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. 2021 Mar 17;6(12):8717–8725. doi: 10.1021/acsomega.1c00697

Promoting Visible Light Generation of Hydrogen Using a Sol–Gel-Prepared MnCo2O4@g-C3N4 p–n Heterojunction Photocatalyst

Maha Alhaddad †, Reda M Mohamed †,‡,*, Mohamed H H Mahmoud §
PMCID: PMC8015085  PMID: 33817535

Abstract

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The production of hydrogen using a new type of heterogeneous photocatalyst under visible light is considered a remarkable essential pathway for sustainable, pure energy not only on the laboratory scale but also on a bigger scale. Hence, a new nanocomposite of mesoporous MnCo2O4, g-C3N4, and MnCo2O4@g-C3N4 was produced utilizing a sol–gel method with variable MnCo2O4 contents. The crystal structure of MnCo2O4 was effectively confirmed by the X-ray diffraction pattern and integrated onto the g-C3N4 structure. The MnCo2O4 nanoparticles were displayed as spherical particles by TEM images and dispersed in a uniform way inside the g-C3N4 nanosheet. The synthesized nanocomposites in the form of MnCo2O4@g-C3N4 were examined as a new effective photocatalyst against glycerol as a source for H2 production with visible light. The MnCo2O4 contents indicated a corroborative impact for the photocatalytic action related to the H2 production process. A maximum H2 production molecular value was observed (21,870 μmol·g–1·h–1) for a 1.5 wt % MnCo2O4@g-C3N4 nanocomposite as a considerable increase in its photocatalytic activity. The yields of H2 are ∼55 and 23 times higher than those of g-C3N4 and MnCo2O4, respectively. Up to five times cycles of visible lighting were the maximum number of repeated cycles by which the 1.5 wt % MnCo2O4@g-C3N4 product showed higher stability and durability.

1. Introduction

The recent technological development in the field of photocatalytic activity of a variable type of new material based on solar light is reflected in a widely convenient way to convert energy for environmental curing and even for the organic synthesis methodology.1,2 A particular one is the reaction of H2 production in the presence of nanomaterials containing different semiconductor types with variable approximate electron donor types. The major renewable source for clean energy depends on the specific type of material such as carbohydrates and alcohol. This fact has been recognized in the existence of a visible light illumination source.3,4 In general, any developed oxidation process such as the photocatalytic effects of any heterogeneous catalyst reaction is most likely wastewater processing for pollutant regression.5,6 In a more enjoyable manner, the photocatalytic reaction utilizes the sunlight renewable energy source together with the existence of a heterogeneous catalyst to produce a clean environment without any additional minor pollutants. This is mainly due to the complete mineralization process in water, carbon dioxide, and any other suitable inorganic ions from its primary organic pollutants.7 The functional wastewater processing in the presence of variable types of semiconductors for the disintegration of almost all forms of nondegradable toxins based on sunlight irradiation has been widely illustrated.8,9 A 2-D material, namely, graphitic carbon nitride, with a general abbreviation (g-C3N4), represents a good example of a heterogeneous photocatalyst that gains much more treatment decontamination development worldwide in the past two decades. The g-C3N4 nanomaterial has been found for more than a decade and utilized mainly to produce H2 through photocatalytic activity. It has also been globally studied over an extremely wider number of applications in a variety of fields. These include organic pollutant degradation, NO oxidation, catalyst support, CO2 reduction, solar cells, synthetic organic reactions, etc.6,10 g-C3N4 is also classified as a metal-free-type semiconductor. It absorbs a considerable section of visible range due to its reasonable band gap value (2.7 eV).6,11 In addition to a huge number of insoluble properties in water and other common solvents, this is mainly referring to the interaction between graphitic layers via van der Waals forces. It is also showing higher stability in both acidic and basic media with increasing temperature until 600 °C.12,13 Moreover, g-C3N4 carries a polymer-like structure that assists in generating vigorous, active radicals through the diversified electron excitation. g-C3N4 can also dynamically reduce oxygen to a superoxide radical in the O2·– form due to its powerful reduction capability (−1.13 eV versus NHE). On the other hand, some disadvantages are also found in the g-C3N4 nanomaterial, which is also faced by individual photocatalysts, including high reincorporation of photoinduced charges, inadequate light response, and low surface area.14−17 Hence, the researchers think of improving those drawbacks via the possible coupling with any other semiconductor that can possess narrow band gap energy in the form of surface amendment.16,18 For this purpose, as fundamentally known, the surface area increases while increasing the porous surface in the form known as a porous material. Thus, this is responsible for increasing the active substrate sites for adsorption probability. Mesoporous silica is included in a large number of methods, which are indicated as templates for the fabrication of the porous structure and treatment of acid to boost the generation of pores via ammonia gas loss.19,20 But such a way fetches a higher toxic prospect in addition to its uncomfortable procedures.21 A quite good way to generate a porous and large-surface-area material through a thermal method classified as a facile route to yield g-C3N4 was proposed by Wang et al. in 2009. The synthesized material was able to absorb visible light easily and effectively decompose Rhodamine B using a photocatalytic process.10 An ultrathin g-C3N4 was prepared by Kadi et al. using thermal treatment of bulk g-C3N4 for 6 h as a long-time procedure. Kadi et al. evidenced that this expansion provides a higher-surface-area material and shows a 22-fold increase in the production of H2 in comparison to the use of the commercial one.22 Apart from using g-C3N4 nanomaterials, bismuth-containing materials might also be utilized as eco-friendly materials for environmental curing. Photocatalysts containing AB2O4 spinel oxides also have a polarization effect and crystal facet, which lowers the electron–hole reincorporation in the photocatalytic reaction.23−25 Recently, Wang et al. showed a noticeable improvement in the CO2 reduction activity, activated functional groups, and local photogenerated charge carrier separation centers via preparation of MnCo2O4 microspheres.24 In this regard, the produced electrons and holes were able to join as an attachment to diverse charges due to the surface polarization, driving the separation of the electron–hole pairs.26,27 MnCo2O4 is easily coupled to any other nanomaterial such as g-C3N4 via composite formation due to the difference of their band gap edge levels. A proper route of photogenerated charge carriers has been supplied via different band gap energies.28−37 MnCo2O4 has a band energy gap of ∼1.7 eV.30,31 Tetracycline and hexavalent chromium have been felicitously decomposed by CoO@MnCo2O4 based on visible lighting.38 Coming from the great behavior of porous MnCo2O4 and g-C3N4, herein, we synthesized mesoporous MnCo2O4@g-C3N4 nanocomposites. The enhanced performance of mesoporous MnCo2O4@g-C3N4 nanocomposites for the hydrogen production process was evaluated. As above, the mesoporous MnCo2O4 and g-C3N4 showed similar performances, cooperation, and quite an increase in the efficiency of photocatalytic activity that may be credited to the less carrier recombination besides the wider absorption range of visible light. The previous methods for the preparation of MnCo2O4@g-C3N4 produced a small surface area.39 However, in our study, a simple method was used to produce a high surface area of MnCo2O4@g-C3N4 mesoporous photocatalysts for the promoted hydrogen generation.

2. Experimental Section

2.1. Materials

EO106-PO70-EO106 (F127), with an average molecule weight of 12,600 g·mol–1, was used as a surfactant. Hydrochloric acid, ethanol, CH3COOH, Co(NO3)2·6H2O, urea manganese acetate, and dicyandiamide were taken from Sigma-Aldrich. A Millipore Milli-Q system was used to purify water.

2.2. Mesoporous MnCo2O4 Preparation

A sol–gel procedure was used to prepare the mesoporous MnCo2O4 nanoparticles. F127 was also used as a structure-directing agent tri-block copolymer. The desired product was synthesized by the required 1:0.02:50:2.25:3.75 molar ratios attributed to MnCo2O4 /F127/C2H5OH/HCl/CH3COOH. In a general procedure, under constant stirring for 60 min, F127 (1.6 g) was added to a solution of ethanol (30 mL). After that, 0.74 mL of HCl and 2.3 mL of CH3COOH were further added to the previous solution under the same constant magnetic stirring for an extra 30 min. Then, Co(NO3)2·6H2O and manganese acetate were considered as the precursors for MnCo2O4. Hence, 0.0125 mol of cobalt nitrate and 0.0075 mol of manganese acetate were added to the previous mesophase of F127-CH3COOH to obtain mesoporous MnCo2O4 after 60 min vigorous stirring, to eliminate ethanol, and to get the required gel. The gel was transferred to a chamber with 40% humidity and temperature up to 40 °C for 12 h. Later, the aging process was carried out at 65 °C for 24 h for that gel, followed by the calcination process, which was carried out in air for 4 h at 550 °C. Throughout this process, a 1 °C/min heating rate and 2 °C/min cooling rates were observed to slip off the surfactant and to obtain the final product as mesoporous MnCo2O4.

2.3. Preparation of g-C3N4

High mesoporous silica (HMS) has a high surface area (ca. 500–1000 m2·g–1) to realize the similar surface texture of g-C3N4. Also, utilize the pyrolysis process of urea and dicyandiamide in air. The preparation of HMS was easily obtained, as reported in the literature.2 About 50 mL of pure water was ultrasonicated for 30 min in the presence of 1 g of HMS. A mixture of 3 g of dicyandiamide and 5 g of urea was further added. Then, a continuous instigation at 80 °C was carried out to enhance the dissolving process of both components pending complete dissolution. Drying overnight at around 80 °C was applied to take off the extra water amount. Calcination procedures were also performed over 4 h at 550 °C. In the end, the materials obtained were immersed in NH4HF2 solution (50 mL, 2 M) with strong stirring up for 24 h to drive out the HMS solid pattern. To release any contaminants adsorbed over the produced g-C3N4 nanoparticles, they are easily cleaned by washing several times with water. After that, the synthesized pure material was heated by drying up to 100 °C for 12 h.

2.4. Mesoporous MnCo2O4@g-C3N4 Nanocomposite Synthesis

A water exfoliation process was used to synthesize the desired MnCo2O4@g-C3N4 photocatalysts as follows: 0.2 g of the previously synthesized g-C3N4 mixed with mesoporous MnCo2O4 of a certain amount in deionized water (400 mL). The mixture was dispersed using sonication of a power of 40 kHz for 3 h until thin layered MnCo2O4@g-C3N4 materials were obtained. The final products were specified as x MnCo2O4@g-C3N4 and formed by centrifugation. The molar content of MnCo2O4 in MnCo2O4@g-C3N4 was represented by “x” (x = 0.5, 1.0, 1.5, and 2.0 wt %).

2.5. Characterizations

The crystalline structures for the produced photocatalysts were measured using a Bruker AXS D4 X-ray diffractogram using Cu Kα radiation. A JEOL JEM-2100F electron microscope, Japan, was used to determine the morphology of the photocatalysts. The FT-IR spectra of the obtained photocatalysts were determined using a PerkinElmer instrument using the KBr technique. A Zahner Zennium electrochemical workstation was used for measuring the photocurrent response of the produced photocatalysts. The surface area of the prepared photocatalysts was examined using a Quantachrome Autosorb instrument. An RF-5301 spectrofluorophotometer, SHIMADZU, Japan, was utilized for examining the photoluminescence spectra of prepared photocatalysts. The V-570 spectrophotometer of Jasco, Japan, was used to determine the band gap energy of the prepared photocatalyst.

2.6. Photocatalytic Tests

A 0.250 L reactor of Pyrex having a window cell and flow system was used to examine the efficiency of MnCo2O4@g-C3N4 photocatalysts or sometimes named as photocatalytic efficiency against H2 emergence with constant magnetic stirring. A certain amount of photocatalyst was added in the presence of an aqueous solution of 10 vol % glycerol (200 mL). Before the photocatalytic process, Ar gas flow was sanitized for 0.25 h to overcome the water’s oxygen. A photoreactor-based 500 W xenon lamp (vertically fixed) in the presence of a UV cutoff screen (λ > 420 nm) was used. The photocatalytic H2 emergence process started by turning the lamp to an on-position. An interval time was used to measure H2 evolution throughout the photocatalytic reaction using an Agilent GC 7890A system gas chromatograph instrument.

3. Results and Discussion

3.1. Depiction of the Materials Obtained

The XRD diffractograms displayed all the expected peaks of g-C3N4 and MnCo2O4@g-C3N4 at different MnCo2O4 contents and were measured as shown in Figure 1A. XRD patterns of pure MnCo2O4 are shown in Figure 1B. The XRD pattern noted as A represents the original peak of the typical g-C3N4 phase existing at 27.4° (JCPDS no. 87-1526), evidencing a diffraction plane indexed as (002) together with 0.325 nm d-spacing.3,4 In MnCo2O4@g-C3N4 samples, there are no observable peaks for MnCo2O4, which may be credited to the high dispersion of small-sized MnCo2O4 on the surface of g-C3N4 or may be due to the lower detection limit of MnCo2O4 determined by the XRD instrument. Also, we noticed that the addition of MnCo2O4 to g-C3N4 leads to reduced g-C3N4 peak intensity. The pattern denoted as B reveals that all XRD peaks are referred to spinal MnCo2O4 (JCPDS no. 01-1130), as illustrated in Figure 1B.39

Figure 1.

Figure 1

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XRD patterns of g-C3N4 and MnCo2O4@g-C3N4 samples (A) and XRD pattern of MnCo2O4 sample (B).

The FT-IR spectra of parent g-C3N4 and 1.5 and 2.0 wt % MnCo2O4@g-C3N4 nanocomposites are illustrated in Figure 2. A peak at 807 cm–1, referring to the triazine unit stretching mode in pure g-C3N4 nanosheets, was observed.40,41 However, a small turn at 807 cm–1 for 1.5 and 2.0 wt % MnCo2O4@g-C3N4 nanocomposites has been observed, indicating a strong coupling between MnCo2O4@g-C3N4 nanoparticles and the g-C3N4 nanosheet.39 It is seen from the FT-IR spectra that the peak intensity is worthily reduced with the content increase of MnCo2O4. Peaks at 1639, 1575, 1439, 1325, and 1250 cm–1 are designated to the CN-heterocyclic stretching mode.42−44 Also, another broad peak suggesting the stretching vibration of the N–H group is mentioned in the range of 3500–3000 cm–1.45 Moreover, after MnCo2O4 nanoparticle incorporation with g-C3N4, there is an apparent confirmation that all the examined peaks are not shifted.

Figure 2.

Figure 2

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FT-IR spectra analysis of produced g-C3N4 photocatalysts compared to 1.5 and 2.0 wt % MnCo2O4@g-C3N4 samples.

Figure 3 displays the XPS spectra for the desired 1.5 wt % MnCo2O4@g-C3N4 products. Three peaks at ∼642.4, 644.1, and 645.6 eV and ∼652.3, 654.5, and 656.5 eV for Mn2p3/2 and Mn2p1/2 are observed correspondingly, as seen in Figure 3A, emphasizing the presence of Mn2+, Mn3+, and Mn4+ ions in MnCo2O4.39 Two peaks at ∼779.1 and 780.2 eV and ∼796.2 and 791.1 eV for Co2p3/2 and Co2p1/2 are observed correspondingly, as seen in Figure 3B, emphasizing the presence of Co2+ and Co3+ ions in MnCo2O4.39 Two peaks at ∼529.6 and 530.8 eV for O1s are observed, as displayed in Figure 3C, emphasizing the existence of oxygen ions in MnCo2O4.39,46 Peaks at ∼287.9 and ∼284.6 eV are two essential peaks depicted in Figure 3D, which are due to the hybridized sp2 carbon enlisted to nitrogen near the C–C bonds in the aromatic structure.47,48 The sp2 hybridization (pyridine carbon) is shown in (Figure 3D). The peak at 398.3 eV is due to the sp2 hybridization of the nitrogen atom. Mostly, the graphite-like C3N4 structure is created by the observation of all the peaks mentioned above.49,50

Figure 3.

Figure 3

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High-resolution XPS of 1.5 wt % MnCo2O4@g-C3N4 nanocomposite showing Mn2p (A), Co2p (B), O1s (C), C1s (D), and N1s (E).

Figure 4A displays the TEM observation of MnCo2O4 nanoparticles. The images indicate that the nanoparticles are symmetric and have average particle sizes of 8–10 nm. Figure 4B depicts the TEM images of g-C3N4, which show a clear graphene-resembling sheet structure as an apparent confirmation for the formation of the g-C3N4 nanosheet. In the 1.5 wt % MnCo2O4@g-C3N4 nanocomposites, TEM images show that MnCo2O4 nanoparticles are distributed uniformly onto the g-C3N4 nanosheet as seen in Figure 3C. The less agglomerated MnCo2O4 nanoparticles also have a size of 10–15 nm and keep the spherical shape. Figure 4D displays the 1.5 wt % MnCo2O4@g-C3N4 HR-TEM picture. The lattice planes of g-C3N4 (002) and MnCo2O4 (111) with 0.320 and 0.479 nm lattice spacing are observed, respectively, as shown in Figure 4D,39 which confirm MnCo2O4 and g-C3N4 heterojunction formation.

Figure 4.

Figure 4

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TEM image of MnCo2O4 (A), g-C3N4 (B), and 1.5 wt % MnCo2O4@g-C3N4 (C) samples and HRTEM image of 1.5 wt % MnCo2O4@g-C3N4 (D) sample.

The N2 adsorption–desorption isotherms of parent MnCo2O4, g-C3N4, and the 1.5 wt % MnCo2O4@g-C3N4 nanocomposite are depicted in Figure 5. The produced materials show mesoporous characteristics, which are mainly estimated by the H3 hysteresis at 0.62–0.88 virtual pressure, as shown in the type IV isotherms. Such results support the formation of porous, clear structures.51,52 The porous feature of g-C3N4 was preserved even after MnCo2O4 was distributed onto the g-C3N4 skim. The surface area values for pure MnCo2O4 and g-C3N4 are 240 and 175 m2/g, respectively. After MnCo2O4 incorporation with g-C3N4, the surface areas of 0.5, 1.0, 1.5, and 2.0 wt % MnCo2O4@g-C3N4 nanocomposites are 192, 203, 205, and 208 m2/g, respectively, as displayed in Table 1. The produced photocatalysts provide a highly efficient photocatalytic process through their obtained higher surface area. Moreover, the diffusion/mass transit is highly supported by the observed mesostructures, which also encourages the presence of light inside the pores that boosts the efficiency of solar energy.

Figure 5.

Figure 5

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Nitrogen adsorption–desorption isotherms of pure MnCo2O4, pure g-C3N4, and 1.5 wt % MnCo2O4@g-C3N4 nanocomposites.

Table 1. BET Surface Area of g-C3N4 and MnCo2O4@g-C3N4 Samples.

sample SBET (m2/g)
g-C3N4 175.00
0.5 wt % MnCo2O4@g-C3N4 192.00
1.0 wt % MnCo2O4@g-C3N4 203.00
1.5 wt % MnCo2O4@g-C3N4 205.00
2.0 wt % MnCo2O4@g-C3N4 208.00
MnCo2O4 240.00
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The spectra of UV–Vis for parent MnCo2O4, g-C3N4, and MnCo2O4@g-C3N4 heterojunctions are presented in Figure 6. The absorption range of the obtained MnCo2O4@g-C3N4 photocatalysts is visibly increased through the supplement of MnCo2O4 to the g-C3N4 sheets. An apparent movement for the g-C3N4 nanosheet absorption edge toward the blue region is mentioned, emulated to the MnCo2O4@g-C3N4 nanocomposites at variable MnCo2O4 contents as listed in Table 2. The 1.5 and 2.0 wt % MnCo2O4@g-C3N4 nanocomposites exhibit the smallest band gap energies (∼1.99 eV) that allow the minimum energy to get the extreme efficient electron transfer compared to pure MnCo2O4 or g-C3N4 (1.70 and 2.70 eV, respectively). Thus, the higher content of MnCo2O4 incorporated onto g-C3N4 resulted in the minimum band gap value of the produced MnCo2O4@g-C3N4 nanocomposites.53,54

Figure 6.

Figure 6

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UV–Vis spectra of pure MnCo2O4, g-C3N4, and different MnCo2O4@g-C3N4 samples.

Table 2. Band Gap of MnCo2O4, g-C3N4, and MnCo2O4@g-C3N4 Samples.

sample band gap (eV)
g-C3N4 2.70
0.5 wt % MnCo2O4@g-C3N4 2.24
1.0 wt % MnCo2O4@g-C3N4 2.12
1.5 wt % MnCo2O4@g-C3N4 1.99
2.0 wt % MnCo2O4@g-C3N4 1.96
MnCo2O4 1.70
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3.2. The Examination of H2 Progression

The molecular H2 generation experimentations were examined in the presence of pure MnCo2O4 nanoparticles, pure g-C3N4, and the heterojunctions of MnCo2O4@g-C3N4 nanocomposites at various MnCo2O4 contents (0.5–2.0 wt %) together with glycerol (10 vol %) as a hole scrounger. The experiments confirm that there is no H2 generation observed without using the photocatalyst or in the dark. Figure 7A displays the molecular H2 production versus overall time course above photocatalysts. A noticeable H2 evolution activity is observed in the parent MnCo2O4 and g-C3N4 under visible lighting at room temperature for 9 h (nearly 972 and 400 μmol·g–1·h–1). This is ascribed to the quick charge carrier’s reincorporation, remarking that parent photocatalysts are both not efficient for photocatalytic H2 production.55 The data show that the variable amounts of MnCo2O4 onto g-C3N4 possess a considerable impact on H2 production. The production of H2 is highly raised by raising the MnCo2O4 contents by 1.5 wt %. The H2 value is highly increased and reaches a maximum of 15,660 μmol·g–1·h–1 in the presence of a 1.5 wt % MnCo2O4@g-C3N4 nanocomposite. The adjustable content of MnCo2O4 nanoparticles, together with high dispersion on the g-C3N4 surface, plays a significant role in getting such an explanation. This agrees with the TEM images (Figure 4C), in which the hole–electron pair separation is highly associated.56 The primary principles for producing the H2 process using the MnCo2O4@g-C3N4 nanocomposite refer to four main factors, including crystallinity, band gap tightening, crystallite sizes, and large surface area. Figure 7B exhibits the impact of the 1.5 wt % MnCo2O4@g-C3N4 photocatalyst dose on H2 yield, which enhances to 10,550, 12,150, 15,660, 19,755, and 21,870 μmol·g–1·h–1 with increasing photocatalyst content from 0.5 to 2.5 g/L. Such results are due to the number of active sites on the 1.5 wt % MnCo2O4@g-C3N4 surface, which is increased. The higher content from the photocatalyst resulted in the decrease in H2 yield (more than 2.5 g/L reduces the yield to 18,144 μmol·g–1·h–1). This has resulted from the reduction in the light infiltration through the photocatalyst surface as the particles’ number is increased.

Figure 7.

Figure 7

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Hydrogen generation using the produced photocatalysts (A). Impact of the dose of the optimized 1.5 wt % MnCo2O4@g-C3N4 nanocomposite (B).

To emphasize the obtained data, the PL spectra and PC responses for all produced samples were examined, as displayed in Figure 8. The findings of the PL spectra exhibit that high emission intensity is observed in g-C3N4. However, a measured reduction in the intensity, and a redshift of peak position, is detected with increasing MnCo2O4 content into the g-C3N4 sheet in the following order: g-C3N4 > MnCo2O4 > 0.5 wt % MnCo2O4@g-C3N4 > 1.0 wt % MnCo2O4@g-C3N4 > 1.5 wt % MnCo2O4@g-C3N4 > 2.0 wt % MnCo2O4@g-C3N4. Pure MnCo2O4 and g-C3N4 show higher PL intensities, which may be investigated by hastening the reincorporation rate of holes and electrons, and this might give rise to a noticeable decrease in the H2 yield. Moreover, the PL intensity is reduced as MnCo2O4 nanoparticles are introduced onto the g-C3N4 sheet. Such behavior resulted from the delay in the charge carrier’s reincorporation rate, which created a high H2 yield. The transient photocurrent responses for all produced photocatalysts are illustrated in Figure 8B. Depending upon the results, it is clearly mentioned that all nanocomposites have high photocurrent densities and the values are noticeably raised with increasing MnCo2O4 contents as follows: g-C3N4 < MnCo2O4 < 0.5 wt % MnCo2O4@g-C3N4 < 1.0 wt % MnCo2O4@g-C3N4 < 1.5 wt % MnCo2O4@g-C3N4 < 2.0 wt % MnCo2O4@g-C3N4. PC and PL data for the 1.5 wt % MnCo2O4@g-C3N4 nanocomposite exhibit reduced PL values and elevate the photocurrent compared with pure g-C3N4. Such findings totally confirm the results mentioned above. The fundamental stumbling block of hastening the rate of reincorporation for holes and photons/electrons in pure g-C3N4 has been completely barred through the production of MnCo2O4@g-C3N4 nanocomposites and wonderfully enhances the photocatalytic efficiency. These measurements have supported the demonstration based on the increase in the photocatalytic rendering joint with the charge carrier reincorporation rate reduction. The most important key agents in this process are based on the photocatalyst stability and durability and the major directions of its workable recyclability. The steadiness of the produced 1.5 wt % MnCo2O4@g-C3N4 nanocomposite has been examined by reusing the photocatalyst five times in the production of H2 photocatalytic activity, as presented in Figure 9. The data also specified no conspicuous reduction in the photocatalytic H2 evolution activity for the produced material. Visible light illumination is exposed to five times cycles as the maximum number of cycles, and such an effect proposes that the 1.5 wt % MnCo2O4@g-C3N4 nanocomposite demonstrates high stability through visible lighting. The most common notified empirical equations for determining both valence and conduction band positions of photocatalysts are shown below, as the literature illustrates.57

3.2. 1
3.2. 2

Figure 8.

Figure 8

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PL spectra (A) and transient photocurrent measurements (B) of pure MnCo2O4, g-C3N4, and varied MnCo2O4@g-C3N4 photocatalysts.

Figure 9.

Figure 9

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Impact of the 1.5 wt % MnCo2O4@g-C3N4 photocatalyst on the hydrogen production, showing the stability for five consecutive runs.

The band gap values of both pure MnCo2O4 and g-C3N4 are presented as Eg, while Ee is the hydrogen scale-free electron energy equal to 4.5 eV. The electronegativity of pure MnCo2O4 nanoparticles and g-C3N4 is presented as X, and the values are 4.77 and 4.72 eV, respectively. The conduction band positions of pure MnCo2O4 nanoparticles and the g-C3N4 sheet acquired from eqs 1 and 2 are −0.58 and −1.13 eV, respectively. The valence bands of MnCo2O4 nanoparticles and g-C3N4 sheets amount to +1.12 and +1.57 eV, respectively. The attained values of valence bands and conduction bands acquired a display that the expected mechanism for the production of H2 in the presence of the MnCo2O4@g-C3N4 nanocomposite supports a heterojunction as the literature illustrates, as shown in Scheme 1.58 In such a scheme, as the MnCo2O4@g-C3N4 surfaces are attacked by the photons, a wide range of visible light is collected by g-C3N4, followed by a logical excitation for the electrons to the conduction band from its valence band, which consequently move to the MnCo2O4 conduction band for H2 formation through the reduction of H+. Therefore, the photogenerated holes departed in the g-C3N4 valence band consumed for the glycerol oxidation, as illustrated in Scheme 1. Such a process led to the segregation of photogenerated electrons and holes; hence, the reincorporation rate turns relatively weak.

Scheme 1. Photocatalytic Process of 1.5 wt % MnCo2O4@g-C3N4 Photocatalyst for Hydrogen Production.

Scheme 1

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4. Conclusions

In summary, a sol–gel technique has been applied to prepare MnCo2O4, g-C3N4, and MnCo2O4@g-C3N4 nanocomposites at variable MnCo2O4 amounts for promoting H2 production. XRD shows the successful formation of MnCo2O4 that is integrated onto the g-C3N4. The MnCo2O4 nanoparticles show 8 nm spherical shapes from TEM images. The nanoparticles are also spread uniformly inside the g-C3N4 nanosheet. The surface-active sites, as well as the light gathering, are significantly increased by the MnCo2O4 nanoparticles. The MnCo2O4@g-C3N4 nanocomposites show a significant increase in the H2 evolution yields about ∼55 and 23 times than those observed for g-C3N4 and neat MnCo2O4, respectively. The confirmed increase in the photocatalytic efficiency regarding its H2 production is mainly emphasized by a narrow band gap, better charge carrier separation, and more surface-active sites. An insignificant decrease in the photocatalytic competence for the optimized MnCo2O4@g-C3N4 photocatalyst has been observed after five times photocatalytic cycles. Such observation enhances the higher probability of its stability and durability via the visible lighting process.

Acknowledgments

The authors would like to thank Taif University Researchers Supporting Project (number TURSP-2020/158), Taif University, Taif, Saudi Arabia, for supporting this work.

The authors declare no competing financial interest.

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