1 3
Topics in Current Chemistry (2020) 378:3
recombination of electrons and holes and the resulting loss of energy during the process typically reduce the photoactivity of the sample. The electrons and holes generated can initiate a wide range of reactions in both gas and liquid media, which
include the production of energy vectors (CO 2 reduction, water splitting, alcohol
conversion) [15–22], selective organic synthesis of high-value products [14, 23–29],
and elimination/degradation of undesired chemicals [4, 30–37] and microbial pathogens in air and water [38–42]. Obviously, the final efficiency of the process depends
on the semiconductor used. In this regard, TiO 2 -based materials are the most widely
used photocatalysts, as described in numerous reports in the literature [5, 7, 9,
43–45], although ZnO- and g-C 3 N 4 -based materials and many others have shown
outstanding results in a variety of photocatalytic applications [46–52].
This contribution focuses on waste-derived materials which have been employed
or can be considered as a potentially attractive option for photocatalytic reactions.
Throughout the manuscript, two main concepts are exploited: the reuse of waste as a
source of components (e.g. titanium, zinc, sulfur, carbon) for the photocatalyst, and
the valorization of waste for use in as catalyst in the synthetic protocol (e.g. sacrificial templates). Figure 1 presents a schematic illustration of all the possibilities that
different types of waste, including orange peel, rice husk, spent coffee grounds, and
lignocellulosic and electronic residues, to name just a few, can offer for the design
of highly sustainable materials. In addition, the optoelectronic, morphological, and
textural properties can be tuned by employing these residues using various synthetic
strategies, with the ultimate goal of obtaining efficient photocatalytic materials. The
concept of waste transformation in photocatalytic applications is itself highly ecofriendly, and the use of green methodologies and alternative sustainable technologies will be considered as well and highlighted in the following sections.
2 TiO 2 ‑based Materials: Ti from Waste
As mentioned above, TiO 2 is by far the most commonly used photocatalyst, showing competitive results in a wide range of photocatalytic applications. Pure TiO 2 and
TiO 2 -based samples have both been widely studied from a photocatalytic, chemical,
morphological, and optical point of view [7, 9, 43, 44]. In fact, some authors highlight TiO 2 as the most extensively studied transition-metal oxide material [44].
Titanium and Ti-related materials have found application in many areas, from
aerospace, marine, and automobile industries, to chemical plant materials, medical
equipment, buildings, and several consumer products (spectacle frames, golf clubs,
etc.) [53]. As is common, during the production of the final product, Ti-containing
waste is usually generated. Indeed, there are numerous opportunities for the recovery
of Ti and Ti-related compounds in Ti smelting and Kroll processes [53]. One area in
which researchers have focused is on the reuse-extraction of Ti or Ti-containing slag
[54]. As described by Liu et al., approximately 53% of the Ti is carried through processing into iron-rich concentrations for feeding blast furnaces. The slag generated
contains up to 25% of TiO 2 . Given that it is produced at a rate of three million tons
per year, this is a clear opportunity for the production of a cheap photocatalyst [55].
Extraction, however, has many limitations. From an environmental point of view,
3
Reprinted from the journal
Topics in Current Chemistry (2020) 378:3
recombination of electrons and holes and the resulting loss of energy during the process typically reduce the photoactivity of the sample. The electrons and holes generated can initiate a wide range of reactions in both gas and liquid media, which
include the production of energy vectors (CO 2 reduction, water splitting, alcohol
conversion) [15–22], selective organic synthesis of high-value products [14, 23–29],
and elimination/degradation of undesired chemicals [4, 30–37] and microbial pathogens in air and water [38–42]. Obviously, the final efficiency of the process depends
on the semiconductor used. In this regard, TiO 2 -based materials are the most widely
used photocatalysts, as described in numerous reports in the literature [5, 7, 9,
43–45], although ZnO- and g-C 3 N 4 -based materials and many others have shown
outstanding results in a variety of photocatalytic applications [46–52].
This contribution focuses on waste-derived materials which have been employed
or can be considered as a potentially attractive option for photocatalytic reactions.
Throughout the manuscript, two main concepts are exploited: the reuse of waste as a
source of components (e.g. titanium, zinc, sulfur, carbon) for the photocatalyst, and
the valorization of waste for use in as catalyst in the synthetic protocol (e.g. sacrificial templates). Figure 1 presents a schematic illustration of all the possibilities that
different types of waste, including orange peel, rice husk, spent coffee grounds, and
lignocellulosic and electronic residues, to name just a few, can offer for the design
of highly sustainable materials. In addition, the optoelectronic, morphological, and
textural properties can be tuned by employing these residues using various synthetic
strategies, with the ultimate goal of obtaining efficient photocatalytic materials. The
concept of waste transformation in photocatalytic applications is itself highly ecofriendly, and the use of green methodologies and alternative sustainable technologies will be considered as well and highlighted in the following sections.
2 TiO 2 ‑based Materials: Ti from Waste
As mentioned above, TiO 2 is by far the most commonly used photocatalyst, showing competitive results in a wide range of photocatalytic applications. Pure TiO 2 and
TiO 2 -based samples have both been widely studied from a photocatalytic, chemical,
morphological, and optical point of view [7, 9, 43, 44]. In fact, some authors highlight TiO 2 as the most extensively studied transition-metal oxide material [44].
Titanium and Ti-related materials have found application in many areas, from
aerospace, marine, and automobile industries, to chemical plant materials, medical
equipment, buildings, and several consumer products (spectacle frames, golf clubs,
etc.) [53]. As is common, during the production of the final product, Ti-containing
waste is usually generated. Indeed, there are numerous opportunities for the recovery
of Ti and Ti-related compounds in Ti smelting and Kroll processes [53]. One area in
which researchers have focused is on the reuse-extraction of Ti or Ti-containing slag
[54]. As described by Liu et al., approximately 53% of the Ti is carried through processing into iron-rich concentrations for feeding blast furnaces. The slag generated
contains up to 25% of TiO 2 . Given that it is produced at a rate of three million tons
per year, this is a clear opportunity for the production of a cheap photocatalyst [55].
Extraction, however, has many limitations. From an environmental point of view,
3
Reprinted from the journal