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Article

Bryophyte-Bioinspired Nanoporous AAO/C/MgO Composite for Enhanced CO2 Capture: The Role of MgO

by
Paulina Jaqueline Cortés-Valadez
1,
Esperanza Baños-López
2,
Yazmín Mariela Hernández-Rodríguez
1,* and
Oscar Eduardo Cigarroa-Mayorga
1,*
1
Department of Advanced Technologies, UPIITA—Instituto Politécnico Nacional, Av. IPN 2580, Mexico City C.P. 07340, Mexico
2
Academia de Química, Universidad Autónoma del Estado de Hidalgo (UAEH), Carretera Pachuca-Tulancingo Km. 4.5., Pachuca C.P. 42184, Mexico
*
Authors to whom correspondence should be addressed.
Nanomaterials 2024, 14(8), 658; https://doi.org/10.3390/nano14080658
Submission received: 24 February 2024 / Revised: 4 April 2024 / Accepted: 8 April 2024 / Published: 10 April 2024
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Abstract

:
A composite material composed of anodized aluminum oxide (AAO), carbon (C), and magnesium oxide (MgO) was developed for CO2 capture applications. Inspired by the bryophyte organism, the AAO/C/MgO composite mirrors two primary features of these species—(1) morphological characteristics and (2) elemental composition—specifically carbon, oxygen, and magnesium. The synthesis process involved two sequential steps: electroanodization of aluminum foil followed by a hydrothermal method using a mixture of glucose and magnesium chloride (MgCl2). The concentration of MgCl2 was systematically varied as the sole experimental variable across five levels—1 mM, 2 mM, 3 mM, 4 mM, and 5 mM—to investigate the impact of MgO formation on the samples’ chemical and physical properties, and consequently, their CO2 capture efficiency. Thus, scanning electron microscopy analysis revealed the AAO substrate’s porous structure, with pore diameters measuring 250 ± 30 nm. The growth of MgO on the AAO substrate resulted in spherical structures, whose diameter expanded from 15 nm ± 3 nm to 1000 nm ± 250 nm with increasing MgCl2 concentration from the minor to major concentrations explored, respectively. X-ray photoelectron spectroscopy (XPS) analysis indicated that carbon serves as a linking agent between AAO and MgO within the composite. Notably, the composite synthesized with a 4 mM MgCl2 concentration exhibited the highest CO2 capture efficiency, as determined by UV-Vis absorbance studies using a sodium carbonate solution as the CO2 source. This efficiency was quantified with a ‘k’ constant of 0.10531, significantly higher than those of other studied samples. The superior performance of the 4 mM MgCl2 sample in CO2 capture is likely due to the optimal density of MgO structures formed on the sample’s surface, enhancing its adsorptive capabilities as suggested by the XPS results.

1. Introduction

Alongside transitioning from fossil fuels to renewable energies like hydrogen, carbon recycling—utilizing CO2 as a resource—emerges as a viable strategy for achieving a carbon-neutral society [1]. Fujishima et al. introduced a method involving hydrogen extraction through water electrolysis using electricity from high-efficiency solar cells [2] (methodology currently used and highly studied [3]), followed by combining this hydrogen with CO2 emitted by power plants and factories to produce methanol, a potential energy source [4]. This process transforms carbon-containing gases like CO2 from greenhouse contributors into valuable resources, substituting oil and natural gas [5]. This concept harnesses the principles of artificial photosynthesis, which involves a nanostructured device designed to capture solar energy and convert CO2 emissions into fuel. Typically, an artificial photosynthesis device comprises complex materials that facilitate the energy conversion process [6]. Within the realm of artificial photosynthesis, a critical aspect is the development of materials geared towards the efficient physisorption of CO2. The focus is on synthesizing an active compound that can effectively capture CO2 through physical adsorption mechanisms [7].
Carbon dioxide (CO2) is identified as a significant greenhouse gas (GHG), contributing approximately 66% to radiative forcing due to long-lived GHGs [8,9]. Its presence in the atmosphere decreases the total loss of infrared radiation to space by absorbing and reflecting radiation emitted from the Earth’s surface, while oceans and ecosystems absorb the remainder [10]. The main causes include fossil fuel combustion, anthropogenic activities, coal energy, gasoline combustion, and population growth, leading to global warming [11]. The World Meteorological Organization reported atmospheric CO2 levels at 413.2 ppm ± 0.2 ppm in 2020, with atmospheric fractions showing significant variability over the past 60 years, ranging from 0.2% to 0.8% [12]. On the one hand, ocean absorption has led to an increase in sea surface temperatures and a decrease in pH due to CO2 uptake, which also slows down the meridional overturning circulation, contributing to the melting of sea ice [13]. Additionally, the deficiency in CO2 capture by nature is highlighted, attributed to the reduced ability of ecosystems and natural systems to adequately absorb and retain CO2 emissions from human activities such as fossil fuel combustion and deforestation [14,15]. Key factors behind this deficiency include deforestation and vegetation loss, land use changes, ocean acidification, wildfires and ecosystem degradation, atmospheric CO2 saturation, and changes in climate patterns [16]. These factors underscore the importance of addressing CO2 capture deficiency, adopting measures to protect and restore natural ecosystems, and reducing global CO2 emissions. Mitigating climate change and promoting sustainable practices are crucial for balancing the carbon cycle and limiting the adverse impacts of global warming [17].
In the context of escalating environmental challenges, notably the surge in greenhouse gases and climate change, bionics emerges as a promising field [18]. Defined by Lodato as the assimilation of engineering principles observed in natural systems and their application to the design or enhancement of technological systems or materials, bionics focuses on the technical transformation and application process to structures, methodologies, and principles of biological systems [19]. Biological systems are characterized by their sensitivity, high degree of flexibility, and adaptive function to various physical environments due to their high reliability. These features provide a vast research field to inspire biological systems for engineering applications, innovation, and solving day-to-day problems [20]. Particularly, bryophytes are represented by around 12,800 species worldwide, with 984 species and varieties recognized in Mexico [21]. The genus Sphagnum, commonly known as peat moss, was selected for this study. It includes between 150 to 350 different species. The specific species used in this work has not been identified, due to the need for comparison with species located in other countries. In this regard, this study focused on bryophytes from the moss genus, which absorb significantly more CO2 and water than other plants [22,23]. Carbon, oxygen, and magnesium (Mg) are elements that play a crucial role in CO2 adsorption and conversion into energy within green plants [24]. Furthermore, contemporary theoretical studies have suggested that the oxidation states of Mg significantly impact CO2 capture capabilities, with particular emphasis on the efficacy of the MgO phase in this process [25].
In this work, characterization techniques such as optical microscopy, SEM, FTIR, and XPS were utilized to investigate the morphology, physical properties, and chemical composition, aiming to replicate the key chemical and morphological features of bryophyte towards CO2 capture. Thus, anodized aluminum oxide (AAO) was synthesized utilizing the electroanodization technique, followed by employing a hydrothermal method in a glucose solution mixed with magnesium chloride (MgCl2) to fabricate the AAO/C/MgO composite. The concentration of MgCl2 was systematically varied to explore its impact on the Mg content’s oxidation states, as well as the physical and chemical properties, and ultimately, the CO2 capture efficiency. This research thoroughly examines the effect of MgCl2 concentration on the formation of the MgO phase and its correlation with CO2 capture efficiency. In addition, the relevance and novelty of this study are anchored in three pivotal areas: (1) the investigation of bryophyte’s chemical properties using advanced techniques such as X-ray photoelectron spectroscopy (XPS), (2) the detailed study of the AAO/C/MgO composite’s formation process, and (3) the comprehensive analysis of how the MgO phase influences the physical and chemical properties of the AAO/C/MgO composite, particularly in relation to CO2 capture efficiency.

2. Materials and Methods

2.1. Materials and Reagents

Aluminum foil (Al, product number: 326860-3.6G), glucose (C6H12O6, product number: PHR1000-1G), magnesium chloride (MgCl2, product number 449172-10G), oxalic acid (C2H2O4, product number: 247537-500G), perchloric acid (HClO4, product number: 311421-250ML), phosphoric acid (H3PO4, product number: 695017-500ML), sodium hydroxide (NaOH, product number: 306576-100G), and deionized water (H2O, product number: W4502-1L with a resistivity of 18 MΩ) were procured from Sigma-Aldrich (St. Louis, MI, USA). These reagents were employed as provided, without any further purification, ensuring the consistency of experimental conditions.

2.2. Bryophyte Study for Bioinspired Elements Selection

A sample of 0.2 mg of bryophyte was subjected to comprehensive characterization through optical microscopy, scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), and X-ray photoelectron spectroscopy (XPS) to elucidate its morphological, physical, and chemical attributes that contribute to bryophytes’ well-documented CO2 capture capabilities [23]. Prior to these analyses, the bryophyte sample was dehydrated using a 10% formalin solution for 24 h and subsequently dried under a nitrogen flow to prepare it for examination. The findings, as detailed later, identified two key characteristics that informed this research: (1) the high density of pores and (2) the presence of carbon, oxygen, and magnesium in the bryophyte’s chemical composition. These critical attributes, derived from the bryophyte’s characterization, underpin the design of the AAO/C/MgO composite, positioning it as a material inspired by natural processes [19]. This bioinspired approach leverages the inherent properties of bryophytes to enhance CO2 capture efficiency, reflecting a novel integration of natural mechanisms into material science.

2.3. Synthesis of the Nanoporous AAO/C/MgO

The anodized aluminum oxide (AAO) substrate was synthesized using the established two-step anodization method [26,27]. Briefly, aluminum foil was cut into 10 mm diameter circles and underwent mechanical polishing to achieve a mirror finish. Electropolishing followed, utilizing a potentiostat in a 4:1 mixture of perchloric acid and ethanol, applying a steady 20 V for one minute at room temperature. Residual acid was thoroughly removed via four ultrasonic cleaning cycles in deionized water and methanol, with subsequent nitrogen drying. The substrate was then anodized in 3 M oxalic acid at 20 V for two hours at 4 °C to form the initial alumina layer. This layer was dissolved in 0.5 M phosphoric acid for 20 min, succeeded by a second anodization for six hours to refine the pore structure. A final cleaning in six ultrasonic cycles and nitrogen drying completed the process, yielding an alumina structure with enlarged pores and increased surface area. An aqueous solution of 10 mM glucose was prepared and combined with varying concentrations of MgCl2 aqueous solution. The mixture was subjected to magnetic stirring for 30 min to ensure thorough mixing. MgCl2 concentrations of 1 mM, 2 mM, 3 mM, 4 mM, and 5 mM were investigated, keeping the glucose concentration constant at 10 mM. To facilitate the formation of the C/MgO complex, the established hydrothermal method was utilized [28]. Thus, the mixture was transferred into a cylindrical stainless-steel vial, equipped with a Teflon liner to prevent sample contamination from reactions with the stainless-steel interior. Additionally, 0.2 mg of AAO was added to the mixture to aid in forming the AAO/C/MgO complex. The system was then heated to 180 °C for 4 h to conduct hydrothermal synthesis. Post synthesis, the samples were cleaned with two ultrasonic baths in deionized water and dried under a nitrogen flow. To ensure reproducibility, the samples were synthesized in triplicate across separate experiments. Please note that the choice of 4 °C for the synthesis of AAO was deliberately made to achieve small pores within the alumina substrates. This is because, as documented in the literature [26], higher temperatures tend to result in larger pore diameters, whereas lower temperatures favor the formation of smaller pores. This principle is well established in the anodization method for alumina synthesis from aluminum, underscoring the critical role of temperature control in tailoring the pore size to desired specifications.

2.4. Physical and Chemical Characterization

The samples underwent a comprehensive characterization process. Initially, optical microscopy was employed for initial visualization. The vibrational modes were thoroughly analyzed using Fourier-transform infrared spectroscopy (FTIR), where samples mixed with potassium bromide (KBr) were subjected to a temperature of 150 °C for 24 h. High-resolution measurements were carried out with a Varian 660-IR spectrometer (Agilent, Santa Clara, CA, USA), set to 1 cm−1 resolution, and comprising 20 scans per measurement to guarantee the accuracy and reliability of the spectral data. For morphological analysis, field-emission scanning electron microscopy (FE-SEM) was performed using a JEOL 7401F microscope (Tokyo, Japan), enabling detailed observation of the samples’ surface and structure. Additionally, chemical composition and atomic-level studies were conducted using X-ray photoelectron spectroscopy (XPS) on a Thermo Fisher Scientific K-alpha model (Waltham, MA, USA). This utilized a monochromatic Alkα X-ray source for excitation, ensuring precise compositional analysis. All measurements were executed at room temperature to ensure consistent and reliable conditions across the analyses.

2.5. Evaluation of CO2 Capture

The CO2 capture efficiency of AAO/C/MgO nanostructures was rigorously evaluated following the methodology outlined by Mendoza-Sánchez et al. [7]. This involved exposing the samples to CO2 generated from the reaction of sodium carbonate (Na2CO3) with hydrochloric acid (HCl), yielding sodium chloride (NaCl), water (H2O), and carbon dioxide (CO2). The samples’ interaction with CO2 was conducted within a hermetically sealed quartz cell, engineered to specifically assess their CO2 adsorption potential. A 186 mM sodium carbonate solution was used for these tests. CO2 adsorption was indirectly quantified by observing changes in the absorbance of the solution over time, employing a UV–Vis spectrophotometer (i3 UV-VIS SPECTROPHOTOMETER, Hanon Instruments, Shanghai, China) at room temperature. Absorbance spectra were captured every 15 min over a span of 180 min to ensure comprehensive data collection.

3. Results and Discussion

3.1. Bryophyte Characterization

Optical microscopy revealed a heterogeneous appearance with dark brown areas indicating potential decomposition or water stress, alongside green zones suggestive of ongoing photosynthetic activity (Figure 1a). This diversity in color and texture could denote various growth stages or environmental stress responses, particularly as the sample underwent drying for integration purposes [29]. Such features are typical of bryophytes, non-vascular plants renowned for their resilience in moist environments and their pivotal role in ecosystems, contributing to soil formation and serving as environmental quality indicators. Microscopic analysis unveiled a net-like structure composed of predominantly hexagonal, polygonal cells outlined by dark lines, possibly representing cell walls, and featuring a light, uniform color across the sample. A detailed examination (Figure 1b) highlighted smaller, more compactly organized structures with grayish-brown coloration. Additionally, it showed a mesh-like structure with irregularly sized and shaped polygonal cells, where the demarcating lines were thicker and darker, and the cell interiors varied from light to dark brown, implying a more textured or three-dimensional aspect [30]. Particularly noteworthy are the larger, lighter hyaline cells unique to Sphagnum (bryophyte), essential for water retention [31]. Chloroplasts, vital for photosynthesis, were observed in the smaller green cells, termed chlorophyllic cells. The elongated cells, arranged in a regular pattern with significant intercellular spaces, are likely hyaline cells specializing in water retention, as illustrated in Figure 1b. Figure 1c shows an FESEM image of a Sphagnum surface. It highlights the openings or pores on the surface of the hyaline cells, characteristic of Sphagnum. The fibers or hanging structures could be remnants of cells or structural elements of the moss. Therefore, the high density of pores observed in bryophytes may serve as a critical morphological feature for CO2 adsorption, as it enhances the physical accessibility of CO2 molecules to the active sites involved in the capture process. The FTIR spectrum is shown in Figure 1d, where the band at 989 cm−1 is attributed to cellulose C-O stretching, the band at 1013 cm−1 to pectin groups, the band at 1126 cm−1 to polysaccharides, and the band at 3321 cm−1 to cellulose O-H bending [32,33,34,35]. This suggests a significant presence of organic molecules in the sample as expected. On the other hand, the XPS analysis, as illustrated in Figure 1e, revealed the elemental composition of the sample, identifying the presence of oxygen, carbon, calcium, nitrogen, silicon, and magnesium. This was determined through the respective energy binding (EB) bands for each element: O1s, C1s, Ca2p, N1s, Si2p, and MgKl1 [36,37,38,39,40]. The analytical results reveal both the atomic percentage and the weight percentage of elements detected in the sample: 68.1% of the atoms in the sample are carbon (C1s), followed by oxygen (26.7%) and nitrogen (2.9%). Minor quantities of silicon (1.1%), calcium (0.8%), sulfur (0.2%), sodium (0.2%), and magnesium (0.2%) were also detected. Overall, the sample predominantly consists of carbon and oxygen, with only a minor fraction of atoms belonging to elements other than C, O, and N, typically attributed to plant samples [41]. Consequently, calcium (Ca) and magnesium (Mg) might be integral to the active components within the structures facilitating CO2 absorption in the bryophyte. Two primary factors appear to enhance the bryophyte’s CO2 capture efficiency: (1) its substantial porosity and (2) its chemical composition. Inspired by these findings, a strategy was adopted to utilize a porous substrate and investigate the role of magnesium in synthetic samples, aiming to analyze its impact on CO2 capture efficiency.

3.2. Influence of MgCl2 on the Morphology of MgO on AAO

Once the AAO/C/MgO complex was synthesized, it was characterized by FESEM to understand the achieved morphology across MgCl2 variations. Figure 2a shows the AAO substrate as obtained. As can be seen, the porosity on the sample is extended across the entire surface of the sample with regular size. After the hydrothermal synthesis was complete, small structures were attached to the entire surface of the sample. Figure 2b shows the sample obtained after the hydrothermal synthesis with a 1 mM solution of MgCl2. On the other hand, Figure 2c shows the AAO after the hydrothermal synthesis employing the aqueous solution of MgCl2 with a concentration of 5 mM, where the presence of larger structures on the entire surface of the AAO can be seen. A close-up of the samples allows the comparison of the AAO before and after the hydrothermal process. Figure 2d shows the pores in the AAO (black circles). The pores have well-aligned tubular pores (which is in good agreement with the AAO obtained by the anodization method [42]) with a diameter of 250 ± 30 nm (measured by FESEM analysis). Please note that AAO is a well-established material, renowned for its characteristic formation of well-aligned cylindrical pores [26,27]. After the hydrothermal synthesis, the sample obtained with the minor MgCl2 concentration that was explored (1 mM) depicts a relatively smooth surface sprinkled with particles of varying sizes. These particles exhibit an irregular morphology and are randomly distributed across the surface. These particles comprise small nanoparticles (diameter 15 nm ± 3 nm) of a combined phase of C and MgO present on the entire alumina surface (see Figure 2e). Once the MgCl2 concentration is increased in the hydrothermal synthesis, the particles increase in size and form flakes until the formation of microparticles (diameter of 1000 nm ± 250 nm) attached to the surface of the AAO when the maximum explored MgCl2 concentration (5 mM) is employed (see Figure 2f). Thus, the particles now exhibit an elongated and fibrous morphology, standing out against the AAO substrate. Please note that the appearance of the AAO in Figure 2b,e might seem distinct from that in Figure 2a,c,d,f. This discrepancy is attributed to charge effects encountered during FESEM (Field Emission Scanning Electron Microscopy) analysis, as AAO (anodized aluminum oxide) is an electrically insulating material. Analyzing such materials with electron microscopy techniques poses a significant technical challenge due to these charge effects. However, this does not indicate that the pore diameter of the AAO is altered during the synthesis process. The stability of alumina is well documented to remain consistent across various temperatures and chemical reactions, ensuring the integrity of the AAO structure throughout our study. In addition, the MgCl2 has a direct influence on the morphology of the AAO/C/MgO composite. Thus, at lower concentrations of MgCl2, the MgO structures formed on the AAO are typically smaller and more uniformly distributed. As the concentration of MgCl2 increases, the MgO structures begin to grow in size and change shape.
Optical images of the top view of the AAO/C/MgO complex synthesized with 1 mM, 2 mM, 3 mM, 4 mM, and 5 mM solutions of MgCl2 are shown in Figure 3a, Figure 3b, Figure 3c, Figure 3d, and Figure 3e, respectively. The initial formation starts at lower MgCl2 concentrations (1 mM), and the hydrothermal synthesis initiates the deposition of C-MgO complexes on the AAO surface. This process results in the formation of small, irregularly shaped particles that are randomly distributed across the AAO substrate. These initial formations are characterized by their nanoparticle size (approximately 15 nm ± 3 nm), suggesting that the low MgCl2 concentration facilitates the nucleation of discrete C-MgO composite particles without significantly altering the inherent porosity of the AAO substrate. For the 2 mM of MgCl2 solution, a regular end uniform cover of the C/MgO complex is deposited on the AAO substrate. Then, as the concentration of MgCl2 is increased in the hydrothermal process, there is a notable transition in the morphology of the deposited structures. The particles begin to increase in size and adopt larger flat-like structures with morphology. This change indicates that higher MgCl2 concentrations promote the aggregation of C-MgO complexes, leading to the formation of larger structures while still maintaining the distribution of these formations across the porous AAO surface. At the highest investigated MgCl2 concentrations (5 mM), the hydrothermal synthesis process culminates in the formation of microparticles with diameters of approximately 1000 nm ± 250 nm. These larger structures exhibit an elongated and fibrous morphology, significantly distinct from the smaller particles formed at lower concentrations. This advanced stage of formation suggests that the increased MgCl2 concentration facilitates the growth and coalescence of C-MgO complexes into larger microparticles. Figure 3f shows a scheme that represents the growing formation of the AAO/C/MgCl2 complex. In addition, the increase in MgCl2 concentration during the hydrothermal procedure systematically influences the morphology of the C-MgO structures formed on the AAO substrate.

3.3. Chemical Nature of Bindings in AAO/C/MgO

To corroborate the chemical composition of the AAO/C/MgO composite previously proposed, XPS spectra analysis was used to identify the elements present in the samples. Figure 4a shows the comparison of XPS survey spectra obtained from the samples, revealing the presence of key elements such as aluminum, magnesium, carbon, and oxygen. Carbon and oxygen are highlighted in Figure 1a, where peaks corresponding to the C1s binding energy (BE) and the O1s BE bands are clearly identified, indicating the successful incorporation of these elements into the samples [43,44]. In Figure 4b, the O1s BE band region from all the investigated samples is presented, showing a noticeable shift towards higher energies in the spectra following the C/MgO incorporation into the AAO substrate. This shift suggests an increase in chemisorbed oxygen within the samples [26,45], implying that oxygen atoms play a significant role in the interaction with the C/MgO complex on the AAO support. Such an increase in chemisorbed oxygen could enhance the materials’ properties, particularly in applications where oxygen reactivity is crucial. Further analysis in Figure 4c compares the C1s BE band across the samples, where a distinct shift of 0.75 eV towards higher energies is observed post-hydrothermal treatment for the AAO/C/MgO complex formation. This shift indicates a transformation in the hybridization state of carbon atoms from sp2 to sp3 following the hydrothermal process [46]. This transformation corroborates the critical role of carbon in facilitating the linkage between AAO and MgO, suggesting modifications in the electronic structure and bonding environment of carbon that could influence the overall functionality of the synthesized material. Additionally, Figure 4d presents the Mg2p BE band comparison. Notably, the Mg2p signal is absent in untreated AAO samples, whereas in post-hydrothermal treatment, the signal emerges for all samples.
The presence of the Mg2p EB band is indicative of the MgO phase [47], highlighting the successful integration of MgO into the AAO framework. The emergence of this signal further supports the formation of the AAO/C/MgO complex and suggests that the hydrothermal process effectively induces the incorporation of magnesium into the structure. The XPS data underwent Gaussian fitting to elucidate the oxidation states of Mg, O, and C more clearly, as shown in Figure 5. For the C1s BE band, two main components were identified (refer to Figure 5a). In the case of the AAO sample, a component located at 284.5 eV corresponds to C-C bonding [48], while the other component was identified as a background signal, likely resulting from fluorescence [49]. Following the hydrothermal procedure (Figure 5b–f), a second component emerging at 288.7 eV was observed, which is attributed to C-O bonding [48]. This finding suggests the involvement of carbon atoms in linking the AAO substrate with the MgO phase. On the other hand, the O1s BE band is composed of the oxygen species present in the sample. These species include oxygen vacancies (OV), chemisorbed oxygen (OC), and lattice oxygen vacancies (OL) [50]. Applying a Gaussian fit to the O1s EB data within this study has enabled the detailed identification of OV, OC, and OL species. The O1s binding energy (BE) band, depicted in Figure 5a, reveals the presence of oxygen vacancies (OV) and chemisorbed oxygen (OC) within the sample, as indicated by the component peaks at 530.93 eV and 531.35 eV, respectively. Following the hydrothermal treatment, a notable shift in the intensity ratios of these components is observed (Figure 5b–f), indicating a change in the oxygen species composition within the samples. Specifically, the samples synthesized with 2 mM and 4 mM concentrations of MgCl2 demonstrated the presence of all three oxygen-related components, including lattice oxygen vacancies (OL), which is particularly significant. Particularly, the OL EB is indicative of oxygen atoms bonded to Mn or Fe family metals such as Mn, which reinforces the idea about the presence of the MgO phase [51]. Finally, the Mg2p component was subjected to Gaussian fitting to analyze the Mg oxidation states associated with the samples. As anticipated, the AAO sample did not display an Mg2p binding energy (BE) band (see Figure 5a), which is consistent with the absence of MgCl2 addition during its preparation. Conversely, following the C/MgO incorporation process, two distinct components were identified within the Mg2p BE band, located at approximately 44.8 eV and 53.6 eV (see Figure 5b–f). These components have been attributed to the Mg1+ species and Mg-O bonding, respectively [52]. In addition, the detection of the Mg-O-related signal confirms the formation of the MgO phase, while the signal associated with the Mg1+ species indicates that Mg predominantly contributes to the linkage of C to the MgO structures adhered to the AAO surface. Notably, the sample synthesized with a 4 mM MgCl2 concentration exhibited the highest Mg-O-to-Mg1+ ratio (as shown in Figure 5e), implying that this sample possesses the greatest concentration of MgO structures available on the AAO surface.

3.4. CO2 Adsorption of the AAO/C/MgO

Figure 6a displays a series of absorbance curves for the sample labeled AAO/C/MgO-40 during a 180 min exposure to CO2, as detailed in Section 2.4 of the experimental procedure. The curves exhibit a progressive decrease in absorbance over time, which is indicative of the sample’s CO2 absorption on its surface. This diminishing trend suggests that as the CO2 interacts with the surface of the sample, it is steadily captured, thereby reducing the amount of light absorbed at specific wavelengths—this being a direct measure of the CO2 concentration around the sample. The same procedure was meticulously applied to all samples to construct a comprehensive kinetic profile of CO2 interaction with the various synthesized materials. By comparing the rate and extent of absorbance reduction across different samples, the relative efficiencies of CO2 capture can be ascertained. This comparative analysis is crucial for understanding the dynamics of CO2 adsorption and for identifying which samples exhibit the most promising characteristics for effective CO2 sequestration. The kinetics derived from these experiments are essential for modeling the adsorption process and for scaling up the technology for practical CO2 capture applications. Figure 6b illustrates the CO2 capture kinetics for different samples. The x-axis represents time in minutes, while the y-axis shows the relative intensity, which might correlate with the amount of CO2 absorbed. Each line represents a sample treated with different concentrations of MgCl2, ranging from 1 mM to 5 mM, labeled as AAO/C/Mg-10 to AAO/C/Mg-50, respectively, with AAO serving as a control. The upward trend of the lines suggests that CO2 capture increases over time. The steeper the slope, the more efficient the CO2 capture. It is evident that samples treated with higher MgCl2 concentrations (AAO/C/Mg-50, for example) demonstrate a more pronounced increase in relative intensity over time, indicating a potentially higher CO2 capture efficiency compared to those with lower MgCl2 concentrations (AAO/C/Mg-10). Based on the literature, the k constant was computed [53]. The rate constant ‘k’ for CO2 capture was meticulously computed, revealing a notable dependence on the magnesium content within the samples, as detailed in Table 1. The tabulated data underscore the direct correlation between Mg concentration and CO2 capture efficacy, with varying degrees of influence observed across the range of samples tested. From the values reported in Table 1, it is evident that the Mg content plays a pivotal role in the adsorption process. The AAO/C/MgO-40 sample, synthesized using a 4 mM MgCl2 solution, exhibited the highest ‘k’ constant of 0.10531, significantly outperforming its counterparts. This result positions the AAO/C/MgO-40 composite as the most proficient in CO2 sequestration, suggesting that this specific MgCl2 concentration provides an optimal balance that enhances the adsorptive interactions between CO2 and the material’s surface. By contrast, the pristine AAO sample, devoid of MgO integration, registered the lowest ‘k’ constant at a mere 0.00028, underscoring the dramatic effect of C/MgO incorporation on CO2 capture capabilities. A gradual increase in efficiency is observed with the addition of MgCl2, as evidenced by the samples AAO/C/MgO-10 and AAO/C/MgO-20, which show ‘k’ constants of 0.00159 and 0.00239, respectively. However, upon further incrementing the MgCl2 concentration beyond the optimal level to 50 mM, as in the AAO/C/MgO-50 sample, the ‘k’ constant decreases to 0.00481. This suggests a diminishing return on efficiency, potentially due to an over-concentration of Mg leading to a detrimental aggregation effect or a decrease in available active sites for CO2 binding. The K constant displayed by the sample AAO/C/MgO-40 is approximately 376 times greater than that of pure AAO, positioning it as the most efficient material for CO2 capture among all the samples analyzed. This significant enhancement in CO2 capture efficiency can be attributed to two primary factors: (a) the quantity of MgO present on the surface of the samples, which increases the availability of active sites for CO2 adsorption, and (b) the distinct morphology of the MgO structures, which is directly influenced by variations in MgCl2 concentration according to the presented results.
A notable trend was observed in the hydrothermal synthesis process: when utilizing a solution with a concentration higher than 4 mM MgCl2, the CO2 capture efficiency unexpectedly decreased. This reduction in efficiency could be due to a loss of material from the substrate surface, suggesting that the optimal threshold for a homogeneous coating on the AAO surface is achieved at this particular MgCl2 concentration. Beyond this point, the additional material may not adhere as effectively, leading to a potential shedding or blocking of active sites crucial for CO2 adsorption. The supposition is supported by optical observations detailed in Figure 3. The images likely show that at concentrations higher than 4 mM, there is a visible change in the surface morphology, indicating an oversaturation that does not contribute to—or may even detract from—CO2 capture capabilities. It is plausible that the maximum capacity for adsorption is reached with the 4 mM solution, and any further addition of MgCl2 leads to a superfluous layer that hinders the accessibility of CO2 to the adsorptive sites. This observation is critical for understanding the material’s adsorption dynamics and for optimizing the synthesis protocol for AAO/C/MgO composites intended for CO2 sequestration. It emphasizes the delicate balance between the quantity of active material and its distribution across the substrate, which is essential for maximizing CO2 capture efficiency. Future investigations could explore the precise mechanisms that cause the decline in capture efficiency beyond the 4 mM MgCl2 concentration, potentially leading to refined strategies for material synthesis in carbon capture applications.
MgO plays a pivotal role in enhancing CO2 capture efficiency, as evidenced by the comprehensive data and analysis provided in this work. This idea is highlighted by the strong efficiency in CO2 capture exhibited by the AAO/C/MgO-40 sample, the sample with the maximum availability of MgO structures on the surface of AAO. Thus, the incorporation of MgO on the anodized aluminum oxide (AAO) matrix, facilitated through a hydrothermal synthesis process involving magnesium chloride (MgCl2), significantly influences the composite’s physicochemical properties and its ability to adsorb CO2. The presence of MgO introduces additional active sites for CO2 adsorption, as indicated by the shift in binding energies observed in XPS analyses, attributed to its high surface reactivity and affinity for CO2 molecules. The optimization of MgCl2 concentration, particularly at 4 mM, results in an ideal distribution and morphology of MgO structures on the AAO surface, maximizing the composite’s CO2 capture capacity. This optimized MgO content not only enhances the physical accessibility of CO2 to the adsorption sites but also contributes to the formation of bonds between CO2 molecules and the composite surface. Therefore, MgO’s role in the AAO/C/MgO composite is critical for achieving high CO2 capture efficiency. Conversely, while porosity is recognized as a crucial factor for CO2 capture [54], this work did not extensively investigate this parameter, focusing instead on maximizing control over the primary variable of interest: MgCl2 concentration in the synthesis of the composite. Simultaneously, the expected values for porosity and surface area were anticipated to be comparable across all samples, given that the AAO substrate—which significantly contributes to both porosity and surface area—was synthesized under consistent conditions for each sample. This approach ensured a standardized baseline from which the influence of MgCl2 concentration on CO2 capture efficiency could be accurately assessed, without the variability in porosity and surface area confounding the results. Moreover, when comparing the CO2 capture efficiency achieved by the AAO/C/MgO-40 composite with other materials documented in the literature, it becomes evident that the results of this study are on par with, if not superior to, those of other materials, especially considering the minimal sample quantity used for the CO2 capture tests in this research. Table 2 presents a comparison of various materials recently synthesized and designed for CO2 capture, highlighting their capture efficiencies. Consequently, MgO emerges as a promising candidate for the development of materials tailored for CO2 capture applications, demonstrating its potential in significantly enhancing efficiency in this critical area. This comparative analysis further establishes the AAO/C/MgO-40 composite’s notable performance and positions MgO as a key component in the advancement of CO2 capture technology.
Despite the various materials reported in the literature with high CO2 capture efficiencies, this study introduces a pioneering investigation into the role of magnesium oxide (MgO) in enhancing CO2 adsorption within the AAO/C/MgO composite. Through the precise manipulation of magnesium chloride (MgCl2) concentration during the synthesis process, this research highlights the pivotal role of MgO in improving the composite’s CO2 adsorption efficiency. The discovery of an optimal MgCl2 concentration of 4 mM, which promotes the formation of MgO structures on the AAO surface with superior CO2 capture properties, illuminates the previously overlooked effects of MgO’s oxidation states and its interaction with carbon and aluminum oxide in the composite. This breakthrough provides valuable insights into the fundamental mechanisms by which MgO contributes to CO2 capture, opening new avenues for the development of more effective CO2 adsorption materials inspired in biological systems. The innovative findings emphasize the potential of bioinspired materials to address environmental challenges and represent a notable step forward in harnessing MgO for CO2 capture applications.

3.5. Future Prospects of the AAO/C/MgO Composite

To further enhance the CO2 capture efficiency of this already efficient composite in future research, the following recommendations are proposed:
Optimization of MgCl2 concentration: while the 4 mM MgCl2 concentration has proven to be effective, further fine-tuning this concentration within a narrower range could potentially identify a more precise optimal point that maximizes the MgO structure’s efficacy in CO2 adsorption.
Surface modification: Investigating surface treatments or modifications to the AAO/C/MgO composite that could increase the active surface area or introduce additional functional groups that facilitate CO2 adsorption. This could involve doping with other metals or non-metals that synergize with MgO’s adsorption properties.
Composite structure refinement: exploring variations in the hydrothermal synthesis parameters, such as temperature, duration, and the glucose-to-MgCl2 ratio, could lead to a more controlled growth of MgO structures, potentially yielding even more effective CO2 adsorbing materials.
Pore size optimization: since the efficiency of CO2 capture can also be influenced by the pore size of the AAO substrate, further studies could focus on optimizing the electroanodization conditions to produce AAO with tailored pore sizes that match the kinetic diameter of CO2 molecules more closely.
Hybrid composites: investigating the incorporation of other bioinspired materials or nanomaterials with the AAO/C/MgO composite could lead to hybrid materials that leverage multiple mechanisms for CO2 capture, potentially enhancing efficiency beyond the capabilities of single-component systems.
By focusing on these areas for further research, it is anticipated that the efficiency of the AAO/C/MgO composite for CO2 capture can be significantly improved, contributing to the development of more effective and sustainable solutions for addressing CO2 emissions.

4. Conclusions

The AAO/C/MgO complex was achieved by an accessible hydrothermal method. In addition, the rate constant ‘k’ revealed a pronounced effect of Mg content on capture efficiency. Our findings suggest that there is an optimal MgCl2 concentration in the synthesis process that maximizes CO2 capture efficiency. Specifically, the sample synthesized with a 4 mM MgCl2 solution exhibited the highest ‘k’ value at 0.10531, indicating superior CO2 sequestration compared to other concentrations. However, it is essential to note that beyond this optimal point, an increase in MgCl2 concentration to 5 mM resulted in a decrease in ‘k’ value to 0.00481, suggesting a threshold for the beneficial impact of Mg on the CO2 capturing process. FESEM characterization of the synthesized AAO/C/MgO composites further supported these findings. The samples treated with 4 mM MgCl2 solution showed homogeneous coverage on the AAO surface with well-aligned tubular nanopores, which is crucial for effective CO2 adsorption. The synthesis of AAO/C/MgO composites for CO2 capture is highly dependent on the precise concentration of MgCl2 used in the hydrothermal process. The optimization of magnesium chloride (MgCl2) concentration during the hydrothermal synthesis process revealed that a 4 mM concentration is optimal for creating MgO structures on the anodized aluminum oxide (AAO) surface, significantly enhancing CO2 adsorption capabilities. This optimal concentration fosters the development of MgO available on the AAO substrate surface. The knowledge gained from this study is invaluable for the future development of CO2 capture technologies, providing a foundation for optimizing material design and synthesis to combat the increasing levels of CO2 in the atmosphere.

Author Contributions

Conceptualization, P.J.C.-V., E.B.-L., Y.M.H.-R. and O.E.C.-M.; methodology, P.J.C.-V. and E.B.-L.; software, P.J.C.-V.; validation, Y.M.H.-R. and O.E.C.-M.; formal analysis, P.J.C.-V. and E.B.-L.; investigation, P.J.C.-V. and E.B.-L.; resources, Y.M.H.-R. and O.E.C.-M.; data curation, Y.M.H.-R. and O.E.C.-M.; writing—original draft preparation, P.J.C.-V.; writing—review and editing, Y.M.H.-R. and O.E.C.-M.; visualization, P.J.C.-V. and E.B.-L.; supervision, Y.M.H.-R. and O.E.C.-M.; project administration, Y.M.H.-R. and O.E.C.-M.; funding acquisition, Y.M.H.-R. and O.E.C.-M. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the ‘Secretaría de Investigación y Posgrado of the Instituto Politécnico Nacional’ and the APC was funded by the ‘Secretaría de Investigación y Posgrado of the Instituto Politécnico Nacional’.

Data Availability Statement

All data used in this research are available upon request.

Acknowledgments

The authors acknowledge the Secretaría de Investigación y Posgrado of the Instituto Politécnico Nacional for their financial support. The research was carried out at UPIITA-IPN’s Laboratorio de Sistemas para Diagnóstico y Tratamiento de Cáncer. Hernández-Rodríguez is thankful for the support in the postdoctoral program funding from the Consejo Nacional de Humanidades, Ciencias y Tecnologías CONAHCYT. No conflicts of interest have been reported in relation to this study.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Optical microscope image of bryophyte, (b) zoomed area, (c) FESEM close-up. (d) FTIR and (e) XPS spectra of bryophyte.
Figure 1. (a) Optical microscope image of bryophyte, (b) zoomed area, (c) FESEM close-up. (d) FTIR and (e) XPS spectra of bryophyte.
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Figure 2. FESEM image of front view of (a) pristine AAO, (b) AAO post-C-MgO integration with 1 mg MgCl2, and (c) AAO post-C-MgO integration with 5 mg MgCl2. Images (d–f) provide detailed close-ups of (a), (b), and (c), respectively.
Figure 2. FESEM image of front view of (a) pristine AAO, (b) AAO post-C-MgO integration with 1 mg MgCl2, and (c) AAO post-C-MgO integration with 5 mg MgCl2. Images (d–f) provide detailed close-ups of (a), (b), and (c), respectively.
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Figure 3. Optical microscope images showcase AAO/C/MgO synthesized with varying MgCl2 concentrations: (a) 1 mM, (b) 2 mM, (c) 3 mM, (d) 4 mM, and (e) 5 mM. Image (f) provides a schematic lateral view of the samples, illustrating changes across the MgCl2 gradient in the synthesis process. Please note that the blue arrow shows the explored MgCl2 concentration.
Figure 3. Optical microscope images showcase AAO/C/MgO synthesized with varying MgCl2 concentrations: (a) 1 mM, (b) 2 mM, (c) 3 mM, (d) 4 mM, and (e) 5 mM. Image (f) provides a schematic lateral view of the samples, illustrating changes across the MgCl2 gradient in the synthesis process. Please note that the blue arrow shows the explored MgCl2 concentration.
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Figure 4. XPS analysis of samples: (a) overall survey; close-ups of (b) 520–540 eV, (c) 280–292 eV, and (d) 290–340 eV ranges.
Figure 4. XPS analysis of samples: (a) overall survey; close-ups of (b) 520–540 eV, (c) 280–292 eV, and (d) 290–340 eV ranges.
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Figure 5. Gaussian fitting of the XPS spectra for O1s BE, C1s BE, and Mg2p BE bands of the samples (a) AAO, (b) AAO/C/MgO-10, (c) AAO/C/MgO-20, (d) AAO/C/MgO-30, (e) AAO/C/MgO-40, and (f) AAO/C/MgO-50.
Figure 5. Gaussian fitting of the XPS spectra for O1s BE, C1s BE, and Mg2p BE bands of the samples (a) AAO, (b) AAO/C/MgO-10, (c) AAO/C/MgO-20, (d) AAO/C/MgO-30, (e) AAO/C/MgO-40, and (f) AAO/C/MgO-50.
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Figure 6. UV-VIS absorbance spectra for Na2CO3 dissolution with (a) AAO/C/MgO synthesized with MgCl2 4 mM over time. (b) Kinetic comparison of CO2 adsorption between samples.
Figure 6. UV-VIS absorbance spectra for Na2CO3 dissolution with (a) AAO/C/MgO synthesized with MgCl2 4 mM over time. (b) Kinetic comparison of CO2 adsorption between samples.
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Table 1. Comparison between k constants computed at 180 min.
Table 1. Comparison between k constants computed at 180 min.
SampleK Constant
AAO0.00028
AAO/C/MgO-100.00159
AAO/C/MgO-200.00239
AAO/C/MgO-300.00537
AAO/C/MgO-400.10531
AAO/C/MgO-500.00481
Table 2. Comparison of CO2 capture efficiency achieved by the sample AAO/C/MgO-40 against other materials reported in the literature.
Table 2. Comparison of CO2 capture efficiency achieved by the sample AAO/C/MgO-40 against other materials reported in the literature.
MaterialAmount of CO2 AdsorptionTimeReference
ZIF-8-W/[TEPA][MIm]2.22 mol/mol ILs65 min[55]
Ti2C-MXene/activated carbon nanocomposite67.83 cm3/g6 h[56]
Amine-grafted on boron-modified SBA-150.79 mmol/g300 min[57]
Tetraethylenepentamine-modified Cu2(OH)PO40.67 molCO2/molN22 min[58]
Ultramicroporous carbon microspheres0.24 cm3/g6 h[59]
AAO/C/MgO-401.66 mmol/g180 minReported in this work
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Cortés-Valadez, P.J.; Baños-López, E.; Hernández-Rodríguez, Y.M.; Cigarroa-Mayorga, O.E. Bryophyte-Bioinspired Nanoporous AAO/C/MgO Composite for Enhanced CO2 Capture: The Role of MgO. Nanomaterials 2024, 14, 658. https://doi.org/10.3390/nano14080658

AMA Style

Cortés-Valadez PJ, Baños-López E, Hernández-Rodríguez YM, Cigarroa-Mayorga OE. Bryophyte-Bioinspired Nanoporous AAO/C/MgO Composite for Enhanced CO2 Capture: The Role of MgO. Nanomaterials. 2024; 14(8):658. https://doi.org/10.3390/nano14080658

Chicago/Turabian Style

Cortés-Valadez, Paulina Jaqueline, Esperanza Baños-López, Yazmín Mariela Hernández-Rodríguez, and Oscar Eduardo Cigarroa-Mayorga. 2024. "Bryophyte-Bioinspired Nanoporous AAO/C/MgO Composite for Enhanced CO2 Capture: The Role of MgO" Nanomaterials 14, no. 8: 658. https://doi.org/10.3390/nano14080658

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