Visible-light-drived high photocatalytic activities of Cu/g-C₃N₄ photocatalysts for hydrogen production

Abstract

Cu/g-C₃N₄ photocatalysts have been synthesized using a facile method. The composition and morphology of the prepared samples were characterized by a variety of analytical methods. The results indicate that the Cu nanoparticles were uniformly loaded onto the surface of g-C₃N₄. In addition, photocatalytic activity experiments were carried out by investigating H₂ production under visible light irradiation. The results reveal that the composites exhibited excellent performance for H₂ evolution in the absence of a cocatalyst, which demonstrates that Cu nanoparticles could trap photogenerated electrons and act as a cocatalyst effectively. Thus, it was effective in transferring the interfacial photogenerated charge carriers and efficiently enhanced the photocatalytic activity.

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1. Introduction

Currently, energy issues attract widespread attention leading people to explore new energy sources. Fortunately, photocatalytically splitting water to release H₂ has attracted tremendous interest for the direct conversion of solar energy to useful fuels.^1–3 Semiconductors are essential in this method due to their appropriate conduction band potential.^4–7 A large number of photocatalytic semiconductor materials have been investigated for hydrogen evolution, such as TiO₂, CdS, WO₃ and BiVO₄ etc.^8–11 However, these catalysts mostly possess a wide band gap, which restrict their photo-response in the UV region. Thus, this drawback could severely limit their development.

Metal-free polymeric graphitic carbon nitride (g-C₃N₄) has attracted much attention due to its high thermal and chemical stability.^12–15 The g-C₃N₄ material can be obtained from many carbon materials, such as urea, thiourea, melamine and cyanamide.^16–21 In addition, g-C₃N₄ can be used in the photocatalytic splitting of water, organic pollutant degradation, and CO₂ reduction under visible-light irradiation, owing to its appropriate conduction band and band gap.^22,23 However, the high electron–hole recombination rate of the photocatalytic reactions affects its photocatalytic performance. To improve its photocatalytic activity, many methods have been developed, such as sensitizing the photocatalyst using narrow band gap semiconductors, doping with metal and non-metal atoms etc.^24–27 Doping with metal can not only reduce the recombination rate of electron–hole pairs effectively, but also further broaden the absorption capacity of the photocatalyst. The overlap of orbitals or electronic connection between the two partner metals is considered.²⁸ Based on this method, some metal/semiconductor photocatalysts have been investigated. Tian has synthesized Ag/g-C₃N₄ composites, which exhibit excellent photoreactivity under visible light irradiation.²⁹ The metals used in this method were mostly precious metals like Ag, Au etc., which limits practical application.^30,31 Recently, copper (Cu) has been used as an alternative to expensive noble metals, due to its strong localized surface plasmon resonance absorption in the visible light range and its excellent catalytic activity in many reactions.^32,33 For example, Xiao has developed Cu nanoparticles-TiO₂ with a remarkably high photocatalytic H₂ generation rate under UV light irradiation.²⁸ Therefore, using Cu nanoparticles as the cocatalyst is expected to enhance the photocatalytic activity of g-C₃N₄. Cu, compared to Ag, Ru and Au, has nearly the same work function. Furthermore, the possible formation of a Schottky junction between Cu and g-C₃N₄ could enhance the separation of photogenerated electrons and holes. To the best of our knowledge, Cu nanoparticle doped g-C₃N₄ has not been reported.

Herein, a series of g-C₃N₄ materials decorated with different amounts of Cu nanoparticles have been prepared via a facile method. In addition, photocatalytic activity was measured by investigating photocatalytic water splitting for H₂ production. The results indicate that the rate of H₂ evolution of the optimized sample was 5 times that of pure g-C₃N₄.

2. Experimental section

All chemical reagents in this work were of analytical grade and were used without further purification.

2.1 Preparation of single g-C₃N₄

The raw material of melamine was dried at 70 °C for two days to make it dry completely, and g-C₃N₄ was synthesized using thermal treatment. Firstly, melamine was placed in an alumina crucible and covered. Then, the alumina crucible was heated to 550 °C at a heating rate of 2.3 °C min⁻¹ in a muffle furnace and this was maintained for 4 h.

2.2 Preparation of Cu/g-C₃N₄

Firstly, 1 g g-C₃N₄ and the corresponding mass percentage of Cu(NO₃)₂ (0, 2%, 3%, 4%, 10%, and the samples were named as CCN0, CCN2, CCN3, CCN4, CCN10, respectively) were dispersed in water and the reaction mixture was allowed to stir in a water bath to make the water evaporate. After fully milling the precursor, the obtained power was placed in a porcelain boat in a mixture of H₂ (5 vol%) and Ar at 45 °C for 3 h.

2.3 Characterization

All of the obtained samples were identified by X-ray diffraction (XRD, Bruker D8 Advance diffractometer, 50 kV, 300 mA) using Cu-Kα radiation. The morphologies and sizes of the obtained samples were characterized by transmission electron microscopy (TEM), and TEM images were visualized on a F20 S-TWIN electron microscope (200 kV). Energy-dispersive X-ray spectroscopy (EDS) was used to investigate the composition and structure of the samples. UV-vis spectra of the products were obtained on a UV-vis spectrophotometer (UV2450, Shimadzu, Japan) and BaSO₄ was used as the reference. High-resolution-X-ray photoelectron spectroscopies (XPS) were analyzed by a PHI Quantum 2000 XPS system (Al Kα). The electron spin resonance (ESR, Bruker ECS106 X-band spectrometer) signals of the radicals were trapped using a spin trap reagent, 5,5-dimethy-1-pyrroline-N-oxide (DMPO, Sigma Chemical Co). Photoelectric current (PC) response measurements were performed using a CHI 660B electrochemical workstation with a standard three electrode cell at room temperature. Photoluminescence spectra (PL) were obtained on a F4500 (Hitachi, Japan) photoluminescence detector.

2.4 Photocatalytic degradation of dyes

Rhodamine B (RhB) was used as a model organic pollutant to evaluate the photocatalytic activity of the samples under visible light irradiation. The photodegradation of RhB was carried out at 308 K in a photochemical reactor containing a 50 mg sample and 100 mL of a 10 mg L⁻¹ RhB solution. To exclude the influence of physical adsorption, the reactor was kept in the dark for 60 min to reach the adsorption equilibrium. A 250 W xenon lamp as the light source was located approximately 8 cm to one side of the contained solution, which has a glass filter to remove the UV light. At each time interval, the photocatalysts were separated by centrifugation at 10 000 rpm for 5 min, and the light absorption of a clear solution for the different samples was measured using an UV-vis spectrophotometer (Hitachi U-4100).

2.5 Photocatalytic hydrogen production

Photocatalytic H₂ production was tested using a Lab-H₂ photocatalytic system. A Xe lamp (λ > 400 nm) was applied as the light source and placed parallel with the reactor. In a typical photocatalytic H₂ production experiment, 0.05 g photocatalyst was dispersed in 200 mL 25% methanol aqueous solution with vigorous stirring and was stirred continuously to ensure uniform irradiation of the catalyst suspension during the whole process. Before irradiation, the system was vacuumized to remove the dissolved oxygen in water. The generated gas was collected once an hour, and the amount of hydrogen content was analyzed by gas chromatography (GC-14C, Shimadzu, Japan, TCD, with argon as a carrier gas). In addition, the apparent quantum efficiency (QE) was measured in a dark room. A specific wavelength (420 nm) LED (250 W) was applied as the light source. The LED was positioned 1 cm away from the reactor in a vertical direction. The QE was calculated by the following formula:

3. Results and discussion

3.1 XRD analysis

In order to determine the crystal phase of the semiconductor photocatalysts, the obtained samples were carefully checked using X-ray diffraction (XRD). As can be seen in Fig. 1, the strong interplanar stacking peak at around 14.3° and in-plane repeat units at around 27.6° can be found clearly, which correspond to the (100) and (002) planes of g-C₃N₄, respectively.¹² The results confirmed that the g-C₃N₄ was synthesized successfully. However, no Cu peaks were detected in the XRD patterns of the samples when the content of Cu was less than 10%. This result revealed that the Cu nanoparticles in those samples were well dispersed on the surface of g-C₃N₄.^34–36 In the patterns of the CCN10 sample, the peak at around 43.3° and 50.4°, corresponding to the (111) and (200) planes of Cu, can be detected.^28,32,33 These results confirmed that the Cu nanoparticles was synthesized successfully. In addition, compared to pure g-C₃N₄, there are no obvious differences in the intensities and widths of the XRD peaks from the Cu/g-C₃N₄ samples. This result indicates that the g-C₃N₄ crystal structure does not significantly change with Cu modification.

Fig. 1 XRD patterns of the pure g-C₃N₄ photocatalysts, and with various amounts of Cu.

3.2 XPS analysis

To investigate the surface chemical composition and the chemical state, the CCN3 composite was analyzed by X-ray photoelectron spectroscopy (XPS). As can be seen in Fig. 2a, the XPS survey spectrum shown that the C, N and Cu elements can be detected, which reveal that the Cu element exists on the surface of g-C₃N₄. The high-resolution C_1s peak (Fig. 2b) at 284.6 eV corresponds to the C–C bond, while the peak at 288.1 eV can be attributed to the sp³-bonded N–C=N of g-C₃N₄. In addition, the high resolution N_1s XPS spectra of the composites are displayed in Fig. 2c. As can be seen in the image, the binding energy at 398.3 eV can be attributed to sp² hybridized N–C=N, and the two peaks at 399.7 eV and 400.8 eV are related to amino functional groups (C–N–H) and tertiary nitrogen groups (N–(C)₃), respectively.^37–40 Fig. 2d shows that the Cu element spectra has a strong binding energy of Cu_2p3/2 at around 932.2 eV and Cu_2p1/2 at around 952.1 eV, respectively, which perfectly matches the binding energy of Cu⁰ state.³³

Fig. 2 X-ray photoelectron spectroscopy of the CCN3 photocatalyst: (a) survey spectrum; (b) high resolution C_1s spectrum; (c) high resolution N_1s spectrum; (d) high resolution Cu_2p spectrum.

3.3 Characterization of morphology and structure

The morphology of the samples was viewed using transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). The pattern of g-C₃N₄ shows a nanosheet structure with a smooth surface, as shown in Fig. 3a. In comparison, Fig. 3b exhibits an image of Cu/g-C₃N₄ (CCN3), which consists of the g-C₃N₄ nanosheets with the Cu nanoparticles loaded onto the surface. In addition, the HRTEM image (Fig. 3c) of the composite shows the lattice spacing between the cohesive interfaces of Cu and g-C₃N₄. The lattice spacing of 0.255 nm corresponds to the (111) plane of the Cu phase. Thus, the results indicate that Cu is successfully coated onto the surface of g-C₃N₄ and it has a diameter of about 30 nm. In addition, Fig. 3d depicts the EDS image of the as-prepared CCN3 photocatalyst. Cu, C and N can be directly observed, which demonstrates that the Cu nanoparticles are loaded onto the surface of g-C₃N₄ again. Additionally, the real content of Cu atomicity (%) was calculated by EDS, which is shown in Table 1.

Fig. 3 TEM image of (a) pure g-C₃N₄; (b) Cu/g-C₃N₄. (c) HRTEM of a Cu/g-C₃N₄ photocatalyst sample. (d) EDS image of a Cu/g-C₃N₄ photocatalysts sample.

Table 1 The real contents of the samples

Element atomicity (%)	CCNO	CCN2	CCN3	CCN4
C	44.86	43.29	45.29	42.72
N	55.14	54.95	52.39	54.2
Cu	0	1.76	2.32	3.08
Total	100	100	100	100

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3.4 Optical absorption studies

To examine the optical properties, UV-vis diffuse reflectance spectra were analyzed for the prepared samples. As can be seen from Fig. 4, the UV-vis absorption of g-C₃N₄ has a clear strong absorption region at 400–450 nm, which corresponds to a band gap of 2.5 eV.¹² After loading the Cu nanoparticles on the surface of the g-C₃N₄ nanosheets, the absorption spectra exhibit a greatly enhanced ability for light absorption in the visible-light region. With increasing the content of Cu, the Cu/g-C₃N₄ composites displayed a red shift in the band gap transition and an enhanced absorption in the visible-light region. This phenomenon may be caused by the localized surface plasmon resonance (LSPR) of the Cu nanoparticles.^41–43 In addition, the optical absorption of CCN4 is weaker than that of CCN3, indicating a decline in optical absorption as the amount of Cu nanoparticles is increased from 3% to 4%. It is possible for the excess of Cu(NO₃)₂ to lead to the formation of much larger Cu nanoparticles, which have smaller specific surface areas and a weaker ability to absorb light.^32,33 These results indicate that the Cu/g-C₃N₄ composites have a wider absorption region and stronger visible light absorption ability than pure g-C₃N₄.

Fig. 4 UV-vis diffuse reflectance spectra of the pure g-C₃N₄ photocatalyst and Cu/g-C₃N₄ composites with various amounts of Cu.

3.5 Photocatalytic degradation of organic pollutants

The photocatalytic activities of all the samples were evaluated by investigating the photocatalytic degradation of rhodamine B (RhB) under visible light irradiation. Pure g-C₃N₄ shows a lower photocatalytic degradation efficiency of 46.3% for RhB in 60 min (Fig. 5a). However, the degradation effect was obviously improved, when the Cu nanoparticles were loaded on the surfaces of g-C₃N₄. Additionally, there exists an optimum Cu content (3%), which exhibits the best photocatalytic degradation efficiency of 87.3% in 60 min. However, with increasing the content of the Cu nanoparticles, the photocatalytic activity was decreased. Fig. 5b shows that the photocatalytic degradation of RhB by the obtained samples fits the pseudo-first-order kinetics. As can be seen from the figures, the time and −ln(C/C₀) shows an obviously linear relationship, which reveals the above degradation experimental results again.

Fig. 5 Photocatalytic degradation of RhB (a) and pseudo-first-order kinetics (b) using the pure g-C₃N₄ photocatalyst and Cu/g-C₃N₄ composites with various amounts of Cu.

3.6 Photocatalytic hydrogen productions

To investigate the photocatalytic activities of the samples, we have executed hydrogen evolution experiments with a 25% methyl alcohol solution as the sacrificial agent. As shown in Fig. 6b, in the absence of cocatalyst, the pure g-C₃N₄ shows a lower H₂ evolution rate of 4.5 μmol g⁻¹ h⁻¹ under visible light irradiation. However, with the Cu nanoparticles loaded on the g-C₃N₄ nanosheets, the samples showed an obviously enhanced photocatalytic activity compared with pure g-C₃N₄. The enhancement was likely due to the fact that Cu nanoparticles can trap the photogenerated electrons and act as the cocatalyst effectively. Thus, it was the benefit of the interfacial photogenerated charge carriers that transfer so efficiently that suppressed the charge recombination and enhanced the photocatalytic activity. The highest degradation rate was obtained when the Cu content was increased to 3% (Fig. 6a). For this sample, the photodegradation activity was enhanced by 6 times compared to the pure g-C₃N₄. However, the photocatalytic activity decreased with a further increase in the Cu content. It is possible for the photoexcited electrons in g-C₃N₄ to transfer to the Cu nanoparticles, and the excess Cu(NO₃)₂ will lead to the formation of much larger Cu nanoparticles covering the surface of g-C₃N₄, and then reduce the numbers of g-C₃N₄ active sites available for the photocatalytic reaction. In addition, the apparent quantum efficiency (QE) of the CCN3 sample was 0.35% at 420 nm, which has a gap comparable to a noble metal based cocatalyst/g-C₃N₄.¹² However, the composite can still avoid using high cost precious metals and increase the photocatalytic activity of the g-C₃N₄.

Fig. 6 Plots of the amount of photocatalytic H₂ evolution (a) and comparison of the H₂ evolution rates (b).

3.7 Photoluminescence spectroscopy studies

To further prove the above conclusion, electron spin resonance (ESR) spectra were measured for the pure g-C₃N₄ and CCN3 samples. As shown in Fig. 7a, the spectrum shows that the CCN3 sample possesses a higher intensity of the characteristic peaks for superoxide radicals than the pure g-C₃N₄, which indicates that this sample was favorable for the formation of a higher redox potential.⁴ In addition, to further demonstrate that the CCN3 samples had suppressed the recombination of electron–hole pairs, the transient photocurrent response was measured under visible light irradiation. As can be seen from Fig. 7b, pure g-C₃N₄ shows a weaker photocurrent density when the light source was switched on. However, with Cu nanoparticles loaded on the surface of g-C₃N₄, the transient photocurrent response was enhanced drastically. Thus, the experimental results demonstrate that the charge was transferred and separated effectively again. The photoluminescence spectroscopy (PL) data was further analyzed to determine the transfer and separation efficiency of the photogenerated charge carriers, as shown in Fig. 7c. CCN0 shows a strong emission peak in the range of 430–500 nm and all of the Cu/g-C₃N₄ samples have weaker PL intensity than pure NaNbO₃, which demonstrates that the Cu/g-C₃N₄ samples can suppress the electron–hole pair recombination efficiently.³⁶ In addition, the PL intensity agreed strongly with the photocatalytic reaction results.

Fig. 7 Electron spin resonance spectra, photocurrent versus time (I–t) curves and photoluminescence spectroscopy for the samples.

3.8 Photocatalytic degradation mechanism

Based on above experimental results, a possible mechanism for H₂ production was proposed for the Cu nanoparticle modified g-C₃N₄ composite, as shown in Fig. 8. First, under visible light irradiation, electrons in the valence band of the g-C₃N₄ nanosheets are excited to the conduction band, and then the photogenerated holes are accumulated on the valance band. Secondly, the photoexcited electrons will flow to the Cu nanoparticles easily, due to the Fermi level of Cu nanoparticles being lower than the conduction band of g-C₃N₄.⁴⁴ In addition, the excellent electric conductivity of Cu and the strong interaction between Cu and g-C₃N₄ will create a Schottky barrier, which will further facilitate the photoelectron transfers from the g-C₃N₄ to the Cu.²⁸ Thereafter, the aggregation of electrons in the Cu nanoparticles can be trapped by water in the reaction system to produce H₂ and the holes can be consumed by a sacrificial agent. Moreover, the Cu can act as a cocatalyst to accelerate the photocatalysis, and the enriched photoelectrons on the Cu could also promote H⁺ reduction to produce H₂. Thus, the samples can suppress the recombination of photogenerated electrons and holes effectively, which leads to the enhanced photocatalytic H₂ generation by this composite.

Fig. 8 Schematic illustration of the charge transfer.

4. Conclusions

In this study, the Cu/g-C₃N₄ composites were synthesized using a facile method. The results show the significantly enhanced photocatalytic activity for H₂ evolution under visible light irradiation. For the composite, the Cu nanoparticles can trap the photogenerated electrons and act as a cocatalyst, which benefits the transfer of the interfacial photogenerated charge carriers, so that it can suppress the charge recombination and enhance the photocatalytic activity efficiently. Moreover, our results have demonstrated that the sample has a higher H₂ evolution rate in the absence of a cocatalyst like Pt. Thus, we can avoid the use of high cost precious metals and increase the stability of the photocatalyst for H₂ evolution. This new photocatalyst has broad and promising prospects in the field of environmental applications and H₂ evolution.

Acknowledgements

We gratefully acknowledge the financial support of the National Natural Science Foundation of China (21276116, 21477050, 21301076 and 21303074), Excellent Youth Foundation of Jiangsu Scientific Committee (BK20140011), Chinese-German Cooperation Research Project (GZ1091), Program for High-Level Innovative and Entrepreneurial Talents in Jiangsu Province, Program for New Century Excellent Talents in University (NCET-13-0835), Henry Fok Education Foundation (141068) and Six Talents Peak Project in Jiangsu Province (XCL-025).

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