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.