Vertically Aligned Titanate Nanotubes Hydrothermally Synthesized

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Vertically Aligned Titanate Nanotubes Hydrothermally Synthesized from Anodized TiO2 Nanotube Arrays: An Efficient Adsorbent for the Repeatable Recovery of Sr Ions Love Kumar Dhandole,† Hee-Suk Chung,‡ Jungho Ryu,*,§ and Jum Suk Jang*,† †

ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by UNIV OF SUNDERLAND on 10/31/18. For personal use only.

Division of Biotechnology, Advanced Institute of Environment and Bioscience, College of Environmental and Bioresource Sciences, Chonbuk National University, Iksan 54596, Korea ‡ Analytical Research Division, Korea Basic Science Institute, Jeonju, Jeollabuk-do 54907, Korea § Geologic Environment Research Division, Korea Institute of Geoscience and Mineral Resources, Daejeon 34132, Korea S Supporting Information *

ABSTRACT: Vertically aligned titanate nanotubes (VA-TNTs) are prepared for the repeated and effective recovery of Sr ions. Two-electrode electrochemical anodization with a 50 V potential was used to prepare the TiO2 nanotube arrays on a Ti metal foil. The growth and inner diameters of the TiO2 nanotubes were measured as a function of the anodization potential and reaction time. VA-TNTs structures are synthesized via onestep alkaline hydrothermal (HT) reaction of the as-prepared anodized TiO2 nanotube arrays (anodic TiNA). The titanate nanotube synthesis is successfully optimized, revealing that a low calcination temperature and a low HT reaction time allowed for the synthesis of a vertically aligned homogeneously dispersed “Grass”-type morphology among the titanate nanotubes. The equilibrium isotherm and kinetic data are used for model fitting. High-resolution transmission electron microscopy images and X-ray energy dispersive spectroscopy elemental characterizations provided a detailed chemical composition and surface analysis of the VA-TNTs. The recyclability of the VA-TNTs for repeated metal ion adsorption−desorption is demonstrated successfully. The crystallinity of the VA-TNTs after each repeated desorption cycle was dramatically improved by Na treatment. This structural reformation (Na treatment) step increased the number of possible high-yield metal ion recovery cycles. Also, the VA-TNTs have great potential for removing toxic heavy metal ions in an easy, economic, and environmentally friendly way. KEYWORDS: Anodization, TiO2 nanotube arrays, Vertically aligned titanate nanotube, Adsorption−desorption, Metal ions recovery, Recyclability

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INTRODUCTION

Seawater has abundant resources of minerals that include dissolved metals in their ionic form with different concentrations. Strontium metal ions are one of the rare earth metals used widely in industrial applications, such as manufacturing of CRT screens, glass, ceramics, alloy in small engines, and fireworks. The concentration of strontium metal ions in seawater is approximately 7 mg L−1.43 Compared with conventional extraction processes of land mining of mineral ores, the recovery of strontium metal ions from seawater has been easy, cost-effective, and beneficial to eco-systems.44 Few studies have been focused on strontium recovery by using TNTs or composites of titanate materials.42,45−48 Interestingly, the ion exchange properties of TNTs render them proficient adsorbents for the removal of heavy metal ions from wastewater45,49,50 or for the recovery of rare metal ions

Since titanate-derived nanotubes were first developed by Kasuga,1,2 titanate nanotubes (TNTs) have attracted a significant amount of research due to their unique physicochemical properties, including their good structural stability, large specific area, high pore volume, and ionexchange capability.3−6 These features of TNTs suggest their potential utility in a variety of applications in catalysis,7 photocatalysis,8−17 solar hydrogen generation,18,19 solar water splitting,20−25 gas sensors,26 and lithium-ion batteries.27−29 Conventional titanate nanostructures (e.g., belts,30 fibers,31 flowers,32 rods,33 ribbons,34 tubes,35 sheets,36 and wires37) are synthesized using commercially available titanium precursors or Degussa P25 as a seed material via a hydrothermal (HT) process.38,39 Titanate-based materials have been widely investigated for the removal (or recovery) of rare earth metal ions, heavy metal ions, and radioactive pollutants from the various water sources.39−42 © XXXX American Chemical Society

Received: June 13, 2018 Revised: September 20, 2018 Published: October 19, 2018 A

DOI: 10.1021/acssuschemeng.8b02805 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 1. Schematic diagram showing the transformation of the anodic TiNAs into VA-TNTs.

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from diluted water.39 In the case of powdered nanoadsorbents, they are difficult to separate from contaminants during recycling steps, which require the efficient recovery or removal of metal ions.38,39 There is an alternate approach we suggested by using Ti foil-based TNTs. Ti foil-based TNTs are more readily recycled and easily separated than the powdered nanoadsorbents. Few studies have reported on the synthesis of Ti foil-based TNTs.51−53 It is the first time we reported vertically aligned, homogeneous, and well dispersed TNTs on the foil-based system for efficient recovery of metal ions. In a first approach, we prepared an anodic titanium oxide nanotube array (anodic TiNA) by electrochemical anodization. The anodic potential and anodization reaction times for anodic TiNA were optimized using a two-electrode electrochemical process. The vertically aligned tubular morphology and crystallinity of the anodic TiNA were analyzed via scanning electron miscroscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD). Furthermore, this anodic TiNA was used to synthesize a vertically aligned TNTs (VA-TNTs) via a one-pot alkali HT synthesis. After screening the parameters for the TNT, such as anodization condition, TiNA calcination, and alkali concentration, the HT condition was optimized to grow highly ordered VA-TNTs. A low calcination temperature and low HT reaction time allowed for the synthesis of a vertically aligned homogeneously dispersed “Grass”-type morphology of TNTs that adhered well to the Ti metal-foil surface. These VA-TNTs were used to recover or remove metal ions. A maximum metal ion adsorption efficiency (i.e., 100%) and adsorption equilibrium for Sr metal ions were observed over a period of 10 h. Adsorption isotherm and kinetic test data fit well to the Langmuir isotherm model and a pseudo-first-order model, respectively. The recyclability of the VA-TNTs was improved by applying an Na treatment, which increased the number of consecutive adsorption cycles that could be applied. Increasing the number of adsorption cycles of the VA-TNT enabled the efficient recovery or removal of metal ions in an easy, economic, and environmentally beneficial way.

EXPERIMENTAL SECTION

Chemicals and Reagents. Titanium foil (0.127 mm thick, 99.7%), ammonium fluoride (≥98%), and strontium chloride hexahydrate were purchased from Sigma-Aldrich. Glycerol (99%) was purchased from Junsei. Acetone (99.5%) and ethyl alcohol (94.5%) were purchased from Samchun chemicals. Sr2+ standard solutions (Sr-100) were purchased from Kanto chemicals. A 5 M NaOH (Samchun, ≥ 98.0%) solution was prepared by dissolving NaOH in deionized water (DI, CBNU, pH 7). Synthesis of the Anodic TiNA and VA-TNTs. In a typical electrochemical anodization process, a 3 × 3 cm2 Ti foil was first washed and degreased via ultrasonication treatment in acetone, ethanol, and water for 10 min each. The washed Ti foil was dried under nitrogen gas flow, and the cleaned Ti foil was electrochemically anodized on one side using a conventional two-electrode electrochemical cell connected to a DC power supply under an applied potential of 30−50 V over different durations. The cleaned Ti foil and Pt coil were used as the working electrode and counter electrode, respectively, and were immersed into a 50 mL electrolyte solution. All experiments were carried out using a mixed glycerol−water (9:1) electrolyte solution containing NH4F. The anodized samples were cleaned by ultrasonication to remove the extra organic solvent from the surfaces of the anodized nanotubes. The as-synthesized anodic TiNA was then calcined at 400 or 500 °C in a box furnace for 2 h. VA-TNTs were synthesized using a one-step HT method. During this process, the calcined TiO2 nanotubes were HT-synthesized in 10 M concentrated NaOH solutions. The anodic TiNA sample was kept inside an alkali solution that occupied 70% of the volume of the Teflon reactor at 160 °C over different reaction times. After cooling the HT reactor, the synthesized VA-TNT was rinsed with DI water several times to remove any NaOH remaining on the electrode surface. The dried TNTs were then used for metal ion adsorption. Characterization. XRD structural analysis was performed using a PANalytical X’pert Pro MPD diffractometer equipped with a Cu Kα radiation source (wavelength Kα1 = 1.540598 Å and Kα2 = 1.544426 Å) operated at 40 kV, 30 mA at a scan rate of 0.03° 2θ s−1 with a 2θ angle of 5−80°. SEM observations were carried out using a Carl Zeiss SUPRA 40VP with field emission equipped with an X-ray energy dispersive spectrometer (EDS). TEM was performed using a JEOL ARM-200F operated at 200 kV. The TEM sample was prepared by placing a drop of the sample suspension in ethanol on a standard carbon-coated copper grid. The Sr2+ recovery analysis was then carried out using a Profile Plus highly dispersed inductively coupled plasma (ICP) (Teleayne Leeman Laboratories) instrument. B


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ACS Sustainable Chemistry & Engineering Batch Adsorption Tests. Batch experiments were performed to examine the adsorption of metal ions over the VA-TNT. The ion adsorption experiment was performed in a 100 mL volume beaker. A stock solution was prepared by dissolving a quantified amount of a 10 ppm concentration of metal precursors in DI water. The VA-TNT was immersed into a stock solution for 12 h. The concentration of the metal ions was measured by sampling at different time intervals and was determined using an ICP instrument. The ion uptake by the titanate nanomaterials was calculated using the following equation: C=

(C0 − Ce) × 100 C0

ordered anodic nanotubular structures that were further used in this study. HT Synthesis of Highly Ordered Anodic TiNA. A typical synthesis for forming anodic TiNA over a Ti foil was carried out. The VA-TNTs were synthesized from anodic TiNA using a one-pot HT method. A schematic diagram of the process of synthesizing the TNTss is illustrated in Figure 1. Initially, a bare Ti metal foil was synthesized under a HT reaction. The cleaned Ti metal foil was vertically placed into a Teflon-lined reactor, and a 10 M NaOH aqueous solution was added to 70% of the volume of the Teflon-lined reactor. The HT reactor was kept in a hot air oven at 160 °C for 48 h. After the HT reaction, the as-synthesized TNTs were washed by dipping into DI water, and the rinsed foil was dried under a nitrogen stream. SEM images of the hydrothermally synthesized TNTs from direct Ti metal foil are shown in Figure 3. Figure 3 shows that the film (or layer) of the as-synthesized TNTs could be peeled away from the Ti metal foil when dipped into the DI water. SEM images of the peeled part and the Ti foil are shown in Figures. 3C−E. The peeled surface of the as-synthesized film contained tubular nanostructures (shown in Figure 3E). A rough surface was observed under the detached area (as shown in Figure 3C); however, the assynthesized film displayed numerous cracks and discontinuities on its surface. The effects of peeling out of the titanate film from the surface of the as-synthesized sample were controlled by using the anodic TiNA in a HT synthesis. Anodization at 50 V for 4 h permitted the vertical growth of TiO2 nanotube arrays over the surfaces of the Ti metal foil. Anodic conditions of 50 V applied over 4 h were set for the Ti foil anodization used for HT synthesis, which proceeded in a 10 M aqueous NaOH solution at 160 °C, under the varying reaction time intervals to optimize the process. SEM images of the samples synthesized over different HT reaction time intervals are shown in Figure S2. SEM images of the bottom surfaces of the VA-TNT with varying HT reaction time intervals (3, 6, 9, 12, and 24 h, respectively) and the corresponding morphologies are shown in Figure S2A−E. These VA-TNT films obtained from anodized samples adhered well to the Ti metal foil compared to the unanodized film, which is shown in Figure 3. Short lengths of nanotubes and an agglomerated tubular morphology were observed in the 3 h sample. The thickness and length of the tubular structures are the function of HT reaction time, which increases as the HT reaction time increases (samples A−E); however, cracks and irregularities were still observed on the surfaces of the titanate sample films. The most detached surface was observed in the titanate synthesized for 3 and 24 h, whereas a thick layer of organic residues was observed on the surface of the sample synthesized for 12 h. The sample with the cleanest surface and lowest number of cracks that hydrothermally synthesized at 160 °C for 6 h was used in further experiments. The surface regularity and adhesion of the titanate film over the Ti metal foil were improved by calcine the anodized sample prior to conducting the HT reaction. First, the anodic TiNA samples are calcined at 400 and 500 °C, and they were further hydrothermally synthesized at 160 °C for 6 h in a 10 M NaOH solution. The SEM images shown in Figure 4A(a, b, and c and d, e, and f) display the bottom, middle, and top surface morphologies of the TNTs synthesized at 400 and 500 °C, respectively. These samples displayed numerous cracks on the metal foil surfaces. These surface irregularities (cracks and

(1)

where C denotes the percentage (%) of ion uptake by the VA-TNT, and C0 (mg L−1) and Ce (mg L−1) indicate the initial and equilibrium concentrations of the metal ions, respectively.

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RESULTS AND DISCUSSION Synthesis and Optimization of the Anodic Conditions for the Ti Metal Foil. The anodic oxidation of the titanium metal foil involved the field-assisted migration of ions through an oxide film.54 The electrochemical formation of the titanium oxide film depended on the nature of the electrolyte, concentration of the electrolyte, temperature, and anodization voltage, which controlled the film growth and properties. An anodization setup is illustrated in the schematic diagram shown in Figure 1. The Ti metal foil was anodized using a twoelectrode anodization method, widely used to obtain a highly ordered array of TiO2 nanotubes. In a typical anodization process, the cleaned Ti foil was anodized in a glycerol, ammonium fluoride, and water-based electrolyte at different potentials over a given time interval. Figure S1A−D shows SEM images of the top surface morphology of the Ti metal foil anodized under different reaction conditions. The subfigures (A−D) in Figure S1 were collected under the following conditions: 30 V for 30 min, 40 V for 30 min, 50 V for 30 min, and 50 V for 4 h, respectively. Smaller and irregular tubular TiO2 structures were grown at a low potential and short anodization time, as shown in Figure S1A. In a comparison, a higher applied potential produced broader and thicker tubular structures than a low applied potential. However, unwanted particles were grown on the surface of the Ti metal foil during the anodization process (subfigures B and C in Figure S1). These unwanted as-grown particles covered most of the top surfaces of the TiO2 nanotubes. Thick, broader, and highly ordered tubular structures on the Ti metal foil were obtained at 50 V for 4 h anodization (as shown in Figure S1D). Figure 2A,B presents the top and cross-sectional morphologies of the anodic TiNA that were anodized at 50 V for 4 h, respectively. The diameter and length of the anodic TiNA were 100−180 nm and 1.8−2.2 μm, respectively. These anodization conditions of the Ti metal foil were fixed to obtain highly

Figure 2. SEM images of (A) the top view and (B) the cross-sectional view of the anodic TiNA prepared at 50 V over 4 h. C


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Figure 3. HT synthesis of bare Ti foil at 160 °C at 24 h in 70 mL of NaOH (10 M) solution.

Figure 4. (A) SEM images of HT synthesis conditions (A) 160 °C/6 h under 10 M NaOH/40 mL solution and (B) 160 °C/6 h under 5 M NaOH/40 mL solution, where samples (a, b, and c) and (d, e, and f) calcined at 400 and 500 °C, respectively, after the 50 V/4 h of anodization, ((a, d) bottom, (b, e) middle,and (c, f) top-side surface morphologies).

samples that were calcined at 400 °C, such as the vertical “Grass”-type tubular structures (Figure 4B(a−c)). Figure 4B(f) clearly reveals that the vertically aligned nanotubes were not properly grown on the surface of the Ti metal foil. This finding indicated that the NaOH concentration and the calcination temperature played important roles in the synthesis of the VATNTs. Effect of the HT Reaction Time on the Calcined Sample. The effect of the HT reaction time on the synthesis of the VA-TNTs was investigated using anodized TiO2 nanotubes calcined at 400 and 500 °C, hydrothermally synthesized in 5 M NaOH at 160 °C over different reaction time intervals, as shown in Figure S3a−c. SEM images of the samples hydrothermally synthesized for 6 h are shown in Figure S3a, which presents the bottom, middle, and top-side morphologies of the VA-TNTs prepared from anodic TiNA calcined at 400 °C (A, B, and C) (VA-6TNT-400) and 500 °C (D, E, and F) (VA-6TNT-500), respectively. The bottom regions of the TNTs were crack free and displayed good

peeling effects) of the TNTs were controlled by using low concentration (5 M) NaOH for HT reaction after the calcination process. Figure 4B(a, b, and c and d, e, and f) shows SEM images of the bottom, middle, and top surface morphologies of the TNTs hydrothermally synthesized (in 5 M NaOH) from the anodic TiNA, calcined at 400 and 500 °C, respectively. The bottom (a) and top (c) surface images of the sample hydrothermally synthesized at 5 M NaOH (from TiO2 nanotubes calcined at 400 °C) displayed no irregularities, as shown in Figure 4B(a−c). The morphology of the tubular VATNTs sample resembled the “Grass”-type aligned structure, as shown in the high-resolution image (Figure 4B(b)). Even homogeneous morphology was observed throughout the surfaces of the VA-TNTs. Further, a good attachment was observed between the TNT layer and the Ti metal foil for this sample. The samples calcined at 500 °C (5 M; d, e, and f), surface layer of the TNTs was strongly attached over the Ti metal foil, and no surface cracks were observed. However, the morphology of these samples was not similar to the previous D


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ACS Sustainable Chemistry & Engineering adhesion to the surface of Ti metal foil in the 400 °C calcined sample. Moreover, high-order dispersed homogeneous aligned titanate morphologies were observed in VA-6TNT-400, whereas agglomerated morphologies were observed in VA6TNT-500 (as shown in the inset). Other VA-TNTs were hydrothermally synthesized for 12 and 24 h, as shown in Figure S3b,c. A highly vertically aligned tubular morphology was observed in the VA-12TNT-500 sample, (as shown in the inset of Figure S3bE); however, surface cracks were also displayed in the VA-12TNT-500 sample, unlike the VA-12TNT-400 sample (as shown in Figures. S3b(A,D)). Figure S3c presents SEM images of the VA-24TNT-400 and 500 samples. The bottom surface of the VA-24TNT-500 sample peeled away, and an irregular surface was observed in the VA-24TNT-400 sample. These results indicated that a high calcination temperature and a long HT reaction time affected the surface properties, while a low calcination temperature (400 °C), low concentration of NaOH (5 M), low HT reaction temperature (160 °C), and shorter reaction time (6 h) were favorable for transforming the anodic TiNA into VA-TNTs (VA-6TNT-400). Finally, SEM and TEM images of the samples synthesized under the optimal conditions are shown in Figure 5. Figure

Figure 6. XRD patterns of (a) bare Ti foil, (b) anodic TiNA (50 V/4 h), (c) anodic TiNA prepared by anodization (50 V for 4 h) and calcination (400 °C for 2 h), and (d) VA-TNTs hydrothermally synthesized at 160 °C for 6 h from the sample described in (c).

at 400 °C for 2 h produced new diffraction peaks. These new XRD diffraction peaks at 25.38, 48, and 55° were assigned to the anatase phase of the TiO2 nanotubes. The diffraction peaks of the VA-6TNT sample at 9.9, 24.16, 28.5, and 48° corresponded to the titanate crystal structure and agreed with the titanate reference peaks (JCPDS no. 72-0148). Sorption Behavior. Figure 7a shows the time profiles of the Sr adsorption over the VA-TNTs hydrothermally synthesized at different reaction times (e.g., 6, 12, 24, and 48 h). The Sr sorption efficiency of the VA-6TNT-400 sample reached 60% during the first 2 h and showed the highest uptake among the samples tested. This fast adsorption profile indicated direct interactions between the sodium ions of the TNTs and the Sr ions, which could be ascribed to the fact that the Na ions of the TNTs were replaced by Sr ions through an ion-exchange mechanism.39,49 These results indicated that the nanotube structure grew densely in proportion to the HT reaction time and retarded Sr adsorption inside the nanotube structures during the initial sorption period. After 4 h, the Sr uptake slowed, possibly due to the saturation of the ion exchange process and internal diffusion resistance of the Sr ions inside the VA-TNT structures. At a time interval between 4 and 10 h, only a 20% uptake was observed, and an equilibrium adsorption process was achieved within 8 h. However, after 10 h, approximately 100% Sr adsorption efficiency was obtained at the given concentration. Other titanate arrays prepared during different HT reaction times (12, 24, and 48 h) displayed similar trends in their adsorption efficiencies, and the equilibrium adsorption properties of all samples were similar after an 8 h adsorption period. These results suggested that the HT reaction time for the preparation of TNTs was not proportionally correlated with the Sr adsorption capacity and could be optimized at 6 h. The adsorption behavior of the VA-6TNT-400 sample was investigated by applying two kinetic models to the Sr uptake over time. Pseudo-first-order and pseudo-second-order kinetic models are described by eqs 2 and 3, respectively,

Figure 5. SEM images of the (A) top view and (B) cross-sectional view, and (C, D) TEM images of the VA-TNTs hydrothermally synthesized at 160 °C for 6 h.

5A,B presents top and cross-sectional SEM tubular morphologies of the VA-TNTs hydrothermally synthesized at 160 °C for 6 h (VA-6TNT-400). Figure 5C,D presents TEM images of VA-6TNT-400. The interlayer space was observed to be 8.4 Å along the 100 plane. The length and diameter of the VA6TNTs-400 were observed to be 2.4 μm and 10−18 nm, respectively. X-ray diffraction patterns of the bare Ti metal foil, anodized TiO2 nanotubes, and VA-TNTs are shown in Figure 6. The XRD peaks of the bare sample corresponded to 35, 38, 40, 53, 63, 71, and 77° and could be attributed to the characteristic diffraction peaks of the Ti metal foil. These diffraction peak patterns were observed throughout all samples, and their peak positions were similar to those of the parental bare sample. In the anodized TiO2 sample, no new peaks were identified except for the characteristic diffraction peaks of the bare Ti metal foil. Calcining the anodized TiO2 nanotube array E

ln[qe − q] = ln qe − kt

(2)

t t t = + q qe Kqe2

(3) DOI: 10.1021/acssuschemeng.8b02805 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Figure 7. (a) Time-dependent Sr adsorption by the VA-TNTs synthesized at a fixed HT temperature of 160 °C with different HT reaction times (6, 12, 24, or 48 h), and a (b) pseudo-first-order kinetic model fit of the Sr adsorption onto the VA-6TNT-400 sample. The inset table shows the corresponding kinetic model coefficients. ([Sr]0 = 10 mg·L−1, V (volume) = 80 mL, and m (mass) = unit area of 3 × 3 cm2).

Figure 8. (A) Adsorption isotherm of Sr ions onto the VA-6TNT-400 sample, the data were fit to the Langmuir adsorption isotherm model, and the inset table shows the corresponding Langmuir isotherm model coefficients. (m (mass) = unit area of 3 × 3 cm2, V (volume) = 50 mL, and contact time = 12 h). (B) Sr adsorption over the VA6TNT-400 sample at different pH.

where q and qe are the quantities of adsorbed solute (mg· unit−1, unit area = 3 × 3 cm2) at time t and at the equilibrium time (min), respectively, and k and K represent the pseudofirst-order rate constant and pseudo-second-order rate constant (unit·(mg·min)−1), respectively. Figure 7b shows the Sr uptake at 6 h as a function of the adsorption time and the pseudo-firstorder kinetic model fit. The obtained kinetic parameters are summarized in the inset table. The time-dependent Sr adsorption onto the TNT array could be fit to both kinetic models, but the pseudo-first-order kinetic model displayed a higher linear regression coefficient, R2 = 0.9924, than the pseudo-second-order kinetic model (see Figure S4). These results indicated that the reaction was more inclined toward physisorption.55−57 When the initial concentration (C0) of the solute was high, the adsorption kinetics obeyed the pseudofirst-order model, whereas if C0 were not too high, the sorption kinetics fit better to a pseudo-second-order model.58 A 10 mg· L−1 initial Sr concentration appeared to be high for the electrode-type TNT structure in this study, compared to the powder-type TNT sample. The qe value was calculated to be 0.9661 mg·unit−1 and was very close to the value derived from the experimental data. Figure 8A shows the Sr adsorption isotherm of the VA6TNT-400 sample during a fixed contact time of 12 h. The Sr adsorption increased in proportion to the equilibrium Sr concentration and reached a plateau above 20 mg·L−1. Two

isotherm models were used to analyze the Sr sorption onto the 6 h sample, the Langmuir and Freundlich isotherm models. The Langmuir isotherm model describes monolayer adsorption onto the homogeneous surface of an adsorbent as follows (eq 4): qe =

qeKLCe 1 + KLCe

(4) −1

−1

where qe (mg·unit ) and Ce (mg·L ) are the equilibrium adsorption capacity and equilibrium Sr concentration, respectively, and qm (mg·unit−1) and KL are the maximum monolayer adsorption capacity and the Langmuir constant related to the free energy of adsorption, respectively. The Freundlich isotherm model is considered to be a semiempirical equation, as follows (eq 5): qe = K f Ce1/ n

(5) −1

where Kf (mg·unit ) is the Freundlich constant related to the adsorption capacity of the adsorbent, and n is the heterogeneity factor, which indicates the adsorption intensity of the adsorbent. Sr adsorption onto the TNT array could be better fit to the Langmuir model with a higher linear regression coefficient (R2 = 0.9048) than to the Freundlich model, as shown in Figures 8A and S5. The Langmuir adsorption model assumed the homogeneous adsorption of the adsorbate onto F


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ACS Sustainable Chemistry & Engineering the substrate with monolayer formation,39 whereas the Freundlich model assumes an empirical heterogeneous adsorption. Sr adsorption onto the TNT array is better described as homogeneous adsorption, corresponding to the adsorption mechanism associated with ion exchange. The maximum adsorption capacity derived from the Langmuir isotherm was 1.5 mg·unit−1 on the basis of a unit area (3 × 3 cm2) of the VA-TNT. The optimized VA-6TNT-400 sample was tested for Sr uptake in varying pHs at 2, 7, and 11, as shown in Figure 8B. Sr adsorption was hindered in an acidic condition (of low pH 2), while high pH supports the Sr ion adsorption process. This could be explained by two aspects, electrostatic repulsion at acidic pH and ion exchange by protons, leading to a decrease of Sr adsorption. This result indicated that low pH of the Sr ion mixed solution forced Na ions of titanate to leach out from intercalates of nanotubes to the solution by a protonation (H+) process. Due to the high affinity of H+ ions in an ion-exchange process over Na ions, Sr2+ ions cannot adsorb on to the aligned titanate. In a high pH (high concentration of Na ions), extra Na ions (of the solution) penetrate into the interlayer of titanate of the as-prepared VA-6TNT-400 sample, which can further participate in the ion-exchange process with Sr ions. In Figure 8B, the Sr adsorption process reaches the high uptake efficiency within half of an hour, and then uptake is decreased to a certain period of time. This indicated that Na ions of the TNTs initially exchanged with the Sr ions of the solution, and due to the instant interaction of Sr and Na ions, the ionexchange process slowed down for a certain period of time and was further enhanced according to the interaction of TNTs with excess Na ions (of solution pH 11) over the time period. These results are in good agreement with the previously reported work, where Sr adsorption efficiency is a function of pH.39 TNTs (powder) show a higher Sr adsorption efficiency when the pH varies from 2 to 10. Recyclability of the VA-TNTs. The recyclability of the VA-TNTs was evaluated by characterizing consecutive adsorption and desorption performances over 3 cycles. Figure 9a shows the time-dependent profiles of the Sr adsorption onto the optimized VA-6TNT sample over 3 cycles. In the first cycle, Sr adsorption efficiency was 99.7% observed; however, Sr adsorption efficiency decreased remarkably to 44% during the second adsorption cycle, and a further decrease was observed during the third cycle (35%). In this repeating test, the adsorbed Sr ions were desorbed using 0.1 M HCl via an ion-exchange reaction (between Sr and protons). The desorption efficiency was reciprocal to the adsorption efficiency, as shown in Figure S6a. The observation that the Sr adsorption efficiency decreased significantly over the course of the cycles could be explained in terms of the structural deformations of the TNTs due to ion-exchange between the Na ions of the titanate lattice and the corresponding ions, such as Sr or protons.39 These results suggested that presence of Na ions in the lattice of titanate was crucial for maintaining the original titanate-layered structure and for providing inherent ion exchange properties, suggesting an appropriate method for improving the repeatability of the VA-TNTs. Accordingly, we investigated the effects of the Na treatment on the adsorption efficiency during consecutive adsorption cycles. After each adsorption and desorption cycle, the VA-TNT was immersed in 0.1 M NaOH and stirred continuously for 30 min. The Na treated sample was rinsed with DI water to remove excess amount of NaOH and N2 gas stream dried before the next

Figure 9. Recyclability test applied to the VA-6TNT-400 sample for Sr uptake (a) without or (b) with Na treatment.

cycle. Figure 9b clearly shows that the Sr adsorption profiles during 3 successive cycles with Na treatment were nearly identical without a decrease in the efficiency. It is interesting to note that the Sr adsorption rate onto the Na treated sample (the second and third cycles) was about 20% faster than the adsorption rate onto the as-prepared sample (the first cycle) during the initial 2 h. The corresponding desorption performances fit perfectly to the adsorption efficiency values (as shown in Figure S6b), which further supports that Na treatment improved the recyclability of the TNT array. The structural changes of the titanate due to the sorption performance was further investigated by analyzing the crystal structures of the titanate samples prepared under different experimental conditions, using XRD. Figure 10 shows the XRD diffraction patterns of (a) the as-prepared TNT array, (b) the Sr-adsorbed sample, (c) the HCl-treated sample, and (d) the Na-treated sample. The diffraction peaks at 2θ = 9.7, 24.3, 28.1, and 48.3° agreed well with those of a typical titanate crystal phase (PDF no. 72-0148), and different TiO2 phases were observed (PDF nos. 89-5009, 89-4921, 89-4920, and 470124).26,42 The characteristic peaks at 9.7 and 28.1° could be assigned to the 100 and 003 planes of titanate, respectively.39 The intensities of the diffraction peaks at 2θ = 9.7 and 28.1° weakened in the Sr-adsorbed and protonated sample upon HCl treatment, suggesting structural deformations caused by the ion-exchange reaction between Na and Sr (or protons). Notably, the intensity of the diffraction at 28.1° significantly G


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The high-resolution XPS spectra of the VA-6TNT-400 sample used for Sr uptake then followed by HCl treatment for desorption of the Sr ions and regeneration of interlayer Na ions of the tubular structures by Na treatment is shown in Figure 11. The high-resolution XPS spectra of Ti 2p consist of doublet peaks of Ti 2p3/2 and Ti 2p1/2 spin orbit, confirming that titanium ascribed the Ti4+ oxidation state.59,60 The O 1s peak around ∼530 eV is in good agreement with all samples without any variation in its binding energies. The Na 1s peak in the as-prepared VA-6TNT-400 sample appeared at a binding energy around 1071 eV.61 However, this peak disappeared after the Sr uptake (as shown in the XPS Sr 3d spectra of Figure 11), which indicated that Na ions of asprepared TNTs are replaced with Sr ions followed by an ionexchange process. In HCl treatment, sodium contents were also less counted by elemental analysis of EDS, as shown in Figure S7. Further, when the acid-washed sample was treated with Na, the peak reappeared around 1071 eV, as shown in Figure 11. The XPS peak of Sr ion-adsorbed sample contains doublet peaks, which correspond to the Sr 3d5/2 and Sr 3d3/2.62 Figure S8 represents the FTIR spectra of as-prepared, Sradsorbed, HCl-treated, and Na-treated samples. The band at 3231−3366 cm−1 can be assign to the stretching vibration of −OH of the H2O absorbed on TNTs. The peak around 1635 cm−1 corresponds to the bending vibration of O−H.63 The bands at 1417 and 1335 cm−1 are asymmetric and symmetric stretching vibrations of carboxylate groups −COOH and the characteristic vibration of C−O−C bond, respectively.64,65 The peak at 891 cm−1 is ascribed to the stretching modes of the shortest Ti−O bonds.66,67 This peak, after ion-exchange, becomes weaker, suggesting the partial replacement of Na+ by the targeted cations. The peak at 891 cm−1 disappeared after

Figure 10. X-ray diffraction patterns obtained from (a) the VA-6TNT sample, (b) VA-6TNT sample after Sr uptake, (c) VA-6TNT sample treated with HCl, and (d) VA-6TNT sample treated with Na.

decreased in the protonated sample and increased again in the sample regenerated by Na treatment. These results suggested that the structural deformations experienced by the TNTs could be recovered after Na treatment, and Na ions present in the TiO6 octahedra interlayer played an important role in stabilizing the overall layered structure of the titanate and favoring the ion-exchange reaction. The titanate sample was regenerated using excess Na ions that reconstructed the original layered structure, confirming the repeatability of the titanate-based adsorbent. Further XPS and FTIR characterization has done to understand the elemental composition and structural behavior of these nanostructures.

Figure 11. XPS spectra for elemental analysis of the (a) as-prepared sample, (b) Sr-adsorbed sample, (c) HCl treatment, and (d) Na-treatment VA6TNT-400 samples. H


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ACS Sustainable Chemistry & Engineering HCl treatment, due to the existence of a large amount of H+ in the interlayer, and reconstructed after Na treatment. Figure 12 shows high-resolution TEM (HR-TEM) and annular dark field-scanning TEM-EDS (ADF-STEM-EDS)

titanate nanostructure. The interpores of the TNTs of the Sr adsorbed sample (Figure 12B) did not undergo distinct changes upon Sr adsorption. EDS mapping clearly indicated the presence of adsorbed Sr ions onto the titanate. TEM images of the HCl treated sample revealed that the tubular structures appeared very thin compared to the as-prepared samples, as shown in Figure 12C. The nanotube interlayer was disrupted by the removal of sodium and adsorbed Sr ions from the titanate interlayer. These results confirmed that the disrupted titanate interlayers deformed the tubular structures, which was in agreement with the previous XRD results, as shown in Figure 10. A negligible amount of sodium was found to be present. The concentration of the sodium ions is listed in Figure S7. TEM images of the Na treated sample are shown in Figure 12D. The interlayer of the nanotubes were regenerated in the Na treated sample. The sodium content clearly increased, which was shown in the EDS for this sample (as shown in Figure S7). Five consecutive cycles of Sr ion adsorption were tested to show the consistent performance of the VA-6TNT sample over the repeatable recovery of metal ions, as shown in Figure S9. The two optimized VA-6TNT samples were used for cyclic adsorption. The first sample was treated with Na (regenerated) immediately after the Sr desorption (i.e., HCl treatment), and another sample was used directly for the next adsorption cycles after desorption. The Na treatment sample showed excellent performance compared to nontreated sample for repeatable Sr adsorption after the initial adsorption. These samples were further characterized by SEM to understand the morphological changes after the successive uptakes. Most of the nanotubes were agglomerated coarsely after the 5-time uptake process. However, no morphological changes have appeared when compared with the as-prepared samples (high-resolution SEM images), as shown in Figure S10. Further, SEM-EDAX mapping and elemental composition of these samples were characterized as shown in Figure S11. According to Figure S7, for STEM-EDS elemental analysis, the as-prepared sample is a good fit with its elemental compositions, where sodium was obtained at 8.6% (weight). After the 5-cycle repeatable recovery of Sr ions, the sodium composition drastically decreased to 1 wt % in both the with and without Na-treatment samples. However, the key point of this analysis is Sr composition over both of the samples, where the Na-treatment sample shows a high Sr ion content compared to that of the sample without treatment. The Na treatment sample has the capability to adsorb more Sr ions by an ion-exchange process. To understand, the crystallinity and phase of these samples were analyzed for XRD, as shown in Figure S12. The diffraction peaks at 2θ = 9.7 and 28.1° represent titanate and interlayers of titanate diffractions, respectively, and are in good agreement with the PDF card number 72-0148 of titanate. The titanate peaks of the asprepared sample at 2θ = 9.7° were reduced for both (with/ without treated) of the samples, and the diffraction peak at 2θ = 28.1° was suppressed without the Na treated sample, while the treated sample attained the similar crystallinity of the particular diffraction peak. These results indicated that the changes in the low angle XRD diffraction peak intensity are due to the crystalline changes, and the morphology of both of the samples was not affected by the repeatable adsorption process. The improved adsorption after the excess Na treatment was verified by following the process with another heavy metal

Figure 12. HR-TEM and ADF-STEM-EDS mapping images of (A) the VA-6TNT sample, (B) Sr-adsorbed VA-6TNT sample, (C) VA6TNT sample treated with HCl, and (D) VA-6TNT sample treated with Na.

images of the as-prepared, ion-exchanged (Sr adsorbed), HCl treatment (Sr-desorbed), and Na treatment (regenerated) samples. TEM images of the as-prepared sample clearly revealed that the product featured a tubular morphology and a layered lattice structure, as shown in Figure 12A. The ADFSTEM analysis and the corresponding EDS maps show the chemical compositions of TNTs. A uniform distribution of Ti, O, and Na was observed and confirms the Na incorporated I


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ACS Sustainable Chemistry & Engineering element, cadmium (Cd). Figure 13 shows the repeated adsorption of Cd onto the VA-TNT-400 sample with or

nanotubes underwent a morphological change under prolonged HT treatment. A low HT temperature (160 °C) and short reaction time (6 h) allowed for the growth of VA-TNT structures that displayed good adhesion and homogeneous dispersion to the surfaces of the Ti metal foil. The VA-TNT was successfully used to adsorb Sr and Cd ions. The adsorption kinetics and adsorption isotherm of Sr were well fitted to a pseudo-first-order kinetic model and a Langmuir isotherm model, respectively. Sr ion adsorption was tested in varing pH conditions. The desorption of Sr was achieved by applying an acid treatment, although Sr adsorption decreased significantly during subsequent cycles. The recyclability of the VA-TNT samples could be improved by applying excess Na treatment, which was strongly supported by the observation that Sr and Cd adsorption after Na treatment conserved the maximum efficiency in consecutive adsorption−desorption cycles. The VA-TNT is potentially useful as an easy environmentally friendly approach for adsorbing and removing heavy toxic metal ions.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b02805. SEM images, time profile curve, Freundlich isotherm model, desorption of Sr ions, EDS data, FTIR, recycle test, SEM, EDAX, XRD, desorption of Cd ions, and cyclic voltammogram for surface area measurement (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

Figure 13. Recyclability test for Cd uptake (a) without and (b) with Na-treatment samples.

ORCID

Jungho Ryu: 0000-0001-5651-2230 Jum Suk Jang: 0000-0001-6874-8216

without Na treatment. Repeated adsorption cycles in the absence of the Na treatment revealed that the Cd adsorption efficiency gradually decreased to 77 and 68% in the second and third cycles, respectively (Figure 13a). On the other hand, the Cd adsorption did not change significantly over the course of repeated Na treatment cycles, as shown in Figure 13b, and the corresponding desorption efficiency profile of Cd indicated that the VA-TNT sample performance improved upon application of the Na treatment cycles (Figure S13). These results were similar to those obtained from the Sr adsorption tests and strongly supported the effectiveness of Na treatment for regenerating the VA-TNTs. These findings encouraged the development of an economical and clean recyclable adsorbent for metal ions recovery using titanate-based materials.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Basic Research Project of the Korea Institute of Geoscience and Mineral Resources (Project 18-3413), funded by the Ministry of Science and ICT, BK21 plus, and the Basic Science Research Programs (2012R1A6A3A04038530), funded by the Korean National Research Foundation (NRF) and the Korean Ministry of the Environment (MOE) as part of the Public Technology Program based on Environmental Policy (2014000160001). We thank the Jeonju Center of the Korea Basic Science Institute for technical assistance with TEM measurements.

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CONCLUSIONS In summary, two electrode anodization techniques were used to grow a highly ordered 1D crystalline anodic TiNA layer over a Ti metal foil substrate. SEM and TEM techniques were used to characterize the hierarchy of the high pore diameter anodic TiNA. An extended postcalcination (400 °C) step successfully removed the inherent crystal defects from the anodic TiNA. The anodic TiNA was used to synthesize TNT structures, which were subjected to an alkaline HT treatment. The TiO2

ABBREVIATIONS anodic TiNA, anodized TiO2 nanotube arrays; VA-TNTs, vertically aligned titanate nanotubes. REFERENCES

(1) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Formation of titanium oxide nanotube. Langmuir 1998, 14 (12), 3160−3163.

J


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hydrogen production under visible light illumination. Nanotechnology 2014, 25 (3), 035603. (20) Cai, J. M.; Zhu, Y. M.; Liu, D. S.; Meng, M.; Hu, Z. P.; Jiang, Z. Synergistic effect of titanate-anatase heterostructure and hydrogenation-induced surface disorder on photocatalytic water splitting. ACS Catal. 2015, 5 (3), 1708−1716. (21) Momeni, M. M.; Hakimian, M.; Kazempour, A. Preparation and characterisation of manganese−TiO2 nanocomposites for solar water splitting. Surf. Eng. 2016, 32 (7), 514−519. (22) Momeni, M. M.; Ghayeb, Y.; Shafiei, M. Preparation and characterization of CrFeWTiO2 photoanodes and their photoelectrochemical activities for water splitting. Dalton Trans 2017, 46 (37), 12527−12536. (23) Momeni, M. M.; Ghayeb, Y. Visible light-driven photoelectrochemical water splitting on ZnO−TiO2 heterogeneous nanotube photoanodes. J. Appl. Electrochem. 2015, 45 (6), 557−566. (24) Momeni, M. M.; Ghayeb, Y. Fabrication, characterization and photoelectrochemical performance of chromium-sensitized titania nanotubes as efficient photoanodes for solar water splitting. J. Solid State Electrochem. 2016, 20 (3), 683−689. (25) Momeni, M. M.; Ghayeb, Y.; Ezati, F. Fabrication, characterization and photoelectrochemical activity of tungsten-copper cosensitized TiO2 nanotube composite photoanodes. J. Colloid Interface Sci. 2018, 514 (-), 70−82. (26) Han, C. H.; Hong, D. W.; Kim, I. J.; Gwak, J.; Han, S. D.; Singh, K. C. Synthesis of Pd or Pt/titanate nanotube and its application to catalytic type hydrogen gas sensor. Sens. Actuators, B 2007, 128 (1), 320−325. (27) Hong, Z. S.; Zheng, X. Z.; Ding, X. K.; Jiang, L. L.; Wei, M. D.; Wei, K. M. Complex spinel titanate nanowires for a high rate lithiumion battery. Energy Environ. Sci. 2011, 4 (5), 1886−1891. (28) Lan, Y.; Gao, X. P.; Zhu, H. Y.; Zheng, Z. F.; Yan, T. Y.; Wu, F.; Ringer, S. P.; Song, D. Y. Titanate nanotubes and nanorods prepared from rutile powder. Adv. Funct. Mater. 2005, 15 (8), 1310−1318. (29) Li, J. R.; Tang, Z. L.; Zhang, Z. T. Layered hydrogen titanate nanowires with novel lithium intercalation properties. Chem. Mater. 2005, 17 (23), 5848−5855. (30) Wang, J. X.; Liu, W. X.; Yin, G. H.; Li, H. D.; Liu, H.; Zhou, W. J.; Wang, H. L.; Wang, Z. De laminated sodium titanate nanobelts in synergy with cationic polyacrylamide to induce flocculation on kaolin clay. Colloids Surf., A 2012, 414 (-), 9−16. (31) Zhu, H. Y.; Lan, Y.; Gao, X. P.; Ringer, S. P.; Zheng, Z. F.; Song, D. Y.; Zhao, J. C. Phase transition between nanostructures of titanate and titanium dioxides via simple wet-chemical reactions. J. Am. Chem. Soc. 2005, 127 (18), 6730−6736. (32) Huang, J. Q.; Huang, Z.; Guo, W.; Wang, M. L.; Cao, Y. G.; Hong, M. C. Facile synthesis of titanate nanoflowers by a hydrothermal route. Cryst. Growth Des. 2008, 8 (7), 2444−2446. (33) Stengl, V.; Bakardjieva, S.; Subrt, J.; Vecernikova, E.; Szatmary, L.; Klementova, M.; Balek, V. Sodium titanate nanorods: Preparation, microstructure characterization and photocatalytic activity. Appl. Catal., B 2006, 63 (1−2), 20−30. (34) Viana, B. C.; Ferreira, O. P.; Souza Filho, A. G.; Mendes Filho, J.; Alves, O. L. Structural, morphological and vibrational properties of titanate nanotubes and nanoribbons. J. Braz. Chem. Soc. 2009, 20 (1), 167−175. (35) Niu, H. Y.; Wang, J. M.; Shi, Y. L.; Cai, Y. Q.; Wei, F. S. Adsorption behavior of arsenic onto protonated titanate nanotubes prepared via hydrothermal method. Microporous Mesoporous Mater. 2009, 122 (1−3), 28−35. (36) Takezawa, Y.; Imai, H. Bottom-up synthesis of titanate nanosheets with hierarchical structures and a high specific surface area. Small 2006, 2 (3), 390−393. (37) Xu, C. Y.; Zhang, Q.; Zhang, H.; Zhen, L.; Tang, J.; Qin, L. C. Synthesis and characterization of single-crystalline alkali titanate nanowires. J. Am. Chem. Soc. 2005, 127 (33), 11584−11585. (38) Dhandole, L. K.; Ryu, J.; Lim, J. M.; Oh, B. T.; Park, J. H.; Kim, B. G.; Jang, J. S. Hydrothermal synthesis of titanate nanotubes from TiO2 nanorods prepared via a molten salt flux method as an effective

(2) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Sekino, T.; Niihara, K. Titania nanotubes prepared by chemical processing. Adv. Mater. 1999, 11 (15), 1307−1311. (3) Bavykin, D. V.; Walsh, F. C. Elongated Titanate Nanostructures and Their Applications. Eur. J. Inorg. Chem. 2009, 2009, 977−997. (4) Wang, B. X.; Shi, Y.; Xue, D. F. Large aspect ratio titanate nanowire prepared by monodispersed titania submicron sphere via simple wet-chemical reactions. J. Solid State Chem. 2007, 180 (3), 1028−1037. (5) Chen, Q.; Zhou, W. Z.; Du, G. H.; Peng, L. M. Trititanate nanotubes made via a single alkali treatment. Adv. Mater. 2002, 14 (7), 1208−1211. (6) Kolen’ko, Y. V.; Kovnir, K. A.; Gavrilov, A. I.; Garshev, A. V.; Frantti, J.; Lebedev, O. I.; Churagulov, B. R.; Van Tendeloo, G.; Yoshimura, M. Hydrothermal synthesis and characterization of nanorods of various titanates and titanium dioxide. J. Phys. Chem. B 2006, 110 (9), 4030−4038. (7) Torrente-Murciano, L.; Lapkin, A. A.; Bavykin, D. V.; Walsh, F. C.; Wilson, K. Highly selective Pd/titanate nanotube catalysts for the double-bond migration reaction. J. Catal. 2007, 245 (2), 272−278. (8) Ou, H. H.; Liao, C. H.; Liou, Y. H.; Hong, J. H.; Lo, S. L. Photocatalytic oxidation of aqueous ammonia over microwaveinduced titanate nanotubes. Environ. Sci. Technol. 2008, 42 (12), 4507−4512. (9) Peng, Y. P.; Lo, S. L.; Ou, F. H.; Lai, S. W. Microwave-assisted hydrothermal synthesis of N-doped titanate nanotubes for visiblelight-responsive photocatalysis. J. Hazard. Mater. 2010, 183 (1−3), 754−758. (10) Xiong, L.; Sun, W. L.; Yang, Y.; Chen, C.; Ni, J. R. Heterogeneous photocatalysis of methylene blue over titanate nanotubes: Effect of adsorption. J. Colloid Interface Sci. 2011, 356 (1), 211−216. (11) Momeni, M. M.; Ghayeb, Y.; Ghonchegi, Z. Visible light activity of sulfur-doped TiO2 nanostructure photoelectrodes prepared by single-step electrochemical anodizing process. J. Solid State Electrochem. 2015, 19 (5), 1359−1366. (12) Momeni, M. M.; Ahadzadeh, I.; Rahmati, A. Nitrogen, carbon and iron multiple-co doped titanium dioxide nanotubes as a new highperformance photo catalyst. J. Mater. Sci.: Mater. Electron. 2016, 27 (8), 8646−8653. (13) Momeni, M. M.; Ghayeb, Y. Synthesis and characterization of iron-doped titania nanohoneycomb and nanoporous semiconductors by electrochemical anodizing method as good visible light active photocatalysts. J. Mater. Sci.: Mater. Electron. 2015, 26 (7), 5509− 5517. (14) Momeni, M. M. Fabrication of copper decorated tungsten oxide−titanium oxide nanotubes by photochemical deposition technique and their photocatalytic application under visible light. Appl. Surf. Sci. 2015, 357 (-), 160−166. (15) Momeni, M. M.; Ghayeb, Y.; Ghonchegi, Z. Fabrication and characterization of copper doped TiO2 nanotube arrays by in situ electrochemical method as efficient visible-light photocatalyst. Ceram. Int. 2015, 41 (7), 8735−8741. (16) Momeni, M. M.; Ghayeb, Y.; Gheibee, S. Silver nanoparticles decorated titanium dioxide-tungsten trioxide nanotube films with enhanced visible light photo catalytic activity. Ceram. Int. 2017, 43 (1), 564−570. (17) Momeni, M. M.; Ghayeb, Y.; Davarzadeh, M. Electrochemical construction of different titania−tungsten trioxide nanotubular composite and their photocatalytic activity for pollutant degradation: a recyclable photocatalysts. J. Mater. Sci.: Mater. Electron. 2015, 26 (3), 1560−1567. (18) Zhao, W. X.; Wang, X. T.; Sang, H. X.; Wang, K. Synthesis of Bi-doped TiO2 Nanotubes and Enhanced Photocatalytic Activity for Hydrogen Evolution from Glycerol Solution. Chin. J. Chem. 2013, 31 (3), 415−420. (19) Long, L. Z.; Yu, X.; Wu, L. P.; Li, J.; Li, X. J. Nano-CdS confined within titanate nanotubes for efficient photocatalytic K


Research Article

ACS Sustainable Chemistry & Engineering adsorbent for strontium ion recovery. RSC Adv. 2016, 6 (100), 98449−98456. (39) Ryu, J.; Kim, S.; Hong, H. J.; Hong, J.; Kim, M.; Ryu, T.; Park, I. S.; Chung, K. S.; Jang, J. S.; Kim, B. G. Strontium ion (Sr2+) separation from seawater by hydrothermally structured titanate nanotubes: Removal vs. recovery. Chem. Eng. J. 2016, 304 (-), 503−510. (40) Liu, S. S.; Lee, C. K.; Chen, H. C.; Wang, C. C.; Juang, L. C. Application of titanate nanotubes for Cu (II) ions adsorptive removal from aqueous solution. Chem. Eng. J. 2009, 147 (2−3), 188−193. (41) Wang, T.; Liu, W.; Xu, N.; Ni, J. Adsorption and desorption of Cd (II) onto titanate nanotubes and efficient regeneration of tubular structures. J. Hazard. Mater. 2013, 250−251 (-), 379−386. (42) Yang, D.; Zheng, Z.; Liu, H.; Zhu, H.; Ke, X.; Xu, Y.; Wu, D.; Sun, Y. Layered titanate nanofibers as efficient adsorbents for removal of toxic radioactive and heavy metal ions from water. J. Phys. Chem. C 2008, 112 (42), 16275−16280. (43) Angino, E. E.; Billings, G. K.; Andersen, N. Observed variations in the strontium concentration of sea water. Chem. Geol. 1966, 1 (C), 145−153. (44) Bardi, U. Extracting minerals from seawater: an energy analysis. Sustainability 2010, 2 (4), 980−992. (45) Li, N.; Zhang, L.; Chen, Y.; Fang, M.; Zhang, J.; Wang, H. Highly efficient, irreversible and selective ion exchange property of layered titanate nanostructures. Adv. Funct. Mater. 2012, 22 (4), 835− 841. (46) Yang, D.; Liu, H.; Zheng, Z.; Sarina, S.; Zhu, H. Titanate-based adsorbents for radioactive ions entrapment from water. Nanoscale 2013, 5 (6), 2232−2242. (47) Merceille, A.; Weinzaepfel, E.; Barre, Y.; Grandjean, A. The sorption behaviour of synthetic sodium nonatitanate and zeolite A for removing radioactive strontium from aqueous wastes. Sep. Purif. Technol. 2012, 96 (-), 81−88. (48) Moon, J.; Kim, K.; Jung, C. H.; Shul, Y. G.; Lee, E. H. Preparation of organic-inorganic composite adsorbent beads for removal of radionuclides and heavy metal ions. J. Radioanal. Nucl. Chem. 2000, 246 (2), 299−307. (49) Sun, X. M.; Li, Y. D. Synthesis and characterization of ionexchangeable titanate nanotubes. Chem. - Eur. J. 2003, 9 (10), 2229− 2238. (50) Yang, D. J.; Zheng, Z. F.; Zhu, H. Y.; Liu, H. W.; Gao, X. P. Titanate nanofibers as intelligent absorbents for the removal of radioactive ions from water. Adv. Mater. 2008, 20 (14), 2777−2781. (51) Chu, M.; Tang, Y.; Rong, N.; Cui, X.; Liu, F.; Li, Y.; Zhang, C.; Xiao, P.; Zhang, Y. Hydrothermal synthesis, and tailoring the growth of Ti-supported TiO2 nanotubes with thick tube walls. Mater. Des. 2016, 97 (-), 257−267. (52) Wang, C.; Zhang, X.; Zhang, Y.; Jia, Y.; Yang, J.; Sun, P.; Liu, Y. Hydrothermal growth of layered titanate nanosheet arrays on titanium foil and their topotactic transformation to heterostructured TiO2 photocatalysts. J. Phys. Chem. C 2011, 115 (45), 22276−22285. (53) Guo, Y.; Lee, N. H.; Oh, H. J.; Yoon, C. R.; Park, K. S.; Lee, W. H.; Li, Y.; Lee, H. G.; Lee, K. S.; Kim, S. J. Preparation of titanate nanotube thin film using hydrothermal method. Thin Solid Films 2008, 516 (1), 8363−8371. (54) Allam, N. K.; Shankar, K.; Grimes, C. A. Photoelectrochemical and water photoelectrolysis properties of ordered TiO2 nanotubes fabricated by Ti anodization in fluoride-free HCl electrolytes. J. Mater. Chem. 2008, 18 (20), 2341−2348. (55) Fierro, V.; Torne-Fernandez, V.; Montane, D.; Celzard, A. Adsorption of phenol onto activated carbons having different textural and surface properties. Microporous Mesoporous Mater. 2008, 111 (1− 3), 276−284. (56) Vardhan, C. M. V.; Srimurali, M. Preparation of lanthanum impregnated pumice for defluoridation of water: batch and column experiments. J. Environ. Chem. Eng. 2018, 6 (1), 858−865. (57) Simonin, J. P. On the comparison of pseudo-first order and pseudo-second order rate laws in the modeling of adsorption kinetics. Chem. Eng. J. 2016, 300 (-), 254−263.

(58) Azizian, S. Kinetic models of sorption: a theoretical analysis. J. Colloid Interface Sci. 2004, 276 (1), 47−52. (59) Ohno, T.; Mitsui, T.; Matsumura, M. Photocatalytic activity of S-doped TiO2 photocatalyst under visible light. Chem. Lett. 2003, 32 (4), 364−365. (60) Li, D.; Haneda, H.; Labhsetwar, N.; Hishita, S.; Ohashi, N. Visible-light-driven photocatalysis on fluorine-doped TiO2 powders by the creation of surface oxygen vacancies. Chem. Phys. Lett. 2005, 401 (4−6), 579−584. (61) Dhandole, L. K.; Mahadik, M. A.; Kim, S.; Chung, H. S.; Seo, Y.; Cho, M.; Ryu, J.; Jang, J. S. Boosting photocatalytic performance of inactive rutile TiO2 nanorods under solar light irradiation: synergistic effect of acid treatment and metal oxide co-catalysts. ACS Appl. Mater. Interfaces 2017, 9 (28), 23602−23613. (62) Tanigawa, S.; Takashima, T.; Irie, H. Enhanced visible-lightsensitive two-step overall water-splitting based on band structure controls of titanium dioxide and strontium titanate. J. Mater. Sci. Chem. Eng. 2017, 05 (01), 129−141. (63) Tian, B.; Qian, Yu.; Hu, B.; Sun, J.; Du, Z. Synthesis and characterization of nitrogen-doped titanium dioxide nanomaterials derived from nanotube sodium titanate precursor. J. Mater. Res. 2012, 27 (18), 2408−2416. (64) Inam, M.; Khan, R.; Park, D.; Lee, Y. W.; Yeom, I. T. Removal of Sb(III) and Sb(V) by ferric chloride coagulation: Implications of Fe solubility. Water 2018, 10 (4), 418. (65) Van Viet, P.; Trung, N. C.; Nhut, P. M.; Van Hieu, L.; Thi, C. M. The fabrication of the antibacterial past based on TiO2 nanotubes and Ag nanoparticles-loaded TiO2 nanotubes powders. J. Exp. Nanosci. 2017, 12 (1), 220−231. (66) Moreira, A. P. D.; Sader, M. S.; Soares, G. D.; Leao, M. H. M. Strontium incorporation on microspheres of alginate/β-tricalcium phosphate as delivery matrices. Mater. Res. 2014, 17 (4), 967−973. (67) Neto, J. M.; Bellato, C. R.; de Souza, C. H.; da Silva, R. C.; Rocha, P. A. Synthesis, characterization and enhanced photocatalytic activity of iron oxide/carbon nanotube/Ag-doped TiO2 nanocomposites. J. Braz. Chem. Soc. 2017, 28 (12), 2301−2312.

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Vertically Aligned Titanate Nanotubes Hydrothermally Synthesized

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