Methods
Materials
The Cu foil (99.8%, metal basis, thickness 25 μm, No. 046986) and poly-
vinylpyrrolidone (PVP; molecular weight 55,000) were purchased from
Alfa Aesar Chemical Reagent Co. Ltd (Tianjin, China). Cu(111), Cu(110)
and Cu(100) single crystals were obtained from Hefei Kejing Materi-
als Technology Co., Ltd (Hefei, China). 1-DT (analytical reagent, AR),
CuCl 2 ·2H 2 O (AR), glucose (AR), oleylamine (80−90%) and oleic acid
(90%) were purchased from Aladdin Reagent Co., Ltd (Shanghai, China).
Sodium formate (HCOONa), sodium sulfide (Na 2 S), copper sulfate
(CuSO 4 ·5H 2 O), sodium hydroxide (NaOH), diethyldithiocarbamate
(DDTC), sodium ethylenediamine tetraacetate (EDTA), ammonium
citrate, ethylene glycol (EG), DMF, carbon tetrachloride (CCl 4 ), ace-
tone, ethanol and other reagents were purchased from Sinopharm
Chemical Reagent Co., Ltd (Shanghai, China). Brass foil (H68), bronze
foil, Cu wire (99.9%, diameter 2 mm) and Cu mesh (99.9%, thickness
50 μm) were obtained from Shenzhen Xinming Metal Material Co., Ltd
(Shenzhen, China). All reagents were used as received without further
purification. The water used in the studies was ultrapure water with
a resistivity of ≥18 MΩ cm provided by a Millipore water purification
system (Bedford, USA).
Hydrothermal and solvothermal formate treatments of copper
(Cu-FA)
The following three methods were developed to treat Cu surfaces with
formate for effective anti-oxidation passivation.
Method I. Hydrothermal treatment of Cu in the presence of formate.
In a typical treatment, a 2 cm × 2 cm × 0.025 mm Cu foil or mesh or
a 10-cm-long Cu wire is placed with 500 mg HCOONa in 10 ml of
nitrogen-saturated H 2 O. The mixture is sealed in a 50-ml stainless-steel
autoclave. The mixture is then heated from room temperature to 200 °C
in 40 min and kept at 200 °C for 24 h to prepare Cu-FA. The Cu-FA
products are collected and rinsed with deionized water to remove
free HCOONa.
Method II. Solvothermal formate treatment of Cu in the presence of
oleylamine (Cu-FA). In a typical treatment, a 2 cm × 2 cm × 0.025 mm
Cu foil or mesh or a 10-cm-long Cu wire and 200 mg HCOONa in 1 ml
H 2 O are dissolved in 12 ml DMF by sonication. Then, 1 ml oleylamine is
added into the mixture and sonicated for another 3 min. The mixture is
sealed in a 50-ml stainless-steel autoclave. The mixture is then heated
from room temperature to 160 °C in 30 min and kept at 160 °C for 16 h
to prepare Cu-FA. The Cu-FA products are collected and rinsed three
times with deionized water and ethanol alternately to remove free
HCOONa and oleylamine.
Method III. Solvothermal formate treatment of Cu in the pres-
ence of oleylamine and Cu(HCOO) 2. In a typical treatment, a
2 cm × 2 cm × 0.025 mm Cu foil or mesh or a 10-cm-long Cu wire,
0.1 mg Cu(HCOO) 2 ·4H 2 O and 200 mg HCOONa in 1 ml H 2 O are mixed
in 12 ml DMF by sonication. Then, 1 ml oleylamine is added into the
mixture and sonicated for another 3 min. The mixture is sealed in a
50-ml stainless-steel autoclave. The mixture is then heated from room
temperature to 120 °C in 20 min and kept at 120 °C for 12 h to prepare
Cu-FA. The Cu-FA products are collected and rinsed three times using
deionized water and ethanol alternately to remove free HCOONa and
oleylamine.
Formate treatments of Cu single crystals
In a typical treatment, two species of 1 cm × 1 cm × 0.5 cm Cu single
crystals with exposure surfaces of Cu(110), Cu(100) or Cu(111) were
placed in 10 ml H 2 O containing 200 mg HCOONa (N 2 -purged). The
mixture was sealed in a 50-ml stainless-steel autoclave. The sample
was then heated from room temperature to 100 °C in 20 min and kept
at 100 °C for different times to prepare Cu(hkl)-FA. The single crystals
were collected and rinsed with deionized water to remove free HCOONa.Preparation of copper foils, meshes and wires co-stabilized by
formate and 1-DT (Cu-FA/DT)
The as-prepared Cu-FA samples were dried in vacuum before use. In
order to prepare Cu-FA/DT, the Cu-FA samples were immersed in 20 ml
1 mM DT/EtOH solution and kept still for 5 min, and were then washed
with pure ethanol three times to produce Cu-FA/DT.Preparation of graphene-coated Cu foils (Cu-G)
The Cu-G samples were grown by the chemical vapour deposition (CVD)
method^31. Cu foils (25 μm thick, 99.8%, Alfa Aesar) were placed on a
flat oxide substrate and then loaded into a CVD system (Hefei Kejing
Company, OTF1250). The system was heated to 1,000 °C in 1 h with
Ar (500 standard cubic centimetres per minute, sccm), followed by
annealing in additional H 2 (10 sccm) for 40 min. Then, CH 4 gas (0.1 sccm)
was introduced as the carbon source for graphene growth. Finally,
the system was cooled naturally with Ar (500 sccm) and H 2 (10 sccm).Preparation of Cu NWs
In a typical synthesis of Cu NWs^32 , CuCl 2 ·2H 2 O (680 mg, 4 mmol) and
glucose (792 mg, 4 mmol) were dissolved in 80 ml of deionized water by
stirring in a beaker. In another beaker, 8 ml oleylamine and 80 μl oleic
acid were dissolved in ethanol by stirring. Afterwards, these two solu-
tions were put into a 500-ml beaker and diluted to 400 ml with water,
followed by magnetic stirring for 16 h at 50 °C. After the colour of the
mixture changed from blue to cyanish grey, the mixture was transferred
to a stainless-steel autoclave and then heated from room temperature to
120 °C in 20 min. After heating at 120 °C for 6 h, the mixture was cooled
to room temperature. The upper yellow suspension was discarded and
the bottom reddish-brown Cu NWs were obtained. The Cu NWs were
then centrifuged and washed with a 1:1 ethanol:water mixture (100 ml,
8,000 rpm, 2 min) and pure ethanol. Finally, the purified Cu NWs were
dispersed into 100 ml ethanol to obtain a Cu–nanowire dispersion with
a content of ~1 mg ml−1.Preparation of Cu NWs-FA and Cu NWs-FA/DT
In a typical synthesis, 10 mg Cu NWs, 0.1 mg Cu(HCOO) 2 ·4H 2 O and
200 mg HCOONa in 1 ml H 2 O were dissolved in 12 ml EG by sonication
to form a homogeneous solution. Then, 1 ml oleylamine was added
into the mixture and sonicated for another 3 min. The mixture was
sealed in a 50-ml stainless-steel autoclave, and was heated from room
temperature to 120 °C in 20 min and kept at 120 °C for 12 h to prepare
Cu NWs-FA. Following the same protocol, when the EG was replaced
by 12 ml DMF, Cu NWs-FA were also readily prepared by heating the
mixture from room temperature to 160 °C in 20 min and then main-
taining it at 160 °C for 16 h. The Cu NWs-FA products were collected
and rinsed three times with ethanol and deionized water alternately
to remove free HCOONa and oleylamine. The as-prepared Cu NWs-FA
was dried in vacuum before use. To prepare Cu NWs-FA/DT, 10 mg of the
as-prepared Cu NWs-FA was dispersed in 20 ml EtOH containing 1 mM
DT by magnetic stirring for 10 min and was then centrifuged with a 1:1
ethanol:water mixture (100 ml, 8,000 rpm, 2 min) and pure ethanol.Preparation of Cu nanoparticles (Cu NPs)
Cu NPs were synthesized using a previously reported method^33. 11.1 g
PVP (K30) and 4 g sodium hypophosphite were mixed into 40 ml EG
inside a round-bottom flask while vigorously stirring at room tem-
perature under ambient atmosphere. The mixture was heated to
90 °C at a rate of 5 °C min−1. Then, 10 ml of a 1 M solution of copper
sulfate (CuSO 4 ·5H 2 O) in EG at 90 °C was rapidly added into the PVP–
sodium hypophosphite solution while stirring vigorously. As reduction
occurred, the colour of the suspension turned from green to reddish