Academician Liu Zhongfan’s team "ACS Nano": Highly conductive, high-quality nitrogen-doped vertically-oriented graphene film

In recent years, graphene has attracted great attention due to its superior performance and various applications in high-performance electronics, transparent electrodes, supercapacitors, energy storage and conversion
.
In order to avoid the transfer process after the graphene is prepared on the metal surface with high catalytic performance, many studies have realized the direct synthesis of graphene on the functional insulating substrate
.
For example, low-cost, highly transparent glass materials can become an ideal platform for direct manufacturing of graphene-based electronic products
.
However, graphene deposited on a non-catalytic insulating substrate at a low temperature (about 600°C) generally has a high defect density, poor crystalline quality, and conductivity
.
Therefore, it is very important to develop a low-temperature growth strategy for graphene that is compatible with low-cost glass substrates while maintaining the high crystal quality of graphene
.
The team of Academician Liu Zhongfan and Researcher Zhang Yanfeng of Peking University used radio frequency plasma enhanced chemical vapor deposition (rf-PECVD) to grow high-quality vertically-oriented graphene (VG) films on high borosilicate glass substrates
.
Also based on the synthesis strategy of methane/acetonitrile precursor, the carrier concentration is adjusted by nitrogen doping
.
When the transparency of the nitrogen-doped VG film is 88%, its resistance can be reduced to about 2.
3kΩ·sq-1, and the conductivity is more than doubled compared to conventional PECVD products based on methane precursors
.
This synthesis method can produce 30-inch-scale uniform nitrogen-doped graphene glass, thereby promoting its application in high-performance switchable windows
.
In addition, this nitrogen-doped VG film can also be used as an effective electrocatalyst for the electrocatalytic hydrogen release reaction
.
The research was published in "ACS NANO" as a paper entitled "Highly Conductive Nitrogen-Doped Vertically Oriented Graphene toward Versatile Electrode-Related Applications"
.
? [Preparation of nitrogen-doped VG film] The author prepared a nitrogen-doped VG film on a glass substrate by using the rf-PECVD method (Figure 1a)
.
The methane and acetonitrile molecules are first decomposed into several carbon and nitrogen active substances by radio frequency glow discharge.
These active substances can be adsorbed on the dangling bonds and cracks on the glass substrate due to ion bombardment
.
By extending the growth time, the active material migrates on the horizontal layer and forms small graphene nanosheets
.
Then, due to the vertical electric field and internal stress effects, the graphene nanosheets grow vertically on the entire glass surface
.
To be application-oriented, the author also obtained a 30-inch graphene glass sample (Figure 1e) through this method, which exhibited good transparency
.
Therefore, the growth strategy of nitrogen-doped VG films developed by rf-PECVD can be further extended to an industrial scale, which is conducive to practical applications
.
Figure 1 Synthesis of large-scale uniform nitrogen-doped VG film on high borosilicate glass? [Smart window based on nitrogen-doped VG film] The author uses nitrogen-doped VG film on high borosilicate glass to make a switch Windows (Figure 2a)
.
A layer of liquid crystal material (PDLC) is sandwiched between two layers of VG glass composites.
By applying an appropriate electric field, the liquid crystal molecules can be switched from the unaligned state to the aligned state, so that the window changes from opaque to transparent
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Figure 2b is a 6mm×10cm switchable window glass sample (transmittance?88%; sheet resistance?2.
3kΩ·sq-1)
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By switching the applied voltage from 0 V to 30 V, the window immediately changed from the scattering state (about 1% transmittance at 550 nm) to the transparent state (about 70% transmittance at 550 nm) (Figure 2c) )
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Therefore, due to the characteristics of low sheet resistance, high transparency, high contrast, and good durability, nitrogen-doped VG glass composites are expected to be used as transparent electrodes in switchable smart windows
.
Figure 2 Smart window based on nitrogen-doped VG film? [Electrocatalytic application of nitrogen-doped VG film] The high-quality nitrogen-doped VG nanosheets in this study have good conductivity and many active catalytic sites (N Atoms, edges, etc.
), can enhance its hydrogen release reaction (HER) activity (Figure 3a)
.
The author grows a series of VG films with different nitrogen doping concentrations directly on glassy carbon electrodes as electrocatalysts
.
The nitrogen-doped VG film exhibits higher electrocatalytic activity in HER than other nitrogen-doped graphene materials
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After 2000 CV cycles, the electrocatalytic current density remained almost unchanged from the initial state (Figure 3f)
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This nitrogen-doped VG film has higher HER performance than the undoped VG film, and can even be comparable to the HER performance of graphene doped with metal atoms, and is expected to be used in HER electrocatalytic applications
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? Figure 3 Electrocatalytic performance of nitrogen-doped VG film in HER