Tailoring the morphology through alkali treatment: Dynamics of pattern formation

Abstract

Diversified technological applications necessitate the tailoring of morphology of materials at nanoscale. This paper reports morphological modifications of boron carbide (BC) at nanoscale through alkali (NaOH) treatment. The multilayered hollow microspheres of BC, obtained by the refluxion of castor oil with boric acid, are subjected to exfoliation by sonicating it in the ethanol–water mixture. The attractive morphologies evolved in the sonication process as a result of changing the BC concentration, auxiliary agent and sonication time are analysed. The multilayered hollow structure on sonication with NaOH yields flower-like morphology that seems to evolve from the basic spindle structure. The morphological tuning makes the study relevant in the areas like bleaching, where the surface area is crucial.

Keywords

Liquid-phase exfoliation boron carbide morphology tuning sodium hydroxide refluxion bleaching

Introduction

In the development of the hardest materials, the third hardest material, boron carbide (BC), has a leading role in a variety of applications. Boron carbide is an essential refractory semiconductor with unique physical, mechanical and chemical properties such as low density, high modulus, high melting point, chemical durability, and excellent neutron absorption [1 -3]. These properties together make it a strong material for high energy and technology applications in the automobile industry, nuclear, microelectronics, military, space and medical industries [4 -6].

The quality and also the quantity of boron carbide depend upon the boron and carbon sources and techniques used for synthesis [7 -9]. The commercial production of boron carbide includes carbothermic reduction of boric anhydride at higher temperature, i.e. above 2000°C. The disadvantage of this method is the presence of a large amount of free carbon in the final product [10,11]. An alternative synthesis method uses a solution-based approach which yields powdered boron carbide at a lower temperature. The condensation reaction is one such most crucial method for the synthesis of boron carbide at low temperature [10,12 -14]. In spite of the difficulties in getting the morphology with unusual shapes like spheres, fibres, spindle, and flower-like, the emulsion-based synthesis overcomes these limitations. We have already reported the synthesis of multilayered hollow microspheres of boron carbide at low temperature by the emulsion-based synthesis method using castor oil and boric acid [15].

In the long term, the synthesis of tailored microstructures has significant interest due to their potential applications in electronic, optical, drug and gene delivery, and catalysis [16,17]. The fabrication of particles as individual units with the same form and function has research attention due to their enhanced properties such as narrow size distribution, porosity, high surface area and shell thickness as compared to larger one [18]. There are several techniques for the production of individual units with different sizes and shapes. The commonly used methods of exfoliation are chemical method, chemical vapour deposition, epitaxial growth, and liquid-phase exfoliation. It is reported that the nanoplates generated in the chemical exfoliation method contain defects which dramatically modify the electrical and physical properties [19]. The substrate-based methods such as chemical vapour deposition (CVD) and epitaxial growth suffer from the control on dimension and higher cost [20]. Liquid phase exfoliation, a top-down method, involves a stable dispersion of particles in organic/ionic solvents exfoliates via sonication or shear mixing. The method is proven to give good results in the exfoliation of boron carbide [21] and boron based nanosheets [22,23].

The solvents used for exfoliation plays a significant role in separating the layers or particles depending on their nature. For example, water can act as an ideal dispersion medium for exfoliating hydrophobic materials. The non-volatile organic solvents with high boiling points used for exfoliation include N,N-Dimethyl formamide, dimethyl sulfoxide, and ortho-dichlorobenzene. The use of such solvents necessitates drying by heating to recover the material. This process leads to the formation of composites due to the possible reaction between the material and the solvent. The use of solvents like ethanol [24,25], acetone, isopropanol and chloroform [26 -28] with a low boiling point for the exfoliation process as it enables the recovery of material by simple air drying overcomes the formation of composites. The efficiency of liquid exfoliation can be further be enhanced by the addition of secondary agents, like organic and inorganic salts or by increasing the sonication time. Liu et al. [29] have reported the use of sodium hydroxide for improving the level of exfoliation efficiency. The paper betells methods of tuning the morphology of material through alkali (sodium hydroxide-NaOH) treatment on boron carbide (BC) sample as an example for enhancing the effective surface area through liquid-phase exfoliation for potential surface area dependent applications [30 -34]. For this, the previously reported multilayered hollow microspheres of boron carbide are systematically subjected to exfoliation. The role of effective surface area on the physical property is exemplified through bleaching studies as an example.

Materials and methods

Among various processing methods available for the synthesis of boron carbide, condensation reaction helps to reduce the synthesis temperature. In the present work, boric acid is refluxed with castor oil in the ratio 1:5. The constant temperature maintained during refluxion also facilitates the removal of water from the mixture. The process results in a thermodynamically unstable emulsion which attains spherical shape. The detailed synthesis procedure is given in our earlier paper [15]. The process results in a transparent product which on cooling appears like a white solid. This sample is heated to 300°C for 1 h and subjected to structural and morphological characterisations. The ultrasonication of the sample in ethanol–water mixture yields boron carbide nanostructures of different morphology. The ultrasonication is carried out by varying the concentration (S1 – 0.8 mg ml⁻¹, S2 – 0.5 mg ml⁻¹ and S3 – 0.2 mg ml⁻¹) and sonication time (1 and 2 h). The addition of sodium hydroxide (NaOH) as the secondary agent in the ratio 1:2 (S_i:NaOH) improve the dispersion of sample in ethanol–water solution. For understanding the influence of NaOH on tailoring the morphology, samples are prepared by varying the amount of NaOH in the ratio 1:1, 1:2, and 1:3. The effect of sonication time along with NaOH is also studied for 1 and 2 h. The morphologies of samples are analysed by field emission scanning electron microscope (FESEM – Nova Nano FESEM).

The morphological modification greatly influences the properties of the material depending on surface area. The surface area modifications can easily be brought out through the bleaching action of the sample on the dye methylene blue (MB). For carrying out MB dye degradation by the BC samples (S1, S2, and S3 exfoliated in the presence of NaOH), dye solutions are prepared by dissolving 0.02 mg MB in 5 ml distilled water at room temperature. To study the dye degradation efficiency of BC, MB is added to the samples and the absorption spectrum is recorded (using UV–VIS – Perkin Elmer scan lambda 950) at three different intervals of time as 0, 41, and 65 h.

Results and discussions

In the present work, we report how the morphology can be tailored for various applications depending on surface morphology. For this boron carbide synthesised are subjected to exfoliation through sonication for tailoring its morphology. The sample synthesised is sonicated and investigated the effect of (i) concentration, (ii) the auxiliary agent NaOH, and (iii) sonication time on morphology. The morphology and X-ray diffraction (XRD) pattern of the multilayered hollow microspheres of boron carbide, synthesised by the refluxion of castor oil with boric acid, is reproduced in Figure 1 [15] for better understanding and clarity. The analysis of the XRD pattern shown in Figure 1, detailed in our earlier publication [15], reveals the peaks at 20.1°, 28.4°, and 40.1° corresponding to the planes (021), (901), and (271), respectively of orthorhombic boron carbide.

Figure 1

(a) FESEM image and (b) XRD pattern of the boron carbide sample(Reproduced with permission from Ref. [15].

Effect of concentration

To study the role of concentration in exfoliation by sonication three samples are prepared by varying the boron carbide concentration as 0.2, 0.5, and 0.8 mg ml⁻¹. From the FESEM images of the samples shown in Figure 2, it can be seen that the multilayered hollow spheres are broken down on sonication for 1 h. The FESEM images also reveal that for the higher concentration (0.8 mg ml⁻¹) though exfoliation does not happen upon sonication, the hollow microspheres get break down. The thick multilayered broken piece of the boron carbide (Figure 1) hollow sphere can be seen in Figure 2(a). When the concentration is reduced to 0.5 mg ml⁻¹ and 0.2 mg ml⁻¹ and sonicated, the thick layered piece of broken hollow sphere starts separating into flakes (evident from Figure 2(b,c)). This study suggests that the sample concentration influences the exfoliation by sonication for a given sonication time.

Figure 2

FESEM images of exfoliated boron carbide with varying concentration (a) 0.8 mg ml⁻¹, (b) 0.5 mg ml⁻¹, and (c) 0.2 mg ml⁻¹.

Effect of auxiliary agent, NaOH

From literature, it can be seen that the pH of the dispersing solution places a significant role in the exfoliation process [35]. In the present work, NaOH is used as an auxiliary agent to enhance the dispersion ability [36 -38] and to assist the exfoliation process. The samples (S1–S3) sonicated for 1 h in the presence of the auxiliary agent (taken in the ratio 1:2) shows attractive evolution of flower-like morphology as shown in Figure 3. This study also reveals that lowering of concentration significantly influences the exfoliation process. Figure 3(a) shows a flower-like structure with the petals as an aggregation of some basic structure and is given in the inset. For the sample S2, the exfoliation results in a structure with a morphology resembling a garden of flowers and buds. When the concentration of the sample is further reduced, the agglomerated structures of the petals, as seen in Figure 3(a) get separated to form a beautiful flower-like morphology (Figure 3(c)). The dynamics of evolution of morphology is depicted in Figure 4, which shows a spindle structure as the base unit which combines to form a flower-like morphology. The aggregation of the spindle structure may also be due to Ostwald ripening.

Figure 3

FESEM images of samples (a) S1, (b) S2, and (c) S3 exfoliated in the presence of NaOH.

Figure 4

Dynamics of formation of flower-like morphology.

The above studies revealed better result for exfoliation resulting flower-like morphology for the sample concentration of 0.2 mg ml⁻¹. Investigations are further carried out to find the effect of varying the amount of auxiliary agent, NaOH. Accordingly, the sample-NaOH ratios are taken as 1:1, 1:2 and 1:3 and sonicated for 1 h. Surprisingly the attractive morphologies get destroyed for the ratios except 1:2. The resultant morphologies are shown in Figure 5. Thus it can be concluded that for tailoring beautiful morphologies, it is advisable to fix the auxiliary agent to vary the amount of sample as evidenced by Figure 3.

Figure 5

FESEM images of sample S3 exfoliated with different NaOH concentrations (a) 1:1, (b) 1:2, and (c) 1:3.

The sample shown in Figure 3(c) is dropped on a quartz glass plate, dried and washed repeatedly with water. The FESEM image shown in Figure 3(c) is found to get modified, as shown in Figure 6. The morphology undergoes a transformation from flower-like to pineapple-like structure. This may be due to the rejoining of the petals shown in Figure 3(c) initiated by the surface tension of water.

Figure 6

FESEM image of sample S3 sonicated for 1 h along with NaOH after washing.

Effect of sonication time

Sonication time is another critical parameter in the process of exfoliation. The lower concentration sample, S3 with NaOH in the ratio 1:2 is sonicated initially for 1 h and then for 2 h. The FESEM image revealing flower-like morphology is obtained for sonication time of 1 h and is shown in Figure 3(c). Dwindling effect is observed on the morphology on increasing the sonication time. The petals found in Figure 3(c) get separated, as shown in Figure 7 and the beauty of morphology is lost. For other concentrations of the sample, no significant effect could be observed in the morphology on sonication.

Figure 7

FESEM images of sample S3 exfoliated for 2 h in the presence of NaOH with concentrations (a) 1:1, (b) 1:2, and (c) 1:3.

Degradation of methylene blue

The dye degradation studies are highly significant in waste water treatment. In this work, the degradation of one of the widely used neurotoxic dye MB by the exfoliated BC samples is studied. The study reveals the effect of morphology of the sample in the bleaching action. The UV–VIS absorption spectrum of MB and sample incorporated MB at different time (0, 41, and 65 h) are shown in Figure 8.

Figure 8

UV–VIS absorption spectrum of – (a) methylene blue; BC-MB sample (b) at t = 0 h, (c) at t = 41 h, and (d) at t = 65 h.

The intensity of the absorption peak around 665 nm, seen in Figure 8(a), gets reduced by the BC samples. This is due to the decomposition of chromo-phoric group of dye into the intermediate molecules [32], here the BC sample. From Figure 8, the percentage degradation of dye can be calculated using the relation where A_i and A are the absorbance of MB and BC-MB mixture at 665 nm respectively. The percentage degradation of dye by the BC samples, shown in Figure 9, reveals the initial faster rate of dye degradation up to 41 h and a near saturation later. From Figure 9, it can also be seen that the dye degradation efficiency is the highest for the sample S2 exfoliated in the presence of NaOH, for which the effective surface area seems to be the highest from Figure 3(b). Thus the study confirms the morphology dependent bleaching action of the exfoliated BC samples.

Figure 9

Percentage degradation vs. time for the samples S1, S2, and S3 exfoliated in the presence of NaOH.

Conclusions

Wide range of industrial applications demands morphological modifications of material to suit a particular application. (i) In this work, we have reported simple, cost-effective methods of tailoring the morphology through alkali (NaOH) treatment. (ii) The multilayered hollow microspheres of BC, synthesised by refluxing castor oil with boric acid, are subjected to ultrasonication by varying (a) concentration, (b) auxiliary agent, and (c) time. (iii) The process is found to evolve attractive flower-like morphologies from the broken layered flakes of multilayered hollow microspheres of BC, with a spindle-like structure as the basic unit. (iv) The degradation of the dye MB by the exfoliated BC samples clearly reveals the morphology dependent surface area in bleaching. (v) Thus, the study suggests the possible methods of varying the surface area by tailoring its morphology for surface area dependent potential applications.

Footnotes

Disclosure statement

No potential conflict of interest was reported by the author(s).

References

Mondal

Banthia

AK.

Low-temperature synthetic route for boron carbide. J Eur Ceram Soc. 2005; 25:287–291. doi: 10.1016/j.jeurceramsoc.2004.08.011

Bose

Nair

Gupta

CK.

Production of high purity boron carbide. High Temp Mater Processes. 1986; 7:133–140. doi: 10.1515/HTMP.1986.7.2-3.133

Suri

Subramanian

Sonber

Synthesis and consolidation of boron carbide: a review. Int Mater Rev. 2010; 55:4–40. doi: 10.1179/095066009X12506721665211

Thevenot

Boron carbide – a comprehensive review. J Eur Ceram Soc. 1990; 6:205–225. doi: 10.1016/0955-2219(90)90048-K

Zakharova

Mednikova

Rumyantsev

Synthesis of boron carbide from boric acid and carbon-containing precursors. Proceedings of the international conference on Nanomaterials: Appl Prop 2013;2:03AET09 (4pp).

Chang

Gersten

Adams

Preparation of boron carbide nanoparticles by carbothermal reduction method. Mater Res Soc Symp Proc. 2005;848. https://doi.org/10.1557/PROC-848-FF9.28.

Saritha Devi

Swapna

Raj

Natural cotton as precursor for the refractory boron carbide—a hydrothermal synthesis and characterization. Mater Res Express. 2018; 5:015603. doi: 10.1088/2053-1591/aaa367

Saritha Devi

Swapna

Ambadas

Hydrothermal development and characterization of the wear-resistant boron carbide from Pandanus: a natural carbon precursor. Appl Phys A. 2018; 124:297. doi: 10.1007/s00339-018-1733-z

Saritha Devi

Swapna

Ambadas

Low-temperature green synthesis of boron carbide using aloe vera. Chin Phys B. 2018; 27:107702. doi: 10.1088/1674-1056/27/10/107702

10.

Rafi-ud-din

Asghar

Ethylene glycol assisted low-temperature synthesis of boron carbide from borate citrate precursors. J Asian Ceramic Soc. 2014; 2:268–274. doi: 10.1016/j.jascer.2014.05.011

11.

Wang

Xing

Low-temperature synthesis of high-purity boron carbide via an aromatic polymer precursor. J Mater Res. 2018; 33:1659–1670. doi: 10.1557/jmr.2018.97

12.

Tahara

Kakiage

Yanase

Effect of addition of tartaric acid on synthesis of boron carbide powder from condensed boric acid-glycerin product. J Alloys Compd. 2013; 573:58–64. doi: 10.1016/j.jallcom.2013.03.255

13.

Pilladi

Ananthansivan

Anthonysamy

Synthesis of boron carbide from boric oxide-sucrose gel precursor. J Pow Tec. 2013; 246:247–251. doi: 10.1016/j.powtec.2013.04.055

14.

Kakiage

Tominaga

Yanase

Synthesis of boron carbide powder in relation to composition and structural homogeneity of precursor using condensed boric acid-polyol product. J Pow Tec. 2012; 221:257–263. doi: 10.1016/j.powtec.2012.01.010

15.

Saritha Devi

Swapna

Ambadas

Low temperature synthesis of multilayered-hollow microspheres of boron carbide from castor oil for photonic applications. J Appl Phys. 2018; 124:065303. doi: 10.1063/1.5040681

16.

Sainsbury

Satti

May

Oxygen radical functionalization of boron nitride nanosheets. J Am Chem Soc. 2012; 134:18758–18771. doi: 10.1021/ja3080665

17.

Narayan

Kim

SO.

Surfactant mediated liquid phase exfoliation of graphene. Nano Convergence. 2015; 2:20. doi: 10.1186/s40580-015-0050-x

18.

Mutua

Lin

Koech

Surface modification of hollow glass microspheres. Mater Sci Appl. 2012; 3:856.

19.

Nuvoli

Valentini

Alzari

High concentration few-layer graphene sheets obtained by liquid phase exfoliation of graphite in ionic liquid. J Mater Chem. 2011; 21:3428–3431. doi: 10.1039/C0JM02461A

20.

Cao

Xue

Liquid-Phase exfoliation of graphene: an overview on exfoliation media, techniques, and challenges. Nanomaterials. 2018; 8:942. doi: 10.3390/nano8110942

21.

Qiu

Luo

Liang

B₄c nanosheets decorated with in situ-derived boron-doped graphene quantum dots for high-efficiency ambient N₂ fixation. Chem Commun. 2019; 55:7406–7409. doi: 10.1039/C9CC03413G

22.

James

Jasuja

Chelation assisted exfoliation of layered borides towards synthesizing boron based nanosheets. RSC Adv. 2017; 7:1905–1914. doi: 10.1039/C6RA26658D

23.

John

Anappara

AA.

Aqueous dispersions of highly luminescent boron-rich nanosheets by the exfoliation of polycrystalline titanium diboride. New J Chem. 2019; 43:9953–9960. doi: 10.1039/C9NJ01502G

24.

Liu

Xia

Wang

Exfoliation and dispersion of graphene in ethanol-water mixtures. Front Mater Sci. 2012; 6:176–182. doi: 10.1007/s11706-012-0166-4

25.

Navik

Gai

Wang

Curcumin-assisted ultrasound exfoliation of graphite to graphene in ethanol. Ultrason Sonochem. 2018; 48:96–102. doi: 10.1016/j.ultsonch.2018.05.010

26.

Xiao

Bergeret

Preparation of polylactide/graphene composites from liquid-phase exfoliated graphite sheets. Polym Compos. 2014; 35:396–403. doi: 10.1002/pc.22673

27.

Sun

Feng

Solvothermally exfoliated fluorographene for high-performance lithium primary batteries. Nanoscale. 2014; 6:2634–2641. doi: 10.1039/C3NR04609E

28.

Chen

Liu

Chen

Synthesis of disordered and highly exfoliated epoxy/clay nanocomposites using organoclay with catalytic function via acetone−clay slurry method. Chem Mater. 2004; 16:4864–4866. doi: 10.1021/cm048922e

29.

WeiáLiu

NongáWang

Direct exfoliation of graphene in organic solvents with addition of NaOH. Chem Commun. 2011; 47:6888–6890. doi: 10.1039/c1cc11933h

30.

Hartley

Axelrod

HD.

Observations of a boron carbide electrode. J Electroanal Chem Interfacial Electrochem. 1968; 18:115–121. doi: 10.1016/S0022-0728(68)80165-2

31.

Echeverria

Dong

Peterson

Semiconducting boron carbides with better charge extraction through the addition of pyridine moieties. J Phys D: Appl Phys. 2016; 49:355302. doi: 10.1088/0022-3727/49/35/355302

32.

Singh

Kaur

Singh

Specially designed B₄C/SnO₂ nanocomposite for photocatalysis: traditional ceramic with unique properties. Appl Nanosci. 2018; 8:1–9. doi: 10.1007/s13204-018-0662-7

33.

Sheng

Meng

An improved carbothermal process for the synthesis of fine-grained boron carbide microparticles and their photoelectrocatalytic activity. Ceram Int. 2018; 44:1052–1058. doi: 10.1016/j.ceramint.2017.10.047

34.

Kim

Jun

Lee

MK.

Particle size-dependent pulverization of B4C and generation of B4C/STS nanoparticles used for neutron absorbing composites. Nucl Eng Technol. 2014; 46:675–680. doi: 10.5516/NET.06.2014.015

35.

Ricardo

Sendecki

Liu

Surfactant-free exfoliation of graphite in aqueous solutions. Chem Commun. 2014; 50:2751–2754. doi: 10.1039/c3cc49273g

36.

Sonia

Jayram

Kumar

Effect of NaOH concentration on structural, surface and antibacterial activity of CuO nanorods synthesized by direct sonochemical method. Superlattice Microst. 2014; 66:1–9. doi: 10.1016/j.spmi.2013.10.020

37.

Moazzen

Borghei

Taleshi

Change in the morphology of ZnO nanoparticles upon changing the reactant concentration. Appl Nanosci. 2013; 3:295–302. doi: 10.1007/s13204-012-0147-z

38.

Anandhavelu

Thambidurai

Effect of zinc chloride and sodium hydroxide concentration on the optical property of chitosan–ZnO nanostructure prepared in chitin deacetylation. Mater Chem Phys. 2011; 131:449–454. doi: 10.1016/j.matchemphys.2011.10.003