Photoelectrocatalysis of Rhodamine B and Solar Hydrogen Production by TiO2 and Pd/TiO2 Catalyst Systems
Introduction
Since the discovery of TiO2’s capability for water-splitting and the photocatalytic degradation of organic compounds, almost every suitable semiconductor has been analysed for environmental and energy applications [1], [2]. The most heavily investigated is still TiO2 due to its chemical stability, low cost and good photocatalytic efficiency [3]. The main drawback of TiO2 is its limited optical response. Due to its large band gap (Eg ∼3.2 eV), the photoresponse of TiO2 is limited to UV light which comprises only 5% of solar energy [4]. There have been extensive studies on a variety of modifications of TiO2 to improve wavelength range response to boost charge generation and induce efficient charge separation to avoid recombination [5]. Techniques for modifications of TiO2 include metal loading [6], [7], ion doping [8], semiconductor coupling [9], [10] and dye sensitisation [11].
Precious metals or rare earth metals deposition on semiconductors is one of the most commonly studied methods for photocatalytic enhancement of TiO2. There are two main reasons responsible for such achievement: formation of Schottky junction for efficient charge separation and localised surface plasmon resonance (LSPR) for higher charge generation due to the absorption of visible light. For TiO2, Au [12], [13], [14] and Ag [15], [16], [17] are the most popular noble metals to enhance the photocatalytic activity but plasmonic photocatalysis with Pt, [18], [19] Pd [20], [21], [22] and Ru [23] has also been reported. Comparative studies on the performance of TiO2 with different metals have been done where the discussions of photoreactivity were based on the work function of the metals [24], [25].
However, metal deposition on TiO2 does not necessarily result in a enhancement of photoelectrochemical performance. Declines in photoactivity and photocurrent by the addition of Ag [26] and Au [27] on TiO2 have also been reported. This is related to the complex charge transfer mechanism between the metal, TiO2, substrate and the surrounding electrolyte solution. The relative positions of band levels of the semiconductor, work function of the deposited metal and redox potentials of the solution are the key aspect for understanding the mechanism. There are two features of electron transfer between TiO2 and deposited metal nanoparticle. First mechanism is the transfer of electrons from conduction band of TiO2 to metal nanoparticles which is thermodynamically favourable as Fermi level of the metal has lower energy than that of conduction band of TiO2. These electrons are the excited within band gap of TiO2 by UV light and further injected to the electrolyte solution by the metal nanoparticles. The charge separation induced by the metal in this way surpasses the recombination [28], [29]. Second one is the migration of electrons oscillated by the surrounding localised plasmon resonance from the metal nanoparticles to the conduction band of TiO2 [6]. These electrons are then transferred to the FTO coated glass substrate and the counter electrode Pt via a conducting wire to form the photocurrent. Charge trapping role of the deposited metals influences the photocurrent as the electrons scavenged by the nanoparticles end up not being transferred to the counter electrode results in decreased photocurrent [26]. The extent of metal loading should also be discussed because an excessive amount can have detrimental effects on the resulting photoreactivity as higher amount of loading may create recombination centres [30], [31]. The active sites of the semiconductor are also decreased by the excessive amount of metal deposition on the surface which limits the charge generation and charge transfer at the interface of the semiconductor [5].
Here, we report the synthesis of a hybrid Pd/TiO2 photocatalyst system where metallic Pd nanoparticles were photochemically deposited onto TiO2 nanorods hydrothermally grown on FTO coated glass substrate. The photoelectrochemical performance of the bare TiO2 and hybrid Pd/TiO2 samples were compared through the decolourisation of a standard commercial textile dye Rhodamine B (RhB) and water splitting to produce photogenerated hydrogen gas in different electrolyte solutions at various applied voltage values. Hybrid photocatalyst Pd/TiO2 showed enhanced photoelectrocatalytic activity in decolourisation of aqueous RhB solution. For the case of solar hydrogen production, Pd/TiO2’s photoelectrochemical performance was superior with methanol solution whereas bare TiO2 samples produced a higher amount of hydrogen in 0.01 M Na2SO4 and aqueous solutions under same conditionsThe results are discussed with proposition of possible charge transfer mechanisms with an emphasis on the charge trapping role of Pd nanoparticles. The key point in understanding the difference between photoelectrocatalytic performance of TiO2 and Pd/TiO2 lies in the fate of photoelectrons trapped by the Pd nanoparticles. Whether the trapped electrons by Pd do reduction chemistry with the solution on the surface prior to recombination with the holes determines the performance of the Pd/TiO2 catalyst system.
Section snippets
TiO2 Nanorods Synthesis
A hydrothermal method was followed to grow titania rods on FTO coated glass substrates [32], [33]. The substrates were washed with acetone and isopropanol for 15 minutes subsequently and rinsed with DI water. A solution of 180 mL HCl (6 M) (VWR International, 37%) and 4.2 ml titanium (IV) butoxide (Sigma Aldrich, Reagent Grade 97%) was prepared and stirred vigorously under room temperature for 30 minutes. The solution was transferred to a Teflon container which was placed in a stainless steel
Materials Characterisation
The morphology of TiO2 nanorods hydrothermally grown on FTO coated glass was analysed under SEM imaging (Fig. 1). The SEM micrographs show a highly uniform and densely packed array of nanorods. TiO2 nanorods can be seen to have rectangular shapes in Fig. 1d. The length of the nanorods range between 2.2 μm (±0.2 μm) with diameters of 80–100 nm. Pd nanoparticle deposition onto TiO2 nanorods did not affect the morphological structure (Fig. 1b). Pd NPs couldn’t be observed under SEM imaging or EDX due
Conclusions
A hybrid Pd/TiO2 photocatalyst system was developed by depositing metallic Pd nanoparticles onto TiO2 nanorods hydrothermally grown on FTO substrate. The existence of Pd was confirmed by characterisation methods such as TEM and XPS. The samples were photoelectrochemically characterised by electrochemical impedance spectroscopy and subsequently derived Mott-Schottky plots. There was a positive shift in flat band potential due to relative position of Pd’s to Fermi level of TiO2 which induced a
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