Generation of Hydrogen Gas Using CuCr2O4-g-C3N4 Nanocomposites under Illumination by Visible Light.

In this research, nanocomposites made of CuCr2O4-g-C3N4 accommodating distinct contents of CuCr2O4 (1-4 wt %) nanoparticles (NPs) were endorsed for hydrogen gas production after illumination by visible light in the presence of aqueous glycerol solution. The ultrasonication-mixture method was applied to assure the homogeneous distribution of CuCr2O4 NPs over synthesized mesoporous g-C3N4. Such nanocomposites possess suppressed recombination between the photoinduced charges. High-resolution transmission electron microscopy and X-ray photoelectron spectroscopy examinations affirmed the formation of CuCr2O4-g-C3N4 heterojunctions. The separation between the induced charges and the photocatalytic performance with the CuCr2O4 NP amount were investigated. The CuCr2O4-g-C3N4 heterojunction of 3 wt % CuCr2O4 content was documented as the optimal heterojunction. Upgraded hydrogen gas generation was attained over the optimal heterojunction with the extent of ten and thirty times as those registered for pure CuCr2O4 and g-C3N4 specimens, respectively, under illumination by visible light. The photocatalytic performance acquired by the diverse synthesized specimens was assessed not only by their effectiveness to absorb light in the visible region but also by their potential to separate the photoinduced charges.

Introduction

In recent years, numerous investigations were accomplished regarding impressive generation of hydrogen gas as a clean source of energy, using photocatalysis methods.[1] Water is considered one of the most important sources of hydrogen gas production that could be used to generate hydrogen gas with the aid of photocatalysis. Nevertheless, such a process is characterized by the production of a low yield of hydrogen gas owing to the large capability of reversible reaction to occur between the produced gases. Consequently, many efforts have been performed to find another adequate and efficient source for hydrogen production. Among the efficient hydrogen sources that exhibited powerful results in hydrogen gas production via photocatalysis are biomass-originated compounds like bio-oil, sugars, alcohols, etc.[1−3]

In the same context, glycerol was used as an efficient and economical source of hydrogen generation via the aid of photocatalysis owing to its great abundance. The first trials to generate hydrogen gas from the biomass-originated compounds by photocatalysis methods were achieved over metal oxide semiconductors (like titania) decorated with noble metals.[4,5] Nevertheless, confined efficiency was attained using these photocatalysts under illumination by visible light. So, efforts were directed to explore alternatives for such systems, and it was concluded that graphitic carbon nitride (g-C3N4), an n-type semiconductor, exhibited excellent efficiency toward the photocatalytic generation of hydrogen gas owing to its admirable characteristics like finite band gap energy, a reasonable ability to absorb visible light, large stability, and excellent electronic characteristics.[6]

Numerous kinds of g-C3N4 semiconductors of improved characteristics (surface, electronic, and morphological) were attained experimentally by different methods where each experimental process produces a certain kind of g-C3N4 semiconductor.[7] Although acceptable properties are possessed by the synthesized g-C3N4 semiconductor, its photocatalytic performance when subjected to visible light is still confined owing to the ease of recombination amid the photoinduced charges in addition to its rather limited surface area.[7,8] Consequently, the creation of g-C3N4-containing compounds acquiring enhanced photonic efficiency was of great interest to many researchers, which was accomplished via various pathways such as mixing g-C3N4 with another semiconductor and decoration of g-C3N4 with noble metals.[4,9,10]

Construction of heterojunctions made of g-C3N4 mixed with another semiconductor and preparation of a g-C3N4 semiconductor of improved surface texture via selecting the suitable preparation method are the most advantageous pathways to attain upgraded photocatalytic characteristics of g-C3N4, which, by their roles, increase the capability of the photocatalyst to absorb visible light. Briefly, improvement of the surface texture of g-C3N4 could be attained via either preparing g-C3N4 of a mesoporous structure utilizing an adequate template or via exfoliating.[11−14] On the other hand, the progression of heterojunctions of boosted photocatalytic performance could be realized via mixing g-C3N4 with another semiconductor.

The basic principle of the latter method depends mainly on suppressing the recombination amid the photoinduced charges as well as enhancing the charges’ speed.[15] g-C3N4 could be mixed with various materials like phosphides, selenides, oxides, and sulfides as adequate semiconductors to construct heterojunctions of advanced photocatalytic performance.[16−20] On the other hand, owing to the admirable properties possessed by CuCr2O4 such as confined band gap energy (1.40 eV), it was used as a successful semiconductor of p-type for diverse photocatalytic applications like supercapacitors, ceramic pigments, photocatalytic decomposition of some pollutants, some selective conversion reactions, and photocatalytic hydrogen generation.[21−29] Consequently, the construction of heterojunctions made of g-C3N4 nanosheets decorated with diverse CuCr2O4 proportions is expected to introduce an advanced photocatalyst that is capable of absorbing a large quantity of visible light when illuminated, separating effectively the induced charges and enhancing the photocatalytic reaction rate.

In this investigation, novel CuCr2O4-g-C3N4 nanocomposites were synthesized via decoration g-C3N4 with diverse proportions of CuCr2O4, aiming to boost visible-light absorption and to accomplish enhanced photocatalytic generation of hydrogen gas from aqueous glycerol solution. Various techniques were applied to characterize and identify the synthesized specimens. Crystallinity and phase structure of the prepared samples were explored by X-ray diffraction (XRD), energy dispersive X-rays (EDX), and Fourier transform infrared spectroscopy (FTIR) analyses. Meanwhile, the surface area of the synthesized specimens was measured by applying the Brunauer–Emmett–Teller (BET) formula. The morphology and microstructure of the prepared specimens were explored by transmission electron microscopy (TEM) investigation. Chemical states of the diverse specimens were assessed by X-ray photoelectron spectroscopy (XPS) evaluations. UV–vis diffuse reflectance spectroscopy (UV–vis DRS) was performed to discover the optical characteristics of the synthesized specimens in addition to the estimation of the band gap values. On the other hand, the separation between the photoinduced charges was estimated by photoluminescence (PL) evaluation. The photocatalytic performance of the synthesized photocatalysts toward photocatalytic hydrogen gas production from glycerol was assessed.

Materials and Experimental Routine

Materials

All chemicals and reagents were delivered from Sigma-Aldrich, and they were utilized in this examination without subsequent treatment. The main materials involved in this research were urea, chromium nitrate(III) nonahydrate, copper nitrate trihydrate, and dicyandiamide. On the other hand, a nonionic surfactant triblock copolymer (Pluronic L-64) with a molecular weight of 2900 g/mol was utilized as a template. Furthermore, the following materials were utilized as reagents: cyclohexane, absolute ethyl alcohol, acetic acid, hydrochloric acid, and formic acid.

Construction of Mesoporous CuCr2O4

This section describes the construction of mesoporous CuCr2O4 NPs with Pluronic L-64 as a nonionic surfactant. Initially, a mixture of absolute ethyl alcohol (30 mL) and Pluronic L-64 (0.4 g) was stirred for 1 h to assure homogeneous distribution of the template. Then, 5.5 and 15.4 g of copper nitrate trihydrate and chromium nitrate nonahydrate, accordingly, were introduced to the surfactant solution accompanied by prolonged stirring. After that, 2.5 and 0.8 mL of acetic acid and hydrochloric acid were therefore added to the previously attained system. The produced sol was kept at a temperature of 40 °C in a humidity of about 60%. Finally, the dried sol was maintained at 65 °C for 12 h to attain extra drying followed by heating for 3 h at 550 °C in air, and so, the mesoporous CuCr2O4 NPs were produced.

Construction of the Mesoporous g-C3N4 Nanosheets

In this part, mesoporous g-C3N4 nanosheets were synthesized via the aid of the previously prepared MCM-41 hard template.[30] The method of preparation of the mesoporous g-C3N4 nanosheets is principally dependent on the calcination of a mixture containing dicyandiamide and urea in the presence of MCM-41 for 4 h at 550 °C. Initially, deionized water (50 mL) was used to assure the homogeneous dispersion of the MCM-41 template (1 g) via the aid of sonication for 30 min. After that, 5 and 3 g of urea and dicyandiamide, accordingly, were introduced gradually to the previously obtained mixture accompanied with stirring for 3 h at 80 °C. Then, the previous mixture was heated at 80 °C to remove water completely. Finally, the system was burnt at 550 °C in air for 4 h. The MCM-41template was removed from the product via leaching the previously produced powder by 2 M NH4HF2 (50 mL) accompanied with prolonged stirring overnight. Finally, the unreacted species were removed from the produced mesoporous g-C3N4 nanosheets after washing with deionized water followed by drying at 100 °C overnight to attain the pure mesoporous g-C3N4 nanosheets.

Construction of the Mesoporous CuCr2O4-g-C3N4 Heterojunctions

In this section, mesoporous CuCr2O4-g-C3N4 heterojunctions accommodating 1, 2, 3, and 4 wt % CuCr2O4 NPs were obtained via dispersing CuCr2O4 nanoparticles on the surface of g-C3N4 nanosheets. Initially, a mixture of C6H14 (10 mL) and ethanol (20 mL) was utilized to distribute the mesoporous g-C3N4 nanosheets (mixture A). At the same time, cyclohexane was used to disperse a definite quantity of CuCr2O4 nanoparticles (mixture B). Then, the mixtures (A and B) were mixed together accompanied by stirring for 3 h. The solid product was separated from the resultant mixture via centrifugation for 15 min. The mesoporous CuCr2O4-g-C3N4 heterojunctions were obtained after drying the previously obtained solid powder for 12 h at 110 °C. Similarly, CuCr2O4-g-C3N4 heterojunctions accommodating other portions of CuCr2O4 nanoparticles were produced.

Identification and Characterization of the Prepared Materials

Crystallinity and phase structure of the prepared specimens were explored by X-ray diffraction (XRD). XRD analysis was performed via a Bruker X-ray diffractometer with Cu Kα radiation. Fourier transform infrared spectroscopy (FTIR) analysis was performed using a PerkinElmer Fourier transform infrared spectrometer (FTIR) in the range of 400–4000 cm–1. Meanwhile, N2 adsorption–desorption isotherms of some selected specimens were attained by utilizing a Chromatech instrument (Nova 2000 series). Furthermore, the surface area of the synthesized specimens was measured by applying the Brunauer–Emmett–Teller (BET) formula from the obtained isotherms. The morphology and microstructure of the prepared specimens were explored by transmission electron microscopy (TEM) investigation using a TEM instrument (JEOL-JEM-1230). Chemical states of the diverse specimens were assessed by X-ray photoelectron spectroscopy (XPS) evaluations that were attained by the application of a Thermo Scientific K-ALPHA spectrometer. UV–vis diffuse reflectance spectroscopy (UV–vis DRS) was performed to discover the optical characteristics of the synthesized specimens in addition to the estimation of the band gap values derived from Tauc’s method. DRS was performed by utilizing a Jasco V-570 spectrophotometer (Japan). On the other hand, separation between the photoinduced charges was estimated by photoluminescence (PL) evaluation that was performed using a Shimadzu RF-5301 spectrophotometer (a Xe lamp as an excitation source). The transient photocurrent of the synthesized samples was evaluated using a Zahner Zennium electrochemical system.

Photocatalytic Action Tests

The photocatalytic performance of the synthesized CuCr2O4-g-C3N4 photocatalysts against photocatalytic hydrogen production in the presence of glycerol was assessed under illumination by visible light. In this test, a definite amount (50 mg) of the examined photocatalyst was dispersed in a 250 mL Pyrex reactor containing 200 mL of aqueous glycerol solution (10 vol %). The system was subjected to Argon gas bubbling to attain the desired stirring in addition to getting rid of the oxygen gas from the photocatalytic system. The system was left for a certain interval in the dark to permit the adsorption equilibrium to be reached. The photoreactor was then illuminated by visible light via light sourced from a Xe lamp (500 W) using a UV cutoff filter to assure the existence of radiation of a longer wavelength (>420 nm) only. The quantity of hydrogen gas produced at definite intervals (each 1 h) was estimated by utilizing a gas chromatograph (Agilent GC 7890A system) all over the reaction.

Results and Discussion

Identification and Characterization of the Synthesized CuCr2O4-g-C3N4 Nanocomposites

Phase composition of the synthesized specimens was discovered from the interpretation of the X-ray diffraction patterns. Evidently, Figure demonstrates X-ray diffraction patterns of the heterojunctions containing 0, 1, 2, 3, and 4 wt % CuCr2O4. The appearance of the peaks characterizing the (100) and (002) planes of C3N4 nanosheets at 2θ = 13.0 and 27.4°, respectively, in the X-ray diffraction patterns of pure g-C3N4 as well as of synthesized heterojunctions confirms the existence of pristine g-C3N4 within these specimens (JCPDS# 087-01526).[7,8] The small proportions of the included CuCr2O4 NPs and/or their homogeneous distribution might be interpreted as the absence of the diffraction patterns of CuCr2O4 NPs within the X-ray diffractograms of the prepared heterojunctions. It could be derived from the data of Figure that g-C3N4 retains its peaks’ position even after incorporation with CuCr2O4 NPs in the constructed heterojunctions, affirming that g-C3N4 conserves its graphitic layer arrangement. Nevertheless, exfoliation of g-C3N4 graphitic layers in the heterojunctions during sonication owing to the inclusion of CuCr2O4 NPs could be confirmed from the suppression of peak intensity assigned to the (002) plane of g-C3N4.[31−34] The inset X-ray diffractogram of neat CuCr2O4 displays the peaks characterizing the planes assigned to the crystalline CuCr2O4 planes (JCPDS# 05-0657).[29,30]

Figure 1

XRD patterns of the synthesized heterojunctions of 0–4 wt % CuCr2O4. The inset is the XRD pattern of CuCr2O4.

FTIR spectral curves of the prepared specimens could provide extra knowledge about their phase composition (Figure ). Neat g-C3N4 displays FTIR spectral bands at 808, 1639, 1567, 1407, 1319, 1240, and 3300 cm–1. The occurrence of triazine units is declared from the existence of a spectral band at 808 cm–1. Meanwhile, the presence of C–N–H and C–N heterocycles is asserted from the appearance of spectral bands at 1639, 1567, 1407, 1319, and 1240 cm–1.[35,36] The spectral peak at 3300 cm–1 affirms the presence of the stretching vibration of −NH2 or NH bonds. On the other hand, the spectral bands identifying pure g-C3N4 are retained in the FTIR spectral curves of the diverse synthesized CuCr2O4-g-C3N4 heterojunctions without the appearance of the bands assigned to CuCr2O4, confirming the conservation of the graphitic C–N lattice in the synthesized heterojunctions after incorporating g-C3N4 with CuCr2O4.[26] Furthermore, the inclusion of higher contents of CuCr2O4 in the CuCr2O4-g-C3N4 heterojunctions leads to the appearance of g-C3N4 spectral bands with declined intensities. This observation demonstrates that the functional groups (−NH2, −NH–, and =N−) of the g-C3N4 surface are affected owing to the inclusion of CuCr2O4 in such a way that permits the binding of the CuCr2O4 NPs on the g-C3N4 surface.[36,37]

Figure 2

FTIR spectral curves of the synthesized heterojunctions of 0–4 wt % CuCr2O4.

XPS of the Cu 2p, Cr 2p, O 1s, C 1s, and N 1s energy levels could be used to identify the valence as well as the chemical states within the specimen made of 3 wt % CuCr2O4-g-C3N4 heterojunctions (Figure ). XPS of the Cu 2p energy level shows that Cu2+ ions are the main chemical states of the Cu element that could be affirmed from the existence of Cu 2p3/2 and Cu 2p1/2 energy levels of the peaks at 932.3 and 952.2 eV (Figure A). In addition, the inclusion of a peak with declined intensity in between the previously mentioned energy levels confirms the presence of Cu as Cu2+ ions (XPS of Figure A).[22,38] On the other hand, XPS of the O 1s energy level shows that oxygen anions are the main chemical states of the oxygen element in CuCr2O4 that could be affirmed from the existence of the O 1s energy level of the peak at 531.3 eV (Figure B).[39] Furthermore, the existence of the Cr metal in the form of Cr3+ oxidation state has been affirmed from the inclusion of the peaks at the binding energies of 576.5 and 586.4 eV that are connected with Cr 2p3/2 and Cr 2p1/2 energy levels (Figure C).[38] Meanwhile, the description of XPS of the C 1s energy level concludes the existence of sp2 hybridization of C=C or CN2 bonds owing to the appearance of the peak at 287.8 eV in addition to the existence of sp2 hybridization of N–C=N bonds owing to the occurrence of the peak at 286.0 eV. Also, the existence of a C–C bond is confirmed from the occurrence of the peak at 284.6 eV, accordingly (Figure D).[39−41] The occurrence of the peaks assigned to C–N bonds with relatively lower intensities affirms the inclusion of CuCr2O4 NPs to the g-C3N4 structure. XPS of the N 1s energy level shows the existence of C–N=C and N–C3/C–NH2 groups within the structure of the heterojunction that could be affirmed from the existence of the peaks at 400.6 and 398.4 eV, accordingly along with little displacement in comparison to the same peaks for XPS of neat g-C3N4.[42,43] Latter information supports that CuCr2O4 NPs are successfully incorporated to the g-C3N4 surface.

Figure 3

XPS spectra of the 3 wt % CuCr2O4-g-C3N4 sample for Cu 2p (A), O 1s (B), Cr 2p (C), C 1s (D), and N 1s (E).

Knowledge about the microstructure and morphology of the prepared 3% CuCr2O4-g-C3N4 nanocomposites as well as those of the neat ingredients could be provided by interpreting the TEM images of the examined specimens (Figure A–C). The successful synthesis of the nanosheet structure of C3N4 of partial aggregation along with some extent of porosity, owing to the utilization of an MCM-41 hard template in the preparation step, is affirmed from the TEM image of Figure A. The development of irregular particles with the dimension of 12–16 nm is clear in the TEM image of neat CuCr2O4 (Figure B). These particulates are partially aggregated to form clusters of a dimension larger than 100 nm. Figure C clarifies the successful homogeneous distribution of the spherical CuCr2O4 NPs (6–8 nm) over the exfoliated g-C3N4 surface within the TEM image of the heterojunction containing 3 wt % CuCr2O4. Actually, the suppressed particle dimension possessed by the dispersed CuCr2O4 NPs within the nanocomposite when connected with that of the neat ones is assigned to the sonication technique applied in the preparation process. The HRTEM image of the 3 wt % CuCr2O4-g-C3N4 heterojunction could provide information about the lattice planes of its components (Figure D). The existence of the (002) lattice plane of g-C3N4 is affirmed from the appearance of its characteristic lattice spacing of 0.280 nm. On the other hand, the existence of the (111) lattice plane of CuCr2O4 is confirmed from the appearance of its distinct lattice spacing of 0.476 nm. In addition, the successful construction of heterojunctions between the examined ingredients could be confirmed from their close connection in the illustrated image. The concluded remarks of TEM and HRTEM analyses agree with those of XRD investigation.

Figure 4

(A–C) TEM images for pure g-C3N4, pure CuCr2O4, and the 3 wt % CuCr2O4-g-C3N4 heterojunction. (D) HRTEM image of the 3 wt % CuCr2O4-g-C3N4 heterojunction.

Surface and textural characteristics of the heterojunction containing 3 wt % CuCr2O4 as well as those of neat ingredients could be provided from the description of adsorption–desorption isotherms illustrated in Figure . In addition, Table shows the surface area of the prepared specimens as considered from the BET formula. A complex mesoporous matrix of neat g-C3N4 is confirmed from the progression of a type IV isotherm along with an H3 hysteresis loop. Interestingly, Table shows that neat g-C3N4 possesses a surface area of 180 m2/g, which is much larger than that possessed by the conventional one.[42] On the other hand, mesoporous texture of neat CuCr2O4 NPs that possess a large surface area (205 m2/g), owing to the utilization of a Pluronic L-64 template during their synthesis, could be affirmed from the data of Figure and those of Table . The occurrence of pores with a cylindrical shape within CuCr2O4 NPs is illustrated from the presence of a well-identified H3 hysteresis loop in their isotherm. The isotherm acquired by the synthesized heterojunction is of a similar feature as that of neat g-C3N4 with suppressed hysteresis. Such observation illustrates the mesoporous texture with wedge-shaped pores of the heterojunction that might have progressed during exfoliation. The upgrading of the specific surface area acquired by the heterojunction containing a larger quantity of CuCr2O4 NPs is clear from the magnitudes of Table . Such conclusions agree with those attained by previous analyses, and all of them support the successful exfoliation of g-C3N4 layers in addition to the successful interaction of CuCr2O4 NPs with g-C3N4.

Figure 5

Adsorption/desorption isotherms of pure g-C3N4, CuCr2O4, and 3 wt % CuCr2O4-g-C3N4 specimens.

Table 1

BET Surface Area of the Prepared Specimens

samples	SBET (m2/g)
g-C3N4	180.00
1.0 wt % CuCr2O4@g-C3N4	184.00
2.0 wt % CuCr2O4@g-C3N4	185.00
3.0 wt % CuCr2O4@g-C3N4	187.00
4.0 wt % CuCr2O4@g-C3N4	190.00
CuCr2O4	205.00

UV–vis DR spectra of the synthesized specimens could provide information about the optical absorption response of these specimens (Figure ). An intensified absorption band in the visible range with an absorption edge at 460 nm is obvious within the spectral curve of g-C3N4. Figure affirms that absorption possessed by the synthesized heterojunctions is not only advanced toward a longer wavelength (visible light) but also occurred with greater intensity owing to the connection between CuCr2O4 NPs and g-C3N4. This advancement is magnified with the increase in the CuCr2O4 portion and optimized at the ratio of wt % CuCr2O4.

Figure 6

UV–vis DRS of the synthesized specimens.

Tauc’s method was applied to formulate the band gap energy of the diverse specimens to provide knowledge about the heterojunctions established between g-C3N4 and CuCr2O4 within the constructed CuCr2O4-g-C3N4 heterojunctions (Table ).[21] It is indicated that the band gap acquired by neat g-C3N4 is 2.70 eV. Meanwhile, this value is suppressed after the incorporation with CuCr2O4 to formulate the novel nanocomposites, and the suppression is obvious for the specimens containing large portions of CuCr2O4 NPs. This observation affirms the successful establishment of heterojunctions of lower band gap energy and so enhanced photocatalytic performance in the visible region.

Table 2

Band Gap Magnitudes of the Synthesized Specimens

samples	band gap, eV
g-C3N4	2.70
1.0 wt % CuCr2O4@g-C3N4	2.45
2.0 wt % CuCr2O4@g-C3N4	2.34
3.0 wt % CuCr2O4@g-C3N4	2.20
4.0 wt % CuCr2O4@g-C3N4	2.18
CuCr2O4	1.40

Capability of the Photocatalytic H2 Gas Generation

In this section, the capability of the synthesized photocatalysts toward hydrogen gas generation when exposed to visible light is shown in Figure . The decreased capability of neat g-C3N4 and neat CuCr2O4 toward hydrogen gas generation is obvious from the data of Figure , which could be credited to the great ability of the induced charges to be recombined in these specimens. On the contrary, the capability of hydrogen gas generation is boosted over the CuCr2O4-g-C3N4 heterojunctions in comparison to those of g-C3N4. Furthermore, the heterojunctions of large CuCr2O4 portions (3 wt %, 10,776 μmol/g) display the greatest photocatalytic capability of hydrogen gas generation in comparison to other specimens. Subsequent inclusion of CuCr2O4 (greater than 3 wt %) displays no further improvement in the capability of hydrogen gas generation. Actually, the heterojunction containing 3 wt % CuCr2O4 is capable of photocatalyzing hydrogen gas generation (10,776 μmol/g) thirty times of that possessed by pure g-C3N4 (360 μmol/g).

Figure 7

H2 generation capability adopting the diverse prepared specimens.

The consequence of the photocatalyst dose (0.4–2.4 g/L) for the optimized heterojunction (3% CuCr2O4-g-C3N4) on the ability of hydrogen gas generation is assessed (Figure A). The capability of the optimized heterojunction to produce hydrogen gas is improved continuously, reaching its maximum magnitude (12,600 μmol/g), with the increase in the photocatalyst dose up to 1.6 g/L. This optimized dose (1.6 g/L) represents the greatest number of exposed active sites without pronounced hindrance of visible-light absorption that might take place owing to the possible particle aggregation when a higher dose is applied.[44]

Figure 8

(A) Consequence of the 3 wt % CuCr2O4-g-C3N4 load. (B) Stability of the 3 wt % CuCr2O4-g-C3N4 heterojunction.

The capability of the regenerated 3 wt % CuCr2O4-g-C3N4 photocatalyst to photocatalyze the hydrogen generation reaction many times (5 runs) was assessed (Figure B). The data illustrates that excellent efficiency is attained by the recycled photocatalyst even after application for five times, affirming the greater stability and applicability of such a nanocomposite.

Photocatalytic Performance and the Proposed Mechanism of CuCr2O4-g-C3N4 Heterojunctions

Obviously, the improved photocatalytic performance of the examined heterojunctions toward hydrogen generation when exposed to visible light is connected with their boosted visible-light absorption (Figure ), which is supported by their declined band gap values (Table ). These factors operate in the same direction, which is the enhancement of the photocatalytic performance of the progressed heterojunctions owing to the improvement of the generation of the photoinduced charges (especially for the specimens of higher CuCr2O4 contents).

Not only the capability of the photocatalyst to absorb visible light and/or to induce the generation of the charges but also their performance to separate the induced charges govern the photocatalytic efficiency. So, PL emission spectra of the examined photocatalysts were assessed (Figure ). The improved ability of the CuCr2O4-g-C3N4 heterojunctions to separate the induced charges and to hinder their recombination in comparison to those of the pure components is obvious from the suppressed PL emission spectra attained by these heterojunctions (Figure ).[45,46] The CuCr2O4 content within the formulated heterojunction plays an important role in the PL emission suppression, which becomes more pronounced for the heterojunction of 3 wt % CuCr2O4. So, the developed heterojunctions between the pure components (g-C3N4 and CuCr2O4 NPs) of the CuCr2O4-g-C3N4 nanocomposite, optimized at the 3 wt % CuCr2O4 content, succeed to hinder the recombination between the induced charges.

Figure 9

(A) PL spectra of the prepared specimens. (B) Transient photocurrent of the synthesized specimens.

The photocurrent response of the prepared specimens was assessed in order to examine the separation between the induced charges (Figure B). Enhanced photocurrent density is acquired by the specimens made of CuCr2O4-g-C3N4 heterojunctions of various contents of CuCr2O4 NPs in comparison to those of the pure components.[47] Furthermore, the improvement of the photocurrent response is found to be controlled by the CuCr2O4 content that becomes maximum at 3 wt % CuCr2O4. Subsequent inclusion of CuCr2O4 NPs displays no extra improvement in the photocurrent density values, affirming that the nanocomposites that accommodate 3 wt % CuCr2O4 NPs are capable of separating between the induced charges with the maximum efficiency in comparison to other specimens.

Previous investigations affirm that the surface area of the examined photocatalysts, separation between the induced charges, dispersion of the CuCr2O4 NPs, recombination between the induced charges, and the capability of the photocatalysts to absorb visible light are the most essential parameters that govern the photocatalytic performance.

The constructed CuCr2O4-g-C3N4 heterojunction is composed of two semiconductors: an n-type semiconductor (g-C3N4) and a p-type semiconductor (CuCr2O4), and so, two mechanisms are proposed to describe the separation between the induced charges. The first proposed mechanism is type II p-n3. Meanwhile, the second mechanism is known as an S-scheme or Z-scheme mechanism.[48,49] Evidently, the potentials possessed by the valence and conduction bands govern immensely the pathway of the induced charges and so affect the redox reaction mechanism involving hydrogen production from glycerol solution.[50] The band edge position of both valence (EVB) and conduction (ECB) bands of the components of the CuCr2O4-g-C3N4 nanocomposite could be assessed from the following formula[49]where Eg is the band gap of the semiconductor, X is the electronegativity, and Ee is the energy of the free electron (4.5 eV). The assessed potentials of valence bands of g-C3N4 and CuCr2O4 are in the sequence of +1.56 and +1.92 eV, while those of the conduction bands are in the sequence of −1.12 and +0.52 eV. The potential possessed by the conduction band of CuCr2O4 is lower than that of g-C3N4, and so, the photoinduced electrons are transferred from the conduction band of g-C3N4 to that of CuCr2O4. The proposed mechanism of such transference could be stated as a Z-scheme rather than a type II p-n heterojunction one. This conclusion could be attributed to the fact that the redox potential of hydrogen ion–hydrogen conversion reaction is much larger than that of the transferred electrons on the CuCr2O4 conduction band (Figure ). We can summarize the proposed mechanism as follows: when the nanocomposite is exposed to visible light, electrons are induced and moved from the valence bands of the pure ingredients to the conduction bands, and at the same time, holes are preserved in the valence bands. After that, induced electrons are moved from the conduction band of g-C3N4 to that of CuCr2O4, whereas holes are moved from the valence band of CuCr2O4 to that of g-C3N4, bringing about enhanced separation as well as suppressed recombination between the photoinduced charges. So, hydrogen gas could be generated via reduction of hydrogen ions by the accumulated electrons in CuCr2O4 or via oxidation of glycerol via the accumulated holes in g-C3N4. In conclusion, the synthesized heterojunctions displayed upgraded photocatalytic performance toward hydrogen production from glycerol solution.

Figure 10

Proposed Z-scheme mechanism displayed by CuCr2O4-g-C3N4 heterojunctions.

Conclusions

A simple ultrasonication-assisted routine was adopted to synthesize CuCr2O4-g-C3N4 heterojunctions of varied proportions of CuCr2O4. The synthesized heterojunctions displayed upgraded physicochemical and photocatalytic features like expanded surface area, adequate dispersion of the CuCr2O4 NPs over the g-C3N4 surface, a large capability to absorb visible light, and suppressed recombination as well as boosted separation between induced charges. The quantity of CuCr2O4 within the heterojunction played an important role in controlling the photocatalytic characteristics of the synthesized heterojunctions reaching its optimum content at 3 wt %. The prepared heterojunctions displayed upgraded performance toward hydrogen generation from glycerol solution when subjected to visible light in comparison to those of the pure ingredients. The 3 wt % CuCr2O4-g-C3N4 heterojunction was able to photoproduce hydrogen from glycerol solution thirty times of that produced by pure g-C3N4. The transference of the induced charges through the developed heterojunctions could be described by a Z-scheme mechanism.