there are two possible sources of absorbance in the as-prepared (and aged as-prepared) films: metallic titanium Ti and/or titanium carbide Ti
C. Both of these have similar absorption coefficients and their relative amounts in the films decrease over time (see Figure 4) as the films are aging and oxidizing. This decrease corresponds to the decreasing absorption of the films over time. On the other hand, any possible surface contamination by hydrocarbons should not affect absorbance of the films in the visible region of the spectra. Nevertheless, since XPS is only a surface sensitive method, we cannot directly conclude which of the components is more responsible for the decrease in absorption in the visible spectral range. However, since the XRD measurement 20942

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The Journal of Physical Chemistry C showed that the as-prepared nanoclusters are composed mainly of metallic titanium, we can assume that the carbide forms only part of a shell on the surface of the clusters. Thus, the main cause of absorption of the as-prepared films would be free electrons in mostly metallic titanium nanoclusters. As the metal clusters oxidize in time, electrons are bound in the oxidized state and absorbance of the films decreases (transmittance increases). The transformation of titanium clusters to titanium dioxide is completed after the annealing process and the films become transparent in visible light. Therefore, the presence of carbon/carbides/hydrocarbons in the samples and their removal from the samples by annealing did not relevantly modify optical characteristics of the studied films. As can be seen in Figure 6b, as the oxidation of metallic titanium proceeds over time, the transmittance of the films at 450 nm gradually increases from about 45% to about 60%. The increase (and thus the oxidation) is the fastest in the first days after the preparation (extraction from vacuum to ambient air) while it tends to saturate after about 2 weeks. For a low-temperature oxidation, a longer period is required for the transformation of Ti to TiO2 as a result of slower oxygen diffusion kinetics.24,29,30 On the other hand, annealing at higher temperatures promotes further oxidation of the titanium nanoclusters and crystallization of the amorphous phase resulting in crystalline titanium dioxide nanoclusters. This is evidenced by the step-change in the values of transmittance at 450 nm from about 60% to about 85%. The process of oxidation of nanoclusters is then completed and the transmittance of the annealed films remains almost constant over time. Transmittance in the visible spectral region does not change though adsorption of hydrocarbon contamination on the surface after some time after the annealing was proven by XPS analysis (see Figure 3). This is most probably due to very limited thickness of the contamination layer and the fact that typical organic molecules (besides organic semiconductors, dyes, and various doped and conjugated molecules that are unlikely to be adsorbed on these samples) absorb light only in the UV spectral region.31

4. CONCLUSION Structure and composition of titanium nanocluster films prepared by gas aggregation cluster source were described. It was found that titanium nanoclusters oxidize immediately when exposed to the ambient atmosphere and contain substantial hydrocarbon contamination originating in both the deposition process and ambient air. The structure and properties of titanium nanoclusters change on storage under typical room conditions. It was shown that aging of titanium nanoclusters is mainly connected with further oxidation and adsorption of more hydrocarbon contamination. Transformation from metal to oxide structure of the nanoclusters upon annealing at 420 °C terminates in anatase crystal form of TiO2 with traces of rutile crystal form. Color of the nanocluster films changes from matte black to milky white as the metallic nanoparticles oxidize. Optical properties depend on the composition and nanostructure of the films. ’ ASSOCIATED CONTENT Supporting Information. Dependence of film composition on the cluster source pressure and the magnetron current; time evolution of high resolution XPS of films prepared at a magnetron current of 0.1 A and cluster source pressure of 100 Pa.

bS

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’ AUTHOR INFORMATION Corresponding Author

*Tel.: (+420) 2-2191-2257. Fax: (+420) 2-2191-2350. E-mail: [email protected]ff.cuni.cz.

’ ACKNOWLEDGMENT This work is part of the Research Plan MSM0021620834 and the Grant No. 1M06031 financed by the Ministry of Education of the Czech Republic and was partly supported by the Grant Agency of the Academy of Sciences of the Czech Republic under contract KAN101120701. ’ REFERENCES (1) Chen, X.; Mao, S. S. Chem. Rev. 2007, 107, 2891. (2) Sato, M.; Slamovich, E. B.; Webster, T. J. Biomaterials 2005, 26, 1349. (3) Fujishima, A.; Rao, T. N.; Tryk, D. A. J. Photochem. Photobiol. C 2000, 1, 1. (4) Zhu, Y.; Zhang, L.; Yao, W.; Cao, L. Appl. Surf. Sci. 2000, 158, 32. (5) O’Regan, B.; Graetzel, M. Nature 1991, 353, 737. (6) Loebl, P.; Huppertz, M.; Mergel, D. Thin Solid Films 1994, 251, 72. (7) Zhang, Q.; Gao, L.; Guo, J. J. Eur. Ceram. Soc. 2000, 20, 2153. (8) Wang, C. C.; Ying, J. Y. Chem. Mater. 1993, 11, 3113. (9) Ting, C. C.; Chen, S. Y.; Liu, D. M. J. Appl. Phys. 2000, 88, 4628. (10) Binns, C. Surf. Sci. Rep. 2001, 44, 1. (11) Wegner, K.; Piseri, P.; Vahedi Tafreshi, H.; Milani, P. J. Phys. D: Appl. Phys. 2006, 39, R439. (12) Barborini, E.; Kholmanov, I. N.; Conti, A. M.; Piseri, P.; Vinati, S.; Milani, P.; Ducati, C. Eur. Phys. J. D 2003, 24, 277. (13) Kholmanov, I. N.; Barborini, E.; Vinati, S.; Piseri, P.; Podesta, A.; Ducati, C.; Lenardi, C.; Milani, P. Nanotechnology 2003, 14, 1168. (14) Podesta, A.; Bongiorno, G.; Scopelliti, P. E.; Bovio, S.; Milani, P.; Semprebon, C.; Mistura, G. J. Phys. Chem. C 2009, 113, 18264. (15) Shyjumon, I.; Gopinadhan, M.; Helm, C. A.; Smirnov, B. M.; Hippler, R. Thin Solid Films 2006, 500, 41. (16) Haberland, H.; Mall, M.; Mosseler, M.; Qiang, Y.; Rainers, T.; Turner, Y. J. Vac. Sci. Technol. A 1994, 12, 2925. (17) Drabik, M.; Choukourov, A.; Artemenko, A.; Kousal, J.; Polonskyi, O.; Solar, P.; Kylian, O.; Matousek, J.; Pesicka, J.; Matolinova, I.; Slavinska, D; Biederman, H. Plasma Process. Polym. 2011, 8, 640. (18) Drabik, M.; Choukourov, A.; Artemenko, A.; Matousek, J.; Polonskyi, O.; Solar, P.; Pesicka, J.; Lorincik, J.; Slavinska, D.; Biederman, H. Surf. Coat. Technol. 2011, 205, S48. (19) National Institute of Standards and Technology, X-Ray Photoelectron Spectroscopy database, http://srdata.nist.gov/xps/ (accessed Feb 8, 2011). (20) The International Centre for Diffraction Data, Powder Diffraction File, http://www.icdd.com/ (accessed April 14, 2011). (21) Pouilleau, J.; Devilliers, D.; Groult, H.; Marcus, P. J. Mater. Sci. 1998, 32, 5645. (22) Vaquila, I.; Vergara, L. I.; Passeggi, M. C. G., Jr.; Vidal, R. A.; Ferron, J. Surf. Coat. Technol. 1999, 122, 67. (23) Choukourov, A.; Solar, P.; Polonskyi, O.; Hanus, J.; Drabik, M.; Kylian, O.; Pavlova, E.; Slavinska, D.; Biederman, H. Plasma Process. Polym. 2010, 7, 25. (24) Kofstad, P.; Hauffe, K.; Kjoellesdal, H. Acta Chem. Scand. 1958, 12, 239. (25) Scherrer, P. G€ottinger Nachr. Gesell. 1918, 2, 98. (26) Spurr, R. A.; Myers, H. Anal. Chem. 1957, 29, 760. (27) McNeil, L. E.; French, R. H. Acta Mater. 2000, 48, 4571. 20943

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