We present zinc oxide (ZnO) particles obtained inside a porous silicon matrix in the same electrolytic process using a p-type silicon wafer in a hydrofluoric acid (HF) solution containing formaldehyde (CH2O) and hydrated zinc sulfate as additives. The X-ray diffraction pattern of the sample confirmed the presence of ZnO with a hexagonal-type wurtzite structure. Photoluminescence (PL) spectra of the samples, before and after the functionalization process, were measured to observe the effect of ZnO inside the porous silicon. The PL measurements of porous silicon functionalized with ZnO (ZnO/PS) revealed infrared, red, blue, and ultraviolet emission bands. The ultraviolet region corresponds to the band-band emission of ZnO, and the visible emission is attributed to defects. The results of the nitrogen adsorption/desorption isotherms of the PS and ZnO/PS samples revealed larger BET surface areas and pore diameters for the ZnO/PS sample. We conclude that ZnO/PS can be obtained in a one-step electrolytic process. These types of samples can be used in gas sensors and photocatalysis.
In recent years, zinc oxide (ZnO) has been investigated as a functional material for electronic and optoelectronic devices such as gas sensors [
ZnO films on silicon substrates result in a large stress between the ZnO and silicon due to the significant mismatch in the thermal expansion coefficients and lattice constants of these materials [
In this study, PS and ZnO/PS were obtained via electrochemical anodization. The PS was characterized by PL, nitrogen adsorption/desorption isotherm, and gravimetry. The ZnO/PS was characterized by X-ray measurements (XRD), PL, nitrogen adsorption/desorption isotherm, and gravimetry. The ZnO/PS was made without a catalyzer.
The PS was obtained using wafers of p-type crystalline silicon with an orientation of <100> and an electrical resistivity of 0.01–0.02 Ω·cm in a teflon reactor in which the cathode was a tungsten wire. We used a mixture of hydrofluoric acid (HF) (J.T.Baker 48%; J.T.Baker Chemical Company, Mexico) and CH2O formaldehyde (37%, Merck KGaA; Merck Company, Mexico) in a ratio 1 : 1 as an electrolyte. We used a current density of 33.50 mA/cm2, and the anodization process took 16.66 minutes. The porosity and thickness of PS were 50% and 50
We used XRD to investigate the crystal phase of the samples. Figure
(a) XRD patterns of ZnO/PS. The inset shows the expanded view of the (110), (103), and (200) diffraction peaks corresponding to the ZnO planes. (b) Multi-Gaussian fitting of the XRD patterns between 67° and 72°. The green dotted line corresponds to the experimental data, and the red line is the sum of the two contributions, which consist of the crystalline silicon substrate peak (C-Si) and the PS layer.
The black line in Figure
(a) Room temperature PL of PS and ZnO/PS. (b) Multi-Gaussian fitting of PL spectra from the PS sample: the red line is the sum of the three contributions (blue, green, and pink lines), and the black line corresponds to the experimental data. (c) Multi-Gaussian fitting of PL spectra from the ZnO/PS sample: the red line is the sum of the tree contributions (blue, green, and pink lines), and the black line corresponds to the experimental data.
In order to explain our results, in this oxygen-rich condition, the amount of oxygen that diffuses into the sample increases. Therefore, the types of defects most frequently found in ZnO grown under O-rich conditions are intrinsic defects such as interstitial oxygen (Oi), zinc vacancies (VZn), and antisite oxygen (OZn).
The red line in Figure
When the PS is functionalized with ZnO, the PL intensity in the ultraviolet and red-infrared regions increases (Figure
The PS and ZnO/PS samples were texturally characterized by nitrogen physisorption. Figures
Nitrogen adsorption/desorption (ads./des.) isotherms at 77 K, (a) PS and (b) ZnO/PS.
In addition, the adsorption/desorption isotherms exhibit a type H1 hysteresis cycle that is associated with materials with pores with homogeneous shapes and sizes and narrow pore size distributions. The pore size distribution was evaluated using the Barret-Joyner-Halenda (BJH) and density functional (DFT) methods. This work confirmed the presence of mesoporosity in the PS and ZnO/PS materials. Considering that multilayer adsorption results in capillary condensation on a cylindrical mesopore at a given relative pressure, the pore size can be determined applying the Kelvin equation [
Using the constants for nitrogen, equation (
The Kelvin radius is the radius of the meniscus of the adsorbable condensed within the cylindrical pore at a relative pressure
From the adsorption data determined experimentally, we also calculated the surface specific of the material using the Brunauer-Emmett-Teller (BET) model [
Data obtained from the adsorption/desorption isotherms.
Sample | Pore diameter (nm) | Porosity (%) | Surface area (m2/g) |
---|---|---|---|
PS | 5.86 | 50 | 237.90 |
ZnO/PS | 8.19 | 50 | 294.00 |
We used the gravimetric method to obtain the porosity and etching rate of the PS samples with and without ZnO [
Porosity as a function of the applied current density for PS and ZnO/PS.
Etching rate as a function of the applied current density for PS and ZnO/PS.
Hole injection is well documented for the oxidation and dissolution of silicon in metal-assisted chemical etching of Si [
A similar behavior can be observed for both electrolytes if increasing the current density increases the porosity (Figure
ZnO was deposited and pores were formed simultaneously during the electrochemical anodization. The presence of ZnO inside the PS matrix was confirmed by XRD. Ultraviolet and red emissions of ZnO inside the PS were verified by PL. We suggest that the optical properties of ZnO/PS were enhanced by the functionalization process. Pore formation was demonstrated for nitrogen adsorption/desorption isotherms, proving the formation of a mesoporous material. Obtaining PS functionalized with ZnO was carried out at RT in step one of the electrolytic process.
The data used to support the findings of this study are available from the corresponding author upon request.
The authors declare that they have no conflicts of interest.
R. Juárez-Nahuatlato acknowledges CONACyT for a fellowship (CVU: 88178); funding was provided by VIEP-BUAP projects.