Photoactivity improvement of TiO2 electrodes by thin hole transport layers of reduced graphene oxide
Introduction
Photoelectrochemistry is a powerful tool to elucidate the performance and working mechanism of materials and interface components used in layered optoelectronic devices, such as thin film solar cells. The resulting data reveal critical information about electronic properties such as conduction and valence band limits, as well as trapping, distribution, separation, recombination and transport of charges [1,2]. Using a simple electrochemical three-electrode configuration, photoelectrochemical measurements enable the elucidation of processes occurring at the working electrode, covering phenomena in bulk materials, across solid-solid interfaces for layered systems, as well as across the solid-liquid interface. Importantly, the acquired information provides valuable feedback for interface engineering towards optoelectronic device structures with improved performance.
Materials of great current interest for solar energy conversion and storage are metal oxides such as ZnO [3] or TiO2 [2], as well as carbon nanomaterials (graphene-based materials and carbon nanotubes) [4]. Employed as photoelectrodes [5], electron transport layers (ETLs) [6] or hole transport layers (HTLs) [7], they constitute important components in optoelectronic devices. Especially TiO2, graphene, and their hybrid materials are widely studied for this purpose [2,[8], [9], [10], [11], [12]]. In dye-sensitized solar cells, TiO2 is often used as the active layer, working as a photoanode [13,14]. In the case of perovskite [15] and organic solar cells [6], TiO2 can constitute ETL [16,17], enabling the transfer of photogenerated electrons from the photoactive material to the conducting substrate. In new generation solar cells, reduced graphene oxide (rGO) layers can act as electron or hole acceptors [18,19], protective layers [18], layers to improve the adhesion between polymeric and oxide layers [20,21], and sensitizers absorbing light in the visible region [[22], [23], [24], [25], [26]]. However, systematic studies on thickness effects to elucidate the working principle of thin layers of reduced graphene oxide are yet missing.
This work investigates the photoelectrochemical properties of nanostructured TiO2 films covered with electrochemically reduced graphene oxide (ECrGO) layers of different thicknesses. The films are used as photoelectrodes in a three-electrode electrochemical cell. Methanol in an alkaline solution is used as effective probe (effective hole acceptor enabling the direct methanol photo-oxidation process) for elucidating the electronic processes across the solid-liquid and solid-solid interfaces of the working electrode. Cyclic voltammetry under dark and illuminated conditions, time-dependent photocurrent and photovoltage measurements, as well as electrochemical impedance spectroscopy clearly reveal that ECrGO layers act as acceptors of photo-induced holes from TiO2 resulting in the formation of a p-n junction at the interface between ECrGO and TiO2 comprising a space charge region of about 10 nm. For ECrGO layers up to 10 nm in thickness, this region extends into the solid-liquid interface, enhancing the photoelectroactivity of the electrode, thus acting as hole transport layer (HTL). Contrariwise, for thicker ECrGO layers photo-generated holes cannot reach anymore the solid-liquid interface and get blocked. Hence ECrGO layers with higher thicknesses act as hole blocking layer (HBL). These findings give us, for the first time, a benchmark for the use of ECrGO as either HTL or HBL in order to achieve optoelectronic structures with improved performance.
Section snippets
Materials and equipment
Commercial TiO2 nanoparticles (Aeroxide P25, Evonik) were used in this work. Graphite flakes were purchased from Aldrich (ref. 332461). Reagent grade NaOH was bought from Sigma Aldrich. Ethanol and isopropanol (p.a. grade) were obtained from Panreac. Methanol (analytical reagent grade) was acquired from Fisher Scientific. Soda lime glass substrates, coated with fluorinated tin oxide (FTO, 70-100 Ω/sq, thickness of 80 nm, cut in 2.5 × 1 cm pieces) were shopped from Solems, Palaiseau, France. A
Physical characterization
Aeroxide® P25 TiO2 is a well-described standard material [32], while a complete characterization of GO and rGO materials can be found in our previous works [27,28,[33], [34], [35]]. Surface topography and optical absorption response, features of direct concern to the performance of the ECrGO/TiO2 photoelectrodes, are detailed in the following.
An AFM image of the blank TiO2 electrode (Fig. 1A) shows a quite uniform porous structure for TiO2 films, with aggregates of about 300–400 nm uniformly
Conclusions
Photoelectrochemical measurements in the presence of methanol show that a thin ECrGO covering layer significantly improves the photopotential (up to 270 mV), halfwave potential (up to 120 mV) and photocurrent (up to ca. 16%) of nanostructured TiO2 electrodes. These results reveal that the mechanism behind this process is based on (a) the formation of a favorable solid-liquid interface. This is ensured by the use of methanol as well adsorbing hole acceptor; (b) the establishment of a p-n
Acknowledgements
The authors thank Prof. M.T. Martínez for AFM measurements and the Analysis Service of the Instituto de Carboquímica (ICB-CSIC) for XPS measurements. This work has received funding from the Spanish MINEICO (project grant ENE 2016-79282-C5-1-R), Gobierno de Aragón (Grupo Reconocido DGA T03_17R), and associated EU Regional Development Funds. S.V. acknowledges Spanish MINEICO for her PhD grant (BES2014-068727 and associated EU Social Funds).
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