On the origin of the surface superhydrophobicity of rough-textured inorganic materials with intrinsic hydrophilicity

Abstract

In mechanism, surface superhydrophobicity is often thought to be the result of the trapped air within the grooves of a superhydrophobic surface, leading to a composite solid–liquid–air interface. However, the mechanism cannot reasonably reflect why the rough surfaces of intrinsically hydrophilic materials are capable of showing hydrophilicity or superhydrophobicity. In this work, several typical rough-textured inorganic materials (i.e. metal oxide, sulfide, selenide and halide) endowed with intrinsic hydrophilicity are taken as examples to reveal the superhydrophobic origin of intrinsically hydrophilic materials. The wettability of these rough-textured surfaces is usually hydrophilic when dried in N₂, while it is hydrophobic or superhydrophobic when dried in O₂. This distinct difference in wettability is closely related to anion vacancies of anions such as oxide, sulfide and halide ions. From the generation of H₂O₂ in water droplets on the rough-textured surfaces, it is found that the H₂O₂ yield increases with an increase in their hydrophobicity and decreases with an increase in their hydrophilicity, indicating an evident dependence of their surperhydrophobicity on the absorption of O₂²⁻ on their surfaces. DFT calculation shows that introducing V_a can give a higher adsorption-energy for molecular oxygen species (especially O₂²⁻) than N₂ and H₂O. Hence, we propose that the presence of abundant V_a on intrinsically hydrophilic inorganic materials can result in the formation of preferential O₂²⁻ adsorption layer in their grooves, which endows these inorganic materials with superhydrophobicity. Our results can provide further insight into the origin of superhydrophobicity in intrinsically hydrophilic materials and guide the design of water-proof functional surfaces.

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1. Introduction

A solid surface with superhydrophobic wettability is an ever-fresh scientific topic in surface chemistry and materials science and closely related with many chemical, physical, and biological processes. Superhydrophobic wettability is found in nature, e.g., in self-cleaning lotus leaves, directionally adhesive butterfly wings, antifogging mosquito eyes, and underwater non-wetted spider abdomen.^1–3 Natural superhydrophobic phenomena are derived from the combination of low-energy epicuticular wax and unique rough micro-nanostructures.^4,5 Inspired by natural surface characteristics, in the past decades, many artificial superhydrophobic surfaces have been manufactured via a variety of rough-textured techniques, which gives them great values in self-cleaning,⁶ metal corrosion protection,⁷ anti-icing,⁸ oil–water separation,⁹ and microfluidic devices.¹⁰

Generally, superhydrophobicity can be obtained either by preparing micro-/nano-scale roughness structures on hydrophobic materials or by modifying a rough-textured surface with organic chemical coating. In this regard, many theoretical hypotheses and models have been proposed to interpret natural and artificial superhydrophobic phenomena. Among them, the Wenzel and Cassie–Baxter theoretical models, which propose a basic hypothesis that the wettability of solid surfaces is determined both by surface roughness and surface energy, have been widely adopted. According to the models, superhydrophobic behaviors are attributed to the ability to trap air within the grooves of a rough surface, leading to a composite solid–liquid–air interface under water droplets (i.e. a trapping-air layer mechanism). In recent years, many advances have been made in the construction of water-proof functional surfaces using intrinsically hydrophilic inorganic materials such as metal oxide, sulfide, selenide and halide, which have been proven to have wide applications in electro- or photo-controllable water permeation, superhydrophobic antibacterial materials, water-resistant electronic devices, and self-cleaning solar cells.^11–15 Although some efforts have been devoted to interpret their superhydrophobic behaviors over recent years, such as hollow structure¹⁶ and re-entrant-textured structure,^17,18 the trapping-air layer mechanism cannot reasonably explain how the rough surfaces of these intrinsically hydrophilic materials are superhydrophobic. For example, why is the trapping-air layer on their rough surfaces stable under water, given that the intrinsic hydrophilicity tends to facilitate the formation of a water adsorption layer on rough surfaces rather than a trapping air layer? Why do their rough-textured surfaces with superhydrophobicity generally showcase a phenomenon of high viscosity “pegging” to water droplets? And why can their rough-textured surfaces with the same roughness exhibit either superhydrophobicity or superhydrophilicity?^19–21 Therefore, shedding new light on the origin of superhydrophobicity in intrinsic hydrophilic materials would be of great significance to further understand the superhydrophobic phenomenon as well as to design novel superhydrophobic materials for future applications. In many reports, the micro-/nano-textured inorganic materials are endowed with abundant anion vacancies (V_a)^22–25 (i.e., oxygen-, sulfide- and halogen-ion ones) both on the surface and in the bulk, and the V_a can play a pivotal role in tailoring their functionalities, such as electrical conductivity,²⁶ photocurrent,²⁷ gas sensitivity,²⁸ pH detection²⁹ and catalytic and photocatalytic performance.³⁰ Singh et al.³¹ reported that the wettability of some nano-textured metal oxides strongly depended on the oxygen-related defects. Yan and Zhang et al.^32,33 found that the introduction of oxygen vacancies can promote the adsorption of O₂ on catalytic materials and offer additional benefits for OER.

In the previous work,³⁴ we focused on the correlation between the wetting behavior and the oxygen vacancies for intrinsically hydrophilic metal oxides and found that the presence of oxygen vacancies makes the roughness on their surfaces more capable of adsorbing O₂ than N₂ and H₂O, which resulted in an oxygen-preferred adsorption layer in the grooves (namely, O₂-adsorption layer mechanism) rather than the trapping-air layer as commonly thought. The O₂-adsorption layer mechanism can not only explain the origin of superhydrophobicity and the “pegging” phenomenon for intrinsically hydrophilic oxide materials but also conduct the design of excellent superhydrophobicity with low adhesion to the water droplet. But though our research has advanced the understanding of the superhydrophobic mechanism for intrinsic hydrophilic metal oxides, some fundamental questions remain unclear when the mechanism of the O₂-adsorption layer is further extended to the intrinsic hydrophilic inorganic materials: Can the presence of V_a on their micro-/nano-textured surfaces cause a preferential O₂-adsorption rather than the air-trapped layer, as oxygen vacancies? How do the V_a with positive charge adsorb electrically neutral O₂ molecules to form a stable O₂-adsorbed layer within the grooves of their rough surfaces under water-droplet? Recent researches have proved that the harvesting of H₂O₂ in the water droplet on the intrinsic hydrophilic inorganic materials strongly depends on their superhydrophobicity, which suggests that their superhydrophobicity can be closely related to the absorption of O₂²⁻ on their rough-textured surfaces. If so, what is the role of adsorbed O₂²⁻ ions on their superhydrophobic behaviors?

In this work, we take a variety of representative hydrophilic inorganic materials including sulfide, selenide, oxide and halide (Table S1†) as model materials to reveal their superhydrophobic origin. These inorganic materials were prepared with different topographic structures, such as nanorods, nanoplates, nanoparticles, microrods and microparticles, and the drying and water contact angle (WCA) test were performed in different atmospheres, namely, air, nitrogen (N₂) and oxygen (O₂). Surface anion defects and related O₂ adsorption were investigated with X-ray photoelectron spectroscopy, Raman signals and spontaneous H₂O₂ generation. Combining the adsorption energies of H₂O, N₂ and O₂ on ideal and defected crystal plane calculated by density functional theory (DFT), we proposed a new understanding for the origin of superhydrophobicity of intrinsically hydrophilic inorganic materials based on O₂²⁻-dominant O₂-adsorption layer.

2. Results and discussion

A series of metal compounds of group VIA and group VIIA elements, including ZnS, CuS, Ni₃S₂, PbSe, CaF₂, AgCl, AgBr and AgI, with different topographic structures was prepared by conventional hydrothermal method or precipitation method with aqueous precursors. As shown in Fig. 1a, the respective topographic structures are oriented hexagonal nanorods of ZnS with diameters of ∼60 nm, oriented heazlewoodite nanoplates of Ni₃S₂ with 30 nm average thickness, covellite microrods of CuS with diameters of 50–100 nm, clausthalite PbSe microparticles with diameters of 1–4 μm, fluorite CaF₂ powder with mixture of sizes ranging from the nanoscale to the microscale, rocksalt nanoparticles of AgCl with average grain sizes of 100 nm, bromargyrite microparticles of AgBr with grain sizes of about 5 μm, and AgI microclusters consisting of iodargyrite nanoparticles, whose corresponding X-ray diffraction (XRD) analysis data are shown in ESI Fig. S1.† Given that these metal compounds of group VIA and group VIIA elements were synthesized via water-soluble cations and anions, the freshly prepared samples easily imbibed water droplets on their surfaces when dried by natural air (seeing ESI Fig. S2†), indicating their water affinity and intrinsic superhydrophilicity. To investigate the influence of drying atmospheres on the wettability of these metal compounds of group VIA and group VIIA elements with different topographic structures, the prepared samples were dried in air, O₂ and N₂, respectively, and WCA measurement was conducted in corresponding atmospheres. Fig. 1b shows that after drying in air, these compounds with different structures are hydrophobic or superhydrophobic with WCA of 155° for ZnS, 120° for Ni₃S₂, 122° for CuS, 118° for PbSe, 97° for CaF₂, 128° for AgCl, 127° for AgBr, 129° for AgI, respectively. These values were partially reported by previous reports.^35–37 When the drying atmosphere was changed into N₂, these compounds exhibit their intrinsic superhydrophilicity, as evidenced by the spreading of water droplets on their surfaces, as shown in Fig. 1c. It is worth noting that their hydrophobicity or superhydrophobicity reappears when the drying atmosphere is switched to O₂, as seen in Fig. 1d. More importantly, the WCAs on these inorganic materials dried in O₂ were 160° for ZnS, 137° for Ni₃S₂, 133° for CuS, 132° for PbSe, 130° for CaF₂, 140° for AgCl, and 136° for AgBr, 132° for AgI, and all were increased compared to those in air (Fig. 1b). Similar wettability transformation has also been observed on metal oxides in our previous work.³⁴ These results revealed that the same inorganic samples dried in N₂ and O₂, two main components of air, exhibited opposite wetting behaviors, and samples with different rough structures showed similar wettability variation in the switching of drying atmospheres. It can be concluded that the interaction of material surface and ambient O₂, rather than roughness, was directly responsible for the hydrophobic or superhydrophobic behaviors of rough-textured intrinsically hydrophilic inorganic materials.

Fig. 1 Topographic structures of the prepared inorganic materials and their surficial wettability toward water droplets depending on drying atmospheres. (a) FE-SEM top view images of samples, scale bars: 5 μm. (b, c and d) Optical photographs of water droplets on corresponding samples dried for 12 h in air, N₂, and O₂. The volume of water droplets is 3 μL. The white, light blue and faint yellow backgrounds denote air, O₂ and N₂ atmosphere, respectively. Scale bars = 5 μm.

To investigate the effect of oxygen on the surface state, we studied the surface defects and related oxygen adsorption on ZnS nanorods and AgCl nanorods, as representative specimens, via X-ray photoelectron spectroscopy and Raman spectroscopy. Fig. 2a and c shows the S 2p and Cl 2p XPS spectra of ZnS nanorods and AgCl nanoparticles, respectively. The asymmetric S 2p spectrum can be deconvoluted into four peaks centered at 161.2 eV, 162.2 eV, 162.4 eV and 163.4 eV. The binding energies (BE) of 161.2 eV and 162.4 eV were derived from S 2p_3/2 and S 2p_1/2 of lattice S atoms in the ZnS crystal, while 162.2 eV and 163.4 eV were considered to be derived from S atoms in the vicinity of S vacancies (V_s).^38,39 The obvious shoulder XPS peaks at 162.2 eV and 163.4 eV indicate the presence of abundant V_s defects on the surface of ZnS nanostructures, which has been confirmed by Jaewon et al.^39,40 For Cl 2p, two peaks were observed at binding energies of 197.7 eV and 199.4 eV, which moved to a lower energy compared with BE of 198.6 eV and 200.2 eV of ideal lattice Cl in the AgCl crystal.⁴¹ This movement of Cl 2p peaks has been widely reported at reduced AgCl surface with Cl removal, namely Cl vacancy (V_Cl).^25,42–44 Accompanied by the emergence of V_s and V_Cl defects, the O 1s peaks were both observed in the XPS spectra of ZnS nanorods (Fig. 2b) and AgCl nanoparticles (Fig. 2d). These results demonstrate that plenty of V_a defects exist on the surface of inorganic micro/nano-structures, which might induce the adsorption of oxygen species from air.

Fig. 2 The correlation between anion defects and O₂ adsorption for ZnS and AgCl. (a and b) S 2p and O 1s XPS spectra of ZnS nanorods. (c and d) Cl 2p and O 1s XPS spectra of AgCl nanoparticles. (e and f) Raman spectra of ZnS nanorods and AgCl nanoparticles dried in O₂ and N₂ atmospheres.

The interaction between V_a-enriched inorganic materials and ambient atmospheres, mainly O₂ and N₂, was studied by Raman spectroscopy analysis, which is shown in Fig. 2e and f. The Raman peaks at 280 cm⁻¹, 335 cm⁻¹ and 350 cm⁻¹ in Fig. 2e were attributed to the first-order phonon modes of wurtzite ZnS nanorods,⁴⁵ and the Raman signals at 95 cm⁻¹, 143 cm⁻¹, 233 cm⁻¹, 344 cm⁻¹ and 410 cm⁻¹ in Fig. 2f belonged to the face-centered cubic AgCl.⁴⁶ In comparison with the Raman spectra obtained in N₂, several obvious new peaks centered at 840 cm⁻¹, 862 cm⁻¹ were found in the O₂-dried ZnS sample, while the peaks at 938 cm⁻¹ were found in the O₂-dried AgCl sample. As reported in the present work, the peaks located at 840 cm⁻¹ and 862 cm⁻¹ were assigned to the O–O stretching of the adsorbed peroxide species (O₂²⁻) around Zn²⁺ with different degrees of defect aggregation.^34,47 On the defective AgCl surface, the Raman peak centered at 938 cm⁻¹ was attributed to the stretching of O₂²⁻.^46,47 In addition, in the Raman spectra of the O₂-dried samples, a weak peak centered at 1548 cm⁻¹ and a broad peak ranging from 1050 cm⁻¹ to 1170 cm⁻¹ were observed in the case of ZnS, while a weak peak centered at 1550 cm⁻¹ and a broad peak spanning from 1020 cm⁻¹ to 1180 cm⁻¹ were observed in the case of the AgCl sample. The Raman peaks at 1548 cm⁻¹ or 1550 cm⁻¹ generally corresponded to the O–O stretching vibrations of adsorbed O₂ molecules, with a slight difference in the wavenumbers attributed to the varying adsorption environments.⁴⁸ The two broad peaks might be attributed to the stretching and bending vibrations of a small amount of O₂⁻ around Zn²⁺ and Ag⁺, respectively.⁴⁷ These Raman results indicated that inorganic materials with V_a-defective surface favored the adsorption of O₂ species rather than N₂.

Recently, Zare et al. reported that hydrogen peroxide can be generated in water droplets on the surface of hydrophobic materials, and their works show that the yield of hydrogen peroxide is related to the WCAs of water droplets.^49–51 In order to clarify the relationship between H₂O₂ yield and the hydrophobic properties of material surfaces, we recorded the yield of H₂O₂ on inorganic materials with different wettability. Fig. 3a, b, and c show the microscopic images and H₂O₂ yields of condensed water on ZnS nanorods, AgCl nanoparticles, and ZnO nanowires (their morphologies and XRD data are shown in ESI Fig. S3†), respectively, after being dried in different atmospheres. It can be seen that the condensed water spreads over the samples dried in N₂ but beads up on the samples dried in air. The hydrophilicity of these inorganics in N₂ and the hydrophobicity of the same samples in air are consistent with the results shown in Fig. 1. To quantify the concentration of H₂O₂ generated in water droplets on their surfaces, we measured the absorption spectrum of aqueous potassium titanium oxalate (PTO, K₂TiO(C₂O₄)₂·H₂O) that had reacted with the condensed water. According to the calibration plot of the absorbance at 400 nm (see ESI S4†), the H₂O₂ concentrations in the condensed water on hydrophilic ZnS nanorods, AgCl nanoparticles and ZnO nanowires, which were dried in N₂, are all approximately 0 μM L⁻¹ (gray lines). On the other hand, for the corresponding air-dried hydrophobic samples, the H₂O₂ concentrations were ∼71.15 μM L⁻¹, ∼52.05 μM L⁻¹, and ∼63.24 μM L⁻¹ (blue lines), respectively. Furthermore, O₂ and H₂ heating procedures were performed on the ZnO nanowires to adjust its wettability in air. The insets of water droplets in Fig. 3c3 show that H₂ heat-treatment results in an enhanced superhydrophobicity with a CA of ∼162° compared to the unheated samples (∼150°), but the O₂ heat-treatment induces a wettability transition from superhydrophobicity to hydrophilicity (CA ≈ 75°). For the H₂-annealed sample, the H₂O₂ concentration in the condensed water droplet increases to 77.38 μM L⁻¹ (purple line in Fig. 3c3) compared to the untreated sample (63.24 μM L⁻¹). On the contrary, the H₂O₂ concentration decreases to 49.47 μM L⁻¹ (orange line in Fig. 3c3) in the water droplet on the O₂-annealed sample. These results demonstrate that the yield of H₂O₂ on these inorganic materials increases with the enhancement of their hydrophobicity, while it decreases with the reduction of their hydrophobicity. Zhang et al. have reported that O₂²⁻ is an important intermediate in the formation of H₂O₂ during 2e⁻ oxygen reduction reaction (ORR).^49,52,53 Also, we have confirmed that O₂ heat-treatment is capable of reducing V_o defects on the surface of ZnO nanomaterials, whereas H₂ heat-treatment increases their occurrence.³⁴ Therefore, it can be inferred that the hydrophobicity of inorganic materials is related to the amount of O₂²⁻ adsorbed on the V_a defects.

Fig. 3 Spontaneous generation of H₂O₂ in water droplets on the surface of ZnS nanorods, AgCl nanoparticles and oriented ZnO nanowires. [(a1 and 2), (b1 and 2) and (c1 and 2)] Microscopic images of condensed water on ZnS nanorods, AgCl nanoparticles and ZnO nanowires dried in N₂ and air. [(a3), (b3) and (c3)] The absorption spectrum of aqueous PTO solution reacted with condensed water on corresponding samples dried in different atmospheres, scale bars: 50 μm.

DFT calculations were further performed to study the hydrophobicity of inorganic materials induced by O₂ adsorption at V_a. The effect of V_a on the adsorption energies (E_a) of O₂ species (O₂, O₂⁻, O₂²⁻), N₂ and H₂O molecules was calculated using wurtzite ZnS, rocksalt AgCl and wurtzite ZnO as representatives of sulfides, halides and oxides, respectively. Herein, the ZnS-(101̄0) plane of wurtzite, AgCl-(100) plane of rocksalt and ZnO-(101̄0) plane of wurtzite, which are predominantly exposed planes grown by common solution approaches, were selected as model unit cells for periodic slab.^20,54,55 The defective surfaces with V_a concentration of 1/6 monolayer were modeled by removing a sulfur atom, a chlorine atom or an oxygen atom at the top atomic layer from the corresponding ideal crystal surface. The most stable adsorption configurations for O₂²⁻, N₂, and H₂O molecules on ideal and defective ZnS-(101̄0), AgCl-(100) and ZnO-(101̄0) surfaces, as well as their adsorption energies (E_a), are given in Fig. 4 (the E_a differences of O₂, O₂⁻ and O₂²⁻ shown in ESI S5† manifested that O₂²⁻ exhibited the most stable adsorption on the V_a sites of the three materials). Fig. 4a1 and c1 show that the ideal ZnS-(101̄0) and AgCl-(100) crystal surfaces exhibit strong adsorption capability for H₂O, with an E_a of −0.584 eV and −0.712 eV respectively, but display very weak adsorption for both O₂²⁻ and N₂. This result is consistent with the wetting characteristics of intrinsically hydrophilic inorganic materials. When V_a defects are present on ZnS-(101̄0) and AgCl-(100) surfaces, the E_a for H₂O at the defect sites are −0.410 eV and −0.516 eV, respectively, which are lower than those at ideal surfaces. The adsorption of H₂O is suppressed at V_a defects mainly due to the repulsion between positively charged H atoms and the electropositive metal atoms surrounding the V_a sites. It is worth noting that the adsorption of O₂²⁻ is significantly enhanced at the V_a sites, with its E_a on V_S, V_Cl, and V_o is much larger than that of H₂O on both V_a and ideal crystal planes as well as that of N₂ (the E_a of H₂O and N₂ on ideal and defective ZnO-(101̄0) plane has been reported in our previous work³⁴). These calculated thermodynamic data demonstrated that the presence of V_a led to a transition of the inorganic crystals from its original H₂O affinity to O₂²⁻ affinity. Furthermore, the O₂²⁻-adsorbed V_a sites were also capable of adsorbing other O₂ species, O₂ and O₂⁻; the E_a are listed in ESI S6.† Therefore, the presence of V_a defects caused the inorganic materials to have stable O₂²⁻-predominant O₂ molecular adsorption layer, which would prevent the wetting of water on the inorganic surfaces. This defect-induced hydrophobicity is consistent with the variation in the hydrophobicity of ZnO and ZnS nanomaterials caused by the tuning of vacancy concentration, as shown in Fig. 3c3 and Fig. S7.†

Fig. 4 The most stable configurations and adsorption energy for O₂²⁻, N₂ and H₂O on ideal ZnS-(101̄0), AgCl-(100), ZnO-(101̄0) planes and on V_a sites of their defective planes with a vacancy concentration of 1/6 monolayer. (a) Ideal ZnS-(101̄0). (b) V_s-ZnS-(101̄0). (c) Ideal AgCl-(100). (d) V_Cl-AgCl-(100). (e) O₂²⁻ on ideal and defective ZnO-(101̄0). The yellow, gray, green, cyan and red spheres denote S, Zn, Cl, Ag and O atoms, respectively.

According to the above experimental results and theoretical calculations, we provided a new understanding toward the origin of superhydrophobicity in rough-textured inorganic materials, which is schematically shown in Fig. 5. (1) The abundant V_a defects on the surface of rough-textured inorganic materials gave rise to the preferential adsorption of O₂²⁻ from the air compared to N₂ and H₂O. This preferential O₂²⁻ adsorption enabled the defective inorganic surface to anchor O₂²⁻ from air, leading to the formation of an O₂²⁻-predominant O₂ molecular adsorption layer on the micro-/nano-structures. The preferentially adsorbed O₂ molecular layer hindered the contact of the water droplet on the inorganic surfaces and endowed the intrinsically hydrophilic inorganic materials with the property of hydrophobicity. (2) The adsorbed O₂ layer was stable under water and hard to be replaced by H₂O because of the larger adsorption energy of O₂²⁻ than H₂O, which ensured the stability of hydrophobicity. (3) The rough-textured structures led to an increase of the V_a content per unit area and promoted the stability of O₂²⁻-predominant O₂ molecular adsorption layer within the grooves. The hydrophobicity of inorganic materials was further enhanced to superhydrophobicity by the rough textured micro-/nano-structures. This understanding of superhydrophobicity focused on the critical role of V_a-induced O₂²⁻ adsorption, which furthered the formation of the O₂ adsorption layer on the V_a site rather than the air-trapped layer, as commonly believed, and provided beneficial development of the traditional superhydrophobicity mechanism.

Fig. 5 Schematic of the wettability origin of intrinsically hydrophilic inorganic materials.

3. Conclusion

In summary, we have demonstrated a further understanding of the superhydrophobic behavior for the micro-/nano-structured inorganic materials with intrinsic hydrophilicity. Our results show that their superhydrophobicity is attributed to the presence of the V_a on the rough-textured surface. On the one hand, the V_a can give the preferential adsorption of O₂²⁻ species relative to N₂ and O₂. On the other hand, the V_a have the ability to remarkably enhance the adsorption energy of O₂²⁻ and reduce the affinity for H₂O. This results in the formation of a stable O₂²⁻-dominated oxygen species adsorption-layer within the grooves of the rough-textured surfaces under water-droplet, which is responsible for their superhydrophobicity. This new insight into the origin of superhydrophobicity for intrinsic hydrophilic materials is of great significance to further understand the wettability mechanism of natural and artificial rough-textured surfaces and may open up avenues towards the design of desirable wettability based on anion-vacancy engineering.

4. Experimental

This section is available in the ESI.†

Author contributions

Ruihang Wen: synthesis, formal analysis, calculation, visualization. Xiaobing Chen: synthesis, wettability characterization, data curation. Wenbin Li: validation, formal analysis. Gaocan Qi: conceptualization, calculation, formal analysis, foundation, writing. Zhihao Yuan: supervision, formal analysis, writing & editing.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Nature Science Foundation of China (no. 21501132), the Natural Science Foundation of Tianjin City (no. 20JCZDJC00280), and the National Key R&D Program of China (no. 2017YFA0700104) We are grateful to Dr Jia He, Dr Xudong Zhao, and Dr Yuehu Wang from Tianjin University of Technology for their technical assistance and valuable discussions on DFT calculations.

References

1. T. M. Schutzius, S. Jung, T. Maitra, G. Graeber, M. Köhme and D. Poulikakos, Spontaneous droplet trampolining on rigid superhydrophobic surfaces, Nature, 2015, 527, 82–85.
2. X. Gao, X. Yan, X. Yao, L. Xu, K. Zhang, J. Zhang, B. Yang and L. Jiang, The Dry-Style Antifogging Properties of Mosquito Compound Eyes and Artificial Analogues Prepared by Soft Lithography, Adv. Mater., 2007, 19, 2213–2217.
3. Z. Tang, P. Wang, B. Xu, L. Meng, L. Jiang and H. Liu, Bioinspired Robust Water Repellency in High Humidity by Micro-meter-Scaled Conical Fibers: Toward a Long-Time Underwater Aerobic Reaction, J. Am. Chem. Soc., 2022, 144, 10950–10957.
4. P.-G. Gennes, F. Brochard-Wyart and D. Quéré, Capillarity and Wetting Phenomena: Drops, Bubbles, Pearls, Waves, SpringerNew York, NY, New York, 2004.
5. P. Roach, N. J. Shirtcliffe and M. I. Newton, Progess in superhydrophobic surface development, Soft Matter, 2008, 4, 224–240.
6. H. Li, Q. Jin, H. Li, H. Tong, K. Wang, S. Chen, G. Ouyang, Z. Wang and Y. Li, Transparent Superamphiphobic Material Formed by Hierarchical Nano Re-Entrant Structure, Adv. Funct. Mater., 2024, 34, 2309684.
7. Y. Liu, M. Wu, Z. Zhang, K. Luo, J. Lu, L. Lin, K. Xu, H. Zhu, B. Wang, W. Lei and Y. Fu, Fabrication of wear-resistant and superhydrophobic aluminum alloy surface by laser-chemical hybrid methods, Phys. Fluids, 2023, 35, 052108.
8. L. Wang, D. Li, G. Jiang, X. Hu, R. Peng, Z. Song, H. Zhang, P. Fan and M. Zhong, Dual-Energy-Barrier Stable Superhydrophobic Structures for Long Icing Delay, ACS Nano, 2024, 18, 12489–12502.
9. M. Liu, X. Liu, S. Zheng, K. Jia, L. Yu, J. Xin, J. Ning, W. Wen, L. Huang and J. Xie, Environment-Friendly superhydrophobic sponge for highly efficient Oil/Water separation and microplastic removal, Sep. Purif. Technol., 2023, 319, 124060.
10. J. Tan, Z. Fan, M. Zhou, T. Liu, S. Sun, G. Chen, Y. Song, Z. Wang and D. Jiang, Orbital Electrowetting-on-Dielectric for Droplet Manipulation on Superhydrophobic Surfaces, Adv. Mater., 2024, 36, 2314346.
11. G. Parisi, P. K. Szewczyk, S. Narayan and U. Stachewicz, Photoresponsive Electrospun Fiber Meshes with Switchable Wettability for Effective Fog Water Harvesting in Variable Humidity Conditions, ACS Appl. Mater. Interfaces, 2023, 15, 40001–40010.
12. Y. Yu, W. Cui, L. Song, Q. Liao, K. Ma, S. Zhong, H. Yue and B. Liang, Design of Organic-Free Superhydrophobic TiO₂ with Ultraviolet Stability or Ultraviolet-Induced Switchable Wettability, ACS Appl. Mater. Interfaces, 2022, 14, 9864–9872.
13. N. Liu, N. Li, C. Jiang, M. Lv, J. Wu and Z. Chen, Perovskite Single Crystals with Self-Cleaning Surface for Efficient Photovoltaics, Angew. Chem., Int. Ed., 2024, 63, e202314089.
14. V. K. Manivasagam, G. Perumal, H. S. Arora and K. C. Popat, Enhanced antibacterial properties on superhydrophobic micro-nano structured titanium surface, J. Biomed. Mater. Res., Part A, 2022, 110, 1314–1328.
15. B. You, Y. Kim, B.-K. Ju and J.-W. Kim, Highly Stretchable and Waterproof Electroluminescence Device Based on Superstable Stretchable Transparent Electrode, ACS Appl. Mater. Interfaces, 2017, 9, 5486–5494.
16. Y. Lai, X. Gao, H. Zhuang, J. Huang, C. Lin and L. Jiang, Designing Superhydrophobic Porous Nanostructures with Tunable Water Adhesion, Adv. Mater., 2009, 21, 3799–3803.
17. A. Tuteja, W. Choi, G. H. Mckinley, R. E. Cohen and M. F. Rubner, Design Parameters for Superhydrophobicity and Superoleophobicity, MRS Bull., 2008, 33, 752–758.
18. A. Tuteja, W. Choi, M. Ma, J. M. Mabry, S. A. Mazzella, G. C. Rutledge, G. H. McKinley and R. E. Cohen, Designing superoleophobic surfaces, Science, 2007, 318, 1618–1622.
19. C. Badre, T. Pauporté, M. Turmine and D. Lincot, A ZnO nanowire array film with stable highly water-repellent properties, Nanotechnology, 2007, 18, 365705.
20. X. Feng, L. Feng, M. Jin, J. Zhai, L. Jiang and D. Zhu, Reversible Super-hydrophobicity to Super-hydrophilicity Transition of Aligned ZnO Nanorod Films, J. Am. Chem. Soc., 2004, 126, 62–63.
21. M. Guo, P. Diao and S. Cai, Highly hydrophilic and superhydrophobic ZnO nanorod array films, Thin Solid Films, 2007, 515, 7162–7166.
22. F. C. Tompkins, Superficial Chemistry and Solid Imperfections, Nature, 1960, 186, 3–6.
23. C. Xie, D. Yan, H. Li, S. Du, W. Chen, Y. Wang, Y. Zou, R. Chen and S. Wang, Defect Chemistry in Heterogeneous Catalysis: Recognition, Understanding, and Utilization, ACS Catal., 2020, 10, 11082–11098.
24. K. Greben, S. Arora, M. G. Harats and K. I. Bolotin, Intrinsic and Extrinsic Defect-Related Excitons in TMDCs, Nano Lett., 2020, 20, 2544–2550.
25. X. Shi, X. A. Dong, Y. He, P. Yan, S. H. Zhang and F. Dong, Photoswitchable Chlorine Vacancies in Ultrathin Bi₄O₅Cl₂for Selective CO₂ Photoreduction, ACS Catal., 2022, 12, 3965–3973.
26. Z. Zhang, W. Chen, X. Jiang, J. Cao, H. Yang, H. Chen, F. Yang, Y. Shen, H. Yang, Q. Cheng, X. Chen, X. Tang, S. Kang, X.-M. Ou, C. J. Brabec, Y. Li and Y. Li, Suppression of phase segregation in wide-bandgap perovskites with thiocyanate ions for perovskite/organic tandems with 25.06% efficiency, Nat. Energy, 2024, 9, 592–601.
27. A. Hötger, W. Männer, T. Amit, D. Hernangómez-Pérez, T. Taniguchi, K. Watanabe, U. Wurstbauer, J. J. Finley, S. Refaely-Abramson, C. Kastl and A. W. Holleitner, Photovoltage and Photocurrent Absorption Spectra of Sulfur Vacancies Locally Patterned in Monolayer MoS₂, Nano Lett., 2023, 23, 11655–11661.
28. S. Zeb, Z. Yang, R. Hu, M. Umair, S. Naz, Y. Cui, C. Qin, T. Liu and X. Jiang, Electronic structure and oxygen vacancy tuning of Co & Ni co-doped W₁₈O₄₉ nanourchins for efficient TEA gas sensing, Chem. Eng. J., 2023, 465, 142815.
29. H. Zhou, G. Qi, W. Li, Y. Wang and Z. Yuan, Electrolyte-gated SrCoOx FET sensor for highly sensitive detecting pH in extreme alkalinity solution, Nano Res., 2024, 17, 3079–3086.
30. T. Liu, W. Zhu, N. Wang, K. Zhang, X. Wen, Y. Xing and Y. Li, Preparation of Structure Vacancy Defect Modified Diatomic-Layered g-C₃N₄ Nanosheet with Enhanced Photocatalytic Performance, Adv. Sci., 2023, 10, 2302503.
31. K. Yadav, B. R. Mehta, K. V. Lakshmi, S. Bhattacharya and J. P. Singh, Tuning the Wettability of Indium Oxide Nanowires from Superhydrophobic to Nearly Superhydrophilic: Effect of Oxygen-Related Defects, J. Phys. Chem. C, 2015, 119, 16026–16032.
32. W. Yan, Z. Shi, H. Feng, J. Yu, W. Chen and Y. Chen, Crystalline metal phosphide-coated amorphous iron oxide-hydroxide (FeOOH) with oxygen vacancies as highly active and stable oxygen evolution catalyst in alkaline seawater at high current density, J. Colloid Interface Sci., 2024, 667, 362–370.
33. K. Zhang, Q. Pu, J. Wang, D. Li, L. Xu, M. Xie and J. Cao, Promoted oxygen adsorption on porous CeO₂ cubes with abundant oxygen vacancies for efficient gaseous formaldehyde removal, Chemosphere, 2024, 361, 142576.
34. G. Qi, X. Liu, C. Li, C. Wang and Z. Yuan, The Origin of Superhydrophobicity for Intrinsically Hydrophilic Metal Oxides: A Preferential O₂ Adsorption Dominated by Oxygen Vacancies, Angew. Chem., Int. Ed., 2019, 58, 17406–17411.
35. W. Jia, B. Jia, X. Wu and F. Qu, Self assembly of shape-controlled ZnS nanostructures with novel yellow light photoluminescence and excellent hydrophobic properties, CrystEngComm, 2012, 14, 7759–7763.
36. H. Fu, L. Yang, Y. Wang, C. Yang and W. Jiang, Preparation of AgCl Particles with Different Superwettabilities by Particle Size Regulation, Langmuir, 2019, 35, 7944–7953.
37. F. Zhang, R. Zhao and Y. Wang, Superwettable Surface-Dependent efficiently electrocatalytic water splitting based on their excellent liquid adsorption and gas desorption, Chem. Eng. J., 2023, 139513.
38. G. Wang, B. Huang, Z. Li, Z. Lou, Z. Wang, Y. Dai and M. H. Whangbo, Synthesis and characterization of ZnS with controlled amount of S vacancies for photocatalytic H2 production under visible light, Sci. Rep., 2015, 5, 8544.
39. L. Jaewon, H. Sooho, C. Dayeon and J. Du-Jeon, Facile fabrication of porous ZnS nanostructures with controlled amount of S vacancies for enhanced photocatalytic performances, Nanoscale, 2018, 10, 14254–14263.
40. E. M. Jubeer, M. A. Manthrammel, P. A. Subha, M. Shkir, K. P. Biju and S. A. Alfaify, Defect engineering for enhanced optical and photocatalytic properties of ZnS nanoparticles synthesized by hydrothermal method, Sci. Rep., 2023, 13, 16820.
41. V. K. Kaushik, XPS core level spectra and Auger parameters for some silver compounds, J. Electron Spectrosc. Relat. Phenom., 1991, 56, 273–277.
42. X. Lin, H. Liu, B. Guo and X. Zhang, Synthesis of AgCl/Ag/AgCl core-shell microstructures with enhanced photocatalytic activity under sunlight irradiation, J. Environ. Chem. Eng., 2016, 4, 4021–4028.
43. H. Daupor and S. Wongnawa, Urchinlike Ag/AgCl photocatalyst: Synthesis, characterization, and activity, Appl. Catal., A, 2014, 473, 59–69.
44. L. Dong, D. Liang and R. Gong, In Situ Photoactivated AgCl/Ag Nanocomposites with Enhanced Visible Light Photocatalytic and Antibacterial Activity, Eur. J. Inorg. Chem., 2012, 2012, 3200–3208.
45. J. H. Kim, H. Rho, J. Kim, Y-J. Choi and J-G. Park, Raman spectroscopy of ZnS nanostructures, J. Raman Spectrosc., 2012, 43, 906–910.
46. I. Martina, R. Wiesinger, D. Jembrih-Simburger and M. Schreiner, Micro-Raman characterization of silver corrosion products: Instrumental set-up and reference database, e-Preserv. Sci., 2012, 9, 1–8.
47. Z. Wu, M. Li, J. Howe, H. M. Meyer 3rd and S. H. Overbury, Probing defect sites on CeO₂ nanocrystals with well-defined surface planes by Raman spectroscopy and O₂ adsorption, Langmuir, 2010, 26, 16595–16606.
48. Z. Z. Ho, W. M. K. P. Wijekoon, E. W. Koenig and W. M. Hetherington, Coherent anti-stokes raman scattering spectrum of O₂ adsorbed on a ZnO optical waveguide, J. Phys. Chem., 1987, 91, 757–760.
49. M. A. Mehrgardi, M. Mofidfar and R. N. Zare, Sprayed Water Microdroplets Are Able to Generate Hydrogen Peroxide Spontaneously, J. Am. Chem. Soc., 2022, 144, 7606–7609.
50. J. K. Lee, K. L. Walker, H. S. Han, J. Kang, F. B. Prinz, R. M. Waymouth, H. G. Nam and R. N. Zare, Spontaneous generation of hydrogen peroxide from aqueous microdroplets, Proc. Natl. Acad. Sci. U. S. A., 2019, 116, 19294–19298.
51. J. K. Lee, H. S. Han, S. Chaikasetsin, D. P. Marron, R. M. Waymouth, F. B. Prinz and R. N. Zare, Condensing water vapor to droplets generates hydrogen peroxide, Proc. Natl. Acad. Sci. U. S. A., 2020, 117, 30934–30941.
52. X. Zhang, H. Su, P. Cui, Y. Cao, Z. Teng, Q. Zhang, Y. Wang, Y. Feng, R. Feng, J. Hou, X. Zhou, P. Ma, H. Hu, K. Wang, C. Wang, L. Gan, Y. Zhao, Q. Liu, T. Zhang and K. Zheng, Developing Ni single-atom sites in carbon nitride for efficient photocatalytic H₂O₂ production, Nat. Commun., 2023, 14, 7115.
53. J. Zhang, B. Huang, Q. Shao and X. Huang, Highly Active, Selective and Stable Direct H₂O₂ Generation by Monodispersive Pd-Ag Nanoalloy, ACS Appl. Mater. Interfaces, 2018, 10, 21291–21296.
54. S. Biswas and S. Kar, Fabrication of ZnS nanoparticles and nanorods with cubic and hexagonal crystal structures: A simple solvothermal approach, Nanotechnology, 2008, 19, 045710.
55. S. Wu, X. Shen, Z. Ji, G. Zhu, H. Zhou, H. Zang, T. Yu, C. Chen, C. Song, L. Feng, M. Zhao and K. Chen, Morphological syntheses and photocatalytic properties of well-defined sub-100 nm Ag/AgCl nanocrystals by a facile solution approach, J. Alloys Compd., 2017, 693, 132–140.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4qi02059f

‡ These authors contributed equally to this work.

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