Understanding and Controlling Nucleation and Growth of TiO2

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Understanding and Controlling Nucleation and Growth of TiO2 Deposited on Multi-wall Carbon Nanotubes with Atomic Layer Deposition Yucheng Zhang, Carlos Guerra, Ivo Utke, Johann Michler, Marta D Rossell, and Rolf Erni J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp511004h • Publication Date (Web): 20 Jan 2015 Downloaded from http://pubs.acs.org on January 26, 2015

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The Journal of Physical Chemistry

Understanding and Controlling Nucleation and Growth of TiO2 Deposited on Multi-wall Carbon Nanotubes with Atomic Layer Deposition Yucheng Zhang1*; Carlos Guerra-Nuñez2, Ivo Utke2; Johann Michler2; Marta D. Rossell1; Rolf Erni1 *

Corresponding author’s email: [email protected]

*

Corresponding author’s telephone: 0041 587656552

1

Electron Microscopy Center, EMPA, Swiss Federal Laboratories for Materials Science and

Technology, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland 2

Laboratory of Mechanics of Materials and Nanostructure, EMPA, Swiss Federal Laborato-

ries for Materials Science and Technology, Feuerwerkstrasse 39, CH-3602 Thun, Switzerland KEYWORDS: MW-CNT, TiO2, ALD, aberration-corrected TEM, nucleation and growth

ABSTRACT: Controlled deposition of thin conformal oxide films on carbon nanotubes (CNTs) by atomic layer deposition (ALD) for applications in solar energy and photo-catalysis is still challenging as the early stages of nucleation and subsequent growth are not yet well understood. Here we adopted ALD to grow TiO2 on multiwall carbon nanotubes (MWCNTs). The effect of deposition temperature (120°C-240°C), number of ALD cycles (20750), and surface pre-treatment of the MW-CNTs with oxygen plasma, on morphology and 1 ACS Paragon Plus Environment


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crystallinity of TiO2 was systematically studied using transmission electron microscopy (TEM). By tuning the deposition conditions, controllable nucleation and growth of TiO2 on CNT can be achieved. In particular, high quality crystalline anatase conforming to CNT is obtained with an ALD growth temperature as low as 200°C. Direct observation using aberrationcorrected atomic-resolution TEM imaging operated at 120 keV reveals an island structure of crystalline TiOx at the very early stage of nucleation followed by coalesced growth of crystalline anatase at this temperature. The study also paves the way to understand the interface between the two materials on an atomic level.

1. Introduction:

Nano-composites of TiO2 nano-particles and CNTs can exploit the unique properties of both components. As an environment-friendly semiconductor which can be readily obtained, TiO2 has been one of the most important materials in many applications such as photo-catalysis and solar energy1-3. It has been long discovered to be useful for electrochemical photolysis of water4. On the other hand, the large surface area, superior electrical conductivity and high electron-storage capacity of CNTs are advantageous to enhance the photo-catalytic performance of TiO2, which can benefit many areas such as acquisition of sustainable energy5,6 and prevention of environment pollution7,8. Tremendous research interests have been provoked to synthesize composites of TiO2 and CNTs7,9-11, to improve the photo-catalytic efficiency5,12 and to understand the enhancement mechanisms8, 13,14. So far the mostly adopted methods to synthesize the nano-composite are sol-gel and hydrothermal deposition7-11. In comparison, atomic layer deposition (ALD) can offer several advantages15,16. The technique relies on self-limiting surface reactions of gases which are alternately introduced into and purged out of the reaction chamber, and consequently is capable to achieve conformal deposition with an atomic precision on nano-structural architectures with a 2 ACS Paragon Plus Environment

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large aspect ratio, such as CNTs. The challenge is to provide initial chemisorption sites on the relatively inert surface of CNTs by modifying the chemical (and electronic) structure of CNTs through functionalization or surface defects and by optimized growth conditions, so as to achieve controlled nucleation and growth. Progress has already been made to form conforming oxide layers on CNTs using ALD17-22. Homogenous oxide layers such as Al2O3, ZnO and TiO2 with thickness less than 10 nm can be achieved. The layers usually have an amorphous form. Achieving a crystalline conformal layer is still difficult. More importantly, a detailed study on the early stage of ALD deposition on CNTs is, up to our knowledge, yet to be done. In this article, we present a systematic characterization of TiO2 on MW-CNTs at the early stage of nucleation using transmission electron microscopy (TEM), complementary to the study on the conformal growth of TiO223. The present work includes an atomic-scale study of crystalline nuclei grown at a high temperature of 200 °C using an aberration-corrected TEM. Extra care has been taken by using a low acceleration voltage of 120 keV to avoid any electron-beam-induced crystallization24 or amorphization through knock-on damage25. Through the TEM characterization, the effect of the temperature, the number of ALD cycles, and the pre-treatment of CNT surface on the nucleation and growth of TiO2 on MW-CNT has been studied, which allows for achieving controlled morphology and crystallinity that may be desired for technological applications based on the ensemble. Revealing the atomic structure of the nuclei at the early stage of deposition provides crucial information about the fundamental processes underlying nucleation and growth of ALD-deposited TiO2 on CNTs. 2. Experimental Section: 2.1 ALD deposition Powders of chemical vapor deposition (CVD) grown MW-CNTs purchased from Nanocyl were used in this study for ALD deposition of TiO2. The CNT powders were dispersed onto 3 ACS Paragon Plus Environment


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TEM holey carbon-film grids in ethanol droplets. A selection of the samples on the TEM grid was pre-treated with Ar2/O2 plasma for 30s. The samples were then transferred into an ALD reaction chamber for TiO2 deposition at various temperatures of 120°C, 160°C, 200°C and 240°C for 20, 100, 200 and 750 cycles. Titanium-isopropoxide Ti(OCH(CH3)2)4 (TTIP) and H2O were used as precursors for Ti and O respectively, and Ar was used as purge and carrier gas. 2.2 TEM characterization After the deposition, the samples were studied with extensive TEM techniques. A JEOL-2200 FS microscope was used for selected area electron diffraction (SAD) and high-resolution imaging in the TEM mode, as well as high angle annular dark field (HAADF) imaging and electron energy loss spectroscopy (EELS) in the scanning TEM (STEM) mode, in order to examine the crystallinity and morphology of each sample. Atomic-resolution TEM imaging on an aberration-corrected microscope JEOL-200 ARM was performed to investigate the samples of 20 ALD cycles where small TiO2 nuclei are formed. A low acceleration voltage of 120 kV was used to minimize electron beam damage on the nuclei and CNTs. To obtain the atomic structure of the nuclei, HR-TEM simulations were performed using a multislice method in JEMS26. The parameters chosen for the defocus and the thickness are close to the experimental value, -10 µm for C3, 1 mm for C5 and 0.2 eV for energy spread, which gives a contrast transfer function (CTF) similar to the one used in the experiment. 3. Results and discussion 3.1 Early stage nucleation of TiO2 on MW-CNTs Figure 1 shows STEM-HAADF images of TiO2 on MW-CNT deposited for 20-cycles at 160°C, 200°C and 240°C respectively. With such a low number of cycles, small TiO2 nuclei of less than 2 nm are formed. Due to the small size, the TiO2 nuclei are hardly differentiable 4 ACS Paragon Plus Environment

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from the amorphous carbon on the MW-CNT surface in HR-TEM images. The amorphous carbon arises from the CVD-growth of the CNT. In contrast to HR-TEM imaging, the Zcontrast in STEM-HAADF imaging can readily distinguish TiO2 due to its higher atomic number in comparison to C. The images in Figure 1 show that while the size of the nuclei is similar at all temperatures, the density is clearly decreasing with increasing temperatures. The high density of the nuclei at 160°C appears to cover most of the MW-CNT surface while at 240°C only sporadic nuclei appear. It indicates that the dissociative chemisorption of the precursors on MW-CNTs is more likely to occur at a lower temperature. The similar size of the nuclei at different temperatures is an indication that the growth per cycle (GPC) during the nucleation period is not dependent on temperature, but rather that the temperature limits the number of available reactive (OH containing) surface groups on MW-CNTs for TTIP to chemisorb and nucleate.

Figure 1: TiO2 nucleation on MW-CNTs after 20 ALD cycles at different temperatures characterized with STEM-HAADF imaging. The top row shows an overview of the sample and the

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bottom row zooms in on individual CNTs. The nuclei densities decrease from 160°C to 240°C while the nuclei size remains similar.

To further investigate the nucleation of TiO2 on MW-CNTs at the early stage, atomicresolution TEM images were taken from the samples of 20 ALD cycles using a third-order aberration-corrected microscope. A small negative value of about -10 µm for the third-order aberration coefficient, along with a small over-focus of about 6 nm, was used in order to optimize the phase contrast in respect of the small positive fifth-order spherical aberration27. That is, the contrast of the image can be enhanced such that both Ti and O atoms can be resolved directly28. Two samples, one deposited at 120°C and another at 200°C, were compared, as shown in Figure 2. It shows the difference between the two samples in terms of crystallinity. The TiO2 nuclei deposited at 120°C are mostly amorphous (Figure 2(a)) while they are crystalline at 200°C (Figure 2(b) and (c)). Since both samples were imaged at the same condition, the possibility of electron beam induced crystallization can be ruled out. The size of the nuclei is similar in both samples, less than 2 nm. In some regions, coalescence of the nuclei can be observed, leading to an island structure of about 7~8 monolayers high (less than 2 nm) and up to ~10 nm wide (More evidence see Figure S1 in Supporting Materials). All the nuclei, whether amorphous or crystalline, seem to be associated with an atomic step on the surface of MW-CNT, as pointed out by the arrows. This is direct evidence that the nucleation is preferred at a defective site where possible C dangling bonds and -OH groups are present. The probability that the nucleation takes place on these sites is dependent on the temperature, as indicated by Figure1. Figure 2(c) shows a region where a crystalline particle sits on a dislocation in the MW-CNT.


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Figure 2: Aberration-corrected HR-TEM images of (a) TiO2 on pristine CNTs deposited at 120°C after 20 ALD cycles, and (b) deposited at 200°C after 20 ALD cycles. The arrows denote the atomic steps on the CNT surface. (c) A digitally magnified image of a different region from the sample in (b) that has been processed with a Wiener filter. The left image in the inset is a magnified image of the area in the dotted box. The dotted line measures the spacing between two Ti atoms, ~3.0 Å, consistent with a TiO rock salt structure. The right one is a simulated image overlaid with Ti atoms (green) and O atoms (red). The arrows point at the alternative stacking of Ti atoms (green) and O atoms (red) in the (111) plane.

To reveal the atomic structure of the nuclei, HR-TEM image simulations using JEMS has been used to compare with the experiment image (further details seen in Figure S2 in Supporting Materials). Simulations were run on major zone axes of anatase and rutile TiO2 including [100], [001], [101], [110] and [111], but none of them matched the experimental image in Figure 2(c) in terms of symmetry and lattice distance. However, good match was found for the [110] zone axis of a rocksalt TiO structure29. Vacancies in the oxygen column could potentially explain the intensity variation of the atomic columns in the image28. The {111} lattice planes are entirely composed of either Ti or O, hence the cubic TiO may facilitate the 7 ACS Paragon Plus Environment


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layer-by-layer growth in ALD, which explains non-hemispherical shape of the nuclei observed frequently at the early stage. Other reduced titanium oxides such as hexagonal phase TiO and Ti2O3 may also be present at the early nucleation stage since they both contain the alternative stacking of Ti and O planes that corresponds well to the ALD sequence. The TEM observation of the reduced TiOx is also consistent with the X-ray photoelectron spectroscopy (XPS) analysis which shows a stronger signal from reduced Ti species at the early nucleation stage (see Figure S3 in Supporting Materials). While the XPS measurement results from an averaged area containing a large amount of TiOx and CNTs, the HR-TEM images pinpoint the exact location of the nuclei. The formation of the reduced TiOx at the early stage of nucleation may be due to the interaction at the interface between TiOx and the tube wall, that is, C-O bonds may form at the interface1, 30. In comparison, Felten has deposited pure Ti metal on CNTs using e-beam evaporation and found O impurities at the interface which weakens the strong interaction of Ti and CNTs via Ti-C bonds30. In our case, the abundance of O allows more oxygenated sites on the surface of CNTs, facilitating nucleation of TiOx. Less Ti-C interaction is expected in our ALD samples. Especially at a low ALD temperature a weak van der Waals force may also be present at the interface.

3.2 Effect of ALD temperature on the growth of TiO2 on MW-CNTs

Figure 3 shows TEM images of TiO2/MW-CNT after 750 ALD cycles at 120 °C and 200 °C respectively, together with the SAD patterns. Both samples show a conformal coalesced TiO2 layer of about 20 nm thickness along the MW-CNTs. However, the TiO2 film is deposited in amorphous structure at the low temperature, while at high temperature it is deposited crystalline. The diffraction pattern of the crystalline TiO2 indicates an anatase phase, which is also confirmed by X-ray diffraction (see Figure S4 in Supporting Materials). The high temperature therefore promotes growth of crystalline TiO2, and a conforming layer is obtained once the 8 ACS Paragon Plus Environment

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anatase nano-crystals coalesce. Similar observations of the influence of ALD temperature on the crystallinity of TiO2 on MW-CNT and graphene31,32 as well as on planar silicon substrates33 have been reported. However, our work reports a substantially lower deposition temperature of 200 °C at which a conforming layer of crystalline TiO2 is obtained.

Figure 3: TEM images and SADPs of 750 ALD cycles of TiO2 on MW-CNT. (a) and (b) are at 120°C. (c) and (d) are at 200°C. The SADP in (d) is indexed and shows crystal planes of anatase in the reciprocal space, while the SADP in (b) shows a diffusive pattern indicating an amorphous TiO2.



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3.3 Effect of oxygen plasma pre-treatment of MW-CNT and ALD cycle number The morphology and crystallinity of TiO2 grown on MW-CNTs can be controlled by tuning the ALD conditions as well as pre-treatment of the tubes. Figure 4 compares TiO2 ALD films obtained at 200°C for 20, 200, and 750 cycles on as-received and O2 plasma treated MWCNTs. The samples were prepared in the same ALD run for a given cycle number. The effect of oxygen plasma treatment on the nucleation and growth is clearly visible. After 20 ALD cycles, TiO2 is nucleated on the surface. The density of the nuclei is much higher in the sample with the plasma pre-treatment and the nuclei have an amorphous morphology similar to the one grown at the temperature of 160°C. It is expected that the plasma pre-treatment introduces additional reactive defects on the surface of the MW-CNT, which facilitates the ALD surface reaction to form the nuclei34-36. While the SAD pattern shows a diffusive background, there are a few reflections which may indicate the presence of larger crystalline nuclei, as shown in the inset of Figure 4(a) and (d). The rings observed in the SAD pattern are due to the diffraction of the MW-CNT. After 200 cycles, crystalline TiO2 in the amorphous titanium oxide matrix is clearly visible, as indicated by the lattice fringes in the HR-TEM images (also see Figure 5(a)), consistent with the SAD pattern in Figure 4 (b) and (e). The morphology of the TiO2 crystallites in the two samples, with and without the plasma pre-treatment, is quite different. Without the plasma pre-treatment, the TiO2 nuclei grow to large crystals with an average size of 7.4 nm, but do not fully coalesce at this stage (200 cycles) due to their low surface density. With the plasma pre-treatment, a layer of TiO2 conforming onto the MW-CNT is formed due to the full coalescence of the numerous laterally growing nuclei. Both morphologies can be exploited in applications based on the nano-composites. The former morphology is beneficial for photo-catalysis where the small particles TiO2 enhance the total surface area and can improve the efficiency of light absorption in TiO2 and electron collection in 10 ACS Paragon Plus Environment

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CNTs14,37. The latter can be favorable for anode electrodes used in dye-sensitized solar cells (DSSC) where a coalesced TiO2 layer prevents direct contact between CNT and electrolyte, and consequently reduces recombination of photon-induced charges38. In both applications, a crystalline structure of TiO2 may have a beneficial effect on the device performance by facilitating charge transport. At 750 ALD cycles a conformal layer of TiO2 is also obtained for pristine MW-CNTs, as shown in Figure 4(c). For the 750 cycle films on both pristine and oxygen plasma treated MW-CNTs the film thickness is the same and is about 20 nm (corresponding to a GPC of 0.26 Å/cycle) and large crystalline particles constitute the film morphology (also see Figure 5(b)). The SAD pattern indicates the presence of the anatase phase.



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Figure 4: TEM images of TiO2 on MW-CNTs deposited at 200°C. (a), (b) and (c) are after 20, 200, 750 cycles respectively. (d), (e) and (f) are on the CNT pre-treated with O2 plasma and subsequently deposited for the same cycles. The inset shows SADP from the corresponding sample. The scale bar is 5 nm-1. The top histograms are the size distributions of TiO2 particles measured from the samples in (a) and (b) respectively. For the other samples, a coalesced layer is formed and hence similar measurement is not possible.

Figure 5 shows the HR-TEM images and the EELS spectra of the Ti_L2,3 edge for the samples after 200 and 750 cycles on pristine CNTs. Spectra including O_K edges of the samples can be found in Figure S5 in Supporting Materials. The images show that both layers consist of crystalline particles whose facets are outlined, while the 200-cycle sample also contains amorphous particles, which is consistent with the SAD patterns shown in Figure 4. Using the fine structure near the Ti_L2,3 edge in EELS, the crystallinity and chemical state of the TiO2 at the various stages can also be analyzed. It exploits the fact that the lowest un-occupied energy levels of crystalline TiO2 have discrete orbitals due to crystal field, resulting in a split within the Ti_L2 and L3 peaks reflecting the energy difference of the eg and t2g orbitals39. In amorphous TiO2, the split is missing because of the absence of the crystal field40. In addition, the split can also be obscured by the different Ti oxidation states, because the position of the Ti_L2 and L3 peaks shifts with the oxidation state of Ti atoms41. The spectra for both samples in Figure3 (b) and (d) show clearly the resolved fine structure of Ti_L2,3 edge. For the samples of 20 and 100 cycles, the EELS signal is too weak to resolve the fine structure (see Figure S5 in Supporting Materials). For the 200-cycle sample, a low intensity shoulder at the energy range of 457~459 eV appears on the Ti_L2,3 edge, indicating the contribution from both amorphous and crystalline TiO2. The peak split due to the presence of the eg and t2g orbitals is clearly observed for the 750-cycle sample, indicating that the amount of crystalline TiO2 is significantly more. Probably all the particles are crystalline. The observation can also be interpreted as the presence of more mixed oxidation states of Ti atoms in the 200-cycle sample, since the peaks from Ti of different valences such as reduced Ti2+ and Ti3+ may convolute with that of Ti4+, obscuring the peak split. Overall, the STEM-EELS study shows that the sample of 750 cycles contains large particles of a completely crystalline and stoichiometric 12 ACS Paragon Plus Environment

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anatase phase. It is noted that the fine structure of Ti_L2,3 edge of anatase is slightly different from those reported in literature, in particular the intensity of the t2g and eg peaks39, 42. This can be explained by the composition of the layer which may still contain a small amount of titanium oxides other than anatase including amorphous and reduced oxides. Indeed, the peak intensity corresponds well with that from the titania-based nanotubes which were shown to have different long range connectivity in the arrangement of TiO6 octahedra from anatase and rutile42, 43. In addition, the experiment conditions used in our work including energy resolution and collection angle etc. may also result in different peak intensities.

Figure 5: Evolution of amorphous versus crystalline TiO2 with ALD cycle numbers on pristine CNTs at 200°C, characterized using HR-TEM imaging and EELS: (a) and (b) are after 200 cycles; (c) and (d) are after 750 cycles. Faceted crystalline particles are outlined with the dot-



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ted lines in the images. The arrows in (b) point at the peak shoulder indicating the presence of amorphous TiO2.

3.4 Phenomenological picture of nucleation and growth of anatase Based on these observations, the early stage nucleation and subsequent growth of TiO2 on MW-CNTs can be well understood. The mechanism is schematically illustrated in Figure 6. At a very early stage of nucleation, e.g. after 20 ALD cycles or even less at 200°C, both TiO2 (amorphous and crystalline) and reduced TiOx including TiO, and Ti2O3 with the size less than 2 nm can nucleate as 2D islands on defective sites of CNTs such as the atomic step on the surface. The dominant species is probably the reduced TiOx. This is evidenced by the frequent observation of the atomic structure similar to that in Figure 2(c). The structure can also be revealed with STEM-BF and STEM-HAADF imaging (an example shown in Figure S1 in Supporting Materials). In addition, XPS analysis indicates more pronounced peaks for Ti2+ and Ti3+ at the early nucleation stage. Not only depending on the temperature, may the formation of the initial crystalline TiOx nuclei also depend on the substrate, which determines the bonding at the interface. However, it is not known yet if the reduced TiOx transforms to TiO2 or remains at the interface at a later growth stage, the knowledge of which can be crucial to understand the interfacial bonding of the ensemble and the charge transport in applications based on them. As the deposition proceeds, the size of the nuclei increases and TiO2 particles become dominant. After 200 cycles for instance, crystalline particles of TiO2 with an average size of 7 nm are formed, competing with the growth of amorphous TiO2 particles, as sketched in Figure 6(b). The crystalline particles have hemispherical shape with facets and are distributed on the surface of CNTs. After an extended number of cycles, e.g. 750 cycles, the particles grow large enough to coalesce, as in Figure 6(c). The coalesced layer therefore shows the profile of the facets of the anatase. The amount of the amorphous TiO2 is significantly suppressed, leading to a completely anatase. Since it took over 30 hours to run 750 cycles in our 14 ACS Paragon Plus Environment

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ALD experiment, thermal annealing at 200°C could have transformed the amorphous particles to the crystalline ones. A separate annealing experiment in which deposition of TiO2 is performed at a low temperature and then subject to a high temperature annealing to further control the crystallinity and conformity of the TiO2 layer is ongoing. The number of cycles required to achieve a coalesced crystalline layer can be reduced if the MW-CNT is pre-treated with O2 plasma which increases significantly the density of nucleation sites. However, in this case amorphous TiO2 remains in the coalesced layer. The surface pretreatment is complementary to the previous work where a thin layer of Al2O3 was deposited before ALD of TiO2 on CNTs and graphenes22. Functionalization of the inert surface of the carbon-based nanomaterials provides a better control over the morphology and conformity of the deposition layer. In addition, our current work is a step forward in controlling the crystallinity of TiO2 through the deposition temperatures.



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Figure 6: Schematic illustration of the early stage nucleation and subsequent growth of ALDdepositedTiO2 on MW-CNTs at 200°C: (a) After 20 cycles, the small nuclei can exist as reduced TiO (the left particle), anatase (the middle particle) and amorphous TiO2 (the right particle). Some of them are coalesced to form islands of less than 2 nm in height and up to 10 nm in width. The unit cell of cubic TiO and anatase is shown. (b) After 200 cycles, faceted anatase particles and amorphous TiO2 particles grow up. The average size of the anatase particles is about 7 nm. (c) The 3D growth of the anatase particles results in a coalesced conforming layer on MW-CNTs with the layer thickness ~20 nm at around 750 cycles.

4. Conclusions In conclusion, we have systematically studied nucleation and growth of ALD-deposited TiO2 using TEM characterization. Controlled morphology and crystallinity can be achieved by tuning the parameters in ALD such as temperature, number of cycles as well as surface pretreatment of the MW-CNT. At the temperature > 200°C, crystalline anatase on CNT can be obtained, either in the form of individual particles with an average size of ~7 nm after 200 ALD cycles or a conforming layer comprising of coalesced particles with the thickness about 20 nm after 750 cycles. At temperatures below 200°C, high nuclei density results in thin compact amorphous titanium oxide films. Oxygen plasma treatment also enhances density of 16 ACS Paragon Plus Environment

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nuclei and gives compact anatase layers at >200°C ALD at lower thicknesses than on pristine MW-CNTs. State-of-art aberration-corrected TEM imaging was used to directly resolve the atomic structure of small TiOx nuclei on CNTs. The atomic-scale study has revealed crystalline TiOx (x

Understanding and Controlling Nucleation and Growth of TiO2

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