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Films of brookite TiO2 nanorods/nanoparticles deposited by MAPLE as A. P. Caricato, M.Cesaria,G. Leggieri, A. Luches, M. MartinoDipartimento di Fisica, Universit`a del Salento, Italy The ability to reveal trace amount of NO2 is of into a vacuum chamber on a target holder, while primary importance for detecting explosives, such being cooled with liquid nitrogen to maintain a as EGDN, TNT, PETN, RDX, at concentrations low and constant temperature (-160 C). The vac- on the order of 50 ppm and 30 ppb, while the uum chamber was evacuated down to 5 × 10−4 detection limit for TNT and nitroglycerin should Pa. The frozen solution was then irradiated with reach as low as 1 ppm and 1 ppb, respectively.
a KrF (λ=248 nm, τ =20 ns) excimer laser at the Oxide semiconductors are well recognized mate- repetition rate of 10 Hz. The laser beam was rials to be used as resistive gas sensors, as they attenuated and focused (with a rectangular spot exhibit resistance variations induced by the pres- of 0.075 cm2) to obtain an energy density of 350 ence of ambient gases. However, in order to in- mJ/cm2 on the target. The target was placed crease the sensitivity and the selectivity of sen- in front of the substrate at a distance of 40 mm sors, new active materials and structuring strate- and rotated at the frequency of 3 Hz to allow gies are requested. This is why innovative sen- uniform erosion. 6000 laser pulses were used to sors based on nanoparticles, nanorods and nan- deposit a single film. The films were deposited otubes are being extensively studied. The matrix- on < 100 > Si substrates, silica slabs and car- assisted pulsed laser evaporation (MAPLE) is a bon coated Cu grids to perform the different char- laser-based technique that has recently been de- acterizations. Films very adherent to their sub- veloped for the deposition of organic and bio- strates were deposited, as demonstrated by scotch logical materials. Moreover, it is very promis- tests. The morphological and structural proper- ing for the deposition of colloidal nanoparticles ties of the nanostructured films were investigated to fabricate thin active films for sensing applica- by scanning (SEM) and transmission (TEM) elec- tions. In the MAPLE technique the material of tron microscopy. UV-Vis transmittance and re- interest (polymer, biological material or colloidal flectance spectra of films deposited on silica sub- nanoparticles/nanorods) is diluted/suspended in strates were recorded at room temperature by a a volatile solvent, at a concentration of up to a few spectrophotometer equipped with an integrating weight percents, and frozen at the liquid nitro- gen temperature, forming the target to be laser-irradiated. The laser radiation is mainly absorbedby the solvent, which vaporizes entangling the so-lute particles and promotes their deposition onsuitable substrates. The MAPLE technique, asa derivative of the pulsed laser deposition (PLD)method, can successfully be used for the deposi-tion of materials onto plastic and other thermo-lable substrates, as well as on rough and flexiblesupports at room temperature.
Size-tunable brookite (orthorhombic) TiO2 nanorods, covered with an oleate/oleyl aminecapping layer, were synthesized by a colloidalnonhydrolytic sol-gel route based on surfactant- assisted aminolysis of titanium oleate complexesat 280 C under air-free conditions. After the syn-thesis and the extraction/purification procedures,the nanocrystals were suspended in toluene at a Figure 1. Representative BF TEM image of the concentration of 0.016 wt %. The solution was first treated in an ultrasonic bath for 10 minutesto prevent aggregation, after which it was frozenat the liquid nitrogen temperature and mounted The TiO2 nanorod layer to be used as a sensor was deposited onto an Al2O3 substrate equipped Table 1Experimental and theoretical lattice spacings cor- responding to the diffraction rings in the SAEDpattern of fig. 2 (c) nanostructured TiO2 film deposited onto a Si sub- spacing of 0.325 nm, which is not compatible with strate (a) along with the histogram of the diame- ter distribution of the spherical structures in the Table 1 reports the indexing of the diffraction film (inset). BF TEM image (b) and SAED pat- pattern and compares the experimental interpla- tern (c) of the film deposited on carbon-coated nar spacings with the theoretical values belonging to the rutile and brookite phases. the average vol-ume of the nano-spheres is approximately doublewith respect to that of the nanorods. The unex-pected observation of TiO2 spheres suggests oc- with Pt interdigitated contacts and a Pt heater on currence of a laser-induced process, which induces the backside. A total flow of 100 sccm was fixed a melting/coalescence of the nanorods, driving their transformation into most thermodynami- anisotropic brookite TiO2 nanocrystals as build- cally stable spherical shapes. A decrease of the ing blocks for thin films suitable for gas-sensing actual melting temperature has been already re- purposes. A representative BF TEM image of ported for low-dimensional solids, compared to brookite TiO2 nanorods used by us is shown in their corresponding bulk materials. Therefore, Fig. 1. The diameter and length dimensions are although the laser fluence used in this work was 3 - 4 nm × 20 - 50 nm intervals. A high- quite low (350 mJ/cm2) and the fusion tempera- resolution SEM image of a typical TiO2 MAPLE- ture of bulk TiO2 is very high ( 1850o C), lower deposited film onto a Si substrate starting from thermal loss rates could be associated with the a toluene solution of TiO2 nanorods is presented nanorods due to their nanometer-scale dimen- in Fig. 2a. The image evidences the simultane- sions, which could in turn cause them to in- ous presence of rod-like and spherical nanostruc- tures. The spherical structures have an average can reliably explain the observed modification of diameter of 134 nm, with a broad size distribu- the TiO2 nanorod morphology in terms of laser- tion, as shown in the inset of Fig. 2a. Examples of nanorods and spheres are more clearly visible at higher magnification in the TEM image of Fig.
2b, which displays an area of TiO2 films deposited deposited films are shown in Fig. 3. High optical by MAPLE onto the carbon film of a TEM grid.
transparency can be noted over a wide wavelength They have an average length of about 25 nm and range (300 nm- 1500 nm). The visible interference a diameter of 3 nm. The SAED pattern demon- effects are due to the small film thickness ( 150 strates a prevalence of the brookite phase (Fig.
nm). The steep profile of the transmittance spec- 2c). Nevertheless, the presence of rutile phase trum with a sharp cut off at around 350 nm (3.5 can also be admitted due to the presence of a eV) suggests a quite high value of the band gap diffraction maximum, corresponding to a lattice for TiO2 in agreement with theoretical and ex- Figure 4. (a) Sensor response for different work-ing temperatures under 1 ppm NO2 exposure; Figure 3. Total transmittance (a) and the specu- (b) typical dynamic response to the NO2 pres- lar reflectance (b) UVvis spectra of the MAPLE ence/absence at the working temperature of 300 perimental works, which report for the brookite ence of interfering gases. To this purpose, pre- phase higher band gap values with respect to the liminary measurements have put in evidence typ- ical reducing behavior, since the conductivity of the thin sensing layer increased in the presence of plex morphology and high surface to volume ra- 50 ppm of CO gas mixed in dry-air. Moreover, tio have revealed very promising performances for measurements to investigate the possible benefi- sensing applications. The sensor response to 1 cial effects of light illumination in a wavelength region close to the absorption edge of TiO2 have peratures is given in Fig. 4a. The gas response been performed. An increased sensor response of the sensor is calculated as Ra/Rg, where Rg is and dynamics of interactions were interestingly the current of sensor in the target gas and Ra is the current value in dry air. The TiO2 nanorod-based thin films were subjected to several cyclesof 1 ppm of NO2 exposure in dry air, separatedby a recovering flux of dry air.
A typical dynamic response curve recorded at the working temperature of 300 C (at which thesensor response was the highest) is shown in Fig.
4b, where quite stable and reversible sensing sig-nals can be appreciated. From the dynamic re-sponse curves, response and recovery times ofabout 2 min and 13 min have been estimated,respectively. Further tests are in progress to op-timize the performances of the sensing materialto detect lower that 1 ppm NO2 gas concentra-tions, as required for detection of explosives and,at the same time, to analyze the sensitivity andreduce the response and recovery times. Stud-ies are in progress also to analyze the responseof this material towards reducing gases like CO,in order to investigate its selectivity in the pres-

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