Evolution of the Composition and Suspension Performance of

Article pubs.acs.org/JPCC

Evolution of the Composition and Suspension Performance of Nitrogen-Doped Graphene Saad A. Hasan,*,†,§ Eleni K. Tsekoura,† Victoria Sternhagen,‡ and Maria Strømme† †

Nanotechnology and Functional Materials, The Ångström Laboratory, Uppsala University, 751 21 Uppsala, Sweden The Ångström Microstructure Laboratory, Uppsala University, 751 21 Uppsala, Sweden

‡

S Supporting Information *

ABSTRACT: Nitrogen functionalization of graphene enables it to be used for catalysis and targeted adsorption of biomolecules in both the solid state and in suspension. Thus, we sought to characterize the functional groups and suspension charge behavior of nitrogen-doped graphene (NDG) prepared in the absence of hydrazine, a highly toxic reagent. The hydrothermal reaction of graphite oxide (GO) with ammonia was shown to effectively remove oxygen and to restore the graphitic framework within the resulting NDG sheets. The enhanced graphitic character of the NDG materials was verified using X-ray photoelectron spectroscopy, thermogravimetic analysis, and electrical conductivity measurements. With six hours of reaction time (sample NDG-6), up to 9.6 wt % (7.1 atomic %) of nitrogen could be introduced into the graphene. All the NDG materials exhibited excellent dispersibility in water allowing their surface charge to be probed by measuring zeta potential (ζ) as a function of suspension pH. The NDG-6 material could hold surface charge ranging from ζ = −50 mV to ζ = +20 mV, which is, to the best of our knowledge, the widest range of surface charges measured on a colloidal graphene material.

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INTRODUCTION The ability to functionalize carbon nanomaterials (CNMs) serves several important purposes. Functionalization can improve their dispersibility in solvents and their mixing with polymers when preparing composites.1−5 Functional groups can also provide local sites for highly specific interactions with other molecules allowing CNMs to be used as sensors and catalysts.6−8 One topic in this field that continues to receive interest is the functionalization of CNMs with nitrogen. Doping with nitrogen, which possesses one more valence electron than carbon, modifies the electronic band structure by introducing new energy levels in the lower part of the conduction band for graphitic, sp2-bonded CNMs such as carbon nanotubes and graphene.9−11 These new energy levels have been shown to be catalytically active in reactions such as the electrochemical reduction of oxygen at fuel cell cathodes.12−14 CNMs with nitrogen-containing groups may also be useful as robust, solid base catalysts. For the industrial production of chemicals, electron-donating (Lewis base) catalysts can facilitate a wide range of reactions including isomerization, condensation, and alkylation.15 Solid catalysts such as CNMs offer over liquid catalysts the advantage of facile separation and recovery from the reaction products. Recently, amine-modified carbon nanotubes were shown to outperform commercially available catalysts in the conversion of triglycerides to methyl esters, a form of biodiesel.16 CNMs featuring amine chemistry are also broadly relevant to areas such as biotechnology as demon© 2012 American Chemical Society

strated by the use of graphene-poly(ethylenimine) complexes to transfect DNA into cells.17 Two strategies may be employed for inserting nitrogen into CNMs. In the first approach, a nitrogen source (e.g., ammonia or melamine) is introduced during the growth of the material. This approach can be applied to carbon nanotubes18 and to other CNMs produced by various methods including pyrolytic growth,19 chemical vapor deposition,20 and solvothermal synthesis.21 In the second approach, already-formed CNMs are chemically modified using nitrogen-containing reagents. The second approach is suitable when starting with CNMs of a specific morphology, so that chemical groups may be introduced selectively without drastically altering the morphology of the starting material. Here, we report on the conversion of graphene oxide (GO) to nitrogen-doped graphene (NDG) via hydrothermal reaction with ammonia. GO is a precursor with several advantages: it is readily produced at low cost from graphite, it is rich in reactive sites because of its oxygen functionalities, and its sheet morphology and high surface area can be preserved over a wide range of reaction conditions.22 The high surface area of individually dispersed sheets suggests that functionalized graphene can be an effective support for catalytically active molecules and moieties. Thus, in addition to probing the Received: November 1, 2011 Revised: February 20, 2012 Published: February 23, 2012 6530

dx.doi.org/10.1021/jp210474x | J. Phys. Chem. C 2012, 116, 6530−6536

The Journal of Physical Chemistry C

Article

Velleman DVM1500 multimeter with one probe connected to each stainless steel rod. Aqueous suspensions of the powders (0.5 mg/mL) were prepared by bath sonication to facilitate several measurements. Samples for X-ray photoelectron spectroscopy (XPS) analysis were prepared by drop casting the suspended graphene onto silicon wafers with native SiO2 layer. The XPS measurements were performed using a PHI Quantum 2000 scanning ESCA microprobe equipped with a monochromatic Al Kα source. A 50 W, 200 μm spot size was used for data acquisition along with a neutralizing electron current of 10 μA. Survey scans were acquired for the range 0−1300 eV using a pass energy of 187.85 eV. High-resolution scans of the C 1s and N 1s regions were acquired using a pass energy of 23.5 eV. Data analysis and peak fitting were performed using the CasaXPS software package. The −NH2 groups theorized to be present in the NDG samples as primary amines were quantified by a Kaiser assay30 using some of the modifications described by Sarin et al.31 In this two-reagent assay, originally designed to monitor free amino groups during peptide synthesis, the reaction of ninhydrin with the sample’s amine groups yields a blue chromophore that is used to quantify the sample’s amine concentration. Reagent A contained 1.0 g phenol and 50 μL of hydrindantin (0.01 M in EtOH) in 2.5 mL pyridine and 250 μL EtOH. Reagent B contained 50 mg ninhydrin in 1 mL EtOH. A known mass of powdered graphene sample was suspended in a vial containing a known volume of 60% EtOH (in water). An identical volume of 60% EtOH was dispensed in a separate vial for use as the reagent blank. To each of these vials were added 100 μL of reagent A and 25 μL of reagent B. The vials were sealed and heated in a 100 °C oil bath for 10 min. Afterward, the graphene solids were separated from the liquid using a syringe filter (pore size 0.45 μm). The absorbance of this liquid, containing the ninhydrin reaction product, was measured against the reagent blank at 570 nm. A molar extinction coefficient of 1.5 × 104 M−1 cm−1 was used to calculate the chromophore concentration.31 From this concentration, the quantity of −NH2 groups in the graphene sample (in mol/g) was calculated using the known volume of liquid and the known mass of graphene. The morphologies of the materials were probed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). SEM of the drop cast materials on silicon was carried out using a Zeiss LEO 1550 operating at 1 kV. To prepare samples for AFM, the graphene suspensions were diluted to 0.05 mg/mL and were cast onto freshly cleaved mica substrates. Samples were scanned using a Park Systems XE-150 operating in noncontact mode. The AFM images were analyzed with the XEI and Gwyddion software packages. Colloidal characterization of the materials was performed using 0.05 mg/mL aqueous suspensions. The pH of the suspensions was adjusted using HCl and KOH. Zeta potentials were calculated using the Smoluchowski approximation from electrophoretic mobility measurements acquired with a Malvern Instruments Zetasizer.

structure and composition of NDG, we provide the first report of the surface charge on these materials by investigating their zeta potential in aqueous suspension. The ability to control the dispersed graphene’s charge is important for modulating its interaction with biomolecules like DNA17 as well as for directing its assembly when fabricating multilayered coatings.23 A previous study on the insertion of nitrogen into GO used hydrazine along with ammonia in a hydrothermal reaction.24 Although that report indicated the successful preparation of Ndoped graphene, we note two rationales for excluding hydrazine in our study. First, hydrazine is a highly toxic reagent whose use should be minimized or avoided especially if the synthesis is scaled up to large quantities. Second, the use of hydrazine may be redundant given that ammonia is a nitrogen source and that either hydrothermal treatment25 or water with elevated temperature26 has been shown to reduce graphene oxide and to restore the graphitic framework.

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EXPERIMENTAL METHODS Materials and Synthesis. Graphene oxide (GO) was prepared from powdered graphite (SP-1, Bay Carbon, Inc., nominal particle size 30 μm) using a modified Hummers method.27,28 All other reagents were purchased from SigmaAldrich. The as-synthesized powder of GO was washed with dilute HCl and deionized water on a filter and then was dried with the aid of vacuum. All further references to water will mean deionized water. For the reaction with ammonia, GO (100 mg) was combined with water (10 mL) in a plastic centrifuge vial and was sonicated for 2 h to yield a dark brown suspension. A 25% ammonia solution (5 mL) was added to the dispersion followed by 0.25 h of sonication. This mixture was transferred to a Teflon-lined autoclave (Parr Instrument Co.), which was sealed and placed in an oven heated to 110 °C. The reaction time inside the oven was either 3 or 6 h. At the end of the reaction, the autoclave was removed from the oven and was allowed to cool. After 0.75 h of cooling, the contents were transferred to centrifuge vials. The solids were isolated by centrifugation at 2000g for 0.5 h. The solids were washed with water and then were dried by vacuum filtration. These products are designated as NDG-n, nitrogen-doped graphene resulting from n hours of reaction time. As a control, the above protocol was followed with 6 h inside the oven except that the ammonia solution was replaced with 5 mL of water. The product of these control trials is designated as hydrothermally treated graphene oxide (HTGO). Characterization. The precursor GO and the products in the form of dried powders were characterized by X-ray diffraction (XRD) using a Siemens D5000 diffractometer with a Cu Kα source. XRD scans were collected in 0.05° steps. The thermal stability of the powders was assessed using a Mettler Toledo thermogravimetric analyzer (TGA) in which samples were heated from 25 to 1000 °C at a heating rate of 2.5 °C/min under the flow of air at 40 mL/min. CHN elemental analysis was performed by Mikro Kemi AB (Uppsala, Sweden). Electrical conductivity measurements of the powders were performed in a homemade cell similar to the one described by Stankovich et al.29 Inside a glass tube (inner diameter, 6 mm), a known mass of powder was confined between two stainless steel rods and was compressed into a pellet of thickness ∼ 3 mm. The exact thickness of the pellet was determined using a digital caliper. The resistance of the pellet was measured using a

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RESULTS AND DISCUSSION GO was subject to hydrothermal treatment at 110 °C in the presence of ammonia solution for 3 and 6 h yielding the nitrogen-doped graphene products NDG-3 and NDG-6, respectively. To probe the effect of the hydrothermal environment itself, control trials were performed in which 6531

dx.doi.org/10.1021/jp210474x | J. Phys. Chem. C 2012, 116, 6530−6536

The Journal of Physical Chemistry C

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These shifts suggest a decrease in the quantity of basal plane oxygen atoms as a result of the reaction with ammonia even for the shorter reaction time of 3 h. An examination of the peaks at 2θ value ∼43°, which can be indexed to the 100 reflection, provides some hints about the evolution of the structure within the graphene sheets. Relative to the intensity of the dominant 002 peak, the intensity of the 100 peak increases in the order GO < HTGO < NDG-3 < NDG-6. In GO, the sp3 bonding between carbon and oxygen results in the vertical displacement of carbon atoms from the planar arrangement of sp2 bonded graphene sheets weakening the intensity of the in-plane 100 reflection. The stronger intensities of the 100 reflection in NDG-3 and NDG-6 compared to GO and HTGO suggest a restoration of the inplane ordering of carbon atoms and their associated sp2 bonding. The concurrent decrease in sp3 bonding would correspond to the decrease in oxygen content that we inferred from the evolution of the 002 reflections. From the 002 reflection in the XRD measurements, the thickness of the graphene sheets and their relative oxygen content could be inferred. The thickness of individual sheets could be probed directly using AFM. Figure 3 shows AFM

GO was subject to identical treatment for 6 h in the absence of ammonia yielding the hydrothermally treated product HTGO. Structural Changes. A survey by SEM showed that all the materials retain the sheetlike morphology of GO after reaction at 110 °C (Figure 1). More detailed information about their

Figure 1. Scanning electron micrographs of (a) GO, (b) HTGO, (c) NDG-3, and (d) NDG-6.

Figure 3. Atomic force micrographs of singly dispersed sheets of (a) GO, (b) HTGO, (c) NDG-3, and (d) NDG-6. Z-direction scale bars are provided to the right of each panel.

scans of the four different graphene materials atop mica substrates. These images confirm that the flat sheet morphology is retained in all the materials. Further, height profile measurements of individual sheets (see Supporting Information) corroborate the progressive decrease in sheet thickness suggested by the XRD measurements going from GO to NDG-3 and to NDG-6. Averages calculated from multiple sheets in each type of sample showed thickness values for individual sheets of 11.9 ± 0.6 Å for GO, 8.0 ± 0.2 Å for HTGO, 7.9 ± 0.4 Å for NDG-3, and 5.2 ± 0.2 Å for NDG-6. Together, the XRD and AFM data confirm the decreased thickness of NDG sheets compared to the GO sheets. This decrease is associated with a loss of oxygen atoms from above and below the basal plane and implies a restoration of the graphitic sp2 framework. The nature of the carbon framework in the graphene samples was verified by probing their chemical bonding as described in the next section. Chemical Composition. The composition of the graphene materials was measured by CHN elemental analysis. Table 1 reports the composition in terms of both weight fractions and the corresponding atomic fractions. The fractions were calculated from at least two independent measurements for each type of graphene. It is evident that GO has the highest oxygen content (51.9 wt %), while HTGO possesses a

Figure 2. Powder X-ray diffraction patterns of the graphene. Graphene sheet spacing values are indicated.

structures was obtained from XRD measurements (Figure 2). Peaks in the 2θ region between 10° and 26° can be indexed to the 002 reflection corresponding to the spacing between the graphene sheets. GO exhibits a strong signal at a sheet spacing of 7.9 Å indicative of the material’s oxidation. Many of the oxygen atoms in GO are positioned above and below the basal plane,32−34 and their presence increases the spacing between the basal planes compared to the 3.3 Å separation of graphene sheets in pristine graphite. A weak signal at 4.0 Å suggests the presence of partially oxidized graphite in the GO as well.35 For HTGO, the dominant 002 peak appears at 7.8 Å indicating that the GO’s structure is largely unchanged after 6 h of hydrothermal treatment. For the NDG materials, the dominant 002 reflections are shifted to higher angles indicative of lower sheet spacing values, 6.7 Å for NDG-3 and 3.7 Å for NDG-6. 6532

dx.doi.org/10.1021/jp210474x | J. Phys. Chem. C 2012, 116, 6530−6536

The Journal of Physical Chemistry C

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Table 1. Elemental Composition of Graphene Samples weight fraction (wt %)

a

atomic fraction (at %)

sample

carbon

oxygena

nitrogen

carbon

oxygena

nitrogen

GO HTGO NDG-3 NDG-6

45.7 48.2 58.2 64.4

51.9 48.9 32.4 24.0