Fabrication of hierarchically mesoporous CuO nanostructures and their role as heterogenous catalysts and sensors

Abstract

Tween-80 templated mesoporous CuO (mpCuO) nanostructures were explored via a facile, environmentally friendly and scalable sol–gel route for heterogeneous catalysis and sensor technology. Silica nanoparticles (Si-NPs) were used as structural directing agents (SDAs) for the shape selective morphological changes of porous materials which resulted in the fabrication of nanoflowers of CuO/Si-NPs along with a great change in surface area. The mpCuO nanostructures were well characterized by FTIR, TGA, PXRD, FESEM-EDX, TEM, BET techniques. Interestingly, the as-synthesized mpCuO nanostructures showed excellent catalytic activity against the direct hydrogenation of acetone to isopropanol in the presence of sodium borohydride (NaBH₄). In addition, mpCuO nanostructures have proved to be excellent sensor materials for simultaneous detection of some metals and biomolecules.

---

1. Introduction

In recent times mesoporous metals and metal oxides have been receiving great attention due to their excellent surface properties like high surface area, large pore volume and well-ordered pore channels. These porous materials can find use in vast applications such as gas adsorption,¹ sensing,² catalyses,^3–6 removal of hazardous ions,^7,8 optoelectronics,^9,10 sustainable energies,^11,12 and many more. These materials are remarkably active as catalysts for various organic transformations, such as acid^13,14 or base-catalyzed reactions,^15,16 redox reactions,^17,18 and size and shape-selective isomerization.¹⁹ The well-ordered mesopores and high aspect ratio of these materials permit organic molecules to access the active sites without facing much diffusional restrictions.²⁰

Carbonyl compound reduction is a fundamental transformation in synthetic organic chemistry and selectivity of hydrogenation reaction is important in both homogeneous and heterogeneous catalytic process.²¹ The specific target in hydrogenation process is unsaturated bonds (C=C, C≡C, or C=O), leaving other unsaturated bonds within the molecule (or in other molecules) unaffected.²² The selective hydrogenation of organic compound also having different functional groups to be hydrogenated is not an easy goal in synthesis of fine chemical.²³ Among different methods, catalytic hydrogenation is most reliable, versatile and environmentally benign method for hydrogenation of carbonyl compounds. Isopropanol is widely regarded as essential commodity in fine chemical synthesis which is used as a solvent in industry and academia.²⁴ Fortunately, isopropanol is in high demand in direct isopropanol (2-propanol) (DIPA) fuel cells because of their high energy production, much lower crossover current, ease to oxidize, production of more hydrogen, and being less toxic than other alcohols such as methanol and ethanol.²⁵ In this regard the production of isopropanol via different methods is demonstrated by researchers since times but the production of isopropanol is expensive, time consuming with toxic by-product formation. Hence, scientists are still quite busy in developing greener methods with high efficiency for the production of isopropanol. Acetone hydrogenation is a cost effective and environmentally benign alternative method for the production of isopropanol because of its easy reduction of C=O to convert into C–OH.^26,27 Although the basic hydrogenation of crude acetone via RANEY® is highly efficient but is useful only when being coupled with cumene process which is complicated multistep process.^26,28 In addition, the production of an explosive intermediate (cumene hydroperoxide) and benzene contamination by the above approach has high impact on the environment.²⁹ The use of other alternatives like metal supported catalysts such as Pd on silica, Au and Pt on Al₂O₃, Pt on SiO, Pt on TiO microspheres and Cu, Pt, Pd and Rh on kieselghur has been reported for the hydrogenation of acetone to isopropanol.^30–33 But this method requires high temperature and complicated experimental set-up which are its big draw backs. Unsupported Pt, Ni, Fe, and W catalysts also have been used for isopropanol production which being efficient has disadvantages of less selectivity hence produces low molecular weight hydrocarbons (C1–C4).^32,34–35 Recently, nanoporous palladium a costly metal has also been used for the hydrogenation of acetone to isopropanol using NaBH₄.²⁸

In this work we have reported the fabrication of nanoporous CuO and CuO/Si-NPs monoliths using nonionic surfactant Tween-80 as porogen by the modified green sol gel route. Silica nanoparticles (Si-NPs) were used to change the morphology and pore dimensions of CuO monolith to increase its surface area which has a direct impact on the heterogeneous catalysis. CuO is an exceptional member of the generally, rocksalt family as it deviates both structurally and electronically from others as one traverses the periodic table from MnO to CuO.³⁶ Unlike other members of the 3d transition oxides (TMO) which crystallizes in the cubic rocksalt structure (with possible rhombohedral distortions),^37–39 tenorite (CuO) crystallizes in the lower symmetry monoclinic (C_62h) crystal structure. The typical electronic arrangement (Fig. S1†) shows Cu with one unpaired electron which is antiferromagnetic and CuO bearing four Cu–O covalent bonds.⁴⁰

To the best of our knowledge, non-ionic surfactant Tween-80 has been first time used for the synthesis of mesoporous CuO monoliths without the involvement of any acid or base at low temperature. The synthesized samples were evaluated as catalysts for the hydrogenation of acetone to isopropanol in presence of sodium borohydride (NaBH₄) and showed an excellent heterogeneous catalytic activity. In addition, the mpCuO monoliths were examined as sensors by electrochemical study and exhibited an excellent sensor activity.

2. Experimental details

2.1 Materials

All chemicals like copper nitrate trihydrate CuNO₃·3H₂O (50 wt%, Sigma Aldrich) as a precursor, soft template (Tween-80, MW = 1310 g, Sigma Aldrich), Ludox [silica nanoparticles Si-NPs (As-40, colloidal silica, 40 wt% suspension in water, Sigma-Aldrich)] were of analytical grade and used as received without further purification. Acetone and NaBH₄ were purchased from CDH, private limited, New Delhi, India. Double distilled water was used to prepare the solutions.

2.2 Synthesis of nanoporous CuO and CuO/Si-NPs

In a typical synthesis 1 g Tween-80 was dissolved in 10 g of water with stirring on a magnetic stirrer at room temperature and in other beaker 1 g Cu(NO₃)₂·3H₂O was dissolved in minimum amount of water (almost 2 ml). After this the solution in first beaker was added slowly in the Cu(NO₃)₂·3H₂O solution. The mixture was stirred on magnetic stirrer for about 40 minutes at 40 °C till light blue sol is formed which on further 20 min stirring resulted in blue gel of CuO/Tween-20. In case of CuO/Tween-80/Ludox catalyst 0.21 g Ludox (4.03 × 10² M, 40 wt% suspension in water) to above protocol separately to CuO/Tween-80 as shown in Scheme 1. The gels were aged for 72 hours at room temperature and then calcined at 650 °C at a heating rate of 10 °C minute⁻¹ followed by cooling at a rate of 10 °C minute⁻¹ to room temperature in an ELITE furnace to get the nanoporous CuO.

Scheme 1 Protocol for the synthesis of CuO and CuO/Si-NPs monoliths.

2.3 Sample characterizations

Characterizations were carried out in two parts. FTIR (Fourier transform infra-red spectroscopy) and TGA (thermogravimetric analysis) were carried out before calcination and other characterizations after calcination. FTIR studies were carried on Shimadzu-8400S spectrometer. TGA curves were obtained on Perkin-Elmer thermal analyzer using alumina reference crucible at the heating rate of 10 °C min⁻¹. X-ray diffraction patters were obtained for information regarding the phases and crystallinity of the resultant powders and the measurements were carried out on Bruckner D-8 advance diffractometer in the diffraction angle range from 2θ = 10–80°, using Cu Kα radiation (λ = 0.154056 Å) at 40 kV and 40 mA. The surface morphology with porous structure were analyzed using fast scanning electron microscopy (NOVA NANOSEM 450) and transmission electron microscopy (TEM-FEI-TECHNAI G2 30S Twin electron microscope). Elemental composition and mapping was determined by energy dispersive X-ray spectroscopy attached to FESEM. BET surface area and pore volume distribution were analyzed by measuring nitrogen adsorption–desorption measurements (Sorptometer ASAP 2026 Micromeritics USA) at 77 K.

2.4 Catalytic activity

The catalytic test of the as synthesized samples was carried out by hydrogenation of acetone. The reaction was worked out by taking 20 ml of mixture containing 5 × 10⁻² mol L⁻¹ acetone and 0.6 × 10⁻³ mol L⁻¹ NaBH₄ into a glass vial in which 0.004 g of synthesized catalyst was added and subjected to stirring at 300 rpm at 308 K temperature. The reaction was studied for 10 minutes. The catalytic effect of the synthesized catalysts for the hydrogenation of acetone was observed by taking almost 3 ml at time intervals of 2 minutes and then transferred to quartz cuvette for recording absorption spectrum. The absorption measurements were obtained on (Systronics UV-vis double beam spectrophotometer, 2201).

Further the isopropanol formation in the acetone hydrogenation was confirmed by gas chromatography. The yield of isopropanol was calculated from the change in acetone concentration using the relation

But to get C₀ and C_f, we first plotted the standard absorption calibration graph for acetone in the concentration range of 0.01–0.1 mol L⁻¹ at its λ_max of 265 nm against water as standard as shown in Fig. 1 and a straight line of y = mx + C was also plotted (inset in Fig. 1).

Fig. 1 Optical absorption spectra graph for acetone with varying concentration (0.01 M–0.1 M). Inset is the standard calibration graph for acetone that is plotted from absorbance.

2.5 Electrochemical study

All the electrochemical measurements were performed on computer controlled NOVA software version 1.10.1.9 Metrohm Autolab PGSTAT128N equipped with a conventional three electrode system consisting of carbon paste modified (CuO NP) and unmodified as working electrode, an Ag/AgCl (saturated KCl) reference electrode and a platinum wire as counter electrode.

2.5.1 Fabrication of modified and unmodified carbon paste electrode (CPE) electrode. A suspension of the as-synthesized materials was prepared in 1 : 1 DMF and acetone. The suspension was sonicated for 15 minute followed by centrifugation at 1500–2000 rpm. The supernatant was decanted off and the remaining material was added with appropriate amounts of graphite powder and 5 ml of 1 : 1 solvent solution (DMF and acetone). The solution was again sonicated for 15 minutes. The excess solvent was evaporated and the remaining mixture obtained was graphite powder and synthesized material. A paste was formed by adding paraffin oil to the above obtained mixture. This paste was filled in polyethylene syringes (2 mm diameter) having a pre-inserted Cu wire for external contact. The paste was mechanically pressed from the top to obtain new surfaces for each measurement. The smoothing of surface was achieved by rubbing on a smooth, clean, weighing paper and washed with doubly distilled water prior to each measurement. The unmodified CPE was prepared by the same procedure without addition of synthesized material.

3. Results and discussion

3.1 FTIR and TGA

FTIR and TGA of as synthesized samples were carried out before calcination. FTIR analysis was carried out to confirm the complexation of the metal precursor and Tween-80. FTIR spectrum of Tween-80 as shown in Fig. 2A(a) shows typical peaks at 2900 cm⁻¹, 1730 cm⁻¹ and strong absorption peak at 3500 cm⁻¹ which correspond to symmetric stretching of methylene (–CH₂), C=O stretching of ester, OH stretching respectively. Further the peaks at 1647 cm⁻¹ and 1470 cm⁻¹ corresponds to deformation vibration of interlayer water and symmetric C–H vibration of CH₃.^41,42 The presence of prominent peak near 1100 cm⁻¹ can be assigned to (–CO–O–CH₂). The presence of above assigned peaks in the synthesized samples CuO and CuO/Si (Fig. 2Ab and c) confirms the presence of Tween-80 which suggests the complexation of metal precursor and surfactant Tween-80. For comparison we also recorded the FTIR spectrum of calcined sample with uncalcined ones. As shown in Fig. 2A(d) there is appearance of no prominent peaks upto 1200 cm⁻¹ which are due to surfactant Tween-80 which is completely removed on calcination.

Fig. 2 (A) Fourier infra red (FTIR) spectroscopic studies of as synthesized samples before and after calcination (a) Tween-80 (b) CuO/Tween-80 (c) CuO/Tween-80/Si-NPs (d) calcined sample. (B) TGA profiles of the synthesized samples before calcination.

The thermogravimetric analysis of as synthesized catalysts is also carried out before calcination to understand the process of weight losses during calcination. The TGA depicts weight loss percentage with increase in temperature. Fig. 2B is the thermogram of as synthesized samples. The thermogram depicts the weight loss of 23–25% below 150 °C corresponding to physically absorbed water.⁴³ The weight drop of almost (8–10%) from 150 to 190 °C corresponds to decomposition of Cu(II) nitrate to CuO.^40,44,45 Further weight loss upto the temperature of 380 °C can be attributed to the thermal degradation of surfactant leaving 20–23% of CuO and CuO/Si behind respectively. Silica nanoparticles are not removed upto 650 °C which was later on also confirmed by EDAX analysis.

3.2 PXRD and EDAX analysis

PXRD was carried out to get the composition and phase purity of as synthesized samples. As shown in Fig. 3 diffraction patterns revealed that the calcined samples depict diffraction peaks which are indexed to (110), (002), (200), (202), (020), (202), (113), (311), (220), (311) and (004) planes of monoclinic CuO (JCPDS no. 4.783). The PXRD results further confirm the high crystallinity of the synthesized samples. The diffraction peak at 2θ value of 20° can be attributed to the presence of silica nanoparticles in CuO/Si-NPs. In addition write the heading PXRD and EDAX analysis as PXRD study. The dominant peaks between 2θ values of 20° and 80° clearly indicate that pure monoclinic CuO is formed with no characteristic peak of impurities like Cu₂O and Cu(OH)₂.⁴⁶ The crystallite size of the catalysts was calculated from the PXRD characteristic peak at 35.7° (002) (crystallite size given in Table 1) by using Scherrer formula (eqn (2)) where shape factor (K) equal to 0.9 (Spurr and Myers 1957;⁴⁷ Cullity and Stock 2001),⁴⁸ λ correspond to wavelength of Cu Kα (0.15406 nm) and W full width half maximum (FWHM) at reflection maximum.

Fig. 3 PXRD pattern of calcined samples.

Table 1 Physicochemical properties of synthesized catalysts

Catalyst	XRD crystallite size (nm)	TEM crystallite size (nm)	d spacing (nm)	Surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)	Pore diameter (nm)
CuO	24.3	26.1	0.22	25	0.321	35
CuO/Si-NPs	08.4	09.8	0.23	86	0.423	10

---

3.3 Morphology study

The morphological study was carried out by taking FESEM and TEM analysis to observe the difference between CuO and CuO/Si-NPs. From the FESEM images it is quite clear that the use of Tween-80 has led to the formation of nano-scale ligament like surface of CuO as clear from Fig. 4a. The surface of CuO shows the formation of pores with average pore size belongs to mesopore range as given in Table 1, hence mesoporous CuO was formed using Tween-80 surfactant as reducing agent. As use of Tween-80 surfactant has led to the formation of ligament shaped structure but when Si-NPs were used in addition to surfactant, flower type CuO was formed as shown in Fig. 4b. The formation of nanoflowers of CuO using Tween-80 and silica nanoparticles are being reported first time here. The use of silica nano particles not only changed the morphology but also decreased the pore size to the minimum. The average pore diameter of CuO/Si-NPs is given in (Table 1). The presence of silica was confirmed by EDAX as given in Fig. 4d. Elemental mapping of CuO and CuO/Si-NPs was also done as shown in the insets of respective EDAX.

Fig. 4 Field emission scanning electronic microscopic images of (a) CuO (c) CuO/silica-NPs. (b) EDAX showing presence of Cu and O in CuO (d) EDAX Cu, O and Si-NPs in CuO/Si-NPs. Insets in respective EDAX show elemental mapping.

The morphology of the synthesized mesoporous copper oxides was further investigated by TEM. The TEM images Fig. 5(a and d) depicted ligament and nanoflower like shapes clearly which is in close agreement with their FE-SEM results. The high-resolution (HR-TEM) image Fig. 5(b and e) of CuO and CuO/Si-NPs exhibit the interplanar distances of 0.25 nm and 0.22 nm respectively which are same as d spacing obtained by PXRD corresponding to (202) plane. The clearly visible fringes in the respective HRTEM images are the indicative of high crystalline nature of synthesized copper oxides. Also from the TEM results it was observed that the crystallite size decreased as Si-NPs were added to CuO (Table 1). The selected area electron diffraction (SAED) patterns (Fig. 5c and f) of the samples calcined at 650 °C were indexed to different planes of monoclinic CuO with polycrystalline nature. The insets in the respective (HRTEM) results are FFT pattern of the samples.

Fig. 5 (a) TEM image of CuO (b) HRTEM image of CuO (c) SAED pattern of CuO (d) TEM image of CuO/silica-NPs. (e) HRTEM of CuO/silica-NPs. (f) SAED pattern of CuO/silica-NPs. Insets in respective SAED pattern shows the corresponding FFT pattern of samples.

3.4 N₂ adsorption/desorption isotherms and BET surface area measurement

The inner porous nature of CuO catalysts was further analyzed by nitrogen adsorption/desorption measurements. The nitrogen adsorption/desorption isotherms and the pore size distribution curves of ligament and nanoflower shaped porous architectures of CuO are given in Fig. 6(a and b). The surface area of the catalysts was measured by BET (Brunauer–Emmett–Teller) method and pore size distribution determined from pore volume distribution curves (given in insets in respective isotherms) using BJH (Barrett–Joyner–Halenda) method.

Fig. 6 BET isotherms (a) CuO (b) CuO/silica-NPs. Insets in isotherms show the pore size distribution of respective samples.

As seen from (Fig. 6a and b) the sorption isotherms of CuO and CuO/Si-NPs exhibit type IV isotherm of IUPAC classification with hysteresis loop which mostly correspond to mesoporous nature of materials. The use of silica nanoparticles has resulted increase both in surface area and pore volume with profound decrease in pore diameter as compared to CuO. The surface area increases to 86 m² g⁻¹ in CuO/Si-NPs as compared to 25 m² g⁻¹ in CuO. From the BET results it is clear that the surface area showed an increase with decrease in crystallite size (Table 1) and also there is profound change in pore volume with increase in surface area. Further the pore diameter is 10 nm in nanoflowers of CuO/Si-NPs as compared to 24 nm in CuO which well matches with the FESEM results. The respective pore volumes are given in Table 1.

4. Catalytic hydrogenation of acetone

Hydrogenation of acetone to isopropanol was carried out to evaluate the catalytic properties of as synthesized porous CuO nanostructures. In a typical reaction 14.7 ml 0.05 M acetone and 0.3 ml 0.06 M NaBH₄ were mixed in a glass vial and the reaction progress was monitored by UV-vis spectrophotometry in the presence and absence of synthesized mesoporous CuO catalysts. The decrease in the absorbance peak of acetone at 265 nm confirms the conversion of acetone to isopropanol. The hypo- and hypsochromic effect revealed a reduction of concentration of acetone and selective formation of isopropanol, respectively, by sodium borohydride.^33,49

The reaction was mainly carried out under the following cases (i) optimization of NaBH₄ concentration used for hydrogenation of acetone in absence of catalyst (ii) using the optimized concentration of NaBH₄ for hydrogenation of acetone in the absence and presence of catalysts (iii) effect of catalyst doses and (iv) reusability of the catalyst. Hydrogenation of acetone was carried out by varying the concentration of NaBH₄ from 0.1 mM to 2 mM. It can be seen from the UV-vis spectra of acetone under variation of NaBH₄ concentration as shown in Fig. 7a that there is a drastic decrease in the absorbance of acetone at 265 nm when concentration of NaBH₄ increases from 0.1 to 0.6 mM and after that the decrease in concentration is not so profound. The inset in the Fig. 7a shows the percentage yield of isopropanol formed on varying the concentration of NaBH₄. It is seen that yield at NaBH₄ of 0.6 mM is as high as 28% and further increases only 2–3% on increasing the concentration of NaBH₄ even upto 2 mM. This limited increase in the yield on increasing the NaBH₄ concentration despite NaBH₄ being the hydrogen source in the hydrogenation of acetone can be associated with the steric effect arising due to oversaturation of NaBH₄ that restricts further hydrogenation.²⁸ Therefore, to increase the yield of isopropanol to very high the catalyst is required in the hydrogenation of acetone inpresence of optimized concentration of NaBH₄ (0.6 mM).

Fig. 7 (a) UV-vis spectra for hydrogenation of acetone with varying concentration of NaBH₄ at fixed concentration of acetone (0.05 M) in absence of catalyst. Inset shows the percentage yield of isopropanol with varying concentration of NaBH₄ (b) UV-vis spectra of hydrogenation of acetone using 0.06 mM L⁻¹ NaBH₄ with 0.05 M acetone in absence of catalyst. Inset shows the percentage yield of isopropanol with 0.06 mM L⁻¹ of NaBH₄.

Next the reaction was carried out in absence of catalyst using 0.6 mM NaBH₄ with stirring for 10 minutes. As shown in the Fig. 7b, the absorbance peak at 265 nm decreases gradually with increase in reaction time and from this absorbance spectrum of acetone percentage yield of isopropanol was calculated and found to be only 30%. The same reaction was also carried out inpresence of synthesized CuO mesoporous catalysts (0.004 g). The comparison between the catalytic potentiality of CuO and CuO/Si-NPs was studied by using the catalysts separately in the above reaction. As shown in Fig. 8a, in presence of CuO, the gradual decrease in the absorbance peak at 265 nm with reaction time is much higher as compared to the blank reaction and the isopropanol formation was found to be increased to 50% and when the same reaction was carried out using nanoflowers of CuO/Si-NPs, the isopropanal yield calculated from the absorbance graph Fig. 8b was increased to 65%. This high yield percentage of isopropanol formation in 10 minutes clearly shows the excellent catalytic activity of nanoflowers of CuO/Si-NPs that of CuO. The higher catalytic performance of nanoflowers of CuO/Si-NPs may be due the binding of silica on the surface of CuO which results in high surface area as heterogeneous catalysis mainly depends on surface area. The catalytic performance comparison of CuO/Si-NPs catalyst for the hydrogenation of acetone with already reported results in literature is given in Table 2.

Fig. 8 (a) UV-vis spectra of hydrogenation of acetone using 0.06 mM L⁻¹ NaBH₄ with 0.05 M acetone in presence of CuO. Inset shows the percentage yield of isopropanol in reaction time of 10 minutes. (b) UV-vis spectra of hydrogenation of acetone using 0.06 mM L⁻¹ NaBH₄ with 0.05 M acetone in presence of CuO/Si-NPs. Inset shows the percentage yield of isopropanol.

Table 2 Comparison of catalytic activity for hydrogenation of acetone over different catalysts

Sample	Catalyst	Mass of catalyst	Temperature (°C)	Conversion (%)	Selectivity (%)	Reference
a Temperature is used along with hydrogen gas purging.b NaBH4 is used as hydrogen source.
1	NiO, CoO4	0.5 g	200	50	100	33^a
2	Co/Al2O3	0.5 g	200	64	80	33^a
3	RANEY®	2.9 g	80	99.2	100	55^a
4	RANEY®	7.8 g	120	82.5	99	55^a
5	CuO/Si-NPs	0.01 g	35	95	100	Present work^b

---

4.1 Plausible mechanism

The high and selective hydrogenation of acetone to isopropanol using NaBH₄ as hydrogen source reported in this paper can be attributed to the high porosity, flower shape and large surface area of CuO/Si-NPs. In broader sense, the hydrogenation of acetone is an adsorption phenomenon in which H from NBH₄ and acetone adsorbs on the catalyst surface which leads to conversion of ketone to enol form and finally desorbs isopropanol. It is assumed that the Cu–O distance for adsorbed acetone is the van der Waals separation of 2.93 Å and would appear to be a reasonable estimate for the Cu–O separation that suggests the molecule interacts only weakly with the substrate and is not strongly chemisorbed.⁵⁰ Hence, the product is desorbed easily as the temperature is increased. However, the plausible mechanism of reaction proceeds through hydride ion mechanism as shown in Scheme 2. Initially, the reactants acetone and NaBH₄ adsorb on the CuO surface to form a partial metal–NaBH₄ and metal–carbon (acetone) bond.⁵¹ Then the hydride ions (H⁻) from NaBH₄ form bond with carbonyl carbon partially attached to surface of catalyst. Finally, lone pairs on oxygen (O) attacks proton (H) of water (H₂O) to form isopropanol which leads to the desorption of isopropanol from the catalyst surface and leaving vacant space for the adsorption of new molecules of acetone.⁵² This hydrogenation reaction of acetone in presence of synthesized porous catalysts is very fast and completes just within 10 minutes.

Scheme 2 Plausible mechanism for the hydrogenation of acetone using NaBH₄ with CuO as catalyst.

4.2 Effect of catalyst dose and reusability

From the above studies, it is clear that among the synthesized catalysts CuO/Si-NPs nanoflowers showed excellent catalytic activity, so we did further studies by using CuO/Si-NPs.

Inorder to increase the percentage yield of isopropanol we investigated the effect of increase in catalyst dose of CuO/Si-NPs. As we increased the catalyst dose the percentage yield of isopropanol increased linearly as given in Fig. 9a and that can be directly attributed to the higher number of active sites available as the concentration of catalyst is increased. So the optimized catalyst dose of CuO/Si-NPs where the complete hydrogenation of acetone occurs and isopropanol yield reaches to 95% is 0.01 g. To check further superiority of the synthesized catalyst we carried out the recyclability test of the used catalyst (CuO/Si-NPs). In the test, the used catalyst was separated with centrifugation, washed with distilled water, dried and employed for next cycle. From the Fig. 9b it is clear that the catalyst can be used again and again upto 6^th cycle with meager loss in activity. Since the catalyst is solid can be easily separated on low rpm centrifugation and can be used again and again.

Fig. 9 (a) Effect of CuO/Si-NPs concentration on the percentage yield of isopropanol at a fixed concentration of acetone (0.05 M) and NaBH₄ (0.06 mM). (b) Reusability of CuO/Si-NPs for the conversion of acetone to isopropanol, acetone (0.05 M) and NaBH₄ (0.06 mM).

5. Surface study

The area of both unmodified and modified CPE were determined by performing cyclic voltammetric measurements using 1.0 mM K₃Fe(CN)₆ probe in 0.1 M KCl electrolyte at different scan rates as shown in Fig. 10. The surface area was calculated from the slope of the plot of I_pa versus ν^1/2 taken from Randles–Sevcik equation.^53,54where I_pa is anodic peak current, n is number of electrons transferred, A₀ is surface area of electrode, D₀ is diffusion coefficient and C is concentration of K₃Fe(CN)₆ respectively, ν is scan rate. For 1.0 mM K₃Fe(CN)₆ in 0.1 M KCl at T = 298 K, n = 1 and D₀ = 7.6 × 10⁻⁶ cm² s⁻¹. In this study the surface area of bare CPE was calculated to be 0.081 cm² and that of modified CPE was found to be 0.124 cm² for CuO/CPE and 0.430 cm² for CuO/Si-NPs/CPE.

Fig. 10 Cyclic voltammogram of 1 × 10⁻³ M K₃Fe(CN)₆ probe in 0.1 M KCl electrolyte (a) bare CPE (b) CuO/CPE (c) CuO/Si-NPs/CPE electrode.

5.1 Simultaneous electrochemical analysis of metal ions and bio-molecules

The sensor activity of synthesized porous materials was performed by electrochemically analyzing some metal ions like cadmium (Cd), lead (Pb), zinc (Zn) and biomolecules like ascorbic acid (AA) and uric acid (UA).

To explain the electrochemical behavior of modified electrodes, DP voltammograms of 4 × 10⁻⁶ mol L⁻¹ of Cd, Pb and Zn and 5 × 10⁻⁶ mol L⁻¹ of AA and UA of 1 × 10⁻⁶ mol L⁻¹ on the surface of bare CPE, CuO/CPE and CuO–Si-NPs/CPE in phosphate buffer solution (pH = 4.0 ± 0.05) were recorded. Fig. 11A shows the DP voltammograms of CPE and modified CPE in phosphate buffer solution containing 4 × 10⁻⁶ mol L⁻¹ Cd, Pb and Zn solutions. At the bare CPE, the oxidation peaks of the three metal ions are not well defined and well resolved, hence simultaneous determination is not so possible as given in Fig. 11A(a). However, at modified CPE's, the peaks for the corresponding metal ions were well separated with much increase in peak current and on adding flower shaped CuO/Si-NPs, the peak current showed a remarkable increase in comparison with CPE and CuO/CPE (Fig. 11A(b and c)) and the peaks were well resolved.

Fig. 11 DP voltammograms (A) for the simultaneous determination of metal ions using bare and modified CPE (a) CPE (b) CuO/CPE (c) CuO/Si-NPs/CPE. (B) For the simultaneous determination of bio-molecules using bare and modified CPE (a) bare CPE (b) CuO/CPE (c) CuO/Si-NPs/CPE.

Similarly, as above the simultaneous determination of AA and UA using CuO and CuO/Si-NPs modified CPE is quite interesting in comparison to bare CPE. As can be seen from Fig. 11B, the DPV of AA and UA using CPE and modified CPE, the peak current showed a maximum value on using modified CuO/Si-NPs/CPE and displayed two separate peaks for AA and UA. The use of CuO/Si-NPs modified CPE as electrochemical sensor with better sensitivity and selectivity was evaluated by performing DVP measurements for simultaneous determination of AA and UA at different concentrations (Fig. S2A and B†). The limit of detection (, where s is standard deviation of calibration plot and m is the slope of calibration plot) where evaluated from the calibration plot (inset in Fig. S2A and B†) and compared with reported literature as presented in Table 3. The results show that the sensitivity of the modified electrode is superior to previously reported electrode materials which is reflected by the lower detection limits obtained using CuO/Si-NPs–CPE.

Table 3 Comparison of detection limits and linear ranges of different modified electrodes for the determination of AA and UA with the present study

Analyte	Electrode material	Linearity range (μM)	Correlation coefficient	LOD (μM)	Ref.
AA	Chitosan–graphene GCE	50–1200	—	50.0	56
UA		2–65	—	2.0
AA	Poly(caffeic acid) GCE	20–1000	0.998	7.0	57
UA		5–300	0.997	0.6
AA	Poly EBT GCE	150–1000	0.991	10	58
UA		10–130	0.993	1.0
AA	Pd/CNF-CPE	50–4000	—	15	59
UA		2–200	—	0.7
AA	Pt–Au hybrid	103–165	—	103	60
UA		21–336	—	21
AA	ZnO/RM	15–240	—	1.4	61
UA		50–800	—	4.5
AA	NG/GCE	5–1300	—	2.2	62
UA		0.1–20	—	0.045
AA	CuO/Si-NPs CPE	10–100	0.996	3.340	Present work
UA		0.1–1.0	0.998	0.115

---

Hence, from the above study it is clear that the synthesized porous materials showed excellent electro-catalytic activity for the electrochemical analysis of metals and biomolecules. The remarkable results obtained for CuO/Si-NPs can be attributed to its high surface area and flower like shape.

6. Conclusion

The design of mpCuO nanostructures and their corresponding morphologies have been explored successfully via modified green sol gel route. The conversion of acetone to isopropanol within 10 minutes (95% yield) is the attribution of enhanced surface area of mpCuO by the addition of Si-NPs and disseminating the great attraction of the present work for industrial applications. The electrochemical results obtained reveal that the mpCuO/Si-NPs modified CPE exhibited high electro-catalytic activity towards the analysis of metal ions and biomolecules, promising a great potential of the mpCuO–SiNPs monoliths for the environmental remediation, electrocatalysis and energy storage devices etc.

Acknowledgements

The authors acknowledge UGC and DST New Delhi, for financial assistance. We are also thankful to SIL and Department of Chemistry, Dr H. S. Gour University, Sagar for providing instrumental facilities.

References

1. S. Choi, J. H. Drese and C. W. Jones, ChemSusChem, 2009, 2, 796–854.
2. M. Pashchanka, R. C. Hoffmann, A. Gurlo and J. J. J. Schneider, Mater. Chem., 2010, 20, 8311–8319.
3. A. Corma, Chem. Rev., 1997, 97, 2373–2419.
4. I. Tamiolakis, I. N. Lykakis, A. P. Katsoulidis, M. Stratakis and G. S. Armatas, Chem. Mater., 2011, 23, 4204–4211.
5. S. Dutta, S. De, A. K. Patra, M. Sasidharan, A. Bhaumik and B. Saha, Appl. Catal., A, 2011, 409, 133–139.
6. Z. Z. Wei, D. C. Li, X. Y. Pang, C. Q. Lv and G. C. Wang, ChemCatChem, 2012, 4, 100–111.
7. D. Chandra, S. Mridha, D. Basak and A. Bhaumik, Chem. Commun., 2009, 17, 2384–2386.
8. A. K. Patra, A. Dutta and A. Bhaumik, J. Hazard. Mater., 2012, 201, 170–177.
9. S. Nejati and K. K. S. Lau, Nano Lett., 2011, 11, 419.
10. Q. Li, L. Zeng, J. Wang, D. Tang, B. Liu, G. Chen and M. Wei, ACS Appl. Mater. Interfaces, 2011, 3, 1366–1373.
11. Y. Ren, L. J. Hardwick and P. G. Bruce, Angew. Chem., Int. Ed., 2010, 49, 2570–2574.
12. G. J. A. A. Soler-Illia and O. Azzaroni, Chem. Soc. Rev., 2011, 40, 1107–1150.
13. M. Nandi, J. Mondal, K. Sarkar, Y. Yamauchi and A. Bhaumik, Chem. Commun., 2011, 47, 6677–6679.
14. A. E. R. S. Khder, H. M. A. Hassan and M. S. El-Shall, Appl. Catal., A, 2012, 411, 77–86.
15. P. F. Fulvio, R. I. Brosey and M. Jaroniec, ACS Appl. Mater. Interfaces, 2010, 2, 588–593.
16. N. Kumar, E. Leino, P. Maki-Arvela, A. Aho, M. Kaldstrom, M. Tuominen, P. Laukkanen, K. Eranen, J.-P. Mikkola, T. Salmi and D. Y. Murzin, Microporous Mesoporous Mater., 2012, 152, 71–77.
17. S. E. Dapurkar, A. Sakthivel and P. Selvam, New J. Chem., 2003, 27, 1184–1190.
18. S. Dzwigaj and M. Che, Catal. Today, 2011, 169, 232–241.
19. A. Dhakshinamoorthy, M. Alvaro, H. Chevreau, P. Horcajada, T. Devic, C. Serre and H. Garcia, Catal. Sci. Technol., 2012, 2, 324–330.
20. A. K. Patra, A. Dutta and A. Bhaumik, ACS Appl. Mater. Interfaces, 2012, 4, 5022–5028.
21. M. T. Shah, A. Balouch, K. Rajar, Sirajuddin, I. A. Brohi and A. A. Umar, ACS Appl. Mater. Interfaces, 2015, 7, 6480–6489.
22. J. Silvestre-Albero, G. Rupprechter and H.-J. Freund, J. Catal., 2005, 235, 52–59.
23. O. Domínguez-Quintero, S. Martínez, Y. Henríquez, L. D'Ornelas, H. Krentzien and J. Osuna, J. Mol. Catal. A: Chem., 2003, 197, 185–191.
24. A. Rahman, Bull. Chem. React. Eng. Catal., 2010, 5, 113–126.
25. D. X. Cao and S. H. Bergens, J. Power Sources, 2003, 124, 12–17.
26. A. J. Papa, Propanols, in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, Germany, 2005.
27. P. Sumodjo, E. da Silva and T. Rabockai, J. Electroanal. Chem. Interfacial Electrochem., 1989, 271, 305–317.
28. A. Balouch, A. A. Umar, A. A. Shah, M. M. Salleh and M. Oyama, ACS Appl. Mater. Interfaces, 2013, 5, 9843–9849.
29. S. B. Jing, Z. L. Wang, W. C. Zhu, J. Q. Guan and G. J. Wang, React. Kinet. Catal. Lett., 2006, 89, 55–60.
30. L. Gandia, A. Diaz and M. Montes, J. Catal., 1995, 157, 461–471.
31. L. M. Gandía and M. Montes, J. Mol. Catal., 1994, 94, 347–367.
32. B. Sen and M. A. Vannice, J. Catal., 1988, 113, 52–71.
33. S. Narayanan and R. Unnikrishnan, J. Chem. Soc., Faraday Trans., 1998, 94, 1123–1128.
34. A. Farkas and L. Farkas, J. Am. Chem. Soc., 1939, 61, 3396–3401.
35. C. Stoddart and C. Kemball, J. Colloid Sci., 1956, 11, 532–542.
36. C. E. Ekuma, V. I. Anisimov, J. Moreno and M. Jarrell, Eur. Phys. J. B, 2014, 87,  DOI:10.1140/epjb/e2013-40949-5.
37. L. F. Mattheiss, Phys. Rev. B: Condens. Matter Mater. Phys., 1972, 5, 290–306.
38. K. Terakura, T. Oguchi, A. R. Williams and J. Kubler, Phys. Rev. B: Condens. Matter Mater. Phys., 1984, 30, 4734–4747.
39. W. A. Harrison, Phys. Rev. B: Condens. Matter Mater. Phys., 2007, 76, 054417.
40. G. Naikoo, R. A. Dar and F. Khan, J. Mater. Chem. A, 2014, 2, 11792–11798.
41. E. Nyankson, O. Olasehinde, V. T. John and R. B. Gupta, Ind. Eng. Chem. Res., 2015, 54, 9328–9341.
42. T. K. Jain, M. Varshney and A. Maitra, J. Phys. Chem., 1989, 93, 7409–7416.
43. M. U. D. Sheikh, G. A. Naikoo, M. Thomas, M. Bano and F. Khan, J. Sol-Gel Sci. Technol., 2015, 76, 572–581.
44. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 1992, 359, 710–712.
45. F. Khan and S. Mann, J. Phys. Chem. C, 2009, 113, 19871–19874.
46. Y. X. Zhang, M. Huang, M. Kuang, C. P. Liu, J. L. Tan, M. Dong, Y. Yuan, X. L. Zhao and Z. Wen, Int. J. Electrochem. Sci., 2013, 8, 1366–1381.
47. R. A. Spurr and H. Myers, Anal. Chem., 1957, 29, 760–762.
48. B. D. Cullity and S. R. Stock, Elements of X-ray diffraction, Prentice Hall, Upper Saddle River, 3^rd edn. 2001.
49. T. M. Yurieva, Catal. Today, 1999, 51, 457–467.
50. S. M. Johnston, A. Mulligan, V. Dhanak and M. Kadodwala, Sur. Sci.e, 2004, 548, 5–12.
51. A. Shokrolahi, A. Zali and M. H. Keshavarz, Green Chem. Lett. Rev., 2011, 4, 195–203.
52. G. Guella, C. Zanchetta, B. Patton and A. Miotello, J. Phys. Chem. B, 2006, 110, 17024–17033.
53. J. C. Abbar and S. T. Nandiberwoor, In. Eng. Chem. Res.h, 2011, 51, 111–118.
54. U. J. Pandit, I. Khan, S. Wankar, K. K. Raj and S. N. Limaye, Anal. Methods, 2015, 7, 10192–10198.
55. K. S. P. M. Hayes, J. W. Mitchell, A. Niak and M. G. Turcotte, Hydrogenation of Acetone, US Pat., 7041857B1, May 9, 2006.
56. D. Han, T. Han, C. Shan, A. Ivaska and L. Niu, Electroanalysis, 2010, 22, 2001–2008.
57. W. Ren, H. Q. Luo and N. B. Li, Biosens. Bioelectron., 2006, 21, 1086–1092.
58. H. Yao, Y. Sun, X. Lin, Y. Tang and L. Huang, Electrochi.a Acta, 2007, 52, 6165–6171.
59. J. Huang, Y. Liu, H. Hou and T. You, Biosens. Bioelectron., 2008, 24, 632–637.
60. S. Thiagarajan and S. M. Chen, Talanta, 2007, 74, 212–222.
61. C. F. Tang, S. A. Kumar and S. M. Chen, Anal. Biochem., 2008, 380, 174–183.
62. Z. H. Shenga, X. Q. Zheng, J. Y. Xu, W. J. Bao, F. B. Wang and X. H. Xia, Biosens. Bioelectron., 2012, 34, 125–131.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra07307g

---