Hierarchical zinc oxide nano-tips and micro-rods: hydrothermal synthesis and improved chemi-resistive response towards ethanol

Abstract

In the present work, a novel hierarchical architecture of zinc oxide has been synthesized through a spherical carbon template assisted two step hydrothermal process. The architecture can be represented visually as a combination of secondary phase nano-tips grown on initially prepared zinc oxide micro rods. An attempt has been made here to understand the mechanism for the formation of the said zinc oxide architecture. As compared to the zinc oxide micro rods, the enhanced surface activity of the hierarchical architecture has been reflected during its improved chemi-resistive response towards ethanol. The sensing characteristics are measured by varying the sensor operating temperature (250–350 °C) and ethanol concentrations (50–200 ppm). The prepared micro rods and hierarchical architecture show 47 and 59% responses towards 50 ppm ethanol at their optimum operating temperature of 325 °C.

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1. Introduction

Zinc oxide (ZnO), an important wide-band-gap (∼3.2 eV) semiconductor has been investigated extensively for various applications including but not limited to piezoelectric transducers, optical waveguides, surface acoustic wave filters, chemical/gas sensors, catalysts, light-emitting diodes, solar cells etc.^1,2 The morphology of zinc oxide plays crucial role in modulating their performances for many of these applications (e.g. catalytic, sensing, sensitized type solar cells etc.). A variety of synthesis methods (e.g. chemical/physical vapour deposition, metal–organic vapour phase epitaxy, solgel, wet chemical etc.) have already been reported for preparing different ZnO architectures (e.g. rods, wires, sheets, tubes, hollow spheres, dumbbell like, tetrapod like, waxberry-like, feather-like, needle-like, shuttle-like, obelisk-like, rotor-like, flower-like particles etc.).^3–8 In recent years, such artistically impressive and functionally promising shapes of ZnO are attracting immense attention from the scientific community particularly to realize their growth mechanisms as well as to know their convenience towards multifunctional applications. For instance, the mechanism for the hydrothermal growth of ZnO nanowire has been studied by N. J. Nicholas et al.⁹ D. Iqbal has synthesized ZnO nano-rod arrays grown along (0001) direction (i.e. along c-axis) at room temperature and investigated its photo-catalytic activity for the decomposition of organic dyes.¹⁰ Similar type growth (along c-axis) of ZnO nano-wires on Si substrate through hydrothermal synthesis route is investigated by T. Y. Olson et al.¹¹ H. Huang et al. reported the high performance H₂S detection of ZnO nanowires grown within interdigitized metal comb-electrodes using hydrothermal method.¹² ZnO nanorod arrays prepared through mechano electrospinning-assisted process in conjunction with hydrothermal growth process are utilized as NO₂ sensors by X. Wang et al.¹³ T. Wang et al. synthesized unique ZnO nanopencil and investigated their performances for photo-chemical water splitting.¹⁴ A. Resmini et al. demonstrated the gas sensing application of ZnO nano-rod networks prepared through cost-effective chemical procedures.¹⁵ D. Gedamu et al. has investigated UV response features of interpenetrated ZnO nano-tetrapod networks.¹⁶ As reviewed, c axial growth of 1D ZnO and their multifunctional properties are adequately reported in literature. Generally, the anisotropic crystal structure (hexagonal wurzite with Zn²⁺ and O²⁻ terminated polar surfaces) of ZnO is considered for facilitating its growth along c axis.

In the present work, a novel hierarchical architecture composed of nano-tips on micron size ZnO rod like structure has been prepared. Unlike the aforementioned either nano or micron scale ZnO morphologies, the present architecture is an aesthetic conjugation of both micron and nano scale structures. More specifically, ZnO nano-tips are grown over micron scale hexagonal rod like structure through carbon template assisted two step hydrothermal procedures. An attempt has been made here to understand the formation of the developed ZnO architecture through the rigorous analyses of electron microscopy images. Due to the growth of the nano-tips on micro-rod, the surface area of the conjugated structure is improved than the simple micro-rod based structures. As compared to simple micro-rod based structure, the enhanced surface activity of the conjugated architecture has been reflected during its improved chemi-resistive response towards ethanol. The growth of secondary ZnO nano-tips/nano needles orthogonal to the primarily grown (along c axis) rod like structures is not very frequent in literature. The formation of such conjugated hierarchical structures through two stage chemical route is highlighted previously by T. L. Sounart et al.¹⁷ Herein, not only a unique morphology of ZnO is introduced, but its improved ethanol sensing characteristics are also demonstrated.

2. Experimental

The simple micro-rod and nano-tips decorated micro-rod based hierarchical morphologies of ZnO are prepared through hydrothermal route. For synthesis, first 0.25 mole zinc nitrate is dissolved in distilled water. Then ammonium hydroxide is added slowly to the solution that initially results white precipitate. The addition of excess ammonium hydroxide to the mixture ultimately produces a transparent solution. Next, spherical carbon templates are added to the solution and the whole mixture is kept under ultra-sonication for 15 min. The carbon templates are prepared from dextrose through the hydrothermal procedure described elsewhere.¹⁸ The ultrasonic treatment results a homogeneous suspension which is transferred equally into two separate but similar Teflon lined autoclaves namely A-I and A-II. The autoclave A-I is kept at 160 °C for 24 h whereas autoclave A-II is kept at 160 °C for first 12 h and then maintained at 190 °C for another 12 h. Both the autoclaves are then cooled under ambient atmosphere. The obtained mass is centrifuged and the precipitate is collected. The precipitate is washed with plenty of water and then dried in laboratory oven at 80 °C. The dried mass is calcined at 450 °C to get crystalline ZnO. The synthesis procedure is summarised briefly in schematic Fig. 1.

Fig. 1 Experimental procedure for the synthesis of zinc oxide micro-rod and nano-tips decorated micro-rod hierarchical structure.

To measure the sensing characteristics, the sensing elements are prepared by coating thick films of the A-I and A-II derived products on alumina substrates using mixture of ethyl cellulose and terpineol as binder. The sensing elements are then heat treated at 400 °C for 1 h to remove the organics. Ag-paste based planner electrodes (separated by ∼2 mm) are used to take out the electrical response from the sensing elements. The surface current of the sensor is estimated by applying a fixed voltage (∼20 V) to one of the electrode. From the measured value of the equilibrated surface current in air (I_air) and in vapour (I_vap), the response% (S) of the sensor is estimated using the following relation

S = (I_vap − I_air)/I_vap × 100 (1)

The sensing characteristics are measured in a static flow gas sensing chamber described elsewhere.¹⁹ The set-up is equipped with a temperature controller (2216e, Eurotherm, USA) and a source metre (2612A, Keithley, USA) interfaced with the PC through GPIB interface and operated through LabTracer 2 software. Before starting the sensing measurements, the sensing elements are aged at the respective operating temperature for half an hour to achieve equilibrated surface current of the sensor in air (I_air). The recovery of the sensor is attained by opening the circular lid (diameter 8 cm) attached with the sensing chamber.

3. Results and discussions

As envisaged from the Fig. 1, the morphology of the products derived from A-I and A-II are different. Specifically, hexagonal micron scale rod like structure is resulted from A-I whereas nano-tips are grown on micron sized rod in products resulted from A-II. Since dissimilar morphologies are resulted when same volume of the reaction mixtures are kept under similar condition (except the schedule of reaction temperature) and for equal duration, the temperature schedule of hydrothermal reaction can be proposed as the main factor for creating products with different morphologies. The underlying mechanisms which facilitate the formation of such morphologies particularly the hierarchical aesthetic combination of ZnO nano-tips and micro-rod is not very clear to us till date.

Towards understanding the formation mechanism of hierarchical ZnO structure, the morphologies of carbon templates and hierarchical ZnO could provide significant information. In Fig. 2(a), the electron microscope image of prepared carbon spheres is shown. As reflected in figure, the diameters of hydrothermally prepared spheres are in the range of 1–2 μm. The cluster of ZnO micro-rod prepared through single stage (heat treatment at 160 °C for 24 h) hydrothermal process is shown in Fig. 2(b). In Fig. 2(c) and (d), the FESEM images of nano-tips decorated hierarchical ZnO prepared through double stage (heat treatment at 160 °C for 12 h followed by at 190 °C for 12 h) hydrothermal process are presented.

Fig. 2 FESEM images of prepared (a) spherical carbon templates (b) zinc oxide micro-rod assembly. (c) and (d) Two different architechtures of zinc oxide nano-tips decorated micro-rod assembly. Inset of (c) shows the zinc oxide micro-rod hexagonal face without nano-tip decoration. Inset of (d) shows the nano-tips decorated hexagonal face of zinc oxide micro-rod.

Critically investigating these figures, it can be stated that the morphology shown in Fig. 2(c) and (d) are different. Fig. 2(c) contains the hierarchical ZnO where the hexagonal face is not decorated with nano-tip like morphology (highlighted in dotted circle as well as inset figure). On contrary, an attractive decoration of nano-tips is observed in the hexagonal face of micro-rod ZnO shown in Fig. 2(d). For more clarity, nicely decorated typical hexagonal face of hierarchical ZnO is shown in the inset of Fig. 2(d). As shown in the figure, the nano-tips are grown around a circular area above the hexagonal face of ZnO micro-rod. Keeping in mind the experimental conditions and investigating rigorously the relevant electron microscope images, a plausible mechanism has been proposed here. The proposed schematic pathway for the growth of micro-rod shaped and hierarchical ZnO is presented in Fig. 3. In aqueous solution, Zn(NO₃)₂ first decomposes into zinc cations (Zn²⁺) and nitrate (NO₃⁻) ions. The spherical carbon templates having surfacial hydrophilic C=O and –OH groups (confirmed in FTIR shown in ESI Fig. S1† and also reported by X. Sun et al. and Y. Z. Jin et al.) then anchor the Zn(II) ions.^20,21 Under hydrothermal condition (at fixed temperature of ∼160 °C), such anchoring initially produces giant spherical structure of micro-rod assembly. The schematic of giant spherical structure of ZnO micro-rod assembly (product A) and the individual building block of hexagonal shaped ZnO micro-rod (grown along c axis) are shown in Fig. 3. During the process of hydrothermal reaction, heat treatment schedule when changed after 12 h, the kinetics of growth alters. Thus a secondary phase of nano-tips grow on the initially formed micron length rod when the reaction mixture is kept at 160 °C for 12 h followed by at 190 °C for another 12 h under hydrothermal condition. Depending on the availability of carbon-templates in the vicinity of the micro-rod shaped ZnO, two different hierarchical ZnO (product B and C in Fig. 3) may form. In case of product B, the growth of nano-tips occur only on the wall of ZnO micro-rod. The hexagonal face of ZnO micro-rod is not decorated by nano-tips for product B. On the other hand, a well organised decoration of nano-tips on the hexagonal face as well as on the micro-rod wall is observed for product C. It is difficult to physically isolate the product B and C from the mixture and in the present work no attempt is made to isolate these products. We have found only the two different type of morphology (product B and C) while acquiring the FESEM images. It is also noteworthy that we found mostly the product C in the mixture under the FESEM. However, the presence of product B can not be avoided in the mixture. The electron microscope images of product B and C are already shown in Fig. 2(c) and (d) respectively.

Fig. 3 Schematic mechanism for the formation of zinc oxide micro-rod and nano-tips decorated micro-rod.

For product C, the typical SEM images captured by raising the magnifications are shown in Fig. 4(a)–(d). From Fig. 4(d), the growth of nano-tips on hexagonal face as well as hexagon wall is distinguished clearly.

Fig. 4 (a)–(d) FESEM images for zinc oxide architectures composed of nano-tips decorated micro-rod.

The morphology of the products is further studied using transmission electron microscope. For A-II derived products, the typical TEM images captured in different magnifications are shown in Fig. 5(a) and (b) where the growth of ZnO nano-tips on the micro-rod is identified. The lengths of the nano-tips are found in the range of 50–100 nm whereas the micro-rods are of 12–15 μm. The lattice fringe (corresponding to (101)) of the hierarchical structure is shown in Fig. 5(c). The fringes are captured from the highlighted portion of the structure shown in inset of Fig. 5(c). Because of the poor crystalline nature, the lattice fringe of the nano-tip is not identified. The SAED pattern of the hierarchical structure is shown in Fig. 5(d). During capturing the SAED pattern, it is difficult to restrict the diffraction of electrons on single microrod/nano-tip. As a consequence the phase related features for the hierarchical structures arise in the SAED pattern which resembles the pattern for polycrystalline materials. Indexing the “ring pattern” of the SAED, it is confirmed that the hierarchical structures are crystallized into a hexagonal wurzite phase which is further verified in the XRD pattern shown in ESI Fig. S4.† The compositions of the nano-tips as well as micro-rod are separately studied using the energy dispersive X-ray spectroscopy (EDS) facility associated with the FESEM instrument. Compositional analyses using point scanning mode exhibit that the zinc (Zn) and oxygen (O) ratio is higher for micro-rod than nano-tips (shown in ESI Fig. S2†) which further supports that the crystallinity of the micro-rod is better than the nano-tips. The surface area of the products derived from A-I and A-II are estimated from the N₂ adsorption–desorption isotherm. The N₂ adsorption–desorption isotherms for products A-I and A-II are shown in ESI Fig. S3.† The estimated surface area of product derived from A-II (7.2 m² g⁻¹) is found considerably higher than the product derived from A-I (3.9 m² g⁻¹). The higher surface area, better porosity as well as nano-zigzag surface pattern of semiconducting ZnO can be effective to improve its performance for catalysis, gas/vapour sensing, adsorption and related applications.

Fig. 5 (a) and (b) TEM images with increasing magnifications for zinc oxide nano-tips decorated micro-rod. (c) Lattice fringe of zinc oxide micro-rod. Inset figure highlights the selected part of micro-rod from where the fringe pattern is acquired. (d) SAED pattern of zinc oxide nano-tips decorated micro-rod structure.

As a case study, we have compared here the chemi-resistive sensing performance of the sensors prepared using the semiconducting ZnO powders derived from A-I and A-II. The principle of chemi-resistive sensors is simply based on their resistance change when exposed to reducing gas/vapour.²²

During the sensing process, first, at elevated temperature oxygen is chemi-adsorbed on the sensor surface either in atomic (O⁻) or in molecular (O₂⁻) forms described in eqn (2) and (3).²³

O₂ + e → O₂⁻ (2)

O₂ + 2e → 2O⁻ (3)

Such chemi-adsorbed oxygens (O₂⁻/O⁻) electronically deplete the sensing surface and forms a Schottky potential barrier for electron conduction leading to decrease in surface current of the sensor. In exposure to reducing vapours (e.g. ethanol), the chemi-adsorbed oxygens oxidizes these vapours (shown in eqn (4)) and the released electrons come back to the conduction band of the sensor. As a consequence, the width of electron depleted layer decreases leading to increase in the output surface current from the sensor.

C₂H₅OH + 6O⁻ → 3H₂O + 2CO₂ + 6e⁻ (4)

The chemi-adsorption of oxygen followed by the formation of electron depleted layer over A-I (ZnO micro-rod) and A-II (nano-tips decorated ZnO micro-rod) based sensing elements and the formation of subsequent products by the reactions of ethanol with the chemi-adsorbed oxygen are presented in Fig. 6.

Fig. 6 Schematic illustration on the ethanol sensing phenomenon on ZnO micro-rod and nano-tips decorated micro-rod.

Due to the growth of additional nano-tips, the A-II sensor facilitates chemi-adsorption of more oxygen than A-I sensor which ultimately creates more opportunity for the interaction of ethanol with chemi-adsorbed oxygens over A-II sensor. Viewing in the same line, it is presumed that the A-II sensor should exhibit better response towards ethanol than A-I sensor. The results are verified by measuring experimentally the ethanol sensing characteristics of A-I and A-II sensors. For A-I (micro-rod) and A-II (hierarchical) derived ZnO based sensing elements, the transient current at different operating temperature (250–350 °C) for sensing 50 ppm ethanol are shown in left and right panel of Fig. 7 respectively. The operating temperature and the estimated response% are mentioned in the figures. As revealed from the figures, the sensors restore its original value of output current during recovery. From the operating temperature dependent response of A-I and A-II based sensor towards fixed concentration (50 ppm) of ethanol (shown in Fig. 8(a)), it is observed that both the sensors show maximum response at ∼325 °C. The response of semiconducting metal oxide (SMO) based chemi-resistors are generally achieved at an optimum operating temperature. Beyond the optimum temperature, the response of the sensor decreases mainly because of enhanced desorption probability of the oxygen from sensor surface. This phenomenon can again be explained using the depleted layer width (L_D) concept described elsewhere.²⁴ The response time (time taken to achieve the 90% of the equilibrated current after the exposure of ethanol) for both A-I and A-II derived samples are measured at the studied operating temperature range. For the detection of 50 ppm ethanol, the estimated response time is found high (∼100 s) at low operating temperature (<300 °C). However, beyond 300 °C, the response times for these sensors are found in the range of 50–55 s.

Fig. 7 Transient surface current of the zinc oxide micro-rod (left panel) and hierarchical (right panel) sensing elements towards 50 ppm ethanol.

Fig. 8 (a) Operating temperature dependent response (%) of simple and nano-tips decorated micro-rod based sensing elements towards 50 ppm ethanol. (b) Transient surface current of simple and nano-tips decorated micro-rod based sensing elements towards 50–200 ppm ethanol at 325 °C.

However, at the studied operating temperature range, the A-II product based sensing element show better response than A-I based sensor. The transient surface current of these sensors (kept at ∼325 °C) obtained by varying the concentration (50–200 ppm) of ethanol vapour are shown in Fig. 8(b). As reflected from the figure, the distinct change in the surface current of these sensors for detecting 50–200 ppm ethanol is identified. The estimated response% values of these sensors for detecting 50–200 ppm ethanol are compared in the Fig. 9. Comparing the response% of these sensors, it can further be stated that the A-II sensor is more efficient for the detection of low concentration (50–100 ppm) of ethanol. Critically investigating the Fig. 9, it is observed that response of A-II sensor does not show linear variation with the vapour concentration. The linear response of the sensor is expected when the adsorption, desorption, diffusion and interaction of vapours with sensor surface changes proportionally with the vapour concentrations. More specifically, the linearity of the response depends on the dynamics of the interaction between the vapour phase and sensor. At this stage, it is difficult to comment on the actual dynamics of the vapour and sensor interaction. However, based on the observed results we feel that the surface morphologies of A-I and A-II derived materials could play important role in controlling the linear response of the sensors. The decoration of nano-tips on micro-rod for A-II sample creates uneven surface which may lead to the disorder interaction of vapour with sensor resulting non-linearity of the response with the vapour concentration.

Fig. 9 Variation of response% of simple and nano-tips decorated micro-rod based sensing elements with ethanol concentration for detection of 50–200 ppm ethanol.

The sensing performances of prepared hierarchical ZnO structures are again studied in presence of other volatile organics (e.g. acetone, formaldehyde and methanol). The variation of response% for hierarchical ZnO sensor towards the detection of 50 ppm ethanol, acetone, formaldehyde and methanol is shown in Fig. 10. As reflected from the figure, the hierarchical ZnO sensor show better response towards ethanol compared to other common volatile organics. The superior response of hierarchical ZnO sensor can be attributed due to its higher surface area as well as zigzag surface pattern of ZnO structure which provides better surface area as well as more binding sites for the interaction of gases on the sensor surface.

Fig. 10 Variation of response% of nano-tips decorated hierarchical ZnO sensor for detecting 50 ppm ethanol, acetone, formaldehyde and methanol at ∼325 °C.

4. Conclusion

In summary, a novel hierarchical structure of ZnO is prepared from zinc nitrate hexahydrate through spherical carbon template assisted two step hydrothermal process. The structure can be represented as an aesthetic combination of ZnO nano-tips and micro-rod. While the single step 24 h heat treatment at 160 °C under hydrothermal condition yields simple micro-rod structure, double step heat treatment (12 h at 160 °C followed by at 190 °C for another 12 h) produce nano-tips over micro-rod. The spherical carbon templates help to grow artistically the nano-tips on the hexagonal face of micro-rod. An attempt has been made to understand the mechanism for the formation of the synthesized products through the rigorous analyses of electron microscopy images. Due to the growth of nano-tips (length ∼ 50–100 nm) over micro-rod (length ∼ 12–15 μm), the hierarchical ZnO structure is composed of zigzag surface pattern and posses higher surface area than the single step derived micro-rod ZnO. As compared to micro-rod ZnO, the superior surface activity of the hierarchical structure is reflected through its higher chemi-resistive response for sensing ethanol. The sensing characteristics are measured by varying the operating temperature (250–350 °C) of the sensing elements and concentrations (50–200 ppm) of ethanol vapour. Within the studied operating temperature range, both the sensing elements shows maximum response at ∼325 °C. At this optimum operating temperature, micro-rod and hierarchical ZnO based sensing elements exhibit 47 and 59% response towards 50 ppm ethanol. The hierarchical structure also reveals better response towards ethanol than other common volatile organics (e.g. acetone, formaldehyde and methanol).

Acknowledgements

The authors express their sincere thanks to Director, CSIR-CMERI for his kind support to carry out the work. KM thanks DST, Govt. of India for providing him Inspire Faculty fellowship (Ref. DST/IFA12-CH-43) and associated research grant. PD is thankful to DST, Govt. of India for supporting her fellowship from Inspire Faculty research grant.

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Footnote

† Electronic supplementary information (ESI) available: FTIR of spherical carbon templates, FESEM images in conjunction with EDX spectra and N₂ adsorption–desorption isotherms of simple and nano-tips decorated micro-rods and XRD of ZnO nano-tips decorated micro-rods. See DOI: 10.1039/c5ra23203a

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