Growth of potassium sulfate crystals in the presence of organic dyes: in situ characterization by...

*Corresponding author. Tel.:#39-0270635120; fax:#39-0270635288.

E-mail address: [email protected] (M. Moret)

Journal of Crystal Growth 208 (2000) 599}614

Growth of potassium sulfate crystals in the presence of organicdyes: in situ characterization by atomic force microscopy

Andrea Mauri, Massimo Moret*

Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Universita% di Milano, Via G. Venezian 21, I-20133 Milano, Italy

Received 21 June 1999; accepted 26 July 1999Communicated by R. Kern

Abstract

In situ atomic force microscopy (AFM) has been used to observe potassium sulfate crystals growing in the presence ofacid fuchsin and pyranine. These polysulfonated dyes are well known for their ability to adsorb onto the M1 1 0N andM0 1 0N (pyranine only) crystal faces. Using AFM, we analyzed the changes in surface micromorphology induced by theadditives on advancing steps for the M1 1 0N and M0 1 0N surfaces. In situ AFM showed that layers grow by step #ow atpre-existing steps by the addition of growth units at the step edges. It has been found that dye concentrations as low as&2]10~6 M for pyranine and &4]10~4 M for acid fuchsin produce signi"cant changes in the step morphology andgrowth rates. The additive molecules attach to the terraces and pin the growing front. As a consequence, the edges of thegrowing steps become jagged as the dye molecules are adsorbed onto the crystal surface. At critical dye concentrationscrystal growth is heavily hampered or even stopped along certain crystallographic directions producing, on a macro-scopic scale, strong habit modi"cations. The formation of dye inclusions by means of macrosteps overgrowing thepoisoned surface was also imaged. Interestingly, comparison of the in situ AFM experiments with previous habitmodi"cation studies showed acid fuchsin is also able to enter the M0 1 0N surfaces, a previously unnoticed phenom-enon. ( 2000 Elsevier Science B.V. All rights reserved.

PACS: 61.16.Ch; 61.72.!y; 81.10.Aj; 81.10.Dn

Keywords: In situ atomic force microscopy; Crystal growth; Habit modi"ers; Potassium sulfate; Surface topography

1. Introduction

The advent of scanning probe microscopy [1,2]has allowed the study of surface features with un-precedented resolution. The atomic force micro-

scope (AFM) has been, since its invention in 1986[2], successfully applied to the analysis of surfacestructure and topography of a great variety of non-conducting materials [3]. Improving the basic in-strumental set-up with liquid cells has introducedthe possibility to image surfaces (e.g. of crystals)in situ under controlled ambient conditions. There-fore, crystal growth/dissolution phenomena, play-ing a key role in "elds such as geochemistry (oreformation, mineral weathering), materials science

0022-0248/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved.PII: S 0 0 2 2 - 0 2 4 8 ( 9 9 ) 0 0 4 0 1 - 7

(tailoring of materials, corrosion) and chemicaltechnology (control of polymorphism/crystal habitof pharmaceuticals) are accessible through AFMimaging. Moreover, owing to AFM's high spatialresolution, fundamental crystal growth theories canbe assessed directly on a nanoscale.

In general, the interface between a bulk solutionand a crystal face is hard to access by directmeasurement, but several authors have recentlydemonstrated that in situ AFM studies are usuallynot a%icted with surface artifacts (such as growthfeatures due to fast and uncontrolled evaporationof the mother solution when extracting the crystalsfrom the growth cell [4]) or su!er less than ex situanalysis where adsorbed water can stimulate stepsmovement [5,6]. The relatively slow temporal res-olution of AFM (tens of seconds per image) limitsits applications to growth/dissolution processeswith slow enough kinetics. Recently, some progressto decrease the time needed to capture images dur-ing crystal growth has been made [7]. Thus, withthe analysis of crystal growth in real time andin situ, AFM surpasses the previous in situ tech-niques of optical microscopy and interferometry.

In recent years, an increasing number of studieshave paid attention to the micromorphology ofsingle-crystal surfaces grown in situ at low or mod-erate supersaturations. Pioneering experimentsdealt with calcite [8], gypsum [9], biomolecules[10] and inorganic salts [11]. Interestingly, theinteraction of the growing surface with additives(habit modi"ers or growth poisons) appeared earlyamong the studied systems, especially for the indus-trially important calcite [12]. Indeed, a thoroughknowledge of the crystal growth processes down tothe nanoscale is valuable in understanding how toinhibit crystallization (scale deposits in pipes) [13]or control the crystal morphology (biomineraliz-ation, caking, drug polymorphism) [14]. Signi"cantresults in this "eld could help reconcile growthmodels based on a thermodynamical approach(such as the PBC analysis [15] or attachment en-ergy models [16]) with kinetically driven crystalli-zation.

Generally, the in#uence of additive adsorptionand incorporation on the particular growth mecha-nism for each crystal face has not yet been fullyelucidated. Recently, several authors brought to

prominence the seminal work by Bunn [17], Whet-stone [18] and particularly Buckley [19] in the "rstdecades of this century about the interactions be-tween inorganic salts and organic dyes. A greatamount of phenomenological data has been ac-quired in those times but, without enough struc-tural and stereochemical information derived fromX-ray di!raction, this kind of research remainedforgotten until recently [20].

Among many historically relevant systems in therealm of organic inclusions by inorganic salts, forwhich there have recently been a renaissance, po-tassium sulfate grown in the presence of sulfonateddyes (acid fuchsin and pyranine) is a well suited caseowing to the amount of data already obtained byBuckley [19], and Kahr and coworkers [21}23].The latter research group has demonstrated theincorporation of organic molecules within inor-ganic host crystals of simple salts can producesolid-state dye lasers with linearly polarized light[24]. Similar interactions can produce promisingcandidates for use in nonlinear optical devicesthrough local symmetry reduction [25]. Moreover,crystallization of K

2SO

4in the presence of foreign

substances is an important process still under care-ful examination by several research groups[26}28].

In this study we present the results of in situAFM experiments on the growth of potassium sul-fate crystals in the presence of organic dyes, inorder to characterize their in#uence on the growthmechanism and crystal habit, and identify the sur-face}molecule interaction mechanism.

2. Experimental procedure

2.1. Specimen preparation

Reagent grade potassium sulfate (Fluka), acidfuchsin disodium salt (Fluka), polysulfonatedpyrene dyes (pyrene-1,3,6,8-tetrasulfonic acid tetra-sodium salt and pyrene-1-hydroxy-3,6,8-trisulfonicacid trisodium salt) (Lambda Fluoreszenztech-nologie, Graz, Austria) were used without furtherpuri"cation and dissolved in deionized water. Allsolutions were "ltered through cellulose nitratemembrane "lters with 0.2 lm pore size (Whatman

600 A. Mauri, M. Moret / Journal of Crystal Growth 208 (2000) 599}614

Ltd., Maidstone, UK). Crystals suitable for mor-phological analysis and in situ growth experimentswithin the AFM #uid cell were obtained by slowevaporation of saturated solutions at room temper-ature or by slow cooling of saturated solutions.

Determination of dye content within the coloredgrowth sectors was accomplished by cutting witha razor blade, weighting and dissolving the zonalsectors followed by UV}VIS photometry.

2.2. Atomic force microscopy

For in situ AFM analysis pure K2SO

4crystals

were extracted from the crystallization vessel andquickly transferred to a standard #uid cell set-upcomprising a stainless-steel cup topped with anO-ring seal. The #uid cell provided a watertightchamber of about 50 ll, able to accommodate crys-tals with thickness up to 3 mm. Before starting anin situ growth experiment surface regeneration wasaccomplished directly in the #uid cell in order torestore the surface morphology and diminish thechance of introducing artifacts (even if crystalscarefully withdrawn from the mother liquor andquickly dried with "lter paper showed no signi"-cant surface damage or alteration of the typicalmorphology).

A Nanoscope III AFM (Digital Instruments,Santa Barbara, CA) equipped with a J-type piezoscanner was operated in contact mode. The x andy sensitivities of the scanner were calibrated byusing a reference grid with 10 lm pitch along x andy. The z piezo sensitivity has been calibrated by thesame grid bearing 180 nm deep grooves; for mo-lecular scale features a (0 0 1) muscovite surface wasetched with HF [29]; the steps obtained were usedto correct nonlinearity e!ects of the piezo responseand to calibrate the z sensitivity. Silicon or siliconnitride cantilevers with integrated tips and nominalspring constants of 0.06}0.12 N/m were used. Thetip}sample contact force was reduced as much aspossible down to few nN to minimize artifacts andsurface erosion phenomena. The scan frequencywas in the range 3}6 Hz; integral and proportionalgains chosen allowed z measurement together withde#ection (error) signal for better image contrast.All images shown in this paper represent un"lteredde#ection data.

2.3. In situ growth experiments

Owing to the high solubility of K2SO

4in water

(about 11% w/w at 253C) [30], low supersatura-tions are needed in order to obtain low crystalgrowth rates, otherwise step movement is too fastcompared to the AFM image acquisition time.Moreover, temperature changes due to laser radi-ation in the volume surrounding the cantileverwere observed by us and previously reported byother authors [31,32] and caused severe problemswhen trying to grow K

2SO

4crystals. In fact, within

the sealed #uid cell and with a stagnating saturatedK

2SO

4solution, we observed dissolution of the

crystal in the area irradiated by the laser spot(comprising therefore the scan area) while crystalgrowth occurred for the other regions of the crystalsurface. We estimated 3}43C is the typical tem-perature increase in the volume surrounding thecantilever in accordance with the values of 3.93Cdeduced from cantilever deformation by Wenzleret al. [33] and '43C by Kipp et al. [31]. Todissipate the laser-generated heat we grew the crys-tals by #owing the solution through the #uid cell.A supersaturated K

2SO

4solution #owed continu-

ously from a 250 ml thermostatted reservoirthrough the cell, using a gravity feed with control ofthe #ow rate. Flow rates of 5}20 ll/s through theAFM cell were enough to partially compensate thelaser thermal e!ect. No degradation of the imagequality was observed during #ow. The solutionsupersaturation was estimated from the temper-ature dependence of K

2SO

4solubility [30]; nom-

inal p"0.02}0.04 was set with p"(C!C%2

)/C%2

,where C and C

%2are the actual and the equilibrium

concentration, respectively. The actual super-saturation in the #uid cell was reduced by an un-known amount by the aforementioned thermale!ects. By using this experimental set-up we wereable to keep the composition of the solution con-stant for pure K

2SO

4growth experiments while it

was possible to add the dye at the desired concen-tration.

2.4. Time-series images analysis

Analysis of the AFM frames was performed withSPIP software [34] for height, distance and angle

A. Mauri, M. Moret / Journal of Crystal Growth 208 (2000) 599}614 601

measurements. Surface microtopography imageswere acquired during growth experiments, sub-sequently converted into animated sequences anddisplayed and analyzed by means of the programImage Tool [35].

3. Results

3.1. Crystal morphology and surfacemicrotopography

b-K2SO

4crystallizes at room temperature with

an orthorhombic structure [36] (space group Pmcn,no. 62, axes chosen to agree with Buckley [19] andKahr [21]) with a hexagonal pseudo-symmetrydown the c-axis which originates aragonitic twinn-ing [37]. At temperatures around 840 K b-K

2SO

4undergoes a phase transition to hexagonal a-K

2SO

4(space group P6

3/mmc, no. 194). For b-K

2SO

4the

usually observed forms for crystals grown slowlyfrom aqueous solutions are M0 0 1N, M0 1 1N, M0 2 1N,M0 3 1N, M0 1 0N, M1 1 0N, M1 3 0N, M1 0 0N, M1 1 1N andM1 1 2N [37], usually with M0 2 1N, M1 1 0N, M0 1 0N,and to a lesser extent M1 3 0N, being the dominantforms.

The presence of such a variety of forms makesthis substance very interesting for face speci"c de-termination of surface topography and growth fea-tures. The various faces exhibit di!erent growthrates and topographies according to their di!erentstructure at the molecular level and peculiargrowth mechanism. The main features observed onthe dominant faces are described here, whilst a de-tailed analysis of the growth features for K

2SO

4crystals will be the subject of a forthcoming paper[38]. The M1 1 0N and M0 1 0N surfaces are typicallysmooth in appearance and of good optical quality,while M0 2 1N faces on a macroscopic scale alwaysexhibit striations [39]. AFM showed a complexM0 2 1N surface with heavily interwoven small tri-angular terraces. The steps were too closely spacedto follow individual steps from scan to scan duringin situ growth experiments. Due to its microtopo-graphic complexity and the absence of interactionswith organic dyes this face will not receive furthercomment in the present paper. Optical microscopywith re#ected-oblique illumination and scanning

electron microscopy showed M1 1 0N and M0 1 0Nsurfaces grew by a spiral mechanism together withstep nucleation (birth and spread of new terraces)on the edges and corners of the crystals, possiblywith wrapping of steps from adjacent faces acrosscrystal edges [40]. During in situ AFM experi-ments at low supersaturations spontaneous 2D sur-face nucleation on terraces has never beenobserved, a step-#ow growth mechanism was pres-ent, with species deposition leading to a lateralgrowth of the steps on the surface. Step nucleationwas sometimes observed, but only at topographicimperfections such as cracks, foreign particles or onsurfaces exposed to fast evaporation of the solventwhen the crystal was extracted from the solution[4]. Emergence of screw dislocations providedcontinuous sources of steps obviating the needfor nucleation of new layers. Usually M1 1 0N andM0 1 0N faces displayed few active dislocations (nor-mally only one or two on each face) spreading stepson the whole surface. On crystals grown at lowsupersaturation, such as in the present case, a singlehillock usually dominated the entire crystal face.The step trains originating at defect centers couldnot be imaged entirely even in a 150]150 lm2 scanarea from the source region towards the endboundaries. Vicinal faces of growth hillockschanged to macrosteps when moving away fromthe center of the outcrop; macrosteps were com-monly observed on both M1 1 0N and M0 1 0N, butwere more evident on M1 1 0N due to the greatermorphological importance of this face compared toM0 1 0N which seldom becomes very large. The ter-races between macrosteps are shown by AFM tocontain step trains with height of a few nanometersdown to the basic layer thickness of 5.007 and 5.035(d

0 2 0) As for M1 1 0N and M0 1 0N, respectively.

The distribution of growth centers, the orienta-tion of steps, step height and spacing, and "nallythe density of kink sites de"ne an overall surfacetopography which is characteristic of each of theforms appearing in the crystals. In Figs. 2 and 5 arethe topographic features repeatedly observed onthe M1 1 0N and M0 1 0N faces. In situ AFM hasshown layer growth on M1 1 0N faces occurs bytangential movement of rounded terraces severallm wide. On a larger scale these faces are domin-ated by terraces de"ned by macrosteps. The M0 1 0N


Fig. 1. K2SO

4crystals showing the progressive habit modi"cation produced by increasing amounts of acid fuchsin (dye/salt molar

ratio). (a) pure salt; (b) 1 : 2000; (c) 1 : 1000; (d) 1 : 500; (e) 1 : 250; (f )'1 : 250. Figs. 1a}b were taken almost perpendicularly to M0 2 1Nwhile Figs. 1c}e shows the M0 1 0N face parallel to the plane of paper. In Fig. 1b acid fuchsin was included only in the M1 1 0N sectors, whilestarting from Fig. 1c the dye adsorbed also on M0 1 0N. Vertical orientation of the [1 0 0] direction is maintained throughout Figs. 1a}e.

form, on the contrary, exhibits smaller terraceselongated on [1 0 0] and mimicking the overallcrystal morphology of pure K

2SO

4(Fig. 1a).

3.2. K2SO4 crystal growth in the presence of dyes

b-K2SO

4has been carefully studied by Buckley

during his work on the interactions between in-organic crystals and organic molecules, focusedparticularly on dye incorporation and habit modi"-cation (Ref. [19], Chapter 10). In many cases dyeadsorption gives rise to the so-called organic hour-glass inclusions (HIs), i.e., the dye is adsorbed onto

speci"c growth sectors having the appearance ofhourglasses or Maltese crosses, depending on thecrystal symmetry and forms involved. Buckley no-ticed a strong adsorption of acid fuchsin into theM1 1 0N growth sectors only. He also observed anincreasing morphological importance of M0 1 0Ndue to increasing amounts of acid fuchsin in solu-tion, not followed, however, by dye incorporationinto the corresponding growth sectors. Buckley wasthe "rst to attempt a stereochemical correlationbetween the host crystal structure and the func-tional groups present on the habit modi"er. Sup-posing that the stereochemical key to enter the


K2SO

4lattice is the presence of negatively charged

sulfonate groups together with a good geometricalmatch between the guest molecule and the un-derlying 2D surface periodicity [22,41], other dyesare potentially able to enter the K

2SO

4lattice. In-

deed, for polysulfonated pyrene dyes like pyra-nine (pyrene-1-hydroxy-3,6,8-trisulfonic acid) andpyrene-1,3,6,8-tetrasulfonic acid, the interaction be-tween the organic molecule and the host lattice iseven stronger than acid fuchsin and involves boththe M1 1 0N and M0 1 0N growth sectors [22]. It must berecalled that these forms are pseudosymmetric with2D rectangular meshes of 1.607

*1 11 0+/7.478

*0 0 1+and 5.770

*1 0 0+/7.478

*0 0 1+As , respectively (doubling

the 5.770 As parameter of M0 1 0N, gives a 0.6% dif-ference between the two unit meshes). The highera$nity of sulfonated pyrenes for the M1 1 0N andM0 1 0N sectors compared to acid fuchsin canbe ascribed to their rigid molecular frameworkwhich gives a better structural "tting betweenthe SO~

3groups and the SO2~

4ions of the host

lattice.The di!erent behavior of acid fuchsin and pyra-

nine can be quantitatively evidenced by their segre-gation coe$cient, i.e. the ratio [dye]/[K

2SO

4]

observed in the crystal sectors over the ratio insolution (due to the large volume of solution com-pared to the amount of crystals extracted it isreasonable to assume that the [dye]/[K

2SO

4]

ratio in solution remained constant during the crys-tallization process). For acid fuchsin the [dye]/[K

2SO

4] ratio in the M1 1 0N crystal sector is about

half the ratio in solution for the range of dye con-centrations explored, while for pyrene dyes theratio found in the crystal is always higher than inthe solution. During crystal growth in the presenceof pyranine a fading of the #uorescent yellow of thesolution was observed as the crystals continuedto grow. A similar result has been observed forK

2SO

4and dye National Fast Wool Blue (Ref.

[19], p. 371), with K2SO

4acting like a sponge and

capturing all the dye molecules available in solu-tion.

3.3. The ewect of acid fuchsin on crystal habit

Before attempting in situ AFM experiments were-examined the e!ect on crystal habit of crystalliz-

ing K2SO

4in the presence of the aforementioned

dyes. Analogously to other HIs, acid fuchsin, pyra-nine and pyrenetetrasulfonate inclusion crystals actlike good single crystals for X-ray di!raction, evenif pyrene dyes cause a substantial lattice straingiving sectorial contrast in X-ray topography[42,43] and an increase in the number of macro-scopic imperfections.

Our growth experiments were designed to spana range of [dye]/[K

2SO

4] ratios large enough to

reach poisoning and completely stop growth of thecrystal forms involved in dye incorporation. Foracid fuchsin this happens at about 1 dye moleculeper 250 K

2SO

4units. In Fig. 1 the typical crystal

habit is shown for increasing concentrations of acidfuchsin. Fig. 1a shows a pure K

2SO

4crystal slight-

ly elongated on [1 0 0] and displaying large M1 1 0Nand M0 2 1N faces; upon addition of acid fuchsin atdi!erent concentrations (Figs. 1b}f ) the crystals de-velop increasingly colored M1 1 0N sectors togetherwith a reduction of the [1 0 0]/[0 0 1] aspect ratio.When reaching a fuchsin/K

2SO

4ratio of about

1 : 250 the crystals which were previously wellshaped start to show some imperfections and ap-pear as many as 2}5 times more elongated on[0 0 1] than [1 0 0] (Fig. 1e). With a ratio exceeding1 fuchsin molecule per 250 K

2SO

4units only sev-

eral centimeters long and thin needles are produc-ed, the long morphological axis being again [0 0 1](Fig. 1f ). As will be discussed later, growth stops onM1 1 0N and M0 1 0N faces while only M0 2 1N growthcontinues unperturbed, giving rise to needles withdominant M1 1 0N and M0 1 0N faces. Thus, acidfuchsin produces M1 1 0N sectorial zoning anda marked modi"cation of the crystal habit, espe-cially for the crystal aspect ratio along the [1 0 0]and [0 0 1] directions.

3.4. In situ AFM growth experiments

In order to elucidate the mechanism of interac-tion between acid fuchsin or pyranine and the crys-tal forms forming hourglass inclusions, severalgrowth experiments were performed in the AFM,with e!orts to estimate the minimum dye concen-tration necessary to induce a signi"cant variationof the microtopography. All combinations ofM1 1 0N and M0 1 0N forms and additives were


checked, even if for acid fuchsin and M0 1 0N there isno previous report about HIs formation. Severalexperiments showed that the alteration of thegrowth processes observed and discussed in thenext sections were reproducible.

3.4.1. Acid fuchsin and the M1 1 0N surfaceAccording to macroscopic observation on crys-

tals grown in the presence of acid fuchsin theremust be a signi"cant interaction between theM1 1 0N faces and this dye. During typical in situAFM growth experiments, the fuchsin concentra-tion was in the range 0}3.76]10~4 M, correspond-ing to a maximum fuchsin/K

2SO

4molar ratio of

1 : 1700.As shown in the sequence in Fig. 2 (scan area

45]45 lm2), K2SO

4growing at low supersatura-

tion exhibited large terraces separated by 3}5 lmand step height of &0.5}3 nm (Fig. 2a), i.e. stepscomposed of 1}6 elementary layers. We observeda step #ow mechanism with tangential movementof the terraces while growth units were incorpor-ated at pre-existing steps. On adding acid fuchsin3.35]10~4 M (i.e., a dye/salt molar ratio of1 : 1900) we noticed a progressive slowing down ofthe tangential step motion (Fig. 2b) especially along[0 0 1], while growth persisted for a longer time

parallel to [1 1 0] (the projection of [1 0 0] on theM1 1 0N face). This anisotropic response to the dyegave rise to terraces elongated parallel to the crys-

tallographic [1 1 0] direction. The in#uence of acidfuchsin appeared also on the pro"le of the ledgeswhich became increasingly rougher (Figs. 2c}g).The terraces surface was smoother before the ac-tion of the dye and changed as a consequence of theroughening of the areas between the steps. Indeed,in the presence of acid fuchsin, several &0.5 nmsteps appeared, corresponding to d

1 1 0within ex-

perimental error, but at the same time bunchingproduced steps &10}15 nm high. After the intro-duction of acid fuchsin (Fig. 2a) the crystal surfacewas rapidly poisoned and after 691 s (Fig. 2g) thesurface growth was completely stopped. In particu-lar, the same area has been scanned for more than1 h (Fig. 2h, at 3880 s) and, apart from some drift ofthe whole area, there has been no modi"cation ofthe "nest details in the scan area of Fig. 2. The

observation of long sequences of images of poi-soned surfaces, with steps immobile or moving veryslowly, con"rmed, despite the potential erosion ofthe surface by the tip especially at the step edgeswhere molecules are less tightly bound to the sur-face, there was no disturbance of the crystal surfaceby the scanning tip. The possibility of a diminishedor increased growth within the scanned area wasrejected after enlarging the scan area at the end ofthe experiment.

The complete poisoning of the thin M1 1 0N terra-ces in the presence of acid fuchsin 3.35]10~4 M iscoherent with the strong adsorption of acid fuchsinonto M1 1 0N with formation of colored growth sec-tors. When the solution #owing through the cellwas exchanged with a pure K

2SO

4solution (Fig.

2i) growth did not restart from the poisoned ledgesbut from the groove in the middle top of Fig. 2h,where probably several sites not blocked by theadditive were still available for the incorporation ofgrowth entities. As shown in Fig. 2i, a macrostep(&70 nm high) emerged from the inner surface ofthe groove, grew laterally and upward, reached thepoisoned surface and then continued to grow tan-gentially at high speed, overgrowing in a few min-utes the imaged area, while in the meantime all thesmaller steps remained immobile. This kind of pro-cess could be responsible of the permanent uptakeof fuchsin molecules after their adsorption on thesurface because macrosteps can still grow in a high-er dye concentration, as long as there are free andactive growth sites.

Another growth sequence under similar condi-tions is shown in Fig. 3 (fuchsin 3.76]10~4 M,dye/salt molar ratio 1 : 1700). Several terraces werepresent with average step height of 2 nm togetherwith some elementary 0.5 nm steps. Several pits arealso seen in Fig. 3a which were left behind whensteps coalesced during the initial surface regenera-tion. These pits were slowly "lled and disappeared.As can be noted in Fig. 3a there are several terraceswith gently rounded ledges while multiple bunchedsteps (data not shown here) were polygonized.When fuchsin entered the AFM cell (Fig. 3b) thesmall grooves still present continued to annealwhile the simple terraces slowed down, becamejagged and elongated on [1 1 0], and at the sametime their separation widened. Analogously to Fig. 2


Fig. 2. AFM images of a K2SO

4M1 1 0N surface (scan size: 45]45 lm2). Images were taken in constant force mode while displaying the

de#ection error signal for better contrast. (a) 0 s; (b) 68 s; (c) 147 s; (d) 214 s; (e) 283 s; (f) 350 s; (g) 691 s and (h) 3880 s after acid fuchsin3.35]10~4 M reached the AFM cell; (i) at 3959 s pure K

2SO

4was fed through the cell. The [0 0 1] crystallographic direction is aligned

vertically. During imaging thermal drift moved the sample surface downwards relative to the scanned area.

the in#uence of impurities was least pronounced forstep advancement in the [1 1 0] direction and therewere perturbations of the areas between steps.Measuring height before and after additive addi-

tion it was found that steps bunched together giv-ing rise to step risers 15}20 nm high. Also in thiscase complete poisoning of the surface was ob-served for over 1 h while continuously feeding the



4M1 1 0N surface (scan size: 80]80 lm2; de#ection signal). (a) 0 s; (b) 68 s; (c) 147 s; (d) 214 s; (e) 283 s; (f)

350 s after acid fuchsin 3.76]10~4 M reached the AFM cell. The [0 0 1] crystallographic direction is aligned vertically.

cell with fuchsin 3.76]10~4 M. After exchangingthe #owing solution with pure salt, regrowthstarted again with steps having a normal pro"le,indicating that dye adsorption could be at terracesinstead of steps, as was proposed for calcite poi-soned with diphosphonates (see discussion).

3.4.2. Pyranine and the M1 1 0N surfaceFig. 4 is a M1 1 0N face growing in the presence

of pyranine (1.0]10~5 M, dye/salt molar ratio1 : 63000). On the pure K

2SO

4surface (not shown

here) there were 0.5}2 nm high steps together withhigher bunched steps (15}100 nm). 109 s after theintroduction of pyranine into the cell (Fig. 4a) the"rst signi"cant e!ects on the face microtopographyappeared. The elemental steps on the left of Fig. 4astarted to bunch and correspondingly the averagestep height increased to 5}30 nm; at the same time,the ledge pro"le, which started gently rounded and

smooth at the resolution used, soon became veryrough and sinuous (Figs. 4b}c). Fig. 4c evincesa strong alteration of the advancing terraces, being#at previous to the pyranine e!ect and roughenedafter jagging of the ledges pro"le. The tangentialmotion of the terraces is a!ected more along [0 0 1]than [1 1 0] and hence they became elongated(Figs. 4a}d). This low pyranine concentration isenough to disturb signi"cantly the elementary stepswhose tangential speed is abruptly reduced. On thecontrary, the bunched steps on the right side ofFigs. 4a}d were disturbed by the additive, theirpro"le roughened but their tangential growth wasless perturbed. Reducing the scan area from 52 to25 lm allowed us to see that elemental steps werestill slowly moving at &10 nm/s (Figs. 4e}f andsubsequent frames not reported here); indeed,terraces at the base of bunched steps moved, evenif with irregular pro"les, and reached the next



4M1 1 0N surface (scan size: 52]52 lm2 a}d; 25]25 lm2 e}f; de#ection signal). (a) 109 s; (b) 218 s; (c)

327 s; (d) 880 s; (e) 1119 s and (f ) 1162 s after pyranine 1.0]10~5 M reached the AFM cell. The [0 0 1] crystallographic direction isaligned vertically.

multiple step which was moving with a lower speed.This process thus generated a continuous exchangeof elementary layers between adjacent bunchedsteps keeping the number of constituting steps, andtheir height, unchanged. The observation of sucha behavior could be related again to a preferentialadsorption of the additives onto the terraces in-stead of being incorporated directly at the steps.

Similarly to acid fuchsin, when we reduced thepyranine concentration to zero in the AFM cell,surface growth started from the bunched stepswhile elementary step risers were more inactive andcovered by the lateral expansion of the macrosteps.

3.4.3. Acid fuchsin and the M0 1 0N surfaceIn a pseudo-hexagonal crystal such as b-K

2SO

4one would expect to "nd similarities between the

behavior of M1 1 0N and M0 1 0N. However, it haslong been known that there is no visible adsorptionof fuchsin on M0 1 0N, as reported previously[19,22]. Therefore, it has been interesting to ob-serve the growth of M0 1 0N faces in the presence ofacid fuchsin. The growth sequence in Fig. 5 refersto K

2SO

4initially grown without any dye and

subsequently in the presence of acid fuchsin3.76]10~4 M, analogously to the experiment de-scribed in Section 3.4.1. Before the introduction ofacid fuchsin the terraces (with steps 40}60 nm high)were advancing tangentially with v

*1 0 0+'v

*0 0 1+.

When the dye reached the cell (Fig. 5b) the ledgesadvancement was hindered by the adsorbed additivemolecules and the steps tried to move through thefence of stoppers causing the jagging of the ledgesfront. In the case of M1 1 0N the same amount of dye



4M0 1 0N surface (scan size: 75]75 lm2; de#ection signal). (a) 0 s; (b) 461 s; (c) 1051 s; (d) 2107 s and (e)

2637 s after acid fuchsin 3.76]10~4 M reached the AFM cell; (f ) at 2811 s pure K2SO

4was fed through the cell. The [1 0 0] and [0 0 1]

crystallographic directions are aligned vertically and horizontally, respectively.

produced a signi"cant deceleration of the stepmotion, particularly for the [0 0 1] direction, due tothe additive molecules pinning the steps. Also onM0 1 0N, owing to an anisotropy of poisoning, theterraces were more decelerated parallel to [1 0 0],with bulbous structures expanding on [0 0 1] dueto the squeezing of solute molecules through theblocked sites. Analogously to M1 1 0N, new stepsappeared between the pre-existing ones during theattempt to overcome the barrier of adsorbed mol-ecules. Upon feeding with fresh solute, growthcould restart directly from the steps regaininga higher velocity on [1 0 0] (Fig. 5f ).

Another experiment provided further detailsabout the interaction between M0 1 0N and acidfuchsin. Introducing fuchsin 2.11]10~4 M duringgrowth produced minor changes on the advancingsteps, which were still basically free to integrate new

growth units at the kink sites. Upon increasingthe additive concentration up to 2.53, 3.14 and3.76]10~4 M a progressive intensi"cation of themotion impeding e!ect by acid fuchsin was ob-served, which resulted in the jagging of steps al-ready cited. Parallel to these e!ects a substantialdissociation of bunched steps into thinner or ele-mentary steps was present for dye concentrationsgreater than 2.5]10~4 M.

Therefore, in the presence of acid fuchsin&4]10~4 M with a supersaturation &1}2%, theM0 1 0N face could continue to grow even if at a re-duced rate. This results agree with the macroscopicobservation of acid fuchsin adsorption only onM1 1 0N sectors and the increased morphologicalimportance of M0 1 0N described by Buckley [19].At this stage it would seem correct to conclude thatthis dye is adsorbed reversibly onto M0 1 0N, with


a deceleration of the step tangential motion, butwithout being incorporated into the host lattice.This fact could be substantiated by observing that,eliminating acid fuchsin from the AFM cell, growthcould start again from the jagged steps. As pointedout in Section 3.2, M1 1 0N and M0 1 0N possess veryclose 2D periodicities with di!erences in the anionsarrangement (assuming nonreconstructed surfaces).Because AFM showed acid fuchsin was able tointeract signi"cantly with the M0 1 0N surface, wehave started new crystallization experiments in thepresence of acid fuchsin. Interestingly, we foundthat increasing the dye/salt molar ratio, acidfuchsin enters the M0 1 0N growth sectors also, evenif to a lesser extent compared to M1 1 0N. For threetypical dye/salt ratios, 1 : 2000, 1 : 1000 and 1 : 500,fuchsin inclusions switch from `classicala (i.e. onlyon M1 1 0N, Fig. 1b) to an incipient adsorption onM0 1 0N (Fig. 1c) to strong inclusions onto M0 1 0N(Fig. 1d). As the dye/salt ratio increases, the overallcrystal morphology re#ects the relative decrease inthe normal face growth for the three dominantforms M1 1 0N, M0 1 0N and M0 2 1N, the latter beingthe only one not interacting with the dye and there-fore still growing at high dye/salt ratios (Fig. 1f ).

3.4.4. Pyranine and the M0 1 0N surfaceThe M0 1 0N face of K

2SO

4was studied in situ in

the presence of pyranine, with concentrations in therange 1.0}8.0]10~6 M. When the M0 1 0N face grewwith pyranine 1.0]10~6 M (R

$:%@4!-51 : 630 000)

there were no signi"cant e!ects either on the mor-phology or on the step velocity (growth sequencenot shown here). After raising the pyranine concen-tration to 2.0]10~6 M the "rst e!ects appeared,the pro"le of terraces a few nm high starting tochange. When pyranine reached 4.0]10~6 Mthe edge pro"les were signi"cantly roughened,the average size of the terraces diminished and thewhole surface became irregular. Concurrently, thestep advancement was gradually reduced. Onreaching 8.0]10~6 M growth virtually stopped.When a fresh K

2SO

4solution was #ushed through

the sample cell, the immobile surface started togrow again, with the original step pro"le quicklyreappearing.

Compared to acid fuchsin, polysulfonatedpyrenes have more intense e!ects on the growth of

K2SO

4, at concentrations as low as 10~6}10~5 M.

In Fig. 6 are some snapshots taken from growth ofM0 1 0N in the presence of pyranine. The 22]22 lm2 area contained mainly 100}300 nm highbunched steps having well polygonized pro"les andlong straight ledges parallel to [0 0 1]. Soon afterintroducing 5.0]10~6 M pyranine the macrosteppattern started to show heavy alterations, both inheight and step pro"le. During the next 15 minthere was an impressive reduction of the mean stepheight (down to a few nm) and distance betweenadjacent steps (Fig. 6f ). The completion of the dyeadsorption process produced the total dissociationof previously bunched steps into elementary ora few unit cells high layers with ill-de"ned ledges.The resulting surface was quite complex showingbulbous features of reduced average size (Fig. 6g).Also in this case we observed that elementarysteps were almost immobile while the higher onescould continue to move very slowly. After about50 min from the introduction of pyranine aging ofthe terraces led to a complete stop of the surface,indicating that e!ectiveness of the dye dependsalso on the time during which the surface is exposedto its disruptive action. The experiment showedin the next 30 min that the growth of the facewas totally frozen, as exempli"ed by the smallchanges seen in Fig. 6i with respect to Fig. 6h ona 10]10 lm2 area. Thus, pyranine was con"rmedon a microscale to strongly interact with the M0 1 0Nsurface [23].

4. Discussion

AFM imaging of K2SO

4crystals growing in the

presence of acid fuchsin &10~4 M and pyranine&10~6 M con"rmed the strong in#uence of thesedyes on the host crystal. It was found that on theM1 1 0N and M0 1 0N faces the ledges of elemental ormultiple steps became jagged as the dye reached thecrystal surface, causing the gradual deceleration ofsteps as the dye concentration increased. At a givensupersaturation there was a critical value of the dyeconcentration above which crystal growth wasgreatly disturbed or stopped along particularcrystallographic directions (dead zone). We ob-served bunching of steps as a result of absorption



4M0 1 0N surface (scan size: 22]22 lm2 a}g; 10]10 lm2 h}i; de#ection signal). (a) 0 s; (b) 154 s; (c) 374 s;

(d) 592 s; (e) 755 s; (f ) 919 s; (g) 2090 s; (h) 2163 s and (i) 4949 s after pyranine 5.0]10~6 M reached the AFM cell. The [0 0 1] and [1 0 0]crystallographic directions are aligned vertically and horizontally, respectively. During imaging thermal drift moved the sample surfacedownward relative to the scanned area.

of impurities binding to the crystal surface. How-ever, debunching was also observed in the caseof M0 1 0N due to pyranine strongly a!ecting thissurface.

The question of whether the additives present inthe growth medium adsorb at the terraces betweensteps or at kink sites on the steps deserve somecomments. Determining the adsorption mechanism


is not directly accessible by either bulk growthmonitoring or conventional microscopic tech-niques. In situ AFM growth studies can provideuseful information about the molecular level originof the mechanism by which surfaces interact withthe additives/impurities. For example, correlatingthe distribution of foreign molecules inside the hostcrystal with the microtopography can help to "ndthe relationships between surface structure, growthmechanism and additive incorporation. For layergrowth the speci"c a$nities for dye incorporationare associated with the detailed structural featuresof the growing surface, i.e. steps, ledges or terraces.Steps occurring on nonequivalent crystallographicdirections generally have di!erent structural prop-erties and may possess adsorption sites that di!erin their a$nities toward the additive, an impuritybeing preferentially taken up or rejected by thegrowing surface.

The high rate of crystal growth prevented, in ourcase, molecular resolution images being acquiredduring growth. Observation on a lm scale of steppro"le changes upon addition of the additive mol-ecules cannot shed light on the positions whereadsorbed molecules bind to the surface. Adsorptionon the terraces or on the steps can give the sameragged steps pattern. Therefore, AFM images ofjagged steps do not allow us to distinguish betweenimpurity adsorption at steps or onto terraces fol-lowed by pinning of the step motion according toCabrera and Vermilyea's (C&V) model for immo-bile impurities [44].

Several authors suggested that analysis of re-growth morphology after #ushing the sample withfresh supersaturated solution could provide evid-ence for terrace- or step-bound additive molecules.Preferential adsorption of additives at steps wassuggested for calcite [45] because regrowth ofa previously poisoned surface did not restart atsteps. In that case growth did not resume uniform-ly, but instead it began at a limited number ofsurface sites with "ngers advancing between thepoisoned sites. On the contrary, for K

2SO

4we

observed regrowth from the steps front with resto-ration of the normal morphology or, in some cases,surface overgrowth by bunched steps. In C&V'smodel [44] an immobile impurity when adsorbedon a smooth terrace cannot be passed by a straight

step, because impurities act as local pinning pointsfor the moving step front which, trying to squeezethrough the fence of impurities, must bend in circu-lar cusps. Accordingly, the step velocity is reducedcompared to that of straight steps in the absence ofimpurities by the Gibbs}Thomson factor 1!r

#/r (r

#is the 2D critical radius and r the actual radius ofcurvature). For average distances between impuritiesless than the critical diameter 2r

#the step will be

completely blocked. Therefore, a strong argument infavor of adsorption at surface terraces is the observa-tion of dead supersaturation zones, in which growthis greatly hampered or entirely stopped [46].

Lumpy step pro"les having a roughness on a lar-ger scale than the average impurity separation havebeen observed for K

2Cr

2O

7[47] and KAP [48] or

predicted on the basis of Monte Carlo simulationson Kossel crystals [49]; these altered pro"les havebeen associated with blocking of steps by isolated,immobile impurities. Similar pro"le patterns ob-served in our system are in agreement with terraceadsorption. For example Figs. 4e}f report bunchedsteps after the action of pyranine on M1 1 0N but alsothin steps slowly expanding, as if moving throughthe additive molecules sitting on the surface. More-over, acid fuchsin and sulfonated pyrene are strongpolyelectrolytes which can adsorb on terraces ow-ing to the high number of simultaneous attachingpoints on a molecule (three/four sulfonated groups)strongly interacting with the surface via long-dis-tance electrostatic forces. This class of additivesshould remain practically immobile once adsorbedonto the surface due to the strong interactions withthe host surface, provided that sulfonate}sulfonatedistances reasonably match the distance betweenthe sulfate ions on the surface host lattice and withsulfonate groups substituting for surface anions.This should be particularly true for pyraninethanks to its conformational rigidity, and indeed itse!ects on K

2SO

4are signi"cantly stronger than

those produced by acid fuchsin.

5. Conclusions

In this work we were concerned with the growthof K

2SO

4crystals in the presence of organic dyes.

The conclusions we draw are based on the analysis


of surface microtopographic features rather thanon quantitative kinetic data. Nevertheless, in situAFM studies of this highly soluble salt, even iflimited by the AFM's time resolution and di$cul-ties for temperature control of the scanning area,allowed us to investigate on a microscopic scale therole of additives on the growth interface. We haveobserved the inhibitory action of the polysul-fonated dyes acid fuchsin and pyranine which givehourglass inclusions on M1 1 0N and M0 1 0N growthsectors. The inhibition of growth implied adsorp-tion of additive molecules at terraces between steps.Modi"cation of the step morphology produced jag-ged steps given by the growth units squeezingthrough pinned areas of the surface. The stronginhibition of M1 1 0N and M0 1 0N growth upon addi-tion of acid fuchsin is coherent with the macro-scopic habit change from prismatic elongated on[1 0 0] to thin [0 0 1] needles. This con"rms thatAFM can probe directly at the nanoscale level thein#uence of additives on crystal habit. In this re-spect, AFM gives better statistics than ex situ ex-amination of mature crystals. Indeed, the results ofour in situ AFM experiments allowed us to recog-nize that acid fuchsin can enter the M0 1 0N growthsectors, provided the dye concentration duringcrystallization is su$ciently high.

Unfortunately, even AFM cannot, in general,represent the ultimate tool to solve the problem ofstep versus terrace binding of additives, but com-parison of bulk growth experiments, in situ AFMgrowth and computational studies can give birth tointeresting stimuli [50,51].

Acknowledgements

Dr. M.P. Wilson (Curtin University of Techno-logy, Perth, Australia) is kindly acknowledged formany useful suggestions about the manuscript. Weare also grateful to Prof. A.A. Chernov for stimulat-ing discussions during ISSCG-10 held in Rimini in1998.

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Growth of potassium sulfate crystals in the presence of organic dyes: in situ characterization by...

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