g-C₃N₄/TiO₂ NANOCOMPOSITES AND THEIR APPLICATION IN PHOTOCATALYTIC CO₂ REDUCTION: A MINIREVIEW

M. MANRIQUE-HOLGUÍN, J.J. ALVEAR-DAZA,
J.A. RENGIFO-HERRERA and L.R. PIZZIO

Laboratory of Advanced Oxidation Processes and Photocatalysis (LAPh), Centro de Investigación en Ciencias Aplicadas “Dr. Jorge J. Ronco” (CINDECA) (CCT La Plata CONICET, UNLP, CICPBA), Departamento de Química, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, 47 No. 257, La Plata, Argentina

lrpizzio@quimica.unlp.edu.ar; julianregifo@quimica.unlp.edu.ar

Cite this article as:

Manrique-Holguín, M., Alvear-Daza, J.., Rengifo-Herrera, J.A., Pizzio, L.R.. (2023) “g-c3n4/tio2 nanocomposites and their application in photocatalytic co2 reduction: a minireview”, Latin American Applied Research, 53(1) pp 71-76.

Abstract-- g-C₃N₄/TiO₂ nanocomposites seem to be promising materials for photocatalytic reductive applications such as water splitting and CO₂ reduction. The g-C₃N₄ is known as a metal-free semiconductor exhibiting a high reductive conduction band (CB) (-1.3 V vs. NHE) and visible light absorption (E_g =2.7 eV), while TiO₂ is the most popular photocatalyst. However, both semiconductors show high electron/hole recombination, and in the case of TiO₂, lack of visible light absorption. Both problems could be overcome by designing type II heterojunctions or a direct Z-scheme between g-C₃N₄ and TiO₂. These strategies make these composites suitable for CO₂ photocatalytic reduction and solar fuel production. Herein, the main aspects related to photocatalytic CO₂ reduction in aqueous media to obtain solar fuels such as methane and methanol, synthesis of g-C₃N₄/TiO₂ nanocomposites, and their reactivity will be addressed and reviewed.

Keywords-- g-C₃N₄, TiO₂, heterojunctions, direct Z-scheme, CO₂ reduction, solar fuels

I. INTRODUCTION

The presence of greenhouse gases such as CO₂, methane (CH₄), fluorinated gases, and NO_x on the planet´s atmosphere causes a detrimental effect on the global climate (global warming). Data from the US Environmental Protection Agency (USEPA) revealed that CO₂ accounts for 80% of the total greenhouse gas emissions from the combustion of nonrenewable energy sources (oil derivatives and charcoal) (US Environmental Protection Agency, 2022). Between 2001 and 2014, the World Bank reported a rise of CO₂ emissions at global scale from 22 to 36 million kt, while in 2016 Argentina’s CO₂ emissions amounted to around 200.000 kt (Banco Mundial, 2022).

Due to the above, the interest in reducing CO₂ emissions has been steadily increasing, and one of the strategies is the production of chemical compounds from CO₂. Heterogeneous catalytic processes such as heterogeneous photocatalysis, heterogeneous electrocatalysis, and heterogeneous photoelectrocatalysis exhibit interesting features to convert CO₂ into solar fuels such as CH₄ and CH₃OH (Tu et al.,2014; Xie et al., 2021). Applications of TiO₂ based photocatalytic technologies have been historically addressed to employ the oxidant capacity of photoinduced valence band holes (h⁺_VB) to remove or anic pollutants and bacteria from water and air.

Figure 1: Number of publications per year about CO₂ photoreduction by using Scopus database.

g However, the first study reporting the photocatalytic CO₂ reduction on TiO₂ nanoparticles was published in the late 70´s by Inoue et al. (1979). But the necessity to find novel strategies to mitigate CO₂ in the atmosphere has led to a rebirth of this technology in the last 10 years (Fig. 1).

TiO₂ would mimic the process occurring in a plant leaf through photosynthesis, where CO₂ molecules are captured and then converted into more complex substances via several photoinduced reactions (Abdullah et al., 2017). The photocatalytic CO₂ reduction shows another interesting feature besides the production of solar fuels: the possibility to use solar light, a renewable energy source, to excite the photocatalyst. However, TiO₂ nanoparticles do not exhibit visible light absorption since their band gap energy (E_g) is around 3.2 eV, allowing only UV wavelengths, less abundant on the planet´s surface (4%-7%), to photoinduce charge carriers. This fact makes the development of visible-light TiO₂ nanomaterials, which are able to take better advantage of more abundant wavelengths such as visible light (50%), an attractive strategy to overcome this drawback.

Graphitic carbon nitride (g-C₃N₄), a metal-free semiconductor with visible light absorption (E_g= 2.7 eV), has a conduction band redox potential able to reduce CO₂ molecules, but a high electron-hole recombination and low specific surface area limit its use as photocatalyst in these applications. Nevertheless, heterojunctions between TiO₂ and g-C₃N₄ seem to be an attractive strategy to overcome the lack of visible light absorption of TiO₂ and the limitations of g-C₃N₄ mentioned above (Wen et al., 2017).

Figure 2: Primary events occurring in illuminated TiO₂ nanoparticles.

Figure 3: Redox potentials of CO₂ reduction vs. semiconductor band positions.

This minireview is devoted to showing the main aspects linked to the photocatalytic reduction of CO₂ and its conversion into solar fuels by using visible-light active g-C₃N₄/TiO₂ composites.

II. PRIMARY EVENTS OCCURING IN ILLUMINATED TiO₂ NANOPARTICLES

A. TiO₂ nanoparticles under UVA irradiation

When TiO₂ nanoparticles are irradiated by UVA light (λ< 400 nm), whose wavelengths have enough energy to overcome their E_g, electrons (e^-) located in the valence band (VB) are promoted to the conduction band (CB), generating charge separation and inducing electron-hole pairs (this event takes place in the first nanoseconds (ns)) (Fig. 2) (Ward et al., 1983).

Unfortunately, most of these charge carriers (~90%) undergo a fast recombination within the first nanoseconds, dissipating the energy excess in the form of heat (Mohamed and Bahnemann, 2012). Those charge carriers that survived from recombination can be trapped on different surface defects present on the metal oxide surface (in the scale of microseconds (µs)). For instance, electrons are mostly trapped onto pentacoordinated Ti⁴⁺ sites commonly named as oxygen vacancies (V_o), while photoinduced holes are trapped on Ti-OH sites. These trapped charges can react further with suitable donors and acceptors (in the scale of milliseconds (ms)) (Schneider et al., 2014).

Suitable electron acceptors must have a more positive

Figure 4: Steps to achieve CO₂ photoreduction on a photocatalyst.

potential than that of conduction band photoinduced electrons (e^-_CB) (around -0.5 V vs. NHE at pH 7.0), while suitable electron donors should exhibit a more negative potential than that of photoinduced h⁺_VB (+2.7 V vs. NHE at pH 7.0) (Zhang et al., 2012; Rengifo-Herrera et al., 2022) (Fig. 3).

Considering the time of each primary event occurring after UVA light irradiation, it is clear that there is a strong competition between the generation of electron-hole pairs and recombination, since both events occur in the same time scale (nanoseconds) and for this reason this is one of the main limitations of TiO₂ photocatalytic processes.

III. PHOTOCATALYTIC REDUCTION OF CO₂ IN AQUEOUS MEDIA

A. Mimicking nature

Artificial photosynthesis seeks to mimic the natural photosynthesis process carried out in a plant leaf (Remiro-Buenamañana and García, 2019). A suitable photocatalyst for CO₂ reduction must fulfill some key features such as good carbon dioxide adsorption, high pair electron-hole generation, charge-carrier separation, and CO₂ reduction (Fig. 4) (Gong et al., 2022).

The initial pH solution has a relevant role in the photocatalytic CO₂ reduction. Depending on pH, both the VB and CB of TiO₂ can undergo band bending. This fact is caused by an excessive adsorption of H₃O⁺ or OH^- on the TiO₂ surface, producing a magnetic field that changes the position of Fermi energy level (E_f). At acidic or alkaline pH values, the excess of positive or negative charge in the solid-liquid interface makes these bands undergo a downward or upward band bending respectively (Schneider et al., 2014). For instance, in TiO₂ nanoparticles each pH increase raises the redox potential of the CB by 59 mV, making the material more reductive (Moser and Gratzel, 1983).

When CO₂ reduction is carried out in aqueous media, the pH also plays an important role because CO₂ solubility in water is low (0.48 mg L^-1 at 25 °C). In the presence of NaOH (0.2 mol L^-1), the photocatalytic CO₂ reduction is enhanced, since CO₂ solubility is highest because its acidic properties and the presence of OH^- ions may serve as a strong hole scavenger (Koci et al., 2009). Moreover,

Figure 5: Types of CO₂ adsorption occurring on metal oxide semiconductors.

at alkaline pH or modifying the photocatalyst surface (adding alkaline groups to the surface), CO₂ adsorption (either as HCO₃^- or CO₃⁼) on the photocatalyst surface is boosted (Gong et al., 2022). Some studies about CO₂ adsorption on TiO₂ surfaces have proposed physisorption (adsorption as a linear molecule) or chemisorption (adsorption as a partially charged species of CO₂^δ●-). This issue is key since CO₂ reduction shows a very negative redox potential (-1.9 V vs, NHE) making this process on TiO₂ or g-C₃N₄ semiconductors thermodynamically unfavorable. However, CO₂ adsorption leads to the formation of charged structures with a geometrical distortion resulting in a lower barrier for accepting electrons (Vu et al., 2019). In Fig. 5, coordination schemes of CO₂ chemisorption are shown, where CO₂ or TiO₂ surface atoms behave either as Lewis acids or bases. Figure 5a shows how the O atoms of CO₂ play the role of Lewis bases, donating electron pairs to the Lewis acid centers of the photocatalyst. The opposite occurs in Fig. 5b, where the positively charged C atoms take the role of Lewis acids receiving electrons from Lewis basic centers of the photocatalysts. Moreover, Figure 5c shows a mechanism where either C or O atoms from CO₂ act as Lewis acids or bases. It is important to highlight the importance of chemisorption and the formation of CO₂^δ●- species on the photocatalyst surface since the latter weakens the linear symmetry of CO₂ molecule (a highly stable molecule with a dissociation energy of C=O bond of around 750 KJ mol^-1) enhancing the production of reduced by-products. Thus, increasing the number of active sites of the photocatalyst that is able to form CO₂^δ●- species (by chemisorption) would make CO₂ reduction more feasible.

The presence of a hole scavenger or a sacrificial electron donor (SED) is also a key factor in the photocatalytic CO₂ reduction (Shehzad et al., 2018). The trapping of photoinduced valence band holes by SED plays an important role since it allows a better charge separation and its oxidation produces the formation of protons that are essential to generate CO₂ reduced by-products.  Often, in photocatalytic CO₂ reduction by TiO₂ materials, water is used as SED due to its abundance and low cost. However, water exhibits two important disadvantages: (i) O₂ evolution when water oxidation takes place (Fig. 3) since molecular oxygen can compete with CO₂ molecules for photoinduced conduction band electrons, and (ii) water reduction (water splitting) is thermodynamically allowed in these photocatalytic systems (E= 0.00 V compared to CO₂ reduction E= -1.90 V vs. NHE)) leading to H₂ evolution reactions (HER). These drawbacks can be overcome by a suitable photocatalyst design where reductive and oxidative sites are separated by adding co-catalysts or designing heterojunctions or Z-scheme strategies with different semiconductors. Moreover, the use of nonaqueous solvents (where CO₂ solubility can be higher than in water) or the presence of SED, such as alcohols, amines (which exhibit a higher reductive potential), benzyl-dihydronicotinamide (BNAH imitating the role of NADH in the natural photosynthesis) and ascorbic acid/thiols, can also enhance the photocatalytic CO₂ reduction.

To summarize, the overall performance of solar fuel generation by photocatalytic processes depends on the reaction medium (pH, solvent, solid/aqueous or solid/gas interfaces, etc.) the concentration of SEDs, and the photocatalyst nature. For instance, if the photocatalytic CO₂ reduction is carried out in aqueous media, the contact between CO₂ and the photocatalyst is easier to achieve than in a solid/gas interface. Moreover, in aqueous media, a high diversity of products is obtained, while in solid/gas interface it is possible to obtain CO and CH₄ only.

B. Visible-light absorbing TiO₂ based materials for CO₂ reduction

TiO₂ is a material used in different industrial applications such as cosmetics, foods, medicine, fibers, paper, resins, plastics, and paints, and its global market is estimated to be around USD 15.76 billion, making this product easily available and abundant (Parrino and Palmisano, 2021). Since the Fujishima-Honda effect was reported in the early 70s (Fujishima and Honda, 1972), when TiO₂ was used as heterogeneous photocatalyst to promote water splitting to generate H₂, its photocatalytic applications to remove chemical and biological pollution in solid/water and solid/air interfaces, for H₂ production, organic synthesis, and CO₂ reduction to produce solar fuels (CH₄ and CH₃OH) have increased in the last decades (Nahar et al., 2017). TiO₂ offers several advantages as photocatalyst such as abundance, physicochemical stability, and suitable conduction (-0.5 V vs. NHE) and valence (+2.7 V vs. NHE) band redox potentials able to produce highly oxidant reactive oxygen species (ROS) and e^-_CB with enough redox potential to reduce H₃O⁺ and CO₂. Photocatalytic CO₂ reduction by-products obtained in aqueous TiO₂ suspensions or gas phase exhibit differences. For instance, the presence of water in the media leads to the formation of by-products such as CH₃OH, formic acid (COOH), and formaldehyde (CH₂O), while under gas phase the main by-products are CO and CH₄ (Nahar et al., 2017).

Despite these advantages, the TiO₂ photocatalyst exhibits some drawbacks linked to high electron/hole recombination and large bandgap energy, the latter limiting its use in solar applications.

There are several strategies to overcome these limitations. For instance, the modification of TiO₂ with noble metal nanoparticles can generate Schottky barriers where photoinduced conduction band electrons can easily migrate to the metal nanoparticles, enhancing the charge carrier separation and reducing the hole/electron recombination. Moreover, the presence of noble metal nanoparticles such as silver or gold with sizes smaller than 100 nm on TiO₂ surfaces can promote localized surface plasmon resonance (LSPR), allowing the composite TiO₂/noble metal visible light absorption at wavelengths comprised between 360 and 500 nm (depending on the size of noble metal nanoparticles) (Zhang et al., 2013). Modification of TiO₂ surfaces with organic or inorganic dyes (photosensitization) can also confer visible light absorption. Dye may behave as an antenna absorbing visible light photons and generating excited states able to transfer electrons to the TiO₂ CB. These e^-_CB can participate in redox reactions in the presence of molecular oxygen and generate ROS that can destroy the organic or inorganic dye eliminating the visible light absorption of these materials (Rengifo-Herrera et al., 2022).

On the other hand, there are also several reports about the use of heterojunctions or Z-scheme strategies with other metal oxides or metal-free semiconductors. Often, heterojunctions (type I or II) are carried out with other semiconductors that exhibit different positions of VB or CB than those of TiO₂. The main aim of heterojunctions is to create an intimate contact between semiconductors in order to efficiently separate the photoinduced electrons or holes, leaving the holes and electrons in the VB and CB with the highest oxidant and reductive power respectively. Thus, the literature reports the existence of type I heterojunctions where the conduction and valence bands of semiconductor 1 are respectively higher and lower than those of semiconductor 2, and type II heterojunctions where the CB and VB of semiconductor 1 are higher than those of semiconductor 2. The most common are the type II heterojunctions; however, unfortunately in this case, photoinduced h⁺ remain in the VB with the less positive redox potential, while electrons are left in the most positive CB. These features negatively affect the photocatalytic performance of the nanocomposite materials (Qi et al., 2017).

The direct Z-scheme, which was first reported by Yu et al. (2013), is an interesting strategy to prepare nanocomposites of TiO₂ with other metal oxides or free-metal semiconductors. In this case, the intimate contact between semiconductors (i.e., through chemical bonds) can achieve the effective charge separation as well. The magnetic field achieved by the intimate contact between semiconductors drive electrons to the most reductive CB and holes to the most oxidative VB, generating composite materials with high photocatalytic activity.

In this regard, graphitic carbon nitride (g-C₃N₄), which is a metal-free semiconductor composed of heptazine polymeric units, offers interesting properties to be used as semiconductor in either heterojunctions or direct Z-schemes with TiO₂. Its semiconducting properties were firstly reported by Wang et al. (2009) and it exhibits visible light absorption (E_g = 2.7 eV) and a high reductive CB position (-1.3 V vs. NHE) (Wen et al., 2017). Some few studies in the literature have reported type II heterojunctions and direct Z-scheme nanocomposites between TiO₂ and g-C₃N₄ with efficient ability to reduce CO₂ into solar fuels (Adekoya et al., 2017; Wang et al., 2020). These g-C₃N₄ structures can be easily synthesized by thermal condensation of urea, melamine, and thiourea at temperatures beyond 400 °C. In a first stage, Urea is transformed to biuret at temperatures ranging between 300 and 350 °C, which further cyclizes to form cyanuric acid. The latter reacts with ammonia coming from urea thermal decomposition to form ammelide and subsequently, melamine. Polycondensation of melamine generates a polymer composed of melem units and finally, at temperatures around 500 °C melem undergoes high polymerization obtaining g-C₃N₄ structures (Dai et al., 2015).

C. Synthesis and characterization of g-C₃N₄/TiO₂ nanocomposites

There are several strategies to synthesize g-C₃N₄/TiO₂ heterojunctions such as sol-gel method, hydrothermal method, solvothermal and microwave-assisted synthesis (Acharya and Parida, 2020). However, the sol-gel synthesis has been the favorite method to obtain g-C₃N₄/TiO₂ nanocomposites. The sol-gel method is a very versatile wet synthesis where acid or base catalyzed hydrolysis of a titanium alkoxide or titanium (IV) chloride is achieved in order to obtain a gel. Parameters such as initial pH, water concentration, and organic additives allow controlling the synthesis and obtaining a material with different physicochemical properties. The obtained gel must be further annealed at temperatures beyond 400 °C to produce TiO₂ with a well-defined crystalline structure (anatase or rutile). The addition of urea, melamine or thiourea during the titanium alkoxide hydrolysis allows obtaining TiO₂/g-C₃N₄ nanocomposites, but the calcination must be carried out at temperatures of 500 °C to obtain highly polymerized g-C₃N₄ structures (Pérez-Obando et al., 2019).

The characterization of g-C₃N₄/TiO₂ nanocomposites requires the use of bulk and surface techniques such as X-ray diffraction (XRD), diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), surface FT-IR techniques (ATR and DRIFT), and transmission electron microscopy (TEM). A study reported by some of us revealed that the synthesis of g-C₃N₄/TiO₂ by acid catalyzed sol-gel method using urea (30% w/w) and annealing temperatures of 400 °C for 1 h produced g-C₃N₄/TiO₂ nanocomposites. XRD diffraction patterns did not show evidence of the presence of metal-free semiconductor g-C₃N₄, apparently due to its high dispersion on the TiO₂, but several peaks corresponding to TiO₂-anatase crystalline structure were found. Materials exhibited visible light absorption comprised between 400 and 500 nm, matching very well the optoelectronic properties of the metal-free semiconductor. The presence of g-C₃N₄ onto the nanocomposite was evidenced by XPS and DRIFT-FTIR measurements. N 1s and C 1s XPS signal deconvolution showed a component at 399 eV and 288 eV respectively, attributed to C=N-C bonds of heptazine rings, while the IR spectrum revealed the presence of signals in the region comprised between 1200 and 1650 cm^-1 from the formation of extended C-N=C networks (Pérez-Obando et al., 2019). Most recently, we also reported the possible formation of direct Z-scheme in TiO₂ nanorods in the presence of g-C₃N₄. TiO₂ nanorods were prepared by thermal treatment of H-titanate nanotubes impregnated with urea (H-titanate:urea ratio, 1:4) and annealed at 450 °C for 1 h. In this material, unlike the previously reported, the XRD diffraction pattern showed evidence of g-C₃N₄ structures by the appearance of a peak at 27.4 ° typically assigned to this metal-free semiconductor. Moreover, by XPS a new N 1s signal at 397.5 eV was detected, probably due to the formation of Ti-N bonds. TEM micrographs also revealed an intimate contact between anatase TiO₂ and g-C₃N₄. All these findings allowed suggesting the existence of direct Z-scheme in this nanocomposite since H-titanates may show an important presence of Ti^IV sites allowing an interaction with formed g-C₃N₄ (through the formation of Ti-N bonds) (Osorio-Vargas et al., 2022).

D. Photocatalytic CO₂ reduction using g-C₃N₄/TiO₂ nanocomposites

Specific surface area, crystalline structure, and C/N ratio (of g-C₃N₄ structures) play an important role in photocatalytic CO₂ reduction using photocatalysts based on g-C₃N₄/TiO₂ nanocomposites (Ong et al., 2016).

Specific surface area is important since it could enhance CO₂ adsorption on the photocatalyst, making its photocatalytic reduction feasible. Moreover, the C/N ratio may allow controlling the g-C₃N₄ band gap energy, given that some authors have reported that materials with a high C/N ratio exhibit a band gap decrease. In addition, g-C₃N₄ materials with a low C/N ratio show a poor charge carrier separation and charge transport, both factors negatively affecting the photocatalytic activity (Ong et al., 2016). TiO₂ crystalline structure is another important characteristic to obtain g-C₃N₄/TiO₂ nanocomposites with high photocatalytic activity to reduce CO₂. It is well known that anatase TiO₂ presents the highest photocatalytic activity since its conduction band redox potential is suitable, so that CO₂ reduction can be a thermodynamically allowed reaction. Moreover, high crystallinity is also required to obtain TiO₂ materials with high electron/hole mobility. In most of the studies on g-C₃N₄/TiO₂ nanocomposites with high photocatalytic activity to produce solar fuels, anatase has been reported as the main TiO₂ crystalline structure.

There are just few studies about the use of g-C₃N₄/TiO₂ nanocomposites to photocatalytically reduce CO₂ in water (Acharya and Parida, 2020). For instance, Zhang et al. (2018) reported the synthesis of hollow g-C₃N₄/TiO₂ with high surface area and its evaluation in CO₂ photocatalytic assisted reduction under visible light irradiation. This nanocomposite showed an interesting methanol production due to an efficient charge separation occurring in the nanocomposite, probably due to a type II heterojunction.

Adekoya et al. (2017) found that modification of g-C₃N₄/TiO₂ with copper created islands of CuO and metal copper onto the nanocomposite surface, generating Schottky barriers (with metallic copper) and CuO acting as an electron trapping site for CO₂ adsorption, which enhanced the charge separation and charge transfer to CO₂. The main reduced products detected were methanol and formic acid. Wang et al. (2020) prepared g-C₃N₄/TiO₂ with direct Z-scheme containing gold nanoparticles. These materials showed an efficient production of light-induced electrons able to reduce CO₂ molecules into CH₄ and CO. The selectivity of CH₄ product was around 66%.

IV. CONCLUSIONS

Nanocomposites of g-C₃N₄/TiO₂ either as type II heterojunctions or direct z-scheme can overcome several drawbacks exhibited by TiO₂ and g-C₃N₄ separately, such as high electron-hole recombination, lack of visible light absorption, and low specific surface area.

These nanocomposites seem to be promising to be applied in photocatalytic reductive processes such as CO₂ in water interfaces and are an interesting strategy to help reduce greenhouse gases and produce solar fuels such as methanol.

ACKNOWLEDGEMENTS

Authors thank to National Scientific and Technical Research Council (CONICET) (Grant PIP 1492 and PIO 024), National University of La Plata (Grant X879) for their financial support and especially to R. Manrique-Holguín (ricardo.manrique@ucp.edu.co) for his contribution and support in image designing.

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Received: August 29, 2022

Sent to Subject Editor: September 3, 2022

Accepted: December 13, 2022

Recommended by Subject Editor Laura Briand