1 Introduction

Superhydrophilic surfaces (SHS) with water contact angle (WCA) less than 5° have been a center of attention in research and industries mainly due to their applications in self-cleaning and antifogging effects [1,2,3]. Generally, the wettability of a surface is influenced by its surface energy and roughness. Hence, SHS can be accomplished by maneuvering the chemical composition and morphology of the surface [4,5,6]. It is a well-known fact that SHS has high surface energy. This makes them susceptible to adhesion of contaminants, which deplete the superhydrophilic nature of the surface. Therefore, it is desirable to have SHS, which can photocatalytically degrade contaminants by natural means. There are reports of achieving superhydrophilicity of metal oxide semiconductors (TiO2, ZnO) by the generation of surface oxygen vacancies upon photoirradiation [7,8,9]. The hydroxyl group of atmospheric water adsorbed over the surface oxygen vacancies present in the photocatalyst leads to the formation of SHS [10]. It must be noted that hydroxyl groups on the surface stimulate the photocatalytic activity of metal oxides by chemical degradation of organic pollutants. In this way, the photocatalyst involves in breaking down the pollutant from the surface, whereas the superhydrophilic character causes the water to stretch over it, which carries away the filth and impurities.

TiO2 has been widely studied because of its photocatalytic activity [2]. Among the metal oxides, ZnO is a suitable alternative to TiO2 because its photodegradation mechanism is similar to that of TiO2 and ZnO has approximate equal band gap energy like TiO2. However the photocatalytic efficiency of single-phase ZnO or TiO2 semiconductor is significantly low because of the quick recombination between photogenerated electrons and holes. ZnO (Eg = 3.37 eV) and SnO2 (Eg = 3.6 eV) are wide band gap, n-type semiconductor, which are studied for their widespread application in antimicrobial, sensors, photocatalyst, and self-cleaning surfaces [11,12,13,14,15,16]. Their photocatalytic property is particularly studied because on coupling them together their efficiency gets increased considerably as compared with individual form [17,18,19,20,21]. This is mainly due to the formation of heterostructure that inhibits charge recombination of electron and holes. Although there are many published reports of ZnO/SnO2 heterostructures for degradation of organic pollutants in powder form, depositing it in the form of films gives the added benefit to it by being solid and can be recycled easily [22,23,24,25]. Recently, we have demonstrated the significant improvement in wettability and photocatalytic efficiency of ZnO/SnO2 thin films with annealing temperature [26]. But charge ion diffusion from the substrate during annealing may alter the properties of films [27]. So in order to prevent that we have prepared the bilayer ZnO/SnO2 heterostructure and comparative study was made with monolayer component. The wettability studies reveal that ZnO/SnO2 thin films exhibit superhydrophilic character with a very high degree of transparency. The photocatalytic activity of the films was assessed by measuring the degradation of an aqueous solution of methylene blue (MB), methyl orange (MO), and their mixture. Apart from degradation of dyes in aqueous form, degradation of a thin layer of stearic acid adsorbed on the film surface was also assessed by measuring the change in WCA as a function of photoirradiation time. The transparent superhydrophilic ZnO/SnO2 films being photocatalytically active will have potential application in antifogging and self-cleaning.

2 Experimental

Borosilicate glass was used as a substrate to prepare nanocrystalline films of SnO2, ZnO, and ZnO/SnO2 (ZnO on top and SnO2 at the bottom) using sol–gel method. SnO2 sol was prepared by dissolving SnCl2·2H2O in ethanol [15]. After stirring for 1 h at 70 °C, polyethylene glycol was added to the solution and was stirred continuously for 2 h at the same temperature to obtain a clear sol. ZnO sol was prepared by the method described elsewhere [28]. Sols of SnO2 and ZnO were spin coated on BSG and annealed at 500 °C to obtain SnO2 and ZnO thin films. In case of ZnO/SnO2 thin films, SnO2 sol is first spin coated on BSG and annealed at 500 °C followed by deposition of ZnO sol and subsequent heating at 500 °C.

A UV-Vis (Agilent Cary 60) spectrophotometer scanning over the wavelength range of 200–800 nm was used to investigate the optical properties and photocatalytic activity of the synthesized films. Contact angle meter (DMs-401, Kyowa Interface Science Co Ltd, Japan) via sessile drop method was used to examine the wetting properties. The Owens and Wendt geometric mean two liquid method was used to calculate surface energies of thin films. An X-ray diffractometer (Rigaku Ultima IV) with Cu Kα radiation of wavelength 0.15405 nm in the 2θ range of 20°–80° was used for phase and crystal structure analysis. Field Emission Gun Scanning Electron Microscope (FEG-SEM, JSM-7600F) joint with energy dispersive X-ray (EDX) was used to determine the surface morphology and elemental composition of the films. The high-resolution transmission electron microscope (HRTEM) images were acquired by a Tecnai 20 G2 STwin, FEI electron microscope, operated at 200 kV. Atomic force microscope (AFMNT-MDT NTEGRA Vita) was used for investigating the surface roughness of the films. Nanoindentation technique (Agilent technologies, model: G 2000) was used to examine the nanomechanical properties with maximum load of 500 mN.

3 Results and discussion

The X-ray diffraction (XRD) pattern of SnO2, ZnO, and ZnO/SnO2 is presented in Fig. 1. It is evident from XRD pattern that SnO2 films over BSG crystallized into the tetragonal structure (JCPDS 88-0287), whereas ZnO films show hexagonal wurtzite structure (JCPDS 89-1397). The presence of SnO2 and ZnO crystal phase in ZnO/SnO2 thin films is evident from the coexistence of both hexagonal wurtzite ZnO and tetragonal SnO2. Scherrer’s formula was used to calculate the average crystallite size from full-width half maxima. It can be seen that the diffraction peaks of SnO2 are fairly broad, which gives a small average crystallite size of 5 nm. ZnO exhibits sharp diffraction peaks, which result an increase in average crystallite size of 16.8 nm. However, in ZnO/SnO2, the average crystallite size of ZnO reduces to 14.3 nm, which is smaller than that of pure ZnO. In contrast, the average crystallite of SnO2 was found to be 5.1 nm, which is similar to that of pure SnO2. This indicates that SnO2 hamper the growth of ZnO nanocrystal in ZnO/SnO2 [24].

Fig. 1
figure 1

X-ray diffraction pattern of SnO2, ZnO, and ZnO/SnO2 thin films

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The surface morphology of SnO2, ZnO, and ZnO/SnO2 thin films are shown in Fig. 2a–c. The particles are uniformly distributed. The measured mean particle sizes were found to be 7.68, 21.9, and 16.5 nm for SnO2, ZnO, and ZnO/SnO2 thin films, respectively. The thickness of the ZnO/SnO2 thin films calculated from cross-sectional FESEM image was found to be in the range of 95 nm as depicted in Fig. 2d. The EDX spectra of SnO2, ZnO, and ZnO/SnO2 thin films are shown in Fig. 3a–c. The existence of preferred elements in SnO2, ZnO, and ZnO/SnO2 films respectively are confirmed by EDX analysis. The appearances of peaks of silicon, calcium, and sodium are due to the borosilicate glass substrate. Moreover, the peaks of carbon and gold are detected due to the use of carbon tape on the stub and gold coating on the sample.

Fig. 2
figure 2

FESEM images of a SnO2, b ZnO, c ZnO/SnO2, and d cross-sectional image of ZnO/SnO2 thin films

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Fig. 3
figure 3

EDX spectra of a SnO2, b ZnO, and c ZnO/SnO2 thin films

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The formation of ZnO/SnO2 heterostructure is further probed through HRTEM. The high crystallinity of the heterostructure is confirmed through the appearance of clear lattice fringes in the HRTEM image as shown in Fig. 4. A region having coexistence of crystalline ZnO and SnO2 nanocrystals are accessed with a combination of FFT-IFFT technique and shown as insets in Fig. 4. The two distinct lattice fringes are observed. The observed fringes correspond to interplaner distances of 3.40 Å and 2.46 Å. The calculated lattice spacing is in agreement with (110) and (101) of the tetragonal SnO2 and hexagonal ZnO respectively indicating the formation of ZnO/SnO2 heterostructure.

Fig. 4
figure 4

High-resolution transmission electron microscope image of ZnO/SnO2 heterostucture. Top and bottom inset shows FFT and IFFT image of selected region. The lattice fringe region of both the crystals line profile with computed length scale is shown in right

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For investigating the surface wettability of SnO2, ZnO, and ZnO/SnO2 thin films, WCA was measured by keeping the drop volume of water as 2 µl. The variation of WCA with r.m.s roughness is shown in Fig. 5. The WCA of as-deposited SnO2 and ZnO thin films is 11.8° and 83.9°. It is interesting to see that ZnO/SnO2 thin films exhibit superhydrophilic character with WCA of 3.4°. The 3D AFM images of SnO2, ZnO, and ZnO/SnO2 thin films are shown in Fig. 6a–c. The r.m.s roughness of SnO2, ZnO, and ZnO/SnO2 films were found to be 46.90 nm, 11.40 nm, and 60.04 nm, respectively. It is evident from Fig. 5 that with an increase in roughness of the film, the WCA decreases, which in agreement with the Wenzel model of wetting [29].

Fig. 5
figure 5

Variation of the water contact angle with r.m.s roughness

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Fig. 6
figure 6

3D AFM images of a SnO2, b ZnO, and c ZnO/SnO2 thin films

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The antifogging performance was evaluated by immersing the samples in cold water for 10 min. The samples were then placed onto a paper to examine the accumulation of mist and tiny drops of water. Due to the contrast in temperature between the cold water and the surrounding, water molecules get condensed forming a fog like layer. The optical comparison was recorded using contact angle meter and mobile lens camera. The pictorial images of glass and superhydrophilic ZnO/SnO2 is shown Fig. 7a, b. Tiny droplets of water (fogging) formed on the untreated glass can be seen from Fig. 7a, whereas superhydrophilic ZnO/SnO2 shows antifogging behavior. The inset of Fig. 7a, b shows that the optical transparency of bare glass is significantly reduced due to the formation of fog. However, for superhydrophilic ZnO/SnO2 with WCA < 5°, the water droplets on the surface will quickly spreads away, which inhibits the formation of fog. Figure 7c depicts the transmittance spectra of the films. The films were fairly transparent with ZnO and ZnO/SnO2 showing transparency of more than 85% in the visible region. Superhydrophilic ZnO/SnO2 films with high degree of transparency will have potential application in self-cleaning and antifogging [30].

Fig. 7
figure 7

Photographic images showing the antifogging effect on a glass and b superhydrophilic ZnO/SnO2 films (insets a and b show the transparency of glass and superhydrophilic ZnO/SnO2 films) and c transmission spectra of SnO2, ZnO, and ZnO/SnO2 thin films

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One of the major challenges for wide use of SHS with antifogging behavior is the lack of mechanical stability. Hence to probe the mechanical property (Hardness and Young’s modulus) of superhydrophilic ZnO/SnO2 film, nanoindentation technique was employed. The films are substantially hard with average hardness and Young’s modulus are 53.8 ± 5.9 and 3.31 ± 0.47 GPa, respectively.

The adsorption of airborne organic contaminants has an adverse effect on the superhydrophilic character [31]. In order to investigate the role of organic pollutants on the superhydrophilic character of the film surface, a thin layer of stearic acid was coated on the surface of the photocatalyst. The formation of a stearic acid layer on the film surface is confirmed by FT-IR as shown in Fig. 8. The peaks at 2915 cm−1 and 2848 cm−1 correspond to –CH3 asymmetrical and –CH2 symmetrical stretching vibrations of stearic acid. The peaks at 1697 cm−1 correspond to the carboxyl group of stearic acid. Metal oxides on modification with stearic acid form a metal stearate bond with the carboxyl group attached to the metal and the alkyl group orients outward forming a self-assembled layer [32]. The surface energy of the films was measured by using Owen–Wendt geometric mean method [33]. The surface energy and WCA of films modified with stearic acid are shown in Fig. 9. The WCA of stearic acid modified SnO2 and ZnO thin films are 125.6° and 108.2°, whereas stearic acid modified ZnO/SnO2 thin films have WCA of 132.2°. The surface energy of stearic acid modified films is shown in Table 1. The surface energy of stearic acid modified SnO2 and ZnO thin films is 27.2 and 34.8 mJ/m2, whereas the surface energy of stearic acid modified ZnO/SnO2 thin films is 14.8 mJ/m2. It is clear from Table 1 that upon modification with stearic acid, the polar component (water-loving) has no contribution in the total surface energy. As a result, the hydrophobicity of films is entirely due to the contribution of the dispersive component (nonpolar) in total surface energy [15].

Fig. 8
figure 8

FT-IR spectra of a ZnO/SnO2, b ZnO/SnO2 modified with stearic acid, and c ZnO/SnO2 modified with stearic acid after UV irradiation

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Fig. 9
figure 9

Water contact angle and surface energy of stearic acid modified SnO2, ZnO, and ZnO/SnO2 thin films

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Table 1 Surface energy of stearic acid modified SnO2, ZnO, and ZnO/SnO2 thin films
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A self-cleaning photocatalyst can only be realized if the organic pollutants are degraded from the surface of the films by natural means. The degradation of stearic acid from the surface of the films was carried out by irradiating the stearic modified films with UV light. The degradation of stearic acid is confirmed by FT-IR (Fig. 8) with the decrease in intensity of stearic acid peaks. The variation of WCA upon UV irradiation is shown in Fig. 10a. It is seen that the hydrophobic stearic modified films become hydrophilic after exposure to UV light for 60 min. To understand the rate of degradation of stearic acid, hydrophilic conversion rates were calculated from the slope of the inverse of WCA plotted against UV irradiation time (Fig. 10b). Table 2 shows the hydrophilic conversion rate of SnO2, ZnO, and ZnO/SnO2 thin films. The hydrophilic conversion rate was found to be 6.85 × 10−4 deg−1 min−1, 5.90 × 10−4 deg−1 min−1, and 12.70 × 10−4 deg−1 min−1 for SnO2, ZnO, and ZnO/SnO2 thin films.

Fig. 10
figure 10

Variation of a water contact angle and b inverse of water contact angle of SnO2, ZnO, and ZnO/SnO2 thin films modified with stearic acid under UV light

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Table 2 Hydrophilic conversion rate, rate constant, and photocatalytic efficiency of SnO2, ZnO, and ZnO/SnO2 thin films
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The photocatalytic activities of the films were further carried out by the degradation of cationic (MB), anionic (MO) dye. and their mixture (MO–MB) under UV light (254 nm, Philips Hg germicidal lamp) for 3 h. The absorbance spectra of MB, MO, and MO–MB as a function of irradiation time are shown in Fig. 11. The photocatalytic efficiency of the films was calculated by using the following equation:

$$\eta = \left(\frac{{C_o - C}}{{C_o}}\right) \times 100,$$
(1)

where Co is the initial concentration of the dye and C is the concentration after UV-irradiation time.

Fig. 11
figure 11

Absorbance spectra of degradation MB (a–c), MO (d–f), and MO–MB (g–i) by SnO2, ZnO, and ZnO/SnO2 thin films

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The photocatalytic efficiency of SnO2, ZnO, and ZnO/SnO2 thin films is shown in Fig. 12a–c. As inscribed in Fig. 12, SnO2 degrades 85% of MB and 37% of MO, whereas ZnO degrades 59% and 34% of initial MB and MO concentration, respectively. The degradation efficiency of SnO2 is higher than ZnO, which is in agreement with the earlier report [23]. The efficiency is highest for ZnO/SnO2, which degrades 95% of MB and 51% of MO. To understand the rate at which the degradation was taking place, rate constant was calculated using the following pseudo first-order equation:

$$\ln \left(\frac{{C_o}}{C}\right) = - kt.$$
(2)
Fig. 12
figure 12

% Photocatalytic efficiency of a SnO2, b ZnO, and c ZnO/SnO2 thin films

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The kinetics of photodegradation of MB and MO by SnO2, ZnO, and ZnO/SnO2 thin films are shown in Fig. 13a, b. The slope of ln(Co/C) as a function of UV-irradiation time gives the rate constant. The rate constant of SnO2, ZnO, and ZnO/SnO2 thin films using MB and MO as model organic pollutants is shown in Table 2. The degradation rate of MB and MO for SnO2 was found to be 5.0 × 10−3 min−1 and 1.12 × 10−3 min−1, whereas for ZnO was 2.26 × 10−3 min−1 and 1.05 × 10−3 min−1. The rate constant of ZnO/SnO2 thin films in degrading MB and MO was found to be 7.36 × 10−3 min−1 and 1.81 × 10−3 min−1, respectively. The recyclability of ZnO/SnO2 thin films as a photocatalyst was carried out by performing the photodegradation of MB and MO for five cycles. The % photocatalytic degradation efficiency of ZnO/SnO2 after five cycles is shown in Fig. 13c. The results show that the ZnO/SnO2 films remain stable after five cycles of photodegradation of MB and MO.

Fig. 13
figure 13

Kinetics of photodegradation of a MB and b MO by SnO2, ZnO, and ZnO/SnO2 thin films, and c recyclability of ZnO/SnO2 heterostructure in photodegradation of MB and MO

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The rate constants calculated for various photocatalysts showed that the degradation of MB is faster than MO. The absorbance spectra (Fig. 11g–i) for mixed dye (MO–MB) also confirm that MB degrades faster as compared with MO. This is due to the presence of the azo group in MO, which makes it more stable, hence delaying the degradation rate as compared with MB [34]. Moreover, the negatively charged photocatalyst (n-type) increases the electrostatic attraction with cationic MB because of their opposite charge polarity. This leads to the easy adsorption of cationic MB on the film surface as compared with anionic MO. As a result, the degradation of MB is faster in comparison with MO.

The improved photocatalytic performance of ZnO/SnO2 can be understood from the energy band diagram shown in Fig. 14. The band edge location of the conduction band (CB) and valence band (VB) of the semiconductor was calculated by using the following equations:

$$E_{\mathrm{CB}} = \chi - E_{\mathrm{e}} - 0.5E_{\mathrm{g}},$$
(3)
$$E_{\mathrm{VB}} = E_{\mathrm{g}} - E_{\mathrm{CB}},$$
(4)

where χ is the absolute electronegativity of the semiconductor (χ = 5.79 eV for ZnO and χ = 6.25 eV for SnO2 [35]), Ee is the energy of free electron on the hydrogen scale (Ee ~ 4.5 eV) and Eg is the band gap of the semiconductor. The position of CB edges of SnO2 and ZnO versus normal hydrogen electrode was calculated by using Eq. (3). The \(E_{\mathrm{CB}}^{\mathrm{SnO}_{2}}\) and \(E_{\mathrm{CB}}^{\mathrm{ZnO}}\) for SnO2 and ZnO was found to be −0.15 eV and −0.39 eV, respectively. Similarly, the position of VB edge, i.e., \(E_{\mathrm{VB}}^{\mathrm{SnO}_{2}}\) and \(E_{\mathrm{VB}}^{\mathrm{ZnO}}\) calculated by using Eq. (4) for SnO2 and ZnO was found to be +3.65 eV and +2.98 eV, respectively. Since the CB edge of ZnO is more negative and its VB edge is less positive than SnO2, ZnO/SnO2 forms type-II heterostructure [36]. According to type II band alignment, when SnO2 and ZnO form a heterostructure, the electron transfer occurs from ZnO to SnO2 and the hole from SnO2 to ZnO until a stable equilibrium is attained by the system (Eq. 5). This leads in the effective separation of electron and hole pairs. The holes are trapped by hydroxyl group (OH−) or physisorbed water molecules at the ZnO surface to produce a hydroxyl radicals (OH*) and the electrons are captured by dissolved O2 to generate superoxide anions (\(\mathrm{O}_2^{ \ast - }\)) as shown in Eqs. (6) and (7), respectively. Further, the hydroxyl radicals (OH*) and superoxide ions \((\mathrm{O}_2^{ \ast - })\) are particularly reactive to decompose organic pollutants present on the surface of the films because of their strong oxidizing and reducing ability (Eq. 8). Thus, the effective separation of charge carrier is responsible for the higher photocatalytic activity of the ZnO/SnO2 films as compared with the monocomponent thin films. The possible reaction for the photocatalytic activity of ZnO/SnO2 catalyst is listed below

$$(\mathrm{ZnO/SnO}_{2}) + \mathrm{h}\nu \to \mathrm{ZnO}(\mathrm{h}_{\mathrm{VB}}^ + ) + \mathrm{SnO}_2(\mathrm{e}_{\mathrm{CB}}^ - ),$$
(5)
$${\rm{SnO}}_2({\rm{e}}_{{\rm{CB}}}^ {-}) + {\rm{O}}_2 \to {\rm{SnO}}_2 + {\rm{O}}_2^{\ast^{_-}},$$
(6)
$$\mathrm{ZnO}\left(\mathrm{h}_{\mathrm{VB}}^{+}\right) + \mathrm{H}_{\mathrm{2}}{\mathrm{O}} \to \mathrm{OH}^{\ast} + \mathrm{H}^{+},$$
(7)
$$\mathrm{Pollutants} + \mathrm{OH}^\ast /\mathrm{O}_2^{\ast-} \to \mathrm{CO}_2 + \mathrm{H}_{\mathrm{2}}{\mathrm{O}}.$$
(8)
Fig. 14
figure 14

Energy band diagram showing the charge-transfer process in the ZnO/SnO2 heterojunction

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4 Conclusion

This work presents the wettability and photocatalytic studies of SnO2, ZnO, and ZnO/SnO2 thin films deposited on glass substrate by sol–gel spin coating technique. The result shows that ZnO/SnO2 thin films exhibited superhydrophilic character with a very high degree of transparency. The photocatalytic activity of the films was studied using model pollutants in a solid and liquid phase. It was shown that the efficiency of ZnO/SnO2 thin films was superior in comparison to ZnO and SnO2 thin films. This is attributed to the formation of heterostructure between ZnO and SnO2, which suppresses the charge recombination. We believe a superhydrophilic film with high transparency can have potential application in antifogging and self-cleaning.