Journal of Materials Science & Technology, 2020, 48(0): 1-8 DOI: 10.1016/j.jmst.2019.10.040

Research Article

Superhydrophobic diamond-coated Si nanowires for application of anti-biofouling’

Wenjing Longa,b,1, Haining Lia,b,1, Bing Yang,a,*, Nan Huanga, Lusheng Liua, Zhigang Gaic, Xin Jiang,a,*

a Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang, 110016, China

b Department of Materials Science and Engineering, University of Science and Technology of China, Hefei, 230026, China

c Institute of Oceanographic Instrumentation, Qilu University of Technology (Shandong Academy of Sciences), Shandong Provincial Key Laboratory of Marine Monitoring Instrument Equipment Technology, National Engineering and Technological Research Center of Marine Monitoring Equipment, Qingdao, 266100, China

Corresponding authors: * E-mail addresses:byang@imr.ac.cn(B. Yang);xjiang@imr.ac.cn(X. Jiang).

First author contact: 1 These authors contributed equally to this work.

Received: 2019-08-14   Accepted: 2019-10-24   Online: 2020-07-1

Abstract

The effect of the surface wettability of plasma-modified vertical Si nanowire array on the bio-fouling performance has been investigated. The Si nanowires prepared by a metal-assisted chemical etching technique exhibit a super-hydrophilic surface. The treatment in CH4/H2 gas plasma environment leads to the decoration of graphite and diamond nanoparticles around Si nanowires. The detailed interface between graphite/diamond and Si nanowire was characterized by HRTEM technique. These surface-modified nanowire samples show an increased water contact angle with ultrananocrystalline diamond decorated ones being superhydrophobic. The immersion test in chlorella solution reveals that the diamond-coated Si nanowires possess the least attachment of chlorella in comparison with other Si nanowires. This result confirms that the coating of Si nanowires with diamond nanoparticles shows the best behavior in anti-biofouling. Importantly, this work provides a method fabricated super-hydrophobic surface for the application of biofouling prevention.

Keywords: Silicon nanowires ; Plasma treatment ; Super-hydrophobic ; Diamond

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Cite this article

Wenjing Long, Haining Li, Bing Yang, Nan Huang, Lusheng Liu, Zhigang Gai, Xin Jiang. Superhydrophobic diamond-coated Si nanowires for application of anti-biofouling’. Journal of Materials Science & Technology[J], 2020, 48(0): 1-8 DOI:10.1016/j.jmst.2019.10.040

1. Introduction

Bio-fouling in marine equipment has attracted much attention in the past few years since it causes many problems, such as increased fuel consumption, corrosion of marine devices and so on. To prevent the attachment of fouling organisms, the coating of anti-fouling materials is always employed besides the approaches of the mechanical cleaning and UV irradiation [[1], [2], [3], [4], [5], [6], [7], [8]]. The coatings containing biocides, such as Sn [3,7], Cu [7,9,10] and Ag [11] related compounds, exhibit an effective protection against micro-organism attachment. However, the release of the biocides into seawater leads to the deterioration of marine environment, and thus these coatings are no longer used nowadays. A non-toxic alternative to biocide antifouling is the painting of enzyme-based organic compounds, which inhibits biofilm formation to reduce the adhesion of marine foulants [2,12]. However, the potential self-degradation of enzyme limits the application lifetime of this type of coating. In addition, the coating of non-toxic conductive materials (such as polyaniline) was investigated with the blocking of the foulant attachment by electrically charging [13]. But the fouling-release property remained poor. Also, the materials with a low-friction, ultra-smooth surface and high elastic modulus, such as fluoropolymers [14] and polysiloxanes [15,16], are applied to improve the non-stick efficiency of marine organisms. The shortcoming of these materials is easy to damage and of poor mechanical properties.

Recently, many studies [[17], [18], [19]] reported that the super-hydrophilic or super-hydrophobic surface showed an excellent anti-fouling behavior by mimicking the rough surface nanostructure of lotus or killer whale. For example, Galopin et al. [20] found heterogeneously wetted Si NWs exhibited the attachment of fewer bacteria than flat silicon wafers. Piret et al. [21] revealed that the amount of attached mammalian cells on superhydrophobic surface was much less than that on super-hydrophilic ones. These results confirmed that the nanowires with super-hydrophobic surface showed great potential in the area of biofouling reduction and prevention. Fabrication of super-hydrophobic nanowires was always achieved by the coating of low surface-energy materials on the nanowires surface (such as siloxane) [[22], [23], [24]]. However, under the coating of low surface-energy organic materials, the strength of modified nanowires is low, which limits the anti-fouling performance. It is well-known that diamond materials possess high mechanical strength, low friction coefficient and chemical stability [[25], [26], [27], [28]]. In addition, nanocrystalline diamond is reported to exhibit a remarkable non-toxicity [29] and anti-bacteria behavior [30] for the bio-application. This means that the coating of diamond on the nanowire surface could dramatically increase the mechanical properties of the nanowires. Understanding of the anti-fouling performance of diamond-coated nanowires will eventually lead to the design of the robust nanowire materials for the application of anti-fouling.

In this work, Si nanowires (SiNWs) prepared by a metal-assisted chemical etching (MACE) method were employed to conduct surface modification for the investigation of their anti-fouling performance. The coating of SiNWs with diamond and other carbon materials was achieved in CH4/H2 plasma CVD system. The microstructure of the surface-modified nanowires was characterized by SEM, Raman, XPS and High-resolution TEM (HRTEM). The water contact angle was measured by sessile-drop method, and antifouling performance of surface-modified SiNWs was tested using chlorella culture medium.

2. Experimental

2.1. Materials

Hydrofluoric acid (HF, 40%), nitric acid (HNO3, 65%) and hydrogen peroxide (H2O2, 30%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Chlorella culture medium and nutrient solution A (NaNO3 and NaH2PO4·H2O), B (micronutrient working stock solution) and C (vitamin working stock solution) were purchased from the Institute of Oceanology, Chinese Academy of Sciences. Single-crystalline N-type silicon (100) wafers (0.008-0.02 Ω m) were purchased from China Electronic Technology Group Corporation Forty-sixth Research Institute. Silver nitrade (AgNO3, 99.9999%) was purchased from SIGMA-ALDRICH, USA. De-ionized water from Millipore (R≥18.2 MΩ m) was used for all experiments. All the chemicals were used without further purification.

2.2. Preparation of surface-modified silicon nanowires (SiNWs)

Si nanowires (SiNWs) were fabricated by a metal-assisted chemical etching method, as previously reported [24]. Prior to the etching process, the Si wafers with dimension of 2 mm × 1 mm were cleaned in acetone and ethanol for 10 min, and then dried by nitrogen flow. Afterwards, Si wafers were immersed in concentrated nitric acid for 60 s. During the etching process, the Si wafers were firstly soaked into a HF/AgNO3 solution for 60 s to the deposition of Ag nanoparticles. Then the wafers were immersed in HF/ H2O2 etchant solution for 2 h at room temperature to fabricate Si nanowires (as-prepared Si nanowires, SiNWs-0). Subsequent washing in concentrated nitric acid was to remove residual Ag nanoparticles on the nanowire surface.

A part of SiNWs-0 samples were treated in microwave plasma chemical vapor deposition system with different gas plasma (pure H2 and CH4/H2 mixed gas). The corresponding samples are named as SiNWs-h and SiNWs-g, respectively. For the coating of diamond, another part of Si NWs was firstly seeded with diamond seeds and then treated in CH4/H2 mixed plasma. As control groups, the as-prepared Si wafers with or without the treatment of H2 gas plasma were prepared (Si and Si-h, respectively). The parameters of plasma treatment were listed in Table 1.

Table 1   Parameters of surface treatment of Si wafer and nanowires.

Microwave power (kW)Gas pressure (mbar)Processing time (h)H2 flow rate (sccm)CH4 flow rate (sccm)
Si-h63024000
SiNWs-h63024000
SiNWs-g630240012
SiNWs-d630240012

New window| CSV


2.3. Microstructure characterization and mechanical property measurement

The detailed microstructure and chemical composition of surface-modified SiNWs were characterized with scanning electron microscope (SEM, Hitachi SU-70), Raman spectroscopy (Horiba, LabRAM HR Evolution, a 325 nm laser), X-ray photoelectron spectra (XPS, ESCALAB250) and high-resolution transmission electron microscope (HRTEM, FEI Tecnai G2 F20). The water contact angle (CA) of surface-modified SiNWs was measured using sessile drop method with a Dataphysics 15Pro instrument. The mechanical properties of different SiNWs were quantitatively measured by the scratching experiment in the tribometer (MTF-5000).

2.4. Anti-fouling evaluation

33.3 g of sea crystals and 1 mL of solution A, B and C were mixed with 1 L of distilled water to simulate a sea environment, where the chlorella disposable medium was cultured. After the culture at room temperature for 11 days, the amount of chlorella cells reached a steady state. Then the culture medium with chlorella was used for anti-biofouling research. All the samples of surface modified Si nanowires were immersed in the chlorella culture for 2 days (fluorescence microscopy) and 14 days (direct observation). The fluorescence analysis was conducted in optical microscope (Olympus BX53).

3. Results and discussion

3.1. Morphology characterization of modified SiNWs

The morphology of the as-prepared and plasma-modified Si nanowires (SiNWs) is shown in Fig. 1. Due to the chemical etching of Si substrate through Ag nanoparticles, the as-prepared Si NWs sample (SiNWs-0) features a rough and porous surface (Fig. 1(a)). As observed from the cross-sectional SEM image in Fig. 1(b), the nanowires are ordered and vertical to the Si substrate, with an average length of about 30 μm. As reported in previous investigation [24,31], the fabrication process of Si nanowires from smooth Si wafer can be divided into two steps. The first is the deposition of Ag nanoparticles. When Si wafer is immersed in the AgNO3 solution, Ag+ ions are reduced and preferentially nucleated at near-defective sites (around the dopants) of Si surface. As a result, Ag nanoparticles deposit on the whole area of Si wafer, which makes it possible to obtain Si NWs in large-area scale. The second step is the etching process, in which Ag nanoparticles act as a cathode while the Si atoms beneath Ag nanoparticles, as an anode, are oxidized and dissolved quickly under the role of H2O2. Therefore, the samples of Si nanowires feature a porous morphology, different from normal Si wafer. In addition, the average surface porosity is estimated to be 24% from the image contrast. The average nanowire density in the sample is thus calculated to be about 7.6 × 106 mm-2 based on the estimated nanowire diameter from the TEM results (shown in the following section). Fig. 1(c) and (d) show the morphology of the Si nanowires treated in the H2 plasma (SiNWs-h). It is noteworthy that the length and agglomeration of Si nanowires remain unchanged in comparison to the as-prepared sample of SiNWs-0. The growth of graphite (Fig. 1(e) and (f), SiNWs-g) and diamond nanoparticles (Fig. 1(g) and (h), SiNWs-d) on Si nanowires are conducted with the addition of CH4 gas in the H2 plasma. No variation of the nanowire width is observed in these samples, implying that a thin layer of diamond or graphite nanoparticles are deposited on the surface of Si nanowires. The result reveals that the plasma treatment using H2 or CH4/H2 gas has a slight effect on the morphology of surface-modified Si nanowires.

Fig. 1.

Fig. 1.   SEM morphology and cross section of surface-modified sample of Si NWs: (a) and (b) As-prepared sample by MACE methods (SiNWs-0); (c) and (d) Treatment of Si NWs by H2 gas plasma (SiNWs-h); (e) and (f) Treatment of Si NWs by CH4/H2 gas plasma forming graphite-coating nanowires (SiNWs-g); (g) and (h) Treatment of Si NWs by CH4/H2 gas plasma forming diamond-coating nanowires (SiNWs-d).


3.2. Compositional and structural characterization

In order to study the crystalline structure of the deposited layer in the SiNWs-g and SiNWs-d samples, the combination of Raman spectroscopy with HRTEM was employed. In the Raman spectra of Fig. 2, only two peaks centered at 1370.65 cm-1 (D band) and 1546.53 cm-1 (G band) are observed in the SiNWs-g sample (Red curve). These peaks are identified as the phase of pure sp2 carbon [32]. It implies that graphite is formed at the surface of Si nanowires. For the sample of SiNWs-d, besides the two peaks related to D and G bands, the diamond Raman peak located at 1322.38 cm-1 is observed, which indicates the formation of diamond phase around the nanowires [[32], [33], [34], [35]]. The peak at 1468.92 cm-1 corresponds to the presence of trans-polyacetylene (TPA), which is typical feature of the ultra-nanocrystalline diamond (UNCD). This means that the shell of diamond phase is crystallized in nano-size particles. In addition, compared to the standard unstressed diamond (1332 cm-1), the diamond Raman peak shifts downwards to the low wavenumber, indicating that tensile stress is generated in the diamond phase [32].

Fig. 2.

Fig. 2.   Raman spectra of samples of SiNWs-d and SiNWs-g.


Fig. 3 shows the typical TEM images of the modified nanowires in the SiNWs-g and SiNWs-d samples. For the sample of SiNWs-g in Fig. 3(a) and (b), there are lots of depression and bulges formed on the surface of the nanowires. The formation of bulges is generated during two sub-sequential processes. Firstly, the electrochemical etching around the side surface of the nanowires occurs under the migration of Ag nanoparticles during the MACE process. It leads to small-size pores along the side surface, as reported previously [31]. Secondly, the diameter of bulges enlarged under the etching of high-energy hydrogen plasma during the treatment in MWCVD [36]. The average nanowire diameter is about 201 nm, which are close to that of the as-prepared nanowires. At this magnification, the phase of nano-graphite is invisible due to the two-dimension effect. The interface between nano-graphite and Si nanowire was characterized by HRTEM technique, as shown in Fig. 3(c). The HRTEM image is recorded under the [110] zone axis of Si nanowires. The measured inter-spacings of about 0.271 nm and 0.313 nm are indexed to be (002) and (1 $\bar{1}$ 1) plane of silicon crystal, respectively. It implies that the growth direction of the nanowires is along the [002] of Si. This is in accordance with the etching of (001) Si wafer along the [001] depth direction, forming [002]-grown Si nanowires. In addition, it is observed that the side surface of Si nanowires is composed of {2 $\bar{2}$ 0} plane. The measured inter-spacing of about 0.364 nm corresponds to the (0002) basal plane of graphite phase. This graphite shell has a thickness of about 6 nm. One can see that the (0002) plane of graphite is parallel to the {2 $\bar{2}$ 0} side surface of Si nanowires. This special orientation between graphite and Si leads to a slight increase in the diameter of Si nanowires.

Fig. 3.

Fig. 3.   TEM and HRTEM images of surface-modified samples of Si NWs: (a), (b) and (c) SiNWs-g; (d), (e) and (f) SiNWs-d.


For the sample of SiNWs-d, it is observed that the nanowires exhibit a rougher side surface with a large number of bulges inside, as shown in the bright field images of Fig. 3(c) and (d). The bulges formed at the nanowire surface exhibit an average size of about 15 nm. Meanwhile, diamond nanoparticles are nucleated and grew around the nanowire, which leads to the increase in the nanowire roughness. In Fig. 3(f), the HRTEM image is taken from the [010] zone axis of Si nanowires. The average size of diamond nanoparticles is about 5 nm, which is in good agreement with the result of the formation of ultra-nanocrystalline diamond in Raman spectra (Fig. 2). The inter spacing of 0.271 nm and 0.2075 nm corresponds to the (002) plane of Si and the (111) plane of diamond phase, respectively. It is observed that the deposited diamond nanoparticles are orientated with Si nanowires with an angle of 27° between (111)D and (002)Si. The lattice mismatch between (111)D and (002)Si leads to tensile stress formed in diamond phase, in accordance with the Raman analysis.

In conclusion, the TEM results reveal that ultra-nanocrystalline diamond and graphite particles are deposited around the side surface of Si nanowire during the plasma treatment.

3.3. Evaluation of surface chemical bonds

To evaluate the chemical bonds of the modified Si NWs after different plasma treatment, XPS was employed, as summarized in Fig. 4. For the as-prepared nanowires (Fig. 4(a)), the Si 2p spectrum is curve-fitted into four peaks centered at ∼99.6, ∼102.28, ∼103.4 and ∼104.33 eV, which attribute to the bonds of Si-Si3, H-Si-O-Si, O = Si = O and H-Si = O, respectively [37,38]. These multiple chemical bonds are caused by the complex reaction during the MACE etching in oxidation solution [31].

Fig. 4.

Fig. 4.   Si 2p XPS spectra of surface-modified samples of Si NWs: (a) SiNWs-0 and (b) SiNWs-h; C1s XPS spectra of the samples of diamond-decorated (c) and graphite-decorated (d) Si NWs.


After H2 plasma treatment (Fig. 4(b)), besides the above-mentioned peaks, a peak centered at 99.93 eV is present, which is indexed to the bond of Si-H [38]. This means that parts of Si atoms are bonded with H atoms. In addition, the bond of H2-Si-O-Si formed at 102.9 eV is transformed from the H-Si-O-Si bond, implying that there are more H atoms bonded at the nanowire surface [37]. Based on the presence of Si-O related bonds, it is concluded that only a part of Si nanowires is transformed from oxygen into hydrogen termination due to the treatment of H2 plasma.

For the samples of SiNWs-g and SiNWs-d, the types of Si-related bonds at the nanowire surface are similar to that in the SiNWs-h sample, since the reactive plasma is rich in H2 gas. In addition, the coating of carbon phase lead to its related bonds on the nanowire surface. The C1s spectra are recorded to measure the variation of C-related bonds in these nanowires (Fig. 4(c) and (d)). In Fig. 4(d), the peaks located at 284.8 eV and 284.4 eV are identified as the bonds of sp3 and sp2 carbon, respectively. It implies that the formation of nanocrystalline diamond along the surface of SiNWs-d, in agreement with the results of Raman spectra in Fig. 2. In addition, chemical bond between C and H exhibits a peak at 285.2 eV [[39], [40], [41]]. This means that the surface of SiNWs-d is terminated with hydrogen. The quantitative calculation reveals that the content of sp3 and sp2 carbon is 29% and 44.2%, indicating that grain boundaries account for the major phase in the diamond layer. This means that the diamond shell is crystallized in nano-level, which is consistent with the results of Raman spectra and HRTEM images in Fig. 2, Fig. 3. As for SiNWs-g, only the bonds of sp2 C—C and C-H are observed (Fig. 4(c)). It implies that no diamond phase is formed, which is caused by the absence of pre-treatment in diamond seeding. As a result, the treatment in H2/CH4 gas plasma leads to the formation of H-termination at the Si nanowires.

3.4. Surface wettability of modified Si NWs

The influence of different gas plasma on the wettability of Si NWs was studied by measuring the contact angle (CA) of water on the surface, as shown in Fig. 5. It is observed that the as-prepared Si nanowires (SiNWs-0) show a CA of about 5.5° (Fig. 5(c)). It implies that as-prepared Si NWs exhibits a super-hydrophilic surface. The introduction of oxygen termination on the nanowires during the MACE process contributes to the formation of this super-hydrophilic surface. For the treatment under the H2 gas plasma, the CA of SiNWs-h increases to about 42°. With increasing the treatment time, the CA remains unchanged. This value is much smaller than that in the previous work [42]. The CA of Si NWs was reported to increase with the treatment time in H2 gas plasma and reached a maximum value of 146°. Generally, the surface terminated with H bond exhibits a weaker film-water interaction and tends to be more hydrophobic than the polar O termination (serving as water wetting center) [43]. It can infer that the smaller CA of Si NWs in our work is caused by the low power density of H2 gas plasma, which is not enough to fully transform the oxygen-related bonds to hydrogen-related bonds. The oxygen-related bonds are present in the XPS spectra (Fig. 4(b)), which supports the above elucidation. With increasing microwave power, the CA of Si NWs increases close to a superhydrophobic nature owing to the formation of higher content of hydrogen-related bonds, which is not shown here. As control, the Si wafer shows a slight increased CA value from 56.5° in the as-prepared sample to 61.9° in the hydrogen-plasma treated sample (Fig. 5(a) and (b)).

Fig. 5.

Fig. 5.   Water contact angle images of the as-prepared (a) and surface-modified (b) Si wafer; (c)-(f) water contact angle images of as-prepared and surface modified Si NWs: (c) SiNWs-0, (d) SiNWs-h, (e) SiNWs-g, and (f) SiNWs-d.


For the case of surface modification in the mixed CH4/H2 gas plasma, the CA of Si NWs increases to be about 133.6° (SiNWs-g, Fig. 5(e)) and 150.7° (SiNWs-d, Fig. 5(f)), respectively. This means that the surface of diamond-decorated Si nanowires is super-hydrophobic. The reason why the CA of the nanowires deposition of diamond phase increases dramatically can be summarized into two factors: roughness and surface chemical termination. For the first one, the effect of roughness on surface wettability could be estimated according to the equation cosθr = R ∙ cosθs [44], where θr and θs are the CA on the rough and flat surface, respectively (Fig. S1 in Supporting Materials). R is the roughness factor defined as the ratio of the real surface area to the projected area (R > 1 for a rough surface). As a result, the value of θr is smaller than that of θs in hydrophilic materials while θr is larger than θs in hydrophobic materials. The super-hydrophilic or super-hydrophobic surface could be obtained by the design of surface nanostructure. Since hydrogen-terminated diamond is always hydrophobic [45,46], the coating of diamond on the rough Si nanowires makes the surface more hydrophobic. With increasing the thickness of diamond layer, the roughness of Si nanowires is decreased, which leads to the decrease of their CA (Fig. S2 in Supporting Materials).

For the second factor, there are silicon-related (Si-H, H-Si-O-Si, H2-Si-O-Si) and carbon-related bonds (C—C, C-H) formed on the surface of diamond or graphite coated nanowires. Considering that the Si nanowires treated under pure H2 gas plasma at the same power density remain still hydrophilic, it implies that the effect of Si-related bonds is not decisive in the CA variation. The presence of C—C and C-H bonds plays an important role in the increase of CA in the SiNWs-g and SiNWs-d samples. The difference in CA between these two samples could be understood by their different surface energy. The surface energy (γS) of hydrogen-terminated diamond film could be calculated through the following equation by a two-fluid method [43]:

$\frac{1+\text{cos}\theta }{2} \bullet\frac{{{\gamma }_{\text{L}}}}{\sqrt{\gamma _{\text{L}}^{\text{n}}}}=\sqrt{\gamma _{\text{S}}^{\text{p}}}\sqrt{\frac{\gamma _{\text{L}}^{\text{p}}}{\gamma _{\text{L}}^{\text{n}}}}+\sqrt{\gamma _{\text{S}}^{\text{n}}}$

where θ is contact angle of test liquid, γL is surface energy of test liquid, $γ_{L}^{ p }$ and $γ_{L}^{ n }$ are polar force and non-polar force of test liquid, respectively. In our work, the test liquid of formamide and α-bremnaphthalene was employed. As reported previously [46], $γ_{L}^{ p }$ and $γ_{L}^{ n }$ of formamide are 19.6 mJ/m2 and 39.4 mJ/m2, respectively; and $γ_{L}^{ n }$ for α-bremnaphthalene is 44.4 mJ/m2. Based on the above equation, the surface energy of diamond is calculated to be about 44 mJ/m2, which is similar to the value mentioned in the literature [43,47]. As for the graphite film, the value of surface energy cannot be obtained through experiment since the droplet of test liquid is easy to seeps into the graphite fabricated by MWCVD devices. This value was reported to be (85 ± 11) mJ/m2 in the previous reference [48]. This means that the surface energy of diamond is smaller than that of graphite, which leads to the increased CA in the SiNWs-d sample. As a result, the Si nanowires coated with diamond nanocrystals tend to more hydrophobic than those with graphite.

3.5. Anti-biofouling performance of Si NWs

The biofouling performance of the as-prepared and surface-modified Si NWs is evaluated in the chlorella culture medium. Fig. 6 shows the morphology of all the samples before (up row) and after immersion (down row) test for 14 days. The planar Si wafer with and without the modification of hydrogen plasma is used for comparison, as shown in Fig. 6(a-d). Observation of the white-contrast region at the wafer surface in Fig. 6(b-d) reveals the attachment of chlorella organism. One can see that there is less chlorella attached on the hydrogen-modified wafer than that of the as-prepared one. For the as-prepared nanowires (SiNWs-0), the samples before immersion are in black color due to the strong light adsorption induced by the presence of rough or porous nanostructure. After immersion test, observation of deeper yellow-green contrast implies the attachment of thick layer of chlorella on the nanowire surface (Fig. 6(e) and (f)). With the treatment of hydrogen plasma on the Si nanowires (SiNWs-h), the organism attached on the surface is less than that on the SiNWs-0 sample, which is demonstrated by the presence of lighter yellow-green contrast (Fig. 6(g) and (h)). With regard to the samples of SiNWs-g and SiNWs-d, they remain in black color after 14 days. The difference in contrast between these two samples is negligible. It implies that the amount of attached chlorella is the least in these two types of nanowires. In order to evaluate the anti-fouling behavior of these surface-modified Si nanowires in detail, fluorescence microscopy was employed to observe the attachment of chlorella on the sample surface, as shown in Fig. 7(a-d). Since the average size of chlorella is about 2.5 μm (Fig. S3 in Supporting Materials), the long-time culture (such as 14 days) leads to the whole-area attachment and overlapping of chlorella organisms, which is difficult for the quantitative evaluation. The culture time is thus reduced to 2 days. It is observed from Fig. 7(a) that the SiNWs-0 sample with super-hydrophilic surface exhibits large-area fouling of chlorella. The observation of different-size chlorella is caused by the de-focusing of this organism distributed along different depths. With the treatment by the H2 or H2/CH4 gas plasma, the chlorella adhered on the surface-modified SiNWs is much reduced in comparison to the sample of SiNWs-0. The SiNWs-d sample shows the least fouling of chlorella (Fig. 7(d)). In addition, the water contact angle increases gradually from the SiNWs-0 sample to SiNWs-h, and finally to SiNWs-g and SiNWs-d sample. This means that when the water contact angle is increased, the attachment of chlorella is gradually decreased in these surface-modified nanowires. Statistical investigation at numerous regions based on the fluorescence images reveals the same trend of chlorella attachment on these SiNWs samples (Fig. 7(e)). The super-hydrophilic SiNWs-0 sample exhibits the highest number of attached chlorella. In comparison, the amount of chlorella attached on the SiNW-g and the SiNW-d sample is decreased by 6.4 fold and 12.9 fold, respectively. This result implies that the super-hydrophobic nanowires coated with nano-crystalline diamond possess the best anti-fouling behavior among all the samples. Therefore, it is concluded that the anti-biofouling behavior of the nanowires is associated with the surface wettability.

Fig. 6.

Fig. 6.   Biofouling performance of Si wafers and different surface-modified nanowires before immersion (0d) and after immersion for 14 days: (a) and (b) As-prepared Si wafer; (c) and (d) Modified Si wafer by H2 gas plasma, Si-h; (e) and (f) As-prepared Si NWs by MACE, SiNWs-0; (g) and (h) Modified Si NWs by H2 gas plasma, SiNWs-h; (i) and (j) Graphite-coated Si NWs, SiNWs-g; (k) and (l) Diamond-coated Si NWs, SiNWs-d.


Fig. 7.

Fig. 7.   (a-d) Fluorescence microscopy images of chlorella after 2 days of culture on different surface-modified SiNWs: (a) As-prepared Si NWs by MACE, SiNWs-0; (b) Modified Si NWs by H2 gas plasma, SiNWs-h; (c) Graphite-coated Si NWs, SiNWs-g; (d) Diamond-coated Si NWs, SiNWs-d. (e) Quantitative evaluation of the number of adhered chlorella on different surface-modified SiNWs.


Since surface wettability is related with the roughness and chemical termination, the fouling result indicates that the performance of anti-biofouling may be affected by surface chemical termination. For Si wafer, the samples treated by hydrogen plasma exhibits less attachment of chlorella organism than the as-prepared one. In comparison to the hydrophilic samples of SiNWs-0, the nanowires coated with nano-diamond tend to be superhydrophobic and exhibits the least attachment of chlorella. This means that super-hydrophobic nanowires (SiNWs-d) are more effective for the antifouling of chlorella than super-hydrophilic nanowires (SiNWs-0). It is similar to the mechanism in solid-water-biological material system [49]. When immersed in chlorella medium, an air film is formed between the nanowire surface and chlorella cells in water, making them repelled from surface. Thus, the probability for the adhesion of chlorella in water to the SiNWs-d sample is dramatically decreased.

Scratching experiment was employed to evaluate the mechanical property of Si nanowires with different surface modification. Under a load of 1 N, the friction force indirectly reveals the mechanical properties of the nanowires. Fig. 8(a) shows the results of different surface-modified Si nanowires. It is observed that the friction force in the samples of the SiNWs-h and SiNWs-g is similar to that in the SiNWs-0 sample. This means that the plasma treatment under pure H2 gas or the deposition of graphite leads to no improvement in the mechanical strength of Si nanowires. The SiNWs-d sample exhibits the friction force 8 times as large as the other sample, which indicates that the coating of diamond remarkably increases the mechanical strength of the nanowires. The SEM images of as-prepared Si NWs (SiNWs-0) and diamond decorated ones (SiNWs-d) after immersion for 14 days are shown in Fig. 8(b) and (c), respectively. For the sample of SiNWs-0, large areas of nanowire exhibit a behavior of bending with part of nanowires peeled off from the sample (Fig. 8(b)). The remaining broken nanowires are still coated by attached chlorella cells, which feature a shallow contrast as marked by the arrows. As for SiNWs-d (Fig. 8(c)), the morphology after immersed for 14 days is similar to that before immersion. This result means that there is less attachment of chlorella on the surface of the sample. It also implies that the nanowires coated with ultra-nanocrystalline diamond particles are of good robustness apart from the best anti-biofouling performance. Since diamond exhibits the outstanding properties of chemical stability and mechanical strength, the coating of diamond increases the chemical inertness and robustness of Si nanowires. As a result, the morphology of SiNWs-d sample remains in better condition after chlorella attachment with comparison to the other samples. Based on the above results, it is concluded that the Si nanowires decorated with diamond phase possess the best performance of anti-fouling of chlorella and good robustness for mechanical damage. The surface modification of Si nanowires in CH4/H2 plasma could be used in the fabrication of superhydrophobic surface for the application in the anti-biofouling marine area.

Fig. 8.

Fig. 8.   (a) Mechanical properties of different surface-modified SiNWs samples under the load of 1 N using scratching experiment. High friction force in the SiNWs-d sample demonstrates that the Si nanowires coated with diamond possess better mechanical strength in comparison with other samples. The SEM morphology of SiNWs-0 (b) and SiNW-d (c) samples after immersion for 14 days. The unchanged morphology of SiNWs-d also implies good robustness of the nanowires with the coating of diamond.


4. Conclusion

In this work, the wettability and anti-fouling behaviors of surface modified Si nanowires were investigated. The as-prepared Si nanowires with a MACE technique are super-hydrophilic due to the formation of O termination, as confirmed by the XPS result. The treatment of Si nanowires in hydrogen plasma of MPCVD system leads to an increase of water contact angle to about 41°. For the coating of graphite and diamond nanoparticles on Si nanowires, the water contact angle of the samples increases up to 133.6° and 150.7°. This result implies that a super-hydrophobic surface is formed in diamond-coated Si nanowires. XPS, Raman and HRTEM images confirmed that the formation of diamond with H-termination plays a key role in the super-hydrophobicity beside the presence of porous morphology. Immersion test in chlorella culture medium demonstrates that diamond-coated Si nanowires shows good antifouling and mechanical performance compared to other treatments. As a result, the coating of H-terminated diamond could be applied to convert the surface of Si nanowires from super-hydrophilic to super-hydrophobic, which are highly demanded in the application of anti-fouling.

Acknowledgements

The work was finically supported by the National Natural Science Foundation of China (No. 51872294). The authors thank Ms Yuan Zhang and Prof. Lei Yang from Northeastern University for the experiment of fluorescence imaging of anti-fouling.

Appendix A. Supplementary data

Supplementary material related to this article can be found, inthe online version, at doi:https://doi.org/10.1016/j.jmst.2019.10.040.

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