Regular ArticleIn-situ growth of TiO2 imbedded Ti3C2TA nanosheets to construct PCN/Ti3C2TA MXenes 2D/3D heterojunction for efficient solar driven photocatalytic CO2 reduction towards CO and CH4 production
Graphical abstract
Introduction
Photocatalytic reduction of CO2 to various chemicals and fuels, through artificial photosynthesis, is a promising approach to solve greenhouse effect and to partially fulfill energy issues [1], [2]. For this purpose, TiO2 has been commonly employed as one of the promising semiconductors photocatalyst due to its numerous benefits which include low cost and high stability, but it can be used under UV-light irradiation only due to wider band gap (Ebg ~ 3.20 eV for anatase TiO2) [3], [4], [5]. In the recent development of visible light responsive semiconductors, two dimensional (2D) graphitic carbon nitride has been widely studied due to its characteristics of metal free with low cost, easy preparation by thermal heating, non-toxic and have high photostability. More importantly, g-C3N4 has band gap energy of ~2.8 to 2.7 eV, thus, it can be used for solar energy assisted CO2 reduction and water splitting process [6]. More importantly, g-C3N4 conduction band (CB) is sufficiently negative (CB ~ −1.10 to −1.30 eV) and it would be beneficial for the reduction of CO2 to various desirable products such as CO, H2, CH4, CH3OH, HCHO and CH3COOH. The limitations by using single g-C3N4 material has lower photocatalytic performance due to easy recombining of photogenerated charge carriers [7].
The performance of semiconductors can be maximized through creating defects, structure alternation, morphology tuning, surface oxygen vacancies, loading metals as cocatalysts and constructing heterojunctions [8], [9]. For example, 2D g-C3N4 coupled with 3D hierarchical TiO2 was constructed and observed selective CO2 reduction to CO/CH4 [10]. In another report, efficient CO2 reduction with CO and CH4 production over g-C3N4/TiO2 composite was attained under UV-light irradiation [11]. Similarly, performance of g-C3N4 was obviously increased when doped with cobalt and coupled with TiO2 to construct Co-doped 0D/2D g-C3N4/TiO2 heterojunction [12]. Efficient production of CO and H2 over Ag-La co-doped g-C3N4 during dry reforming of methane was evidenced by Beenish and coworkers [13]. Excellent CO2 adsorption with efficient CO2 reduction to CO was achieved over structured Cu-NPs-doped g-C3N4 photocatalyst [14]. In another work, WO3/g-C3N4 heterojunction with proficient evolution rate of CO/CH4 during the process of CO2 photoreduction was obtained [15]. ZnV2O4/g-C3N4 composite exhibited higher methanol production during liquid phase photoreduction of CO2 under visible light [16]. The appreciable amounts of CO, CH4 and O2 were obtained over Z-scheme Cu2V2O7/g-C3N4 composite during CO2 photoreduction process [17]. However, expensive metals-based semiconductors hindered their development for commercial acceptance level of this technology for the conversion of greenhouse gas CO2 to renewable fuels. Thus, constructing noble metals free cocatalysts composite would be a promising to stimulate photocatalytic activity, thus, would be a meaningful approach.
Recently, organic–inorganic materials are under exploration as they would not only promote charge carrier separation, but also provides pathways to increase reaction kinetics [18], [19]. In this perspective, noble metals free materials, MXenes, a class of transition metal carbides/nitrides/carbonitrides, have been under exploration [20], [21]. Among the 2D MXene materials, titanium carbide (Ti3C2) has been widely investigated in CO2 reduction and water splitting applications due to its benefits of high electronic conductivity and good chemical/thermal stability [22], [23]. Numerous efforts have been devoted in this field to construct MXene based composite materials for several photocatalytic applications [24], [25], [26]. For instance, Tang et al. synthesized alkalinized MXene Ti3C2 attached with g-C3N4 and obtained higher photoactivity for the production of CO/CH4 during CO2 photoreduction process [27]. Similarly, 2D/2D heterojunction of Ti3C2/g-C3N4 exhibited enhanced photocatalytic hydrogen production [28] and CO2 reduction through photo-electrocatalytic process [29]. In another work, TiO2/Ti3C2 composite with the production of solar fuels such as CH4 has been investigated [30].
The morphology of Ti3C2 can be altered through different preparation methods, in addition of controlled growth of TiO2 NPs through oxidation process. In general, titanium aluminum carbide (Ti3AlC2) MAX can be converted to Ti3C2 MXenes through hydrofluoric acid (HF) treatment [31]. Our group reported enhanced photocatalytic performance of exfoliated 2D MAX Ti3AlC2 dispersed with TiO2 for stimulating CO2 photoreduction through dry reforming of methane [32]. So far, several Ti3C2-based composite photocatalysts have been proved as a promising cocatalyst in energy applications. In the current development, Ti3C2 derived carbon doped TiO2 was obtained through high temperature calcination coupled with g-C3N4 to construct nanocomposite for efficient photocatalytic H2 production [33]. Similarly, titanium precursor was used to produce TiO2 over Ti3C2 nanosheets and this synthesis process was conducted in two steps process, while employing higher temperature [34]. According to available literature, controlled growth of anatase TiO2 NPs and their intimate contact with exfoliated Ti3C2 MXene to synthesize porous g-C3N4 attached Ti3C2TA MXene heterojunction for photocatalytic CO2 reduction with water to CO and CH4 has never been described. The TiO2 NPs imbedded over Ti3C2 MXene multilayers would be promising to provide bridge between two materials for promoting their photocatalytic efficiency.
Herein, well designed porous 2D g-C3N4 nanosheets anchored with exfoliated 3D Ti3C2 MXenes to fabricate 2D/3D composite photocatalyst with in-situ growth of TiO2 NPs to promote charge carrier separation for solar energy assisted CO2 reduction has been explored. The samples were successfully constructed using thermal decomposition of melamine and etching of MAX phase Ti3AlC2 with HF under different controlled conditions. The TiO2 NPs decoration over the multilayers Ti3C2 were successfully achieved after etchant times of 24–96 h. After 48 h of etching time, the growth of TiO2 NPs degrading Ti3C2 and the final amount of Ti3C2 produced was significantly reduced. Specifically, effect of different 3D Ti3C2 structures were coupled with porous g-C3N4 and role of TiO2 NPs were explored. It was observed that using TiC-48/PCN composite, highest CO and CH4 evolution rate of 317 and 79 µmol g−1 h−1, respectively, were obtained due to proficient charge carrier separation. The photocatalytic CO2 reduction experiments were further conducted under different reducing agents and their performances were systematically compared for the production of CO and CH4. Using CO2-water system, highest CO was evolved, however, CO2 reduction in the presence of methanol enables more CH4 formation due to more formation of protons (H+). Among the UV and visible light irradiation, highest productivity was observed under visible light irradiation, enabling more CO and CH4 evolution. Finally, stability of optimized composite catalyst was tested and found continuous CO and CH4 production in multiple cycles.
Section snippets
Synthesis of exfoliated g-C3N4
A facile approach was used for the synthesis of porous g-C3N4 through stirring and hydrothermal approach. For this purpose, melamine was used as the precursor for the preparation of g-C3N4 bulky materials. Typically, specific amount of melamine (5 g) was placed at the bottom of ceramic crucible before heating at 550 °C for 2 h in a muffle furnace under air atmosphere. After calcination, the yellowish product was obtained. The product was collected and milled to get bulk graphitic carbon
FESEM and TEM analysis
The morphology of Ti3AlC2, and Ti3C2 with different reaction times were investigated using SEM and the results are demonstrated in Fig. 3. SEM image in Fig. 3 (a) shows stacked layers of Ti3AlC2 MAX structure. However, selective etching of MAX phase Ti3AlC2 with concentrated acid (HF ~ 39%), enables the removal of Al layers with the formation of TiO2 NPs over Ti3C2 multilayers. Different growth of TiO2 NPs could be seen by varying HF etching time in a continuous stirred tank reactor integrated
Conclusions
In conclusion, successful fabrication of multilayers Ti3C2 nanosheets imbedded with TiO2 NPs and anchored with porous g-C3N4 was achieved using controlled chemical and ultrasonic approach. The Ti3C2 MXene is a promising cocatalyst due to its outstanding electron conductivity. The controlled growth of TiO2 NPs embedded with Ti3C2 found beneficial for transporting electrons from CB of PCN towards the MXene surface, enabling proficient separation of charge carriers. Highest CO and CH4 production
CRediT authorship contribution statement
Muhammad Tahir: Conceptualization, Data curation, Formal analysis, Funding acquisition, Writing - review & editing. Beenish Tahir: Methodology, Writing - original draft.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The financial supports for this work were provided by University Technology Malaysia, Malaysia, under Fundamental Research (UTMFR, Q.J 130000.2551.21H66).
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