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Self-Assembly of Metal Oxide Nanosheets at Liquid-Air Interfaces in Colloidal Solutions Huiyu Yuan, Melvin Timmerman, Marijn van de Putte, Pablo Gonzalez Rodriguez, Sjoerd A. Veldhuis, and Johan E. ten Elshof J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b07961 • Publication Date (Web): 21 Oct 2016 Downloaded from http://pubs.acs.org on October 25, 2016

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The Journal of Physical Chemistry

Self-Assembly of Metal Oxide Nanosheets at Liquid-Air Interfaces in Colloidal Solutions Huiyu Yuan, Melvin Timmerman, Marijn van de Putte, Pablo Gonzalez Rodriguez, Sjoerd Veldhuis,† Johan E. ten Elshof * MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, the Netherlands.

ABSTRACT: The oxide nanosheet concentration at the liquid-air interface (LAI) is a key parameter in the formation of Langmuir-Blodgett (LB) deposited nanosheet films. Knowledge of the oxide nanosheet concentration at the LAI as a function of process conditions is needed to understand the relevant processes and achieve better control over the LB fabrication process. In this study, the concentration of Ti0.87O2δ- titanate nanosheets at the LAI was investigated by considering the trend in the lift-up point (LUP) in the surface pressure-surface area isotherm of an LB compression process as a function of time and exfoliation agent. The oxide nanosheet concentrations in the bulk solutions were studied using UV-Vis spectroscopy. The results show that the restacking process in the bulk solution does not significantly retard the occurrence of nanosheets at the LAI. The nanosheet concentration changes in the bulk and at the LAI occur on different time scales. Short exfoliation times yield higher nanosheet concentrations at the LAI than longer exfoliation times, in contrast to the bulk where the nanosheet concentration increases

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in the course of time. We found the same behavior for other metal oxide nanosheet solutions, i.e. iron-doped titanate (Ti0.6Fe0.4O20.4-) and calcium niobate (Ca2Nb3O10-) nanosheets. The reason behind this phenomenon is likely related to the high degree of adsorption of surfactant molecules on the nanosheet surface after short exfoliation times.

INTRODUCTION Two-dimensional (2D) materials known also as nanosheets have large potential for application in optoelectronics, spintronics, catalysis, chemical and biological sensors, supercapacitors, solar cells, and lithium ion batteries.1-4 Their 2D nature provides new paths for researchers to design and develop novel advanced materials and devices.5-7 The research interest in 2D materials has increased largely over the last decade.1-2, 5 To date, a large number of 2D materials have been discovered and synthesized on laboratory-scale, such as graphene,8 metal oxides,9 layered metal dichalcogenides,10 clays,11 layered double hydroxides,12 and MXenes.4 In order to bring the application of 2D materials on industrial scale forward, a facile synthesis method to fabricate them in large quantities is needed.13 Among the several available synthesis methods to make 2D materials, liquid exfoliation has been identified as the most promising strategy for large-scale production.14 Ion intercalation exfoliation, one of the liquid exfoliation methods, has shown to function well for the formation of a large number of layered ionic compounds and yields high quality monolayer nanosheets.14-15 One of its important advantages compared to other exfoliation techniques is that monolayer nanosheets obtained by ion intercalation exfoliation can be transferred from the liquid-air interface (LAI) to a solid substrate by deposition techniques such as the Langmuir-Blodgett (LB) method.16-18 The resulting nanosheet films have found useful applications in crystal engineering for oriented thin film growth.19-21


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The mechanism of exfoliation has been intensively studied for layered titanates (H0.7Ti1.824O4·H2O and H1.07Ti1.73O4·H2O).16, 22-25 For layered titanates, exfoliation is normally achieved by mixing protonated layered titanates with tetrabutylammonium hydroxide (TBAOH). The exfoliation of layered titanates is triggered by an acid-base reaction between TBAOH and protonated layered titanates.24 This exfoliation process is very rapid and can yield single layer nanosheets in a minute or less.24 The same exfoliation mechanism also holds for other protonated layered oxides, e.g. HCa2Nb3O10, HxTi2-yFeyO4, and many others. The molar ratio TBAOH/H+ (where H+ refers to the number of protons in the protonated titanate or other layered oxide) is one of most important parameters in the exfoliation process. For titanates, the yield of nanosheets is low and the obtained nanosheets have been found to be relatively defective when the molar ratio TBAOH/H+ is as low as 1/16.16 At molar ratios TBAOH/H+ above 1/2, isolated nanosheets form but restacking also occurs to some degree. The restacking process, which may ultimately lead to precipitation, reduces the yield of isolated colloidal nanosheets, making the synthesis window for titania nanosheets rather narrow.16, 23, 25 In the intermediate range, where TBAOH/H+ is 0.5–2, oxide nanosheet dispersions are dominated by unilamellar sheets. Under these conditions, high quality monolayer films were successfully made using the LB method.16 It was concluded that the surface pressure during LB deposition is the key parameter that controls the coverage of the film. Nevertheless, despite its importance for the LB film formation process, little systematic information is available on the concentration of nanosheets at the LAI under different processing conditions. In our previous work, we showed that even at low TBAOH/H+ ratios monolayer nanosheet films could be formed.16 However, no quantitative information about nanosheets at the LAI was available. Here, we used a semi-quantitative method to evaluate the nanosheet yield at


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the LAI by monitoring the lift-up point (LUP) in the surface pressure-surface area isotherm of LB films as they are being formed. For dispersions consisting of unilamellar nanosheets, the formation process of interfacial LB nanosheet films usually goes through three distinct stages, as illustrated in Figure 1. In a very dilute nanosheet solution, the concentration of nanosheets at the LAI is so low that the surface pressure between the barriers increases only slowly upon compression (stage 1). When the barriers have been compressed to the point that the concentration of nanosheets between the barriers is so high that they start to interact considerably, the surface pressure increases more rapidly with compression (stage 2). Further compression leads to a dense monolayer nanosheet film (stage 3). In stage 3, the surface pressure tends to saturate; the dense nanosheet film may even collapse if the compression process proceeds too far. The LUP is determined by extrapolation of the surface pressure – surface area curve in stage 2 to zero pressure as shown in Figure 1. In the present work, we studied the influence of (i) molar ratio TBAOH/H+ and (ii) reaction times on the yield of nanosheets at the LAI using a well-known and thoroughly studied model material, the layered titanate H1.07Ti1.73O4·H2O (HTO).22,

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The same experiments were

repeated with protonated iron-titanate (H0.8Ti1.2Fe0.8O4) and protonated calcium niobate (HCa2Nb3O10) to confirm the universality of our findings, see supporting information. The nanosheet concentration at the LAI was evaluated by considering the trends in the LUP as a function of process conditions. UV-Vis spectroscopy was employed for the determination of the corresponding nanosheet concentration in the bulk of the solutions for the sake of comparison. It is noted that LB deposition is a well-known and effective method for the formation of metal oxide nanosheet films,18 and the morphology and thickness of nanosheet films by this method have been discussed elsewhere.16-18, 24


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EXPERIMENTAL METHODS Materials. Titanium(IV) dioxide TiO2 (Riedel-de Haën), molybdenum(VI) oxide MoO3 (SigmaAldrich), anhydrous potassium carbonate K2CO3 (Fluka), and lithium carbonate Li2CO3 (Riedelde Haën) had a purity of 99.0% or higher and were used as received. Tetra-(n-butyl)ammonium hydroxide TBAOH (40 wt% in H2O, Alfa Aesar) was used as received. Demineralized water was used throughout the experiments. Preparation of lepidocrocite-type protonated layered titanate and its nanosheet suspension The K0.80Ti1.73Li0.27O4 (KLTO) precursor was obtained with a flux method developed by Tanaka et al.28 TiO2, K2CO3, Li2CO3, and MoO3 (1.73:1.67:0.13:1.27 molar ratio) were heated to 1150°C, held at that temperature for 30 min, and then slowly cooled down to 950 °C at a speed of 4 °C/h. The oven was then allowed to cool further to room temperature by natural cooling. The resulting KLTO powder was washed 3 times in 250 mL water to remove K2MoO4. Then KLTO powder was dispersed in a 2 mol/L HNO3 solution (250 mL) at room temperature while stirring. The acidic solution was replaced daily by a fresh one via decantation. After treatment for 3 days, the acid-exchanged crystals were collected by filtration and washed with a copious quantity of demineralized water, then air dried to get HTO powder. Samples were prepared by mixing 0.1 g of HTO with water and TBAOH with a TBAOH/H+ molar ratio of 1/32, 1/16, 1/8, 1/4, 1/2, 1/1, 2/1, 4/1, 6/1 and 8/1 to a total volume of 20 mL solution. The mixtures were stirred for 2 h. The solutions were then directly used for UV-visible absorption spectroscopy and LB deposition experiments. The time dependency experiments were made using solutions with TBAOH/H+ molar ratios of 1/1 and 4/1. In a total volume of 20 mL solution, 0.1 g of HTO was used for each sample while


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stirring for different periods of time, and then used directly as described below to estimate the nanosheet concentrations at the LAI and in the bulk. Evaluation of nanosheet concentration at the liquid-air interface. To obtain a suspension for surface pressure-surface area isotherm experiments, 2 mL of the above solutions after reaction for the given amount of time were diluted to 500 mL by adding demineralized water. The diluted suspensions were then left standing for 1 h to allow any possible precipitates to settle. For the pressure-surface area isotherm experiments, 50 mL of solution was taken from the middle or upper part of the nanosheet suspension using a syringe. After the separated suspension was poured into a Langmuir-Blodgett trough (KSV Minimicro, a Teflon trough with an active trough surface area of 100 cm2, L195 x W51 x D4 mm3 and a dipping well L10 x W28 x D28 mm3, trough volume 48 cm3), a stabilization time of 10 min was used to equilibrate and stabilize the surface pressure before compression. The surface pressure after stabilization was arbitrarily defined as zero and then the compression process was started using a compression speed of 3 mm/min. The compression process was continued for about 20 min until either a minimum trough area of 30 % of the initial surface area had been reached, or the surface pressure had already saturated at lower degrees of compression. A schematic representation of the experimental process of the LUP measurements is shown in Figure 2. Characterization. UV-Vis spectra of samples were recorded with a Cary 50 UV-Vis spectrophotometer in transmission mode. The original suspensions were diluted 300 times by volume in order to obtain an appropriate range of absorbance. Small-angle X-ray scattering (SAXS) experiments using synchrotron radiation were carried out on the Dutch-Belgian beamline, DUBBLE BM-26B of the European Synchrotron Radiation Facility (ESRF) in Grenoble.29 A home-built solution cycling setup including a SAXS measurement chamber, a


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mixing chamber, a time-controlled injection system and a pump system were used to do in situ SAXS measurements directly upon mixing of the HTO and TBAOH solutions. The cycling flow in the mixing chamber was placed in up and down direction. Background scattering data were recorded using demi-water cycling in the setup. SAXS scattering curves were recorded with 30 s intervals and the data acquisition time for 1 curve was 28.5 s. For further details about the SAXS configuration the reader is referred to ref. 24. pH measurements were performed using an USB DrDAQ recorder with a pH electrode. The data were recorded every 1 minute. In a typical measurement, 15 mL water and 0.1 g HTO were placed in a reaction bottle and stirred. Then, 5 mL TBAOH solution of varying concentration was manually injected into the reaction bottle while simultaneously monitoring the pH. The zeta potential data were measured by a Malvern Zetasizer Nano ZS. Samples were prepared with a nominal nanosheet concentration of 5 g/L and a TBAOH/H+ molar ratio of 4/1 after a reaction times of 1 h and 5 days, respectively. Subsequently, the samples were diluted 10 times by volume for zeta potential measurements. RESULTS AND DISCUSSION The influence of the TBAOH/H+ molar ratio on the concentration of titanate nanosheets at both the LAI and in the bulk of the solution are shown in Figure 3 (See Figure S1 for further details). Figure 3a shows the LUP at different molar ratios TBAOH/H+. It is noted that the LUP for solutions with a nominal molar ratio TBAOH/H+ of 1/32 and 1/16 was ˂ 30%, and therefore out of the detection range under our experimental conditions. At molar ratio TBAOH/H+ = 1/8 the LUP was close to 100 %, which indicates that the nanosheet concentration at the LAI was high under these conditions. The large change of the nanosheet concentration at the LAI between


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TBAOH/H+ = 1/16 and 1/8 is due to the much higher degree of exfoliation at TBAOH/H+ = 1/8, as discussed below. The LUP remained constant with increasing TBAOH concentration until a molar ratio TBAOH/H+ of 2/1, and decreased when the ratio was increased further. The optical absorption spectra (supporting information Figure S1b) show that the absorption peak is located at 265 nm, indicating the presence of nanosheets.16,

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The maximum absorbance at 265 nm

provides an estimate of the nanosheet concentration in the bulk of the solution, see Figure 3b. The absorbance increased with molar ratio TBAOH/H+ until a molar ratio TBAOH/H+ of 1/4, and decreased upon further increase of TBAOH/H+. These optical data are consistent with our previously reported results on layered titanate nanosheets,16 where we showed that the nanosheet yield increases with increasing molar ratio TBAOH/H+ for ratios < 1/4, while some restacking occurs when the TBAOH/H+ ratio is higher than 1/4.16 Our present results, however, suggest that even though restacking occurs at low molar ratios in the order of 1/4, the restacking process in the bulk only has a minor negative effect on the nanosheet concentration at the LAI, even at TBAOH/H+ molar ratios > 2. It is stressed that even at low TBAOH/H+ ratios high nanosheet concentrations were reached at the LAI, where no complete exfoliation occurs in the solution bulk. Although these layers exhibited a tendency towards collapsing (evidenced by the unstable surface pressure during compression), indicating that the monolayer at the LAI was not stable compared to monolayers formed at higher TBAOH/H+ ratios (See Figure S1a). Figure 4 shows the influence of reaction time on LUP and the absorbance of the nanosheet suspensions (experimental details in Figure S2). The LUP had a stable value during the first 6 h. The LUP slightly decreased at t > 6 h, and finally decreased strongly after 3 days. Since the LUP provides an indication of the nanosheet concentration at the LAI, the data indicate that the concentration of nanosheets at the LAI is higher after short reaction times than after prolonged


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reaction times. Moreover, the LUP value close to 100 % after only 30 s, shows that a sufficient number of nanosheets was rapidly present at the LAI to form a saturated monolayer. The formation of a dense nanosheet layer at the LAI after very short reaction times, and a drastic decrease of the interfacial nanosheet concentration after prolonged reaction (exfoliation) times, were also observed with other oxide nanosheets, e.g. calcium niobate and iron titanate (See Figure S3). The observation of fast nanosheet formation is in agreement with our previous findings that the exfoliation of these layered oxides is a rapid process.24 Figure 4b shows the UV-vis absorption spectra of nanosheet solutions after various reaction times. The absorbance increased significantly after > 2 h of reaction time, and reached a maximum after 3 days. The increasing absorbance is thought to be the result of exfoliation of restacked nanosheets by mechanical force introduced by stirring, as discussed in more detail below. Comparison of the trends in Figure 4a and 4b shows that the nanosheet concentration at the LAI and the nanosheet bulk concentration are not positively correlated for longer reaction times. The two regions exhibited independent behavior. For efficient LB deposition, a short period of exfoliation seems to be more suitable to efficiently form a dense monolayer film than a longer exfoliation time. The optical absorption increase after 2 h suggests that the nanosheet concentration in the bulk increased. To confirm this hypothesis, a time-resolved SAXS experiment was carried out to monitor the occurrence of scattering entities in solution. Figure 5 shows SAXS curves at a nominal TBAOH/H+ molar ratio of 4/1 and a nominal nanosheet concentration of 5 g/L. Under the same conditions we have reported very rapid exfoliation and restacking reactions in a previous study.24 Details of the first 2 frames of the in situ SAXS curves in Figure 5a indicate that rapid exfoliation and the emergence of Bragg-like correlation peaks suggests restacking.


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However, the restacked structure exfoliated again after more than 2 h, as evidenced by the gradual vanishing of these correlation peaks in Figure 5b. To the best of our knowledge, this long term re-exfoliation phenomenon has not been reported before, although the preceding restacking reaction at TBAOH/H+ = 4 is a well-known phenomenon.24-25 Our current data show that since the restacked structures in the bulk seem to be re-exfoliated, an increasing nanosheet concentration develops in the bulk of the solution, as evidenced by the trend in absorbance in Figure 5b. In contrast to the steadily increasing nanosheet concentration in the bulk, the nanosheet concentration at the LAI decreased (Figure 4a). This phenomenon was investigated further in order to understand the behavior of nanosheets at the LAI and to achieve better control over the fabrication of inorganic nanosheet LB thin films. First, we ruled out the possibility that TBA+ ions would contribute to the surface pressure by performing a reference experiment in which TBAOH but no HTO was present. The surface pressure remained very low even after compression of the trough area to 30 % of the initial surface area (Figure S4). Another experiment was performed to study the influence of TBAOH concentration on the Langmuir film formation of titanate nanosheets. The TBAOH/H+ molar ratio was 1/1 and the nominal nanosheet concentration was 5 g/L. The dependence of the LUP and corresponding pH on reaction time are shown in Figure 6. We found that the LUP started to decrease appreciably after 1 day (Figure 6a). The LUP decrease started after 3 days at higher TBAOH concentration (TBAOH/H+ molar ratio 4/1) (Figure 4a), which indicates that TBAOH somehow helps to stabilize the titanate nanosheets at the LAI. The pH change in Figure 6b demonstrates that the acidity of the solution increased with time. The pH was stable during the first 2 days after the rapid acid-base reaction and then decreased. We hypothesized that the absorption of CO2 from


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air into the solution may contribute to the decrease of pH (noting that it has been known that the release of protons from nanosheets after rapid exfoliation also contributes to the decrease of pH24). For example, it has been reported that tetramethylammonium hydroxide (TMAOH) can react with CO2 as a strong base to form tetramethylammonium carbonate.31 Similarly, TBAOH could react with CO2 following reaction (1), and serve as a carbon dioxide absorbant:32-33 2TBAOH + CO2 → (TBA)2CO3 + H2O

(1)

This reaction consumes hydroxide ions present in solution and introduces some protons. As the concentration of protons increases in the solution, the exchange between TBA+ associated with nanosheets and protons in solution is likely because protons are smaller in size and can initiate electrostatically more favorable interactions with the negative nanosheets. We therefore performed a reference experiment in a glovebox in N2 atmosphere to avoid the reaction between TBAOH and CO2 from the ambient. We observed that the period of high nanosheet concentration at the LAI was not prolonged in the absence of CO2 (Figure S5), so the reaction between TBAOH and CO2 is not a driving force for the decreasing nanosheet concentration at the LAI. Another possible reason for the time-dependent concentration of nanosheets at the LAI may be related to its dynamic electrostatic interactions with counter-ions. Since air presents a hydrophobic medium to the species in aqueous solution, the hydrophobicity of nanosheets covered with TBA+ has been recognized as a key factor in the process of LB deposition of nanosheets.18 Higher nanosheet concentrations at the LAI may therefore indicate more hydrophobic nanosheets (i.e. nanosheets adsorbing more TBA+). If this proposed mechanism is


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correct, then the experimental data actually suggest that nanosheets are more hydrophobic after short exfoliation times than after extended exfoliation times. We hypothesized that nanosheets may have a varying chemical surrounding depending on exfoliation time. In order to validate this hypothesis, zeta potential measurements were performed with solutions after 1 h and 5 days of exfoliation reaction. Although the two samples had similar pH values, i.e. pH 12.23 for the 1 h sample and pH 12.20 for the 5 day sample, their zeta potentials were significantly different, i.e. –28.7 and –58.3 mV for the 1 h and 5 day sample, respectively. The 1 h sample was unstable during zeta potential measurements and its zeta potential gradually shifted to a higher value. The high zeta potential of the 1 h sample demonstrates that it had more counter-ions (TBA+ and possibly protons) associated to the nanosheets than the sample aged for 5 days. These results clearly show that the chemical properties of colloidal nanosheets vary with exfoliation time. The time dependence of the LUP and the time-dependent zeta potential suggest that the TBA-nanosheet complexes (simplified here as TBA-TO, where TO refers to the nanosheet) went through an ionization process in the course of time, as schematically shown in equation (2): TBA-TO → TBA+ + TO-

(2)

The increased proton concentration has a negligible influence on the equilibrium because the pH > 12 indicates a very low proton concentration. To validate whether reaction (2) is reversible, additional experiments were carried out, in which the nanosheet concentration at the LAI was compared before and after the addition of extra TBA+. In order to avoid a change of pH upon addition of TBAOH, tetrabutylammonium bromide (TBABr) was used instead. First, a sample solution with TBAOH/H+ = 4 was left for 3 weeks and the LUP was measured. Then an


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equimolar amount of TBABr to TBAOH was added to the solution. The solution was stirred overnight and the LUP was measured again. The results shown in Figure S6 demonstrate that the LUP after TBABr addition was not higher than without TBABr, suggesting that reaction (2) is not reversible. CONCLUSIONS With increasing molar ratio TBAOH/H+, the nanosheet concentration at the LAI and in the bulk of the solution increased, and the stability of nanosheet monolayers formed at the LAI was higher at higher ratios TBAOH/H+. Even though restacking occurred in the bulk solution at high molar ratio TBAOH/H+, the restacking process did not influence the buildup of nanosheets at the LAI. Short time reaction periods were sufficient for the development of a high concentration of nanosheets at the LAI. Reaction times of days led to lower nanosheet concentrations at the LAI even though the nanosheet concentration in the bulk solution kept increasing with reaction time. Our investigation suggests that the high nanosheet concentrations at the LAI after short exfoliation times are caused by a high degree of adsorption of TBA+ on the negatively charged titanate nanosheets, yielding relatively hydrophobic nanosheets which tend to accumulate at the LAI. These TBA-nanosheets complexes gradually ionized, which resulted in a decreasing nanosheet concentration at the LAI. Our results are important for the understanding of the formation of metal oxide nanosheets at the LAI and to guide further research on the assembly of 2D materials using deposition techniques such as Langmuir-Blodgett deposition. ASSOCIATED CONTENT


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The_Supporting_Information is available free of charge at http://pubs.acs.org. Details concerning synthesis of calcium nibate and iron titanate nanosheets and their LUP measurements, pressurearea isotherms and UV-Vis spectra curves and additional data. AUTHOR INFORMATION Corresponding Author * Correspondence to: [email protected]. Phone +31 53 489 2695. Present Addresses †

Energy Research Institute at Nanyang Technological University (ERI@N), Research Techno

Plaza, X-Frontier Block Level 5, 50 Nanyang Drive, Singapore 637553, Singapore Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge financial support from the China Scholarships Council (CSC, No.2011704003). We thank The Netherlands Organization for Scientific Research (NWO) for beam time at the ESRF DUBBLE beamline. We thank Dr. G. Portale, Dr. D. Hermida Merino and Dr. W. Bras from DUBBLE for support and on-site assistance. REFERENCES


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Figure captions Figure 1. Surface pressure – surface area isotherm of a dilute nanosheet solution. The lift-up point in this curve is at 67%. In stage 1, free floating isolated nanosheets at the LAI; surface pressure is low and increases slowly with decreasing working surface area; In stage 2, the nanosheets are so close to each other that they interact at the interface by steric effects; surface pressure increases more rapidly with decreasing working surface area; In stage 3, a dense nanosheet film has formed at the LAI; surface pressure is more or less constant. Figure 2. The schematic of experimental procedure of LUP measurements. Figure 3. (a) Lift-up point (LUP) and (b) UV-vis absorbance at 265 nm of diluted suspensions at TBAOH/H+ molar ratios of 1/32 to 8/1 after a reaction time of 2 h. Figure 4. (a) The lift-up point and (b) UV-Vis absorbance of suspensions after different reaction times. The TBAOH/H+ molar ratio was 4/1. Figure 5. Time-resolved SAXS curves in the q-range 0.2 – 4 nm-1 at TBAOH/H+ molar ratios of 4/1 and a nominal nanosheet concentration of 5 g/L. (a) first 3 frames: the injection time was 8 s and started at t = 0, the red and blue curves were recorded at t = 2 and 32 s, respectively; (b) the full data set covering 250 min. Figure 6. (a) The lift-up point and (b) pH of suspensions after different reaction times in air. The TBAOH/H+ molar ratio was 1/1. Nominal nanosheet concentration 5 g/L.


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Figure 1. 85x67mm (300 x 300 DPI)



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Figure 2 203x76mm (300 x 300 DPI)


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Figure 3 121x49mm (300 x 300 DPI)



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Self-Assembly of Metal Oxide Nanosheets at ... - ACS Publications

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