from vaterite spheroids and shells to ZnO flakes and hexagonal

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Precipitation of inorganic phases through a photoinduced pH jump: from vaterite spheroids and shells to ZnO flakes and hexagonal plates Tan-Phat Huynh, Carsten Pedersen, Nina Kølln Wittig, and Henrik Birkedal Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.8b00093 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 7, 2018

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Crystal Growth & Design

Precipitation of inorganic phases through a photoinduced pH jump: from vaterite spheroids and shells to ZnO flakes and hexagonal plates Tan-Phat Huynh, Carsten Pedersen, Nina Kølln Wittig, Henrik Birkedal* Department of Chemistry & iNANO, Aarhus University, 14 Gustav Wieds Vej, 8000 Aarhus, Denmark

ABSTRACT. This report demonstrates a new way to precipitate inorganic phases through pH jumps driven by optical excitation of a photobase. The level of the pH jumps is manipulated by either the wavelength range or illumination time used. The method is demonstrated by precipitation of CaCO3 and ZnO. Vaterite spheroids and shells for CaCO3 as well as flakes and hexagonal plates for ZnO are formed under various controllable conditions. Notably, ZnO films could be formed and patterned directly on quartz substrates using a photomask. The film thickness was easily controlled by illumination time. This method is an important step towards self-organized crystallization with spatio-temporal control fully in the hands of the experimenter.

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Controlled crystallization remains a challenge. New crystallization pathways have emerged recently, including crystallization through particle aggregation1, polymeric precursors2, amorphous precursor phases3-5, self-organization phenomena6-8, or through the use of additives. Additives may become occluded into the crystal to profoundly alter its properties9,10 or drastically modify the crystallization pathway11-13. Driving crystallization can effectively be done through a change in supersaturation caused by changes in physical parameters such as temperature or pressure.14,15 However, for compounds whose speciation in solution depends on pH, a pH change can be a very effective way of inducing supersaturation. This can be done rapidly by a pH jump16. Typically, such jumps have been done by rapid mixing of the target solution with concentrated acid or base16-20. Another way to induce rapid changes in pH in solutions is by the use of photoacids21 or -bases22-25. These molecules release either protons or hydroxide ions upon excitation by light. A notable advantage of the use of photoinduced pH jumps is that the light beam can be localized in space and time allowing for spatio-temporal control, thus potentially opening a new door for self-organized crystallization akin to light driven reaction diffusion systems26. For example, a photoacid was recently used to control the selfassembly behavior of carboxylic acid/carboxylate coated nanoparticles through inducing a change in the protonation state (and hence surface charge) of the nanoparticles in methanol27. To the best of our knowledge, however, this type of chemistry has not been harnessed in aqueous chemistry or for the formation of inorganic materials before. Similar research using photobases to increase pH is even more limited. Here we use the photobase malachite green carbinol base (MGCb) (Figure S1) to induce a pH jump in aqueous solution through the release of a hydroxyl ion under UV irradiation (Eqn. 1).

MGCb → MGCb+ + OH

(1)


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We use this process to drive the formation of two inorganic materials, vaterite CaCO3, that we show can be then etched into porous shells in situ, and ZnO hexagonal plates that we demonstrate can be formed as patterned films by using spatial control over the illumination. The MGCb photobase has several absorption bands as shown in Figure 1a. Broad wavelength (280-450 nm) illumination leads to a rapid increase in pH from 6.2 to 9.6 (Movie 1), but accompanied by a significant temperature increase from 20 to over 36 °C in the span of 1 min of irradiation.

Figure 1. Behavior of the photobase MGCb used in this study to induce pH jumps. (a) Absorption spectrum of MGCb in water at 20 °C, pH 6.2. The stripes in the figure indicate how different wavelength ranges result in pH changes as indicated by the maximum pH (pHmax) and temperature values induced by illumination. (b) The dependence of pH on time for a MGCb solution illuminated for 1 min with 280-340 nm light. The inserted images show the correlation between the color and pH of the MGCb solution.

Using filters to restrict the wavelength range (and reduced the light intensity), the temperature increase could be avoided. Indeed, using an optical filter of 350-450 nm (Figure 1a, yellow band) gave only a temperature increase but no appreciable pH change. Restricting the illumination with


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an optical filter of 280-340 nm, on the other hand, gave a pH jump to 9.2 with almost no temperature increase (Figure 1a, green band); therefore all subsequent experiments employed this wavelength range to ensure that pH jumps could be induced without concomitant heating effects. Following illumination for 1 min, the pH of MGCb increased rapidly over a few minutes, Figure 1b, to a maximum of 9.2, corresponding to a thousand-fold increase in hydroxide concentration. This is sufficient to drive many precipitation reactions. When the light was turned off, pH continued to increase (with the rate of 1.25 min-1) for another 5 min because the hydroxide liberation is a slow process compared to the optical excitation.22 Thereafter, the pH value slowly dropped (with a rate of -0.04 min-1) as MGCb reformed under consumption of hydroxide.28 This recombination was much slower than the original pH increase and not complete within an hour. This is an advantage as it allows for rapid pH increases followed by a slow decline leaving time for harvesting and/or processing products (vide infra). MGCb is the key component in solution and is responsible for the maximum pH of the solution; without MGCb in the solution, no pH change occurred (data not shown). We used the photoinduced base generation to drive the formation of CaCO3 following Eqn. 2

+ MGCb +Ca + HCO → MGCb + CaCO3 (s) + H2 O

(2)

We worked with 4 mM solutions of calcium and hydrogen carbonate. Such solutions are undersaturated with respect to all calcium carbonate polymorphs at pH 6.2 and become supersaturated with respect to vaterite, aragonite and calcite at pH 7.3, 6.9 and 6.8, respectively. At pH 9.2, the relative supersaturation with respect to vaterite is 9·103 reflecting the huge change in reactivity that can be attained by the present method. In a first series of experiments, we varied


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the target pH by varying the concentration of MGCb (Figure 2) and keeping the irradiation time constant at 4 min. Concentrations of 1.2 mM, 2.4 mM and 3.6 mM MGCb were used. Because MGCb in itself is a weak base, the starting pH was 0.3 and 0.5 pH units higher for 2.4 mM and 3.6 mM solutions, respectively, compared to the 1.2 mM solution. As a result, the solution became little opaque at MGCb concentrations of 2.4 and 3.6 mM due to some formation of CaCO3, which we removed by centrifugation, prior to illumination. Similar crystalline products were obtained after irradiation in all cases as determined by SEM and FTIR (Figure 2). The products are mostly spheroidal vaterite particles29 with a few rhombohedral calcite crystals30,31 (Figure 2a-c). The calcite likely results from transformation of preformed vaterite particles This transformation of vaterite to calcite can be explained by a surface-controlled process driven by the growth of the calcite.32 During transformation, the amount of vaterite decreased and calcite crystals formed on the surface of the vaterite aggregates. This has been shown32 to occur through a dissolution and reprecipitation mechanism. Due to the very low amount of calcite in the overall precipitates (Figure S2), only vaterite absorption bands 33,34 at 878 and 748 cm-1 were seen in IR spectra (Figure 2d). The spheroidal vaterite morphology has been observed by others in experiments where pH was controlled by classical base addition methods30,31, through stabilization by various polymers35,36, or via direct precipitation from supercritical CO237 or rapid mixing of aqueous solutions38. We then studied the dependence of phase formation on illumination time (Figure S3). At the irradiation time less than 2 min (Figures S3a and S3b), only vaterite was formed, i.e. the pH jump was not high enough to drive the formation of calcite. When the irradiation time was about 4 min, either using continuous (Figure S3c) or pulsed (Figure S3d) illumination, some calcite


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together with large portions of vaterite were observed. This illustrates the potential of driving polymorph selection at least to some degree through timing of the illumination profile.

Figure 2. SEM images of CaCO3 precipitates featuring spheroidal vaterite particles and rhombohedral calcite crystals prepared from solutions at (a) 1.2 mM, (b) 2.4 mM, and (c) 3.6 mM MGCb. Scale bar in SEM images is 5 µm. (d) IR spectra of CaCO3 precipitates prepared from solutions at (1) 1.2 mM, (2) 2.4 mM, and (3) 3.6 mM MGCb.

Due to the quasi-reversibility of MGCb, the pH of the solution will slowly return to neutral after UV-illumination is stopped (Figure 1b) with the precipitation solution becoming neutral in pH after 30 min. The rapid pH increase, leading to particle formation followed by a slower pH decrease, allows a new type of control over particle morphology through what we term dynamic etching: The vaterite particles were etched in a controlled way by remaining in the solution for a selected amount of time after illumination as shown in Figure 3. The particles


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etched from the inside out. This phenomenon is attributed to dissolution of the central region of the vaterite spheroids.39 Hollow shells with open top and bottom were obtained after 2 h etching, while 5 h etching resulted in particle belts (Figure 3b and 3c). The etching is accompanied by an overall reduction in the amount of vaterite as evidenced by FTIR spectra where the carbonate bands progressively lose intensity as the etching time increases (Figure 3d). This self-etching behavior of the solution affords a way of controlling the morphology of the particle aggregates simply through the residence time in solution after illumination.

Figure 3. SEM images of CaCO3 precipitates prepared from solutions at (a) zero, (b) 2 h, and (c) 5 h delay time. Scale bar in SEM images is 5 µm. (d) IR spectra of CaCO3 precipitates collected from solutions at (1) zero, (2) 2 h, and (3) 5 h delay time.

Due to the dynamic etching in water, the vaterite spheroids manifest themselves as a potential material for uptake and release of molecules such as the model-cargo calcein, which is


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a fluorescent dye.40 Upon dispersing calcein-bound vaterite spheroids (Figure 4a and see the Experimental section for more details) into water, calcein was quickly released as probed by fluorescence upon excitation at 500 nm (Figure 4b). The emission intensity increased for higher concentrations of calcein in the precursor solution prior to photoinduced precipitation.

Figure 4. Release of calcein bound to vaterite particles. (a) Suspension of calcein-binding vaterite particles in 2-propanol and (b) fluorescence spectra of corresponding aqueous solutions after injection of calcein-bound vaterites into water in which particles with 0 (1), 0.2 (2) and 1 mM (3) calcein present during particle synthesis. Excitation at 500 nm.

We then extended the present method beyond CaCO3 formation by forming ZnO from zinc acetate Zn(OAc)2. We chose Zn(OAc)2 as a precursor instead of e.g. Zn(NO3)2 or ZnCl2 due to its weak acidity that helps stabilizing MGCb without requiring adjustment of pH of the solution. Illumination resulted in the formation ZnO precipitates both in the solution and on the wall of the quartz cuvette. SEM revealed that flake-like ZnO precipitates with a few nm in thickness (Figure 5a and S4) formed in solution, a common product from ZnO precipitation from Zn(OAc)2.41,42 Interestingly, the crystals grown on the quartz substrate, on the other hand, had


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hexagonal plate- and drum-like structures (Figure 5b). The hexagonal plates, which in some cases were so thin that they were almost electron transparent in the SEM (Figure 5b, arrow) are believed to appear at first. These hexagonal plates might then combine with each other towards development of the drum-like structures structures.43-45

Figure 5. Photoinduced pH jump formation of ZnO. SEM image of (a) a drop-cast collection of flake-like ZnO precipitates from obtained solution and of (b) a film of the plate- and drum-like ZnO formed crystals on the quartz substrate. Scale bar is 5 µm. (c) Patterned crystallization on a substrate. By inserting a photomask (left), a macroscopic pattern spelling AU of precipitates formed on the surface of the quartz cuvette was obtained (right). The film was stable under cleaning conditions, such as washing the cuvette with water, ethanol, or acetone.


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The ZnO film that formed on the quartz substrate after irradiation (Figure 5c) was not seen in the case of CaCO3 because the concentration of its precursor was much lower (4 mM for CaCl2·2H2O and NaHCO3) than that of Zn(OAc)2 (0.1 M). This formation of a ZnO film suggests that it will be possible to use the method for patterned precipitation, i.e. direct writing of desired patterns of crystallization onto a substrate or in a medium. To demonstrate this ability, we used a photomask containing the letters “AU” as a pattern. As a result, “AU” ZnO was patterned when light passed through the photomask (Figure 5c). Varying the exposure time lead to patterned precipitates with a particle density in the crystal-containing regions that scaled with the illumination time showing that it is possible to locally control supersaturation to a degree that allows ‘writing’ patterns of crystals. The resolution of the patterns formed in the present experiments are modest (Figure 5c) due to the large scale of the pattern used in this demonstration experiment. We expect much better resolution to be attainable by using masks with finer details, thinner quartz walls to reduce scattering effects and possibly using focused light to ensure that the pH increase only will be initiated just behind the cuvette wall. These effects will be explored in future work. In conclusion, we demonstrated the usefulness of the present method of photobase driven precipitation for two different inorganic compounds. The pattern formation observed for ZnO shows that this method will be very useful for generating patterns. By incorporation focal point control, we expect that it should be possible to control crystallization in 3D by scanning the focal point through a medium as known e.g. from two photon polymerization46-48. Furthermore, the combination of the present photoinduced pH jumps with interfacial crystallization systems, such as Langmuir-Blodgett films or self-assembled monolayers, can be expected to provide spatialtemporal control of crystallization at a new level. We are therefore convinced that this method is


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an important step towards self-organized crystallization with spatio-temporal control fully in the hands of the experimenter. ASSOCIATED CONTENT The following files are available free of charge.

Procedures for precipitation of inorganic phases, UV-irradiation setting, spectroscopic characterization, SEM, TEM, and chemical formula of MGCb (PDF).

AUTHOR INFORMATION Corresponding Author *[email protected]. ACKNOWLEDGMENT T.P.H. acknowledges the Carlsberg Foundation's Distinguished Postdoctoral Fellowship (Grant No. 21903). Funding from the Villum Foundation (Grant No. 17553) is gratefully acknowledged. We also thank Assoc. Prof. Victoria Birkedal for access to the fluorimeter. Affiliation with the integrated materials research center (iMAT) at Aarhus University is gratefully acknowledged. REFERENCES 1. De Yoreo, J. J.; Gilbert, P. U. P. A.; Sommerdijk, N. A. J. M.; Penn, R. L.; Whitelam, S.; Joester, D.; Zhang, H.; Rimer, J. D.; Navrotsky, A.; Banfield, J. F.; Wallace, A. F.; Michel, F. M.; Meldrum, F. C.; Cölfen, H.; Dove, P. M.; Crystallization by particle attachment in synthetic, biogenic, and geologic environments Science 2015, 349, aaa6760.


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For Table of Contents Use Only Precipitation of inorganic phases through a photoinduced pH jump: from vaterite spheroids and shells to ZnO flakes and hexagonal plates Tan-Phat Huynh, Carsten Pedersen, Nina Kølln Wittig, Henrik Birkedal*

Inorganic phases are precipitated through pH jumps driven by optical excitation of a photobase. We use the method to form vaterite spheres and shells as well as ZnO flakes and hexagonal plates under various controllable conditions. Patterned ZnO films (images to the right) were formed using a photomask. This method is an important step towards self-organized crystallization with spatio-temporal control.


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from vaterite spheroids and shells to ZnO flakes and hexagonal

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