Ultrafast Photodeposition of Size‐Controlled PbS Quantum Dots on TiO₂

Abstract

At present, narrow gap semiconductor quantum dot (QD)-loaded mesoporous TiO 2 nanocrystalline films (mp-TiO 2 ) are the key material in the photoelectrochemical (PEC) devices for the conversion of solar energy to electric and chemical energy. [1,2] To prepare such coupling systems, the self-assembled monolayer (SAM) method for immobilizing QDs on the TiO 2 surface through bifunctional coupling agents (BCAs) has conveniently been used.[3] However, the loading capacity of QDs is small because their diffusion into the interior of the mesopores is difficult, while a strict size control is possible. Also, the insulating BCA molecules intervening between QD and TiO 2 could be a barrier for the interfacial electron transfer.[4] On the other hand, the photodeposition (PD) technique which takes advantage of the photoinduced properties-that is photocatalysis and surface superhydrophilicity-of TiO 2 , is applicable to synthesize the coupling systems on the nanoscale. [5][6][7][8][9][10] We have recently shown that the PD technique has the following unique and important features in the application to the mp-TiO 2 :[11] i) the electron transfer from QDs to TiO 2 is inherently guaranteed, ii) a large amount of QDs can be deposited on-not only-the external surfaces, but also the inner surfaces of mp-TiO 2 without pore-blocking, iii) the band energy of QDs are tunable by irradiation time due to the size quantization. When QD/mp-TiO 2 is applied to the PEC cells, these features should increase the light-harvesting efficiency and the electron-collection efficiency. An additional merit of the PD technique is that it affords a simple one-pot process with excellent reproducibility and this heightens its practical value. Consequently, a power conversion efficiency as high as 2.5 % has been achieved for a QD-sensitized solar cell (QD-SSC) using the CdS QD/mp-TiO 2 photoanode prepared by the PD technique.[11] A subject to be solved in the PD technique is the control of the QD size, which is of great importance for increasing the interfacial electron-transfer efficiency.[12] Additionally, while the light absorbed by CdS having a band gap (E g ) of 2.4 eV is limited to the wavelength region at l < 520 nm, PbS (E g = 0.41 eV) can absorb the light in the whole visible region, and thus, is very attractive from the viewpoint of efficient solar energy utilization. Very recently, the coupling of PbS QDs with TiO 2 nanotubes has been shown to increase the visible light photocatalytic activity for dye decomposition, [13] and the incident photon-to-current conversion efficiency (IPCE) over 35 % have been attained for PbS QD-SSC in a wide wavelength region.[14] Also, the PD of PbS on TiO 2 has been reported in the particulate system by Zhukovskiy et al.; however, the particle size distribution is fairly wide, and the detailed reaction mechanism remains unknown.[15]Here we report the ultrafast size-controlled PD of PbS QDs on TiO 2 using mercaptoacetic acid (MAA) as a surface modifier. To our knowledge, this is the first report on the surface modifi...

Near‐Infrared Colloidal Quantum Dots for Efficient and Durable Photoelectrochemical Solar‐Driven Hydrogen Production

“…[ 14 ] Near-infrared (NIR) PbS QDs, which allow fast/effi cient charge separation/injection in the whole UV, Vis, and NIR, [ 15 ] can be easily grown inside a mesoporous TiO 2 fi lm via in situ deposition techniques, yielding a solar to H 2 conversion effi ciency of 1.15% in PEC systems, with a photocurrent density of 1.5 mA cm −2 and an H 2 generation rate of 12.5 mL cm −2 d −1 . [ 16 ] However, PbS as a sensitizer suffers from problems of stability and high charge recombination. [ 17 ] Recently, PbS QDs synthesized by a successive ionic layer adsorption and reaction (SILAR) approach and further coated by a CdS layer on TiO 2 mesoporous fi lm have been used as a photocatalyst to produce H 2 at 60 mL cm −2 d −1 with a photocurrent density of 6 mA cm −2 under one-sun simulated solar illumination.…”

Facile synthesis and catalytic activity of MoS2/TiO2 by a photodeposition-based technique and its oxidized derivative MoO3/TiO2 with a unique photochromism

“…This technique taking advantage of the photocatalysis and the photoinduced superhydrophilicity of TiO 2 [15] possesses two unique features: one is that the efficient interfacial charge transfer is inherently guaranteed, and the other is that NGS QDs can be deposited on not only the external surfaces but also the inner surfaces of the mesopores of mp-TiO 2 [8]. So far, most of the NGSs prepared are metal chalcogenides including CdSe [9], CdS [10][11][12], PbS [13,14], and CuS x [13], which were photodeposited from a solution containing the two precursors of metal ions and chalcogens. On the other hand, the heteronanostructured metal oxide/TiO 2 systems also have promising applications including photocatalysis [2,3,16], photoinduced surface superhydrophilicity [17], and energy storage materials [18].…”

“…Their crucial point in common is enhancing the charge separation through the efficient interfacial charge transfer between NGS and TiO 2 , which needs both the intimate and large-area contact of the semiconductors [5,6] and the control of the band offset between NGS and TiO 2 [7,8]. Currently, the photodeposition (PD) technique is being revealed to have the possibility of building various directly nanocoupled NGS/TiO 2 systems [9][10][11][12][13][14]. This technique taking advantage of the photocatalysis and the photoinduced superhydrophilicity of TiO 2 [15] possesses two unique features: one is that the efficient interfacial charge transfer is inherently guaranteed, and the other is that NGS QDs can be deposited on not only the external surfaces but also the inner surfaces of the mesopores of mp-TiO 2 [8].…”

Optofluidic Sensor: Evaporation Kinetics Detection of Solvents Dissolved with Cd3P2 Colloidal Quantum Dots in a Rolled‐Up Microtube

“…To investigate the optical sensing capability of other solvents, we also carried out the measurements on solvents such as water and THF, which were dissolved with Cd 3 P 2 QDs. [ 39 ] Therefore, a precise knowledge of the time constant can certainly help us to optimize the preparative procedures of photoanodes. [ 36 ] The logarithmic dependence of mode shifts on evaporation time was also observed, and different rate constants were measured.…”

“…Complete evaporation can reduce the recombination of photoelectrons, and can enhance the effi ciency of photo to electron. [ 39 ] Therefore, a precise knowledge of the time constant can certainly help us to optimize the preparative procedures of photoanodes.…”