Hematite nanoparticle monolayers on mica preparation by controlled self-assembly

Journal of Colloid and Interface Science 386 (2012) 51–59

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Hematite nanoparticle monolayers on mica preparation by controlled self-assembly _ Magdalena Oc´wieja ⇑, Zbigniew Adamczyk, Maria Morga, Elzbieta Bielan´ska, Adam We˛grzynowicz Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, 30-239 Cracow, Poland

a r t i c l e

i n f o

Article history: Received 11 April 2012 Accepted 17 June 2012 Available online 2 August 2012 Keywords: Deposition of hematite particles on mica Hematite nanoparticle films Kinetics of hematite particle deposition Monolayers of hematite nanoparticles on mica Self-assembly of hematite particles

a b s t r a c t A stable suspension of a-Fe2O3 (hematite) was synthesized according to the method of Matijevic and Scheiner by an acidic hydrolysis of ferric chloride. The average size of the particles was determined by dynamic light scattering (DLS) and atomic force microscopy (AFM) and was 22 nm. The electrophoretic mobility and zeta potential of particles were determined as a function of ionic strength and pH. The zeta potential of the hematite particles was positive for pH < 8.9 (isoelectric point) and negative otherwise. Using the suspension, systematic studies of particle deposition kinetics on mica were carried out. The coverage of self-assembled particle monolayers was determined by AFM and SEM imaging. Particle deposition was diffusion controlled, with the initial rate proportional to the bulk concentration of particles. On the other hand, for long times, the saturation coverage was attained, increasing systematically with ionic strength. The deposition kinetic runs were adequately reflected by the random sequential adsorption (RSA) model. Additionally, particle desorption kinetics, from previously formed monolayers, were studied using the AFM and SEM methods. It was confirmed that hematite particle desorption was practically negligible within the time period of 60 h. Our experimental data proved, therefore, that it is feasible to produce uniform and stable hematite particle monolayers of desired coverage in self-assembly processes controlled by the bulk suspension concentration and the ionic strength. Ó 2012 Elsevier Inc. All rights reserved.

1. Introduction Iron oxide nanoparticles are of great importance for advanced technologies and various industries. Hematite, (a-Fe2O3), is thermodynamically the most stable of iron oxides, showing weak ferromagnetic properties at room temperatures [1]. It is a semiconducting material with a relatively narrow band gap of 2.0–2.2 eV [2]. Moreover, it can be easily doped by divalent or tetravalent additives, for example, niobium, which converts hematite into an attractive material used in photovoltaic cells (water photo-oxidation and photo-electrochemical hydrogen production [3]). Hematite is applied in industry in the form of suspensions as red pigment and anti-corrosion agent. It also has significance for catalysis in the Haber process [1,4], the Fisher–Tropsch synthesis [5], and the desulfurization of natural gas [1]. Most practical applications of hematite involve thin films deposited on various conducting surfaces, for example, those which serve as gas, alcohol or humidity sensors [6,7], electrodes in lithium batteries [6], photo-anodes, etc. Usually these hematite films are produced via physical or chemical methods such as sputtering [8], laser ablation [9], ⇑ Corresponding author. Fax: +48 124251923. E-mail addresses: [email protected] (M. Oc´wieja), [email protected] (Z. Adamczyk), [email protected] (M. Morga), [email protected] (E. Bielan´ska), [email protected] (A. We˛grzynowicz). 0021-9797/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.jcis.2012.06.056

electrodeposition [10], spray pyrolysis (SP) [11,12], ion beam induced chemical vapor deposition (IBICVD) [13], plasma enhanced vapor deposition (PECVD) [14], and the aerosol-assisted chemical vapor deposition (AACVD) [15]. However, these ‘‘dry’’ methods require sophisticated and expensive apparatus. The process of films formation is tedious, and the coverage degree and film structure cannot be regulated in a systematic manner. More convenient and flexible methods of hematite film production based on controlled self-assembly of nanoparticles from aqueous phase hematite suspensions (sols). Obviously, the first step of film preparation involves a synthesis of stable hematite sols of well-defined surface properties and appropriate particle size. The most common method of preparing hematite nanoparticle suspensions is the acid hydrolysis of iron salts [16–18], which was recently improved by introducing the gel–sol procedure described by Sugimoto et al. [19–22]. Other methods consist of the calcinations of iron oxyhydroxide (b-FeOOH) [23,24] and the ionic liquid-assisted synthesis [25]. A major advantage of aqueous hematite suspensions is that the particles exhibit a positive charge (zeta potential) for pH < 9 [26], which facilitates their deposition on solid substrates promoted by electrostatic attraction. Therefore, such aqueous phase hematite suspensions can be efficiently used for preparing hematite coatings and films on solid supports. However, despite the major significance of wet methods, there are few systematic works devoted to this subject. As one of the few exceptions should be mentioned the work of Björkstèn

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M. Oc´wieja et al. / Journal of Colloid and Interface Science 386 (2012) 51–59

et al. [27] who described preparation of nanocrystalline hematite films directly from a colloidal a-Fe2O3 suspension for photoelectrochemical studies. They used conducting glass, which was prepared by spray pyrolysis, and then, the hematite suspension was deposited on this substrate using a Pasteur pipette. Afterward, in order to obtain stable hematite film, the layer was dried for prolonged time. Wang et al. [28] described the preparation of thin, uniform hematite monolayers from a suspension of iron oxide nanoparticles, using the Langmuir–Blodgett technique. Based on earlier works [29,30], where this technique was applied to form a-Fe2O3 films, the authors modified the surface of hematite nanoparticles using three different procedures such as a solvent exchange process, a surfactant-assisted phase transfer process, and a thermal evaporation process. Kuhnen et al. [31] studied deposition of large (120 nm) hematite nanoparticles on quartz sand media under flow rate. Hematite nanoparticles deposition dynamics was investigated by conducting a series of column transport experiments at different conditions. Authors show how the maximum surface concentration of particles depends on the ionic strength, colloid concentrations, and flow velocities. Experimental results were successfully interpreted in terms of the random sequential adsorption (RSA) model. In view of the deficit of experimental results, the goal of our paper is developing a reliable and efficient method of producing hematite monolayers by a colloid self-assembly carried out from aqueous suspensions under diffusion transport conditions. The coverage and structure of hematite monolayers is systematically controlled by the deposition time, the suspension concentration, pH, and ionic strength variations. An advantage of our method, as compared to the Wang’s work [28] and our previous studies devoted to silver particle self-assembly [32], is that no modification of particles or substrate surfaces are required prior to particle deposition. Besides practical aspects, our measurements possess significance for basic colloid science, enabling one to test the validity of theoretical approaches for predicting electrostatically driven deposition of nanoparticles. 2. Experimental 2.1. Materials All materials were analytic reagents used without further purification. The ferric chloride (FeCl36H2O), hydrochloric acid (HCl), sodium chloride (NaCl), and sodium hydroxide (NaOH) were purchased from POCH. Natural ruby mica sheets supplied by Continental Trade Ltd., Poland, were used as substrate surfaces for nanoparticle deposition. Thin sheets were freshly cleaved and used in each experiment without any pretreatment. Ultrapure water, used throughout this investigation, was obtained using the Milli-Q Elix & Simplicity 185 purification system from Millipore SA Molsheim, France. 2.2. Synthesis of hematite (a-Fe2O3) nanoparticles The hematite nanoparticles were synthesized according to the modified Matijevic´ method [16] based on forced hydrolysis of ferric chloride in an acid solution. A stock solution of ferric chloride (0.25 M) was first prepared by dissolving FeCl36H2O in 0.004 M HCl. Before starting the synthesis, the solution was purified through 0.22 lm Millipore filters to remove any particular contaminations. Then, while stirring, 12 ml of this solution was rapidly added into a boiling solution of 0.004 M HCl to yield a final Fe3+ concentration of 5 mM. The reaction mixture was kept at boiling

temperature for 20 min and then cooled to room temperature. The colloid iron oxide suspension was purified from ionic excess using a stirred membrane filtration cell with a regenerated cellulose membrane (Millipore, NMWL: 100KDa). The washing procedure was repeated until the electric conductivity of the suspension reached a minimum value of c.a. 15 lS/cm. 2.3. Methods Characterization of the hematite suspension and the monolayers on mica was performed using various techniques. The weight concentration of the particle suspension was determined using the high precision densitometer: DMA 5000M (Anton Paar). The crystal structure and purity of a dried hematite sample was determined by X-ray photoelectron spectroscopy (XRD), which was performed using a X’PERT PRO diffractometer (Cu Ka, 40 kV, 30 mA), working in the Bragg–Brentano geometry, fixed slits, X’CELERATOR detector. The size (hydrodynamic diameter) and the zeta potential of the nanoparticles were determined by dynamic light scattering (DLS) and microelectrophoresis, respectively, using the Zetasizer Nano ZS Malvern instrument. The measurement range was 3 nm to 10 lm for zeta potential and 0.6 nm to 6 lm for particle size. Additionally, the zeta potential of particles was measured using the Brookhaven Zeta Pals apparatus. The size and the morphology of nanoparticles were also investigated using the JEOL JSM-7500F microscope working in transmission mode. Samples for this examination were prepared by dispersing a drop of the hematite suspension on a copper grid covered by a carbon film. The surface concentration of hematite monolayers on the mica substrate was quantitatively determined using the atomic force microscopy (AFM) and the scanning electron microscope (SEM). AFM measurements were carried out using the NT-MDT Solver Pro instrument with the SMENA SFC050L scanning head. The imaging was done in semicontact mode using composite probes possessing a silicon body, polysillicon levers, and silicon high resolution tips. Independently, SEM measurements of iron oxide monolayers were carried out using the JEOL JSM-7500F Field Emission microscope at 15 kV. To ensure a sufficient conductivity during the measurement, the hematite nanoparticles were treated with a thin layer of chromium before the examination. The number of particles per unit area of the substrate was determined from these AFM pictures and SEM micrographs using the image analysis software. Zeta potential of mica was determined via streaming potential measurements using a home-made cell previously described [33,34]. The main part of the cell was a parallel plate channel, having dimensions of 2bc  2cc  L = 0.027  0.29  6.2 cm, formed by mica sheets separated by a perfluoroethylene spacer. The streaming potential Es was measured using a pair of Ag/AgCl electrodes as a function of the hydrostatic pressure difference DP, which was driving the electrolyte flow through the channel. The cell electric conductivity Ke was determined using Pt electrodes. Knowing the slope of the Es vs. DP dependence, the apparent zeta potential of substrate surface (fi) can be calculated from the Smoluchowski relationship

fi ¼

    DEs gK e DEs ¼ 4ebc cc Re DP e DP

gL

ð1Þ

where g is the dynamic viscosity of the solution, e is the dielectric permittivity, and Re is the electric resistance of the cell governed mainly by the specific conductivity of the electrolyte in the cell.

M. Oc´wieja et al. / Journal of Colloid and Interface Science 386 (2012) 51–59 Table 1 The Bragg reflections peaks and the assigned lattice planes (h k l).

3. Results and discussion 3.1. Hematite nanoparticles characterization

qp ðqs  qe Þ qs ðqp  qe Þ

10

0 0

250

Count

200

150

100

50

0 20

30

40

50

60

70

80

2θ Fig. 1. The XRD pattern of the a-Fe2O3 (hematite) nanoparticles sample.

012 104 110 006 024 116 214 300 1 0 10 220

20

10

20

30

40

50

dH [nm]

3

300

hkl

23.9 33.2 35.5 39.8 49.6 53.9 62.8 64.1 71.9 75.6

30

ð2Þ

where qp = 5.26 g cm is the specific density of hematite particles [1]. It was determined in these experiments that the weight fraction of hematite in the stock suspension, after the cleaning procedure, was 2.8  103, which corresponds to 2800 ppm (parts per million by weight). The well-characterized stock suspension was diluted prior to each experiment to a desired concentration, usually varying between 10 and 20 ppm in the deposition experiments and 50– 100 ppm in the DLS and microelectrophoretic measurements. The size distribution of hematite nanoparticles was determined from AFM images (a typical example of hematite monolayer is shown in Fig 2). The particle diameter were determined from their height with respect to the surface, the values were calculated with the use of Nova 1152 software that is coupled directly with AFM microscope. A few hundred particles were counted in this way to

Peak position 2H (°)

40

Count

The chemical composition and crystallinity degree of hematite nanoparticles were examined using powdered XRD measurements. The hematite suspension was dried at 100 °C for a prolonged time period, and the obtained powder was examined using a X’PERT PRO diffractometer, working in the Bragg–Brentano geometry, fixed slits, X’CELERATOR detector. The measurement parameters were 40 kV, 30 mA. The XRD spectrum of the powder is shown in Fig. 1. All of the observed peaks can be attributed to the rhombohedral structure of a-Fe2O3 (space group R3c). The narrow sharp peaks suggest that the obtained product is highly crystalline. No characteristic peaks indicating impurities like c-Fe2O3 and other inorganic ions were observed. A list of Bragg reflections and assigned lattice planes (h k l) was shown in Table 1. The hematite weight concentration in the stock suspension after the cleaning procedure was determined by the densitometer providing relatively precise of measurements 5  106 g cm3. The density of the stock hematite suspension qs and the supernatant solution qe acquired by membrane filtration were measured using this device. Then, the weight fraction w of colloid hematite in the sol was determined from the formula

w¼

53

Fig. 2. The histogram of hematite particle size distribution determined from the AFM image, scan size 2  2 lm, see the inset. The average diameter of particles is 22 ± 5 nm.

obtain a histogram also shown in Fig. 2. The mean diameter of particles was 22 nm with the standard deviation of 5 nm. Hematite nanoparticle size was also determined via diffusion coefficient (D) measurements performed using the DLS technique. Knowing the diffusion coefficient, one can determine the hydrodynamic diameter of particles using the Stokes–Einstein relationship:

dH ¼

kT 3pgD

ð3Þ

where dH is the hydrodynamic diameter, k is the Boltzman constant, and T is the absolute temperature. The hydrodynamic diameter can be interpreted as the size of an equivalent sphere having the same hydrodynamic resistance coefficient as the particle. The advantage of using this quantity in comparison with the diffusion coefficient is that it is independent of temperature and liquid viscosity, so it is an appropriate parameter for analyzing suspension stability under various conditions. The dependence of the hydrodynamic diameter of hematite particles on ionic strength varied between 3  105–101 M and pH = 5.5 is shown in Fig. 3a. As can be seen, for ionic strength up to 3  102 M, the hydrodynamic diameter was practically constant, attaining an average value of 22 nm with a standard deviation of 5 nm, that is, identical to that previously determined by AFM. A small increase in the hydrodynamic diameter was only observed for ionic strength approaching 0.1 M. In Fig. 3b, the dependence of dH on pH regulated by the addition of an appropriate amount of HCl or NaOH for a fixed ionic strength of 102 M is shown. A considerable increase in the hydrodynamic diameter of hematite particles was observed for pH above 8,

M. Oc´wieja et al. / Journal of Colloid and Interface Science 386 (2012) 51–59

54

30

30

dH [nm]

40

dH [nm]

(a) 40

20

10

20

10

0 10-5

10-4

10-3

10-2

10-1

0 0

300

600

900

1200

1500

1800 18000 19000 20000 21000

t [min]

I [M]

(b) 250

Fig. 4. The dependence of the hydrodynamic diameter of hematite nanoparticles on the time, determined by DLS for pH = 3.5 I = 3  102 M and bulk concentration 40 ppm.

200 100 90

dH [nm]

80 70 60 50 40 30 20 10 0 2

4

6

8

10

12

pH Fig. 3. (a) The dependence of the hydrodynamic diameter of hematite nanoparticles on ionic strength, determined by DLS for pH = 5.5, the dashed line represents a constant value of the hydrodynamic diameter of 22 nm. (b) The dependence of the hydrodynamic diameter of hematite nanoparticles on pH determined by DLS for I = 102 M, (cb = 40 ppm); the solid line represents non-linear fits of experimental data.

suggesting that the suspension became unstable. However, for pH > 10, the hydrodynamic diameter again attained the value of 22 nm, pertinent to a stable hematite suspension. This significant growth of hydrodynamic diameter observed for 8 < pH < 10 suggests that the hematite particles aggregated within this pH range. It is interesting to mention that analogous results were obtained by Schudel et al. [35] for two hematite samples having an average particle size of 69–84 nm (determined by time-resolved DLS). However, the range of pH above 9 was not studied it his work. Additionally, the stability of the hematite suspension upon prolonged storage times was checked. The results shown in Fig. 4 (pH = 3.5, I = 3  102 M and the bulk suspension concentration of 40 ppm) indicate that the suspension was stable up to the time of 300 h, even at such a high ionic strength. Obviously, the same trend was observed for lower ionic strength if the pH of the suspension was kept below 8. To fully characterize the hematite suspension, the electrophoretic mobility of particles le was also determined as described above. It is a quantity of major significance for characterizing particle stability and interactions with surfaces leading to deposition. Knowing the electrophoretic mobility one can calculate the zeta potential of particles using the Henry–Smoluchowski formula

fp ¼

3g l 2ef ðjdp Þ e

ð4Þ

where fp is the zeta potential of particles, f(j dp) is the  ekTcorrection 1=2 function of the dimensionless parameter, j dp, j1 ¼ 2e is the 2I thickness of the electric double layer, e is the elementary charge, P I ¼ 12 i ci z2i is the ionic strength, ci is the ion concentration, zi is the ion valency, and dp is the characteristic dimension of the particle, for example, the hydrodynamic diameter dH. The dependencies of electrophoretic mobility and zeta potential of hematite nanoparticles on the pH, determined for a fixed ionic strength of 103 M and 102 M, are shown in Fig. 5. As can be seen, the mobility is highly positive for the low pH range, attaining the value of 4.01 lm cm (V s)1 for pH = 3.5 (I = 103 M) and 3.43 lm cm (V s)1 for pH = 3.5 (I = 102M). This corresponds to the zeta potential of 47 and 38 mV, respectively. For higher pH values, the electrophoretic mobility and the zeta potential of hematite decrease monotonically and vanish at pH = 8.8–9 (for I = 103 M and 102 M, respectively). For higher pH, both values become negative. Thus, the zeta potential attained the value of 47 mV for I = 103 M and 42 mV for I = 102 M (pH = 10.7). Our data indicate that hematite particles exhibit a clearly defined isoelectric point (i.e.p.) at pH 8.9. This is the value at which the electrophoretic mobility of the particles vanishes. It should be mentioned that our result agrees with that reported by Zhang and Buffle [26] (9.2) and Schudel et al. [35] (8.8–9.2). However, other values were also reported. For example, He and coworkers [36] reported pH = 7.8–8.8 as the isoelectric point of hematite nanoparticles of various sizes. Even a lower value of pH = 7–7.5 was reported by Delgado and Gonzalez-Cabállero [37] and Bentaleb et al. [38]. Variations in the isoelectric point value for various hematite sols were also observed by Matijevic´ and Scheiner [16]. It is interesting to mention that the electrophoretic mobility data (Fig. 5) correlate well with the hematite suspension stability range above shown in Fig. 3, which suggest that the stability is mainly governed by electrostatic interactions. Additionally, in a separate series of experiments, the dependence of the particle zeta potential on the ionic strength varied within 3  105 and 3  102 M for a fixed pH = 5.5 was determined (Fig. 6). As can be seen, the zeta potential of hematite particles decreased slightly with ionic strength, from 52 mV for I = 3  10–5 M to 38 mV for I = 3  102 M. From these measurements, one can conclude that the hematite particles exhibit a high positive zeta potential for a broad range of pH and ionic strength, which is expected to promote their efficient deposition on negatively charged substrates.

M. Oc´wieja et al. / Journal of Colloid and Interface Science 386 (2012) 51–59 60

(a) -5 -4

40

-3 20

-1 0

0

ζ [mV]

-1

μe [(μm cm)(Vs) ]

-2

1

3.3. Kinetics of hematite particle deposition -40

4 5

-60 2

3

4

5

6

7

8

9

10

11

12

pH 60

(b) -5 -4

40

-3 -2

20

-1 0

0

ζ [mV]

-1

μe [(μm cm)(Vs) ]

potential of mica on pH (adjusted with the addition of HCl or NaOH), for three different ionic strengths (103 M, 102 M and 3  102 M), is shown in Fig. 7. As can be noticed, the zeta potential was negative for the entire range of pH studied (3–11) for both ionic strength values. For pH = 3.5, it was 20 mV and 40 mV, for I = 3  102 and I = 102 M, respectively. The zeta potential decreased monotonically with pH, attaining 45 mV and 70 mV, for I = 3  102 and 102 M, respectively (pH = 7.4).

-20

2 3

1 -20

2 3

-40

4 5

-60 2

3

4

5

6

7

8

9

10

11

12

pH Fig. 5. The dependence of the electrophoretic mobility and zeta potential of hematite particles on pH. (a) I = 103 M, (b) I = 102 M.

54

4.0

52

3.8

50 3.6

48

-1

46

3.4

44 3.2

ζ [mV]

μe [(μm cm)(Vs) ]

55

42

After a thorough characterization of the system, a series of measurements were performed with the aim of quantitatively determining the kinetics of hematite particle deposition. In the first stage of these measurements, freshly cleaved mica sheets were immersed in a thermostated diffusion cell containing a hematite suspension of a desired concentration. The diffusion cell was a cylindrical glass vessel with a capacity of 6 ml placed in thermostated chamber. The vessel was equipped with a glass element that allows for vertical immersion of mica sheets in the hematite suspension. Particle deposition was allowed to proceed over a prescribed time under diffusion-controlled transport. Next, the sheets were rinsed with distilled water and air dried. The surface concentration of deposited particles was determined by a direct enumeration carried out using AFM and SEM imaging. The number of particles deposited on 8–10 equal-sized surface areas (typically having the dimensions of 2 lm  2 lm or 2.3 lm  1.7 lm for AFM and SEM, respectively) was determined. The overall number of particles counted was larger than 1000, which ensured a relative precision of these measurements better than 5%. Knowing the average number of particles and the surface area DS, the particle coverage was calculated as the number of particles per square micrometer, denoted hereafter by NS. Typical AFM and SEM images of hematite particle monolayers deposited from a suspension of the concentration equal to 20 ppm (I = 102 M and pH = 3.5) are shown in Figs. 8 and 9, respectively. As can be noticed, NS increased monotonically with deposition time. Knowing NS as a function of deposition time, particle deposition kinetic for various experimental conditions can be determined. Typical examples of such kinetic runs obtained from the bulk suspension concentration of 10 and 20 ppm (I = 102 M NaCl, pH = 3.5) are shown in Fig. 10. Note that the square root of deposition time t1/2 was used as an independent variable rather than

40

3.0

38 20

2.8 2.6 10 -5

36 10 -4

10 -3

10 -2

0

34 10 -1

-20

I [M]

-40

ζ [mV]

Fig. 6. The dependence of the electrophoretic mobility and zeta potential of hematite particles on ionic strength for pH = 5.5.

-60 -80

3.2. Mica substrate characterization

-100

In our hematite particle deposition experiments, mica sheets were used as a well-defined substrate. There are many advantages connected with using mica because it is a chemically stable and molecularly smooth material, characterized by a uniform and homogeneous surface charge distribution [39]. Zeta potential of freshly cleaved mica sheets was determined by the streaming potential method according to the procedure described in previous works [33,40]. The dependence of the zeta

-120 -140 3

4

5

6

7

8

9

10

11

pH Fig. 7. The dependence of the zeta potential of mica determined by streaming potential measurements on pH. The points denote experimental results obtained for: (1) (s) I = 3  102 M, (2) (h) I = 102 M, (3) (}) I = 103 M.

M. Oc´wieja et al. / Journal of Colloid and Interface Science 386 (2012) 51–59

56

Fig. 9. SEM images of hematite nanoparticles monolayers on mica obtained from colloidal hematite suspension: 20 ppm, I = 102 M NaCl, pH = 3.5, (a) particle surface concentration Ns = 163 lm2, (b) particle surface concentration Ns = 765 lm2.

Fig. 8. AFM images (scan size 2 lm  2 lm) of hematite particles monolayers on mica. Deposition conditions: 20 ppm, I = 102 M NaCl, pH = 3.5, (a) particle surface concentration Ns = 175 lm2, (b) particle surface concentration Ns = 258 lm2.

500

 1=2 D NS ¼ 2 t 1=2 nb

p

ð5Þ

-2

NS [μm ]

400

the deposition time. As shown in our previous work [32], this is an appropriate transformation to analyze diffusion-controlled deposition processes. As can be seen in Fig. 10, for adsorption time t1/2 < 30 min1/2 (t < 900 min), particle deposition was described by a linear function of t1/2, with the slope two times larger for cb = 20 ppm and for 10 ppm. This behavior was in accordance with an analytical formula, pertinent to bulk controlled diffusion transport [41,42]

300

200

100

0 0

5

10

15

20

25

30

t1/2 [min1/2]

where nb = cb/mp is the number concentration of particles in the 3 bulk, mp ¼ p6 dm qp is the effective mass of one particle. An agreement of our results with Eq. (5) proves that the particle deposition process was solely controlled by diffusion and the surface blocking effects were negligible. As a result, Eq. (5) can be used for selecting the appropriate bulk concentration and deposition time to obtain hematite monolayers of desired coverage. Moreover, the results shown in Fig. 10 can be used to determine the

Fig. 10. The kinetics of hematite particle deposition on mica determined by AFM (full points) and SEM (hollow points) on the square root of adsorption time t1/2. Deposition conditions: pH = 3.5, I = 3  102 M, T = 293 K, (1) (d, s) cb = 20 ppm, (2) (j, h) cb = 10 ppm. The lines denote linear fits of experimental results.

‘‘diffusion’’ diameter of hematite particles, which is given by the previously derived expression [32]

M. Oc´wieja et al. / Journal of Colloid and Interface Science 386 (2012) 51–59

dp ¼

12

qp p

!2=7 

2s D

kT 3g

1=7 ð6Þ

where sD is the slope of the empirical dependence of NS/cb on t1/2. From the results shown in Fig. 10, one can calculate that the average slope sD = 1.34 ppm1 lm2 min1/2 = 1.42  1013 cm g1 s1/2. Substituting this value in Eq. (6) and T = 293 K, g = 0.01 g (cm s)1, qp = 5.26 g cm3, one obtains from dp = 21.6 nm. This is in agreement with the hydrodynamic diameter determined by DLS and the average particle size determined by AFM. However, as demonstrated in previous work concerning silver particle deposition [43] for longer times, the surface blocking effects start to play a significant role reducing particle deposition rates and the maximum concentration of deposited particles. In order to study this effect, long-lasting deposition experiments for hematite nanoparticles were also performed. In Fig. 11 results obtained for cb = 20 ppm, pH = 3.5 and various ionic strength are shown. The surface concentration of particles was determined by both AFM (full points) and SEM (hollow points). Characteristic features of these kinetic runs are a linear increase in NS with t1/2 for shorter times and then, after reaching some critical time, an abrupt stabilization of the surface concentration at a constant value. The latter is referred to as the maximum surface concentration of particles, denoted by N Smx . It is interesting to observe that the maximum coverage of hematite particles increased significantly with the ionic strength assuming 469 lm2 for I = 103 M, 853 lm2 for I = 102 M and 1033 lm2 for I = 3  102 M. It is convenient, as is commonly done, to express these maximum concentration values in terms of the dimensionless coverage defined as

Hmx ¼ pd2p NSmx =4

ð7Þ

Taking dp = 22 nm, as determined above, one obtains Hmx = 0.18 for I = 103 M, Hmx = 0.33 for I = 102 M and Hmx = 0.41 for I = 3  102 M. Hence, deposition of hematite nanoparticles at an ionic strength of 3  102 M creates the most favorable conditions for producing uniform monolayers of the highest coverage. However, at higher ionic strengths, the hematite suspension become less stable (see Fig. 3a); therefore, it is impractical to attain surface coverage higher than Hmx = 0.41. Using the dimensionless coverage also facilitates a comparison of experimental results with theoretical models developed to

57

quantitatively describe nanoparticle deposition processes [42,44]. In this case, the most convenient model is the random sequential adsorption (RSA) model, successfully applied before for describing irreversible adsorption (deposition) of colloid microparticles (polystyrene latexes) [45,46], nanoparticles [32,47–51], and proteins [40,52]. The method is also extensively discussed in the book [42] and recent reviews [39,53–55]. According to the generalized RSA model, the constitutive equation describing the adsorption flux of particles to interfaces for an arbitrary adsorption mechanism is given by

ja ¼

1 dh kd ¼ ka nðda ÞBðhÞ  h Sg dt Sg

where ja is the adsorption flux, Sg is the characteristic cross-section area of the particle, t is the adsorption time, ka and kd are the adsorption and desorption kinetic constants, n(da) is the concentration of particles at the adsorption boundary layer of the thickness da, and B(h) is the surface blocking function. It should be mentioned that Eq. (8) is coupled with the convective diffusion equation describing particle transport from the bulk to the diffusion boundary layer [42,43]. For spherical particles, the RSA blocking function is described by the expression [42]

  3 BðhÞ ¼ 1 þ a1 h þ a2 h2 þ a3 h3 1  h

ð9Þ

where  h ¼ h=hmx is the normalized coverage of particles, hmx is the maximum coverage of particles usually dependant on the ionic strength [32,48,55], and a1–a3 are the dimensionless coefficients equal to 0.812, 0.426 and 0.0717, respectively. The maximum coverage increases with ionic strength because lateral electrostatic interactions among particles are screened [42,43]. Exploiting the RSA approach one can formulate a useful analytical expression connecting the maximum coverage with measured quantities

hmx ¼ h1 

1 

1 þ 2h =dp

ð10Þ

2

where H1 is the maximum coverage for hard (non-interacting) particles equal to 0.547 for spheres [42,52] and h is the effective interaction range characterizing the repulsive double-layer interactions particles, which can be calculated from the formula 

2h =dp ¼

   1 / 1 / ln 0  ln 1 þ ln 0 jdp 2/ch jdp 2/ch

Fig. 11. The kinetics of hematite particle deposition at mica determined for various ionic strengths using AFM (full points) and SEM (hollow points). Particle deposition conditions: pH = 3.5, T = 293 K, cb = 20 ppm. The points denote experimental results obtained for: (1) (d, s), I = 3  102 M, (2) (j, h) I = 102 M, (3) (, }) I = 103 M, (4) (.) I = 104 M. The solid lines denote the theoretical results calculated from the RSA model.

ð8Þ



ð11Þ

 2 2 fp e is the characteristic interaction where /0 ¼ 16pedp kTe tanh 4kT energy of particles and /ch is the scaling interaction energy, close to the kT unit. As discussed in Ref. [42], Eq. (11) is a good approximation of exact numerical results for relatively thin double-layers, where jdp > 2. In our case of diffusion-controlled transport, Eqs. (8)–(10) together with the bulk transport equation can be integrated numerically using the efficient finite-difference method [32,42,43]. The theoretical results obtained in this way are shown in Fig. 11 as solid lines. The agreement with experimental data is satisfactory given the irregular shape of hematite particles and the polydispersity of the suspension. Additionally, in Fig. 12, the maximum coverages calculated from Eq. (10) for various ionic strengths are compared with experimental data. Again, one observes an agreement of theoretical and experimental results which suggests that Eq. (10) can be used for predicting the coverage of the hematite monolayer for an arbitrary ionic strength range, exceeding 104 M. Therefore, by adjusting the ionic strength, one can in a fully controlled way produce hematite nanoparticle monolayers of arbitrary coverage between 0.1 and 0.41.

M. Oc´wieja et al. / Journal of Colloid and Interface Science 386 (2012) 51–59

58

4. Conclusions 0.5

0.4

θ

0.3

0.2

0.1

0.0 10 -4

10 -3

10 -2

10 -1

I [M] Fig. 12. The dependence of the maximum coverage of hematite particles on mica on the ionic strength. The points denote experimental data obtained by SEM or AFM, and the solid line shows the theoretical results calculated from Eq. (7) by adopting the effective hard particle concept.

Another issue that was studied in our work was the stability of hematite monolayers, which was determined by performing controlled desorption experiments. Thus, in the first stage of these experiments, a hematite monolayer of a defined coverage was produced as described above. Afterward, the hematite suspension was replaced by pure electrolyte of a defined pH and ionic strength and the particles were allowed to desorb under diffusion-controlled transport for a prescribed period of time. Their final coverage of particles remaining on the surface was determined ex situ by direct SEM and AFM enumeration. A typical kinetic desorption run obtained for the initial coverage of hematite particles Ho = 0.25, pH = 3.5, I = 102 M, is presented in Fig. 13. As can be seen for the desorption time of 3600 min (60 h), the decrease in the initial coverage of the monolayer was only 4%. Analogous results were obtained in other desorption runs performed for an ionic strength range of 104 to 3  102 M and a pH range of 3.5–7.4. In all cases, desorption of hematite particles was negligible for times up to 60 h, which suggests hematite monolayers were indeed stable under these conditions. More extensive studies on the electrokinetic properties of hematite monolayers and their stability under dynamic conditions (flow) are presented in the forthcoming works.

1.2

1.0

θ/θ0

0.8

0.6

0.4

0.2

0.0 0

20

40

60

t1/2 [min1/2] Fig. 13. Kinetics of hematite particle desorption determined by SEM, pH = 3.5, I = 102 M, T = 293 K and initial coverage of particles Ho = 0.25. The solid line denotes a linear fit of experimental results.

The nanosized hematite particle suspension, synthesized by an acidic hydrolysis of ferric chloride was stable for a broad range of ionic strength and bulk concentration up to 2800 ppm. For pH < 8.9 (isoelectric point), the hematite particles were positively charged, which facilitated their deposition driven by electrostatic interactions on negatively charged substrates. This suspension was used to produce hematite monolayers on mica via a controlled self-assembly process carried out under diffusion controlled transport. The absolute coverage of deposited particles was quantitatively determined by AFM and SEM imaging. It was confirmed that the deposition was diffusion controlled, with the initial rate proportional to the bulk concentration of particles. On the other hand, for long adsorption times, the saturation coverage was attained, increasing systematically with ionic strength. The kinetic runs were adequately reflected by the RSA model. Additionally, particle desorption measurements confirmed that for a higher coverage range, hematite particle desorption was practically negligible within the time period of 60 h, indicating that the monolayers were stable. Our experimental data proved, therefore, that it is feasible to produce uniform and stable hematite nanoparticle monolayers (films) of desired coverage in self-assembly processes controlled by the bulk suspension concentration and the ionic strength. Such monolayers may find practical applications as substrates for selective protein and nanoparticle deposition, sensors, and photoelectrodes in various electrocatalytic applications. Acknowledgments This work was financially supported by the Research Grant: POIG 01.01.02-12-028/09-00. The authors are grateful to Katarzyna Luberta – Durnas´ for her help in performing the XRD analysis. References [1] A.S. Teja, P.Y. Koh, Prog. Cryst. Growth Charact. Mater. 55 (2009) 22–45. [2] S.M. Ahmed, J. Leduc, S.F. Haller, J. Phys. Chem. 92 (1988) 6655–6660. [3] J.A. Glasscock, P.R.F. Barnes, I.C. Plumb, A. Bendavid, P.J. Martin, Thin Solid Films 516 (2008) 1716–1724. [4] Ch. Li, Y. Shen, M. Jia, S. Sheng, M.O. Adebajo, H. Zhu, Catal. Commun. 9 (2008) 355–361. [5] Y. Wang, B.H. Davis, Appl. Catal., A 180 (1999) 277–285. [6] Ch. Wu, P. Yin, X. Zhu, Ch. OuYang, Y. Xie, J. Phys. Chem. B 110 (2006) 17806– 17812. [7] A. Esteban-Cubillo, J.M. Tulliani, C. Pecharroman, J.S. Moya, J. Eur. Ceram. Soc. 27 (2007) 1983–1989. [8] E.L. Miller, D. Paluselli, B. Marsen, R.E. Rocheleau, Thin Solid Films 466 (2004) 307–313. [9] F. Zhou, S. Kotru, R.K. Pandey, Thin Solid Films 408 (2002) 33–36. [10] Y.S. Hu, A. Kleiman-Shwarsctein, A.J. Forman, D. Hazen, J.N. Park, E.W. McFarland, Chem. Mater. 20 (2008) 3803–3805. [11] A. Duret, M. Grätzel, J. Phys. Chem. B 109 (2005) 17184–17191. [12] F. Le Formal, M.M. Grätzel, K. Sivula, Adv. Funct. Mater. 20 (2010) 1099–1107. [13] L. Yuberto, M. Ocaña, A. Justo, L. Conttreras, A.R. González-Elipe, J. Vac. Sci. Technol., A 18 (2000) 2244–2248. [14] B.J. Kim, E.T. Lee, G.E. Jang, Thin Solid Films 341 (1999) 79–83. [15] A.A. Tahir, K.G. Upul Wijayantha, S. Saremi-Yarahamadi, M. Mazhar, V. McKee, Chem. Mater. 21 (2009) 3763–3772. [16] E. Matijevic´, P. Scheiner, J. Colloid Interface Sci. 63 (1978) 509–524. [17] E. Matijevic´, R.S. Sapieszko, J.B. Melville, J. Colloid Interface Sci. 50 (1975) 567– 581. [18] R.S. Sapieszko, R. Patel, E. Matijevic, J. Phys. Chem. 81 (1977) 1061–1074. [19] T. Sugimoto, K. Sakata, J. Colloid Interface Sci. 152 (1992) 587–590. [20] T. Sugimoto, M.M. Khan, A. Muramatsu, Colloids Surf., A 70 (1993) 167–169. [21] T. Sugimoto, M.M. Khan, A. Muramatsu, H. Itoh, Colloids Surf., A 79 (1993) 233–247. [22] T. Sugimoto, K. Sakata, A. Muramatsu, J. Colloid Interface Sci. 159 (1993) 372– 382. [23] T. Sugimoto, Y. Wang, H. Itoh, A. Muramatsu, Colloids Surf., A: Physicochem. Eng. Aspects 134 (1998) 265–279.

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