J. Phys. Chem. C 2008, 112, 9717–9722
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Impact of Thermal Oxidation on the Adsorptive Properties and Structure of Porous Silicon Particles Karyn L. Jarvis,† Timothy J. Barnes,† Alexander Badalyan,‡ Phillip Pendleton,‡ and Clive A. Prestidge*,† Ian Wark Research Institute, UniVersity of South Australia, Mawson Lakes, SA 5095, Australia, Center for Molecular and Materials Sciences, Sansom Institute, UniVersity of South Australia, City East, SA 5000, Australia ReceiVed: January 31, 2008; ReVised Manuscript ReceiVed: April 6, 2008
A combination of gas and probe molecule adsorption from aqueous solution have been applied to determine the adsorptive and structural properties of porous silicon (pSi) particles as a function of thermal oxidation in the range 473-1073 K. Gaseous nitrogen adsorption has shown a decrease in the BET specific surface area, mesopore volume, and diameter due to the increasing molecular volume of the oxidized silicon layer within the pores. Methylene blue was used as an adsorbate to establish the apparent surface area of pSi particles in aqueous solution and has shown adsorption capacity independent of oxidation in the range of 473-873 K. Comparable adsorption capacities at low oxidation temperatures are due to methylene-blue-induced oxidation which has been established by X-ray photoelectron spectroscopy, while increased adsorption at high temperatures is due to increased surface silanol concentrations. Comparisons between the nitrogen- and methylene-blue-determined surface areas demonstrate a difference in the adsorptive properties of gas and solution phase molecules onto pSi. These differences can be attributed to the different adsorption mechanisms, that is, physisorption for nitrogen and electrostatic attraction for methylene blue. Nitrogen and methylene blue adsorption probe different features of pSi, demonstrating modification of the structural and adsorptive properties with oxidation as well as highlighting the improved characterization that can be obtained from the combination of solution and gas phase adsorption. Introduction 1950s;1
Porous silicon (pSi) was first discovered in the however, it was not until after 1990 when pSi was found to emit visible photoluminescence at room temperature2 that detailed research into its physicochemical properties was undertaken. Initially pSi was developed for use in light-emitting devices and optoelectronics;3 however, more recent interests have focused on exploring a range of biomaterial applications due to its biocompatible nature and particularly that cells adhere to its surface.4 A recent application for pSi is drug delivery where drug molecules5 or proteins6,7 are loaded into the porous matrix and released into the body as the matrix degrades.8 A variety of mesoporous silicon and silica materials are under development for drug delivery applications. The loading capacity of mesoporous silica has been linked to pore size since release rates decrease with a decrease in pore size.9 Increasing the silica surface area directly increases the loading capacity and release kinetics of capropril.10 Modification of the silica surface results in sustained ibuprofen release while an increase in the number of functional groups produces lower loading capacities and release rates.11 Sustained release from pSi occurs when the unloaded dissolution of a drug is high, while enhanced release is demonstrated for pSi loaded with poorly soluble drugs.12 Thermal oxidation and carbonization are two methods routinely used to modify pSi surface chemistry, while high temperature * To whom correspondence should be addressed. Tel.: +61 8 8302 3569. Fax: +61 8 8302 3683. E-mail:
[email protected]. † Ian Wark Research Institute, University of South Australia. ‡ Center for Molecular and Materials Sciences, University of South Australia.
thermal annealing can be used to increase the average pSi pore size prior to thermal carbonization or oxidation. Despite their similar pore diameters, annealed thermally carbonized pSi exhibits a greater ibuprofen release rate than annealed thermally oxidized pSi, indicating that surface chemistry and pore size have a combined effect on release rate.13 Characterization of adsorbents (e.g., for drug delivery applications) can be carried out via the adsorption of methylene blue, a well-established probe molecule for ascertaining physical and chemical properties.14 Methylene blue adsorption typically occurs by electrostatic attraction in its cationic form.15 For negatively charged adsorbents, an increase in pH enhances methylene blue adsorption15–18 due to increased numbers of negatively charged surface (typically hydroxyl) sites available for methylene blue adsorption.17,18 The specific surface area and pore diameter of pSi is typically determined via nitrogen (N2) adsorption and the BrunauerEmmett-Teller (BET) specific surface area ranges from less than 1 m2/g for macroporous silicon to up to 800 m2/g for microporous silicon.19 Mesoporous silicons have BET specific surface areas in the range of 200-315 m2/g.12,20 Specific surface area can also be determined via methylene blue adsorption, a molecule of well-defined shape (1.7 nm × 0.76 nm × 0.33 nm) and adsorbed area per molecule of 1.3 nm2, assuming the molecule adsorbs with its largest face flat on the surface.21 Methylene blue adsorption has previously been used to determine the surface area of clay minerals22 and sludge ash23 in aqueous solution, with comparable specific surface areas to those obtained from N2 adsorption observed. To our knowledge no such comparative investigations have been reported for methylene blue adsorption on pSi.
10.1021/jp800950j CCC: $40.75 2008 American Chemical Society Published on Web 06/11/2008
9718 J. Phys. Chem. C, Vol. 112, No. 26, 2008
Figure 1. SEM image of unoxidized porous silicon particle.
Figure 2. Structure of methylene blue molecule.
The oxidation of pSi improves its photoluminescence24 and enhances its stability for sensor applications.25 Oxidation has also recently received attention in drug delivery applications where it stabilizes pSi via the formation of surface oxide layers. A range of oxidation methods have been investigated, including thermal,26 dry and humid atmospheres,27 water and alcohol,28 and anodization.29 Oxidation has a significant effect on the structure of pSi, with oxidation temperature a crucial parameter of the process. The average pore diameter decreases with increasing oxidation temperature,30,31 which can be attributed to the increased molecular volume of Si-O-Si compared to Si-Si, resulting in expansion of the silicon oxide layer,32 i.e., oxidation “fills up” micropores32,33 and therefore reduces the surface area.34,35 For example, thermal oxidation at 820 °C reduced the BET surface area from 253.5 to 71.5 m2/g.13 However, it remains unclear whether BET surface area values are relevant for the loading of molecules from aqueous solution. This investigation utilizes N2 adsorption to determine the BET specific surface area, mesopore volume, and pore size distribution of unoxidized and thermally oxidized pSi particles. Methylene blue has been employed as an adsorbate from aqueous solution to probe interactions with pSi and the influence of the porous structure. The analysis of oxidized pSi demonstrates the effect of oxidation on methylene blue adsorption and can aid in the prediction of drug interactions and loading capacities of molecules that possess similar functionalities. Methylene blue adsorption has established the affinity and capacity of adsorption of both unoxidized and oxidized pSi particles. From the equilibrium isotherms the area occupied by methylene blue has been calculated, hence direct comparisons with N2 adsorption have been made to demonstrate the difference in the adsorptive properties of gas and solution phase molecules onto pSi particles. Materials and Methods Adsorbent. Porous silicon layers were prepared from p+ silicon wafers (0.005-0.020 Ωcm) by electrochemical anodization using hydrofluoric acid/ethanol as the electrolyte. The current density was fixed to give an average porosity of 70%. The porous layer is detached from the underlying silicon substrate electrochemically, resulting in a free-standing pSi membrane with a typical thickness of 150 µm. Particles were produced from the membranes by a jet milling process with the particles subsequently classified to give an average particle
Jarvis et al. diameter of 50 µm. A SEM image of an individual pSi particle is presented in Figure 1. Adsorbates. Methylene blue (Figure 2) was purchased from Sigma-Aldrich and used as received. Gaseous N2 and helium were of high purity grade (99.999%) and used as supplied by BOC Gases (Australia). Oxidation. pSi was oxidized by heating in air at 40 K min-1 to a specific oxidation temperature in the range 473-1073 K for 1 h, before convective cooling to ambient temperature. N2 Adsorption. Nitrogen adsorption was carried out using a custom built automatic manometric gas adsorption apparatus.36 Samples were preheated to 473 K and evacuated to a pressure of 10-4 Pa for 8 h to remove any physically adsorbed water vapor and gases. Dead-volume measurements were carried by exposure of the samples at 77 K to gaseous helium. Nitrogen adsorption measurements were also carried out at 77 K. The BET specific surface area (BET-SSA) was calculated from the nitrogen adsorption data in the relative pressure range from 0.05 to 0.32 P/P0. The combined standard uncertainty in the BETSSA of each pSi sample was evaluated using a weighted meanleast-squares method.37 XPS Characterization. X-ray photoelectron spectroscopy (XPS) of pSi particles was carried out with a Kratos Axis Ultra spectrometer. A monochromated Al KR´ radiation source (hυ ) 1486.7 eV) was used to excited surface electrons and was operated at 25 kV and 10 mA. To determine the various silicon states, high resolution Si 2p spectra were collected using a pass energy and resolution of 20 and 0.1 eV, respectively. The high resolution Si 2p spectra were fitted by CASA XPS software, with paired fitting curves representing the 2p1/2 and 2p3/2 energy levels for each silicon state. The fitted curves were set with the same full width at half-maximum, height proportion of 1:2 and separated by 0.6 eV38 with only the 2p3/2 peaks presented in the spectra. The binding energies of the Si 2p peaks were normalized by modifying the binding energy of the C 1s peak for C-C to 285 eV. The shift between 285 eV and the binding energy was determined experimentally and applied to all the other peaks. Methylene Blue Adsorption. Methylene blue (MB) adsorption was investigated using a fixed mass of pSi (0.0025 g) weighed into scintillation vials and prewet with 0.04 mL of methanol. The total volume was adjusted to 2.5 mL by the addition of a methylene blue solution with predetermined initial concentrations (10-200 mg/L). The vials were sealed and placed on a suspension mixer for 18 h at 25 °C to reach equilibrium. The resulting suspensions were centrifuged for 20 min at 14 000 rpm to separate the pSi and solution phases. The equilibrium MB concentration was determined using a UV-vis spectrophotometer (Cary 1E) at λ ) 665 nm. The amount of MB adsorbed onto the pSi was determined by the difference between the initial and remaining concentrations of dye solution. The mass of methylene blue adsorbed per unit mass of pSi (qe) was calculated from
qe )
(Ci - Cf)V m
(1)
where Ci and Cf are the initial and final concentrations of the MB respectively (mg/L), V is the total solution volume (L), and m is the mass of the adsorbent (g). The specific surface area (MB-SSA) determined from methylene blue adsorption isotherms was calculated using the equation
Thermal properties and structures of porous silicon oxidation
MB-SSA )
qeNAA MrMB
J. Phys. Chem. C, Vol. 112, No. 26, 2008 9719
(2)
where NA is Avogadro’s number (mol-1), A is the cross-sectional adsorbed area of a methylene blue molecule (assumed ) 1.3 nm2/molecule) and MrMB is the MB molecular weight. The number of interacting silanol groups (Si-OH) per unit area (NSiOH) can be calculated assuming that one molecule of methylene blue binds with one silanol group, according to the following equation
NSiOH )
qeNA MrMBBET-SSA
(3) Figure 4. Mesopore size distributions of unoxidized and thermally oxidized porous silicon particles.
Results and Discussion Gas Phase Probe Molecule Adsorption. N2 adsorption isotherms for both unoxidized and oxidized pSi particles (shown in Figure 3) are classified as type IV isotherms with H1 hysteresis loops.39 This type of isotherm denotes a mesoporous material with the hysteresis loop being associated with capillary condensation of nitrogen within the mesopores. All nitrogen adsorption isotherms level off at relative pressures between 0.9 and 1.0. Loop closure at these relative pressures give an indication that the mesopores are likely to be filled with a liquid-
Figure 3. Nitrogen adsorption (closed symbols) and desorption (open symbols) isotherms at 77 K of (a) unoxidized and (b) thermally oxidized porous silicon particles.
TABLE 1: Specific Surface Areas Determined from N2 Adsorption (BET-SSA), Mesopore Volume and Mean Mesopore Diameter of Unoxidized and Thermally Oxidized Porous Silicon ox temp (K)
BET-SSA (m2/g)
pore volume (mL/g)
pore diameter (nm)
473 673 873 1073
325.7 ( 0.4 312.5 ( 0.6 247.7 ( 2.3 243.8 ( 2.5 175.4 ( 0.3
0.77 ( 0.002 0.74 ( 0.004 0.61 ( 0.003 0.48 ( 0.003 0.38 ( 0.002
10.1 10.1 9.6 9.3 10.7
like adsorbate. A trend is observed in the reduction of the amount of nitrogen adsorbed (at 0.93P0) from 0.917 to 0.492 mL/g for pSi samples oxidized at 473 and 1073 K, respectively. Similar behavior has been observed for silica after calcination40 and silicon after oxidation.41 Such a decrease in the amount of adsorbed nitrogen can be attributed to the collapse of the mesopore structure40 and expansion of the oxide which results in the blockage of pores.41 To calculate the BET-SSA, a BET plot is constructed between 0.05 and 0.32 P/P0 with a straight line observed for all pSi samples. The fitted Pearson correlation coefficients varied from 0.9998 to 0.9999. The BET-SSA values are presented in Table 1 and show a clear trend of decreasing BET-SSA values with increasing oxidation temperature, which has been well documented in the literature.33,42 The volume of nitrogen adsorbed within the mesopores is calculated by the difference in the amount adsorbed at 0.93 P/P0 and 0.40 P/P0, also shown in Table 1. The mesopore volume decreases with increasing oxidation temperature and is reduced by approximately half as oxidation temperature increases from 473 to 1073 K. Nitrogen adsorption is the most appropriate adsorbate for detailed mesopore analysis.39,43 There are many methods available to evaluate mesopore size distribution; however the Barrett-Joyner-Halenda (BJH) method is the most widely accepted and popular.43 To use the BJH method, a nonporous material with a similar surface chemistry to the adsorbent under investigation should be used for the construction of an apparent adsorbed layer thickness plot.44 The mesopore size distributions as a function of oxidation temperature derived from desorption curves are shown in Figure 4. As the oxidation temperature increases from 473 to 873 K the mesopore diameter was reduced from 10.1 to 9.3 nm (see Table 1). Pores with diameters of 3 to 6 nm were also observed upon oxidation at 473 K (Figure 4), with their number steadily decreasing as oxidation temperatures increase to 1073 K. Such a decrease in average pore diameter and an increase in the number of smaller pores are likely to be attributed to the expansion of the structure due to SiO2 formation. The expansion of the structure also accounts for the reduction in the number of pores with diameters of 6-14 nm. The reduction in the number of smaller pores (
