Virtual substrate method: synthesis and growth kinetics of 2D metal oxide nanosheets

We present the experimental decomposition kinetics for the synthesis of metal oxide 2D nanosheets by the virtual substrate method. In this method, acetates of Mg and Cu were used as precursors for the growth of prototype MgO and CuO nanosheets, while the filter paper was utilized as a virtual substrate. The synthesized samples were characterized by X-ray diffraction for the phase analysis of the nanostructures, which confirms the cubic phase of MgO and monoclinic phase of CuO; with a minor Cu2O phase. Field emission scanning electron microscopy was used to determine the surface morphology of the nanosheets. Fourier transform infrared spectroscopy was utilized to identify the vibrational modes of the metal oxides and the functional groups present in the sample. Thermogravimetric analysis revealed the evolution of combustive oxidation of filter paper with the thermal decomposition of acetates situated on its surface.


Introduction
Two-dimensional (2D) nanosheets are freestanding nanomaterial with a high lateral size-to-thickness ratio.Its remarkable electrical, mechanical and catalytic characteristic distinguishes it from bulk counterparts or nanoparticles.This is due to their distinct structure, peculiar surface chemistry, and quantum size effect [1][2][3].Following the discovery of graphene, 2D nanosheets have been produced and used extensively as fillers in polymer composite [4].In many applications, including energy storage, catalysis, and sensing, metal oxide nanosheets have drawn a lot of interest; for example, the use of nanosheets in Li-ion battery anodes [5], in sensor materials with high selectivity toward formaldehyde and acetone against interfering gases [6], and in development of synthetic photosynthetic systems [7].Even though the potential and significance of the applications of metal oxide nanosheets are escalating, there are only limited synthesis techniques.These techniques generally require a lot of efforts, time, expenditure and a need of pre-and post-processing [8][9][10][11][12][13].Recently an effortless, low cost and sustainable method for the synthesis of copper oxide (CuO) and magnesium oxide (MgO) nanosheets was reported, namely virtual substrate method, in which ashless filter paper was utilised as the virtual substrate [14].
Both CuO and MgO have their own remarkable application level importance in the field of nanomaterials.For instance, CuO nanosheets serve as a promising material to make high sensitive gas sensors [15], exhibits an incredible electrocatalytic response to H2O2 [16], photocatalytic activity [17].The MgO is used in catalysis, as an additive in refractory, paint and superconducting products [18].Besides, the mixed CuO-MgO shows highly efficient and excellent photocatalytic properties [19].
IOP Publishing doi:10.1088/1757-899X/1300/1/012037 2 Our present work elucidates the experimental decomposition kinetics for synthesis of copper oxide and magnesium oxide 2D nanosheets by virtual substrate method.Here, ashless filter paper acts as a supporting material for the growth of metal oxide nanosheets.

Materials
For the formation of metal oxide nanosheets, 99% pure copper acetate monohydrate (Cu(CH3COO)2.H2O or Cu(AcO)2) and magnesium acetate tetrahydrate (Mg(CH3COO)2.4H2Oor Mg(AcO)2) were used as precursors.A virtual substrate, specifically Whatman Grade 41 ashless filter paper with an ash residue rating of 0.007%, was utilized to facilitate the growth of the nanosheets.Deionized water was used as a solvent.

Synthesis
Three samples using precursors and one control sample were prepared.The first two samples of CuO (S1) and MgO (S2) were prepared using 0.2 M aqueous solutions of pure copper acetate and magnesium acetate respectively.A mixture of 0.2:0.1 M concentration of copper acetate and magnesium acetate was used in the synthesis of third sample (S3).All the three solutions were independently loaded to the ashless filter papers and allowed to dry in normal atmospheric conditions, according to the literature [14].Dried precursor loaded filter papers were combusted in air, leaving metal oxide ash residues.These residues were collected and used for further characterization.Bare filter paper was used as control sample.

Characterization
X-ray diffraction (XRD) measurement of thoroughly grinded samples were performed to examine phase purity and crystallinity of the synthesized samples.XRD was done using GNR APD 2000 PRO powder X-ray diffractometer with Cu-kα radiation of wavelength 1.5406 Å in step of 0.1°from 30° -85° of 2θrange.Investigation of surface morphology of the synthesized samples were done using a CARL ZEISS SUPRA 55 field-emission scanning electron microscope (FE-SEM) with an operating voltage of 5 kV.The FE-SEM samples were ultrasonically processed in ethanol, then placed on Si-substrate and coated by gold.Fourier transform infrared (FTIR) measurement was performed on Bruker Vertex 80v spectrometer in the range of 300 to 4000 cm -1 to determine the purity and functional group present in the sample.Potassium bromide (KBr) pellet technique was used to measure the infrared spectra.The thermogravimetry and differential thermal analyses was carried out by using a TGA / DSC 3+ combined differential thermal analysis and thermogravimetric analyzer supplied by M/S.Mettler Toledo, Switzerland.The temperature calibration of thermogravimetric analyzer was carried out by the method of fixed melting points employing International Confederation for Thermal Analysis and Calorimetry (ICTAC) recommended standards such as indium, aluminum and gold.This equipment has a mass sensitivity of 0.1 µg and a temperature sensitivity of 0.01 K.A platinum crucible of 70 µl capacity was used as the sample container.For each experiment 5 mg of the sample was used.The temperature was measured using a Pt-10 % Rh thermocouple which was in firm contact with the sample holder on which the platinum sample container was placed.The inner muffle of the furnace was constantly purged with air, the flow of which was controlled by a mass flow controller.A constant gas flow of 20 sccm, a predetermined rate of heating of 5 K/min and a temperature range of 303 -1073 K were used to carry out dynamic experiments.

Results and Discussion
The XRD patterns of the produced powdered samples S1, S2, and S3 are illustrated in Figure 1.It shows monoclinic phase of CuO and a small Cu2O phase in Cu containing samples S1 and S3 (Figure 1(a, c)).As demonstrated in Figure1 (b, c), the XRD pattern of S2 and S3 reveals the cubic phase of MgO. Figure 1(c) confirms the existence of both phases of CuO and MgO in the sample S3.Peaks profile indicates towards good crystallinity as well as purity in the synthesized samples.The XRD pattern of all samples indicates that composites have no detectable secondary phase or impurity peaks, except for a minor Cu2O phase in Cu containing samples.
FE-SEM micrographs of the samples are shown in Figure 2. It displays the surface morphology of each samples as layered structures for CuO (S1), thin sheets for MgO (S2), and folded sheet like structures for CuO-MgO (S3) samples.Notably, in contrast to the completely sheet-like structure found in the S1 (Figure 2(a)) and S2 (Figure 2(b)) samples, S3 contains granular, fibrous as well as highly porous features.The filter paper is made of fibres of cellulose.Therefore, when it is drenched into a solution containing the precursors, it gets on its surface fully coated by these precursors.In case of S1 and S2, precursors were coated on surface of cellulose and result into formation of metal oxide nanosheets followed by combustive heating.The mixed sample S3 has two different kind of nanostructure formation; that of MgO nanosheets and CuO nanoparticles.In a previous report, it is established that it happens because the magnesium precursor has higher solubility than copper acetate [14].Thus, it confirms the role of filter paper's (virtual substrate) surface characteristics in determining the product morphology.The functional groups present on the surface and chemical structure of S3 were further investigated using the FTIR spectrum, as shown in Figure 3. Bands at 432 and 862 cm -1 indicates the stretching vibrations of Mg-O bond [20] and peaks at 482, 532, 592 cm -1 confirms the stretching and bending vibrations of the Cu-O bonds [21].Presence of the bending (1455, 1633 cm -1 ) and the stretching modes (3434 cm -1 ) of hydroxyl group indicate the absorption of water molecules from the air by the nanosheets [22,23].
TG and DTG thermograms of four samples, copper acetate loaded filter paper (Cu/FP), magnesium acetate loaded filter paper (Mg/FP), mixed loaded filter paper (Cu-Mg/FP) and bare filter paper (FP), are shown in Figure 4.While considering TG curve, around 373-450 K, all the thermograms show a mass loss of around 9-10%, which is related to the loss of moisture and the production of anhydrous acetates [24,25].As we heat further, there is a huge and an evident decrease in sample's weight between 495 K and 750 K.It is due to the decomposition of precursors [24] as well as depolymerisation and volatilization of organic components in filter paper [25,26].Table 1 shows the values of the initial temperature (Ti), the temperature at which the reaction rate is maximum (Tm), and the final temperature (Tf) of the major decomposition process.It is observed that the Cu/FP has the lowest thermal stability, as it starts to decompose at the lowest temperature when compared with other samples, including the bare filter paper.The decomposition temperature range 453-573 K of Cu(AcO)2 [27] is lower than that 548-613 K of Mg(AcO)2 [28] and 573-873 K of filter paper [29].This may be the reason behind the accelerating decomposition pattern of Cu/FP sample.The exact decomposition pattern can be obtained from the DTG curve, as shown in and the other one in the range 700-750 K corresponds to the gradual decomposition (combustion) [29].
For Cu/FP, there is a series of many steps, probably corresponding to intermediate decomposition product formation processes from 534-588 K [27], as can be clearly observed in the DTG curve (Figure 4(b)).This indicates that initially the copper acetate starts to decompose forming intermediates around 530-574 K.This is then closely followed by the loss of cellulose which happens at around 581 K.We propose that while the cellulose is still decomposing into carbon, this newly formed carbon will take part in the formation of Cu2O.In this process, the carbon will be catalytically consumed by the copper intermediates.Because of the carbon influence in the formation of Cu2O, gradual combustion phase of filter paper is absent in Cu/FP.On the other hand, this phase of gradual combustion is present in both the FP and Mg/FP specimens, which do not have any copper content, corroborating the proposed mechanism.Further increase in the temperature around 590 K, oxidization of Cu2O into CuO occurs, during which mass increases considerably.This is observed in the DTG curve (Figure 4(b)), in agreement with the literature value of 573-673 K [27].
In the case of Mg/FP, the decomposition of magnesium acetate around 599 K followed by the loss of cellulose occurring around 603 K.However, these temperature values are very close and it can be stated that these two events occur almost simultaneously, with a clear precedence for the Mg(AcO)2 decomposition.Here, the gradual decomposition of filter paper is visible around 712 K, as observed in FP (Figure 4(b)).This is because carbon has no interference in the formation of magnesium oxides, as previously discussed in the case of copper containing precursors.From the above discussion, it is clear that Cu and Mg cations can alter the thermal stability of filter paper.Also, the temperature of FP to initiate decomposition i.e. 573 K (from Table 1), is higher than the other samples.This means that before the decomposition of FP, the process of decomposition of acetates to form oxides gets started.This prevents the agglomeration of the newly formed nanostructures [14].

Conclusion
We have synthesised metal oxide nanosheets by virtual substrate method and examined the detailed thermal decomposition kinetics of metal oxide and bare filter paper by TG/DTG analysis.The study indicates that the decomposition temperatures of acetates are less than that of filter paper, which supports the formation of nanosheets and prevents their agglomeration.The DTG analysis shows the involvement of carbon in the formation of Cu2O in copper containing samples while a carbon independent decomposition process is noticed in case of magnesium acetate.It indicates the effects of the substrate on the growth process, which provides a significant insight into the fundamental mechanisms governing the growth of nanosheets on filter paper.

Figure 3 .
Figure 3. FTIR spectrum of S3 (CuO-MgO).Inset graph showing the -OH functional group present at the surface of nanosheets.

Figure 4 .
Figure 4. (a) TG and (b) DTG curves of precursor loaded filter paper and bare filter paper.DTG curve used to examine the major decomposition in the temperature range 520 -880 K.
Fig 4 (b).For filter paper there are two decomposition phases.The one around 593 K corresponds to the loss of cellulose,

Table 1 .
Characteristic temperatures from TG curves