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Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Zn2+ Induced Self-Assembled Growth of Octapodal CuxO−ZnO Microcrystals: Multifunctional Applications in Reductive Degradation of Organic Pollutants and Nonenzymatic Electrochemical Sensing of Glucose Siddarth Jain,† Suryakant Mishra,‡,§ and Tridib K. Sarma*,† Discipline of Chemistry, ‡Discipline of Physics, Indian Institute of Technology Indore, Simrol, Khandwa Road, Indore, Madhya Pradesh-453552, India

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S Supporting Information *

ABSTRACT: Development of solution based synthetic methodology for the controlled growth of hyperbranched oxide nanomaterials is important for practical applications in nanotechnology. A binary metal oxide nanocomposite CuxO− ZnO with octapod microstructures was fabricated in a one pot hydrothermal method using a water−polyethylene glycol mixture as a solvent. The highly oriented growth of the microstructures emerged from controlled mixing of the precursor metal salts without any involvement of external shape-directing agents. An investigation into the organized conformation and geometric architectural evolution revealed that the reaction temperature and concentration ratio of the precursor metal salts played critical roles on the morphology of the composite structure. The obtained octapod architecture was characterized by various spectroscopic as well as microscopic techniques. A time dependent study demonstrated the evolution of octapod morphology from self-assembled cubic seeds in a template-free pathway. The hyperbranched mixed metal oxide composite showed effective catalytic activity for the reduction of 4-nitrophenol with a rate constant (kapp) of 2.1 × 10−2 s−1 and an activity factor (K) of 2100 s−1 g−1. Additionally, the composite oxide material showed high efficiency toward the reduction of common organic dye pollutants under ambient condition. Moreover, the CuxO−ZnO octapodal microcrystals showed excellent activity toward electrocatalytic glucose oxidation with a sensitivity of 2091 μA/mmol/cm2 when a potential of 0.6 V was applied. The high efficiency of the microparticles in these catalytic applications could be attributed to the coexistance of CuxO and ZnO phases in the matrix and the synergistic electronic effect owing to the formation of heterojunctions, which allowed efficient movement of holes and free electrons. The template-free structural evolution of the CuxO−ZnO materials into octapodal geometry through a simple hydrothermal pathway might encourage promising applications for sensing and organic decontamination. KEYWORDS: Octapodal microstructures, Polyol, mixed metal oxides, Dye degradation, Glucose sensing

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INTRODUCTION In the rapidly expanding area of nanocatalysis, development of highly efficient catalytic systems through size, shape and composition selective synthesis of nanoparticles is a fundamental issue. Especially, nanoparticles with defined shapes, often decorated with exposed surface planes of the crystal facets, are known to have a favorable impact on the activity and selectivity of the nanocatalyst.1−3 Shape controlled self-assembled growth of ordered micrometer sized superstructures is considered as an important tool for the creation of greater versatility for potential applications in optics, electronics, sensing, photovoltaics, nanomechanics and catalysis etc.4−9 The enhanced functionality of these self-assembled systems can be attributed to the increased structural complexity originated from an assembly of nanocrystals with ordered hierarchical structures. Highly branched, multipodal nano© XXXX American Chemical Society

architectures are of particular interest for catalysis and sensing applications and the presence of a large number of exposed edges and corners ensures enhanced specific surface area resulting in higher activity. Various strategies have evolved toward the synthesis of such highly branched metallic and metal oxide structures that includes tripod, tetrapod, octapod, and other multipod nanocrystals.10−17 Several classes of nanomaterials including PbS, CdS, CdSe, CdTe, PbSe, Pt, FePt, Cu2O, and Fe3O4 have been reported to form nanocrystals with higher number of branches.18−25 A high degree of thermodynamic and/or kinetic control is required for the formation of these multibranched structures from a starting Received: February 21, 2018 Revised: June 15, 2018 Published: June 26, 2018 A

DOI: 10.1021/acssuschemeng.8b00838 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

Herein, the morphological evolution of CuxO−ZnO microparticles with octapodal geometry through a facile hydrothermal method involving a polyol mediated synthesis is reported.48,49 These hierarchical superstructures were obtained simply by controlling the precursor Cu2+ and Zn2+ metal salts and reaction temperature without additional structure directing reagents. In absence of any Zn2+ salts, CuxO formed recrystallization induced mesoporous cubical microstructures. Octapodal CuxO−ZnO microcrystals with enhanced surface area were obtained by the simple addition of a controlled amount of Zn2+ salts into the reaction system, thereby confirming the critical role of dopant ions in the rate and direction of CuxO crystal growth. A systematic time-dependent morphological evolution study was carried out to understand the Zn2+ ion directing growth mechanism of the multipodal CuxO−ZnO structures. The catalytic efficacy of the obtained microstructures was investigated for the reductive degradation of 4-nitrophenol (4-NP) and commonly available organic dyes such as methylene blue (MB), rhodamine B (RhB), and methyl orange (MO). These dyes act as major pollutants in industrial and agricultural wastewater, soil, air, and effluents. Further, a high performance nonenzymatic glucose sensor could be developed by using CuxO−ZnO multipodal microparticles as electrocatalyst, taking advantage of their large surface area and effective electron transfer on the surface. The enhanced catalytic efficiency could be appropriated to the formation of a well-controlled p−n heterojunctions during the evolution of the microcrystals, forming highly effective charge transferable interfaces. The evolution of Zn2+ ion induced hierarchical octapodal CuxO−ZnO microstructures are expected to bring a new shape evolution pathway for multimetallic oxide superstructures with optimal control of the interfacial properties for multidimensional applications.

nanocrystal seed, which involves rapid growth along the [111] directions as compared to other planes. Selective etching and template directed growth are the most common methods employed to control the surface reactivity inducing directional growth, leading to these superstructures. However, due to certain limitations associated with the template directed approaches, such as material compatibility and process complexities, template-free methods working under “one-pot” conditions are highly attractive. Doping of transition metal oxide nanostructures with other compatible metal salts is a common method for the enhancement of activity in various applications. The introduction of defects through dopant incorporation often alters the electronic, optical, and magnetic properties of the semiconductors that can be harnessed for potential applications.26−32 Copper oxides (Cu2O and CuO) have shown high chemical activity for various applications including photocatalysis, photovoltaic power generation, gas sensors, lithium ion batteries, electrocatalysis, etc.33−36 Subsequently, CuxO micro/nanostructures with controlled shape have been generated through various methods to obtain materials with improved performances. For example, recrystallization induced self-assembly was used for the formation of Cu2O superstructures, where the crystallization into various shapes was controlled by balancing the rate of hydrolysis and recrystallization.37 Cu2O multipod frameworks could be generated in an ethanol−water mixture in the presence of formic acid.24 It is well-known that the electronic properties of a semiconductor such as CuxO can be tuned easily by doping with other metals. Zn2+ ions can be easily integrated into a CuxO lattice, as the radius of Zn2+ ion closely matches to that of Cu+. Therefore, several studies have been carried out, both theoretically and experimentally, to realize the potential formation of defects such as vacancies in CuxO, while doped with Zn2+ and the subsequent changes in electronic characteristics.38,39 Along with the compositional variation while doping, the dopant ions might also influence the crystal morphology evolution in the resulting nanocrystals, dictating a viable pathway for shapecontrolled synthesis. Heng et. al reported the shape evolution of Cu2O polyhedrons by Zn doping inducing various crystal morphologies such as 50-facet, 26-facet, and 8-facet.40 Further, porous and hierarchical Cu2O microparticles were developed in a Zn2+ ion mediated synthesis.41 On the other hand, due to balanced energy-band structure between CuO and ZnO, there is a considerable interest in the mixed metal oxide nanostructures, where well-controlled heterogeneous interfaces might enhance the synergistic interactions of CuO with ZnO. Charge-transferable heterojunctions could be engineered where coexistence and intergrowth of p-type (CuxO) and ntype (ZnO) takes place, inducing faster diffusion of free electrons and holes.42 Therefore, a variety of synthetic strategies have been adopted for the generation of CuO− ZnO composites for potential applications in the area of catalysis, photocatalysis, hydrogen production, photovoltaics and sensors etc.43−45 However, there are only a handful of reports demonstrating the effect of dopant ions in the evolution of highly branched crystal morphology.46,47 The growth of composite multipodal superstructures simply by doping with another metal ion not only might provide a reliable and inexpensive template-free method for materials with controlled morphology and composition but also can influence the formation of well-constructed nanoheterojunctions with enhanced catalytic performances.

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EXPERIMENTAL SECTION

Materials. Cu(NO3)2·3H2O and Zn(NO3)2·6H2O were obtained from TCI chemicals. Polyethylene glycol 200 (PEG 200), 4nitrophenol (4-NP), rhodamine B (RhB), methyl orange (MO), and methylene blue (MB) were purchased from Sigma-Aldrich. Sodium borohydride (NaBH4), sodium hydroxide (NaOH), sodium chloride (NaCl), potassium chloride (KCl), glucose, uric acid, sucrose, and ascorbic acid were obtained from Sisco Research Laboratories (SRL), India. Ultrapure water from a Milli-Q system was used in all experiments. Characterization. Powder X-ray diffraction spectra (XRD) was performed using a Rigaku Smart lab X-ray diffractometer with Cu Kα radiation (λ = 0.154 nm) in the range of 20−80°. Field-emission scanning electron microscopy (FE-SEM) images were obtained from a Supra55 Zeiss apparatus with an energy-dispersive X-ray (EDX) attachment. TEM images were obtained from a JEOL JEM-2100 instrument operating at an accelerating voltage of 200 kV. The samples were prepared by drop-casting their colloidal solutions on the carbon coated copper grids and dried at room temperature. Fourier transform infrared (FTIR) was recorded on a Bruker Tensor 27FTIR spectrometer using KBr pellet. X-ray photoelectron spectroscopy (XPS) was carried out using a PHI 5000 Versa Prob II, FEI Inc. instrument using Mg Kα radiation photoemission. UV−visible absorption spectra were recorded at room temperature using quartz cuvette (10 mm × 10 mm) on a Varian UV−visible spectrophotometer (Carry 100 Bio). Cyclic voltammetry (CV) experiments were performed on an electrochemical workstation, CHI620D (CH Instruments, USA) using a three electrode system. The Brunauer− Emmett−Teller (BET) surface area analysis was conducted on an Autosorb iQ, version 1.11 (Quantachrome Instruments). Raman spectra were obtained from a Jobin Yvon Horiba LABRAM-HR system equipped with a 632.8 He−Ne laser beam. A spectro-analytical B


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Figure 1. Scanning electron microscopy (FESEM) images of self-assembled microcrystals generated under hydrothermal conditions at 180 °C at various Cu2+:Zn2+ molar ratios: (a) CuxO (1:0), (b) ZnO (0:1), (c) CuO−ZnO (1:1), and (d) ZnO−CuxO (1:7.5). cuvette (path length 1 cm). To the resulting solution was added 10 μL of freshly prepared CuxO−ZnO catalyst. The reaction was monitored by following the time-dependent changes in UV visible absorption spectra of the reaction mixture at room temperature (200−500 nm wavelength range). The comparative catalytic activities of pure CuxO, ZnO, and ZnO−CuxO were also scrutinized following a similar procedure. Similarly, UV−visible absorption studies were carried out to monitor the reductive degradation of organic dyes. To a 2.5 mL of aqueous dye solution (the dye studied were RhB (0.03 mM), MO (0.1 mM), and MB (0.03 mM)) was added a freshly prepared NaBH4 (1.0 M, 125 μL) in a standard quartz cell. Then 10 μL (1 mg mL−1 stock solution) of the catalyst (CuxO−ZnO) was added. The kinetics of the reaction was followed by UV−visible studies of the reaction mixture at different time interval. All the reactions were carried out in the 200−800 nm range at room temperature. Controlled experiments were performed without using any catalysts under similar reaction conditions. Electrochemical Measurements. An enzyme-free electrochemical sensor was fabricated by coating nafion impregnated CuxO−ZnO powder onto a glassy carbon electrode (GCE). Before modification, the bare GCE surface was cleaned using alumina powder and washed in deionized water and ethanol ultrasonically for several times. A homogeneous CuxO−ZnO suspension in ethanol (1 mg mL−1) was dropped onto the clean GCE surface followed by the addition of 20 μL of Nafion solution (0.1%). The modified electrode (CuxO−ZnO/ GCE) was then dried at room temperature. A standard threeelectrode system was used for all the electrochemical experiments at room temperature. The CuxO−ZnO/GCE electrode acted as the working electrode and platinum wire and Ag/AgCl wire was used as the counter and reference electrode, respectively. All the cyclic voltammetry and chrono-amperometry measurements were performed in 40 mL of 0.05 M NaOH with glucose aqueous solutions having variable concentrations. The amperometric measurements were performed under continuous stirring to ascertain the complete mixing of glucose in NaOH aqueous solution. The electrochemical impedance spectroscopy (EIS) measurements were recorded over a frequency range of 0.01 Hz to 70 kHz at a DC perturbation signal of

simultaneous ICP spectrometer (model ARCOS) was used for ICPAES measurements. Synthesis of Metal Oxide Superstructures. For the synthesis of metal oxides of variable compositions and shapes, a hydrothermal synthesis route through a modified polyol method was used at variable temperature. For example, pure CuxO (x = 1 or 2) microparticles was obtained first by dissolving Cu(NO3)2·3H2O (200 mg, 50 mM) in 6 mL of water followed by the addition of 10 mL of PEG 200. The reaction mixture was transferred in a Teflon sealed autoclave and subjected to hydrothermal treatment in a hot-air oven at 180 °C for 16 h. The precipitate obtained after cooling was washed several times with water and ethanol, followed by calcination at 300 °C under air for 2 h. Similarly, pure ZnO was synthesized using Zn(NO3)2·6H2O (235 mg, 50 mM) as a precursor through a similar procedure in a water−PEG 200 mixture. For the synthesis of the mixed metal oxides, the metal precursor salts were dissolved in water, then mixed with PEG 200 and subjected to hydrothermal treatment at various temperatures. For example, CuxO−ZnO composite was obtained from a mixture of Cu(NO3)2·3H2O (50 mM) and Zn(NO3)2·6H2O (7 mM) under hydrothermal conditions. Similarly, ZnO−CuxO was obtained by using a mixture of Cu(NO3)2·3H2O (6.5 mM) and Zn(NO3)2·6H2O (50 mM). The ratio of Zn2+ and Cu2+ salts was optimized to obtain various microstructures in a series of experiments. The reproducibility of the morphology controlled synthesis was ascertained by repeating the synthesis in multiple batches under controlled reaction conditions. No external shapedirecting agents were added in any synthetic procedures, and morphology control was dictated by doped metal only. Evaluation of Catalytic Performance. The catalytic studies of the synthesized CuxO−ZnO composite was carried out by performing a model reaction of 4-nitrophenol (4-NP) reduction to 4-aminophenol using NaBH4 as a reducing agent at room temperature. In addition, similar reductive degradation reactions of commercially available organic dyes such as methylene blue (MB), methyl orange (MO), and rhodamine B (RhB) were studied. For the reductive degradation of 4-nitrophenol (4-NP), an aqueous solution of 4-NP (1.0 mM), catalyst (1 mg mL−1), and NaBH4 (1.0 M) was freshly prepared. 250 μL of 4-NP stock solution, 125 μL of NaBH4, and 2.1 mL of water were added into a standard quartz C


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Figure 2. (a) FESEM Image of octapodal CuxO−ZnO obtained under hydrothermal conditions at 180 °C at Cu2+:Zn2+ molar ratio of 7:1 (scale bar 2 μm); (b) high magnification FESEM image of CuxO−ZnO (scale bar 200 nm); (c) elemental mapping and (d) HRTEM image of CuxO− ZnO microcrystals. 10 mV in 0.05 M NaOH solution, both in the presence and absence of glucose.

Multilayered hexagonal microplates were obtained (ZnO− CuxO) at a Zn2+/Cu2+ ratio of 7.5:1. The microplates were clustered to form multilayers and are nanoporous as shown in Figure 1d. The EDX spectroscopy suggested the presence of both Zn and Cu in the matrix and elemental mapping showed that both the metals were uniformly distributed in the hexagonal matrix (Figure S2). Considering the fact that there were no external shape-directing agents for the phase-selective growth, the formation of these hierarchical microstructures of different shapes simply by controlling the molar ratio of salts in a binary oxide is indeed intriguing. Similarly, interesting structural evolution of CuxO occurred when small amount of Zn2+ was added to a higher concentration of Cu2+ salt and heated hydrothermally at 180 °C. At an optimum Cu2+/Zn2+ ratio of 7:1, octapodal morphology having dimension in the range of 5−10 μm was obtained (CuxO−ZnO). Figure 2a,b shows the scanning electron microscopy (SEM) images of the CuxO−ZnO microstructures. Three-dimensional star-like morphology was observed in the as-prepared composite that consists of octa-symmetric arms extending from the center with a length of 5 μm. In each of the arm structures, small nanoparticles with an average diameter of 20 nm were attached randomly on the surface. The energy dispersive X-ray (EDX) studies for CuxO−ZnO as shown in Figure S3 validated the presence of Cu, Zn, and O. The distribution of Cu, Zn, and O was analyzed by elemental mapping, which revealed that all these elements are homogeneously present in the octapod structure (Figure 2c). The effect of temperature on the formation of these microstructures was further monitored by performing the hydrothermal synthesis at 200 °C for 12 h, keeping the concentration of the metal salts and other reagents similar. The formations of the octapodal microstructures were evident by SEM studies (Figure S4a) under these conditions also;

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RESULTS AND DISCUSSION Structural and Compositional Characterization. The growth of hierarchical nanocomposites was carried out using polyethylene glycol (PEG 200) as a solvent in a one-pot hydrothermal pathway at varying ratio of precursor Cu2+ and Zn2+ salts without any other additives. The metal salts were dissolved in water and added to PEG 200. In all cases, water to PEG 200 ratio was kept constant (H2O/PEG 200 0.6 v/v). During the polyol synthesis, the reaction is initiated through the nucleation into a solid, when sufficient aqueous precursors are heated in a high-boiling alcohol such as PEG 200.14 In the absence of Zn2+, the as-prepared CuxO is in the form of cubical assemblies with a rough external surface having an average dimension of 5−7 μm, while heated hydrothermally at 190 °C. Figure 1a represents the scanning electron microscopy of these assembled structures. High resolution imaging of the microcubes showed that the overall microstructure is an assembly of small cubic shaped nanoparticles (Figure S1). On the other hand, hydrothermal treatment of Zn(NO3)2, in the absence of Cu2+ salts, led to the formation of hollow spherical particles with an average size of 500 nm under similar reaction conditions (Figure 1b). The hollow architectures were confirmed by cracked ZnO sphere, as marked out. The nanoparticles possess a rough external surface and are nanoporous in nature. The hierarchical growth of microscopic length scale of variable shapes was observed when both Cu2+ and Zn2+ were simultaneously added and subjected to nucleation and growth. When the Cu2+/Zn2+ ratio was 1:1, a mixture of spherical and cubical microstructures was obtained (Figure 1c). On the other hand, drastic morphological changes occurred in the structure of ZnO in the presence of a small amount of Cu2+. D


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Figure 3. (a) Powder X-ray diffraction patterns of various microstructures obtained under hydrothermal conditions at 180 °C, and after calcination at 300 °C; (b) X-ray photoelectron spectroscopy (XPS) spectra of octapodal CuxO−ZnO (i) survey spectra and high resolution spectrum of (ii) Cu 2p, (iii) Zn 2p and (iv) O 1s (binding energy in the XPS spectra was calibrated using C 1s (284.6 eV)).

the semiconducting behavior of the material (Figure S6). The phase characteristics of CuxO−ZnO as well as pure CuxO and ZnO nano/microstructures were obtained from X-ray diffraction (XRD) patterns as shown in Figure 3a. In the case of copper oxide microcubes, a mixture of cuprite and cuprous oxides was obtained. Diffraction peaks at 2θ of 32.4, 35.4, 38.6, 48.7, 58.2, 61.4, 66.2, 68, and 73.4 correspond to the (110), (002), (111), (−202), (202), (−113), (311), (220), and (311) lattices in CuO (JCPDS no. 45-0937). The reflections at 2θ of 29.5, 36.4, and 42.3 correspond to the (110), (111), (200) lattices in Cu2O (JCPDS no. 78-2076). In the case of ZnO nanospheres, the X-ray diffraction peaks can be attributed to hexagonal ZnO (JCPDS no. 36-1451). The diffraction peaks of the CuxO−ZnO samples can be assigned to a mixture of Cu2O−CuO along with low intensity diffraction peaks owing to ZnO. The dominant diffraction peak of CuxO compared to ZnO in CuxO−ZnO octapodal microparticles suggested better crystallinity of CuxO in the sample. However, the crystallinity in ZnO−CuxO shows the reverse trend, demonstrating significantly different arrangement of CuxO and ZnO in the two samples. FTIR studies confirmed the formation of a mixed metal oxide and vibrational peaks of CuO (624 cm−1) and ZnO (455 cm−1) were observed (Figure S7).52 Further X-ray photoelectron spectroscopy (XPS) studies were performed to analyze the surface composition and chemical state of the elements present in the CuxO−ZnO (Figure 3b). The binding energy is calibrated using C 1s (284.6 eV) in the XPS spectra. From the survey spectrum, the presence of Cu, Zn, and O can be validated. In the high resolution spectrum of Cu, two dominant peaks at 933.4 eV (Cu 2p3/2) and 953.3 eV (Cu 2p1/2) were obtained, which can be fitted into Cu2+ (934.1 and 954.0 eV) and Cu+ (932.6 and 952.6 eV), respectively. Two satellite peaks at ∼942 and ∼962 eV, corresponding to Cu2+ oxidation state, validated the presence of CuO as a component.53 Further, the peaks at ∼1021.5 eV (Zn 2p3/2) and ∼1044.7 eV (Zn 2p1/2) indicate that Zn was present in +2 oxidation state in the matrix.54 The asymmetrical peak of O 1s (Figure 3b) suggested that the oxygen at the surface comprises two components. One component located at ∼529.2 eV can be assigned to the O2− ions bonded with Zn2+ or Cu+/Cu2+ions, whereas the second component at ∼531 eV is attributed to the O2− ions in oxygen

however, the surface on the arms were much rougher as compared to those obtained at 180 °C. The arm structure appeared as if small cubes were stacking on each other to form the arms, probably due to rapid growth. Octapodal microstructure formation was not observed when the temperature during the hydrothermal synthesis was increased to 220 °C. Instead, a mixture of cubes and smaller particles was observed (Figure S4b). It is well-known that the formation of metal oxide nanoparticles involves hydroxide intermediates. To observe the morphological evolution of the composite structures in the presence of OH− ions, NaOH was added purposefully to the metal salts during hydrothermal synthesis. When a mixture of Cu2+ and Zn2+ salts (maintaining Cu2+/ Zn2+ ratio of 7:1) was added to a 10 mM NaOH aqueous solution and exposed to hydrothermal treatment at 180 °C in a PEG−water mixture, this resulted in the formation of microspheres with blooming flower-like morphology (Figure S5). EDX studies showed the formation of pure CuxO microparticles in the present case, as no Zn2+ was detected (Figure S5). High resolution transmission electron microscopy (HRTEM) studies were performed to have an insight into the spatial distribution of CuxO−ZnO sample. As shown in Figure 2d, lattice fringes with interplanar spacing of 0.282, 0.247, and 0.237 nm were obtained, corresponding to the (100) atomic plane of ZnO, (111) plane of Cu2O, and (111) plane of CuO, respectively.50 This clearly revealed the integration of monoclinical CuO/Cu2O and zincite ZnO in the resulting hybrid nanostructure. Further, no obvious break or disorder in the mismatched lattice arrangements suggest intimate contacts between the constituents resulting in the formation of highly crystalline heterojunctions between CuxO and ZnO phases. Considering the different semiconducting behavior of CuxO and ZnO, the formation of heterointerfaces present distinct possibilities for p−n junction formation on the surface.43,51 This leads to an alteration of electron distribution in the nanocomposite facilitating easier charge transfer across the interface. UV−visible diffused reflectance spectra of CuxO−ZnO was recorded, and from the Tauc plot we obtained optical band gaps that could be assigned to CuO (1.9 eV), Cu2O (2.4 eV), and ZnO (3.7 eV), respectively. The results are consistent with E


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Figure 4. (a) Schematic presentation of the morphology evolution of CuxO−ZnO octapodal microstructures and (b−e) SEM images of the mesocrystals obtained after (b) 4 h, (c) 8 h, (d) 12 h, and (e) 16 h. The growth of mesocrystals was performed at 180 °C under hydrothermal condition using PEG−H2O solvent mixture.

high yield. A time-dependent study, as shown in Figure 4b−e, depicts the evolution of these multiarmed microstructures, where crystal branching was initiated along all orientations of the [111] plane, resulting in the formation of eight-pod framework. The branching started from a cubic microcrystal as a seed. The migration of adatoms resulted in the formation of a concave cube structure (after 4 h, Figure 4b) and centralized crystal porosity was generated in a close-up process (dice-like crystal). This was transformed into a truncated cube after 8 h that showed a “cube-within-cube” arrangement and eight small cubes were developed, whose dimension is eight times smaller than its initial building block (Figure 4c). Overgrowth from the corners of the eight cubes are observed along the [111] planes with time (Figure 4d). This resulted in the gradual growth of multibranched octapodal crystal aggregates after 16 h. The octapod microcrystals are decorated with low Miller Index crystal planes of [110] (Figure 4e). Each pod structure is bound with six [110] facets and the growing tips along the [111] facet are terminated with three [100] planes. However, the [110] surfaces in the multipodal structures are not smooth and small nanocrystallites of average dimension ∼20 nm were decorated all over the surfaces. The octapodal morphology of CuxO−ZnO was found to have higher surface area and higher surface energy as compared to cubic structures. Therefore, a kinetic growth process might influence the formation of the octapod structures along with temperature-driven crystal facet engineering. Kinetically controlled growth is achieved through manipulation of the rate at which the generated atoms are grown on the surface of a nucleating seed. In a H2O−PEG mixture, metal salts are coordinated to H2O molecules, whereas water is bridged through intermolecular interaction with PEG. When the reaction proceeds at high temperature, the adatoms migrate at a reduced rate during the first few minutes of synthesis due to absence of water. The growth of the nanopods takes place following a kinetic growth mechanism as reported for multipodal metal nanocrystals.15 The anisotropic growth results due to reduced precursor concentration in the mother liquor coupled with a large diffusion distance in the polyol medium. Thus, the multibranched octapodal structures were obtained through an oriented growth along the [111] direction during Ostwald ripening and recrystallization induced growth. For shape control of oxide nanoparticles, usually organic capping molecules are used that selectively bind on particular facet allowing the growth to take place on other facets resulting

deficient regions. The results suggest the presence of oxygen vacancy defects on the sample surface.55 An average Zn/Cu ratio was calculated from the areas under the peak of Cu 2p and Zn 2p. The atomic ratio of Zn/Cu was found to be ∼0.1 for the CuxO−ZnO composite, which matches well with the ratio calculated from EDX analysis. Possible Growth Mechanism. The growth of both ZnO and CuO nano/microparticles into variable shapes using polyol methods is well reported, where the shape selectivity is mainly governed by additional shape directing groups that adheres on particular phases allowing the growth to take place in a selective manner.15 However, the influence of other metal ions in shape-selectivity in a multimetallic oxide material is not wellknown. Taking into consideration the formation of unusual octapodal morphology in a CuxO−ZnO microstructure above a certain critical temperature (180 °C), we performed a time and temperature dependent study to look into the growth mechanism. It is well reported that the presence of water in the medium plays a critical role in controlling the particle size of oxide nanoparticles during polyol synthesis.56 In order to simplify the understanding of growth mechanism, we kept the water to PEG ratio constant for the development of oxide materials of various compositions. Several phenomena could be observed during the shape evolution of CuxO−ZnO microcrystals under variable reaction time and temperature. In the polyol method, a solid seed is initially formed when sufficient aqueous metal precursors are treated in a high boiling alcohol at elevated temperature. The shape evolution occurs due to the competitive growth of nuclei from the solvent in the immediate vicinity of the parent seed. Below 150 °C, the nanocrystals are immediately covered with the polyol after nucleation, resulting in small, spherical nanoparticles (Figure S8a). On the other hand, at an elevated temperature (>150 °C), pronounced facet engineering might occur resulting in the migration of adatoms on the crystal surface with the minimization of the surface energy, in a process known as Ostwald ripening. When a mixture of Cu2+ and Zn2+ salts (Cu2+:Zn2+ = 7:1) was treated hydrothermally in a PEG−water mixture at 150 °C, the particle size enhanced to the microscopic length scale, resulting in the formation of polyhedral cubic microstructures decorated with low-index facets (Figure S8b). Interesting hierarchical morphology evolution arises when the temperature of the reaction medium was further elevated to 180 °C, when octapodal microstructures were obtained in F


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reducing agent in water. UV−visible spectroscopy can be used to monitor the course of the reaction. The heterogeneous catalytic reaction contains several steps.62 The first step involves the adsorption of BH4− ions on the nanoparticle surface forming surface-hydrogen species through BH4− hydrolysis. In the second step, diffusion of 4-nitrophenolate ions from solution and adsorption on the catalyst surface takes place. Subsequently, 4-nitrophenol are reduced by the surfacehydrogen species and the final reaction step involves desorption of the reaction products and their diffusion away from the surface. As the concentration of 4-nitrophenol is much lower as compared to NaBH4, the kinetic equation for the catalytic degradation reaction can be expressed as shown in eq 1:

in the progressive elongation. In the present case, the evolution of octapodal superstructures occurred when Zn2+ was used as a dopant. Although the role of Zn2+ salts in the shape-selective evolution of CuxO−ZnO microstructures could not be fully understood, the uniform distribution of Zn in the overall structure as observed by EDX studies suggested that the Zn2+ ions were convoluted during the whole process of CuxO formation. Based on the observations, a conversion mechanism can be postulated, where reconstructive transformation of CuO occurred in the solution resulting in crystallization of Cu2O upon reduction in a polyol medium. During this process, the hierarchically oriented growth and oriented attachment of Cu(OH)2 took place throughout the process. The presence of strongly acidic Zn2+ might influence the anisotropic growth along various crystal planes having different free energies. The highly oriented growth along the [111] plane further suggest that the Zn2+ precursor might selectively stabilize the [110] facet allowing the reconstructive growth along the [111] facet.45 The multivalency of Cu in the overall composite can be easily understood from considering the presence of both reducing (PEG) and oxidizing (NO3− ions) in the reaction environment. The evolution of multiarmed microstructure resulted in a larger specific surface area as demonstrated by Brunauer− Emmett−Teller (BET) nitrogen (N2) adsorption experiments (Table 1, Figure S9). Compared to the cubic CuxO−ZnO

ln(Ct /Co) = ln(A t /Ao) = K appt

Here, Ct/Co denotes the relative concentration of 4-nitrophenol at time t to its starting value (at t = 0), which corresponds to the relative intensity of absorbance (At/Ao) in the UV−visible spectrum and kapp is the rate constant. Under neutral condition, 4-NP shows an absorption peak at 317 nm (Figure S10), which is shifted to 400 nm when NaBH4 is added, suggesting the generation of 4-nitrophenolate ions. There was no obvious change in the intensity of the peak at 400 nm even after 6 h, clearly indicating that no reduction occurred in the absence of a catalyst, even after using NaBH4 in large excess (C(4-NP)/C(NaBH4) = 1:500 (Figure S11). The catalytic activity of the pure CuO, ZnO and various compositions of CuxO−ZnO was studied by monitoring the time-dependent decrement in the peak intensity at 400 nm. For this, 10 μg of the metal oxide catalyst was added to a 2.5 mL solution containing NaBH4 (50 mM) and 4-NP (0.1 mM). Hollow spherical ZnO nanoparticles were incapable of initiating the catalytic degradation of 4-NP even after prolonged time (Figure 5a). In the case of cubic CuxO microparticles, the peak intensity at 400 nm gradually decreases with time and finally diminished, suggesting that the catalytic reaction started without any induction period. The complete reduction of 4-NP required only 300 s, showing the high efficiency of Cu based oxide materials for reductive degradation reactions (Figure 5b). In this process, a new peak at 295 nm appears, characteristic of 4-aminophenol. The UV− visible spectra also show an isosbestic point at 313 nm, depicting no byproduct formation during the catalytic reaction. Highly efficient degradation of 4-NP was observed when octapodal CuxO−ZnO microcrystals was used as catalyst, and complete degradation took place within 90 s (Figure 5c). A linear graph between ln(Ct/C0) and reaction time was plotted that showed a pseudo-first-order kinetics (Figure 5d). The apparent rate constant (kapp) was calculated to be 2.2 × 10−2 s−1 and 0.7 × 10−2 s−1 for CuxO−ZnO and CuxO, respectively (Table S1). Considering the fact that ZnO has very little catalytic activity for the model reaction, the high efficiency of CuxO−ZnO composite compared to pure CuxO was unexpected. This can be attributed to a combination of factors. The higher surface area of CuxO−ZnO compared to cubic CuxO and exposure of catalytically active planes in the multiarmed composite results in higher accessibility of reactants on the surface. Further, HRTEM studies (Figure 2d) suggested the formation of a continuous hybrid nanostructure at the CuxO and ZnO interphase, satisfying the condition for enhanced interfacial charge transfer. CuxO (x = 1, 2) is a narrow band gap p-type

Table 1. Specific Surface Area of Various Microstructures Obtained under Hydrothermal Conditions at 180 °C Microstructures

Specific surface area (m2 g−1)

Cubic CuxO−ZnO CuxO−ZnO ZnO−CuxO

5.901 11.609 21.086

(1)

microparticles (5.9 m2 g−1), the surface area of the octapodal CuxO−ZnO structure enhanced significantly (11.2 m2 g−1). This suggests that the multipodal microstructures with exposed crystal planes offer better possibility for the substrate accessibility through the voids, a fundamental criterion for better catalytic applications. Interestingly, the ZnO−CuxO hexagonal multilayered microstructures had the highest specific surface area among all the materials synthesized with enhanced porosity. Catalytic Performance of the CuxO−ZnO Composites for Pollutant Degradation. Toxic materials such as nitrobenzene and organic dyes, e.g., methyl orange (MO), methylene blue (MB), and rhodamine B (RhB), are common pollutants in soil, industrial water and effluents, and are known to have detrimental effect on human health as well as aquatic organisms. Noble metal nanocatalysts such as Au, Ag, Pd, etc.57−61 show high efficiency for the catalytic degradation of these pollutants at room temperature. However, high cost and scarcity associated with these metals prompted search for alternative low cost non-noble metal catalysts for the degradation of these materials. Metal oxide nanomaterials have demonstrated high activity toward reduction reactions. Taking into consideration the availability of a large number of catalytically active facets in the synthesized CuxO−ZnO microstructures, we explored the efficacy of these materials as catalysts for the degradation of pollutants. First, the 4nitrophenol (4-NP) reduction to 4-aminophenol (4-AP) was studied as a model reaction in the presence of NaBH4 as a G


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Figure 5. (a−c) Time-dependent UV−visible spectra of the reaction mixtures containing 4-NP aqueous solutions in the presence of NaBH4 as a reducing agent and ZnO, CuxO, and CuxO−ZnO as catalyst, optical image of 4-NP to 4-AP (inset c); (d) calibration curves as a function of ln(Ct/ C0) vs reaction time for the reaction mixtures containing aqueous solutions of NP and catalyst in the presence of NaBH4 as a reducing agent, derived from the spectra (a−e); (e) time-dependent UV−visible absorption spectra of the reaction mixtures containing MB aqueous solutions in the presence of NaBH4 as a reducing agent and CuxO−ZnO as a catalyst; (f) calibration curves as a function of ln(Ct/C0) vs reaction time for the reaction mixtures containing aqueous solutions of dyes MB, MO, and RhB in the presence of NaBH4 as a reducing agent and CuxO−ZnO as a catalyst.

of O from the CuO surface. Thus, the reduction of Cu2+ to Cu+ becomes easier in the CuxO−ZnO matrix, which is clearly reflected in the powder XRD and Raman spectroscopy studies of the recovered catalysts after their participation in the 4nitrophenol reduction (Figure S13). Powder XRD studies of the recovered CuxO−ZnO catalysts showed that the intensity of the diffraction peaks at 2θ of 35.4° and 38.6° corresponding to the (002) and (111) planes of CuO was reduced and peak at 36.4° owing to Cu2O (111) intensified. The results suggest that the Cu(I)/Cu(II) ratio is significantly affected during the catalytic process (Figure S13a). The original composition of the CuxO−ZnO catalysts can be regained upon calcination at 300 °C for 2 h in air and the recovered catalyst showed an almost similar XRD pattern as that of the pristine catalyst. The pristine and recovered CuxO−ZnO microparticles were further investigated by Raman spectroscopy. As illustrated in Figure S13c, the Raman spectra of the pristine CuxO−ZnO showed major peaks at about 288, 338, and 628 cm−1, attributed to the vibration of CuO, along with three inherent band at about 210, 401, and 481 cm−1 corresponding to vibrational mode of Cu2O. In the case of the recovered CuxO−ZnO, the band at

oxide (CuO; Eg = 1.2−2.0 eV, Cu2O; Eg = 1.9−2.5 eV). On the other hand, ZnO shows the characteristics of a n-type semiconductor with a wide band gap (Eg = 3.2−3.4 eV). A strong synergistic interaction occurs at the two-phase interface upon formation of a p−n junction. In the hybrid system, the electron density of Cu2+ increases due to transfer of electrons from Zn2+. This results in the decrement in electron density in Zn2+, generating equal number of holes at the interface. Thus, efficient electron transfer from ZnO to CuxO and hole transfer in the reverse direction can be attained leading to the spatial separation of the charged species at the two-phase intergrowth system (Figure S12).42,63 Upon induction of NaBH4, electrons are continuously injected into the system and the Fermi level of the nanocatalyst is raised. The electron−hole recombination probability in CuxO−ZnO will be greatly reduced due to prompt shifting of electrons from ZnO to CuxO due to lower barrier height. This results in increasing the electronacceptance process by various dyes. As the Cu2+ center becomes more negatively charged due to the presence of Zn2+ in the composite, the Cu2+ and O2− bonding will be weakened. This will facilitate the easy removal H


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Figure 6. (a) Electrochemical response of CuxO−ZnO/GCE and nafion/GCE toward 2.0 mM glucose in a 0.05 M NaOH solution at a scan rate of 50 mVs−1; (b) calibration curve for the CuxO−ZnO/GCE electrode at varying concentration of glucose (0.2−6.0 mM), (inset) linear range of calibration of the CuxO−ZnO/GCE electrode; (c) oxidation of glucose at variable scan rate for CuxO−ZnO/GCE electrode, (inset) variation of reduction and oxidation peak current at different scan rate for 2.0 mM glucose; (d) amperometric responses of the CuxO−ZnO/GCE electrode with successive addition of glucose (100 μM) at +0.6 V vs Ag/AgCl; (e) amperometric studies showing responses to interfering molecules (NaCl, KCl, uric acid, ascorbic acid, and sucrose); (f) EIS of the CuxO−ZnO/GCE electrode in 0.05 M NaOH solution (in presence and absence of glucose).

210 cm−1 was significantly enhanced and that at 288 cm−1 was reduced, suggesting that Cu(II) was converted to Cu(I) during the catalytic reaction. However, there was no morphological alteration in the recovered catalyst as the octapodal structure of CuxO−ZnO was retained as observed by the SEM studies (Figure S13d). Further, inductively coupled plasma-atomic emission spectroscopy (ICP-AES) of the pristine CuxO−ZnO and recovered CuxO−ZnO catalysts was performed to obtain the Zn/Cu molar ratio. There was a minor change in the Zn/ Cu ratio from 0.096 to 0.0974 in the case of recovered CuxO− ZnO catalysts, suggesting that the elemental proportion did not change appreciably in this hybrid structure after their participation in catalysis. EDX analysis of the recovered catalyst further supported the insignificant changes in the Zn/Cu ratio as compared to the pristine catalyst. Surprisingly, the hexagonal multilayered ZnO−CuxO microparticles showed the complete reduction of 4-NP within 180 s under similar reaction conditions (Figure S14). The apparent rate constant (kapp) was calculated to be 1.0 × 10−2 s−1 for ZnO−CuxO and showed better catalytic efficiency than CuxO (kapp = 0.7 × 10−2 s−1). Considering the much lower amount of catalytically active CuxO in the matrix, the results further emphasize the role of ultrafast electron transfer in the CuxO− ZnO heterojunction on the catalytic activity. A variety of nanostructures have been used for the catalytic degradation of organic substrates. In order to compare the efficiency of the octapodal CuxO−ZnO microparticles with the earlier reports, we evaluated the activity factor K, calculated as the ratio of kapp to the catalyst amount (K = kapp g−1). The activity factor for the octapodal CuxO−ZnO was found to be 2100 s−1 g−1, which was higher than previous reported Cu

based nanomaterials such as nanocubes, nanoplates, microsphere and nanoparticles.64 The catalyst showed better performance than reported noble metal nanocatalysts, such as Ag/SNTs-4, Au/graphene and Pd/SPB-PS (Table S2). The results clearly demonstrate that octapodal CuxO−ZnO represents one of the highly efficient and low cost unsupported catalytic systems for reductive degradation reactions. Encouraged by the high catalytic efficiency of CuxO−ZnO microcrystals for reduction reactions, we further studied its activity for the reductive degradation of organic dyes. For these studies, commonly used methyl orange (MO), methylene blue (MB), and rhodamine B (RhB) were tested as model dyes. When a mixture of CuxO−ZnO catalyst, NaBH4, and an aqueous solution of a dye were taken, rapid reduction of the dye could be observed visually within a very short period, resulting in decolorization. Time-dependent UV−visible spectroscopy studies confirmed the fast degradation and a gradual decrease in the intensity of the prominent absorbance peak of the dyes was recorded. The dye degradation was completed within 90−120 s (Figure S15). Figure 5f shows a linear dependence between the reaction time and ln(Ct/C0), indicating that the degradation reactions followed a pseudofirst-order kinetics. The rate constants were found to be 3.6 × 10−2 s−1 (K = 3600 s−1 g−1), 3.5 × 10−2 s−1 (K = 3500 s−1 g−1), and 2.7 × 10−2 s−1 (K = 2700 s−1 g−1) for MB, MO, and RhB, respectively (Table S3). In a controlled reaction, it was observed that NaBH4 alone could not initiate the reduction of dyes in absence of catalyst even after a prolonged period (Figure S11). It is well-known that the rate of dye degradation on various catalytic surfaces depend on the size and reactivity of the dye molecules, hence variable reaction rates for different I


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significantly higher compared to these interfering species in blood serum. Therefore, high selectivity of the electrode for glucose sensing even in the presence of these biomolecules is desirable. The studies were carried out amperometrically by first injecting aliquots of glucose to a 0.05 M NaOH solution at an applied potential of +0.6 V, followed by the addition of 10 μM uric acid, ascorbic acid, and sucrose sequentially. As shown in Figure 6e, the electrode showed very low response toward other interfering molecules as compared to glucose. This contrast confirms high selectivity of the CuxO−ZnO/GCE electrode toward glucose sensing. The perm selective Nafion could act as a barrier to negatively charged ascorbic acid and uric acid, along with its activity as an ionomer in the electrocatalytic system.67 Further, the presence of common salts such as NaCl (0.15 M) or KCl (0.15 M) in the medium did not interfere in the sensing behavior, suggesting that the electrode system can be used for real blood samples. Electrochemical impedance spectroscopy (EIS) was carried out to further support the glucose-sensing induced enhancement in electrical conductivity in the CuxO−ZnO modified electrode. The conductivity measurements were carried out in the presence or absence of glucose under an open circuit potential with an input frequency from 0.01 Hz to 70 kHz in 0.05 M NaOH. A Nyquist plot (Figure 6f) shows a single depressed semicircle and an inclined line in the high-frequency and low frequency region, respectively. The charge transfer resistance of CuxO−ZnO electrode is very low, implying that the conductivity of the CuxO−ZnO electrode is high and good enough to be used for various electrocatalytic applications. The injection of glucose in the electrolyte minimizes the resistance due to the enhancement in the number of free OH − ions or electrons during glucose oxidation. The enhancement in the conductivity upon glucose addition as observed from the Nyquist plot can be exploited as another electrochemical sensing way for the presence of glucose. For the application of CuO based electrodes for the nonenzymatic glucose sensing, it is commonly postulated that Cu(III) oxohydrides play a major role in the glucoseoxidation mechanism.68 In the electrocatalytic process, CuII is initially oxidized to CuIII, which subsequently reacts with glucose, regenerating CuII species and the oxidized product gluconic acid. However, recently Barragan et al. provided an alternative mechanistic insight into the electrochemical oxidation of glucose and other carbohydrates on CuO electrodes. It has been demonstrated that CuIII is not involved in the oxidation of glucose. Instead, the semiconductive properties of the electrode material and hydroxyl ion adsorption play the most influential role.65 On applying sufficient anodic potential, the vacancies (h+), which act as the major charge carriers, are accumulated on the electrode surface. This facilitates a partial electron transfer from the oxygen atoms of the adsorbed OH− ions to CuxO, forming a vacancy-adsorbed hydroxyl pair, which can be represented as follows

dyes are quite apparent. The results further demonstrate the enhancement in catalytic efficiency through synergistic effect between compatible components in a bimetallic oxide nanomatrix. Electrochemical Glucose Sensing. There has been an increasing attention toward the fabrication of cost-effective nonenzymatic electrochemical sensors for various biomolecules, especially for glucose detection with high sensitivity, selectivity, stability, and repeatability. An exquisite material choice and rigorous control over the structure and morphology holds the key for a high performance electrochemical glucose sensor through exploitation of available surface area and efficient electron transfer. In order to realize the multifunctionality of the CuxO−ZnO superstructures, their applicability in the electrochemical sensing of glucose was studied. Cyclic voltammetry (CV) measurements were performed using a three-electrode system where glassy carbon electrode (GCE) coated with octapodal CuxO−ZnO microparticles acted as the working electrode. Pt wire and Ag/AgCl (1 M KCl) were used as counter and reference electrodes, respectively. Figure 6a shows the electrochemical response of CuxO−ZnO/GCE and Nafion/GCE electrodes toward 2.0 mM glucose dissolved in a 0.05 M NaOH solution at a scan rate of 50 mVs−1. The Nafion/GC electrode (without catalyst loading) did not show any oxidation peak. On the other hand, CuxO−ZnO/GCE showed oxidation peaks at +0.6 V, which could be associated with the oxidation of OH− to O2 on the surface of the electrode.65 In the absence of NaOH, the CuxO−-ZnO/GCE showed only a little enhancement in the current response, suggesting that the presence of OH− ions is necessary for significant sensing (Figure S16). High intensity peaks were obtained after the addition of glucose, and the peak intensity also increased with the enhancement in glucose concentration. Figure S17 shows the CV curves of the CuxO− ZnO/GCE electrode on varying glucose concentration (0.2− 6.0 mM). The corresponding calibration curve for the CuxO− ZnO/GCE electrode is shown in Figure 6b (inset; linear range from 0.2 to 2.0 mM). The current response of the modified electrode exhibited a linear dependence on concentration of glucose. A detection limit of 7.5 μM was calculated using the equation 3σ/s, where σ is the standard deviation and s is the slope of the curve, respectively. CV responses of CuxO−ZnO/GCE electrode were further measured for 2.0 mM glucose in the presence of 0.05 M NaOH, while the scan rate was varied from 25 to 150 mV s−1 (Figure 6c). It was observed that that with increasing scan rates, the peak potential shifted toward more positive values. There was a linear increase in the current of the glucose oxidation peak with increasing scan rates (inset Figure 6c). This linear relationship suggests an electrochemical kinetics being regulated by the rate of adsorption of glucose on the electrode surface.66 The amperometric response of CuxO− ZnO/GCE electrode at successive addition of glucose (0.05 M NaOH, +0.6 V) is shown in Figure 6d. During these experiments, the solution was stirred continuously to ensure a proper mixing of glucose and NaOH. A fast response (less than 3 s) for the change in glucose concentration was observed using the electrode and the sensing current also increased with glucose addition. A high sensitivity of 2091 μA mM−1 cm−2 could be achieved using CuxO−ZnO/GCE electrode for glucose oxidation. Several other easily oxidizable biomolecules, such as ascorbic acid, uric acid, and sucrose, are also present in human blood serum. However, the concentration of glucose is

OHads ‐ + h+ → (OHads ‐)(h+)

(2)

The accumulation of energy during the combination of the electroadsorbed OH− species with the vacancies on the electrode surface might be high enough to drive the oxidation of glucose. The glucose oxidation takes place through their adsorption on the surface and abstraction of labile hydrogen atom. During glucose oxidation, gluconolactone is formed initially via two electron transfer, which subsequently underJ


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catalytic performance for the reduction of organic pollutants such as 4-nitrophenol and commonly available dyes. Further, the multifunctionality of the octapodal microstructures was explored for nonenzymatic electrochemical sensing of glucose, which showed high sensitivity and selectivity. High stability and repeatability of the CuxO−ZnO immobilized modified electrode ensured their applicability for glucose sensing in blood serum samples. The formation of p−n junctions at the interface of CuxO and ZnO in the hybrid nanostructure results in effective charge separation and ultrafast electron transfer that played the central role in enhancement of multidimensional catalytic activity. Overall, the controlled growth of various hierarchical microcrystals, simply by controlling the precursor metal salt concentration using a polyol method might provide an applicable strategy for the generation of shape-selective superstructures in a template-free one-pot pathway for multifunctional applications.

goes hydrolysis leading to the formation of gluconate ions. Considering the improved electrocatalytic efficiency of the CuxO−ZnO in comparison to CuxO electrode for glucose oxidation, it becomes logical to think that a similar mechanism is followed, where the oxidation is highly dependent on the semiconducting behavior of the electrode material and adsorbed hydroxyl ions on the surface. Due to the p−n junction formation at the CuxO and ZnO interface in the nanocomposite, vacancies are predominantly present on the surface, making them facile for the adsorption of hydroxyl ions. Further, the presence of a large number of exposed facets in the octapodal structure is expected to have an impact on the performance of CuxO−ZnO electrodes. The [111] facets have higher surface energy compared to the [100] facet, thus showing better electrocatalytic activity. The [111] facet contains a large number of Cu dangling bonds, making them more positively charged and thus stimulating effective electron transfer during glucose oxidation.69 Additionally, the octapodal structural features resulted in high surface area, which will inevitably enhance the glucose absorption on the surface. Repeatability, stability, and lifetime of the modified electrode are vital parameters for practical applications of sensing devices.70,71 For the lifetime measurement of CuxO−ZnO modified electrode, cyclic voltammetry was performed in a mixture of 2 mM glucose and 0.05 M NaOH for 1000 cycles (Figure S18a). The peak current response retained 89% of its initial value even after continuous 1000 cycles, showing high lifetime of the electrode system. The repeatability of the CuxO−ZnO modified electrode was studied in 6 different samples, each one containing 2 mM glucose in 0.05 M NaOH; the results are shown in Figure S18b. The peak current of CV response of the CuxO−ZnO electrode showed relative standard deviation (RSD) of 2.9%. Further, long time I−t curve analysis showed that the amperometric response of the CuxO−ZnO electrode retained its initial value even after continuous analysis for 1200 s (Figure S18c). Finally, the practical applicability of the CuxO−ZnO based electrochemical sensor was tested for the measurement of glucose in real blood serum samples, employing the standard addition method. Table S4 summarizes the results of a series of experiments where two known blood samples (collected from a diabetic patient from a local pathological clinic) were impaled with specific amount of glucose, and the glucose concentration was measured on the basis of repeated responses (triplicate) showing good recoveries (Figure S19). Therefore, the asprepared CuxO−ZnO electrode can be used for quantitative glucose detection in real blood samples.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b00838. Additional SEM images, EDX spectrum, elemental mapping, UV−visible spectra of dye reduction, N2 adsorption/desorption isotherms spectra, FTIR spectrum, CV plots, Raman spectra and experimental results in tabular form (PDF)

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AUTHOR INFORMATION

Corresponding Author

*T. K. Sarma. Email: [email protected]. ORCID

Suryakant Mishra: 0000-0002-9331-760X Tridib K. Sarma: 0000-0002-5168-6327 Present Address

§ (S.M.) Department of Chemical and Biological Physics, Weizmann Institute of Science, Rehovot, 76100, Israel.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank IIT Indore for laboratory and infrastructure facilities and SIC IIT Indore for instrumentation facilities. We are grateful to SAIF NEHU, Shillong for providing HRTEM facility, and ACMS, IIT Kanpur for XPS facility. S.J. thanks SERB, DST, India for DST-INSPIRE fellowship. Research funding from SERB, Department of Science and Technology, India (SR/S1/PC-32/2010) is acknowledged. We thank Dr. Rajesh Kumar, Dr. Sonam Mandani, Dr. Biju Majumdar, and Ms. Daisy Sarma for helpful scientific discussions.

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CONCLUSION In summary, a variety of hierarchical microstructures of a binary oxide CuxO−ZnO could be generated in a template-free approach through a one-pot hydrothermal method using PEG−H2O mixture as solvent. The morphology of the superstructures could be tuned by carefully controlling the synthetic parameters such as reaction temperature and concentration of precursor metal salts. Of particular interest was the formation of branched octapodal microcrystals through the faster facet oriented growth along the [111] plane from cubic seeds under strict thermodynamic and kinetically controlled regime using a controlled ratio of Cu2+ and Zn2+ precursor salts. The high surface area and the presence of a large number of exposed catalytically active facets in the octapodal CuxO−ZnO composite facilitated excellent

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REFERENCES

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