Hierarchical ZnO/zeolite nanostructures: synthesis, growth mechanism and hydrogen detection

Abstract

An improved hydrothermal method has been contrived for the synthesis of highly uniform, ordered, and monodispersed zeolite nanocrystals. An efficient impregnation of zinc on the outer surface of zeolite was carried out using aqueous solution at room temperature followed by high temperature calcination to form an interpenetrating network of hierarchical ZnO/zeolite-nanostructured assembly. The hierarchical structures offer high surface area, a porous structural network, a stimulated surface for catalytic and redox reactions, and better electron transport properties. Morphological and comprehensive structural analysis was carried out by advanced techniques including XRD, FESEM, TEM, STEM-EDS, BET and XPS, respectively. A plausible growth mechanism has been proposed for the formation of zeolite and ZnO/zeolite hierarchical nanocrystals. The hydrogen gas sensor made with these ZnO/zeolite hierarchical nanocrystals exhibited a very fast response of ∼10 s with slow recovery.

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1. Introduction

Zeolites are usually made up of a framework of tetrahedral TO₄ building units (T = Si, Al, etc.) bonded with each other by sharing oxygen atoms to form three-dimensional crystalline, porous building blocks.^1,2 Their tetrahedral framework results in variety of rings that are responsible for the formation of zeolite cages, porous networks, and multichannel windows.² The 3-dimensional (3D) framework, with its linked channel structures and very distinct micropores, offers an open porosity that potentially gives rise to remarkably high surface area.³ The porous materials have attracted substantial consideration due to their high surface area, highly uniform pore size distribution, ultra-high ordered pore arrangement, and conceivable surface engineering.^4–7

Compared to micro-structured zeolites, advanced nanostructured zeolites offer higher surface area, better porosity, and incredible surface engineering possibilities.^8–10 More recently, much attention has been paid to the synthesis of nanosized zeolites that can serve as model system for the basic understanding of nucleation and growth and that exhibit better performance in terms of ion exchange, sorption, separation, catalysis, and the capability to act as hosts in advanced materials.^11,12 The fundamental feature of the porous material for sensor applications is the large surface area provided for gas access. Porous metal oxide nanostructures have mostly been used as sensing materials.^13–20 Usually, metal oxide nanostructures are decorated with noble metals to enhance the response of sensor devices.^21–24

Recently, numerous hierarchical nanostructures have been investigated for gas sensor applications.^25–27 However, these hierarchical structures were limited by slow sensor response.²⁸ It is predicted that bifunctional complex materials based on metal oxide/zeolite nanostructures (MOZN) composed of n-type semiconductor ZnO and microporous zeolite nanocrystal framework could be potential candidates for improved gas-sensing nanodevices. This promising material may show synergic effect of the semiconductor ZnO and the ordered, uniform, porous zeolite nanocrystals. ZnO nanocrystals grown on the external surface of zeolite potentially improve the electrical and structural properties of the sensing layer, building an interpenetrating network with zeolite on the substrate that can provide active sites to interact with reactive gas molecules and change the electrical resistance.

Gas sensors with semiconducting metal oxides strongly depend on the nature of the materials and the structures of the sensing layer with respect to bulk, surfaces, porosity, grain boundaries, and interfaces between the contacts and the substrate. The zeolite framework provides high surface area with uniform porosity.^29–34

Hydrogen is a potential fuel for vehicles and can be converted into electricity in fuel cells. It is also used in space exploration and the production of industrial chemicals, petroleum refining, and food products. Safety is a matter of concern when using hydrogen gas. An explosive blend can form if hydrogen leaks into the air from a tank or valve, posing a hazard to drivers, operators, or others present in the neighboring environment.

In this work, hierarchical ZnO/zeolite nanocrystals (ZZNs) synthesized by hydrothermal process and in aqueous solution were used to fabricate a hydrogen gas sensor. The aim of this work is to understand the structure–property relationship of ZnO/zeolite hierarchical structures and their performance in resistor-based hydrogen gas sensors. Also, detailed structural characterization was performed to understand the formation of the heterostructure. A conceivable growth mechanism for the formation of zeolite and ZnO/zeolite hierarchical structures is proposed. It is expected that this new device configuration not only offers performance that is comparable to the initial devices, but also provides a faster, reliable response and long-term stability in gas sensors.

2. Experimental details

Zeolite Y was synthesized using clear solutions. Aluminum isopropoxide (3.5 g), tetramethyl ammonium hydroxide (TMAOH (8 g)) solution, and water (40 ml) were mixed and stirred until the reaction mixture became clear. Next, tetraethyl orthosilicate (TEOS (7.9 g)), an additional amount of TMAOH solution (8 g), and water were added to the clear solution. The mixture was stirred overnight to ensure complete hydrolysis of the aluminum and silicon sources. The final clear synthesis gel had the following composition: 0.07 Na : 2.4 TMAOH : 1.0 Al : 2.0 Si : 132 H₂O : 3.0 i-PrOH : 8.0 EtOH. A thorough washing was carried out with water to remove excess sodium.

The reaction temperature and time were optimized to between ∼80–110 °C, and 46 h, respectively. The reaction temperature and duration of hydrothermal treatment (autoclave vessels) were found to be critical for the growth of Y type zeolite. Later, the product was centrifuged followed by drying in an oven overnight and calcination for 16 h at 500 °C. However, in this hydrothermal synthesis experiment, relatively low yield of nanocrystals was obtained. Several reactions were carried out in order to obtain the zeolite nanocrystals.

Sodium ions in the zeolite structure were ion exchanged for protons to convert the zeolite to the acid form. The sodium product from above is placed in an Erlenmeyer flask and stirred with 12.60 ml of 1 M aqueous ammonium hydroxide for 24 h. After that, the zeolite was collected by Buchner filtration, which was repeated for 5 washings. The product was also washed with small amounts of acetone and several times with distilled water and dried in an oven.

The above ion-exchanged zeolitic nanocrystals were then mixed with zinc acetate hydrate (0.5 mM) in ethanol (50 ml) in an Erlenmeyer flask and kept under stirring for 24 h at room temperature. Then, the solution was washed in a Buchner funnel several times and dried in an oven. The final product was calcined for 16 h at 500 °C in a muffle furnace. The product (ZnO/zeolite) was finally characterized by different techniques.

3. Characterization

The X-ray powder diffraction (XRD) patterns of the samples were collected using a Riguka X-ray diffractometer with CuKα radiation. Surface morphology and chemical composition of zeolite were analyzed by FESEM equipped with EDX. Microstructural analysis was carried out by transmission electron microscopy (TEM). High angle annular dark-field STEM (HAADF-STEM) line profile was used to confirm the elemental composition of the composite structure. The BET-specific surface area, micropore volume, and external surface area were measured by BET (Micrometrics), and comprehensive chemical composition was further investigated by X-ray photoelectron spectroscopy (XPS).

3.1 Deposition of sensing layer on IDE

5–10 mg of ZZNs were dispersed in 1.5 ml of ethanol and sonicated for 5–10 min. in order to obtain a well-dispersed solution. This solution was drop-casted on an interdigital electrode at 70 °C. The drop-casting was carried out on a hot plate to ensure complete evaporation of ethanol and formation of a uniform sensing layer on the interdigital fingers.

3.2 Gas sensing measurements

The interdigital electrode (IDE) was placed in the gas sensing chamber for electrical resistance measurement. Two probes coupled to an Agilent (B1500) multimeter were well adjusted and connected to two different electrode pads. Water cooling system was started to ensure proper chamber cooling. The chamber was connected to the gas line from one side and with the exhaust from the other side. Mass flow controllers (MFCs) were used to dilute the detected gas (hydrogen) in nitrogen (N₂). The electrical resistance was measured at different temperatures by diluting hydrogen (H₂) in N₂. The response and recovery were monitored by an Agilent multimeter (B 1500). Initially, hydrogen gas (H₂) was detected using standard SnO₂ nanoparticles. Similarly, the sensor response for ZZNs was measured.

4. Results and discussion

4.1 Surface morphology structural and compositional analysis

Fig. 1 shows XRD spectrum of Y zeolite nanocrystals, ZnO/zeolite composite structures, and ZnO nanoparticles. The XRD spectrum of Y zeolite matches well with the Faujasite zeolite (JCDPS card no. 00 038-0238). The XRD spectrum of the ZnO/zeolite nanostructure indicated additional peaks, which correspond to ZnO 1010, 0002, 1011 and 1120, respectively. However, there is a shift and integration of peaks observed in the spectrum. This may be due to calcination in the presence of zinc salt, which might have distressed the framework of silicon and aluminum. Also, XRD analysis of ZnO nanoparticles synthesized in ethanol medium is presented. Beside ZnO wurtzite hexagonal peaks, some additional peaks (*) were seen in the XRD spectrum due to unreacted zinc salt in ethanol medium, which is well documented in the literature. The surface morphology of the samples was examined by FESEM. Fig. 2(a–d) shows low and high magnification FESEM micrographs of zeolite Y nanospheres. These zeolite nanospheres are highly uniform in their size and shape, with high monodispersity. The size of each sphere is around 100–150 nm. The surfaces of the nanospheres are very smooth, and small, glittery nanoparticles can be seen due to the gold coating. Fig. 3(a–d) shows low and high magnification FESEM images of the ZnO/zeolite composite structure. It is clearly seen that ZnO nanocrystals are attached to the surface uniformly. No surface coating is applied in this ZnO/zeolite sample, verifying the attachment of ZnO nanocrystals; a slightly charging effect is also observed, which can be seen in the in low and high magnification FESEM micrographs (Fig. 3). Chemical composition of the sample was analyzed by EDX to determine the ratio of Al/Si (Table 1). These results are close to the expected atomic ratio of aluminum and silicon. Table 2 shows their EDX compositional analysis, which confirms the presence of Zn, Al, Si and O, respectively. Fig. 4 shows the nitrogen adsorption/desorption of zeolite nanocrystals. The BET-specific surface area of zeolite nanocrystals was found to be 517 m² g⁻¹ and slightly decreased for ZnO/zeolite. The pore volume of zeolite nanocrystals was 0.393807 cm³ g⁻¹, with no significant decrease for ZZNs, and the pore size of zeolite nanocrystals was found to be 30.4250 Å.

Fig. 1 The XRD spectrum of zeolite Y nanocrystals.

Fig. 2 (a–d) Low and high magnification FESEM images of zeolite Y nanocrystals.

Fig. 3 (a–d) Low and high magnification FESEM micrograph of ZnO/zeolite.

Table 1 Chemical composition of Al and Si in Y-type zeolite nanocrystals were taken from different positions of same sample

Element	Atomic%
O K	50.39
Na K	3.47
Al K	13.59
Si K	32.56

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Table 2 Chemical composition of Al and Si in Y-type zeolite nanocrystals were taken from different positions of same sample

Spectrum	O	Al	Si	Zn	Total
Spectrum 1	51.48	12.81	31.78	3.93	100.00
Spectrum 2	48.25	13.60	34.01	4.13	100.00
Spectrum 3	50.47	13.45	32.22	3.86	100.00
Spectrum 4	47.44	13.30	34.04	5.22	100.00
Spectrum 5	48.88	13.36	33.63	4.13	100.00
Spectrum 6	46.60	13.46	35.09	4.85	100.00

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Fig. 4 Nitrogen adsorption/desorption of zeolite nanocrystals.

The detailed structural and compositional analysis for the formation of ZnO/zeolite nanostructures was carried out by TEM and STEM-EDS. Fig. 5(a–d) shows high and low magnification TEM micrographs of the composite. Very clearly, small ZnO nanocrystals are anchored and penetrated on the surface of zeolite due to ion exchange reactions. The circled particles clearly indicate the interpenetrating network of ZnO/zeolite structures. These TEM results are highly consistent with FESEM investigations. Fig. 5(e and f) shows HRTEM images of ZnO/zeolite. HRTEM results also confirmed good dispersion of ZnO nanoparticles on the zeolite matrix surface, and no self-aggregation or cluster formation is observed. This might be due to the close interaction of zeolite active sites with the ZnO surface, which has relatively high surface energy, smaller size, and better impregnation during the reaction. The detailed chemical compositions were further scrutinized by STEM-EDS (Fig. 6), which depicts the interpenetrated ZnO nanocrystals on the surface, anchored on zeolitic nanocrystals. Fig. 7(a–d) shows the chemical composition of ZZNs by XPS. All the peaks corresponding to Zn, O, Si and Al were observed in the spectrum.

Fig. 5 (a–f) Low and high magnification TEM micrographs and HRTEM of ZnO/zeolite.

Fig. 6 (a–d) Low and high magnification STEM–EDS profile of ZnO/zeolite composition.

Fig. 7 XPS analysis of (A) Si2p, (B) Al2p, (C) Zn2p, and (D) O1s.

4.2 Growth mechanism for the formation of zeolite and ZnO/zeolite nanocrystals

Fig. 8 shows the schematic diagram of the growth mechanism of zeolite and ZnO/zeolite composite structures. Generally, the formation of new crystalline units from the growth solution starts through the process of nucleation.³⁵ Nucleation is a phase of sequential atomic or molecular processes through which atoms and molecules or reactants reorganize into the building block of the product, with great potential to grow irretrievably to a macroscopically bigger size. The general synthesis of zeolite nanocrystals is a multifaceted process. Growth depends on various parameters, including crystallization rate, nature of product formation, and nanocrystal properties (shape morphology, crystal size, and crystal uniformity).³⁵ These parameters also include the crystallization conditions and chemical compositions: e.g. growth temperature, seeding, gel ageing, pH of solution, water content, ratio of Si/Al, template concentration, surfactant, reaction vessel pressure, and ionic strength.³⁶ For the growth of zeolite nanocrystal, induction firstly ensues, in which the aluminosilicate solution gradually forms the building units of the tiny gel (a). Then, addition of organic template to the aluminosilicate gel enhances the formation of amorphous agglomerates (b). Nucleation of zeolite is a very intricate process, as it entails conversion of the amorphous gel into a good crystalline framework. In the amorphous phase, there is extensive growth in the small-range structural order. In this step, an arbitrary number of structured portions may attain the size of nucleus and start to grow into macroscopic crystals (c). This is due to high supersaturation within the gel or amorphous framework, and most prominently the optimized reaction conditions, which might mutually act as driving force for the primarily controlled nucleation process (c).³⁷ The formation of seeded nuclei conceivably triggers the process of nucleation of the remaining amorphous network in the solution, followed by secondary nucleation through Ostwald ripening. At elevated temperature (∼80–110 °C) and autogenous pressure in hydrothermal vessels, crystal growth proceeds, forming the highly uniform zeolite nanocrystals (d and e). The sodium contents in the zeolite nanocrystals are ion-exchanged by ammonium hydroxide. Then, zeolite nanocrystals are treated with zinc salt at room temperature followed by high-temperature calcination, resulting in hierarchical ZZNs (f), which is confirmed by different advanced characterization techniques (STEM-EDS and XPS).

Fig. 8 Growth mechanism for the formation of zeolite and ZnO/zeolite nanocrystals.

4.3 Hydrogen sensing behavior of ZnO/zeolite nanocrystals

Fig. 9 shows the schematic diagram of a gas sensor made of ZnO/zeolite hierarchical nanostructures. Fig. 10(a and b) shows the hydrogen sensor response of ZZNs. The samples showed relatively high resistance, but this substantially decreased with increasing temperature. The ZZN-based sensor showed a reversible response to hydrogen gas. The electrical resistance of the samples swiftly decreased upon injection of hydrogen gas (1000 ppm). Also, when the flow of H₂ gas stopped, the resistance of the sample slowly reached the initial point. The hierarchal ZnO/zeolite showed fast response, about 10 s, at 400 °C and slow recovery compared with recent literature (see ESI†). Also, the response of ZZNs towards hydrogen gas at different concentrations is monitored, which shows expected behavior (ESI†). However, at lower temperature, metal oxides on the surface of insulating zeolite crystals were not activated. At the higher operating temperature, the response is more notable and swift, which is typically attributed to the intensified reaction between hydrogen and the adsorbed oxygen with the increased temperature on the surface.³⁸ The fast response might be attributed to the fine tuning of hierarchical ZZNs. The STEM-EDS micrograph (Fig. 6(a)) clearly shows the highly homogenous and monodisperse distribution of ZnO nanocrystals on the surface of zeolite nanocrystals. For matrix (zeolite)-stabilized metal oxides, the overall structure of the metal oxide, structural defects, and conductivity is highly influenced by preparation techniques.³⁹

Fig. 9 Schematic illustration of the sensor device made up of ZnO/zeolite nanocrystals.

Fig. 10 (A and B) Gas sensor response of ZnO/zeolite nanocrystals for hydrogen gas at 400 °C.

The presence of a reducing gas causes a transfer of electronic charge at the surface, predominantly contingent on both gas concentration and operating temperature. Here, hierarchal ZnO/zeolite (matrix) nanocrystals showed fast response towards hydrogen gas due to the well-maintained matrix structure, enhanced reactions at grain boundaries, and complete depletion of carriers.^40,41 Since the matrix-decorated ZnO nanocrystals are n-type semiconductors with plenty of oxygen vacancies on the surface offering donor states, the electrical conductivity of the ZZNs is heightened by the constant concentration of adsorbed oxygen ions through the chemical dynamics of redox properties and diffusion.⁴² Hydrogen, with its small molecular size, can possibly penetrate the surface layer. This semiconductor oxide-decorated matrix surface reacts swiftly with negatively charged oxygen adsorbates, resulting in water and free electrons.⁴³ As a result, the conductivity of ZZNs increases swiftly.

5. Conclusion

A simple modified hydrothermal method was developed for the synthesis of porous, highly ordered zeolite nanocrystals. A two-step hybrid method was developed for the synthesis of hierarchical ZZNs. We furthermore used various advanced techniques to investigate the growth of zeolite and the formation of ZnO/zeolite hierarchical nanostructures. A sensor array of ZZNs was fabricated for hydrogen gas detection. The sensor showed a fast response of 10 s with slow recovery time. From an environmental insight, the synthesis reported in this work is also convenient, fast, and requires low energy, since the nanocrystals can be straightforwardly synthesized at low temperature, in a reproducible manner and the shortest possible reaction time, compared to the methods reported in literature. Consequently, the present synthesis approach offers most useful opportunities for both fundamental studies and potential applications of hierarchical assembly. The abundance of ZZN sensor fabrication materials and the low-cost assembly of the electronics for the sensing array shows good potential for future hydrogen-sensing miniature devices.

Acknowledgements

The authors would like to acknowledge the support provided by King Abdul-Aziz City for Science and Technology (KACST) through the Science & Technology Unit at King Fahd University of Petroleum & Minerals (KFUPM) for funding this work through project 09-NAN772-04, as part of the National Science, Technology and Innovation Plan.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra15497e

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