Alkyl group-decorated g-C₃N₄ for enhanced gas-phase CO₂ photoreduction

Abstract

With excellent physical/chemical stability and feasible synthesis, g-C₃N₄ has attracted much attention in the field of photocatalysis. However, its weak photoactivity limits its practical applications. Herein, by easily planting hydrophobic alkyl groups onto g-C₃N₄, the hydrophilicity of g-C₃N₄ can be well regulated and its specific surface area be enlarged simultaneously. Such a modification ensures enhanced CO₂ capture and increased active sites. In addition, the introduction of alkyl groups endows g-C₃N₄ with abundant charge density and efficient separation of photoinduced excitons. All these advantages synergistically contribute to the enhanced photocatalytic CO₂ reduction performance over the optimized catalyst (DCN90), and the total CO₂ conversion is 7.4-fold that of pristine g-C₃N₄ (CN).

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Introduction

Photocatalytic CO₂ reduction into high value-added chemical fuels is able to simultaneously mitigate the increasingly severe energy crisis and environmental problems, which is considered as one of the photochemical reactions with the most potential.^1–7 To apply photocatalytic CO₂ reduction technology in practice, the main research aim is to seek the proper photocatalyst. In recent decades, polymeric g-C₃N₄ has received great attention as a promising photocatalyst because of its excellent physical and chemical stability, easy synthesis methods, low cost, abundant resources, and visible-light response, etc.^8–10 Nevertheless, due to the fast recombination of photogenerated charge carriers, insufficient active sites, and weak CO₂ adsorption, the CO₂ reduction performance of g-C₃N₄ is commonly unsatisfactory.^11–14

To promote the photocatalytic activity of g-C₃N₄, plentiful strategies have been developed so far. For example, crafting heterojunctions or loading cocatalysts is an effective approach to boost the separation of photogenerated charge carriers.^15–17 To provide more active sites, designing unique nanostructures is obviously crucial, which also favors the process of mass transfer including CO₂ capture and the desorption of reduction products.¹⁸ Recently, upon introducing electron donor or acceptor units into the framework of g-C₃N₄, the obtained donor–acceptor-based g-C₃N₄ has demonstrated enhanced light-harvesting capacity and efficient separation of charges due to the strong electron push–pull effect.^19,20

Although these methods play a positive role in improving photocatalytic CO₂ reduction performance, there still exists an imperative scientific issue that is usually neglected. As is known, during gas-phase CO₂ photoreduction, the simultaneous adsorption of H₂O and CO₂ molecules on the surface of the photocatalyst jointly determines the reaction rate.^21,22 Specifically, the oxidation of H₂O molecules to provide protons is considered as a rate-determining step. A beneficial H₂O adsorption process can accelerate the oxidation of H₂O for the fast supplementation of protons. However, excessive H₂O adsorption will occupy the CO₂ adsorption sites, resulting in the weakening of the CO₂ hydrogenation process.^23,24 Therefore, a good adsorption balance between CO₂ and H₂O molecules is very significant, which is rarely investigated now.

Herein, we achieve the facile regulation of the hydrophilicity of g-C₃N₄ by planting hydrophobic alkyl groups. It is found that the decline in the hydrophilicity of g-C₃N₄ trades off against the enhanced CO₂ adsorption. In addition, the specific surface area of g-C₃N₄ is also enlarged, thus providing more active sites. Most importantly, the introduction of alkyl groups increases the electron density of g-C₃N₄ and promotes the dissociation of photogenerated charge carriers. These enhancements synergistically improve the photocatalytic CO₂ reduction performance, and the total CO₂ conversion over the optimal sample (DCN90) is 7.4-fold that of pristine g-C₃N₄. This research offers a straightforward and effective strategy for the design of highly efficient g-C₃N₄-based photocatalysts for energy conversion.

Results and discussion

Fig. 1a shows the synthesis route of alkyl group-functionalized g-C₃N₄. Under illumination, a series of radical reactions occur on the surface of g-C₃N₄ (CN), which can trigger the occurrence of additional reactions between CN and 1-decene (Fig. S1†),²⁵ thus successfully obtaining hydrophobic alkyl group-grafted g-C₃N₄ (DCNT, T is the irradiation time, T = 30, 60, 90, and 120 min). The X-ray diffraction (XRD) patterns and Fourier transform infrared spectra (FTIR) were recorded to investigate the chemical structures of the samples (Fig. S2 and S3†). Compared with CN, the XRD peak position and the FTIR spectra of DCNT show no obvious change, indicating that the chemical structure of CN is well retained after planting alkyl groups.²⁶ Notably, the DCNT samples possess wider XRD peaks (∼27°) than those of CN, which might result from the insertion of alkyl groups into the interlayer of CN.²⁷

Fig. 1 (a) Synthesis route of alkyl group-functionalized g-C₃N₄ with controlled hydrophilicity. High-resolution XPS (b) C 1s and (c) O 1s spectra of CN and DCN90.

To further analyze the chemical structures of the samples, X-ray photoelectron spectra (XPS) were recorded. Fig. S4a† shows that both CN and DCN90 mainly consist of the C, N, and O elements and the O content in the DCN90 sample is lower than that of CN. As shown in Fig. S4b,† the high-resolution N 1s spectra of CN and DCN90 can be fitted into three peaks, respectively, attributed to sp²-hybridized nitrogen (C–N=C), tri-coordinated nitrogen (N–C₃), and the terminal amino groups (–NH_x).^28–30 From Fig. 1b, the high-resolution C 1s spectrum of CN consists of three sub-peaks, indexed to sp²-hybridized carbon (N–C=N), C–OH from adsorbed H₂O, and adventitious carbon impurities, respectively.^31–33 As for the DCN90 sample, a new peak located at 283.9 eV appears, which is assigned to sp³-hybridized C–H from the alkyl groups.³⁴ Meanwhile, the peak assigned to the adsorbed H₂O disappears. These results suggest that the hydrophobic alkyl groups have been successfully grafted onto the surface of CN via photoinduced grafting, thus leading to a decrease in H₂O adsorption, which can be further confirmed by the weaker high-resolution O 1s spectrum for DCN90 (Fig. 1c).³⁵ In addition, the peak positions of N–C=N and C–N=C shift to lower binding energies for DCN90 compared with CN, which is attributed to the increased electron density of g-C₃N₄ after grafting alkyl groups.

The introduction of hydrophobic alkyl groups could exert an imperative effect on the physical and chemical properties of CN. Therefore, the hydrophilicity of the samples was first explored. As shown in Fig. 2a, with prolonged photoinduced grafting time, the water contact angles of the samples gradually increase from 60° (CN) to 83° (DCN90). This result indicates a slightly decreased H₂O adsorption capacity. The field emission scanning electron microscopic (FESEM) images show that CN exhibits a nanosheet-like morphology (Fig. S5a†). After grafting the alkyl groups, the morphology of CN is still retained (Fig. S5b–e†), which can be further confirmed by the transmission electron microscopic (TEM) images (Fig. S6†). Atomic force microscopy (AFM) was applied to measure the thickness of the samples.³⁶Fig. 2b provides the AFM image of CN and the corresponding height profile shows a thickness of 61.7 nm (the inset of Fig. 2b). Compared with CN, the DCN90 sample is thinner with a thickness of around 10 nm (Fig. 2c and the inset of Fig. 2c). This can be explained by the insertion of alkyl groups into the interlayer of CN, resulting in the exfoliation of CN, which is consistent with the results of XRD.

Fig. 2 (a) Water contact angles of the samples. AFM images and the corresponding height profiles (insets) of (b) CN and (c) DCN90. (d) N₂ adsorption–desorption isotherms, the corresponding pore size distribution curves (inset of (d)), and (e) the CO₂ uptake curves of CN and DCN90.

In general, the thinner thinness of g-C₃N₄ signifies the larger specific surface area (S_BET).³⁷ To confirm this point, the N₂ adsorption–desorption isotherms of CN and DCN90 were measured. As shown in Fig. 2d, both CN and DCN90 exhibit type-IV isotherms with an H3-type hysteresis loop.³⁸ The corresponding pore size distribution curves of CN and DCN90 mainly display the mesopores and macropores. Compared with CN, the DCN90 sample possesses a higher N₂ adsorption capacity and pore volume (inset of Fig. 2d), thus showing a larger S_BET for DCN90 (66 m² g⁻¹) than for CN (48 m² g⁻¹). As is known, the larger S_BET favors providing more adsorbed and active sites.^39,40 Accordingly, the CO₂ adsorption performance of CN and DCN90 was further tested. Fig. 2e shows that the CO₂ uptake amount of DCN90 is 0.28 mmol g⁻¹ at a relative pressure (P/P₀) of 1, obviously higher than that of CN (0.16 mmol g⁻¹). The slightly decreased hydrophilicity and enhanced CO₂ adsorption might boost the adsorption balance between the H₂O and CO₂ molecules,²¹ which is beneficial for the progress of the interfacial CO₂ reduction reaction.

To analyze the electronic structures of the samples, the UV–vis diffuse reflectance spectra (DRS) were first recorded to investigate the light-harvesting ability of the samples. As shown in Fig. 3a, upon grafting the hydrophobic alkyl groups, all the samples show enhanced light absorption in the UV region. However, in the visible region, the light absorption performance of samples shows no obvious change. In addition, the absorption edges of samples even display a slight blue-shift, which can be assigned to the typical quantum confinement effect,⁴¹ resulting from the decreased thickness of the samples by inserting the alkyl groups into the interlayer of g-C₃N₄. The absorption edges of CN and DCN90 are respectively 450 and 439 nm, and the corresponding band gaps (E_g) are 2.75 and 2.82 eV. The Mott–Schottky curves were analysed to determine the conduction band potential (E_CB) of the samples (Fig. S7†). The values of E_CB for CN and DCN90 are respectively −1.29 and −1.44 V (vs. Ag/AgCl, pH = 6.7), which can be transformed to −1.11 and −1.26 V (vs. NHE, pH = 7) using the conversion formulae S1 and S2.⁴² Based on their E_g, the valence band potential (E_VB) of CN and DCN90 is 1.64 and 1.56 V, respectively. Fig. 3b illustrates the band structure of CN and DCN90. The more negative E_CB for DCN90 allows for a stronger electron driving force for CO₂ photoreduction.⁴³

Fig. 3 (a) UV–vis DRS spectra of various samples. (b) Electronic band structure, (c) EPR spectra, (d) time-resolved PL spectra, (e) steady-state PL spectra, and (f) transient photocurrent response of the CN and DCN90 samples.

As is known, the separation of photoinduced charge carriers plays a critical role in a photocatalytic reaction.⁴⁴ Consequently, the electron paramagnetic resonance (EPR) spectra were first recorded to investigate the charge density of the CN and DCN90 samples. Both CN and DCN90 exhibit a single Lorentzian line (g = 2.009) signal (Fig. 3c), attributed to the unpaired electrons from the aromatic heptazine ring.⁴⁵ Compared with CN, the EPR signal of DCN90 is obviously enhanced. This is due to the introduction of alkyl groups because the alkyl groups act as electron donors for supplying more delocalized electrons.⁴⁶ Furthermore, the time-resolved photoluminescence (PL) spectra were recorded to investigate the PL lifetime of the samples (Fig. 3d). According to the tri-exponential decay fit (Table 1), the PL lifetimes of CN and DCN90 are calculated to be 28.2 and 22.1 ns, respectively. The decreased lifetime indicates a faster electron transfer for DCN90 than CN, which could be attributed to the establishment of a built-in electric field between alkyl groups and g-C₃N₄.⁴⁷ The steady-state PL spectra show that the PL intensity of DCN90 is obviously weakened compared with CN (Fig. 3e), suggesting the more efficient separation of photogenerated charges for DCN90, which is attributed to the decreased thickness of DCN90 for quick electron transfer from the bulk to the surface.^48–50 In addition, the PL peak of DCN90 shows a blue shift compared with CN, resulting from the quantum confinement effect due to the decreased thickness of DCN90. The photocurrent tests show that DCN90 possesses a stronger photocurrent signal than CN. When the photocurrent signal is steady, DCN90 shows a slower decay of the photocurrent signal compared with CN whether the light source is on or off (Fig. 3f). This result indicates that DCN90 possesses a slower recombination of photogenerated charges than CN. The decreased electrochemical impedance for DCN90 further confirms the improved transfer of photogenerated charges after grafting alkyl groups (Fig. S8†).⁵¹ The more delocalized electrons and efficient separation of charges demonstrate the higher photocatalytic performance.

Table 1 Summary of the photoluminescence decay time (τ) and their pre-exponential factor (B) of the CN and DCN90 samples. The calculation formula of average lifetime is as follows: τ_ave = (B₁τ₁² + B₂τ₂² + B₃τ₃²)/(B₁τ₁ + B₂τ₂ + B₃τ₃)

Samples	Decay time (ns)			Pre-exponential factor (%)			τ ave (ns)
	τ 1	τ 2	τ 3	B 1	B 2	B 3
CN	1.59	7.44	35.56	20.87	47.70	31.43	28.2
DCN90	1.08	5.29	27.67	20.41	48.55	31.04	22.1

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To evaluate the photocatalytic activity of the samples, CO₂ reduction tests were carried out under full spectrum irradiation without any cocatalyst or sacrificial reagent. Fig. 4a shows that CO is the primary CO₂ reduction product, along with a small amount of CH₄. This is because the reduction of CO₂ into CO follows the two-electron reduction process, which is easier than the eight-electron reduction of CO₂ into CH₄. Unmodified CN shows a very low photocatalytic performance, and the yields of CO and CH₄ are 2.61 and 0.20 μmol h⁻¹ g⁻¹, respectively. After grafting hydrophobic alkyl groups onto g-C₃N₄, the photocatalytic activity of samples is greatly enhanced. In particular, DCN90 exhibits an optimal CO₂ reduction performance with yields of 19.77 μmol h⁻¹ g⁻¹ for CO and 1.09 μmol h⁻¹ g⁻¹ for CH₄, which are higher than many reported results (Table 2). The total CO₂ conversion of DCN90 is 7.4-fold that of CN.

Fig. 4 (a) Photocatalytic CO₂ reduction performance of various samples. (b) Cycle tests and (c) control experiments of CO₂ photoreduction over the DCN90 sample.

Table 2 Comparison of the photocatalytic activity and the reaction conditions with other g-C₃N₄-based photocatalysts for CO₂ reduction (LS, CS, TOR, SR, AOP, TEOA, and MeOH represent the light source, the CO₂ source, the type of reaction, the sacrificial reagent, the amount of photocatalyst, triethanolamine, and methanol, respectively)

Materials	LS	CS	TOR	SR	AOP (mg)	Product yield (μmol h^−1 g^−1)	Ref.
DCN90	300 W Xe lamp (full spectrum)	CO2 gas	Gas–solid	—	50	CO: 19.77	This work
						CH4: 1.09
g-CN-0.01Dbc	300 W Xe lamp	CO2 gas	Liquid–solid	—	20	CO: 2.4	19
g-C3N4/7Ag/m-CeO2	8 W UV lamp	CO2 gas	Liquid–solid	TEOA	100	CO: 1.39	52
						CH4: 0.74
OCCN0.25	4 W UV lamp (254 nm, 40 μW cm^−2)	CO2 gas	Liquid–solid	—	20	CO: 8.74	53
CMN	300 W Xe lamp	CO2 gas	Gas–solid	—	30	CO: 18.8	54
						CH4: 1.8
10TC	300 W Xe lamp (λ ≥ 420 nm)	NaHCO3 + H2SO4	Gas–solid	—	20	CO: 5.19	16
						CH4: 0.044
CFC-0.2	350–780 nm	CO2 gas	Gas–solid	—	50	CO: 8.182	55
						CH4: 0.0805
2Au-CN	300 W Xe lamp (full spectrum)	CO2 gas	Gas–solid	—	50	CO: 6.585	56
						CH4: 1.55
3% Ni/NiO/g-C3N4	300 W Xe lamp (full spectrum)	CO2 gas	Gas–solid	—	50	CO: 13.955	57
						CH4: 2.08
Ni5-CN	300 W Xe lamp (λ ≥ 420 nm)	CO2 gas	Gas–solid	—	25	CO: 8.6	58
						CH4: 0.5
CABB@C3N4-82%	AM1.5G (150 mW cm^−2)	CO2 gas	Liquid–solid	MeOH	15	CO: <0.75	59
						CH4: <1.35
5BSCN	300 W Xe lamp (full spectrum)	NaHCO3 + H2SO4	Gas–solid	—	50	CO: 8.2	15
BiCN-0.6	AM1.5G	CO2 gas	Liquid–solid	—	50	CO: 3.78	60
						CH4: 1.65
CZTS-CN	400 W Xe lamp (λ ≥ 420 nm)	CO2 gas	Gas–solid	—	100	CO: 2.402	61
						CH4: 0.752

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In addition, photocatalytic cycle tests were conducted to investigate the stability of DCN90. After three consecutive runs, 81.7% of the total CO₂ conversion performance of DCN90 is still retained with no obvious change in morphology, hydrophilicity, and chemical structure (Fig. 4b and S9–S11†). A certain degree of decreased activity might be caused by the occupation of active sites due to the adsorption of intermediates on the surface of DCN90. Finally, control experiments were carried out to determine the carbon source of the reduction products. From Fig. 4c, without a photocatalyst and under a N₂ atmosphere, a little CO is observed and no CH₄ is detected. Furthermore, with DCN90 irradiated under a N₂ atmosphere, a very similar result is obtained. The small amounts of CO produced might originate from the pollution of the apparatus.⁶² However, when DCN90 is irradiated under a CO₂ atmosphere, a rapidly enhanced CO yield is observed, and CH₄ is also detected. These results strongly affirm that CO and CH₄ are produced by CO₂ photoreduction.

Based on the above results of CO₂ photoreduction and the characterization of the physical and photoelectrochemical properties of the samples, a rational photocatalytic mechanism is proposed. First, under irradiation, the electrons in VB are excited and jump into CB. As for the hydrophobic alkyl group-decorated DCN90, a more negative E_CB allows for a stronger electron reduction ability. Furthermore, the electron-rich alkyl groups can increase the charge density of g-C₃N₄ and boost the separation of photogenerated charge carriers. In addition, due to the insertion effect of the hydrophobic alkyl groups, g-C₃N₄ becomes thinner, which enlarges the specific surface area to provide more active sites. Meanwhile, the hydrophilicity of g-C₃N₄ is also regulated to optimize the simultaneous adsorption of H₂O and CO₂ molecules. All these factors synergistically endow DCN90 with a more favorable photoelectronic nature, thus leading to greatly enhanced photocatalytic activity compared with pristine g-C₃N₄.

Conclusions

In summary, hydrophobic alkyl group-decorated g-C₃N₄ is prepared by an easy photoinduced grafting method. The optimal photocatalyst (DCN90) demonstrates excellent photocatalytic CO₂ reduction activity. The yields of CO and CH₄ reach 19.77 and 1.09 μmol h⁻¹ g⁻¹, respectively, and the total CO₂ conversion is 7.4-fold that of pristine g-C₃N₄. The promotion of CO₂ photoreduction performance is mainly attributed to the grafting of hydrophobic alkyl groups. On the one hand, alkyl groups regulate the band structure, increase the electron density, and boost the separation of photoinduced charge carriers of g-C₃N₄. On the other hand, alkyl groups are inserted into the interlayer of g-C₃N₄, enlarging the specific surface area to provide more active sites and optimizing the adsorption of H₂O and CO₂ molecules. All these factors synergistically endow hydrophobic alkyl group-modified g-C₃N₄ with enhanced photocatalytic activity. This work offers a straightforward yet effective strategy to design highly efficient g-C₃N₄-based photocatalysts for photocatalytic energy conversion.

Author contributions

Chao Yang: conceptualization, experiments and data processing, formal analysis, and writing – original draft. Yanting Hou: experiments and formal analysis. Guoqiang Luo: resource and writing – review & editing. Jiaguo Yu: resource and writing – review & editing. Shaowen Cao: formal analysis, resource, writing – review & editing and supervision.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was financially supported by the Key-Area Research and Development Program of Guangdong Province (2021B0707050001), the National Natural Science Foundation of China (51922081, 51961135303, and 51932007), and the Self-Innovation Research Funding Project of Hanjiang Laboratory (HJL202202A001).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2nr02551e

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