N and F Codoped TiO₂ Thin Films on Stainless Steel for Photoelectrocatalytic Removal of Cyanide Ions in Aqueous Solutions

Abstract

N-F codoped TiO₂ films were immobilized on stainless steel sheets through a combined approach involving a dip-coating technique and a hydrothermal treatment, followed by calcination at 400°C in the presence of air. Photocatalyst characterization was conducted using XRD, Raman and UV-VIS spectroscopy as well as SEM. The films were tested in a three-electrode cell for the photoelectrocatalytic degradation of CN-containing compounds. The results showed that the increase in the degradation rate of CN-containing compounds is both influenced by a synergistic effect of the doping agents and strongly dependent on the concentration of CN-containing compounds in the solution. Nitrogen contributed to the enhanced photoactivity under visible light due to the generation of localized states within the band gap of TiO₂, whereas the presence of fluoride improved the superficial properties of the film, which resulted in higher amounts of CN-containing compounds that were degraded by direct charge transfer through the photogenerated holes.

Keywords:
Codoped TiO₂; non-metallic doping; photoelectrocatalysis; cyanide oxidation

1. Introduction

Currently, contamination of natural water sources due to industrial toxic waste is one of the major environmental and social global issues¹. In this sense, the gold mining industry is one of the largest water polluter in the world, due to its heavy use of toxic substances such as cyanide, which are disposed of into natural water sources. Conventional methods for the removal of cyanide involve chemical oxidation with various substances, which include hydrogen peroxide, chloride dioxide, ozonation, and SO₂/air, among others. However, the limitations associated with these methods, such as high cost of reagents and/or equipment as well as the generation of new products with high toxicity², has motivated the study of photoelectrocatalysis as an alternative technique for the detoxification of cyanide waters.

In this context, photoelectrocatalysis (PEC) using semiconductor materials such as titanium oxide (TiO₂), which is low-cost, chemically stable and quantum efficient³, has emerged as one of the most promising advanced oxidation processes for the removal of contaminants from natural water sources. In order to perform TiO₂ PEC, photon illumination with a wavelength equal or less than 388 nm is required, which corresponds to the ultraviolet (UV) region and promotes the generation of electron-hole pairs that contribute to the redox process^3-5. However, the sunlight spectrum has its maximum radiation between 400-700 nm, and only a small fraction of this range corresponds to UV light. This hinders the ability to use solar radiation in PEC processes on TiO₂ films. Therefore, in order to address this problem and avoid the use of artificial UV lamps, it is essential to develop strategies towards the reduction of the band-gap of the semiconductor^6-8.

In this context, doping with non-metallic elements such as N, F, C, S and B^9-20 has become an attractive alternative because this type of modification results in materials with a low recombination rate, great chemical stability and good response to visible light, relative to metallic doping^{3, 21}. Recently, it has been suggested that the use of codoped systems such as N and F^13,15,19,20, N and B¹⁴, and N and V¹⁶ allow to compensate for the excess charge from substitutional doping with N. Additionally, these codoped materials are effectively used in the charge separation of photogenerated electrons and holes, improving photoelectrocatalytic activity^13-16,19,20. In this regard, the combination of N and F appears as an interesting doping approach, since it would not only increase the TiO₂ response due to the presence of N^13,15,19,20, but also would improve the superficial and crystallinity properties of the material due to modification with F^13,15,19,20. Nonetheless, the mechanisms of action of N and F as doping agents in TiO₂ are strongly dependent on the methods for the synthesis and preparation of the photocatalysts.

Furthermore, even though the use of this type of semiconductor materials for the photoelectrocatalytic oxidation of cyanide represents a promising technology for the decontamination of natural water sources, it has hardly been investigated^5,22. To our knowledge, the studies reported in the literature primarily deal with the electrooxidation of CN^- by direct electron transfer^23,24 using expensive electrodes, which compromises the economical feasibility of the process. In the present work, we attempt to gain deeper insights into the CN^- photoelectrocatalytic oxidation processes that involve the use of TiO₂ semiconductor films^5,22 supported on low-cost substrates. Simultaneous doping of TiO₂ with N and F was performed and its effect on photoactivity under visible light was evaluated. Moreover, the influence of e^--h⁺ pairs and oxidative species such as OH* radicals on process efficiency was also assessed. The photoelectrochemical studies presented here lay important groundwork needed to accurately understand the mechanisms involved in the photoelectrolytic oxidation of cyanide towards the development of low-cost and efficient technologies for the removal of CN^- from contaminated natural water sources.

2. Experimental

2.1. Preparation of N-F codoped TiO₂ films

Semiconductor films were prepared as previously described¹². Briefly, 5 mL of titanium tetraisopropoxide (TTIP) were dissolved in 5 mL of isopropanol, followed by combination with a second solution consisting of 5 mL of acetyl acetone (AcAc) and 0.6 mL of HNO₃ (65%) in 30 mL of distilled water. The resulting mixture was stirred for 12 h at room temperature. After this, 5 mL of triethylamine (TEA) and 0.93 g of NH₄F were added as nitrogen and fluorine sources, respectively. Stirring was kept for an additional 12 h. AISI 304 stainless steel substrates were prepared by abrasion with SiC #600 paper followed by cleaning in an ultrasonic bath with ethanol and then acetone, 15 min each time. The films were formed on the metallic substrates by dip-coating and then placed into an autoclave for 4 h at 17 psi and 125 °C. Finally, the photocatalyst films were annealed at 400 °C for 1 h. Following similar procedures as the one described above, four types of films were prepared: i) without doping agents (pristine TiO₂), ii) with TEA (TiO₂-T), iii) with NH₄F (TiO₂-NF), and iv) codoped TiO₂ (TiO₂-TNF). Although the stainless steel is prone to corrosion, it can be used to manufacture photoanodes as long as the semiconductor film deposited on such a substrate is homogeneous, stable chemically and has corrosion protective properties, such as occurs with sol-gel TiO₂ coatings^25-27.

2.2. Characterization of the supported photocatalysts

Grazing angle X-ray diffraction (GAXRD) with an incidence angle of 3° was performed on a BRUKER D8 diffractometer. Additionally, the films were characterized by Raman spectroscopy using a HORIBA Scientific HR LabRAM Evolution equipped with a green laser (532 nm) and a 50X objective. Attenuated reflectance infrared spectroscopy (ATR-FTIR) analyses were performed on a NICOLET IS50 (Thermo Scientific) at a resolution of 4 cm^-1 with 32 scans and an interferometer speed of 0.4147 cms^-1. Moreover, the morphology of the supported photocatalysts was examined by field-emission scanning electron microscopy (FE-SEM) using a FEI QUANTA FEG 650 microscope, and film roughness was evaluated with an AFM Park NX10 microscope using contact mode in 20 µm × 20 µm areas.

2.3. Photoelectrocatalytic oxidation of cyanide solutions

The experiments were performed using a conventional three-electrode cell. AISI 304 stainless steel sheets (4 cm²) coated with the semiconductor films served as the working electrode. A saturated calomel electrode [SCE: Hg/Hg₂Cl₂ (saturated KCl)] in a Luggin capillary was used as the reference electrode, and the counter electrode consisted of a high purity graphite rod (AGKSP grade, Alfa Aesar). All measurements were carried out on a GAMRY 600 potentiostat. Illumination was provided by a 150 W metal-halide lamp (Phillips MHN-TD UV block) with an integrated UV light filter. 25 mL of solution [NaCN (50, 100 and 250 ppm CN^-) + 0.1 M Na₂SO₄] of pH 11 were used. Solution pH was kept constant by addition of NaOH. The levels of cyanide were measured by duplicate using titration with silver nitrate in a Tritroline Easy automatic titrator (SI analytics). All solutions were prepared using deionized water, and all reagents were analytic grade (Merck).

3. Results and discussion

3.1. Characterization of the photocatalyst films

In the context of photoelectrochemistry, and especially for macroelectrolysis purposes, it is essential to ensure that the working electrode is stable and exhibits appropriate structural properties. This is particularly crucial when working with semiconductor films supported on substrates of different nature, as it is the case in the present studies (TiO₂/stainless steel). Therefore, SEM characterization was performed in order to evaluate the morphology of the synthesized films (Figure 1). The representative SEM images in Figure 1 evidence the superficial line patterns associated with SiC paper abrasion during preparation of the metal substrates. Despite this, all four types of films appeared to have a crack-free, homogeneous and compact structure, features that are required for their use as photoanodes since they favor their stability and increase corrosion resistance of the metal substrate^25,26,28,29.

Figure 1
SEM images for (a) pristine TiO₂, (b) TiO₂-T, (c) TiO₂-NF and (d) TiO₂-TNF films.

Additionally, AFM images obtained for the films in contact mode (20 µm × 20 µm area) are shown in Figure 2. As it can be noted, polishing patterns caused by abrasion with SiC paper (# 600) are present on all the films. However, some agglomerates are observed as a function of the composition of the films. Average (R_a) and root mean squared (R_q) roughnesses are tabulated in Table 1. The addition of TEA seems to promote the increase of roughness (TiO₂-T and TiO₂-TNF); while NH₄F appears to favor the formation of more homogeneous and thicker films, which increases their corrosion protective character²⁷.

Figure 2
AFM images for the synthesized films on stainless steel: a) pristine TiO₂; b) TiO₂ T; c) TiO₂ NF and d) TiO₂ TNF.

Table 1
Roughness measurements for TiO₂ and modified TiO₂ films supported on stainless steel.

Material	Roughness
	Ra (nm)	Rq (nm)
Pristine TiO2	44.1	59.4
TiO2-T	112.7	140.5
TiO2-NF	28.2	34.7
TiO2-TNF	57.2	68.9

Grazing angle X-ray diffraction (GAXRD) patterns for the different films are shown in Figure 3. All four samples were subjected to hydrothermal treatment for 4 h followed by calcination at 400°C. As it can be observed in Figure 3, anatase is the only TiO₂ crystalline phase that is present, which is known as the phase with the highest activity in photocatalytic processes³. Moreover, the intense peak associated with Fe content in stainless steel was detected, due to the thin nature of the TiO₂ films. Some additional diffraction peaks associated with iron nitride (FeN_0.0324) were also observed for the TiO₂-NF sample.

Figure 3
X-ray diffraction patterns for pristine TiO₂, TiO₂-T, TiO₂-NF and TiO₂-TNF films after hydrothermal treatment and annealing at 400 °C.

The optical response of semiconductor materials can be related to the presence of energetic states within their band-gap, which are induced by defects and/or the effect of doping elements on the structure of TiO₂. Such localized states are generated above the lower limit of the valence band (type p doping) or below the upper limit of the conduction band (type n doping) and are responsible for the absorption of less energetic light. This allows the activation of TiO₂ under visible light^9-20. Figure 4 shows the UV-VIS spectra for the films, which reveal that modifications with the doping agents significantly affected the visible light absorption of the TiO₂-NF and TiO₂-TNF samples. Using the Kubelka-Munk modified function (see Figure 4 insert) the band-gap values for each photocatalyst were estimated³⁰ as follows: 3.2 eV for pristine TiO₂, 3.1 eV for TiO₂-T and 2.9 eV for both TiO₂-NF and TiO₂-TNF. These results indicate that when TEA is used alone, the insertion of N in the oxide network is not stable to heat treatment, which leaves anionic vacancies in the material. On the other hand, modification with either NH₄F or TEA and NH₄F resulted in more thermally stable doping of TiO₂, and therefore, produced a material that can effectively use visible light in photo-assisted electrochemical processes for the degradation of contaminants.

Figure 4
UV-VIS spectra for TiO₂ pristine, TiO₂-T, TiO₂-NF and TiO₂-TNF films after hydrothermal treatment and annealing at 400 °C.

Figure 5 shows the ATR-FTIR spectra for the synthesized films. TiO₂ characteristic bands can be identified for all samples. The band at 3350 cm^-1 can be attributed to the stretching of the OH groups from residual alcohol, hydroxyl groups (Ti-OH)^31-33 as well as from water that was trapped in the oxide network. The band between 1800-1500 cm^-1 represents the contribution due to water proton absorption^31-33, and the band between 871-479 cm^-1 corresponds to Ti-O-Ti bonding^31-33.

Figure 5
ATR-FTIR spectra for TiO₂ pristine, TiO₂-T, TiO₂-NF and TiO₂-TNF films after hydrothermal treatment and annealing at 400 °C.

Additionally, for the modified samples (TiO₂-T, TiO₂-NF and TiO₂-TNF) a signal at 1458 cm^-1 was detected. This band is associated with -NO_x groups (-Ti-O-N-Ti-), which indicates incorporation of N in the TiO₂ structure. As expected, such signal was very weak for the TiO₂-T film due to the lack of thermal stability of the N inserted in this sample. This is consistent with what has been previously reported in the literature, which indicates that the N contribution from TEA can leave from the oxide structure at calcination temperatures around 400 °C³⁴. Furthermore, a small band at 916 cm^-1 was only detected for the samples that contained fluoride (TiO₂-NF and TiO₂-TNF) which is attributed to TiO₆ octahedral distortion resulting from N and F incorporation in the TiO₂ oxide network^31-33.

The results from Raman characterization are shown in Figure 6. As it can be observed, all the film samples exhibited the characteristic bands for the vibrational modes of the anatase phase: E_g (148 cm^-1), B_1g (396 cm^-1), A_1g (513 cm^-1) and E_g (638 cm^-1)³⁵. Relative to pristine TiO₂, a slight shift in the main band of the E_g mode (148 cm^-1) was observed for the doped samples. This could be due to the presence of structural defects in the TiO₂ network. The red shift observed for TiO₂-T (E_g = 146 cm^-1, see Figure 6 insert) can be associated with a variation in the crystal size^35,36. On the other hand, the blue shift detected for TiO₂-NF (E_g=152 cm^-1) and TiO₂-TNF (E_g=153 cm^-1) has been previously attributed to distortions in the TiO₂ crystal network caused by the presence of N and F, which increase the vibration energy of the E_g mode^35,36.

Figure 6
Raman spectra for TiO₂ pristine, TiO₂-T, TiO₂-NF and TiO₂-TNF films after hydrothermal treatment and annealing at 400 °C.

3.2. Semiconducting properties

In order to evaluate the impact of the modifying agents on the semiconducting properties of the synthesized thin films, Mott-Schottky (M-S) plots were generated at a frequency of 400 Hz (Figure 7)³⁷. A positive slope in the linear region of the M-S plots can be observed for all cases, which indicates the n-type behavior of the films. The linear region in Figure 7 was fitted to equation (1) (solid line) in order to estimate the flat-band potential (E_fb) and donor density (N_d) of the formed films, which are summarized in Table 2:

(1) $\frac{1}{{\mathit{C}}_{\mathit{sc}}^2}=\frac{2{\mathit{N}}_{\mathit{A}}}{{\mathit{N}}_{\mathit{d}}\mathit{F}{\epsilon }_{\mathit{r}}{\epsilon }_0}.\left({\mathit{E}}_{\mathit{m}}-{\mathit{E}}_{\mathit{fb}}-\frac{\mathit{RT}}{\mathit{F}}\right)$

Figure 7
(a) Mott-Schottky plots for the synthesized films obtained experimentally at a frequency of 400 Hz in a 0.1 M Na₂SO₄ (pH 11), and (b) Band diagram derived by assuming the flat band potential of the films as the potential of the conduction band (CB). Valence band (VB) position was estimated by adding the measured band gap energy (Figure 4).

Table 2
Donor density (N_d) and flat band potential (E_fb) estimated from Figure 7.

Sample	Nd × 1021 cm-3	Efb mV Vs SCE
TiO2	2.4	-33
TiO2-T	11.7	-49
TiO2-NF	1.4	61
TiO2-TNF	1.1	71

where N_A is Avogadro's number (6.02 × 10²³ mol^-1), N_d (cm^-3) is the donor density, F is the Faraday constant (~ 9.65 × 10⁴ C mol⁻¹), ε_r is the relative permittivity (50)³⁷, ε₀ is the permittivity of vacuum (8.8542×10^-14 F cm^-1), E_m (V) is the potential at which the measurement was carried out, E_fb (V) is the flat-band potential, R is the gas constant (8.314 JK^-1 mol^-1), and T is the absolute temperature (~ 298 K). The third term in parentheses can be assumed to be negligible at room temperature, relative to the other terms.

As seen in Table 2, relative to pristine TiO₂, TEA modification of TiO₂ (TiO₂-T) resulted in a slight shift towards more negative values of E_fb with a nearly five-fold increase in the donor density (N_d), which indicated the generation of defects such as oxygen vacancies in the material^9,10. On the other hand, NH₄F doping (TiO₂-NF) displaced the flat band potential towards positive values, with a slight decrease in N_d, compared to what was measured for the pristine samples. This behavior could be related to the compensating effect associated with the presence of F, which prevents the generation of new defects such as oxygen vacancies when N and F are simultaneously incorporated in the TiO₂ structure¹³. Moreover, it is well known that fluorine chemisorption onto a TiO₂ surface changes the surface acidity of the material, which could explain the variation in the E_fb values relative to pristine TiO₂. Furthermore, band diagrams were sketched from E_fb measurements, see Figure 7b. The valence band position (VB), derived by adding the band-gap energy (measured from UV-vis spectra in Figure 4), shows a slight displacement toward lower potential values for N-modified films. This behavior indicates that N2p states are being generated by the incorporation of N in the TiO₂ lattice, as it has been previously proposed⁵.

3.3. Photoelectrocatalytic oxidation of cyanide

Synthesized films were used as photoanodes in a conventional three-electrode cell to perform the photoelectrocatalytic oxidation of cyanide in synthetic solutions. All experiments were carried out in solutions of pH 11. Solution pH was kept constant by addition of 0.1 M NaOH to avoid the generation of hydrogen cyanide. In order to determine the most favorable potentiostatic conditions for the photoelectrocatalytic oxidation of cyanide, TEA and NH₄F modified TiO₂ films (TiO₂-TNF) were used to degrade 100 ppm of CN^- at different potentials for 120 min (see Figure 8). As shown in Figure 8, the percentage of degraded CN^- increases proportionally with the imposed potential, exceeding 50% cyanide oxidation when a potential of 0.97 V Vs SCE is reached. However, beyond this point, cyanide degradation drops drastically to undetectable levels at 1.1 V Vs SCE. When looking at the currents that were registered during the experiment, a different trend is observed: there is a monotonic increase of the current followed by a change in the slope of the curve in the interval of potentials where maximum percentage CN^-degradation is achieved (0.95 to 0.97 V). This response indicates that potentials above 0.97 V favor an additional process that reduces the efficiency of CN^- degradation, which is possibly the oxidation of water. Based on these findings, a potential of 0.97 V Vs SCE was selected for the photoelectrocatalytic oxidation of the cyanide solutions.

Figure 8
Photoelectrocatalytic oxidation of CN^- (100 ppm, pH=11) on TiO₂ TNF film during 120 minutes.

Once the appropriate potential was selected, four different film samples were prepared (pristine TiO₂, TiO₂-T, TiO₂-NF and TiO₂-TNF) for the PEC oxidation of 100 ppm of CN^- during 150 min, as shown in Figure 9.

Figure 9
Photoelectrocatalytic oxidation of CN^- (100 ppm, pH=11) on pristine TiO₂, TiO₂-T, TiO₂-NF and TiO₂-TNF films during 150 minutes: a) kinetic curves and b) current transients.

The kinetic curves in Figure 9a reveal that the highest degradation rate was achieved on the TiO₂ films modified with TEA and NH₄F (TiO₂-TNF), which allowed for up to 64% CN^- degradation after 2 hr. The remaining TiO₂-NF, TiO₂-T and pristine TiO₂ samples provided a 55 %, 48 % and 32 % CN^- removal, respectively. Furthermore, Figure 9b shows the cronoamperograms generated during the PEC experiments. In all cases, there is a drastic current drop followed by an increase until a maximum current value is reached, and then it slowly falls down to the end of the test. In previous studies, it was observed that the variation of the current with time is related to the morphology of the films, i.e. their thickness and porosity⁵. Nonetheless, for the present studies, the magnitude of the current varied significantly among samples despite the fact that all films exhibited similar morphology. As shown in Figure 9b, the oxidation process occurs at a higher rate on the TiO₂-TNF and TiO₂-NF films, relative to pristine TiO₂ and TiO₂-T. These results are consistent with what was observed for the kinetic curves and confirm that simultaneous modification of TiO₂ with TEA and NH₄F results in a codoped material with a higher ability for photoelectrocatalytic oxidation of CN^-, relative to pristine or individually modified TiO₂.

As seen in Figure 10a, increasing CN^- concentrations increased the quantity of degraded cyanide in both photoelectrocatalytic (PEC) and electrocatalytic (EC) processes. Figure 10 shows that the amount of oxidized cyanide was directly related to the magnitude of the registered currents, according to Faraday's law of electrolysis. Moreover, regardless of the contaminant initial concentration, illumination resulted in higher CN^- removal and therefore higher currents were generated, relative to the process without illumination (Figure 10b). This is associated with the generation of electron-hole pairs on the surface of the semiconductor^5,38. As it has been previously reported in the literature^5,22-24, photocatalytic oxidation of CN^- on TiO₂ occurs primarily due to direct charge transfer.

Figure 10
(a) Photoelectrocatalytic (On) and electrocatalytic (Off) oxidation of CN^- at different concentrations vs. time, and (b) current transients. (c) ln(C/C₀) vs. Time for photoelectrocatalytic process at different cyanide concentrations. (d) Effect of the cyanide concentration over the kinetic constant (k) for pseudo-first order reaction⁵.

Furthermore, ln C/C₀ vs time curves obtained at different initial cyanide concentrations, and the kinetic constant for a pseudo-first order reaction versus the initial cyanide concentration are plotted in figures 10c and 10d, respectively. The higher k values were obtained at the lowest cyanide concentration, which indicates that photoelectrocatalytic process could be employed more efficiently at a low pollutant concentration, as it has been stated for photocatalytic processes³.

Since TiO₂ modification with fluoride generally results in an increased generation of OH^* radicals for both photocatalytic (PC) and photoelectrocatalytic (PEC) processes, the effect of OH^* radicals on PEC cyanide degradation was evaluated. In order to do this, 80 mM of tertbutanol (t-BuOH) was added to the CN^- solution³⁸. A constant potential of 0.97 V Vs SCE under illumination for 150 min was used as well. Figure 11 shows the results for the TiO₂-TNF and TiO₂-T films. The kinetic curves indicate a small decrease in CN^- degradation when OH^* radicals are inhibited by t-BuOH, due to the fact that oxidation occurs primarily by direct electron transfer.

Figure 11
Effect of t-BuOH scavenger on the rate of CN^- degradation on TiO₂-TNF and TiO₂-T films.

Several researchers have associated the photocatalytic oxidation of CN^- on TiO₂ with a direct charge transfer process^39-43. Nonetheless, it has also been reported that such photocatalytic oxidation could also take place through indirect mechanisms at the semiconductor/electrolyte interface. These indirect mechanisms are due to the presence of adsorbed hydroxyl ions that form OH^* radicals or caused by a homogeneous phase reaction with the OH^* radicals that diffuse into the cyanide solution³. These OH^* radicals react with the CN^- ions to give cyanate ions (OCN^- and formamide (HCONH₂)³⁹. However, most authors report that for the carbon balance after degradation, the sum of CN^- and OCN^- ion concentrations is constant and properly fits the mass balance^39,40. For this reason, indirect oxidation mechanisms do not seem feasible, or do not take place at significant magnitudes during the photocatalytic oxidation of CN^-.

4. Conclusions

TiO₂ modification using nitrogen and fluoride precursors (TEA and NH₄F) revealed a higher degree of crystallinity of the anatase phase and superior absorption of visible light, relative to pristine TiO₂. This modification probably favored a direct oxidation mechanism through the holes trapped on the catalyst (≡Ti-O*), which resulted in a more efficient use of visible light to generate electron-hole pairs.

The results from the PEC experiments showed that TEA and NH₄F modified TiO₂ photoanodes (TiO₂-TNF) achieve a significant degradation percentage of cyanide (> 60%) in diluted solutions (100 ppm) in just 150 min, when an optimal potential of 0.97 V Vs. SCE is used. Furthermore, it was found that the generation of OH* radicals during TiO₂ modification with fluoride does not have a significant effect on the PEC degradation of CN^- ions, which takes place by direct electron transfer. Cumulatively, these results indicate that the increase in percentage degradation of cyanide can be associated with a more efficient use of visible light due to a reduction in the band-gap of TiO₂ in the presence of NH₄F and TEA.

5. Acknowledgements

The authors would like to acknowledge financial support from COLCIENCIAS (Project 1102-521-28875) and Universidad Industrial de Santander (DIEF Ingenierías Fisicoquímicas, Project 9416). The authors would also like to thank Dr. Hugo A. Estupiñán from Universidad Nacional de Colombia (Medellín) for technical assistance with AFM measurements.

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Publication Dates

• Publication in this collection
  23 Feb 2017
• Date of issue
  Mar-Apr 2017

History

• Received
  10 Mar 2016
• Reviewed
  30 Dec 2016
• Accepted
  25 Jan 2017