In this paper, the surface roughness and wettability of Ti nanofilms were characterized after Ar‑H₂ plasma cleaning at different durations. Combined with the chemical structure and elemental composition of the Ti nanolayer following 120-second Ar‑H₂ plasma activation, the activation mechanism of Ar‑H₂ plasma on Ti nanofilms was analyzed and summarized.

Surface Roughness Analysis

Ti nanofilms were cleaned and activated by Ar‑H₂ plasma for different periods (0 s, 30 s, 60 s, 90 s, 120 s, and 150 s). Atomic force microscopy (AFM) was employed to characterize the three-dimensional surface morphology, with results presented in Figure 1.

As shown in Figure 1(a), the as-deposited sample without plasma treatment exhibits scattered columnar protrusions with heights ranging from several nanometers to over ten nanometers. These protrusions are likely organic contaminants and particulate impurities, leading to relatively high surface roughness. After 30 seconds of Ar‑H₂ plasma exposure (Figure 1(b)), the number of protrusions decreases significantly, their heights drop below 3 nm, and features taller than 10 nm disappear completely, reducing the surface roughness to 0.641 nm. At this stage, most surface contaminants are removed via plasma bombardment, yet scattered residual impurities remain, resulting in strong image contrast and uneven surface undulation.

For plasma treatment times exceeding 60 s (Figures 1(c)–(f)), columnar protrusions are nearly eliminated, image contrast becomes uniform, surface flatness is enhanced, and contaminants are fully removed. When activation time surpasses 90 s, densely distributed needle-like structures emerge on the Ti nanofilm surface, likely induced by chemical structural modifications under plasma interaction. Notably, 120-second Ar‑H₂ plasma activation yields the most uniform peak height distribution and a relatively low surface roughness of 0.574 nm. Therefore, 120-second Ar‑H₂ plasma treatment effectively removes surface contaminants, improves three-dimensional morphological flatness, induces chemical reactions with the Ti nanofilm, and reduces surface roughness.

Figure 1 Effect of plasma cleaning and activation time on surface morphology of Ti nanofilms

Surface Chemical State Analysis

To investigate the chemical modification mechanism of Ar‑H₂ plasma on Ti nanofilms, X-ray photoelectron spectroscopy (XPS) was performed to analyze Ti and O species before and after plasma treatment, with peak-fitting results displayed in Figure 2.

Under ambient conditions, pristine Ti spontaneously forms a 3–5 nm thick native oxide layer upon exposure to air. Consequently, more than 80% of the Ti 2p signal before activation corresponds to TiO₂, accompanied by 10.79% Ti₂O₃, 6.06% Ti⁰, and 2.72% TiO. After Ar‑H₂ plasma activation, the relative intensities of TiO₂ characteristic peaks at 458.5 eV and 465 eV in the Ti 2p spectrum slightly increase, accounting for 87.40% of the total Ti 2p signal and establishing TiO₂ as the dominant surface component. The relative proportion of Ti⁰ 2p peaks at 454 eV and 460.1 eV decreases from 6.06% to 3.88%.

The O 1s spectrum reveals that the TiO₂ fraction rises from 58.74% to 60.17% after activation, indicating increased TiO₂ content and enhanced chemisorbed oxygen on the surface, accompanied by a reduction in the relative intensity of the Ti–OH characteristic peak. N 1s spectra confirm that nitrogen detected on the Ti nanofilm surface before and after activation originates solely from adsorbed nitrogen atoms, without forming chemical bonds with titanium. Furthermore, the signal intensity of surface nitrogen weakens after plasma treatment.

Figure 2 XPS spectra of the surface before and after 90 s Ar‑H₂ plasma cleaning and activation

Surface Wettability Analysis

To reveal the influence of Ar‑H₂ plasma on wettability, Ti nanofilms were activated for varying durations (0 s, 30 s, 60 s, 90 s, 120 s, 150 s), and wettability was characterized immediately afterward. Droplet spreading profiles under different conditions are shown in Figure 3.

The water contact angle of the untreated Ti nanofilm is 80.7°. With increasing plasma treatment time, water droplets gradually spread across the surface and achieve complete spreading once activation exceeds 90 s. Although the contact angle of pure TiO₂ (a primary component of the Ti nanofilm) is 70.7° without treatment, Ar‑H₂ plasma treatment drastically improves surface wettability. The main mechanisms include:

l Removal of organic contaminants and impurities from the Ti nanolayer surface;

l Coordination of water molecules with oxygen vacancies (V_O) generated by Ar⁺ bombardment, promoting dissociative and physical adsorption of water;

l Adsorption of hydroxyl (–OH) hydrophilic groups at defect sites induced by Ar⁺ interaction.

Since contact angle measurements were conducted immediately after plasma activation, abundant surface hydroxyl groups remain, contributing to strong hydrophilicity.

Figure 3 Droplet spreading profiles on Ti nanofilms after plasma surface treatment for different durations

Plasma Cleaning and Activation Mechanism

Ar‑H₂ plasma cleaning and activation of Ti nanofilms achieves the following effects:

l Elimination of surface contaminants and surface cleaning;

l Reduction of surface titanium oxides, generating Ti³⁺ and Ti²⁺ via Ar⁺ bombardment and metallic Ti through reduction by H radicals;

l Adsorption of water molecules and hydroxyl groups under Ar⁺ bombardment, enhancing surface wettability and surface energy;

l Reduction in the electrical resistivity of Ti nanofilms and improved electrical properties.

A schematic illustration of this plasma activation mechanism is presented in Figure 4.

Figure 4 Schematic diagram of the Ar‑H₂ plasma cleaning and activation mechanism