Visible Light Absorption Characteristics of M-Nitrogen (M= Mn, Fe, Co, Ni, Cu) Co-Doped Monodisperse TiO 2 Microparticle
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Visible Light Absorption Characteristics of M-Nitrogen (M= Mn, Fe, Co, Ni, Cu) Co-Doped Monodisperse TiO2 Microparticles John E. Mathis1 1 Embry-Riddle Aeronautical University, 600 Clyde Morris Blvd., Daytona Beach, FL 32114 ABSTRACT There is great interest in improving TiO2’s photocatalytic activity in the visible portion of electromagnetic spectrum. Recent work has shown that co-doping mesoporous TiO2 microparticles with a transition metal and nitrogen, hereby designated as (M,N) TiO2, significantly increases its visible light absorption. However, the hydrothermal method used to produce the microparticles creates a wide distribution in the size of the microparticles, which could affect the absorption properties. Recently, it has become possible to produce monodisperse, mesoporous TiO2 microparticles with engineered sizes using a hybrid sol-gel/hydrothermal technique. Further, it has also been shown that the size of monodisperse TiO2 microparticles affects the the photocatalytic activity. This study investigated whether using mondodisperse (M,N) TiO2 microparticles would further increase visible-light absorption for (M,N)TiO2. The first-row transition metals chosen for this study - Mn, Fe, Co, Ni, and Cu – have been characterized in the earlier (M,N) TiO2 UV-vis study, which was used as a baseline. The doping levels of the transition metals samples were set at the 2.5 percent level previously shown to be optimum for photocatalytic activity.
INTRODUCTION TiO2 has many attractive features as material for energy production as is well-known to researchers in this field. Its chief advantages are that it is inexpensive, non-toxic, stable in air, and corrosion resistant to highly basic and acidic environments. The photovoltaic process begins when a photon of sufficient energy strikes a semiconducting material, promoting an electron from the valence band to the conduction band. The energy of the photon must be greater than the energy gap between the valence and conduction bands for this to occur. For TiO2, the valence band arises from the O 2p orbitals, and the conduction band arises from the Ti 3d orbitals [1]. The vacancy left behind during the process is called a hole. However, there are problems that need to be addressed. First, TiO2 has a large band gap of 3.2 eV, which means it absorbs only in the UV portion of the spectrum. Another problem is the recombination of electrons and holes before they either have a chance to reach an external circuit, in the case of photovoltaic devices, or effect an oxidation-reduction reaction in the case of photocatalysts. The recombination can take place at grain boundaries, surface sites, or other defects in the crystal [1]. In addition TiO2 suffers from high electrical resistance and low ionic conductivity [2]. One solution to these problems is to use metal-nitrogen co-doping. Here, a transition metal and nitrogen act in concert to effect substitution into the TiO2 lattice. Normally, metals have to overcome a sizeable energy barrier of about 2 eV to substitute for titanium, and nitr
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