Hexagonal K2W4O13 Nanowires for the Adsorption of Methylene Blue

Article Cite This: ACS Appl. Nano Mater. 2019, 2, 3802−3812

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Hexagonal K2W4O13 Nanowires for the Adsorption of Methylene Blue Qi-Su Huang,† Wei Wei,† Jing Sun,†,‡ Shun Mao,†,‡ and Bing-Jie Ni*,†,‡ †

State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai 200092, PR China ‡ Shanghai Institute of Pollution Control and Ecological Security, Shanghai 200092, P.R. China

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ABSTRACT: In this study, novel hexagonal K2W4O13 (hK2W4O13) nanowires were strategically synthesized via a facial hydrothermal method, which exhibited excellent adsorption capacities for wastewater treatment. The inorganic agent K2SO4 was used as a structure-directing agent to scaffold the tunnel structure of h-K2W4O13 and form the one-dimensional structure. Through increasing the relative molar ratio of K 2 SO 4 to Na 2 WO 4 precursor, the pure-phase h-WO 3 nanorods and h-K2W4O13 nanowires were obtained, attributing to the competitive electrostatic adsorption between K+ ions and Na+ ions on h-WO3 nuclei. With a smaller hydrated radius in the solution (dK+ = 3.31 Å, dNa+= 3.58 Å), K+ exhibited superior affinity compared to Na+ with the negatively charged h-WO3 nuclei because of a larger charge density, resulting in the formation of h-K2W4O13. Adsorption experimental results showed that 89.4% of methylene blue was removed by h-K2W4O13 in the first 5 min (99% in 1 h) and the maximum uptake capacity reached 204.08 mg g−1. In addition, the novel h-K2W4O13 exhibited acid or alkali resistance and good reusability, revealed by the stable adsorption capacity in a wide pH range of 3.0−11.0 and five-run recycle tests. The large specific area, high proportion of effective pore volume, and abundant hydroxyl groups of the synthesized h-K2W4O13 resulted in excellent adsorption performance for methylene blue. KEYWORDS: self-assembled, hexagonal K2W4O13, competitive electrostatic adsorption, wastewater treatment, structure directed

1. INTRODUCTION Self-assembly is well-known as an intelligent strategy for nanostructure preparation. Driven by the forces such as van der Waals, π-stacking, electrostatic attraction, and hydrophobic interaction, it occurs spontaneously to achieve a thermodynamic minima and form stable, robust, and well-defined aggregates.1−3 A series of nanomaterials with precisely designed structure that are directed by self-assembly have been reported, such as urchin-like WO3 nanoparticles, αFeOOH hollow spheres, flowerlike iron oxide, and MnO2 hollow nanoparticles.4−7 As a typical transition metal element, tungsten-based nanomaterials are widely used as photocatalysts,8,9 sensors,10 chromic devices,11 and solar filters12 because of their specific band structure, vast surface, and rich valence state. The tungsten-grouped crystals are basically built up by corner and edge sharing of [WO6] octahedra. Because of slight displacement of tungsten and oxygen atoms in the crystal cells, the [WO6] octahedra twists and rotates to different angles, forming various crystal phases.13 Among them, hexagonal phase structure is expected for its natural tunnel structure, which can function as carriers for guest ions and initiate enhanced optical and electric performance.14 However, the hexagonal © 2019 American Chemical Society

structure is verified to be thermodynamically metastable and often coexists with other crystal phases.13 Therefore, additional assistance is required to obtain pure-phase hexagonal products. Dosing structure-directing agent in the hydrothermal reaction system was found to be an efficient self-assembly method to fabricate pure hexagonal phase materials.15 The formed hexagonal tunnels are commonly negatively charged because of the surface oxygen-containing groups (e.g., WO, O−H).16 Thereby, Na+ has been widely used as structuredirecting agent to support and stabilize the tunnel structure through the electrostatic attraction between them.17 For example, Li et al.18 successfully synthesized h-WO3 nanotube-based bundles determined by NaHSO4. Na+ ions were utilized as stabilizer for the formation of h-WO3. The electrostatic repulsion between Na+ ions and H+ ions promoted the curl of WO3 slices, and SO42− ions bridged WO3 nanotubes together to form well-organized bundles. It was reported that K+ was a kind of alkali metal with smaller hydrated radius than Na+ in actual water.19 We hence Received: April 11, 2019 Accepted: May 17, 2019 Published: May 22, 2019 3802

DOI: 10.1021/acsanm.9b00674 ACS Appl. Nano Mater. 2019, 2, 3802−3812

Article

ACS Applied Nano Materials Scheme 1. One-Step Synthetic Route for Pure h-K2W4O13 Nanowire Preparation

hypothesized that K+ ions would be more effective in scaffolding tunnel structure because of their larger charge density than Na+ ions. In this work, K2SO4 was hence used as the structuredirecting agent to synthesize pure hexagonal potassium tungsten oxide (h-K2W4O13). In previous studies, K2SO4 was also used to assist the growth of h-K2W4O13 in hydrothermal reaction with the presence of tungsten oxide precursor (e.g., WO3·0.33H2O, WO3. H2O,).20,21 However, most of these studies prepared the precursor and the desired h-K2W4O13 product separately in sequence, which required complex operations and caused a waste of time and energy. In addition, the significance of sulfate ions from K2SO4 were regularly overemphasized in determining the growth of h-K2W4O13 through selectively adsorbing on specific crystal planes,12 while the roles of K+ ions were barely appreciated or investigated. To remedy the limits of these studies, a simplified, one-step method that directly mingled K2SO4 and Na2WO4 precursor before hydrothermal reaction was used. The roles of K+ ions in the preparation of h-K2W4O13 were thoroughly investigated by comparing the structure and morphology changes of the samples when the molar ratios of K2SO4 to Na2WO4 increased. Pure phase h-K2W4O13 nanowires were successfully obtained when the molar ratio was 2:1. The mechanism revealed that the formation of h-K2W4O13 was attributed to the superior affinity of K+ ions for h-WO3 nuclei compared to Na+ ions during the synthesis process (Scheme 1). Similar to other tungsten-based materials, h-K2W4O13 has been verified with satisfying optical and electric performance.21−23 Nevertheless, to the best of our knowledge, the possible application of h-K2W4O13 in environmental decontamination has never been documented. In this work, the potential of h-K2W4O13 as an efficient adsorbent was then explored systematically using cationic dye methylene blue as the representative pollutant. The relevant adsorption mechanism was also proposed.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Sodium tungstate dihydrate (Na2WO4·2H2O) and methylene blue (MB) were purchased from Aladdin. Hydrochloric acid (HCl, 37%), sodium hydroxide (NaOH), and potassium sulfate (K2SO4) were purchased from the Sinopharm Chemical Reagent Co. Ltd. All the reagents were analytical grade that were used without further purification. 2.2. Strategic Synthesis of h-K2W4O13 Nanowires. In a typical experiment, 1.98 g of Na2WO4.2H2O and a certain amount of K2SO4 were dissolved in 35 mL of deionized water. Then, 3 mol·L−1 HCl was added dropwise to maintain a pH value of 1.1−1.2. The resulting solution was stirred for 30 min before being transferred to a Teflonlined stainless-steel autoclave with a capacity of 50 mL. The autoclave was sealed and hydrothermally treated at 180 °C for 4 h. After the autoclave cooled to room temperature naturally, the obtained precipitates were centrifuged (12 000g, Bioridge TG16-WS), washed with deionized water, and finally dried at 60 °C under vacuum. The products were strategically prepared under the conditions of increasing amounts of K2SO4, with relative molar ratio of K2SO4 to Na2WO4 increased from 0:1 to 2:1, and the corresponding products were labeled as TBM-1 (0:1), TBM-2 (0.25:1), TBM-3 (0.5:1), TBM-4 (1:1), and TBM-5 (2:1). 2.3. Characterizations. X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance diffractometer with Cu Kα radiation (λ = 1.5406 Å) at 40 kV, 40 mA with the scanning rate of 10 °C min−1. Morphology of the specimens were observed using a scanning electron microscope (SEM, Nova Nano SEM 450) at 1 kV and field-emission scanning electron microscope (FESEM, Hitachi 4800) at 5 kV. Structural analysis was conducted by a transmission electron microscopy (TEM and HRTEM, TF20, Joel 2100F, 200 kV). X-ray energy-dispersive spectroscopy (EDS) was performed on Hitachi 4800 at 20 kV. The N2 adsorption−desorption isotherm analysis was performed using a Quantachrome ASiQwin at 77K, and the specific surface area was calculated using the Brunauer−Emmett− Teller (BET) method. X-ray photoelectron spectroscopy (XPS) measurements were determined on an Escalab 250Xi and Fourier transform infrared (FTIR) spectroscopic analysis was conducted using a Nicolet 5700. 2.4. Methylene Blue Adsorption Experiments. Methylene blue (MB) adsorption tests using the synthesized products (i.e., TBM-1, TBM-2, TBM-3, TBM-4 and TBM-5) were performed in a series of batch experiments. Briefly, 50 mg of the prepared products was well-dispersed into 50 mL of MB solution (100 mg L−1 at pH of 7) by constantly stirring in the dark. The adsorption process was 3803


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ACS Applied Nano Materials

Figure 1. (a) XRD patterns and (b) FT-IR spectra of the as prepared products.

WO3·0.33H2O (JCPDS No. 35−0270). The (200) and (100) diffraction peaks of h-WO3 on these two patterns are both sharp and intense, wheras the (111) peak of o-WO3·0.33H2O is relatively weak, indicating both TBM-1 and TBM-2 were composed of h-WO3 and a small amount of o-WO3·0.33H2O. For TBM-3, the (111) peak disappears and no impurities were identified, confirming pure phase h-WO3 was successfully produced. With more K2SO4 added (1:1), a broad and amorphous like peak in the range of ∼25−32° and a very sharp peak at 23.0° coexist in the XRD pattern for TBM-4, which was defined as a transition state between crystalline and amorphous phase of K2W4O13.25 As to TBM-5, only two obvious peaks at 23.0° and 47.0° appeared, highly matching (001) and (002) peaks stemming from hexagonal K2W4O13 (JCPDS No. 20−0942). The hexagonal WO3 is metastable phase and Na+ ions as the stabilizer can promote the formation of h-WO3 by inserting into the bulk of h-WO3 nuclei.15 In this work, Na+ ions from the precursor predominated in adjusting the crystal phase with low content (0.5:1) of K2SO4 incorporated. The added K+ ions would collaborate with Na+ ions to remove the thermodynamically stable o-WO3·0.33H2O impurities. Pure phase hexagonal WO3 would not be obtained until enough cations were supplied. In this case, 0.3 mol L−1 of Na+ and 0.15 mol L−1 of K+ in the reacting solution were adequate for the complete phase transformation and to obtain pure h-WO3 (TBM-3). It is well-known that the crystallographic radius of K+ (1.38 Å) is larger than that of Na+ (1.01 Å).26 Most ions are hydrated by water molecules through solvation and surrounded by multilayer hydration shell in an aqueous environment.27 The hydrated ion radius of K+ (3.31 Å) has been verified as being smaller than that of Na+ (3.58 Å) because of the weaker interaction with water molecules,19 resulting in a larger charge density. The positive charged K+ hydrated ions can adhere to the negative charged hexagonal tunnel through electrostatic attraction,17 and then intercalate into the crystal lattice of h-WO3, initiating the formation of hexagonal K2W4O13. In this work, hexagonal K2W4O13 would not be synthesized until the molar ratio of K2SO4 to Na2WO4 increased to 1:1, where the contents of K+ and Na+ were equal and both cations determine the product structure simultaneously. Further dosing more K2SO4, K+ ions would predominantly occupy the limited binding sites of the tunnel structure, explaining the absence of h-WO3 and the formation of pure hexagonal K2W4O13 for TBM-5.

recorded by a microplate reader (Thermo Scientific Varioskan LUX) at given time intervals and the absorbance value of the resulting solution at 664 nm was regarded as the indicator of MB concentration in solution. To evaluate the uptake capacity of the adsorbents for MB, we mixed 10 mg of the adsorbents with 50 mL of MB solution of different initial concentrations varying from 20 to 200 mg L−1. After adsorbing for 12 h at room temperature, MB concentration in the solution was measured. To estimate the effect of solution pH on the removal of MB by the products, similar processes were conducted except that pH of MB solution was adjusted to a specific value (i.e., 3, 5, 7, 9, and 11) by adding 1 mol L−1 HCl and 1 mol L−1 NaOH solution at the beginning of each experiment. The removal percentage of MB and uptake capacity of adsorbents in the adsorption tests were calculated as the following equations:24

remove percentage (%) = (C0 − Ct )/ C0100

(1)

uptake capacity qe = (C0 − Ce)V /w

(2)

where C0 and Ce are the initial and equilibrium concentrations of MB (mg L−1), Ct is the concentration of MB (mg·L−1) at time t, V is the working volume of MB solution (L), and w was the mass of absorbent (g). 2.5. Desorption and Recycle Test. A five-run recycle test was conducted to evaluate the stability of h-K2W4O13 nanowires using HCl solution as the regenerant. Typically, the MB-loaded h-K2W4O13 nanowires were rinsed in the 1 mol L−1 HCl solution for 8 h at room temperature. After centrifugal separation and being washed and dried, the regenerated h-K2W4O13 was used for MB removal again, as detailed in previous section. The removal efficiency was calculated and compared among different run tests.

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology. The initial molar ratios of K2SO4 to Na2WO4 were adjusted to 0:1, 0.25:1, 0.5:1, 1:1, and 2:1 by increasing the content of K2SO4 with a fixed amount of Na2WO4. White precipitates were produced instantly as HCl added. After hydrothermal reaction, the offwhite TBM-1 (0:1), TBM-2 (0.25:1), and TBM-3 (0.5:1) and the pale-yellow TBM-4 (1:1) and TBM-5 (2:1) were obtained, respectively. The structure and morphology of all the five prepared products were then comprehensively characterized. Figure 1a illustrates the XRD patterns of the synthesized products. It shows that crystal phases of the products were closely relevant to the content of K2SO4. With no or a low level (0.25:1) of K2SO4, the XRD patterns have a number of peaks well indexed to the hexagonal WO3 (JCPDS No. 75−2187) and a characterized peak at 18.1° ascribed to the orthorhombic 3804


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Figure 2. (a, b) SEM images of TBM-1 and the corresponding enlarged picture of the rectangular region; (c−f) SEM images of TBM-2, TBM-3, TBM-4, and TBM-5, respectively.

Figure 3. (a) TEM, (b, c) HRTEM, (inset of c) SAED and (d) EDS images of K2W4O13 nanowires (TBM-5).

by the XRD pattern of TBM-3, possibly due to a quite low content. Both TBM-4 and TBM-5 have strong peaks of ν(H− OH) from weakly adsorbed water at 3423 cm−1, suggesting that they are covered by much surface water. For TBM-1, TBM-2, and TBM-3, peaks from δ(H−OH) bending vibrations split into two peaks at 1622 and 1601 cm−1, corresponding to the weakly adsorbed water and the strongly coordinated water, respectively.19 Only one single peak at 1615 cm−1 appears for TBM-4 and TBM-5, which stems from the surface water but is a little blue-shifted from the other products, possibly induced by the formation of K+···O−H from h-K2W4O13.16 It should be pointed out that, similar to ν(H− OH) vibrations, the peaks of δ(H−OH) become stronger with more K2SO4 added. N2 adsorption−desorption measurements were also conducted to disclose the surface nature of the

Sulfate ions are known for selectively absorbing on the facets with high surface energy, retarding the growth of these facets and promoting the growth along the c-axis.12 Sulfate ions here induce the preferential growth of h-K2W4O13 along (001) and (002) facets. As such, the high content of K2SO4 manipulates the crystal phase of the as-prepared products. The FT-IR spectra shown in Figure 1b also reveal the structural peculiarities of the products. Absorption peaks located in 3750−3000 cm−1 contain infrared bands from various hydroxyl groups, including surface W−OH modes in the region of 3750−3480 cm−1 and H−OH modes in the region of 3460−3000 cm−1.28,29 In this study, peaks of TBM-1, TBM-2 and TBM-3 at 3602 and 3535 cm−1 are corresponding to the ν(W−OH) stretching vibrations originated from WO3· 0.33H2O, though o-WO3·0.33H2O was failed to be captured 3805


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Figure 4. (a) Full range XPS spectrum and (b−d) high-resolution XPS spectrum of O1s, K2p, W4f for the as-prepared K2W4O13 nanowires (TBM5).

synthesized products. The BET surface and the total pore volume increased from 31.67 m2 g−1 and 0.063 cm3 g−1 for TBM-1 to 132.42 m2 g−1 and 0.414 cm3 g−1 for TBM-5, respectively, with the increase in K2SO4. A vast surface provides more adsorbed sites for water molecular, revealed by the strong absorption peaks of δ(H−OH) and ν(H−OH) in the FT-IR spectra for TBM-4 and TBM-5. Strong absorption peaks below 1000 cm−1 are attributed to the W−O vibrations.30 In Figure 1b, peaks below 1000 cm−1 for TBM-1, TBM-2, and TBM-3 are relatively flat, whereas the peaks split up into four different peaks at 823, 739, 661, and 592 cm−1 for TBM-4 and TBM-5, indicating the W−O bonds in different chemical environments,31 likely due to the formation of K+···OW from h-K2W4O13.16 The FT-IR results strongly confirm the observations from the XRD patterns. The scanning electron microscopy (SEM) images were then used to characterize the morphology of the synthesized products. Figure 2a displays that TBM-1 composes of flowerlike structure with the diameters of 4−10 μm, assembled by numerous irregular nanoplatelets (about 90 nm thick). Electrostatic attraction between negative-charged h-WO3 and Na+ ions likely initiates the self-assembly of nanoplatelets into microflower structure.17 A spot of nanorods, which are considered to be o-WO3·0.33H2O, vertically distributed on the surface of the nanoplatelets (Figure 2b), consistent with the interspatial structure between h-WO3 and o-WO3·0.33H2O depicted in the previous study.32 When K2SO4 was added, interestingly, the flowerlike structure completely collapsed, and a mixture of stacked nanoparticles and bulks was obtained for TBM-2 (Figure 2c). When the K2SO4 amount is further increased, a onedimensional (1D) structure is formed. Longer and thinner 1D structures would be obtained with more K 2 SO 4

incorporated. The nanorods with aspect ratios of 4.2−6.3 were obtained in TBM-3, although some nanoparticles could still be observed (Figure 2d). For TBM-4, nanoparticles disappeared and the aspect ratio of nanorods increased to 6.9− 9.7 (Figure 2e). Nanowires with diameters of about 5 nm and length up to 400 nm composed TBM-5 (Figure 2f). The selective adsorption property of sulfate ions made it as capping agents to promote the growth along the c-axis, which usually turns out to build up the 1D nanomaterials.33 The findings of this work convincingly support this hypothesis. To gain more insight into the novel h-K2W4O13 (TBM-5) with large BET surface and ideal nanowire structure, TEM, HRTEM, EDS, and XPS analysis were further performed on product TBM-5. Figure 3a shows a typical TEM image of the synthesized K2W4O13 nanowires, in which a batch of nanowires with diameters of 3−7 nm piled up disorderly. The HRTEM images (Figure 3b, c) indicate the fringe spacing of the nanowire is 0.385 nm, concurred well with (001) crystal facet of K2W4O13 (JCPDS No. 20-0942). Thus, it can be concluded that h-K2W4O13 nanowires grow preferentially along the [001] direction. The characteristic single-crystal spots indexed to the (001) and (002) planes can also be observed in the corresponding selected area electron diffraction (SAED) pattern in the inset of Figure 3c. The EDS image ascertains that there are only three types of elements in the nanowires: K, W, and O (Figure 3d). The K/W ratio and elemental oxidation state of the novel K2W4O13 nanowires (TBM-5) were also determined by XPS. The full-range XPS spectrum (Figure 4a) verifies the presence of W, K, and O, calibrated by C1s (BE = 284.6 eV). The calculated molar ratio of K/W was 0.32, smaller than the stoichiometric value of h-K2W4O13 as 0.5, possibly because K+ ions prefer to drop inside the tunnel of h-K2W4O13, whereas the detected signals of XPS are mainly from the surface.34 3806


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Figure 5. (a) A typical hexagonal structure with tungsten−oxygen framework that made up of layers containing corner-sharing [WO6] octahedral, and (b) Illustration of the proposed competetive adsorption between Na+ and K+ on hexagonal nuclei.

Figure 4b displays the high-resolution O1s peaks of h-K2W4O13. A dominant peak at 531.8 eV and shoulder peak at 530.6 eV stem from the crystal lattice oxygen (WO, W−O−W) and surface adsorbed water (H−OH), respectively.16 Figure 4c and Figure 4d present the elemental state of K2p and W4f. Peaks centered at 35.8 and 37.9 eV are assigned to W4f 7/2 and W4f 5/2 for W6+ and peaks centered at 293.1 and 295.8 eV are attributed to K2p 3/2 and K2p 1/2 for K+, respectively.34 3.2. Formation Mechanism of h-K2W4O13 Nanowires. In this work, a facial route was designed to synthesize the hK2W4O13 nanowires for wastewater treatment. With the increase in K2SO4, which acted as structure-directing agent, pure phase h-WO3 and h-K2W4O13 were obtained in turn. Figure 5a illustrates a basal hexagonal tunnel structure of hWO3, which is made up of layer tungsten−oxygen framework containing corner-sharing [WO6] octahedra. Hexagonal and trigonal tunnels are formed by sharing the equatorial and axial corner oxygen of the octahedra.35 On the basis of the characterization results, a potential mechanism was proposed in this work to interpretate the formation of one-dimensional h-WO 3 and h-K 2 W 4 O 13 regarding the competitive adsorption between Na+ and K+

for h-WO3 nuclei. Fristly, H2WO4 precipitate was formed via eq 3 once hydrochloric acid was mixed with Na2WO4 precursor through ion excharge at room temperature. Under high temperature and pressure conditions, the supersaturated H2WO4 was then forced to hydrolyze into metastable h-WO3 nuclei via eq 4.36 A small part of H2WO4 dehydrated and generated WO3·0.33H2O (eq 5) during the heating procedure.16 Na 2WO4 + 2HCl → H 2WO4 (white) + 2NaCl

(3)

H 2WO4 → WO3 (off‐white) + H 2O

(4)

H 2WO4 → WO3 ·0.33H 2O(white) + H 2O

(5)

Without K2SO4 incorporated, h-WO3 nuclei were mostly surrounded by Na+ ions from the precusor. Driven by electrostatic attraction, Na+ ions were first adsorbed on the surface of negative h-WO3 nuclei and then penetrated into the bulk to scaffold the tunnel structure of h-WO3. Orthorhombicphase tungsten oxide hydrate was observed to coexist with hWO3, which could be attributed to the epitaxial growth of stable phase on the surface of metastable hexagonal phase for 3807


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ACS Applied Nano Materials Table 1. EDS Analysis of the Samples with Different Content of K2SO4 Added sample

W/atom (%)

Na/atom(%)

K/atom (%)

Na/W

K/W

(Na+K)/W

TBM-1 TBM-3 TBM-5

20 22 20

3 2 1

not detected 3 4

0.15 0.09 0.05

0 0.14 0.20

0.15 0.23 0.25

Figure 6. (a) Adsorption spectra and (b) color change captured by camera of MB solution in the presence of the synthesized samples. (c) Fitting adsorption isotherms with Langmuir on the as-prepared samples at room temperature and (d) fitting adsorption kinetic curve with pseudo-secondorder model. (e) Effect of pH on the removal rate of MB on the as-prepared samples, and (f) relationship between the zeta potential and pH value of the synthesized products.

the matching lattice fringes.37 The Na+ ions anchored on the surface also function as bridges to link the neighboring h-WO3 nuclei and promote nuclei aggregation.17 Growing along the preferential directions, nanoplatelets were primarily obtained and then self-assembled around Na + ions to form a microflower structure to further minimize surface energy.36 With K2SO4 added, the viscosity of solutions increased, retarding the epitaxial growth of nuclei. Microscared materials collapsed and the nanoscared materials, such as nanoparticles, nanorods, and nanowires formed instead. In addition, more sulfate ions would promote the growth along [001] direction

and render the obtained building blocks featuring large aspect ratio, concurs well with the SEM images. With a larger charge density, K+ ions are considered more affinitive with h-WO3 nuclei. When content of K+ ions is low (e.g., 0.5:1), active adsorbed sites of h-WO3 nuclei are sufficient and not totally occupied. Both Na+ ions and K+ ions are permitted to locate on the surface of h-WO3 nuclei, functioned as bridges or scaffolds to determine the structure or morphology simultaneously. However, continuely increasing the K2SO4 amount (e.g., 1:1), K+ ions compete with Na+ ions for the limited active adsorbed sites. K+ ions with the stronger affinity prefer to adhere to h-WO3 nuclei compared to Na+ 3808


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ACS Applied Nano Materials ions. As illustrated in Figure 5b, ionized K+ would replace the adjacent bonded Na+ ions and combine with the residual oxygen atom of the tunnel of h-WO3 when active sites were limited, eventually generating a novel material h-K2W4O13 (TBM-5). To verify the formation mechanism of h-K2W4O13 nanowires proposed, we performed EDS measurements. Table 1 displays the atomic percentage and the calculated atomic ratio of TBM1, TBM-3, TBM-5. The value of Na/W and K/W are considered as the indicators about the donation of Na+ ions and K+ ions to stabilize the products, respectively. The value of (Na+K)/W indicates the total donation of cations. For TBM1, Na+ is the only cation directing the structure. With K2SO4 incorporated, the Na/W value decreases steadily, accompanied by a consistent increase of the K/W value. For TBM-5, the Na/W value is extremely low and neglectable, whereas K+ ions are predominant in structure directing. Notably, with a certain amount of K2SO4 incorporated (i.e., 0.5:1), the (Na+K)/W value increases from 0.15 to 0.23 rapidly. Nevertheless, it is nearly unchanged when dosing K2SO4 increased to 2:1, implying the saturation of active adsorbed sites. These EDS results clearly supported our proposed formation mechanism for h-K2W4O13 nanowires. 3.3. Adsorption Performance for Wastewater Treatment. To assess the potential of h-K2W4O13 nanowires applied in wastewater treatment, we selected the common cationic dye methylene blue (MB) as the model organic dye pollutant. By monitoring the characteristic absorption behavior for MB, adsorption capacities of the as-prepared products and possible adsorption mechanism were investigated. Figure 6a, b presents the MB removal efficiency and the digital photographs of MB wastewater at different contact time, respectively. All the synthesized products adsorb MB much faster in the first 5 min, and 60 min is adequate to reach the adsorption equilibrium. However, the uptake properties among the products differ significantly. The TBM-1 and TBM-2 only remove a small part (less than 20%) of the pollutant. For pure h-K2W4O13 (TBM-5), 89.4% of MB are removed dramatically in the first 5 min respectively, and more than 99% of MB is removed after adsorption for 1 h. To better understand the adsorption kinetic of the asprepared materials for MB, we applied the pseudo-first-order and pseudo-second-order model to fit the experimental data.16 The fitting results are exhibited in Figure 6c and Figure S1a, and the kinetic parameters are listed in Table S1. The results show that the predicted equilibrium uptake capacity (qe) from the pseudo-second-order agrees well with the experimental qe. The values of correlation coefficient (i.e., R2) in all cases are higher than 0.999, indicating the pseudo-second-order model can describe the adsorption kinetic process accurately. The pseudo-first-order model is not suitable to explain the adsorption kinetic behavior of MB, suggested by the relatively low value of R2 (R2 < 0.86). Adsorption isotherms (i.e., Langmuir and Freundlich adsorption model) are employed to evaluate the adsorption capacities of the as-prepared products.38 Figure 6d illustrates the linear-fitting results (R2 > 0.90) from Langmuir adsorption model and the parameters calculated from the experimental data are listed in Table S2. The estimated maximum adsorption capacities of the as-prepared products for MB increase with more K2SO4 incorporated. Excitingly, the maximum adsorption capacities (qm) of pure h-K2W4O13 nanowires (TBM-5) reaches to 204.08 mg g−1, much higher

than that of other relevant reported adsorbents (Table S3). The fitting-results and related equation parameters of the Freundlich adsorption model are showed in Figure S1b and Table S2. Apparently, the Freundlich isotherms failed to capture the experimental data with all R2 value smaller than 0.88. To apply in wastewater treatment, the novel adsorbents should be applicable under different pH conditions. As shown in Figure 6e, uptake capacities of the as-prepared products for MB did not show significant change in the wide experimental pH range. The uptake capacity of the novel h-K2W4O13 nanowires (TBM-5) for MB stabilized at 103.7 ± 2.4 mg g−1 from pH 3 to pH 11, revealing that the novel h-K2W4O13 nanowires as the powerful adsorbent have the great ability of acid or alkali resistance. Solution pH is vital to the species of the adsorbate and surface charge of the adsorbent. MB is known for a cationic dye.39 Zeta potential measurements (Figure 6f) ascertain that the surface of the synthesized materials are all negatively charge from pH 3 to 11, which explaind the stable removal of MB in a wide range of pH by the materials. It can also be observed that zeta potentials decrease with the increase in pH values because of the deprotonation of O−H groups on the surface. In general, adsorbent with a more negative surface will attract more positive charged adsorbates, leading to an enhanced uptake capacity.16 Nevertheless, different results were obtained, revealing that electrostatic attraction could not be the only force responsible for the interaction between MB and the materials. 3.4. MB Adsorption Mechanism. To clarify the key functional groups for MB removal, FT-IR spectroscopies were performed for the as-prepared products after MB adsorption. As shown in Figure 7b, new peaks (compared with Figure 7a) stemmed from MB appeared, including peaks at 2924, 1599, 1489, 1441, 1392, 1338, 1248, 1223, 1176, and 1144 cm−1, ascribed to ν(CH3), ν(CC), ν(CS+), δ(CH), δ(CH), ν(C−N), δ(CH)/γ(CH), ν(C−C), δ(CH) and δ(C−N) vibrations, respectively.40 It is obvious that the peaks of MB become stronger for TBM-4 and TBM-5, implying more MB was adsorbed. The peak at 1622 cm−1 of the TBM-1, TBM-2, and TBM-3 and the peak at 1615 cm−1 of TBM-4 and TBM-5 disappeared after adsorption, both of which originate from the weakly bonded water. Peaks at 1601 cm−1 from water coordinated with Lewis sites of WO3 and the peaks below 1000 cm−1 from W−O vibrations display minor change. The FT-IR analysis indicates that −OH from the weakly bonded water on the surface of the products play a key role in MB adsorption. Pore size distribution (PSD) is an important property for the porous adsorbent, because it determines the effective pore volume assessable to the molecule of given size.41 This method is commonly used in investigating sorption mechanism of carbon-based materials with abundant pore structure.42 N2 adsorption−desorption isotherms (Figure S2) in this study all display hysteresis loops, suggesting that the mesoporous structure. The Barrett−Joyner−Halenda (BJH) method, a model appropriate for mesoporous material, was then used to calculate PSD from the adsorption branch of the N 2 adsorption−desorption isotherms, and the results are showed in the inset of Figure S2. PSD of different materials concentrated in the range of 3−10, 3−13, 3−16, 3−35, and 3−57 nm for TBM-1, TBM-2, TBM-3, TBM-4, and TBM-5, respectively. Obviously, more mesoporous interstices with 3809


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volume result in poor uptake properties for the materials formed with less K2SO4. Interaction between metal oxides and dyes are commonly ascribed to electrostatic attraction.38 The synthesized h-WO3 and h-K2W4O13 are verified as being negatively charged by the zeta potential results because of the functional groups such as −OH and WO. These oxygen-containing functional groups can also enhance hydrophilic nature of the materials. Electrostatic attraction between the adsorbents and the cationic dyes would occur both on the surface and inner pore structure of the adsorbents. Thus, a large BET surface and a high proportion of effective pore volume available to MB molecule would provide more active sites for electrostatic interaction. Hexagonal K2W4O13 (TBM-5) here has the largest specific surface/total pore volume and the lowest proportion of ineffective pore, interpreting its most powerful uptake capacity for MB from wastewater among the five synthesized products. Varying solution pH from 3 to 11, removal efficiency of different products for MB has rarely changed, although the zeta potentials of different products decrease at the higher pH values. This indicates that electrostatic attraction may be not the only reason responsible for MB uptake. From FT-IR analysis, the peaks stemmed from weakly adsorbed water disappeared after adsorption. Thus, the −OH from the weakly bond water should also play an important role in MB adsorption. Particularly, characteristic peaks of the original MB (Figure S3) have slight blue-shifts after adsorption. For instance, the peak attributed to vibration of aromatic ring at 1599 shifts to 1591 cm−1 after adsorption, which may be due to the formation of the MB-WO3 or the MB-K2W4O13. The similar conclusion has been reported by Xiong et al., where the MB-TiO2 nanotube composite was formed after adsorption.44 In fact, an obvious pH decrease after adsorption was observed. The initial pH value of adsorption system is 6.14 and it decrease to 4.45 after adsorption, revealing that H+ from the weakly adsorbed water are exchanged with MB cations and form the MB- adsorbent composition. Cohesion of the coordinated water with the adsorbents is so strong that H+ from the strongly adsorbed water would not be replaced by MB cations, which is supported by the FT-IR spectra. Moreover, adsorption process follows the pseudo-second order kinetic, in accordance with the formation of new chemical composition.45 3.5. Desorption and Recycle Peformance. To be practically useful in actual wastewater decontamination, the stability and reusability of an adsorbent should be considered to reduce the cost. A five-run of adsorption−desorption test using 0.1 mol L−1 HCl as the regenerant was conducted to evaluate the sustainability of h-K2W4O13 nanowires (Figure S4). Overall, the h-K2W4O13 nanowires exhibited excellent reusability. The removal efficiency of MB was satisfying in each cycle and maintained over 88% even in the fifth cycle. Therefore, h-K2W4O13 nanowires with good adsorption stability is a promising material for practical applications on MB removal from water.

Figure 7. FT-IR spectra of the synthesized products (a) before adsorption and (b) after adsorbing MB.

larger sizes were formed by random stacks of nanorods/ nanowires. Because of the molecular sieving effect, pores with narrow or unfit shape would not allow adsorbates to diffuse into. The dimensions of MB are 1.43 nm × 0.61 nm × 0.4 nm.41 Bartonet et al.43 demonstrated that pores narrower than 3 nm were ineffective for MB adsorption. It was found that adsorption process would be very slow if the pore width was close to the adsorbate size.41 Considering the contact time of MB with the as-prepared products is relatively short in this work, we postulate the diameter of pores smaller than 4−5 nm are difficult for MB molecule to penetrate into. Calculating the percentage of the pores narrower than 4 or 5 nm, as listed in Table 2, it is found that TBM-1 and TBM-3 have a higher proportion of narrow pores than pure h-K2W4O13 (TMB-5) that are useless in MB removal. Furthermore, these two products tend to have smaller BET surface and total pore volume. In short, small specific surface and low effective pore Table 2. BET Surface and Pore Properties of the Prepared Products as-prepared product 2

−1

BET (m g ) total pore volume (cm3 g−1) pore size

Hexagonal K2W4O13 Nanowires for the Adsorption of Methylene Blue

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