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The role of capping agents in the fabrication of nano-silver-decorated hydrothermal carbons
Journal of Environmental Chemical Engineering 7 (2019) 103415
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Journal of Environmental Chemical Engineering journal homepage: www.elsevier.com/locate/jece
The role of capping agents in the fabrication of nano-silver-decorated hydrothermal carbons
T
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Hamza Simsira, Nurettin Eltugrala, , Selhan Karagozb a b
Department of Metallurgical and Materials Engineering, Karabuk University, 78050, Karabuk, Turkey Department of Chemistry, Karabuk University, 78050 Karabuk, Turkey
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrothermal carbons Silver nanoparticles Capping agent Stabilizer Ligand Hybrid nanostructure
In this work, silver-decorated hydrothermally grown carbons were fabricated by introducing either silver nitrate, or silver nanoparticles (Ag NPs) which were coated differently with polyvinyl pyrrolidone (PVP), polyvinyl alcohol (PVA), gelatin, and glucose. Hybrid nanostructures were prepared by methods which involve either in situ formation under hydrothermal conditions or mixing of as-prepared hydrothermal carbons (HTCs) with Ag NPs. With these approaches, hybrid nanostructures could be fabricated with some differences in their morphologies. Interestingly, a dense silver core at the center of the HTCs was observed after hydrothermal processing of glucose with gelatin-stabilized Ag NPs while particles were observed to attach to the HTCs surface under mixing conditions. PVP-stabilized Ag NPs were shown to form hybrid products where particles were attached to the surface rather than encapsulated at the center. On the other hand, PVA-stabilized ones were hardly observed on the HTCs upon mixing for 48 h, and they seemed not to produce any hybrid HTC-Ag under hydrothermal processing. Besides, glucose-stabilized Ag NPs were also subjected to the hydrothermal process and HTCs produced with interesting surface characteristics revealed that Ag NPs induce their morphology.
1. Introduction Hydrothermal carbonization has become a prominent research topic since valuable carbonaceous materials, called hydrothermal carbons (HTCs), can be produced at low temperatures (180–250 °C) [1–6] through an environmentally friendly route. This simple, economic, and sustainable method has recently opened new frontiers in science and technology to develop novel metal/HTC hybrid nanostructures that could endow the HTCs with specific catalytic, magnetic, electronic, optical and optoelectronic properties, and allow them to be considered as an alternative feedstock in a wide range of related applications [7–15]. Significant efforts have been spent on fabricating functional metal/ HTC hybrid nanocomposites using noble metallic nanoparticles of Ag, Au, Pd, and Pt. Metallic nanoparticles exhibit a characteristic surface plasmon resonance owing to their unique properties principally originated from size and shape. When they are exposed to visible light, the surface plasmons are excited and consequently they selectively absorb the light at certain wavelengths [16,17]. Ag NPs could be immobilized on the carbon support via a variety of different routes. These routes mainly involve (1) in situ formation via hydrothermal carbonization of carbon precursor together with a metal
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salt in the presence or absence of a ligand which both acts as a reducing agent and stabilizer, (2) subsequent reduction of metal salt by as prepared carbon’s surface groups at room temperature or by heating under reflux. For instance, Yu and coworkers, fabricated various types of metal/carbon nanohybrids with different architectures including nanocables, hollow tubes, and hollow spheres by mixing silver salt (AgNO3) under hydrothermal conditions with commercial starch. Besides, addition of silver salt could increase the carbonization efficiency of starch from 18 to 54 wt% indicating formerly grown tiny Ag NPs [5]. In a recent study, Ag NPs were incorporated on N-doped carbon composites, where chitosan was utilized as the precursor for Ndoped carbon structure. Similarly, AgNO3 aqueous solutions at different concentrations (4–16 mM) were mixed thoroughly with chitosan and subjected to hydrothermal carbonization process at 180 °C for 12 h. The obtained composites were shown to act as a catalyst for the reduction of 4-nitrophenol in the presence of NaBH4 [18]. In another report, sandwich-like Ag-C-Ag nanoparticles were prepared under hydrothermal conditions using glucose precursor with AgNO3 aqueous solution [19]. Hydrothermal carbonization was carried out in an autoclave at different temperatures (180, 190 and 200 °C). The effect of temperature on the composite morphology was investigated. Silver nanoparticle growth on carbon support was shown be temperature dependent that sandwich-
Corresponding author. E-mail address: [email protected] (N. Eltugral).
https://doi.org/10.1016/j.jece.2019.103415 Received 20 June 2019; Received in revised form 6 September 2019; Accepted 13 September 2019 Available online 19 September 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
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darker yellow silver nanoparticle solution was kept in the dark at ambient temperature for better long-term stability.
like Ag-C-Ag nanostructures were observed at 190 °C. Ag NPs were observed to be encapsulated at the center of carbon sphere, distributed throughout the shell as well as on the surface of the carbon. At 200 ℃ nanoparticles were only encapsulated at the center of the carbon spheres in the form of core/shell structure [19]. Besides, encapsulation of Ag NPs at the center of carbon spheres were also reported, in which the preparation method first involves the synthesis of hydrothermal carbon spheres from glucose precursor, and then followed by mixing the prepared carbon spheres with AgNO3 in water [20]. In another report, Jiang et al. prepared carbon composites wherein Ag NPs embedded. Nano-silver-loaded carbon composite was synthesized by treating glucose, silver nitrate and trioctylamine (TOA) via the hydrothermal process at 180 °C. TOA was added as stabilizer to avoid aggregation, and to control growth of Ag NPs. The average particle diameter of dispersed Ag NPs embedded in carbon was 5 nm. In addition, they were shown to have good catalytic performance in photocatalytic degradation of methylene MB [21]. Ag NPs are prone to self-aggregate because of high surface energy and Van der Waals forces likely exist between particles which induces aggregation. Many organic capping agents (also called as coating groups, ligands or stabilizers) have been used to prevent aggregation of nanoparticles and attain them colloidal stability. Polyvinyl pyrrolidone (PVP), polyvinyl alcohol (PVA), polyethylene glycol (PEG), gelatin, glucose and other polysaccharides are common ligands preferred for coating Ag NPs [22–26]. Although some of them have been utilized individually in previous reports as mentioned here, the data available are sparse and, to the best of our knowledge, there is no a comprehensive study in the role of capping agents in fabricating hydrothermally grown metal-supported hybrid carbon nanostructures. In this study, we report the role of different capping agents including PVP, PVA, gelatin, and glucose for the fabrication of nanosilver-decorated carbon structures through different routes. For comparison purpose, metal/HTC hybrid nanostructure was also synthesized from AgNO3 salt instead of its corresponding nanoparticle. Glucose and Ag NPs with different coatings were subjected to hydrothermal carbonization process at 200 °C for 48 h. Besides, glucose-derived HTCs were first obtained and then treated with nanoparticle suspension at room temperature for 48 h. The obtained products were characterized.
2.2.2. Synthesis of Ag@PVP and Ag@PVA NPs To 10 ml of 1 × 10−3 M AgNO3 solution, was added 0.2 g polyvinyl alcohol (PVA, ≥99.9%, Sigma-Aldrich) and magnetically stirred for 15 min at 60 °C to obtain a clear solution. Then, the solution was cooled down until the temperature dropped below 5 °C in an ice bath. Then, 10 ml of 3 × 10−3 M NaBH4 aqueous solution was added dropwise at a rate of 1 drop per second. The obtained nanoparticle (Ag@PVA) solution was kept in the dark at ambient temperature. A similar procedure was applied for the synthesis of Ag@PVP by replacing PVA with polyvinyl pyrrolidone (PVP, ≥99.9, % Sigma-Aldrich). 2.2.3. Synthesis of Ag@Glu NPs To 30 ml of 0.4 M aqueous solution of glucose being heated under reflux condition, was added 10 ml of 3 × 10−2 M AgNO3 aqueous solution dropwise with vigorous stirring. The reaction was refluxed for 3 h and resulting glucose-capped silver nanoparticles (Ag@Glu) were cooled down to room temperature. 2.3. Preparation of hybrid HTC-Ag hybrid nanostructures
2. Experimental section
A comparative study of different physicochemical treatments was employed to prepare hybrid HTC-Ag nanostructures by four different approaches: (1) 0.035 g of glucose-derived HTCs were mixed in 10 ml of 2 × 10−4 M Ag NPs suspension (i.e, Ag@Gel, Ag@PVP, and Ag@PVA) and in aqueous solution of AgNO3 under continuous stirring for 24 h at room temperature. These samples were denoted as HTC-Ag@Gel, HTCAg@PVP, and; HTC-Ag@PVA (2) To 20 ml of 1 × 10−4 M nanoparticle suspension (Ag@Gel, Ag@PVP, and Ag@PVA), was added 1.5 g glucose and mixed until glucose dissolved thoroughly. This mixture was directly subjected to hydrothermal carbonization at 200 °C in the autoclave for 48 h. The prepared samples were denoted as HTC/Ag@Gel, HTC/Ag@ PVP, and HTC/Ag@PVA; (3) 20 ml of Ag@Glu (prepared in Section 2.2.3) was directly subjected to hydrothermal carbonization (denoted as HTC/Ag@Glu); (4) 20 ml of 0.4 M glucose aqueous solution was placed in a Teflon-lined autoclave, then 0.66 ml of 0.3 M AgNO3 aqueous solution was added dropwise and mixed vigorously. This mixture was then subjected to hydrothermal carbonization.
2.1. Synthesis of HTCs
2.4. Characterization
In a typical synthesis, 1.5 g of D(+)-glucose (Merck) was completely dissolved in 20 ml of deionized water and heated in a 50 ml Teflon-lined autoclave to 200 °C and kept at this temperature for 48 h. After the reaction was complete, it was cooled to room temperature and the resulting product was filtered off and the solid residue washed thoroughly with deionized water several times and dried at 105 °C for 3 h. In our previous report, we carried out the hydrothermal carbonization of glucose, cellulose, chitin, chitosan, and woodchips at 200 °C and processing times between 6 and 48 h [27]. It was seen that carbon content reaches to the its highest value at 200 °C and the processing time of 48 h. Thus, the optimum processing time and temperature were set as 48 h and 200 °C, respectively.
The optical properties of as-prepared colloidal silver suspensions were characterized using an Agilent Cary 60 UV–Vis spectrophotometer. UV–Vis absorbance spectra were measured using a double beam spectrophotometer over the range of 300–700 nm. The structural and morphological properties of the samples were characterized by a Carl Zeiss Ultra Plus Gemini FESEM scanning electron microscope, at accelerating voltage in the range of 5–10 kV, equipped with an energydispersive X-ray (EDX) spectrometer. Samples were coated with gold prior to analysis to avoid charging during the interaction of irradiated electrons with the sample. Transmission electron microscope (TEM) images were taken on a Philips CM 300 FEG/UT at 300 kV. Functional chemical analysis was performed on a Bruker ALPHA Platinum FTIRATR spectrometer equipped with a single reflection diamond ATR accessory. Samples were investigated in the wavenumber 4000–600 cm−1 using 24 scans at a spectral resolution of 4 cm−1.
2.2. Synthesis of Ag NPs 2.2.1. Synthesis of Ag@Gel NPs Gelatin-stabilized silver nanoparticles (Ag@Gel) were prepared according to Tollens process [28–30]. 10 ml of 5 × 10−2 M NaBH4 (≥98.0%, Sigma-Aldrich), 20 ml of 1.25 × 10−2 M NH3 and 10 ml of 5 × 10−2 M NaOH aqueous solutions were mixed in a reaction flask followed by addition of 1 g gelatin (Fluka) and further stirred for 30 min upon addition of 10 ml of 1 × 10-3 M AgNO3 (≥99.9%, Sigma-Aldrich), aqueous solution dropwise at a rate of 1 drop per second. The resulting
3. Results and discussion 3.1. Characterization of as-prepared Ag NPs Ag NPs were prepared using four different types of stabilizing ligands including gelatin, PVP, PVA and glucose. Gelatin is a natural polymer and composed of peptides and proteins [31]. It has amine 2
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shift in the wavelength of the plasmon absorption band could be attributed to the formation of bigger particles [26]. Compared to NaBH4, glucose has a lower reducing ability at ambient temperature. Therefore, Ag@Glu synthesis was performed under reflux conditions with the temperature control at around 80 °C to increase the reaction rate and thus to produce smaller particles having a surface plasmon band similar to the formers. Parameters other than the stabilizers are relatively important to observe the overall effect on the size and morphology issues during the synthesis of nanoparticles as well as the formation of hybrid HTC/Ag structures [41]. For instance, metal salt concentration, temperature, alkalinity of the reaction mixture, accelerator, etc. were studied previously in detail under different processing conditions to investigate the details of the mechanism of nanoparticle formation. It has been demonstrated that the surface plasmon absorption band could be blue shifted either by increasing the amount of NaOH in the reaction mixture or decreasing the AgNO3 concentration while keeping the glucose concentration unchanged [39,42]. Notably, this study exclusively focused on the role of the different type of stabilizing ligands in the fabrication of nano-silver-decorated hydrothermal carbons. Thus, no attempts were made to study reaction conditions of synthesized nanoparticles. Gelatin, PVA and PVP have relatively higher molecular weight and more functional groups such as -H which promotes formation of a protective coating on the surface of growing nanoparticles. Thus, they can effectively stabilize the dispersed nanoparticles at smaller sizes [32]. Another reason could be ascribed to the fact that NaBH4 favors the formation of smaller nanoparticles with respect to glucose itself [43]. Fig. 2 represent the TEM images of all the four differently capped Ag NPs. All prepared suspensions displayed spherical particles with comparable sizes. Judging from the TEM images, the estimated average particle sizes for Ag@Gel, Ag@PVA, Ag@PVP and Ag@Glu were in the range of 8.4 ± 2.0, 9.2 ± 1.6, 14.1 ± 4.8, and 18.3 ± 6.3 nm, respectively. Our findings from TEM investigations are in good agreement with what we presumed from the UV–vis analysis.
Fig. 1. A typical absorbance spectrum of the differently capped silver nanoparticle colloidal dispersions.
groups which readily donate electrons or coordinate to the nano-metal. Besides, functional groups on the gelatin (i.e. COO−) carrying a charge avoid the particle aggregation between the formed nanoparticles. Overall, gelatin is considered as a much better stabilizer than synthetic polymers, i.e., PVP [26,28]. UV spectra and corresponding plasmon absorption band suggested successful synthesis of the Ag@Gel. PVP is an important water-soluble synthetic polymer which is composed of a hydrophobic back bone baring hydrophilic side groups, i.e., highly polar amide groups. Hence, it can act as a stabilizer for coating Ag NPs via strong affinity of carbonyl oxygen atom of the amide group towards the silver [32–35]. The Ag@PVP produced a narrow, well defined strong absorption band around 400 nm (Fig. 1), which indeed, confirms formation of nanoparticles stabilized by PVP. PVA is also a water-soluble, nontoxic and biodegradable synthetic polymer and used as stabilizing ligand for inorganic nanoparticle synthesis. PVA stabilizes the metal core via interfacial compatibility through non covalent interactions [36]. The prepared Ag@PVA exhibited a peak profile similar to that of Ag@PVP with an absorption band centered at 399 nm in the corresponding UV spectrum, which indicates those particles have similar morphology with those of Ag@ PVP. The obtained Ag@PVA present a UV spectrum which also well fits with the findings reported in the literature [37]. Apart from those stabilizing ligands (gelatin, PVP, and PVA), glucose is encountered as well in nanoparticle synthesis which serves as both stabilizer and reducing agent. Apparently, all the nanoparticle suspensions showed colloidal stability throughout the experiments. The prepared Ag@Gel, Ag@PVP and Ag@PVA exhibited a strong plasmon absorption band at a wavelength around 400 nm which is characteristics for colloidal Ag NPs in the diameter of several tens of nanometers (Fig. 1) [28,38]. Glucose is a simple monosaccharide. Here in the synthesis of Ag@Glu, glucose not only serves as a reducing agent, but also acts as a stabilizer and trapping agent for Ag NPs. The reaction mechanism involves oxidation of glucose to gluconic acid and the released electrons reduces to silver ion to metallic silver and progressively Ag NPs are formed by coating the surface with gluconic acid moieties [26]. Moreover, it is reported in previous studies that glucose concentration is shown to be non-effective in determining the size. Instead, the ratio of the stabilizer, i.e. glucose, to the metal salt plays an important role in determining the size of the nanoparticles. As this ratio increases, smaller nanoparticles are formed with a blue shift in their surface plasmon absorption peak profile [39,40]. The prepared Ag@Glu revealed a broader absorption band at 422 nm which was slightly red shifted with respect to the formers. Since the position of the plasmon absorption band is very sensitive to size and surface interactions, this
3.2. Characterization of hybrid HTC-Ag nanostructures Different preparation protocols were employed to fabricate hybrid HTC-Ag nanostructures. Firstly, Ag NPs, which were coated differently with gelatin, PVA and PVP were mixed with HTCs at room temperature for 24 h. No significant changes after further increments in time was observed so the residence time was fixed to 24 h. The obtained samples were both visually and morphologically characterized by SEM and TEM, respectively. It can be seen from a representative SEM image (see Supplementary information Fig. S1) that, a typical HTC morphology appears as spheres mostly in the range between 400–600 nm, while few are around 1.2 μm. However, there was no clear indication of immobilized nanoparticles of Ag@Gel, Ag@PVA and Ag@PVP on the surface of the HTCs. Since nanoparticles were somewhat smaller to visualize by SEM, a close view of the samples was required to see them. To further check whether the nanoparticles were anchored on the HTC surface, TEM was employed. It can be seen from Fig. 3a that Ag@Gel could be detected as black dots on the surface of the HTCs. The nanoparticles were observed to agglomerate on the carbon surface with an average particle size of approximately 40 nm. Similarly, Ag@PVP were attached and distributed around the HTCs and particle size ranged between 13–25 nm (Fig. 3b). However, Ag@PVA were hardly detected (Fig. 3c). This observation indicated that ligand type could selectively affect the affinity of nanoparticles towards the HTCs via physicochemical interaction of surface moieties. It is believed that intermolecular hydrogen bonding occurs between functional groups of the gelatin such as -OH, -C=O, and -NH2, and the rich oxygen containing groups at the HTCs surface. Furthermore, amine groups of the gelatin may also favor to some extent the formation of hydrogen bonding between the surfaces. Consequently, this may lead to cleavage of gelatin from the nanoparticle because metal nanoparticles were stabilized 3
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Fig. 2. Representative TEM images of differently capped Ag NPs: (a) Ag@Gel; (b) Ag@PVA; (c) Ag@PVP and (d) Ag@Glu.
surface and Ag NPs were shown to aggregate with a size of approximately 170 nm (Fig. 3d). Size of the silver particles formed on the HTCs in the absence of any stabilizer molecule was rather big when compared to that of the prepared Ag NPs. This ensures that capping agents are of great importance in producing hybrid HTC-Ag nanostructures with a controlled size. Hybrid HTC/Ag nanostructures was synthesized by following the hydrothermal carbonization process for glucose with differently coated Ag NPs. For this purpose, glucose was dissolved in differently coated nanoparticle suspensions (Ag@Gel, Ag@PVP and Ag@PVA) and placed in the autoclave, then subjected to hydrothermal carbonization. The spherical carbons could be clearly seen from the SEM images of the obtained HTC/Ag@Gel hybrid nanostructure while the inner portion
through the amine functional groups of the gelatin backbone [26] which in turn may result in nanoparticle agglomeration on the HTCs. In the case of HTC-Ag@PVA interaction, it can be stated that PVA molecule is less prone to attach on the HTCs since it has no additional amine or oxygen containing groups. Besides, the OH groups of PVA possibly interacts with water molecules thereby inducing the cleavage process from the surface of the nanoparticles and allowing the release of the nanoparticles from HTC [37]. When the prepared HTCs were mixed thoroughly with aqueous AgNO3 solution, it was observed that a hybrid HTC-Ag nanostructure was obtained. The white spots were proved to be silver particles from the EDX analysis (Fig. S2). Here, the reduction of silver ion was achieved by the rich oxygen containing groups present on the HTCs
Fig. 3. TEM images of HTCs treated with Ag NPs: a) HTC-Ag@Gel, b) HTC-Ag@PVP, c) HTC-Ag@PVA, and d) SEM image of HTC-AgNO3. 4
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measured to be around 600 nm, and the shell thickness of the produced hybrid HTC/Ag@Gel nanostructures was around 300 nm. Furthermore, the size of the immobilized Ag NPs outside of the core was between 8–16 nm. It can be stated that hydrothermal carbonization of glucose may occur by surrounding and concentrating the remarkable amount of Ag@Gel to produce hybrid HTC/Ag@Gel nanostructures. Thus, a dense silver core inside the carbon spheres was obtained, which was protected by a thick carbonaceous shell. When we replaced Ag@Gel with Ag@PVP in the autoclave during the hydrothermal carbonization of glucose, the situation was rather different, and we could not detect any core/shell type structure. Apparently, Ag@PVP were immobilized as individual dark dots throughout the produced hybrid HTC/Ag@PVP (Fig. 5c). The estimated nanoparticle size in the hybrid structure was between 14–22 nm according to the corresponding TEM images. Also, there were no significant differences between the hybrid HTC-Ag nanostructures which were prepared under physical mixing conditions and in the autoclave. On the other hand, hydrothermal carbonization of glucose in the presence of Ag@PVA did not produce any hybrid nanostructure which was not observed from TEM images (Fig. 5d). A plausible explanation of the possible formation mechanism under hydrothermal conditions for the hybrid HTC/Ag nanostructures can be made based on the nucleation phenomena of the subsequent formation of HTC spheres. Nanoparticles of Ag@Gel selectively undergo in situ hydrothermal carbonization acting as nuclei for the growth of carbon spheres [19,21]. Glucose does not transform into carbon spheres below 140 ℃, and HTCs are formed from glucose the temperature between 160 and 180 °C [20]. It is proposed that gelatin surrounding the Ag@Gel may prevent nanoparticles from being disrupted in the reaction vessel under elevated temperatures. When Ag@Gel alone was exposed to the same hydrothermal processing conditions, there was still a trace of characteristic absorption profile of the remaining nanoparticle solution after the treatment, where a red shifted in the absorption maxima was observed, which indicates formation of larger particles during the in situ hydrothermal carbonization process (see Fig. S5). This finding indicates that gelatin could resist to complete disruption of the nanoparticles thus making them still remained under hydrothermal conditions. And these larger particles assumed to form the basis for the nucleation step of the
Fig. 4. A SEM and line EDX for HTC/Ag@Gel.
was hard to discriminate (Fig. S3). To further investigate the evidence of Ag NPs inside the HTC/Ag@Gel, line EDX analyses were employed for several individual carbon spheres. Fig. 4 shows a SEM image in which individual carbon spheres were examined by the line EDX analysis. A line crosses over an individual HTC/Ag@Gel and, the corresponding C and Ag signals were recorded accordingly through the line (Fig. 4). Interestingly, C signal dominates the signal from Ag at the edge of the HTC (see the inset in Fig. 4). However, this behavior reverses at the center of the scanned carbon sphere. This observation revealed a dense deposited silver in the core of HTC/Ag@Gel. This significant observation was not casual for the rest samples (HTC/Ag@PVP or HTC/Ag@PVA) even though they were proved to contain some spot like particles distributed on the carbon spheres. Noticeably, the dense silver layer immobilized almost at the center of the HTC/Ag@Gel is seen from the TEM images (Fig. 5a and b) as well, while some small dark spots, which were Ag NPs, were distributed in the inner part of the carbon spheres and at the surface as well. From the representative TEM images, an average metal core diameter was
Fig. 5. Representative TEM images samples from hydrothermal processing of glucose in Ag NPs suspensions: a) and b) HTC/Ag@Gel, c) HTC/Ag@PVP, and d) HTC/ Ag@PVA. 5
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carbonization yielding a dense metal embedded at the center of the formed HTCs. Ag@PVP alone also showed resistance towards disruption of the nanoparticles to some extent and a slight red shift in absorption peak occurred as it was not significant as for Ag@Gel (Fig. S6). Therefore, it was thought that hydrothermal processing of Ag@PVP in glucose solution produced only spot like nanoparticles embedded on the HTCs rather than a core/shell type hybrid nanostructure. However, a complete disruption of the PVA@Ag does occur when they undergo hydrothermal treatment at the same operating conditions (Fig. S7). These findings may suggest that since Ag@PVA might have been disrupted and collapsed at the bottom of the reaction vessel, it was not possible to obtain any hybrid HTC/Ag structures from the hydrothermal processing of Ag@PVA in the presence of glucose. It is noteworthy that surface coating groups selected for the synthesis of Ag NPs as well as the method to prepare functional hybrid HTC/Ag nanostructures significantly govern the final composition of the material which might be intended for ultimate purposes. Nevertheless, gelatin acts as the best candidate to stabilize Ag NPs which can be further utilized to produce core/shell type hybrid HTC/Ag nanostructures with a controlled fashion. Interesting morphologies were obtained upon direct hydrothermal treatment of glucose-capped silver nanoparticles. Spherical HTC structures with different surface morphologies can be seen from SEM image (Fig. 6a). Ag@Glu undergo cleavage of glucose molecules from the nanoparticle surface under hydrothermal condition and the hydrothermal carbonization took place. TEM analysis of the carbon spheres did not reveal any nanoparticles embedded within the structure. It is considered that as the hydrothermal process went on, metallic core of the silver nanoparticle could not be stabilized within the inner hydrophobic part of the growing carbon spheres. So, they ran out of the structure leaving some apertures behind on the carbon sphere surface which indeed caused the different surface morphologies. On the other hand, carbon spheres with smooth surfaces were most probably due to carbonization of glucose molecules that were assumed cover the silver without undergo a change or might be attributed to the ones which did not attach to nanoparticle surface formerly. When glucose and AgNO3 were hydrothermally treated together, both micrometer-sized Ag NPs and spherical HTCs could be visible from the corresponding SEM images (Fig. 6b) Here, nanoparticles aggregated independently elsewhere outside of the carbon spheres. It was not possible to obtain such carbonaceous spheres embedded with Ag NPs. TEM investigations did not reveal any indication of nanoparticles embedded and dispersed throughout the HTCs (Fig. S4). Although similar attempts were employed for different metal precursors and hybrid HTC/Ag nanostructures were achieved in previous studies. This is believed to be selectively depended on the additives, and other reaction parameters such as reaction temperature, reaction time, and mole ratio of the metal to carbon precursor [41,44]. The interaction of coating ligands with the nanoparticle surfaces, and of nanoparticles incorporated inside carbon spheres was studied through the FTIR spectroscopy as well. Undoped hydrothermal carbons
Fig. 7. FTIR spectra: a) Gelatin, b) Ag@Gel, c) HTC-Glu, d) HTC-Ag@Gel, and e) HTC/Ag@Gel.
displayed an absorption profile including a series of peaks (Fig. 7). A broad band observed at 3310 cm−1 signifies the presence of hydroxyl functional group. The absorption band appeared at 2921 cm−1 is attributed to aliphatic C-H stretching. Furthermore, the double peaks at 1695 cm−1 and 1610 cm−1 are due to C=O and C=C stretching, respectively. The peaks at 1200 cm−1 and 1288 cm−1 correspond to the vibrations of C-O-C and C= C-O- stretching, respectively [21,27]. Ag@Gel clearly possess the characteristic peaks of the gelatin which indicated the existence of gelatin on the surface of Ag NPs (Fig. 7). In this manner, the absorption band at 1628 cm−1 is attributed to C=O stretching of amide; the band at 1522 cm−1 is assigned for C-N-H bending, and the broad band at 3268 cm−1 is due to N-H structural vibrations. The band seen in pure gelatin at 800 cm−1 could be attributed to NH2 out of plane vibration and this peak is weakened noticeably, and further broadening of the band at 3286 cm−1 at some extent occurred when gelatin coated the nanoparticle surface. This observation suggests that gelatin covers the nanoparticle surface by the interaction of amine groups with the metal surface [45–47]. When HTC is mixed with Ag@Gel, the overlapping amide carbonyl peak of the gelatin at 1622 cm−1 can be clearly identified for HTC-Ag@Gel (Fig. 7d). However, significant changes occur in the absorption profiles at certain band intervals when glucose was hydrothermally treated in the presence of Ag@Gel. The intensity of the bands 1000-1288 cm−1 could be assigned to C-O-C and C = C–O vibration. Aromatic C-N stretching, was suppressed while some new peaks appeared at 2100 cm−1 and 3600 cm−1. Moreover, the intensity of the amide carbonyl peak of gelatin residue becomes predominant in the range of 15001700 cm−1 compared to that seen in the case of the HTC-Ag@Gel. This
Fig. 6. SEM images of products obtained: a) HTC/Ag@Glu and b) HTC/AgNO3. 6
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Fig. 8. FTIR spectra: a) PVP, b) Ag@PVP, c) HTC-Glu, d) HTC-Ag@PVP, and e) HTC/Ag@PVP.
Fig. 9. FTIR spectra: a) PVA, b) Ag@PVA, c) HTC-Glu, d) HTC-Ag@PVA, and e) HTC/Ag@PVA.
reveals that more Ag@Gel are embedded within carbon spheres under hydrothermal conditions than the ones obtained by mixing of HTC with Ag@Gel. This observation is consistent as well with the corresponding SEM and TEM analysis. In Fig. 8, Ag@PVP revealed the main characteristic peaks of PVP which indicates that nanoparticle surface was occupied by PVP (Fig. 8a). Pure PVP spectrum presents distinct peaks at 2921 cm−1, 1656 cm-1 and 1270 cm-1 belong to C-H asymmetric stretching, C=O, and C-N stretching vibrations, respectively [48]. The latter two peaks observed to be shifted to 1643 cm−1 and 1284 cm−1, respectively in the case of Ag@PVP. This observation could be ascribed to the coordination of oxygen and nitrogen atoms of PVP with the surface atoms of the Ag NPs [49]. When HTCs were mixed with Ag@PVP to obtain HTC-Ag@ PVP, FTIR spectrum of the resulting sample presented the overlapping of C=O groups of HTCs and PVP where only seemed a single peak at between 1600 and 1700 cm−1. Similarly, the intensity of the peak observed at 1284 cm−1 increased due to the combination of the absorption of C-N groups of PVA and C-O-C vibration seen in HTC alone. This, in turn, reveals the presence of Ag@PVP on the HTCs. Accordingly, the HTC/Ag@PVP hybrid structure, fabricated through the hydrothermal treatment of glucose in the presence of Ag@PVP, yielded an FTIR spectrum which was quite similar to that of HTCs alone except for some peaks. The peaks at 1600-1700 cm−1 split again into two as seen for HTCs alone. This may indicate possible rupture of partial carbonyl groups of the pyrrolidone and implying the formation of C-O bonds. Furthermore, C=O peak of HTC/Ag@PVP is weakened with respect to that of HTCs alone. This may indicate interaction of the nanoparticles through the carbon media [21]. PVA has several characteristic FTIR peaks (Fig. 9a). A broad peak appeared at 3286 cm−1 and the bands 1000–1420 cm−1, correspond to OH stretching. The peaks at 2913 cm−1 and 1420 cm−1 correspond to C-H stretching and C-H bending vibration of CH2, respectively. Main characteristic peaks of PVA, which were observed for Ag@PVA, signatures the presence of PVA on the surface of the Ag NPs. It is noteworthy that the intensities OH stretching and OH bending bands of PVA decreased and these bands shifted to 1562 cm−1 for OH stretching and 1402 cm−1 for OH bending. The changes of the peaks indicates that OH groups of PVA interacts with the metal surface through formation of CO-Ag [50]. The FTIR spectrum of HTC-Ag@PVA is shown in Fig. 9d. Comparing with the spectrum of HTCs alone, the intensity of the OH band at 3330 cm−1 increased due to contribution from Ag@PVA. OH bending in Ag@PVA observed at 1562 cm−1 overlapped with C=C band of HTCs alone. This resulted an increased intensity of the band at
1608 cm−1 which implies the presence of Ag@PVA inside the carbon spheres. On the contrary, HTC/Ag@PVA, prepared by treating glucose in the presence of Ag@PVA under hydrothermal conditions revealed almost the same FTIR spectrum as that of HTCs alone with enhanced peak intensities. This observation indicates that since there is no significant sign of Ag@PVA, no detectable nanoparticle deposited on HTCs. These observations conform with SEM and TEM results as well. Finally, FTIR spectrum of Ag@Glu is shown in Fig. 10. Two distinct absorption bands were observed. The broad peak at 3291 cm−1 refers to OH groups of gluconic acid and glucose as well, and the one located at 1636 cm−1 is assigned to C=O group. Glucose reduces the silver salt and is converted to gluconic acid. Hence, gluconic acid covers the surface of metallic silver by means of interactions occur between the metal surface and oxygen bearing groups of gluconic acid [39,42,51]. Glucose molecules which were not undergo reduction of silver salt may also acts as capping agent to some extent for silver surface through its OH groups. When Ag@Glu was subjected to hydrothermal carbonization, HTC/Ag@Glu, displayed the characteristic peaks of HTC alone at certain bands which are appeared at 1608 cm−1, 1691 cm−1, and 3314 cm−1. The peak at 1636 cm−1 for Ag@Glu was completely disappeared upon hydrothermal carbonization treatment. This may indicate, gluconic acid undergo the same hydrothermal conversion path
Fig. 10. FTIR spectra: a) HTC-Glu, b) Ag@Glu, c) HTC/Ag@Glu, and d) HTC/ AgNO3. 7
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as glucose itself. Hydrothermal treatment of glucose with silver nitrate yielded almost the same spectrum as in the case of HTC/Ag@Glu where only the intensity of the band in between 1600-1700 cm−1 range for HTC/AgNO3 was higher than that of HTC/Ag@Glu.
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4. Conclusions Hybrid HTC-Ag nanostructures could be prepared with different morphologies by incorporating differently coated Ag NPs. Ag@Gel were successfully loaded into the HTCs more like a core/shell type under hydrothermal conditions. A dense silver core of about 600 nm at the centers of the HTCs was observed. Thickness of the carbon shell was about 300 nm. On the other hand, Ag@Gel were attached to HTCs surface as particles with a size around 40 nm when they were mixed with as-prepared HTCs at ambient conditions. Ag@PVP were shown to attach to HTCs with a particle size between 13–25 nm. However, Ag@ PVA were not shown to selectively attach to HTCs. Results showed that Ag NPs with different degrees of dispersing capabilities on the HTCs could be obtained by controlling the ligand type and the preparation method as well. Our findings reveal that capping agents are of great importance in producing hybrid HTC-Ag nanostructure with a controlled nanoparticle size. Acknowledgment This work was supported by the Karabuk University (Project ID number: KBU BAP-17-DR-047). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jece.2019.103415. References [1] M. Sevilla, A.B. Fuertes, The production of carbon materials by hydrothermal carbonization of cellulose, Carbon 47 (2009) 2281–2289. [2] M.-M. Titirici, A. Thomas, M. Antonietti, Back in the black: hydrothermal carbonization of plant material as an efficient chemical process to treat the CO 2 problem? New J. Chem. 31 (2007) 787–789. [3] M.M. Titirici, A. Thomas, S.-H. Yu, J.-O. Müller, M. Antonietti, A direct synthesis of mesoporous carbons with bicontinuous pore morphology from crude plant material by hydrothermal carbonization, Chem. Mater. 19 (2007) 4205–4212. [4] M.-M. Titirici, A. Thomas, M. Antonietti, Replication and coating of silica templates by hydrothermal carbonization, Adv. Funct. Mater. 17 (2007) 1010–1018. [5] S.-H. Yu, X.J. Cui, L.L. Li, K. Li, B. Yu, M. Antonietti, H. Cölfen, From starch to metal/carbon hybrid nanostructures: hydrothermal metal-catalyzed carbonization, Adv. Mater. 16 (2004) 1636–1640. [6] H. Simsir, N. Eltugral, S. Karagoz, The effects of acidic and alkaline metal triflates on the hydrothermal carbonization of glucose and cellulose, Energy Fuels 33 (2019) 7473–7479. [7] A. Chen, Y. Yu, Y. Zhang, W. Zang, Y. Yu, Y. Zhang, S. Shen, J. Zhang, Aqueousphase synthesis of nitrogen-doped ordered mesoporous carbon nanospheres as an efficient adsorbent for acidic gases, Carbon 80 (2014) 19–27. [8] Y.-M. Lu, H.-Z. Zhu, W.-G. Li, B. Hu, S.-H. Yu, Size-controllable palladium nanoparticles immobilized on carbon nanospheres for nitroaromatic hydrogenation, J. Mater. Chem. A 1 (2013) 3783–3788. [9] K.M. Steel, W.J. Koros, Investigation of porosity of carbon materials and related effects on gas separation properties, Carbon 41 (2003) 253–266. [10] A.V. Nakhate, G.D. Yadav, Cu2O nanoparticles supported hydrothermal carbon microspheres as catalyst for propargylamine synthesis, Mol. Catal. 451 (2018) 209–219. [11] J. Wu, C. Jin, Z. Yang, J. Tian, R. Yang, Synthesis of phosphorus-doped carbon hollow spheres as efficient metal-free electrocatalysts for oxygen reduction, Carbon 82 (2015) 562–571. [12] M. Endo, C. Kim, K. Nishimura, T. Fujino, K. Miyashita, Recent development of carbon materials for Li ion batteries, Carbon 38 (2000) 183–197. [13] C. Galeano, J.C. Meier, M. Soorholtz, H. Bongard, C. Baldizzone, K.J. Mayrhofer, F. Schüth, Nitrogen-doped hollow carbon spheres as a support for platinum-based electrocatalysts, ACS Catal. 4 (2014) 3856–3868. [14] H. Simsir, N. Eltugral, R. Frohnhoven, T. Ludwig, Y. Gönüllü, S. Karagoz, S. Mathur, Anode performance of hydrothermally grown carbon nanostructures and their molybdenum chalcogenides for Li-ion batteries, MRS Commun. (2018) 1–7. [15] E. Unur, S. Brutti, S. Panero, B. Scrosati, Nanoporous carbons from hydrothermally treated biomass as anode materials for lithium ion batteries, Microporous
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[49] D. Kaur, A. Gupta, N. Singh, S.R. Dhakate, Anti-emetic drug delivery for cancer patients through electrospun composite nanofibers transdermal patch: in-vitro study, Adv Mater Lett. 6 (2014) 33–39. [50] S. Tang, Y. Tang, S. Vongehr, X. Zhao, X. Meng, Nanoporous carbon spheres and their application in dispersing silver nanoparticles, Appl. Surf. Sci. 255 (2009) 6011–6016. [51] S. Suvarna, R. Nairy, K.C. Sunil, Y. Narayana, Cytotoxicity studies of functionalized gold nanoparticles using yeast comet assay, J. Clin. Toxicol. 7 (2017) 2161–0495.
[46] A.L. Daniel-da-Silva, A.M. Salgueiro, T. Trindade, Effects of Au nanoparticles on thermoresponsive genipin-crosslinked gelatin hydrogels, Gold Bull. 46 (2013) 25–33. [47] N. Zahra, Lead removal from water by low cost adsorbents: a review, Pak. J. Anal. Environ. Chem. 13 (2012) 8. [48] R.K. Gangwar, V.A. Dhumale, D. Kumari, U.T. Nakate, S.W. Gosavi, R.B. Sharma, S.N. Kale, S. Datar, Conjugation of curcumin with PVP capped gold nanoparticles for improving bioavailability, Mater. Sci. Eng. C 32 (2012) 2659–2663.
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The role of capping agents in the fabrication of nano-silver-decorated hydrothermal carbons
T
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Hamza Simsira, Nurettin Eltugrala, , Selhan Karagozb a b
Department of Metallurgical and Materials Engineering, Karabuk University, 78050, Karabuk, Turkey Department of Chemistry, Karabuk University, 78050 Karabuk, Turkey
A R T I C LE I N FO
A B S T R A C T
Keywords: Hydrothermal carbons Silver nanoparticles Capping agent Stabilizer Ligand Hybrid nanostructure
In this work, silver-decorated hydrothermally grown carbons were fabricated by introducing either silver nitrate, or silver nanoparticles (Ag NPs) which were coated differently with polyvinyl pyrrolidone (PVP), polyvinyl alcohol (PVA), gelatin, and glucose. Hybrid nanostructures were prepared by methods which involve either in situ formation under hydrothermal conditions or mixing of as-prepared hydrothermal carbons (HTCs) with Ag NPs. With these approaches, hybrid nanostructures could be fabricated with some differences in their morphologies. Interestingly, a dense silver core at the center of the HTCs was observed after hydrothermal processing of glucose with gelatin-stabilized Ag NPs while particles were observed to attach to the HTCs surface under mixing conditions. PVP-stabilized Ag NPs were shown to form hybrid products where particles were attached to the surface rather than encapsulated at the center. On the other hand, PVA-stabilized ones were hardly observed on the HTCs upon mixing for 48 h, and they seemed not to produce any hybrid HTC-Ag under hydrothermal processing. Besides, glucose-stabilized Ag NPs were also subjected to the hydrothermal process and HTCs produced with interesting surface characteristics revealed that Ag NPs induce their morphology.
1. Introduction Hydrothermal carbonization has become a prominent research topic since valuable carbonaceous materials, called hydrothermal carbons (HTCs), can be produced at low temperatures (180–250 °C) [1–6] through an environmentally friendly route. This simple, economic, and sustainable method has recently opened new frontiers in science and technology to develop novel metal/HTC hybrid nanostructures that could endow the HTCs with specific catalytic, magnetic, electronic, optical and optoelectronic properties, and allow them to be considered as an alternative feedstock in a wide range of related applications [7–15]. Significant efforts have been spent on fabricating functional metal/ HTC hybrid nanocomposites using noble metallic nanoparticles of Ag, Au, Pd, and Pt. Metallic nanoparticles exhibit a characteristic surface plasmon resonance owing to their unique properties principally originated from size and shape. When they are exposed to visible light, the surface plasmons are excited and consequently they selectively absorb the light at certain wavelengths [16,17]. Ag NPs could be immobilized on the carbon support via a variety of different routes. These routes mainly involve (1) in situ formation via hydrothermal carbonization of carbon precursor together with a metal
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salt in the presence or absence of a ligand which both acts as a reducing agent and stabilizer, (2) subsequent reduction of metal salt by as prepared carbon’s surface groups at room temperature or by heating under reflux. For instance, Yu and coworkers, fabricated various types of metal/carbon nanohybrids with different architectures including nanocables, hollow tubes, and hollow spheres by mixing silver salt (AgNO3) under hydrothermal conditions with commercial starch. Besides, addition of silver salt could increase the carbonization efficiency of starch from 18 to 54 wt% indicating formerly grown tiny Ag NPs [5]. In a recent study, Ag NPs were incorporated on N-doped carbon composites, where chitosan was utilized as the precursor for Ndoped carbon structure. Similarly, AgNO3 aqueous solutions at different concentrations (4–16 mM) were mixed thoroughly with chitosan and subjected to hydrothermal carbonization process at 180 °C for 12 h. The obtained composites were shown to act as a catalyst for the reduction of 4-nitrophenol in the presence of NaBH4 [18]. In another report, sandwich-like Ag-C-Ag nanoparticles were prepared under hydrothermal conditions using glucose precursor with AgNO3 aqueous solution [19]. Hydrothermal carbonization was carried out in an autoclave at different temperatures (180, 190 and 200 °C). The effect of temperature on the composite morphology was investigated. Silver nanoparticle growth on carbon support was shown be temperature dependent that sandwich-
Corresponding author. E-mail address: [email protected] (N. Eltugral).
https://doi.org/10.1016/j.jece.2019.103415 Received 20 June 2019; Received in revised form 6 September 2019; Accepted 13 September 2019 Available online 19 September 2019 2213-3437/ © 2019 Elsevier Ltd. All rights reserved.
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darker yellow silver nanoparticle solution was kept in the dark at ambient temperature for better long-term stability.
like Ag-C-Ag nanostructures were observed at 190 °C. Ag NPs were observed to be encapsulated at the center of carbon sphere, distributed throughout the shell as well as on the surface of the carbon. At 200 ℃ nanoparticles were only encapsulated at the center of the carbon spheres in the form of core/shell structure [19]. Besides, encapsulation of Ag NPs at the center of carbon spheres were also reported, in which the preparation method first involves the synthesis of hydrothermal carbon spheres from glucose precursor, and then followed by mixing the prepared carbon spheres with AgNO3 in water [20]. In another report, Jiang et al. prepared carbon composites wherein Ag NPs embedded. Nano-silver-loaded carbon composite was synthesized by treating glucose, silver nitrate and trioctylamine (TOA) via the hydrothermal process at 180 °C. TOA was added as stabilizer to avoid aggregation, and to control growth of Ag NPs. The average particle diameter of dispersed Ag NPs embedded in carbon was 5 nm. In addition, they were shown to have good catalytic performance in photocatalytic degradation of methylene MB [21]. Ag NPs are prone to self-aggregate because of high surface energy and Van der Waals forces likely exist between particles which induces aggregation. Many organic capping agents (also called as coating groups, ligands or stabilizers) have been used to prevent aggregation of nanoparticles and attain them colloidal stability. Polyvinyl pyrrolidone (PVP), polyvinyl alcohol (PVA), polyethylene glycol (PEG), gelatin, glucose and other polysaccharides are common ligands preferred for coating Ag NPs [22–26]. Although some of them have been utilized individually in previous reports as mentioned here, the data available are sparse and, to the best of our knowledge, there is no a comprehensive study in the role of capping agents in fabricating hydrothermally grown metal-supported hybrid carbon nanostructures. In this study, we report the role of different capping agents including PVP, PVA, gelatin, and glucose for the fabrication of nanosilver-decorated carbon structures through different routes. For comparison purpose, metal/HTC hybrid nanostructure was also synthesized from AgNO3 salt instead of its corresponding nanoparticle. Glucose and Ag NPs with different coatings were subjected to hydrothermal carbonization process at 200 °C for 48 h. Besides, glucose-derived HTCs were first obtained and then treated with nanoparticle suspension at room temperature for 48 h. The obtained products were characterized.
2.2.2. Synthesis of Ag@PVP and Ag@PVA NPs To 10 ml of 1 × 10−3 M AgNO3 solution, was added 0.2 g polyvinyl alcohol (PVA, ≥99.9%, Sigma-Aldrich) and magnetically stirred for 15 min at 60 °C to obtain a clear solution. Then, the solution was cooled down until the temperature dropped below 5 °C in an ice bath. Then, 10 ml of 3 × 10−3 M NaBH4 aqueous solution was added dropwise at a rate of 1 drop per second. The obtained nanoparticle (Ag@PVA) solution was kept in the dark at ambient temperature. A similar procedure was applied for the synthesis of Ag@PVP by replacing PVA with polyvinyl pyrrolidone (PVP, ≥99.9, % Sigma-Aldrich). 2.2.3. Synthesis of Ag@Glu NPs To 30 ml of 0.4 M aqueous solution of glucose being heated under reflux condition, was added 10 ml of 3 × 10−2 M AgNO3 aqueous solution dropwise with vigorous stirring. The reaction was refluxed for 3 h and resulting glucose-capped silver nanoparticles (Ag@Glu) were cooled down to room temperature. 2.3. Preparation of hybrid HTC-Ag hybrid nanostructures
2. Experimental section
A comparative study of different physicochemical treatments was employed to prepare hybrid HTC-Ag nanostructures by four different approaches: (1) 0.035 g of glucose-derived HTCs were mixed in 10 ml of 2 × 10−4 M Ag NPs suspension (i.e, Ag@Gel, Ag@PVP, and Ag@PVA) and in aqueous solution of AgNO3 under continuous stirring for 24 h at room temperature. These samples were denoted as HTC-Ag@Gel, HTCAg@PVP, and; HTC-Ag@PVA (2) To 20 ml of 1 × 10−4 M nanoparticle suspension (Ag@Gel, Ag@PVP, and Ag@PVA), was added 1.5 g glucose and mixed until glucose dissolved thoroughly. This mixture was directly subjected to hydrothermal carbonization at 200 °C in the autoclave for 48 h. The prepared samples were denoted as HTC/Ag@Gel, HTC/Ag@ PVP, and HTC/Ag@PVA; (3) 20 ml of Ag@Glu (prepared in Section 2.2.3) was directly subjected to hydrothermal carbonization (denoted as HTC/Ag@Glu); (4) 20 ml of 0.4 M glucose aqueous solution was placed in a Teflon-lined autoclave, then 0.66 ml of 0.3 M AgNO3 aqueous solution was added dropwise and mixed vigorously. This mixture was then subjected to hydrothermal carbonization.
2.1. Synthesis of HTCs
2.4. Characterization
In a typical synthesis, 1.5 g of D(+)-glucose (Merck) was completely dissolved in 20 ml of deionized water and heated in a 50 ml Teflon-lined autoclave to 200 °C and kept at this temperature for 48 h. After the reaction was complete, it was cooled to room temperature and the resulting product was filtered off and the solid residue washed thoroughly with deionized water several times and dried at 105 °C for 3 h. In our previous report, we carried out the hydrothermal carbonization of glucose, cellulose, chitin, chitosan, and woodchips at 200 °C and processing times between 6 and 48 h [27]. It was seen that carbon content reaches to the its highest value at 200 °C and the processing time of 48 h. Thus, the optimum processing time and temperature were set as 48 h and 200 °C, respectively.
The optical properties of as-prepared colloidal silver suspensions were characterized using an Agilent Cary 60 UV–Vis spectrophotometer. UV–Vis absorbance spectra were measured using a double beam spectrophotometer over the range of 300–700 nm. The structural and morphological properties of the samples were characterized by a Carl Zeiss Ultra Plus Gemini FESEM scanning electron microscope, at accelerating voltage in the range of 5–10 kV, equipped with an energydispersive X-ray (EDX) spectrometer. Samples were coated with gold prior to analysis to avoid charging during the interaction of irradiated electrons with the sample. Transmission electron microscope (TEM) images were taken on a Philips CM 300 FEG/UT at 300 kV. Functional chemical analysis was performed on a Bruker ALPHA Platinum FTIRATR spectrometer equipped with a single reflection diamond ATR accessory. Samples were investigated in the wavenumber 4000–600 cm−1 using 24 scans at a spectral resolution of 4 cm−1.
2.2. Synthesis of Ag NPs 2.2.1. Synthesis of Ag@Gel NPs Gelatin-stabilized silver nanoparticles (Ag@Gel) were prepared according to Tollens process [28–30]. 10 ml of 5 × 10−2 M NaBH4 (≥98.0%, Sigma-Aldrich), 20 ml of 1.25 × 10−2 M NH3 and 10 ml of 5 × 10−2 M NaOH aqueous solutions were mixed in a reaction flask followed by addition of 1 g gelatin (Fluka) and further stirred for 30 min upon addition of 10 ml of 1 × 10-3 M AgNO3 (≥99.9%, Sigma-Aldrich), aqueous solution dropwise at a rate of 1 drop per second. The resulting
3. Results and discussion 3.1. Characterization of as-prepared Ag NPs Ag NPs were prepared using four different types of stabilizing ligands including gelatin, PVP, PVA and glucose. Gelatin is a natural polymer and composed of peptides and proteins [31]. It has amine 2
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shift in the wavelength of the plasmon absorption band could be attributed to the formation of bigger particles [26]. Compared to NaBH4, glucose has a lower reducing ability at ambient temperature. Therefore, Ag@Glu synthesis was performed under reflux conditions with the temperature control at around 80 °C to increase the reaction rate and thus to produce smaller particles having a surface plasmon band similar to the formers. Parameters other than the stabilizers are relatively important to observe the overall effect on the size and morphology issues during the synthesis of nanoparticles as well as the formation of hybrid HTC/Ag structures [41]. For instance, metal salt concentration, temperature, alkalinity of the reaction mixture, accelerator, etc. were studied previously in detail under different processing conditions to investigate the details of the mechanism of nanoparticle formation. It has been demonstrated that the surface plasmon absorption band could be blue shifted either by increasing the amount of NaOH in the reaction mixture or decreasing the AgNO3 concentration while keeping the glucose concentration unchanged [39,42]. Notably, this study exclusively focused on the role of the different type of stabilizing ligands in the fabrication of nano-silver-decorated hydrothermal carbons. Thus, no attempts were made to study reaction conditions of synthesized nanoparticles. Gelatin, PVA and PVP have relatively higher molecular weight and more functional groups such as -H which promotes formation of a protective coating on the surface of growing nanoparticles. Thus, they can effectively stabilize the dispersed nanoparticles at smaller sizes [32]. Another reason could be ascribed to the fact that NaBH4 favors the formation of smaller nanoparticles with respect to glucose itself [43]. Fig. 2 represent the TEM images of all the four differently capped Ag NPs. All prepared suspensions displayed spherical particles with comparable sizes. Judging from the TEM images, the estimated average particle sizes for Ag@Gel, Ag@PVA, Ag@PVP and Ag@Glu were in the range of 8.4 ± 2.0, 9.2 ± 1.6, 14.1 ± 4.8, and 18.3 ± 6.3 nm, respectively. Our findings from TEM investigations are in good agreement with what we presumed from the UV–vis analysis.
Fig. 1. A typical absorbance spectrum of the differently capped silver nanoparticle colloidal dispersions.
groups which readily donate electrons or coordinate to the nano-metal. Besides, functional groups on the gelatin (i.e. COO−) carrying a charge avoid the particle aggregation between the formed nanoparticles. Overall, gelatin is considered as a much better stabilizer than synthetic polymers, i.e., PVP [26,28]. UV spectra and corresponding plasmon absorption band suggested successful synthesis of the Ag@Gel. PVP is an important water-soluble synthetic polymer which is composed of a hydrophobic back bone baring hydrophilic side groups, i.e., highly polar amide groups. Hence, it can act as a stabilizer for coating Ag NPs via strong affinity of carbonyl oxygen atom of the amide group towards the silver [32–35]. The Ag@PVP produced a narrow, well defined strong absorption band around 400 nm (Fig. 1), which indeed, confirms formation of nanoparticles stabilized by PVP. PVA is also a water-soluble, nontoxic and biodegradable synthetic polymer and used as stabilizing ligand for inorganic nanoparticle synthesis. PVA stabilizes the metal core via interfacial compatibility through non covalent interactions [36]. The prepared Ag@PVA exhibited a peak profile similar to that of Ag@PVP with an absorption band centered at 399 nm in the corresponding UV spectrum, which indicates those particles have similar morphology with those of Ag@ PVP. The obtained Ag@PVA present a UV spectrum which also well fits with the findings reported in the literature [37]. Apart from those stabilizing ligands (gelatin, PVP, and PVA), glucose is encountered as well in nanoparticle synthesis which serves as both stabilizer and reducing agent. Apparently, all the nanoparticle suspensions showed colloidal stability throughout the experiments. The prepared Ag@Gel, Ag@PVP and Ag@PVA exhibited a strong plasmon absorption band at a wavelength around 400 nm which is characteristics for colloidal Ag NPs in the diameter of several tens of nanometers (Fig. 1) [28,38]. Glucose is a simple monosaccharide. Here in the synthesis of Ag@Glu, glucose not only serves as a reducing agent, but also acts as a stabilizer and trapping agent for Ag NPs. The reaction mechanism involves oxidation of glucose to gluconic acid and the released electrons reduces to silver ion to metallic silver and progressively Ag NPs are formed by coating the surface with gluconic acid moieties [26]. Moreover, it is reported in previous studies that glucose concentration is shown to be non-effective in determining the size. Instead, the ratio of the stabilizer, i.e. glucose, to the metal salt plays an important role in determining the size of the nanoparticles. As this ratio increases, smaller nanoparticles are formed with a blue shift in their surface plasmon absorption peak profile [39,40]. The prepared Ag@Glu revealed a broader absorption band at 422 nm which was slightly red shifted with respect to the formers. Since the position of the plasmon absorption band is very sensitive to size and surface interactions, this
3.2. Characterization of hybrid HTC-Ag nanostructures Different preparation protocols were employed to fabricate hybrid HTC-Ag nanostructures. Firstly, Ag NPs, which were coated differently with gelatin, PVA and PVP were mixed with HTCs at room temperature for 24 h. No significant changes after further increments in time was observed so the residence time was fixed to 24 h. The obtained samples were both visually and morphologically characterized by SEM and TEM, respectively. It can be seen from a representative SEM image (see Supplementary information Fig. S1) that, a typical HTC morphology appears as spheres mostly in the range between 400–600 nm, while few are around 1.2 μm. However, there was no clear indication of immobilized nanoparticles of Ag@Gel, Ag@PVA and Ag@PVP on the surface of the HTCs. Since nanoparticles were somewhat smaller to visualize by SEM, a close view of the samples was required to see them. To further check whether the nanoparticles were anchored on the HTC surface, TEM was employed. It can be seen from Fig. 3a that Ag@Gel could be detected as black dots on the surface of the HTCs. The nanoparticles were observed to agglomerate on the carbon surface with an average particle size of approximately 40 nm. Similarly, Ag@PVP were attached and distributed around the HTCs and particle size ranged between 13–25 nm (Fig. 3b). However, Ag@PVA were hardly detected (Fig. 3c). This observation indicated that ligand type could selectively affect the affinity of nanoparticles towards the HTCs via physicochemical interaction of surface moieties. It is believed that intermolecular hydrogen bonding occurs between functional groups of the gelatin such as -OH, -C=O, and -NH2, and the rich oxygen containing groups at the HTCs surface. Furthermore, amine groups of the gelatin may also favor to some extent the formation of hydrogen bonding between the surfaces. Consequently, this may lead to cleavage of gelatin from the nanoparticle because metal nanoparticles were stabilized 3
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Fig. 2. Representative TEM images of differently capped Ag NPs: (a) Ag@Gel; (b) Ag@PVA; (c) Ag@PVP and (d) Ag@Glu.
surface and Ag NPs were shown to aggregate with a size of approximately 170 nm (Fig. 3d). Size of the silver particles formed on the HTCs in the absence of any stabilizer molecule was rather big when compared to that of the prepared Ag NPs. This ensures that capping agents are of great importance in producing hybrid HTC-Ag nanostructures with a controlled size. Hybrid HTC/Ag nanostructures was synthesized by following the hydrothermal carbonization process for glucose with differently coated Ag NPs. For this purpose, glucose was dissolved in differently coated nanoparticle suspensions (Ag@Gel, Ag@PVP and Ag@PVA) and placed in the autoclave, then subjected to hydrothermal carbonization. The spherical carbons could be clearly seen from the SEM images of the obtained HTC/Ag@Gel hybrid nanostructure while the inner portion
through the amine functional groups of the gelatin backbone [26] which in turn may result in nanoparticle agglomeration on the HTCs. In the case of HTC-Ag@PVA interaction, it can be stated that PVA molecule is less prone to attach on the HTCs since it has no additional amine or oxygen containing groups. Besides, the OH groups of PVA possibly interacts with water molecules thereby inducing the cleavage process from the surface of the nanoparticles and allowing the release of the nanoparticles from HTC [37]. When the prepared HTCs were mixed thoroughly with aqueous AgNO3 solution, it was observed that a hybrid HTC-Ag nanostructure was obtained. The white spots were proved to be silver particles from the EDX analysis (Fig. S2). Here, the reduction of silver ion was achieved by the rich oxygen containing groups present on the HTCs
Fig. 3. TEM images of HTCs treated with Ag NPs: a) HTC-Ag@Gel, b) HTC-Ag@PVP, c) HTC-Ag@PVA, and d) SEM image of HTC-AgNO3. 4
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measured to be around 600 nm, and the shell thickness of the produced hybrid HTC/Ag@Gel nanostructures was around 300 nm. Furthermore, the size of the immobilized Ag NPs outside of the core was between 8–16 nm. It can be stated that hydrothermal carbonization of glucose may occur by surrounding and concentrating the remarkable amount of Ag@Gel to produce hybrid HTC/Ag@Gel nanostructures. Thus, a dense silver core inside the carbon spheres was obtained, which was protected by a thick carbonaceous shell. When we replaced Ag@Gel with Ag@PVP in the autoclave during the hydrothermal carbonization of glucose, the situation was rather different, and we could not detect any core/shell type structure. Apparently, Ag@PVP were immobilized as individual dark dots throughout the produced hybrid HTC/Ag@PVP (Fig. 5c). The estimated nanoparticle size in the hybrid structure was between 14–22 nm according to the corresponding TEM images. Also, there were no significant differences between the hybrid HTC-Ag nanostructures which were prepared under physical mixing conditions and in the autoclave. On the other hand, hydrothermal carbonization of glucose in the presence of Ag@PVA did not produce any hybrid nanostructure which was not observed from TEM images (Fig. 5d). A plausible explanation of the possible formation mechanism under hydrothermal conditions for the hybrid HTC/Ag nanostructures can be made based on the nucleation phenomena of the subsequent formation of HTC spheres. Nanoparticles of Ag@Gel selectively undergo in situ hydrothermal carbonization acting as nuclei for the growth of carbon spheres [19,21]. Glucose does not transform into carbon spheres below 140 ℃, and HTCs are formed from glucose the temperature between 160 and 180 °C [20]. It is proposed that gelatin surrounding the Ag@Gel may prevent nanoparticles from being disrupted in the reaction vessel under elevated temperatures. When Ag@Gel alone was exposed to the same hydrothermal processing conditions, there was still a trace of characteristic absorption profile of the remaining nanoparticle solution after the treatment, where a red shifted in the absorption maxima was observed, which indicates formation of larger particles during the in situ hydrothermal carbonization process (see Fig. S5). This finding indicates that gelatin could resist to complete disruption of the nanoparticles thus making them still remained under hydrothermal conditions. And these larger particles assumed to form the basis for the nucleation step of the
Fig. 4. A SEM and line EDX for HTC/Ag@Gel.
was hard to discriminate (Fig. S3). To further investigate the evidence of Ag NPs inside the HTC/Ag@Gel, line EDX analyses were employed for several individual carbon spheres. Fig. 4 shows a SEM image in which individual carbon spheres were examined by the line EDX analysis. A line crosses over an individual HTC/Ag@Gel and, the corresponding C and Ag signals were recorded accordingly through the line (Fig. 4). Interestingly, C signal dominates the signal from Ag at the edge of the HTC (see the inset in Fig. 4). However, this behavior reverses at the center of the scanned carbon sphere. This observation revealed a dense deposited silver in the core of HTC/Ag@Gel. This significant observation was not casual for the rest samples (HTC/Ag@PVP or HTC/Ag@PVA) even though they were proved to contain some spot like particles distributed on the carbon spheres. Noticeably, the dense silver layer immobilized almost at the center of the HTC/Ag@Gel is seen from the TEM images (Fig. 5a and b) as well, while some small dark spots, which were Ag NPs, were distributed in the inner part of the carbon spheres and at the surface as well. From the representative TEM images, an average metal core diameter was
Fig. 5. Representative TEM images samples from hydrothermal processing of glucose in Ag NPs suspensions: a) and b) HTC/Ag@Gel, c) HTC/Ag@PVP, and d) HTC/ Ag@PVA. 5
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carbonization yielding a dense metal embedded at the center of the formed HTCs. Ag@PVP alone also showed resistance towards disruption of the nanoparticles to some extent and a slight red shift in absorption peak occurred as it was not significant as for Ag@Gel (Fig. S6). Therefore, it was thought that hydrothermal processing of Ag@PVP in glucose solution produced only spot like nanoparticles embedded on the HTCs rather than a core/shell type hybrid nanostructure. However, a complete disruption of the PVA@Ag does occur when they undergo hydrothermal treatment at the same operating conditions (Fig. S7). These findings may suggest that since Ag@PVA might have been disrupted and collapsed at the bottom of the reaction vessel, it was not possible to obtain any hybrid HTC/Ag structures from the hydrothermal processing of Ag@PVA in the presence of glucose. It is noteworthy that surface coating groups selected for the synthesis of Ag NPs as well as the method to prepare functional hybrid HTC/Ag nanostructures significantly govern the final composition of the material which might be intended for ultimate purposes. Nevertheless, gelatin acts as the best candidate to stabilize Ag NPs which can be further utilized to produce core/shell type hybrid HTC/Ag nanostructures with a controlled fashion. Interesting morphologies were obtained upon direct hydrothermal treatment of glucose-capped silver nanoparticles. Spherical HTC structures with different surface morphologies can be seen from SEM image (Fig. 6a). Ag@Glu undergo cleavage of glucose molecules from the nanoparticle surface under hydrothermal condition and the hydrothermal carbonization took place. TEM analysis of the carbon spheres did not reveal any nanoparticles embedded within the structure. It is considered that as the hydrothermal process went on, metallic core of the silver nanoparticle could not be stabilized within the inner hydrophobic part of the growing carbon spheres. So, they ran out of the structure leaving some apertures behind on the carbon sphere surface which indeed caused the different surface morphologies. On the other hand, carbon spheres with smooth surfaces were most probably due to carbonization of glucose molecules that were assumed cover the silver without undergo a change or might be attributed to the ones which did not attach to nanoparticle surface formerly. When glucose and AgNO3 were hydrothermally treated together, both micrometer-sized Ag NPs and spherical HTCs could be visible from the corresponding SEM images (Fig. 6b) Here, nanoparticles aggregated independently elsewhere outside of the carbon spheres. It was not possible to obtain such carbonaceous spheres embedded with Ag NPs. TEM investigations did not reveal any indication of nanoparticles embedded and dispersed throughout the HTCs (Fig. S4). Although similar attempts were employed for different metal precursors and hybrid HTC/Ag nanostructures were achieved in previous studies. This is believed to be selectively depended on the additives, and other reaction parameters such as reaction temperature, reaction time, and mole ratio of the metal to carbon precursor [41,44]. The interaction of coating ligands with the nanoparticle surfaces, and of nanoparticles incorporated inside carbon spheres was studied through the FTIR spectroscopy as well. Undoped hydrothermal carbons
Fig. 7. FTIR spectra: a) Gelatin, b) Ag@Gel, c) HTC-Glu, d) HTC-Ag@Gel, and e) HTC/Ag@Gel.
displayed an absorption profile including a series of peaks (Fig. 7). A broad band observed at 3310 cm−1 signifies the presence of hydroxyl functional group. The absorption band appeared at 2921 cm−1 is attributed to aliphatic C-H stretching. Furthermore, the double peaks at 1695 cm−1 and 1610 cm−1 are due to C=O and C=C stretching, respectively. The peaks at 1200 cm−1 and 1288 cm−1 correspond to the vibrations of C-O-C and C= C-O- stretching, respectively [21,27]. Ag@Gel clearly possess the characteristic peaks of the gelatin which indicated the existence of gelatin on the surface of Ag NPs (Fig. 7). In this manner, the absorption band at 1628 cm−1 is attributed to C=O stretching of amide; the band at 1522 cm−1 is assigned for C-N-H bending, and the broad band at 3268 cm−1 is due to N-H structural vibrations. The band seen in pure gelatin at 800 cm−1 could be attributed to NH2 out of plane vibration and this peak is weakened noticeably, and further broadening of the band at 3286 cm−1 at some extent occurred when gelatin coated the nanoparticle surface. This observation suggests that gelatin covers the nanoparticle surface by the interaction of amine groups with the metal surface [45–47]. When HTC is mixed with Ag@Gel, the overlapping amide carbonyl peak of the gelatin at 1622 cm−1 can be clearly identified for HTC-Ag@Gel (Fig. 7d). However, significant changes occur in the absorption profiles at certain band intervals when glucose was hydrothermally treated in the presence of Ag@Gel. The intensity of the bands 1000-1288 cm−1 could be assigned to C-O-C and C = C–O vibration. Aromatic C-N stretching, was suppressed while some new peaks appeared at 2100 cm−1 and 3600 cm−1. Moreover, the intensity of the amide carbonyl peak of gelatin residue becomes predominant in the range of 15001700 cm−1 compared to that seen in the case of the HTC-Ag@Gel. This
Fig. 6. SEM images of products obtained: a) HTC/Ag@Glu and b) HTC/AgNO3. 6
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Fig. 8. FTIR spectra: a) PVP, b) Ag@PVP, c) HTC-Glu, d) HTC-Ag@PVP, and e) HTC/Ag@PVP.
Fig. 9. FTIR spectra: a) PVA, b) Ag@PVA, c) HTC-Glu, d) HTC-Ag@PVA, and e) HTC/Ag@PVA.
reveals that more Ag@Gel are embedded within carbon spheres under hydrothermal conditions than the ones obtained by mixing of HTC with Ag@Gel. This observation is consistent as well with the corresponding SEM and TEM analysis. In Fig. 8, Ag@PVP revealed the main characteristic peaks of PVP which indicates that nanoparticle surface was occupied by PVP (Fig. 8a). Pure PVP spectrum presents distinct peaks at 2921 cm−1, 1656 cm-1 and 1270 cm-1 belong to C-H asymmetric stretching, C=O, and C-N stretching vibrations, respectively [48]. The latter two peaks observed to be shifted to 1643 cm−1 and 1284 cm−1, respectively in the case of Ag@PVP. This observation could be ascribed to the coordination of oxygen and nitrogen atoms of PVP with the surface atoms of the Ag NPs [49]. When HTCs were mixed with Ag@PVP to obtain HTC-Ag@ PVP, FTIR spectrum of the resulting sample presented the overlapping of C=O groups of HTCs and PVP where only seemed a single peak at between 1600 and 1700 cm−1. Similarly, the intensity of the peak observed at 1284 cm−1 increased due to the combination of the absorption of C-N groups of PVA and C-O-C vibration seen in HTC alone. This, in turn, reveals the presence of Ag@PVP on the HTCs. Accordingly, the HTC/Ag@PVP hybrid structure, fabricated through the hydrothermal treatment of glucose in the presence of Ag@PVP, yielded an FTIR spectrum which was quite similar to that of HTCs alone except for some peaks. The peaks at 1600-1700 cm−1 split again into two as seen for HTCs alone. This may indicate possible rupture of partial carbonyl groups of the pyrrolidone and implying the formation of C-O bonds. Furthermore, C=O peak of HTC/Ag@PVP is weakened with respect to that of HTCs alone. This may indicate interaction of the nanoparticles through the carbon media [21]. PVA has several characteristic FTIR peaks (Fig. 9a). A broad peak appeared at 3286 cm−1 and the bands 1000–1420 cm−1, correspond to OH stretching. The peaks at 2913 cm−1 and 1420 cm−1 correspond to C-H stretching and C-H bending vibration of CH2, respectively. Main characteristic peaks of PVA, which were observed for Ag@PVA, signatures the presence of PVA on the surface of the Ag NPs. It is noteworthy that the intensities OH stretching and OH bending bands of PVA decreased and these bands shifted to 1562 cm−1 for OH stretching and 1402 cm−1 for OH bending. The changes of the peaks indicates that OH groups of PVA interacts with the metal surface through formation of CO-Ag [50]. The FTIR spectrum of HTC-Ag@PVA is shown in Fig. 9d. Comparing with the spectrum of HTCs alone, the intensity of the OH band at 3330 cm−1 increased due to contribution from Ag@PVA. OH bending in Ag@PVA observed at 1562 cm−1 overlapped with C=C band of HTCs alone. This resulted an increased intensity of the band at
1608 cm−1 which implies the presence of Ag@PVA inside the carbon spheres. On the contrary, HTC/Ag@PVA, prepared by treating glucose in the presence of Ag@PVA under hydrothermal conditions revealed almost the same FTIR spectrum as that of HTCs alone with enhanced peak intensities. This observation indicates that since there is no significant sign of Ag@PVA, no detectable nanoparticle deposited on HTCs. These observations conform with SEM and TEM results as well. Finally, FTIR spectrum of Ag@Glu is shown in Fig. 10. Two distinct absorption bands were observed. The broad peak at 3291 cm−1 refers to OH groups of gluconic acid and glucose as well, and the one located at 1636 cm−1 is assigned to C=O group. Glucose reduces the silver salt and is converted to gluconic acid. Hence, gluconic acid covers the surface of metallic silver by means of interactions occur between the metal surface and oxygen bearing groups of gluconic acid [39,42,51]. Glucose molecules which were not undergo reduction of silver salt may also acts as capping agent to some extent for silver surface through its OH groups. When Ag@Glu was subjected to hydrothermal carbonization, HTC/Ag@Glu, displayed the characteristic peaks of HTC alone at certain bands which are appeared at 1608 cm−1, 1691 cm−1, and 3314 cm−1. The peak at 1636 cm−1 for Ag@Glu was completely disappeared upon hydrothermal carbonization treatment. This may indicate, gluconic acid undergo the same hydrothermal conversion path
Fig. 10. FTIR spectra: a) HTC-Glu, b) Ag@Glu, c) HTC/Ag@Glu, and d) HTC/ AgNO3. 7
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as glucose itself. Hydrothermal treatment of glucose with silver nitrate yielded almost the same spectrum as in the case of HTC/Ag@Glu where only the intensity of the band in between 1600-1700 cm−1 range for HTC/AgNO3 was higher than that of HTC/Ag@Glu.
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4. Conclusions Hybrid HTC-Ag nanostructures could be prepared with different morphologies by incorporating differently coated Ag NPs. Ag@Gel were successfully loaded into the HTCs more like a core/shell type under hydrothermal conditions. A dense silver core of about 600 nm at the centers of the HTCs was observed. Thickness of the carbon shell was about 300 nm. On the other hand, Ag@Gel were attached to HTCs surface as particles with a size around 40 nm when they were mixed with as-prepared HTCs at ambient conditions. Ag@PVP were shown to attach to HTCs with a particle size between 13–25 nm. However, Ag@ PVA were not shown to selectively attach to HTCs. Results showed that Ag NPs with different degrees of dispersing capabilities on the HTCs could be obtained by controlling the ligand type and the preparation method as well. Our findings reveal that capping agents are of great importance in producing hybrid HTC-Ag nanostructure with a controlled nanoparticle size. Acknowledgment This work was supported by the Karabuk University (Project ID number: KBU BAP-17-DR-047). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jece.2019.103415. References [1] M. Sevilla, A.B. Fuertes, The production of carbon materials by hydrothermal carbonization of cellulose, Carbon 47 (2009) 2281–2289. [2] M.-M. Titirici, A. Thomas, M. Antonietti, Back in the black: hydrothermal carbonization of plant material as an efficient chemical process to treat the CO 2 problem? New J. Chem. 31 (2007) 787–789. [3] M.M. Titirici, A. Thomas, S.-H. Yu, J.-O. Müller, M. Antonietti, A direct synthesis of mesoporous carbons with bicontinuous pore morphology from crude plant material by hydrothermal carbonization, Chem. Mater. 19 (2007) 4205–4212. [4] M.-M. Titirici, A. Thomas, M. Antonietti, Replication and coating of silica templates by hydrothermal carbonization, Adv. Funct. Mater. 17 (2007) 1010–1018. [5] S.-H. Yu, X.J. Cui, L.L. Li, K. Li, B. Yu, M. Antonietti, H. Cölfen, From starch to metal/carbon hybrid nanostructures: hydrothermal metal-catalyzed carbonization, Adv. Mater. 16 (2004) 1636–1640. [6] H. Simsir, N. Eltugral, S. Karagoz, The effects of acidic and alkaline metal triflates on the hydrothermal carbonization of glucose and cellulose, Energy Fuels 33 (2019) 7473–7479. [7] A. Chen, Y. Yu, Y. Zhang, W. Zang, Y. Yu, Y. Zhang, S. Shen, J. Zhang, Aqueousphase synthesis of nitrogen-doped ordered mesoporous carbon nanospheres as an efficient adsorbent for acidic gases, Carbon 80 (2014) 19–27. [8] Y.-M. Lu, H.-Z. Zhu, W.-G. Li, B. Hu, S.-H. Yu, Size-controllable palladium nanoparticles immobilized on carbon nanospheres for nitroaromatic hydrogenation, J. Mater. Chem. A 1 (2013) 3783–3788. [9] K.M. Steel, W.J. Koros, Investigation of porosity of carbon materials and related effects on gas separation properties, Carbon 41 (2003) 253–266. [10] A.V. Nakhate, G.D. Yadav, Cu2O nanoparticles supported hydrothermal carbon microspheres as catalyst for propargylamine synthesis, Mol. Catal. 451 (2018) 209–219. [11] J. Wu, C. Jin, Z. Yang, J. Tian, R. Yang, Synthesis of phosphorus-doped carbon hollow spheres as efficient metal-free electrocatalysts for oxygen reduction, Carbon 82 (2015) 562–571. [12] M. Endo, C. Kim, K. Nishimura, T. Fujino, K. Miyashita, Recent development of carbon materials for Li ion batteries, Carbon 38 (2000) 183–197. [13] C. Galeano, J.C. Meier, M. Soorholtz, H. Bongard, C. Baldizzone, K.J. Mayrhofer, F. Schüth, Nitrogen-doped hollow carbon spheres as a support for platinum-based electrocatalysts, ACS Catal. 4 (2014) 3856–3868. [14] H. Simsir, N. Eltugral, R. Frohnhoven, T. Ludwig, Y. Gönüllü, S. Karagoz, S. Mathur, Anode performance of hydrothermally grown carbon nanostructures and their molybdenum chalcogenides for Li-ion batteries, MRS Commun. (2018) 1–7. [15] E. Unur, S. Brutti, S. Panero, B. Scrosati, Nanoporous carbons from hydrothermally treated biomass as anode materials for lithium ion batteries, Microporous
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