Final Report Summary - HETMAT (Heterostructure Nanomaterials for Water Splitting)
The Heterostructure Nanomaterials for Water Splitting (HETMAT) is a multidisciplinary project that covers activities related to the synthesis of novel semiconductor nanoparticles, water splitting studies, and photocatalytic degradations of organic contaminants. During the course of this project various semiconductor materials have been explored in the form of single phases or heterostructures. In addition, nanocomposites were another class of materials that were also studied in depth. The materials which were studied namely include: zin oxide (ZnO), silver (Ag) decorated ZnO, tungsten trioxide (WO3), iron oxide (Fe2O3), copper oxide (CuO), ZnO decorated copper tungstate (CuWO4), cadmium chalcogenides (CdX, X=S, Se) decorated ZnO, and a novel semiconductor based on lead tellurium vanadate (PbTeV2O8). Some of these materials were used in the form of nanostructured thin films while others used in the form of nanoparticles. The materials used in the form of nanoparticles include ZnO, Ag decorated ZnO and ZnO/CuWO4 nanocomposite. These nanomaterias were used for degradation of model organic pollutants such as terephtalic acid (TA) and methyl orange (MO). The synthesis of of the above mentioned materials include combinations of methods such as solvothermal synthesis, electrodeposition, co-precipitation, heat treatment in tube furnace or combinations of methods like solvothermal synthesis combined with heat treatment. For example, the preparation of textured WO3 thin films involve solvothermal synthesis of tungsten (W) nanoparticles combined with heat treatments at elevated temperatures (above 450°C). On the other side, the synthesis of a novel PbTeV2O8 nanoparticles include co-precipitation approach coupled with heat treatment. All of the synthesized inorganic materials were used in photocatalytic degradation of model organics or photoelectrochemical (PEC) water splitting studies. For the purpose of PEC studies the semiconductors were deposited in the form of nanostructured thin films onto substrates. The semiconductors that were used in PEC studies include WO3, Fe2O3 and CdX decorated ZnO. PEC experiments were conducted with artificial light source generating 1.5 AM simulated sunlight. The efficiency of water splitting were estimated from the photoinduced current for water splitting. The highest achieved photocurrent was obtained with textured CdSe decorated ZnO thin films which was in the order of ~3.1 mA/cm2 at 1.23 V vs RHE. This value can be estimated to be in the order of ~4 % solar-to-hydrogen conversion efficiency. Here, the fabrication of porous ZnO thin films is achieved by electropolymerization of simonkollite (Zn5(OH)8Cl2(H2O)) thin films and further heat treatment at 450°C. The proposed approach for preparation of Zn5(OH)8Cl2(H2O) thin films has been found to be useful for the fabrication of porous films based on intermediate metal hydroxides. Another study which has been carried out covers the fabrication of porous WO3 thin films. The preparation method is very simple and is based on the synthesis of amorphous W nanoparticles. The prepared W nanoparticles dissolved in organic solvents are very stable against aggregation since these are coated with hydrophobic molecules. For the first time spin-coating of hydrophobic W nanoparticles was used to prepare porous WO3 thin films. The spin-coating allows to produce WO3 films which are with homogenous thickness and are optically transparent. Moreover, the WO3 films are firmly attached to the substrate which is another plus for practical applications like PEC water splitting studies. Similar, studies has been conducted for the preparation of porous Fe2O3 thin films. The preparation steps involve synthesis of core/shell Fe/Fe-oxide nanoparticles. Followed by spin-coating of the Fe/Fe-oxide onto conductive substrate such as fluorine doped tin oxide (FTO). Heat treatment of the coated Fe/Fe-oxide films at elevated temperatures (800°C) produces porous Fe2O3 thin films. The obtained films were tested in PEC water splitting studies. In general the stocks solutions containing W and Fe/Fe-oxide nanoparticles have been found to exhibit long shelf lifetime like more than one year. The extreme stability against aggregation makes them promising
Regarding the electrodeposition studies one of the most relevant study explored in the HETMAT activities is the electrodeposition of copper halogenides (CuY, Y=Cl, Br) thin films. The study cover a unique approach for the fabrication of CuY thin films and their conversion to CuO thin films. For the first time, it was demonstrated the electrodeposition of copper chloride (CuCl) thin films. Although, electrodeposition approaches are well know in the literature there has been missing details about the electrodeposition of compounds such as MY (M=Cu, Hg, Pb, Y=Br, Cl). Moreover, the CuY (Y=Cl, Br) thin films were used as intermediates for the preparation of porous CuO thin films. Although, there has been reports regarding the use of CuO electrodes for water splitting without a protective coating layer it was demonstrated that bare CuO alone can not be used in water splitting since this material suffer from photocorrosion. This necessitates use of coating layer on top of CuO semiconductor.
Nanocomposites are among the most studied systems for photoinduced charge carrier separation. Developing materials with long carrier lifetime is essential in photocatalytic applications. Photoexcitation of a nanocomposite based on ZnO/CuWO4 has been demonstrated to exhibit very long carrier lifetime. It was experimentally measured photoinduced charges in the range of microseconds by using transient absorption spectroscopy technique. The prolonged carrier lifetime open a door for the development of photocatalytically active materials using the concept of nanocomposites. It has been found that the ZnO/CuWO4 exhibit better photocatalytic activity than the industry benchmark titanium dioxide (TiO2) Degussa P25.
The exploration of efficient photocatalyst has not been limited only to known semiconductors but also goes beyond this horizon. During the HETMAT project it was discovered a novel semiconductor based on lead tellurium vanadate (PbTeV2O8). This materials has been synthesized both in bulk form and also in the form of nanoparticles. While, the bulk material do not give any any sign of photoactivity the PbTeV2O8 nanoparticles show good photoactivity for degradation of MO under UV illumination. The advantage of the PbTeV2O8 semiconductor over other materials is due to the low synthesis temperature (~400 °C).To tune the optical activity of this material there are plenty of possibilities such as cation or anion doping. The low synthesis temperature can be an advantage for industry applications where low processing temperatures are required. Useful information about the HETMAT projec can be found on the following website: www.hetmat.com.
Regarding the electrodeposition studies one of the most relevant study explored in the HETMAT activities is the electrodeposition of copper halogenides (CuY, Y=Cl, Br) thin films. The study cover a unique approach for the fabrication of CuY thin films and their conversion to CuO thin films. For the first time, it was demonstrated the electrodeposition of copper chloride (CuCl) thin films. Although, electrodeposition approaches are well know in the literature there has been missing details about the electrodeposition of compounds such as MY (M=Cu, Hg, Pb, Y=Br, Cl). Moreover, the CuY (Y=Cl, Br) thin films were used as intermediates for the preparation of porous CuO thin films. Although, there has been reports regarding the use of CuO electrodes for water splitting without a protective coating layer it was demonstrated that bare CuO alone can not be used in water splitting since this material suffer from photocorrosion. This necessitates use of coating layer on top of CuO semiconductor.
Nanocomposites are among the most studied systems for photoinduced charge carrier separation. Developing materials with long carrier lifetime is essential in photocatalytic applications. Photoexcitation of a nanocomposite based on ZnO/CuWO4 has been demonstrated to exhibit very long carrier lifetime. It was experimentally measured photoinduced charges in the range of microseconds by using transient absorption spectroscopy technique. The prolonged carrier lifetime open a door for the development of photocatalytically active materials using the concept of nanocomposites. It has been found that the ZnO/CuWO4 exhibit better photocatalytic activity than the industry benchmark titanium dioxide (TiO2) Degussa P25.
The exploration of efficient photocatalyst has not been limited only to known semiconductors but also goes beyond this horizon. During the HETMAT project it was discovered a novel semiconductor based on lead tellurium vanadate (PbTeV2O8). This materials has been synthesized both in bulk form and also in the form of nanoparticles. While, the bulk material do not give any any sign of photoactivity the PbTeV2O8 nanoparticles show good photoactivity for degradation of MO under UV illumination. The advantage of the PbTeV2O8 semiconductor over other materials is due to the low synthesis temperature (~400 °C).To tune the optical activity of this material there are plenty of possibilities such as cation or anion doping. The low synthesis temperature can be an advantage for industry applications where low processing temperatures are required. Useful information about the HETMAT projec can be found on the following website: www.hetmat.com.