What Does Nitric Acid Really Do to Carbon ... - ACS Publications

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Article pubs.acs.org/JPCC

What Does Nitric Acid Really Do to Carbon Nanofibers? S. Sainio,† D. Nordlund,‡ R. Gandhiraman,§ H. Jiang,∥ J. Koehne,§ J. Koskinen,⊥ M. Meyyappan,§ and T. Laurila*,† †

Department of Electrical Engineering and Automation, School of Electrical Engineering, Aalto University, Espoo 02150, Finland Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo Park, California 94025, United States § Center for Nanotechnology, NASA Ames Research Center, Moffett Field, California 94035, United States ∥ Department of Applied Physics, School of Science, Aalto University, Espoo 02150, Finland ⊥ Department of Materials Science, School of Chemical Technology, Aalto University, Espoo 02150, Finland ‡

S Supporting Information *

ABSTRACT: Understanding the chemical nature of the surface of carbon nanofibers (CNF) is critical in assessing their fundamental properties and tailoring them for the right application. To gain such knowledge, we present here a detailed X-ray adsorption spectroscopy (XAS) study accompanied by high resolution transmission electron microscopy (TEM) micrographs of two morphologically different CNF pairs (tetrahedral amorphous carbon (ta-C) grown “open structured” fibers and traditional bamboo-like “closed structured” fibers), where the surface chemical properties and structural features of the fibers are investigated in depth and the effects of nitric acid treatment on the fibers are revealed. The morphology of the fiber and/or the original seed- and adhesion layers markedly affect the response of the fibers to the acid treatment. Results also show that the nitric acid treatment increases the observed sp2 intensity and modifies the two types of fibers to become more-alike both structurally and with respect to their oxygen functionalities. The XAS and HRTEM results confirm that a short nitric acid treatment does not remove the Ni catalyst particle but, instead, oxidizes their surfaces, especially in the case of ta-C grown fibers.

1. INTRODUCTION Carbon nanofibers (CNF) are used in a wide variety of applications due to their versatile properties. Such applications include polymer additives, gas storage, catalyst support1 and sensors.1−3 CNFs have been used to successfully detect a wide range of neurotransmitters such as dopamine (DA) and serotonin (HT-5) in a background of ascorbic acid (AA).4 CNFs can also be used for other biosensing applications such as a base for DNA detection.5 Interestingly, CNFs can detect DA, HT-5, and AA simultaneously6 unlike many other carbon-based materials such as ta-C, graphite and glassy carbon. At the same time, CNFs are capable of detecting in vivo level concentrations of these neurotransmitters, especially DA.6,7 Carbon nanotube (CNT) based sensors have been used in many studies investigating sensor materials for neurotransmitter detection.4 CNFs and especially multiwalled carbon nanotubes (MWCNTs) have a lot in common as they are both formed from graphene sheets. However, the alignment of the sheets is often dissimilar between the CNT and CNF. Different acid treatments are commonly used in order to enhance the electrochemical (sensing) properties of the carbon material. Example of such treatment is using HNO3 to significantly increase the sensitivity of the electrode toward DA.7 Nitric acid treatment has been discussed widely in the literature including the effects to the carbon structure of the fiber itself and the oxygen functional groups on the surface of the fibers/tubes.8−15 There are many studies in the literature for treating CNTs and CNFs bulk material (several grams) with © 2016 American Chemical Society

concentrated acids for extended periods of time (30 min and longer).16,17 However, no studies to date have investigated the effects of short concentrated acid treatments for applicationready sensor surfaces with vertically aligned integrated carbon nanofibers. Also the role of the metal particle and how they are affected by the acids are often neglected and the studies are concentrated mainly on the oxygen functionalities. Moreover, results from the literature are highly scattered and even contradictory to some degree. Thus, there is a clear need for a systematic in-depth investigation especially for short nitric acid treatments. In order to understand why the CNFs are selective and sensitive toward the given neurotransmitters, e.g. why the HNO3 treatment increases the sensitivity of the sensor toward DA,7 it is necessary to know (i) the type and amount of oxygen functional groups on the surface of the as-grown fibers, (ii) which of these functional groups are affected by short acid treatment, and (iii) the role of the metallic seed particles. Thus, we present here an in-depth study of two different types of carbon nanofibers and their response to short nitric acid treatment. One of the fibers represent the almost classic bamboo-like structure18,19 whereas the other one is a recently described novel structure where tetrahedral amorphous carbon (ta-C) thin film is used as an extra carbon source of the CNF Received: June 23, 2016 Revised: August 22, 2016 Published: September 15, 2016 22655

DOI: 10.1021/acs.jpcc.6b06353 J. Phys. Chem. C 2016, 120, 22655−22662

Article

The Journal of Physical Chemistry C growth as well as means to integrate the CNFs firmly to the desired location on a Si wafer.20,21 Although there are a significant number of different variants of CNF described in the literature we have focused our investigation on two representing “closed” (bamboo) and “open” (ta-C) CNF structures. We apply X-ray absorption spectroscopy (XAS) to the local electronic structure of carbon, oxygen and nickel and scanning transmission electron microscopy (STEM), by which the structural properties of the fibers are assessed. This provides both detailed chemical information about the changes occurring in the CNF structures and direct morphological observations of the treated structures. Changes in the total amount of oxygen and especially in the nature of the oxygen functional groups are shown to be significantly different depending on the fiber structure (e.g., closed vs open).

peaks with the exception of the sp2 peak were first given a fixed full width half max (fwhm) of 1.5 eV. After the initial fit converged without errors (such as negative intensity peaks) all the pre-IP peaks were released to constraints where the fwhm was forced to be 1.5 eV or lower. (Most peaks are obviously naturally sharper, but we constrained the fwhm to have an upper limit of 1.5 eV which is rarely observed in the pre-IP region. By doing this we could avoid unphysical mathematical widths and get reliable relative intensity differences across the data set.) Initial peak and IP locations were justified by double differentiating the spectra, using the minima from the second derivative as peak positions. The fit was accepted when the χ2test result was less than 10. High-resolution transmission electron microscopy (HRTEM) was performed using a double-aberration corrected JEOL JEM-2200FS (JEOL, Japan) microscope operating at 200 kV. A Gatan 4k × 4k UltraScan 4000 CCD camera was employed for digital recording of the HRTEM images. The same microscope was used to record the energy dispersive spectroscopy (EDS) data.

2. EXPERIMENTAL WORK A Black Magic (Aixtron, Germany) reactor was used to grow all the samples with the same recipe. Detailed description of the growth process is reported in.20 Briefly, the ta-C grown fibers are grown from Si + 20 nm Ti + 7 nm ta-C + 20 nm Ni (see ref 22 for further details on the ta-C process), and the bamboo-like fibers are grown on Si + 80 nm Cr + ∼24 nm Ni based substrate. A set of both samples were treated with nitric acid (68% concentration) for 5 min by immersing the whole sample into a glass Petri dish. Sample was removed after 5 min and rinsed by immersing the sample to clean deionized (DI) water for three consecutive times for 10 s, same time stirring the water with the sample. Sample was dried between every step by placing it edge-down on a kimwipe for few seconds. After the last immersion to DI water and placing the sample edge down on the kimwipe, the sample was left to dry in a Petri dish inside a fumehood. Acid treatment, DI water immersion, and drying was done at room temperature. For each batch there was an as-grown and nitric acid treated sample that was measured using XAS. Each sample was scanned from three different locations from the sample surface. This study was carried out at beamline 10−1 at the Stanford Synchrotron Radiation Lightsource (SSRL). To observe any angular dependency, spectra were collected at different incident angles (20°, 54° and 90°). Incident beam size was