Quantum Dot Monolayer Sensitized ZnO Nanowire‐Array Photoelectrodes: True Efficiency for Water Splitting

Abstract

Increasing demand for clean energy has motivated considerable effort to exploit the properties of various materials in photovoltaics and related solar-harvesting devices. [1] Splitting of water by sunlight to generate hydrogen is one of the forms of energy production with the most potential. Metal oxides such as TiO 2 , ZnO, and WO 3 with various morphologies have been investigated for use in splitting water. [2][3][4][5][6] However, most of these metal oxides have large band gaps, which limit light absorption in the visible region and overall efficiency. To reduce the band gaps of nanostructured metal oxides, doping and utilization of transition metals, carbon, or nitrogen have been investigated. [7][8][9] One possibility is the use of semiconductor nanocrystals, known as quantum dots (QDs), as an alternative to photosensitive dyes. Quantum dots generally offer various significant advantages over dyes. [10] It was recently established that QDs generate multiple electronhole pairs per photon, improving device efficiency. [11] Quantum dot sensitized nanostructures are widely studied for use in solar cells. [12,13] However, little work has been done on metal oxide and semiconductor QD-based composite structures for use in water-splitting nanodevices.To elucidate this fundamental issue, we examined a combination of CdTe QDs and ZnO nanowires for splitting water photoelectrochemically (Scheme 1). One-dimensional nanostructures offer the additional potential advantage of improved charge transport over zero-dimensional nanostruc-tures such as nanocrystals. [6] Additionally, the typical electron mobility in ZnO is 10-100 times higher than that in TiO 2 , so the electrical resistance is lower and the electron-transfer efficiency higher. [14] However, since the overall water-splitting reaction is tough, sacrificial reagents are commonly adopted to evaluate the photocatalytic activity for water splitting. When the photocatalytic reaction is carried out in an aqueous solution that contains a reductant, electron donors, or hole scavengers such as sulfide ions or selenium ions, photogenerated holes irreversibly oxidize the reductant rather than the water. Employment of CdTe QDs in water splitting system has major advantages. CdTe with a more favorable conduction band energy (E CB = À1.0 V vs. NHE) can inject electrons into ZnO faster than CdSe (E CB = À0.6 V vs. NHE). In addition, monolayer deposition of CdTe QDs on the surface of ZnO nanowires would further improve the stability in electrochemical reaction, by avoiding anodic decomposition/corrosion of CdTe and thus enhancing the overall watersplitting performance. During the photoirradiation of CdTe, two reactions can be expected to dominate after initial charge separation [Eqs. (1) and (2)]. [15] Anodic decomposition : CdTe ðe þ hÞ ! Cd 0 þ Te 0Anodic corrosion : CdTe ðhÞ ! Cd 2þ þ TeC ÀMonolayer deposition of QDs can facilitate transfer from CdTe to ZnO and improve QD stability, so that the efficiency in the overall water-splitting reaction can be exactly measured in aqu...

High-Efficiency Photochemical Water Splitting of CdZnS/CdZnSe Nanostructures

“…The band gap corresponding to the absorption edge of this electrode was 1.87 eV (662 nm) [23]. This value is in the range of 1.23-2.0 eV, validating it as a suitable photocatalyst for water splitting [5,[9][10][11][12][13][14][15][16][17][18]. The existence of Cd, Zn, S, Se, Ti, and O components in the as-prepared electrode was further confirmed from energy dispersive spectroscopy (EDS) results (Figure 1(b)).…”

“…Cathode: produced a noteworthy limiting photocurrent density value of 9.7 mA cm −2 at −0.9 V versus SCE, which was superior to those produced by most QDs-sensitized electrodes [10][11][12][13][14][15][16][17][18]. Such a high photocurrent density can be attributed to the cascade structure of CdZnS (5) /CdZnSe (2) .…”

“…To further quantify the performance of PECs incorporating TiO 2 /CdZnS (5) /CdZnSe (2) electrodes, their IPCE values were acquired in the wavelength range of 400-700 nm (Figure 3(d)), [10,16,27,28]. The IPCE values were determined from [16] …”

“…Photoelectrochemical cells (PECs) incorporating QDssensitized TiO 2 electrodes provide several advantages: (i) ease of fabrication, (ii) generation of multiple electron/hole [19], (iii) high visible light harvesting capability in solar spectrum, and (iv) tunable band gaps due to the quantum size effect of QDs. Remarkably, CdTe-sensitized ZnO nanowire electrodes provided a superior of 1.83% relative to that of ZnO nanowires (0.66%) in a nonsacrificial electrolyte [10]. Under white light illumination of 100 mW cm −2 , CdS/CdSe QDssensitized ZnO nanowire electrodes and TiO 2 electrodes had high photocurrent densities of 12 and 14.9 mA cm −2 , respectively [14][15][16][17][18].…”

“…Quantum dots (QDs) such as CdTe [10], CdS [11][12][13], and CdSe [14][15][16][17][18] have been anchored to TiO 2 and ZnO electrodes to harvest visible light for more efficient water splitting. Photoelectrochemical cells (PECs) incorporating QDssensitized TiO 2 electrodes provide several advantages: (i) ease of fabrication, (ii) generation of multiple electron/hole [19], (iii) high visible light harvesting capability in solar spectrum, and (iv) tunable band gaps due to the quantum size effect of QDs.…”