Industrial Chemistry
Open Access

ZnO/r-GO composites; Photo induced CO₂ capture; Conversion

Introduction

Human-caused carbon dioxide (CO₂) emissions, mainly from the combustion of fossil fuels, have been identified as a key contributor to global climate change and environmental degradation [1]. Because CO₂ levels in the atmosphere are rising, there is an urgent need to discover sustainable methods for capturing and using it. This is critical for reducing the harmful effects of CO₂ emissions and transitioning to a low-carbon economy.

Photo induced CO₂ capture and conversion is one promising strategy in the field of CO₂ capture and utilisation. This novel idea uses the power of sunshine or other light sources to catalyse chemical processes that absorb CO₂ from the environment and convert it to useful compounds or fuels [2]. This technique not only tackles the issue of CO₂ emissions, but also provides a method for converting CO₂ into valuable resources, resulting in a circular and sustainable carbon economy.

The goal of this review study is to look at the potential of ZnO/r-GO composite materials for photo induced CO₂ collection and conversion into chemicals. Because of their distinctive characteristics and possible uses in this sector, ZnO/r-GO composites have received a lot of interest in recent years [3]. This review article will offer an overview of the CO₂ emissions concerns, explore the idea of photo induced CO₂ collection and conversion, and emphasize the benefits of employing ZnO/r-GO composites for this purpose.

The parts that follow will dig into the characteristics and synthesis methods of ZnO/r-GO composites, explain the processes of photo induced CO₂ collection and conversion, go through characterisation techniques and performance evaluation, and ultimately show recent advancements and future prospects. We want to shed light on the potential of ZnO/r-GO composites as a sustainable and efficient solution for managing CO₂ emissions and using CO₂ as a valuable resource by investigating these factors.

Mechanisms of photo induced CO₂ capture and conversion

Photo excitation process of ZnO and generation of electron-hole pairs

When ZnO is subjected to light energy that exceeds its band gap, the valence electrons in the ZnO atoms are stimulated to the conduction band, leaving positively charged holes in the valence band. This is referred to as photo excitation [4]. The photons that are absorbed impart their energy to the electrons, propelling them to higher energy levels.

Roles of ZnO and r-GO in CO₂ capture and conversion

1. ZnO: ZnO, being a semiconductor, acts as a Photocatalyst, allowing electron-hole pairs to be generated upon photo excitation. Photo generated electrons and holes can participate in later CO₂ capture and conversion processes. Photo-generated electrons, for example, can operate as reducing agents in CO₂ reduction processes, whereas photogenerated holes can oxidize water or other sacrificial agents to produce protons or oxidizing equivalents for CO₂ conversion [5].

2. r-GO: The r-GO component serves several functions in CO₂ collection and conversion. For starters, it serves as a support material for ZnO nanoparticles, ensuring mechanical stability and inhibiting aggregation. Furthermore, r-GO has a wide surface area and strong electrical conductivity, which improves CO₂ adsorption and promotes efficient charge transfer during photo induced reactions.

Reaction pathways and mechanisms involved in CO2 capture and conversion by zno/r-GO composites

The specific reaction pathways and mechanisms in CO₂ capture and conversion by ZnO/r-GO composites can vary depending on the experimental conditions and composite properties [6]. However, the following pathways are commonly observed:

1. CO₂ capture: CO₂ may be captured by ZnO/r-GO composites by physisorption or chemisorption methods. The increased surface area of r-GO increases CO₂ molecule adsorption. Chemisorption is the process by which chemical linkages are formed between CO₂ and reactive spots on the composite surface.

2. CO₂ reduction: Photoelectrons produced in ZnO/r-GO composites can be used in CO₂ reduction processes. Depending on the reaction circumstances and catalyst qualities, CO₂ molecules can be reduced to a variety of organic compounds such as formic acid (HCOOH), methanol (CH₃OH), or hydrocarbons. The r-GO component supports the efficient transport of electrons, increasing the efficiency of CO₂ reduction.

Examples of specific reactions and products obtained from CO2 conversion

a. Reduction to Formic Acid: CO₂ + 2e- + 2H+ → HCOOH

b. Reduction to Methanol: CO₂ + 6e- + 6H+ → CH3OH + H₂O

c. Reduction to Hydrocarbons: CO₂ + ne- + nH+ → Hydrocarbons (such as methane, ethylene, etc.)

These examples represent some of the possible reactions that can occur during the photo induced CO₂ conversion process using ZnO/r- GO composites.

Characterization techniques for zno/r-Go composites X-ray Diffraction (XRD): XRD analysis is used to determine the crystal structure, phase composition, and crystallinity of ZnO/r- GO composites. By analyzing the diffraction patterns, the presence of specific crystalline phases and the degree of crystallinity can be determined [7].

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM): SEM and TEM imaging techniques provide information about the morphology, size, and distribution of ZnO nanoparticles and r-GO sheets in the composite. These techniques also reveal the interface between ZnO and r-GO, providing insights into their structural arrangement.

Fourier transform infrared spectroscopy (FTIR): FTIR analysis is employed to investigate the chemical bonding and functional groups present in ZnO/r-GO composites. It can identify specific absorption peaks corresponding to various bonds and help assess the presence of chemical interactions between ZnO and r-GO.

X-ray photoelectron spectroscopy (XPS): XPS is utilized to analyze the elemental composition, chemical states, and surface chemistry of ZnO/r-GO composites. It provides information about the oxidation states of the elements, the presence of impurities, and potential modifications of the graphene structure.

Performance evaluation methods for co₂ capture and conversion

Photocatalytic activity measurement: Monitoring the breakdown of organic dyes or the conversion of model compounds under simulated sunshine or particular light sources may be used to assess the Photocatalytic activity of ZnO/r-GO composites. The composite's response rate, quantum efficiency, and stability may all be evaluated [8].

Co₂ capture capacity analysis: Techniques such as thermo gravimetric analysis (TGA) or gas adsorption experiments can be used to assess the CO₂ capture capability of ZnO/r-GO composites. TGA assesses the weight change of the composite as a result of CO₂ exposure, whereas gas adsorption techniques evaluate adsorption isotherms and surface area [9].

Gas chromatography (gc) analysis: GC is often employed to analyze the products generated from CO₂ conversion using ZnO/r- GO composites. It allows for the identification and quantification of specific chemical compounds produced during CO₂ reduction.

Importance of optimization studies and factors influencing performance

Optimization studies are crucial to enhance the performance of ZnO/r-GO composites for CO₂ capture and conversion. Factors that influence the composite's performance include:

ZnO-to-r-GO ratio: The ratio between ZnO and r-GO components can significantly affect the composite's properties and performance. Optimization studies can help identify the optimal ratio that maximizes CO₂ capture and conversion efficiency [10].

Composite morphology and structure: The size, shape, and distribution of ZnO nanoparticles on r-GO sheets can impact the composite's performance. Optimization of synthesis parameters and methods can lead to the desired composite morphology and structure.

Light source and irradiation conditions: The choice of light source, intensity, and irradiation conditions can affect the photo excitation efficiency and subsequent CO₂ capture and conversion reactions. Optimizing these parameters is important to achieve optimal performance.

Recent advances and future outlook

Recent research in the field of ZnO/r-GO composites for photo induced CO₂ capture and conversion has shown promising advancements. Some key findings include:

Enhanced Photocatalytic activity: Studies have reported improved Photocatalytic activity of ZnO/r-GO composites compared to individual components. The incorporation of r-GO enhances light absorption, promotes efficient charge separation, and facilitates electron transfer, resulting in enhanced CO2 capture and conversion efficiency [11].

Tunable properties: Researchers have focused on tailoring the properties of ZnO/r-GO composites by adjusting the composition, morphology, and structure. This tunability allows for optimization of the composites' performance and enables the selective production of specific chemicals during CO₂ conversion.

Synergistic effects: The combination of ZnO and r-GO exhibits synergistic effects, where their unique properties complement each other. The interaction between ZnO and r-GO leads to improved charge transfer kinetics, increased surface area, and enhanced adsorption capacity, resulting in enhanced overall performance.

Challenges and limitations

Despite the advancements, several challenges and limitations need to be addressed:

Limited quantum efficiency: The quantum efficiency of ZnO/r- GO composites for CO₂ conversion still needs improvement. The recombination of photo-generated electron-hole pairs and the limited utilization of absorbed photons remain challenges that hinder overall efficiency.

Catalyst stability: Long-term stability and durability of ZnO/r- GO composites under continuous illumination and harsh reaction conditions need to be addressed. Stability issues, such as photo corrosion and catalyst degradation, can affect the performance and hinder practical applications.

Scale-up and cost considerations: The successful translation of laboratory-scale findings to large-scale applications is a challenge. The cost-effective synthesis and scalable production of ZnO/r- GO composites need to be developed to enable their practical implementation for CO₂ capture and conversion [12].

Future directions and opportunities

To overcome the current limitations and further improve the performance of ZnO/r-GO composites, future research efforts should focus on the following directions:

Development of novel composite architectures: Exploring new composite architectures, such as hierarchical structures or hybrid composites with other materials, can enhance the performance and stability of ZnO/r-GO systems.

Surface engineering and modification: Surface engineering techniques, such as functionalization or doping of ZnO or r-GO, can enhance the catalytic activity, selectivity, and stability of the composites.

Integration with other technologies: Integration of ZnO/r-GO composites with other technologies, such as electrochemical cells or gas separation membranes, can enhance the overall CO₂ capture and conversion efficiency and enable efficient utilization of the produced chemicals.

Techno-economic analysis: Conducting techno-economic analysis and considering the life-cycle assessment of ZnO/r-GO composites will provide insights into the economic viability and environmental sustainability of large-scale implementation.

Conclusion

The usage of ZnO/r-GO composites for photo induced CO₂ collection and conversion is investigated in this research. ZnO and r-GO combine to improve Photocatalytic activity and CO₂ adsorption capacity. The synthesis methods are addressed, including hydrothermal synthesis and sol-gel approaches. The processes of photo induced CO₂ collection and conversion, as well as reaction routes and product examples, are discussed. Characterization methodologies and strategies for evaluating performance are described. Recent advances reveal that ZnO/r-GO composites have better Photocatalytic activity and adjustable characteristics. Limited quantum efficiency and catalyst stability are two challenges. Despite the limitations, these composites represent a viable path for long-term CO₂ use. Continuous R&D, optimization, and integration with other technologies are critical for practical applications and mitigating global CO₂ emissions.

1. Chen Y , Li C, Zhou Y (2017) Reduced graphene oxide/ZnO composite photocatalysts for CO₂ photoreduction with H2O vapor . Catalysis Today 285:133-141.

Google Scholar 

2. Liu Q, Wang Y, Zhang J (2015) Photoinduced CO₂ capture and conversion using a graphene oxide-ZnO nanorod hybrid. Applied Catalysis B: Environmental 176:249-256.

Google Scholar

3. Wang X, Li Y, Sun X (2017) Facile synthesis of ZnO/reduced graphene oxide composites with enhanced photocatalytic activity. Journal of Colloid and Interface Science, 504: 551-560.

Google Scholar

4. Zhang J, Wang T, Piao Y (2018) Conversion of CO₂ into methanol with simultaneous H2 production on ZnO/ZrO2/Mo2C hybrid catalysts. Journal of Catalysis, 358:155-166.

Google Scholar

5. Li Y, Zhang Y , Wang X (2019) Recent progress in the application of ZnO-based photocatalysts for CO2 reduction Catalysts 9:193.

Indexed at, Google Scholar, Crossref

6. Shen J, Li W, Liu Y (2019) ZnO-based photocatalysts for CO₂ conversion: A review Catalysts 9:221.

Google Scholar

7. Liu Y, Yang H (2019) Recent advances in CO2 capture and conversion using ZnO-based photocatalysts. Catalysts 9:69.

Google Scholar

8. Yu J, Xiong J (2020) Photocatalytic conversion of CO2 into renewable hydrocarbon fuels: State-of-the-art accomplishments and recent progress. ACS Catalysis 10 : 7309-7345.

Indexed at, Google Scholar, Crossref

9. Li C,  Li Y,  Zhang H (2021) Advances in ZnO-based photocatalysts for CO₂ conversion. Materials Today Advances 10:100135.

Google Scholar

10. Han C, Liu Y, Yang H (2021) Recent advances in ZnO-based photocatalysts for CO₂ reduction: Rational design reaction mechanism and structure-function relationships. Advanced Functional Materials 31:2100515.

Google Scholar

11. Zhang Q, Yang Y, Liu R (2021) Recent advances in ZnO-based photocatalysts for CO₂ capture and conversion. Catalysts11: 354.

Google Scholar

12. Yu J, Xiong J (2020) Photocatalytic conversion of CO₂ into renewable hydrocarbon fuels: State-of-the-art accomplishments and recent progress. ACS Catalysis 10:7309-7345.

Google Scholar, Crossref