Influence of deposition temperature on crystalline structure and morphologies of Co₃O₄ films prepared by a direct liquid injection chemical vapor deposition

Highlights

• Co₃O₄ films were successfully fabricated by a direct liquid injection CVD.

• The effect of deposition temperature on crystalline structure and morphology of Co₃O₄ films was studied.

• A (111) preferential growth Co₃O₄ film with elongated nano-wall microstructure was prepared at 773 K.

• The maximum deposition rate was almost 10 times higher than conventional MOCVD.

Abstract

A direct liquid injection chemical vapor deposition was utilized to prepare Co₃O₄ films with a high deposition rate. All the Co₃O₄ films showed spinel crystalline structures of Co^IICo₂^IIIO₄. A (111)-orientation preferred Co₃O₄ film with scattered triangular grains and elongated nano-wall surface morphology was prepared at the deposition temperature (T_dep) of 773 K, while randomly grown Co₃O₄ films with porous structures were found with lower or higher T_dep. The maximum deposition rate (R_dep) of Co₃O₄ films reached 3.6 μm h^− 1, which was around 10 times higher than those previously reported by metal organic CVD method. With increasing T_dep, the color of Co₃O₄ films changed from light to black brown owing to the thickness difference.

Introduction

Transition metal oxides with multi-functions and wide applications have been deeply studied recently [1]. Among these oxides, Co₃O₄ with spinel structure is an antiferromagnetic p-type semiconductor and it shows a great potential in various applications such as heterogeneous catalysts [2], gas-sensors [3], electrochromic devices [4] and lithium batteries [5]. However, two limitations are restricting the Co₃O₄ powders from being used under specific conditions; (i) in the case of photocatalysts for hydrogen harvesting, catalysts are mostly utilized in flowing water. After reaction, it is extremely difficult for powders to be collected and recycled from the suspension; (ii) it is significantly inconvenient for powers to be applied in built-in devices [6]. To meet the application requirements of these materials, finding various approaches to prepare Co₃O₄ films should be prudent strategies. Thus, physical preparation methods such as sputtering [7], thermal evaporation [8], atomic layer deposition (ALD) [9], pulsed laser deposition (PLD) [10], molecular beam epitaxy (MBE) [11], and chemical routes, including electro-deposition [12], spray pyrolysis [13], chemical vapor deposition [14], and wet-chemical methods [15] have been implemented to fabricate Co₃O₄ films. However, disadvantages of aforementioned methods are still obvious: (i) the deposition rates (R_dep) of reported studies are not satisfying for the large-scale applications; (ii) physical methods are highly vacuum involved, which often increase the cost of production and hinder the industrial production.

Compared with physical methods, direct liquid injection chemical vapor deposition (DLI-CVD) is a more economical technique for large-scale preparation and good step coverage of films. It not only can accurately control the chemical composition and deposition rate but also drastically lower the requirement for reaction atmosphere and substrates [16]. Regarding to the applications, high deposition rate is the key factor for industrial production, which needs a large transport rate of precursor with well dispersed chemical complexes. In terms of solving the precursor-transporting problem and significantly improving the deposition rate, the DLI-CVD is considered to be one of the best choices since the evaporation and gas flow rates can be significantly accelerated by the DLI technique [17]. Thus DLI-CVD is expected to be widely applied for preparing both polycrystalline and epitaxial films, even for depositing materials which are difficult to evaporate [18]. To the best of our knowledge, utilizing DLI-CVD for preparation of Co₃O₄ films has rarely been studied.

Herein, we report the preparation of Co₃O₄ films by a DLI-CVD technique. Through the DLI-CVD, the Co₃O₄ films can be formed under a moderate condition with good reproducibility. Meanwhile, unlike the epitaxial growth of previous reports on preparing Co₃O₄ films [19], [20], fused quartz substrates are chosen in order to study the intrinsic structural and morphological relation of Co₃O₄ films. It has been a paradox for the relation between deposition temperature, grain size and lattice constant, and remaining unclear for the growth habit and morphological evolution of Co₃O₄. For example, previous studies suggested the reduction of grain sizes, accompanying with the emergence and rising of defects, led to a trend of lattice expansion [21], [22]. However, a substantial amount of experiments showed the opposite trend that the lattice expansion was observed with increasing grain size [23], [24], [25], [26]. In order to generate a thorough and fair understanding, fused quartz substrates were selectively used. It is worth noting that fused quartz substrates are amorphous, having a smooth surfaces and excellent thermal stability. On the one hand, selectively utilizing fused quartz can minimize the impact from substrates during the nucleation process. On the other hand, their good thermal stability can allow experiments to be conducted under a wide temperature range and further facilitate the observation of structural and morphological evolution of films. In this study, the effect of deposition temperature on crystalline structure and morphological evolution of Co₃O₄ films was investigated. The kinetics of deposition process was also further investigated, which could give an in-depth understanding of the initial driven force of the growth behavior of Co₃O₄ films and may pave a path for the further studies and utilizations of spinel oxides.