10.1039/c5tc00196j
A facile fabrication of large-scale reduced graphene oxide-silver nanoparticle hybrid film as a highly active surface-enhanced Raman scattering substrate
Natural graphite flake (99.8% purity), silver nitrate (AgNO3, >99%), and sodium citrate tribasic dehydrate (>=99.0% purity) were purchased from Sigma-Aldrich. R6G and MA were obtained from J&K Scientific Ltd (Beijing, China). H2SO4 (95-98 wt%) and KMnO4 (99.5% purity) were purchased from Beijing Chemical Co., Ltd (Beijing, China). H3PO4 (85% purity) and H2O2 (30% aqueous solution) were obtained from Xilong Chemical Co., Ltd. All chemicals used in this work were of analytical reagent grade and obtained from commercial sources and directly used without additional purification. The water used was purified through a Millipore system (~18.2 MΩ cm-1). Carbon-coated Cu grids for transmission electron microscopy (TEM) characterization were purchased from Plano GmbH (Wetzlar, Germany). 
GO was synthesized by the oxidation of natural graphite flakes using a modified Hummers method to produce graphite oxide.31 In detail, 3 g of graphite flakes were mixed with a mixture of concentrated H2SO4/H3PO4 (9:1) under stirring at room temperature; KMnO4 (18 g) was added slowly by stirring, and the mixture was incubated and stirred in the thermostatic water bath for 12 h (50 degC). The mixture was poured slowly onto ice crush (200 mL) and 10% hydrogen peroxide (10 mL) when the temperature was decreased to room temperature. Then the mixture was centrifuged (4000 rpm for 0.5 h), and the remaining solid material was washed in succession with 400 mL of water, 400 mL of 30% hydrochloric acid, and 400 mL of ethanol (2x). The remaining material was filtered and vacuum-dried overnight at room temperature. Finally, 5.8 g of the product was obtained. 
AgNPs were synthesized according to the literature previously reported.32 In a typical synthesis, 100 mL of aqueous AgNO3 (0.5 mmol) was added into a 250 mL flask and heated to boil under punchy stirring in an oil bath. After that, 10 mL aqueous sodium citrate (1%) was added, and the color of the solution reached to yellow after 1 h, which indicates the formation of AgNPs.
Natural graphite flake
purity
99.8%
%
silver nitrate (AgNO3
>99%
%
sodium citrate tribasic dehydrate
purity
>=99.0%
%
H2SO4
95-98 wt%
wt%
KMnO4
purity
99.5%
%
H3PO4
purity
85%
%
H2O2
30%
%
water
purified through a Millipore system
~18.2 MΩ cm-1
MΩ cm-1
graphite flakes
mixed
3 g
g
H2SO4/H3PO4
9:1
KMnO4
added
18 g
g
mixture
stirred
12 h
h
mixture
stirred
50 degC
degC
ice crush
200 mL
mL
hydrogen peroxide
10%
%
hydrogen peroxide
10 mL
mL
mixture
centrifuged
4000 rpm
rpm
mixture
centrifuged
0.5 h
h
water
400 mL
mL
30% hydrochloric acid
400 mL
mL
hydrochloric acid
30%
%
ethanol
400 mL
mL
product
obtained
5.8 g
g
AgNO3
added
100 mL
mL
AgNO3
added
0.5 mmol
mmol
flask
250 mL
mL
sodium citrate
added
10 mL
mL
sodium citrate
added
1%
%
solution
reached to yellow
1 h
h