Generation of silver-anatase nanocomposite by excimer laser-assisted processing

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I. INTRODUCTION
Materials based on the anatase phase of TiO 2 semiconductor have been widely used for various photo-catalytic applications owning to their low cost, chemical and biological inertness, non-toxicity etc.However, the anatase may not respond to light excitations within the visible region due to its wide energy band gap (3.3 eV ∼ 386 nm), and thus absorbs only a fraction of the solar spectrum. 1,2 wever, due to its amazing properties and economy, it is desired to extend its applications domain from ultraviolet to visible region (400-700 nm).For this purpose, plenty of research has been pursued to generate anatase in the form of nanoparticles, nanotubes and nanowires etc.However, these efforts could not lead to a much significant improvement in photo-catalytic properties of the anatase. 3,4 ][6] Ag NPs possess a quality of high photo-absorption in the visible region owing to the surface plasmon resonance (SPR) effect. 7][10][11] In this instance, generally photo-chemical and thermo-physical methods have been adopted. 12,13  a typical photo-chemical process, Ag ions are directly adsorbed on anatase film/nanoparticles from their aqueous solution and photo-chemically reduced into metallic state. 14This involves extensive pre/post-processing steps e.g.furnace heating cycles and UV light exposure, which also restricts the deposition of Ag-TiO 2 film on lower melting substrate such as polymers and plastics.A flexible and time efficient process has always been desired for developing uniform Ag-TiO 2 nanocomposite structures with required properties on variety of substrates.6][17] In the present research, we have applied a quite similar excimer laser-assisted process (ELAP) to modify Ag-TiO 2 films on the glass substrate.By this approach, plain TiO 2 films with self-adsorbed Ag 2+ /Ag 1+ ions were successfully transformed into nanocrystalline silveranatase nanocomposite structure by pulsed ultraviolet laser beam.It combines the benefit of both photo-thermal and photo-chemical phenomenon in a single-step and thus saves considerable time and effort.In addition, the process is conducted at room temperature because there is no requirement for high temperature equipment.

II. EXPERIMENTAL
The recipe for making sol-gel TiO 2 precursor film was acquired from our previous work. 18Asdried TiO 2 precursor films were immersed into a silver nitrate (AgNO 3 , 0.01M) aqueous solution for 15 minutes.Films were subsequently rinsed with de-ionized water (DI) and dried in air.A pulsed KrF excimer laser was irradiated on the film at optimized parameters as listed in Table I.Excimer laser keeps a rectangular beam shape and top-hat profile, however, it is essential to use more uniform central portion of the raw beam to increase reproducibility of results.The laser beam size was reduced to 5×5 mm 2 on the sample for this purpose, which was obtained by masking inhomogeneous edges of the raw beam through a steel aperture.A typical experimental arrangement for ELAP treatment is shown in Fig. 1.
The crystalline structure of films before and after laser irradiation was determined by XRD (Philips X'pert-APD grazing incidence angle XRD machine).Spectra were obtained from 20-70 • 2theta with a step size of 0.05 and scan time/step of 6 seconds.A glancing angle of 4 • was selected to increase the signals from the thin film.The anatase grain size was calculated from Scherrer's method by using the following relation.
Where, 'D' is the average particle size in angstroms, 'B' is the width of the peak at half the peak height in radians, 'λ' is the X-ray wavelength in angstroms, and θ b is the Bragg's angle in degrees.Analytical simulation of temperature induced by a single laser pulse interaction with plain TiO 2 film was compiled and run in Wolfram Mathematica (8.0.1).A one-dimensional heat equation model was used for this purpose to simply complex calculations.Nanostructure imaging at high resolution and chemical analysis was carried out by STEM coupled with EDX detector and selected area electron diffraction (SAED).Sample for STEM was prepared by immersing the film in DI water and scraping off fragments, which were picked on carbon-coated grid and dried on filter paper in the oven at 80 o C before examination.

A. Imaging and Chemical Analysis by STEM
A STEM image obtained from Ag-TiO 2 film fragment after 50 laser pulses at 85 mJ/cm 2 fluence is shown in Fig. 2. It reveals distinct areas consisting of a matrix phase (light) and a dispersion of Ag NPs (dark) over it.The SAED from corresponding area revealed a crystalline pattern as shown by the inset in Fig. 2(a), which closely matched with the anatase (JCPDS-00-21-1272) phase of TiO 2 .
Moreover, an EDX line scan performed across the film produced strong signals from Ag, Ti and O confirming the Ag NPs formation during the laser treatment.Additional peaks of C, Si and Cu originated from the carbon film and STEM sample grid.
By close observation, the anatase grains were sintered together at edges and Ag NPs were self-adhering to the anatase grains.Roughly, an average grain size of 38 nm was obtained from the anatase, whereas Ag NPs were approximately 9.6 nm across as determined from STEM image.Ag NPs size distribution was verified by performing an image analysis and plotting a histogram as shown in Fig. 2(b) inset.It is evident that most of the Ag NPs synthesized by ELAP were within 5-10 nm range.
High resolution TEM (HRTEM) images obtained from the same Ag-TiO 2 film are shown in Figs.2(c) and 2(d).The lattice fringes are visible in the matrix and correspond to (101) plane of the anatase (JCPDS-00-21-1272) as shown by HRTEM filter image in Fig. 2(c) inset.The particles appearing in dark shade (arrow and square) were identified by their corresponding FFT and revealed a cubic (JCPDS-01-071-4613) as well as hexagonal (JCPDS-01-071-5025) crystal structure as shown in Figs.2(c) and 2(d) insets.A crystallite size of cubic silver NP as-observed from HRTEM image was approximately 8.5 nm, whereas hexagonal crystal was 11 nm across its edges.
The ultraviolet laser treatment of Ag-TiO 2 film leads to a promising solution to develop silveranatase nano-composites on glass or glazed surfaces.However, certain aspects still remain to be further investigated to explain dual symmetries of Ag NPs after laser irradiation.Generally, it may be ascribed to non-equilibrium phase transformation by the laser interaction with Ag ions or other processing factors.The silver-anatase structure is generated at optimized laser fluence i.e. 85 mJ/cm 2 with 50 shots at 15 pulses/sec repetition rate in order to suppress the rutile formation.However, effect of laser parameters on Ag NPs generation and transformation needs further steps of experiments and characterization.

B. XRD Analysis and the Crystallite Size
XRD results of the plainTiO 2 and Ag-TiO 2 films before and after ELAP are shown in Fig. 3.The as-dried TiO 2 film (350 • C, 30 minutes) exhibited a halo curve shape indicating amorphous state.However, several crystallographic peaks were generated after irradiation with 50 laser pulses at 85 mJ/cm 2 fluence.The d-spacing values obtained from various peaks corresponded to the anatase phase of TiO 2 (JCPDS-00-21-1272).Additional peak at 27.8 degrees was associated with the rutile phase of TiO 2 (JCPDS-00-21-1276).On the other hand, Ag-TiO 2 film prepared at the same parameters, revealed anatase structure without any mixing of the rutile phase.In this sample, intensity of (101) planes of the anatase was greater than the plain TiO 2 , which indicates that a greater crystallization volume of anatase was obtained in the former after ELAP.
The excimer laser keeps a very short pulse of 13-20 ns only and a total interaction time of 50 pulses equals 1000 ns, which is still quite low from materials point of view.This narrow lasermaterial interaction is responsible for a very narrow heat affected zone (HAZ) and thermal diffusion is limited.This may in part explain the reason of lower crystallization volume of the anatase in plain TiO 2 film.Whereas, a higher degree of crystallization in Ag-TiO 2 film can be associated with higher thermal conductivity of Ag NPs compared to plain TiO 2 , which aided photo-thermal diffusion induced by the laser into the anatase matrix.
Moreover, a stabilization effect to formation of the anatase was revealed by Ag-TiO 2 film after the laser irradiation.When the laser pulses were increased up to 200 at 85 mJ/cm 2 fluence, the anatase was still observed to be dominant as indicated in our previous report. 19It may be attributed to the laser absorption by Ag NPs in addition to TiO 2 resulting in laser energy distribution among these species and thus less energy was available to the anatase for phase transformation.Furthermore, Ag NPs may also induce a pinning effect to growth of the anatase by distributing alongside the grain boundaries of TiO 2 under influence of the laser.
The anatase grain size obtained from various films is listed in Table II.A larger crystallite size obtained in plain TiO 2 film is due to significant anatase grain growth and subsequent transformation  A slight difference may result from the machine intrinsic broadening and may be neglected.

C. Numerical Simulation
It is well known that crystallization is a thermo-physical phenomenon, which is dependent on the surrounding temperature of the material.A nanosecond laser-material interaction is more specialized case in which temperature is generated by laser-induced excitations at the atomic level. 20n the present study, incident high energy photons may trigger thermal as well as photochemical modification within amorphous TiO 2 film.Generally, crystallization of the anatase requires a temperature exceeding 400 • C by thermal and/or photo-thermal mechanism.However, ultraviolet photons of the excimer laser are capable to introduce photochemical transformation inside the film at room temperature.
Here, we will consider a 350 nm deep plain TiO 2 film on glass to numerically simulate the photo-thermal changes.The temperature attained during interaction of a 13 ns laser pulse with the film surface can be calculated by solving the standard three-dimensional heat equation.However, for large area irradiation (unfocussed), the temperature remains more or less uniform within z plane and the temperature rise can be calculated by one-dimensional heat equation. 21,22 Where, 'β' determines the temporal pulse shape, 'z' the depth of film in nm, 't' the time in ns, 'τ ' the laser pulse width in ns, 'T' the temperature in • C, 'c p' the specific heat, 'k c' the thermal conductivity, 'α' the absorption coefficient,'R' the reflectance and 'ρ' the density of TiO 2 film.Standard thermo-physical properties of TiO 2 anatase were used against each of these parameters where feasible as shown in Table III.Equation (2) was numerically solved to calculate the temperature rise and supplemented with specific boundary conditions to obtain the final surface temperature as follows.
T T i O 2 = 273 + T (x, y, z, t) at t = 0 s and z = 0 nm (5)   Simulation results of calculated temperature against the laser pulse interaction time are plotted in Fig. 4(a).The inset shows an exaggerated part of the curve near the peak temperature and differentiates between various applied fluence values.Results indicate that a peak surface temperature of 852, 941 and 1030 • C is generated at the surface of plain TiO 2 film after interaction with a single laser pulse at 65, 75 and 85 mJ/cm 2 fluence respectively.The temperature levels off quickly to 400 • C after 100 ns due to a very short interaction time of the laser pulse.In addition, temperature variations across the film thickness after a single laser pulse at varying fluence are shown in Fig. 4(b).It reveals that by 85 mJ/cm 2 laser fluence, a thermal field is generated that diffuses as deep as 350 nm into the material, which is also the thickness of TiO 2 film, but the temperature drops to 439 • C at that point.A smooth thermal gradient is thus set up during interaction of a single pulse of the excimer laser with TiO 2 film.
Interestingly, for all intensities, calculated peak temperature remained well below the melting point (1870 • C) of TiO 2 film. 2 The excimer laser melting threshold of TiO 2 film has been reported to occur at 250 mJ/cm 2 fluence according to a recent study. 24It can therefore be established that there was no melting or ablation of TiO 2 during the ELAP treatment at 85 mJ/cm 2 fluence.However, the temperatures are significantly high and coincide with the sintering temperature of TiO 2, which is promoted by solid-state diffusion.Although, sintering temperature lies within the rutile stability range, anatase is still crystallized as a dominant phase after ELAP treatment as witnessed by XRD and STEM results.The effect of multiple laser pulses and interaction with Ag NPs cannot be accomplished due to the present software limitations.However, a more detailed discussion on this topic will follow in our next publication.

IV. CONCLUSIONS
In summary, nanocrystalline silver-anatase nanocomposite thin films were successfully developed on glass by ELAP treatment.Laser energy and the number of pulses were optimized to avoid melting/ablation yet to increase the temperature of Ag-TiO 2 film photo-thermally to crystallization and sintering of the anatase.At the same time, silver ions were reduced and metallized photochemically into Ag NPs.The photothermo-chemical transformation can be flexibly customized by changing laser parameters to obtain the desired ratio of constituents (anatase, rutile etc.).Both cubic as well as hexagonal silver crystals were formed and a uniform size distribution was achieved by ELAP without any surfactants.Crystallization of the anatase during laser irradiation was significantly improved with silver addition compared to the plain TiO 2 .This is a new advance in laser processing for photochemical generation of NPs, which may lead to UV laser-assisted metallization of metal NPs on various substrates especially in photovoltaic devices.

FIG. 2 .
FIG. 2. STEM image of Ag-TiO 2 thin film fragment (inset is a SAED pattern) after 50 laser pulses at 85 mJ/cm 2 (a) a corresponding EDX with inset showing the Ag NPs size distribution histogram (b) HRTEM images with relative FFT insets of cubic and hexagonal Ag NPs (c) and (d).

FIG. 3 .
FIG.3.XRD spectra of plain TiO 2 and Ag-TiO 2 thin film before and after ELAP treatment.

FIG. 4 .
FIG. 4. Numerical simulation of temperatures attained during a single laser pulse interaction with TiO 2 film surface a) and across the depth b) at a range of laser fluence.

TABLE I .
The laser operating window for processing of Ag-TiO 2 thin film.
FIG. 1.A typical ELAP setup for treatment of Ag-TiO 2 thin films, inset shows the GSI excimer laser system.

TABLE II .
Anatase grain size obtained from various TiO 2 films by XRD.
into the rutile phase.The anatase grain size (38.4 nm) is quite close to STEM observations (40 nm).

TABLE III .
Standard thermo-physical data of TiO 2 used for numerical simulation.