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In dieser Studie wird eine bahnbrechende Methode zur plasmagetriebenen Trockensynthese von Natriumnitratkristallen auf Cellulose-Nanofibrillenfilmen vorgestellt, die einen neuartigen, lösungsmittelfreien Ansatz zur Stickstofffixierung bietet. Die Forschung kombiniert TEMPO-oxidierte Cellulose-Nanofibrillen (TCNFs) mit diffusem Plasma zur Entladung der Oberflächenbarriere (DCSBD) unter Umgebungsbedingungen, um kristallines NaNO3 erfolgreich zu synthetisieren. Die Studie zeigt drei kritische Stadien morphologischer und chemischer Transformationen: die anfängliche Protonisierung von TCNF-Carboxylgruppen, die Nitratbildung auf der Filmoberfläche und die Bildung einer homogenen dünnen Schicht NaNO3 nach längerer Plasmabehandlung. Die Bildung und das Wachstum von Natriumnitratkristallen folgen einem dreistufigen Mechanismus, der durch verschiedene Komponenten der Plasmaentladung beeinflusst wird. Dieses mechanistische Verständnis unterstreicht das Zusammenspiel zwischen den Eigenschaften von Plasmaentladung und Substratreaktivität und eröffnet Wege für fortschrittliche Anwendungen in den Bereichen thermische Energiespeicherung, Katalyse, Stickstofffixierung in der Landwirtschaft und funktionalisierte Cellulose-Nanomaterialien als Festelektrolyte für Li-Ionen-Batterien. Die Fähigkeit, die Bildung von NaNO3 räumlich und zeitlich präzise zu kontrollieren, eröffnet neue Möglichkeiten für plasmafunktionalisierte Cellulose in nachhaltigen, leistungsstarken Materialanwendungen. Künftige Arbeiten werden sich auf die Skalierbarkeit und Integration in elektrochemische Geräte konzentrieren und damit weiteres Potenzial für plasmafunktionalisierte Cellulose in verschiedenen Branchen erschließen.
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Abstract
Fixation and direct capture of atmospheric gases into stable, reusable products is highly desirable for environmental remediation and sustainable resource utilization. Traditionally, catalytic materials based on heavy metals or metal oxides have been extensively employed. This work studies the potential of TEMPO-oxidized cellulose nanofibrils (TCNFs) combined with non-thermal plasma for atmospheric nitrogen fixation. We specifically investigate how sodium counterions on the TCNF surfaces influence interactions with plasma-generated reactive nitrogen species in a half-diffuse coplanar surface barrier discharge (HDCSBD). Structural chemical analyses using Scanning Electron Microscopy (SEM), X-ray Photoelectron Spectroscopy (XPS), Attenuated Total Reflection Fourier-transform infrared spectroscopy (ATR-FTIR), and ultraviolet–visible (UV–Vis) spectroscopy reveal that reactive plasma species, coupled with an electric field, drive selective nitrate formation. We show that sodium counterions play a crucial role in facilitating the direct formation of crystalline particles on the surface, uncovering a previously unreported plasma-driven pathway for selective nitrogen fixation. These findings provide new insights into the interplay between plasma chemistry and surface counterions, paving the way for developing functionalized cellulose nanomaterials for energy storage, catalytic applications, or agriculture.
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HDCSBD
Half-diffuse coplanar surface barrier discharge
TCNF
TEMPO-oxidized cellulose nanofibril
CNF
Cellulose nanofibril
CNC
Cellulose nanocrystals
XPS
X-ray photoelectron spectroscopy
SEM
Scanning electron microscopy
ATR-FTIR
Attenuated total reflection Fourier-transform infrared spectroscopy
UV–Vis
UV–visible spectroscopy
Introduction
Cellulose, the most abundant biopolymer on Earth (Li et al. 2021) has shown promise for sustainable materials design and engineering because of its renewability, biodegradability, non-toxic nature and chemical versatility. Cellulose can be sourced from a diversity of plant- and animal-derived biomasses, including wood, cotton, agricultural crops, marine animals, and bacteria, among others.
Cellulose is a linear homopolysaccharide of glucose repeating units bonded via β-1,4-linkages (French 2017). Due to the presence of three hydroxyl groups (–OH) per glucose unit, linear cellulose chains assemble via strong intra- and intermolecular hydrogen bonding into semi-crystalline units known as elementary fibrils (or microfibrils). These fibrils are ~ 3–4 nm wide in wood (Isogai et al. 2011). Bundles of elementary fibrils form larger units called macrofibrils (~ 15–60 nm wide), which in turn assemble into cellulose fibers (e.g., 20–50 μm wide and 1–4 mm long) (Isogai et al. 2011).
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The potential of cellulose fibers for sustainable materials design and engineering has been significantly extended with the exploitation of their nano- to micro-scale structures (i.e., microfibrils). According to recently established ISO standards on cellulose nanomaterials (ISO/TS 20477:2023), cellulose fibrils with a width < 100 nm are referred to as cellulose nanofibrils (CNFs). CNFs can be produced from biomass by high-shear mechanical treatment. Additional mechanical (e.g., refiner), chemical (e.g., TEMPO-mediated oxidation) and/or enzymatic (e.g., endoglucanase) pretreatments can be combined with the primary high shear treatment to ease the extraction method of CNFs (i.e., to lower the energy consumption and processing time) and/or modify the surface chemistry of cellulose (Nechyporchuk et al. 2016). CNFs are semi-crystalline, flexible, entangled network-forming fibrils of high specific surface area and high aspect ratio (> 50) (Kelly et al. 2021). By subjecting the biomass to a controlled acid hydrolysis treatment, highly crystalline, rod-shaped particles, so-called cellulose nanocrystals (CNCs), can instead be produced. Unlike CNFs, CNCs have widths varying between 3 to 30 nm and lengths between 0.05 to 0.5 μm (for plant biomass) (Vanderfleet et al. 2021).
Cellulose nanomaterials have been investigated for a multitude of applications, spanning from composites and insulation foams to packaging films and coatings, wound dressing and biomedical scaffolds (Lin et al. 2014). However, such application development often requires surface chemical modification of cellulose to equip these nanomaterials with specific functionalities (e.g., hydrophobicity, thermal resistance (Aziz et al. 2022)) and/or enable their compatibility with other materials (e.g., inorganics, hydrophobic polymers (Sivério et al. 2013)). Typical methods of surface modifications are focused on altering surface functional groups, either via chemical treatment (e.g. carboxymethylation or esterification) or physical modifications (e.g. surface adsorption or grafting) (Rol et al. 2019; Ghasemlou et al. 2021).
Plasma-based surface modification represents an alternative, energy-efficient, environmentally benign approach to conventional chemical methods (Varshney et al. 2011). Cold atmospheric plasmas, such as dielectric barrier discharge (DBD), provide a scalable, non-thermal approach to induce molecular, microscopic, and macroscopic transformations through controlled ion bombardment and radical-driven reactions (Kusano et al. 2009). Among different DBDs, diffuse surface barrier discharge (DCSBD) configuration holds substantial promise for its cost-effectiveness, durability, and adaptability (Černák et al. 2009). Its ability to generate a high-density cold atmospheric plasma under ambient conditions makes it well-suited for large-scale cellulose functionalization (Prochádzka et al. 2018; Krumpolec et al. 2020). DCSBD plasma has also shown promise for wood modification, further expanding its potential in bio-based applications (Talviste et al. 2019). Previous studies have also demonstrated the benefits of plasma treatments to enhance the wettability, chemical reactivity, and interfacial bonding of cellulosic materials without modifying or altering the bulk properties of cellulose (Ranjha et al. 2023; Kolarova et al. 2013). Most studies focus on the introduction of polar groups to increase surface energy via oxygen-containing (e.g. -OH; (Gerullis et al. 2022; Calvimotes et al. 2011)) or nitrogen-containing (e.g. -C-ONO, -C-N; (Dimic-Misic et al. 2019; Kusano et al. 2018)) groups. Additionally, studies suggest the possibility of tuning the refractive index and optical band gap of the cellulose films with plasma treatment (Mahmoud et al. 2016). Beyond direct surface modifications, plasma processes like volume DBD have been employed to deposit functional layers onto cellulose substrates. Recent studies have demonstrated multilayer coatings on cellulose-based hand sheets and porous composites significantly enhance electrochemical performance in battery applications (Profili et al. 2023; Rousselot et al. 2023; Hamdan et al. 2023).
Plasma treatments in the air have also been recently studied for nitrogen fixation in liquid solutions. Plasma treatments in air generate reactive oxygen and nitrogen species (RONS), including nitrate (NO₃⁻), nitrite (NO₂⁻), hydroxyl radicals (•OH), hydrogen peroxide (H₂O₂), ozone (O₃), and atomic oxygen (•O) (Trunec et al. 2022; Prochádzka et al. 2018). Such species play a crucial role in nitrogen fixation processes at the plasma/liquid interface due to their ability to permeate and dissolve in the liquid. In the case of plasma-activated water (PAW), plasma-generated RONS facilitate the conversion of atmospheric nitrogen (N₂) into valuable nitrogen-containing compounds (e.g. nitrite and nitrate (Galmiz et al. 2025)), representing a sustainable route for nitrogen fixation (Janda et al. 2025). Despite growing interest in plasma-assisted nitrogen fixation, nitrogen fixation on solid material remains an unexplored substrate beyond conventional nitrogen functional group (amine, amide) grafting (i.e. low-temperature plasma-assisted nitriding for metallurgical surface modifications (Czerwiecz et al. 2000) or plasma-induced surface functionalization of polymeric biomaterials to induce amino groups on the surface enhancing biocompatibility (Schröder et al. 2001; Vesel et al. 2021)). Regarding cellulose, only a few studies have reported plasma-induced nitrogen functionalization of cellulose, primarily by introducing functional groups such as amines and amides on the surface (Pertile et al. 2010).
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However, recent modifications on cellulose have highlighted the possibility of creating a low-cost and sustainable nanomaterial able to capture greenhouse gases such as CO₂ (Akaya et al. 2025). The authors describe amine-functionalized cellulose leveraging their surface functionalization to facilitate chemisorption with gaseous species. In this sense, the use of cellulose could offer synergy to catalyze the nitrogen fixation of gaseous species activated during the plasma process.
In this work, we explore the potential of cellulose nanomaterials as substrates for nitrogen gas trapping, specifically investigating the interactions between cellulose surfaces and reactive plasma species during treatment. This study focuses on understanding the surface modification of TCNF films by diffuse coplanar surface barrier discharge (DCSBD) plasma and the specific role of TCNF sodium counterions on plasma-induced surface changes. To accomplish this, we investigated extended plasma exposures, i.e., up to 10 min, focusing on the film’s interaction with reactive gas species. Therefore, our study is divided into two parts. The first part discusses the dynamic plasma treatment of TCNF films for uniform treatment of the TCNF film surface. The second part focuses on the static plasma treatment of the films to differentiate the effects of various discharge regions on the film’s modification. The findings establish a sustainable, solvent-free and tunable pathway for nitrogen fixation and expand the potential of plasma-functionalized cellulose nanofibrils in energy storage, catalysis, and bio-based electronics.
Materials and methods
Materials
Laboratory grade chemicals, including sodium hydroxide (NaOH), 2,2,6,6–tetramethylpiperidinyl-1-oxy radical (TEMPO), sodium borohydride (NaBH4), sodium bromide (NaBr), and a 10–15% sodium hypochlorite solution (NaClO) were all purchased from Sigma Aldrich (USA).
From TEMPO-oxidized cellulose nanofibril suspension to TCNF films: preparation and characterization
TEMPO-oxidized CNFs with a surface charge density of 1.2 mmol of carboxyl groups per dry gram of cellulose was produced from southern bleached softwood kraft pulp (SBSK) market pulp (with a 0.12 mmol/g of surface carboxyl groups) by TEMPO-mediated oxidation, as described by Saito et al. (2006).
In brief, 5 g of dry SBSK pulp was reacted with 0.08 g TEMPO, 0.5 g NaBr and 5 ml NaClO in deionized water at pH 10. The reaction was conducted for 3 h with regular additions of 0.5 M NaOH to maintain the pH at 10. The TEMPO-oxidation was followed by a reduction process with the addition of 0.5 g NaBH4 at pH 10 for 3 h. The resulting TEMPO-oxidized pulp was thoroughly washed with deionized water until neutral pH.
The TCNFs were then produced by subjecting a 0.5 wt.% TEMPO-oxidized pulp through an aqueous counter collision equipped with a 100-µm nozzle (ACC, Sugino, Ltd) for 5 passes at a pressure jet of 200 MPa.
TCNF films were finally prepared by pouring the aqueous suspension into polystyrene Petri dishes and let dry for 3–5 days in ambient conditions. Once dried, the films were kept in a temperature-controlled room at 23 °C and ~ 35% relative humidity (RH) before characterization and further functionalization.
Atmospheric plasma treatment
A cold atmospheric plasma produced by DCSBD-like geometry (ROPLASS s.r.o. and Masaryk University, Czech Republic) was used to functionalize the TCNF films, as illustrated in Fig. 1a. The discharge generated by this electrode is typically filamentary in coplanar geometry (Fig. 1b)1. The plasma has two distinct components: filament channels bridging the electrode gap and diffuse spots over the electrode surface. The used electrode was covered by 96% alumina (Kyocera, Japan). The discharge was powered by continuous AC voltage with a peak-to-peak amplitude of 16 kV and frequency of ~ 20 kHz, generating a plasma layer with a surface area of 9 × 10 cm2 and an effective thickness of ~ 0.3 mm. The total power in the discharge was approximately 185 W, and the temperature of the dielectric substrate remained within the interval of 60—65 °C. A built-in temperature probe and cooling unit ensured temperature stability to prevent thermal degradation of the cellulose films. The plasma was operated in ambient air at ~ 23 °C and humidity between 30 and 50%.
Fig. 1
a Schematic of the experimental setup used for the treatment of TCNF films, and b detail of the coplanar geometry with filament components
Due to the filamentary character of the discharge, two treatment modes were employed: dynamic and static. In the dynamic mode, samples (dimensions ~ 30 × 20 mm2) were mounted on a holder that allowed smooth movement through the plasma region, ensuring homogeneous exposure. Specimens were positioned 0.25 mm from the dielectric layer and exposed to plasma for durations ranging from 30 s to 10 min. This distance was selected as optimal for the reactor: decreasing the gap quenches gas exchange between the ceramic barrier surface while increasing it reduces treatment intensity and weakens direct interaction with the filament channel (Ondráčková et al. 2008).
In the static mode, samples were treated without movement at a fixed 0.25 mm distance, allowing discharge pattern formation on the film surface. This mode was used exclusively for TCNF films to investigate nitrate growth mechanisms and the role of discharge components in pattern formation. Samples were exposed to plasma for 1, 2, and 5 min, corresponding to the time range in which a distinct pattern appeared on the film surface. Optical measurements were then performed to analyze the patterns.
To separately investigate the effect of the discharge’s electric field on Na⁺ diffusion, without the influence of surface etching or reactive species, the electrode was covered with a Kapton tape to prevent discharge formation while maintaining the same voltage amplitude as in the active plasma experiments. This configuration simulated the external electric potential of the discharge under identical conditions, and samples were exposed for 10 min. The approximate value of the electric field strength was in the range of 1–30 kV/cm.
Surface characterization
Morphological changes induced by plasma were examined by field emission scanning electron microscopy (FE-SEM) with Crossbeam 540 (Zeiss NTS GmbH) using the in-lens detector. The operating voltage ranged from 0.5 to 1.5 kV, with lower voltages used for untreated films and higher for films identified with nitrate crystals. Before imaging, all films were coated with 5 nm of gold to ensure no charging effects during SEM analysis. The average sizes of the structures were calculated using the Gwyddion software (Nečas et al. 2012).
The topography of untreated and plasma-treated films was assessed using an Asylum Research atomic force microscope (AFM, Oxford instrument, USA) in tapping mode. A silicon cantilever (AC240TS, Olympus) with a resonance frequency of ~ 300 kHz and a tip radius < 10 nm was used. The surface roughness (mean roughness, Sa) of the films was determined using the Gwyddion program (Nečas et al. 2012) from 5 × 5 μm2 scans in triplicate. Photos of the discharges and treated films were taken by a Nikon D7500 camera using an EX Sigma DG macro lens with teleconverter Soligor C/D7.
The impact of plasma treatment on change in the surface chemistry was examined through Attenuated Total Resonance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) and X-ray Photoelectron Spectroscopy (XPS).
The FTIR measurements were performed on the CARY 660-FTIR system (Agilent Technologies, USA) with a silicon ATR crystal at an angle of 45°. For each sample, three spectra were taken at evenly spaced locations with a resolution of 4 cm−1, and each was averaged over 128 scans. All analyses were conducted in an ambient atmosphere between 4000 and 400 cm−1.
The XPS measurements were carried out using an ESCALAB 250Xi (Thermo Scientific, UK) spectrometer with a monochromized Al-K alpha X-ray source and a spot size of 650 µm. Overview spectra were obtained at a pass energy of 100 eV with a step size of 1 eV, while high-resolution spectra were collected at a pass energy of 20 eV with a step size of 0.1 eV. The spectra were calibrated using the C–C and C-H bond energies at 284.8 eV as a reference. Curve fitting was performed with ThermoFisher Scientific Avantage software, using Gauss-Lorentz peak fitting and a Shirley baseline.
The optical properties of the films before and after plasma treatment were evaluated using a Cary 60 UV–Vis spectrometer (Agilent Technologies, USA) in transmittance mode. The films were supported on glass/quartz substrates (Ossila Technologies, UK) to ensure stability and alignment perpendicular to the beam. Baseline measurements were performed without blank substrates to avoid potential artefacts. Transmittance spectra were recorded in the 200–900 nm wavelength range with a step size of 1 nm.
X-ray Diffraction (XRD) analysis was used to identify any changes in the crystalline structure and crystallinity of the cellulose nanomaterials after plasma treatment. The examination was conducted using a D8 Advance system (Bruker, USA) in the 2θ range of 3° to 50°, with a step size of 0.01° and a scan rate of 1°/min. The crystallinity index (CI) was determined from the XRD patterns using the Segal method (French 2014), according to the equation:
In this equation, I200 represents the intensity of the crystalline peak at the 200 plane, and Iam corresponds to the minimum intensity at 2θ ~ 18.5°. Although this method has been reported to yield higher CI values than other experimental and data analysis approaches, such as peak deconvolution of XRD spectra, Raman spectroscopy or 13C Nuclear Magnetic Resonance (Park et al. 2010), this study discusses CI by XRD to capture the qualitative changes in crystallinity of the films induced by plasma treatment. However, the Segal CI is also known to depend on crystallite size, which may be modified by plasma treatment through peak broadening and overlap (French et al. 2013), so the reported CI values should be regarded as purely qualitative indicators of the relative magnitude of changes between different treatments rather than as absolute crystallinity fractions.
Results and discussion
Dynamic plasma treatment on TCNFs
Figure 2 presents the SEM images of TCNF films subjected to plasma treatment under dynamic conditions for different durations. The untreated TCNF film (Fig. 2a) appears smooth, without any visible fibrils or surface defects, suggesting uniform consistency fibrillation of the pulp fibers.
Fig. 2
Surface morphology of TCNF films before a and after plasma treatment for b 30 s c 60 s d 5 min and e–f) 10 min at × 5 k and × 25 k magnification, respectively and in the inserts
After 30 s of plasma exposure (Fig. 2b), notable structural alterations highlighted by dark patches of ~ 275 ± 71 nm in diameter can be seen on the TCNF surfaces. These variations in contrast indicate possible alterations in both surface morphology and chemistry. One plausible explanation is the exposure of crystalline cellulose regions to reactive oxygen species, leading to uneven surface etching, a phenomenon previously reported in plasma-treated polymer films (Gerullis et al. 2022; Dimic-Misic et al. 2021).
After 60 s of plasma treatment (Fig. 2c), the film develops an irregular granular pattern with particle diameters ranging from 100 nm to up to 400 nm. While individual particle sizes remain too small to correlate with discharge filaments directly, the clustering of particles aligns with the filament channel diameter of ~ 10 μm (Hoder et al. 2009), suggesting that direct filament interaction plays a key role in initiating particle formation.
After 5 min of exposure (Fig. 2d), the particles have grown evenly over the entire surface of the TCNF film, with smaller particles clustering together to form larger structures. They form a thin, continuous, rough layer, uniformly covering the film. Still, parts of the TCNF substrate remain visible, showing clear morphological changes indicative of etching. These observations suggest the possibility of two concurrent phenomena occurring during plasma treatment: an uneven etching of the film’s topography and the growth of particles.
After 10 min of plasma exposure (Fig. 2e and f), the area is covered with a granular coating of larger, evenly distributed particles. These structures exhibit nucleation centers with underlying pillar-like protrusions. Nanometric bumps on the particle surfaces are also visible, which may be created from the etching of cellulose and the related carbon redeposition rather than being parts of the original particle growth.
According to Table 1, the average surface roughness (Sa) after 30 s of plasma treatment quadrupled. After 60 s of exposure, the roughness continued to rise, reaching 45 nm, representing a 20-fold increase over the untreated film. For higher times, the Sa was not obtaineda. These trends align with the SEM images, which revealed larger particles and noticeable surface etching (Gerullis et al. 2022; Kusano et al. 2018). AFM measurements further confirm surface etching at shorter plasma exposure times (< 60 s) and particle accumulation at longer durations. The AFM scans from all measured conditions can be found in the Supplementary Information in Fig. S1.
Table 1
Morphological and optical properties of TCNF film before and after air plasma treatment. Sa is the surface roughness of the films, measured by AFM, T350 and T700 are the transmittance values of the films reported at 350 and 700 nm, respectively
Plasma treatment duration [s]
Sa [nm]
T350 [%]
T700 [%]
0
2.55 ± 0.08
85
89
30
11.47 ± 0.51
84
89
60
44.13 ± 1.27
58
81
300 (5 min)
N/Aa
2
32
600 (10 min)
N/Aa
14
16
aFor plasma exposures lasting over 60 s (5 and 10 min) the surface becomes less stable because of powder-like nature of the particle layer, making it difficult for subsequent AFM analysis
Plasma-treated films exhibit a noticeable milky-white appearance, suggesting reduced optical transparency. Figure 3 and Table 1 compare the transmittance at two wavelengths (350 nm and 700 nm) of the films before and after plasma exposure. Up to 30 s of exposure, transmittance remains nearly unchanged, indicating that initial roughness increases are too low to affect optical properties. However, after 60 s, a 33% decrease in transmittance was observed at 350 nm, likely connected to both etching and the start of particle formation. After 5 min of exposure, the transmittance decreases by more than 97%, confirming the observed formation of a thick and uniform particle layer. Interestingly, after 10 min of plasma exposure, the transmittance slightly increased to ~ 16% of its initial value. This slight rise may be attributed to the observed modifications in particles’ structure on the SEM images (Fig. 2e), responsible for affecting light scattering and transmission. The change in optical transparency cannot be caused by over-oxidation, as visible from reference transmittance measurement of non-TEMPO oxidized CNFs in Supplementary information in Table S1. Therefore, an explanation by a different structural change must be found.
Fig. 3
Transmittance of TCNF films a measured after different plasma exposure times and b corresponding visual change
XRD analysis was performed to investigate plasma-induced etching (Fig. 4) further. Plasma exposure often preferentially affects amorphous regions due to their weaker intermolecular bonding (Dimic-Misic et al. 2019). The XRD diffractograms of all samples show the main peaks consistent with the cellulose Iβ structure at Miller indices (1–10), (110) and (200), as reported by French et al. (2014) and Garvay et al. (2005). As visible in Fig. 4, in our case the first two are superimposed into one peak at ~ 16°, followed by a clear peak 22.8°.
Fig. 4
XRD patterns of TCNF films after different plasma exposure times
The crystallinity index (CI) value of ~ 70% was calculated for the untreated TCNF films (see Eq. 1), which is consistent with values reported in the literature for TCNF films made by the casting of a suspension of carboxyl charge density of 1.7 mmol/g (Isogai et al. 2011). The CI of the films slightly dropped with increasing exposure time (69% for 5 min treatment) but remained at ca. 72% after 10 min of treatment. These minor changes in CI suggest that either the etching process occurred in a non-selective way with no differentiation between crystalline and disordered regions of the films or the etching occurred in a thin surface layer below the sensitivity limit of XRD. However, taking the limitation of Segal method into account, the calculated values could also be affected by changes in crystallite size due to plasma treatment. As a result, determination of the etching behavior purely due to changes CI values are not feasible.
Interestingly, after 10 min of plasma treatment, new crystalline peaks can be observed on the XRD spectrum, with the most prominent one at 29.5°, followed by lower intensity peaks at 32°, 35.4°, 39°, 42.6°, 47.9° and 48.4°. The presence of these peaks after 10 min plasma exposure was assigned to the formation of sodium nitrate crystals on the films’ surface. Peak identification also revealed the presence of the characteristic nitrate signal at 23°, which may overlap with the (200) cellulose signal. While the intensity of this nitrate peak is reported to be generally low (Schmahl et al. 1989), it may have contributed to the increase in CI observed after 10 min of plasma exposure (Table 2).
Table 2
Calculated crystallinity indices (CI) of TCNF films, estimated by the Segal method
Plasma treatment duration [s]
CI [%]
0 (neat TCNF film)
71.2
30
71.1
60
69.8
300 (5 min)
69.3
600 (10 min)
72.3
We believe that the presence of nitrate was initiated by the discharge, as no nitrate peaks were detected in the XRD of the untreated TCNF films. Furthermore, 5 min of exposure may have generated insufficient nitrate concentration for XRD detection, explaining why peaks become prominent only after 10 min.
ATR-FTIR analyzed the chemical changes of the TCNF film surfaces and the dynamics of nitrate synthesis (Fig. 5) to confirm these hypotheses.
Fig. 5
FTIR spectra a of TCNF films with highlighted cellulose bands (in gray) and nitrate bands (in pink). Detail of carboxyl b and nitrate c bands are shown, respectively
The ATR-FTIR spectra of the TCNF films subjected to plasma reveal the characteristic cellulose fingerprint regions between 1800 and 800 cm⁻1 and 3800–2800 cm⁻1, consistent with previous studies (Adel et al. 2011; Gerulis et al. 2022; Haniffa et al. 2017). The band at 900 cm⁻1 corresponds to the C-H out-of-plane ring stretching, often attributed to the disordered regions of cellulose (Poletto et al. 2013). The area.
Between 1200 and 1000 cm⁻1 encompasses vibrations related to the pyranose ring stretching, ether vibrations (~ 1060 cm⁻1), C–C and C-O stretching, and asymmetric C–O–C valence vibrations (~ 1111 cm⁻1 and ~ 1160 cm⁻1). In the 1500–1300 cm⁻1 range, distinct bands appear at 1315 cm⁻1 from the CH2 rocking vibration, 1340 cm⁻1 from the C–OH in-plane bending, 1370 cm⁻1 from the CH deformation, and 1412 cm⁻1 from the CH2 symmetric scissoring in the pyranose ring. Additionally, strong symmetric and asymmetric C-H stretching vibrations of methyl and methylene groups are observed at 2900 cm⁻1, while a broad O–H stretching band appears at 3400 cm⁻1. Another distinct feature is the band at 1605 cm⁻1, assigned to the C = O stretching vibrations of the carboxylate groups (Shimizu et al. 2013), characteristic of TCNFs.
After the 30 s of plasma exposure, a new band at 1735 cm⁻1 appeared in the carboxyl group region, indicating the protonation of the carboxyl groups (formation of –COOH) (Fujisawa et al. 2011, Lavoine et al. 2017). This functionalization suggests an interaction between the surface carboxylate groups and acidic plasma-generated species, such as HNO2 and HNO3, which are known byproducts of air-based DCSBD discharge (Trunec et al. 2022). Given the pKa of the TCNF carboxyl groups is 3.6 (Jiang et al. 2016), the acidic environment generated by plasma likely facilitated the carboxyl protonation. Additionally, ions (e.g. H+) created in the discharge could also play a role in the observed change, supporting more electrostatic pathway of surface functionalization.
At 60 s of plasma exposure, the protonation effects persist. In addition, a slight decrease in the intensity of the β-glucosidic linkage (C–O–C) at 896 cm⁻1 is observed, suggesting selective degradation within the amorphous regions of cellulose (Polettto et al. 2013). Plasma treatment is also known to cleave certain chemical bonds, such as the glycosidic bonds in cellulose, leading to depolymerization (Rumi et al. 2022). In the present study, although a slight decrease in the intensity of the β-glucosidic linkage was observed, one may infer that no substantial deterioration of the cellulose in the TCNF films occurred upon plasma treatment as supported by the XRD analysis.
At the 5-min mark, new bands emerge at 838 cm⁻1, 1356 cm⁻1, and 1790 cm⁻1, corresponding to NO3⁻ out-of-plane bending, asymmetric, and symmetric stretching vibrations, respectively (Zhang et al. 2014). These bands, whose intensity increased with increasing exposure time, confirm the formation of nitrate species on the TCNF film surfaces. After 10 min of plasma exposure, the nitrate bands are significantly more pronounced, indicating further growth of NaNO3.
Notably, the broadening of the primary nitrate band (visible in Fig. 5c) at 1356 cm⁻1, particularly around 1407 cm⁻1, suggests the formation of hydrated nitrate species, likely due to moisture absorption or hydrogen bonding with the cellulose matrix (Shetty et al. 2021; Habib et al. 2019). A shoulder at 1450 cm⁻1, coupled with the decreased intensity of the carboxylate band and a decrease in the carboxylate band intensity, may also indicate mild thermal degradation of the substrate. While significant decarboxylation of TCNFs typically occurs above 250 °C (Lavoine et al. 2017), localized ion bombardment and etching effects during plasma treatment may have induced minor surface-level degradation despite the overall film temperature remaining within (80–100 °C). This could be related to carbon redeposition, as carbon-containing species2 (e.g., CO2) are etched from the substrate. These species are ionized in the plasma into more reactive species, like atomic oxygen and CO, reacting with both cellulose nanomaterials and nitrate crystals.
Since FTIR provides insights into bulk modifications of our films, XPS was used to examine surface chemical changes induced by plasma treatment. The XPS survey spectra, summarized inTable 3,3 confirms the presence of carbon (C), oxygen (O), sodium (Na), and trace nitrogen (N) in the neat TCNF films. The nitrogen content in the untreated films was negligible, as the trace likely comes from exposure to the atmosphere. The untreated TCNF films had an initial O/C ratio of 0.77, which is consistent with prior reports reporting typical O/C ratios between 0.61 and 0.83 for untreated cellulose surfaces (Belgacem et al. 1995; Gerullis et al. 2022; Klarhöfer et al. 2010).
Table 3
Summary of XPS overview spectra results in atomic percentages for TCNF films at different plasma exposure times
%
0 s
30 s
60 s
5 min
10 min
C
55.44 ± 0.65
52.74 ± 0.72
51.15 ± 0.35
43.03 ± 2.26
18.56 ± 0.03
O
42.57 ± 0.43
43.97 ± 0.75
44.95 ± 0.12
46.66 ± 1.04
51.94 ± 0.05
Na
1.63 ± 0.04
2.77 ± 0.02
3.25 ± 0.36
6.55 ± 1.06
15.76 ± 0.17
N
0.36 ± 0.03
0.57 ± 0.04
0.66 ± 0.13
3.74 ± 0.15
13.74 ± 0.20
After 30 s of air plasma treatment, the surface sodium content nearly doubled, accompanied by a slight decrease in carbon. A similar effect was observed in, e.g. the study by Kusano et al. (2018), who attributed this increase in sodium content to the preferential etching of the organic cellulose components, highlighting the resistance to the discharge of the present inorganic ions (e.g. Ca). Simultaneously, oxygen incorporation increases, suggesting the introduction of oxygen-containing functional groups, such as COOH (due to protonation) or other low molecular weight oxidation (LWMO) functional groups on the surface (Siegel et al. 2008, Carlson et al. 1991).
At the 60 s mark, sodium levels increased even more along with the measurement’s standard deviation, suggesting an uneven growth of sodium-containing particles, as observed in the SEM images (Fig. 2c). A further increase in oxygen and nitrogen accompanied by a further decrease in carbon was also observed, with O/C ratio increasing to 0.88.
After 5 min of plasma exposure, a rapid rise in sodium (100%) and nitrogen (450%) concentrations were observed. The oxygen levels remained relatively stable, suggesting that the surface was partially covered by sodium nitrate (NaNO₃) crystals. This hypothesis is further supported by the substantial drop in carbon content, which aligns with the decrease in transmittance (Fig. 3) and the formation of particles atop the cellulosic substrate visible in the SEM images (Fig. 2d). At 10 min of plasma treatment, the entire TCNF surface is covered with NaNO3, as evidenced by the near absence of carbon and the dominance of oxygen, sodium, and nitrogen in the survey spectra. The O/C ratio reached 2.77, aligning with the complete coverage of the TCNFs. Furthermore, the Na/N ratio of 1.15 closely matches the theoretical stoichiometry of sodium nitrate. However, the O/Na ratio of 3.3 is slightly higher than the theoretical value of 3, suggesting the presence of residual oxygen-containing functionalities, potentially from unreacted hydroxy groups or surface-bound moisture. This hypothesis aligns with the asymmetric NaNO₃ band at 1356 cm⁻1 in the FTIR spectrum (Fig. 5), indicating the presence of hydrated nitrate species.
Further insight is provided by high-resolution XPS spectra (Fig. 6), which shows the surface chemistry dynamic (C, O, Na, and N) during plasma treatment.
Fig. 6
High resolution XPS spectra of TCNF films before (0 s) and after dynamic plasma treatment (60 s and 10 min), with a carbon, b oxygen, c sodium and d nitrogen peaks shown. The data in d) 60 s are 4-times enhanced (compared to the rest of the graphs) to highlight chemistry changes
The C 1 s spectrum of untreated TCNF films (Fig. 6a) exhibits peaks corresponding to the C–C/C-H bonds (284.8 eV) within the cellulose matrix. The most prominent peak, observed at 286.5 eV, is attributed to the C–O functional groups, primarily in the form of C–OH and C–O–C bonds. This region also overlaps with the C-N bonds that can form after plasma exposure. The peak at ~ 287.7 eV can be assigned to the O–C–O and C = O bonds in the TEMPO-oxidized cellulose. Finally, the peak at 288.5 eV represents the O–C = O bonds in the TCNF carboxyl groups.
After plasma treatment, the most notable changes were observed in the C–C peak (284.8 eV), whose intensity progressively decreased with plasma exposure. Conversely, the intensity of the C = O peak (287.7 eV) increased during plasma treatment, particularly at lower exposure times (< 60 s). This trend suggests that air plasma primarily affected the C–C–O bonds, which may have led to cellulose fragmentation and the formation of new carbon–oxygen crosslinks.
After 10 min of plasma exposure, a new XPS peak at 290.5 eV assigned to C = O-OR appeared, which can be caused by new functionalities (NaNO3) occurring in O–C = O environment, shifting the binding energy. At this stage, a slight increase in C–C bonds and a corresponding decrease in C–O bonds suggest that while cellulose oxidation slows after nitrate formation, crosslinking (creation of C–C/C-H bonds) may occur beneath the nitrate layer. All in all, these observations, along with FTIR data, imply that while cellulose fragmentation dominates at short plasma exposure, the formation and growth of the layer of NaNO3 crystals on the films’ surface prevent the bulk chemistry of the TCNF films from further alteration and fragmentation.
The O 1 s spectra (Fig. 6b) further support these transformations. The intrinsic cellulose-related C–O (530.5 eV) and C = O/C–OH (531.5 eV) peaks remain relatively stable, with only a minor intensity increase in the C–O peak at early plasma treatment stages, consistent with oxygen functionalization. By 10 min, a distinct N–O peak at 536 eV appears, confirming successful nitrate formation. In addition to these intrinsic peaks, the sodium Auger peak at 536 eV became increasingly pronounced with plasma exposure due to the rise in sodium concentration (see Table 3).
The Na 1 s spectra (Fig. 6c) further supported the dynamic surface transformations during plasma treatment. In the untreated films, a single peak at 1071.4 eV was detected, corresponding to the Na⁺ counterions of the carboxylate groups (COO⁻), characteristic of TCNFs produced by TEMPO/NaBr/NaClO at pH 10 (Tan et al. 2002; Hammond et al. 1981). After 60 s of plasma exposure, a second Na peak emerged at 1072.2 eV, attributed to the Na⁺ ions forming ionic bonds with NO3⁻. This NaNO3 peak intensifies with prolonged plasma treatment (> 60 s), while the carboxylate-associated Na peak diminishes, indicating progressive Na⁺ migration and reaction with plasma-generated NOₓ species to form crystalline sodium nitrate. This observation supports electrostatic mechanism of NaNO3, with NO3 anions likely being a result of three-body reactions as suggested in modeling studies of DCSBD discharge (Trunec et al. 2022).
The N 1 s spectra (Fig. 6d) showed the most dramatic transformation, reflecting the gradual accumulation of nitrate species on TCNFs over time. After 60 s of plasma exposure, two distinct nitrogen peaks emerged: the first, centered at ~ 400 eV, likely corresponds to nitrogen-containing species (e.g., C–N or NHₓ groups), while the second peak at 406 eV is characteristic of the NO3⁻ groups. With prolonged plasma treatment (> 60 s), the 406 eV nitrate peak became dominant, reinforcing the hypothesis of progressive NaNO3 growth and crystallization, alongside a smaller contribution at even higher energy (~ 410 eV), likely from the carbon-based moieties due to redeposition. These observations strongly support the hypothesis that sodium nitrate crystals are induced by interacting plasma-generated reactive species and TCNF surface sodium counterions.
Static treatment on TCNF films
In the second part, we investigated the impact of discharge filaments and their electric field on the diffusion of ions and the formation of sodium nitrate. To achieve this, TCNF films were placed stationary on the ceramic barrier surface and exposed to the discharge. Within 1 min of exposure, a faint striped pattern began to emerge, becoming more distinct after 2 and 5 min (Fig. 7).4 The white stripes appeared above the electrode gaps, where the filament channels are located, while the transparent stripes were associated with the more diffuse region above the electrodes.
Fig. 7
Images of electrode pattern on TCNF film after static plasma exposure for 2 and 5 min, with red markings indicating the locations of the electrodes
This pattern formation can be understood by considering the three-dimensional structure of the coplanar discharge. The plasma filaments, which appear in the gaps between the electrodes, propagate as ionized channels into the air—extending up to 300 μm above the ceramic surface. The diffuse regions, on the other hand, are created when the ionizing wave interacts with the dielectric surface and spreads outward at a decreasing velocity (Hoder et al. 2009; Jánský et al. 2021). Previous studies have shown that certain reactive plasma species, such as OH radicals (Prochádzka et al. 2018) and molecular nitrogen (Hoder et al. 2010), are not evenly distributed in the discharge but instead accumulate preferentially near the electrode edges. Considering our previous findings, we can conclude that there is a connection between the decrease in transparency and the presence of NaNO3, suggesting a strong link between the filament channel and crystal growth.
The morphology of these two regions after 60 s of exposure was analyzed under SEM (Fig. 8) to support the hypothesis that the white regions were related to the crystalline areas. In the transparent region (Fig. 8c), mild etching and increased roughness were observed, likely caused by the higher concentration of reactive species in this part of the discharge (Prochádzka et al. 2018). In contrast, the white region (Fig. 8c-d) showed extensive surface modification, with the emergence of micrometer-sized clusters resembling those observed in dynamic conditions (Fig. 2c-d). These morphological differences strongly support the hypothesis that nitrate growth is linked to the presence of discharge filaments.
Fig. 8
SEM images of TCNF films after static plasma exposure: a-b transparent region at × 5000 and × 25,000 magnification, respectively; c-d), white region × 5000 and × 250,000 magnification, respectively
FTIR analysis confirmed the sequential chemical transformations during plasma exposure, particularly the evolution of carboxyl protonation and nitrate formation (Fig. 9). After 1 min, the transparent regions displayed an increase in carboxyl protonation (1735 cm⁻1). In contrast, the white regions showed reduced protonation and the initial appearance of the NaNO3 band at 1356 cm⁻1. By 2 min, nitrate band intensities increased in both regions, while protonation remained dominant in the transparent areas. After 5 min, the transparent regions exhibited a significant decrease in protonation. In comparison, the white areas displayed an FTIR spectrum nearly identical to that observed in 10 min dynamic treatment, confirming complete nitrate formation.
Fig. 9
FTIR spectrum of TCNF under static condition after 1 min, 2 min and 5 min of plasma exposure
Lastly, XPS measurements5 further support this mechanistic understanding, revealing apparent differences in chemical composition between the white and transparent regions over time (Table 4). After 1 min, oxygen concentration remained nearly identical in both regions, but the white areas contained notably higher sodium and nitrogen levels, consistent with early-stage NaNO3 nucleation.6 At 2 min, Na and N concentrations continued to rise in the white region, while sodium content in the transparent region decreased. By 5 min, the white regions exhibited a composition closely resembling the 10-min dynamic treatment results, with almost no sodium in transparent regions. This redistribution aligns with the hypothesis that Na⁺ migration is driven by the localized electric field of discharge.
Table 4
The results of XPS overview spectra for statically treated samples
Treatment
C [%]
O [%]
Na [%]
N [%]
1 min—White
37.85 ± 0.23
46.9 ± 0.63
9.41 ± 0.46
5.78 ± 0.06
1 min—Transparent
48.3 ± 0.25
47.2 ± 0.20
3.34 ± 0.50
1.17 ± 0.05
2 min—White
22.06 ± 1.08
50.30 ± 0.08
16.23 ± 0.23
11.36 ± 0.78
2 min—Transparent
62.64 ± 3.07
30.02 ± 0.05
5.06 ± 0.07
2.28 ± 0.16
5 min—White
16.24 ± 0.11
53.73 ± 0.01
16.23 ± 0.10
13.45 ± 0.30
5 min—Transparent
67.3 ± 7.42
28.01 ± 3.56
2.85 ± 2.28
1.86 ± 1.58
E-field (No Plasma)
45.16 ± 3.17
47.94 ± 0.70
4.70 ± 0.91
2.2 ± 1.56
This hypothesis was further explored by exposing the TCNF films to an external electric field alone (without plasma discharge with approximate strength 1–30 kV/m). The XPS analysis showed increased sodium content -on the surface (Table 4) compared to neat TCNF film (Table 3). However, no protonation or NaNO₃ bands were observed in the FTIR spectrum (SI, Fig. S7), indicating that while the electric field facilitates Na⁺ mobility, reactive plasma species are essential for nitrate formation.
These findings suggest a two-step chemical process: (1) an initial protonation of carboxyl groups, prominent for shorter plasma exposures, followed by (2) sodium nitrate crystallization, enabled by Na+ diffusion, which dominates after prolonged exposure. Notably, the protonation-to-nitrate transition occurs earlier in the white regions, supporting previous observations from the dynamic part on the effect of nitrate formation on nanofibril functionalization. While these results confirm that protonation and nitrate growth are interconnected, it remains unclear whether they represent sequential steps in a single process, or two independent processes influenced by plasma parameters. Further investigations focusing on plasmochemical processes between ambient air and cellulose are necessary to determine the pathways behind the formation of NO3−.
Conclusion
This study introduces a unique method of plasma-driven dry synthesis of sodium nitrate crystals on cellulose nanofibril films, offering a novel, solvent-free approach for nitrogen fixation.
Combining TEMPO-oxidized cellulose nanofibrils (TCNFs) and diffuse coplanar surface barrier discharge (DCSBD) plasma under ambient conditions, we successfully synthesized crystalline NaNO3. This achievement highlights the potential of plasma technologies to address sustainability challenges in chemical synthesis by eliminating solvents and relying on reactive oxygen and nitrogen species (RONS).
Our results revealed three critical stages marked by distinct morphological and chemical transformations. First, less than 1 min of plasma exposure led to the protonation of TCNF carboxyl groups. Next, for treatments longer than 1 min, we observed nitrate formation on the film’s surface. Finally, after 10 min of plasma treatment, we showed evidence of the formation of a homogeneous thin layer of NaNO₃ on the films’ surface, as confirmed by XRD, FTIR, and XPS analyses.
Formation and growth of sodium nitrate crystals followed a three-step mechanism, each influenced by different plasma discharge components: (1) protonation of the carboxyl groups on the films’ surface driven by RONS in the diffuse plasma regions, (2) sodium ion diffusion facilitated by the field of the discharge, and (3) sodium nitrate crystallization, initiated in the filamentary regions of the discharge with localized-etching induced nucleation.
This mechanistic understanding sheds light on the underlying processes of plasma-enabled dry nitrogen fixation and highlights the interplay between plasma discharge characteristics and substrate reactivity for new applications. The ability to control NaNO3 formation with spatial and temporal precision opens pathways for advanced applications, including thermal energy storage, catalysis, nitrogen fixation in agriculture, and functionalized cellulose nanomaterials as solid electrolytes for Li-ion batteries. Future work will focus on scalability and integration into electrochemical devices, unlocking new opportunities for plasma-functionalized cellulose in sustainable, high-performance material applications.
Acknowledgments
The authors would like to express their gratitude to Daniel Chartrand and Thierry Maris for their assistance with XRD measurements and to Monika Stupavská for her invaluable help with XPS analysis. The authors also thank Manon Saget for her help with film preparation and Gaétan Laroche and Pascale Chevalier for their insightful discussions. This research was made possible through funding from the Mitacs Globalink program and the NSERC Alliance project in Canada. The authors also gratefully acknowledge the support of the CEPLANT project (LM2023039), funded by the Ministry of Education, Youth, and Sports of the Czech Republic, for financial support toward developing the plasma system and measurements conducted at Masaryk University.
Declarations
Conflict of interests
The authors declare no competing interests.
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The presence of NaNO3 under static conditions was further confirmed by HR XPS measurements, as shown in Supplementary information (Fig. S6).
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