Fresh or aged: Short time anodization of titanium to understand the influence of electrolyte aging on titania nanopores

doi:10.1016/j.jmst.2021.11.050

Abstract

Abstract:

Electrochemical anodization (EA) is a simple and cost-effective technique to fabricate controlled nanostructures on Ti substrates, such as TiO₂ nanotubes and nanopores. Electrolyte aging of organic EA electrolytes (repeated EA using non-target Ti before EA of target Ti) is recognized to influence the characteristics of the anodized nanostructures. However, there is limited information about how surface topography and electrolyte aging dictate the formation and characteristics of the anodized nanostructures. In the current study, short-time EA (starting at 10 s) of micro-machined Ti substrates was performed with electrolytes of various ages (fresh/unused, 15 h aged and 30 h aged), followed by evaluation of the TiO₂ nanopores (TNPs) characteristics in terms of topography, chemistry, stability and protein adhesion. The results showed that aligned TNPs were obtained earlier (120 s) with fresh electrolyte as compared to the aged electrolyte EA (600 s). Interestingly, TNPs fabricated using fresh electrolyte (at lower EA times) showed favorable wettability, protein adhesion capacity and mechanical properties compared with aged electrolyte counterparts. The findings of the study demonstrate how nanopore formation differs between fresh and aged electrolytes when performing EA of micro-machined Ti, which provides an improved understanding of electrolyte aging and its influence on anodized nanostructures.

Key words: Titanium, Titania nanopores, Electrochemical anodization, Electrolyte aging, Surface modification

Tianqi Guo, Sašo Ivanovski, Karan Gulati. Fresh or aged: Short time anodization of titanium to understand the influence of electrolyte aging on titania nanopores[J]. J. Mater. Sci. Technol., 2022, 119: 245-256.

Figures/Tables 9

Fig. 1. Schematic representation of electrochemical anodization of micro-rough titanium using various aged electrolytes (fresh, 15 h aged and 30 h aged) for short durations (10-600 s). Time of anodization and age of electrolyte dictates the formation of TiO2 nanopores (stability, alignment and dimensions).

Fig. 2. Characterization of fresh and aged electrolytes. (A) conductivity, (B) pH, (C) Ti ion concentration and (D) free F- ions concentration. *Significantly different as compared to fresh electrolyte.

Fig. 3. SEM images showing topography of titania nanopores (TNPs) fabricated using various electrolyte ages at 60 V for different times. Results showed that fresh (0 h) electrolyte enabled TNPs fabrication at 10 s (A), but no significant nanostructures were formed on anodization by aged electrolyte until 120 s (B,C, E,F, H,I). On fresh electrolyte (0 h) anodized TNPs, cracks started to form at 300 s and significantly compromised its stability at 600 s (M, P). TNPs fabricated using aged electrolytes (15 h and 30 h) remained stable between 120 s and 600 s anodization (K, L, N, O, Q, R).

Table 1. Topographical characteristics of titania nanopores (TNPs) anodized by different aged electrolytes for various anodization times. *: Significant difference with the 10 s-anodized samples by the same electrolyte. #: Significant difference with the 120 s-anodized samples by the same electrolyte (only presented for the pore diameters and porosity rates of 15 h/30 h anodized TNPs).

Anodization time (s)	Surface characteristics	Pore diameter (nm)	Porosity (%)	Roughness
Anodization time (s)	Surface characteristics	Pore diameter (nm)	Porosity (%)	S_q (nm)	S_a (nm)
Fresh (0 h) electrolyte
10	Aligned TNPs	36.07±9.63	6.53±0.69	35.37±9.28	29.54±9.53
30		36.90±9.76	7.22±1.06	38.81±10.11	31.25±8.36
60		41.75±10.06*	7.20±2.42	38.17±10.35	29.44±7.89
120		62.20±10.74*	9.48±2.03*	48.81±7.30*	40.01±6.33*
300	Cracks	35.75±6.26	6.25±1.00	42.23±12.21	31.16±6.33
600	Cracks	36.94±9.37	6.04±0.52	47.48±8.79*	36.66±5.04*
15 h aged electrolyte
10	No Nanopores	N/A	N/A	29.93±10.75	20.96±6.67
30				34.29±7.24	27.66±3.95
60				36.09±13.37	28.28±11.18
120	Aligned TNPs	32.17±6.77	3.70±0.77	45.61±7.38*	35.97±7.81*
300		41.51±8.76^#	5.18±0.50^#	58.81±8.01*	47.89±6.73*
600		47.49±7.93^#	7.38±0.42^#	64.57±9.05*	51.54±6.53*
30 h aged electrolyte
10	No Nanopores	N/A	N/A	31.42±5.27	22.67±3.76
30				27.06±7.09	21.22±5.18
60				32.66±11.60	23.70±8.41
120	Aligned TNPs	27.10±5.45	2.84±0.48	46.18±11.13*	36.31±8.86*
300		36.46±7.85^#	4.41±0.35^#	47.36±6.00*	37.05±5.55*
600		36.70±5.81^#	4.85±0.50^#	63.07±7.48*	51.47±6.21*

Fig. 4. AFM images showing the surface roughness of various anodized Ti surfaces (10-600 s) using different aged electrolytes.

Fig. 5. Change of current density vs time for anodization of Ti using various electrolyte ages. (A) 0-60 s and (B) 0-600 s. SEM images showing the TNP-0h (black frame) and TNP-30h (red frame) at 10 s, 30 s, 120 s and 600 s of EA. Scale bars represent 100 nm.

Fig. 6. The hydrophilicity of various short-time anodized Ti substrates. *: significant difference (p<0.05) with the TNPs by fresh electrolyte at the same EA time. #: significant difference (p<0.05) with the TNPs anodized at 10 s via the same electrolyte. Electrolyte age: 0 h (fresh), 15 h or 30 h; time of anodization: 10-600 s.

Table 2. Mechanical characteristics (elastic modulus and hardness) of various anodized TNPs. (*, p< 0.05 compared with the 0 h-120 s group at the same indentation depth). Electrolyte age: 0 h (fresh), 15 h or 30 h; time of anodization: 120 s or 600 s.

Indentation depth (nm)	Young's modulus (GPa)
Indentation depth (nm)	0 h-120 s	15 h-120 s	30 h-120 s	15 h-600 s	30 h-600 s
50	86.16±17.72	96.26±10.89	130.98±15.95*	58.01±11.42*	70.91±14.49
100	87.43±20.07	109.10±9.96	127.41±8.38*	57.12±11.72*	72.06±13.63
150	87.38±14.75	113.78±12.90*	124.43±11.31*	58.70±16.59*	70.01±14.35
200	88.04±9.48	115.23±16.98*	119.01±7.85*	58.27±15.42*	71.83±15.18
250	93.63±7.64	112.08±13.16	120.97±17.60*	58.53±14.20*	73.23±11.87
Indentation depth (nm)	Hardness (GPa)
Indentation depth (nm)	0 h-120 s	15 h-120 s	30 h-120 s	15 h-600 s	30 h-600 s
50	3.01±0.58	2.71±0.52	4.77±0.85*	2.16±0.40*	3.06±0.37
100	3.10±0.59	3.23±0.31	5.25±1.09*	2.14±0.33*	2.79±0.35
150	3.29±0.99	3.72±0.18	5.14±0.87*	2.08±0.56*	2.72±0.77
200	3.42±0.40	4.00±0.51	5.03±0.79*	2.05±0.54*	2.70±0.64
250	3.49±0.59	4.17±0.44	4.85±0.68*	2.00±0.53*	2.75±0.62

Fig. 7. Protein adhesion capacities of various Ti and anodized Ti surfaces. *: Significant difference (p<0.05) with the non-anodized Rough-Ti/Micro-Ti substrates. TNPs: titania nanopores; electrolyte age: 0 h (fresh), 15 h or 30 h; time of anodization: 120 s or 600 s.

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