A controlled growth strategy for CAU-17 MOF thin films via two-step synthesis

Abstract

Sputtering-assisted synthesis is being investigated for the first time as a novel approach for the fabrication of CAU-17 MOF thin films. This method enables the production of MOF thin films without the risk of forming impurities or requiring harsh conditions, unlike conventional powder-based techniques. In this work, the synthesis of CAU-17 was achieved directly on different substrates via a two-step sequential process. Initially, a thin film of bismuth oxide was deposited onto the substrate using reactive DC magnetron sputtering. This oxide layer was subsequently transformed into a CAU-17 MOF through coordination with an organic linker under controlled conditions. The synthesized material was characterized using several advanced characterization techniques. The influence of synthesis conditions, such as growth temperature and formation on the surface of various support materials, was also investigated. This approach paves the way for advanced fabrication of MOF thin films with potential uses in sensing, catalysis, and other cutting-edge technologies.

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Introduction

Metal–organic frameworks (MOFs) are a class of porous crystalline materials that exhibit permanent porosity.^1,2 They were first conceptualized by Dr Omar Yaghi in 1999 with the discovery of MOF-5, commonly known as IRMOF-1.³ It is a hybrid material composed of organic and inorganic components.^4,5 The organic component, known as a linker, acts as a connector between the inorganic units, which are referred to as metal nodes.^6,7 These materials are renowned for their structural diversity, tunable porous nature, and extremely large surface area.^8–10

Recently, bismuth-based MOFs have gained the attention of researchers as promising materials for advanced nanoscience and technology applications. One of the main reasons for this is the safe and non-hazardous nature of bismuth. Additionally, it is one of the Earth's most abundant elements, which makes it easily accessible and affordable at an industrial level.^11–13 To date, a limited number of Bi-based MOFs have been synthesized and characterized, and among them, only a few exhibit permanent porosity, i.e., the family of CAU (CAU-7, CAU-17, and CAU-35), SU-101, and NOTT-220. In this classification of Bi-MOFs, so far, only CAU-17 has been synthesized cheaply using trimesic acid.¹⁴ This MOF was first reported by Inge et al. in 2016¹⁵ and was subsequently widely found to have emerging applications in several areas, such as electrochemical CO₂ reduction to formate/formic acid, electrochemical sensors for detecting heavy metal ions, and the development of advanced batteries with higher conductivity and stability.^16–21

Interestingly, the CAU-17 MOF has a more complex structure than many other MOFs. In terms of its topology, 54 unique nodes with 135 edges have been reported. Its structure consists of infinite linear chains of Bi-oxo rods formed by edge-sharing helical BiO₉ polyhedra.²² In CAU-17, many framework species take part in asymmetric units, i.e., 9 Bi³⁺ cations, 9 BTC³⁻ anions, and 9 coordination water units, where each 9 Bi³⁺ ions connect with 8 O atoms of the BTC³⁻ and 1 O atom from coordinated water molecules.²³ Each Bi-cation shares 4 O anions with adjacent cations, forming helical/spiral rods. These rods develop joints with BTC³⁻ anions, resulting in an intricate 3D framework of CAU-17.¹⁵ In short, its crystal structure consists of left and right helical/spiral edge-sharing BiO₉ polyhedra linked with BTC³⁻ ions, forming channels of hexagonal, rectangular, and triangular shapes.²⁴

One unique fact about CAU-17 is that it is highly sensitive to synthesis conditions. A minor change in synthesis parameters, such as temperature, time, or solvent, can result in a completely different structure. Nguyen et al. reported a study in which the impact of the solvent on CAU-17 crystallization was investigated.²⁵ They found that using DMF, methanol, or even a mixture of DMF and methanol as the solvent led to crystallization into different structures. Köppen et al. determined the optimal synthesis temperature for CAU-17.²⁴ In their work, they employed a microwave-assisted approach and identified 120 °C as the optimal condition for preparing the CAU-17 structure. They also noted that synthesizing above or below this temperature resulted in the formation of unknown impurities. Dong et al. investigated the influence of the synthesis technique on CAU-17 morphology.²⁶ In that study, flower- and rod-like morphologies of CAU-17 with distinct crystalline structures were obtained using sonication and conventional solvothermal methods. Similar behavior was also reported by Huang et al. and Lee et al., who observed almond-flake-like and sheet-like morphologies of CAU-17.^19,27

Until now, only three techniques have been reported for the synthesis of CAU-17, to the best of our knowledge: solvothermal, microwave-assisted, and sonochemical techniques. The major drawback of these methods is the formation of impurities and secondary phases during CAU-17 formation. For instance, previous reports have shown that solvothermal and microwave-assisted routes often resulted in the formation of unknown impurities, primarily due to the addition of bismuth(III) nitrate during the synthesis process.^24,25 Another significant challenge is the unintentional formation of a Bi(btc) MOF instead of the desired CAU-17. Although both Bi-MOFs are synthesized using the same organic linker and chemical constituents, the key distinction between these two structures lies in their morphology: the Bi(btc) MOF is denser and less porous, whereas CAU-17 features a highly porous framework, which is crucial for applications requiring high surface area. It is well established that a dense phase occurs when CAU-17 crystallizes too rapidly or during prolonged synthesis times.^20,25,28 Additionally, powder-based MOFs have previously demonstrated many challenges, such as poor bonding with the substrate, processability, and mechanical strength for advanced applications.^29–31

The transition from powder-based technology to thin films is an attractive choice to overcome the above-stated problem, as it offers more precise control over the synthesis conditions and results in the formation of high-quality thin films with improved crystalline nature and a higher purity level.³² Furthermore, the direct synthesis of MOFs from metal oxides is considered a simple and cost-effective approach with strong adhesion between the substrate and MOF, along with admirable structural stability.^33,34

The primary advantage of using magnetron sputtering for the synthesis of CAU-17 thin films is the use of a metallic target as a source of metal ions. This approach offers unique benefits over metal salts, as it effectively overcomes challenges related to the formation of unknown impurities or undesirable secondary phases. Additionally, this technique is greener, cost-effective, and scalable, in contrast to the powder-based technique previously employed for CAU-17 synthesis.^35,36 Other advantages of this technique include strong adhesion strength, synthesis under mild conditions, and the formation of a uniform coating with great control over thickness.^37,38 Aside from that, it has great potential for synthesizing a wide range of MOF thin films by modifying the organic linker, metal target, and synthesis conditions.

In this work, the CAU-17 MOF thin film was synthesized directly on the substrate surface via a two-step sequential process. Initially, a thin film of bismuth oxide was deposited onto the substrate using reactive DC magnetron sputtering. This oxide layer was subsequently transformed into CAU-17 through coordination with the organic linker under controlled conditions. Furthermore, the study systematically examined the formation of the CAU-17 thin film on various substrate materials and assessed the effect of synthesis temperature on the crystallization behavior and structural properties of the resulting CAU-17 MOF thin films.

Experimental work

Materials

A 99.99% pure bismuth target (2″ diameter and 0.125″ thickness bonded on Cu back plate) was purchased from Semiconductor Wafer Inc. 1,3,5-Benzenetricarboxylic acid (H₃BTC) was purchased from Thermos Scientific. Ethyl alcohol (C₂H₆, absolute) was purchased from DUKSAN Pure Chemicals. Dimethylformamide (C₃H₇NO, ≥99.5%) was purchased from Fisher Scientific. All chemicals in this work were used as received without any purification. The solution of 0.01 M HCl was prepared by diluting the analytical grade HCl with distilled water.

Characterization techniques

X-ray diffraction (XRD) analysis was performed on a Rigaku Mini X-ray diffractometer operating at 30 kV and 15 mA with a Cu Kα wavelength of 1.54 Å. The morphology of thin films was observed by field emission scanning electron microscopy (FESEM, Tescan Lyra 3) operating at 20 kV coupled with energy-dispersive X-ray spectroscopy (EDX). Raman spectroscopy was performed using a DXR Raman microscope (Thermo Fisher, Wisconsin, USA). The analysis was performed using a 10× confocal microscope objective equipped with 1200 lines per mm diffraction. The system was operated at a laser power and wavelength of 3 mW and 455 nm, respectively. Raman signals were acquired in 5 accumulations of 50 seconds from 400 to 2000 cm⁻¹. FT-IR analysis was performed on a PerkinElmer Spectrum 3. The analysis was recorded between the wavelengths of 400 and 4000 cm⁻¹ with a spectral resolution of 4 cm⁻¹. The surface composition of the thin films was examined using the X-ray photoelectron spectroscopy technique (XPS, Thermo Scientific, ESCALAB 250Xi).

Bi₂O₃ thin film deposition

The sputtered Bi₂O₃ thin films were synthesized on a quartz substrate using a DC reactive sputtering approach. Before deposition, the substrates were ultrasonicated in acetone, ethanol, and DI water separately for 10 min and then dried on a hotplate (100 ± 5 °C). After cleaning, the substrates were mounted on the substrate holder, and the deposition chamber was evacuated to a base pressure of 4.14 × 10⁻⁵ Torr. The bismuth target of 99.99% purity was sputtered by DC magnetron sputtering (NSC–4000 sputter coater) by using argon (Ar) and oxygen (O₂) as sputtering gases. Bi₂O₃ thin films were deposited onto the quartz substrate with a cathode power of 50 W for 15 min at room temperature at an Ar : O₂ flow rate (sccm) of 35 : 35. During synthesis, the substrate table was rotated to achieve uniform thickness. Similarly, the same deposition conditions were used for the synthesis of Bi₂O₃ thin films on the FTO substrate and carbon paper.

Sputtered Bi₂O₃ conversion into the CAU-17 thin film

The conversion of the sputtered amorphous Bi₂O₃ thin film into the CAU-17 thin film is achieved by immersing the coated substrate into the reaction medium that consists of 5 mL of DMF solution, 100 mg of H₃BTC, and 1 mL of 0.01 M HCl in a glass vial. Afterwards, the solution was sonicated until it became transparent, and then heated at 105 °C for 2 days. The synthesized CAU-17 thin film was then cleaned multiple times with an ethanol solution and dried in an oven (100 °C ± 10 °C) until the solvent had fully evaporated, as part of the solvent exchange process. For investigating the influence of temperature on CAU-17 formation, fresh samples were prepared in a wider range of temperatures (30 °C, 80 °C, 105 °C, 120 °C, and 130 °C) for 2 days. The complete synthesis procedure of the CAU-17 thin film is illustrated in Scheme 1. For the synthesis of CAU-17 on the surface of the FTO substrate and carbon paper coated with amorphous Bi₂O₃, a similar solution was prepared. The conversion of the Bi₂O₃-coated FTO substrate into CAU-17 occurs at 130 °C for 2 days, while for the carbon paper-coated Bi₂O₃ thin film, it takes place at 105 °C for 2 days.

Scheme 1 Synthesis procedure of the CAU-17 MOF thin film.

Results and discussion

Formation of CAU-17 MOF thin films

XRD analysis of CAU-17 MOF thin films was performed to confirm their structure, phase purity, and crystalline nature. XRD spectra of the sputtered Bi₂O₃ thin film and the CAU-17 thin film are shown in Fig. 1. As can be observed, no crystalline peaks were observed in the diffraction pattern of the as-deposited sputtered Bi₂O₃ thin film, indicating the amorphous nature of the thin film. In contrast, the XRD pattern of the CAU-17 thin film displayed several crystalline peaks. Among them the most intense peaks of CAU-17 were determined at 8.35°, 10.49°, 12.68°, 16.51°, 24.75°, and 33.13°. The average crystallite size of CAU-17 was about 62 nm, calculated by applying Scherrer's equation to the most intense (hkl) peak:
where D is the crystallite size, β is the full width at half maximum (FWHM) measured in radians, λ is the wavelength of incident X-ray (0.15406 nm), K is the shape factor (0.9), and θ represents the X-ray diffraction angle.³⁹ The XRD pattern of CAU-17 displayed distinct peaks in comparison to the reported simulated pattern of CAU-17.¹⁵ The main reason for obtaining a different diffraction pattern of CAU-17 is due to its high sensitivity towards synthesis conditions and preparation approaches.

Fig. 1 XRD pattern of the as-deposited amorphous Bi₂O₃ thin film via reactive DC magnetron sputtering and the CAU-17 thin film prepared at 105 °C for 2 days.

FESEM results revealed that the surface of the Bi₂O₃ thin film transformed into CAU-17 nanorods with homogenous distribution, as shown in Fig. 2(a and b). The surface morphology of the CAU-17 thin film was also consistent with that reported in the literature.^14,40,41 The width of the nanorods was determined by using the open-source image-processing software ImageJ (1.41v) developed by the NIH. The width of the nanorods was measured by considering 50 nanorods chosen randomly from the sample. The mean calculated width of the CAU-17 nanorods was 3.719 µm.

Fig. 2 FESEM images of the (a) as-deposited Bi₂O₃ thin film and (b) CAU-17 thin film, along with (c) EDX elemental mapping of the CAU-17 thin film prepared at 105 °C for 2 days.

EDX elemental mapping of CAU-17 nanorod thin films, shown in Fig. 2(c), confirms the existence of bismuth (Bi), carbon (C), and oxygen (O). The homogenous distribution of these elements throughout the nanorods represents the well-formed structure of CAU-17. Additionally, it was also observed that the nanorod morphology consists of a high bismuth content, indicating a nearly complete transformation of the Bi₂O₃ thin film into CAU-17. To further confirm whether the synthesized material belongs to CAU-17 or not, additional spectroscopy techniques were employed.

To confirm the molecular structure of the CAU-17 thin film, Raman spectroscopy was used. The Raman spectra of H₃BTC and CAU-17 are shown in Fig. 3(a), which exhibits multiple peaks corresponding to their respective functional groups. The Raman spectrum of the CAU-17 MOF thin film was highly consistent with the reported literature.⁴² The CAU-17 spectrum showed the characteristic peaks of H₃BTC within the range of 700–2000 cm⁻¹ and Bi–O-related bonds at 50–600 cm⁻¹.¹⁴ The characteristic bands at 425.73 and 490.33 cm⁻¹ belong to the Bi–O bond stretching modes. The stretching and bending modes of C–H bonds were observed at 731.39, 838.42, and 1205.79 cm⁻¹. The C–O–O symmetric and asymmetric stretching vibration modes were determined at 1448.78 and 1554.85 cm⁻¹, respectively.⁴³ The peaks at 1003.30, 1133.48, and 1608.84 cm⁻¹ represent the symmetric stretching of C=C bonds from the aromatic ring of H₃BTC.^43–45 The band at 1358.14 was assigned to the in-plane vibration of the C–O bond.⁴⁶ The presence of C–O–O and Bi–O bonds in the CAU-17 spectrum indicates the existence of required functional groups and confirms metal–linker coordination.

Fig. 3 (a) Raman spectra of H₃BTC and the CAU-17 MOF thin film and (b) FT-IR spectra of H₃BTC and the CAU-17 MOF thin film.

FT-IR spectroscopy was applied to determine the existence of intended functional groups in the structure of the CAU-17 thin film. Fig. 3(b) shows the FT-IR results of H₃BTC and the CAU-17 thin film. The FT-IR spectrum of H₃BTC shows the characteristic peaks of O–H stretching, C=O stretching, and O–H bending groups at 3000–2500 cm⁻¹, 1763–1569 cm⁻¹, and 996–764 cm⁻¹, respectively.²⁶ In the FT-IR spectrum of CAU-17, both characteristic peaks of the Bi–O and carboxylate groups were observed. The new peaks appeared within the range of 500 to 700 cm⁻¹, corresponding to the Bi–O group, indicating the coordination of the COOH group of the organic linker with the Bi³⁺ ions through electrostatic interaction.²¹ The characteristic peaks at 536 and 720 cm⁻¹ correspond to Bi–O stretching.⁴⁷ The carboxylate groups were observed at 1352 and 1432 cm⁻¹, attributed to the asymmetric vibration of the –COOH group.²⁶ The disappearance of the characteristic peaks of the H₃BTC linker, along with the appearance of new bands corresponding to the coordination between the linker and Bi³⁺ ions, confirms the successful formation of the CAU-17 thin film.

XPS analysis was performed to determine the surface composition, chemical states, and functional groups of the CAU-17 MOF thin film. Fig. 4(a) shows the survey scan that confirms the existence of the main constituents of CAU-17, i.e., Bi, C, and O elements in the synthesized thin film, consistent with the aforementioned EDX mapping results. The presence of Sn is due to the synthesis of the CAU-17 thin film on the surface of the FTO substrate, while the existence of N at ∼399 eV corresponds to N–C bonding, which occurs due to the entrapment of a trace amount of DMF within the pores of CAU-17.^48,49 For the correction of binding energies, the C 1s peak of carbon (BE = 284.6 eV) was used as a reference.⁵⁰

Fig. 4 XPS analysis of the CAU-17 thin film: (a) survey scan; high-resolution spectra of (b) Bi 4f, (c) O 1s, and (d) C 1s.

The high-resolution Bi 4f spectrum of CAU-17 is shown in Fig. 4(b), which consists of two binding energy peaks at 158.08 eV and 163.18 eV corresponding to Bi³⁺ 4f_7/2 and Bi³⁺ 4f_5/2, respectively.^26,51 These peaks confirm the existence of Bi³⁺ ions in the CAU-17 thin film.⁵² The O 1s spectrum displayed three peaks (Fig. 4(c)), where the intense peak at 530.08 eV corresponds to Bi–O bonding, while the low intensity peaks at 531.68 eV and 532.98 eV belong to the O–H bond in H₂O and O in the bismuth-oxo clusters of the CAU-17 thin film, respectively.^16,26,53 The C 1s spectrum of CAU-17, shown in Fig. 4(d), consists of three peaks at 284.38, 286.28, and 287.68 eV corresponding to C=C, C–C, and O–C=O of the H₃BTC linker in the CAU-17 framework, respectively.^26,54,55 These findings were also consistent with the aforementioned Raman and FT-IR spectra.

By considering the spectra of Bi 4f and O 1s, and also the structure of CAU-17 {9 Bi³⁺, 9 BTC³⁻ and 9 H₂O}, it was observed that the Bi³⁺ ions are coordinated with 9 O atoms in the prepared CAU-17 thin film.^16,23 In this coordination system, 8 O atoms belong to the BTC³⁻ ligands and 1 O atom from water molecules, resulting in the formation of a porous framework of CAU-17.²¹

Influence of synthesis temperature

Many researchers previously confirmed that temperature plays a major role in the synthesis of MOFs and has a great influence on their structure and morphology.^56–58 Additionally, temperature significantly influences thermodynamics, reaction kinetics, and coordination chemistry of MOFs.⁵⁹ The influence of synthesis temperature on CAU-17 formation was investigated by heating at 30 °C, 55 °C, 80 °C, 105 °C, and 130 °C, and the corresponding XRD results are depicted in Fig. 5. The synthesis of CAU-17 at 30 °C displayed two weak XRD signals at 8.66° and 12.98°, indicating a poor crystalline nature and an early stage of nucleation. Increasing the synthesis temperature to 55 °C results in improved peak intensity; however, the CAU-17 thin film still exhibits low crystallinity, displaying a sharp peak at 8.58° along with a weak signal at 10.69°, 13.15°, 25.08°, and 33.46°. The main reason behind this is that Bi³⁺ ions belong to the group of metal ions that possess high valence. It has previously been reported that synthesizing MOFs using trivalent-metal ions at room temperature is challenging and usually requires external energy for their formation.^60,61 Due to this reason, the synthesized CAU-17 at a lower temperature displayed weak signals. Further increasing the temperature from 80 °C to 130 °C displayed the formation of several XRD peaks, suggesting a strong temperature-dependent formation of the CAU-17 MOF thin film. In this temperature range, all three spectra of CAU-17 were highly consistent with each other and displayed the most intense peaks of CAU-17 at 8.35°, 10.49°, 12.68°, 16.51°, 24.75°, and 33.13°.

Fig. 5 XRD pattern of the CAU-17 thin film synthesized at different temperatures.

The FESEM images of CAU-17 thin films fabricated at different temperatures are shown in Fig. 6, which reveals that temperature has a significant impact on the formation of CAU-17 morphology. An increase in the temperature from 30 °C to 130 °C resulted in the transformation of small nanorods into larger nanorods. Fig. 6(f) shows the influence of temperature on the CAU-17 nanorods’ width, measured using ImageJ software. For the samples prepared at 30 to 80 °C, individual nanorods were analyzed. However, for samples synthesized at higher temperatures (105 °C to 130 °C), only a single nanorod was considered, where multiple measurements were taken for improving accuracy. With the increase in temperature from 30 °C to 130 °C, the width of nanorods was found to increase from 0.15 to 6.86 µm. Moreover, it was also observed that the growth of CAU-17 nanorods was random and non-directional.

Fig. 6 FESEM images of CAU-17 MOF thin films synthesized at different temperatures: (a) 30 °C, (b) 55 °C, (c) 80 °C, (d) 105 °C, and (e) 130 °C. (f) Influence of synthesis temperatures on the CAU-17 nanorods’ width.

The synthesis of CAU-17 at 30 °C displayed the formation of multiple nanorods, as shown in Fig. 6(a). Additionally, it was also observed that multiple nanorods arranged themselves in one specific crystallographic direction. In the intermediate temperature range of 55 to 80 °C, it was observed that multiple nanorods grew along the same direction, which further fused into large single nanorods at higher temperatures, as observed in Fig. 6(b–e). Such growth behavior belongs to “oriented attachment,” conceptualized by Penn and Banfield et al.⁶² According to this model, the same crystallographic-oriented nanoparticles (i.e., one-, two-, and three-dimensional) undergo fusion to form a single crystal.⁶³ This process consists of three stages: (1) self-organization of adjacent nanoparticles towards a specific crystallographic direction; (2) the fusion of nanoparticles and formation of a smooth and continuous interface; and (3) maintaining nanoparticle alignment during the entire growth process.^64,65 Similarly, the same trend was observed in the CAU-17 nanorod formation, as evidenced by the FESEM image shown in Fig. 6(a), where smaller nanorods aligned themselves towards the specific crystallographic direction at a relatively low temperature. The interaction between nanorods takes place via van der Waals forces, and due to their thermal energy, they try to arrange themselves towards a specific crystallographic direction to find the lowest energy configuration.⁶³ As the thermal energy of the material increases with temperature, the nanorods align more quickly and gain more freedom to form larger-sized nanorods at higher temperatures (80 °C to 130 °C). Herein, the oriented attachment growth mechanism takes place due to the in situ growth of CAU-17 on the quartz substrate.

Effect of the substrate on CAU-17 formation

Previously, it was determined that the formation of MOFs directly on the substrate surface plays an important role in their growth along a specific direction and strongly affects their crystal structure and orientation.^66,67 Falcaro et al. reported direct synthesis of a MOF from the metal–hydroxide surface by reacting it with an organic linker. The resultant MOF formation takes place via heteroepitaxial growth, displaying directional growth along different crystallographic axes.⁶⁸ Similarly, Scheurle et al. emphasized the crystal orientation of MOF-74 via vapor-assisted conversion, synthesized using different metal ions, on different substrates, where different degrees of order were achieved depending on the substrate nature.⁶⁹

In this investigation, the formation of CAU-17 on the surface of different substrates was studied. Fig. 7(a) confirms the synthesis of CAU-17 on the FTO surface. The sharp intense peaks of CAU-17 emerged at 8.64°, 10.8°, 13°, 16.84°, 25.08°, and 33.48° while peaks at 27°, 34.16°, 38.2°, 51.92°, 54.96°, and 61.96° belong to the FTO substrate. The surface morphology of CAU-17 on the FTO surface is displayed in Fig. 7(b), showing the formation of well-defined nanorods. The XRD results of CAU-17 on the surface of carbon paper revealed four peaks at 8.31°, 10.5°, 12.7°, and 33.18°, while peaks at 18.22°, 26.64°, 42.38°, 44.64°, and 54.68° correspond to carbon paper, as shown in Fig. 7(c). The CAU-17 surface morphology on the carbon paper substrate is displayed in Fig. 7(d), showing the formation of nanorods. On both substrates, the synthesis of CAU-17 showed the formation of randomly oriented nanorods.

Fig. 7 XRD pattern and FESEM images of the CAU-17 MOF thin film prepared at 130 °C on the FTO substrate (a) and (b) and at 105 °C on the carbon paper (c) and (d), both thin films were synthesized for 2 days.

Proposed mechanism for CAU-17 thin film growth

The formation of Bi₂O₃ takes place via reactive DC magnetron sputtering through the following reaction (eqn (1)–(3)).

e⁻ + Ar_(g) → 2e⁻ + Ar⁺ (1)

Ar⁺ + Bi_(s) → Bi⁰ + e⁻ (2)

4Bi⁰ + 3O₂ → 2Bi₂O₃ (3)

Firstly, the ionization of Ar ions takes place, which results in the formation of Ar⁺ ions. These ionized Ar⁺ ions further bombarded the surface of the pure Bi target, leading to the ejection of Bi⁰ atoms from the surface of the target. These atoms further interact with the reactive O atoms in the sputtering chamber and result in the formation of Bi₂O₃ by consolidating on the surface of the substrate. In this study, XRD analysis confirmed that the sputtered Bi₂O₃ thin film deposited on different substrates is amorphous in nature. This observation could be attributed to the deposition of Bi₂O₃ thin films at room temperature, low working gas pressures, and low deposition power.⁷⁰ Additionally, Ibrahim et al.,⁷¹ and Hernández et al.,⁷² reported that under these conditions, DC reactive sputtering typically led to the formation of an amorphous Bi₂O₃ thin film.

The formation of CAU-17 involves three primary reactions: (1) dissolution of Bi³⁺ ions; (2) deprotonation of H₃BTC; and (3) coordination reaction between Bi³⁺ ions and deprotonated H₃BTC, as illustrated in Fig. 8. In this study, CAU-17 {Bi(BTC)(DMF)} is formed in the solution of DMF and H₃BTC, where pH was ranged between 4.80 and 7.48 with the addition of 0.01 M HCl. Prior studies also revealed that the addition of HCl results in a speed-up of the rate of the reaction during MOF synthesis and ensures phase-purity and reproducibility of results.^73,74 In the present work, the dissolution of Bi₂O₃ was carried out in a slightly acidic medium, as shown in eqn (4).

Bi₂O₃ + 6H⁺ → 2Bi³⁺ + 3H₂O (4)

Fig. 8 Schematic illustration of the formation mechanism of the CAU-17 thin film.

In the synthesis of CAU-17, H₃BTC plays the role of a structure-directing agent and acts as a hard base because of its carboxyl groups (–COOH) that bind with Bi³⁺ ions, forming a stable structure.²⁷ The DMF solution functioned as a deprotonation agent.⁷⁵ CAU-17 is formed when the deprotonation of H₃BTC takes place in the DMF solution.⁷⁶ The –COOH group of H₃BTC lost its three protons (H⁺), resulting in the formation of a negatively charged carboxylate ion (–COO⁻).^76,77 H₃BTC is a type of multifunctional ligand that consist of a benzene ring with three carboxylate groups at 1,3, and 5 positions. It is known for its ability to construct the coordination architecture with metal ions through adopting different coordination modes due to its fully or partially deprotonation ability.⁷⁸ Depending on the pH of the solution, H₃BTC can be either fully or partially deprotonated.⁷⁹ Typically, the partial deprotonation of H₃BTC takes place at a pH of 5, and full deprotonation takes place at a pH of 7 and 9.⁸⁰

The deprotonation of H₃BTC in this investigation occurs in a two-step reaction (eqn (5) and (6)). Initially, partial deprotonation of H₃BTC leads to the formation of HBTC²⁻ ions, followed by full deprotonation that results in the formation of BTC³⁻ ions.

H₃BTC + 2e⁻ → HBTC²⁻ + H₂ (5)

HBTC²⁻ → BTC³⁻ + H⁺ (6)

Bi³⁺ + BTC³⁻ → [Bi(BTC)](CAU-17) (7)

The charge difference of Bi³⁺ and BTC³⁻ ions encourages the formation of the CAU-17 framework, where Bi³⁺ serves as a central metal ion that coordinates with the –COO⁻ group of deprotonated H₃BTC to form a coordination bond (eqn (7)).⁷⁷

Conclusions

Herein, we reported a novel two-step sequential synthesis approach of the CAU-17 MOF thin film based on the controlled transformation of the Bi₂O₃ thin film into CAU-17. By employing advanced materials characterization techniques, the main constituent elements and functional groups in the synthesized thin film were successfully confirmed. This method enables the synthesis of CAU-17 thin films under relatively mild conditions without any risk of forming secondary phases and impurities, which was unachievable with previous powder-based techniques. This work also highlights the formation of CAU-17 at temperatures above and below 120 °C, which previous studies considered as an ideal temperature for its synthesis. The formation of CAU-17 on different substrates further increases its application prospects in fields like sensing, catalysis, and other cutting-edge technologies.

Conflicts of 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.

Data availability

The data that support the findings of this study are available from the corresponding author, Khaled Hassanein, upon reasonable request.

Acknowledgements

The authors would like to acknowledge the support provided by the H₂ Future Consortium at King Fahd University of Petroleum & Minerals (KFUPM) through the project H2FC2402.

References

1. J. M. Zamaro, N. C. Pérez, E. E. Miró, C. Casado, B. Seoane, C. Téllez and J. Coronas, Chem. Eng. J., 2012, 195–196, 180–187.
2. K. S. Lin, A. K. Adhikari, C. N. Ku, C. L. Chiang and H. Kuo, Int. J. Hydrogen Energy, 2012, 37, 13865–13871.
3. H. Furukawa, K. E. Cordova, M. O’Keeffe and O. M. Yaghi, Science, 2013, 341, 1230444.
4. X. X. Yang, N. Li, C. Li, Z. Bin Jin, Z. Z. Ma, Z. G. Gu and J. Zhang, J. Am. Chem. Soc., 2024, 146, 16213–16221.
5. N. Baig, I. Abdulazeez, B. Salhi, H. A. Asmaly, A. I. Osman and K. H. Ahmed, Chem. Eng. J., 2025, 504, 158628.
6. M. Safaei, M. M. Foroughi, N. Ebrahimpoor, S. Jahani, A. Omidi and M. Khatami, TrAC, Trends Anal. Chem., 2019, 118, 401–425.
7. R. Zhai, Y. Zhu, L. Chang, Z. Gu and J. Zhang, Jiegou Huaxue, 2022, 41, 2209074–2209079.
8. I. Ahmed, G. Lee, H. J. Lee and S. H. Jhung, Chem. Eng. J., 2024, 488, 151022.
9. Z. Z. Ma, Q. H. Li, Z. Wang, Z. G. Gu and J. Zhang, Nat. Commun., 2022, 13, 6347.
10. L. M. Chang, Q. H. Li, P. Weidler, Z. G. Gu, C. Wöll and J. Zhang, CCS, Chemistry, 2022, 4, 3472–3481.
11. J. Pang, Q. Han, W. Liu, Z. Shen, X. Wang and J. Zhu, Appl. Surf. Sci., 2017, 422, 283–294.
12. V. M. Alexander, R. P. Bhat and S. D. Samant, Tetrahedron Lett., 2005, 46, 6957–6959.
13. Y. Wang, H. Sun, Z. Yang, Y. Zhu and Y. Xia, Carbon Neutralization, 2024, 3, 737–767.
14. Y. Ying, B. Khezri, J. Kosina and M. Pumera, ChemSusChem, 2021, 14, 3402–3412.
15. A. K. Inge, M. Köppen, J. Su, M. Feyand, H. Xu, X. Zou, M. O’Keeffe and N. Stock, J. Am. Chem. Soc., 2016, 138, 1970–1976.
16. F. Li, G. H. Gu, C. Choi, P. Kolla, S. Hong, T. S. Wu, Y. L. Soo, J. Masa, S. Mukerjee, Y. Jung, J. Qiu and Z. Sun, Appl. Catal., B, 2020, 277, 119241.
17. L. Liu, K. Yao, J. Fu, Y. Huang, N. Li and H. Liang, Colloids Surf., A, 2022, 633, 127840.
18. S. Li, W. Shan, T. Rao, Y. Qi, Y. Xiong, Z. Lou, H. Yu, J. Cui and X. Feng, Microchem. J., 2023, 191, 108861.
19. D. Lee, K. S. Oh, Y. Lee, J. Jin, S. Y. Lee, Y. Jho and J. H. Park, Chem. Eng. J., 2024, 496, 153825.
20. S. Frank, E. Svensson Grape, E. D. Bøjesen, R. Larsen, P. Lamagni, J. Catalano, A. K. Inge and N. Lock, J. Mater. Chem. A, 2021, 9, 26298–26310.
21. Q. Huang, X. Sha, R. Yang, H. Li and J. Peng, ACS Appl. Mater. Interfaces, 2024, 16, 13882–13892.
22. M. Köppen, V. Meyer, J. Ångström, A. K. Inge and N. Stock, Cryst. Growth Des., 2018, 18, 4060–4067.
23. Q. X. Wang and G. Li, Inorg. Chem. Front., 2021, 8, 572–589.
24. M. Köppen, A. Dhakshinamoorthy, A. K. Inge, O. Cheung, J. Ångström, P. Mayer and N. Stock, Eur. J. Inorg. Chem., 2018, 3496–3503.
25. V. H. Nguyen, L. Van Tan, T. Lee and T. D. Nguyen, Sustainable Chem. Pharm., 2021, 20, 100385.
26. S. Dong, L. Wang, W. Lou, Y. Shi, Z. Cao, Y. Zhang and J. Sun, Ultrason. Sonochem., 2022, 91, 106223.
27. R. Huang, Z. Zhou, X. Lan, F. K. Tang, T. Cheng, H. Sun, K. Cham-Fai Leung, X. Li and L. Jin, Mater. Today Bio, 2023, 18, 100507.
28. H. Yang, Y. Zhao, Y. Guo, B. Wu, Y. Ying, Z. Sofer and S. Wang, Small, 2025, 20, 2307484.
29. D. Chakraborty, A. Yurdusen, G. Mouchaham, F. Nouar and C. Serre, Adv. Funct. Mater., 2023, 34, 2309089.
30. D. Bazer-Bachi, L. Assié, V. Lecocq, B. Harbuzaru and V. Falk, Powder Technol., 2014, 255, 52–59.
31. B. Huang, Y. Li and W. Zeng, Chemosensors, 2021, 9, 226.
32. A. Baptista, F. Silva, J. Porteiro, J. Míguez and G. Pinto, Coatings, 2018, 8, 402.
33. Y. Wang, H. Chen, X. Wang, B. Meng, N. Yang, X. Tan and S. Liu, Ind. Eng. Chem. Res., 2020, 59, 15576–15585.
34. X. Liu, M. Ding, P. Ma, C. Duan and J. Yao, Sep. Purif. Technol., 2022, 295, 121336.
35. J. Liang, Q. Liu, T. Li, Y. Luo, S. Lu, X. Shi, F. Zhang, A. M. Asiri and X. Sun, Green Chem., 2021, 23, 2834–2867.
36. P. Borowski and J. Myśliwiec, Coatings, 2025, 15, 922.
37. F. Wang, H. Zhao, J. Liang, T. Li, Y. Luo, S. Lu, X. Shi, B. Zheng, J. Du and X. Sun, J. Mater. Chem. A, 2020, 8, 20260–20285.
38. X. Tian, Z. Hu, C. Jia, H. Wang and X. Wei, J. Environ. Chem. Eng., 2023, 11, 111516.
39. A. Monshi, M. R. Foroughi and M. R. Monshi, World J. Nano Sci. Eng., 2012, 02, 154–160.
40. P. Lunca Popa, S. Sønderby, S. Kerdsongpanya, J. Lu, N. Bonanos and P. Eklund, J. Appl. Phys., 2013, 113, 046101.
41. B. Zhang, S. Cao, Y. Wu, P. Zhai, Z. Li, Y. Zhang, Z. Fan, C. Wang, X. Zhang, J. Hou and L. Sun, ChemElectroChem, 2021, 8, 880–886.
42. H. A. Le Pham, D. T. Nguyen, V. C. Nguyen and T. Ky Vo, Inorg. Chem. Commun., 2024, 159, 111822.
43. S. A. Siddiqui, A. Prado-Roller and H. Shiozawa, Mater. Adv., 2022, 3, 224–231.
44. W. Moloto, P. Mbule, E. Nxumalo and B. Ntsendwana, J. Photochem. Photobiol., A, 2021, 407, 113063.
45. H. Sun, X. Han, K. Liu, B. Shen, J. Liu, D. Wu and X. Shi, Ind. Eng. Chem. Res., 2017, 56, 9541–9550.
46. L. Yaqoob, T. Noor, N. Iqbal, H. Nasir, M. Sohail, N. Zaman and M. Usman, Renew, Energy, 2020, 156, 1040–1054.
47. S. R. Zhu, M. K. Wu, W. N. Zhao, P. F. Liu, F. Y. Yi, G. C. Li, K. Tao and L. Han, Cryst. Growth Des., 2017, 17, 2309–2313.
48. F. Khosravi, M. Gholinejad, J. M. Sansano and R. Luque, Appl. Organomet. Chem., 2022, 36, 6749.
49. S. Karimi, M. Gholinejad, R. Khezri, J. M. Sansano, C. Nájera and M. Yus, RSC Adv., 2023, 13, 8101–8113.
50. S. Han, R. A. Ciufo, M. L. Meyerson, B. K. Keitz and C. Buddie Mullins, J. Mater. Chem. A, 2019, 7, 19396–19406.
51. J. Sun, W. Zheng, S. Lyu, F. He, B. Yang, Z. Li, L. Lei and Y. Hou, Chin. Chem. Lett., 2020, 31, 1415–1421.
52. P. Zhang, Y. Huang, Y. Rao, M. Chen, X. Li, W. Ho, S. Lee and J. Cao, Chem. Eng. J., 2021, 406, 126910.
53. L. Zhao, Z. Pan, L. Cai, S. Wang, B. Lu, S. Lv, Y. Qiu and G. Wang, Desalination, 2024, 576, 117360.
54. M. Stawowy, P. Jagódka, K. Matus, B. Samojeden, J. Silvestre-Albero, J. Trawczyński and A. Łamacz, Catalysts, 2020, 10, 108.
55. C. Fan, H. Dong, Y. Liang, J. Yang, G. Tang, W. Zhang and Y. Cao, J. Cleaner Prod., 2019, 208, 353–362.
56. W. Xuan, R. Ramachandran, C. Zhao and F. Wang, J. Solid State Electrochem., 2018, 22, 3873–3881.
57. G. Zahn, P. Zerner, J. Lippke, F. L. Kempf, S. Lilienthal, C. A. Schröder, A. M. Schneider and P. Behrens, CrystEngComm, 2014, 16, 9198–9207.
58. P. Mahata, A. Sundaresan and S. Natarajan, Chem. Commun., 2007, 4471–4473.
59. Y. X. Sun and W. Y. Sun, Chin. Chem. Lett., 2014, 25, 823–828.
60. P. Zhang, X. Kang, L. Tao, L. Zheng, J. Xiang, R. Duan, J. Li, P. Chen, X. Xing, G. Mo, Z. Wu and B. Han, CCS Chem., 2023, 5, 1462–1469.
61. S. Dai, F. Nouar, S. Zhang, A. Tissot and C. Serre, Angew. Chem., Int. Ed., 2021, 60, 4282–4288.
62. R. L. Penn and J. F. Banfield, Geochim. Cosmochim. Acta, 1999, 63, 1549–1557.
63. Q. Zhang, S. J. Liu and S. H. Yu, J. Mater. Chem., 2009, 19, 191–207.
64. F. Cheng, L. Lian, L. Li, J. Rao, C. Li, T. Qi, Z. Zhang, J. Zhang and Y. Gao, Adv. Sci., 2019, 6, 1802202.
65. K. Wen and W. He, Nanotechnology, 2015, 26, 382001.
66. J. Park, H. R. Moon and J. Y. Kim, Mater. Chem. Front., 2023, 7, 5545–5560.
67. M. M. Sabzehmeidani, S. Gafari, S. jamali and M. Kazemzad, Appl. Mater. Today, 2024, 38, 102153.
68. P. Falcaro, K. Okada, T. Hara, K. Ikigaki, Y. Tokudome, A. W. Thornton, A. J. Hill, T. Williams, C. Doonan and M. Takahashi, Nat. Mater., 2017, 16, 342–348.
69. P. I. Scheurle, A. Mähringer, A. Biewald, A. Hartschuh, T. Bein and D. D. Medina, Chem. Mater., 2021, 33, 5896–5904.
70. K. Wang, Z. Cheng, G. Wu and Q. Qin, ChemCatChem, 2024, 16, 202400415.
71. S. Ibrahim, P. Bonnet, M. Sarakha, C. Caperaa, G. Monier and A. Bousquet, Mater. Chem. Phys., 2020, 243, 122580.
72. G. Orozco-Hernández, J. Olaya-Flórez, C. Pineda-Vargas, J. E. Alfonso and E. Restrepo-Parra, Surf. Interfaces, 2020, 21, 100627.
73. F. Bigdeli, M. N. A. Fetzer, B. Nis, A. Morsali and C. Janiak, J. Mater. Chem. A, 2023, 11, 22105–22131.
74. R. S. Forgan, Chem. Sci., 2020, 11, 4546–4562.
75. R. Ediati, M. A. Setyani, D. O. Sulistiono, E. Santoso, D. Hartanto and M. M. Al Bakri Abdullah, J. Water Process Eng., 2021, 39, 101670.
76. Y. Devi, I. Ang, F. E. Soetaredjo, S. P. Santoso, W. Irawaty, M. Yuliana, A. E. Angkawijaya, S. B. Hartono, P. L. Tran-Nguyen, S. Ismadji and Y. H. Ju, J. Mater. Sci., 2020, 55, 13785–13798.
77. J. Bai, J. H. Jia, Y. Wang, C. C. Yang and Q. Jiang, Nano-Micro Lett., 2025, 17, 60.
78. T. Zelenka, M. Baláž, M. Férová, P. Diko, J. Bednarčík, A. Királyová, Ľ. Zauška, R. Bureš, P. Sharda, N. Király, A. Badač, J. Vyhlídalová, M. Želinská and M. Almáši, Sci. Rep., 2024, 14, 15386.
79. Y. F. Zhou, B. Y. Lou, D. Q. Yuan, Y. Q. Xu, F. L. Jiang and M. C. Hong, Inorg. Chim. Acta, 2005, 358, 3057–3064.
80. R. Seetharaj, P. V. Vandana, P. Arya and S. Mathew, Arabian J. Chem., 2019, 12, 295–315.

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