Electrochimica Acta 83 (2012) 288–293 Contents lists available at SciVerse ScienceDirect Electrochimica Acta jou rn al h om epa ge: www.elsevier.com/locate/electacta Formation behavior of nanoporous anodic aluminum oxide films in hot glycerol/phosphate electrolyte Truong Nhat Nguyen a , Doohun Kim b , Dae-Yeong Jeong a,b,∗ , Min-Woo Kim b,c , Jong Uk Kim b a Korea Electrotechnology Research Institute Campus, University of Science and Technology, 70 Bulmosan-gil, Seongju-dong, Changwon 641-120, Republic of Korea b Korea Electrotechnology Research Institute (KERI), 70 Bulmosan-gil, Seongju-dong, Changwon 641-120, Republic of Korea c Pukyong National University, 100 Yongdang-dong, Nam-gu, Busan 608-739, Republic of Korea a r t i c l e i n f o Article history: Received 16 June 2012 Received in revised form 6 August 2012 Accepted 6 August 2012 Available online 14 August 2012 Keywords: Anodic aluminum oxide films High temperature anodization Mesosponge structure K2HPO4–glycerol electrolyte solution a b s t r a c t Formation behavior of anodic aluminum oxide (AAO) films was studied when aluminum foils were anodized in hot glycerol electrolyte containing 10 wt.% K 2 HPO 4 in temperatures of 120–200 ◦ C. The char- acteristics of the present anodization including the current density change with time, the effect of applied voltage and electrolyte temperature on anodization behavior, and the morphology and the porous struc- ture resulting from anodization in temperatures of 120–160 ◦ C indicated that the present anodization regime is very similar to the conventional one for formation of nanoporous AAO films near room temper- atures. Two-step anodization under 50 V at 160 ◦ C resulted in an AAO film with relatively well-ordered pore channels. Anodization at 180 ◦ C, on the other hand, led to formation of an AAO film with a new mesosponge structure that is consisted of nanopores and nanochannels. The mechanism related to the structure transformation is not known at present. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction After the discovery of nanoporous anodic aluminum oxide (AAO) films with a highly ordered hexagonal array of pore chan- nels by Masuda and Fukuda [1], tremendous studies have been conducted to investigate their formation in other electrochem- ical systems and to find their applications [2–8]. As a result of such efforts, it has been found that a well-ordered pore array forms in regimes of sulphuric acid at 25 V for an interpore dis- tance (D int ) of 63 nm [9,10], oxalic acid at 40 V for D int = 100 nm [1,10,11], and phosphoric acid at 195 V for D int = 500 nm [12,13]. Also a new regime of anodization, so-called “the 2nd generation hard anodization” for formation of self-ordered AAO films in high voltages with much higher growth rate than that in the conven- tional mild anodization was found in oxalic and sulphuric acid electrolytes [14,15]. Up to now, all the anodization studies men- tioned above has been performed near room temperatures which range from -15 to 50 ◦ C [16,17]. Anodization in high temperatures had been performed in tan- talum used for capacitors for guaranteeing the maximum use temperature long time ago [18–20]. Melody et al. [21] in 1998 anodized tantalum using a hot glycerol electrolyte containing ∗ Corresponding author at: Korea Electrotechnology Research Institute, 70 Bulmosan-gil, Seongju-dong, Changwon 641-120, Republic of Korea. Tel.: +82 55 280 1671; fax: +82 55 280 1590. E-mail address: dyjeong@keri.re.kr (D.-Y. Jeong). dibasic potassium phosphate in temperatures of 90–190 ◦ C, which had been identified as a proper solution in aspects of low resistivity, thermal stability and so on. They introduced a new electrochemical anodization process that leads to growth of non-thickness lim- ited tantalum oxide layer in high temperatures. Later, Lu et al. [22] demonstrated that the phenomenon of non-thickness lim- ited (NTL) growth of anodic films on tantalum and aluminum in the hot phosphate/glycerol electrolyte is due to the formation of porous films. In their work, however, the anodic metal oxide films formed were only several hundred nanometers thick. Using the same electrolyte, recently, formation of novel TiO 2 films thicker than 50 m with a highly mesoporous nano-features so called TiO 2 mesosponge layer was reported [23,24]. Also there were reports that thick mesoporous Nb 2 O 5 films [25] and WO 3 films [26] were successful formed by using the same electrolyte. Moreover, Lee et al. recently showed that uniformly self-aligned porous TiO 2 [27] and Ta 2 O 5 [28] structures could be formed under the optimized anodization conditions. Even with the successful formation of oxide films in high temperatures in series, however, it has been a still dif- ficult task to electrochemically synthesize thick and robust oxide films due to the fact that many unveiled phenomena and defensible mechanisms has not been characterized. According to the reports to date on the conventional mild and hard anodization, anodization of aluminum in high temperatures is not likely to be a feasible approach owing to rapid chemical etch- ing of the formed aluminum oxide layer in hot acidic electrolytes. The present study was performed to investigate the formation behavior of AAO films in the hot phosphate glycerol electrolyte 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.electacta.2012.08.019