Eco-friendly conversion of carbon dioxide into solar fuels via artificial photosynthetic routes: A review

Abstract

The greenhouse gas carbon dioxide (CO₂) affects the environment drastically. Nowadays, the reduction of the CO₂ level from the environment is a challenging task for researchers. Solar light-driven CO₂ reduction via artificial photocatalysis is an auspicious method for achieving carbon neutrality. In the last few decades, a lot of research work has been done for photocatalytic CO₂ reduction via photo-electrochemical and electrochemical methods. Herein, we summarise the recent advances in photocatalyst-biocatalyst coupled artificial photosynthetic systems for CO₂ reduction and solar chemical production. In addition, we provide the application of produced solar chemicals for future perspectives. This study provides a clear background for the future progression of highly efficient CO₂ reduction in an eco-friendly manner.

Keywords

energy storage artificial photosynthesis solar fuels production carbon dioxide reduction green chemistry catalysis

Introduction

Carbon dioxide (CO₂) is the most copious greenhouse gas in the environment today. The concentration of CO₂ is currently found at ∼400 ppm by volume, while other gases such as CH₄ and N₂O) that have maximum potency for global warming are also present in the environment. Furthermore, CO₂ is the non-condensing, most radiative forcing, and longest-lived greenhouse gas in the environment.¹ For the construction of a green and sustainable environment, the reduction of CO₂ levels is highly desirable. As we know, plants have unique properties of photosynthesis that convert environmental CO₂ to chemical energy. Photosynthesis is widely acknowledged as a process through which photosynthetic organisms transform solar energy into ATP and NADPH which are necessary for carbon dioxide fixation. {1 solar energy NP-AP} The molecular mechanisms involved in natural photosynthesis can be classified into two categories; dependent and light-independent processes, which are sometimes referred to as the "light" and "dark" reactions. The group of reactions that generate ATP and NADPH is commonly referred to as the reactions and can be summarized as such.

\begin{aligned} 2 H_{2} O + 2 {NADP}^{+} + 3 ADP \to 3 P_{i} O_{2} + 2 NADPH + 3 ATP \end{aligned}

Where inorganic phosphate is P_i, which is a negatively charged free phosphate ion. In simplified form, the solar energy conversion into chemical form takes place by involving two consecutive photoinduced charge separation processes, shown in Figure 1. Initially, water splitting in PS II and transferring the photoexcited electrons to PS I via an electron transport chain takes place simultaneously which produces a chemiosmotic potential. The generated potential provides a sufficient amount of energy for the synthesis of ATP. After the solar light absorption and charge separation in PS I, the electrons are further transferred to ferredoxin–NADP⁺ oxidoreductase through ferredoxin for the regeneration of NADPH, as shown in Figure 1. The charge separation pathway in the natural photosynthetic system inspired for construction of artificial photosynthetic systems based on organic/ inorganic photosensitizers for converting solar energy into chemical form.²

Figure 1.

Diagrammatic representation of solar light-driven natural photosynthetic mechanism.

The ability of ATP and NADP to be easily carried allows for these reactions to happen in a location, from where light energies gathered. In the last step, the mechanism utilizes the Calvin Benson cycle to capture carbon dioxide (CO₂) and convert it into carbohydrates.³ As shown in Figure 2, natural photosynthesis and artificial photosynthesis are quite similar, both are uphill reactions.

Figure 2.

Comparative illustration of natural and artificial photosynthetic systems.

To mimic natural photosynthesis, a new cascade is developed by the researchers called “Artificial Photosynthesis”. It is a process that transforms solar energy into chemical energy. It replicates the phenomena of natural photosynthesis in an eco-friendly manner. In the mechanism of artificial photosynthesis, solar light harvesting material relates to the enzymes via a cascade system to convert solar energy to chemical form.⁴ Here, solar energy is converted into chemical energy through light stimulation of electron/hole pairs. The artificial photosynthetic cascade consists of photocatalysts, sacrificial agents (donor) such as ascorbic acid (AsA), triethanolamine (TEOA), electron mediators, NAD⁺, and redox enzymes. Sacrificial agents are hole scavengers that reduce the recombination rate of electrons and holes.⁵ In addition, the organometallic rhodium complex is used as an electron mediator that helps enhance the rate of chemical regeneration.⁴

Numerous methods have been employed to examine artificial photosynthesis, but the most recent studies have focused on simultaneous light-induced CO₂ reduction. In the mechanism of artificial photosynthesis, firstly solar light harvesting material absorbs the sunlight that results in the excitation of electrons. The excited electron now jumped from the valence band (VB) to the photocatalyst's conduction band (CB). This process generates excited electron-hole pairs in the VB which are quenched by the sacrificial agent. Then after the excited electron transfers from the CB of photocatalyst to NAD⁺ via electron mediator (M) and regenerates NADH followed by the conversion of CO₂ into solar chemicals in the presence of redox enzymes, shown in Scheme 1.⁶ Some examples of redox enzymes are F_ateDH (Formate dehydrogenase) and GDH (Glutamate dehydrogenase).^7–9

Scheme 1.

General mechanism of CO₂ reduction using solar light harvesting photocatalyst.

For solar chemical production from CO₂, the reaction medium consists of AsA (310 μL), rhodium complex (62 μL), NAD(P)⁺ (124 μL), Enzyme (3 units) and photocatalyst (15.5 μL) in 3.1 mL of sodium phosphate buffer (100 mM, pH 7.0) in the presence of CO₂ (flow rate: 0.5 mL/min).^9–11

Synthesis of electron mediator (m)

In most of the artificial photosynthetic systems, the Rh complex is used as an electron mediator. For its synthesis, took 5 ml of distilled methanol and add 25 mg (pentamethylcyclopentadienyl) rhodium (III) dichloride dimer into it. Stir the solution for about 2–3 min. The reaction mixture was again stirred for 30 min under nitrogen atmosphere. Then after, 2 eq. of 2, 2′-bipyridyl (13 mg) was added into the above stirred reaction mixture. After addition, the reaction mixture was again allowed for stirring. The yellow colour Rh complex (electron mediator) was precipitated out after the excessive addition of diethyl ether. The precipitate was filtered and dried at room temperature (Figure 3).¹²

Figure 3.

Synthesis of electron mediator.

Solar light harvesting templates in artificial photosynthetic systems

In artificial photosynthetic systems, graphene, carbon nitride, covalent organic framework, quantum dots, organic dyes etc. are mostly used as a photocatalyst for CO₂ reduction.¹³ Some researchers used metal-organic framework, TiO₂ etc. as a photocatalyst for CO₂ reduction.¹⁴ The use of inexpensive polymeric semiconductor materials such as graphene, carbon nitride, covalent organic framework, quantum dots, organic dyes etc. emerges as a new benchmark example of photocatalyst due to its high electronic conductivity and excellent physical properties. For example, carbon nitride has high thermal and chemical stability due to the presence of strong covalent bonds in the conjugated layer structure. Its band gap can easily be tuned.^15–17 In addition, COFs also have a porous structure, high stability, significant surface area, and tunable band structure that makes them a good candidate for photocatalysts in artificial photosynthetic systems. An organic dye with strong absorption coefficients and high molar extinction coefficients, such as metalloporphyrin, phthalocyanines, and ruthenium polypyridyl complexes, are also frequently used as a solar light harvesting material in artificial photosynthetic systems. Some inorganic dyes, such as cadmium sulphide and cadmium selenide are used as solar light harvesting material.

Study of CO₂ reduction using various solar light harvesting photocatalysts

The current state of carbon dioxide (CO₂) emissions poses a critical challenge, contributing significantly to global warming and climate change.¹⁸ CO₂ emissions, primarily from fossil fuel combustion, cement manufacturing, and other industrial processes, are major contributors to greenhouse gas (GHG) emissions, leading to environmental deterioration.^19,20 This environmental impact emphasizes the urgency of finding sustainable solutions to mitigate CO₂ emissions.

Efforts to address this issue have led to the exploration of artificial photosynthetic routes for the eco-friendly conversion of CO₂ into solar fuels. These innovative approaches aim to emulate natural photosynthesis, harnessing solar energy to drive the conversion of CO₂ into valuable fuels.²¹ The potential of artificial photosynthesis lies in its ability to offer a clean and renewable energy source while simultaneously reducing CO₂ levels in the atmosphere. In natural photosynthesis, photocatalytic CO₂ reduction also takes place with the help of sunlight.²² This review paper delves into the advancements and challenges in utilizing artificial photosynthesis for the eco-friendly conversion of CO₂ into solar fuels, providing a comprehensive analysis of the current landscape. We are now discussing photocatalysts that can be utilized for CO₂ reduction.

CO₂ reduction via COF-based photocatalysts

A fascinating new class of microporous polymers known as covalent organic frameworks (COFs), is used as a light-harvesting material in the artificial photosynthetic systems. This is because it has a vast number of properties which were never seen in organic material chemistry (Figure 4). Typically, they are constructed from inflexible, geometrically specified organic building blocks that produce strong, two or three-dimensional crystalline networks bound by covalent bonds.²³ COFs provide a bunch of unique properties like low density, materials used in the construction of COFs are light elements, which means that they should provide excellent gravimetric performance for guest molecules and energy storage, COF-108 has a density of just 0.17 g cm⁻3, which is lowest of any other crystalline solid. Strong covalent bonds bind COFs together, which provides them with greater stability than MOFs. Because of their crystalline structure, COFs provide a method for positioning of functional groups in a highly regulated and predictable way. Since full access to the pores is needed for gas separation or catalysis, COFs periodic and uniform porosity allows for improved performance in these processes.²⁴

Figure 4.

Diagrammatic illustration of the properties of COF.

In addition, these materials have high physiochemical stabilities, structural tunability and diversities, and some interesting semiconducting and charge separation properties.²⁵ Firstly, solar chemical formic acid via CO₂ reduction was synthesized in 2016 when J.-O. Baeg et. al. studied the photocatalytic CO₂ reduction property of triazine-based COF (2D-CTF Film). At that time, the photocatalyst's formic acid synthesis rate was 881.3 µmol g⁻¹ h⁻¹, which established it as a remarkable example of photocatalytic CO₂ reduction via COF.²⁶

Later, two azine-linked 2D COFs (ACOF-1 and N₃-COF) were identified by Zhu and co-workers. In 24 h, 13.7 µmol g⁻¹ of methanol was formed over N₃-COF, which was more than that of g-C₃N₄ (4.8 µmol g⁻¹) and ACOF-1 (8.6 µmol g⁻¹). Results from the density functional theory (DFT) calculation verified that N₃-COF's HOMO and LUMO electronic distributions were well separated, which was advantageous for intramolecular charge transfer.²⁷ Thomas was the first one to develop a covalent triazine framework (CTFs). They were polymerized from aromatic nitriles and since then it has become an innovative idea for research.²⁸ Cao et al. used a CTF material based on pyridine to attach the Re(CO)₃Cl complex. Under full light irradiation, the obtained Re-CTF-py photocatalyst showed the highest CO evolution rate of 353 µmol g⁻¹ h⁻¹ with a TON of 4.8 over 10 h. This work explored the possibility and development of COFs for photocatalytic CO₂ reduction and brought out the immense potential of CTF materials as high-performance platforms to anchor single active sites for heterogeneous photocatalysis.²⁹

Furthermore, Rhenium-(1) bipyridine (bpy) complexes in particular, fac- [Re¹ (bpy)(CO)₃ (L)]ⁿ⁺ (L = Cl) (n = 0); PR₃ (n = 1), have special and high yield photocatalysis, allowing them to serve as both a catalyst and a photosensitizer, with CO being the main result of CO₂ reduction. In the past, the most effective photocatalyst for CO₂ reduction in a homogenous system that specifically produces CO with a quantum yield of 0.38 was fac- [Re (bpy)(CO)₃{P(OEt)₃}]⁺.³⁰ A 15-min induction period in acetonitrile solution with triethanolamine (TEOA) as a sacrificial reducing agent allowed Re-COF to steadily produce approximately 15 mmol CO br per g of Re-COF for 20 h, accounting for a TON of 48 and 22-fold better activity than its homogenous counterpart. This was substantially due to the effective electron transfer from COFs to Re(bpy)(CO)₃Cl and the inhibition or detention of charge recombination in the Re-COF.³¹ Cooper and co-workers have produced a novel crystalline, porous sp²c-COF that contains bipyridine. To improve the photocatalytic CO₂ reduction efficiency, a rhenium complex was added post-synthetically. Re-Bpy-sp²c-COF during 17.5 h of illumination produced 1040 mmol g⁻¹ h⁻¹ of CO with 81% selectivity over H₂. Its performance was boosted up to 84% over a 5-h period by Dye-sensitization, which resulted in producing 1400 mmol h⁻¹ g⁻¹ of CO with an 86% selectivity over H₂. Re-Bpy-sp²c-COF is more stable than the homogenous catalyst [Re(bpy)(CO)₃Cl].³² Similarly, Qizhao Wang and co-workers created a Z-scheme hybrid structure between metalloporphyrin block-based COF and TiO₂. After the CO₂ reduction, the production rate of CO given by the prepared TiO₂-INA@CuP-Ph was 50.5 µmolg⁻¹ h⁻¹ which is almost 24.5 times of pristine TiO₂ and COF.³³

A 2,2′-bipyridine-based COF with single Ni sites (Ni-TpBpy) has been reported by Zhigang Zhu and colleagues as a synergistic catalyst for the selective photoreduction of CO₂ to CO. Chelation with bipyridine binding units allowed for the effective insertion of single Ni sites into TpBpy. High photocatalytic selective reduction of CO₂ to CO in both pure and diluted CO2 is demonstrated by Ni-TpBpy. In a 5-h process, Ni-TpBpy yields 4057 µmol g⁻¹ of CO with 96% selectivity over H₂ evolution. More notably, 76% selectivity for CO generation is still attained even when the CO₂ partial pressure is lowered to 0.1 atm.³⁴

To achieve photocatalytic CO₂ reduction, Ya-Qian Lan and co-workers have successfully built a TMI- modified COF system that exhibits great durability and efficiency for converting CO₂ to CO and HCOOH when exposed to visible light. The main purpose of DQTP COF (2,6-diaminoanthraquinone – 24,6-triformylphloroglucinol) as a platform is that, due to the ordered π electronic pathway, COFs in columnar orientations offer a very effective charge carrier transport, which enhances electron transfer from COF to metal moiety due to which reactivity is enhanced. The selectivity of CO₂ reduction is significantly influenced by the type of open metal active species present in COFs. Therefore, the highest CO and formic acid generation were shown by DQTP COF-Co and Zn, with 1.02×10³ and 152.5 µmol h⁻¹ g⁻¹, respectively. To explain variations in activity and selectivity a “two-pathway” mechanism was proposed.³⁵

The comparative chart of produced solar chemicals after CO₂ reduction is shown in Table 1. The general mechanism of solar chemical production via COF is shown in Scheme 2.

Scheme 2.

General mechanism of solar chemical production via COF.

Table 1.

Summary of solar chemicals produced via COF.

COFs	Solar fuel	Yield	Reference
2D-CTF Film	HCOOH	881.3 µmol g⁻¹ h⁻¹	26
N3-COF	CH₃OH	13.7 µmol g⁻¹	27
ACOF-1	CH₃OH	8.6 µmol g⁻¹	27
Re-CTF-py	CO	353 µmol g⁻¹ h⁻¹	29
Re-Bpy-sp²c-COF	CO	1040 mmol g⁻¹ h⁻¹	32
Re-Bpy-sp²c-COF-dye	CO	1400 mmol h⁻¹ g⁻¹	32
TiO₂-COF	CO	50.5 µmolg⁻¹ h⁻¹	33
Ni-TpBpy	CO	4057 µmol g⁻¹	34
DQTP-COF-Co	CO	1020 µmolg⁻¹ h⁻¹	35
DQTP-COF-Zn	HCOOH	152.5 µmol h⁻¹ g⁻¹	35

Selective CO₂ reduction via graphene-based photocatalyst

Advanced carbon nanomaterials include graphene is a sheet of single carbon atoms in two dimensions. Graphene shows various properties like high thermal conductivity, high mechanical strength, and high optical transparency (Figure 5). Another great quality of graphene is that it has a large surface area.³⁶ Acoustic phonons are what make single-layer graphene able to conduct heat in normal circumstances. This thermal conductivity happens because of the suppression of Umklapp processes and because there isn't any crystal defect. If you look at graphene crystals closely, they are usually well-kept and very stable. In terms of their mechanical qualities, they are a high-strength material. Regarding its physical properties, graphene is a million times thinner than a sheet of paper, it is transparent.

Figure 5.

Diagrammatic representation of the properties of graphene.

The scientific and technical community has witnessed an exponential increase in graphene research since Novoselov et al. in 2004 did a firsthand characterization and observation of a mechanically exfoliated graphene monolayer.³⁷ Nanocomposites based on the less faulty solvent-exfoliated graphene demonstrate a much higher improvement in CO₂ photoreduction, especially in the presence of visible light, as demonstrated by Yu Teng Liang and colleagues. Their greater electric mobility, which promotes the diffusion of photoexcited electrons to reactive sites, is responsible for this unanticipated outcome.³⁸ Due to its exceptional qualities, graphene offers plenty of options for creating various composite materials with exceptional properties for CO₂ reduction by photocatalysis. To meet the practical need for CO₂ reduction, strong and fine-tuned photocatalytic material based on graphene must be created.³⁹

Jiaguo Yu and colleagues have created highly active (reduced graphene oxide) RGO-CdS nanorod composite photocatalysts for the photocatalytic conversion of CO₂ to CH₄ by the successful development of a one-step hydrothermal procedure supported by microwaves. RGO has a major impact on the activity and was initially demonstrated to be a useful co-catalyst for improving the photocatalytic reduction of CO₂ to CH₄. The ideal RGO concentration was found to be 0.5 weight per cent, and the associated CH₄ production rate was 2.51 µmol h⁻¹ g⁻¹, ten times higher than that of pure CdS nanorods. This is because RGO can improve CO₂ molecule adsorption and destabilization in addition to producing more photogenerated electrons on its surface, which improves the CO₂ reduction efficiency of the RGO-CdS composite photocatalyst. This work shows the possibility of using an affordable carbon material in place of noble metals in the photocatalytic reduction of CO₂.⁴⁰ Wenguang Tu and co-workers fabricated TiO₂-graphene hybrid nanosheets in situ using a unique simultaneous reduction-hydrolysis (SRH) process in a binary ethylenediamine (En)/H₂O solvent. In the presence of water vapour, CO₂ gets converted into valuable hydrocarbons CH₄ and C₂H₆ with yields of 8 µmol h⁻¹ g⁻¹ and 16.8 µmol h⁻¹ g⁻¹ respectively, which confirms the excellent photocatalytic activity of the G-TiO₂ hybrid. The formation of C₂H₆ is favoured by the synergistic impact of the surface-Ti³⁺ sites and graphene, and the yield of C₂H₆ increases with the amount of integrated graphene.⁴¹ In addition, a graphene-based photocatalyst (CCG-BODIPY) was developed and synthesized by No-Joong Park and co-workers. It is composed of a light-harvesting BODIPY molecule (1-picolyamine-2-aminophenyl-3-oxy-phenyl-4,4′-difluoro-1,3,5,7-tetramethyl-2,6-diethyl-4-bora-3a,4a-diaza-s-indacene-triazine) and chemically converted graphene (CCG). A highly effective solar light harvesting BODIPY system could be the cause of the CGC-BODIPY's exceptional photocatalytic activity, which results in a high NADH regeneration yield of 54.02% and exclusive synthesis of formic acid with a yield of 144.2 µmol from CO₂. Current studies clearly show that the graphene-based photocatalyst (CCG-BODIPY) is superior to previously published photocatalysts like graphene-TPP.⁴²

A quick and eco-friendly one-step hydrothermal method was developed by Xueshan Li and co-workers to create ZnO-RGO composites for photo-catalysis applications. Methane, methanol, and certain higher hydrocarbons are among the products of the photo-catalytic reduction reaction, which occurs when CO₂ combines with light and a photo-catalyst. Methanol was selected as the target product to assess the items’ capacity for photo-reduction. Under simulated sunshine irradiation for 10 h, the highest yields of methanol were obtained for ZnO and ZnO-RGO photo-catalysts, with respective yields of 26.2 and 45.8 µmol/g-cat. It is evident from the data that adding the RGO sheets can significantly enhance ZnO's photo-catalytic reduction capabilities.⁴³ Mechanism for the production of solar chemicals via graphene-based photocatalyst is shown in Scheme 3.

Scheme 3.

General mechanism of the solar chemical production via graphene-based photocatalyst.

By using the basic microwave method, Kuei-Hsien Chen and co-workers were able to synthesize a series of Cu-NP-modified GO photocatalysts that showed a notable improvement in photocatalytic activity to produce solar fuel. When exposed to visible light for photocatalytic CO₂ reduction, the Cu/GO-2 composite with 10% weight percentage Cu showed the highest rate of methanol synthesis (6.84 µmol g-cat⁻¹ h⁻¹). The rate of photocatalytic CO₂ reduction obtained in this instance is 60 times more than that of the pure GO and 240 times higher than that of commercial P-25 when exposed to visible light. It is anticipated that adding Cu-NPs to GO effectively tunes its work function, improving charge separation and boosting CO₂ photocatalytic reduction.⁴⁴

Developing materials and technologies to improve the effectiveness of CO₂ photoreduction is one of the major challenges of the twenty-first century. To effectively convert CO₂ into CH₄, Md. Arif Hossen et al. developed novel ternary photocatalysts, in their study which consist of reduced graphene oxide (RGO), gold (Au) nanoparticles, and TiO₂ nanotubes arrays (TNTAs). The visible light-responsive RGO/Au-TNTAs composite was created by simply electrochemically depositing Au nanoparticles and then submerging RGO nanosheets into the TNTAs. The structural and functional properties of the composite were evaluated by a comprehensive characterization process that involved the use of FESEM, HR-TEM, XRD, XPS, FT-IR, UV-VIS DRS, and PL analyzer. The composite outperformed pure TNTAs, Au-TNTAs, and RGO-TNTAs, with a maximum CH₄ production of 35.13 ppm/cm² under visible light after 4 h. The combination of Au nanoparticles and RGO leads to increased CO₂ photoreduction efficiency in the RGO/Au-TNTAs composite. Au nanoparticles utilize surface plasmon resonance (SPR) to enhance visible light absorption, while the RGO layers help in efficient electron transport, effectively separating electron-hole pairs. This newly developed composite shows significant potential for use as a photocatalyst in various applications in the future.⁴⁵

The photocatalytic efficiency of conventional semiconductors is of great interest, and graphene has surfaced as a possible addition to increase this activity. In the study by Diego Mateo et al., it has been shown that NiO/Ni nanoparticles (NPs) supported on few-layer graphene (fl-G) function as a potent photocatalyst for CO₂ methanation at about 200^οC, with apparent quantum yields of 1.98%. They achieved specific CH₄ formation rates of 642 µmol.g_Ni⁻¹.h⁻¹, which is nearly twice the rate observed for Ni NPs supported on high surface area silica-alumina.⁴⁶

The synthesis of GO/MAPbBr₃ hybrid and MAPbBr₃ quantum dots (QDs) for electrochemical and photochemical CO₂ reduction was reported by Qinglong Wang et al. via ligand-assisted re-precipitation. According to their research, the GO/MAPbBr₃ hybrid that was created in-situ performed noticeably better at photoelectrochemical CO₂ reduction than the bare MAPbBr₃. In particular, the MAPbBr₃ yielded 0.268 µmol cm⁻² h⁻¹ of CO, which was significantly increased to 1.05 µmol cm⁻² h⁻¹ after hybridization with GO. This enhancement is explained by the improved electron extraction capability of the conductive GO, which makes it easier for photoelectrons to go from MAPbBr₃ QDs to CO₂. Concerning photoelectrochemical CO₂ reduction in organic solvents, this work expands the application of perovskite-based hybrids to more unstable configurations than previously documented.⁴⁷

Won-Chun Oh et al. used ultrasonic techniques to create WSe₂-graphene-TiO₂ ternary nanocomposites. They conducted detailed characterization using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The composite has a band gap of 1.62 eV, making it suitable for photodegradation in both UV and visible light. When tested for their ability to reduce CO₂ to CH₃oh, the WSe₂-G-TiO₂ composite with 8% graphene exhibited the highest photoactivity, producing a total CH₃OH yield of 6.3262 µmol g⁻¹ h⁻¹ after 48 h. This exceptional performance is attributed to the synergistic interaction between the WSe₂/TiO₂ and graphene components in the heterogenous system.⁴⁸ The summary for the solar chemical production via graphene-based photocatalyst is shown in Table 2.

Table 2.

Summary of the different solar chemical production via graphene-based photocatalyst.

Graphene-based photocatalysts	Solar chemicals	Yield	Reference
RGO-CdS	CH₄	2.51 µmol h⁻¹ g⁻¹	40
G-TiO₂	CH₄	8 µmol h⁻¹ g⁻¹	41
G-TiO₂	C₂H₆	16.8 µmol h⁻¹ g⁻¹	41
CCG-BODIPY	HCOOH	144.2 µmol	42
RGO-ZnO	CH₃OH	45.8 µmol/g-cat	43
GO-Cu	CH₃OH	6.84 µmol g-cat⁻¹ h⁻¹	44
RGO/Au-TNTAs	CH₄	35.13 ppm/cm²	45
NiO/Ni-(fl-G)	CH₄	642 µmol.g_Ni⁻¹.h⁻¹	46
GO/MAPbBr₃	CO	1.05 µmol cm⁻² h⁻¹	47
WSe₂-graphene-TiO₂	CH₃OH	6.3262 µmol g⁻¹ h⁻¹	48

Efficient reduction of CO₂ via graphitic carbon nitride-based photocatalyst

Graphitic carbon nitride is a conjugated polymer made up of carbon and nitrogen which exists in two dimensions. Researchers are now very interested in graphitic carbon nitride because it has many great qualities such as being cheap, being earth-abundant, having a lot of surface area, not containing any metal, having a fast electron transfer π-π conjugation structure, being biocompatible and having catalytic properties (Figure 6). Due to its great biocompatibility and catalytic properties scientists are now inspired to use this material in various sensors and biosensors.⁴⁹

Figure 6.

Diagrammatic representation of the properties of carbon nitride.

Carbon nitride's (CN) distinct, optical and electrical structure, ease of fabrication, great chemical and thermal stability, affordability and “earth-rich” environment have made it an emerging star in scientific research. A promising catalyst with the right energy band gap (i.e., 2.7 eV) was found when Wang et al. used CN in photocatalysis in 2006. As a result of the numerous uses of CN in photocatalysis, scientists from all over the world have been interested in creating new CN-based catalysts to enable the effective use of solar energy. Stated differently, the development of gC₃N₄-based heterostructure will allow CN to become a new class of next-generation visible-light responsive photocatalysts.⁵⁰

Due to the stability of CO₂, it is quite challenging to lower it at room temperature. Using a hard-template method, Jianlin Shi and co-workers developed and synthesized mesostructured CeO₂/graphite carbon nitride (m-CeO₂/g-C₃N₄). In addition to promoting charge carrier separation and transfer efficiency, the heterogenous nanocomposites demonstrated a much-improved responsiveness to solar light. As a result, they have significantly improved their CO₂ photoreduction efficiency. The CO₂ reduction on 50 mg nanocomposite photocatalyst after an hour of room temperature irradiation produced CO and CH₄ with yields of 0.590 and 0.694 µmol respectively.⁵¹

MoS₂ /g-C₃N₄ heterojunctions were effectively synthesized by Hao Qin and co-workers by using the hydrothermal deposition method and calcination. The setup of Z-scheme transfer involved the proper connection of two semiconductor photocatalysts. The hybrid heterojunction consisting of 10% MoS₂ /g-C₃N₄ demonstrated the most significant conversion efficiency in the reduction of CO₂ to CO during a 7-h reaction with the yield of 58.59 µmol(g-cat)⁻¹ which was approximately 2.94 times greater than that of unmodified g-C₃N₄.⁵²

Liqiang Jing et al. have effectively produced nanocomposites consisting of SnO₂-coupled boron and phosphorous co-doped g-C₃N₄ (SO/B-P-CN). These nanocomposites serve as cost-effective visible-light photocatalysts that are efficient in converting CO₂ and degrading pollutants, without the need for co-catalysts. The results demonstrate that the SO/B-P-CN nanocomposite, which has been tuned for the quantity, displays improved visible-light capabilities in converting CO₂ to CH₄ from CO₂-containing water. The conversion efficiency is increased by approximately 9 times, resulting in a yield of 49 µmol. Additionally, it has been verified that the significant increase in photocatalytic activity can be due to the substantial production of hydroxyl radicals on the SO/B-P-CN catalyst. The quantum efficiency (2.02%) of the improved SO/B-P-CN nanocomposite for photocatalytic CO₂ conversion at a wavelength of 420 nm is significantly higher compared to earlier reported results.⁵³

Sheng Zhou et al. conducted an in-situ synthesis of composites consisting of graphitic-C₃N₄ and N-TiO₂ using a simple heating technique. The incorporation of g-C₃N₄ and nitrogen species into the TiO₂ subcomponent in the composite photocatalysts results in excellent sensitivity to visible light and efficient separation of charges during light irradiation. The photocatalysts that were synthesized exhibit favourable photocatalytic activity and photostability when used for the photoreduction of CO₂ in the presence of water vapour. Notably, the optimized sample achieved the largest quantity of CO evolution, measuring 14.73 µmol, after being exposed to light for 12 h.⁵⁴

Under normal weather conditions, the most stable form of all the carbon nitride allotropes is graphitic carbon nitride (g-C3N4). Ke Wang et al. focused on creating sulfur-doped g-C3N4 by directly heating thiourea at 520°C. The resulting sulfur-doped g-C3N4 (TCN) could absorb light up to 475 nm, or 2.63 eV, which is slightly lower than the 2.7 eV band gap of the undoped g-C3N4 (MCN). To examine the electrical characteristics, spin-polarized density functional theory was used to calculate the theoretical partial density of states for TCN and MCN based on first principles. The test results indicated that the TCN sample contained impurities, although the band gaps for TCN and MCN were identical. Electrons generated by light can move more freely through these impurities, either from the valence band to the impurity state or from the impurity state to the conduction band. The samples’ photocatalytic performance was assessed through CO₂ reduction experiments. TCN achieved a methanol (CH₃OH) yield of 1.12 µmol g⁻¹, while MCN yielded 0.81 µmol g⁻¹.⁵⁵

A promising approach to address the energy crisis and lessen the greenhouse effect at the same time is photocatalytic CO₂ reduction to fuels. Using a straightforward calcination process, Haiwei Guo et al. created a direct Z-scheme NiTiO₃/g-C₃N₄ (NT/GCN) photocatalyst to increase the efficiency of CO₂ photoreduction. Without sacrificial agents or cocatalysts, the NT/GCN40 variant remarkably obtains the greatest CH₃OH yield of 13.74 μmol·g⁻¹·h⁻¹, which is about 3.29 times more than that of pure g-C₃N₄.⁵⁶

The role of heat input in traditional photocatalytic processes is rarely studied. In this study, Ag nanoparticles supported by thermal energy were created by a photochemical process and used for photocatalytic CO₂ reduction. The photocatalytic characteristics were examined at temperatures up to 200°C under LED light (420 nm, similar to solar intensity). The results showed that the reaction temperature significantly affected the rate of CO evolution. For the 20% Ag-loaded CN at 190 °C, the maximum CO generation reached 179.6 μmol•g⁻¹•h⁻¹, approximately 4.2 times greater than that of CN. However, there was very little photocatalytic activity under light illumination at ambient temperature.⁵⁷

Baorong Duan and colleagues successfully developed a photocatalyst using a hydrothermal method and a colloidal solution containing a mix of Fe₂O₃ and g-C₃N₄. This photocatalyst showed high activity due to the collaboration between the two components in a Z-scheme configuration. While Fe₂O₃ alone couldn't produce methanol due to its low conduction band, and g-C₃N₄ only yielded a moderate amount of methanol from CO₂ photoreduction, the combination significantly increased methanol production under the same reaction conditions. The highest methanol yield achieved was 1768 μmol•g⁻¹ with a 15% Fe₂O₃/g-C₃N₄ catalyst. This improved performance highlights the effectiveness of the heterojunction in enhancing photocatalytic activity.⁵⁸ Table 3 shows the comparative chart of carbon nitride-based photocatalysts for solar fuel production. The mechanism for the CO₂ reduction via carbon nitride-based photocatalyst is shown in Scheme 4.

Scheme 4.

General mechanism of CO₂ reduction via carbon nitride-based photocatalyst.

Table 3.

Summary of CO₂ conversion to solar fuels via carbon nitride-based photocatalyst.

Carbon nitride-based photocatalysts	Solar fuel	Yield	Reference
CeO₂-g-C₃ N₄	CO	0.590 µmol	51
CeO₂-g-C₃ N₄	CH₄	0.694 µmol	51
MoS₂ /g-C₃ N₄	CO	58.59 µmol(g-cat)⁻¹	52
SnO₂ /B-P/g-C₃N₄	CH₄	49 µmol	53
g-C₃N₄-N-TiO₂	CO	14.73 µmol	54
sulfur-doped g-C₃N₄ (TCN)	CH₃OH	1.12 µmol g⁻¹	55
NiTiO₃/g-C₃N₄ (NT/GCN)	CH₃OH	13.74 μmol·g⁻¹·h⁻¹	56
Ag-loaded CN	CO	179.6 μmol•g⁻¹•h⁻¹	57
Fe₂O₃-(g-C₃N₄)	CH₃OH	1768 μmol•g⁻¹	58

Conclusion

Inspired by natural photosynthesis, artificial photosynthesis is at the forefront of cutting-edge approaches to CO₂ management and solar chemical production. By mimicking the phenomenon of natural photosynthesis, artificial photocatalysis presents a viable way to slow down global warming and make the environment greener. The synthesis of highly efficient photocatalysts is a key challenge for CO₂ reduction. Various types of photocatalysts such as graphene or carbon nitride-based nanocomposite, covalent organic frameworks have been synthesized by the researchers for constructing artificial photocatalytic systems. In this review article, we study the various types of artificial photocatalytic systems that produce solar fuels by the reduction of CO₂ with good yield. The study conclude that an artificial photocatalytic system emerges as a new benchmark for the reduction of the level of harmful greenhouse gases from the environment. This technology reduces dependency on fossil fuels and helps in the reduction of greenhouse gas emissions.

Footnotes

Acknowledgements

This work was supported by Chandigarh University, Mohali and Deen Dayal Upadhyaya Gorakhpur University, India.

ORCID iD

Surabhi Chaubey

Funding

The authors received no financial support for the research, authorship, and/or publication of this article.

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

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Eco-friendly conversion of carbon dioxide into solar fuels via artificial photosynthetic routes: A review

Abstract

Keywords

Introduction

Synthesis of electron mediator (m)

Solar light harvesting templates in artificial photosynthetic systems

Study of CO2 reduction using various solar light harvesting photocatalysts

CO2 reduction via COF-based photocatalysts

Selective CO2 reduction via graphene-based photocatalyst

Efficient reduction of CO2 via graphitic carbon nitride-based photocatalyst

Conclusion

Footnotes

Acknowledgements

ORCID iD

Funding

Declaration of conflicting interests

References

Study of CO₂ reduction using various solar light harvesting photocatalysts

CO₂ reduction via COF-based photocatalysts

Selective CO₂ reduction via graphene-based photocatalyst

Efficient reduction of CO₂ via graphitic carbon nitride-based photocatalyst