# Increasing hydrophobicity of ceramic membranes by post-deposition nitrogen annealing of molecular layer deposition grown hybrid layers

Harpreet Sondhi, Michiel Nijboer, Elissa Makhoul, Arian Nijmeijer, Fred Roozeboom, Mikhael Bechelany, Alexey Kovalgin, Mieke Luiten-Olieman

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## ► To cite this version:

Harpreet Sondhi, Michiel Nijboer, Elissa Makhoul, Arian Nijmeijer, Fred Roozeboom, et al. Increasing hydrophobicity of ceramic membranes by post-deposition nitrogen annealing of molecular layer deposition grown hybrid layers. Applied Surface Science, 2025, 683, pp.161790. 10.1016/j.apsusc.2024.161790. hal-05086521

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Submitted on 9 Jul 2025

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Full Length Article

# Increasing hydrophobicity of ceramic membranes by post-deposition nitrogen annealing of molecular layer deposition grown hybrid layers

Harpreet Sondhi<sup>a</sup>, Michiel Nijboer<sup>a</sup>, Elissa Makhoul<sup>c</sup>, Arian Nijmeijer<sup>a</sup>, Fred Roozeboom<sup>a</sup>,
Mikhael Bechelany<sup>c,d</sup>, Alexey Kovalgin<sup>b</sup>, Mieke Luiten-Olieman<sup>a,*</sup>

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## ARTICLE INFO

### Keywords:

- Molecular layer deposition

- Ceramic membranes

- Surface functionalization

- Hybrid layers

- Post-deposition N₂ annealing

- Hydrophobicity

## ABSTRACT

Molecular layer deposition is increasingly used to functionalize the surface of planar and 3D porous substrates. This study explores functionalized 'alucone' hybrid layers grown on ceramic substrates to enhance their surface hydrophobicity. The layers were made from trimethyl-aluminum as a precursor with five different alcohols as a co-reactant: aliphatic (ethane-1,2-diol, 2,2-oxydiethanol, 1,6-hexanediol), and aromatic (4-aminophenol, benzene-1,4-diol). After post-deposition annealing in N₂ atmosphere at 250 and 350 °C, the changes in the physical properties and the chemical affinity of the layers were studied and compared to those of the as-deposited hybrid layers. Differential thermal analysis of the hybrid layers showed higher decomposition temperatures for the layers grown from aromatic alcohols than for those grown from aliphatic alcohols. Additionally, Fourier-transform infrared and X-ray photoelectron spectroscopy showed that an increase in annealing temperature decreased the concentrations of surface hydroxyl-groups and caused changes in the carbon-related groups. These results could be correlated to the hydrophobicity of the layers: higher water contact angles (>90°) were measured for annealed samples, compared to those (<90°) of the as-deposited samples grown at 150 °C. These findings confirm that the proposed method of MLD functionalization and post-deposition annealing can be used to tune the surface hydrophobicity of ceramic membranes.

## 1. Introduction

In many filtration processes, polymeric membranes are used to treat process streams [1]. Over the last few decades, industrial streams containing organic solvents have increased [2,3]. Most of these streams are upgraded using energy-intensive processes like distillation at elevated temperatures [4]. An energy-efficient alternative to distillation is membrane filtration [5,6]. However, presently used polymeric membranes are still insufficient: when the complex mixtures of solutes and solvents supplied come into contact with this type of membranes, the polymeric matrix starts swelling, hampering the membranes' performance (like selectivity) and adversely affects them [7-9].

Oxidic ceramic types of membranes remain far less affected during organo-solvent nanofiltration (OSN) [10–12]. Ceramic membranes [13] enable chemically and physically more robust system with a long shelf-life [14–16]. A major challenge remaining is that ceramic membranes

are generally hydrophilic [10]. Literature confirms that metal oxide surfaces often terminated with OH-groups, can be tailored to fabricate films, opening up novel applications (for e.g. OSN membranes) [17]. Their chemical structure and effective adsorption sites make them suitable for further functionalization with suitable organic reactants to make surface hydrophobic [18,19]. A common approach to make ceramic membranes hydrophobic [20,21] is by using grafting such as by silanization. This method is still facing challenges such as achieving consistent and uniform hydrophobicity across the entire surface. Variations in silanization conditions (such as reagent concentration, reaction time, and temperature) can impact the degree of hydrophobic conversion [22,23].

Organic constituents like carbon are known to make a substrate surface hydrophobic, so the molecular layer deposition (MLD) technique was selected as a method to functionalize a substrate surface and tune its hydrophobicity. MLD is a conformal coating approach [24], a subset of

<footnote><sup>a</sup> Inorganic Membranes, University of Twente, 7500 AE Enschede, The Netherlands</footnote>

<footnote><sup>b</sup> Integrated Devices and Systems, University of Twente, 7500 AE Enschede, The Netherlands</footnote>

<footnote><sup>c</sup> Institut Européen des Membranes, IEM, UMR-5635, Université Montpellier, ENSCM, CNRS, Place Eugène Bataillon, 34095 Montpellier, France</footnote>

<footnote><sup>d</sup> Functional Materials Group, Gulf University for Science and Technology (GUST), Mubarak Al-Abdullah 32093, Kuwait</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: m.w.j.luiten@utwente.nl (M. Luiten-Olieman).</footnote>
atomic layer deposition (ALD) [25], which can be used to deposit inorganic-organic hybrid layers. One example of such hybrid layers is formed by alternating inorganic vapor precursor with organic co-reactants [26–32]. Hybrid layers grown from trimethyl-aluminum (TMA) precursor and a variety of co-reactants are called alucone layers. In this work, the degree of hydrophobicity of membranes coated with alucone layers grown from TMA with ethane-1,2-diol (EG), 2,2-oxydiethanol (DEG), 1,6-hexanediol (HD), and aromatic, 4-aminophenol (AP), and benzene-1,4-diol (HQ) has been investigated. This method is combined with post-deposition annealing in flowing nitrogen.

## 2. Experimental methods

### 2.1. Materials

Alpha-alumina (α-Al₂O₃) supports (average pore size of 80 nm) from Pervatech, Netherlands [33], silicon (100) wafers (SUMCO, Japan), α-Al₂O₃ beads for thermogravimetric analysis (TGA) (Saint-Gobain, France) were used. 97 % trimethyl-aluminum (Al₂(CH₃)₆), 99.8 % ethylene glycol (CH₂OH)₂, 99 % di-ethylene glycol (C₄H₁₀O₃), 98 % 4-aminophenol (C₆H₇NO), 99 % 1,6-hexanediol (C₆H₁₄O₂), 99 % benzene-1,4-diol (C₆H₆O₂), 99.999 % lanthanum (III) nitrate hexahydrate (La(NO₃)₃-(H₂O)₆), 97 % aluminum-tri-sec-butoxide (Al[OCH(CH₃)-C₂H₅]₃) and Mowiol 8-88 ([-CH₂CHOH-]ₙ) were procured from Sigma-Aldrich. All chemicals were of analytical grade.

### 2.2. Principles of MLD

MLD is based on self-limited half-reactions between a gas-phase inorganic precursor and an organic co-reactant [34]. The precursors are injected sequentially, with the first half-reaction (labelled as I in Fig. 1) introducing a precursor on the surface until the surface is saturated, followed by a co-reactant half-reaction (labelled as III in Fig. 1) step that completes the first mono-layer deposition [35]. Inert gas is purged (II and IV) after each half-reaction (I and III) to remove unreactive species. This cycle can be repeated until the desired thickness is achieved. Most MLD processes are based on a binary reaction sequence [35], with only two successive surface reactions per cycle. Fig. 1 illustrates the process of contacting the surface with the precursor and co-reactant.

In our MLD experiments, lanthanum-doped gamma-alumina (La/γ-

Al₂O₃, modified alpha-alumina, α-Al₂O₃) as the starting substrate for hybrid layer depositions was used. γ-Al₂O₃ is rich in OH-groups compared to α-Al₂O₃, thus providing more reaction sites for surface functionalization [36]. Here, TMA was selected as a precursor, and EG, DEG, HD, AP, and HQ were selected as co-reactants with a focus on differences in properties of aliphatic (EG, DEG, HD) and aromatic (AP, HQ) alcohols [25]. The selection of co-reactants depends on their bifunctional nature, cost, availability, and high reactivity, which is widely reported in MLD studies [37]. First, the TMA pulse is introduced to cover the support with a methylated surface layer [26]. Subsequently, the EG pulse is introduced, which reacts with the already deposited methylated surface to form a monolayer of TMA-EG, as illustrated in Fig. 2(A). This cycle is repeated until the desired thickness of the layer is reached.

After the film fabrication, as shown in Fig. 2(B), the membranes were annealed in a nitrogen environment at 250 °C and 350 °C, respectively. This leads to the hybrid network's formation and the partial decomposition of the organic part. As per literature, temperature and environment (annealing atmosphere) do influence the decomposition of the hybrid networks and play a decisive role in the formation of functionalized layers [38,39]. Temperature and environment are crucial for tailoring the properties of the membrane to specific applications. Fig. 2 (B) shows an overview of nanopore fabrication on a ceramic support; at the start of the process, partial pore filling takes place and a dense layer over the membrane surface is gradually formed, resulting in an organometallic thin film over the surface [40]. Next, annealing is carried out in a nitrogen environment to partially decompose the organic part, as compared to air (where more complete decomposition can take place, due to oxidation reactions) [39].

The main objective of the work is to investigate the surface hydrophobicity of different hybrid layer membranes after deposition and subsequent heat annealing in N₂ for application in industrial streams containing organic solvents.

### 2.3. Support preparation

#### 2.3.1. Aluminum oxide layer on planar silicon (100) substrate (Al₂O₃-coated Si)

Silicon (100), p-type, low B-doped with 10¹⁵ B-atoms.cm⁻³), with a thickness of 525 µm, resistivity of 5–10 Ω.cm) and coupon size 1 × 1 cm², coated with a 10 nm aluminum oxide (Al₂O₃) pre-layer grown with

<figure><img src="image_5.png" /><figcaption>**Fig. 1.** General scheme of MLD cycling for the preparation of one monolayer of material.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Fig. 2.** Schematic representation of (A) fabrication of one monolayer of TMA and EG on La/γ-Al₂O₃ support and (B) layer deposition and nanopore fabrication on ceramic support.</figcaption></figure>

conventional thermal ALD from TMA and water [41,42] were used to measure the film thickness of hybrid layers deposited on top by using spectroscopy ellipsometry (SE). The Al₂O₃ pre-layer formation process is shown below (Eqs. 1–2). Once the first half-reaction is completed (1a–1c), water is introduced in the second half-reaction (2a)–(2c) [41]. The total reaction is written as follows:

$$ \|\text{OH} + \text{Al(CH}_3)_3 \rightarrow \|\text{O-Al(CH}_3)_2 + \text{CH}_4 \quad (1a) $$

$$ \|\text{OH} + \|\text{O-Al(CH}_3)_2 \rightarrow (\|\text{O})_2\text{-Al(CH}_3) + \text{CH}_4 \quad (1b) $$

$$ \|\text{OH} + (\|\text{O})_2\text{-Al(CH}_3) \rightarrow (\|\text{O})_3\text{-Al} + \text{CH}_4 \quad (1c) $$

$$ \|\text{O-Al(CH}_3)_2 + \text{H}_2\text{O} \rightarrow \|\text{O-Al(CH}_3)\text{OH} + \text{CH}_4 \quad (2a) $$

$$ \|\text{O-Al(CH}_3)\text{OH} + \text{H}_2\text{O} \rightarrow \|\text{O-Al(OH)}_2 + \text{CH}_4 \quad (2b) $$

$$ (\|\text{O})_2\text{-Al(CH}_3) + \text{H}_2\text{O} \rightarrow (\|\text{O})_2\text{-Al(OH)} + \text{CH}_4 \quad (2c) $$

$\|\text{O}$ indicates the species on the substrate surface.

### 2.3.2. Preparation of La/γ-Al₂O₃ membranes

Planar membrane supports (α-Al₂O₃, 2 mm thickness, average pore width of 80 nm) were used as starting substrates. To reduce their pore size below 20 nm, they were coated with a La/γ-layer as illustrated in Fig. 3. First, boehmite was prepared from aluminum-tri-sec-butoxide (Al [OCH(CH₃)-C₂H₅]₃). By adding polyvinyl alcohol (PVA) (Mowiol 8-88 ([CH₂CHOH]-n) (MW ~68 kDa)), and lanthanum-(III) nitrate hexahydrate (La(NO₃)₃(H₂O)₆) solution is activated for dip-coating the α-Al₂O₃. Lanthanum is added to stabilize the layer for high temperatures and avoid phase changes in the layer. The dip-coated support is annealed at 650 °C for 3 hrs [36].

<figure><img src="image_7.png" /><figcaption>**Fig. 3.** Schematic of preparation of La/γ-Al₂O₃ and pictorial description of an OH-terminated substrate.</figcaption></figure>
## 2.4. Preparation of hybrid layers from TMA with EG, DEG, HD, AP, HQ

Al₂O₃-coated silicon and La/γ-Al₂O₃ were selected as starting substrates for the deposition from TMA as a precursor with different co-reactants (EG, DEG, HD, AP, HQ). A custom-built low-pressure MLD reactor (as described in the detail in the Supplementary Material (SM) was utilized for the deposition of hybrid layers, see Fig. S1. The precursor and organic co-reactants were supplied as vapors from canisters held at room temperature for TMA and elevated temperatures for the co-reactants, as listed in Table 1. Argon (Ar) was used as an inert purging gas. All depositions were carried out under a 2–3 mbar pressure. The pulse (0.4 s), exposure (10 s), and Ar purge time (30 s) of TMA were kept constant, while the co-reactant pulse, exposure, and purge time were changed depending on vapor pressure to achieve the required layer thickness (Table 1). Relatively long purges were used to ensure the presence of only one chemical reactant at a time. A plausible schematic of hybrid layer formation using different co-reactants in combination with TMA is shown in Fig. 4 (derived after [37,43]). The OH-group in each co-reactant reacts with the methyl group of TMA, releasing CH₄ as a byproduct, while the OH/NH₂ groups on the opposite end can react to functionalize further or act as cross-linker [37,43,44]. Based on these reports, MLD was conducted at a temperature set at 150 °C for each co-reactant in combination with TMA as precursor. After successive depositions, samples were annealed at 250 °C and 350 °C under nitrogen flow (50 ml/min).

## 2.5. Characterization

A Semilab Spectroscopic GES-5E ellipsometer was used ex-situ to measure the film thickness of films grown on planar Si-substrates. Thermogravimetric analysis (TGA) on TA instruments' SDT-Q600 tool was used to investigate the influence of annealing temperatures on the mass loss and decomposition patterns of hybrid layers. An optical tensiometer (OT) (Kruss DSA30) was used to measure the water contact angle (WCA, ΘA). Here, Milli-Q purified water droplets were measured at three points on each substrate, and the standard deviation was calculated. Fourier-transform infrared spectroscopy (FTIR) was used for surface chemistry and reaction mechanism studies, and spectra were collected on a (PerkinElmer-Spectrum Two FT-IR) spectro-photometer within the 400–4000 cm⁻¹ wavenumber range. X-ray photoelectron spectroscopy (XPS) analysis was used to identify the nature of chemical bonds, and to measure the atomic composition of the films using a PHI QUANTES tool with Al Kα (1486.6 eV) radiation. The C 1s peak due to ambient hydrocarbon contamination was calibrated at 284.8 eV [45]. The binding energies of the photoelectron lines of the hybrid layers were charge-referenced to this C 1s line. A Zeiss Merlin High-resolution scanning electron microscopy (HR-SEM) system (field emission source and resolution of 1.2 nm) was used for cross-section analysis of hybrid layers.

### 2.5.1. Hybrid layer thickness studies on Al₂O₃-coated Si

The layer thickness is critical in tuning the hydrophobicity and

<table><caption>**Table 1**
Overview of the process parameters for MLD growth at 150°C.</caption><thead><tr><th>Co-reactant</th><th>Pulse time (s)</th><th>Exposure time (s)</th><th>Argon purge time (s)</th><th>Cannister temperature (°C)</th></tr></thead><tbody><tr><td>Ethylene glycol (Ethane-1,2-diol)</td><td>7</td><td>20</td><td>120</td><td>100</td></tr><tr><td>Diethylene glycol (2,2-oxydiethanol)</td><td>7</td><td>20</td><td>120</td><td>100</td></tr><tr><td>1,6-Hexanediol</td><td>7</td><td>10</td><td>60</td><td>85</td></tr><tr><td>4-Aminophenol</td><td>7</td><td>10</td><td>60</td><td>145</td></tr><tr><td>Hydroquinone (Benzene-1,4-diol)</td><td>10</td><td>10</td><td>60</td><td>130</td></tr></tbody></table>

nanopore sizes after annealing. First, to achieve hybrid films with the same layer thickness (set at 30 nm), the number of cycles had to be adjusted for each precursor/co-reactant combination based on the data listed in Table 1. SE was used to verify hybrid layer thicknesses (as measured for films grown on planar Si-substrates and described in detail in SM (Table S1)).

### 2.5.2. Influence of annealing temperatures on hybrid layers deposited on α-Al₂O₃ beads

Each hybrid layer was annealed at 250 °C and 350 °C, respectively, to optimize the temperature required to decompose the organic co-reactants. The inflection point using TGA-DTA (differential thermal analysis), where the mass loss starts, is crucial in determining a suitable range for the subsequent annealing steps, providing key insights into the stability of the decomposition patterns and hybrid layers [39].

### 2.5.3. Degree of hydrophobicity on planar Al₂O₃-coated Si and La/γ-Al₂O₃ membrane

An optical tensiometer was used to analyze the surface hydrophilicity of the hybrid layers. The hybrid layer's hydrophobicity depends on the surface wettability; the higher the contact angle (ΘA) value, the lower the wettability. Values above ΘA ~90° are termed hydrophobic, and below are hydrophilic [46,47]. These classifications directly affect various surface chemistry applications, making our research highly relevant and applicable in NF membranes.

### 2.5.4. Surface and bulk composition on La/γ-Al₂O₃

Transmission mode FTIR was used to analyze the surface of both the hybrid layers (after deposition and annealing at 250 °C and 350 °C) and the pristine La/γ-Al₂O₃ support. It fingerprints the characteristic deformations and vibration modes of functional groups (carbon-related or other).

### 2.5.5. XPS depth profile and composition on Al₂O₃-coated Si

XPS analysis was used in depth profiling mode to analyze the composition of TMA-EG and TMA-HQ hybrid layers on planar Al₂O₃-coated Si. These were selected on the basis of their structure (aliphatic (EG) versus aromatic (HQ)). A key aspect of this analysis is the determination of the C:O ratios for each combination, as it underscores the significance of the number of carbon groups present in the layer. In addition, depth profile studies were conducted to observe the change in distribution for Al, C, and O from the surface of the layer to the interface with the silicon substrate.

### 2.5.6. Degree of porosity and uniformity of Al₂O₃-coated Si and La/γ-Al₂O₃ membrane

HR-SEM analysis in cross-section mode was used to investigate the layer thickness, degree of porosity and uniformity of the hybrid layers grown using MLD. It elucidates the change in porosity from the starting substrate to the MLD-grown hybrid layers (described in detail in SM (section S2)).

## 3. Results and discussion

### 3.1. TGA-DTA analysis

The TGA-DTA measurements were conducted on the alucone hybrid layers in a nitrogen environment in the temperature range from 25 to 800 °C to determine the influence of post-deposition annealing conditions. TGA spectra of TMA-EG, TMA-DEG, TMA-HD, TMA-AP, and TMA-HQ are shown in Fig. 5(A). It is possible to detect the region associated with changes in the material mass on the TGA spectra. At the range between 200 °C and 400 °C, the major mass loss takes place.

As per the DTA analysis, distinct weight loss peaks can be observed for each combination of hybrid layers, depending on the type of co-reactant used, as shown in Fig. 5(B)-5(C). More particular, from the
<figure><img src="image_8.png" /><figcaption>Fig. 4. Plausible surface functionalization schemes for alucone hybrid layer formation using MLD of TMA with different co-reactants viz. EG, DEG, HD, AP, and HQ.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>Fig. 5. TGA and DTA spectra of functionalized hybrid layers grown on α-Al₂O₃ beads. (A) TGA spectra, (B) DTA spectra (—TMA-EG; —TMA-DEG; —TMA-HD; and (C) TMA-AP; —TMA-HQ).</figcaption></figure>

inflection points around 250 °C (TMA-EG), 225 °C (TMA-DEG and TMA-HD), 350 °C (TMA-AP), and 300 °C (TMA-HQ), 250 °C and 350 °C were selected as distinct post-deposition annealing temperatures. It is evident that the layers grown from aliphatic groups (Fig. 5(B)) are less thermally stable than the layers grown from aromatic groups (Fig. 5(C)), with their characteristic benzene ring. See also the XPS results in section 3.4.

## 3.2. Optical tensiometer (OT) analysis

WCA measurements were carried out to elucidate the surface hydrophobicity of hybrid layers. Al₂O₃-coated Si samples (ΘA = 52°) were taken as a reference for comparison with the corresponding hybrid layers, each grown with a targeted thickness of 30 nm before annealing. A visual representation of the hybrid layer structure is shown in Fig. 6 (A), with X being the co-reactant used in TMA-X, and La/γ-Al₂O₃ layer (3 µm) on α-Al₂O₃ (20 nm thickness). As shown in Table 2 and Fig. 6(B)-6(F), each co-reactant significantly improved the hydrophobicity of the hybrid layers. The ΘA values changed from ≤90° after deposition to >90° after annealing at 250 °C and 350 °C, respectively. This change in ΘA values is attributed to the difference in concentrations and nature of organic species, as further clarified by using FTIR (see section 3.3). Each co-reactant has a certain number and chemical class of carbon groups.

Thus, one can expect that carbon removal will depend on the inter- and intra-molecular (de-) bonding during nitrogen annealing, as already experimentally shown by TGA-DTA (see section 3.1). Hybrid layers with aliphatic co-reactants were decomposed at lower temperatures (<300 °C) as compared to aromatic co-reactants (>300 °C). Each individual alucone layer will have its co-reactant bonded with TMA-precursor and will show different amounts of termination groups at the surface.

As shown in Fig. 6(B)-6(F) all hybrid layers show a ~2 – 8 % change in ΘA values after annealing at 250 °C and 350 °C. The hybrid layers on both the Al₂O₃-coated Si and La/γ-Al₂O₃ substrates showed similar trends, apart from slight differences due to different surface OH-concentrations, surface roughness and porosity, and carbon contamination. However, it is clear that the choice of chemical functionalization (i.e. the co-reactant for the MLD of alucone) and physical (post-deposition) treatment of different hybrid layers can be used for tailoring the hydrophobicity of planar, porous substrate surface layers.

## 3.3. FTIR analysis

FTIR measurements were conducted to study the structural behavior of functional surface groups of hybrid layers after deposition and subsequent annealing. FTIR spectra of the pristine (La/γ-Al₂O₃) substrate
<figure><img src="image_10.png" /><figcaption>**Fig. 6.** Surface functionalization of La/γ-Al₂O₃ membrane supports with hybrid layers TMA-X to enhance surface hydrophobicity. (A) Basic layer structure of hybrid layer, TMA-X (30 nm), MLD grown on La/γ-Al₂O₃ layer (3 µm) on α-Al₂O₃ (20 mm) thickness. (B), (C), (D), (E), (F). Comparison of WCA on silicon (■) and La/γ-Al₂O₃ (■) after deposition (150 °C), annealing in N₂ (250 °C and 350 °C, respectively).</figcaption></figure>

<table><caption>**Table 2**

Overview of water contact angle values (WCA, Θ<sub>A</sub>) of hybrid layers after deposition at 150 °C, and subsequent annealing at 250 °C and 350 °C, respectively.</caption><thead><tr><th rowspan="3">Hybrid MLD layer</th><th colspan="3">La/γ-Al<sub>2</sub>O<sub>3</sub> samples (Θ<sub>A</sub>)</th><th colspan="3">Al<sub>2</sub>O<sub>3</sub>-coated silicon samples (Θ<sub>A</sub>)</th></tr><tr><th>After deposition</th><th>After annealing</th><th>After annealing</th><th>After deposition</th><th>After annealing</th><th>After annealing</th></tr><tr><th>@ 150 °C</th><th>@ 250 °C</th><th>@ 350 °C</th><th>@ 150 °C</th><th>@ 250 °C</th><th>@ 350 °C</th></tr></thead><tbody><tr><td>TMA-EG</td><td>52 ± 2</td><td>96 ± 1</td><td>89 ± 2</td><td>62 ± 2</td><td>102 ± 2</td><td>100 ± 2</td></tr><tr><td>TMA-DEG</td><td>39 ± 2</td><td>98 ± 2</td><td>92 ± 1</td><td>28 ± 2</td><td>100 ± 2</td><td>98 ± 2</td></tr><tr><td>TMA-HD</td><td>80 ± 2</td><td>92 ± 2</td><td>90 ± 3</td><td>76 ± 1</td><td>97 ± 1</td><td>94 ± 1</td></tr><tr><td>TMA-AP</td><td>91 ± 1</td><td>96 ± 1</td><td>94 ± 1</td><td>85 ± 3</td><td>95 ± 1</td><td>92 ± 3</td></tr><tr><td>TMA-HQ</td><td>88 ± 2</td><td>93 ± 1</td><td>90 ± 2</td><td>65 ± 0</td><td>98 ± 1</td><td>95 ± 1</td></tr></tbody></table>

and the hybrid layers on top are shown in Fig. 7. La/γ-Al₂O₃ exhibits characteristic vibration and deformation band of OH-groups at 3400 cm⁻¹, the characteristic vibrations of covalent bonds of H₂O, OH at 1640 cm⁻¹, 1500 cm⁻¹ and 1400 cm⁻¹ and Al-O, Al-OH vibrations at 1080 cm⁻¹, 840 cm⁻¹ confirms the formation of La/γ-Al₂O₃ [48] as shown in Fig. 7(A). The spectrum of the TMA-EG hybrid layer annealed at 250 °C exhibits enhanced OH-groups as indicated by the broad vibration peak at 3320 cm⁻¹ compared to 350 °C (diminished), as shown in Fig. 7(B). Then, the broad band around 3400 cm⁻¹, pointing to -OH groups, and vibrations at 2920-2850 cm⁻¹, being indicative for C—H stretching modes (aldehydes or carboxylic groups), the peaks at 1640 cm⁻¹, and 1415 cm⁻¹ (carboxylic/carbonyl groups), and C—C (acyl groups) at 1100 cm⁻¹ respectively, all confirm the formation of a TMA-EG hybrid layer [39] with vibration signals far different from those of pristine layer. The spectra show the strong influence of the annealing temperature on the relative amounts of carbon and OH-groups present: the peak intensities are reduced by increasing the heat annealing temperatures from 250 °C to 350 °C (see also ref. [39]). Similarly, for TMA-DEG, TMA-HD, TMA-AP, and TMA-HQ [37], as shown in Fig. 7(C)-7(F) and listed in Table 3, the vibrations of the broad OH band and the

covalent bonds of aldehydes or carboxylic, acyl, carboxylic/carbonyl, and carboxylic groups all point to the formation of hybrid layers, respectively. The FTIR spectra confirm the types of different relevant carbon groups in the layer. Sharp peaks and pronounced vibrations are observed for aromatic compared to the aliphatic co-reactant hybrid layers.

### 3.4. XPS analysis

XPS is used to determine the composition of the hybrid layers. Fig. 8 provides a visual representation of the XPS spectra of an Al₂O₃-coated-Si sample that was carefully modified by MLD of a TMA-EG hybrid layer. The conclusion drawn from the data shown in Fig. 8(A) and Table 4, it is clear that the surface region contains 48 and 41 at. % C, 36 and 37 at. % O, and 16 and 22 at. % Al for 250 °C and 350 °C respectively, for annealed TMA-EG (aliphatic) hybrid layer. The elemental components at the surface are in the ratio of C:O in decimals is 1.3 and 1.1, O:Al is 2.2 and 1.7, and C:Al is 2.9 and 1.9. A complete hybrid layer is observed, effectively covering the surface. The significant atomic % of C 1s and O 1s suggest that the hydrophobicity on the top surface of the hybrid layer
<figure><img src="image_11.png" /><figcaption>**Fig. 7.** FTIR spectra of (A) α-Al₂O₃ (—) and pristine La/γ-Al₂O₃ support (—), and functionalized hybrid layers (B), (C), (D), (E), (F) after deposition on La/γ-Al₂O₃ at 150 °C (—), and subsequent annealing at 250 °C (—) and 350 °C (—), respectively.</figcaption></figure>

<table><caption>**Table 3**

Overview of main FTIR vibrational signals for the different membrane samples.</caption><thead><tr><th>Hybrid layer</th><th>Aldehydes or carboxylic (cm<sup>-1</sup>)</th><th>Acyl (cm<sup>-1</sup>)</th><th>Carboxylic/ carbonyl (cm<sup>-1</sup>)</th><th>OH (cm<sup>-1</sup>)</th><th>Carboxylic (cm<sup>-1</sup>)</th></tr></thead><tbody><tr><td>α-Al<sub>2</sub>O<sub>3</sub></td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>La/γ-Al<sub>2</sub>O<sub>3</sub> [48]</td><td>—</td><td>—</td><td>—</td><td>3400</td><td>1080, 840</td></tr><tr><td>TMA-EG [39]</td><td>2920, 2850, 1640</td><td>1100</td><td>1510, 1415</td><td>3320</td><td>1070, 820, 720</td></tr><tr><td>TMA-DEG</td><td>2920, 2850, 1630</td><td></td><td>1470, 1390</td><td>3420</td><td>1095, 810, 720</td></tr><tr><td>TMA-HD</td><td>2960, 2920, 2850, 1630</td><td>1260, 1155</td><td>1465</td><td>3380</td><td>1050, 810, 740</td></tr><tr><td>TMA-AP [49]</td><td>2970, 2930, 2850, 1590</td><td>1250, 1030</td><td>1460</td><td>3400</td><td>1080, 800, 730</td></tr><tr><td>TMA-HQ [37]</td><td>2965, 2920, 2850, 1560</td><td>1260, 1220</td><td>1500</td><td>3440</td><td>1090, 800, 750</td></tr></tbody></table>

is primarily due to the presence of a carbon enriched at surface layer. Moreover, depth profile tests were conducted to compare the variation in the amount of C 1s and O 1s present within (the bulk of) the hybrid layer in contrast to its top surface layer. This comparison is particularly intriguing as the surface consistently harbours more carbon (known as adventitious carbon [50]). Upon closer examination, the sputter profile yield for the TMA-EG hybrid layer exhibited a significant shift from the top surface to bulk, as depicted in the Fig. 8(B)-8(C). As expected, a substantial decrease in carbon content was observed. This can be attributed to the porous nature of the hybrid layer, where during annealing EG (organic part) is removed, thereby creating nanopores. However, in comparison to the hybrid layer annealed at 250 °C, a higher

amount of carbon is removed at 350 °C, thereby reducing the C:O ratio. Similarly, the TMA-HQ (aromatic) hybrid layer was analysed for samples annealed at 250 °C and 350 °C, respectively. In contrast to EG, HQ contains a stable benzene ring, making it challenging to completely decompose HQ from the TMA-HQ hybrid layer. This leads to higher C:O ratios for samples annealed at 250 °C and 350 °C, as listed in Table 4 and shown in Fig. 8(D)-8(F). The thermal stability of the aromatic rings on HQ and AP was also clear from the TGA-DTA measurements (section 3.1) and will result in a higher residual C-content for these films upon annealing in comparison with the hybrid layers containing aliphatic groups.

### 3.5. Highlights of experimental results and outlook

The objective was to accurately follow the structural and hydrophobicity properties of the hybrid layers used to functionalize ceramic membranes with La/γ-Al₂O₃ layers on α-Al₂O₃ support, and their subsequent annealing.

Post-deposition annealing was carried out in a nitrogen environment at 250 °C and 350 °C, respectively, to understand the influence of temperature on functional groups (e.g. the relative amounts of carbon and OH) contained by the hybrid layers. In all cases, a uniform surface coverage for all alucone hybrid layers formed with tunable surface hydrophobicity was observed, as confirmed with WCA measurements. Using a combination of ex-situ TGA, OT, FTIR, and XPS measurements, the surface hydrophobicity of the hybrid layers as measured by their WCA could be correlated to the relative amounts of carbon and OH-groups species formed after post-deposition annealing at 250 °C and 350 °C. Different compositional changes upon thermal treatment of the hybrid layers were also detected by TGA; the respective inflection points in mass change were different for each hybrid layer within a range of
<figure><img src="image_12.png" /><figcaption>**Fig. 8.** XPS spectra TMA-EG and TMA-HQ hybrid layers after annealing at 250 °C and 350 °C, respectively. Regular binding energy spectra for (A) TMA-EG and (D) TMA-HQ; depth profile spectra at sputter rate, 5 nm/min for (B) TMA-EG and (E) TMA-HQ annealed at 250 °C; depth profile spectra at sputter rate, 10 nm/min for (C) TMA-EG and (F) TMA-HQ at 350 °C. Data are based on peak areas (C 1s, Al 2p, O 1s) in XPS spectra.</figcaption></figure>

<caption>**Table 4**</caption>

Overview of the carbon, oxygen and aluminum contents (in at. %) of the top surface and bulk of hybrid layers deposited on Al₂O₃-coated Si. Surface is defined as the upper 5 nm of each film; bulk is the remaining thickness of the nominally 30 nm thick films. Data are based on peak areas (C 1s, Al 2p, O 1s) in XPS spectra.

<table><thead><tr><th rowspan="2">Hybrid layer</th><th colspan="2">Carbon (at %)</th><th colspan="2">Aluminum (at %)</th><th colspan="2">Oxygen (at %)</th><th colspan="2">Ratio (C:O) in decimal</th></tr><tr><th>Surface</th><th>Bulk</th><th>Surface</th><th>Bulk</th><th>Surface</th><th>Bulk</th><th>Surface</th><th>Bulk</th></tr></thead><tbody><tr><td>TMA-EG (250 °C)</td><td>48</td><td>11</td><td>16</td><td>38</td><td>36</td><td>51</td><td>1.3</td><td>0.2</td></tr><tr><td>TMA-EG (350 °C)</td><td>41</td><td>10</td><td>22</td><td>42</td><td>37</td><td>48</td><td>1.1</td><td>0.2</td></tr><tr><td>TMA-HQ (250 °C)</td><td>54</td><td>28</td><td>17</td><td>32</td><td>29</td><td>40</td><td>1.9</td><td>0.7</td></tr><tr><td>TMA-HQ (350 °C)</td><td>51</td><td>15</td><td>19</td><td>38</td><td>30</td><td>47</td><td>1.7</td><td>0.3</td></tr></tbody></table>

200 to 400 °C. FTIR studies also revealed that pronounced characteristic vibrations are observed for hybrid layers grown from aromatic as compared to aliphatic co-reactants.

XPS further confirmed that the depth profiles for aliphatic (EG) differ from those for aromatic (HQ) molecules in the hybrid layer. Aliphatic molecules comprise σ-bonds only, while aromatics have a more stable benzene ring (with conjugated σ- and π-bonds), which will require more energy to break during annealing. This will make the carbon—carbon bonds in the bulk of hybrid layer more difficult to decompose as observed in XPS measurements. Furthermore, HR-SEM measurements confirmed the change in porosity and high uniformity for the MLD-grown hybrid layers.

These surface-tuned membranes can be used for industrial separations. Industries such as pharmaceuticals, agro- and petrochemicals, etc., rely on specialty chemicals to make their products. However, the separations in these chemical process flows are highly energy-intensive. Functionalized ceramic membranes, under such harsh conditions, hold the potential to significantly reduce energy consumption by separating molecules based on size exclusion. The ability to tailor the porous support surface may facilitate the synthesis of ceramic OSN membranes.

## 4. Conclusions

In this work, a detailed study on the surface functionalization of La/γ-Al₂O₃ membrane layers by coating them with alucone hybrid layers grown by MLD from TMA as precursor and different co-reactants (EG, DEG, HD, AP, and HQ) has been performed. Planar Al₂O₃-coated Si substrates were used as 2D reference samples in the thin-film processing (deposition and subsequent annealing) and α-Al₂O₃ beads were used as a reference in TGA-DTA analysis. Our results demonstrate that MLD of alucone films from TMA, and organic co-reactants can be used to coat ceramic oxide membranes, with well-controlled surface area coverage (as planar and porous substrates with different sizes were used) and mitigates many of the complexities inherent to solution phase depositions. The implications of this alternative vapor-phase approach of tuning the hydrophobicity by functionalization using MLD coating and careful subsequent annealing could be far-reaching, as it opens avenues for ceramic OSN membranes with tunable modified surface properties and for separation of solutes from organic solvents, but also deepens our understanding of surface reaction engineering.
CRediT authorship contribution statement

**Harpreet Sondhi:** Writing – review & editing, Writing – original draft. **Michiel Nijboer:** Writing – review & editing. **Elissa Makhoul:** Writing – review & editing. **Arian Nijmeijer:** Writing – review & editing. **Fred Roozeboom:** Writing – review & editing. **Mikhael Becheblany:** Writing – review & editing, Conceptualization. **Alexey Kovalgin:** Writing – review & editing, Conceptualization. **Mieke Luiten-Olieman:** Writing – review & editing, Conceptualization.

Funding

This publication is part of the project 'Re-use of industrial solvent process streams with ceramic nanofiltration membranes with adaptable surface properties (ReSolve)' with project number 18993 of the Open Technology research program which is partly financed by The Dutch Research Council (NWO).

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.

Acknowledgements

This work was supported as part of the Inorganic Membranes (IM) group, part of Department of Chemical Engineering, and Integrated Devices and Systems, part of Department of Electrical Engineering, MESA + institute, University of Twente, Enschede, and Institut Européen des Membranes, IEM, UMR-5635, Université Montpellier, ENSCM, CNRS, Place Eugène Bataillon, 34095 Montpellier, France.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.apsusc.2024.161790.

Data availability

No data was used for the research described in the article.

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