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The effect of electrolyte re-utilization in the growth rate and morphology of TiO2 nanotubes
Materials Letters 171 (2016) 224–227
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
The effect of electrolyte re-utilization in the growth rate and morphology of TiO2 nanotubes José D. Costa a,1, Paula Quitério a,1, Arlete Apolinário a,b,n, Célia T. Sousa a, João Azevedo a,b, João Ventura a, Luísa Andrade b, Adélio Mendes b, João P. Araújo a a IFIMUP and IN – Institute of Nanosciences and Nanotechnology, Dep. de Física e Astronomia, Faculdade de Ciências da Universidade do Porto, Rua do Campo Alegre, 678, 4169-007 Porto, Portugal b LEPABE-Laboratory for Process Engineering, Environment, Biotechnology and Energy, Dep. Engenharia Química-Faculdade de Engenharia da Universidade do Porto, R. Dr. Roberto Frias, 4200 465 Porto, Portugal
art ic l e i nf o
a b s t r a c t
Article history: Received 19 January 2016 Accepted 20 February 2016 Available online 22 February 2016
In an industrial context, the re-utilization of resources is very important to monetize processes, reducing the production costs without overlooking a proper performance of the devices. For specific applications such as dye-sensitized solar cells (DSC), a high-aspect-ratio configuration of the produced TiO2 nanotubes (NTs) is essential to achieve higher conversion efficiencies. In this work, we report the effect of electrolyte re-utilization in the diameter, length and morphology of TiO2 nanotubes (NTs) produced by electrochemical anodization. A detailed study of the bias voltage effect on the anodic NTs for new and reutilized electrolytes was performed. Also, the possibility of obtaining NT arrays with ribs morphology just by re-using the electrolyte was unveiled. & 2016 Published by Elsevier B.V.
Keywords: TiO2 nanotubes Anodization Growth rate Electrolyte Re-utilization
1. Introduction Vertically oriented titanium dioxide (TiO2) nanotubes (NTs) have unique physico-chemical properties, making them attractive for potential applications in emerging fields, such as dye-sensitized solar cells (DSC) [1–3]. Since they provide a more efficient charge transport, the NTs geometry ensures a faster electron transfer towards the counter-electrode, minimizing the chance for charge trapping and electron-hole recombination [3,4]. Among the various synthesis methods, electrochemical anodization is an easy and low-cost approach to fabricate NT arrays. Since the NT properties are closely related with the geometric surface areas, it is important to control NT parameters such as length, pore size and wall thickness. Such tailoring can be easily achieved with electrochemical anodization by simply changing the anodizing parameters [3,5–7]. The fabrication of self-assembled TiO2 NTs was introduced by Zwilling et al. in 1999 [8]. However, this first generation of NTs was produced in aqueous hydrofluoric acid based electrolytes which has a high TiO2 chemical dissolution rate, limiting the NTs growth n Corresponding author at: IFIMUP and IN – Institute of Nanosciences and Nanotechnology, Dep. de Física e Astronomia, Faculdade de Ciências da Universidade do Porto, Rua do Campo Alegre, 678, 4169-007 Porto, Portugal. E-mail address: [email protected] (A. Apolinário). 1 These authors have contributed equally to this work and both should be considered as first authors.
http://dx.doi.org/10.1016/j.matlet.2016.02.085 0167-577X/& 2016 Published by Elsevier B.V.
to lengths of up to 500 nm [9,10]. Subsequently, the NT length was increased (up to 4.4 mm) by controlling the anodization electrolyte pH and thus reducing the chemical dissolution effect [11,12]. To suppress local concentration fluctuations and pH bursts during anodization, viscous organic electrolytes were introduced in the third generation of NTs, leading to smoother NTs with several micrometers or even millimeters [3,13,14]. A common feature to all this anodic approaches is that the outer walls of the tubes show considerable thickness variation ribs, also known as bamboo morphology [6,15,16]. The growth of ribs around the nanotubes has been related with periodical pH burst at the pore bottom (in fluoride-based electrolytes) [13,17], with the dehydration process of the hydroxide layer (double layer wall of NTs), by the fluoriderich layer in the outer wall of the NTs [16,18] or, more recently, as a consequence of the chemical dissolution of the surface layer (at the NTs tops) [19,20]. This structure was also obtained using alternating voltage cycling instead of potentiostatic anodizations, proving to be useful in the optimization of DSC structures [6,15,21,22]. Ti anodizations are usually performed using fresh electrolytes and the effect of electrolyte re-utilization in the TiO2 NTs morphology is still not understood. The re-use of the electrolyte (electrolyte aging) was earlier reported to be critical in electrolytes with low water contents (r1.0 wt%) [22,23]. The increase in the water amounts (moisture absorption) during the anodization process lead to an increase of the NTs organization and growth rate. However, it is with fresh ethylene glycol electrolyte solutions
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with higher amounts of water (2 wt%) and 0.3 wt% NH4F that the best NTs growth is achieved, since a faster growth rate is obtained [3]. Therefore, to make this nanostructured material a low cost choice, the question of electrolyte re-use can be key for their success in industrial applications. Here, we studied the effect of electrolyte re-utilization at different bias voltages (V) on the anodization process and on the NTs morphology (NT diameters and growth rate). A systematic comparative study is reported between experimental measurements and resulting anodization curves. We also report the possibility of achieving the ribs morphology just by re-using the electrolyte, with practical interest in DCS applications.
2. Experimental Prior to anodization, the Ti foils (99.99 þ %, 0.127 mm, SigmaAldrich) were ultrasonically cleaned sequentially with de-ionized water and ethanol, for 10 min. The anodizations were performed using a home-made set-up, in 98% ethylene glycol, 2% DI water and 0.3 wt% NH4F electrolyte solution [17,24,25]. Different samples were prepared with bias voltages ranging from 20 to 60 V for fresh and re-used electrolytes [25]. Three sets of experiments were performed: for each potential we anodized a new Ti foil using (i) a fresh electrolyte; (ii) the previously used electrolyte (second use) and (iii) a third use electrolyte. After each anodization, the samples were sequentially rinsed with ethanol. The morphology of the resulting TiO2 NT templates was characterized by Scanning Electron Microscopy (SEM; FEI Quanta 400FEG Field Emission Scanning Electron Microscopy). The measurements of the TiO2 NT template geometries, such as length or diameter, were performed using the ImageJ software [26].
3. Results and discussion The behavior of the NTs growth rate as a function of V can be identified in the anodization curves (Fig. 1). The 3 h anodization curves for the set performed featuring different V (ranging from 20 to 60 V) is represented in Fig. 1a. The curves show the typical anodization current transients for TiO2 for all voltages [24,27]. After a steep decrease of the current density (j), indicative of oxide formation and growth, such j-decrease slows down due to the pore nucleation onset, reaching a minimum value of j (jmin). Afterwards, a j increase is seen, associated with the continuous oxide dissolution (pore structure formation) until a local maximum (jmax) is reached. Finally, as the anodization proceeds, the NTs vertical growth is revealed by the progressive j-decay during the remaining anodization time, characteristic of Ti non-steady state anodization with faster oxidation than dissolution at the NTs bottom [24]. It is clear from Fig. 1a that the j-values increase with V. The applied potential is directly correlated with the ionic flowing current, which is responsible for the field-enhanced oxidation and dissolution that affect the mechanisms involved in the oxide formation, nucleation and pores growth. Thus, for lower V the oxide formation and pore nucleation stages occur later, and a longer time is needed to reach jmin and jmax. The increase of the electric field strength (higher V) promotes a faster ionic diffusion that in turn leads to a faster oxide formation and nucleation period as visible in the j(t) curves, with jmin and jmax being reached sooner as the potential increases. Within the studied range, 60 V is the most favorable regime for pore growth. According to our previous work, the potentials tested above 60 V resulted in more irregular anodizations curves and NT morphologies, typical of hard anodization
Fig. 1. (a) Anodization curves during the growth of TiO2 NT arrays for different values of bias voltage. (b) Mean value of current density as a function of the applied voltage. (c) Anodization curves for 1st, 2nd and 3rd electrolyte use (at 60 V).
conditions [25]. Fig. 1b shows the mean current density (jmean; average taken from jmax to the end of anodization) as a function of V for different electrolyte uses. The jmean increases with V due to the easier ionic diffusion and thus higher NT formation rate promoted by higher voltages. In addition, there is an overall current
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formation, NT nucleation, jmin and jmax, and lower jmean. The resulting NTs diameter and template thickness with variable V are shown in Fig. 2a and b respectively, taken from the SEM images illustrated in Fig. 2c. The outer tube diameter varies linearly from 61 to 155 nm for voltages from 20 to 60 V (Fig. 2a) with a slope of 2.5 nm/V. All electrolyte uses revealed similar outer tube diameters since they only depend on the applied voltage [5,12]. The resulting thickness of the NTs template (L) for each V is shown in Fig. 2b for 3 h anodizations. A continuous L(V) increase was observed due to the enhanced ionic transport. In Ti anodizations, the oxidation and dissolution processes occurring at the bottom of the NTs are given respectively by: Tiþ2H2O-TiO2 þ4H þ þ4e and TiO2 þ 6F þ4H þ -TiF62 þ 2H2O
Fig. 2. (a) Diameter and (b) length of the TiO2 NTs as a function of the applied voltage for fresh and re-used electrolytes (3 h anodizations). (c) SEM images of TiO2 NTs bottom and cross-section view (inset), anodized for 3 h at 60 V with fresh electrolyte (1st use), representing the measurements for diameter and length parameters.
decrease for re-used electrolyte (Fig. 1b). This is further confirmed in Fig. 1c where we show the anodization curves for V¼ 60 V for 1st, 2nd and 3rd electrolyte use. Analyzing the j(t) curves we conclude that the electrolyte re-utilization leads to slower oxide
As the anodization voltage increases, both processes are enhanced due to an increased ionic flow at the NTs bottom. This fact, allied with the higher mobility of F anions through the electrolyte for higher bias voltages, leads to higher NT formation rates [3,5]. The presence of sufficient F ions is very important to promote this mobility, enhancing the chemical dissolution of the TiO2 barrier layer which contributes to the growing of the NTs [28]. However, for 2nd and 3rd electrolyte use, an L decrease was observed for potentials above 40 V, with a higher dispersion of the L values (Fig. 2b). We believe that this is a consequence of the electrolyte auto-acidification that increases the dissolution at the top of the NTs. Also, the decrease of the fluoride concentration with successive anodizations leads to fluctuations in the NT arrays thickness [11,12]. Since the electrolyte re-utilization affects the anodization curves and the NT length, it may also affect the NT morphology. In fact, we verified the formation of oxide ribs for re-utilized electrolyte at the bias voltage of 60 V [16,18,29]. Here, we show SEM images of the NTs for 1st, 2nd and 3rd electrolyte use (Fig. 3a, b and c, respectively). It is clear that for the 1st electrolyte use the walls are smooth. However, for the 2nd use some ribs on the surface of the NTs already appear, an effect that is much more pronounced for the 3rd utilization of the electrolyte. It was also observed that the ribs become more evident and thicker, and thus the average spacing between them goes from 120 nm (2nd use) to 100 nm (3rd use). The formation of these structures can be explained by a mechanism that is related with the fluoride-rich layer in the outer wall of the NTs [16]. The electrolyte, when in contact with the F -layer, leads to the separation of adjacent NTs. This gap accompanies the tubes growth, allowing electrolyte and ions to diffuse between them. When the oxide residual layer at the NTs gap is sufficiently thin, it allows the electromigration of O2 near the Ti metal interface forming a thin oxide film (at the outer walls of the NTs), the so-called rib. The ribs formation was related with the amount of NH4F in the electrolyte, being observed for NH4F concentrations above 0.4 wt% only [20]. These higher amounts of NH4F enhance the electronic current with a strong O2 release, affecting the NT tops by chemical dissolution and allowing the electrolyte to reach the NTs gaps [19,20]. However, the amount of NH4F is not the only determinant factor for ribs formation. In fact, electrolytes with higher H2O concentrations (5–25%) showed NTs with increased number of ribs [16]. Here, the formation of rib structures is related with the increase of the H þ concentration (resulting from the oxidation process) that leads to local auto-acidification of the electrolyte (pH decrease) during anodization, due to its re-utilization. With a decreased pH, the dissolution rate in the gap between NTs increases
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Fig. 3. Cross-sectional view of TiO2 NTs obtained by potentiostatic anodization with 60 V and (a) first electrolyte use, (b) second electrolyte use and (c) third electrolyte use.
and the formation of the rib arises. Achieving this morphology just by re-using the electrolyte is a simple process while maintaining the high etch rate of the viscous organic electrolytes.
4. Conclusions The usual linear relation between tube diameter and bias voltage was verified for all electrolyte uses. For all experiments, the dependence of the outer diameter relies only on V, not being significantly affected by the electrolyte re-utilization. The electrolyte re-utilization results in a decrease of the growth rate above 50 V, due to chemical dissolution effects on the NT tops, caused by the electrolyte acidification through successive anodizations. This acidification was also confirmed by the increase of the nucleation period as the electrolyte is re-used, due to the progressive depletion of F ions and H þ accumulation. The electrolyte re-utilization also leads to the formation of ribs in the tube walls caused by a local pH decrease. This type of morphology is important to increase the exposed surface area of the NT arrays, with potential high practical interest in applications such as the DCS.
Acknowledgements The authors acknowledge funding from FEDER and ON2 through project Norte-070124-FEDER-000070, FCT through the Associated Laboratory – IN. and project PTDC/EQU–EQU/107990/ 2008. J.D. Costa, P. Quitério, J. Azevedo and C. T. Sousa are thankful to FCT for Grants SFRH/BD/79393/2011, SFRH/BD/110698/2015, SFRH/BD/79207/2011 and SFRH/BPD/82010/2011, respectively. A. Apolinário is grateful to PECDEMO project, co-funded by Europe's Fuel Cell and Hydrogen Joint Undertaking (FCH JU) under Grant 621252, for financial support. J. V. acknowledges financial support through FSE/POPH.
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[7] D. Regonini, F.J. Clemens, Anodized TiO2 nanotubes: effect of anodizing time on film length, morphology and photoelectrochemical properties, Mater. Lett. 142 (2015) 97–101. [8] V. Zwilling, M. Aucouturier, E. Darque-Ceretti, Anodic oxidation of titanium and TA6V alloy in chromic media. An electrochemical approach, Electrochim. Acta 45 (1999) 921–929. [9] R. Beranek, H. Hildebrand, P. Schmuki, Self-organized porous titanium oxide prepared in H2SO4/HF electrolytes, Electrochem. Solid-State Lett. 6 (2003) B12–B14. [10] G.K. Mor, O.K. Varghese, M. Paulose, N. Mukherjee, C.A. Grimes, Fabrication of tapered, conical-shaped titania nanotubes, J. Mater. Res. 18 (2003) 2588–2593. [11] J.M. Macák, H. Tsuchiya, P. Schmuki, High-aspect-ratio TiO2 nanotubes by anodization of titanium, Angew. Chem. Int. Ed. 44 (2005) 2100–2102. [12] Q. Cai, M. Paulose, O.K. Varghese, C.A. Grimes, The effect of electrolyte composition on the fabrication of self-organized titanium oxide nanotube arrays by anodic oxidation, J. Mater. Res. 20 (2005) 230–236. [13] J.M. Macak, H. Tsuchiya, L. Taveira, S. Aldabergerova, P. Schmuki, Smooth anodic TiO2 nanotubes, Angew. Chem. Int. Ed. 44 (2005) 7463–7465. [14] M. Paulose, H.E. Prakasam, O.K. Varghese, L. Peng, K.C. Popat, G.K. Mor, et al., TiO2 nanotube arrays of 1000 mm length by anodization of titanium foil: phenol red diffusion, J. Phys. Chem. C 111 (2007) 14992–14997. [15] X. Luan, D. Guan, Y. Wang, Facile synthesis and morphology control of bamboo-type TiO2 nanotube arrays for High-efficiency dye-sensitized solar cells, J. Phys. Chem. C 116 (2012) 14257–14263. [16] A. Valota, D.J. LeClere, P. Skeldon, M. Curioni, T. Hashimoto, S. Berger, et al., Influence of water content on nanotubular anodic titania formed in fluoride/ glycerol electrolytes, Electrochim. Acta 54 (2009) 4321–4327. [17] P. Quiterio, A. Apolinario, C.T. Sousa, J.D. Costa, J. Ventura, J.P. Araujo, The cyclic nature of porosity in anodic TiO2 nanotube arrays, J. Mater. Chem. A 3 (2015) 3692–3698. [18] Z. Su, W. Zhou, Formation mechanism of porous anodic aluminium and titanium oxides, Adv. Mater. 20 (2008) 3663–3667. [19] B. Chong, D.-l. Yu, M.-Q. Gao, H.-W. Fan, C.-Y. Yang, W.-H. Ma, et al., Formation mechanism of gaps and ribs around anodic TiO2 nanotubes and method to avoid formation of RiBs, J. Electrochem. Soc. 162 (2015), H1244-H50. [20] Y. Zhang, H. Fan, X. Ding, Q. Yan, L. Wang, W. Ma, Simulation of anodizing current-time curves and morphology evolution of TiO2 nanotubes anodized in electrolytes with different NH4F concentrations, Electrochim. Acta 176 (2015) 1083–1091. [21] D. Guan, P.J. Hymel, Y. Wang, Growth mechanism and morphology control of double-layer and bamboo-type TiO2 nanotube arrays by anodic oxidation, Electrochim. Acta 83 (2012) 420–429. [22] S.P. Albu, D. Kim, P. Schmuki, Growth of Aligned TiO2 bamboo-type nanotubes and highly ordered nanolace, Angew. Chem. Int. Ed. 47 (2008) 1916–1919. [23] K.S. Raja, T. Gandhi, M. Misra, Effect of water content of ethylene glycol as electrolyte for synthesis of ordered titania nanotubes, Electrochem. Commun. 9 (2007) 1069–1076. [24] A. Apolinario, C.T. Sousa, J. Ventura, J.D. Costa, D.C. Leitao, J.M. Moreira, et al., The role of the Ti surface roughness in the self-ordering of TiO2 nanotubes: a detailed study of the growth mechanism, J. Mater. Chem. A 2 (2014) 9067–9078. [25] A. Apolinário, P. Quitério, C.T. Sousa, J. Ventura, J.B. Sousa, L. Andrade, et al., Modeling the growth kinetics of anodic TiO2 nanotubes, J. Phys. Chem. Lett. 6 (2015) 845–851. [26] W.S. Rasband, ImageJ U.S. National Institutes of Health, Bethesda, Maryland, USA, 〈http://imagej.nih.gov/ij/〉, 1997–2012. [27] C.T. Sousa, D.C. Leitao, M.P. Proenca, J. Ventura, A.M. Pereira, J.P. Araujo, Nanoporous alumina as templates for multifunctional applications, Appl. Phys. Rev. (2014) 031102. [28] Y. Xue, Y. Sun, G. Wang, K. Yan, J. Zhao, Effect of NH4F concentration and controlled-charge consumption on the photocatalytic hydrogen generation of TiO2 nanotube arrays, Electrochim. Acta 155 (2015) 312–320. [29] F. Zhang, S. Chen, Y. Yin, C. Lin, C. Xue, Anodic formation of ordered and bamboo-type TiO2 anotubes arrays with different electrolytes, J. Alloy. Compd. 490 (2010) 247–252.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
The effect of electrolyte re-utilization in the growth rate and morphology of TiO2 nanotubes José D. Costa a,1, Paula Quitério a,1, Arlete Apolinário a,b,n, Célia T. Sousa a, João Azevedo a,b, João Ventura a, Luísa Andrade b, Adélio Mendes b, João P. Araújo a a IFIMUP and IN – Institute of Nanosciences and Nanotechnology, Dep. de Física e Astronomia, Faculdade de Ciências da Universidade do Porto, Rua do Campo Alegre, 678, 4169-007 Porto, Portugal b LEPABE-Laboratory for Process Engineering, Environment, Biotechnology and Energy, Dep. Engenharia Química-Faculdade de Engenharia da Universidade do Porto, R. Dr. Roberto Frias, 4200 465 Porto, Portugal
art ic l e i nf o
a b s t r a c t
Article history: Received 19 January 2016 Accepted 20 February 2016 Available online 22 February 2016
In an industrial context, the re-utilization of resources is very important to monetize processes, reducing the production costs without overlooking a proper performance of the devices. For specific applications such as dye-sensitized solar cells (DSC), a high-aspect-ratio configuration of the produced TiO2 nanotubes (NTs) is essential to achieve higher conversion efficiencies. In this work, we report the effect of electrolyte re-utilization in the diameter, length and morphology of TiO2 nanotubes (NTs) produced by electrochemical anodization. A detailed study of the bias voltage effect on the anodic NTs for new and reutilized electrolytes was performed. Also, the possibility of obtaining NT arrays with ribs morphology just by re-using the electrolyte was unveiled. & 2016 Published by Elsevier B.V.
Keywords: TiO2 nanotubes Anodization Growth rate Electrolyte Re-utilization
1. Introduction Vertically oriented titanium dioxide (TiO2) nanotubes (NTs) have unique physico-chemical properties, making them attractive for potential applications in emerging fields, such as dye-sensitized solar cells (DSC) [1–3]. Since they provide a more efficient charge transport, the NTs geometry ensures a faster electron transfer towards the counter-electrode, minimizing the chance for charge trapping and electron-hole recombination [3,4]. Among the various synthesis methods, electrochemical anodization is an easy and low-cost approach to fabricate NT arrays. Since the NT properties are closely related with the geometric surface areas, it is important to control NT parameters such as length, pore size and wall thickness. Such tailoring can be easily achieved with electrochemical anodization by simply changing the anodizing parameters [3,5–7]. The fabrication of self-assembled TiO2 NTs was introduced by Zwilling et al. in 1999 [8]. However, this first generation of NTs was produced in aqueous hydrofluoric acid based electrolytes which has a high TiO2 chemical dissolution rate, limiting the NTs growth n Corresponding author at: IFIMUP and IN – Institute of Nanosciences and Nanotechnology, Dep. de Física e Astronomia, Faculdade de Ciências da Universidade do Porto, Rua do Campo Alegre, 678, 4169-007 Porto, Portugal. E-mail address: [email protected] (A. Apolinário). 1 These authors have contributed equally to this work and both should be considered as first authors.
http://dx.doi.org/10.1016/j.matlet.2016.02.085 0167-577X/& 2016 Published by Elsevier B.V.
to lengths of up to 500 nm [9,10]. Subsequently, the NT length was increased (up to 4.4 mm) by controlling the anodization electrolyte pH and thus reducing the chemical dissolution effect [11,12]. To suppress local concentration fluctuations and pH bursts during anodization, viscous organic electrolytes were introduced in the third generation of NTs, leading to smoother NTs with several micrometers or even millimeters [3,13,14]. A common feature to all this anodic approaches is that the outer walls of the tubes show considerable thickness variation ribs, also known as bamboo morphology [6,15,16]. The growth of ribs around the nanotubes has been related with periodical pH burst at the pore bottom (in fluoride-based electrolytes) [13,17], with the dehydration process of the hydroxide layer (double layer wall of NTs), by the fluoriderich layer in the outer wall of the NTs [16,18] or, more recently, as a consequence of the chemical dissolution of the surface layer (at the NTs tops) [19,20]. This structure was also obtained using alternating voltage cycling instead of potentiostatic anodizations, proving to be useful in the optimization of DSC structures [6,15,21,22]. Ti anodizations are usually performed using fresh electrolytes and the effect of electrolyte re-utilization in the TiO2 NTs morphology is still not understood. The re-use of the electrolyte (electrolyte aging) was earlier reported to be critical in electrolytes with low water contents (r1.0 wt%) [22,23]. The increase in the water amounts (moisture absorption) during the anodization process lead to an increase of the NTs organization and growth rate. However, it is with fresh ethylene glycol electrolyte solutions
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with higher amounts of water (2 wt%) and 0.3 wt% NH4F that the best NTs growth is achieved, since a faster growth rate is obtained [3]. Therefore, to make this nanostructured material a low cost choice, the question of electrolyte re-use can be key for their success in industrial applications. Here, we studied the effect of electrolyte re-utilization at different bias voltages (V) on the anodization process and on the NTs morphology (NT diameters and growth rate). A systematic comparative study is reported between experimental measurements and resulting anodization curves. We also report the possibility of achieving the ribs morphology just by re-using the electrolyte, with practical interest in DCS applications.
2. Experimental Prior to anodization, the Ti foils (99.99 þ %, 0.127 mm, SigmaAldrich) were ultrasonically cleaned sequentially with de-ionized water and ethanol, for 10 min. The anodizations were performed using a home-made set-up, in 98% ethylene glycol, 2% DI water and 0.3 wt% NH4F electrolyte solution [17,24,25]. Different samples were prepared with bias voltages ranging from 20 to 60 V for fresh and re-used electrolytes [25]. Three sets of experiments were performed: for each potential we anodized a new Ti foil using (i) a fresh electrolyte; (ii) the previously used electrolyte (second use) and (iii) a third use electrolyte. After each anodization, the samples were sequentially rinsed with ethanol. The morphology of the resulting TiO2 NT templates was characterized by Scanning Electron Microscopy (SEM; FEI Quanta 400FEG Field Emission Scanning Electron Microscopy). The measurements of the TiO2 NT template geometries, such as length or diameter, were performed using the ImageJ software [26].
3. Results and discussion The behavior of the NTs growth rate as a function of V can be identified in the anodization curves (Fig. 1). The 3 h anodization curves for the set performed featuring different V (ranging from 20 to 60 V) is represented in Fig. 1a. The curves show the typical anodization current transients for TiO2 for all voltages [24,27]. After a steep decrease of the current density (j), indicative of oxide formation and growth, such j-decrease slows down due to the pore nucleation onset, reaching a minimum value of j (jmin). Afterwards, a j increase is seen, associated with the continuous oxide dissolution (pore structure formation) until a local maximum (jmax) is reached. Finally, as the anodization proceeds, the NTs vertical growth is revealed by the progressive j-decay during the remaining anodization time, characteristic of Ti non-steady state anodization with faster oxidation than dissolution at the NTs bottom [24]. It is clear from Fig. 1a that the j-values increase with V. The applied potential is directly correlated with the ionic flowing current, which is responsible for the field-enhanced oxidation and dissolution that affect the mechanisms involved in the oxide formation, nucleation and pores growth. Thus, for lower V the oxide formation and pore nucleation stages occur later, and a longer time is needed to reach jmin and jmax. The increase of the electric field strength (higher V) promotes a faster ionic diffusion that in turn leads to a faster oxide formation and nucleation period as visible in the j(t) curves, with jmin and jmax being reached sooner as the potential increases. Within the studied range, 60 V is the most favorable regime for pore growth. According to our previous work, the potentials tested above 60 V resulted in more irregular anodizations curves and NT morphologies, typical of hard anodization
Fig. 1. (a) Anodization curves during the growth of TiO2 NT arrays for different values of bias voltage. (b) Mean value of current density as a function of the applied voltage. (c) Anodization curves for 1st, 2nd and 3rd electrolyte use (at 60 V).
conditions [25]. Fig. 1b shows the mean current density (jmean; average taken from jmax to the end of anodization) as a function of V for different electrolyte uses. The jmean increases with V due to the easier ionic diffusion and thus higher NT formation rate promoted by higher voltages. In addition, there is an overall current
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formation, NT nucleation, jmin and jmax, and lower jmean. The resulting NTs diameter and template thickness with variable V are shown in Fig. 2a and b respectively, taken from the SEM images illustrated in Fig. 2c. The outer tube diameter varies linearly from 61 to 155 nm for voltages from 20 to 60 V (Fig. 2a) with a slope of 2.5 nm/V. All electrolyte uses revealed similar outer tube diameters since they only depend on the applied voltage [5,12]. The resulting thickness of the NTs template (L) for each V is shown in Fig. 2b for 3 h anodizations. A continuous L(V) increase was observed due to the enhanced ionic transport. In Ti anodizations, the oxidation and dissolution processes occurring at the bottom of the NTs are given respectively by: Tiþ2H2O-TiO2 þ4H þ þ4e and TiO2 þ 6F þ4H þ -TiF62 þ 2H2O
Fig. 2. (a) Diameter and (b) length of the TiO2 NTs as a function of the applied voltage for fresh and re-used electrolytes (3 h anodizations). (c) SEM images of TiO2 NTs bottom and cross-section view (inset), anodized for 3 h at 60 V with fresh electrolyte (1st use), representing the measurements for diameter and length parameters.
decrease for re-used electrolyte (Fig. 1b). This is further confirmed in Fig. 1c where we show the anodization curves for V¼ 60 V for 1st, 2nd and 3rd electrolyte use. Analyzing the j(t) curves we conclude that the electrolyte re-utilization leads to slower oxide
As the anodization voltage increases, both processes are enhanced due to an increased ionic flow at the NTs bottom. This fact, allied with the higher mobility of F anions through the electrolyte for higher bias voltages, leads to higher NT formation rates [3,5]. The presence of sufficient F ions is very important to promote this mobility, enhancing the chemical dissolution of the TiO2 barrier layer which contributes to the growing of the NTs [28]. However, for 2nd and 3rd electrolyte use, an L decrease was observed for potentials above 40 V, with a higher dispersion of the L values (Fig. 2b). We believe that this is a consequence of the electrolyte auto-acidification that increases the dissolution at the top of the NTs. Also, the decrease of the fluoride concentration with successive anodizations leads to fluctuations in the NT arrays thickness [11,12]. Since the electrolyte re-utilization affects the anodization curves and the NT length, it may also affect the NT morphology. In fact, we verified the formation of oxide ribs for re-utilized electrolyte at the bias voltage of 60 V [16,18,29]. Here, we show SEM images of the NTs for 1st, 2nd and 3rd electrolyte use (Fig. 3a, b and c, respectively). It is clear that for the 1st electrolyte use the walls are smooth. However, for the 2nd use some ribs on the surface of the NTs already appear, an effect that is much more pronounced for the 3rd utilization of the electrolyte. It was also observed that the ribs become more evident and thicker, and thus the average spacing between them goes from 120 nm (2nd use) to 100 nm (3rd use). The formation of these structures can be explained by a mechanism that is related with the fluoride-rich layer in the outer wall of the NTs [16]. The electrolyte, when in contact with the F -layer, leads to the separation of adjacent NTs. This gap accompanies the tubes growth, allowing electrolyte and ions to diffuse between them. When the oxide residual layer at the NTs gap is sufficiently thin, it allows the electromigration of O2 near the Ti metal interface forming a thin oxide film (at the outer walls of the NTs), the so-called rib. The ribs formation was related with the amount of NH4F in the electrolyte, being observed for NH4F concentrations above 0.4 wt% only [20]. These higher amounts of NH4F enhance the electronic current with a strong O2 release, affecting the NT tops by chemical dissolution and allowing the electrolyte to reach the NTs gaps [19,20]. However, the amount of NH4F is not the only determinant factor for ribs formation. In fact, electrolytes with higher H2O concentrations (5–25%) showed NTs with increased number of ribs [16]. Here, the formation of rib structures is related with the increase of the H þ concentration (resulting from the oxidation process) that leads to local auto-acidification of the electrolyte (pH decrease) during anodization, due to its re-utilization. With a decreased pH, the dissolution rate in the gap between NTs increases
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Fig. 3. Cross-sectional view of TiO2 NTs obtained by potentiostatic anodization with 60 V and (a) first electrolyte use, (b) second electrolyte use and (c) third electrolyte use.
and the formation of the rib arises. Achieving this morphology just by re-using the electrolyte is a simple process while maintaining the high etch rate of the viscous organic electrolytes.
4. Conclusions The usual linear relation between tube diameter and bias voltage was verified for all electrolyte uses. For all experiments, the dependence of the outer diameter relies only on V, not being significantly affected by the electrolyte re-utilization. The electrolyte re-utilization results in a decrease of the growth rate above 50 V, due to chemical dissolution effects on the NT tops, caused by the electrolyte acidification through successive anodizations. This acidification was also confirmed by the increase of the nucleation period as the electrolyte is re-used, due to the progressive depletion of F ions and H þ accumulation. The electrolyte re-utilization also leads to the formation of ribs in the tube walls caused by a local pH decrease. This type of morphology is important to increase the exposed surface area of the NT arrays, with potential high practical interest in applications such as the DCS.
Acknowledgements The authors acknowledge funding from FEDER and ON2 through project Norte-070124-FEDER-000070, FCT through the Associated Laboratory – IN. and project PTDC/EQU–EQU/107990/ 2008. J.D. Costa, P. Quitério, J. Azevedo and C. T. Sousa are thankful to FCT for Grants SFRH/BD/79393/2011, SFRH/BD/110698/2015, SFRH/BD/79207/2011 and SFRH/BPD/82010/2011, respectively. A. Apolinário is grateful to PECDEMO project, co-funded by Europe's Fuel Cell and Hydrogen Joint Undertaking (FCH JU) under Grant 621252, for financial support. J. V. acknowledges financial support through FSE/POPH.
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