Investigations of Sol-Gel ZnO Films Nanostructured by Reactive Ion Beam Etching for Broadband Anti-Reﬂection

A novel ZnO dry etching approach is introduced using reactive ion beam etching of thick sol-gel ZnO layers for controlled nanodisk/nanocone array fabrication. In this approach the same system can be used for the colloidal lithography mask (silica particles) size reduction by a ﬂuorine-based chemistry and etching of the ZnO nanostructures by a CH 4 /H 2 /Ar chemistry. This resulted in a ZnO:SiO 2 etch selectivity of ∼ 3.4 and etch rate of ∼ 56 nm/min. Thick sol-gel ZnO layers, nanodisk arrays and (truncated) nanocone arrays were fabricated and their optical properties analyzed by ﬁnite-difference time-domain simulations and spectrally-resolved total/specular reﬂectivity measurements. The demonstrated broadband omnidirectional anti-reﬂection, controlled nanostructure period/geometry and low absorption in the visible-NIR spectrum makes these sol-gel ZnO nanostructures very interesting for many optoelectronic applications, including photovoltaics. ©

Here, a sol-gel method has been developed and used for the fabrication of (relatively) thick porous polycrystalline ZnO layers. A similar method has already been reported for thin films by, e.g., Gohdsi et al. 34 and Choudhury et al. 44 Here the method has been extended for obtaining well-defined thick ZnO films (up to ∼2 μm) by using a sequential drop-cast approach in a 'vacuum' environment. The 'building' of these sequential ZnO layers to one thick layer provides a method to tune specific layer thicknesses on the micro/nanoscale. Other approaches for obtaining thick porous ZnO layers have been reported. [45][46][47] The sol-gel method is attractive since it is simple, cheap and straight forward. In addition, this method offers the flexibility to provide ZnO layers on a variety of surfaces. Sol-gel layers, consisting of crystals with c-axis orientation vertical to the surface, can be obtained when the substrate material has a similar Wurtzite crystal structure to ZnO, e.g., GaN, SiC and (0001)-sapphire. For other types of substrates dense sol-gel layers, composed of non-specific direction, can still be obtained. In this work, thick ZnO layers have been fabricated and nanostructured on a Si substrate. These layers have been used to fabricate ZnO nanodisk arrays and (truncated) nanocone arrays based on reactive ion beam etching (RIBE); where the same system has been used for the mask (colloidal SiO 2 ) size reduction and the etching of the ZnO nanodisk/cone arrays. The etch process settings have been chosen with regard to the ZnO:SiO 2 selectivity and ZnO etch rate. Etch rates have been determined and compared for both sol-gel and sputtered ZnO layers. The focus of the optical characterization, by spectrophotometry, of the fabricated structures has been on their anti-reflection nature for applications in optoelectronics.
RIBE has been used for the colloidal lithography (CL) mask size reduction step as well as for the etching of the ZnO nanostructures. A CL method, based on SiO 2 colloidal particles, has been used to pattern the surface. A CHF 3 /Ar-based chemistry has been applied to size-reduce the mask particles for obtaining the desired nanostructure diameters; the original size of the colloidal particles determines the hexagonal array period. In the same system, a CH 4 /H 2 /Ar-based chemistry has been developed and used to etch the ZnO nanostructures.
Finite-difference time-domain (FDTD) simulations, using a Lumerical tool, have been used for the electromagnetic modeling and simulation of the fabricated ZnO structures on Si and to determine the most optimal structure for the (truncated) ZnO nanocone array with regard to minimizing surface reflections. The sol-gel ZnO nanocone array is attractive for optoelectronics (e.g., solar cells, photodetectors, LEDs) due to its broadband omnidirectional anti-reflectivity and low absorption in the visible-NIR wavelength range. Spectrally-resolved total/specular reflectivity measurements have been used to characterize the fabricated ZnO structures.

Fabrication
Below the fabrication of the nanostructured ZnO films -thick solgel ZnO films, colloidal lithography, mask size reduction and RIBE of the ZnO nanostructures -are discussed. The optical properties of the fabricated structures analyzed by FDTD simulation and spectrally resolved total/specular reflectance measurements are presented in the Electromagnetic modeling and simulations section and Optical characterization section.  are added to fully dissolve the zinc acetate dehydrate. The solution is stirred at room temperature for at least 1 hour to obtain a uniform solution. Next a drop-cast process in vacuum conditions, by using a desiccator, has been used to produce uniform sol-gel ZnO films, typically ∼200-250 nm thick, with homogeneous area coverage on both silicon (Si) and glass substrates. The substrates (∼4 cm 2 ) were first cleaned by 5 minutes sonication in acetone and isopropanol, respectively, followed by rinsing with DI water and blow drying with N 2 . The surface was treated by oxygen plasma (O 2 flow of 500 sccm, RF power of 1 kW, a pressure of 800 mTorr and reaction time of 10 min) to increase the wettability of the surface. In the next step the sol-gel film was heated at 80 • C for 15 minutes, followed by annealing for 1 hour at 300 • C to crystallize the ZnO. This resulted in a porous, polycrystalline ZnO single film on the Si surface with a thickness of ∼250 nm and a grain size of ∼10-20 nm (see Fig. 1a); whereas on glass the layer thickness was lower (∼200 nm).

Sol-gel
Relatively thick porous sol-gel ZnO films were obtained by repeating the layer formation process several times. Between each deposition the surface was treated by O 2 plasma (reaction time of 5 min) to improve the wettability. The layer which forms on top of a previous layer resulted in an added thickness of ∼200 nm. In this work, five sequential layer depositions have been used on Si surfaces and glass, respectively, resulting in approximately 1 μm thick layers (see Fig. 2a).  A high-resolution X-ray diffraction measurement (HR-XRD; Philips X'Pert mrd) of the ZnO films (see Fig. 3) shows the Zn (100) , Zn (002) and Zn (101) peaks; indicating the Wurtzite crystal structure. The result shown for the sol-gel ZnO layer (∼250 nm) is one for a typical deposited layer and representative for the etched structures. In addition, the HR-XRD data is shown for a sputtered ZnO layer (∼1 μm thick), which shows a crystal growth in the ZnO (002) orientation.
The applied annealing temperature influences the ZnO grain size, wherefore temperatures varying from 300-700 • C the average grain size is indicated to change from ∼10-40 nm, respectively. Representative images used for estimating the average grain sizes are shown in Fig. 4, where Figs. 4a-4c show scanning electron microscopy (SEM) images of the ZnO films annealed at 300, 500 and 700 • C, respectively, and Fig. 4d shows an atomic force microscopy (AFM) image for a sol-gel ZnO layer annealed at 300 • C. The average grain sizes were estimated using the SEM images. For the sol-gel ZnO layer (300 • C) grain sizes were in the range of ∼10-20 nm and the surface roughness was ∼5-10 nm as obtained from the AFM measurement.
Layer thicknesses up to ∼2 μm were obtained in this work due to the choice of substrate size (2 × 2 cm) and the amount of sol-gel solu- tion used. After each single sol-gel ZnO layer deposition, the available surface area for a homogeneous film layer shrinks due to border effects. This resulted in that after reaching a layer thickness of ∼2 μm, the effective surface area with a uniform layer thickness becomes too small. Representative images are shown in Figs. 3b and 3c. This issue could be avoided by using a larger substrate surface area combined with a larger volume of sol-gel precursor for the drop-cast process.
The ZnO layer on glass (∼1 μm thick) was used for transmittance/absorption characterization measurements (see Fig. 1b) and the ZnO layers on Si (∼250 nm and ∼1 μm thick) were used for the fabrication of the sol-gel ZnO nanodisk and (truncated) nanocone arrays for optical characterization with regard to their anti-reflective properties. In addition, a ZnO film (∼1 μm thick) on Si was fabricated by rf-magnetron sputtering to compare the etch rates and the ZnO:SiO 2 etch selectivity for sputtered and sol-gel ZnO layers.
Colloidal lithography and mask size reduction.-Colloidal lithography (CL) was used for patterning both the sol-gel and the sputtered ZnO films, and results in self-assembly of SiO 2 colloidal particles in a hexagonally close-packed array (see Fig. 2b). A mild spincoating process of the colloid solution (size 500 nm; Sigma Aldrich) was applied, resulting in close-packed monolayer coverage in several mm 2 patches on the sample surface. Optimization of the CL process is required to obtain larger (>1 cm 2 ) monolayer area coverages. Before CL a thin (∼50 nm) SiO 2 layer was deposited on the ZnO film by plasma-enhanced chemical vapor deposition (PECVD) to increase the wettability of the surface to improve the monolayer coverage and to provide a well-defined (additional mask) beneath the silica particles. Before CL the surface was cleaned for 5 minutes in acetone and isopropanol, respectively, followed by rinsing with DI water and blow drying by N 2 . Oxygen plasma treatment (reaction time of 10 min) was used to further improve the wettability.
An Oxford Instruments Plasma Technology Ionfab 300 Plus tool was used, with a developed CHF 3 /Ar-based chemistry (CHF 3 /Ar flow of 10/5 sccm, RF power of 500 W, V acc of 500 V, platen angle of 20 • (with regard to perpendicular) and rotation speed of 20 rpm) for the size reduction of the SiO 2 colloidal etch mask. The SiO 2 mask particles were size reduced to ∼370 nm (diameter). The etching is predominantly from the top, with a 'vertical' etch rate of ∼96 nm/min (see Fig. 2c). An indication of the relation between the etch rate and the resulting colloidal particle diameter was determined, based on this predominantly vertical etching process, and is given in Eq. 1.
Where D 0 is the original diameter, D the new diameter, e rate is the etch rate and t the etch time. Additionally the exposed PECVD SiO 2 thin layer was etched away, resulting in a cylindrical shaped SiO 2 hard mask underneath the SiO 2 colloidal particles. This is necessary to avoid undesired etching at the top part of the fabricated nanodisks/cones due to the relatively small contact area of the colloidal particle mask with the underlying surface. 4  Two types of sol-gel ZnO nanostructures were fabricated: ZnO nanodisk arrays and (truncated) nanocone arrays. The nanodisk array, see Fig. 2d, has a hexagonal array with a period of 500 nm, a height of ∼250 nm and a diameter of ∼250 nm. The (truncated) nanocone array (Fig. 2f) has a hexagonal array with a period of 500 nm, a height of ∼1100 nm, a top diameter of ∼110 nm and a bottom diameter of ∼500 nm. Figure 2e shows the intermediate etching for the ZnO nanocone array for which a nanopillar array has a remnant buffer layer. For both the ZnO nanodisks and nanocones the residual SiO 2 mask was retained; typical approaches for SiO 2 removal (e.g., HF treatment) show poor selectivity between ZnO and SiO 2 . Spectrallyresolved reflectivity measurements were done for the fabricated ZnO nanodisk/cone arrays (see Fig. 8) and these will be discussed in the Optical characterization section and Discussion section.

Reactive ion beam etching of ZnO nanostructures.-CH
Additionally, the fabricated sol-gel ZnO nanostructures/layers can be used as a seed layer for the growth of ZnO nanowires (Fig. 6). Figure  6a shows the ZnO nanowires grown on a (thin) ZnO layer on Si and Fig.  6b shows, as an example, the grown ZnO nanowires on the etched ZnO nanodisks. Previously reported results 44 have shown that such hierarchical ZnO nanowire structures can further reduce the surface reflections, though the growth of these nanowire structures are difficult to control with regard to their specific geometries and spacing and are not suitable for the fabrication of relatively thick pillar arrays. In this work, our focus is on well-defined dry-etched arrays of ZnO nanostructures with typical lateral dimensions much larger than grown nanowires.

Electromagnetic Modeling and Simulations
Finite-difference time-domain (FDTD) simulations, using a Lumerical tool, were used for simulating the optical properties (reflection and transmission) of the fabricated ZnO nanodisk/cone array structures. For the ZnO nanodisk array on a c-Si substrate a hexagonal array period of 500 nm, a disk height of 250 nm and a disk diameter of 250 nm were taken and for the (truncated) nanocone array on a c-Si substrate a hexagonal array period of 500 nm, a height of 1100 nm, a top-diameter of 110 nm and a bottom-diameter of 500 nm. In the simulations, a plane wave source, periodic boundary conditions and the known optical constants for bulk ZnO from Palik 48 were used. However, the refractive index of the fabricated ZnO structures could vary with regard to the porosity of the layer. We assume that the porosity is similar in the thick multilayer film. Ellipsometry was used to determine the refractive index of a ∼250 nm thick sol-gel ZnO layer on Si and this resulted in a value of n NIR ≈ 1.76 in the NIR range; where the refractive index from Palik 48 shows a value of n NIR ≈ 1.92. This difference should be taken into account when comparing the data of the simulations (Fig. 8a) and the optical characterization (Fig. 8b). For clarification, the simulation data for the corrected sol-gel refractive index value compared to Palik 48 has been included as inset in Fig.  8a. The data will be further discussed in the Discussion section.
The simulated total reflectance data was determined using a 2D frequency-domain field and power monitor. In addition, simulations of a 250 nm ZnO layer (n = 1.9-2.2 for the NIR-visible wavelength range) on a c-Si substrate has been done to compare the anti-reflection properties of the ZnO nanostructures. All the simulated reflectivity results are shown in Fig. 8a. Simulations including the residual SiO 2 mask showed minor influence on the reflectivity and its effect is thus neglected in the following.
Additionally, FDTD simulations have been used to determine the optimal geometrical and array dimensions for bulk (and sol-gel; inset in Fig. 7a) ZnO (truncated) nanocone arrays on a c-Si substrate for optimal anti-reflection characteristics. For this the hexagonal array period was varied between 500-2000 nm, the height between 500-1500 nm, the top-diameter between 0-500 nm and the bottom-diameter between 50-500 nm. These simulations resulted in an optimal structure consisting of a hexagonal array period of 500 nm, a height of 600 nm, a top-diameter of 50 nm and a bottom-diameter of 500 nm. A 100 nm ZnO buffer layer underneath the (truncated) nanocone structures minimizes the surface reflection further and results in total reflectance values as low as ∼0-5% in the wavelength range of 450-800 nm (see Fig. 7a). The schematic for the optimal structure is shown in Fig. 7b.

Optical Characterization
The Si substrate and ZnO films on Si were characterized by spectrally-resolved total reflectivity measurements (spot size  ∼9 mm 2 ) using a Lambda 950 UV/Vis/NIR spectrophotometer equipped with an integrating sphere; the wavelength range was 300-850 nm. The reflectivity data for the nanocone/disk arrays were obtained using an in-house specular reflection setup (using a supercontinuum source, fiber, lens system and Si detector; the wavelength range was 400-1000 nm) with a spot size of ∼900 μm 2 , compatible with typically obtained areas (∼1 mm 2 ) with monolayer coverage in CL. The simulated and the measured reflectivity data are shown in Figs. 8a and 8b, respectively, for the wavelength range of 400-850 nm.
The same spectrophotometry setup, as for the spectrally-resolved total reflectivity measurement, was used for the transmittance/ absorption measurements (spot size of ∼3 mm 2 ) for the ∼1 μm thick ZnO film on the glass substrate. The transmittance result for this sample is shown in Fig. 1b. From this it can be concluded that the sol-gel ZnO shows a low to no absorption (total reflectance ≈ 8%) in the wavelength range of 400-850 nm. A rough estimate for the optical bandgap of ∼3.5 eV for the sol-gel ZnO was obtained.

Discussion
A sol-gel method has been used to obtain (relatively) thick ZnO films. This approach provides the flexibility to fabricate thick ZnO films from sequential ZnO layer depositions. By tuning/optimizing the settings of the sol-gel process for the separate layer deposition, e.g., deposition volume, viscosity of precursor solution, (air) pressure during deposition/drying, surface conditions (hydrophilicity) of the substrate and drying temperature, a single layer with a predetermined layer thickness can be tailored. Here five sequential layer depositions, each ∼200 nm thick, were used to fabricate a total layer thickness of ∼1 μm on Si and glass; where the first layer on Si had a thickness of ∼250 nm. Furthermore, instead of drop-casting, very thin ZnO layers can be obtained by spin-coating for which the layer thickness was found to be ∼20-30 nm for a spin rate of 3000 rpm.
From the refractive index relation n 1 = √ (n 0 · n s ) for a Rayleigh's film on Si, with n 0 = n air ≈ 1 and n s = n Si ≈ 3.7, the total reflection can be minimized with a film with a refractive index of n 1 ≈ 1.9. This makes the sol-gel ZnO (n ≈ 1.76 in the NIR wavelength range) a very promising candidate for anti-reflection and in general light manipulation functions on most semiconductor materials (Si, III-Vs, etc.). The nanostructuring of the ZnO film(s) enables optical guiding by refractive index engineering. 18 For this the geometry and array spacing can be tuned for optimizing the material-light interaction for the desired wavelength range.
Dry etching of ZnO layers has been reported for inductively coupled plasma reactive ion etching (ICP-RIE) using CH 4 /H 2 /Ar-, Cl 2 /H 2 /Ar-and Cl 2 /Ar-based chemistries. 37,38 Previous work concluded that the ICP-RIE etching with CH 4 /H 2 /Ar-based chemistry shows the highest etch rates due to the higher volatility of the etch products. For the etching of ZnO by CH 4 -and Cl 2 -based chemistries the etch products are typically (CH 3 ) y Zn and ZnCl x , respectively, Figure 8. Spectrally-resolved reflectivity data obtained for the fabricated ZnO nanostructures (at normal incidence). (a) the total reflectivity data from the FDTD Lumerical simulations, where the optical constants 48 for bulk ZnO have been used; Inset: the corrected 'sol-gel' ZnO results. (b) the total reflectivity data (spot size ∼9 mm 2 ) for the plain Si substrate and ZnO layer (∼250 nm) on Si, and the specular reflectivity data (spot size ∼900 μm 2 ) for the ZnO nanodisk/cone arrays on Si.
) unless CC License in place (see abstract where the reported vapor pressure for the (CH 3 ) y Zn is 301 Torr at 20 • C and for ZnCl x 1 Torr at 428 • C. Thus the etch rate drastically decreases with Cl 2 concentration in the Cl 2 -based chemistry. However, for the CH 4 -based chemistry the etch rate increases for a higher CH 4 concentration and an etch rate as high as ∼280 nm/min has been reported. 38 The work by Guo et al. 42 using RIE, with CH 4 /H 2 -based chemistry, showed that the etch rate of ZnO strongly depends on the gas pressure and composition. It is suggested that a volatile metalorganic zinc compound is formed and that the ZnO etch rate linearly increases with RF plasma power. The highest etch rate reported in that work is ∼130 nm/min.
Other chemistries for ICP-RIE etching of ZnO have been reported 39 -example, BCl 3 /Cl 2 /Ar-and BCl 3 /Ar-based chemistries with highest etch rates up to ∼100 and ∼130 nm/min, respectively. By using only BCl 3 plasma an etch rate of ∼150 nm/min was obtained. The BCl 3 chemistry shows to be more effective in etching compared to Ar and Ar/Cl 2 . This is attributed due to the higher volatility of the byproducts, e.g., BOCl, (BOCl) 3 and BO 2 . However due to the presence of B-Cl residues, an additional buffered oxide etch (BOE) treatment is necessary.
Work by Kim et al. 41 reports on CF 4 /Ar etching of ZnO with a highest etch rate of ∼150 nm/min with a relatively low ICP power compared to previous works. The non-volatile by-product of Zn(CF x ) y synthesized during the etching process are removed by Ar + -ion bombardment.
Joo et al. 43 report on ICP-RIE etching of ZnO by N 2 /Cl 2 /Ar-based chemistry. The obtained etch rate and the selectivity to SiO 2 were ∼110 nm/min and ∼3.2, respectively. The N 2 gas had a positive effect due to the formation of N x O y by-products.
In this work, a different approach has been used to dry etch ZnO layers by RIBE. The advantage of this approach is that the same tool can be used for the size reduction of the CL etch mask (SiO 2 ) as well as for etching of the ZnO layer. For the (hard) mask fabrication a combination of SiO 2 colloidal particles (by CL) and a thin SiO 2 layer (deposited by PECVD) underneath was used. The original diameter of the colloidal particle determines the hexagonal array period and the colloidal particle size reduction by CHF 3 /Ar chemistry, determines the diameter of the etch mask. A CH 4 /H 2 /Ar-based chemistry was used for the etching of the ZnO layer due to the advantages noted from previous work discussed above, e.g., high volatility (CH 3 ) y Zn by-products, easy to handle and less toxic and corrosive than chloride and bromide gases. Important factors for the tuning of the RIBE process for the ZnO layer etching were the ZnO:mask selectivity, etch rate and anisotropy. The RIBE tool provides the possibility to combine chemical and physical etching; resulting in the etching of well-defined nanostructures. A further advantage is that the sample stage can be tilted/rotated, providing an additional flexibility and control for etch direction/shaping.
In this work, a compromise was made between the etch selectivity and the etch rate and the aim was not to obtain the highest etch rate. For comparison a sputtered ZnO layer was used for etching, resulting in a lower etch rate (∼30 nm/min) than for the sol-gel ZnO (∼51 nm). The higher etch rate for the sol-gel ZnO is most likely due to the porous nature of the layer.
The spectrally-resolved reflectivity measurements (wavelength range 300-850 nm) by spectrophotometry and the FDTD simulations show similar results (see Fig. 8). For the simulations both bulk and 'sol-gel' ZnO refractive index data (obtained by ellipsometry) were used for the structures. Due to the lower refractive index, the sol-gel ZnO results show a slight (red)shift of the spectrum and the reflectance is observed to be, on average, ∼2% higher compared to bulk. The ZnO Rayleigh's film (∼250 nm) shows an average total reflectivity of ∼20-30% and shows a Fabry Perot-effect. The simulated data for the ZnO nanodisk array shows an average reflectivity of ∼15-20% with dips at wavelengths around ∼433 nm and ∼510 nm due to the array period and Mie resonances (related to the diameter of the nanodisks), respectively. The lack of dips due to Mie resonances for the fabricated nanodisk array is most likely due to disk diameter variations due to the SiO 2 particle sizes and further process-induced variations. The ZnO (truncated) nanocone array, shows an average total reflectivity of ∼5-10% and has the lowest measured surface reflection. Additional simulation data for an optimal structure for the ZnO (truncated) nanocone array on Si shows that the surface reflection can be further minimized to ∼0-5% for the wavelength range 450-800 nm (see Fig. 7).
The broadband omnidirectional anti-reflectivity and low absorption of the well-defined ZnO nanostructures makes them very interesting for optoelectronic applications, e.g., solar cells and photodetectors. In addition, these highly transparent (optical guiding) structures could be useful for LED applications. Furthermore the sol-gel ZnO porous nature and possibility for hierarchical ZnO structuring could also make these structures interesting for (bio/gas) sensing and photo catalysis.

Conclusions
A flexible method is introduced using the fabrication of relatively thick sol-gel ZnO films (up to ∼2 μm) on silicon (Si) which is used for the RIBE etching of well-defined ZnO nanostructures having a good control over their periodicity and nanostructure geometry. This cheap and easy method is interesting since the sol-gel layers can be applied on a variety of substrates. Both the lithography-mask size reduction step and the ZnO etching were performed in the etching chamber. A colloidal lithography method using SiO 2 colloidal particles was applied to pattern the surface with a close-packed hexagonal array. CHF 3 /Ar-based chemistry was used for size reducing the SiO 2 mask. RIBE, using a CH 4 /H 2 /Ar-based chemistry, was used to etch the ZnO films resulting in a ZnO:SiO 2 selectivity of ∼3.4 and an ZnO etch rate of ∼51 nm/min. The sol-gel ZnO etch rate is significantly higher compared to that of sputtered ZnO (∼30 nm/min). Sol-gel ZnO nanodisk arrays, (truncated) nanocone arrays and planar layer(s) were fabricated on Si-substrates and optically characterized for their spectral dependent surface reflectivity. The fabricated nanodisk array has a hexagonal array period of 500 nm, a height of ∼250 nm and a diameter of ∼250 nm. The fabricated (truncated) nanocone array has a hexagonal array period of 500 nm, height of ∼1.1 μm and top-bottom diameter of ∼110-500 nm.
The reflectivity data obtained from FDTD simulations and spectrophotometry measurements show similar results. For the nanodisk array on Si average reflectivity of ∼15-20%, in the wavelength range of 400-850 nm, and for the (truncated) nanocone array on Si reflectivity as low as ∼5-10% were obtained. Simulations regarding the optimal anti-reflective ZnO (truncated) nanocone array structuring on a Si surface indicate further reduction in reflectivity, as low as ∼0-5% over the wavelength range of 450-800 nm.
The broadband omnidirectional anti-reflectivity and low absorption makes the sol-gel ZnO nanostructures very interesting for optoelectronic applications, e.g., solar cells, photodetectors and LEDs. In addition, this sol-gel ZnO (nano)structuring could be useful for (bio/gas) sensing and photo catalysis applications due to its porous nature and high surface-to-volume ratios.