High-Efficiency Photochemical Water Splitting of CdZnS/CdZnSe Nanostructures

We have prepared and employed TiO 2 /CdZnS/CdZnSe electrodes for photochemical water splitting. The TiO 2 /CdZnS/CdZnSe electrodes consisting of sheet-like CdZnS/CdZnSe nanostructures (8–10μm in length and 5–8 nm inwidth) were prepared through chemical bath deposition on TiO 2 substrates.The TiO 2 /CdZnS/CdZnSe electrodes have light absorption over the wavelength 400– 700 nm and a band gap of 1.87 eV. Upon one sun illumination of 100mWcm, the TiO 2 /CdZnS/CdZnSe electrodes provide a significant photocurrent density of 9.7mA cm at −0.9V versus a saturated calomel electrode (SCE). Incident photon-tocurrent conversion efficiency (IPCE) spectrum of the electrodes displays a maximum IPCE value of 80% at 500 nm. Moreover, the TiO 2 /CdZnS/CdZnSe electrodes prepared from three different batches provide a remarkable photon-to-hydrogen efficiency of 7.3± 0.1% (the rate of the photocatalytically produced H 2 by water splitting is about 172.8mmol⋅h⋅g), which is the most efficient quantum-dots-based photocatalysts used in solar water splitting.


Introduction
Developing environmentally clean energy resources from abundant solar energy has attracted considerable attention these years [1].Hydrogen production by photochemical water splitting is a promising route because hydrogen has the highest energy density values per mass (140 MJ kg −1 ) and its oxidation product (H 2 O) is more eco-friendly [2][3][4].Hitherto, many semiconductors with band-gap energy exceeding the oxidation potential of water (1.23 V versus normal hydrogen electrode (NHE)) at pH 1.0 have been employed for water splitting [5].Albeit metal oxides including TiO 2 , ZnO, and their derivatives are the most common photocatalysts used in water splitting, yet they provide low overall photon-to-hydrogen efficiency () attributed to their wide band gaps [6,7].To overcome these limitations, doping other metal or inorganic ions to TiO 2 and ZnO materials has been demonstrated [8].However, this strategy is not quite successful, mainly because their band gaps are greater than 2.0 eV (620 nm) [9], whereas photocatalysts having band gaps less than 2.0 eV can absorb solar light in the visible to nearinfrared region more efficiently.
Therefore, to fabricate highly efficient PECs for water splitting, we prepared TiO 2 /CdZnS/CdZnSe electrodes from Cd(NO 3 ) 2 , ZnSO 4 , Na 2 S, and Na 2 SeSO 3 via chemical bath deposition (CBD) approach [21].Under AM 1.5 illumination (100 mW cm −2 ) and in presence of a sacrificial electrolyte (2 M Na 2 S and 0.25 M Na 2 SO 3 ), the three as-prepared TiO 2 /CdZnS/CdZnSe electrodes provided  of 7.3 ± 0.1%.To the best of our knowledge, this is the most efficient QDssensitized electrode used in solar water splitting.

Preparation of TiO 2
Electrodes.A repetitive dry method was used to prepare the TiO 2 electrodes [21].In brief, TiO 2 paste was prepared by mixing TiO 2 powder (after HNO 3 treatment; 0.6 g), PVP (0.18 g), methyl cellulose (0.06 g), and ultrapure H 2 O (3 mL).The paste (0.05 ml) was then applied to one of the bare edges of a FTO glass; it was flattened by sliding a glass rod over the tape-covered edges.A single-layer TiO 2 electrode was obtained after drying at 50 ∘ C in an oven for 1 h.The process was repeated to obtain a double-layer TiO 2 electrode with a thickness of 20 m.

Preparation of TiO
2 /CdZnS () /CdZnSe () Electrodes.The as-prepared TiO 2 electrodes each with an effective area of 1 cm 2 were immersed into a solution (2 mL) containing Cd(NO 3 ) 2 (0.5 M) and ZnSO 4 (0.75 M) for 5 min, rinsed with ultrapure H 2 O (1 mL), and dried with an air gun.They were then dipped for 5 min into an aqueous solution (2 mL) of 0.5 M Na 2 S, and then rinsed with ultrapure H 2 O (1 mL), and finally dried with an air gun.The process was repeated up to  cycles ( is integer 1∼7).These as-prepared electrodes are designated herein as TiO 2 /CdZnS () electrodes.The TiO 2 /CdZnS () electrodes were further dipped into a solution of Cd(NO 3 ) 2 (0.5 M) and ZnSO 4 (0.75 M) for 5 min at 27 ∘ C and then immersed into aqueous Na 2 SeSO 3 (0.08 M) at 50 ∘ C for 60 min followed by rinsing with ultrapure H 2 O and then dried with an air gun.The process was repeated up to  cycles ( is integer 1∼4).The as-prepared electrodes are designated as TiO 2 /CdZnS () /CdZnSe () electrodes.

Measurements.
The UV-vis absorption spectra of the as-prepared electrodes were recorded using a Cary 100 UV-Vis spectrophotometer from Varian (Palo Alto, CA, USA).Scanning electron microscopy (SEM) images and energy dispersive spectra (EDS) were recorded using an S-2400 SEM system from Hitachi (Tokyo, Japan).Cyclic voltammetry (CV) tests were performed using a CHI 700D electrochemical analyzer (CH Instruments, Austin, TX, USA).CV measurements of the TiO 2 /CdZnS () /CdZnSe () electrodes (effective area: 1.0 cm × 1.0 cm) were performed using a threeelectrode system: a TiO 2 /CdZnS () /CdZnSe () electrode as a working electrode, a Pt counter electrode, and an SCE as a reference electrode.CV measurement was carried out in a solution containing 2.0 M Na 2 S and 0.25 M Na 2 SO 3 .The solution was purged by nitrogen to remove dissolved oxygen before the experiment.The irradiation source was a 450-W xenon arc lamp (Oriel, Stratford, CT, USA) equipped with an AM 1.5 filter.A commercially available silicon-based reference cell (Oriel, Stratford, CT, USA) was employed to measure the light intensity (100 mW cm −2 ).Incident photon-to-electron conversion efficiency (IPCE) spectra were recorded using a PEC-S20 instrument (Peccell Technologies, Kanagawa, Japan).Photocatalytic hydrogen generation experiments have been carried out in a labmade photochemical reactor.A typical glass cell consisted of photoanode (TiO 2 /CdZnS () /CdZnSe () ) and cathode (Pt foil) that were connected by a fine porous glass frit.The H 2 produced in our experiments was measured by applying a gas volume headspace method.

Properties of Electrodes.
The CBD layers of QDs play a significant role in light harvesting [22], so we investigated the role that the amounts (layers) of CdZnS and CdZnSe nanostructures played in determining the  values (Scheme 1).Moreover, upon increasing CdZnS () /CdZnSe () layer numbers, the light absorption increased and reached a maximum value at  = 5, and  = 2 (not shown).The as-prepared TiO 2 /CdZnS (5) /CdZnSe (2) electrode absorbs light in the wavelength range of 400-700 nm (Figure 1(a)).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 1.23-2.0eV, 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)).The scanning electron microscopy (SEM) image (inset to Figure 1(b)) elucidates that the as-resulting electrode was decorated with sheet-like nanostructures, having lengths of 8-10 m and widths of 5-8 nm, respectively.The crosssectional view of the photoelectrode is shown in Figure 2, which reveals that sheet-like nanostructures are not only found on the surface but also deeply embedded within the film as denoted by the dotted circles.
The electrodes were tested for solar water splitting in presence of sacrificial electrolyte [24,25] consisting of Na 2 S and Na 2 SO 3 .The use of Na 2 S/Na 2 SO 3 mixture provided an advantage of negligible photocorrosion effect on QDs.Moreover, Na 2 S in solution acted as a hole scavenger and it was oxidized into S 2 2− , which was reduced back to S 2 − by Na 2 SO 3 [26].Photogenerated holes irreversibly oxidized the reducing agents (Na 2 S/Na 2 SO 3 ) instead of water, providing the photocatalyst electron rich and an enhanced H 2 evolution reaction.The reaction mechanism of each electrode can be described as follows. Anode: 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) .Although electrodes having higher limiting photocurrent densities have been reported [14][15][16][17][18], their relative high applied voltages (0.356 and −0.574 V versus SCE) are disadvantageous over ours.The as-prepared electrode required a very low onset potential (−0.9 V), implying that it required less energy to drive the reaction.The negative shift of the onset potential confirms the enhanced charge transfer which may possibly be due to a high recombination rate and kinetic hindrance by CdZnS (5) /CdZnSe (2) sensitization [16].We further conducted amperometric - measurements to examine the photocatalytic activity and photoresponses of the TiO 2 /CdZnS (5) /CdZnSe (2) electrode (Figure 3(b)).Notably, the potential set at −0.9 V produced stable limiting photocurrent density.Upon illumination, spiked photoresponses of 9.7 mA cm −2 were observed, which was much higher than the dark current density (∼50 A cm −2 ).The reproducibility of the photoresponses was also excellent (relative standard deviation 0.4% from 10 replicated measurements).The photoresponses were fast (<4 s), revealing their promising photocatalytic activity.By using (7) [27], the value of  for the data obtained from Figure 3 is as follows: where,   is the photocurrent density (mA cm −2 ),    0 rev is the total power output,   | app | is the electrical power input, and  light is the power density of incident light (100 mW cm −2 ). 0 rev (1.23 V/NHE) is the standard reversible potential for water splitting.The applied voltage is calculated as 0.48 V using  app = | meas −  aoc |, where  meas is the working electrode potential at which photocurrent was measured under illumination and  aoc is the potential measured at this working electrode under open circuit potential (OCP) and same experimental conditions, respectively.As shown in Figure 3(c), a maximum  value of 7.3% was achieved at an  app of 0.48 V. We also recorded  values at various  app values from three different batches of TiO 2 /CdZnS (5) /CdZnSe (2) electrodes, revealing  of 7.3 ± 0.1%, which is superior to those (e.g., 3.67%) [11][12][13] of other QDs-sensitized TiO 2 electrodes [11][12][13][14][15][16].Relative to some reported electrodes such as TiO 2 /CdS (6.4 mmol⋅h −1 ⋅g −1 ) and CdSe/CdS (40 mmol⋅h −1 ⋅g −1 ) [17,18], our QDs-sensitized electrodes provided a higher H 2 generation rate of 172.8 mmol⋅h −1 ⋅g −1 ( = 3).However, in many PECs competing side reactions dominate, resulting in different products and less than ideal faradaic efficiency.If we consider an average current of 9.7 mA cm −2 flowing through the circuit, one would expect to observe H 2 formation at a theoretical rate of 200.9 mmol⋅h −1 ⋅g −1 (at 25 ∘ C).The observed H 2 rate of 172.8 mmol⋅h −1 ⋅g −1 accounts for 86% of the amount predicted on the basis of current flow.
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] where,  SC is the photocurrent density,  is the incident light wavelength, and  light is the measured irradiance.The TiO 2 /CdZnS (5) /CdZnSe (2) electrodes exhibited a pronounced IPCE value of 80% at 500 nm.We further estimated the  SC values from the following [28]: where  is the electron charge and () is the incident photon flux of the solar light.The value of  SC calculated from the maximum IPCE value at AM 1.5 photon flux was ca.∼9.74 mA cm −2 , which was quite close to the  SC value (∼9.7 mA cm −2 ) measured from LSV measurements.The close agreement between  SC values obtained from IPCE spectra and - curves indicates that the photocurrent was indeed generated from the electrode upon illumination [28].Moreover, photochemical stability of the TiO 2 /CdZnS (5) /CdZnSe (2) electrodes in sacrificial electrolyte was quite appreciable as negligible changes in the - curve was observed over 100 cycles (not shown).Table 1 summarizes the LSV measurements of various electrodes employed for solar water splitting under the same conditions.These results revealed that the cascade structure of CdZnS () /CdZnSe () played a major role in determining the values of .Upon increasing the CdZnS layers, the  value increased and reached a plateau at  = 5, when  was kept 2. The absorption spectra results (Figure 4) validate that the generation of different photocurrent densities is a vital factor responsible for superior light harvesting efficiency of these electrodes.The band gaps of the as-resulting CdZnS and CdZnSe QDs were 2.40 and 1.85 eV, respectively.Thus, the insertion of CdZnS layer between TiO 2 and CdZnSe of the cascade structure elevates the conduction band edge of CdZnSe, and thus provides higher driving force for the injection of excited electrons out of CdZnSe layer.However, upon increasing the values of  from 5 and  from 2 (the thickness of the QDs layer), the photoexcited electrons could not be effectively injected into the TiO 2 conduction band, likely due to the presence of additional QDs.Nevertheless, the position of the absorption peak exhibits a red shift with increase in the number of CdZnSe () layers, mainly due to the size quantization and confinement effects.
To highlight the features of the TiO 2 /CdZnS (5) /CdZnSe (2) electrodes, we have listed some of the most effective electrodes for solar water splitting (Table 1).The as-prepared TiO 2 /CdZnS (5) /CdZnSe (2) electrodes provide the highest  value as a result of their lowest applied voltage and highcurrent density.However, they had a negative applied voltage versus SCE, mainly because their Fermi energy level will be changed due to the band realignment of TiO 2 by CdZnS and CdZnSe sensitization.These results also reveal that the photocurrents and applied voltage are highly dependent on the structure of the photoelectrodes.
To the best of our knowledge, the electrodes provided the highest value of  for the solar water splitting among QDsbased photocatalysts, mainly because they provide high photocurrent density at low applied voltage.Although the TiO 2 /CdZnS (5) /CdZnSe (2) electrodes hold great promise for commercial use in water splitting, further increase in their efficiency is still required.
a Under AM 1.5 (100 mW cm −2 ) illumination of light.b Not available.