Role of NiO Nanoparticles in Enhancing Structure Properties of TiO₂ and Its Applications in Photodegradation and Hydrogen Evolution

Abstract

Pure and modified mesoporous TiO₂ nanoparticles with different loadings of NiO (3–20.0 wt %) were prepared through the surfactant-assisted sol–gel approach with the use of cetyltrimethylammonium bromide as a template. The optical and structural properties of different samples were examined using N₂ adsorption–desorption analysis, energy-dispersive spectroscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, UV–vis spectroscopy, Fourier transform infrared spectroscopy, and photoluminescence (PL) spectroscopy. X-ray diffraction results confirmed the insertion of Ni²⁺ into the lattice of TiO₂, and the crystallite size reduced remarkably after the addition of NiO. The diffuse reflectance spectroscopy spectra displayed obvious red shift in the absorption edges, and new absorption bands appeared in the visible region when NiO was added, which indicates the formation of surface defects and oxygen vacancies. The optical band gap of TiO₂ reduced sharply when the contents of NiO were increased. The increase in the surface defects as well as oxygen vacancies were examined using PL spectroscopy. The photocatalytic performance of the as-synthesized samples was investigated over photodegradation of brilliant green (BG) and phenol and hydrogen generation under visible light. 10% NiO/TiO₂ exhibited the highest photocatalytic efficiency. The photocatalytic activity was improved due to the creation of a p–n junction at the interface of NiO/TiO₂, which efficiently promotes the separation of photogenerated electron/hole pairs and consequently enhances its photodegradation activity. According to the photocatalytic activity results, NiO contents were considered one of the most important factors affecting the photodegradation of BG and phenol and H₂ evolution. Also, we discussed the mechanism of photodegradation, mineralization (total organic carbon), and photocatalytic reaction kinetics of BG and phenol.

1. Introduction

As one of the most promising well-known semiconductors, titanium dioxide (TiO₂) has attracted a great deal of interest in different promising applications such as solar energy conversion, hydrogen generation, the photocatalysis field, gas sensors, and so forth.¹⁻⁴ So far, titanium dioxide (TiO₂) has emerged as a promising photocatalyst because of cost-effectiveness, the lack of secondary pollutants, non-toxicity, photochemical stability, and high efficiency.^5,6 However, of these features, the rapid recombination rate of electron/hole pairs and the wide band gap potentially limit the high performance of TiO₂.^5,7 One of the most promising approaches to synthesize the mesoporous titanium oxide is the sol–gel method due to its excellent product yield, cost-effectiveness, simplicity, and low reaction temperature. There are numerous reports which used organic and inorganic salts in the sol–gel approach for preparation of metal oxides.⁸⁻¹² However, the particles were agglomerated in these approaches, which hinder the wide applications.^13,14 It is highly required to develop a surfactant-mediated synthesis method to overcome this drawback. A number of polymeric surfactants have been used in the sol–gel method.¹⁵⁻¹⁷

To alleviate these critical drawbacks, different techniques were established to reduce the band gap, increase the dissociation of photogenerated electron–hole pairs, and improve the optical absorption to expand into the visible light range,⁶ such as using suitable dopants and coupling with metallic and non-metallic materials,^18,19 noble metals,⁷ metal ion doping, and metal oxides.²⁰ Therefore, the formation of a p–n heterojunction is a promising method to narrow the band gap and hinder the recombination of electron/hole (e^–/h⁺) pairs, leading to prolong the lifetime of charge carrier and enhancing the photocatalytic activity of TiO₂.^5−7,21 The p–n junction was formed through coupling of two semiconductors together; one of them is p-type (hole-rich), while the other one is n-type (electron-rich). Therefore, the joining between the p-type metal oxide dopant and n-type TiO₂ leads to the formation of a p–n heterojunction.⁷

Because of its wide band gap (E_g = 3.5 eV), rapid hole mobility, and high charge carrier concentration and it can be effectively performed to generate an n–p junction with the n-type TiO₂ semiconductor,^22,23 nickel oxide (NiO) is considered one of the more suitable p-type semiconductors. This n–p junction leads to the formation of a space charge region at the interface during the transfers of hole and electron carriers to the n-type and p-type semiconductors, respectively. Thus, for charge equilibrium, an inner electric field is formed, which acts as a potential barrier and enhances the separation and lifetimes of the charge carriers.^5,6,24 Also, the photocatalytic activity of TiO₂ was improved after addition of NiO, attributed to many factors: the anatase/rutile ratio, Ti³⁺, oxygen vacancies, and surface defects. These factors depend on the amount of NiO, indicating that the amount of NiO is the major parameter that controls the enhancement of the photocatalytic activity of TiO₂.

Brilliant green (BG) is a very common dye that has a wide range of applications in modern textile, printed circuit boards, inks, biological staining, rubber and plastic industries, leather industries, and antiseptic preparation (Gram-positive bacteria).^25,26 BG has been reported to cause carcinogenicity, hypersensitivity, and toxicity to living organisms.^25,27 Phenols are also highly toxic organic compounds, and these compounds are found in wastewater coming from the chemical and petroleum industries.²⁸ Different techniques were investigated for dye removal from wastewater, such as biological treatment,²⁹ photodegradation,³⁰ coagulation–flocculation,³¹ and adsorption processes.^10,32−37 Photodegradation is efficient in the industrial wastewater treatment process with more success rates.³⁸

In this work, mesoporous TiO₂ and p–n heterostructure NiO/TiO₂ nanoparticles were synthesized, and then, we examined the effect of NiO content on the optical, structural, and photocatalytic properties over TiO₂ nanoparticles. The activities of the as-synthesized photocatalysts were studied by the photodegradation of BG and phenol and hydrogen evolution. Also, the relation between NiO content, oxygen vacancies, and photocatalytic activity was discussed. NiO/TiO₂ displayed excellent stability, reusability, and photocatalytic performance, suggesting its potential application as an effective photocatalyst.

2. Experimental Section

2.1. Preparation of NiO/TiO₂

Pure and modified mesoporous TiO₂ NPs with different loadings of NiO (3.0–20.0 wt %) were prepared through the surfactant-assisted sol–gel method using cetyltrimethylammonium bromide (CTAB) as a template, where titanium(IV) isopropoxide and Ni(CH₃CO)₂·4H₂O were used as precursors. In a typical preparation,²⁷ 2.0 g of CTAB was dissolved in 100 mL of ethanol under stirring for 3 h; after that, titanium(IV) isopropoxide (11.7 mL) was poured drop by drop to the above solution under constant stirring. The desired amounts of Ni(CH₃COO)₂·2H₂O were dissolved in ethanol and then added dropwise to the solution, and the solution was stirred by magnetic stirring at room temperature 3 h. After that, an ammonia solution (10 mL, 32%) was added to the mixture dropwise, and then, 10 mL of deionized H₂O was added. The solution was transferred and aged in air for 24 h. Then, the as-prepared samples were filtered and washed with distilled H₂O and anhydrous ethanol and dried at 80 °C. Finally, the obtained powders were calcined at 500 °C for 3 h. Mesoporous TiO₂ NPs were synthesized using the same procedure without addition of nickel acetate.

2.2. Characterization of Catalysts

The X-ray diffraction (XRD) pattern of the prepared samples was examined using a PW 150 (Philips) instrument with Cu Kα radiation. The crystallite size was calculated from the Debye–Scherrer formula.¹⁵ Transmission electron microscopy (TEM) (JEOL JEM-2100) and scanning electron microscopy (SEM) (JEOL JSM-6510LV) were used to examine the surface morphology of the prepared photocatalysts. Fourier transform infrared (FTIR) spectra were conducted on a PerkinElmer system 2000 with the KBr-disc technique, and the spectra were recorded in the region of 400–4000 cm^–1. N₂ adsorption–desorption was studied at 77 K using a BELSORP-mini II instrument and used to estimate pore size distribution and surface area (S_BET) of the prepared photocatalysts. A Thermo ESCALAB 250Xi spectrometer with a monochromatic Al Kα (1486.6 eV) radiation was used to measure the sample contents with its oxidation state. Photoluminescence (PL) spectra of the sample were examined over an FP-6500 fluorescence spectrophotometer with an excitation wavelength of 315 nm. A PerkinElmer LAMBDA 950 spectrophotometer was used to study UV–vis diffuse reflectance spectra.

2.3. Photocatalytic Activity

2.3.1. Photodegradation of Organic Pollutants

The photocatalytic activities of the photocatalysts were examined by measuring the degradation of BG and phenol in an aqueous solution. The light source was a 500 W Xe lamp (simulated sunlight irradiation) without using a radiation filter, and the reactor was enclosed by a water-cool system. 50 mg from the as-synthesized catalyst was moved into 50 mL of BG and phenol with an initial concentration (C₀) 10 ppm, and after that, the mixture was moved to a photoreactor. The resultant mixture was first stirred for 30 min in the dark to reach the adsorption–desorption equilibrium of BG on the surface of the catalyst and to attain a good dispersion. During light irradiation, the mixture was irradiated under continuous stirring. Then, 2 mL from the resultant mixture was withdrawn and centrifuged at different time intervals to separate the photocatalyst particles before absorbance measurements using a Shimadzu MPC-2200 UV–vis spectrophotometer. Equation 1(39) was used to calculate the photodegradation percentage of BG and phenol

1

where C₀ is the initial dye concentration and C_t is the dye concentration after irradiation time (t). Also, after photocatalytic reactions, there is a reactive species produced from the reaction, which could be examined by using various scavengers at a concentration (1 mM) including benzoquinone (BQ), isopropanol (IPA), and Na₂EDTA as scavengers of ^•O₂^–, e^–, ^•OH, and h⁺, respectively.^16,40 The mineralization percentage of BG and phenol was measured using the total organic carbon (TOC) approach using a TOC Analyzer Shimadzu 5000 (eq 2)

2

2.3.2. H₂ Production

The photocatalytic activity of the photocatalysts toward the evolution of H₂ gas was analyzed under visible light irradiation (500 W Xe lamp) at room temperature. Typically,^5,41,42 100 mg of the prepared photocatalyst was suspended in aqueous methanol solution (100 mL, 25 vol %). Before the reaction, the air in the reactor must be removed from the system through using a flow of dry N₂ gas for 20 min. The amount of H₂ production was examined by gas chromatography (PerkinElmer Clarus 500 GC), equipped with a thermal conductivity detector detector (N₂ as the carrier gas).

3. Results and Discussion

3.1. X-ray Diffraction Patterns

XRD patterns for unmodified and modified TiO₂ with different amounts of NiO (x wt % NiO/TiO₂) are displayed in Figure 1. The XRD pattern displays distinct peaks at 2θ equal to 25.39, 38.12, 48.16, 54.81, 62.96, 69.12, 70.41, and 75.23°, representing the indices of (101), (004), (200), (105), (204), (116), (220), and (215), respectively. These peaks have been identified as peaks of the tetragonal TiO₂ anatase phase (JCPDS# 21-1272).^19,38,43 Also, a small peak appeared at 27.2°, representing the indices of (110) planes of the rutile phase.⁴⁰ It is worth noting that the intensities of diffraction peaks are gradually decreased and weakened with the increase of the NiO content up to 15 and 20.0 wt % and some peaks disappear at 2θ equal to 38.12 and 48.16°. According to the anatase/rutile ratio results (Table 1) of pure and doped TiO₂, very small changes in the phases were observed with the increase of the NiO contents to 5.0 wt %. Also, new peaks were observed after increasing the contents of NiO to 10.0 wt % at 35.8 and 41.4°, attributed to the rutile phase (JCPDS no. 21-1276),^16,44 and these peaks became more preferential with increasing NiO loadings to 20.0 wt %. This indicates that the increase in the NiO content beyond 5.0 wt % enhanced the anatase to rutile transformation. According to these results, we can conclude that NiO at lower concentrations stabilize the TiO₂ phases, while at higher concentrations, NiO promotes the conversion of the anatase phase to the rutile phase. The reduction of the anatase phase and enhancement of the rutile phase of TiO₂ after doping TiO₂ by NiO may have resulted due to the role of NiO in reducing the phase transformation energy (decreasing the activation energy), which in turn facilitates the change of the anatase phase to the rutile phase.⁴³ Also, the difference in ionic radius and charge between Ni²⁺ and Ti⁴⁺ and the existence of oxygen vacancies in the TiO₂ lattice facilitate the conversion of the anatase phase to the rutile phase.^43,45

Figure 1.

XRD pattern of (a) TiO₂ and (b) 3, (c) 5.0, (d) 10.0, (e) 15.0, and (f) 20.0 wt % NiO/TiO₂.

Table 1. Textural Properties and Optical Properties of the Prepared Photocatalysts.

sample name	Da (nm)	A/R ratiob%	SBET (m2 g–1)	Vp (cm3 g–1)c	Dp (nm)d	BG (eV)e
TiO2	18.5	91.3/8.7	103.4	0.148	5.7	3.1
3.0 wt % NiO/TiO2	16.7	91.1/8.9	120.8	0.194	6.4	2.40
5.0 wt % NiO/TiO2	14.6	90.9/9.1	139.7	0.291	7.5	2.31
10.0 wt % NiO/TiO2	13.9	80.2/19.8	154.1	0.327	8.7	2.18
15.0 wt % NiO/TiO2	12.6	75.3/24.7	147.2	0.315	8.1	2.24
20.0 wt % NiO/TiO2	11.2	67.8/32.2	141.9	0.301	7.6	2.26

^aAnatase/rutile ratio.

^bCrystallite size.

^cTotal pore volume.

^dAverage pore size.

^eBand gap energy.

It is interesting to note that no diffraction peaks corresponding to NiO could be detected with the increase of the amount of NiO up to 10.0 wt %, suggesting a rational dispersion and incorporation of NiO in the lattice and on the surface of TiO₂. However, with the further increase of NiO content up to 15% and 20.0 wt %, new peaks were observed centered at 37.3 and 43.4°, attributed to the NiO crystallites phase,^21,46 indicating a decrease in the solubility of Ni²⁺ in the TiO₂ lattice and consequently the NiO crystals aggregated on the TiO₂ surface.³⁸Figure S1 shows that the positions of anatase peaks (101) were shifted to higher angles with increasing NiO content, indicating the replacement of some Ti⁴⁺ with Ni²⁺ ions, which is an indication of insertion and substitution positions of the Ti⁴⁺ with Ni²⁺ ions in the lattice of TiO₂.^19,47 The shifting to higher angles after addition of NiO indicates that Ni²⁺ entered into TiO₂ via both substitutional and interstitial modes and the large amount of Ni²⁺ entered into the TiO₂ lattice via the substitutional mode.^48,49

The crystalline size of the prepared photocatalyst was calculated using the Scherrer relation and is summarized in Table 1. Obviously, the crystalline size of TiO₂ reduced with the increase of the NiO content, suggesting that Ni²⁺ effectively inhibited the grain growth of TiO₂.^18,47 On the other hand, the insertion of Ni²⁺ ions in the lattice of TiO₂ led to a distortion in the lattice structure due to the difference in ionic radii of Ti cations (0.68 Å) and Ni cations (0.72 Å), which in turn hindered the growth of TiO₂ crystals.¹⁸ Also, the addition of NiO led to the formation of structural defects which in turn prevent particle growth.

3.2. TEM and SEM and Energy-Dispersive Spectroscopy-Mapping Analysis

The surface morphology and particle size of unmodified and modified mesoporous TiO₂ were characterized by TEM images and are displayed in Figure 2a. The images show a spherical shape with the average particles size ranging from 16 to 21 nm. The images of Figure 2b–d show the effect of NiO content on the morphology and the particles size of TiO₂ nanoparticles. The images exhibit that the samples have spherical shapes and the particles size decreased gradually with increasing NiO content compared with pure TiO₂ nanoparticles as illustrated by XRD results. The formation of the p–n junction in the NiO/TiO₂ nanoparticles can be emphasized by high-resolution TEM (HRTEM), as shown in Figure S2.

Figure 2.

TEM images of (a) pure TiO₂, (b) 5.0 wt % NiO/TiO₂, (c) 10.0 wt % NiO/TiO₂, and (d) 20.0 wt % NiO/TiO₂ nanoparticles.

Figures S3 and S4 show the HRTEM images and selected area electron diffraction (SAED) pattern of NiO/TiO₂ samples, respectively. HRTEM clearly demonstrated that pure TiO₂ (Figure S3a) showed lattice planes with a d-spacing at ∼0.35 nm, attributed to the (101) plane of the anatase TiO₂ phase. On the other hand, the HRTEM images of 10.0 and 20.0 wt % NiO/TiO₂ (Figure S3b,c) showed different lattice planes with d-spacings at ∼0.35, 0.32, and 0.20 nm, attributed to the (101) and (110) planes of anatase and rutile of TiO₂ and the (200) plane of NiO, respectively.⁵⁰ The SAED pattern (Figure S4) also showed the same results which emphasize the successful preparation of NiO/TiO₂ nanoparticles, in good agreement with the XRD and TEM analyses.

The surface morphology of the mesoporous TiO₂ and NiO/TiO₂ nanoparticles was measured through SEM micrographs. The images display that the samples are composed of aggregated particles with different size distributions. The SEM image of undoping TiO₂ displays a spherical particle shape (Figure 3a). Figure 3b–d displays the effect of NiO addition on the SEM micrographs of 5.0, 10.0, and 20.0 wt % NiO/TiO₂, respectively. The SEM micrographs illustrate that the amounts of NiO nanoparticles on the TiO₂ surface increased with the increase of NiO loading and NiO nanoparticles were well dispersed on TiO₂ until 10.0 wt % NiO (Figure 3c). However, with the further increase in the NiO content up to 20.0 wt % (Figure 3d), the NiO particles aggregated on the surface of TiO₂, which agrees with XRD results.

Figure 3.

SEM images of (a) pure TiO₂, (b) 5.0 wt % NiO/TiO₂, (c) 10.0 wt % NiO/TiO₂, and (d) 20.0 wt % NiO/TiO₂ nanoparticles.

Mapping and energy-dispersive spectroscopy (EDS) were performed on 10.0 wt % NiO–TiO₂ to map the presence of different elements and are displayed in Figure S5. The figure displays that 10.0 wt % NiO–TiO₂ sample contains titanium (Ti), oxygen (O), and nickel (Ni) in its structure. Also, according to the obtained results, the experimental Ni/Ti atomic ratio was nearly the same as the theoretical value. On the other hand, titanium (Ti), oxygen (O), and nickel (Ni) were homogeneously dispersed as shown in elemental mapping (Figure S5). All these data confirm the successful preparation of NiO–TiO₂.

3.3. FTIR Analysis

FTIR spectra of mesoporous TiO₂ and TiO₂ loaded by different contents of NiO are shown in Figure 4. The spectra revealed two characteristics peaks at 1631 and 3431 cm^–1, which are related to the bending and stretching modes of the O–H vibration of Ti–OH bonding vibration and/or owing to the presence of physically absorbed water.⁵¹ The intensities of these bands increased after the addition of NiO and became broad, and the presence of these groups is very significant to the photocatalytic reactions due to the fact that photoexcited holes generated on the photocatalyst surface react with them and produce hydroxyl radicals.⁵² It is reported that the adsorption of water molecules was improved obviously in the presence of oxygen vacancies over the photocatalyst surface, which might lead to an increase of surface hydroxyl group concentration.⁵³ Also, Figure 4 displays a broad absorption band in the region below 900 cm^–1, assigned to the stretching modes of Ti–O, Ti–O–Ti, Ti–O–Ni, and Ni–O.⁵⁴⁻⁵⁶ The small band observed at 470 cm^–1 is related to the Ni–O bond, indicating the existence of nickel oxide, and the intensity of this band increases with the increase of the NiO content.^20,57 Other bands appeared at 1070 and 1365 cm^–1, attributed to hetero Ti–O–Ni and Ni–O, which confirm the introduction of Ni²⁺ ions in the TiO₂ lattice and the formation of the p–n junction.^24,38,58 On the other hand, Figure 4 shows that the band location in TiO₂ shifted to a higher wavelength with the increase of the NiO content, indicating the existence of strong interactions between Ni²⁺ and TiO₂.

Figure 4.

FTIR spectra of (a) TiO₂ and (b) 3.0 wt % NiO/TiO₂ (c) 5.0 wt % NiO/TiO₂ (d) 10.0 wt % NiO/TiO₂ (e) 15.0 wt % NiO/TiO₂, and (f) 20.0 wt % NiO/TiO₂.

3.4. Brunauer–Emmett–Teller Measurement

The textural properties of pure and modified TiO₂ were examined by N₂ adsorption–desorption isotherms. As seen from Figure 5, all the prepared photocatalysts displayed type IV isotherms according to IUPAC classification for mesoporous materials,^15,40 with type II hysteresis loops.⁶ The surface area (S_BET) and pore volume of TiO₂ and NiO/TiO₂ were calculated by the Brunauer–Emmett–Teller (BET) method and are summarized in Table 1. From Table 1, the values of S_BET and pore volume of TiO₂ increased after doping by NiO until a maximum value was reached at 10.0 wt % NiO/TiO₂ and then reduced with the further increase in the NiO loading. The increase in S_BET of TiO₂ after the addition of NiO may be due to the incorporation of Ni²⁺ into the TiO₂ lattice, which led to a reduction in the crystallite size.⁵⁹ Also, the high dispersion of NiO nanocrystals as amorphous layers on TiO₂ crystals inhibited the grain growth of TiO₂, which act in elevation of the surface area. However, the reduction in the surface area may be due to the accumulation of NiO nanocrystals over the TiO₂ surface, and consequently, partial blockage of small pores could occur.^6,60 Increasing the S_BET leads to promote photocatalytic activity due to the presence of more available reaction sites on the surface of the photocatalyst.⁶¹ The pore size distributions were investigated from the desorption part of the adsorption isotherm as depicted in Figure S6. As seen in Figure S6, there are two types of mesopores with different shapes and sizes. Also, Table 1 displays the pore sizes shifted toward larger sizes after the addition of NiO compared with pure TiO₂ to reach a maximum at 10.0 wt % NiO/TiO₂ nanoparticles.

Figure 5.

Adsorption–desorption isotherm curves of TiO₂ and x wt % NiO/TiO₂ composites.

3.5. X-ray Photoelectron Spectroscopy

The surface composition and chemical state of the photocatalysts were studied using X-ray photoelectron spectroscopy (XPS). The survey spectra of 10% NiO/TiO₂ nanoparticles are displayed in Figure 6. There are four elements, Ti, Ni, C, and O, which can be determined in the sample, and their XPS peaks are detected at 458.37 (Ti 2p), 855.2 (Ni 2p), 286 (C 1s), and 530.2 eV (O 1s), as shown in Figure 6a. The C 1s peak results mainly from environmental contamination. From Figure 6b, the high-resolution scan of Ti 2p region spectra displays the existence of the main doublet composed of two symmetrical peaks at 458.5 and 464.2 eV corresponding to Ti 2p_3/2 and Ti 2p_1/2, respectively, arises from spin–orbit splitting and is consistent with Ti⁴⁺ in the TiO₂ lattice.^44,62 Also, two peaks appeared at 457.3 and 463 eV corresponding to Ti 2p_3/2 and Ti 2p_1/2, respectively, indicating the formation of Ti³⁺.⁶³ There is a slight shift when compared with the pure TiO₂ peaks that are mentioned in the earlier literature,⁶⁴ which indicates the influence of Ni²⁺ addition on the electronic state of TiO₂, due to substitution of some Ti⁴⁺ with Ni²⁺ in the TiO₂ lattice.⁶⁴ The ionic radius of Ni²⁺ is nearly similar to Ti⁴⁺ and could form an octahedron coordination similar to Ti⁴⁺. However, Ni²⁺ ions can replace the Ti⁴⁺ ions in the TiO₂ lattice. Also, another Ni²⁺ enters in the interstitial position in the TiO₂ lattice.^19,47−49 In the same manner, Figure 6c shows the high-resolution scan of the Ni 2p spectrum. The figure exhibits peaks at 855.2 and 861.3 eV (satellite) corresponding to Ni 2p_3/2, and the peak at 873.8 eV is related to Ni 2p_1/2, representing the formation of Ni²⁺ in NiO.^18,44,62 In contrast, the XPS spectrum of O 1s was asymmetric and wide at a higher binding energy (E_b), which indicates that oxygen existed in different forms of O binding states, as displayed in Figure 6d. Deconvolution of the O 1s line produces three main emission peaks, with one of them at 530.2 eV related to the crystal lattice oxygen in TiO₂ and NiO.^19,65 The other one at a binding energy equal to 531.5 eV is assigned to the oxygen vacancy, and the third peak appears at 533.1 eV, attributed to O₂/OH groups that adsorbed on the sample surface.^41,43,63 The difference in the binding energies between Ni 2p_3/2 and Ni 2p_1/2 is 18.6 eV, while the difference in the case of metallic Ni is 17.27 eV and that in the case of NiO is 17.49 eV.¹⁹ The shift of Ti 2p peaks and the change in the Ni 2p peak indicate the reordering of Ni²⁺ ions and Ti⁴⁺ ions, which gives evidence of replacement of some Ti⁴⁺ with Ni²⁺ ions in the TiO₂ lattice.^19,63 From the above discussion, we can conclude that 10% NiO/TiO₂ photocatalyst is composed of Ti⁴⁺, Ni²⁺, O, and oxygen vacancies.

Figure 6.

XPS spectra of 10.0 wt % NiO/TiO₂ photocatalyst: (a) survey scan, (b) Ti 2p, (c) Ni 2p, and (d) O 1s.

3.6. UV–Vis Spectroscopy

The earlier challenge in enhancing the properties of TiO₂ is to shift the absorption spectrum to the visible region for effective solar light photon harvesting. The optical absorption properties of TiO₂ and NiO/TiO₂ samples were examined using UV–vis diffuse reflectance spectroscopy, as displayed in Figure 7. The figure shows a strong absorption edge below 400 nm due to the charge transfer from the valence band (VB) to the conduction band (CB) of TiO₂.⁴⁰ Comparing with pure TiO₂, the addition of NiO into TiO₂ showed a red shift in the absorption to visible light and all the photocatalysts absorbed in the visible region. Also, the samples showed obvious enhancement in the absorption in visible light, and a marked red shift was observed with the increase of the contents of NiO up to 20.0 wt % NiO/TiO₂ nanoparticles, which emphasizes the presence of Ni²⁺ ions inside the TiO₂ lattice and indicates generation of additional energy levels by Ni²⁺ ions in the TiO₂ band gap.^19,40 Also, new absorption bands appeared at 371.5, 422.2, and 523.6 nm after the addition of NiO, indicating the introduction of surface defects, oxygen vacancies, and a new impurity energy level in the TiO₂ energy band.⁶⁵ The oxygen vacancies could generate oxygen vacancy states in the TiO₂ band gap and lead to improvement of the absorption in the visible region.^53,66 On the other hand, it is reported that the oxygen vacancies could act as photogenerated charge carrier traps and enhance the photocatalytic activity.^53,67 Another new broad band appeared in the range of 730–850 nm, assigned to the electron transition in Ni d–d orbits, and might indicate the variation in surface defects and oxygen vacancies.^23,54,65 The intensity of this broad band increased with the increase of the NiO content. The appearance of these bands (370–550 and 600–800 nm) indicates the strong interaction between the TiO₂ and NiO species and the formation of p–n heterojunctions.^4,23 Also, the formation of the rutile phase played a significant factor in the red-shifting to the visible region.^38,40 According to these results, the photocatalytic degradation activity of NiO/TiO₂ under visible-light irradiation should be enhanced markedly compared with undoped TiO₂. We can calculate the band gap energy (E_g) from Tauc’s equation (eq 3)

3

where ν is the wavenumber, h is the Planck constant, α is the absorption coefficient of the photocatalyst at a definite wavelength (λ), E_g is the energy band gap, and A is a constant, in which n = 2 for direct transition and n = 1/2 for indirect transition. The TiO₂ anatase phase is an indirect band gap semiconductor.^6,68 From the plot of (αhν)^1/2 against photon energy (eV), we can calculate the band gap energies of the as-synthesized photocatalysts, as displayed in Figure 7 (inset). The values of band energy can be measured by the extrapolation of the absorption tangent to the abscissa. The E_g of TiO₂ nanoparticles is 3.1 eV, while the E_g of NiO/TiO₂ was reduced with the increase of the contents of NiO, as shown in Table 1. These results support the qualitative observation of the red shift in the absorption edge of NiO/TiO₂ as compared with undoped TiO₂. On the other hand, it is reported that the addition of NiO to TiO₂ led to changes in the band gap type of anatase from indirect to direct due to the increase of the percent of the rutile phase in TiO₂ with the increase of the NiO amount over 10.0 wt % as illustrated in the XRD results where the addition of NiO enhanced the anatase to rutile phase transformation and the rutile phase is the direct band gap.⁶⁹ The reduction of E_g may be due to three reasons: (i) the overlap of the CB of the d-level of the Ni²⁺ ion with the d-level of Ti⁴⁺,⁷⁰ (ii) the partial conversion from the anatase phase to the rutile phase produces shifting of the absorption to the visible region,^71,72 and (iii) the existence of defects and oxygen vacancies, which improves absorption in the visible region.

Figure 7.

UV–vis absorbance spectra and Tauc’s plot for optical band gap calculations of TiO₂ and NiO/TiO₂ (inset).

3.7. PL Spectroscopy

Figure 8 displays PL spectra of pure and modified TiO₂ nanoparticles to identify the defects sites and oxygen vacancies.^54,61 It is clear that pure TiO₂ displayed a clear emission peak at about 426 nm, attributed to the self-trapped exciton and could be due to the band edge free excitons.^42,54 Also, Figure 8 displays that the incorporation of Ni²⁺ ions led to reduction in the PL intensities and causes a significant red shift of emission PL peaks. The PL spectra became broad in the visible region with a wide spectral width, and new bands appeared around 520 and 675 nm, related to the oxygen vacancies.⁵⁴ It is reported that the bands appearing in the visible region are related to surface defects, band edge free excitons, and oxygen vacancies in the TiO₂ lattice.^16,73,74

Figure 8.

PL spectra of TiO₂ and NiO/TiO₂ with different NiO contents.

The red shift in the emission of PL spectra at the longer wavelength region after the addition of Ni²⁺ ions resulted due to introduction of certain surface vacancies and sub-bands in the TiO₂ band gap which facilitate the transfer of electrons from VB to CB.^19,75 It is worthy to note that the PL intensity of NiO/TiO₂ reduced significantly compared with pure TiO₂, which could be assigned to the reduction of the recombination rate of the electrons/holes and lead to a high photocatalytic efficiency.⁴ The concentration of Ni²⁺ ions plays a significant role in the intensity of the emission peaks. From Figure 8, it is obvious that the PL intensity reduced gradually with the increase of the contents of NiO until a maximum value was reached at 10.0 wt % NiO/TiO₂, which means that 10.0 wt % of NiO is the optimum content to inhibit the recombination of e^–/h⁺ pairs and enhance the electron lifetime, which enhances the photocatalytic efficiency. The increase in the NiO content up to 15.0 and 20.0 wt % NiO/TiO₂ was accompanied by the increase in PL intensity. This indicates that the Ni²⁺ ions played as recombination centers after increasing the concentration of Ni²⁺ ions above 10.0 wt % Ni–TiO₂ nanoparticles.

3.8. Photocatalytic Activity

3.8.1. Photodegradation of BG and Phenol

The photocatalytic performance of pure and modified mesoporous TiO₂ was studied under simulated sunlight irradiation in an aqueous solution over BG and phenol, as displayed in Figures 9 and 10. The photodegradation was tested without using any photocatalyst (TiO₂ or NiO/TiO₂). The results showed that there is no appreciable photodegradation of BG and phenol observed in the absence of the catalyst. First, the reaction was studied in the dark to study the effect of adsorption on the photodegradation. The results displayed that NiO/TiO₂ composites showed higher adsorption capacity compared with pure TiO₂, which is in favor of the photocatalytic reaction. Also, Figures 9 and 10 display that NiO/TiO₂ showed much higher photodegradation of BG and phenol compared with pure TiO₂. Moreover, the photodegradation of BG and phenol improved with increasing NiO contents until a maximum value was reached at 10.0 wt % NiO/TiO₂. The improvement in the photocatalytic activity of TiO₂ after addition of NiO is attributed to many factors: the anatase/rutile ratio, p–n junction heterostructures, Ti³⁺, oxygen vacancies, and surface defects. These factors depend on the amount of NiO, indicating that the amount of NiO is the major parameter that controls the enhancement of the photocatalytic activity of TiO₂.

Figure 9.

Photocatalytic degradation of BG over TiO₂ and NiO/TiO₂ nanoparticles vs irradiation time.

Figure 10.

Photocatalytic degradation of phenol over TiO₂ and NiO/TiO₂ nanoparticles vs irradiation time.

The role of these factors for enhancing the photocatalytic activity of NiO/TiO₂ is attributed to the following: (i) the formation of p–n junction heterostructures at the interfaces and effective suppression of the electron–hole recombination;^5,6,41 (ii) prevention of the rapid recombination of photogenerated carriers by providing capturing centers, leading to promotion of electron transfer and photocatalytic reactions; (iii) the formation of new energy levels within the band gap of TiO₂ after insertion of NiO, leading to reduction of the required energy for transfer of photogenerated electrons from VB to CB;⁵⁴ (iv) the increase of the surface area and oxygen vacancies, which play together with NiO in reducing the recombination rate of photogenerated e^–/h⁺; and (v) broadening and enhancement of the light absorption range of photocatalysts in the visible region. (iv) Another reason is the existence of mixed rutile and anatase phases, enhancing the photocatalytic activity compared with the anatase or rutile phase alone, and at 10.0 wt % NiO/TiO₂, the good interaction between mixed rutile and anatase phases occurred, which enhanced the photocatalytic activity.

The decrease in the activity after 10.0 wt % NiO/TiO₂ nanoparticles may be due to the presence of excess amount of Ni²⁺ ions that played in two manners; one is that Ni²⁺ act as recombination centers, which led to enhancing the recombination rate of e^–/h⁺, and the other manner is the formation of NiO layers on the TiO₂ surface, which could prevent the incident light to reach into TiO₂, leading to the decrease of the number of photogenerated e^–/h⁺; the two manners led to the decrease of the photocatalytic performance of NiO/TiO₂ nanoparticles.⁶⁵ Also, the conversion of the anatase phase to the rutile phase increased largely with the increase of the NiO loading beyond 10.0 wt %, which resulted in reducing the photocatalytic activity of NiO/TiO₂ due to the fact that the rutile phase is less active than the anatase phase.³⁸ According to these results, the surface defects, oxygen vacancies, and p–n junction were formed after addition of NiO and increased with the increase of the NiO content and enhanced the photocatalytic performance of NiO/TiO₂. This means that the photodegradation of BG and phenol would increase continuously with the increase of the NiO content. However, it is observed that the photodegradation of BG and phenol reduced after the increase of NiO content to 10.0 wt %, where Ni²⁺ played the opposite role as recombination centers and weakened the role of oxygen vacancies as centers of electron capture.⁶⁵ These results indicate that the amount of NiO was the crucial factor in enhancing and reducing the photodegradation of BG and phenol.

The Langmuir–Hinshelwood first-order kinetic model was applied to study the photodegradation behavior on the prepared photocatalysts by applying eq 4(38)

4

where k is the rate constant and t is the time of photodegradation reaction. Figure 11 shows a straight line when ln(C₀/C) is plotted versus time. The rate constants of all the prepared samples were measured and are tabulated in Table 2. From the figure and table, all the prepared catalysts obey the pseudo-first-order model according to correlation coefficient (R²). From these results, the photodegradation kinetics depends only on the concentrations of BG and phenol at a constant photocatalyst amount. Also, the rate constant (k) was increased with increasing NiO content until a maximum value was reached at 10.0 wt % NiO/TiO₂ and then reduced again.

Figure 11.

Pseudo-first-order kinetics of photodegradation of (a) BG and (b) phenol over TiO₂ and NiO/TiO₂ nanoparticles.

Table 2. Correlation Coefficients and Rate Constants for BG and Phenol Photodegradation.

	BG		phenol
sample name	K1	R2	K1	R2
TiO2	0.01098	0.9923	0.00476	0.98113
3.0 wt % NiO/TiO2	0.01685	0.99516	0.00685	0.99045
5.0 wt % NiO/TiO2	0.02995	0.98617	0.01542	0.98173
10.0 wt % NiO/TiO2	0.05562	0.99438	0.01992	0.98728
15.0 wt % NiO/TiO2	0.02522	0.98924	0.01327	0.9809
20.0 wt % NiO/TiO2	0.02123	0.98381	0.01116	0.99102

The TOC percentages of BG and phenol photodegradation over 10% NiO/TiO₂ nanoparticles for 180 min of irritation are displayed in Figure S7. The results display that the mineralization percentage of both BG and phenol was increased with the increase of the irritation time to reach 86.7 and 74.3%, respectively. From these data, the TOC percentages were lower than the photodegradation percentages, revealing that there are some colorless intermediates undegraded. However, the TOC percentage reached 100% for both BG and phenol at an irritation time equal to 360 min, revealing the complete mineralization of BG and phenol to carbon dioxide and water.

3.8.2. Hydrogen Evolution

The photocatalytic hydrogen evolution of TiO₂ and NiO/TiO₂ was also investigated. Figure 12a displays that H₂ evolution increased with the increase of the contents of NiO and 10.0 wt % NiO/TiO₂ nanoparticles displayed the highest hydrogen evolution rate with 445.1 μmol·h^–1·g^–1. As represented in Figure 12b, with the increase of the irradiation time to 5 h, the amount of H₂ generated was increased and the highest hydrogen evolution was achieved by 10.0 wt % NiO/TiO₂ with 2101.4 μmol·h^–1·g^–1. Also, H₂ production efficiency was studied using various sacrificial agents studied over 10.0 wt % NiO/TiO₂, as displayed in Figure 13. The H₂ production efficiencies for ethylene glycol, methanol, ethanol, and lactic acid were found to be 3100.3, 2100.4, 1800.7, and 1015.8 μmol, respectively. The mixture of 10.0 wt % NiO/TiO₂ with ethylene glycol showed the maximum H₂ production (3100.3 μmol) efficiency as compared to all other sacrificial agents. This is attributed to the faster charge-transfer reaction in 10.0 wt % NiO/TiO₂ with the ethylene glycol system compared to the photogenerated electron–hole recombination process.^76,77 The number of hydroxyl groups, the length of the carbon chain, and dehydrogenation/decarbonylation characteristics of sacrificial agents are the primary features in controlling the H₂ production efficiency. Moreover, the formation of byproducts, adsorption capability on the photocatalyst surface, polarity and electron donating ability, and the selectivity for reaction with photogenerated holes (e.g., decarboxylation process) could also strongly affect the efficiency.⁷⁶⁻⁸¹ On the other hand, Figure S8 shows the role of methanol as a sacrificial agent in enhancing the H₂ evolution rate. It is exciting to note that a trace amount of hydrogen evolution was attained for pure TiO₂ under visible-light irradiation and no hydrogen was produced in the absence of the photocatalyst. From these results, the hydrogen evolution mainly depends on the amount of NiO, revealing the role of NiO in improving the photocatalytic performance of TiO₂.

Figure 12.

(a) Rate of hydrogen evolution and (b) reaction time profiles of photocatalytic hydrogen evolution of TiO₂ and NiO/TiO₂ nanoparticles after 5 h under visible-light irradiation.

Figure 13.

Photocatalytic H₂ production efficiency of 10.0 wt % NiO/TiO₂ using various sacrificial agents.

3.8.3. Photocatalytic Mechanism

The photodegradation of BG was performed in the presence of the radical scavengers. Figure S9 shows the effect of adding radical scavengers on the photodegradation of BG over 10.0 wt % NiO/TiO₂ nanoparticles. The result displayed that the photodegradation of BG decreased sharply after addition of IPA, revealing that the ^•OH radical played the main role in the photodegradation of BG. However, the addition of Na₂EDTA and BQ caused remarked suppression in the case of Na₂EDTA and slight retardation in the case of BQ, suggesting the minor role of h⁺ and ^•O₂^– in the photodegradation of BG. According to these results, ^•OH is the main reactive radical in photocatalytic degradation of BG.

From the last results, a possible photodegradation mechanism of BG and phenol over NiO/TiO₂ nanoparticles can be suggested, as displayed in Scheme 1. Under simulated sunlight irradiation, NiO/TiO₂ is excited and photogenerated electrons in the CB and holes in VB are created of both oxides. According to the work function studies, the work function of NiO is 5.4 eV, while in the case of TiO₂, it is 4.15 eV, which indicate that the Fermi energy level of NiO is lower than that of TiO₂ and thus, the VB and CB positions of TiO₂ are lower than those of NiO.⁶ According to Scheme 1, the photogenerated electrons on the CB of NiO will transfer to the CB of TiO₂ (n-type) driven by the internal field, while the photogenerated holes on the VB of TiO₂ transfer to the VB of NiO (p-type), which leads to the formation the p–n heterojunction, which effectively separates the photogenerated electron–hole pairs.^6,41

Scheme 1. Postulated Mechanism of Electron Transfer in NiO/TiO₂ Nanoparticles.

On the other hand, the H₂ evolution mechanism is displayed in Scheme 1. In this mechanism, water molecules reacted with the photogenerated holes and produced ^•OH and H⁺ and then the resulting ^•OH reacted with methanol (sacrificial agent) and generated H⁺.⁸² Subsequently, the photogenerated electrons reduced the H⁺ ions and generated H₂, where the CB potential of NiO/TiO₂ is more negative than the proton reduction potential.

3.8.4. Reusability of the Catalyst

The reusability of 10.0 wt % NiO/TiO₂ nanoparticles was investigated. Figure 14 shows the photodegradation of the BG dye after five runs under the same photoreaction conditions. The powder was centrifuged after each run to completely separate and then immersed in ethanol for 120 min and then washed with deionized water and dried in an vacuum oven at 80 °C for 12 h.¹⁶ No important loss in the photocatalytic activity was detected after 5 runs, which confirmed the excellent reusability and stability of the NiO/TiO₂ photocatalyst. Furthermore, the stability of the prepared photocatalyst was investigated after five times of reuse using XRD analysis (Figure S10). The results showed no considerable changes in the phases and structural properties of 10.0 wt % NiO/TiO₂ nanoparticles. These experiments reveal that not only can NiO/TiO₂ nanoparticles be used for sustainable photodegradation of dyes but also they have excellent reusability, making them suitable as a stable and environmentally friendly catalyst for photodegradation of organic pollutants. Also, the results gained from our recent work were compared with other literature studies as displayed in Table S1. The results display that our work has excellent performance for BG degradation^{22,23,58,83,84} compared with other catalysts, especially in the range of visible light. Also, the obtained data were compared with data of other literature studies and are displayed in Table S2, which revealed that 10.0 wt % NiO/TiO₂ acts as an excellent photocatalyst in hydrogen production.⁸⁵⁻⁸⁸

Figure 14.

Effect of reusing times of the photocatalyst on photocatalytic degradation of BG over 10.0 wt % NiO/TiO₂ nanoparticles.

4. Conclusions

Pure and modified mesoporous TiO₂ nanoparticles were successfully prepared through the sol–gel method. XRD data displayed that the crystallite size reduced remarkably after addition of NiO. Diffuse reflectance spectroscopy displayed red shift in absorption edges, and a new absorption band in the visible region was observed after addition of NiO. The introduction of NiO into TiO₂ led to the formation of p–n heterojunctions, which improved the separation efficiency of photogeneration (e/h⁺) pairs and improved the activity of NiO/TiO₂. The photodegradation of BG improved significantly after insertion of NiO, and 10.0 wt % NiO/TiO₂ showed the highest degradation efficiency. The TOC percentage reached 100% after 360 min, which approves the complete mineralization of BG and phenol over 10.0 wt % NiO/TiO₂. The photodegradation kinetics of BG and phenol obeyed the pseudo-first order, and the mechanism of photodegradation of BG was studied and schematically illustrated. Also, the photocatalytic hydrogen evolution rate increased with the increase of the NiO content until the maximum value was reached at 10.0 wt % of NiO. The photocatalyst displayed excellent reusability and stability after five runs in photodegradation of BG without significant loss in the activity. According to these results, the photocatalytic activity and physicochemical properties of the NiO/TiO₂ nanoparticles strongly depend on the NiO content.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.1c03693.

• XRD pattern of pure and modified TiO₂ by different weight percents of NiO; HRTEM image showing the p–n junction of NiO/TiO₂; HRTEM image and SAED image of pure TiO₂, 10.0 wt % NiO/TiO₂, and 20.0 wt % NiO/TiO₂; elemental mapping and EDS analysis of 10.0 wt % NiO/TiO₂ nanoparticles; pore size distribution of pure and modified TiO₂ by different weight percents of NiO; % TOC removal and photodegradation of BG and phenol versus time; photocatalytic evolution of H₂ under different conditions; photodegradation of BG over 10% NiO/TiO₂ in the absence and presence of different scavengers under similar reaction conditions; and XRD and TEM analysis of 10.0 wt % NiO/TiO₂ after five times of photocatalytic reuses (PDF)

The authors declare no competing financial interest.