Article Cite This: ACS Appl. Energy Mater. 2018, 1, 3636−3645
www.acsaem.org
Enhanced Visible-Light Photoelectrochemical Conversion on TiO2 Nanotubes with Bi2S3 Quantum Dots Obtained by in Situ Electrochemical Method Denilson V. Freitas,†,∥ Johan R. González-Moya,‡,∥ Thiago A. S. Soares,‡ Richardson R. Silva,† Dyego M. Oliveira,‡ Herman S. Mansur,§ Giovanna Machado,†,‡ and Marcelo Navarro*,† †
Departamento de Química Fundamental, Universidade Federal de Pernambuco, Cidade Universitária, 50670-901 Recife, PE, Brazil Departamento de Nanotecnologia, Centro de Tecnologias Estratégicas do Nordeste (CETENE), 50740-540 Recife, PE, Brazil § Departamento de Engenharia Metalúrgica e Materiais, Universidade Federal de Minas Gerais, 31270-901 Belo Horizonte, MG, Brazil
Downloaded via UNIV PARIS-SUD on May 16, 2019 at 17:12:19 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: A new greener strategy to incorporate Bi2S3mercaptopropionic acid (MPA) quantum dots (QDs) onto TiO2 nanotube (NT) films was developed, using in situ electrochemical synthesis with a graphite/sulfur powder macroelectrode (cathode) and cavity cell. Simultaneously, the obtained QDs were adsorbed on TiO2 NTs, for the photoelectrochemical cell (PEC) assays. After the electrochemical synthesis, different thermal post-treatments were performed for growth and crystallization of obtained nanocrystals. The obtained TiO2−Bi2S3 nanostructured composites were characterized by UV−vis, diffuse reflectance spectroscopy, X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Photoelectrochemical tests were carried out, where TiO2 NTs/Bi2S3 QDs with 15 min post-treatment at 90 °C presented a greater photocurrent density, when irradiated with visible light (427−655 nm region, 10 mW.cm−2) and compared to pristine TiO2 NTs. Such performance can be attributed to the synergetic effect caused by the good interface between TiO2 NTs/Bi2S3 QDs, promoted by the electrochemical method of synthesis employed. The proof of concept evidenced in this work can be potentially extended for the sensitization of TiO2 NTs with other semiconductors for PEC hydrogen generation and solar cell applications. KEYWORDS: photoelectrochemical conversion, electrochemical synthesis, TiO2 nanotubes, in situ sensitization, Bi2S3, quantum dots
1. INTRODUCTION
photovoltaic and photoelectrochemical devices, due to the versatility of the nanoparticle band gap, which varies according to the size.7 For the applications in optoelectronic devices, the most used QDs are based on Cd6,8 and Pb9,10 chalcogenides because the efficient electron transfer can occur under visible light excitation.7 However, these QDs are very toxic and infeasible for large scale application, with the study of QDs based on more abundant and less toxic materials being necessary.11,12 Bismuth-based QDs (V−VI group) are promising because of its low toxicity and useful electronic properties, like a short bulk band gap of 1,3 eV and high absorption coefficient.5 These QDs are a powerful tool in optoelectronic devices due to their absorption in the visible and near-infrared region.13−16 Different methodologies are employed to sensitize the TiO2
Currently, the development of high efficiency photovoltaic and photoelectrochemical devices at low cost and moderate toxicity is extremely important for use on a large scale.1,2 TiO2 is the most studied material for these devices since 1972, when Fujishima and Honda demonstrated the photoelectrochemical properties of TiO2.3 Due to its excellent properties, like low cost and good chemical and photocorrosion stability, TiO2 is described as an excellent candidate for photovoltaic applications.4 As the material morphology has a great influence in the photovoltaic activity, the nanostructured forms (high surface area materials) are being increasingly studied, such as TiO2 nanotubes (NTs) and nanoparticles.5 However, the large band gap of TiO2 confines the photoelectrochemical activity of the material to the ultraviolet region, requiring a sensitization or doping of the TiO2 based materials to enhance the absorption of visible and IR light.6 The sensitization with quantum dots (QDs) is a very efficient and studied way to increase the efficiency of © 2018 American Chemical Society
Received: March 8, 2018 Accepted: July 18, 2018 Published: July 18, 2018 3636
DOI: 10.1021/acsaem.8b00375 ACS Appl. Energy Mater. 2018, 1, 3636−3645
Article
ACS Applied Energy Materials with Bi2S3 semiconductor. One of the most used is the successive ionic layer adsorption and reaction (SILAR) method.17,18 Li et al.17 attached Bi2S3 nanoparticles to TiO2 NT films enhancing the photocurrent when compared to pristine TiO2 NTs. Using the same technique, Zumeta-Dubé et al.19 described the sensitization of the TiO2 NTs with Bi2S3 nanoparticles; however, in the presence of sodium ions the stability of the system decreases, compromising its photovoltaic properties. Electrochemical methods for QDs synthesis have been proposed for the preparation of chalcogen precursors.20−22 The electrosynthesis follows some principles of green chemistry, using milder reaction conditions, avoiding chemical reducing agents and preventing residue formation. Passos et al.21 developed a versatile electrochemical methodology for the synthesis of CdTe quantum dots. The generation of the chalcogenide precursors was carried out in a two-compartment electrochemical cell, separated by Nafion membrane, producing nanoparticles with high size control and low dispersity. Aiming the use of visible light on TiO2 based photoelectrochemical devices and the development of greener materials and processes, TiO2 NT films were sensitized with Bi2S3 QDs by using a simple and versatile in situ electrochemical method. A graphite/sulfur powder macroelectrode was used as cathode in electrochemical cavity cell, and mercaptopropionic acid (MPA) was used as bifunctional linker, stabilizing the Bi2S3 nanoparticles and promoting its adsorption on the TiO2 NT surface. After the electrochemical synthesis, different thermal post-treatments were carried out to optimize the interface between the semiconductors. The photoanodes obtained were characterized by several techniques, and a photocurrent behavior was addressed and discussed.
Figure 1. (a) TiO2 NTs production by electrochemical anodization and (b) electrochemical cavity cell used for the sensitization of TiO2 NTs with Bi2S3 QDs. compartment of the electrochemical cavity cell containing BiCl3/MPA aqueous solution (Figure 1b), before starting the electrolysis. The cathodic compartment was prepared by compressing 58.5 mg (4.88 × 10−3 mol) of graphite powder mixed with 1.63 mg (5.09 × 10−5 mol) of elemental sulfur powder, and the mixture was pressed under P = 3.2 kg·cm−2 for 10 min. A sintered glass (previously sonicated in 0.2 mol·L−1 NaOH aqueous solution), with a diameter equal to the cavity of the cell (1.0 cm), was placed over the cavity to avoid the dispersion of the graphite, and at the same time allowing the migration of sulfide ions (S2−) formed during the electroreduction. A stainless steel grid was used as anode, placed in the anodic compartment containing 0.1 mol·L−1 NaCl solution, and separated from the intermediate compartment by a Nafion membrane. The bismuth precursor solution was prepared by diluting 10.53 mg (3.34 × 10−5 mol) of BiCl3 in 2.0 mL of 1.0 mol·L−1 HCl, with subsequent addition of 1.22 × 10−3 mol (103.2 μL) of MPA (a paleyellow solution was formed). The BiCl3/MPA solution pH was adjusted to 7.0 (0.1 mol·L−1 NaOH) and then diluted to 50 mL by addition of Milli-Q water. The resulting solution was poured into the intermediate compartment together with TiO2 plates/nanotube films. The electrolysis was kept under argon atmosphere to avoid the atmospheric oxygen interference. During the synthesis of Bi2S3-MPA QDs, the precursor molar ratio was fixed to S2−:Bi3+:MPA (2:3:72). Constant current electrolysis (i = −30 mA) was carried out during t = 327 s (Q = 5.09 × 10−5 mol × 2e− × 96.500 F·mol−1 = 9.82 C, and Q = i × t) plus an extended electrolysis time of 173 s (35% of a total time of 500 s) to ensure the theoretical complete electroreduction of the sulfur (i.e., 100% conversion S → S2−). After the electrosynthesis, the TiO2 NTs sensitized with Bi2S3-MPA QDs were rinsed in water and dried with N2 flux for further characterization (Ti_Bi_0 sample). A Ti_Bi_0 sample was transferred to a flask containing 10 mL of the Bi2S3-MPA QDs colloidal solution with pH adjusted to 10 and heated at 90 °C for 15 min (Ti_Bi_15 sample). Finally, another Ti_Bi_0 sample was transferred to a flask containing 10 mL of Bi2S3-MPA QDs colloidal solution (pH 10) in a Teflon-lined stainless steel autoclave, for hydrothermal treatment at 180 °C during 30 min (Ti_Bi_30 sample). These thermal treatments were carried out to improve the interface between Bi2S3-MPA QDs and TiO2 NTs. The TiO2 NTs sensitized with Bi2S3 nanoparticles were referred to as Ti_Bi_time, where time relates to the period of heat treatment performed for that sample. 2.4. Materials and Methods. Controlled-current electrolyses were carried out by using an Autolab PGSTAT 30 potentiostat/ galvanostat and an electrochemical cavity cell (Figure 1b). X-ray diffraction (XRD) patterns were taken on a Bruker X-ray diffractometer model D8 Advance with a Cu Kα radiation (λ = 1.5418 Å) and with the 2θ range from 20° to 60° with a step of 0.02°. The scanning electron microscopy (SEM) images were obtained using a FEI Quanta 200F field emission scanning electron microscope (FESEM) coupled to an electron probe energy disperse X-ray spectrometer (EDS) for mapping surface. The transmission electron
2. EXPERIMENTAL SECTION 2.1. Chemicals. All chemicals were of reagent grade and used without further purification. Elemental sulfur (S0) powder (100 mesh, Aldrich), bismuth(III) chloride (≥98%, Sigma-Aldrich), graphite powder (particle size
