# Copper benzene-1,3,5-tricarboxylate based metal organic framework (MOF) derived CuO/TiO2 nanofibers and their use as visible light active photocatalyst for the hydrogen production

Mitra Saffari, Zahra Hagheh Kavousi, Massomeh Ghorbanloo, Amr A Nada, Maged Bekheet, Djamel Bezzerga, Jisang Hong, Ali Morsali, Yaovi Holade, Roman Viter, et al.

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Mitra Saffari, Zahra Hagheh Kavousi, Massomeh Ghorbanloo, Amr A Nada, Maged Bekheet, et al. Copper benzene-1,3,5-tricarboxylate based metal organic framework (MOF) derived CuO/TiO2 nanofibers and their use as visible light active photocatalyst for the hydrogen production. Applied Surface Science, 2024, 678, pp.161061. 10.1016/j.apsusc.2024.161061. hal-04819551

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# Copper benzene-1,3,5-tricarboxylate based metal organic framework (MOF) derived CuO/TiO₂ nanofibers and their use as visible light active photocatalyst for the hydrogen production

Mitra Saffari<sup>a</sup>, Zahra Hagheh Kavousi<sup>a,b</sup>, Massomeh Ghorbanloo<sup>a,*</sup>, Amr A. Nada<sup>b,d,*</sup>, Maged F. Bekheet<sup>c</sup>, Djamel Bezzerga<sup>f</sup>, Jisang Hong<sup>f</sup>, Ali Morsali<sup>c</sup>, Yaovi Holade<sup>b</sup>, Roman Viter<sup>g</sup>, Heba H. El-Maghrabi<sup>b,h</sup>, Rasha Hosny<sup>i</sup>, Fathi S. Soliman<sup>h</sup>, Igor Iatsunskyi<sup>j</sup>, Emerson Coy<sup>j</sup>, Mikhael Bechelany<sup>b,k,*</sup>

ARTICLE INFO

Keywords:

HKUST-1 MOF

Electrospinning

Photocatalysis

Hydrogen evolution

Water splitting

ABSTRACT

Photocatalytic hydrogen generation through water splitting offers a sustainable and renewable approach to producing green and clean hydrogen fuel. However, it is crucial to explore low-cost semiconductor photocatalysts that can effectively capture a broad range of solar radiation. The present study aims to develop photocatalytic CuO/TiO₂ nanofibers (NFs) with homogeneity of copper with titanium in the heterostructures through the metal-organic framework (MOF) assisted strategy via the electrospinning and subsequent calcination approach. The new methodology enables the fabrication of CuO-modified TiO₂ NFs based on HKUST-1 (copper benzene-1,3,5-tricarboxylate, Hong Kong University of Science and Technology-1) as Cu-MOF varying the amounts of HKUST-1. Ultraviolet-visible light diffuses reflectance spectroscopy (DRS), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), Fourier-transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), photoluminescence spectroscopy (PL), X-ray photoemission spectroscopy (XPS) and Raman spectroscopy are employed to assess the structural and morphological characteristics of the prepared NFs. The characterization revealed that the CuO-TiO₂ NFs heterostructure was uniform. The bandgaps of the CuO-TiO₂ NFs ranged from 3.08 to 2.83 eV based on the Kubelka-Munk function, which is considerable for improving the light-harvesting capacity. The photocurrent technique is used to confirm efficiency in charge separation and transfer, as well as a longer lifetime of photo-generated charge carriers effect in photocatalytic activity. In addition, density functional theory calculations were performed to analyze the impact of CuO on the geometric, electronic structure and photocatalytic properties of TiO₂. The CuO/TiO₂ photocatalyst containing 1.5 mol% of CuO exhibited the highest photocatalytic activity. The hydrogen production rate was exceptional at 45.6 mmol·g⁻¹·h⁻¹, which was a remarkable 400 times higher than that achieved with pure TiO₂ nanofibers. Owing to the one-dimensionality of nanostructures, the prepared CuO-based TiO₂ NFs had high recyclability without lessening the efficiencies of photocatalytic activity.

<footnote><sup>a</sup> Chemistry Department, Faculty of Science, University of Zanjan, Zanjan, P.O. Box: 4537138791, Iran</footnote>

<footnote><sup>b</sup> Institut Européen Des Membranes, IEM, UMR 5635, Univ Montpellier, ENSCM, CNRS, Montpellier, France</footnote>

<footnote><sup>c</sup> Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box: 14155-4838, Tehran, Iran</footnote>

<footnote><sup>d</sup> Department of Analysis and Evaluation, Egyptian Petroleum Research Institute, Cairo 11727, Egypt</footnote>

<footnote><sup>e</sup> Technische Universität Berlin, Faculty III Process Sciences, Institute of Materials Science and Technology, Chair of Advanced Ceramic Materials, Straße des 17. Juni 135, 10623 Berlin, Germany</footnote>

<footnote><sup>f</sup> Department of Physics, Pukyong National University, Busan 48513, Korea</footnote>

<footnote><sup>g</sup> Institute of Atomic Physics and Spectroscopy, University of Latvia, 19 Raina Blvd., LV 1586 Riga, Latvia</footnote>

<footnote><sup>h</sup> Department of Refining, Egyptian Petroleum Research Institute, Cairo 11727, Egypt</footnote>

<footnote><sup>i</sup> Department of Production, Egyptian Petroleum Research Institute (EPRI), Cairo 11727, Egypt</footnote>

<footnote><sup>j</sup> NanoBioMedical Centre, Adam Mickiewicz University, Wszechnicy Piastowskiej 3, 61-614 Poznan, Poland</footnote>

<footnote><sup>k</sup> Gulf University for Science and Technology, GUST, Kuwait</footnote>

<footnote>* Corresponding authors at: Institut Européen des Membranes, IEM, UMR 5635, Univ Montpellier, ENSCM, CNRS, Montpellier, France (A.A. Nada and M. Bechelany).</footnote>

<footnote>E-mail addresses: m.ghorbanloo@znu.ac.ir (M. Ghorbanloo), amr.nada@umontpellier.fr (A.A. Nada), Mikhael.bechelany@umontpellier.fr (M. Bechelany).</footnote>

<footnote>https://doi.org/10.1016/j.apsusc.2024.161061</footnote>

<footnote>Received 12 May 2024; Received in revised form 19 August 2024; Accepted 23 August 2024</footnote>

<footnote>Available online 24 August 2024</footnote>

<footnote>0169-4332/© 2024 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).</footnote>
# 1. Introduction

The rapid growth in energy demand, together with the excessive use of fossil fuels and resulting environmental pollution, like climate change, air pollution, and global warming, accentuates the necessity of detecting solutions based on renewable energy sources. Solar fuels are considered to be hopeful substitutes for fossil fuels in generating environmentally friendly energy [1]. Among solar fuels, one of the most desirable, cleanest, and most sustainable energy is hydrogen (H₂) due to its desirable properties, including high energy per weight (122 kJ/g). Moreover, water is the only by-product in the H₂ combustion process [2,3]. The use of a semiconductor as a photocatalyst in the process enables the generation of H₂ through sunlight-driven water splitting. Therefore, H₂ as a secondary renewable energy source can be a favorable substitution for fossil fuels to fulfill the energy and ecological needs of present and future generations [4].

The process of H₂ generation entails the use of effective catalysts. One of the most frequently utilized catalysts in the photocatalytic processes is TiO₂, owing to its appropriate bandgap structure [1]. Indeed, TiO₂ is widely recognized as the most commonly used n-type metal-oxide semiconductor [5]. Photocatalysis presents a straightforward method for low-cost, large-scale production of H₂ through water splitting. Nevertheless, this method has its own disadvantages, like low levels of quantum efficiency (QE) in the visible range, rapid electron-hole (e⁻/h⁺) pair recombination, and low solar-to-hydrogen efficiencies (less than 0.1 %). As a result, two strategies, namely surface engineering and bulk doping, are employed to promote the state of the photo-generated charge transfer/separation and shorten the distance between photo-induced charge carriers and surface reaction sites [6]. It is widely known that the photocatalytic performance of a semiconductor is significantly influenced by its morphological and structural characteristics. Therefore, one-dimensional (1D) TiO₂ nanostructures are superior to TiO₂ nanoparticles (NPs) in photocatalytic water splitting reactions because they make directional charge transfer possible and consequently reduce charge recombination [7,8]. When compared to the previously mentioned 1D TiO₂ nanostructures, TiO₂ nanofibers (NFs) offer further advantages. TiO₂ nanofibers (NFs) have more appropriate sizes and offer larger specific surface areas, which enhance photocatalytic activity [1]. The exceptional photocatalytic performance of TiO₂ nanofibers can be attributed to mesoporosity and the alignment of nanocrystals. These factors enable efficient charge separation through inter-particle charge transfer along the nanofiber framework [9].

To enhance the performance of the active sites, it is possible to integrate another material, such as a metal or metal oxide, as a co-catalyst onto the primary semiconductor surface, in this case, TiO₂ [6]. The concept of a co-catalyst that integrates the merits of metal and metal oxide can greatly improve the photocatalytic efficiency over of semiconductors [10]. In addition to the built-in electrical field within the metal-semiconductor junction, metal-based co-catalysts can modify the e⁻/h⁺ transfer through the heterojunction structure to boost photocatalytic performance [10]. As for metal oxide co-catalysts, the main functionality is to trap charge carriers to realize effective electron-hole separation [11]. To promote the characteristics of metal-oxide co-catalysts, a number of approaches have been proposed, such as the formation of heterojunctions, surface functionalization, and doping [12]. Using composite semiconductors with p-n heterojunction structures effectively reduces the recombination rate of photo-induced carriers, broadens the range of visible-light absorption, and enhances photocatalytic reactivity [13]. One of the suitable metal-oxides to be used for photocatalytic applications is CuO, as a p-type semiconductor combined with TiO₂ (n-type), which is characterized by low prices, narrow bandgaps, high availability and high environmental acceptability [1].

Metal-organic frameworks (MOFs) and their derivatives are porous materials that possess several intrinsic characteristics. These include desirable pore sizes, well-defined nanoscale cavities, high crystallinity, and customizable chemical properties [14]. Indeed, the derivatives of

metal-organic frameworks (MOFs) can retain their high specific surface area and porous structure, which provides more active sites for photocatalytic reactions. This characteristic promotes the intermediate water splitting process [15]. Despite these advantages, using MOF derivatives in the water splitting process has a drawback, i.e., low levels of efficiency in H₂ production. Eliminating these problems and designing effective structures with MOF derivatives for H₂ generation is quite difficult.

The present study aims to construct titanium dioxide (TiO₂) with copper oxide (CuO) nanocomposites with efficient photocatalytic features. This will be achieved through a simple, scalable, cheap, and adjustable MOF-templated strategy. Although many studies have investigated the utility of MOFs for a variety of applications, including nonlinear optics [16], gas storage [17], catalysis [18], chemical sensing [19], and drug delivery [20], only a limited number of studies have employed MOF templates to synthesize TiO₂-based NFs. In the current study, nano-sized HKUST-1 MOFs are considered as templates to fabricate TiO₂ NFs with appropriate properties for potential solar energy applications. The electrospinning method is used for the synthesis of CuO/TiO₂ (CuO-modified TiO₂ NFs) with Cu-based MOFs, HKUST-1 as a CuO source. The photocatalytic activity of NFs for H₂ evolution was investigated under visible-light irradiation (λ > 400 nm) in the presence of a sacrificial agent (CH₃OH). It is found that the photocatalytic behavior of CuO/TiO₂ NFs (45.6 mmol g⁻¹h⁻¹) is more than 400 times greater than that of pure TiO₂ NPs. MOFs-derived TiO₂ and CuO could significantly enlarge the surface area and increase the number of reaction sites. Simultaneously, the heterojunction formed between TiO₂ and CuO inhibits the recombination of photoexcited electron-hole pairs. The fast separation and migration of electron-hole pairs, along with effective surface reactions, are considered to be the main reasons for the increased photocatalytic activity.

# 2. Experimental

## 2.1. Materials

Cu(NO₃)₂·3H₂O (Mw = 241.60 g/mol), 1,3,5-Benzenetricarboxylic acid (H₃BTC) (Mw = 210.14 g/mol), titanium tetraisopropoxide (TTIP) (Ti(OCH(CH₃)₂)₄, 97 %), polyvinylpyrrolidone (PVP; Mw = 1300000 g/mol), acetic acid (98 %), absolute ethanol (99 %), absolute methanol (99 %), N,N-dimethylformamide (DMF; 99.8 %), Nafion perfluorinated resin solution, were purchased from Sigma Aldrich. All chemicals were used as received.

## 2.2. Preparation of CuO/TiO₂ composite nanofibers

HKUST-1 was synthesized following the procedure described in the literature [21]. Briefly, a solution of benzene-1,3,5-tricarboxylic acid (49.1 g, 0.234 mol) in ethanol (250 mL), and a solution of cupric nitrate hydrate (Cu(NO₃)₂·2·5H₂O; 108.6 g, 0.466 mol) in water (250 mL) were prepared. The prepared solutions were combined and stirred at room temperature for 30 min. The mixture was then transferred into an autoclave and subjected to hydrothermal conditions at 423 K for 18 h. Once the reaction was complete, the autoclave was cooled down to room temperature, and the resulting blue Cu-BTC powder was separated by filtration and subsequently washed with water.

CuO/TiO₂ nanofibers with different mole percentages (mol%) of HKUST-1 relative to TiO₂ (0.5 mol% (TCu0.5), 1 mol% (TCu1), 1.5 mol % (TCu1.5), 2 mol% (TCu2) and 2.5 mol% (TCu2.5)) were fabricated by electrospinning as follows. Briefly, 0.3 g of PVP was added to the TTIP suspension in ethanol (3 mL) and acetic acid (2 mL) under stirring at room temperature for 30 min (precursor solution). Then, HKUST-1 (different mol%) was dissolved in ethanol (2 mL) and DMF (0.5 mL) by stirring for 30 min, and added to the precursor solution. The mixture was vigorously stirred for 1 h, and when homogeneous, it was transferred to a syringe with a stainless-steel needle (0.7 mm in diameter).
Electrospinning was performed with a constant flow rate of 1 mL/h, chamber temperature of 38±5 °C, and high voltage power (1.25 kV/cm). Nanofibers were gathered on an aluminum-coated rotating coil (spinning at a speed of 400 rpm). They were then subjected to calcination in an air furnace at 400 °C, with a heating rate of 5 °C per minute, for a duration of 4 h.

## 2.3. Sample characterization

The nanofiber crystal structure was analyzed using a PANAnalytical Xpert-PRO diffractometer, which was equipped with an X'celerator detector. The diffractometer employed Ni-filtered Cu-radiation, with a Cu $K_{α1}$ radiation wavelength of 0.1540598 nm and a Cu $K_{α2}$ radiation wavelength of 0.1544426 nm. The X-ray source operated at 40 kV and 40 mA. To obtain good quality X-ray diffraction (XRD) patterns, nanofibers were finely ground to powder and then scanned for 2 h at a scanning rate of 0.1° min⁻¹ (step size and time: 0.02° and 12 s) and range of 10–90°. The Fullprof software was used to perform Rietveld refinement of the XRD data in order to determine the crystallite size and lattice parameter of the crystalline phases [22]. The instrument resolution parameters were determined by the Rietveld refinement of XRD data of LaB₆, which served as a reference standard. The morphology and structure of the samples were examined using scanning (SEM; Hitachi S4800, Japan) and transmission electron microscopy (TEM; JEOL ARM 200F) at an accelerating voltage of 200 kV. The composition was analyzed by energy-dispersive X-ray spectroscopy (EDX) and elemental mapping. The Zeiss EVO HD15 microscope and an Oxford X-MaxN EDX detector were used for the analysis. Raman spectra were generated by dispersive Raman spectroscopy (Model Sentera, Bruker, Germany) at 532 nm. The excitation source was a neodymium-doped yttrium aluminum garnet with a power of 60 mW. Fourier transform infrared (FTIR) spectra were recorded using KBr disks and a Bruker FT-IR spectrophotometer. A UV-Vis spectrophotometer (CARY 5E) measured the nanofiber band gap. Photoluminescence spectra were recorded using an optical fiber spectrometer (Ocean Optics usb2000). Elemental analysis was done using Varian Spectra 220 AA spectroscopy. The elemental composition was investigated by X-ray photoelectron spectroscopy (XPS). XPS analysis was performed on a Thermo Electron ESCALAB 250 spectrometer equipped with a monochromatic radiation source Al Mono (Alkα = 1486.6 eV) operating at 150 W (15 kV and 10 mA). The measurements were performed directly on the samples in the form of powder arranged on a support in a chamber under high vacuum (≤10–9 Torr): Survey spectra at 1 eV per step for a pass energy of 150 eV and high-resolution spectra at 0.1 eV per step for a pass energy of 20 eV. Analyzed surface has a 500 µm diameter. The spectra are calibrated in binding energy with respect to the C 1s energy of the C–C component at 284.8 eV. The charge is compensated by a low energy electron beam (−2 eV). Quantification was performed from the peak area of the high-resolution spectra after correction with suitable sensitivity factors. The background signal was removed using the Shirley method [23]. The surface atomic concentrations were determined from photoelectron peaks areas using the atomic sensitivity factors reported by Scofield [24]. Binding energies (BE) of all core levels were referred to the C–C of C1s carbon at 284.8 eV.

## 2.4. Photocatalytic $H_2$ generation

$H_2$ was produced by photocatalysis in an air-tight quartz reactor, in which the reaction temperature was maintained at 25 °C by a cold-water circulation system. The irradiation source was a 500 W linear halogen lamp. The lamp-aqueous solution distance was 10 cm. In the experiment, 50 mg of nanofibers were dispersed in 20 mL of a 20 % methanol aqueous solution, which acted as a scavenger. The mixture was then transferred to the reactor. Before the reaction started, oxygen was eliminated from the reaction medium by purging it with nitrogen for 30 min. This step helped to create an oxygen-free environment. To monitor

the production of hydrogen ($H_2$) over time, gas samples were collected every hour. The measurement of $H_2$ production was conducted using a gas chromatography instrument (GC-1690, Jiedao) equipped with a thermal conductivity detector (TCD). The carrier gas used was argon, and a 5 Å molecular sieve column was employed (measuring 3 m in length and 2 mm in diameter). The gas chromatography analysis allowed to measure and track the amount of hydrogen gas produced during the experiment. Electrochemical impedance spectroscopy (EIS) measurements were performed using a SP-150 potentiostat manufactured by Biologic Science Instruments. In the experimental setup, a three-electrode cell was used with a 1 M KOH (potassium hydroxide) aqueous solution acting as the electrolyte. The counter electrode in the setup was a glassy carbon electrode, measuring 1 cm by 3 cm. The purpose of the counter electrode is to provide a surface for the electrochemical reactions to occur during the experiment. As for the reference electrode, an Hg/HgO electrode was utilized. CuO-based HKUST-1 MOFs coated on carbon paper were used as the working electrode.

## 2.5. Computational methods

The plane-wave DFT calculations were performed using the Quantum ESPRESSO code [25] with the Ultrasoft (US) pseudopotentials [26] used to describe the interaction between the core (O:[He], Ti:[Ar], Cu: [Ar]) and valence electrons. The cut-off for the kinetic energy was optimized at 60 Ry with a charge density cut-off of 720 Ry. We used a k-points grid sampled by Monkhorst-Pack scheme of 16 × 16 × 16 for $TiO_2$ and CuO phases. The density of states (DOS) was determined using plane-wave self-consistent field method, Spin polarized DFT+U [27,28] semi-empirical calculations were performed on CuO and $TiO_2$ to describe its semiconducting nature. The Hubbard potential has been added to the strongly localized 3d electrons with $U=8$ eV and 9 eV for Ti and Cu atoms, respectively, which describes well the periodic solid and indeed is known to reproduce the band gaps of all content accurately in CuO/$TiO_2$. The band gap calculations were performed using unit cells derived from the experimental lattice parameters, with the ions kept at their experimentally determined positions.

## 3. Results and discussion

In the present study, heterojunction between CuO and $TiO_2$, has been shown to enhance the photocatalytic activity for water splitting and $H_2$ production. Once the structure and morphology are characterized, the impacts of these factors on the photocatalytic behavior of $TiO_2$ (titanium dioxide) are evaluated. Methanol ($CH_3OH$) is included as a sacrificial reagent.

### 3.1. Structural and morphological properties of the different composite nanofibers

As shown in Fig. 1, the X-ray diffraction (XRD) patterns of all the prepared $TiO_2$ NFs exhibit characteristic peaks that can be assigned to the anatase tetragonal structures of $TiO_2$ (JCPDS No. 01-084-1285). The XRD reflections observed at 2θ = 25.3°, 37.0°, 37.9°, 38.56°, 48.0°, 53.9°, 55.0°, 62.1°, and 62.7° can be attributed to the (101), (103), (004), (112), (200), (105), (211), (213) and (204) lattice planes of anatase $TiO_2$, respectively [29]. Moreover, the very broad XRD reflections observed at 2θ between 28.6 and 30.7° might be assigned to the (002), (−401), (111), and (400) plane of the monoclinic structure of $TiO_2$-B (JCPDS No. 01-074-1940), as confirmed by TEM characterization (Fig. 4). The interaction of CuO (at different mol%) with $TiO_2$ NFs leads to the reduction of the crystallinity of the anatase $TiO_2$ phase as well as the crystallite size of the anatase phase, as indicated from the increase in the broadening of XRD reflections with increasing CuO amount. The analysis of XRD patterns using Rietveld refinement indicates that the crystallite size is approximately 14.7±0.5 nm for the bare $TiO_2$ sample as well as the CuO-containing $TiO_2$ NFs prepared with
<figure><img src="image_3.png" /><figcaption>Fig. 1. (a) XRD patterns of TCu0, TCu0.5, TCu1, TCu1.5, TCu2 and TCu2.5 nanofibers. The calculated reference pattern of the anatase tetragonal structure of TiO₂ (JCPDS No. 01-084-1285) and monoclinic TiO₂-B structure (JCPDS No. 01-074-1940) are shown at the bottom.</figcaption></figure>

small amounts of CuO (i.e., ≤ 1.5 %). The crystallite size is remarkably decreased to 12.0±0.5 nm and 8.8±0.5 nm for the CuO-containing TiO₂ NFs prepared with 2 % and 2.5 % CuO, respectively. These results also show unchanged lattice parameters and unit cell volumes (136.35±0.05 Å³) of the TiO₂ phase when different amounts of CuO content are used in the preparation, as revealed by Rietveld refinement. Moreover, no XRD reflections corresponding to any crystalline Cu-containing compound are observed in the composite samples compared with the pure TiO₂ sample. It should be noted that the ionic radius of Ti⁴⁺ and Cu²⁺ ((r(Ti⁴⁺) = 60.5 pm, r(Cu²⁺) = 73 pm, both ions are 6-fold coordinated) strongly differs [30]. These results agree with our previous studies [31–33] showing that large ions (e.g., Ce³⁺ and Cs¹⁺) cannot enter the TiO₂ lattice, but may replace Ti⁴⁺ ions on the TiO₂ crystallite surface, limiting the anatase phase growth by reducing the interactions between crystallites and the lattice’s Ti and O atom rearrangement. As a result, Cu–O–Ti bonds are formed on the surface of TiO₂ crystallites, which decreases crystallite interactions and the rearrangement of Ti and O atoms in the lattice and consequently constrains the crystallinity development of the anatase phase, leading to a reduction of the size of the CuO-containing TiO₂ NFs crystallite.

By using X-ray photoelectron spectroscopy (XPS), we were able to gather more information about the influence of CuO on the chemical composition of the TiO₂ nanofibers. The high-resolution XPS spectra of different orbitals like O 1s, Ti 2p, C 1s, and Cu 2p were analyzed, and their results are shown in Fig. 2a–d. XPS provides valuable insights into the elemental composition and electronic states of the constituents in the nanofibers. It helps in understanding the surface chemistry and potential interactions between different elements.

XPS analysis of CuO-TiO₂ physicochemical interactions showed that the two peaks detected at 458.8 eV and 464.3 eV in the Ti 2p XPS spectra corresponded to 2p₃/₂ and 2p₁/₂ (Fig. 2a) [34] which arises from spin orbit-splitting. These peaks are consistent with Ti⁴⁺ in the TiO₂ lattice. The shoulder Tip₁/₂ at a binding energy of 459.4 eV is corresponding to Ti³⁺ in Ti₂O₃, which clearly demonstrates the existence of Ti³⁺ and Ti⁴⁺ ions [35]. Shake-up satellites are seen at 471.8 and 477.8 eV [36]. The presence of CuO in the nanofibers causes a noticeable shift in the characteristic peaks of Ti 2p towards higher binding energies by approximately 0.11 eV. This shift occurs because of a phenomenon called charge transfer, where the Ti⁴⁺ ions transfer some of their electrons to Cu²⁺ ions through the Cu–O–Ti bonding. The higher

electronegativity value of Cu (1.91) compared to Ti (1.54) facilitates this electron transfer. This charge transfer process can affect the electronic structure and properties of the nanofibers. These confirmed interactions between CuO and TiO₂ nanofibers. It also suggests that CuO is connected with the TiO₂ surface [37,38].

Fig. 2-b shows the de-convolution of high-resolution XPS spectra for O 1s. The fitting of asymmetric peak of O 1s in CuO nanofibers shows three components at 529.8, 531.4, and 532.2 eV. The peak at 529.8 eV, which is more intense, can be assigned to the binding energy for lattice oxygen (Oᵢᵣᵝ)² in TiO₂ and/or CuO lattice and is in good agreement with the binding energy of O²⁻ ion in the metal oxide sites (Cu²⁺–O²⁻) [39]. The second peak at 531.4 eV can be assigned to the binding energy for oxygen defects/vacancies (OV)². The last peak at 532.1 eV can be assigned to the binding energy for chemisorbed oxygen species [40] and C-O from residual MOF. In contrast, the fitting of the asymmetric peak of O 1s HKUST-1 shows three components at 531.9 eV, 533.2 eV and 534.1 eV, which belong to C-O, O=C-O and C-OH bonds, respectively [41]. The above results all demonstrate that HKUST-1 has been successfully transformed to CuO [42].

CuO presence on the surface of TiO₂ nanofibers was also confirmed by Cu 2p XPS spectra. As can be seen, the CuO/TiO₂ sample exhibits a typical Cu 2p XPS spectrum with four peaks around 932.2–934.3, 939.2–942.9, 943.3–947.2, 952.1–954.2 eV. The characteristic lines at 932.2–934.3 and 952.1–954.2 eV are ascribed to Cu 2p₃/₂ and Cu 2p₁/₂ of CuO, and the peaks located at 940.2–942.9 eV are attributed to the satellite peaks of CuO (Fig. 2c). The presence of an unsymmetric peak at 934.3 eV is associated with the –Cu–O–Ti–O in the sample [43]. In comparison to bare HKUST-1 (935.08 (Cu 2p₃/₂) 954.8 (Cu 2p₁/₂)), Cu 2p peaks of CuO-containing TiO₂ NFs were shifted to lower binding energy, suggesting the decomposition of HKUST-1 upon calcination and the formation of Cu–O–Cu and/or Cu–O–Ti bondings on the surface of TiO₂ NFs. The presence of characteristic shakeup satellite peaks around 940.2 eV and 960.2–963.5 eV in the case of HKUST-1 provides additional evidence supporting the oxidation state of Cu²⁺. These peaks indicate the presence of copper (Cu) in a + 2 oxidation state (Cu²⁺) [44].

Finally, carbon contamination was confirmed by the peaks at 284.8, 286.2 and 288.8 eV related to the C1s of C–C, O–C=O and C=O groups (Fig. 2-d), agreeing with the results, 288.7 eV (C=O), 285.9 eV (O–C=O) and 284.8 eV (C–C), 290.1 eV (π–π*), that have been reported for HKUST-1 [45].

Fig. 3 depicts the Raman spectra of all the samples. The Raman spectra of all samples (Fig. 3) included the Raman-active modes of TiO₂ anatase phase at 147.2 cm⁻¹ (Eg), 198.2 cm⁻¹ (Eg), 394.3 cm⁻¹ (B₁g), 511.3 cm⁻¹ (A₁g + B₁g), and 636.2 cm⁻¹ (Eg) [46]. For the CuO-containing TiO₂ NFs, the vibrational peaks exhibit a small blue shift and broadening. It is possible that these alterations relate to the presence of few amounts of monoclinic TiO₂-B structure, which was detected by HRTEM and XRD [47].

Fig. S1 shows the results of Fourier-transform infrared spectroscopy (FTIR) employed to study the structural properties of TiO₂ nanocomposites. The FTIR spectra of TiO₂ and TiO₂-CuO exhibit noteworthy differences. In the TCu samples, there is a shift towards lower wavelengths for a peak that is typically centered at 800 cm⁻¹ in TiO₂. The peak observed around ~660 cm⁻¹ is attributed to Cu–O vibrations, while the FTIR peaks observed in the range of 725–900 cm⁻¹ are associated with Ti–O vibrations [48,49]. Besides, the absorption bands observed at 2800–2900 cm⁻¹ can arise from C–H stretching vibrations of the organic part of MOF and PVP residual.

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) are used to examine the morphology and structure of photocatalysts. The results of SEM and TEM are presented in Fig. 4a–f and Fig. 4g–h, respectively. As shown in Fig. 4a–f, SEM images reveal that TCux samples have a highly interconnected network morphology composed of continuous but randomly oriented nanofibers. The nanofiber diameter decreased from 223 nm in pristine TiO₂ to 93 nm in 1.5 % CuO/TiO₂ (Fig. S2). The CuO/TiO₂ nanofibers have smaller diameters
<figure><img src="image_4.png" /><figcaption>**Fig. 2.** High-resolution XPS spectra of Ti 2p (a), O 1s (b), Cu 2p (c) and C 1s (d) for T, TCu1.5 and HKUST-1 (solid lines respect to experimental and dotted lines respect to fitted curve. The curve fitting of the data was carried out after subtracting their background following the Shirley algorithm. The results of peak fitting of the spectra are shown for the all elements).</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Raman spectra of TCu0.5, TCu1.0, TCu1.5, TCu2.0, and TCu2.5 NFs with characteristic vibrational bands of Anatase phases.;</figcaption></figure>

compared with bare TiO₂ NFs. The inability of Cu ions to integrate into the TiO₂ network – possibly due to their larger size – leads to a perturbation in the crystallites of the anatase phase. This disruption in the crystal structure may inhibit the growth of the crystallite structures,

as reported by reference [50].

To elucidate the impact of heterojunction between CuO and TiO₂ on the structural properties of TiO₂ phases, HRTEM was employed (see Fig. 4g and h). The tetragonal anatase structure of TiO₂ was confirmed in samples T and CuT1.5, with measured interplanar spacings of 0.35 ±0.01 nm and 0.235, respectively. The interplanar spacing of 0.35 ±0.01 nm correspond to the (101) crystallographic plane of the anatase structure (refer to Fig. 4g and h, inset). For TCu1.5 nanofibers, lattice spacings of 0.367±0.05 nm (101) and 0.235±0.05 nm (112) pertaining to the anatase TiO₂ phase were detected. Moreover, average d-spacing values of 0.62 and 0.36 nm were observed, corresponding to the (001) and (110) planes of the monoclinic TiO₂-B phase (JCPDS No. 01-074-1940), respectively [51]. The selected area diffraction (SAED) pattern of TiO₂ and TCu1.5 is displayed in the inset of Fig. 4(g and h), showing that the crystallinity changes from pure anatase to a multi-phase mixture by heterojunction between copper oxide and TiO₂. In addition, there is a good dispersion of CuO in TiO₂ nanofibers, as confirmed by the EDX mapping.

The energy dispersive X-ray (EDX) analysis has been performed on several NFs, as shown in Table S1. The results of this analysis validate the existence of copper oxide on TiO₂ NFs. The atomic ratios of Cu²⁺ to Ti⁴⁺ are raised by increasing the amounts of CuO (i.e., 2.5 > 2 > 1.5 > 1 > 0.5 mol%), in agreement with the experimental values. Elemental energy-dispersive X-ray spectroscopy (EDS) mapping of the prepared
<figure><img src="image_6.png" /><figcaption>**Fig. 4.** Scanning electron microscope images of a) HKUST-1, b) TCu0.5, c) TCu1, d) TCu1.5, e) TCu2, f) TCu2.5, and high-resolution TEM images of g) pristine TiO₂ and h) TCu1.5 with the respective selected area diffraction and FFT patterns in the inset.</figcaption></figure>

NFs demonstrates the consistent and even dispersion of CuO on the surface of TiO₂. This is illustrated in Fig. 5, showcasing the homogeneous distribution of CuO.

In order to determine the pore structure and specific surface areas of the samples and investigate the critical role of the MOF templates in increasing the surface area of the photocatalysts, N₂

<figure><img src="image_7.png" /><figcaption>**Fig. 5.** (a) EDS image and distribution of elements (b) C, (c) N, (d) Ti (e) O and (f) Cu of TCu1.5 The vertical bar represents the relative concentration of the element (dark – 0%, pink – 100%).</figcaption></figure>
adsorption-desorption measurements at 77 K were performed on the CuO, TiO₂ and TCu1.5, as shown in Fig. S3. As shown in Fig. S3, all samples exhibited type IV isotherms with H3 hysteresis loops, indicating their mesoporous structure [52]. Owing to the collapse, decomposition, and shrinkage of the framework, linkers, and microporous structure of the HKUST-1 during the calcination process, the BET specific surface areas of CuO exhibited a sharp decrease (from 1500 m²g⁻¹ to 9 m²g⁻¹). The BET specific surface areas, pore volumes and pore size of the TiCu1.5 sample gradually increased (91 m²g⁻¹ and 19.7 nm, respectively). At the same temperature of 400 °C, the TiCu1.5 exhibited a higher specific surface area (~91 m²g⁻¹) than the TiO₂ (45 m²g⁻¹). The obtained TiCu1.5 heterostructure inherited the larger specific surface area and pore structure of HKUST-1.

The BJH analysis further affirmed the mesoporous nature of the CuO/TiO₂ composite nanofibers, since it showed a narrow pore-size distribution for TiO₂ NFs with an average pore size of 2–10 nm and the total pore volumes for the mesoporous are 0.12 cm³g⁻¹, and a broad pore size distribution of between 10–30 nm (Fig. S3(b)) for TCu1.5 NFs and total pore volumes 0.45 cm³g⁻¹. As depicted in Table 1, the BET specific surface area of the composite nanofibers was significantly enhanced with introducing the CuO content. This was consistent with the formation of smaller anatase TiO₂ crystals hence a higher specific surface area, due to the addition of CuO. Because the photocatalytic reactivity depends on the surface properties, the porous structure facilitates mass transfer in surface reactions, and photocatalysts with a larger specific surface area can expose more active sites to the reactants during the catalytic process, which is beneficial to the carrier transport and separation, thereby improving the photocatalytic efficiency. The larger specific surface area of TCu1.5 can facilitate the quick diffusion of reactants and products. This enhanced surface area promotes more contact between the photocatalyst and the substances involved in the photocatalytic reaction. As a result, it can boost the efficiency of the photocatalytic process [53]. It is also found that MOF-based TiO₂ NFs have a surface area 10 times larger than CuO and two times bigger than pure TiO₂ NFs. These findings corroborate the desirable porous properties of CuO-based TiO₂ NFs. This increased surface area may also be beneficial for enhancing the efficiency of carrier transport and separation within the photocatalyst [54]. The higher SBET value of the latter can be explained by the high porosity of MOF-templated CuO.

The Kubelka-Munk formula is commonly used to calculate the bandgap energy (Eg) of fabricated composites [55], it is expressed as follows:

$$F = \frac{(1 - R)^2}{2R} \qquad (1)$$

Fig. 6 presents the absorbance spectra of TCu samples on a semilogarithmic scale. From Fig. 6a–b, we assume that two main absorption areas can be recognized in the samples: 3–3.7 eV (TiO₂ band-band transitions) and 2–2.8 eV (defect levels in TiO₂ or/and CuO absorption). During the deposition of the samples, an increase in the CuO part increased visible light absorption.

The TiO₂ band gap values have been calculated as [55]:

$$(Fhv)^{1/2} = A(hv - Eg) \qquad (2)$$

CuO band gap energy was evaluated by an equation for direct band semiconductors [56]

<table><caption>**Table 1**

Physical properties of CuO, TiO₂ and TCu1.5 samples.</caption><thead><tr><th>Samples</th><th>S<sub>BET</sub><br>(m<sup>2</sup>g<sup>-1</sup>)</th><th>Total pore volume<br>(cm<sup>3</sup>g<sup>-1</sup>)</th><th>BJH<br>Pore diameter<br>(nm)</th></tr></thead><tbody><tr><td>TiO<sub>2</sub></td><td>45</td><td>0.12</td><td>3.5</td></tr><tr><td>CuO</td><td>9</td><td>0.013</td><td>12</td></tr><tr><td>TCu1.5</td><td>91</td><td>0.45</td><td>10–22</td></tr></tbody></table>

$$(Fhv)^2 = A(hv - Eg) \qquad (3)$$

It is worth saying that no linear parts were found for CuO in indirect coordinates.

Urbach tail energy was calculated as reported in [57]

$$\ln(F) = A(hv - Eg)/E_u \qquad (4)$$

where F, hv, Eg, and Eu are the Kubelka–Munk function, photon energy, band gap, and Urbach tail energy, respectively.

The data obtained is summarized in Table 2. It is seen that as-prepared TiO₂ nanofibers exhibit two absorption bands, one at 3.25 eV and another at 1.8 eV. The obtained values are related to TiO₂ anatase and surface defect levels/titanium suboxide [58]. When the annealing temperature is kept low at 400 °C, it is believed that a small amount of titanium suboxide can be formed. Additionally, when CuO is present within the TiO₂ structure, it leads to a gradual decrease in the TiO₂ band gap. The formation of the CuO/TiO₂ interface results in the presence of specific absorption in the visible range related to the CuO modification [59]. The increase of CuO concentration results in the formation of a more defective state in the composite structure and increases Urbach tail energy. The observed optical properties are in good correlation with XRD, where the presence of Cu ions could stimulate poorer crystallinity and phase transitions of TiO₂.

The PL spectra of the samples are shown in Fig. 6c. The TiO₂ PL spectrum displays a wide peak centered at 520 nm, and pure HKUST-1 spectra show a narrow band at 315 nm. The TiO₂ PL emission can be explained by oxygen vacancies and self-trapped excitons [60]. In the photoluminescence spectra of CuO/TiO₂ nanofibers (Fig. 6c), the observed peaks in the wavelength range of visible light corresponded to oxygen vacancies [61]. It is known that CuO and TiO₂ have p-type and n-type conductivity [60,62]. Altering CuO and TiO₂ might form p-n junction, which stimulates photogenerated charge separation [63]. Therefore, CuO-containing TiO₂ NFs display PL quenching, leading to a reduction in photoluminescence.

Furthermore, EIS measurements revealed distinct features that elucidate the photocatalytic performance of our MOF-based CuO/TiO₂ nanocomposites. A smaller semicircle radius in the Nyquist plot indicates a lower charge transfer resistance (Rct), whereas the TCu1.5 showed the smallest radius, suggesting enhanced charge carrier mobility. These impedance spectroscopic features align cohesively with our photoluminescence data, validating the high photocatalytic efficiency of the designed TCux, Fig. 6d.

Fig. 6e shows the transient on/off photocurrent response of TiO₂ and CuO/TiO₂ samples during chronoamperometry tests at a constant potential of 1.3 V vs. RHE under light exposure. The order of photocurrent density is TCu1.5 > TCu1 > TCu2 > TiO₂. The TCu1.5 nanofibers exhibit a much higher photocurrent density compared to the pristine TiO₂ and other CuO/TiO₂ nanofibers. This indicates that TCu1.5 nanofibers have improved efficiency in charge separation and transfer, as well as a longer lifetime of photo-generated charge carriers.

### 3.1.1. H₂ generation reactions

In order to evaluate the photocatalytic performance of the prepared NFs (nanofibers), the production of H₂ in an aqueous solution was monitored using methanol as a scavenger, which helps to prevent the recombination of electron-hole pairs (e⁻/h⁺). By scavenging the holes, methanol molecules reduce electron-hole recombination, allowing the electrons to react with protons and generate H₂ through water splitting processes (Fig. 7a). Wang et al. [53], F. Theng et al. [60], X. Chen et al. [64], P. Karthik et al. [65], C. Dessela et al. and X. Zhang et al. [66,67] have referred to the methanol role in photocatalytic reactions, as shown in Table 3. As for pure TiO₂, low amounts of H₂ are produced (110 µmolg⁻¹h⁻¹). This can be attributed to the rapid recombination between electrons-holes and the wide bandgap of TiO₂ [68]. In addition, the apparent quantum yield (AQY) of TCu1.5 was calculated to be 7.724 %
<figure><img src="image_8.png" /><figcaption>Fig. 6. a,b) Absorption spectra of TiO₂, TCu0.5, TCu1, TCu1.5, TCu2, TCu2.5 nanofibers; c) Photoluminescence spectra of the HKUST-1, TiO₂, TCu0.5, TCu1, TCu1.5, TCu2, TCu2.5 samples d) EIS of TCu1, TCu1.5, TCu2, composites, e) photocurrent data of TiO₂, TCu1, TCu1.5, TCu2 NFs during chronoamperometry tests at a constant potential of 1.3 V vs. RHE under light exposure.</figcaption></figure>

(see supporting information). By increasing the amount of CuO from 0.5 % to 1.5 %, the rate of H₂ generation is raised. An increase in Cu-content seems to lead to the construction of a heterojunction between CuO and TiO₂, facilitating the separation of charges across the interfaces. So, the boosted photocatalytic performance can be attributed to the combined impact of the heterojunction composition, which boosts TiO₂'s ability to

absorb light and reduces the recombination rate of electron-hole pairs (e'/h'). Comparison of the TiO₂ catalysts with different CuO mol% (Fig. 7a) indicated that the best catalytic activity was obtained with the 1.5 % CuO/TiO₂ sample (H₂ evolution rate of 45606 µmol·g⁻¹·h⁻¹), because of its high surface area produced from template of MOF structure. Similar results have been reported by Y.T. Liao et al. about high
<table><caption>**Table 2**
Eg (direct and indirect) and Urbach tail energy.</caption><thead><tr><th>Sample</th><th>Eg, eV (indirect)</th><th>Eg, eV (direct)</th><th>Urbach tail, eV</th></tr></thead><tbody><tr><td>TiO<sub>2</sub> (TCuO)</td><td>3.25±0.01<br>1.80±0.01</td><td>—</td><td>0.19±0.02</td></tr><tr><td>TCu0.5</td><td>3.23±0.02</td><td>2.1±0.01</td><td>0.21±0.01</td></tr><tr><td>TCu1</td><td>3.25±0.02</td><td>2.1±0.02</td><td>0.215±0.02</td></tr><tr><td>TCu1.5</td><td>3.22±0.01</td><td>2.08±0.01</td><td>0.22±0.01</td></tr><tr><td>TCu2</td><td>3.2±0.01</td><td>2.08±0.02</td><td>0.26±0.01</td></tr><tr><td>TCu2.5</td><td>3.19±0.01</td><td>2.08±0.01</td><td>0.29±0.01</td></tr></tbody></table>

<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /><figcaption>**Fig. 7.** A) Photocatalytic performance for H₂ evolution over prepared nanofibers. b) Recycling test for the hydrogen evolution performance of TCu1.5 nanofibers.</figcaption></figure>

surface area of metal oxide produced from MIL-125 MOF [69]. With lower CuO mol%, 1% and 0.5%, photocatalytic activity decreased to 42,152 and 40711 µmol·g⁻¹·h⁻¹, respectively. The 2.0%, 2.5%, CuO/TiO₂ sample gave a H₂ evolution rate of 31788, 25928 µmol·g⁻¹·h⁻¹, respectively. This decrease might be explained by CuO “shielding effect” [69]. The promoted photocatalytic behavior confirms that CuO/TiO₂ nanocomposite is a good catalyst for producing more H₂, presumably due to constructing the heterojunction between TiO₂ and CuO. The potentially created heterojunction created in the nanostructures may positively impact the photocatalytic activity of CuO/TiO₂ nanostructures by reducing the charge recombination rate, increasing the effective e⁺/h⁺ separation at the interfaces, and boosting the TiO₂ optical

absorption in the heterostructure nanocomposites, as discussed by Y.S. Ouyang *et al.* about metal-organic framework-derived CuO@TiO₂ heterojunction toward efficient visible-light-driven hydrogen production [70].

The prepared photocatalysts were also subjected to a reusability and stability evaluation involving four consecutive cycles. Each cycle lasted for six hours under visible light irradiation. This examination aimed to assess how effectively the photocatalysts could be reused without significantly degrading their performance over multiple cycles. The duration of each run and the light source used ensures comprehensive testing of the material's durability and efficiency. The results reveal that after four cycles, there was only a slight decrease in the rate of H₂ generation when 1.5% CuO/TiO₂ NFs were used (Fig. 7b). These findings indicate that the mentioned nanofibers exhibit remarkable stability and recyclability. Their excellent performance over multiple cycles underscores their potential as durable and sustainable materials for various applications.

In contrast to the few studies conducted on CuO-containing TiO₂ for hydrogen generation (as stated in Table 3), the CuO-TiO₂ compound prepared in this study, with a copper oxide content of 1.5 mol%, exhibited a remarkably high activity in generating hydrogen in comparison with CuTi-450 (1485 µmol·g⁻¹·h⁻¹) [5], CuO/TiO₂ derived from Cu-MOF (32766 µmol·g⁻¹·h⁻¹) [71], and CuO@MTs(0.5) (4760 µmol·g⁻¹·h⁻¹) [65]. Even, its performance surpassed that of certain TiO₂ compounds modified with platinum (Pt) and metal-organic frameworks (MOFs), Pt@CuO/TiO₂ [67]. This exceptional ability to catalyze hydrogen production can be primarily attributed to the CuO/TiO₂, known for its substantial surface area of 90.9 m²/g and high porosity. As mentioned before, derivatives of metal-organic frameworks (MOFs) have the advantage of maintaining a high specific surface area and porous structure, which creates numerous reaction sites for photocatalytic reactions. This characteristic greatly facilitates the intermediate water splitting process. Additionally, the heterojunction formed between TiO₂ and CuO plays a crucial role in preventing the recombination of photoexcited electron-hole pairs as detected by EIS and PL measurement. The rapid separation and migration of electron-hole pairs, combined with efficient surface reactions, are considered the primary factors contributing to the enhanced photocatalytic activity.

In addition, the exceptional photocatalytic performance of HKUST-1-derived CuO/TiO₂ nanofibers (NFs), highlighted in comparison with CuO/TiO₂ nanorods [72], was a remarkable 328 times higher than that achieved with CuO/TiO₂ nanorods. This can be attributed to the mesoporosity and alignment of nanocrystals, which enable directional charge transfer and consequently reduce charge recombination. The NFs have more appropriate sizes and offer larger specific surface areas, enhancing photocatalytic activity. These factors facilitate efficient charge separation through inter-particle charge transfer along the nanofiber framework. In contrast, our catalyst showed less catalytic activity compared to Cu-CuO/TiO₂ (286,000 µmol·g⁻¹·h⁻¹) [5]. This high photocatalytic H₂ production activity is predominantly due to the presence of surface-deposited Cu⁰ species and the small-sized heterojunctions (1–2 nm) between CuO and TiO₂, which synergize the interfacial charge carrier transfers from TiO₂ nanoparticles. Even though quantum dots are more significant in photocatalytic reaction studies, our NFs, produced via a simple electrospinning method, showed twice the catalytic activity of CuO quantum dots decorated TiO₂ nanocomposites [73].

In other literature, MOF-derived metal oxides, c-MIL-125–0.4Cu, were prepared from the calcination of two different MOFs, MIL-125 and Cu-BDC, at 400 °C [74]. The hydrogen evolution reaction was carried out in the presence of [Ru(bpy)₃]²⁺ as the photosensitizer and Triethanolamine (TEOA) as the electron sacrificial agent in acetonitrile solution. In comparison, our simple synthesis method and reaction conditions, coupled with higher catalytic activity than c-MIL-125–0.4Cu (4,837 µmol·g⁻¹·h⁻¹), are more noteworthy. Additionally, Cu-MOF-derived cupric oxide, CuO@TiO₂ nanorods, were examined for solar
<table><caption>**Table 3**

Comparative photocatalytic study for hydrogen evolution via water splitting.</caption><thead><tr><th>Catalyst</th><th>Preparation technique</th><th>Output: H<sub>2</sub> production (µmol g<sup>-1</sup>h<sup>-1</sup>)</th><th>Sacrificial agent</th><th>References</th><th>Light source</th></tr></thead><tbody><tr><td>CuO/TiO<sub>2</sub></td><td>Electrospinning</td><td>45606</td><td>MeOH</td><td>This work</td><td>500 W linear halogen lamp</td></tr><tr><td>CuO@TiO<sub>2</sub> (CuTi-450)</td><td>Hydrothermal</td><td>1458</td><td>Triethanolamine</td><td>[5]</td><td>300 W xenon lamp (λ > 420 nm)</td></tr><tr><td>CuO/TiO<sub>2</sub> driven from Cu-MOF</td><td>Hydrothermal</td><td>32766</td><td>KOH electrolyte</td><td>[71]</td><td>500 W halogen lamp with light intensity of 100 mW/cm<sup>2</sup></td></tr><tr><td>Cu/CuO@TiO<sub>2</sub></td><td>Impregnation method</td><td>286000</td><td>MeOH</td><td>[5]</td><td></td></tr><tr><td>NH<sub>2</sub>-MIL-125(Ti)</td><td>Hydrothermal method</td><td>490</td><td>Cu<sup>I</sup>/Cu<sup>II</sup></td><td>[64]</td><td>UV-LEDs with a wavelength of 420 nm (32 W, Shenzhen LAMPLIC Science Co. Ltd. China)</td></tr><tr><td>CuO@MTs(0.5)</td><td>Colloidal method</td><td>4760</td><td>MeOH</td><td>[65]</td><td>Solar simulator equivalent to 1 sun (1000 Wm<sup>-2</sup>)</td></tr><tr><td>Ti<sub>3</sub>C<sub>2</sub>/TiO<sub>2</sub>/UiO-66-NH<sub>2</sub></td><td>Hydrothermal method</td><td>1980 µmol g<sup>-1</sup>h<sup>-1</sup></td><td>Na<sub>2</sub>S, Na<sub>2</sub>SO<sub>3</sub></td><td>[75]</td><td>fulls pectrum</td></tr><tr><td>9.1 mol% CuO-TiO<sub>2</sub></td><td>Impregnation method with P25 as support</td><td>18,500 µmol g<sup>-1</sup>h<sup>-1</sup></td><td>MeOH</td><td>[60]</td><td>400 W high pressure Hg lamp</td></tr><tr><td>CuO/TiO<sub>2</sub> nanorod</td><td>Impregnation method</td><td>139.03 µmol g<sup>-1</sup>h<sup>-1</sup></td><td>Methanol</td><td>[66]</td><td>150 W of 2 metal halide light bulbs</td></tr><tr><td>Pt@CuO/TiO<sub>2</sub></td><td>Impregnation method</td><td>8500 µLg<sup>-1</sup></td><td>Methanol</td><td>[67]</td><td>500 W high-pressure Xe lamp</td></tr><tr><td>c-MIL-125-0.4Cu</td><td>Hydrothermal method</td><td>4837</td><td>TEOA,[Ru(bpy)<sub>3</sub>]<sup>2+</sup></td><td>[74]</td><td>450 nm LED lamp</td></tr><tr><td>CuO/TiO<sub>2</sub> nanorods Drived from MOF</td><td>Hydrothermal and deposition/calcination</td><td>32,766</td><td>1 M KOH and 5 vol% glycerol solution</td><td>[72]</td><td>100 mW/cm<sup>2</sup> power intensity with applied voltage ranging between – 0.7 and 1.2 V in the PEC cell</td></tr><tr><td>CuO QDs decorated on TiO<sub>2</sub> surface</td><td>Molten salt method</td><td>20360</td><td>Glycerol aqueous solution</td><td>[73]</td><td>300 W Xe lamp ORIAL instruments</td></tr></tbody></table>

hydrogen production. The results illustrate the formation of van der Waals type-II heterojunctions with higher visible light absorption and lower charge transfer resistance. The hybrid photocatalyst calcined at 450 °C produced the highest hydrogen production at 32,766 µmol·g<sup>-1</sup>·h<sup>-1</sup>.

## 3.2. Mechanism of interaction in visible light

The band gap is crucial for a photocatalyst function. The photocatalysts TiO₂ and CuO are semiconductors because of their bandgap of 3.21 eV and 1.51 eV, respectively, as shown in (Fig. 8a–b), which is in close agreement with the experimentally measured band gap of present work and comparable to those previously reported of 3.24–3.36 eV, 1.25–1.9 eV for anatase [76,77] and CuO [78,79]. Notably, the CuO/TiO₂ photocatalyst containing 1.5 mol% of CuO displayed a smaller band gap of 3.19 eV (as shown in Fig. 8c based on DOS) compared to anatase TiO₂ due to Cu-3d electrons contribution from CuO, which corresponds with what we obtained from absorption spectra (Fig. 6). The Fig. 8d shows the crystal structures of titanium oxide (left) and copper oxide (right), highlighting the arrangement of Ti and Cu atoms with oxygen atoms in their respective lattices. Based on the position between the conduction band’s minimum (CBM) and the valence band’s maximum (VBM), there exist two types of band gap in photocatalyst, Direct bandgap, and Indirect bandgap. When irradiated with appropriate energy, the valence band (VB) electrons of the oxides receive enough energy to migrate to the conduction band (CB), creating a hole in the valence band, whereas electron-hole pair formation is the fundamental phase behind photoreduction. In the case of a direct bandgap semiconductor (CuO), the CBM and VBM are in the same k-space and the excited electrons can directly jump to the valence band just by releasing photons, which can affect the recombination rate and overall photocatalytic efficiency. Whereas for the Indirect band gap semiconductor (TiO₂), the energy (E) versus the wave vector (k) curve shows that the CBM and VBM do not lie in the same k-space. In this case,

the excited electrons cannot easily return to the valence band just by releasing photons. This misalignment requires additional phonon involvement for electron transitions, which can lead to longer-lived electron-hole pairs and potentially enhanced photocatalytic activity. Similarly, if the electron wants to come to the valence band, it must release a phonon along with photon, as shown in Fig. 8e.

Interestingly, the construction of CuO/TiO₂ heterojunction has also been proven to be an efficient way to increase the activity of the photocatalyst due to the differences in electron-hole recombination processes between direct and indirect bandgap materials. From Fig. 8e, both of the conduction and valence bands of TiO₂ are higher than that of CuO. Upon irradiation in the CuO and TiO₂ semiconductor materials, electron-hole pairs get generated and split between the valence and conduction bands. Due to the locations of the bands, it is thermodynamically favorable for the electrons of TiO₂ to tend to move to the conduction band of CuO. Similarly, the holes in CuO prefer to move to the valence band of TiO₂. This relocation of electron-hole pairs hampers the recombination process as electrons and holes are distributed in two separate materials. This fact shows that the efficiency of a heterostructure is larger than that of a single material, and the following pattern can be established.

## 4. Conclusion

This study reports the synergistic effect of CuO-based HKSUT-1 MOF on TiO₂ photocatalytic activity during water splitting under visible light irradiation. TiO₂ photocatalysts (with/without CuO) were easily and efficiently generated using electrospinning. The analysis of their structures, morphologies, and optical features indicated that CuO-based TiO₂ nanofibers (NFs) exhibited enhanced photocatalytic activity and increased hydrogen (H₂) generation under visible light irradiation, as compared to pure TiO₂ NFs. The band gap was narrower, and the photogenerated e⁻/h⁺ pair recombination was reduced in CuO/TiO₂ compared with pristine TiO₂ nanofibers. The photocurrent data
<figure><img src="image_11.png" /><figcaption>**Fig. 8.** A,b,c) calculated total density of states (DOS) of anatase TiO₂, CuO and TiO₂/CuO nanofibers. (d) Crystal structure of CuO and TiO₂. (e) Mechanism of visible light photocatalysis of TCu1.5.</figcaption></figure>

indicated that TCu1.5 nanofibers have improved efficiency in charge separation and transfer, as well as a longer lifetime of photo-generated charge carriers. The best photocatalytic activity was observed for the composite prepared using 1.5 % CuO/TiO₂ nanofibers, which was 400 times greater than that of pure TiO₂ NFs. The sensing enhancement mechanism of the CuO/TiO₂ nanofibers can be attributed to two main factors. First, the formation of heterojunctions at the interfaces between TiO₂ and CuO allows for efficient charge transfer and separation, as confirmed by DFT calculation. Second, the use of CuO derived from MOF (Metal-Organic Framework) results in high catalytic activity, further contributing to the overall performance of the nanofibers. The proposed approach can be considered a promising technique to fabricate heterogeneous structures that can be used in a wide variety of applications.

CRediT authorship contribution statement

**Mitra Saffari:** Writing – original draft, Validation, Investigation, Formal analysis, Data curation. **Zahra Hagheh-Kavousi:** Writing – original draft, Validation, Investigation, Formal analysis, Data curation. **Massomeh Ghorbanloo:** Writing – original draft, Supervision, Resources, Methodology, Formal analysis, Conceptualization. **Amr A. Nada:** Writing – original draft, Supervision, Resources, Methodology, Formal analysis, Conceptualization, Writing – review & editing, Validation, Investigation, Formal analysis, Data curation. **Maged F. Bekheet:** Writing – review & editing, Validation, Investigation, Formal analysis, Data curation. **Djamel Bezzerga:** Writing – original draft, Validation, Methodology, Formal analysis, Data curation. **Jisang Hong:**
**Writing – review & editing, Validation, Resources, Methodology, Formal analysis, Data curation.**
**Ali Morsali:** Writing – review & editing, Validation, Supervision, Resources, Funding acquisition, Formal analysis.
**Yaovi Holade:** Writing – review & editing, Validation, Supervision, Resources, Formal analysis.
**Roman Viter:** Writing – review & editing, Validation, Resources, Formal analysis, Data curation.
**Heba H. El-Maghrabi:** Writing – review & editing, Validation, Resources, Formal analysis.
**Rasha Hosny:** Writing – review & editing, Validation, Methodology, Formal analysis, Data curation.
**Fathi S. Soliman:** Writing – review & editing, Validation, Funding acquisition, Formal analysis, Data curation.
**Igor Iatsunskyi:** Writing – review & editing, Validation, Resources, Investigation, Data curation.
**Emerson Coy:** Writing – review & editing, Validation, Investigation, Formal analysis, Data curation.
**Mikhael Bechelany:** Writing – review & editing, Validation, Supervision, Resources, Project administration, Methodology, Funding acquisition, Conceptualization.

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.

Data availability

Data will be made available on request.

Acknowledgments

The authors are grateful to the University of Zanjan for financial support of this study. This work was partially supported by the National Research Foundation of Korea (NRF) grant funded by the Korean government (MSIT) (2022R1A2C1004440).

Appendix A. Supplementary material

Supplementary material to this article can be found online at https://doi.org/10.1016/j.apsusc.2024.161061.

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