Single Step Formation of C-TiO 2 Nanotubes : Influence of Applied Voltage and Their Photocatalytic Activity under Solar Illumination

Self-aligned and high-uniformity carbon (C)titania (TiO 2 ) nanotube arrays were successfully formed via single step anodization of titanium (Ti) foil at 30V for 1 h in a bath composed of ethylene glycol (EG), ammonium fluoride (NH 4 F), and hydrogen peroxide (H 2 O 2 ). It was well established that applied voltage played an important role in controlling field-assisted oxidation and field-assisted dissolution during electrochemical anodization process. Therefore, the influences of applied voltage on the formation of C-TiO 2


Introduction
Nowadays, various kinds of environmental contaminants are around all of us, especially organic and inorganic pollutants from industrial wastewaters [1][2][3].Thus, the treatments of such wastewater have become a major concern and it is urgent to develop a sustainable and cost-effective treatment technology to solve global environmental problems [3,4].In recent years, the photocatalyst system has attracted much attention from science community as one of the most promising ways to solve the environmental problems [4][5][6].This system is considered to be an ideal and green environmental solution for our green economy future.
In this case, TiO 2 based nanomaterials have been broadly studied as the most promising photocatalyst for environmental remediation such as air purification, water purification, heavy metals degradation, and hazardous waste remediation [6][7][8][9][10].The reasons are mainly attributed to the nontoxicity, cost-effectiveness, long-term stability, widespread availability, and high stability against photocorrosion with great capacity for oxidation and high photocatalytic property.To date, development of nanoarchitecture of TiO 2 assemblies with precisely controllable nanoscale features has gained significant scientific interest [11,12].Among different nanoarchitectures of TiO 2 , one-dimensional (1D) TiO 2 is considered as a promising candidate in environmental and industrial pollutants degradation due to the well controllable dimensional features such as pore diameter, length, and wall thickness [6][7][8][9][11][12][13][14][15].However, its practical application was limited by poor absorption of photons from the solar illumination and high recombination losses of photoinduced electron-hole pairs [1,4,10,[16][17][18].
Recently, numerous research studies have been paid for the rational design of high efficiency heterostructure C-TiO 2 based nanomaterial for remarkable solar absorption and higher separation efficiency of photo-induced charge carriers [4,[19][20][21][22].In particular, C-TiO 2 based nanomaterials can exert a substantial influence on modifying electronic structure and the construction of heterotomic surface structure that allow higher efficiency photocatalysis reaction under solar irradiation [19,21,23].Several literatures have reported that incorporation of anion C-dopants into TiO 2 crystalline could shift its photoresponse into visible region, which accounts for 40-45% irradiation from the solar light.The main reason was attributed to the narrowing of the band gap energy, which resulted from an increase in the width of the valence band.The reduction of band gap energy was believed to be from the mixing of the -state of C dopants with O 2 and Ti 3 orbital constituting valence band [21,23,24].
In the present study, a detailed study on the C-doped TiO 2 nanotubes via single step of electrochemical anodization technique was performed.An organic electrolyte of ethylene glycol was selected as C source to dope into TiO 2 nanotubes during electrochemical anodization.This study aims to determine the minimal applied voltage to grow the self-aligned and high-uniformity C-TiO 2 nanotubes for better photocatalytic performance.The novelty of this work is to synthesize a potential photocatalyzer, which shows strong absorption range in UV-visible region, strong oxidation ability, costeffective and time saving process.O, J.T. Baker, USA).This composition was selected because it favors the formation of well-aligned and highly ordered nanotubular structure based on our previous studies [25].Electrochemical anodization process was conducted for 1 h with three different applied voltages (e.g., 10 V, 20 V, and 30 V). Anodization was performed in a two-electrode configuration bath with Ti foil as the anode and a platinum rod as the cathode.Subsequently, these samples were annealed at 400 ∘ C for 4 h in an argon atmosphere.

Characterization of Anodic C-TiO 2
Nanotubes.The Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive X-Ray (EDX) were conducted (FEI Quanta 200 FESEM model, USA).The cross-sectional measurements were carried out on mechanically bent samples, where a partial lift-off of the anodic layer occurred to obtain the thickness of the anodic oxide formed on Ti foil.The crystalline phase of the synthesized samples was identified by power X-ray Diffraction (XRD), using a Bruker Xray Diffraction Model D-8, USA, equipped with a Cu K monochromatized radiation source ( = 1.5406Á).The sample with dimension (10 mm × 10 mm) was placed onto a specimen holder and then placed in the rotating unit stage of diffractometer system.The data was collected within 2 of 10 ∘ to 70 ∘ with a step size of 0.034 ∘ at scan time 71.6 sec.Photoluminescence (PL) spectra were obtained at room temperature using an LS 55 luminescence spectrometer (Jobin-Yvon HR 800UV).The photoelectron spectra were obtained through X-ray photoelectron spectroscopy (XPS; JEOL JPS-1000SX) with a dual X-ray source, in which an Al-K (30 kV) anode and a hemispherical energy analyzer were used.The background pressure during data acquisition was maintained at 7.0 × 10 −9 Torr.All binding energies were calibrated using contaminant carbon (C1s = 284.8eV) as a charge reference.

Photocatalytic Degradation of MO Dye Study.
Photocatalytic degradation MO dye was conducted by dipping 4 cm 2 of anodic C-TiO 2 nanotubes samples into a 100 mL of quartz glass tube containing 30 ppm of MO dye.The quartz glass tube was then placed in a custom-made photorecator for photocatalytic degradation studies.A schematic diagram of quartz glass tube custom-made photo-reactor used in photocatalytic degradation MO dye is exhibited in Figure 1.In this study, a blank sample (without anodic sample) was prepared in order to eliminate the effect of light towards the degradation of MO dye solution.All anodic samples were left in a custom-made photo-reactor for 30 min in dark environment to achieve adsorption and desorption equilibrium.Then, samples were photoirradiated using a 150 W Xenon solar simulator (Zolix LSP-X150, China) with intensity of 800 W/m 2 .Three mL of MO dye solution was withdrawn from the quartz glass tube every 1 h to investigate the degradation of MO dye solution under solar irradiation.The concentration of the degraded MO dye solution assisted by the anodic sample was determined using UV-Vis spectrometer (PerkinElmer Lambda 35, USA).It is noteworthy to mention that uniform circular nanotube arrays were successfully formed when minimum voltage of 30 V was applied during electrochemical anodization.The average diameter of 80 nm and thickness of 2 m nanotubes were produced (Figure 2(d)).Therefore, uniform growth of anodic nanotube arrays at lower 30 V of applied voltage was selected and duplicated for the investigation studies.

Chemical Compositions of Anodic C-TiO 2 Nanotubes.
EDX analysis was employed to investigate the chemical stoichiometry of the anodic C-TiO 2 nanotubes after heating treatment at 400 ∘ C in argon atmosphere.As determined through EDX analysis, the anodic C-TiO 2 nanotubes synthesized at 30 V consisted of 60.14 at% of Ti element, 33.68 at% of O element and 6.18 at% of C element (Figure 3).An additional spectrum of C was observed at about 0.  species which gets absorbed on the wall of nanotubes during the electrochemical anodization [19,24,28].These resultant carbonate type species were reduced to C species during heat treatment process and diffused into the TiO 2 lattice [29].In this manner, the single step incorporation of C species to TiO 2 nanotubes could be achieved.Next, XPS measurements were carried out to further investigate the chemical composition and oxidation state of C-TiO 2 nanotubes.XPS survey spectra for the sample synthesized at 30 V are presented in Figure 4.The elements of Ti, O, and C were detected from the XPS survey spectra, which indicated that the carbon species were successfully loaded into the lattice of TiO 2 .It was found that the peak of Ti2p (binding energy of about 460 eV) is attributed to Ti 4+ .Besides, it could be observed that the peak of O1s (binding energy of about 530 eV) showed the presence of oxygen atoms in the TiO 2 structure.Meanwhile, the detection of the peak C1s at about 285 eV can be ascribed to the Ti-C-O bonds of the carbonate species originating from the residual carbon of the organic ethylene glycol electrolyte.

Crystalline Structure of Anodic C-TiO 2
Nanotubes.In the present study, XRD measurements were conducted to reveal the crystal phase transition of the as-anodized and annealed samples.The results are presented in Figure 5. Based on preliminary study, 400 ∘ C was sufficient to convert TiO 2 amorphous structure to high crystallinity of anatase  phase, which exhibited thermodynamically stable and higher surface stability [30][31][32][33].It was found that the presence of Ti phase only was detected from the XRD pattern for the as-anodized sample, which represents the amorphous phase of TiO 2 (Figure 5(a)).However, high crystallinity of anatase phase was detected for the sample subjected to annealing at 400 ∘ C in argon atmosphere (Figure 5(b)).It can be seen that the diffraction peaks of the entire samples are ascribed to the TiO 2 with anatase phase (JCPDS no.21-1272).The diffraction peaks allocated at 25.37 ∘ , 38.67 ∘ , 48.21 ∘ , 54.10 ∘ , and 55.26 ∘ are corresponding to (101), ( 112), ( 200), (105), and (211) crystal planes for the anatase phase, respectively.

Optical Property of Anodic C-TiO 2
Nanotubes.PL emission spectra were broadly used to investigate the competency of charge carrier trapping, migration, and transfer behaviors of anodic C-TiO 2 nanotubes.The information from PL emission spectra is important to understand the state of electron-hole pairs in C-TiO 2 nanotubes photocatalyst since the emission mainly resulted from recombination dynamics of these free carriers.The PL measurement was conducted at room temperatures in the wavelength range of 350 nm to 1000 nm as presented in Figure 6.The dominant PL emission spectrum of the samples prepared at 30 V is clearly observed at 535 nm, implying the visible-light characteristics of the sample.The dominant PL spectrum of anatase phase TiO 2 was closely related to the recombination of self-trapped excitons localized on the TiO 6 octahedral, which results from the interaction of the negative electrons located in the Ti 3 states with the positive holes in the O 2 state [7,16,19].In this case, the peak position at 535 nm might be attributed to the radioactive recombination of self-trapped excitons localized on TiO 6 octahedron.On the other hand, the PL emissions above 535 nm might be attributed to the presence of surface state and intrinsic defects such as oxygen vacancies, which give rise to donor states located below conduction band.Several literatures have reported that the resultant PL emission spectra were attributed to the oxygen-related defects within the sample lattice [34,35].Zhao and coworkers have been reported that the carbon species doping into the lattice of TiO 2 nanotubes favored the formation of oxygen vacancies [36].They proposed that the electrons were easily trapped by those oxygen vacancies, while the holes were trapped by carbon This condition could decrease the PL intensity significantly.Hence, the presence of carbon species could improve the separation extent and restrain the recombination losses of the photo-induced charge carriers.Besides, PL technique has been widely used to investigate the energy levels of samples [23,37].The energy level (  ) of the samples was calculated using the equation   = ℎ/, where   is the band gap energy, ℎ is Planck's constant (4.135667 × 10 −15 eV s),  is the velocity of light (2.997924 × 108 m/s), and  is the wavelength (nm) of PL emission.The   was found at around 2.3 eV, which is relatively low as compared to the   of pure TiO 2 (3.2 eV for anatase phase) [7,10].The result clearly demonstrates that visible emission is likely facilitated by the interstitial C species within the lattice of TiO 2 .

Formation Mechanism of Anodic C-TiO 2
Nanotubes.In the present study, applied voltage played an important role in controlling the field-assisted oxidation of Ti metal to form TiO 2 and field-assisted dissolution of Ti metal ions into electrolyte [38,39].A simple schematic illustration about the formation mechanism of anodic C-TiO 2 nanotube arrays was presented in Figure 7.The compact oxide layer was formed through the hydrolysis of Ti foil (stage A) which was oxidized to TiO 2 forming a barrier type layer under applied voltage.This layer grew predominantly by the inward migration of oxygen ions (O 2− ) through this layer toward Ti/TiO 2 interface with further growth of barrier layer (stage B).The high electric field across the oxide layer subsequently induced the polarization of Ti-O bonding.In this manner, Ti 4+ ions from the barrier layer of TiO 2 will dissolve into the electrolyte and form pits randomly on the surface of oxide layer.The pits will act as nucleation sites for the porosification (stage C).The titanium-fluoro complexes (TiF 6 ) 2− will induce the chemical dissolution and continuous dissolve the pits, which will further enlarge and deepenly the pits into nanotubular structure.The rate of migration of the Ti 4+ , O 2− , and F − ions was higher during the higher voltage electrochemical anodization.The higher electron flow promoted increased field-assisted oxidation and field-assisted dissolution, which leads to an increase in nanotube length and pore size (stage D).The porosity will reach a steady-state, resulting in the formation of uniform of TiO 2 nanotubes [9,11,26].
In this manner, organic EG electrolyte was oxidized to carbonate-species under applied voltage, which get absorbed on the wall of nanotubes.Then, these carbonate-species were reduced to C species during annealing treatment process, which might diffuse into the lattice of TiO 2 [29].Thus, the presence of the C species was found within TiO 2 nanotubes.Valentin and coworkers have proposed that the possible causes for the presence of C atoms in the anatase TiO 2 are due to the substitution of the oxygen lattice with a C atom, the replacement of Ti atoms with a C atom; and stabilization of a C atom at interstitial position of TiO 2 lattice [21].As a matter of fact, the interstitial C atom arises from the pyrogeneration of EG electrolyte, which reduces the band gap marginally or to introduce mid-gap level to increase the light absorption into visible region.In this case, the band gap level reduction was believed to result from an increase in the width of the valence band due to the mixing of the delocalized -state of carbon dopants with O 2.The mixing -state of the C dopants with 2 of O will shift the valence band edge upwards (∼0.8 eV above the valence band) to narrow down the band gap of TiO 2 to ∼2.3 eV [11,19].A comparison between the pure TiO 2 and C-TiO 2 under solar irradiation for the photo-induced electrons transfer pathways was exhibited in Figure 8.Thus, electronic coupling of these states with electronic International Journal of Photoenergy  continuum of TiO 2 valence band state yields a band of surface state at where the electrons could be generated in conduction band by low energy of visible light irradiation [23].

Photocatalytic Decolorization of MO Dye Evaluation.
The significance of C-TiO 2 nanotubes from different applied voltages (10 V-30 V) was applied as the photocatalytic performance evaluation by decolorization of MO dye under visible light irradiation.Figure 9 clearly shows the continuous decrease in the concentration of MO dyes with increasing exposure time.It was found that the decolorization of MO dye follows an apparent first-order kinetic reaction.The decolorization rate order was C-TiO 2 -30 V > C-TiO 2 -20 V > C-TiO 2 -10 V.It could be observed that the C-TiO 2 -30 V exhibited the highest photocatalytic performance of ∼60% after exposure to solar irradiation for 5 h.The dominating factor in decolorization of MO dye is attributed to the active surface area of the catalyst to absorb more photons from the solar irradiation [40,41].In addition, higher active surface area will allow more organic MO molecules to absorb onto the C-TiO 2 surface.Furthermore, higher uniformity of nanotube arrays will result in more rapid diffusion of the organic MO molecules into the inner of nanotubes for photocatalytic reaction [41,42].In this case, C content within the nanotubes could play an important role in reducing the band gap energy of TiO 2 nanotubes (from 3.2 eV to 2.3 eV based on the anatase phase).The visible emission is likely facilitated by the interstitial C species within the lattice of TiO 2 .Thus, the photo-induced charge carriers could be generated effectively under solar illumination.Besides, the photo-induced electrons were easily trapped by this C content and led to higher transportation of charge carriers from bulk to surface of TiO 2 , which retards the recombination losses.It is a well-known fact that MO molecule has one -SO − 3 group and two alkyl groups (-CH 3 ), which can improve the adsorption of the H 2 O or OH − ions on C-TiO 2 surface [43].In this manner, C-TiO 2 nanotubes play an essential role in promoting the adsorption of OH species on the inner and outer wall surface of nanotubes and consequently promoting the photocatalytic reaction.
In the present study, photocatalytic reaction in decolorization of MO dye strongly depends on the ability of catalyst to create photo-induced charge carriers, which generate free radicals (hydroxyl radicals of • OH) able to undergo secondary reactions [6,8,9].Theoretically, the exposure of a photocatalyst to irradiation (ℎV) higher than its   will result in the transition of photo-induced electrons ( − ) from the O 2 state to the Ti 3 state and leave behind photo-induced holes (ℎ + ).Nevertheless, photo-induced  − and ℎ + recombine quickly in ordinary substances.This recombination process can occur either in bulk or on the surface of catalyst by releasing energy in the form of unproductive heat or photons [44].The photo-induced ℎ + escaping from the pair recombination preferentially moves to the photocatalyst/electrolyte interface.The photo-induced ℎ + plays an important role in generation of • OH radicals by the trapping of adsorbed H 2 O or OH − ions from the MO dye, which has strong oxidative decomposing power in decolorization of MO dye.Meanwhile, photo-induced  − escaping from recombination will be trapped on the surface of photocatalyst by reaction with adsorbed oxygen species to generate superoxide anion radicals (O 2 • ), which then will contribute to the formation of • OH radicals by reactions with H 2 O [45].In this case, more • OH radicals will react with organic MO dye to decompose them into harmless substance.

Conclusion
The formation of C-TiO 2 nanotubes via single step electrochemical anodization technique successfully formed selfaligned and high-uniformity C-TiO 2 nanotube arrays with diameter of ∼75 nm and length of ∼2 m with a minimal applied voltage of 30 V. The applied voltage played an important role in enhancing the field-assisted oxidation rate and field-assisted dissolution rate.The reduction of band gap energy level of C-TiO 2 nanotube arrays (3.2 eV to 2.3 eV) was due to the mixing states of the delocalized C 2 and O 2.In photocatalytic evaluation, it was found that C-TiO 2 synthesized at 30 V exhibited the highest decolorization rate under solar irradiation among samples.The reason was mainly attributed to higher active surface area and higher uniformity of nanotube arrays has better photons absorption from solar illumination to trigger more photoinduced charge carriers for decolorization of MO dye.

2 Figure 1 :
Figure 1: Schematic diagram of the experimental setup for quartz glass tube custom-made photoreactor used in photocatalytic degradation MO dye studies.

Figure 3 :Figure 4 :
Figure 3: EDX spectra of self-aligned and high-uniformity C-TiO 2 nanotube arrays formed in an EG electrolyte containing 5 wt% of NH 4 F and 5 wt% of H 2 O 2 for 1 h at 30 V.

Figure 5 :
Figure 5: XRD pattern of self-aligned and high-uniformity C-TiO 2 nanotube arrays formed in an EG electrolyte containing 5 wt% of NH 4 F and 5 wt% of H 2 O 2 for 1 h at 30 V, (a) as-anodized and (b) annealed at 400 ∘ C under argon atmosphere for 4 h (Ti: titanium; A: anatase).

Figure 6 :
Figure 6: The PL emission spectra of self-aligned and highuniformity C-TiO 2 nanotube arrays formed in an EG electrolyte containing 5 wt% of NH 4 F and 5 wt% of H 2 O 2 for 1 h at 30 V.

2 Figure 7 :
Figure 7: Schematic illustration for the formation mechanism of anodic C-TiO 2 nanotube arrays via electrochemical anodization technique.

Figure 8 :
Figure 8: The comparison of the photo-induced electrons transfers pathways for pure TiO 2 and C-TiO 2 under solar irradiation.