Nonstoichiometric Titanium Oxides via Pulsed Laser Ablation in Water
© The Author(s) 2010
Received: 10 January 2010
Accepted: 27 March 2010
Published: 13 April 2010
Titanium oxide compounds TiO,Ti2O3, and TiO2 with a considerable extent of nonstoichiometry were fabricated by pulsed laser ablation in water and characterized by X-ray/electron diffraction, X-ray photoelectron spectroscopy and electron energy loss spectroscopy. The titanium oxides were found to occur as nanoparticle aggregates with a predominant 3+ charge and amorphous microtubes when fabricated under an average power density of ca. 1 × 108W/cm2 and 1011W/cm2, respectively followed by dwelling in water. The crystalline colloidal particles have a relatively high content of Ti2+ and hence a lower minimum band gap of 3.4 eV in comparison with 5.2 eV for the amorphous state. The protonation on both crystalline and amorphous phase caused defects, mainly titanium rather than oxygen vacancies and charge and/or volume-compensating defects. The hydrophilic nature and presumably varied extent of undercoordination at the free surface of the amorphous lamellae accounts for their rolling as tubes at water/air and water/glass interfaces. The nonstoichiometric titania thus fabricated have potential optoelectronic and catalytic applications in UV–visible range and shed light on the Ti charge and phase behavior of titania-water binary in natural shock occurrence.
KeywordsTitanium oxide Nonstoichiometry Structure Optical property Pulsed laser ablation in water TEM
Nanobelts of semiconducting oxides of zinc, tin, indium, cadmium, and gallium were discovered by simply evaporating the desired commercial metal oxide powders at high temperatures . Such nanobelts shed light on a distinctive and common structural characteristic for the family of semiconducting oxides with cations of different valence states and materials of distinct crystallographic structures. The nanobelts were also suggested to be an ideal system for fully understanding dimensionally confined transport phenomena in functional oxides and building functional devices along individual nanobelts . Since then, nanosize and one-dimensional semiconductor oxide nanomaterials, such as zincite (ZnO) nanocrystals with Mn dopant to modify the UV emission  and titania (TiO2) polymorphs with many applications , have received intensive interests regarding their synthesis and applications on novel optical, photoelectricity, catalysis, and piezoelectricity properties. (The titanium oxide polymorphs have particularly attracted research community on their unique physical and chemical properties and wide applications such as paints, plastics, papers, coatings, cosmetics, ceramics, electronics, and photo-catalysts .) A hydrothermal route in the presence of stabilizer and/or acids was commonly adopted for the synthesis of titanium oxide phases with specific crystal structure and novel shape, such as TiO2 rutile nanoparticles , anatase nanotubes , and tubular titanium hydrates with controversial stoichiometries and structures [6–10]. In general, the tailoring of phase structure, particle morphology and hence the properties of the titanium oxides by this wet route were attributed to the precursor used, the presence of anionic species and the pH of the solution which affect the nucleation/growth processes.
Here, we used an alternative route of pulsed laser ablation in liquid (PLAL) to synthesize nonstoichiometric TiO x nanoparticles and nanotubes in a subsequent water-driven assembly process. This stabilizer-free approach is quite different from surfactant/copolymers or other template-assisted assembly of TiO x nanoparticles in a desired manner. The synthesis of such a tubular material from atom clusters and their lamellar derivative is analogous to the fabrication of carbon onions via arc discharge in water  and Au tubes via PLAL and subsequent dwelling in water . We focused on the nonstoichiometry, shape, coalescence, and dense structure, if any, of the TiO x nanocondensates and the phase behavior upon electron irradiation as of concern to the space charge, the surface/interface energetics in terms of unrelaxed or relaxed state and theoretical band gap of such metastable phases [13, 14] for potential optoelectronic applications.
PLAL Synthesis of TiO X Nanocondensates
Laser ablation parameters and resultant phase assemblages of nonstoichiometric titanium oxides via PLAL
1064 nm excitation
Pulsed energy (mJ/pulse)
Beam size (mm2)
Power density (107W/cm2)
1.1 × 104
1.4 × 104
A < T1T2T3
A < T1T2T3
A > T1T2T3
A > T3
Internal stress (GPa)*
The optical absorbance of the as-deposited nanocondensates and further developed MWTs in solution with specified dwelling times were acquired by a UV–Vis spectrophotometer (U-3900H, Hitachi) operating at an instrumental resolution of 0.1 nm in the range of 200 to 800 nm. The powders recovered from such samples were dried for microstructure observations using optical polarized microscopy and scanning electron microscopy (SEM, JEOL JSM-6700F, 10 kV, 10μA). The crystal structure of the MWTs was determined by X-ray diffraction (XRD, Bede D1, Cu Kα, 40 kV, 30 mA, at 0.05° and 3 s per step from 2θ angle for 20° up to 100°). The d-spacings measured from XRD trace were used for least-squares refinement of the lattice parameters with an error ±0.0001 nm using bulk gold reflections as a standard.
Field emission scanning transmission electron microscopy (STEM, FEI Tecnai G2 F20 at 200 kV) with selected area electron diffraction (SAED), and point-count energy dispersive X-ray (EDX) analysis at a beam size of 1 nm was used to study the structure and composition of the nanoparticles and the tubular walls. Z-contrast images and compositional line scanning profiles are acquired by high-angle annular dark-field (HAADF) detector and EDX under STEM mode. Lattice imaging coupled with Fourier transform patterns were used to study the rolling planes of the MWT and their partial epitaxy relationship with the associated TiO x nanoparticles. The Gatan Image Filter (GIF) coupled with electron energy loss spectrum (EELS) by TEM was employed to identify the chemical bonding state of the individual TiO x nanoparticle and that associated with the nanotubes.
Powdery sample mixed with KBr was studied by FTIR (Bruker 66v/S) for the extent of OH− signature on MWT. The powdery MWTs settled on a vitreous SiO2 substrate were studied by micro-Raman in a backscattering geometry by a Jobin–Yvon HR800 system working in the triple-subtractive mode for the estimation of internal stress for the constituting TiO6 and/or TiO4 polyhedra. The same sample was also examined by X-ray photoelectron spectroscopy (XPS, JEOL JPS-9010MX, Mg Kα X-ray source) calibrated with a standard of C 1 s at 284 eV for the determination of Ti2+, Ti3+, and Ti4+ peaks. Photoluminescence (PL) spectra of the powdery samples were recorded using a Jobin–Yvon spectrophotometer (Trix 320) at an excitation wavelength of 325 nm (He-Cd laser) at room temperature.
Phase Identity and General Behavior of the Condensates upon Dwelling in Water
UV–Visible Absorbance of the Colloidal Solutions
Microscopic Observations of the Assembled Nanoparticles
Optical micrographs under open and crossed polarizers for the representative deposits retrieved from sample prepared under a relatively low (i.e. sample 2) and 3 orders-of-magnitude higher power density (i.e. sample 4) showed that the titanium oxides tended to assemble as particles in the former case (Fig. 2b) but tubes in the latter case (Fig. 2c).
EELS and XPS
Vibrational and PL Spectroscopy
The OH− signature of the nanoparticles and microtubes were testified by the IR band near 3430 cm−1 analogous to the fully hydroxylated rutile surface having three distinct absorption bands around 3655, 3530, and 3400 cm−1. The hydroxyl groups were likely associated with a number of different surface sites of the nonstoichiometric titania in the present case to cause various interactions, such as hydrogen bonding between surface OH groups and molecular water with bending vibration near 1640 and 1726 cm−1 after the assignment of ref. , and hence a broad IR band analogous to the case of hydroxyl groups linkage to TiO2 rutile and anatase . The microtubes were much more OH-signified than the nanoparticles upon dwelling in water for a week. The combined results of XRD and Raman shifts indicated that the protonated microtube is structurally different from titanium hydrate, such as H2Ti3O7[6, 7].
The Raman shifts of the nanoparticles in sample 2 are mainly 614 (A1g), 443 (Eg), 248 (due to second order phonon), and 144 cm−1(B1g) with the assigned modes of TiO6 octahedra in parenthesis (Fig. 10b) after the assignment of rutile by ref.  and . These Raman shifts change to 606, 416, 264, and 414 cm−1, respectively, for the microtubes in sample 4. This indicates that the extent of TiO6 distortion in terms of internal stress is different, according to pressure dependence of TiO6 Raman shifts as discussed in Sect. 4. The Ti ion charges/oxygen vacancies are also considerably different in the two cases to change the vibration/stretching behavior as indicated by the presence of Ti2O3 with characteristic band at 344 cm−1 due to Ti3+ occupancy in octahedral site [31, 32] for sample 2 but not for sample 4, although monoxide TiO is not Raman active  to support this point.
Based on the above vibration and PL spectroscopic analyses, the TiO6 octahedral are likely the common units for the present titania nanoparticle/microtubes and the crystalline titanium hydrate nanotubes, such as H2Ti3O7 with long range ordering in the basal layer [6, 7] or other stoichiometries with controversial structures [8–10].
Effect of PLAL Parameter on the Phase Selection of Titanium Oxide
The PLAL by free run with a relatively low power density caused more TiO, Ti2O3, and TiO2 than the amorphous phase (cf. Table 1). By contrast, Q-switch mode with a higher power density caused more oxidized crystallites, i.e. TiO2 rather than TiO and Ti2O3 as in the case of sample 4, besides the predominant amorphous lamellar phase which further assembled and rolled up as microtubes upon dwelling in water. Apparently, PLAL under a higher power density would cause more thorough oxidation of the nanocondensates on the one hand and the amorphous phase on the other hand. In fact, by rapid reactive quenching with water in the liquid-plasma interface, the ablated species can be readily oxidized . As for phase amorphization by a dynamic condensation and very rapid cooling process, it has been demonstrated by the PLA synthesis of amorphous Al2O3 nanocondensates . The cooling rate in the similar PLA process was estimated to be close to 109 K/s for 10-nm-sized Al2O3 as well as α-PbO2 type TiO2 nanocondensates , 4 orders of magnitude higher than that required to quench an amorphous state for oxides . The PLAL process is expected to have an even higher cooling rate under the influence of water rather than air cooling, to quench the present titania nanocondensates as amorphous state under an additional factor of a rather large extent of nonstoichiometrty and associated defect clusters as addressed in next section.
The application of pressure to certain crystalline materials, i.e. so-called pressure induced amorphization, can cause them to become amorphous under certain conditions . An anatase-amorphous transition regime was also reported to occur for TiO2 of very fine crystallite size upon static compression at room temperature using the diamond anvil cell technique . Shock-wave loading and liquid confinement typical for a PLAL process  would also cause the already crystallized nonstoichiometric titania to become amorphous. In this connection, PLAL of TiO2 single crystal and Ti plate targets under the wavelength of 355 nm and maximum laser pulse energy 160 mJ/pulse in de-ionized water was reported to cause mostly amorphous phase . The amorphous lamellae appeared to be formed by a relatively high power density in the present PLAL process and then assembled and rolled up as microtubes upon subsequent dwelling in water. It is not clear, however, whether the amorphous lamellae are in high-density amorphous state .
Stress States of the Nanocondensates and Microtubes
The TiO2 rutile nanoparticles in samples 1 to 4 have a significant internal stress up to 4–5 GPa for the lattice (Table 1) based on SAED lattice parameters and the Birch-Murnaghan equation of state of the rutile with relevant bulk modulus and its pressure derivative .
The Raman shifts provide another estimation of the internal stress in terms of the TiO6 polyhedra shared by the TiO, Ti2O3, and TiO2 and amorphous lamellar phase in the samples. Regarding this approach, the pressure dependence of Raman shifts has been studied for anatase  and rutile-type TiO2. The major Raman modes (Eg(1), B1g(1), A1g + B1g, Eg(3)) of anatase nanocrystals are all well represented by linear increases in Raman shift with pressure up to 41 GPa . (The Raman modes Eg and A1g of the synthetic rutile-type TiO2 single crystal also have a higher wave number under an applied pressure up to 35 GPa ). Using both calibration curves, the TiO6 polyhedra of the present TiO1 − X, Ti2O3 − X, and TiO2 − X and amorphous lamellar have quite different stress state from that of the rutile lattice based on its lattice parameters. The internal compressive stress for the TiO6 polyhedra of the TiO, Ti2O3, and TiO2 nanocondensates in sample 2 is indicated by its A1g mode but not Eg mode being at a higher wave number than the reported ambient value of anatase (513 cm−1)  or rutile (608 cm−1) . In other words, there is a significant distortion of the polyhedra in the present nonstoichiometric titania phases due to the combined effects of quenching a dense state and varied stoichiometry and protonation. The TiO6 polyhedra of the microtubes in sample 4 were quite relaxed based on the A1g and Eg modes being at a much lower wave number (606, 416 cm−1) than the titanium oxide nanocondensates in sample 2 (614, 443 cm−1). This indicates that the TiO6 polyhedra in the amorphous lamellae were relaxed when the lamellae were rolled into microtubes. It should be noted that size, nonstoichiometry, and temperature would also affect the vibration mechanism of the TiO6 polyhedra, as indicated by previous Raman shifts study of rutile [5, 29, 30].
Defect Chemistry due to Hydrolysis and Polymerization of Ti–O–H
The oxygen vacancies and titanium interstitials were suggested to be the dominant defects in TiO2 rutile under low oxygen atmosphere  or doped with cation with lower valence such as Zn2+. The titanium vacancies are however favored in the present Ti2O3 to charge compensate the proton dopant. In fact, Ti2O3 allows a considerable extent of nonstoichiometry with O/Ti ratio ranging from 1.48 to 1.51 in the Ti–O binary at temperatures . The incorporation of proton during PLAL is expected to cause an even higher O/Ti ratio by introducing more titanium vacancies through Eq. 1.
The defect clustering of Ti2+, Ti4+, and H+ co-doped Ti2O3 through Eqs. 1 and 2 may be effective as soon as the oxolation by forming Ti–O–H linkage, typical in a hydrothermal process , caused considerable polymerization and various defect clusters in the crystals as observed by TEM. The hydrolysis and polymerization process to form specific Ti–O–H species is quite effective so that the sedimentation of the nanoparticles is much faster than Au nanoparticles via the same PLAL process . Aside from the detailed defect clustering scheme, the co-existence of Ti2+, Ti3+, Ti4+, and H+ ions and charge/volume-compensating defects in the present nonstoichiometric titanium oxides would account for the Raman shift of rutile Eg band in view of the Ti/O ratio dependence of Raman shift for nanophase TiO2 rutile and anatase .
Rolling and Tubing of the Assembled Nanocondensates
The formation of tubes/pipes in samples 3 and 4 versus NCA or closely packed manner in samples 1 and 2 can be attributed to different growth mechanisms. The former case is analogous to the Au clusters being assembled as lamellae and then rolled up as Au micro- and nano-tubes . To roll up a planar layer, it requires either caps as for carbon nanotubes with single or multiple walls  or intrinsic structural anisotropy as for clay minerals chrysotile . Only intrinsic structural anisotropy is considered here for the protonated microtubes. We suggest that the amorphous titania lamellae stabilized via oxolation in the PLAL process were water/air interface-mediated at the water level and bubbles to become asymmetric on the opposite sides, i.e. more H+ signified on the water side to exert a strain field for rolling up upon dwelling in water.
The H2Ti3O7 nanotube with monoclinic structure (space group C2 1 /m) based on ab initio calculations and TEM examinations has been synthesized by treating TiO2 powders in concentrated NaOH in aqueous solution and then individual trititanate layers are peeled off from the plates and scroll up into nanotubes [6, 7, 55]. It was found that surface tension due to an asymmetry related to H deficiency in the surface layers of the plates is the principle driving force of the cleavage, and the dimension of the nanotubes is controlled by this surface tension together with interlayer coupling energy and Coulomb force . However, H2Ti3O7 was not formed in samples 2 and 4 based on the combined XRD, FTIR, and Raman results. The reasons of forming protonated amorphous titanian microtubes with multiple lamellar wall rather than H2Ti3O7 nanotube in the present PLAL process could be the following. First, the present samples were synthesized under water rather than high concentration of Na+ cations and strongly basic conditions for alkali-treated processes that are essential in the synthesis of the titanium hydrate nanotubes . Second, dynamic high pressure–temperature conditions and very rapid heating/cooling rate via the present PLAL process may favor amorphous lamellae rather than H2Ti3O7 which typically has corrugated ribbons of edge-shared TiO6 octahedrons via a thermal equilibrium synthesis route .
Titania nanotubes have been synthesized and morphologically tailored by acids and electrolytes for potential bioactivity applications . The nonstoichiometric titania nanoparticles and microtubes as fabricated by PLAL here may also have potential optoelectronic and catalytic applications in UV–visible range as indicated by the band gap and the broad photoluminescence from 400 to 700 nm due to the incorporation of Ti ions with various charges and hence varied extent of crystal field stabilization energy under the influence of Jahn–Teller effect . (The Ti3+ ion therefore prefers to occupy octahedral rather than tetrahedral site with the so-called octahedral site preference energy of −28.8 kJ/mole .) It is of interest to explore in the future whether the oxygen deficiency in such PLAL products can be further increased to cause strong absorbance and photoluminescence as the case of titanic acid nanotubes . Finally, the titanium hydrates being not favored under highly activated dynamic conditions by the present PLAL route sheds light on the Ti charge and phase behavior of titania-water binary in natural shock occurrence.
We thank Miss S.Y. Shih for the help on XPS analysis. This research was supported by Center for Nanoscience and Nanotechnology at NSYSU and partly by National Science Council, Taiwan, ROC.
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