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Nitrogen-Doped Alkylamine-Intercalated Layered Titanates for Photocatalytic Dye Degradation Anto Jeffrey A, C. Nethravathi, and Michael Rajamathi* Materials Research Group, Department of Chemistry, St. Joseph’s College, 36 Lalbagh Road, Bangalore 560 027, India
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S Supporting Information *
ABSTRACT: A layered titanate, K2Ti4O9, is intercalated with various n-alkylamines through ion-exchange reaction in aqueous medium. On heating, the intercalated amine is partially deintercalated, yielding nitrogen-doped amine-intercalated titanates. The modified titanates are studied as catalysts in methylene blue degradation under UV irradiation. Heat-treated long-chain amine titanates exhibit better photocatalytic activity in comparison to short chain amine titanates. The improved catalytic activity could be attributed to two factors: (i) increased surface access as the titanate layers are well separated, pillared by the alkylamine chains and (ii) nitrogen doping.
1. INTRODUCTION A layered titanate, K2Ti4O9,1,2 is of interest as a potential material for energy and environmental applicationslithiumion battery electrode,3 photovoltaics,4 and photocatalysis.5−10 It belongs to a class of materials with a general formula M2O· nTiO2 (M = Na, K, Rb, Cs). K2Ti4O9 consists of negatively charged two-dimensional (2D) layers wherein TiO6 octahedral units are connected through shared corners and edges with K+ ions occupying the interlamellar space.5,11,12 K2Ti4O9, structurally related to TiO2, is also a semiconductor with a band gap of 3.54 eV and has been explored as a potential material for various photocatalytic reactionsphenol decomposition,13 methanol oxidation,14 and water reduction.8,15 Though TiO2 and its analogues exhibit good photostability, their photocatalytic efficiency is limited by absorption of only UV rays ( H2Ti4O9 (3.25 eV) > TBA2Ti4O9 (3.00 eV) on exchange of K+ for H+ or TBA+. The band gap reduction has been attributed to two main factors: (i) change in composition of the phases and (ii) in-plane and edge defects caused because of exfoliation. Di Quarto et al.34 predicted that the band gap of a metal oxide increases as a function of the square of the difference in Pauling electronegativities (ENs) of the metal and oxygen components. Hence, exchange of K+ (EN = 0.8) with H+ (EN = 2.2), whose EN is comparable to oxygen (EN = 3.5), leads to a reduction of the band gap.34 A decrease in the band gap has been observed on intercalation of n-alkylamine in layered titanates.33 Layered K2Ti4O9, on hybridization with CdS nanostructures8,9/oxide nanoparticles6,24,35−39 (TiO2, SnO2, CrOx, and SiO2) or on cation-exchange40,41 - (Co2+ and Sn2+), exhibits enhanced photocatalysis. Layered titanates doped with nitrogen, obtained through solvothermal reaction in the presence of triethylamine,6 calcination in the presence of ammonia42/ urea,35 or photo-N doping43 in a N2 atmosphere, also exhibit enhanced photocatalytic activity. Interlayer modified titanates have been largely unexplored as photocatalysts. In this context, n-alkylamine (C4−C18)intercalated titanates are investigated here as photocatalysts toward dye degradation. As the interaction between the interlayer amines and titanate layers, upon heat treatment, is expected to lead to N-doping of the titanate layers, one could anticipate improved catalytic performance from these materials. Received: November 17, 2018 Accepted: January 2, 2019 Published: January 17, 2019 1575
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Figure 1. Schematic representation of formation of nitrogen doped amine titanates.
2. RESULTS AND DISCUSSION Amine titanates with n-alkylamines of varying alkyl chain lengths [n-butylamine (BA), n-hexylamine (HA), n-octylamine (OA), n-dodecylamine (DDA), and n-octadecylamine (ODA)] were prepared through ion exchange as depicted in Figure 1. Amine titanates formed by reacting K2Ti4O9 with n-alkylamines in aqueous medium remain white as K2Ti4O9. When the amine titanates were subjected to heating at 110 °C, they turned brown (Figure 1), indicating nitrogen doping as observed in the literature.1 X-ray diffraction (XRD) patterns of amine titanates before and after heat treatment are shown in Figure 2A and
in the interlayer of the titanates before and after heating (Supporting Information, Figure S2). The scanning electron microscopy (SEM) images (Figure 3a,b) of the as-prepared and heated ODA titanate reveal that
Figure 3. SEM images of the as-prepared (a) and heated (b) ODA titanate.
both the samples are made up of bundles of rectangular rods of lengths in the range of 10−20 μm and, in addition, point to the fact that heat treatment does not affect the morphology of the amine titanates. X-ray photoelectron spectroscopy (XPS) analysis of the asprepared and heated ODA titanate is shown in Figure 4. In the
Figure 2. (A) XRD patterns of (a) pristine K2Ti4O9; (b) as-prepared and (c) heated BA titanate; and (d) as-prepared and (e) heated ODA titanate. (B) UV−visible absorbance spectra of as-prepared and heated ODA titanate in comparison with the XRD pattern of pristine K2Ti4O9.
Supporting Information, Figure S1. The XRD pattern of pristine potassium titanate indicates a layered structure and matches with that reported in the literature for the K2Ti4O9 phase.33 In comparison to K2Ti4O9, BA titanate and ODA titanate [Figure 2A(b,d), respectively] exhibit increased basal spacing, indicating the intercalation of the amines in the interlayer. Though the basal spacing decreases [Figure 2A(c,e)] on heating the amine titanates, the interlayer spacing is still higher compared to pristine K2Ti4O9, indicating a change in the orientation of the alkyl chains of the nalkylamines. HA titanate also exhibits a similar trend (Figure S1). In contrast, OA titanate and DDA titanate (Supporting Information, Figure S1) exhibit no change in basal spacing on heating. Though the as-prepared and heated ODA titanate exhibit red shifts in the UV−visible absorption spectra (Figure 2B) in comparison to pristine K2Ti4O9, the absorption maximum is in the near-UV region. In addition, heated ODA titanate also exhibits strong absorption in the near-infrared (IR) region, probably because of partial carbon doping. In the IR spectra, the presence of broad N−H and weak C− H stretching vibrations at 3400 and 2850−2950 cm−1, respectively (Figure S2), in the as-prepared and heat-treated amine titanates further confirms the presence of n-alkylamines
Figure 4. XPS spectra showing Ti 3p (a,c) and N 1s (b,d) core-level peak regions of the as-prepared and heated ODA titanate, respectively. 1576
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indicated by the XPS analysis (Figure 4). Improved photocatalytic behavior of heat-treated amine titanates when compared to pristine K2Ti4O9 and Degussa P25 is also due to the efficient charge separation induced by the doped nitrogen in the titanate layers.21 In comparison to what has so far been reported in the literature (Table 2), the enhanced activity makes heated ODA titanate a superior photocatalyst. Nitrogen doping seems to be crucial to improve the photocatalytic activity. Doped nitrogen content is expected to increase with increase in temperature.21 Thus, the structural evolution and photocatalytic activity of ODA titanate heated at various temperatures were studied. Figure 5A indicates that
case of the as-prepared ODA titanate, the Ti core-level spectrum (Figure 4a) indicates the presence of Ti−N at 454.6 eV (Ti3+) and Ti−O at 457.95 eV (Ti4+). The N 1s core-level spectrum (Figure 4b) further supports the existence of Ti−N with a peak at 395.7 eV. The intercalated amine exists both as free amine and as an alkyl ammonium ion. The Ti core-level spectrum (Figure 4c) of the heat-treated ODA titanate indicates the presence of only Ti−O at 458.5 eV, and the N 1s core-level spectrum (Figure 4d) indicates the presence of only free amine. The peak due to Ti−N at 394.3 eV (Figure 4d) is attributed to substitutional β-N (N3−), indicating that the titanate layers are doped with nitrogen.21 The photocatalytic activity of the amine titanates toward degradation of aqueous methylene blue (MB) under ultraviolet irradiation was monitored by UV−visible absorption spectroscopy. The efficiencies of the as-prepared amine titanates (Table 1) are similar to each other and to that of K2Ti4O9. All Table 1. Photocatalytic Activity of Various n-AlkylamineIntercalated Titanates toward MB (100 mL of 10 ppm) Degradation under a UV Source time for 100% dye degradation (min) catalyst (15 mg)
heated (100 °C)
as-prepared
MB Degradation in Water Degussa (P25) 183 K2Ti4O9 350 butylamine titanate 390 hexylamine titanate 390 octylamine titanate 385 dodecylamine titanate 370 octadecylamine titanate 365 MB Degradation in Butanol octadecylamine titanate 60
Figure 5. XRD patterns (A) and photocatalytic activity (B) of ODA titanate heated at various temperatures.
183 350 120 105 60 50 25
heating ODA titanate beyond 100 °C causes partial deintercalation of ODA and heating beyond 200 °C results in near complete deintercalation, resulting in decreased surface area, as observed in calcined N-doped TiO2.21,23,44 In tune with the level of deintercalation, the catalytic activity decreases, and heating beyond 400 °C makes the catalyst less effective (Figure 5B) compared to Degussa P25. If nitrogen doping is the only criterion that controls the activity, then all the heated amine titanates should exhibit similar activity, but this is not the case. The interlayer spacing, which increases the access to the catalyst surface, also seems to play a major role. If the interlayer spacing is the major cause for catalytic activity, then as-prepared amine titanates should show good activity. However, all the as-prepared amine titanates show very poor catalytic activitypoorer than even the parent K2Ti4O9. This counterintuitive result could be due to poor wetting of the as-prepared amine titanates. In fact, it was
45
these samples show poorer efficiency compared to commercial Degussa P25. However, the heat-treated amine titanates show improved activity, and the photocatalytic efficiency increases steadily with increase in the chain length of the intercalated alkylamine. Heat-treated long chain amine titanates (OA, DDA, or ODA titanate) degrade the dye much faster compared to heat-treated short-chain amine titanates (BA or HA titanate). This could possibly be because of better surface access due to larger separation between the layers. Alkylamine facilitates nitrogen doping on heating the titanate layers, as
Table 2. Comparison of Photocatalytic Activity of Various Titanate-Based Catalysts (Literature) in the Degradation of MB photocatalytic activity catalyst
morphology
mass of catalyst (mg)
pollutant concentration in ppm & (volume)
N-(ODA)xTi4O9 TiO2−RGO W,N,S-TiO2 TiO2−CNT−CoFe2O4 TiO2−CeO2 N-TiO2−MoS2 Co−Ti4O9 P−TiO2 N-TiO2 C,N-TiO2 N-K2Ti4O9−TiO2 N-titanate
rods composite nanoparticles nanocomposite nanosheet nanostructure lamellar nanoporous nanostructures nanorods core−shell flowers
15 100 500 200 50 50 30 50 100 50 6.8 5
10 (100 mL) 10 (200 mL) 5 (600 mL) 1800 (10 mL) 10 (50 mL) 10 (50 mL) 20 (60 mL) 20 (200 mL) 10 (100 mL) 10 (300 mL) 0.35 (100 mL) 5 (40 mL) 1577
irradiation source UV UV UV UV Xe (300 Xe (300 Xe (300 Xe Xe Xe (500 sun sun
W) W) W)
W)
t (min)
reference
25 240 240 90 180 120 120 100 100 180 180 70
present 45 46 47 48 49 40 50 25 51 35 52
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Figure 6. Degradation of MB was traced through UV−visible absorption spectra of the reaction mixture containing 15 mg of the heated ODA titanate. (a) Evolution of absorption spectra with time, (b) plot of log (absorbance) against time, and (c) efficiency of the catalyst (time taken for complete degradation) in six consecutive cycles.
observed that these materials tend to float in the reaction mixture, suggesting poor wettability. To ascertain the effect of interlayer spacing and to gauge the contribution from nitrogen doping alone, the dye degradation studies were carried out in 1-butanol in which the as-prepared amine titanates is shown to be solvated better. As-prepared and heat-treated ODA titanates degrade the dye completely in 60 and 45 min, respectively (Table 1). These observations suggest that the major cause for improved catalytic behavior of amine titanates is the increased interlayer spacing. Heat treatment leading to nitrogen doping further improves the catalytic activity. Catalytic performance of the heat-treated ODA titanate in the degradation of aqueous MB is summarized in Figure 6a−c. The UV−visible spectrum of MB shows strong absorption at 665 nm (Figure 6a) and decreases with time and disappears within 25 min in the presence of the heat-treated ODA titanate photocatalyst under UV radiation. The log (absorbance) versus time plot (Figure 6b) is linear (R2 = 0.999), suggesting pseudo-first order kinetics. The calculated rate constant from the kinetic plot is 2.3 × 10−3 s−1. The catalytic activity of ODA titanate starts decreasing after four cycles (Figure 6c), probably because of decrease in surface access as a consequence of partial deintercalation of the alkylamine (Supporting Information, Figure S3).
mixture (1:1) and dried at room temperature. A part of the amine titanate was heated at 110 °C for 2 h in air. 4.2. Photocatalytic Degradation of MB. Photocatalytic activity was studied using the as-prepared and heat-treated amine titanate samples as catalysts. MB solution (10 ppm, 100 mL) was used for the catalytic studies. In each case, 15 mg of the catalyst was added to the dye solution, and the mixture was stirred in dark for ∼45 min to equilibrate adsorption/ desorption. The solution with the catalyst was then exposed to ultraviolet (70 W UV lamp) rays. The progress of the reaction was monitored by measuring the absorbance of the dye at 665 nm. 4.3. Characterization. All the solid samples synthesized were characterized by powder XRD using a PANalytical X’pert Pro diffractometer (Cu Kα radiation, secondary graphite monochromator, scanning rate of 1° 2θ/min). IR spectra of the samples were recorded using a PerkinElmer FT-IR spectrometer, Spectrum TWO, UATR TWO. SEM analysis of the hybrid wafers was carried out using a Zeiss Ultra 55 field emission scanning electron microscope. UV−visible spectra of the reaction mixtures were recorded on a PerkinElmer (LS 35) UV−visible spectrometer. XPS measurements were carried out with Kratos Axis Ultra DLD. All spectra were calibrated to the binding energy of the C 1s peak at 285 eV.
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3. CONCLUSIONS Nitrogen-doped layered titanates could be synthesized on heating n-alkylamine-intercalated titanates at 110 °C. In comparison to commercial Degussa, these modified titanates exhibit enhanced photocatalytic activity toward MB degradation under UV irradiation. The enhanced activity is attributed to the interplay between (i) increased surface access as the titanate layers are well separated, pillared by the alkylamine chains, and (ii) nitrogen doping. The results suggest that alkylamine-intercalated layered titanates could be potential photocatalysts for environmental amelioration and open new avenues to explore other propped-up 2D layers as catalysts.
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsomega.8b03207.
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XRD patterns and IR spectra of amine tianates, MB degradation studies of HA, OA and DDA-titanates (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: mikerajamathi@rediffmail.com (M.R.).
4. EXPERIMENTAL SECTION 4.1. Synthesis of n-Alkylamine-Intercalated Titanate (Amine−Titanate). A series of n-alkylamine (BA, HA, OA, DDA, and ODA)-intercalated layered titanates were prepared as reported in our earlier work.33 Aqueous solution of the nalkylamine was adjusted to pH ≈ 7 using dilute HCl (2 N). K2Ti4O9 (0.350 g) was soaked in 100 mL of the freshly prepared n-alkylamine solution for 7 days. The amine titanate product was washed thoroughly with an ethanol−water
ORCID
Michael Rajamathi: 0000-0002-4975-1855 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was funded by DST, New Delhi, India (EMR/ 2015/001982). 1578
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DOI: 10.1021/acsomega.8b03207 ACS Omega 2019, 4, 1575−1580
