Modified-liquid-liquid phase separation: A novel technique to synthesize 2-dimensional CuO nanosheets

doi:10.21203/rs.3.rs-4707290/v1

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Modified-liquid-liquid phase separation: A novel technique to synthesize 2-dimensional CuO nanosheets

https://doi.org/10.21203/rs.3.rs-4707290/v1

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The 2-dimensional materials are seeing an upsurge in research and various applications. In this work, the growth of cupric oxide (CuO) nanostructures in different morphologies is reported by employing modified-liquid-liquid phase separation (m-LLPS) technique. Here, two binary phases copper sulphate-polyethylene glycol (PEG) i.e. C&P and sodium hydroxide-PEG i.e. N&P are used in tandem and reduction of copper salt is employed. Sheet-like agglomerated growth could be restrained into nice flattened disks by merely modifying the interaction of binary phases in time. X-ray diffraction (XRD) for structural information, UV-Visible spectroscopy for optical bandgap estimation, micro-Raman spectroscopy for vibrational levels investigation, and Fourier Transform Infrared spectroscopy (FTIR) as fingerprint for bond/ compound formation have been employed. The flat-disk CuO sample was used for detection of glucose. Limit of detection and limit of quantification levels were estimated and were found to be 4 mM and 13 mM, respectively. The mechanism of the modified-LLPS method leading to oriented growth from disoriented growth is discussed. By taking CuO as an example, a very simple concept of phase separation has been demonstrated to prepare 2-dimensional materials.

CuO

modified-liquid-liquid phase separation

binary system

amperometry

limit of detection and limit of quantification

Copper is the 29th element in the periodic table and exhibits two valence states viz + 1 and + 2. Both the oxides of Cu i.e. cuprous (Cu₂O) and cupric (CuO) have been studied extensively for various applications like electrical, energy, biomedical etc. [1–6]. CuO is a p-type semiconductor with band gap ranging from 1.3 to 2.1 eV and applications ranging from fields like heterogeneous catalysis, sensing devices, solar cells, field emission devices, lithium ion electrode materials, cathode catalyst in fuel cells, etc.[7, 8]. Factors like low cost, environmental stability, non-toxicity and easy processing make it a user-friendly material. Different morphologies like zero-dimensional (0D) nanoparticles, one-dimensional (1D) nano-wires, nano-rods, nano-needles, two-dimensional (2D) nano-sheets and nano-plates, three-dimensional (3D) nano-cubes, etc. have been reported [9–19]. However, few articles have reported toxicity concerns of CuO nanoparticles [20–22]. Thus, if copper oxide nanomaterials get discharged in environment, these could pose a threat to life in the long run [23, 24]. Therefore, architecturally engineered structures should be synthesized that have low or no toxicity originating due to their physical dimensions. Hence, the growth of materials at sub-micron scale needs to be revisited and new methodologies should be developed for growing environmentally benign structures. In this regard, this article explores the possibility of using the concept of liquid-liquid phase separation (LLPS) technique, which is an established technique to separate proteins or other biological specimens, in their liquid phases [25–28]. In general, a homogeneous liquid phase separates into two liquid phases namely the dense and the dilute phase by manipulating parameters like pH of the solution, temperature, concentration, types of liquids, etc [29, 30]. Thus, one of the parameters is changed in order to induce separation within the homogeneous liquid environment. As a result, the macromolecules or proteins which are initially in a homogeneous liquid state, separate into dense and dilute phases. Once the phases separate, the dense phase resembles materials as liquid droplets that can later be filtered via different ways. In this study, a new modified-LLPS (m-LLPS) method has been employed where two different binary liquid systems in tandem have been used to cause reduction of copper salt into CuO. Polyethylene glycol 6000 (PEG) is chosen to be the common liquid in both the sets of binary phases formed by copper sulphate-PEG (C&P) and sodium hydroxide-PEG (N&P). By employing parametric changes during synthesis, different morphologies have been synthesized. This study focusses on the morphological investigations of the grown samples and investigates if binary LLPS systems can be successfully used after modification to develop nanomaterials.

To synthesize CuO structures, copper sulphate pentahydrate (CuSO₄∙5H₂O), sodium hydroxide (NaOH) and polyethylene glycol (PEG 6000) were used. All the chemicals were used as-received from Merck Chemicals. Distilled water (D/W) was used in the synthesis. Two binary liquid phase systems were employed namely, C&P and N&P.

2.1 CuO synthesis

2.1.1 Synthesis using one binary system (C&P)

To begin, 0.5 M CuSO₄∙5H₂O was prepared. Further, aqueous solution of 0.5 wt.% of PEG 6000 was added to this solution gradually which changed the concentration of PEG in the binary phase. The solution was stirred for 30 minutes. This formed the first binary phase C&P. The pH of the reaction mixture was ~ 6.6–6.8 throughout the reaction. For reducing the copper salt, appropriate volume of 1 M NaOH was added drop-wise to the precursor solution under constant stirring. Thereafter, the solution was heated to 60°C and maintained at this temperature for 1 hour. The pH of the solution was 14. A black colored precipitate appeared which was allowed to settle naturally in the same reaction mixture overnight. The precipitate was stored in water (CP-W) and ethanol (CP-E) for a fixed number of hours to convert any residual hydroxides to oxide or dissolution of PEG, respectively. The sample was then collected, repeatedly washed, and finally dried in hot air oven at 80°C. Different synthesis conditions employed in this work are tabulated as Table 1 along with sample labels.

2.1.2 Synthesis using two binary systems (C&P and N&P)

The procedure described above was repeated by employing the new m-LLPS technique by adding N&P in two different ways. In the first m-LLPS sample set (S1), the N&P binary phase was added drop-wise into previously formed C&P. In the second m-LLPS set (S2), both C&P and N&P were simultaneously added drop-wise in a beaker. While in the set S1, the PEG concentration gradually increased in the reaction mixture as more and more N&P was added in the beaker, in S2 the drop-wise addition ensured that the concentration of PEG available in the reaction mixture was limited throughout the addition process because fixed volumes of C&P and N&P were mixed each time. The centrifugation conditions, washing and drying steps were same as described above for CP-W and CP-E.

Table 1

Details of synthesis conditions and sample labels. The concentrations of copper sulphate solution (0.5 M) and PEG (0.5 wt.%) were maintained at 0.5 M, respectively.
Condition	Label
Aged in DI water	CP-W
Aged in ethanol	CP-E
m-LLPS [drop-wise addition of N&P into C&P binary system]	CP-S1
m-LLPS [simultaneous drop-wise addition of N&P and C&P]	CP-S2

2.2 Characterization methods

As-synthesized samples were characterized for their phase formation using X-ray Diffraction (XRD, Bruker D-8 advanced model with monochromatic X-ray beam, Cu-K_α (λ = 1.54178 Å) radiation) technique. Optical properties were studied using UV-Visible Spectroscopy (JASCO v750 from Agilent Technologies) by recording the spectrum in the range of 200–800 nm. Micro-Raman spectroscopy (WiTec alpha 300R) was performed with excitation wavelength of 532 nm and the spectrum recorded between 100–1200 cm^− 1 range. Fourier Transform Infra-Red (FTIR, Agilent Cary 630 FTIR model) spectroscopy was carried out in a range of 650–4000 cm^− 1. The morphological details were investigated by Field Emission Electron Microscopy (FESEM) technique (NanoSEM, model NPEP 303). High-Resolution Transmission Electron Microscopy (JEOL, model JEM 2100F) was used for the detailed morphological investigations. The electrochemical studies were carried out using a three-electrode set-up on a Metrohm Autolab (AUT.MAC204) instrument operated by Nova 2.1 software.

For electrochemical studies, the sample was coated on glassy carbon electrode (GCE, 3 mm diameter). The catalyst ink was prepared by grinding CuO in N-Methylpyrrolidone (NMP) with Nafion binder (5 wt.%) and drop casted on GCE and dried in a hot air oven at 60 ℃. This is referred to as the working electrode CP-S2/Nafion/GCE. Ag/AgCl was used as a reference electrode and platinum wire was used as a counter electrode in 0.1 M NaOH electrolyte. To understand the electrochemical behaviour, ampereometric study was carried out at constant applied voltage of 0.40 V for the glucose concentrations in the range of 0–10 mM.

The following sections first discuss the structural, morphological, optical, and sensing properties. Lastly, growth mechanism using m-LLPS is discussed.

3.1 X-ray Diffraction results

Fig. 1 shows a representative XRD diffractogram of CP-S2 sample. The synthesized CuO exhibits growth in the monoclinic crystalline system. Complete oxidation of copper occurred, and no impurities were detected within the instrument’s detection limit. Further, no peaks corresponding to Cu₂O were detected. The results were found to be in close agreement with JCPDS database (file 00-045-0937) [31, 32]. The crystallite size estimated from (111) peak was found to be ~ 10.9 nm using Scherrer’s formula.

3.2 Optical properties

The optical bandgap was estimated by studying the absorption characteristics from UV-VIS absorption spectroscopy of the sample as shown in Fig. 2(a) and thereafter plotting Tauc’s plot shown in Fig. 2(b). Tauc’s plot showed a direct band gap of 1.25 eV, which is in close agreement with the reported band gap of 1.3 eV [33, 34]. The micro-Raman spectrum shown in Fig. 2(c) shows peaks at 136, 255, and 429 cm^-1 that represent various modes of CuO. PEG also contributes to vibrations at 253 cm^-1 peak which in the present study appear as a broad band at 255 cm^-1 [35]. Vibrations of oxygen atoms appear as A_g and B_g modes which can be seen as minute peaks. The 282 cm^-1 vibration is attributed to A_g [36] and 301 cm^-1 and 316 cm^-1 are due to B_g modes. Similarly, the peak observed at 149 cm^-1 is attributed to the E_g mode [35, 37]. FTIR investigations were used to identify the molecular species present over the surface [38]. Fig. 2(c) shows a representative FTIR spectrum of CP-E while the peak origins are tabulated in Table 2. The dominance of transmission dip at 3390 cm^-1 indicates the presence of PEG or asymmetric O-H stretching vibration due to water molecules adsorbed on surface [3, 39]. Generally, all the metal-oxygen bonding vibrations occur below 1000 cm^-1. Here, Cu-O vibrations were observed at 506 cm^-1 indicating the formation of CuO [40] [41]. Bending type of CuO vibrations were observed at 598 cm^-1 [39] and traces of CO₂ were detected by the bend at 1124 cm^-1 [3]. The origin of carbon dioxide is attributed to oxidation of PEG. The skeletal C-C vibrations were observed at 1089 cm^-1 and can arise from the ethylene chain of PEG [32]. In view of Raman and FTIR analysis, the presence of PEG over the surface of copper oxide structures can be confirmed.

Table 2: Peak positions and their origin as observed by FTIR spectrum

Peak No.	Peak position (cm^-1)	Origin
1	506	CuO
2	598	Cu-O bending vibrations
3	1089	Skeletal C-C bonds attributed to PEG
4	1124	PEG attributed traces of CO₂
5	3390	O-H stretching and PEG

3.3 FESEM and TEM analysis

In this section, the morphological growth of CuO structures is discussed. Fig. 3 shows the FESEM micrograph of CP-W, CP-E, CP-S1 and CP-S2. In Fig. 3(a), the agglomerated structures of 2-dimensional CP-W sheets can be seen. The feather-like tapered tips of the structures are below 20 nm. Thus, thin sheets of CuO have formed which extend in length to about more than 1 µm long yielding an aspect ratio of about 50. Therefore, these structures may exhibit nanoscale properties and at the same time alleviate any toxicity concerns as these are long enough to be filtered and hence contamination of the environment can be easily checked. Fig. 3(b) shows the FESEM micrograph of CP-E. Here, the sheets formed are thicker as compared to CP-W. This is possibly due to the dissolution of PEG in ethanol. PEG is a hydrophilic polymer and serves as surfactant too that helps in keeping the sheets from stacking. However, upon its dissolution in ethanol, the sheets have a probability of stacking together and thus are thicker as compared to CP-W. In case of CP-S1 and CP-S2, the FESEM micrographs are shown in Fig. 3 (c) and (d), respectively. The small and uniform growth in case of CP-S2 is clearly visible as compared to CP-S1. Fig. 3(e) shows the high magnification micrograph of CP-S2. The aspect ratio of such structures has been estimated to be about 3. Fig. 3(f) shows the EDS spectrum confirming the presence of Cu, O and C while the inset depicts the elemental mapping where all the elements are uniformly distrusted overall.

Fig. 4(a) shows the TEM micrograph of CP-S2. Interestingly, the TEM shows the presence of flat 2-dimensional flakes with some slender rod-like structures. Fig. 4(b) shows the selected area electron diffraction (SAED) pattern with faint concentric rings and few bright spots. The faint rings indicate the presence of a polycrystalline sample while bright spots show the presence of crystal planes present in abundance. The SAED pattern, however, has a unique feature of streaks emerging with the bright spots. Though the presence of streaks is interpreted as presence of dislocations or stacking faults in crystals, here these are interpreted to be due to the random pile-up of 2-dimensional CuO sheets on one another on TEM grid while sample preparation. For the electron beam of the TEM this random pile-up appears as stacking faults giving rise to streaks in the SAED pattern. For this reason, the presence of streaks is visible even in the faint concentric rings and is attributed to the sheets’ artefact.

Growth mechanism

The 2-dimensional CuO nanosheets have been grown via various methods in this work. The modified-liquid liquid phase separation (modified-LLPS) employed in this work gives a very easy route to grow materials in 2-dimensions whose native growth structure is not 2-dimensional like MoS₂, WS₂, etc. PEG is used as a common polymer in forming the two binary phases i.e. C&P and N&P binary phases. At the outset it is mentioned that the phase boundaries are influenced by temperature, concentration, etc. parameters [29, 30, 42, 43]. Hence, in all the experiments carried out, the precursor concentrations were fixed and experiments were carried out at room temperature (25 ± 3 °C). The reaction was completed in three fundamentally different ways. In the first method (CP-W and CP-E), the C&P solution was maintained in the single phase and dropwise addition of NaOH was carried out. This ensured reduction of the copper sulphate to CuO was carried out in a single-phase system from initial reaction time T1 to final reaction time T2. Here, the single to bi-phasic boundary was not crossed. In modified-LLPS method of set S1, the C&P binary system’s concentration was maintained in the ratio of 1:3. The phase diagrams of C&P system predicts the phase to be single and homogeneous [43, 44]. To check if the phase separation takes place in C&P system when PEG concentration is increased, a small reaction volume was taken in a vial and PEG was introduced at the base of the vial using a syringe. It could be seen that as the PEG concentration increased, a dense phase formed and is a complex of Cu(OH)₂. Fig. 5(a) shows the phase separation in C&P system on addition of PEG in the vial. For CP-S1, the N&P binary system was gradually added to the single-phase C&P system. This ensures that the concentration of PEG gradually keeps on increasing until the phase separation results. Till this stage, the CuSO₄ is continually reduced to CuO with the help of NaOH. This resulted in CP-S1 morphology where it was observed that thinner but agglomerated CuO sheet-like bundles have formed. These structures had more of a linear growth as the sheets seem to be aligned linearly. In this methodology, the PEG concentration was gradually increased and the system was made to cross the single to bi-phasic line reaction towards the end of the reaction time T2. Lastly, the C&P and N&P binary phases were made to react in a way where overall concentrations of the C&P and N&P remained almost constant. Here, the nature of sulphate and sodium ions plays the role of kosmotropes i.e. these ions promote stronger interaction with H-OH bonds of water. This in turn reduces ₂the effect of hydrogen bonding of water itself. The concentrations of C&P and N&P were maintained so that the kosmotrope effect of Na⁺ and SO₄^- ions interacted stongly with H-OH bond of water. In this process, the H-OH bonds weaken allowing the PEG, which is a hydrophilic polymer attaches self to water molecules. This promotes quick and local formation of Cu(OH)₂ that eventually converts to CuO in the presence of N&P system. This is why the flat sheets formed via the modified-LLPS method exhibit a uniform structure. This type of synthesis methodology ensured that the reaction progressed in the bi-phasic region right from initial time T1 to final time T2. Phase separation could be visibly seen as the black CuO remain suspended towards the top of the reaction mixture and is shown in Fig. 5(b). Therefore, the manner in which the binary phases are made to interact influence reaction pathway and in turn the final morphology. This is depicted in the schematic of Fig. 6. The process pathway for the methodologies adopted for the synthesis are depicted for CP-W, CP-E, CP-S1 and CP-S2, respectively showing how the reaction progresses from single to bi-phasic regions.

In basic chemical reduction, the black precipitate settles at the base of the reaction container, while in this case it is the lighter phase and stays suspended at the top. On the phase diagram, the reaction pathway stays in the bi-phasic region throughout. Finally, in case of CP-S2, the C&P and N&P binary phases are added dropwise. Thus, at all stages of the reaction, the PEG concentration proportionately increases. This results in an environment that remains bi-phasic throughout the reaction and the growth takes place in these small pockets. As the two phases remain separated, the dimensions of the sheets also remain uniform as confirmed by the TEM micrograph. Thus, the m-LLPS technique employed in this work is a very simple and novel way of obtaining 2-dimensional structures.

Electrochemical glucose detector

To check the application of CP-S2 (i.e. 2-dimensional sheets) towards glucose sensing, the material was coated on the glassy carbon electrode (GCE) as described under the experimental section. The working potential for the ampereometric study was selected by checking the CP-S2 and glucose interaction at different applied voltages of 0.40, 0.45, 0.50, and 0.55 V. The experiments revealed at 0.40 V, the sensor gives the linear potential-current signature. Hence, 0.40 V was selected as the set potential for ampereometric study. The ampereometric studies were carried out at 0.40 V and addition of requisite amount of glucose was carried out at fixed time intervals. Different concentrations of the glucose ranging from 0 to 10 mM were then tested. A monotonic rise in the reduction current is observed with respect to the increase in the glucose concentrations. Fig. 7(a) shows the current-potential relationship with the addition of glucose. With the first addition of glucose amounting to 0.05 mM, the sensor current shows a drastic change. Addition of further 0.05 mM of glucose is also detected distinctly by the sensor. As glucose concentration is increased, the current signal gradually shows a lesser response. The limit of detection and limit of quantitation (or quantification) have been estimated as 0.2 mM and 0.6 mM, respectively for glucose concentration less than 0.5 mM. However, the fabricated sensor showed linear response in the range to 1-10 mM. The LoD and LoQ walues in this range have been estimated as 4 mM and 13 mM. This possibly is due to reduction in the active available sites as the sheets could get agglomerated in aqueous media. As glucose gets oxidized, the current response gradually increases from a high negative to a low positive value. This is because when the glucose gets oxidizes, the CuO catalyst gets reduced and transfer of electrons to the sensor takes place. Fig. 7(b) shows the stability of the sensor when 2 µM glucose is added progressively. The response of the sensor can be considered as reasonably stable barring the 4^th and the 5^th additions. Thus, the 2- dimensional CuO sheets show a promising application in glucose detection.

This work reports a very simple and novel way to grow 2-dimensional CuO sheets. The modified-LLPS technique employed here demonstrates how the reaction can be progressed either in single- or bi-phasic regions. These structures have at least one of their dimensions ~ 400 nm or more, can be filtered easily and hence remain environmentally benign. Detailed structural, morphological and optical results have been discussed. Application of CuO nanosheets as glucose sensing has been demonstrated. The results are encouraging as LoD and LoQ of 0.2 mM and 6 mM have been estimated for glucose concentrations below 0.5 mM and 4 mM and 13 mM in the range of 1-10 mM glucose concentration, respectively. This is attributed to reduction in the active catalyst surface area. This work highlights the ease of synthesizing sheet-like materials whose native growth does not mimic sheet like 2-dimensional growth. By restricting the growth in 2-dimensions by the modified-LLPS technique, there is a huge possibility to grow sheet-like structures for other materials too.

Competing Interests: The authors have no competing interests to declare that are relevant to the content of this article.

Funding: Partial funding has been utilized to carry out the research work vide Research Support Fund and MJRP/2024/1393.

Data, Material and/or Code availability: The information shall be shared privately upon request.

CRediT: Methodology, Experiments, analysis, writing [Abha Mahajan], assistance in experiments and analysis [Ajay Piriya], writing- review and editing [Sundara Ramaprabhu], conceptualization, methodology, review and editing [Rupali Nagar]

Acknowledgements: One of the authors AM acknowledges the fellowship support by Symbiosis Centre for Research and Innovation, Symbiosis International (Deemed University), India in the form of SIU-SRF. The authors acknowledge DST and SAIF/CRNTS, IIT Bombay for providing FEG-TEM 200 analytical facility for this research work.

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Modified-liquid-liquid phase separation: A novel technique to synthesize 2-dimensional CuO nanosheets

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Abstract

Figures

1. Introduction

2. Experimental

2.1 CuO synthesis

2.1.1 Synthesis using one binary system (C&P)

2.1.2 Synthesis using two binary systems (C&P and N&P)

2.2 Characterization methods

3. Results and Discussion

3.2 Optical properties

3.3 FESEM and TEM analysis

Conclusion

Declarations

References

Additional Declarations

Status:

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