A decade of graphene research

I. AN INTRODUCTION ABOUT GRAPHENE: a hexagonal structure consisting of sp2 hybridized carbon atoms  
Promising potential applications: longer-lasting batteries, more efficient solar cells, corrosion prevention, circuit boards, display panels,  and  medicinal  technologies  such  as  the  point-of-care detection of diseases

Graphene (top) and related structures: fullerene (bottom left); carbon nanotubes (bottom centre); and graphite (bottom right)

The current methods of large-scale graphene synthesis include many variations of the so-called ‘Hummers’ method, devised by William Hummers in the late 1950s 

The method utilizes powerful oxidizing agents and strong acids to strip apart the graphene layers from a source of graphite – usually a high grade graphite powder available from any good chemical supplier. However, as this method creates graphene oxide, it is necessary to reduce the graphene oxide further to create graphene, termed reduced graphene oxide, which depending on the success of the reduction process can yield near fully reduced graphene oxide (viz. graphene, usually termed rGO) or partially reduced graphene oxide

The best example to date of a chemically reduced graphene was presented in 2008 by Tung et al., who cleverly exploited the powerful reducing ability of hydrazine by immersing graphene oxide paper in pure hydrazine. Reportedly,  after  a few  hours  the  paper  disappears  to  leave  a  suspension  of  hydrazine with  graphene  platelets  dispersed  within.  The  graphene/hydrazine suspension  can  be  spin-coated  upon  a  substrate  such  as  silica  for characterization

The implications of such a highly permitting electron transport material are potentially profound in applications such as field effect transistors (FETs), which, even as of 2010, could operate at frequencies as high as 100 GHz

• a  high  thermal  conductivity  of  5000  W  m^-1/K 
• a high thermal conductivity of 5000 W m^-1/K 
• a high Young’s modulus of ~ 1 TPa
• extraordinarily large specific surface area of 2630 m²/g 

II. GRAPHENE APPLICATION

1. High-speed electronics
high conductivity => high-speed electronics

Graphene is a zero band gap material and hence has yet to make its commercial debut in this manner

One  particular  problem  with  graphene  based  transistors  originates  from  defects  emerging  upon  the  graphene  sheet during  the  fabrication  process  of  the  device.  That  said,  a  literature report  from  2010  emerged  which  utilized  a  self-aligning  Co₂Si–Al₂O₃ nanowire  as  a  gate  in  the  graphene  transistor  which  according  to  their  work  prevented  device  degradation  and  exhibited operational  frequencies  of  100–300  GHz

2. Data storage

Researchers investigating the storage properties of graphene oxides have shown that indium tin oxide electrodes modified with polymers and graphene oxide exhibit the write-read-erase-read-rewrite cycle for a non-volatile memory device

Current/voltage curves typical of the indium tin oxide electrode modified with polymers and graphene oxide. The curves 1–5 represent the relevant stage in the write-read-erase-read-rewrite cycle

3. Smart Windows/OLED displays

4. Supercapacitors
5. Solar cells
6. Electrochemical sensing

Ultralight, multifunctional 3D nitrogen-doped graphene aerogel - Part 2

I. PHYSICAL PROPERTIES

• 10 mg of NGA was able to support 5000 fold its own weight
• The NGA was also fire-resistant
• the NGA was electrically conductive with a high conductivity of 262 S/m

 - the NGA is an ideal candidate for highly efficient separation/extraction of specific substances, such as organic pollutants and oils because of:

 + surface hydrophobicity

 + high porosity

 + mechanically stable

•  NGA showed the capacity to uptake amounts of liquids up to 40 to 156 times of their own weight

 - most of the porous volume was used for oil storage

The reason for this high adsorption capacity was that oils were stored mainly in the interconnected pores formed by the oleophilic walls of the doped graphene sheets. The absorbed oil or solvents can be removed by direct combustion in air for recycled use of the NGA

II. CHEMICAL SENSORS

Ascorbic acid (AA), DA and uric acid (UA) play important roles in inducing cancer, Parkinson’s disease, Huntington’s diseases, schizophrenia, etc => Electroanalytical method has been developed for the determination of these biological substances. Electrochemical determination of these species based on anodic oxidation suffers from the oxidation peaks severely overlapping with solid electrodes

The CV (Cyclic voltammograms) behaviors of AA, DA and UA were investigated

(a) Cyclic voltammograms of 1.0 mM AA, 1.0 mM DA and 1.0 mM UA in 0.10 M PBS (pH 7.4) at NGA/GCE with a scan rate of 100 mV/s. (b) Differential pulse voltammogram (DPV) for 1.0 mM AA, 0.050 mM DA and 0.10 mM UA in 0.10 M PBS (pH 7.4) at NGA/GCE, GA/GCE and GCE. (c) DPV for different concentrations of DA from 0.50 to 160 lM containing 1.0 mM AA and 50 lM UA at NGA/GCE. Inset: plots of the anodic peak current as a function of DA concentrations. (d) Electrochemical impedance spectroscopy of NGA/GCE and GA/GCE in 2.5 mM [Fe(CN)₆]³/[Fe(CN)₆]⁴ containing 0.10 M KCl. The frequency range was selected from 0.01 to 10⁵ Hz with a perturbation amplitude of 5 mV. The initial potential was 0.10 V

•  AA was negatively charged, and the oxidation peak of AA corresponded to the oxidation of hydroxyl groups to carbonyl groups. The formation of hydrogen bonds between the NGA microlayers and AA may increase the electron transfer
•  the hydrogen bonds between the doped nitrogen atoms within the NGA layers and the hydroxyl or amine groups from DA molecule enhanced the electron transfer kinetics
• UA showed quasi-reversible electrochemical behavior with oxidation/reduction peaks on the NGA/GCE, revealing that UA was first oxidized to quinonoid, and then experienced a rapid chemical reaction, which matched to an EC mechanism

Compared to the bare GCE and GA/GCE, the oxidation potential of AA, DA and UA was distinctly negative shifted and well separated, in addition to the much higher oxidation currents using the NGA/GCE.

The difference of their oxidation potentials was large enough to distinguish each other => NGA accelerated the oxidation of AA, DA and UA, and thus decreased their overpotentials, which is the key factor to realize their simultaneous determination.

DPV (Differential pulse voltammogram) is a sensitive way to undertake electrochemical detection

 - the addition of DA into the electrochemical cell did not have significant influence on the peak

currents or peak potentials of the other two biomolecules (Fig. c)

 - the NGA/GCE showed an almost straight tail line and better ability to promote the

electron transfer than that of the GA/GCE due to the nitrogen doping and the particular 3D microstructure with multiple electron paths

=> NGA possessed preferable electroactivity in neutral media and displayed excellent electrocatalytic activity towards the oxidation of AA, DA and UA

Ultralight, multifunctional 3D nitrogen-doped graphene aerogel - Part 1

  * Dopamine (DA) [Dopa (3,4-dihydroxy-L-phenylalanine) + amine groups]    =polymerize=>   Polydopamine (PDA)

 * Novel design of 3D nitrogen (N) doped graphene aerogel (GA) is developed by incorporating

mussel-inspired chemical motif of dopamine

 * Graphene oxide (GO)

I. PROCESS

1. Preparation of ultralight 3D NGA

  DA (15 mg) was added into a GO aqueous dispersion (15 mL, 1mg/mL) with pH adjusted to about 8.0, and sonicated for 40 min [color: pale brown]. The mixture was then sealed in a 20 mL Teflon-lined autoclave and maintained at 180 C for 12 h to form an N-containing gel. After the autoclave was naturally cooled down to room temperature, the as-prepared hydrogel was taken out and washed using ethanol and water and then freeze-dried. Ultralight NGA was obtained after heating the freeze-dried graphene gel at 800 C for 3 h under an Ar atmosphere. NGA (with a range of volumes and densities) was prepared under the same conditions except for using different concentrations of GO (0.2–4.0 mg/mL) and the ratio of GO to DA by weight was kept consistent at 1:1. 

2. Preparation of 3D GA

3D GA was synthesized based on the hydrothermal method. Typically, a 15 mL portion of 1 mg/mL homogeneous GO aqueous dispersion was sealed in a 20 mL Teflon-lined autoclave and maintained at 180 C for 12 h. Then the autoclave was naturally cooled to room temperature and the asprepared hydrogel was taken out, washed with ethanol and water. The hydrogel was then freeze-dried and annealed at 800 C for 3 h under an Ar atmosphere

3. Preparation of pristine graphene

The as-prepared GO sheets were chemically reduced using hydrazine vapor at 90 C for 24 h, followed by vacuum-drying at 160 C for 24 h

II. CHARACTERIZATION
 - Specific surface areas: Brunauer–Emmett–Teller (BET)

 - Pore size distributions: Barrett–Joyner–Halenda (BJH) methods
 - Electronic binding energies: X-ray photoelectron spectroscopy (XPS) analysis
 - Crystallite size: base on the Tuinstra–Koenig relationship L_a (nm) = (2.4x10^-10)λ⁴(I_D/I_G)^-1 [λ is the Raman excitation wavelength  λ = 532 nm]
III. RESULT

 - Two steps:
 + (1)in-situ hydrothermal cross-linking and polymerization of the mixture at 180 C for 12 h to obtain
the 3D hybrid N-containing precursor

 the DA [turned to intermediate dihydroxyindole by liberating protons] formed PDA between individual GO sheets => the intercalating PDA effectively formed a strong adhesive force with the GO sheets 

 + (2) An annealing step at 800 C under an Ar atmosphere for 3 h was carried out to convert the N source obtained from the first step to an N doped character

  DA played multi-functional roles in this method:
 * as a catecholic anchor, it chemisorbed to the surfaces of GO to form an adherent PDA coating, which then acted as a covalentcross-linking unit
 * as a reducing agent, the partial overlapping or coalescence of the GO sheets was readily reduced by DA with simultaneous capping by PDA to prevent the reduced GO sheets from agglomeration or restacking
 * as the direct N source, after the functionalization of the graphene surface, DA then introduced nitrogen atoms onto the graphene sheets upon pyrolysis

 - NGA provided ultra-low densities ranging from 1.9 to 6.0 mg/cm-3

 [increase the concentration of the mixture (the weight ratio of GO to DA was kept at 1:1) or scale up the autoclave => larger volume of NGA]
 - specific high surface area approximating 322.6 m²/g

 - the N atoms were incorporated into the carbon–carbon bonds of the graphene

 Raman spectroscopy is another way to evaluate the doping effect of graphene

(a) XPS spectrum of the NGA and (b) the corresponding high-resolution N1s peak. (c) Schematic illustration of the chemical structure of the NGA. (d) Raman spectra of pristine graphene, GA and the NGA, where D, G and 2D denote the characteristic D band, G band and 2D band of graphene. (e) Magnified spectra of the dashed box of the 2D band (the excitation wavelength is 532 nm). (f) Raman spectra of the NGAwith different initial concentrations of DA

The D band arises because of the structural disorder or defects present in graphitic based materials; whereas the G band arises from sp2 bonded ordered graphitic carbon

The small D peak of pristine graphene (PG) indicated the absence of significant defects

As for the 3D GA, the large amounts of hydroxyl and epoxy groups originating from the hydrothermal step reduced the relative amount of sixfold aromatic rings => increasing the intensity of the D band while decreasing the G band.

 When nitrogen atoms were doped into the graphene sheets, the substitution of nitrogen atoms was usually accompanied by the introduction of defects such as bonding disorders and vacancies in the graphene lattice, and therefore these defects would raise the D band of NGA even higher. 

The ratio of D to G band intensity (ID/IG)is commonly utilized to gauge the degree of structural disorder, with higher values suggesting more disorder along with smaller average graphitic crystalline size

The ID/IGvalues of the PG, GA and NGA were 0.26, 1.06 and 1.21, which corresponded to the calculation of the crystallite size of 74, 18 and 16 nm

 => together with the nitrogen doping, the crystallite size of the NGA decreased as anticipated
 - The increasing initial concentration of DA led to increasing N doped atoms, which caused a concomitant increase of the intensity ratio ofID/IG.

Introduction to graphene aerogel

1. AEROGEL: Ultralight
 - Application: energy storage, supercapacitors, adsorption, catalysis supports, and gas sensors
 - Graphene: extraordinary electron mobility, high surface area, good thermal conductivity and mechanical properties

One efficient approach to assemble graphene sheets into 3D bulk objects is to prepare them as aerogels or foams

2. GRAPHENE PREPARATION: chemical doping is another feasible way to functionalize and tune the properties of assembled graphene
 - Incorporating heteroatoms such as nitrogen into graphene can improve and modulate its physicochemical characteristics, especially its electrochemical performance

 Most nitrogen doped graphene is commonly prepared using chemical vapor deposition, pyrolysis of a nitrogen-containing precursor such as polyaniline or polypyrrole, nitrogen-plasma treatments of graphene and thermal annealing with ammonia or pyrrole

 + Advantage of the method: successfully anchor nitrogen atoms onto the graphene nanosheets => increase their electrocatalytic activity by providing more active sites
 + Disadvantage: require toxic gas precursors, or a large well-ordered template => it is desirable to develop a template-free and low-cost  [to obtain multifunctional, macroscopic nitrogen doped graphene architecture]