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Article

Synthesis of Superior Visible-Light-Driven Nanophotocatalyst Using High Surface Area TiO2 Nanoparticles Decorated with CuxO Particles

1
King Abdulaziz City for Science and Technology, P.O. Box 6086, Riyadh 11442, Saudi Arabia
2
College of Science, King Khalid Military Academy, P.O. Box 22140, Riyadh 11495, Saudi Arabia
3
Advanced Materials Processing and Analysis Center (AMPAC), Nanoscience Technology Center (NSTC), Materials Science and Engineering (MSE) Department, University of Central Florida, Orlando, FL 32816, USA
4
Chemistry Department, College of Science, Al-Imam Muhammad ibn Saud Islamic University, Riyadh 11432, Saudi Arabia
5
Chemistry Department, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
*
Author to whom correspondence should be addressed.
Catalysts 2020, 10(8), 872; https://doi.org/10.3390/catal10080872
Received: 27 May 2020 / Revised: 19 July 2020 / Accepted: 20 July 2020 / Published: 4 August 2020

Abstract

:
This work provides an alternate unique simple methodology to design and synthesize chemically modified nanophotocatalyst based on high surface area TiO2 nanoparticles that can be used efficiently for the photodegradation of organic pollutants under normal visible light rather than complicated UV irradiation. In this study, dual visible light and UV-driven nanophotocatalysts were synthesized via wet chemistry procedures using high surface area TiO2 nanoparticles functionalized with (3-Aminopropyl) trimethoxysilane and attached chemically to the CuXO to improve the charge separation and maintain the non-charge recombination. The successful modification of the TiO2 nanoparticles and the formation of the TiO2-NH2-CuxO nanophotocatalyst were confirmed using different characterization techniques, and the results revealed the synthesis of high surface area TiO2 nanoparticles, and their chemical modification with an amino group and further decoration with copper to produce TiO2-NH2-CuxO nanophotocatalyst. The photocatalytic activity of TiO2 and TiO2-NH2-CuxO nanophotocatalyst were evaluated using methylene blue (MB) dye; as an example of organic pollutants. The resulting TiO2-NH2-CuxO nanophotocatalyst exhibited superior photocatalytic activity for the degradation of MB dye under visible light irradiation, due to the reduction in the energy bandgap. The degradation of the MB dye using the TiO2-NH2-CuxO nanophotocatalyst was investigated using LC-MS, and the results revealed that the hydroxyl free radical is mainly responsible for the cleavage and the degradation of the MB dye.

Graphical Abstract

1. Introduction

The use of TiO2 as the first choice photocatalyst is well known [1], due to many reasons such as low toxicity, availability, cost-effectiveness, photostability [2], as well as the small energy band gap; 3.2 eV. Many comprehensive studies focus on the synthesis of different phases, porosity, and morphology of the TiO2 and TiO2-based materials, which showed that the material porosity plays an important role in finding the active surface area of the photocatalyst during reactions [3,4,5]. The photocatalysis of TiO2 and TiO2-based materials extends to hydrogen production through water splitting [6,7], photoelectrochemistry [8,9], dye-sensitized solar energy conversion [10,11], as well as air and water photocatalytic treatments [12,13]. Moreover, the various structures of TiO2, ranging from nano to microparticles, agglomerates, and single-crystal colloidal particles, exhibited novel photocatalytic degradation efficacy for different types of inorganic and organic pollutants; through oxidation and reduction pathways [14,15].
The anatase phase of the TiO2 has proved superior photocatalytic performance for the disintegration of organic substances in air and water under UV irradiation (wavelength <387 nm) [16]. However, less than 5% of the sunlight falls in the mentioned UV range and the rest of the solar spectrum falls in the visible and infrared range [17]. Hence, it is highly important to develop an efficient photocatalytic material that can work in dual wavelengths (both UV and visible). Making a new solid complex with unique properties to tune the bandgap of TiO2 to permit the absorption of the visible region of the electromagnetic spectrum for photoexcitation is one of the challenging issues in materials science which requires TiO2 to be inlaid with a noble metal [18,19,20].
Also, many research studies showed that non-noble metal such as copper ions can be chemically inlaid with TiO2 nanoparticles and leads to shifting the bandgap and permit the absorption of the visible region of the electromagnetic spectrum. For example, a series of Cu doped titania were synthesized by water-in-oil microemulsion method for the enhancement of photocatalytic evolution of H2 from H2O-methanol/glycerol mixtures in both sunlight and UV–visible irradiation [21]. In another study, Cu–TiO2 nanorod composites were synthesized by microwave-assisted sol–gel method and used for enhanced photocatalytic degradation of bisphenol A (BPA) in the presence of UV and visible lights [22]. Also, copper that was used for the doping of TiO2 semiconductor photocatalyst was prepared via complex precipitation and wet impregnation methods for enhancement of hydrogen generation under UV and visible light irradiation were investigated [23]. Moreover, hybrid nanocomposites of Cu2O/TiO2 nano-powders (NPs) were prepared via solid state reaction followed by 20 h of ball milling and used for the photocatalytic degradation of methylene blue (MB) under both ultraviolet and visible light irradiations [24].
The present study hypothesizes that the surface modification of TiO2 nanoparticles with amine-functional groups and their chemical attachment to Cu ions can permit TiO2 nanoparticles to absorb the visible region of the electromagnetic spectrum and overcome the charge recombination problem. Accordingly, the objective of the present work is to synthesize high surface area TiO2 nanoparticles by maintaining the pore structure, and functionalize them with an amino silane coupling agent and chelate the CuxO as a co-catalyst to improve the efficiency and benefit from both UV and visible wavelengths for the degradation of MB dye; as an example of organic pollutants. The benefit of silane coupling agent surface functionalization is to inoculate carbon and nitrogen and improve the distribution of copper particles on the surface of TiO2, as well as to maintain the non-recombination of the electrons. The effect of the synthesis parameters and surface modification on the structural, morphological, chemical, and photocatalytic properties of the TiO2 nanoparticles are reported in this study.

2. Results and Discussion

2.1. Characterization of the Prepared and Modified TiO2 Nanoparticles

The photocatalytic material development process and degradation for MB dye by TiO2-NH2-CuxO nanophotocatalyst is illustrated in Figure 1. The preparation method includes three main steps: development of porous TiO2 nanoparticles as a core material, modification of the surface of TiO2 nanoparticles, and the bonding of amine groups to CuxO which resulted in a homogeneous distribution of CuxO on the TiO2 surface to improve the photocatalytic degradation process.
The crystallinity phase of the nanophotocatalyst was studied using powder X-ray diffraction, and the results in Figure 2a shows the XRD diffractograms of TiO2, TiO2-NH2, and TiO2-NH2-CuxO nanophotocatalysts. The peaks revealed at 2 theta values are 25.26 (101), 37.90 (004), 47.91 (200), 54.63 (105), 62.21 (213), 69.21 (116), 75.015 (215), and 82.1 (303) represents the existence of the anatase phase TiO2 according to the Joint Committee on Powder Diffraction Standards (JCPDS) card no. 21-1272. The X-ray diffraction (XRD) analysis demonstrates that the chemical modification of the TiO2 nanoparticles did not change the anatase crystalline phase. This observation indicates that the surface modification and addition of copper did not change the TiO2 crystalline phase, which could be due to the low concentration of copper (1.8 at %), and/or the phase intensity of TiO2 and/or the high homogenous dispersion of copper on the TiO2 surface. The calculated crystallite size of TiO2, TiO2-NH2, and TiO2-NH2-CuxO nanophotocatalysts was about 8 ± 1 nm. No change in crystallite size was observed before and after surface modification. This is could be due to the significant interaction between copper and TiO2 via the amine bonding that formed the copper imine groups and hydroxyl groups on TiO2 surface, and this interaction controls or prevents the further growth of crystallization [25]. The crystallite size in TiO2, TiO2-NH2, and TiO2-NH2-CuxO nanophotocatalysts was measured according to the Scherrer Equation
d = kλ/βcosθ,
where k is the Scherrer constant, λ is the wavelength (Cu; 1.5406), β is the line broadening of the XRD peak, and θ is the diffraction angle. The FWHM for the prepared TiO2 nanoparticles was 1.332, and the crystallite size was 6.79 nm. However, when the TiO2 was modified with 3-aminopropyltrimethoxysilane to form TiO2-NH2, the FWHM reduced to 1.135 and the crystallite size increased to 7.97 nm, while when the copper was attached to the surface to form TiO2-NH2-CuxO nanophotocatalyst, the FWHM was 1.177 and the crystallite size 8.92 nm. This variation is owing to the surface modification process as both TiO2-NH2 and TiO2-NH2-CuxO nanophotocatalysts show the increment in crystallite size of ~1 nm. Additionally, the Cu diffraction peaks are not observed in the diffraction pattern due to the low copper content of 1.8 at %, as well as the phase intensity of the TiO2, or due to the similarity in the radius of Ti4+ and Cu2+ (0.068 and 0.073 nm).
For further understanding of the surface chemical modification and the formation of TiO2-NH2-CuxO nanophotocatalyst, Fourier-transform infrared spectroscopy (FTIR) spectra were recorded on freshly synthesized, surface modified, and CuxO added TiO2 nanoparticles (Figure 2b). The peaks of TiO-Ti for the anatase phase were observed at 752 and 1450 cm−1. The peaks at 1650 and 3410 cm−1 are attributed to the OH stretching vibration of the Titania [26]. In the case of TiO2-NH2, several additional peaks are observed for CH2, Si–O from the silane coupling agent, and N–H along with the TiO2 peaks. In detail, the peaks at 2980 and 2888 cm−1 are attributed to CH2 in the silane coupling agent and the peak at 1040 cm−1 is ascribed to Si–O from the silane coupling agent [27]. The peaks at 1550, 3039, and 3450 cm−1 are attributed to N–H. In the case of TiO2-NH2-CuxO nanophotocatalyst, the copper ions attached to the amine on the TiO2 surface have peaks at 756 and 793 cm−1 which belong to CH2 and the peaks at 1047 and 1080 cm−1 are attributed to Si–O. The peak at 1410 cm−1 is ascribed to the OH group. The peaks at 1438, 2923, and 2970 cm−1 are ascribed to CH2 alkanes stretching. The observed bands at 3100 and 3410 cm−1 belong to C–H and N–H, respectively [28]. These results confirm the modification of TiO2 with the silane coupling agent [29]. Furthermore, when the copper was attached to the TiO2 surface, the shift of some bands was observed, such as the band at 649 cm−1, which was shifted to 643 cm−1, the band at 1400 cm−1 was shifted to 1410 cm−1, and the band at around 1609 cm−1, which belongs to N–H, was hidden due to the copper mask. These results showed that the chemical attachment of amine to the TiO2 occurred and the interfacing between the amine group and CuxO succeeded. In addition to FTIR, micro-Raman spectroscopy was also applied to understand the molecular level of bonding in the chemically modified material, as shown in Figure 2c. The peaks at 144, 197, 399, 515, 519, and 639 cm−1 are ascribed to the symmetric anatase phase of TiO2 particles [30]. The band at 1050 cm−1 in TiO2-NH2 and TiO2-NH2-CuxO nanophotocatalyst is attributed to C–C aliphatic chain vibrations, the band at 1380 cm−1 is attributed to CH3. Both FTIR and micro-Raman results confirm the modification of the TiO2 surface with the amine silane coupling agent.
The Brunauer, Emmett and Teller (BET) specific surface area and pore size of the TiO2 and TiO2-NH2-CuxO samples were examined using nitrogen adsorption–desorption isotherms at 77 K, and the calculated values are shown in Table 1. The results revealed that the specific surface area of the as-synthesized TiO2 nanoparticle was 337 m2g−1 which is comparatively higher than other reported TiO2 nanoparticles [31]. The isotherm of TiO2 nanoparticles is illustrated in Figure 3a and shows the multilayer adsorption pursue capillary condensation consequential type IV isotherm, which confirms the mesoporous phase of the synthesized materials. Also, the measured pore volume and pore diameter of the TiO2 material was 0.322 cm3g−1 and 4.77 nm, respectively. Furthermore, the distribution of the pore size in Figure 3a (inset) showed a very narrow distribution which indicates a uniform pore distribution. On the other hand, the surface area of TiO2 nanoparticles decreased from 337 m2g−1 to 97 m2g−1 after surface modification with an amine (TiO2-NH2), and further attachment to CuxO (TiO2-NH2-CuxO). The reduction in surface area was mainly due to the deposition of surface functional groups, and attachment of CuxO on the surface of the TiO2 nanoparticles which could cause a barrier and obstacles to the N2 molecules entering through the pores and cause a non-uniform pore distribution, as shown in Figure 3b.
The surface charge of the TiO2, TiO2-NH2, and TiO2-NH2-CuxO nanophotocatalysts were investigated by performing the zeta potential measurements and the results were presented in Figure 4a. It is very clear that upon the attachment of the CuxO to the amine group of the TiO2, the potential was almost equally shifted to the negative side due to the positive charge of copper, which was also confirmed by measuring the phase over time, as shown in Figure 4b. The higher surface charge was measured for TiO2 and TiO2-NH2 which indicates the stability due to the charge distribution on the particle surface, such that the negatively charged particle surfaces repulse each other in the solution. However, when the CuxO was attached (TiO2-NH2-CuxO) to the amine group, there was a possibility of a hydroxyl group formation on the surface upon dispersion in water, which led to a negative shift in the Zeta potential [32,33]. In all the cases, the nanoparticles were highly suspended in water which is impartment to have direct contact between the photocatalyst and pollutants for greater efficiency.
The surface morphological features and elemental analysis of TiO2-NH2-CuxO nanophotocatalyst were explored using high resolution scanning electron microscope- Energy dispersive X-ray spectroscopy (HR-SEM/EDX) and the results are shown in Figure 5. The HR-SEM image showed a spherical particle of TiO2 in the nanometer range as shown in Figure 5a. The EDX analysis in Figure 5b revealed the distinctive peaks for Ti (Lα 0.45 and, Kα 4.5 KeV), O (Kα 0.53 KeV), Cu (Lα 0.9 and Kα 8.1 KeV), C (Kα, 0.28 KeV), and Si (Kα, 1.8 KeV). The measured weight and atomic percentage of copper are 4.42 and 1.80, respectively. These values were expected as the copper was in the form of copper oxide. To understand the distribution of the elements located on the surface of TiO2-NH2-CuxO nanophotocatalyst, EDX-elemental mapping was performed, and the results are shown in Figure 5c–f. The distribution of copper was proportional to the rest of the elements and the contrast of the color demonstrates that the distribution of copper was uniform on the titanium surface.
Further investigation on the morphological structure was performed on the TiO2, and TiO2- NH2-CuxO nanophotocatalyst using HR-TEM, and selected area electron diffraction (SAED) as shown in Figure 6. The HR-TEM images of TiO2 (a) and its SAED pattern Figure 6b in comparison with TiO2-NH2-CuxO nanophotocatalyst SAED pattern confirmed the formation of additional crystalline structures beside the TiO2 crystalline structure, as compared between Figure 6b and d. The additional diffraction pattern confirms the (1 1 1) and (2 2 0) phases which could be assigned to copper oxide. The intensity of the diffraction pattern Debye rings in Figure 6d was clearly due to the loading of CuxO on the surface, which is consistent with the EDX results.
The thermal behavior of the TiO2, TiO2-NH2, and TiO2-NH2-CuxO nanophotocatalysts was explored and the thermogravimetric analysis curves are shown in Figure 7. The curves revealed that TiO2 nanoparticles show three main areas of weight loss and the total weight loss was 12.4%. The first area of weight loss from 25 °C to 110 °C was due to the evaporation of solvent and water molecules physically adsorbed on the TiO2 nanoparticles. The second weight loss from 110 °C to 298 °C is owed to the removal of the chemically bonded water molecules [34,35]. The weight loss from 298 °C to 850 °C could be attributed to the condensation of hydroxyl groups on the surface of TiO2 nanoparticles. On the other hand, the Thermogravimetric analysis (TGA) curve of TiO2-NH2 shows a total weight loss of 19.4%, which is significantly higher compared to the as-synthesized TiO2 mainly due to the amine loading at the surface of the TiO2 nanoparticles upon modification. This is supported by the evidence of the higher weight loss from 462 °C to 850 °C (6.3%), which was attributed to the combustion of organic materials. The TiO2-NH2-CuxO nanophotocatalyst TGA curve shows many weight loss steps, and the total weight loss was 13.61%, less than the TiO2-NH2 nanoparticles. The less weight loss could be attributed to the presence of CuxO which improves the heat stability [36].
The chemical analysis of TiO2 and TiO2-NH2-CuxO nanophotocatalyst using X-ray photoelectron spectroscopy (XPS) which was performed to identify the chemical bonding relevant to the surface modification and the oxidation state of the copper attached to the TiO2-NH2-CuxO nanophotocatalyst. The recorded XPS spectra are fitted using peak fit software to deconvolute the peaks and the results are shown in Figure 8.
The deconvolute Ti2p and O1s envelop of TiO2 indicate that the synthesized material does not have any contamination before the surface functionalization and the presence of C1s in pure TiO2 is due to the surface adsorbed atmospheric carbon. If the C1s peaks are compared for pure TiO2 and TiO2-NH2-CuxO, we can observe the clear changes in the peak position of deconvoluted peaks. The peaks position of C–C, C–O–C, and O–C=O for pure TiO2 are 284.46, 286.20, and 288.34 eV, respectively. In the case of TiO2-NH2-CuxO, the peaks of C–Si, C–C, C–N, C–O–C, and O–C=O are observed at 282.09, 284.42, 285.71, 286.89, and 288.68 eV, respectively The peaks at the higher binding energies (C–O–C and O–C=O) increased by 0.48 eV and the peak broadening of the same indicates the improvement of electronegativity due to the surface modification of (3-aminopropyl) trimethoxysilane. This result suggests that the electronegativity of TiO2-NH2-CuxO is stronger than that of pure TiO2. This may be attributed to the electrostatic interaction between TiO2 and (3-aminopropyl) trimethoxysilane, suggesting that surface modification is not a simple physical mixture. Cu2p3 envelop in TiO2-NH2-CuxO shows the peaks at 234.9 eV and 233.2 eV indicate the presence of Cu2+ and Cu1+, respectively [36,37].
The relative concentration of each cation state was calculated using the Equation
[Cu x = ∑IA of Cu x/∑IA of Cu],
where Cu x is the Cu atomic concentration of the cation state x, IA of Cu {x} is the integrated area of the corresponding state x, and IA of Cu is the total integrated area of Cu peaks. Upon quantifying the percentage of two Cu states from the XPS spectra, it was found that the percentage of Cu2+ and Cu1+ was 81.91% and 18.09%, respectively. The capability of copper to donate and accept electrons explains its important character in oxidation–reduction reactions [26]. Therefore, the presence of two valences of oxidation states (Cu1+ and Cu2+) could enhance photocatalytic activity due to the possibility of mid-level state formation [38]. N1s spectrum of TiO2-NH2-CuxO shows two peaks centered at 399.2 eV and 400.1 eV. The lower binding energy at 399.2 eV belongs to N–C because the amine groups were chemically attached to the surface of the nanoparticles. The metal cations that combine with amine groups most likely appear at the higher binding energy [39], and according to the structural representation, Cu cations were connected to the amine group. Therefore, it can be concluded that the higher binding energy peak was attributed to N–Cu bonding.
The recorded O1s peak of TiO2 and TiO2-NH2-CuxO is broad with multiple overlapping components as shown in Figure 8. The first peak centered at 529.5 eV marked for the samples belong to O2 (metal oxide) and the second peak, marked around 531.7 eV, may be attributed to the OH species that were mostly present on the surface. The higher binding energy peak centered at around 532.6 eV belongs to surface adsorbed moisture; however, Si–O boding also appears in the same peak position. Hence, it is hard to differentiate the chemically attached Si-O bonding in surface modified TiO2 nanoparticles. Besides, the Si 2p3 peak in TiO2-NH2-CuxO shows the Si–O bonding which confirms the presence of Si.
The XPS spectra of Ti 2p in TiO2-NH2-CuxO displayed two peaks centered at 458.7 eV (2p3/2), and 464.5 eV (2p1/2) which are the predominant state of Ti4+. An additional small peak centered at 460 eV belongs to Ti–O–Si, according to the literature [40], which also confirms the chemical attachment of Si with TiO2 nanoparticles.
UV–vis diffuse reflectance spectra were measured for TiO2 and TiO2-NH2-CuxO nanophotocatalyst to study the optical absorption properties of the samples, as shown in Figure 9a. It is apparent that when Cu chemically attached to the TiO2 via amine complexation the phenomena of redshift are observed in the wavelength which led to a decrease in frequency. Accordingly, this led to a decrease in the value of bandgap energy (Eg). To obtain adequate information about the bandgap energy due to the addition of copper; the direct and indirect band gaps were calculated to infer from the relationship shown in the Equations (3) and (4), respectively.
αhν = A ( − Eg )2,
αhν = A ( − Eg )1/2,
where α is the absorption coefficient, () is the photon energy and A is absorbance. Tauc plots were performed by drawing the (αhν)n vs. Photon energy (eV) and the direct and indirect bandgaps for both TiO2 and TiO2-NH2-CuxO nanophotocatalyst were calculated as it is presented in Figure 9b. Accordingly, the direct bandgap energy for TiO2 nanoparticles was 3.24 eV, whereas the indirect bandgap was 3.10 eV, which is consistent with other obtained results [41]. Moreover, when the TiO2 chemically decorated with Cu the direct bandgap was shifted to 2.98 eV, as well as the indirect bandgap to 2.10 eV. The bandgap for TiO2 is around 3.2 eV for the anatase phase but the observed variation in this study may be due to a difference in pore size and high surface area [42]. Also, it can be seen that the modification of TiO2 has an impact on the absorption range as it shifts to the visible wavelength after copper is attached to the TiO2 surface. The shift in the bandgap towards lower energy is due to the chemically attached Cu on the TiO2 surface which reduces the electron recombination, thus, the difference in bandgap energy between TiO2 and TiO2-NH2-CuxO nanophotocatalyst is (0.58 eV). This result suggests that the modification of TiO2 anatase phase (nanophotocatalyst) allows TiO2 to absorb visible light and indicates that the presence of copper on the surface of the catalyst shifted the bandgap to the visible light (1.6–3.2 eV) which greatly enhanced the degradation of the MB dye molecule in visible light rather than UV light.

2.2. Photocatalytic Assessment

The photocatalytic degradation of the MB dye by the TiO2-NH2-CuxO nanophotocatalyst at different pH was investigated. As shown in Figure 10a, the photocatalytic degradation of the MB dye was greatly enhanced by increasing the solution pH value and the faster degradation rate was observed above pH 7. The maximum degradation was obtained at pH 9, which is mainly due to the generation of hydroxyl radicals (·OH) at a pH higher than 7. At pH lower than 7, there was a decrease in degradation effectiveness due to the coulombic repulsion. Indeed, the surface charge of the TiO2-NH2-CuxO nanophotocatalyst changes, based on the pH variation, and so changes the catalyst reaction potential [43]. By this potential variation, the interaction between the MB dye and surface of the TiO2-NH2-CuxO nanophotocatalysts vary and results in a difference of reaction velocity. Under acidic conditions, the TiO2-NH2-CuxO nanophotocatalyst surface could be pseudo-photon charged whereas, in alkaline conditions, the surface could be negatively charged. In alkaline media, the TiO2-NH2-CuxO nanophotocatalyst’s surface is negatively charged and the cationic MB can be simply adsorbed, consequently, the hydroxyl radicals generated from the surface of the TiO2-NH2-CuxO nanophotocatalyst help to degrade MB dye in the solution. On the other hand, in acid media, the adsorption of the cationic MB dye on the surface of the TiO2-NH2-CuxO nanophotocatalyst is reduced due to the electrostatic repulsion, thus, reduces the photocatalytic degradation rate. This is consistent with another study and indicates that the acid–base property of the synthesized TiO2-NH2-CuxO nanophotocatalyst surface has a significant role in the photocatalytic degradation activity by changing the pH. Therefore, detailed knowledge of the material’s surface properties are highly important to understand and predict their photocatalytic efficiency.
The effect of ionic strength is considered an important factor especially in highly saline water [44]. Therefore, the ionic strength at different ionic concentrations was also investigated by changing the NaCl concentration (0.01, 0.1, and 1M) with/without a TiO2-NH2-CuxO nanophotocatalyst, as shown in Figure 10b. The results reveal that the ionic strength does not have a significant impact on the catalyst activity at the mentioned concentration. From the degradation rate, it can also observe that when a high concentration of NaCl (1M) was applied, the efficiency was only decreased by about 5%. Therefore, this catalyst can work effectively for the photocatalytic applications at high ionic strength and ionic concentration.
To assess the appropriate time and photocatalytic activity for the TiO2-NH2-CuxO nanophotocatalyst to decompose MB dye, an experiment was carried out and the results were presented in Figure 10c, which shows the absorption spectrum for the MB versus degradation time. The inset in Figure 10c shows the photo of the samples taken at specific times after the TiO2-NH2-CuxO nanophotocatalyst was removed from solution using centrifugation. It is obvious that, after 2 h contact time between the TiO2-NH2-CuxO nanophotocatalyst and the MB dye, more than 95% of the MB dye was degraded. We also observed a slight shift in the adsorption spectra which is due to the demethylation of MB occurred at the catalyst surface [45].
Separate experiments were carried out to compare the efficiency of the TiO2-NH2-CuxO nanophotocatalyst for the degradation of MB dye under ultraviolet and visible light and the results were presented in Figure 10d, which indicates that more than over 95% degradation of 40 µM methylene blue under visible light was obtained and the same phenomenon was also observed under ultraviolet light. These results proved the successful tuning of the electronic band of TiO2 toward visible light and showed the ability of the TiO2-NH2-CuxO nanophotocatalyst to work under both UV and visible light, which can show the maximum efficiency under sunlight, as the catalyst worked dually in UV and visible light. This is attributed to the reduction of the electron–hole recombination process by the addition of Cu to the surface of the TiO2 nanoparticles. On the other hand, the high surface area material resulted in higher contact with the MB dye molecules, and Cu attachments to the surface of the TiO2 nanoparticles reduce the electron–hole pair recombination. It is the combination of these two phenomena which improved the photocatalytic degradation efficiency in both UV and visible light.
It was reported that the charge recombination process in semiconducting materials takes place in femtoseconds or picoseconds, and electron trapping on the TiO2 surface occurs in a few femtoseconds, thus, the lifetime of electron–hole pairs is a few nanoseconds [39]. Thus, the rate of electron–hole recombination decreases as charge transfer occurs very fast on the TiO2-NH2-CuxO nanophotocatalyst surface. However, there are several other parameters such as particle size, defects, porosity, and impurities which can affect the electron–hole recombination rate. Therefore, the selection of suitable particle size with high surface area, proper chemical attachment agent and metallic ions are important for the reduction of electron–hole pair recombination and the improvement in photocatalytic reaction efficiency [46].
To assess the efficiency of TiO2-NH2-CuxO nanophotocatalyst for the maximum degradation of the MB dye, various amounts of catalyst (0–60 mg) in 40 µM solution of MB dye were irradiated under visible light for 120 min. A similar experiment was carried out with TiO2 NPs for the comparison and the maximum capacity of TiO2-NH2-CuxO nanophotocatalyst was calculated. Each experiment was repeated three times to get the standard deviation. As shown in Figure 10e, it can be observed that the photocatalyst can break down certain concentrations of the MB dye until it reaches equilibrium with the species in the solution. Also, it is clear that TiO2-NH2-CuxO nanophotocatalyst has the highest degradation capacity and it reaches the equilibrium using 30 mg of the catalyst with the mentioned concentration, the maximum limit of the catalytic degradation capacity was 1.5 mM g−1. This result confirms the high catalytic efficiency and the need for only small weight to conduct the photodegradation process.
The stability and reusability of the TiO2-NH2-CuxO nanophotocatalyst were studied and the results are shown in Figure 10f. The catalyst has maintained its strength after seven cycles of application and the percentage difference between C1 (high recovery) and C7 (low recovery) was less than 5%, which indicates that the TiO2-NH2-CuxO nanophotocatalyst can be recycled several times without losing the activity. Furthermore, there are many reports available on commercial P25 TiO2 material showing the lower photocatalytic degradation efficiency and not suitable for multiple recycling time [47,48,49]. In our case, the degradation efficiency decreased by less than 5% with seven recycling times.
The degradation of the MB dye using the TiO2-NH2-CuxO nanophotocatalyst under visible light was investigated using Liquid chromatography-mass spectrometry (LC-MS) to elucidate the degradation pathway. The chromatograms of the degraded compounds after 40 min and 60 min of irradiation are shown in Figure 11a,b, respectively. The TiO2-NH2-CuxO nanophotocatalyst can degrade MB dye in visible light through multiple steps as shown in Figure 12, which illustrated that the hydroxyl free radical reacted MB dye molecule. After that, the produced molecules start to cleave through the multi-steps due to the free radical’s attack, and it ends with colorless compounds (H2O, CO2, NO2, SO42−).
Mechanism: the mechanism of the photocatalytic activity of the synthesized materials is illustrated in Figure 13, which postulate that the TiO2 within the TiO2-NH2-CuxO nanophotocatalyst releases the electrons from the valence band by exposure to the light and the CuxO works as a reservoir to collect the electrons underneath the conduction band, and therefore minimizes the bandgap energy which makes the catalyst very effective in visible light. Also, it is postulated that the high surface area anatase phase TiO2 decorated with homogeneously distributed CuxO nanoparticles can cause a large surface area exposure to the MB dye and thus increase the photocatalytic degradation effectiveness. In general, the hydroxyl radical and peroxide ion can be initiated due to the existence of holes and electrons in the TiO2 nanoparticle solution bypassing the UV light. These energetic molecules destroy the organic compound through a series of chemical reactions as the potential of ·OH is 2.8 V, which is higher than other oxidant agents such as hypochlorous acid (1.49 V) [39]. Moreover, it is well known that TiO2 is more suitable only for UV light range due to its bandgap, and to make the TiO2 suitable for both the UV and vis ranges, a series of TiO2-NH2-CuxO nanophotocatalyst was developed to overcome this problem by shifting the energy band gap to the visible wavelength region.
The XPS characterization of the nanophotocatalyst showed that the Cu is present in two different oxidation states; Cu2+ which is dominant and Cu1+. It is believed that this changeable oxidation states in transition metals can improve the catalytic activity [50]. It was speculated that when the electron skips from the conduction band, the chance for electron recombination would be very low due to the series which creates charge carriers, electrons, and holes; therefore, a series of chemical reactions will begin to degrade the MB dye as shown in Figure 13.

3. Experimental Section

Materials: (3-Aminopropyl)trimethoxysilane, tetraethyl-titaniate (TEOT), hexadecyltrimethylammonium bromide (95%), hydrochloric acid, acetic acid, and methylene blue (CAS 61-73-4) were obtained from Sigma Aldrich (St. Louis, MO, USA), and copper sulfate was purchased from Merck, Feltham, (UK).
Preparation: Titanium dioxide (TiO2) was synthesized by a sol-gel method using a surfactant cetyltrimethylammonium bromide (CTAB) as an organic template. 25 g (0.11 mol) of TEOT (Ti(OC2H5)4) was dissolved in 250 mL of ethanol, and 6.3 mL of acetic acid (AcOH) (0.11 mol) was added to the reaction solution, while stirring, to control the hydrolysis and condensation reaction. Separately, 50 mL of CTAB 5.21 g, (0.0143 mol) in ethanol was prepared and added to the titanium reaction mixture. A clear and stable sol without any precipitation was obtained and stirred for 24 h at room temperature. Finally, a mixture of acidified water (0.1 M HCl) and ethanol (1:0.5) (125 mL) was added dropwise into the reaction solution. The solution was aged for 24 h and then dried in the oven at 40 °C for 24 h. Finally, the powder was transferred carefully to a Soxhlet reactor to remove the surfactant using organic solvents as follows: ethanol for 24 h, methanol for 24 h, methanol: isopropanol (1:1) for 48 h and then ethanol for 4 days. The powder was dried in vacuum oven at 120 °C for 10 h to be utilized for further studies. The surface modification was carried out using (3-Aminopropyl)trimethoxysilane. 3.0 g of high surface area TiO2 was washed with deionized water and methanol and dried at 110 °C for 6 h. The dried TiO2 powders were re-suspended in 250 mL toluene. The solution temperature was increased to 90 °C, and 3 mL of (3-Aminopropyl) trimethoxysilane was slowly injected. The reaction was left to proceed with stirring for 7 h under a nitrogen environment [26]. When the temperature was reduced to room temperature (25 °C), the modified TiO2 was separated using a high-speed centrifuge. The solid particles were collected and thoroughly rinsed with toluene and methanol before drying under vacuum at 25 °C.
The TiO2-NH2 particles were re-suspended in a 250 mL reaction flask with a solution of 0.1 mol L−1 copper (II) sulfate pentahydrate and stirred for 12 h at room temperature. The solution was then centrifuged to recover a bluish solid complex. The bluish solid complex was washed with deionized water several times until no absorption was detected in the UV–vis above 400 nm. The greenish solid product (TiO2-NH2-CuxO nanophotocatalyst) was then dried under vacuum at 70 °C before use.
Characterization: The IR measurement was recorded under vacuum optics, using a VERTEX 70v BRUKER spectrophotometer (Bruker, Ettlingen, Germany) with a resolution of 4 cm−1 and 64 scans. Energy dispersive X-ray spectroscopy (EDS) was performed using the X-ray detector integrated into the scanning electron microscope (JSM-7800F, JEOL, Akishima, Tokyo, Japan). The sample was prepared by sonicating 10 mg of the powder for 1 min in water then drops of the suspended particles were spread over the aluminum holder and left to dry under vacuum, then the holder was inserted to the SEM chamber. Phase characterization was carried out using the Bruker D8 Advance (Bruker, Karlsruhe, Germany), the source of X-ray was a 2.2 kW Cu anode and the running conditions were 40 kV and 40 mA. The copper oxide nanoparticles were observed and analyzed by transmission electron microscopy (TEM) JEOL JEM-2100F (JEOL, Akishima, Tokyo, Japan operating at 200 kV. The absorbance for the samples and band gaps were determined using diffuse reflectance spectra-UV-3600 from Shimadzu, Kyoto, Japan), and the method was applied as described elsewhere [28]. Micro-Raman analysis was achieved using a laser beam at 514 nm and a power of 40 mW (RENISHAW RM 100B). Zeta potential was accomplished using the Zetasizer Nano series (Malvern Panalytical, United Kingdom). X-ray photoelectron spectroscopy (XPS) (Physical Electronics, USA) analysis was achieved using a PHI 5400 ESCA system equipped with an aluminum anode operating at 15 kV and 300 W.
Photocatalytic assessment: To evaluate the efficiency of the TiO2-NH2-CuxO nanophotocatalyst, experiments were conducted in UV light using a Newport Model 66921 (Newport, Irvine, CA, USA), power 450–1000 W, and 600 W was selected with the temperature maintained at 25 °C using a chiller coupled complained with the system, taking into account the study of varying the pH and contact time. A simulation of the effect of visible rays of MB with and without the catalyst was carried out under sunlight using a UV/IR-Cut filter in a glass vial with stirring, and the temperature was controlled by placing the vials in a basin connected to a chiller to stabilize the temperature at 25 °C. An appropriate weight of the photocatalyst (TiO2-NH2-CuxO) (Ca 30 mg) was dispersed in a 20 mL vial that had 10 mL of (1 mM) of methylene blue in an aqueous solution, and the catalyst was kept suspended using stirring. After the appropriate time had passed, the suspension was centrifuged at 9000 rpm for 30 min and a clear solution was recovered and transferred to a UV–vis cuvette to measure the absorbance of the remaining MB dye.
The products of photocatalytic degradation of MB dye were analyzed using SHIMADZU LCMS-8040 combined with HPLC system (Shimadzu NEXERA X2) (Shimadzu, Kyoto, Japan. 5 µL of the sample was injected into the column (Shim-pack CF-ODS) and the mobile phase was performed using an acidified solvent (a) (ultrapure water, 0.1% acetic acid) and solvent (b) methanol, with the total flow rate being 0.5 mL min−1.

4. Conclusions

The proposed methodology outlined here provides a procedure for the synthesis of superior visible-light-driven Cu-containing surface-modified TiO2 nanophotocatalyst that could be used efficiently for the remediation of organic compounds polluted environment. First, a protocol for the synthesis of high surface area anatase phase TiO2 nanoparticle was optimized (337 m2 g−1), then TiO2 nanoparticles were chemically modified with amine functional groups using a silane coupling agent. The amine-modified TiO2 was chemically decorated with Cu which is a unique process for making a superior photocatalytic material that could be used under visible light. The results revealed the shifting of the modified TiO2 nanoparticles to lower energy band gap upon the attachment of copper which makes the innovative superior nanophotocatalyst work in both UV and visible light. The characterization results revealed the synthesis of high surface area TiO2 nanoparticles, and their chemical modification with an amino group and further decoration with copper to produce TiO2-NH2-CuxO nanophotocatalyst. The photocatalytic activity of TiO2 and TiO2-NH2-CuxO nanophotocatalyst were evaluated using methylene blue (MB) dye; as an example of organic pollutants. The resulting TiO2-NH2-CuxO nanophotocatalyst exhibit superior photocatalytic activity for the degradation of MB dye under visible light irradiation, due to the reduction in the energy band gap which allowed the photocatalyst to work efficiently under visible light. Also, the TiO2-NH2-CuxO nanophotocatalyst can be recycled several times without losing activity. The degradation scheme of MB dye by the TiO2-NH2-CuxO nanophotocatalyst under visible light was investigated using LC-MS, and the results revealed that the hydroxyl free radical is mainly responsible for the cleavage and the degradation of the MB dye.

Author Contributions

N.H.K.: Conceived the Idea, carried out the work, supervise the work, wrote the manuscript with involvement from all authors; W.S.A.: carried out the synthesis work and confirmed the synthesis method; T.S.S.: Help in carried out the photodegradation work and XPS data analysis; D.N.K.: Carried out the modification of TiO2 and reproduce the Figures; M.A.S.: Performed statistical work and review the work; S.A.: Help in supervises the project and review the work; S.I.A.-M.: Review the work helps in edited manuscript; S.S.: Managing and facilitate the lab work. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Acknowledgments

The authors express great appreciation to the King Abdulaziz City for Science and Technology (KACST) for the technical support and facilities provided during the research work period. Also, thanks to the Fulbright Grant Program for the Foreign Scholarship program ID: 68160653.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Overall process of photocatalytic material development and degradation of MB dye.
Figure 1. Overall process of photocatalytic material development and degradation of MB dye.
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Figure 2. (a) XRD pattern of pure TiO2, amine-modified TiO2 and Cu-amine-TiO2; (b) FTIR spectra of pure TiO2, amine-modified TiO2, and Cu-amine-TiO2; (c) Raman spectra for TiO2, TiO2-NH2; and TiO2-NH2-CuxO nanophotocatalyst.
Figure 2. (a) XRD pattern of pure TiO2, amine-modified TiO2 and Cu-amine-TiO2; (b) FTIR spectra of pure TiO2, amine-modified TiO2, and Cu-amine-TiO2; (c) Raman spectra for TiO2, TiO2-NH2; and TiO2-NH2-CuxO nanophotocatalyst.
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Figure 3. Adsorption–desorption isotherms for TiO2 (a) and TiO2-NH2-CuxO (b) impeded the pore size distribution diagram.
Figure 3. Adsorption–desorption isotherms for TiO2 (a) and TiO2-NH2-CuxO (b) impeded the pore size distribution diagram.
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Figure 4. (a) Zeta potential, and (b) phase over the time for TiO2, TiO2-NH2, and TiO2-NH2-Cu nanophotocatalyst.
Figure 4. (a) Zeta potential, and (b) phase over the time for TiO2, TiO2-NH2, and TiO2-NH2-Cu nanophotocatalyst.
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Figure 5. SEM (a) and EDX (b) of TiO2-NH2-CuxO nanophotocatalyst, (c) elemental details measured using EDX, and (dg) EDX elemental mapping of TiO2-NH2-CuxO nanophotocatalyst.
Figure 5. SEM (a) and EDX (b) of TiO2-NH2-CuxO nanophotocatalyst, (c) elemental details measured using EDX, and (dg) EDX elemental mapping of TiO2-NH2-CuxO nanophotocatalyst.
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Figure 6. HR-TEM images of TiO2 (a), and its diffraction pattern (b), HR-TEM of TiO2-NH2-CuxO nanophotocatalyst (c), and its diffraction pattern (d).
Figure 6. HR-TEM images of TiO2 (a), and its diffraction pattern (b), HR-TEM of TiO2-NH2-CuxO nanophotocatalyst (c), and its diffraction pattern (d).
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Figure 7. Thermogravimetric analyses of TiO2 (a), TiO2-NH2 (b), and TiO2-NH2-CuxO (c).
Figure 7. Thermogravimetric analyses of TiO2 (a), TiO2-NH2 (b), and TiO2-NH2-CuxO (c).
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Figure 8. X-ray photoelectron spectroscopy (XPS) for synthesized TiO2 and TiO2-NH2-CuxO nanophotocatalyst.
Figure 8. X-ray photoelectron spectroscopy (XPS) for synthesized TiO2 and TiO2-NH2-CuxO nanophotocatalyst.
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Figure 9. (a) Diffuse reflectance spectra for the TiO2 and TiO2-NH2-CuxO nanophotocatalyst. (b) the Tauc plots for TiO2 and TiO2-NH2-CuxO nanophotocatalysts for direct and indirect energy bandgap.
Figure 9. (a) Diffuse reflectance spectra for the TiO2 and TiO2-NH2-CuxO nanophotocatalyst. (b) the Tauc plots for TiO2 and TiO2-NH2-CuxO nanophotocatalysts for direct and indirect energy bandgap.
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Figure 10. (a) UV–vis absorption spectra of MB-TiO2-NH2-CuxO at pH = 3 (I); pH = 7 (II); and pH = 9 (III). (b) Effect of ionic strength on the MB degradation. (c) Degradation of MB using TiO2-NH2-CuxO nanophotocatalyst as a function of time (T1 = 0 min, T6 = 140 min). (d) Impact of irradiation time on the degradation of MB in UV and visible light. (e) Catalyst capacity. (f) Study of efficiency of the catalyst after reuse for six cycles.
Figure 10. (a) UV–vis absorption spectra of MB-TiO2-NH2-CuxO at pH = 3 (I); pH = 7 (II); and pH = 9 (III). (b) Effect of ionic strength on the MB degradation. (c) Degradation of MB using TiO2-NH2-CuxO nanophotocatalyst as a function of time (T1 = 0 min, T6 = 140 min). (d) Impact of irradiation time on the degradation of MB in UV and visible light. (e) Catalyst capacity. (f) Study of efficiency of the catalyst after reuse for six cycles.
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Figure 11. (a) Mass spectrum of the MB degradation using TiO2-NH2-CuxO nanophotocatalyst at 40 min and (b) after 60 min.
Figure 11. (a) Mass spectrum of the MB degradation using TiO2-NH2-CuxO nanophotocatalyst at 40 min and (b) after 60 min.
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Figure 12. Approach of methylene blue degraded and its products confirmed using GC-MS.
Figure 12. Approach of methylene blue degraded and its products confirmed using GC-MS.
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Figure 13. Mechanism of the catalyst influence for photodegradation of organic pollutant model.
Figure 13. Mechanism of the catalyst influence for photodegradation of organic pollutant model.
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Table 1. Measured surface area, pore size, and pore volume.
Table 1. Measured surface area, pore size, and pore volume.
SubstanceSurface Area,
(BET, m2/g)
Pore Volume
(cm3/g)
Average Pore Diameter, nm
TiO23370.3224.777
TiO2-NH2-CuxO970.1013.215

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Khdary, N.H.; Alkhuraiji, W.S.; Sakthivel, T.S.; Khdary, D.N.; Salam, M.A.; Alshihri, S.; Al-Mayman, S.I.; Seal, S. Synthesis of Superior Visible-Light-Driven Nanophotocatalyst Using High Surface Area TiO2 Nanoparticles Decorated with CuxO Particles. Catalysts 2020, 10, 872. https://doi.org/10.3390/catal10080872

AMA Style

Khdary NH, Alkhuraiji WS, Sakthivel TS, Khdary DN, Salam MA, Alshihri S, Al-Mayman SI, Seal S. Synthesis of Superior Visible-Light-Driven Nanophotocatalyst Using High Surface Area TiO2 Nanoparticles Decorated with CuxO Particles. Catalysts. 2020; 10(8):872. https://doi.org/10.3390/catal10080872

Chicago/Turabian Style

Khdary, Nezar H., Waleed S. Alkhuraiji, Tamil S. Sakthivel, Duaa N. Khdary, Mohamed Abdel Salam, Saeed Alshihri, Sulaiman I. Al-Mayman, and Sudipta Seal. 2020. "Synthesis of Superior Visible-Light-Driven Nanophotocatalyst Using High Surface Area TiO2 Nanoparticles Decorated with CuxO Particles" Catalysts 10, no. 8: 872. https://doi.org/10.3390/catal10080872

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