585
adopted. Metal and non-metal doping, sensitization, and coupling semiconductors
are few commonly known strategies. Each method has its advantage and disadvantage, and the adopted process can vary depending on application and conditions.
4.4.1 Metal and Non-metal Doping
Doping is the intentional introduction of the metal or non-metal in semiconductor
lattice for tuning the optical and electrical properties. In the case of doping in crystalline structures, less amount of species is introduced in the crystal lattice. Norskov
et al. [118] found that upon doping, introduction of new d states near the Fermi level
can take place. Due to the new state, there is variation in electrical and optical properties, which affects the activity and light absorption. Koci et al. [119] found silver
doping on TiO 2 for range of 1–7% and found the shift in the absorption band edge
of TiO 2 from 2.98 to 2.74 eV for 7%Ag-TiO 2 . The product yield has increased with
increased doping content due to the formation of a Schottky barrier at the contact
region of the metal and semiconductor, which prevents charge recombination,
thereby enhancing the activity. Nogueira et al. [120] found that methanol formation
was enhanced due to Nb doping via not modified sputtering technique compared to
that modified by sputtering primarily due to metallic Nb particles promoting the
transportation of electrons. Li et al. [121] studied the effect of various metal dopant
for varied loading amount and found that methane formation is maximum for
0.2 wt% loading of Pt compared to Ag and Au. In the same study, N doping were
found to enhance visible light absorption. The optical properties and synergism with
noble metal determined the overall catalytic performance. From metal and nonmetal doping, the restructuring of the band structure happens which alters the reactivity trends.
4.4.2 Semiconductor Coupling Via z-Scheme or Heterojunction formation
To enhance the activity of semiconductor photocatalyst, coupling with another
semiconductor of an appropriate valance band potential is a promising strategy. The
coupling of two semiconductors can be of two types, either a type II heterojunction
contact formation takes place, or they could form a z-scheme-type of arrangement.
Contact type depends on the relative positions of band edges, Fermi level, and work
functions of parental semiconductors. When z-scheme type arrangement is formed,
it tends to promote spatial charge separations and utilize the benefits of two-component semiconductors [122]. Yu et al. [123] found the binary system of ZnO-C 3 N 4
enhanced the selective methanol formation by 2.5 times due to the extended light
absorption range because of direct z-scheme construction. Tahir et al. [124] found
that the combination of ZnV 2 O 6 /C 3 N 4 gives better selectivity for methanol formation due to 2D–2D nanosheets heterojunction with faster charge separation. Several
other combinations for coupling are possible, which can enhance the activity.
However, the activity depends on environmental and operating conditions;
Perspectives in Carbon Oxides Conversion to Methanol/Dimethyl Ether: Distinctive…
adopted. Metal and non-metal doping, sensitization, and coupling semiconductors
are few commonly known strategies. Each method has its advantage and disadvantage, and the adopted process can vary depending on application and conditions.
4.4.1 Metal and Non-metal Doping
Doping is the intentional introduction of the metal or non-metal in semiconductor
lattice for tuning the optical and electrical properties. In the case of doping in crystalline structures, less amount of species is introduced in the crystal lattice. Norskov
et al. [118] found that upon doping, introduction of new d states near the Fermi level
can take place. Due to the new state, there is variation in electrical and optical properties, which affects the activity and light absorption. Koci et al. [119] found silver
doping on TiO 2 for range of 1–7% and found the shift in the absorption band edge
of TiO 2 from 2.98 to 2.74 eV for 7%Ag-TiO 2 . The product yield has increased with
increased doping content due to the formation of a Schottky barrier at the contact
region of the metal and semiconductor, which prevents charge recombination,
thereby enhancing the activity. Nogueira et al. [120] found that methanol formation
was enhanced due to Nb doping via not modified sputtering technique compared to
that modified by sputtering primarily due to metallic Nb particles promoting the
transportation of electrons. Li et al. [121] studied the effect of various metal dopant
for varied loading amount and found that methane formation is maximum for
0.2 wt% loading of Pt compared to Ag and Au. In the same study, N doping were
found to enhance visible light absorption. The optical properties and synergism with
noble metal determined the overall catalytic performance. From metal and nonmetal doping, the restructuring of the band structure happens which alters the reactivity trends.
4.4.2 Semiconductor Coupling Via z-Scheme or Heterojunction formation
To enhance the activity of semiconductor photocatalyst, coupling with another
semiconductor of an appropriate valance band potential is a promising strategy. The
coupling of two semiconductors can be of two types, either a type II heterojunction
contact formation takes place, or they could form a z-scheme-type of arrangement.
Contact type depends on the relative positions of band edges, Fermi level, and work
functions of parental semiconductors. When z-scheme type arrangement is formed,
it tends to promote spatial charge separations and utilize the benefits of two-component semiconductors [122]. Yu et al. [123] found the binary system of ZnO-C 3 N 4
enhanced the selective methanol formation by 2.5 times due to the extended light
absorption range because of direct z-scheme construction. Tahir et al. [124] found
that the combination of ZnV 2 O 6 /C 3 N 4 gives better selectivity for methanol formation due to 2D–2D nanosheets heterojunction with faster charge separation. Several
other combinations for coupling are possible, which can enhance the activity.
However, the activity depends on environmental and operating conditions;
Perspectives in Carbon Oxides Conversion to Methanol/Dimethyl Ether: Distinctive…