Bidimensional SnSe2-Mesoporous Ordered Titania Heterostructures for Photocatalytically Activated Anti-Fingerprint Optically Transparent Layers

Abstract

The design of functional coatings for touchscreens and haptic interfaces is of paramount importance for smartphones, tablets, and computers. Among the functional properties, the ability to suppress or eliminate fingerprints from specific surfaces is one of the most critical. We produced photoactivated anti-fingerprint coatings by embedding 2D-SnSe₂ nanoflakes in ordered mesoporous titania thin films. The SnSe₂ nanostructures were produced by solvent-assisted sonication employing 1-Methyl-2-pyrrolidinone. The combination of SnSe₂ and nanocrystalline anatase titania enables the formation of photoactivated heterostructures with an enhanced ability to remove fingerprints from their surface. These results were achieved through careful design of the heterostructure and controlled processing of the films by liquid phase deposition. The self-assembly process is unaffected by the addition of SnSe₂, and the titania mesoporous films keep their three-dimensional pore organization. The coating layers show high optical transparency and a homogeneous distribution of SnSe₂ within the matrix. An evaluation of photocatalytic activity was performed by observing the degradation of stearic acid and Rhodamine B layers deposited on the photoactive films as a function of radiation exposure time. FTIR and UV-Vis spectroscopies were used for the photodegradation tests. Additionally, infrared imaging was employed to assess the anti-fingerprinting property. The photodegradation process, following pseudo-first-order kinetics, shows a tremendous improvement over bare mesoporous titania films. Furthermore, exposure of the films to sunlight and UV light completely removes the fingerprints, opening the route to several self-cleaning applications.

Keywords: 2D materials; SnSe2; anti-fingerprint; photocatalysis; titania.

Figure 1

The schematic process of (a) exfoliation process of SnSe₂ powder and (b) preparation of mesoporous ordered titania films containing dispersed nanoflakes of SnSe₂.

Figure 2

(a) High-resolution SEM image of representative SnSe₂ nanoflakes. (b) Statistical analysis of lateral dimension calculated from SEM images. The blue curve represents the Gaussian fit. (c) Representative AFM image of SnSe₂ nanoflakes. The inset shows the height profile of the flake indicated by the segment in white. (d) Statistical analysis of the SnSe₂ flake thickness distribution calculated from AFM images. The blue curve is the log-normal distribution fit.

Figure 3

(a) Raman spectra of bulk SnSe₂ (black line) and exfoliated SnSe₂ nanoflakes (red line) (λ_ex = 532 nm). The blue dot line is a guide for eyes. (b) UV-Vis absorption spectrum SnSe₂ flakes dispersed in ethanol (0.83 g L⁻¹).

Figure 4

(a) UV-Vis spectra and refractive index (b) of m-TiO₂ film (blue line) and m-TiO₂-SnSe₂ film (red line). The two curves overlap. The inset in (a) shows the snapshots of the films deposited on silica glass slides.

Figure 5

(a) FESEM and (b) TEM images of m-TiO₂-SnSe₂ film. (c) Raman comparison of m-TiO₂ film (blue line) and m-TiO₂-SnSe₂ film (red line). (d,e) FESEM image and EDS analysis of SnSe₂ flakes into m-TiO₂ film and (f) GI-XRD pattern of m-TiO₂-SnSe₂ film. The asterisk indicates the (200) diffraction of the Si substrate.

Figure 6

Chemical structure of Rhodamine B (a), Rhodamine 110 (b), and stearic acid (c).

Figure 7

(a) UV–Vis absorption spectra of an aqueous solution of RhB (2.5 × 10⁻⁶ mol L⁻¹) as a function of UV (365 nm) exposure time. (b) UV-Vis absorption spectra of the aqueous RhB solution containing dispersed SnSe₂ flakes (0.07 g L⁻¹) as a function of UV exposure time. (c,d) Percentage absorbance decrease and wavelength shift as a function of the UV exposure time. The data were taken from (b), using the maxima at 560 nm as reference for the calculation of the percentage decrease and wavelength shift. The line is a guide for the eyes.

Figure 8

Optical images of RhB aqueous solutions (a,b) and RhB solution containing dispersed SnSe₂ flakes (c–e) at different UV illumination times, t0 (before exposition), t90 (90 min), and t130 (130 min).

Figure 9

UV-Vis spectra of Rhodamine B deposited on m-TiO₂ (a) and m-TiO₂-SnSe₂ (b) films as a function of UV exposure time.

Figure 10

(a) Optical absorption spectrum of RhB deposited on m-TiO₂ film and curve fit with 2 Gaussian functions. (b) Decrease of the integrated absorbance calculated from the Gaussian function peaked at 565 from the spectra of RhB deposited on m-TiO₂ and m-TiO₂-SnSe₂ films (black squares and red dots, respectively). The lines depict the exponential decay fit of the experimental data.

Figure 11

(a) Degradation rate (lines are a guide for eyes) obtained by measuring the decrease in intensity of the 2945 cm⁻¹ C-H₂ stretching mode and (b) decay law (dashed lines represent the decay fit) of stearic acid deposited on the different samples.

Figure 12

Photographs of human fingerprints on m-TiO₂ (a,b) and m-TiO₂-SnSe₂ (f,g) films before and after exposure to sunlight in air for 24 h. Infrared images of a human fingerprint deposited on m-TiO₂ (c–e) and m-TiO₂-SnSe₂ (h–l) films. The images were taken by integrating the area of the -CH₂ stretching bands of stearic acid. The fingerprint was produced by immersing the finger in a stearic acid solution. The infrared images were detected at different times: as deposited, after 1 h and after 3 h of UV exposure, respectively. The color scale bar shows the intensity scale in false colors; the red and blue colors represent the highest and the lowest absorbance of the infrared signal, respectively.

Figure 13

Hypothesis of the Rhodamine B degradation mechanism in presence of SnSe₂ nanoflakes under UV-Visible light. NHE, normal nitrogen electrode; CB, conduction band; VB, valence band; LUMO, lowest unoccupied molecular orbital; HOMO, highest unoccupied molecular orbital.