High electrocatalytic activity of metal-free and non-doped hierarchical carbon nanowalls towards oxygen reduction reaction

Advancements in Plasma-Enhanced Chemical Vapor Deposition for Producing Vertical Graphene Nanowalls

In recent years, vertical graphene nanowalls (VGNWs) have gained significant attention due to their exceptional properties, including their high specific surface area, excellent electrical conductivity, scalability, and compatibility with transition metal compounds. These attributes position VGNWs as a compelling choice for various applications, such as energy storage, catalysis, and sensing, driving interest in their integration into next-generation commercial graphene-based devices. Among the diverse graphene synthesis methods, plasma-enhanced chemical vapor deposition (PECVD) stands out for its ability to create large-scale graphene films and VGNWs on diverse substrates. However, despite progress in optimizing the growth conditions to achieve micrometer-sized graphene nanowalls, a comprehensive understanding of the underlying physicochemical mechanisms that govern nanostructure formation remains elusive. Specifically, a deeper exploration of nanometric-level phenomena like nucleation, carbon precursor adsorption, and adatom surface diffusion is crucial for gaining precise control over the growth process. Hydrogen's dual role as a co-catalyst and etchant in VGNW growth requires further investigation. This review aims to fill the knowledge gaps by investigating VGNW nucleation and growth using PECVD, with a focus on the impact of the temperature on the growth ratio and nucleation density across a broad temperature range. By providing insights into the PECVD process, this review aims to optimize the growth conditions for tailoring VGNW properties, facilitating applications in the fields of energy storage, catalysis, and sensing.

Nitrogen-doped carbon nanowalls/diamond films as efficient electrocatalysts toward oxygen reduction reaction

Substitution of commercial Pt/C electrocatalysts with efficient carbon-based ones for oxygen reduction reaction (ORR) still remains a huge challenge. For practical ORR applications it is significant to design robust 3D network nanostructures in that they do not require polymer binders. For conventional powder catalysts, they must be combined with substrate, leading to their shedding and degradation. In this work, vertically-aligned N-doped carbon nanowalls/diamond (N-CNWs/D) films are synthesized by means of a microwave plasma chemical vapor deposition technique, where nitrogen doping is conducted during the growth process and a subsequent facile annealing treatment under Ar atmosphere. The obtained Ar treated N-CNWs/D film exhibits an ORR onset potential of 835 mV (versus reversible hydrogen electrode) in 0.1 mol l^-1KOH solution in a four-electron reaction pathway. It also displays excellent tolerance toward methanol crossover and long-term stability (e.g. a current density loss of only 10% even after 16 h measurement). The boosting ORR performance can be attributed to the activated pyridinic N dopant at abundant edge sites and enlarged electrochemical surface areas of N-CNWs/D films. This work not only develops a controllable strategy to fabricate binder-free carbon-based ORR electrocatalysts, but also paves a way to in-depth understand actual active sites in terms of ORR pathway mechanisms.

Review of Vertical Graphene and its Applications

Vertical graphene (VG) is a thin-film complex material featuring hierarchical microstructures: graphene-containing carbon nanosheets growing vertically on its deposition substrate, few-layer graphene basal layers, and chemically active atomistic defect sites and edges. Thanks to the fundamental characteristics of graphene materials, e.g. excellent electrical conductivity, thermal conductivity, chemical stability, and large specific surface area, VG materials have been successfully implemented into various niche applications which are strongly associated with their unique morphology. The microstructure of VG materials can be tuned by modifying growth methods and the parameters of growth processes. Multiple growth processes have been developed to address faster, safer, and mass production methods of VG materials, as well as accommodating various applications. VG's successful applications include field emission, supercapacitors, fuel cells, batteries, gas sensors, biochemical sensors, electrochemical analysis, strain sensors, wearable electronics, photo trapping, terahertz emission, etc. Research topics on VG have been more diversified in recent years, indicating extensive attention from the research community and great commercial value. In this review article, VG's morphology is briefly reviewed, and then various growth processes are discussed from the perspective of plasma science. After that, the most recent progress in its applications and related sciences and technologies are discussed.

Low-temperature low-power PECVD synthesis of vertically aligned graphene

The need for 2D vertical graphene nanosheets (VGNs) is driven by its great potential in diverse energy, electronics, and sensor applications, wherein many cases a low-temperature synthesis is preferred due to requirements of the manufacturing process. Unfortunately, most of today's known methods, including plasma, require either relatively high temperatures or high plasma powers. Herein, we report on a controllable synthesis of VGNs at a pushed down low-temperature boundary for synthesis, the low temperatures (450 °C) and low plasma powers (30 W) using capacitively coupled plasma (CCP) driven by radio-frequency power at 13.56 MHz. The strategies implemented also include unrevealing the role of Nickel (Ni) catalyst thin film on the substrates (Si/Al). It was found that the Ni catalyst on Si/Al initiates the nucleation/growth of VGNs at 450 °C in comparison to the substrates without Ni catalyst. With increasing temperature, the graphene nanosheets become bigger in size, well-structured and well separated. The role of Ni catalysts is hence to boost the growth rate, density, and quality of the growing VGNs. Furthermore, this CCP method can be used to synthesize VGNs at the lowest temperatures possible so far on a variety of substrates and provide new opportunities in the practical application of VGNs.

One-step synthesis of carbon-supported electrocatalysts

Cost-efficiency, durability, and reliability of catalysts, as well as their operational lifetime, are the main challenges in chemical energy conversion. Here, we present a novel, one-step approach for the synthesis of Pt/C hybrid material by plasma-enhanced chemical vapor deposition (PE-CVD). The platinum loading, degree of oxidation, and the very narrow particle size distribution are precisely adjusted in the Pt/C hybrid material due to the simultaneous deposition of platinum and carbon during the process. The as-synthesized Pt/C hybrid materials are promising electrocatalysts for use in fuel cell applications as they show significantly improved electrochemical long-term stability compared to the industrial standard HiSPEC 4000. The PE-CVD process is furthermore expected to be extendable to the general deposition of metal-containing carbon materials from other commercially available metal acetylacetonate precursors.

A Review on the Promising Plasma-Assisted Preparation of Electrocatalysts

Electrocatalysts are becoming increasingly important for both energy conversion and environmental catalysis. Plasma technology can realize surface etching and heteroatom doping, and generate highly dispersed components and redox species to increase the exposure of the active edge sites so as to improve the surface utilization and catalytic activity. This review summarizes the recent plasma-assisted preparation methods of noble metal catalysts, non-noble metal catalysts, non-metal catalysts, and other electrochemical catalysts, with emphasis on the characteristics of plasma-assisted methods. The influence of the morphology, structure, defect, dopant, and other factors on the catalytic performance of electrocatalysts is discussed.

Single-step growth of graphene and graphene-based nanostructures by plasma-enhanced chemical vapor deposition

The realization of many promising technological applications of graphene and graphene-based nanostructures depends on the availability of reliable, scalable, high-yield and low-cost synthesis methods. Plasma enhanced chemical vapor deposition (PECVD) has been a versatile technique for synthesizing many carbon-based materials, because PECVD provides a rich chemical environment, including a mixture of radicals, molecules and ions from hydrocarbon precursors, which enables graphene growth on a variety of material surfaces at lower temperatures and faster growth than typical thermal chemical vapor deposition. Here we review recent advances in the PECVD techniques for synthesis of various graphene and graphene-based nanostructures, including horizontal growth of monolayer and multilayer graphene sheets, vertical growth of graphene nanostructures such as graphene nanostripes with large aspect ratios, direct and selective deposition of monolayer and multi-layer graphene on nanostructured substrates, and growth of multi-wall carbon nanotubes. By properly controlling the gas environment of the plasma, it is found that no active heating is necessary for the PECVD growth processes, and that high-yield growth can take place in a single step on a variety of surfaces, including metallic, semiconducting and insulating materials. Phenomenological understanding of the growth mechanisms are described. Finally, challenges and promising outlook for further development in the PECVD techniques for graphene-based applications are discussed.

Oriented Carbon Nanostructures by Plasma Processing: Recent Advances and Future Challenges

Carbon, one of the most abundant materials, is very attractive for many applications because it exists in a variety of forms based on dimensions, such as zero-dimensional (0D), one-dimensional (1D), two-dimensional (2D), and-three dimensional (3D). Carbon nanowall (CNW) is a vertically-oriented 2D form of a graphene-like structure with open boundaries, sharp edges, nonstacking morphology, large interlayer spacing, and a huge surface area. Plasma-enhanced chemical vapor deposition (PECVD) is widely used for the large-scale synthesis and functionalization of carbon nanowalls (CNWs) with different types of plasma activation. Plasma-enhanced techniques open up possibilities to improve the structure and morphology of CNWs by controlling the plasma discharge parameters. Plasma-assisted surface treatment on CNWs improves their stability against structural degradation and surface chemistry with enhanced electrical and chemical properties. These advantages broaden the applications of CNWs in electrochemical energy storage devices, catalysis, and electronic devices and sensing devices to extremely thin black body coatings. However, the controlled growth of CNWs for specific applications remains a challenge. In these aspects, this review discusses the growth of CNWs using different plasma activation, the influence of various plasma-discharge parameters, and plasma-assisted surface treatment techniques for tailoring the properties of CNWs. The challenges and possibilities of CNW-related research are also discussed.