Discrete Single Crystalline Titanium Oxide Nanoparticle Formation

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Discrete single crystalline titanium oxide nanoparticle formation from 2D nanowelded network Satyanarayan Dhal, Shyamal Chatterjee, Stefan Facsko, Wolfhard Moeller, Roman Boettger, Biswarup Satpati, Satchidananda Ratha, and René Hübner Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00173 • Publication Date (Web): 22 Mar 2017 Downloaded from http://pubs.acs.org on March 27, 2017

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Crystal Growth & Design

Discrete single crystalline titanium oxide nanoparticle formation from 2D nanowelded network

Satyanarayan Dhal1, Shyamal Chatterjee*1, Stefan Facsko2, Wolfhard Möller2, Roman Böttger2, Biswarup Satpati3, Satchidananda Ratha1, René Hübner2 1

School of Basic Sciences, Indian Institute of Technology Bhubaneswar, Bhubaneswar 751007, India

2

Institute of Ion Beam Physics and Materials Research, Helmholtz-Zentrum DresdenRossendorf, 01328 Dresden, Germany 3

Surface Physics & Material Science Division, Saha Institute of Nuclear Physics, Kolkata 700 064, India

*Email: [email protected]

Abstract Nanostructured materials are gaining increasing importance due to their unique properties resulting from the high surface to volume ratio and the altered characteristics of the nanoscaled building blocks. The properties of these materials depend strongly on their microstructure and thus can be controlled by inducing transformation on the nanoscale. In this work a simple low energy ion beam irradiation technique is presented that can be used to weld effectively the hydrogen titanate nanotubes into a large-scale network of nanowires. By varying the ion fluence we are able to fragment the entire nanowire network into uniformly distributed nanocrystaline particles with an average size of 5±2 nm. Computer simulations of the ion irradiation effects on the nanotubes in 3D reproduce most of the experimental findings, and thus confirm that the early development of the system is governed by atomic collision processes. Our study demonstrates that the selective use of ion irradiation can transform metal-oxide nanotubes into large-scale welded networks of nanowires and further into nanocrystalline particles through nucleation and growth.

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1. Introduction: Transforming nanostructured materials offers large possibilities to design and control the properties of these materials for potential applications. Various chemical and physical techniques are effectively used for fabrication and manipulations of nanomaterials. Ion or electron irradiation techniques are also extremely effective for such purpose. For instance, modified surfaces of metal oxide nanowires1, 2, 3 induced by ion irradiation led to enhanced field emission. Apart from surface and structural modifications, ion irradiation techniques have been successfully used to join carbon based4 and ceramic5 nanowires. Very recently laser radiation has been used by different groups to produce joining of silver nanowires (Ag NW)6,7 and Ag NW-TiO2 heterojunctions.8 Several studies have been performed on the fabrication of one-dimensional nanostructures (e.g. nanorods, nanotubes etc) from nanoparticles using various chemical and physical techniques. For instance, hexameric rings on protein blocks form nanotubes by self-aggregation.9 Truncated CdTe tetrahedral nanoparticles self-assemble to form nanoribbons10 or ZnSe nanoparticles form nanorods by self-organization.11 Pachoski et al. showed the formation of ZnO from quasi-spherical ZnO nanoparticles, by dissolution and growth with assistance from Ostwald ripening. A number of studies showed the reverse process, i.e. the formation of nanoparticles from one-dimensional metal nanostructures. For instance, annealing of copper nanowires leads to formation of nanoparticles of dimension 30-50 nm through Rayleigh instability.12 Ion beam induced fragmentation into nanoparticles from cobalt thin films was demonstrated by Lian et al, which occurs due to Rayleigh instability.13 An ion fluence-dependent formation of crystalline Si nanoparticles of spherical shape from a layer of silicon oxide has been studied by Mueller et al.,14 At low fluence isolated Si nanocrystals are formed in between SiO2 layers and at higher fluence it evolves into a spinodal pattern.

In the present work we have studied in detail the fluence-dependent evolution of structure and morphology of hydrogen titanate nanotubes using 15 keV argon ion irradiation. At low fluence of 5×1015 ions.cm-2 the surface of the nanotubes are roughened and bent. With increasing fluence the hollow parts of the nanotubes are filled with atoms and form solid nanowires. Furthermore, the welding of these nanowires is found to happen in large-scale and a network structure appears. At a higher fluence of 3×1016 ions.cm-2 the nanowire network disintegrates into crystalline nanoparticles of rutile phase through nucleation and growth. Supporting TRI3DYN15 computer simulations at fluences up to 1×1016 ions cm-2 demonstrate


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that the early development of the system can largely be attributed to atomic collision processes, causing significant surface roughening, apparent bending of the nanotubes, and welding between two the nanotubes and between the nanotubes and the substrate. Theoretical studies based on molecular dynamics simulation16 have shown joining of carbon nanotubes under ion beam irradiation. However, currently there is no theoretical understanding available to explain the joining mechanism of ceramic nanomaterials such as metal-oxides induced by ion beam irradiation. This work is a first attempt by employing TRI3DYN simulation to explore the joining mechanism of metal-oxide nanotubes. Furthermore, the ion induced formation of crystalline particle from a nanowire network under ion irradiation is also a unique observation in this work. Our study may open up possibilities to design special material, where large reactive surface areas are required. Furthermore, the current study on welding mechanism will also lead to a better understanding of joining nanotubes/nanowires for making nanodevices.

2. Exparimental details: 2.1 Synthesis of nanotubes: Hydrogen titanate nanotubes have been synthesized hydrothermally using anatase TiO2 (99.9%) dissolved in 10 M NaOH aqueous solution. The solution was initially stirred for about 72 hours and then heated in an autoclave at 1500 C for about 12 hours.17 The final product was washed with dilute HCl (1:1) to remove sodium followed by multiple washing in deionized water. The as-synthesized sample comes out in powder form. This powder sample is further dispersed in ethanol and coated on precleaned highly conductive silicon wafer using spin-coating.

2.2 Ion irradiation and characterizations: Ion irradiation was done at the Ion Beam Center of HZDR using 15 keV Ar+ irradiated normal to the sample surface at different ion fluences of 5×1015, 8×1015, 1×1016, and 3×1016 ions.cm-2, respectively. Both the as prepared and irradiated samples have been investigated using scanning electron microscopy (Zeiss Merlin Compact Gemini) at IIT Bhubaneswar to determine the morphology and dimensions of individual nanotubes. To analyse the microstructure of pristine as well as irradiated samples, transmission electron microscopy (TEM) studies have been performed at a 300 KV TEM (FEI, Tecnai F30-ST) in SINP, Kolkata. The TEM specimens were prepared by sonicating the pristine and the irradiated samples dispersed in ethanol for 30 mins. This



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sonicated solution was then put on carbon-coated copper grids. Raman scattering measurements were performed using a Jobin Yvon T64000 Micro-Raman spectrometer equipped with an inverted microscope in IIT Bhubaneswar.

3. Simulation details: In order to investigate the role of atomic collision processes in the observed phenomena, the simulation code TRI3DYN15 has been applied, which describes three-dimensional (3D) atomic-collision induced dynamics for nanostructures of an arbitrary shape and under various ion irradiation conditions in a 3D voxel structure. Atomic collisions of the incident ions and the constituents of the target material are treated in a Monte Carlo concept using the Binary Collision Approximation18. Ions and recoil atoms are traced also in vacuum, so that, redeposition of sputtered atoms is included. In order to minimize the influence of the surface voxel structure on scattering and sputtering, a special algorithm of local surface planarization has been implemented. The program records compositional and atomic density changes in the individual voxels, which occur due to ion-induced transport of recoil atoms and surface sputtering. The resulting deviations from regular atomic densities (given by the predefined atomic volumes of each component) are relaxed by material exchange between the voxels. This enables a 3D description of the compositional and surface contour changes due to ion implantation, ion-induced mixing and preferential sputtering under high-fluence irradiation. It should be noted that the code is unable to describe elastic deformations of the structure, but rather simulates ion-induced plastic flow in a simplified way.

4. Experimental results and discussions: The as-synthesised nanotubes have an average diameter of 15±5 nm and a length of about 70±20 nm (Fig. 1(a) and (b)). The TEM image in Fig. 1(b) shows straight single crystalline nanotubes with a wall thickness of about 5 nm. The high resolution TEM shows two lattice planes in the tube wall (Fig 1(c)). The planes along the axis of nanotube has lattice spacing of 0.8 nm and the same for the planes across the nanotube is about 0.4 nm. These two lattice spacing are corresponding to (200) and (003) planes of hydrogen titanate. The angle between these two planes is about 102O.19 Energy dispersive X-ray spectroscopy (Fig. 1(d)) obtained in the scanning electron microscope (SEM) detects titanium and oxygen and there is


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no trace of other impurities from the precursor material in the system. The silicon signal originates from the substrate. The nanotubes dispersed on silicon substrate have been exposed to 15 keV Ar+ ion irradiation at different fluences (as mentioned in the experimental section). TRIM20 simulation shows that 15 keV Ar+ has an average range of 7 nm and deposits an average 50 eV/Å energy at a distance of 3 nm. This creates about 110 defects per ion in the crystalline nanotubes. At low fluence of 5×1015 ions.cm-2, the nanotubes are no longer straight and the surface gets roughened as observed in the scanning electron micrographs (SEM) and TEM images in Fig. 2. Surface roughening may occur because of local variations in sputtering, i.e. due to preferential sputtering and angle- and curvature-dependent sputtering. With slight increase in ion fluence to about 8×1015 ions.cm-2 (Fig. 2(c)) the tubes are transformed into solid wires and they are randomly bent. The 15 keV Ar+ ions create collision cascades across the tube walls, leading to ejection of atoms from the inner surface and filling of the hollow parts of the tubes. Bending of nanowires due to ion irradiation has been observed earlier in case of ZnO21 and Ge nanowires,22 carbon nanotubes23 and hydrogen titanate nanowires.5 In the present work the nanotubes are lying randomly on the substrate surface. Thus, the nanotubes are not only hit by the primary ions but also by the backscattered and recoiled-ions/atoms from surrounding nanotubes as well as the substrate. Since the distribution of interstitials and vacancies is arbitrary due to the randomness in the direction of collisional events, a differential stress distribution is generated along the nanowire, which leads to arbitrary bending as observed in Fig 2.

The nanotubes are found to be attached to each other even at low fluences used in this study (Fig. 2). At an ion fluence of 1×1016 cm-2, a network is formed through large-scale welding (SEM image in Fig. 3(a)). Continuous and solid joining is observed between the nanowires as seen in the TEM images in Fig 3(b) and (c). The ion-induced modification becomes extremely interesting at an ion fluence of 3×1016 cm-2 .The SEM micrograph ((Fig. 4(a)) apparently shows a network structure. High resolution TEM gives a detailed insight into this network (Fig. 4(b)-(d)). Nanocrystals of size in the range of 2-10 nm appear throughout the network. The lattice spacing of one exemplary nanocrystal (Fig. 4(d)) comes out to be 0.32 nm, which corresponds to the (110) plane of rutile (JCPDS Card No. 78-2485). At high fluence, preferential sputtering becomes considerably high resulting in phase separation. Furthermore, ion-induced recrystallization and grain growth take place at this fluence which lead to the formation of crystalline titanium oxide nanoparticles.14,24 The entire ion fluence



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dependent processes of bending, surface roughening, welding and formation of crystalline nanoparticle is illustrated in Fig. 5. The Raman scattering spectrum (Fig. 6) of pristine titanate nanotube sample shows characteristic lines at around 198, 274, 455 and 663 cm-1.25,26 The irradiated sample at fluence of 1×1016 cm-2 exhibits peaks at around 150 and 191 cm-1 corresponding to the anatase phase. The characteristic peaks of titanate at around 274, 663 and 701 cm-1 are still prominent at this fluence. However a strong peak of rutile appears at around 444 cm-1. At this fluence diffusion of defects triggers recrystallization, leading to formation of phases like anatase and rutile apart from titanate. At further higher fluence of 3×1016 ions.cm-2 the characteristic peaks of titanate at around 274 and 663 cm-1 are still observed. However, the most pronounced peak appears at about 612 cm-1 corresponding to the rutile phase of titanium dioxide.27 Thus the Raman scattering study supports the experimental observation of gradual phase transformation from titanate to titanium oxide with increasing ion fluence. For instance, at ion fluence of 1x1016 ions/cm2 the peaks corresponding to anatase and rutile becomes apparent along with titanate phase. This indicates that ion-atom collisional effects and thermal spike induced rearrangements and recrystallization led to formation of new phases. With further increase of ion fluence (i.e. at 3x1016 ions/cm2) the rutile phase dominates as indicated in the Raman spectrum, which is consistent with the observation in high resolution transmission electron micrograph (Fig. 4(d)).

5. Simulation results and discussions: The model simulation using TRI3DYN is done for argon ion impact on two perpendicularly crossed hydrogen titanate (H2Ti3O7) nanotubes on a silicon substrate (Fig. 7). For the target material, three-dimensional box shape volumes of size 40(depth)×60×60 nm3 have been created with 40×60×60 voxels. The depth coordinate is pointing from surface into the material, whereas Y and Z are lateral coordinates for which periodic boundary conditions are chosen to mimic the influence of close-by neighbour structures. For projectile ions, a broad


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and uniform beam of Ar+ ions (atomic number 18 and mass 39.94 u) with energy 15 keV is used at two different fluences of 4×1015 ions/cm2

and 1×1016 ions/cm2, respectively.

Implanted Ar+ ions are discarded in this simulation. Fig. 7(a) and (b) show atomic fraction of titanium present in the pristine nanotubes for two different orientations. At a fluence of 4×1015 ions.cm-2 the sputtering becomes apparent and the tube structure is deformed (Fig. 7(c) and (d)). The upper nanotube is bent towards the bottom and slight joining between the nanotubes is also visible due to downward transportation of material. Both Ti and O atoms are moved towards the substrate (see also Fig. 8). At a fluence of 1×1016 ions.cm-2 (Fig. 7(e) and (f)) significant sputtering occurs for both the nanotubes and the upper nanotube bends towards the lower one. A downward transport of atoms fills the gaps between nanotubes, narrows the hole in the lower tube, and also spreads on the substrate. A clear joining is observed between the two nanotubes and between the lower nanotube and the substrate. At this fluence, the upper tube becomes porous due to substantial loss of material. Here, the simulation of the structure shape becomes questionable as the mechanisms such as elastic contraction or the release of flakes might be governing. Thus, we refrained from extending the fluence range of the simulation. Simulation results for atomic ratio of Ti and O with increasing ion fluence reveal interesting facts as observed through X-Y cut of the upper nanotube in Fig. 8. The Ti/O atomic ratio for pristine sample is ≈ 0.4 (Fig. 8(a)). At two different ion fluences of 4×1015 and 1×1016 ions.cm-2 the simulation results (figures 8(b) and (c)) demonstrate exchange of material between the two nanotubes, between the nanotubes and the substrate, and between neighbour structures in the periodic arrangement. With increasing ion fluence the Ti/O ratio increases from the pristine sample (≈ 0.4), which indicates more loss of oxygen compared to Ti by means of preferential sputtering. At the fluence of 1×1016 ions.cm-2 the Ti/O atomic ratio exceeds one in the lower nanotube as well as the junction areas. This indicates significant loss of oxygen and downward movement of recoiled Ti atoms and redeposition in the lower nanotube and the junction areas. However, the Ti/O ratio still remains close to 0.4 for the upper nanotube, which indicates the existence of oxygen rich regions even at high fluence. In contrast, the resulting Ti/O atomic ratio in the lower nanotube shows a clear excess of Ti versus the stoichiometric composition of TiO2, which is a prerequisite for the precipitation of TiO2 nanoclusters. The Ti/O atomic ratio of the deposits in the near-surface region of the substrate is nearly uniform at Ti/O ≈ 0.4, which indicates some excess of O against stoichiometric TiO2. This excess, however, can easily be incorporated by reaction with the substrate to a partial formation of SiO2. Thickening of



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junction areas with increasing ion fluence clearly indicates welding between the two nanotubes as well as between the lower nanotube and the substrate. It may be noted that in this whole process there could be some amount of redeposition of silicon atoms to the nanostructures adjacent to the substrate (see Fig. S1 and S2 in the supplementary information).

The above results are obtained based on collisional effects. However, thermal spike related effects may also play a crucial role in this experiment, which is out of scope for the current simulation. For instance large scale welding may also be assisted by localized melting by individual ions.28,29 Furthermore, the phase separation and the particle formation at higher ion fluence is probably indicating the effects of thermal spike. An estimation of average number of particles formed in Fig. 4 comes out to be about 6×1013 cm-2, which is less than the number of ions irradiated per unit area. Therefore, precipitation of individual particles may be caused by energy deposition from individual ions.

6. Conclusions: An effective way to manipulate hydrogen titanate nanotubes by ion irradiation has been presented. We have performed TRI3DYN simulation to investigate the observed results. At comparatively low fluence the nanotube surface is highly roughened and the entire structure bends due to preferential sputtering and ion-induced surface defect formation. With increasing fluence the nanotubes convert into solid nanowires by redisposition of atoms due to collisional cascades. At moderately higher fluence these nanowires are welded together to form a large-scale network structure. Such welding mechanism is possible due to transport and mixing of atoms between two nanotubes as well as between the nanotube and the substrate. However, localized melting due to thermal spike effect may also assist in largescale welding. At even higher ion fluence the welded network fragments into crystalline nanoparticles. The titanium-rich regions promote phase separation and precipitation into rutile phase through nucleation and growth due to thermal spike. Therefore, in summary, we demonstrate the evolution from individual nanotubes to nanowire to nanowire network and further to formation of nanoparticles just by controlling ion fluence keeping the energy of the ions constant and a first theoretical attempt has been made to understand the joining


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mechanism in a metal-oxide system. The large-scale network or mesh structure of titanium oxide-based nanowire/nanotubes is envisaged to be potentially useful as microbial or pollutant filters. It is to be noted that the effects are observed under a broad beam irradiation. However, using focused ion beam (FIB) the joining and particle formation can be observed at specific locations. This will provide precise control of shape and size. Such transformation is unique and opens up opportunities to design special nanostructured architectures. Furthermore, the unique transformation into titanium oxide-based nanoparticles opens up the possibility to design nanoparticle and nanowire network composite structures for photovoltaic30 or sensing applications, where the nanoparticles may yield large surface areas and the network provides easy transport of charge carriers.

Supporting Information TRI3DYN simulation results for Si atomic fraction in hydrogen titanate nanotubes after irradiated with 10 KeV Ar+ ions; EDS spectrum showing presence of Si and Ar in irradiated hydrogen titanate nanotubes.

Acknowledgements: S.C. acknowledges the supports from DST (India) for project grant no. SR/FTP/PS-183/2011 and from IIT Bhubaneswar. Support by the Ion Beam Center (IBC) at HZDR is gratefully acknowledged.

Figure captions: Figure 1. (a) Scanning electron micrograph, (b), (c) HRTEM images and (d) energy dispersive x-ray spectrum (obtained in the SEM) of pristine hydrogen titanate nanotubes.

Figure 2. (a) Scanning electron micrograph of the sample irradiated with 8×1015 ions.cm-2. HRTEM images of samples irradiated with fluences of (b) 5×1015 ions.cm-2 and (c) 8×1015 ions.cm-2, respectively.



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Figure 3. (a) SEM micrograph of the sample irradiated with 1×1016 ions.cm-2 showing large scale joining. HRTEM images in (b) and (c) showing welded junctions formed at this fluence. Figure 4. (a) SEM micrograph of the sample irradiated with 3×1016 ions.cm-2. (b) TEM image showing that the formed network consists of nanoparticles. (c) and (d) HRTEM micrographs showing individual particles. Figure 5. A schematic representation of the process from pristine nanotubes (a) to bending (b) large-scale welding (c) and nanoparticles formation (d) with increasing ion fluence. Figure 6. Raman scattering spectra of pristine and different irradiated samples. Figure 7. Development of the surface contour as obtained from the TRI3DYN simulation: (a), (b) pristine sample, (c), (e) and (d), (f) after 15 keV Ar+ irradiation at ion fluences of 4×1015 ions.cm-2 and 1×1016 ions.cm-2, respectively. The outer diameter of the nanotubes is 15 nm with a wall thickness of 5 nm. The computational volume involved is 40(depth)×60×60 nm3 with 40×60×60 voxels. The surface voxels are coloured according to the local Ti atomic fraction, ranging from 0 to 0.5. Figure 8. TRI3DYN simulation results for X-Y cut-through the axis of the upper tube. The Ti/O atomic ratios are shown in (a) for pristine sample and after 15 keV Ar+ irradiation at ion fluences of 4×1015 ions.cm-2 (b) and 1×1016 ions.cm-2 (c) respectively.


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For Table of Contents Use Only Discrete single crystalline titanium oxide nanoparticle formation from 2D nanowelded network

Satyanarayan Dhal, Shyamal Chatterjee, Stefan Facsko, Wolfhard Möller, Roman Böttger, Biswarup Satpati, Satchidananda Ratha, René Hübner

Synopsis Ion irradiation induced evolution of hydrogen titanate nanotubes is depicted. With increasing ion fluence the nanotubes are bent, softened and a large-scale welded network of nanowires is produced. At higher ion fluence crystalline nanoparticles of titanium oxide are formed from the nanowire network.


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Discrete Single Crystalline Titanium Oxide Nanoparticle Formation

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