Enhancement of self-cleaning properties and durability of super-hydrophobic carbon nitride nanostructures by post-annealing treatment

https://doi.org/10.1016/j.surfcoat.2021.126912Get rights and content

Highlights

  • Carbon nitride nanostructures were grown by RF-PECVD and studied the effects of post-annealing temperatures.

  • The hydrophobic and self-cleaning properties of the samples enhanced as annealing temperature increased.

  • Highest super-hydrophobicity was obtained for the sample that greatly influenced by surface morphology and surface roughness.

  • The durability of the sample annealed at higher temperature maintains its super-hydrophicity after 11th water dropped.

Abstract

In the present work, the effects of annealing temperatures (TA) on the wetting behaviour of carbon nitride (CNx) nanostructures were studied based on static and dynamic contact angle measurements. The contact angle (CA) was exponentially increased from 102.8 ± 1.5° for the as-deposited film to super-hydrophobic behaviour of 158.1 ± 1.5° as TA was increased to 700 °C. The contact angle hysteresis (CAH) of the films was examined by tilted the samples at an angle of about 45° or until the droplet can be detected by an optical camera; and their advancing and receding angles were measured. The acquired result shows that the applied TA to the CNx nanostructures enhances the water repellence of the surfaces. The calculated CAH indicates that as-deposited film demonstrates “sticky” state with higher hysteresis (>55°) compared than film annealed at 700 °C which shows “slippy” state with CAH <5°. The difference in the CAH values was believed to have resulted from the different wetting regimes of both films especially their surface morphology, surface roughness, and its porosity. The durability of the CNx nanostructures was tested by water dropped for 11 times. From the results, film annealed at TA of 700 °C shows excellent durability performance by still maintaining its super-hydrophobic behaviour after water was dropped for more than 10 times. The surface morphology and chemical bonding present in the film seems not much different compared to the first time water was dropped. This indicates that high annealing temperature might remove the defects of the nanostructure, furthermore strengthen it thus, exhibited good water repellences and self-cleaning properties as well as high reusability of the nanostructures.

Introduction

From Bullet Trains that take its inspiration from Kingfisher bird to medical adhesives inspired by spider silk, nature can inspire researchers and offer solutions to countless human problems. Among which, the remarkable non-wetting surfaces with self-cleaning properties inspired by lotus leaf received great attention and numerous technologies have been developed based on these features such as windshields of vehicles [1], body of the vehicles and mirrors in automotive [2]; glasses cameras and lenses in optics [3]; fabrics and sports shoes in textile [[4], [5], [6]]; anti-corrosion coating in marine; woods, marbles and doors in labor sector [7]; and the list is ceaseless [8,9]. One of its special features is the ability to repel water and keep the surface cleaning without any external source as well as durable to the severe environment condition and water droplets. These properties become an important aspect and widely used in coatings such as solar cell panels; which offer great reduction in maintenance cost, elimination of tedious manual effort, and also reduction in the time spent in cleaning works [[10], [11], [12]]. Due to the high water contact angle (>150°), water drops immediately bead up and readily roll off the solar cell panel's surface collecting particles, dirt and debris along the way.

The first instance of a self-cleaning surface was created in 1995 by Paz et al. which created a transparent titanium dioxide (TiO2) film that was used to coat glass and provide the ability for the glass to self-clean [13]. Since then, many approaches have been pursued to develop non-wetting surfaces with super-hydrophobic and self-cleaning properties. Cunqian Wei et al. have discussed the difference of anti-icing performance between the hydrophobic surface imitating lotus leaf and the ultra-smooth slippery liquid-infused porous surface (SLIPS) mimicking pitcher plant [14]. Ci et al. constructed an array of vertically aligned large diameter double-walled carbon nanotubes by a water assisted chemical vapour deposition process [15]. Carbon nitride materials (CNx) have also become one of the promising materials for self-cleaning applications due to its suitability in integrating with other materials such as polymer, metal etc. which increases the number of study especially that involved with self-cleaning applications. For example, Chengkai et al. reported a facile surface modification of graphitic carbon nitride nanosheets (CNNS) functionalize with textiles by directly sprayed onto the surface which creates excellent ability for photocatalytic self-cleaning and degradation of indoor volatile organic pollutants [16]. Meng Yuan et al. studied the polymer carbon nitride (PCN) integrated with polyacrylonitrile (PAN) polymers composite on fabrics which show an excellent stability, long-term durability (50circles for disinfection properties) and self-cleaning properties [17].

The most basic principle of self-cleaning is the properties of forming super-hydrophobic surfaces that can remove the dirt on the surfaces. In this work, the effects of TA have been chosen as a way to improve the characteristics of the nanostructures surfaces. Surfaces with self-cleaning properties are associated with super-hydrophobic behaviour and result from the combination of a few factors such as surface energy, surface porosity, surface morphology, sub-micron scale roughness, chemical material present on it and more [[18], [19], [20], [21], [22]]. Thus, the annealing process might be a potential method that can modify the surfaces in different forms: enhance in crystallinity, increase the film's thickness, enhance the topography of the surface etc. [23,24]. All these may contribute to an improvement of water repellent surfaces and self-cleaning properties. Here in the study, the most influenced factors of self-cleaning properties are discussed.

In our previous study, the vertically aligned CNx nanofibres were successfully produced in a simplified and cost-effective way [25]. Following up from that work, in this present study, the CNx nanostructures were grown on Si (100) substrate under the same conditions by radio frequency plasma enhanced chemical vapour deposition (RF-PECVD) and were annealed at different temperatures using conventional thermal annealing process. The surface morphology and roughness of the as-deposited and annealed CNx nanostructures have been investigated by FESEM microscopy and AFM microscopy, respectively. The effects of post-annealing temperatures on the films; and the durability of the films after 11th water dropped were investigated.

Section snippets

Film preparation

Carbon nitride (CNx) nanostructures were prepared using a parallel plate configuration of PECVD system on p-type silicon (100) wafer at a substrate temperature of 100 °C. The plasma-excitation frequency of the system was set to be 13.56 MHz. The flow rate of CH4 and N2 gases were maintained at 20 and 47 sccm, respectively. The distance between electrodes, deposition pressure, RF power and duration of deposition were kept at 1 cm, 0.8 mbar, 80 W and 90 min, respectively. In order to investigate

The effects of annealing temperatures on the contact angle, contact angle hysteresis, work of adhesion, surface energy and surface porosity

Contact angle (CA) measurements are often used to evaluate the wetting behaviour of the films due to its direct and quick measurements, while contact angle hysteresis (CAH) determined the adhesion of water to the film's surfaces when tilted at a certain angle. These measurements have always been of great interest as it determined the self-cleaning properties of the films that are important for various applications.

Fig. 1 shows water CA and CAH measurements of CNx nanostructures and its

Conclusions

The CNx nanostructures were prepared by PECVD method and were subsequently annealed at annealing temperature (TA) of 400 °C, 600 °C and 700 °C for 1 h in the nitrogen atmosphere. The influence of the annealing treatments on the water contact angle, surface morphology, roughness, surface energy, durability and chemical bonding of the CNx nanostructures have been investigated in this work. From the contact angle (CA) measurement, it showed that high TA enhanced the water repellence properties due

CRediT authorship contribution statement

Shafarina Azlinda Ahmad Kamal: Conceptualization, Methodology, Investigation, Validation, Writing – original draft. Richard Ritikos: Writing – review & editing, Supervision, Project administration, Funding acquisition. Saadah Abdul Rahman: Writing – review & editing, Supervision, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work was supported by the Universiti Malaya Research Grant (UMRG) - AFR (Frontier Science) RG391-17AFR.

References (41)

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