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

A new hybrid photoelectrochemical photoanode is developed to generate H2 from water. The anode is composed of a TiO2 mesoporous frame functionalized by colloidal core@shell quantum dots (QDs) followed by CdS and ZnS capping layers. Saturated photocurrent density as high as 11.2 mA cm−2 in a solar‐cell‐driven photoelectrochemical system using near‐infrared QDs is obtained.


PbS QDs
PbS QDs with diameter ~3.0 nm were synthesized by hot injection method by using OA as ligand. [1] In a three-neck reaction flask, a mixture of lead acetate trihydrate (1 mmol), OA (1.2 mL), TOP (1 mL), and ODE (15 mL) were heated to 150 °C for 1 h. After the system was cooled down to ~100 °C under vacuum for 15 min, 4.8 mL of a sulphur precursor solution prepared by mixing (TMS) 2 S (0.5 mmol) with 0.2 mL of TOP was quickly injected into the reaction flask at 130 °C.

SI-2
Subsequently, the reaction was quenched in cold water. The obtained PbS QDs were precipitated with ethanol, centrifuged to remove unreacted lead oleate and free OA molecules and then re-dispersed in toluene.

PbS@CdS QDs
PbS@CdS QDs were synthesized via a cation exchange method. [2] Typically, CdO (2.3 mmol), OA (2 mL) and ODE (10 mL) were heated to 255 °C under N 2 for 20 min. The clear solution was cooled down to 155 °C under vacuum for 15 min. The flask was then reopened and the N 2 flux was restored. PbS QDs suspension in toluene (1 mL, Absorbance = 3 at the first exciton peak) was diluted in 10 mL toluene, bubbled with N 2 for 30 min and then immediately heated to 100 °C. The Cd/OA mixture was added via a syringe. The solution was maintained at 100 °C for 5 minutes and then cooled down to room temperature with cold water. Then the PbS@CdS was washed by ethanol and re-dispersed in toluene. The re-dispersion-precipitation procedure was repeated two times. Ti-Nanoxide BL/SC (Solaronix) or using TiO x flat film precursor solution, [3] which consists of 0.23 M titanium isopropoxide (Sigma-Aldrich, 99.999%) and 0.013 M HCl solution in isopropanol (Sigma-Aldrich, >99.9%). Then the films were annealed in air at 500 ℃ for 30 min after drying and cooled down to room temperature. Then a 20 nm particle size paste, which was supplied by Dyesol (Queanbeyan, Australia) under the commercial name 18NR-T (paste A), was deposited on the top of FTO by tape casting and dried in the air for 10 min. The photoanodes were then fired on a hot plate at 120 ℃ for 10 min. A blend of active anatase particles (~20 nm) and larger anatase scatter particles (up to 450 nm) paste (18 NR-AO, paste B) was then deposited on the top of paste A, following the same procedure. The electrodes were subsequently sintered  following temperature profile at 325 ℃/5 min, at 375 ℃/5 min, at 450 ℃/15 min and at 500 ℃/30 min, forming film with thickness ∼12 µm, as measured by contact profilometry.

EPD of the QDs on the TiO 2 film.
QDs were dispersed in toluene, with a pair of TiO 2 FTO slides vertically immersed in the QDs solution and facing each other. The distance between them was adjusted at 1 cm. A voltage of 200 V was applied for 120 min. [4] To wash off unbound QDs after the EPD process, the samples were rinsed several times with toluene and dried with N 2 at room temperature. In a typical SILAR deposition cycle, [5,6]  for ZnS for each sample.

Characterization
The morphology of PbS@CdS QDs was characterized by a JEOL 2100F TEM. Absorption spectra were acquired with a Cary 5000 UV-Vis-NIR spectrophotometer (Varian) with a scan speed of 600 nm/min. Fluorescence spectra were taken with a Fluorolog®-3 system (Horiba Jobin Yvon) and the excitation wavelength was set at 430 nm. For the core@shell QDs, the Pb-to-Cd atomic ratio was first determined by using inductively coupled plasma optical emission spectrometry (ICP-OES) (Perkin Elmer Model Optima 7300 DV). Based on this ratio and the overall diameter from TEM images, the diameter of the PbS core and the thickness of the shell were calculated, assuming that all QDs are spherical and contain a uniform shell. The composition of the films was measured on a freshly cleaved cross-section of the TiO 2 layers after EPD, using an Atmospheric Thin Window (ATW) energy dispersive X-ray spectroscopy (EDX) detector in a FEI Sirion high resolution scanning electron microscope (HRSEM) system operated at 10-15 kV accelerating voltage (133 eV resolution at 5.9 keV). The PEC performance of the photoelectrodes  was evaluated in a three-electrode configuration, consisting of a QD-TiO 2 thin film working electrode, a Pt counter electrode, and an saturated Ag/AgCl reference electrode. TiO 2 was printed on FTO and a Cu wire was used to connect FTO using silver paste with outer circuit. An insulating epoxy resin was used to cover the sample's surface except the active area to avoid any direct contact between the electrolyte and the conducting back-contact and/or the connecting wire.
Then the sample is fully immersed in the electrolyte with pH=13, containing 0.25 M Na 2 S and 0.35 M Na 2 SO 3 as the sacrificial hole scavenger to prevent photocorrosion of the QDs. All potentials measured with respect to Ag/AgCl during the electrochemical measurements were converted to the reversible hydrogen electrode (RHE) scale with the following expression V RHE = V Ag/AgCl + 0.197 + pH×(0.059) [7,8] . The photoresponse was measured from a 150 W Xenon lamp used as the light source with an AM 1.5 G filter. The sample is 2 cm far from the window of lamp case (~7 cm far from actual bulb). The light intensity measured by thermopile and 2 cm away from the lamp case is ~100 mW/cm 2 . The working area of the electrode is 0.16 cm 2 All the current versus potential measurements were carried out at a 20 mV/s sweep rate.
The IPCE describes the ratio of photogenerated electrons collected by the electrodes over the number of incident monochromatic photons. To derive the IPCE values, we performed current−voltage measurements using different band-pass optical filters. IPCE can be calculated by using the following equation [7] : Where is the photocurrent density, is the incident radiation intensity at a given wavelength,  . Table S1. Peak position, integrated area and FWHM of PL spectra in Figure 3