Regular Article
In-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

https://doi.org/10.1016/j.jcis.2021.01.099Get rights and content

Highlights

  • New approach for synthesis of exfoliated 2D Ti3C2TA MXenes based PCN composite.

  • In-situ growth of TiO2 NPs imbedded over Ti3C2 obtained through HF etchant of 48 and 96 h.

  • 2D/2D Ti3C2TA MXene/PCN efficiency was 9.9 folds high than PCN under visible light.

  • Photo-induced CO2 reduction with H2/methanol promoted CH4 formation than only water.

  • High stability and reusability in cycles was attained using 2D Ti3C2TA with g-C3N4 catalyst.

Abstract

Constructing efficient structured materials for artificial photosynthesis of CO2 is a promising strategy to produce renewable fuels in addition of mitigating greenhouse effect. In this work, 2D porous g-C3N4 (PCN) coupled exfoliated 3D Ti3C2TA MXene (TiC) nanosheets with TiO2 NPs in-situ growth was constructed in a single step through HF treatment approach. The different exfoliated TiC structures were successfully synthesized for adjusting HF etching time (24 h, 48 h and 96 h). With growing etchant time from 24 to 96 h, the amount of TiO2 produced was increased, but it has adverse effects on CO and CH4 production rate. The maximum production rates for CO and CH4 of 317.4 and 78.55 µmol g−1 h−1 were attained when the 10TiC-48/PCN was employed than using TiC-24/PCN, TiC-96/PCN and PCN composite samples, respectively. The performance of 10TiC-48/PCN composite for CO and CH4 evolution were 9.9 and 6.7 folds higher than using pristine PCN sample, respectively. The possible mechanism is assigned to porous structure with intimate contact enabling efficient charge carrier separation with the role of TiO2 NPs to work as a bridge to transport electrons towards MXene surface. Among the reducing agents, water was favorable for CO evolution, whereas, methanol–water system promoted CH4 production. All these findings confirm that heterojunction formation facilitates charges separation and can be further used in solar energy relating application.

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).

References (56)

  • B. Tahir et al.

    Ag-La loaded protonated carbon nitrides nanotubes (pCNNT) with improved charge separation in a monolithic honeycomb photoreactor for enhanced bireforming of methane (BRM) to fuels

    Appl. Catal., B

    (2019)
  • Z. Sun et al.

    3D porous Cu-NPs/g-C3N4 foam with excellent CO2 adsorption and Schottky junction effect for photocatalytic CO2 reduction

    Appl. Surf. Sci.

    (2020)
  • B. Tahir et al.

    Highly stable 3D/2D WO3/g-C3N4 Z-scheme heterojunction for stimulating photocatalytic CO2 reduction by H2O/H2 to CO and CH4 under visible light

    J. CO2 Util.

    (2020)
  • A. Bafaqeer et al.

    Well-designed ZnV2O6/g-C3N4 2D/2D nanosheets heterojunction with faster charges separation via pCN as mediator towards enhanced photocatalytic reduction of CO2 to fuels

    Appl. Catal., B

    (2019)
  • Y. Zhao et al.

    Two-dimensional photocatalyst design: a critical review of recent experimental and computational advances

    Materials Today

    (2020)
  • T.P. Nguyen et al.

    MXenes: applications in electrocatalytic, photocatalytic hydrogen evolution reaction and CO2 reduction

    Molecular Catalysis 486

    (2020)
  • L. Tie et al.

    In situ decoration of ZnS nanoparticles with Ti3C2 MXene nanosheets for efficient photocatalytic hydrogen evolution

    J. Colloid Interface Sci.

    (2019)
  • Q. Tang et al.

    Decorating g-C3N4 with alkalinized Ti3C2 MXene for promoted photocatalytic CO2 reduction performance

    J. Colloid Interface Sci.

    (2020)
  • J. Low et al.

    TiO2/MXene Ti3C2 composite with excellent photocatalytic CO2 reduction activity

    J. Catal.

    (2018)
  • C. Prasad et al.

    Recent advances in MXenes supported semiconductors based photocatalysts: properties, synthesis and photocatalytic applications

    J. Ind. Eng. Chem.

    (2020)
  • M. Tahir

    Enhanced photocatalytic CO2 reduction to fuels through bireforming of methane over structured 3D MAX Ti3AlC2/TiO2 heterojunction in a monolith photoreactor

    J. CO2 Util.

    (2020)
  • Y. Xu et al.

    In-situ grown nanocrystal TiO2 on 2D Ti3C2 nanosheets for artificial photosynthesis of chemical fuels

    Nano Energy

    (2018)
  • A.A. Khan et al.

    Well-designed 2D/2D Ti3C2TA/R MXene coupled g-C3N4 heterojunction with in-situ growth of anatase/rutile TiO2 nucleates to boost photocatalytic dry-reforming of methane (DRM) for syngas production under visible light

    Appl. Catal., B

    (2021)
  • Y. Li et al.

    Ti3C2 MXene-derived Ti3C2/TiO2 nanoflowers for noble-metal-free photocatalytic overall water splitting

    Appl. Mater. Today

    (2018)
  • Y. Li et al.

    2D/2D/2D heterojunction of Ti3C2 MXene/MoS2 nanosheets/TiO2 nanosheets with exposed (001) facets toward enhanced photocatalytic hydrogen production activity

    Appl. Catal., B

    (2019)
  • A. Raza et al.

    Facile in-situ fabrication of TiO2-Cu2ZnSnS4 hybrid nanocomposites and their photoreduction of CO2 to CO/CH4 generation

    Appl. Surf. Sci.

    (2020)
  • J. Hu et al.

    Ultrathin 2D Ti3C2 MXene Co-catalyst anchored on porous g-C3N4 for enhanced photocatalytic CO2 reduction under visible-light irradiation

    J. Colloid Interface Sci.

    (2021)
  • N.T. Thanh Truc et al.

    The superior photocatalytic activity of Nb doped TiO2/g-C3N4 direct Z-scheme system for efficient conversion of CO2 into valuable fuels

    J Colloid Interface Sci

    (2019)
  • Cited by (74)

    View all citing articles on Scopus
    View full text