Plasma Processing Research Articles

Critical plasma-processing-enabled growth of chirality-predefined single-walled carbon nanotubes from carbon nanorings

The chirality-controlled growth of single-walled carbon nanotubes (SWNTs) has been an ultimate challenge since their discovery. We report a proof-of-principle plasma processing on the chirality-predefined growth of SWNTs originating from single-hoop molecules of carbon six-membered rings (carbon nanorings: CNRs), which can be viewed as simple units of (n, n) metallic SWNTs (n: the number of benzene rings). Plasma-enhanced chemical vapor deposition enables us to find the correlation between the diameter of CNRs and that of SWNTs over n = 6–12, while only the specified CNRs (n = 10, 12) correspondingly grow up to near (10,10) and (12,12) metallic SWNTs at critical low-temperature of 350 °C.

Outside Front Cover: Plasma Process. Polym. 9/2024

Outside Front Cover: Plasma Process. Polym. 9/2024

Effect of radiofrequency bias power on transmission spectrum of flat-cutoff sensor in inductively coupled plasma

Real-time monitoring of plasma parameters at the wafer plane is important because it significantly affects the processing results, yield enhancement, and device integrity of plasma processing. Various plasma diagnostic sensors, including those embedded in a chamber wall and on-wafer sensors, such as flat-cutoff sensors, have been developed for plasma measurements. However, to measure the plasma density on the wafer surface in real-time when processing plasma with bias power, such as in the semiconductor etching process, one must analyze the transmission spectrum of the flat-cutoff sensor in an environment with bias power applied. In this study, the transmission-spectrum and measured plasma-density characteristics of an electrode-embedded flat-cutoff sensor are analyzed via electromagnetic simulations and experiments under applied bias power. Our findings indicate that the flat-cutoff sensor accurately measures the plasma density, which is equivalent to the input plasma density under low bias power. Conversely, under high bias power, the plasma density measured by the sensor is lower than the input plasma density. Also, a thick-sheath layer is formed owing to the high bias power, which may complicate the measurement of plasma parameters using the flat-cutoff sensor. Plasma diagnostics using a flat-cutoff sensor in thick-sheath environments can be achieved by optimizing the flat-cutoff sensor structure. Our findings can enhance the analysis of plasma parameters on-wafer surfaces in processing environments with bias power applied.

On Dusty Plasma Processes in the Physics of Comets

On Dusty Plasma Processes in the Physics of Comets

Improved coloration of hemp fabrics via low-pressure argon plasma assisted surface modification

Interest in hemp as a viable cellulosic fibre for clothing has increased, driven partly by its economic benefits and the importance of natural renewable materials in emerging circular economies. However, the coloration and chemical finishing of lignocellulosic fibres such as hemp typically require large quantities of water and chemicals. Argon plasma pretreatment provides a way of modulating the physical properties of hemp fibres to improve the coloration process without compromising other bulk properties such as tensile strength. Such plasma treatments may contribute to alleviating the negative environmental impacts associated with liquid pretreatments, heating, or the use of auxiliary chemicals. Dyeing of hemp fibres is particularly challenging due to its crystalline chemical structure. In this study, low-pressure argon plasma-assisted surface modification of woven hemp fabrics up to 600 s at 40 and 80 Hz was explored for enhanced dyeability, resulting in enhanced dye-fibre bonding. Fourier-transform infrared spectroscopy and Raman spectroscopy of argon plasma pretreated hemp fabrics produced no noticeable changes in the functional groups of the fibres, but a physiochemical modification was observed in terms of the density of polar groups. Scanning electron microscopy (SEM) images revealed marked morphological changes including nano-etching of the fibre surface at certain argon plasma process conditions. The pretreatment process increased fibre hydrophilicity, and enhanced reactivity of the surficial –OH groups towards fibre-reactive and vat dyes, resulting in higher colour strength in dyed woven hemp fabrics. Overall, we envisage such plasma pretreatments may impact positively on the material and energy efficiency of the hemp fabric dyeing process.

Theoretical simulation study of laser-induced plasma bombardment on bacteria

With the rapid advancement of laser decontamination technology and growing awareness of microbial hazards, it becomes crucial to employ theoretical model to simulate and evaluate decontamination processes by laser-induced plasma. This study employs a two-dimensional axisymmetric fluid dynamics model to simulate the power density of plasma bombardment on bacteria and access its decontamination effects. The model considers the transport processes of vapor plasma and background gas molecules. Based on the destructive impact of high-speed moving particles in the plasma on bacteria, we investigate the bombardment power density under various conditions, including different laser spot sizes, wavelengths, plate’s tilt angles, and plate-target spacing. The results reveal that the bombardment power density increases with a decrease in laser spot size and wavelength. For instance, when the plate is parallel to the target surface with a 1 mm spacing, the bombardment power density triples as the laser spot size decreases from 0.8 mm to 0.5 mm and quadruples as the wavelength decreases from 1064 nm to 266 nm. Notably, when the plate is parallel to the target with a relatively close spacing of 0.5 mm, the bombardment power density at 0° inclination increases sevenfold compared to 45°. This simulation study is essential for optimizing optical parameters and designing component layouts in decontamination devices using laser-induced plasma. The reduction of laser spot size, wavelength, plate-target spacing and aligning the plate parallel to the target, collectively contribute to achieving precise and effective decontamination.

Mechanistic study on 4, 4'-sulfonylbis removal with CO2/Ar gas-liquid DBD plasma

In this study, a single dielectric barrier discharge (DBD) coaxial reactor was used to degrade 4, 4'-sulfonylbis (TBBPS) in water using greenhouse gas (CO2) and argon as the carrier gases. The investigation focused on CO2 conversion, reactive species formation, gas-liquid mass transfer mechanism, and degradation mechanism of TBBPS during the discharge plasma process. With the decrease of CO2/Ar ratio in the process of plasma discharge, the emission spectrum intensity of Ar, CO2 and excited reactive species was enhanced. This increase promoted collision and dissociation of CO2, resulting in a series of chemical reactions that improved the production of reactive species such as ·OH, 1O2, H2O2 and O3. These reactive species initiated a sequence of reactions with TBBPS. Results indicated that at a gas flow rate of 240 mL/min with a CO2/Ar ratio of 1:5, both the highest CO2 conversion rate (17.76%) and TBBPS degradation rate (94.24%) were achieved. The degradation mechanism was elucidated by determining types and contents of reactive species present in treatment liquid along with analysis of intermediate products using liquid chromatography-mass spectrometry techniques. This research provides novel insights into carbon dioxide utilization and water pollution control through dielectric barrier discharge plasma technology.

Plasma and radiation mediated green synthesis of gold Nanoparticles-immobilized bio-template based catalytic reactor ‘AuNICaR’: Mechanistic insights, catalytic performance and reusability analysis

The present work reports a green, non-thermal plasma-based approach for efficient synthesis of a gold nanoparticles (Au NPs) Immobilized Catalytic Reactor “AuNICaR” via a novel, efficient, indigenously developed process termed “Plasma Assisted Synthesis and Simultaneous Immobilization (PASSI)”, using isopropanol to enhance the reducing plasma environment. Au NPs were synthesized and simultaneously immobilized onto a cellulose based bio-template pre-functionalized via radiation induced grafting of 2,3-epoxypropylmethacrylate (EPMA) onto a nonwoven cellulose (NWC) substrate. The process entails minimal wastage of expensive gold precursor and eliminates use of external chemicals (reducing and stabilizing agents). Samples were characterized by 13C NMR, FTIR, XPS, TGA, DSC, SEM-EDS, BET, and EDXRF analysis. Catalytic activity was assessed by spectrophotometrically monitoring the reduction of p-nitroaniline (PNA) in presence of NaBH4. DFT calculations established the interaction between Au NPs and EPMA groups of the bio-template to be occurring in ratios of 1:1 and 1:2, with 1:1 being more stable. ANN modelling was also invoked to investigate the effect of simultaneous variation of plasma process parameters on the catalytic performance. AuNICaR was demonstrated for catalytic reduction of pollutants: PNA, p-nitrophenol (PNP) and methyl orange (MO) azo dye in both batch and continuous flow mode. Over 25000 mL of cumulative pollutant feed with concentrations ranging from 0.06 mM − 2 mM was treated at a flow rate of 540 mL.h−1, using a single catalyst batch after intermittent washing with water. With excellent storage stability of > 12 months and > 92 % retention in catalytic activity post multiple cycles, AuNICaR certifies as a potentially inexhaustible catalytic system for cost-effective pollutant treatment.

Effects of discharge frequency on the conversion of n-hexadecane by pulsed liquid-phase discharge in recycle and batch devices

Converting hydrocarbons can make the fossil fuel industry more flexible in responding to market changes by producing various products to meet market demands. Efficient, clean, and flexible plasma processes are a highly promising technology for hydrocarbon processing and conversion. In this study, the conversion of n-hexadecane was investigated using ethanol solution-assisted pulsed liquid-phase discharge plasma. The effects of recycle and batch devices and discharge frequency on feedstocks conversions and product yields were examined. The use of a recycle device facilitated the conversion of n-hexadecane. Adjusting the frequency enabled the regulation of products concentration. High discharge frequency increased the cracking of n-hexadecane and promoted further cracking of reactants into smaller molecular products, boosting the proportion of H2 and C2 hydrocarbons, and enhancing the yield of gases and light hydrocarbons. Reducing the frequency favored polymerization reactions, resulting in the formation of heavy hydrocarbons. At a frequency of 10.2 kHz, the recycle device achieved a gas production rate of 112.1 mL/min and a gas production efficiency of 87.5 mL/kJ. With an SEI of 3202 kJ/L, the conversion of n-hexadecane was 15.5%, the yield of light hydrocarbons was 717.0 mg, and the light product selectivity was 97.1%. This study offers an efficient approach for the processing and conversion of hydrocarbons in the fossil fuel industry.

Comprehensive study of plasma polymerization parameters on thiol-coated LDPE films for effective fibronectin adsorption targeting biomedical applications

In the realm of biomedical applications, biocompatible materials with optimized surface properties are crucial for facilitating cellular interactions. To attain the perfect balance of these properties, surface modification of non-toxic and stable bulk materials is often required. Within this context, this research aimed to improve the physiochemical and cell-responsive properties of a low-density polyethylene film. This was achieved by depositing thiol-rich coatings using a dielectric barrier discharge plasma reactor operating at medium pressures, with 1-propanethiol serving as polymerization precursor. The study systematically investigated the impact of key plasma polymerization parameters, including DBD chamber pressure, treatment time, and the combination of precursor flow rate and discharge power represented by the Yasuda parameter, to identify optimal plasma processing conditions for the formation of thiol-rich coatings. To characterize the coating thickness, hydrophilicity and surface chemical composition, atomic force microscopy, water contact angle measurements, and X-ray photoelectron spectroscopy were employed. The findings indicated that reducing the chamber pressure to 10 kPa led to more hydrophilic and thicker deposits possessing a higher sulphur content. Deposition time (5 to 15 min) also significantly impacted coating thickness and surface chemistry, where long times increased thickness, but also led to a reduced sulphur and increased oxygen content because of more pronounced etching. The optimal deposition time was therefore set at 10 min resulting in the deposition of dense coatings possessing a high number of sulphur-containing functionalities. The Yasuda parameter (W/FM) analysis demonstrated optimal thiol incorporation in combination with high coating stability at an intermediate W/FM value of 72 MJ/kg. Following the determination of the optimal plasma polymerization parameters, the effectiveness of thiol plasma polymerization and subsequent fibronectin immobilization for enhancing cell adhesion and proliferation was investigated. The thiol-coated substrates were found to exhibit superior protein immobilization, because of the high affinity for protein binding of the available thiol groups. To assess cellular responses, Schwann cells were cultured on uncoated and coated samples before and after fibronectin immobilization. The results revealed a superior cellular response of the thiol-coated samples after fibronectin immobilization, showing the highest cell viability and significantly enhanced cell adhesion and proliferation. Collectively, these results underscore the synergistic effect of thiol plasma polymerization and fibronectin immobilization in promoting the cellular response of LDPE substrates, thus highlighting their potential as a surface modification strategy of biomaterials.

Investigating the effect of gliding discharge plasma on polystyrene and Polyamide6 by positron annihilation lifetime spectroscopy

Polymers are widely used today and the changes in their properties are investigated by different methods such as plasma irradiation. One of the most important methods for modifying polymers is cold plasma, by which the surface properties of the polymers can be changed under atmospheric pressure. In this study, the effect of plasma on Polystyrene (PS) and Polyamide 6 (PA6) samples was investigated. Surface and depth changes of the PS and PA6 have been investigated by various experimental techniques such as Fourier Transform Infrared (FTIR), X-ray Diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM), Contact Angle (CA), and Positron Annihilation Lifetime Spectroscopy (PALS). The results of Attenuated Total Reflectance (ATR)-FTIR show that the chemical bonds of PA6 have been changed, and preserved in PS in the depths. The XRD results show that there are no volume changes for chemical bands. The FESEM analysis results reveal that the plasma causes digging and creates roughness on the surface. The CA measurement confirms the FESEM results and points out an increase in hydrophobicity after plasma processing. The PALS results indicate that the free volumes of the matter are changed after plasma irradiation in the depths of PS and PA6. In addition, the plasma also causes changes in the micrometer depth.

Enhancing electrochemical performance of graphene oxide/polyaniline composite through plasma-assisted chemical cross-linking and surface modification

The composite of graphene oxide (GO) and polyaniline (PANI) is a promising electrode material for supercapacitors. A high loading of PANI can provide large specific capacitance, but it is a pervasive challenge to reconcile long-term cycling stability and high-rate discharge. Herein, we focus on a GO/PANI composite with low PANI content (∼25 wt%), developing a stable network of GO with an even distribution of PANI. Moreover, significantly enhanced electrochemical performance is achieved by utilizing plasma technology. The specific capacitance increases by 99 % to 433 F g−1 at 1.5 A g−1 after Argon (Ar) plasma treatment. Even at 15 A g−1, a capacitance retention rate of 91 % is obtained. After 15,000 cycles, the capacitance retention rate is 93 %, surpassing most previously reported GO/PANI composite electrodes. The further assembled all-solid-state supercapacitors can deliver a high energy density of 13.6 Wh kg−1 and exceptional cycling stability. According to results of characterization, it is demonstrated that Ar plasma imparts GO/PANI better surface morphology and achieves etching, reduction of GO, and cross-linking of GO and PANI, thereby activating the electrochemical activities of GO and PANI while ensuring stability. In this regard, the synergistic effects of cross-linking, reduction, and etching, and the mechanism are theoretically illustrated by dynamic differential equations. This work presents plasma processing unique charm for designing graphene-based materials via a customized strategy, exploiting an effectual approach to drive the applications of graphene-based materials in energy storage.

Multimodal intelligent approach to low-temperature atmospheric plasma processing of apple slices before drying.

This study presents a comprehensive analysis of the impact of plasma treatment on the browning inhibition. A 30 min plasma treatment resulted in a pronounced decrease in the concentration of flavan-3-ols, which play a pivotal role in antioxidant defense and browning prevention. This significant reduction is likely due to plasma-induced oxidative stress, which can lead to the breakdown of these compounds or their conversion into other phenolic structures. Simultaneously, a slight increase in dihydrochalcones and flavonols was observed, suggesting a selective effect of plasma on different phenolic classes. The increase in these compounds could be attributed to the plasma's ability to induce specific reactions that generate these phenolics from other precursors present in the apples. The reduction in flavan-3-ols may affect the antioxidant capacity and health benefits associated with the apples, while the increase in dihydrochalcones and flavonols could have a positive impact on the flavor profile and potential health-promoting properties. Moreover, these modifications could contribute to the extension of shelf-life and maintenance of sensory qualities, making plasma treatment a valuable tool in the food industry for enhancing product stability and consumer appeal.

Origin of reconnecting current sheets in shocked turbulent plasma.

Magnetic reconnection, the rearrangement of magnetic field topologies, is a fundamental plasma process throughout the universe, which converts magnetic energy to plasma kinetic energy and results in particle energization. A current sheet is a prerequisite for the occurrence of magnetic reconnection. It has been well documented that reconnecting current sheets are prevalent in turbulent plasmas. However, how these current sheets are formed remains unclear. Among natural plasmas, the region downstream of the Earth's bow shock, the magnetosheath, is one of the most turbulent. Here, we show that the reconnecting current sheets in the turbulent magnetosheath originate from the waves in the region upstream of the shock. Once excited, the upstream waves are transmitted across the shock, compressed, and then transformed into current sheets in the downstream region. Magnetic reconnection subsequently occurs in these downstream current sheets. This process can be generalized to various shocked plasmas in astrophysical and laboratorial environments where turbulent magnetic reconnection should be common.

In Situ Raman Spectroscopy of Plasma Electrochemical and Plasma Catalytic Reactors

Plasma electrochemistry (PEC) has been under investigation for an expanding number of potential applications such as agriculture, water treatment, decarbonized chemical processing, and nanomaterials synthesis [1–4]. PEC in water leads to the formation of gas-phase species that undergo a cascade of processes in the liquid to produce aqueous species such as OH, H2O2, ONOOH, NO2 -, and NO3 - that are sought after in these applications. Plasma-assisted catalysis (PAC) is another growing research field for some of the same applications and others that occur in the gas phase, such as industrial catalyst decoking, VOC abatement, gas reforming, and CO2 conversion [5]. For such problems, popular catalysts include zeolites, TiO2, and CeO2 [6]. Similar to PEC, in PAC the operating conditions for catalysis become more relaxed (e.g. lower temperature to activate the catalyst). There can be plasma-catalyst synergistic effects that improve the process yield well beyond that using plasma or catalyst alone.In both PEC and PAC, the plasma produces energetic photons, electrons, ions, and excited species, and enhances the electric field near plasma-liquid and plasma-solid interfaces. These strongly non-equilibrium conditions enable the plasma to perform the required processes for these applications in many cases in a single step and without assistance, compared to conventional methods that can require multiple steps, potentially toxic chemical agents (e.g. acids, reducing agents), and/or external heating.A major challenge facing the development of PEC- and PAC-related applications is the direct and detailed experimental investigation of the plasma-liquid and plasma-catalyst interfacial regions, respectively. The majority of existing liquid- and solid-phase diagnostics must be performed ex situ, removed from the plasma reactor and after treatment. To extend the scope of diagnostics, we employ an in situ approach using multiple diagnostic techniques to study a wide range of physical and chemical properties at the plasma-water interface. The centrepiece of this platform is in situ spontaneous Raman microspectroscopy, which is advantageous because of its non-intrusiveness, selectivity, versatility, and straightforward calibration. Shaping the laser beam into a light sheet enables probing of the interfacial region with micron-scale spatial resolution. For PEC in pure water, in situ Raman spectra showed that the concentrations of aqueous H2O2 and NO3 - both exceed those of the bulk liquid at a depth of a few tens of microns from the plasma-liquid interface [7].We will focus on transient changes to Raman spectra that appear in the presence of plasma, then disappear when the plasma is switched off. Such changes are observable in situ, not ex situ, and indicate the energy exchange from the plasma to water or the catalyst surface. The general approach will be to establish relationships between plasma properties and changes to the Raman spectra. Two primary cases will be studied. First, we will study PEC in air plasma-water systems at atmospheric pressure, using both batch reactor and electrospray configurations. We will focus on the spectral profile of the –OH stretch band of water and of probe molecules such as NO3 -. Analysis of the –OH stretch band reveals that the plasma weakens the hydrogen bonding network of water. To help pinpoint the cause, we will track the broadening of the N-O symmetric stretch mode (v 1) of NO3 - at less than 20 µm depth from the plasma-liquid interface. Second, we will investigate a PAC reactor consisting of a low- to medium-pressure CO2 plasma in contact with CeO2 as a catalyst. In this case, the catalyst temperature will be tracked using the Stokes-to-anti-Stokes ratio of the Raman intensities of the first-order optical phonon of CeO2, as well as the spectral shift of this spectral feature. Acknowledgments Financial support: Campus France Franco-Thai PhD fellowship (French Embassy in Thailand, E4C Engie sponsorship), IP Paris Energy4Climate “E4C” Interdisciplinary Center, Campus France PHC Stefanik project (49885SJ), CNRS-IEA “GRAFMET”, EUR PLASMAScience, Fédération Plas@Par, ANR grant ANR-15-CE06-0007-01 References [1] T. Orriere, D. Kurniawan, Y.-C. Chang, D. Z. Pai, and W.-H. Chiang, Nanotechnology 31, 485001 (2020).[2] J.-S. Yang, D. Z. Pai, and W.-H. Chiang, Carbon 153, 315 (2019).[3] S. Kooshki, P. Pareek, R. Mentheour, M. Janda, and Z. Machala, Environmental Technology & Innovation 32, 103287 (2023).[4] D. S. Mallapragada et al., Joule 7, 23 (2023).[5] E. Baratte, C. A. Garcia-Soto, T. Silva, V. Guerra, V. I. Parvulescu, and O. Guaitella, Plasma Chemistry and Plasma Processing s11090-023-10421-z (2023).[6] C. A. Garcia-Soto, E. Baratte, T. Silva, V. Guerra, V. I. Parvulescu, and O. Guaitella, Plasma Chemistry and Plasma Processing s11090-023-10419-7 (2023).[7] D. Z. Pai, Journal of Physics D: Applied Physics 54, 355201 (2021).

Electrocatalytic Conversion of Plasma-Activated CO2 Toward Multicarbon Products

Renewable electricity-powered CO2 reduction reaction (CO2RR) toward chemicals and fuels has been gaining significant attention not only for achieving net-zero carbon emissions but also for enabling long-term energy storage. In electrochemical conversion, CO2 can be upgraded into single- or multicarbon products such as carbon monoxide, formate, and ethylene with remarkable selectivity. However, challenges still exist in terms of high overpotential, limited product range, and low production rates. Moreover, most reported CO2RR to multicarbon products which used Cu as catalysts have predominantly generated C2 products like ethylene or ethanol. However these systems require further performance improvement to target C3+ products which have higher energy density and market price. Another electrified conversion technique involves non-thermal plasma, which can activate CO2 into radical, ionized, and vibrationally excited species at atmospheric pressure and near-ambient temperature. However, plasma processes are non-selective, which poses limitations for controlling reaction pathways and necessitates H2 or light alkanes as co-reactants to produce hydrocarbons.We synergized plasma and electrocatalytic conversion to demonstrate a combined CO2RR process that overcomes the limitations of each individual method. Through the integration of a dielectric barrier discharge plasma and an electrochemical flow reactor, pre-activated CO2 was electrochemically converted on a Cu electrode to produce multicarbon products. The combination enhanced the faradaic efficiency and reaction rate of desirable products and opened new reaction pathways by increasing the activity of CO2 prior to adsorption. The production rates of ethanol, propanol, and acetaldehyde were significantly increased, and new products such as methanol, acetylene, and ethane that were formed only in the combined reaction were detected. The product distribution was studied according to plasma-activated states of CO2 and the electrochemical applied potential. To elucidate the synergistic effects in the combined system, the selectivity and production rates were compared with those in the plasma-only and electrochemical-only reactions. Further control experiments using ground-state reactants were conducted, and the excited states during plasma activation were identified by optical electron spectroscopy. This work represents not only electrocatalysis of pre-activated reactants for enhanced generation of multicarbon products during electrified conversion of CO2, but also demonstrates synergistic convergence of plasma and electrocatalysis for converting reactants possessing strong chemical bonds.

(Invited) Organosilicon Coatings Made By a Cold Plasma Processes at Atmospheric Pressure for Electrochemical Applications

Cell manufacturing is estimated to account for up to 40% of battery-industry and the related market will grow 14.1% in the next five years (108.4 B$ in 2019). In this context, high-power and stationary large-scale energy storage systems, such as lithium-ion batteries (LiBs), will be essential to make electricity quickly accessible to the power grid. This is particularly true when it comes to power plants located in remote areas or the use of renewable energy sources. Despite their advantages, commercial LiBs are today limited by their environmental and safety hazards. These limitations lead researchers around the world to discover and employ new technologies for safe recycling operations and a better control of the cycling processes. Currently, works are focused on the development of new advanced materials, composite design as well as advanced interfacial layers. In this context, surface modification by cold plasma is one of the most promising solutions to control and modify charge transport mechanisms at the interfaces. Although, the synthesis of nano-layers made by plasma at atmospheric pressure has already received a lot of interest, their use to modify electrochemical devices, such as electrochemical cells remain limited. In this work, the synthesis of organosilicon coatings on composite conductive layers by atmospheric pressure DBD is studied in detail. The influence of different organosilicon molecules has been analysed to highlight the effect of the growth mode as well as chemical functionalities on the operation of the final electrochemical device. The physical and chemical modifications of plasma on the conductive layer as well as the water stability of the coatings were investigated by ATR-FTIR, XRD, SEM and contact angle. Figure 1

(Invited) Low Temperature Plasma Synthesis and Functionalisation of Carbon Nanostructures for Energy Conversion and Storage

Low-temperature plasmas are nowadays widely developed and used for synthesis and functionalization of various materials with applications in areas ranging from microelectronics and aeronautical industry to energy conversion and storage. The focus of the research and applications has lately shifted towards carbon and carbon-based materials synthesis. The materials of interest vary from nanoparticles, nanotubes, nanowalls, free standing graphene flakes, vertical graphene rods, sponge structures, nanostructured surfaces, to composites (with conductive or non-conductive polymers, or MoS2, to mention some of the examples that will be presented herein). The interest for direct applications of plasma processes as well as nanostructures in the field of energy conversion and storage has been present since a long time. Recently, however, several interesting fundamental breakthroughs have been observed in terms of better control of processes, diagnostics and several other benefits that are attracting more and more attention: lower temperatures of surfaces needed for synthesis and functionalization (due to active plasma species interacting with surfaces), dry processes (diluted precurors or liquid monomers introduced in processes, without use of solvents), decreased overall pollution, fast processes, to mention just some of the advantages. However, our work must not forget the disadvantages and problems that can occur during application tests, such as stability, adhesion, loss of conductivity, reproducibility, general ageing or recyclability.Different factors can affect the function of the final setup (from photocatalytic elements, to batteries or fuel cells/HER systems, in our case) – from production of the starting materials to the assembly. Therefore, careful synthesis and control of the process (including the chamber and surface conditions for example) is necessary. We present herein several examples of carbon materials developed or in development (as mentioned 1D-3D carbons, carbon in organic and inorganic composits, mesoporous carbons). These materials were synthesized at surface temperatures between room temperature and up to 550°C.We emphasize, for example, the importance of in-situ control of processes, the importance of precise process conditions, the control of the synthesized materials with regard to their electrical conductivity, adhesion, thermal stability or sensitivity to photon irradiation.Acknowledgments:Authors acknowledge the EU Graphene Flagship FLAG-ERA III JTC 2021 project VEGA (PR-11938) and the project PEGASUS (funded by the European Union's Horizon research and innovation programme under grant agreement No 766894. UC and NMS acknowledge the Slovenian Research Agency for the program ARRS No. P1-0417 and project Z2-4467. TS, EK and JB want to thank HZB for the allocation of synchrotron radiation beamtime at the HE-SGM beamline of BESSY II. Thanks goes also to Prof. Wöll from KIT for providing the HE-SGM endstation used for the XPS and NEXAFS measurements. This project has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No 730872 (Nr. 18207084-ST and 18207393- ST). EK and JB acknowledge also support obtained via ARD MATEX Region Centre.

(Invited) Progresses on Plasma Processing of Halide Perovskite Materials for Photovoltaic Applications

Metal halide perovskite (MHPs) solar cells represent a promising newcomer in the front of emerging photovoltaic technologies to address the dramatic energy crisis and climate change that we are facing. The exceptional properties of MHPs derive from their hybrid organic-inorganic nature, which allows also for low-cost and straightforward processing. Solar cells containing MHPs as absorbing layer have already achieved a power conversion efficiency of about 25,7 %, close to the efficiency of silicon-based devices. Nevertheless, a major limitation, still preventing the uptake of the technology, is related to the reduced stability of these materials when exposed to operative conditions, namely temperature, light, and moisture. Herein, an effective defect passivation of MHP surfaces is a key strategy to tackle both the stability and the enhancement of solar cell performances. Although many solution-based approaches 1 have been tested, we propose here an innovative use of plasma, as a solvent-free, scalable, and non-invasive promising strategy to boost MHP solar cells performances 2. As starting point we exposed the surface of Methylammonium Lead Iodide perovskite thin films to different plasma conditions implying the variation of power, gas, and treatment time, both for low-pressure (LP) and atmospheric pressure plasmas (APPs). The impact of Ar, , and LPPs on MAPbI3 optochemical properties and morphology was correlated to the performance of the photovoltaic devices and rationalized by density functional theory calculations3. Herein an interesting improvement in photoluminescence was observed for the Ar and treated films, while an improvement in PCE was observed only for the Ar treated device. This result was ascribed to the efficient removal of the superficial organic component, revealed through X-ray photoelectron spectroscopy (XPS), following suitable surface passivation by the electron transporting layer deposition. Following these results, we tested APP treatments, finding a milder morphological modification than LPP treatments but withstanding good surface passivation, as confirmed through the improved photoluminescence intensity, while the effect in terms of photovoltaic devices is still under investigations. Moving from these encouraging results, we applied plasma surface treatments to tin-based perovskite, finding a very interesting beneficial effect when reductive plasma conditions (forming gas) were used on FaSnI3 thin film.4 The collection of these results already show the great versatility and potential of plasma-based techniques for the intelligent modification of metal halide perovskite for solar energy conversion.

Nitrogen Doped Graphene Aerogel Inserted By Carbon Black As Novel Catalyst Support for Low Temperature Hydrogen Fuel Cells

Carbon supported Pt nanocatalysts, which are commonly used in hydrogen fuel cells, suffer from Pt degradation and carbon corrosion. That causes the high cost and limited lifetime of the fuel cell1. Developing catalyst support materials with high surface area, high electrical conductivity, stability and good interaction with catalyst is an effective solution to address this issue.To enhance the activity and durability of Pt-based catalysts for hydrogen fuel cells, graphene aerogels (GA) are a prospective material with rich porous 3D-structure, large surface area, high electrical conductivity, and good electrochemical stability. They also can improve the tolerance of catalyst CO poisoning and avoid graphene stacking by integrating 2D sheet structure into 3D aerogel structure2, 3. Some research show that the nitrogen doped on the surface of carbon materials could improve ORR activity and electronic conductivity by changing the local electronic structure as well as providing a stable anchor point for pt nanoparticles to enhance the durability of catalysts4, 5.However, most GAs are made from graphene oxide (GO) produced by Hummers method which may lead to safety and environmental issue due to the usage of highly toxic and corrosive chemicals2, 5. Also, there are few articles on the application of GA in fuel cells. It may be limited by the collapse and loss of pore structure caused by compression in a fuel cell setup.In this work, graphene oxide produced by electrochemical exfoliation (EGO) was used to prepare N-doped graphene aerogel (NGA) via hydrothermal treatment with ammonia solution as nitrogen source. Moreover, NGA was mixed with carbon black (CB) as a hybrid support material that can avoid collapse of the macropore and generate a graphene-carbon network to provide large surface area for Pt nanoparticles and improve the performance of the fuel cell. Figure1 shows preliminary characterization results of NGA and performance of NGA-CB supported Pt catalysts with different content of NGA. Rich macro-mesoporous structure of NGA can be observed in the SEM image (Figure 1 (a)). Pt catalyst with 20% NGA has a maximum power density of 838 mw cm-2 which is a 31% improvement over that of Pt/CB (Figure 1 (b)).In conclusion, this study presents a fast, efficient and environmentally friendly way to synthesize nitrogen doped GA. The introduction of NGA into CB as catalyst supports can improve the electrochemical active surface area and performance of fuel cells due to graphene aerogel's high conductivity and porosity. Besides, it is effective to enhance the utilization of Pt and durability of catalysts. This will help to reduce the cost of fuel cells and facilitate their commercialization in various fields of application.Figure 1 (a) SEM images of dried NGA. (b) Performance of Pt/NGA-CB with various content of NGA compared to Pt/CB catalyst in a hydrogen fuel cell at 60 ℃ with 100% RH. References Liu, J.; Wu, X.; Yang, L.; Wang, F.; Yin, J., Unprotected Pt nanoclusters anchored on ordered mesoporous carbon as an efficient and stable catalyst for oxygen reduction reaction. Electrochimica Acta 2019, 297, 539-544.Sarac Oztuna, F. E.; Barim, S. B.; Bozbag, S. E.; Yu, H.; Aindow, M.; Unal, U.; Erkey, C., Graphene Aerogel Supported Pt Electrocatalysts for Oxygen Reduction Reaction by Supercritical Deposition. Electrochimica Acta 2017, 250, 174-184.Wu, Z.-S.; Yang, S.; Sun, Y.; Parvez, K.; Feng, X.; Müllen, K., 3D Nitrogen-Doped Graphene Aerogel-Supported Fe3O4 Nanoparticles as Efficient Electrocatalysts for the Oxygen Reduction Reaction. Journal of the American Chemical Society 2012, 134 (22), 9082-9085.Li, O. L.; Chiba, S.; Wada, Y.; Panomsuwan, G.; Ishizaki, T., Synthesis of graphitic-N and amino-N in nitrogen-doped carbon via a solution plasma process and exploration of their synergic effect for advanced oxygen reduction reaction. Journal of Materials Chemistry A 2017, 5 (5), 2073-2082.Xie, B.; Zhang, Y.; Zhang, R., Coassembly and high ORR performance of monodisperse Pt nanocrystals with a mesopore-rich nitrogen-doped graphene aerogel. Journal of Materials Chemistry A 2017, 5 (33), 17544-17548 Figure 1