Photoluminescence and hydrogen storage properties of gallium nitride hexagonal micro-bricks

Materials Letters 79 (2012) 212–215

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Photoluminescence and hydrogen storage properties of gallium nitride hexagonal micro-bricks Ghulam Nabi a, Chuanbao Cao a,⁎, Sajad Hussain a, Waheed S. Khan a, Tariq Mehmood a, Zahid Usman a, Zulfiqar Ali a, Faheem K. Butt a, Yan Fu a, Jili Li a, M. Zubair Iqbal b a b

Research Centre of Materials Science, School of Material Science and Engineering, Beijing Institute of Technology, Beijing 100081, People's Republic of China Department of Physics, School of Applied Science, University of Science and Technology, Beijing 100083, People's Republic of China

a r t i c l e

i n f o

Article history: Received 9 January 2012 Accepted 28 March 2012 Available online 4 April 2012 Keywords: Semiconductors Defects Chemical vapor deposition Hydrogen storage properties

a b s t r a c t A novel morphology of gallium nitride (GaN) hexagonal micro-bricks has been synthesized at 1250 °C by chemical vapor deposition (CVD) method. Photoluminescence (PL) and hydrogen storage capabilities of hexagonal micro-bricks have been investigated. The PL spectrum exhibits strong near-band-edge emission at 369 nm (3.36 eV). Defect related broad yellow band emission at 556 nm (2.23 eV) has also been observed, which plays significant role in the hydrogen absorption. Maximum hydrogen storage capacity of 1.68 wt.% has been achieved under the pressure of 5 MPa and at 373 K. During desorption process under ambient pressure, about 76% releasing of the stored hydrogen has been noted. Highly reversible absorption/desorption results exhibited by GaN hexagonal micro-bricks are encouraging and promising for hydrogen storage. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Hydrogen is considered as an efficient and clean fuel for the future due to its abundance, easy synthesis, and nonpolluting nature [1]. These significant advantages prefer the use of hydrogen over traditional fossil fuels which pollute the environment, contribute to global warming, and have limited supply. Hydrogen storage plays a key role in the utilization of hydrogen as the energy carrier [2]. In future, usage of hydrogen is very significant as energy source in hydrogen-fueled applications systems and in transportation technologies, such as H2 fuel cell vehicles [3]. For this purpose an economical and safe nano/microstructure hydrogen-storage medium is critically needed. Up to now, different materials such as Zn3N2 [4], Mg3N2, AlN, TiN, ZrN [5], BN [6,7], carbon [8], TiS2 [9], ZnO [10,11] and metal complexes [12] have been studied as hydrogen storage mediums. GaN is one of the most promising semiconductor materials. GaN has attracted a great attention due to its unique physical and chemical properties like wide band gap (Eg = 3.4 eV) at room temperature, high thermal stability and resistance to radiations, high melting point (>2500 °C), high chemical and mechanical stabilities, low electron affinity (2.7–3.3 eV) and high mobility [13,14]. For the synthesis of GaN nano/micro structures different techniques such as sol–gel method [15], sublimation method [16], laser-assisted catalyst growth [17], hydride vapor phase epitaxy [18], molecular beam epitaxy [19], metalorganic chemical vapor deposition [20], and chemical vapor deposition

⁎ Corresponding author. Tel.: + 86 1068913792; fax: + 86 1068912001. E-mail address: [email protected] (C. Cao). 0167-577X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2012.03.113

method [13,14] have been used. So far, different properties of GaN such as photoluminescence (PL), electroluminescence, field emission and electronics properties [13–20] have been investigated abundantly. But hydrogen storage properties of GaN have been studied rarely. Only some theoretical investigations have been done by Van de Walle [21], in which it has been revealed that hydrogen acts as an amphoteric impurity in most semiconductors (GaN, AlN) and form hydrogenimpurity complexes. Experimental investigations of the hydrogen storage properties of GaN materials are very exceptional. In this article we have used simple, facile and low cost catalytic assisted chemical vapor deposition method to synthesize the GaN hexagonal micro-bricks. Hydrogen storage properties of a novel morphology of GaN, hexagonal micro-bricks have been investigated. The effect of hydrogen absorption on PL properties of hexagonal microbricks has also been studied. Hydrogen absorption and desorption results examined for GaN hexagonal micro-bricks are very encouraging. 2. Experimental procedure GaN hexagonal micro-bricks have been synthesized by CVD method from GaN powder locally manufactured from gallium metal in our lab. One gram GaN powder was treated with aqueous ammonia at 150 °C thrice and was loaded in an alumina boat. A cleaned silicon (001) substrate coated with nickel chloride/ethanol solution (0.04 M) was placed on top of alumina boat with a vertical distance 1–2 mm and transferred into horizontal tube furnace (HTF). After sealing the HTF it was heavily flushed with high purity NH3 gas and its flow was set at 200 sccm (standard cubic centimeter per minute). Subsequently, the furnace was heated at ramp rate of 10 °C/min to reach the maximum set

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temperature of 1250 °C. This maximum temperature was maintained for 3 h. After reaction the furnace was allowed to cool down naturally to room temperature. A thick layer of product collected on the substrate was analyzed. The structure and the phase purity of the product were determined by X-ray powder diffraction (XRD, Philips X' Pert Pro MPD) and energy dispersive X-ray spectrometer (EDX). The morphologies of the product were examined by scanning electron microscopy (SEM, TM-1000, Japan). The hydrogen storage properties including hydrogen absorption–desorption kinetics were measured by an isovolumetric method, using a PCT Sieverts-type Gas Reaction Controller. Photoluminescence properties were studied using PL spectrum with Fluorescence Spectrophotometer (F-4500). 3. Results and discussion 3.1. Structural characterization SEM images of as synthesized GaN micro-bricks have been depicted in Fig. 1. It has been observed that most of the bricks are hexagonal with internal angel 120° and having size of 25–40 μm as shown in Fig. 1 (a,b). The thickness of the bricks has been observed from 10 to 15 μm as shown in Fig. 1 (c). The surface of the bricks is very smooth containing multidimensional particles as publicized in Fig. 1(d). Careful observations of high magnification SEM images showed that most of these particles are hexagonal and some are multidimensional as shown inset of Fig. 1(d). XRD pattern of the as synthesized GaN micro-bricks has been depicted in Fig. 2. All of the diffraction peaks in the pattern are indexed to wurtzite GaN with lattice parameters a = 3.188 Å and c = 5.187 Å which are in good agreement with the JCPDS card (no. 076-0703). No other peaks of crystalline impurities, such as Ga or

Fig. 2. XRD pattern of the as-synthesized GaN hexagonal micro-bricks (inset is the corresponding EDX).

Ga2O3 have been detected within the detection limit which shows that the synthesized product is single phased. Quantitative analysis has been achieved by EDX of the product as publicized in the inset of Fig. 2. EDX results verify that the product is composed of the Ga and N elements and the weight ratio of Ga/N was about 1:1 within the experimental errors. A renowned vapor–liquid–solid (VLS) growth mechanism has been proposed for the growth of GaN micro-bricks which has widely been used to interpret the growth of different GaN structures [14]. As we have pre-treated the precursors with aqueous NH3 which provides the favorable conditions and plays an effective role in the growth of the GaN product [13]. Later when the precursor is heated inside a horizontal

Fig. 1. (a–c) SEM images of GaN hexagonal micro-bricks and (d) surface of the brick (the inset on the right hand side is high magnification of the particles on the bricks).

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tube furnace, at high temperature in the atmosphere of NH3 gas the GaN thermally decomposed to generate the Ga vapors as given in Eq. (1) 2GaN →2Ga þ N2 :

ð1Þ

These Ga vapors are evaporated and condensed on the Si substrate on energy favorable sites. During this process when temperature rises, nickel chloride (0.04 M) also decomposes into the nanometersized Ni particles. These Ni nano-particles continuously adsorb the gaseous Ga and N atoms as energy-favorable sites to form the GaN. Thus Ni acts as an effective catalyst in the process of GaN micro-bricks growth [13]. At the same time high temperature causes NH3 gas to decompose gradually into various parts like NH2, NH, N2, N, H2 and H [14]. Thus, NH3 gas acted as a nitriding agent for converting Ga vapors into gallium nitride product (Eq. (2)) and formed the hexagonal GaN micro-bricks like planar nano-clusters. Ga þ N → GaN:

ð2Þ

These nano-clusters also acted as energetically favorable sites for adhesion of newly arriving vapors. The landing species attached themselves with host crystallites present on the six edges in such a way that nano-clusters grow along six sides and this led to the formation of GaN hexagonal micro-bricks. Waheed S. Khan et al. [22] has also explicated such kind of growth mechanism for the growth of hexagonal nano-sheets. 3.2. Hydrogen storage properties Hydrogen absorption/desorption capability of GaN micro-bricks has been investigated at 373 K and up to 5 MPa pressure as publicized in Fig. 3. It has been manifested from the absorption graph that hydrogen storage increases abruptly from 0.1 to 1.5 MPa and then a very slow increase has been observed with the increase of pressure. The maximum value attained for hydrogen absorption in GaN microbricks is 1.68 wt.% at 5 MPa. Hydrogen absorption results of GaN hexagonal micro-bricks are better than the earlier reported results of other compounds such as ball-milled nitrides, 0.472 wt.% for Mg3N2, 0.397 wt.% for AlN, 0.48 wt.% for TiN, 0.476 wt.% for ZrN [5], 0.20 wt.% for bulk BN powder [6], 0.7 wt.% of SWNTS soot, 0.90 wt.% of activated carbon [23], 1.3 wt.% for MmNi4.6Al0.4, 1.6 wt.% for MmNi4.6Fe0.4 [12], 1.05 wt.% for pure ZnO [10], 0.83 wt.% for ZnO nanowires [11], 1.26 wt.% for micro porous carbon [8] and 1.29 wt.% for Zn3N2 [4]. Our reported value is also comparable with 1.74 wt.% of Sb–ZnO nano-spheres,

Fig. 3. Hydrogen adsorption/desorption curves measured at 373 K.

1.179 wt.% for ZnO micro-spheres [10], 1.8 wt.% for multi-walled BN nano-tubes [6] but less than 2.6 wt.% for BN bamboo nano-tubes [6], 2.5 wt.% for TiS2 [9], 2.94 wt.% for Al–ZnO nano-belts [10] and 4.2 wt.% for collapsed BN nano-tubes [7]. The reasonable absorption of hydrogen in GaN hexagonal micro-bricks is believed, due to the surface absorption, defects and Ga vacancies which create interstitial sites for hydrogen atoms. At high temperature and pressure hydrogen adsorbs at solid surfaces and hydrogen atoms also make complexes with GaN, as hydrogen acts an amphoteric impurity in most of the semiconductor materials [21]. Moreover in GaN hexagonal microbricks, Ga vacancies have been observed (as yellow luminescence has been observed in the PL of GaN hexagonal micro-bricks) so at these vacancies one, two, three, or four H atoms can be accommodated. So in GaN hexagonal micro-bricks the absorption of hydrogen is most probably due to the Ga vacancies which is an excellent match with the theoretical results [24]. We have discussed the expected process of hydrogen storage in GaN hexagonal micro-bricks but further study is required to understand the precise mechanism of hydrogen storage in such kind of GaN micro/nano structures. The pressure of the reaction chamber is evacuated gradually to atmospheric pressure for the hydrogen release process and desorption is measured with time as shown in Fig. 3. It is apparent from the desorption graph that most of the hydrogen was desorbed during the first 30 min and after that releasing process was very slow. About 76% of the stored hydrogen can be released under ambient pressure from GaN hexagonal micro-bricks. These results illustrate that the GaN hexagonal micro-bricks have not only better hydrogen storage capacity but they are also considered a promising reversible hydrogen storage media. In future, usage of hydrogen is very significant as energy source in hydrogen-fueled applications systems and in transportation technologies [1]. For this purpose an economical and safe nano/microstructure hydrogen-storage medium is critically needed which must be good enough to satisfy all the criteria of size, efficiency, cost, kinetics and safety required for transportation applications [1–3]. Our reported results are better than many materials, its size is in micrometers and GaN materials can also work on high temperature (>2500 °C). So this study may provide a new platform for the energy storage application systems such as in fuel cells and high energy batteries. 3.3. Photoluminescence properties The influence of hydrogen absorption on the optical behavior of the GaN hexagonal micro-bricks has also been investigated by photoluminescence at room temperature. Ultraviolet (UV) light

Fig. 4. PL spectrum of the GaN hexagonal micro-bricks (as synthesized and with H residual).

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obtained from xenon lamp has been used to excite the product and its excitation wavelength was set at 325 nm. The PL spectrum of GaN micro-bricks before and after absorption has been as shown in Fig. 4. The PL spectrum of as synthesized GaN micro bricks consists of two strong emission peaks: one at 369 nm (3.36 eV), which is attributed to the near-band-edge emission of GaN [13,14], and the other is wellknown yellow luminescence at 556 nm (2.23 eV) which is mostly attributed to the Ga vacancies and defects [17,25]. The PL intensities of both UV emission peak at 369 nm and defects related yellow emission peak at 556 nm has been significantly reduced, when measured after hydrogen storage. As about 24% of hydrogen could not released during desorption, so this residual H will introduce large numbers of nonradiative centers, which reduce the UV emission efficiency of GaN [10,11]. The significant reduction in the PL intensities also shows that hydrogen absorption has direct effect on the PL properties of GaN hexagonal micro-bricks.

Acknowledgments

4. Conclusion

[13] [14]

In conclusion, for GaN hexagonal micro-bricks, hydrogen storage capacity of 1.68 wt.% has been achieved under the pressure of 5 MPa and about 76% of the stored hydrogen could be released under ambient pressure. This significant absorption of hydrogen has been attributed to the defects in the GaN micro-bricks and their large specific surface area. Photoluminescence and hydrogen absorption/ desorption results are consistent with each other. Highly reversible absorption/desorption results exhibited by GaN micro-bricks are encouraging and promising for hydrogen storage. This study may provide a new platform for the energy storage application systems, such as in fuel cells and high energy batteries.

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This work was supported by National Natural Science Foundation of China (50972017) and the Research Fund for the Doctoral Program of Higher Education of China (20060007024). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

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