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A Protocol for High Sensitivity Surface Area Measurements of Nanostructured Films Enabled by Atomic Layer Deposition of TiO

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Stephen M. Ubnoske, Qing Peng, Eric R. Meshot, Charles B. Parker, and Jeffrey T. Glass J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b07458 • Publication Date (Web): 27 Oct 2015 Downloaded from http://pubs.acs.org on November 1, 2015

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The Journal of Physical Chemistry

A Protocol for High Sensitivity Surface Area Measurements of Nanostructured Films Enabled by Atomic Layer Deposition of TiO2 Stephen M. Ubnoske,1* Qing Peng,2 Eric R. Meshot,3 Charles B. Parker,4 and Jeffrey T. Glass4 1

Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708, USA 2 Department of Chemical and Biological Engineering, University of Alabama, Tuscaloosa, AL 35487, USA 3 Physical and Life Sciences Directorate, Lawrence Livermore National Laboratory, Livermore, CA 94551, USA 4 Department of Electrical and Computer Engineering, Duke University, Durham, NC 27708, USA *Corresponding Author. +1 978 223 3968 [email protected]

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ABSTRACT

Due to their nanoscale dimensions, nanomaterials possess a very high specific surface area, which directly informs their properties in energy conversion and storage and catalytic chemical transformation, amongst other applications. However, common laboratory scale samples of nanostructured films have a total surface area that is too small to measure by conventional techniques such as the Brunauer-Emmett-Teller method, although they may have high gravimetric surface area. The methodology presented here allows for accurate measurement of the surface area of nanostructured films of a variety of materials, and involves two steps: uniformly and conformally functionalizing the surface of the nanostructured film under study by an ultrathin titanium oxide adhesion layer through atomic layer deposition, and quantifying the amount of adsorbed dye molecules on the TiO2 coated nanostructure film. Carbon nanostructures, especially nanomaterials making use of the exciting properties of graphene, are under investigation by numerous laboratories around the world, and were therefore chosen as ideal materials for the demonstration of this procedure. In this research, two nanomaterials of high aspect ratio were chosen for this purpose: multi-walled carbon nanotubes and a covalently bonded graphene-carbon nanotube material termed graphenated carbon nanotubes. This method has been successful in studying films with total surface area as low as 20 cm2, and was additionally used to probe the underlying mechanisms of highly effective charge storage in high graphene edge density carbon nanomaterials.


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INTRODUCTION

Nanotechnology, and in particular nanostructured materials, have been at the forefront of research in physics, chemistry, materials science, and various engineering disciplines since the 1980s1. Nanomaterials possess at least one spatial dimension less than 100 nm and often have very high specific surface area. In particular, carbon nanostructures such as fullerenes2, carbon nanotubes3, graphene,4 and hybrid carbon nanotube-graphene materials have received significant attention in recent years for such diverse applications as electrochemical energy storage5-7, lithium ion and metal-air batteries8-10, transparent conductive electrodes11, neural stimulation electrodes12, carbon nanotube field-effect transistors13, and magnetically separable adsorbers and catalyst supports14. These applications are strongly dependent on the surface area of the electrode. Other areas of nanotechnology that rely heavily on accurate characterization and manipulation of surface area include nanotoxicology15-16, gas sensing17-18, hydrogen storage19, and photovoltaics20-21. A particularly active field of research that relies heavily on the careful engineering of the surface area of nanostructured materials is electrochemical doublelayer capacitors (EDLCs)22-23. EDLCs, or supercapacitors, store charge electrostatically using reversible adsorption of electrolyte ions at active nanostructured surfaces. Capacitance (C) is a critical property of the EDLC and depends directly on the surface area by = /, where is the electrolyte dielectric constant, is the permittivity of free space, d is the effective thickness of the double layer, and A is the electrode surface area23. As a result of this relationship, for a certain nanostructured carbon EDLC


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electrode in a particular electrolyte, if one can measure the surface area of the EDLC electrode, it will provide the area specific capacitance = /. can help understand how the other material properties such as the density of graphene edge planes and/or sub-nanometer pores within nanocarbons impact specific capacitance24-27. A high density of graphene edge planes may influence specific capacitance by altering the charge density distribution or allowing for adsorption of pseudocapacitive surface groups28, and sub-nanometer pores may reduce the effective double layer thickness (d) by distortion of the solvation shell of electrolyte ions29. The most common method used to experimentally measure the surface area of nanostructured materials stems from the theory of physical gas adsorption and desorption from solid surfaces known as Brunauer-Emmett-Teller (BET) theory and is employed by many commercial instruments. The typical range of operation for such instruments spans surface areas of 0.1 to 50 m2. However, as is often the case in laboratory-scale research, the samples generally do not have enough total surface area to be precisely measured by BET. This is especially true for synthesis processes geared toward materials development or discovery, where large-scale production methods do not exist and total film surface area is less than 1000 cm2. Here, we describe a highly sensitive method for measuring the surface area of nanostructured films by using atomic layer deposition (ALD) and liquid-phase dye sorption techniques. Quantifying the amount of dye molecules that are adsorbed on mesoporous TiO2 nanoparticle or nanotube scaffolds is a protocol used to characterize the active surface area of dye-sensitized solar cells30-35. However, the application of this method to the characterization of the surface area of other types of nanostructured materials is hindered by limited dye adsorption. In this paper, we solve this challenge by first functionalizing ACS Paragon Plus Environment

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the nanostructured films with TiO2 by atomic layer deposition. Owing to its self-limiting growth mode, ALD may be deposited over the original complex, high aspect ratio nanostructures34 with a conformal and uniform layer of TiO2 while preserving the original nanostructure. With the ALD TiO2 functional coating, it is possible to use the dye adsorption method to estimate the surface area of nanostructured films. The application of this technique to graphene-carbon nanotube hybrid materials developed in our laboratory is also reported, yielding new insight into the improved electrochemical properties of graphene-CNT hybrid materials.

EXPERIMENTAL Carbon Nanostructure Preparation

The substrate used for the deposition of carbon nanostructures was N-type conductive silicon (100) wafers, which were coated with a layer of iron catalyst of 5 nm thickness at RTI International using a CHA electron beam evaporation system. Plasmaenhanced chemical vapor deposition (PECVD) was performed with a 915 MHz microwave PECVD reactor for the growth of carbon nanotubes (CNTs) and graphenated carbon nanotubes (g-CNTs). The growth process is described in detail in previous publications12, 28, 36-37. Briefly, substrates undergo an initial heat-up step during which the substrate is raised to the desired deposition temperature under 100 sccm NH3 flow. The subsequent “pretreatment” stage allows the chamber pressure to rise to 21 torr with NH3 flow at constant temperature, the purpose of which allows the Fe layer to dewet into catalyst nanoparticles on the Si surface. Finally, methane is introduced during the deposition step at a flow rate of 150 sccm CH4 and 50 sccm NH3. For this study, carbon


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nanotubes were deposited at 850 °C substrate temperature and 360 s pretreatment time, with deposition times from 3 min to 10 min to vary film thickness. Graphenated carbon nanotubes were deposited at 1050 °C and 360 s pretreatment time, using deposition times from 2 min to 20 min to vary graphene foliate density. Structures similar to the graphenated carbon nanotubes used in this study have been described in various reports6, 38-40

. The growth mechanism of graphene foliates has been proposed as either a stress

buckling mechanism36 in the CNT sidewalls during growth, or a plasma-induced defect etching mechanism37-39 followed by subsequent carbon deposition at defect sites. In order to ensure the validity of the experimental technique presented in the following section, relatively simple arrays of antimony-doped tin oxide (ATO) nanoparticles were studied for comparison with literature results. ATO films were prepared on a fluorine-doped tin oxide (FTO) glass substrate by colloidal dispersion processing and subsequently coated with TiO2 by atomic layer deposition, thus forming a structure referred to as NanoATO. More details of the nanoparticle coating process have been reported elsewhere34.

TiO2 Atomic Layer Deposition

Atomic layer deposition was performed with a custom-built ALD reactor. ALD is a stepwise, substrate site-limited growth technique capable of producing conformal coatings, even on rough, high aspect ratio nanostructures34. TiO2 was deposited on ATO nanoparticles, CNT, and g-CNT films using TiCl4 and H2O as precursors. The thin TiO2 coating provides adsorption sites for dye molecules. Carbon nanostructured films were raised to the deposition temperature of 135 °C during a 900 s temperature equilibration


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step preceding growth of titanium oxide, and ATO substrates were raised to 300 °C following the process reported previously34. The lower temperature (135 °C) was used for deposition on carbon nanostructures to avoid etching of the few-layered graphene (FLG) foliates on g-CNTs. A negligible difference in the relative intensities of the Raman D and G bands after TiO2 coating strongly suggests that foliates remain intact and undamaged after the coating process. Precursors were alternately introduced during 1.5 s pulses interspersed with 5 s purge steps. 200 such cycles resulted in a conformal TiO2 coating on the nanostructures, measured as ~10 nm by transmission electron microscopy (TEM).

Dye Adsorption

The dye used for this technique development was cis-diisothiocyanato-bis(2,2’bipyridyl-4,4’-dicarboxylato) ruthenium(II) bis(tetrabutylammonium) (Solaronix), more commonly referred to as N719 in the literature. The acetonitrile used in the dye adsorption solution (DrySolv Acetonitrile, VWR International) was

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