In-situ formation of graphene stabilizes the zero-valent copper

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In-situ formation of graphene stabilizes the zero-valent copper nanoparticles and significantly enhances the efficiency of photocatalytic water splitting Menna Hasan, Sarah Tolba, and Nageh K. Allam ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04219 • Publication Date (Web): 24 Oct 2018 Downloaded from http://pubs.acs.org on October 25, 2018

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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ACS Sustainable Chemistry & Engineering

In-situ formation of graphene stabilizes zero-valent copper nanoparticles and significantly enhances the efficiency of photocatalytic water splitting Menna M. Hasan†, Sarah A. Tolba† and Nageh K. Allam†,* †Energy

Materials Laboratory (EML), School of Sciences and Engineering, The American

University in Cairo, New Cairo 11835, Egypt. * Corresponding Author email: [email protected]

KEYWORDS nanofibers; electrospinning; hydrogen production; DFT; rGO; charge density.

ACS Paragon Plus Environment

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ABSTRACT

There is a growing need for new techniques to synthesize metallic copper nanoparticles due to their remarkable use in many advanced technologies. Herein, a novel method to synthesize stable and non-agglomerated zero-valent copper nanoparticles (ZVCNPs) via the in-situ formation of reduced graphene oxide (rGO) during the electrospinning process in the presence of polyvinylpyrrolidone (PVP) as a carbon source is presented. X-ray diffraction (XRD), Raman spectroscopy, electron paramagnetic resonance (EPR), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS) techniques were used to investigate the morphology, structure, and composition of the fabricated materials. The synthesized ZVCNPs were coupled with TiO2 nanofibers and rGO to form an efficient photoactive material to photocatalytically produce hydrogen via water splitting, resulting in 344% increase in the hydrogen yield compared to that of TiO2 nanofibers. The density functional theory (DFT) calculations showed that the ZVCNPs enhance the charge transfer and lower the energy needed for photocatalytic water splitting. This study suggests a novel method for metallic copper stabilization and illustrates the effect of metallic copper as a catalyst for the in-situ formation of rGO.


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INTRODUCTION

Stabilizing copper nanoparticles in the zero-valent state has attracted considerable attention in recent years as they possess the desired properties of noble metals, with the advantage of low processing costs.1 However, it is very challenging to maintain copper in the metallic form, as it is easily oxidized to CuO and/or Cu2O when exposed to air. Therefore, many studies have been devoted to prevent its oxidation by minimizing the contact between copper and air, 2 including polymer coatings1,3–7 and metallic coatings.3,8–11 Nevertheless, there are still some complications due to the cost of the coatings, crystal mismatch between copper and the coating, and the effect of the coating on the charge carriers transport.2,12 Therefore, some recent studies investigate the use of graphene/reduced graphene oxide due to its chemical stability and higher thermal mobility and electrical conductivity than copper.4,13,14 Most of those studies mainly involved coating copper metal with graphene using chemical vapor deposition (CVD) technique.12,15 However, using CVD involves the use of a gas source, which requires a temperature of almost 1000 °C, limiting the number of transition metals that can be used. Besides, in most of the previous studies, the fabrication of graphene and zero-valent copper nanoparticles was performed in two separate steps, then a composite of both materials was prepared and tested in various applications. This often resulted in agglomeration of Cu NPs on graphene surface. Herein,

a

novel

technique

is

presented

to

overcome

copper

nanoparticles

oxidation/agglomeration via the in-situ formation of Cu NPs and rGO using electrospinning technique and PVP as a carbon source. The use of a solid carbon source has the advantage of reducing the temperature needed for the process.16–18 Due to the 3-D folded structure and large surface area of graphene, it is an excellent carrier for copper nanoparticles,19 hindering their


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agglomeration and oxidation.20,21 Also, copper nanoparticles weaken the π-π interaction, thus reducing the stacking of graphene sheets.22 Moreover, to test the utility of the fabricated copper nanoparticles as a catalyst in photocatalytic water splitting, copper nanoparticles were attached to TiO2 nanofibers and the photoactivity was tested and discussed in details. The theoretical calculations using the spin-polarized density functional theory (DFT+U) have been utilized to further confirm and explain the experimental findings.

EXPERIMENTAL SECTION Chemicals.

Titaniumisoprorpoxide,

copper(II)

acetate

monohydrate

(C4H6CuO4·H2O),

Polyvinylpyr-rolidone (PVP, Mw ≈ 1300 000), ethanol absolute, acetic acid. All chemicals were purchased from Alfa Aesar and used as received. Nanofibers Fabrication. The electrospinning technique was used to fabricate TiO2 nanofibers and TiO2-Cu composite nanofibers. For TiO2 nanofibers, 0.5 g of titanium isoprorpoxide was added to 4 g of PVP 10%. The PVP solution was made using polyvinylpyrrolidone (PVP) and ethanol absolute as polymer and solvent, respectively, then 1 g acetic acid was added to the solution, finally the solution was stirred for 2 hours. For pure Cu nanofibers, different weights of copper acetate monohydrate (0.05, 0.075, 0.1, and 0.125) were dissolved in 2.5 g of absolute ethanol, after copper acetate was completely dissolved, 0.25 g of PVP and 5 g of acetic acid were added to the solution, followed by stirring until the polymer is completely dissolved. Ti-Cu composite nanofibers were prepared by mixing the two solutions mentioned before with continuous stirring till complete homogeneity. Finally, each of the three solutions was passed through a 16 G stainless steel nozzle. The distance between the syringe tip and the grounded aluminum foil collector was fixed at 15 cm,


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the voltage used was in the range of 19 to 21 kV, and the feed rate was in the range of 4 to 4.5 ml/h, at humidity of 30% to 40%. The electrospun nanofibers were then annealed in a Lindberg/Blue M tube furnace in argon atmosphere at 500 and 600 ∘C (2 ∘C/min) for 6 hours. Table 1 summarizes the composition of the fabricated TiO2 and TiO2-Cu composite nanofibers annealed in argon atmosphere. Table 1. Composition of the fabricated TiO2 and TiO2-Cu composite nanofibers. Ti (g)

0.5

Cu (g)

Sample ID

0

R1

0.05

R2

0.075

R3

0.1

R4

0.125

R5

Materials Characterization. Zeiss SEM Ultra 60 field emission scanning electron microscope (FESEM) was used to characterize the morphology of the fabricated nanofibers. TGA NETZSCH STA 409 C/CD apparatus was used to elucidate the stability of the nanofibers in air. The crystal structure was identified using x-ray diffractometer (PANalytical X’pert Pro PW3040 MPD) using copper CuKα radiation (λ= 0.15406 nm) in the range of 5° to 80° at a scan rate (2θ) of 3o s-1 and was further confirmed using Raman microscope (Pro Raman-L Analyzer) with an excitation laser beam wavelength of 532 nm. The absorption spectra of the nanofibers were recorded on a Lambda 950 UV/Vis spectrometer. EPR measurements were performed using Bruker EMX 300 EPR spectrometer (Bruker BioSpinGmbH, Silberstreifen 4, Germany). Photoelectrochemical measurements were done in a three-electrode cell, glassy carbon electrode loaded with the nanofiber was used as the working electrode, while platinum foil was used as the counter electrode,


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and saturated calomel electrode (SCE) as the reference electrode in 1.0 M KOH. A 300 W ozonefree xenon lamp with an AM 1.5 G filter at 100 mWcm -2 were used to simulate Sunlight and a scanning potentiostat (Biologic SP-200) was used to measure the photocurrent densities. An innerirradiation reactor was used to test the photocatalytic activity of the fabricated nanofibers that is equipped with a 300 W xenon lamp (CEL, HUL300), a gas collection, a recirculation pump, a vacuum pump, and a water-cooled condenser. 0.1 g of the sample was suspended in 1 M KOH using ultrasonic oscillator. The mixture was then transferred into the reactor and deaerated using the vacuum pump. To maintain the reaction temperature, cooling water was circulated around the light source through a cylindrical Pyrex jacket. During irradiation, the reactor was kept sealed and the evolved hydrogen was monitored using an online gas chromatograph (GC, 7900) equipped with a Porapak-Q column and a thermal conductivity detector (TCD) under high-purity nitrogen carrier. Theoretical Calculations. Spin-polarized density functional theory (DFT)23 with CASTEP code24 was used to perform geometry optimization, energies, and electronic structure calculations. The computational parameters were set as follows: the interaction between electrons and ions was represented using the Ultrasoft pseudopotential,25 and the minimization algorithm was performed using the Broyden-Fletcher-Goldfarb-Shanno (BFGS) scheme.26 A 400 eV plane wave cut-off energy was used and the Monkhorst–Pack mesh,27 for k-points sampling was set with a separation of 0.07 and 0.04 1/A for geometry optimization and DOS calculations respectively. 28 All density functional calculations were performed within general gradient approximation GGA-PW91 functionals to describe exchange and correlation effects.

29

For transition metal oxides, the self-

interaction error is the main drawback of the GGA approach,30 which significantly underestimate the band gap.31 Thus, the DFT+U approach was used to correct or partly correct this problem.32


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The effective Hubbard parameter U was set to 7 eV for d orbitals of Ti and 8 eV for d orbitals of Cu atoms. The choice of these values was semi-empirically where we tested different values for the Hubbard correction and picked up the one that gives results in good agreement with the experimental results. All geometry optimized structures were optimized until the energy change per atom and the force on the atomic nuclei were less than was less than 1 × 10 −5 eV and 0.03 eV/Å, respectively. To test the accuracy of the computed results, the parameters used to optimize the structures in the current work were examined. For bulk TiO2 the optimized lattice parameters are; a= 3.87 and b= 9.84 which are in good agreement with the experimental, 33,34 and previous theoretical calculations.35–37 For the adsorbent, the structure of the isolated gas-phase H2O molecule in a large unit cell (25 × 25 × 25 A˚) was found with 0.977 Å H-O bond length and 104.477° H-O-H bond angle, which agrees with the experimental values.38 For the surfaces models, Figure 1, initially, all atomic positions of ions and lattice parameters were optimized for bulk anatase TiO2 until the absolute values of the ionic forces were < 0.01 eV/Å, then the slab was cute from this optimized structure. TiO2 (1 0 1) surface was represented by a two TiO 2 layers slab structure with 2 × 2 supercell with the formula of Ti8O16, where the upper three layers of atoms were allowed to relax in the geometry optimization calculations to simulate the surface while the lower atoms were constrained with geometric parameters to simulate a bulk-like environment. To minimize the computational effort, a two-layer slab model was used with a vacuum gap between neighboring layers of 15 Å, which helps to eliminate the lateral interaction between adsorbate and distinct slab surfaces. To model Cu/TiO2 (101) surface, a cluster of four Cu atoms was added to the surface as the use of a single atom on the oxide-support might overestimate the activity of the placed metal. On the contrary, the adsorption energy of a cluster with a big number of atoms on the surface depends on the geometry of the cluster and the orientation of the adsorbent on the


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surface, which is computationally expensive to simulate.39 Finally, from the literature findings, a cluster of four or more copper atoms present the catalyst behavior that agrees with the experimental findings while a cluster with less than four atoms does not. First, the initial structure of Cu cluster was first drowned from the literature. The tetramer (Cu4) prefers the tetrahedron (pyramid) compact coordination, where each Cu atom is bound to the other three. 40 Then, we performed DFT calculations for the geometry optimization and the electronic structure calculations.

Figure 1. Structure of (a) TiO2 (101) and (b) Cu/TiO 2 (101) surfaces.

RESULTS AND DISCUSSION Figure 2 shows FESEM images of the fabricated nanofibers before and after annealing in argon atmosphere. The as-electrospun TiO2 nanofibers were smooth with no beaded fibers observed, Figure 2a. Upon annealing, the nanofibers seem to be cross-linked together with the clear emergence of the copper nanofibers on the surface, Figure 2b. The nanoparticles imbedded in the fiber were further investigated using TEM, Figure 2c-f. The measured d-spacing values of the lattice are 3.52 Ao and 2.1 Ao, which correspond to the (101) lattice planes of the anatase phase of TiO2,41 and the (111) lattice plane of metallic copper, Figure 2c,d.42 The particle size of copper nanoparticles was found to be in the range of 23 - 33 nm, which are well distributed among the


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nanofibers, Figure 2e. Moreover, the electron diffraction pattern, Figure 2f, confirms the polycrystalline nature of the material. The UV-Vis spectrum, Figure 3a, showed high absorption intensity and a shift towards the visible region of the light spectrum (≈800 nm) for both TiO2 and TiO2-Cu composite nanofibers, which is different from that usually seen for pure black TiO 2.43 This shift can be related to the formation of rGO in the samples during annealing, which is in agreement with previous studies.44 However, a small peak around 400 nm is still present that is a characteristic of the absorption by TiO2. On the other hand, a recent study by Samani et al. who fabricated black titania core-shell nanoparticles by annealing the synthesized nanoparticles in argon atmosphere at different temperatures attributed the shift in the absorption spectra to the formation of thin Ti 2O3 surface layer.45 Li et al. measured the absorbance of carbon-doped TiO2 and noticed that there was a shift in the absorption up to 700 nm, which enhanced the photocatalytic activity compared to TiO2 nanofibers. They suggested that this shift is due to the formation of Ti3+ defect states.46


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Figure 2. FESEM images of the electrospun titanium-copper nanofibers (a) before annealing, (b) after annealing in argon atmosphere, (c-f) the corresponding HRTEM analysis of the annealed titanium-copper nanofibers.


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Figure 3b shows the XRD patterns of the annealed nanofibers at different temperatures in argon atmosphere. For the titania nanofibers annealed at 500 °C, a characteristic peak of the (101) facet of the anatase phase was observed at 25.4°. However, titania nanofibers annealed at 600 °C showed two peaks at 25.4° and 27.17° corresponding to (101) of the anatase phase and (110) of the rutile phase. This comes in agreement with the results obtained from a recent study, where a mixture of anatase and rutile phases was obtained when TiH 2 powder was annealed in argon atmosphere at 530 °C.47 The rutile percentage in the nanofibers was calculated using Eq. 1. 48,49 Rutile% = [1/(1 + 0.79 IA/IR )] × 100

(1)

where IA and IR are the peak intensities of (101) and (110) reflections for anatase and rutile, respectively. It was found that the anatase-to-rutile ratio is 60:40. For Ti-Cu composite nanofibers (R2) annealed at 600 °C, we noticed the presence of new peaks at 2θ = 43.3°, 50.4°, 74.2°, which correspond to the (111), (200), and (220) facets of face-centered cubic copper, respectively,22,50,51 besides the sharp peak at 25.2° corresponding to the anatase phase. Note that no peak related to the rutile phase was detected, indicating that copper stabilizes the anatase phase and hinders the transformation to the rutile phase, which is very beneficial in photocatalysis as anatase is more photoactive than rutile. For the Ti-Cu composites with higher copper loadings (R3, R4, and R5), we were not able to detect the peaks related to TiO 2, due to the high intensity of the peaks related to copper. This can also be attributed to the interference between the peak related to TiO2 (at 25.2°) and that of rGO (at 25°). However, for the sample with the highest copper loading (R5), a new peak related to rGO was detected at 2θ = 24°. According to previous studies the low intensity of this peak is due to the low crystallinity of the formed rGO and the high intensity of the copper peaks.50


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To confirm the formation of rGO, the Raman spectra were recorded as shown in Figure 3c. The obtained Raman spectra for TiO2 nanofibers showed the major four Raman bands of anatase at 158, 410, 524, and 646 cm−1, which correspond to Eg, B1g, A1g, and Eg active modes, respectively. For the TiO2-Cu composite, the typical peaks of the D and G bands were observed at 1360 cm-1 and 1590 cm-1.2,4,52–55 The D band is attributed to the local defects, while the G band is related to sp2 hybridized graphene domains.56 As shown in Figure 3c, there was an increase in the intensity of the peaks related to the D and G bands for the composites R4 and R5 compared to that of TiO2 nanofibers. There was a significant increase in the intensity of the G and D bands for the composites R2 and R3, indicating an optimum ratio of copper to PVP that facilitates the formation of graphene. Table 2 summarizes the calculated (ID/IG) ratio for the fabricated TiO2 and TiO2-Cu composite nanofibers. Samples R1, R4, and R5 showed low ID/IG values, which indicates relatively low restacking of the graphene sheets, whereas the samples R2 and R3 showed higher ID/IG ratio, indicating a higher degree of stacking of graphene sheets. This might be due to the consumption of a larger amount of carbon in maintaining the excess amount of copper at zero-valent state. This is supported by the results obtained from XRD measurements, Figure 3b, where, the intensity of the peaks related to metallic copper is much stronger than that of rGO. This comes in agreement with a recent study where lignin was used as a carbon source and found that the amount of rGO formed was dependent on the amount of copper present in the sample. 50


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Table 2. ID/IG ratio calculated from the Raman spectra of the fabricated TiO2 and TiO2-Cu composite nanofibers. Sample ID

ID/IG

R1

0.64

R2

0.95

R3

0.95

R4

0.85

R5

0.62

Figure 3. (a) UV-Visible absorption spectra of TiO2 nanofibers (R1) and TiO2-Cu composite nanofibers (R4), (b) XRD patterns of the fabricated nanofibers annealed in argon atmosphere at different temperatures, (c) Raman shift of TiO2 nanofibers (R1) and TiO2-Cu composite nanofibers (R4), and (d) the change in the intensity of the D and G bands with copper loading.


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These findings were further confirmed using XPS analysis as shown in Figure 4a, where two peaks characteristic of Ti 2p were recorded at 258.6 eV and 464.2 eV related to Ti 2p 3/2 and Ti 2p1/2, respectively.57,58 C1s spectrum shows a broad peak in the range 283.5 eV - 287 eV, which is characteristic of C=C (sp2 carbon) and C-C (sp3 carbon) that are usually appear at 284.7 eV and 285.5 eV, respectively, confirming the formation of rGO.56 Note that only sample R3 showed a small peak at 288.4 eV, which is attributed to C=O. 56 O1s spectra of Ti-O was recorded for all samples at 530 eV,59 except for the sample R3, where two humps were observed at 531.6 eV and 533 eV, which are related to C=O and C-OH of GO, respectively.60,61 This comes in agreement with Raman analysis, since the sample R3 showed the highest intensity of D and G bands. For TiO2-Cu composite nanofibers, three main peaks were recorded. The first peak at 932.5 eV is related to Cu 2p3/2 of Cu0, the second peak appeared at 952.4 eV, which is attributed to Cu 2p 5/2 of Cu0, whereas for the sample R3 a small peak was observed at 934.5 eV that may indicate the presence of small amount of Cu2+ species.62


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Figure 4. High resolution XPS scans of (a) Ti 2p, (b) C 1s, (c) O 1s, and (d) Cu 2p. To further confirm the formation of zero-valent Cu nanoparticles, electron paramagnetic resonance (EPR) measurements were performed, Figure 5a. The observed peak at g = 2.008 indicates the presence of carbon centered radicals. For the TiO2-Cu composite, the intensity of the peak was very weak, which can be related to larger amount of the polymer undergoing an oxidative reaction through electron transfer to copper atoms. This also explains the stabilization of copper metal at zero oxidation state.63,64 FTIR analysis was performed to study the chemical structure and bonding of the fabricated nanofibers, Figure 5b. The bands observed at 594 cm -1, 3419 cm-1, 1300 cm-1, and 1700 cm-1 are characteristic of the Ti-O-Ti bond,65 the stretching vibration of the


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hydroxyl groups OH– group, 2,65–67 the C-N vibration,2,67 and the C=O. The broad bands at 1400 and 1230 cm-1 are assigned to the deformation of O–H vibration from C–O and C–OH stretching vibrations in C–OH.68 The peak at 850 cm-1 is characteristic of the epoxide groups,56,69 and the peak at 1560 cm-1 is related to the skeletal vibration of graphene sheets. 56,70 Note the absence of the characteristic peaks of PVP,70 indicating no PVP residues.

Figure 5. (a) EPR spectra of TiO2 nanofibers (R1) and TiO2-Cu composite nanofibers (R4). (b) FTIR spectra of the fabricated nanofibers with different copper loadings. To test the functionality of the fabricated nanofibers, their photocatalytic performance to split water was tested under irradiation with a 300 W xenon arc lamp using a solution of 1M KOH. Figure 6 shows the amount of H2 obtained upon testing the fabricated nanofibers. For TiO 2 nanofibers, the H2 yield was ≈1150 µmol g−1, which was increased with increasing the copper content and reaches the highest H2 yield of 5110 µmol g−1 for the sample containing the highest copper concentration. This could be due to the plasmonic effect of the zero-valent copper nanoparticles, which results in an increase in the density of the generated charge carriers, along with the enhanced charge separation due to the heterojunction formed at the interface between


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copper and TiO2. It is worth to note that these values of hydrogen yield were obtained in absence of hole scavenger species in the electrolyte, making the fabricated TiO 2-Cu composite nanofibers superior in their performance compared to the results reported in previous studies. For example, the catalytic activity of TiO2 nanotube arrays decorated with Cu nanoparticles only showed 15 µmol cm-2.71 To test the stability and long-term photoactivity of the fabricated nanofibers, they were tested over 3 consecutive cycles with no observed decrease in the amount of hydrogen generated, indicating the good durability of the fabricated materials and the importance of the insitu formation of rGO in stabilizing the Cu nanoparticles.

Figure 6. Photocatalytic hydrogen production performance of the fabricated nanofibers.


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To provide an in-depth theoretical understanding of the facile splitting of H 2O on the TiO2 (101) supported with the copper cluster, DFT calculations were performed to understand the fundamental mechanism of the catalytic activity. This investigation is based mainly on four characteristics; the structure, charge distribution, electron density, and reaction energy. After H 2O molecule adsorption, the H2O antibonding orbitals hybridize with the 3d orbital of the metal, leading to an increase of both O-H bond length and H-O-H angle of the H2O molecule. The more the increase in the bond length, the more the interaction between H 2O and the slab is, which plays a vital role in the splitting of the H2O molecule. To get more insights into the electronic structure of the H2O adsorbed on the surface, the spin-polarized charge density difference and the partial density of states (PDOS) were calculated, Figure 7. When H2O molecule is adsorbed on the surface, the interaction between the metal 3d orbitals and the H2O molecule involves effective charge transfer, where the H2O antibonding orbitals are gradually filled via hybridization with the metal 3d orbital. Consequently, the molecular orbitals of the H-O bonds of H2O are depressed and shifted to lower energy compared with free H2O molecule.72 Thus, there is a charge transfer from H2O molecule to the surface that can be estimated from the charge density difference distribution as an electron population on the H2O-metal atom bond, along with a decrease in the charge density on the H2O molecule. Comparing the PDOS of H 2O molecule adsorbed on pure TiO2 surface (H2O@(101)) and that adsorbed on copper-supported TiO2 surface (H2O@Cu/(101)), the copper 3d electrons filled the H2O antibonding orbital more than with the pure surface, indicating that copper atoms introduce higher d-bond centers. Thus, they provide more orbital overlap with the antibonding orbital of H2O and the O-H bond orbitals are weakened. As a result, on Cu/(101) surface, the H2O molecule structure is more stretched and the surface is more depleted from the oxygen atoms of the adsorbed H2O compared to bonding to Ti atom in pure TiO2 (101) surface.


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Also, the Mulliken atomic population analysis shows that H2O-Ti bond population is 0.08 while that of the H2O-Cu bond it 0.1, confirming the higher rate of charge transfer from H 2O molecule to the Cu-modified surface than that to the pure TiO 2 surface.

Figure 7. Partial and total density of states of (a) Free H 2O, (b) H2O@(101), (c) H2O@Cu/(101). The blue areas show where the electron density has been enriched with respect to the fragments and the yellow areas show where the density has been depleted.

From energy point of view, Figure 8 shows a schematic representation of the energy diagram for H2O splitting, where the adsorption energy (Eads) of adsorbate (X= H2O & OH) was calculated using Eq. 2. Note that the more negative Eads indicates stronger and stable adsorption. For the splitting of H2O molecule, the first H atom of the H2O molecule needs to become a free H atom by overcoming certain binding energy, which can be defined by Eq. 3. The more the surface can weaken the binding strength between the H and OH, the more effective it is for water splitting.72,73 As can be seen from Figure 8, the existence of copper cluster on the surface


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significantly decreases the binding strength between H and OH and increased the OH adsorption energy compared to the surface without Cu, which indicates more facile splitting of the H2O molecule. Eads = E(X/slab) – E(X) - E(slab)

(2)

Ebinding = EH2O/slab – EOH/slab – EH = -Esplit

(3)

Figure 8. Adsorption and splitting energy diagram of H2O on (101) and Cu/(101) surfaces. CONCLUSION In this study, TiO2 and TiO2-Cu composite nanofibers were fabricated via a facile one-step method. The morphological, structural, optical and photocatalytic properties of the materials were investigated. Annealing at different temperatures was found to affect the crystallinity of the fabricated materials, where copper nanoparticles were found to stabilize the anatase phase of TiO 2.


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The photocatalytic activity of the fabricated nanofibers to split water was also tested. Metallic copper nanoparticles were stabilized by the carbon present in the nanofiber through the formation of rGO. The presence of rGO shifted the absorption to ≈800 nm. For Ti-Cu nanofibers, there was a 344% increase in the hydrogen yield compared to that of TiO2 nanofibers. This enhancement is attributed to the efficient charge separation due to the presence of copper and rGO. These findings were also confirmed via DFT calculations, indicating the enhanced charge transfer. The existence of copper cluster on the surface was also found to significantly weaken the binding strength between H and OH and increase the OH adsorption energy compared to the surface without Cu. This study introduces a new approach for stabilizing metallic copper nanoparticles and enhancing the efficiency of TiO2 as a photocatalyst for water splitting. AUTHOR INFORMATION Corresponding Author *Author to whom correspondence should be addressed: [email protected] (N.K.A.). Funding Sources Egyptian Academy of Scientific Research and Technology (ASRT) under JESOR grant.

ACKNOWLEDGMENT The authors acknowledge the financial support by the Egyptian Academy of Scientific Research and Technology (ASRT) under JESOR grant. The authors also acknowledge the access to the computational facilities provided by Bibliotheca Alexandrina.


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REFERENCES [1] Brumbaugh, A.

D.;

Cohen,

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Table of Contents Graphic

Graphene protects Cu from oxidation and enhances the photocatalytic water oxidation performance of TiO2-Cu nanofibers in scavengers-free electrolytes


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In-situ formation of graphene stabilizes the zero-valent copper

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