Influence of Different Defects in Vertically Aligned Carbon Nanotubes

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Influence of Different Defects in Vertically Aligned Carbon Nanotubes on TiO2 Nanoparticle Formation through Atomic Layer Deposition Luiz Acauan,*,†,‡ Anna C. Dias,†,§ Marcelo B. Pereira,∥ Flavio Horowitz,∥ and Carlos P. Bergmann† †

Department of Materials, Federal University of Rio Grande do Sul, Porto Alegre, RS 90040-060, Brazil Department of Aeronautics and Astronautics, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States § Department of Chemical Engineering, Federal University of Rio Grande do Sul, Porto Alegre, RS 90040-060, Brazil ∥ Institute of Physics, Federal University of Rio Grande do Sul, Porto Alegre, RS 90040-060, Brazil ‡

ABSTRACT: The chemical inertness of carbon nanotubes (CNT) requires some degree of “defect engineering” for controlled deposition of metal oxides through atomic layer deposition (ALD). The type, quantity, and distribution of such defects rules the deposition rate and defines the growth behavior. In this work, we employed ALD to grow titanium oxide (TiO2) on vertically aligned carbon nanotubes (VACNT). The effects of nitrogen doping and oxygen plasma pretreatment of the CNT on the morphology and total amount of TiO2 were systematically studied using transmission electron microscopy, Raman spectroscopy, and thermogravimetric analysis. The induced chemical changes for each functionalization route were identified by X-ray photoelectron and Raman spectroscopies. The TiO2 mass fraction deposited with the same number of cycles for the pristine CNT, nitrogen-doped CNT, and plasma-treated CNT were 8, 47, and 80%, respectively. We demonstrate that TiO2 nucleation is dependent mainly on surface incorporation of heteroatoms and their distribution rather than structural defects that govern the growth behavior. Therefore, selecting the best way to functionalize CNT will allow us to tailor TiO2 distribution and hence fabricate complex heterostructures. KEYWORDS: atomic layer deposition, vertically aligned carbon nanotubes, titanium oxide, nitrogen-doped carbon nanotubes, plasma treatment, defects

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INTRODUCTION

of adsorbed metals and metal−organic compounds. Thus, the fabrication of TiO2 with nanometer-scale morphology is a research area of considerable interest for those applications. The large surface area, superior electrical conductivity, and high electron-storage capacity of carbon nanotubes (CNT) are advantageous properties for enhancing the photocatalytic performance of TiO2, which could benefit many areas such as the acquisition of sustainable energy8 and the prevention of environmental pollution.9,10 Tremendous research interest has been focused on the synthesis of TiO2/CNT composites11−15

Nanocarbon−inorganic hybrid materials are a new class of functional materials that have gained enormous interest in recent years due to their exceptional thermal, biological, mechanical, electrical, and optical properties. These outstanding properties enable their use in biomaterial, photochemical, catalytic, and electrochemical technologies.1 Another important technological material, and one of the most important transition-metal oxides, is titanium dioxide (TiO2). Its remarkable chemical and physical properties2 make it possible for use in applications such as photocatalysis,3 gas sensing,4 photochromic devices,5 and dye-sensitized solar cells.5−7 Generally, a large specific surface area is crucial to achieve high photocatalytic activities as well as good dispersion © XXXX American Chemical Society

Received: April 4, 2016 Accepted: June 7, 2016

A

DOI: 10.1021/acsami.6b04001 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Figure 1. SEM images from VACNT: (a) PCNT and (b) NCNT. Top view of the PCNT array (c) before and (d) after plasma treatment (OCNT).

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to improve their photocatalytic efficiency and understand the mechanisms of their performance enhancement.16,17 In addition, the possibility of growing highly oriented structures on different substrates18,19 will facilitate the process of packing and handling it, enabling its application in other fields such as energy storage.20,21 Different methods have been used to synthesize these nanocomposites such as sol−gel processing and hydrothermal deposition.9,10,13−15 Among these, atomic layer deposition (ALD) provides the most elegant, efficient, and controllable way to coat high aspect ratio nanostructures and has already been successfully applied for coating CNT.12,21−23 However, film growth using ALD requires the presence of surface functional groups and defect spots that act as anchoring and nucleation sites in the beginning of the film growth.22,23 Therefore, the inert nature of high-quality CNT with a graphitic surface leads to incomplete coatings. Most studies of ALD on CNT defect sites are related to functional groups containing only C−C, C−O, or C−H bonds (ketone, ether, carboxylic acid, etc.) generated by acid or plasma treatment and noncovalent functionalization.22,24−26 However, to the best of our knowledge, structural defects or doped heteroatoms such as C−N bonds on nitrogen-doped CNT still need to be investigated. In this work, we studied the effect of TiO2 deposition by ALD onto three types of CNT: pristine multiwalled carbon nanotubes (PCNT), oxidized plasma-treated carbon nanotubes (OCNT), and nitrogen-doped carbon nanotubes (NCNT). The defects presented on these CNT types were analyzed and compared qualitatively and quantitatively with the formation of the TiO2 nanoparticles.

EXPERIMENTAL SECTION

Growth of Aligned CNT. Vertically aligned multiwalled carbon nanotubes (VACNT) were grown on silicon substrates covered by a thin catalyst layer using a liquid−vapor thermal chemical vapor deposition (CVD) method. Briefly, the substrates were loaded in a quartz boat and placed in the middle of a quartz tube furnace under argon flow. When the furnace reached the synthesis temperature, the tube was filled with hydrogen for 2 min, and then the liquid carbon source was introduced by bubbling argon through the tube. For the PCNT, the synthesis temperature was 800 °C, and hexane (kept at 15 °C) was fed into the reactor for 10 min. For the NCNT, the synthesis temperature was 900 °C, and acetonitrile, the carbon and nitrogen source, was fed into the reactor for 30 min. The substrates were silicon wafers with a thermal oxide layer (100 nm) covered with Al2O3 (20 nm) and iron (1.4 nm) for the PCNT samples, while the NCNT samples had only an iron layer (4.4 nm) on top of the oxide. The samples were cooled naturally inside the furnace under argon flow until the temperature reached 500 °C. Then, the samples were moved to a cold zone and removed at room temperature. The typical PCNT synthesized have an average diameter of 14 nm, while the NCNT have an average diameter of 35 nm. Plasma treatment for CNT arrays (OCNT) was performed in a homemade cylindrical vacuum chamber under a 50/50 argon/oxygen atmosphere at 46 Pa. The plasma treatment was carried out at 150 W for 30 min. Atomic Layer Deposition. The TiO2 coating on the CNT was made by ALD using a Beneq TFS200 reactor. Titanium isopropoxide (Ti(OCH(CH3)2)4, Sigma-Aldrich), heated at 60 °C, was used as the Ti precursor, and deionized water at room temperature was used as the oxidizing agent. The reactor chamber was kept at 250 °C during the film deposition, and the Ti precursor and water were introduced into the reactor chamber by a flow of Ar following this sequence: 500 ms pulse of Ti(OCH(CH3)2)4, 1 s purge, 600 ms pulse of H2O, and a 1 s purge. All depositions were performed with 200 cycles followed by heat treatment at 400 °C for 2 h to crystallize the TiO2 layer. Characterization. CNT array characterization was performed by scanning electron microscopy (SEM) on a JEOL JSM 6060 instrument B


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of defects.29 The decrease in the G′ peak intensity is also evidence of defects caused by the incorporation of N atoms and is characteristic of doped CNT.30 XPS analysis was used to more precisely identify which defects were present and quantify the incorporation of heteroatoms. The only elements appearing in this analysis besides carbon were nitrogen and oxygen. The heteroatom (X) content, which is defined as the (N or O)/(C + N + O) atomic ratio (given as a percentage), was estimated from the area of the N, O, and C 1s peaks, taking into account their relative sensitivities. The estimated atomic percentages are summarized in Table 1.

with a beam voltage of 20 kV at a 30° incidence angle. Sample morphology and crystallinity were characterized through transmission electron microscopy (TEM) images taken using a JEOL JEM2010 instrument operated at 200 kV. Samples were prepared for TEM imaging as follows: an ultrasonic bath was used to disperse the CNTTiO2 composites in acetone, and immediately after, a small amount of the dispersion was deposited on Cu TEM grids with a holey carbon mesh and then allowed to dry. The presence of TiO2 anatase phase as well as the evaluation of defects on the CNT arrays was characterized using a Renishaw inVia microRaman System with a laser wavelength of 532 nm focused onto the sample with a 50× objective lens. Three measurements were taken from different locations on each sample, and the collected data were averaged. The graphitic quality, defect density, and presence of doped atoms on the CNT were investigated by X-ray photoelectron spectroscopy (XPS) using an Omicron-EA125 station with Mg Kα radiation (1253.6 eV). Thermogravimetric analysis (TGA) experiments were conducted in a TA Instruments SDT-Q600 analyzer heated at a rate of 10 °C/min in air to 800 °C.

Table 1. Atomic Percentage of CNT Samples

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N O C

RESULTS AND DISCUSSION Evaluation of CNT defects. The VACNT used in these experiments are shown in Figure 1. The PCNT (Figure 1a) arrays have an average height of 400 μm, while NCNT (Figure 1b) are 90 μm tall. In Figures 1c and 1d, it can be seen that the tips of the PCNT arrays change after plasma treatment to create the OCNT, but the height and diameter remain basically the same. Even if the PCNT forests are longer and denser than the NCNT forests, these factors do not affect in any way the following investigations: the ALD will perform in the same way independently of these factors. Raman analysis was used to identify the presence of defects on the CNT. This technique offers the advantage of being a quicker analysis compared to other techniques. Figure 2

PCNT

NCNT

OCNT

2.15 97.85

2.12 1.05 96.82

13.18 86.82

As indicated in Table 1, the amount of X is much higher for the OCNT compared to the PCNT and NCNT. This trend is reflected in the C 1s peak of each sample (Figures 3a−c). A minor shift to higher binding energies is expected for the NCNT when compared with that of the PCNT,31 but a similar amount of total heteroatoms (N + O) makes this shift negligible. On the other hand, the OCNT shows a large variety of C−X bonds aside from the original C−C graphitic bond. These components were deconvoluted into four bands: PC1, PC2, PC3, and PC4 at 284.6, 285.3, 286.9, and 288.7 eV, respectively, corresponding to the C−C bonds in graphite, sp3hybridized carbon atoms C−C, C−O (e.g., alcohol and ether), and CO (e.g., carboxylic and ester) functionalities.32 The relative areas of the PC1, PC2, PC3, and PC4 bands for the OCNT are 40.5, 30.4, 18.5, and 10.6%, respectively. Figure 3d shows the N 1s peak for the NCNT sample deconvoluted into four bands: PN1, PN2, PN3, and PN4 at 398.0, 400.8, 404.8, and 403.5 eV, respectively, corresponding to the pyridine-like N, substitutional or graphitic-like N, intercalated N2, and N-oxidic species.33 The relative areas of the PN1, PN2, PN3, and PN4 bands for the NCNT are 19.2, 39.0, 36.9, and 4.9%, respectively. Comparing both methods, Raman and XPS, for the characterization of defects, we observe that Raman shows a similar amount of “imperfections” for the NCNT and OCNT, while XPS indicates a presence of defects much higher for OCNT. This is caused by the fact that Raman is very sensitive to many distortions such as CNT diameter,34 defect distribution,35 presence of amorphous carbon or other nanocarbons,27,36 and many other types of structural defects.37 All of these elements affect the D peak differently, while XPS has a more direct response to atom-to-atom bonds (short range). Morphology of TiO2 Crystals on the CNT Surface. Figure 4 shows CNT samples covered by TiO2 (ALD deposition). The PCNT (Figures 4a and b) have the smallest particles among the three samples as well as amorphous TiO2 on some of the CNT walls. The TiO2 coverage on the PCNT is not uniform; many nanotubes show no visible TiO2. From the analysis of many nanotube images, we observe tubes with severe structural defects (wall deformations, open tips, etc.) but no particles attached, while other more “crystalline” tubes show good TiO2 coating.

Figure 2. Normalized Raman spectra of the PCNT, OCNT, and NCNT samples showing the three main peaks from multiwall CNT: D, G, and G′.

compares the main peaks for the three different samples. All spectra were normalized by the G peak intensity, and the ID/IG ratios are specified for each one. The D peak is related to different types of defects in CNT,27 and ID/IG is the most common metric used for comparison. The OCNT have the highest ID/IG value, showing that the plasma treatment was effective at creating defects. This treatment mainly breaks C−C single and double bonds and attaches O atoms on the outer shell of the CNT.28 For the NCNT, the ID/IG is slightly smaller than the value measured for the OCNT, but the D peak is much broader, which also indicates the presence of a large amount and variety C


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Figure 3. C 1s XPS spectra of (a) PCNT, (b) NCNT, and (c) OCNT. (d) N 1s XPS spectrum of NCN.

Figure 4. TEM images from CNT after deposition of TiO2 using ALD (200 cycles) and 400 °C heat treatment for (a and b) PCNT, (c and d) NCNT, and (e and f) OCNT.

Figure 5. Diameter distribution histograms of TiO2 nanoparticles on PCNT (left) and NCNT (right). For the OCNT samples, a coalesced layer was formed, therefore similar measurements were not possible. This distribution was determined from a series of TEM images by measuring the area (A) of each particle and calculating the average diameter (D) by approximating it as a circle (A = πD2/4). Average diameters D ± 0.5 nm were established over 42 particles for the PCNT and 33 particles for the NCNT. D


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Figure 6. Raman spectra in the TiO2 spectral range for the PCNT, OCNT, and NCNT samples. The three spectra were normalized by the G band. The inset shows the PCNT TiO2 peak using a higher laser power and longer signal accumulation (not normalized).

The NCNT shown in Figures 4c and d present a high density of TiO2 particles distributed over all of the nanotubes. The distribution is uniform, and amorphous TiO2 was not identified. There were no specific spots on the nanotubes (tips or “bamboo nodes”, for example) with higher or lower particle densities. On the other hand, the OCNT shown in Figures 4e and f present a morphology different than that of the previous samples. It is not possible to identify single particles; the nanotubes are coated by a layer of crystalline TiO2 formed by “grains”. These grains are monocrystalline, covering the nanotube from one side to another in many regions and measuring dozens of nanometers along the axis of the tube (Figure 4e). As illustrated in Figure 5, single particles deposited over the NCNT have a small size distribution, concentrated mainly in approximately a 7−8 nm diameter range, while the particles on the PCNT have broader distribution. It is evident from the images in Figure 4 that the deposition of TiO2 using ALD is highly dependent on the defect distribution on the nanotube walls, but more precisely, on the presence of carbon-heteroatom bonds. The uniform distribution of nucleation sites (C−N bonds) at the NCNT walls, although in a relatively lower number when compared to that of OCNT, favors a three-dimensional (3D) growth mode where the islands grow during each cycle. The same type of growth can be seen in the PCNT but with an inferior distribution and lower density of surface anchoring sites. On the other hand, the OCNT have a high density of nucleation sites that allow the ALD precursors to react completely with the nanotube wall, promoting a two-dimensional (2D) growth mode.38 Quantification of TiO2 versus Defects. Raman spectra gave us the first indication of the amount of TiO2 for each CNT sample. Figure 6 clearly shows the main peak of the anatase phase (Eg(1)) at 143 cm−1 and gives the ratio between this peak and the G band of the CNT (ITiO2/IG).39 The ITiO2/IG for the PCNT is almost zero and could only be identified using higher laser power and longer data accumulation, indicating the presence of a low amount of TiO2 in the PCNT samples. This value is lower than that expected because the greater part of TiO2 is amorphous, not anatase, decreasing the total signal of ITiO2.

The ITiO2/IG values for the NCNT and OCNT are very similar, though higher for the NCNT, which is different from what was expected. We believe that this behavior is because the G band of N-doped carbon nanotubes is broader but has an intensity lower than that of pristine CNT, masking the ITiO2/IG value.29 A more precise quantification of the amount of TiO2 deposited on the CNT was performed using TGA. The pure CNT showed no residual catalyst traces, which is common for VACNT, where the catalyst stays attached to the substrate. The TiO2 mass fraction was calculated by taking the residual mass after the CNT-TiO2 composites were heated to 800 °C and dividing it by the initial total mass of the sample. As all of the samples were treated previously at 400 °C in air for 2 h, it was expected that no amorphous carbon remained, so all mass lost is assumed to be from the CNT. The results are summarized in Table 2. Table 2. Mass Fraction of CNT/TiO2 Composites PCNT NCNT OCNT

CNT (%)

TiO2 (%)

92 53 20

8 47 80

In agreement with the TEM images, the amount of TiO2 in the NCNT is much higher than that in the PCNT and even higher than that in the OCNT. These results are also proportional to the presence of C−X bonds measured by XPS analysis but not directly related to the indication of defects shown through the Raman D and G′ peak analysis. This indicates that TiO2 nucleates on C−X bonds and not on any kind of defective C−C bonds. In addition, the large D peak for the NCNT is a consequence of distortions provoked by nitrogen atoms in the NCNT inner walls which do not contribute to TiO2 nucleation. This trend can also be seen in Figure 7, where the upper PCNT has a large amount of distortions and conformation defects, while the PCNT on the bottom has straight graphitic walls. Even with such a visible difference of wall deformations, neither sample has TiO2 particles after ALD but only what seem to be a few amorphous TiO2 clusters. E


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Figure 7. Two different PCNT showing no TiO2 particle formation.

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CONCLUSION In this work, it was shown that TiO2 deposition using ALD on CNT is dependent mainly on carbon-heteroatom defects rather than structural defects. Although Raman spectroscopy is an important tool for the identification of a broad range of “CNT imperfections”, XPS was a better tool to investigate the defects that influence metal oxide deposition using ALD. We concluded that the relatively low concentration of heteroatoms on the NCNT and PCNT promote a 2D growth mode, while the highly functionalized OCNT allow a 3D growth mode. It was also shown that the total number of these defects defines the amount of TiO2 deposited. Furthermore, it was shown that doped CNT are a good alternative when the goal is the formation of TiO2 nanoparticles with good size control and uniform distribution over the tube wall. The PCNT with random defects generated a broad dispersion of poorly crystallized particles. On the other hand, the use of plasma treatment was a powerful tool for highly efficient TiO2 deposition over the CNT, especially when a conformal coating layer with large monocrystalline grains is desired.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank the Material Engineering and Physics departments from UFRGS for hosting these experiments and the CNPq for sponsoring our research.

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REFERENCES

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Influence of Different Defects in Vertically Aligned Carbon Nanotubes

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