- Email: [email protected]
CdSe nanoparticles
Electrochimica Acta xxx (2017) 1e9
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Water splitting vs. sulfite oxidation: An assessment of photoelectrochemical performance of TiO2 nanotubes modified by CdS/CdSe nanoparticles Rasin Ahmed, Yin Xu, Giovanni Zangari* Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 November 2017 Received in revised form 29 November 2017 Accepted 11 December 2017 Available online xxx
A major portion of the existing literature on TiO2 nanotubes modified by CdS and CdSe report an enhanced photoelectrochemical (PEC) performance using electrolytes with inorganic sacrificial electron donors where a dominant sulfite oxidation occurs. In this study, we distinguish the PEC performance arising entirely from water splitting to that originated by sulfite oxidation in the TiO2 nanotube system modified by CdS/CdSe nanoparticle depositions. The photocurrent density measured under the AM1.5G simulated spectra, 1 sun (100 mWcm2) intensity for the TiO2 nanotubes modified by CdS/CdSe nanoparticles and using an aqueous alkaline electrolyte (1 M NaOH) was observed to be 0.92 mAcm2 at 1.23 V vs. RHE which was 8-fold lower than in the case for sulfite oxidation (Jph/SO ¼ 7.4 mAcm2 at 1.23 V vs. RHE) using an Na2S/Na2SO3 aqueous electrolyte. A maximum STH efficiency of 0.71% and water splitting efficiency of 12.4% was determined for water oxidation using an aqueous alkaline electrolyte. To rationalize such device efficiency, we propose that the ‘water splitting efficiency’ parameter is much more meaningful since it correlates the PEC activity specifically to water splitting and not to other pathways that produce an artificially enhanced photocurrent through the use of sacrificial reagents. The incident photon-to-current conversion efficiency spectrum measured at 1.23 V (vs. RHE) bias for the TiO2 photoanodes modified by CdS/CdSe nanoparticles revealed that the current conversion efficiency is lower (~17% or less) when absorption occurs solely from the external CdSe layers indicating higher recombination during charge transport. The increase of dark current observed in the linear sweep voltammetry plots for the voltage range of 0.25 V to þ0.15 V vs. RHE was attributed to anodic dissolution of the photoanodes in aqueous electrolytes containing no sacrificial reagents. The photocurrents and onset potentials for the CdS/CdSe modified TiO2 photoanodes improved under acidic conditions showing a slightly increased PEC activity and faster reaction kinetics. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Water splitting Sulfite oxidation Photoelectrochemical CdS CdSe TiO2 SILAR Solar-to-hydrogen (STH) efficiency
1. Introduction Hydrogen production from solar energy driven water splitting is an emerging technology which enables storage of the abundant incident sunlight in the form of hydrogen as an energy vector, via direct conversion of the radiant energy into chemical energy, through a carbon neutral pathway. The stored hydrogen could be successively distributed and used on site to drive a stationary or mobile fuel cell, as well as to power industrial sites [1]. In essence, solar water splitting converts electromagnetic energy from the
* Corresponding author. E-mail address: [email protected] (G. Zangari).
incident solar spectrum by utilizing a photoelectrochemical (PEC) cell where one or both of the electrodes are semiconducting and capable to absorb photons to generate electron-hole pairs (EHPs) that are injected at the electrode/electrolyte interface to perform electrochemical reactions, i.e. producing hydrogen at the cathode and oxygen at the anode. The research efforts focused in the area of photoelectrochemical water splitting relying on renewable resources are aimed at developing a system that utilizes only water and sunlight, without the use of sacrificial species [2]. The realization of a commercially viable PEC water splitting system has been held back due to the lack of suitable materials that could sustain long term water oxidation at the photosensitive anode and produce around 1.7e2.4 eV of energy per EHP necessary to drive the water splitting reaction. The difference between the
https://doi.org/10.1016/j.electacta.2017.12.088 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: R. Ahmed, et al., Water splitting vs. sulfite oxidation: An assessment of photoelectrochemical performance of TiO2 nanotubes modified by CdS/CdSe nanoparticles, Electrochimica Acta (2017), https://doi.org/10.1016/j.electacta.2017.12.088
2
R. Ahmed et al. / Electrochimica Acta xxx (2017) 1e9
figure above and the thermodynamic water splitting energy (1.23 eV per formula unit) is necessary to provide an overvoltage sufficient to run the reaction at a rate of around 10 mAcm2. The material properties that are commonly taken into consideration to design a photosensitive device are extinction coefficients, bandgaps and correct band alignments of the photoactive materials as well as the transport properties of the photogenerated charges. These factors determine the light harvesting efficiency, spectral response and charge injection efficiency of the cells, respectively, all of which are of great importance to produce a high photocurrent required for the fast electrolysis of water. Aside from charge generation and efficient transport of the separated charges, the chemical pathways for redox reactions at the electrolyte/electrode interface can also play a crucial role in achieving higher cell photocurrents under simulated or natural radiation. The use of sacrificial reagents which act as electron donors has been known to enhance the photocurrents, while in some cases the actual photocurrents arising from water oxidation remain unreported [3e6]. These data are essential for designing PEC systems that can run on water and sunlight only. An investigation of the PEC activity of a photoanode or photocathode system using electrolytes with and without sacrificial agents, as well as under different electrolyte pH, would shed light on the maximum conversion efficiency achievable for different chemical pathways, PEC performance trends and stability limits for the semiconductors under study. TiO2 is one of the best studied and commonly used photoanode materials, one of the few that exhibit long term stability under operation. TiO2 may exhibit three distinct phases, of which the most common are anatase and rutile [7,8]. Anatase TiO2 has a conduction band close to the hydrogen reduction potential (Eþ H/ H2 ¼ 0 V vs. NHE) and a much lower valence band level compared to the water oxidation potential (EH2O/O2 ¼ 1.23 V vs. NHE), allowing a variety of oxidation processes to occur [9]. However, its wide bandgap of 3.2 eV limits light absorption within the UV wavelengths (100e400 nm) which comprise less than 5% of the incident solar spectrum. While we attempted to widen the absorption spectrum of TiO2 nanotubes by modification with Cu2O, Fe2O3 or bacteriochlorophyll-C, sensitization with Group IIeVI compounds appears to be more effective [10e12]. In fact, CdS and CdSe, which have direct bandgaps of 2.42 eV and 1.73 eV, respectively, will result in significant broadening of the spectral response of the photoanode well into the visible wavelength region of 400e770 nm [13]. The TiO2 nanotube array modified by CdS and CdSe have been reported by a number of groups following a variety of deposition techniques such as electrodeposition, chemical bath deposition (CBD), sequential CBD, successive ionic layer adsorption and reaction (SILAR), magnetron sputtering, emulsion-based bottom-up self-assembly (EBS) or solvothermal process [14e29]. Amongst the solution-processed techniques, CBD and SILAR are known for their ability to achieve direct deposition of nanoparticles at the electrode surface [30]. Previous studies on SILAR aimed at nanoparticle depositions on porous TiO2 films (~20 nm particles) have reported to cover around 20% of the surface at saturation using 7 deposition cycles of CdS quantum dots and around 30e40% of surface using 6 deposition cycles of PbS quantum dots [31,32]. Thus, the TiO2 nanotubes modified by SILAR deposited CdS and CdSe would benefit from a partial surface coverage which will function as efficient light absorbers in the visible region of the solar spectrum and boost photocurrents owing to a shorter path (approximately half the nanotube thickness) for the photogenerated carriers to be injected in the electrolyte. In this paper, we study a well-known photoanode system such as TiO2 nanotubes, modified by direct deposition of CdS and CdSe nanoparticles using the SILAR technique and focus on the difference observed between PEC activities specific to water splitting and
sulfite oxidation. These figures can in fact vary widely, with sulfite oxidation at the anode enabling a much larger hydrogen current which is not warranted in a sustainable process. We further discuss the efficiency parameters that best describes the energy conversion pathway for water splitting systems, including the stability of CdS/ CdSe nanoparticles in aqueous electrolyte and report a trend of PEC performance improvement for the TiO2/CdS/CdSe photoanodes using a slightly acidic aqueous electrolyte. 2. Experimental section 2.1. Preparation of TiO2 nanotubes Anatase TiO2 nanotubes were prepared on Ti foils using a double anodization method [33,34]. Briefly, the foils were initially cleaned by sonicating for 15 min each in acetone, isopropanol and methanol solutions. A KEPCO BOP-100 power supply unit controlled with LabVIEW software was operated at 50 V to anodize the Ti foils in a 0.3 wt % NH4F (Sigma Aldrich, 99.99% þ trace metal basis) in ethylene glycol (Sigma Aldrich, 99.8% anhydrous) and 2 vol % water solution for one hour. The TiO2 nanotubes grown in the first anodization process were removed by using an adhesive tape and sonicated in ethanol for 10 min to detach excess adhesives from the peeled off nanotube surface. A second anodization was carried out using the same solution and voltage for 30 or 60 min to prepare 5.7 mm and 11 mm long nanotubes, respectively. The prepared samples were left overnight in deionized water to remove excess fluoride ions and later annealed in air at 350 C for 3 h to crystallize TiO2 in the anatase phase. In the final stage of preparation, just prior to SILAR depositions, some of the TiO2 nanotubes underwent electrochemical Li intercalation by applying a potential of 1.55 V vs. SCE for 3 s in 1 M LiClO4 (Sigma Aldrich, ACS Reagent) solution. 2.2. SILAR deposition of CdS and CdSe In order to modify the TiO2 or Li:TiO2 electrodes with CdS and CdSe, two different Cd, S and Cd, Se-precursor solutions pairs were prepared [35]. A 0.02 M Cd(NO3)2$4H2O (Sigma Aldrich, 99.0%) in methanol and 0.02 M Na2S$9H2O (Alfa Aesar, 98.0%) in methanol/ water (1:1 v/v) were used as Cd and S precursor solutions, respectively for the CdS depositions. The Ti foils were masked using single sided Kapton polyimide tapes (Ted Pella Inc.) before deposition so that the CdS could be deposited only onto the TiO2 nanotubes. These masked electrodes were dipped for one minute in the 0.02 M Cd2þ precursor solution followed by one minute of rinsing in a separate methanol solution and one minute of drying in Ar gas flow. The electrodes were then dipped for one minute in the 0.02 M S2 precursor solution for the sulfide ions to grow onto the adsorbed cadmium ions and form CdS nanoparticles. This completed one cycle of SILAR deposition of CdS. By repeating the number of SILAR deposition cycles, the SILAR deposited particles grow in size. A second solution pair of 0.03 M Cd(NO3)2$4H2O in ethanol (abs) and 0.03 M SeO2 (Alfa Aesar, 99.4%, metals basis), 0.06 M NaBH4 (Alfa Aesar, 98.0%) in ethanol (abs) were used as the Cd and Se precursor solutions, respectively for the CdSe depositions. The 0.03 M Se2 precursor solution was prepared in a three neck flask reaction chamber kept under constant Ar gas flow to prevent oxidation of the selenium products. The electrodes were dipped for 30 s alternatively in the Cd and Se precursor solutions followed by one minute of rinsing and drying for the deposition of CdSe nanoparticles on the TiO2 nanotubes. 2.3. Material and device characterizations Chemical composition of the materials was assessed with an
Please cite this article in press as: R. Ahmed, et al., Water splitting vs. sulfite oxidation: An assessment of photoelectrochemical performance of TiO2 nanotubes modified by CdS/CdSe nanoparticles, Electrochimica Acta (2017), https://doi.org/10.1016/j.electacta.2017.12.088
R. Ahmed et al. / Electrochimica Acta xxx (2017) 1e9
Energy Dispersive Spectroscopy (EDS) instrument attached to an FEI Quanta 650 Scanning Electron Microscope (SEM). Besides the FEI Quanta 650, additional SEM images for the CdS/CdSe deposited on TiO2 nanotube surfaces were acquired using a Zeiss Crossbeam 340. A Cary Varian 5E UV-VIS-NIR spectrophotometer was used to record the absorbance spectra for the CdS and CdSe modified photoanodes using the double beam, VW technique. The calibration was performed by using two movable mirrors to record the maximum and minimum baselines in the V and W positions, respectively. The reflectance spectrum of our sample was measured with respect to a low reflectance reference by positioning the Wbeam setup in the reference holder. The Raman spectra for the deposited samples were obtained using a 514 nm LASER as the excitation source coupled with a Raman Spectrometer (Renishaw). The photoelectrochemical performance for the CdS/CdSe-modified working electrodes were measured using a three electrode system consisting of a Pt-mesh counterelectrode and standard calomel electrode (SCE) as reference connected to a potentiostat (Biologic SP-150). All the electrode potentials were referred to the RHE electrode. A 150 W Xenon lamp coupled to an Oriel (Sol1A) class ABB solar simulator was used to produce an AM1.5G spectrum. Linear sweep voltammetry (LSV) plots were recorded under chopped illumination conditions for the electrodes. The incident photon-to-current conversion efficiency (IPCE) for the cell was measured using a monochromator (Princeton Instruments, Acton SP2150) coupled to a 250 W QTH lamp powered by a Princeton Instrument TS-428 PSU. The monochromator output light power was calibrated using a standard Si photodetector (Thorlabs S120B) in the 400e1000 nm wavelength range. The photocurrents for the IPCE test were measured using the Biologic SP-150 potentiostat at an applied potential of 1.23 V vs. RHE. The electrolyte solutions used in this study were (i) 0.35 M Na2SO3, 0.24 M Na2S$9H2O in water (pH ¼ 13.1), (ii) 1 M NaOH in water (pH ¼ 13.3), (iii) 0.08 M K2HPO4, 0.02 M KH2PO4 in water (pH ¼ 7.4), also called the phosphate buffer solution and (iv) 0.2 M Na2SO4 in water (pH ¼ 5.6). 3. Results 3.1. Characterization of the TiO2/CdS/CdSe photoanode The fabricated TiO2 nanotubes had an average inner diameter of 76 nm and average lengths of 5.7 mm and 11 mm, as determined from the SEM images (Fig. S1 in Supplementary material). The deposition of Cd, S and Se on the TiO2 nanotubes were confirmed by EDS where the Cd:S and Cd:Se atomic fraction were found to be very close to 1:1. Raman spectroscopy was performed to verify the formation of CdS and CdSe. The observed Raman peaks at 146 cm1 (Eg), 513 cm1 (A1g) and 637 cm1 (Eg) in Fig. 1a corresponds to three of the six allowed vibrational modes in anatase TiO2 [36]. The 207 cm1 and 407 cm1 peaks observed can be attributed to the longitudinal optical (LO) mode and its overtone, 2LO phonon modes for CdSe [37]. The remaining two peaks at 299 cm1 and 600 cm1 matched the characteristic CdS Raman peaks and were attributed to the 1LO and 2LO modes, respectively [38]. The normalized UV-VIS absorbance plot for the CdS and CdSe deposited (15 cycles each) onto the photoanode is shown in Fig. 1b. The extrapolation of the long wavelength edge revealed the absorption cutoff at 712.2 nm corresponding to the 1.74 eV bandgap of CdSe. Fig. 1c and d shows SEM images for the TiO2 surface and inner cross-section of the nanotube, respectively. The particle size distributions of SILAR deposited nanoparticles have found to be highly dispersed in our previous work [32]. Aside from isolated nanoparticles, there are larger clusters probably due to agglomeration, which were neglected to estimate the particle sizes for each SILAR
3
deposition cycles. In this paper, the deposited CdS/CdSe particles sizes (15 cycles each) were similarly well dispersed within the 7e16.5 nm range (Fig. S2 in the Supplementary material shows the size distribution) and an average particle size of about 10.4 nm could be estimated which corresponds well with the growth rates observed from our earlier study. The available TiO2 surface area, both at the mouth and within the tubes, was found to be partially covered with CdS/CdSe nanoparticles as shown in Fig. 1c and d. EDS measurements confirmed a homogeneous distribution of Cd, S and Se deposition on the TiO2 nanotube arrays (EDS mapping data are included in Supplementary material, Fig. S3eS6). 3.2. Photoelectrochemical response in Na2S/Na2SO3 electrolytes A majority of the studies on TiO2 nanotubes modified by CdS and/or CdSe depositions reported PEC performance using Na2S or Na2S/Na2SO3 electrolytes where the observed photocurrent densities varied in the range of 0.55e9.5 mAcm2 for TiO2/CdS, 0.6e27.5 mAcm2 for TiO2/CdSe and 4e14.9 mAcm2 for TiO2/CdS/ CdSe photoanode systems [14,15,17e26,29,39]. These large variations of the observed photocurrents could be specific to the deposition processes used in the above reports and may also depend on a number of factors such as the amount of semiconductor loading on the TiO2 nanotubes, quality of deposited material that might present a different behaviour with respect to interfacial and trap state recombination, different PEC test conditions, concentration of sacrificial reagents, etc. In this work, we chose an electrolyte consisting of 0.24 M Na2S and 0.35 M Na2SO3 (pH ¼ 13.1) to study sulfite oxidation in the TiO2/CdS/CdSe system following the report by Bühler et al. [40]. Photoanodes (nanotube length of 5.7 mm) with two different CdS to CdSe deposition ratios (1:1 and 1:2) were initially studied using the Na2S/Na2SO3 electrolyte. A 1:1 deposition ratio of CdS and CdSe was achieved by performing 15 cycle SILAR depositions of CdS followed by 15 cycle depositions of CdSe, labelled CdS15/CdS15 in this paper. Similarly, CdS10/CdSe20 would represent a 1:2 SILAR deposition ratio. The photocurrent densities for CdS10/CdSe20 and CdS15/CdSe15 modified electrodes were observed to be 5.5 mAcm2 (Fig. S7 in Supplementary material) and 7.4 mAcm2 (Fig. 2a) at 1.23 V vs. RHE, respectively. The onset potentials for both the photoanode configurations were found to be around 0.35 V vs. RHE. Clearly, the performance claimed in sulfite solutions is highly inflated and is not related to water splitting. Photocurrent transient were observed in the cyclic photoresponse at the onset of the illumination semi-cycle; such transients consisted in a fast current upshot followed by a decay, and were more pronounced at higher applied potentials. In fact, for lower applied potentials in the range of 0.3 V to þ0.3 V vs. RHE, these transients were either absent or showed a small peak intensity, but increased in intensity at higher applied potentials. Specifically, above 0.5 V the observed transients were accentuated which indicated enhanced electron recombination. We hypothesize that this recombination behaviour may be due to charge accumulation and recombination under conditions of higher applied bias, resulting in faster charge carrier migration. The presence of Na2S in the aqueous electrolytes introduces sulfide ions, [S2] and as a consequence sulfide oxidation becomes the dominant anodic process for charge transfer and photocurrent generation. The function of the reducing agent, Na2SO3, consists in the reintroduction of the [S2] in the electrolyte (Eqs. (1) and (2)) and the enhancement of the oxidation rate in order to sustain the hydrogen evolution reaction (HER) at the cathode (Eq. (3)), which would otherwise diminish over time due to disulfide, [S2 2 ] formation (Eq. (1)) [40]. Clearly this test is useful to determine the intrinsic kinetics of charge transfer at the interface and therefore the maximum possible rate of HER at the cathode, but does not
Please cite this article in press as: R. Ahmed, et al., Water splitting vs. sulfite oxidation: An assessment of photoelectrochemical performance of TiO2 nanotubes modified by CdS/CdSe nanoparticles, Electrochimica Acta (2017), https://doi.org/10.1016/j.electacta.2017.12.088
4
R. Ahmed et al. / Electrochimica Acta xxx (2017) 1e9
Fig. 1. (a) Raman peaks detected for TiO2 (anatase) and deposited CdS, CdSe using a 514 nm LASER. (b) Absorbance spectrum for the CdS15/CdSe15 deposited photoanodes. SEM images of CdS15/CdSe15 deposited on (c) the TiO2 nanotube surface and (d) inside of the nanotube columns.
Fig. 2. LSV plot under chopped illumination for photoelectrochemical cells employing TiO2 nanotube electrodes modified by CdS15/CdSe15 SILAR depositions using (a) Na2S/Na2SO3 and (b) NaOH aqueous electrolyte.
provide information on the rate of water oxidation (WO) that may occur at the photoanode, i.e. the reaction of interest. To summarize, the sulfide acts as a sacrificial species with fast kinetics generating electrons that flow at the cathode to enable HER (Eqs. (1)e(3)). Under these conditions, the higher photocurrent density observed in Fig. 2a arise from sulfite oxidation (Jph/SO) and does not reflect the performance of the photoanode system for water splitting.
Anodic reaction : 2S2 ¼ S2 2 þ 2e
(1)
2 2 Anodic reaction : 2S2 2 þ SO2 3 ¼ S 2 O3 þ S
(2)
Cathodic reaction : 2H2 O þ 2e ¼ H2 þ 2OH
(3)
Please cite this article in press as: R. Ahmed, et al., Water splitting vs. sulfite oxidation: An assessment of photoelectrochemical performance of TiO2 nanotubes modified by CdS/CdSe nanoparticles, Electrochimica Acta (2017), https://doi.org/10.1016/j.electacta.2017.12.088
R. Ahmed et al. / Electrochimica Acta xxx (2017) 1e9
3.3. Photoelectrochemical response in NaOH electrolytes Since a higher PEC performance was observed for the CdS15/ CdSe15 modified photoanode, this photoanode configuration was used to perform further PEC activity tests using a 1 M NaOH aqueous electrolyte (pH ¼ 13.3) where the PEC activity and photocurrent contribution would be due solely to water splitting (water oxidation at the anode and hydrogen evolution at the cathode). This electrolyte did not include any reagents that would allow the occurrence of spurious reactions. Fig. 2b shows the LSV plot for TiO2 photoanode (5.7 mm long) modified by CdS15/CdSe15. The photocurrent density due to water splitting, Jph/WS was observed to be 0.92 mAcm2 at 1.23 V vs. RHE which was 8-fold lower compared to the Jph/SO value of 7.4 mAcm2 at 1.23 V vs. RHE measured under similar conditions using the Na2S/Na2SO3 electrolyte. The large variation between our observed photocurrent densities using Na2S/Na2SO3 electrolyte and the NaOH aqueous electrolyte revealed a much slower oxidation rate for the water splitting compared to sulfite oxidation. The enhanced photocurrents using sacrificial reagents commonly reported in literature would thus lead to misleading conclusions about PEC activity of the TiO2/CdS/CdSe system when reporting water splitting efficiencies. The solar-to-hydrogen or STH efficiency (hSTH) is one of the most frequently reported parameters to benchmark the efficiency of water splitting devices; this parameter refers to the energy converted from the net hydrogen produced, divided by the irradiated energy from the full spectrum solar energy. This efficiency can be calculated using Eq. (4) where the parameters J, Vbias and Pin denote the output current for the cell, applied voltage bias and power density of input light, respectively [2]. A maximum hSTH of 6.55% at 0.244 V vs. RHE was calculated for the TiO2 photoanode modified by CdS15/CdSe15 using the Na2S/Na2SO3 electrolytes while the maximum hSTH using NaOH aqueous solution was determined to be 0.71% at 0.36 V vs. RHE. A comparison of the STH efficiencies for the TiO2 photoanodes modified with CdS15/CdSe15 using NaOH and Na2S/Na2SO3 electrolytes (Fig. 3a) at first sight will portray a more efficient hydrogen generation for the latter electrolyte. However, the Na2S/Na2SO3 solutions undergo sulfite oxidation and not water splitting; the use of STH efficiency in this particular case would be erroneous since the process is sustained only by a sacrificial species that should be replenished periodically. To accurately assess the water splitting ability, the ‘water splitting efficiency’ (4ws) is a much more meaningful parameter. 4ws is the ratio between the photocurrent generated from water splitting (Jph/WS) to that originated from sulfite oxidation (Jph/SO). The rationale for this ratio is as follows: the photocurrent generated by a PEC system could be modelled as a product of the maximum theoretical photocurrent
5
(Jmax), the efficiency of charge separation (4CS) and efficiency of the oxidation process (4sulfite for sulfite, 4ws water splitting) as shown in Eq. (5) [33]. The water splitting efficiency for the TiO2 nanotubes (5.7 mm long) modified by CdS15/CdSe15 deposition is shown in Fig. 3b and was found to be around 12.4% at an applied potential of 1.23 V vs. RHE.
hSTH ¼ Jph=WS Jph=SO IPCE ¼
Jð1:23 Vbias Þ Pin
¼
Jmax fCS fws ¼ fWS Jmax fCS fsulfite
Jph hc Jph ¼ 1240:8 e Pin l Pin l
IPCE ¼ he=h htransport hinterface ¼ he=h APCE
(4)
(5)
(6)
(7)
The incident photo-to-current conversion efficiency (IPCE) or external quantum efficiency (EQE) (Eqs. (6) and (7)) is an important diagnostic parameter that describes how well a photoresponsive system can convert light to electricity in the region of UV-VIS-IR wavelengths and this quantity can be experimentally determined using Eq. (6) [41]. This parameter depends on the efficiency of photon absorption (he/h) at the photoactive electrode, efficiency of charge transport towards the electrode/electrolyte interface (htransport) and the efficiency of interfacial charge transfer (hinterface) shown in Eq. (7) [42]. The product of the last two factors are known as the absorbed photon-to-current conversion efficiency (APCE) or the internal quantum efficiency (IQE) of the device, a term which highlights the fraction of charges that contribute to photocurrent generation after absorption occurs. The changes in the IPCE spectrum could be attributed to variations in either the absorption spectrum of the photoanode or the recombination of charges that limits the device photocurrent. The IPCE for the TiO2 electrode (5.7 mm long) modified by CdS15/CdSe15 depositions was measured at an applied potential of 1.23 V vs. RHE and is shown in Fig. 4. The spectrum shows a higher current conversion (~17e44%) in the 400e450 nm range where visible light absorption occurs from both the CdS and CdSe nanoparticles. Beyond the theoretical 512.7 nm (~2.42 eV bandgap of CdS), the photocurrents are produced solely from absorption in the outer CdSe layer. Thus, despite the fact that the current produced from light absorption beyond 510 nm (J>510), which is 58.1% of the total cell current calculated from the IPCE spectrum (JIPCE), the lower IPCE values (<17%) in the wavelength range beyond 510 nm is indicative of a lower APCE
Fig. 3. (a) STH efficiencies using NaOH and Na2S/Na2SO3 added electrolytes and (b) Water splitting efficiency for the TiO2 nanotubes modified by CdS15/CdSe15 depositions.
Please cite this article in press as: R. Ahmed, et al., Water splitting vs. sulfite oxidation: An assessment of photoelectrochemical performance of TiO2 nanotubes modified by CdS/CdSe nanoparticles, Electrochimica Acta (2017), https://doi.org/10.1016/j.electacta.2017.12.088
6
R. Ahmed et al. / Electrochimica Acta xxx (2017) 1e9
reported to dominate over the size quantization effect which caused the apparent lowering of the bandgap. 3.4. Evidence for CdS/CdSe oxidation and dissolution
Fig. 4. IPCE plot for the photoelectrochemical cells employing TiO2/CdS15/CdSe15 photoanodes.
arising from either a lower charge injection or charge collection efficiency for the cell (Eq. (7)). The photons beyond the 513 nm wavelength edge excite electron generation solely in the outer CdSe layer which most likely experiences a higher recombination at the CdS/CdSe, CdS/TiO2 interfaces and at the TiO2 trap sites during electron transport. The IPCE spectrum shows an extended tail beyond the 717 nm theoretical cutoff point of CdSe which has been observed by Rabinovich and Hodes for SILAR deposited nanoparticles [43]. With increase in the crystal size the ‘tail effect’ was
Higher SILAR cycle depositions of CdS30/CdSe30 on TiO2 nanotubes (5.7 mm long) did not improve the photoresponse (Jph ¼ 0.39 mAcm2 at 1.23 V vs. RHE) which is most likely due to optical loss and/or localized and interfacial recombination centres that affect electron transport negatively through a thicker nanoparticle layer (LSV plot included in Supporting material, Fig. S8). For a reverse order deposition of CdSe30/CdS30 a photocurrent density of 0.27 mAcm2 at 1.23 V vs. RHE is shown in Fig. 5a. The slightly reduced PEC performance of this photoanode (CdSe30/CdS30) compared to the CdS30/CdSe30 deposition order could be attributed to Fermi level alignment and favorable electron injection in the TiO2/CdS/CdSe system [22,44,45]. The interesting discrepancy to note in the LSV plot shown in Fig. 5a is the increase of current density between the voltages 0.25 V (vs. RHE) and þ0.15 V (vs. RHE). This increase was attributed to the photo dissolution of CdS and CdSe by the electrolyte. Previous studies on CdS and CdSe stability have reported formations of Cd2þ and elemental S and Se due to photocorrosion in aqueous electrolytes (Eqs. (8)e(10)) [46,47]. This was confirmed by observing the dark current density of the device as shown in Fig. 5b, where the peak current increased with prolonged exposure to the 1 M NaOH electrolyte.
CdX ðX ¼ S; SeÞ þ hy/e þ hþ
(8)
Fig. 5. LSV plot for PEC cell with TiO2 nanotubes (5.7 mm long) modified by CdSe30/CdS30 and using 1 M NaOH aqueous electrolyte under (a) simulated AM1.5G and (b) dark conditions.
Fig. 6. EDS mapping of TiO2 nanotube surface after exposure to 1 M NaOH aqueous electrolyte for over an hour shows Cd and O rich particle coverage along the surface.
Please cite this article in press as: R. Ahmed, et al., Water splitting vs. sulfite oxidation: An assessment of photoelectrochemical performance of TiO2 nanotubes modified by CdS/CdSe nanoparticles, Electrochimica Acta (2017), https://doi.org/10.1016/j.electacta.2017.12.088
R. Ahmed et al. / Electrochimica Acta xxx (2017) 1e9
2e þ 2H2 O/H2 þ 2OH CdX/Cd2þ þ X þ 2e
(9) (10)
A pronounced decrease of Se atomic fraction accompanied with lower Cd and S atomic fractions in the photoanodes after prolonged
Fig. 7. Possible reactions relating to the corrosion of the Cd-compounds.
7
exposure to the 1 M NaOH aqueous electrolyte was confirmed by EDS measurements indicating dissolution of these elements in the solution (Table S1 in Supporting material). SEM images in Fig. 6 show that the nanotube surface was covered by newly formed orthorhombic micron sized particles after one hour of exposure to the 1 M NaOH electrolyte. EDS mapping for these particles (inset in Fig. 6) show a high intensity for Cd and O, suggesting CdS (Se) oxidation, forming on the nanotubes either Cd oxides or hydroxides. The dark current response due to anodic dissolution in the LSV plots were observed between 0.25 V and 0.15 V vs. RHE, a region of potential where the STH efficiency might be overestimated. Since this dark current was proportional to the exposure time in the electrolyte leading to an enhanced pseudo-photocurrent, care should be taken to avoid such overestimations by determination of STH efficiency either by taking LSV measurements with minimum exposure to corrosion conditions or avoiding such potential regions. Fig. 7 shows the relative band alignments under alkaline conditions and possible reactions that could occur at the photoanode in the absence of sacrificial species, resulting in the corrosion of the Cd-compounds. The elemental Se and S formations reported in the literature most likely transform to H2SeO3 and partially to H2SO3, respectively resulting from oxidation at the photoanode which is consistent with the dissolution of Se and S from the photoanodes.
Fig. 8. LSV plots for Li incorporated TiO2 nanotubes (11 mm long) modified with CdS15/CdSe15 SILAR depositions using (a) 1 M NaOH aqueous electrolyte, (b) phosphate buffer solution and (c) 0.2 M Na2SO4 aqueous solution.
Please cite this article in press as: R. Ahmed, et al., Water splitting vs. sulfite oxidation: An assessment of photoelectrochemical performance of TiO2 nanotubes modified by CdS/CdSe nanoparticles, Electrochimica Acta (2017), https://doi.org/10.1016/j.electacta.2017.12.088
8
R. Ahmed et al. / Electrochimica Acta xxx (2017) 1e9
3.5. Photoelectrochemical performance after Li intercalation Li intercalation into the vacant octahedral sites of TiO2 nanotubes (length ¼ 5.7 mm) have been reported to boost PEC performance by passivation of the TiO2 trap states [33,48e50]. Due to trap state passivation, the photogenerated charges experience reduced recombination within the nanotube structure resulting in an unpinning of the Fermi level which is responsible for an increasing photocurrent trend at higher applied potentials. The photocurrent density for the 5.7 mm long Li:TiO2 nanotube modified by CdS15/ CdSe15 using NaOH electrolyte, was observed to be around 1 mAcm2 at 1.23 V vs. RHE which gradually increased to 1.21 mAcm2 at 2 V vs. RHE (Fig. S9 in Supporting material). The onset potential for both the doped and undoped electrodes were 0.36 V vs. RHE which is similar to the 0.35 V observed in the case of sulfide oxidation using Na2S/Na2SO3 electrolyte. The absence of photocurrent spikes at higher applied potentials due to cyclic illumination in the LSV plot is consistent with the trap state passivation and decrease in defect density of TiO2, as discussed in our previously published reports.
OER under alkaline condition : 4OH ¼ O2 þ 2H2 O þ 4e ; Eox ¼ þ0:401 V vs: NHE (11) OER under acidic condition : 2H2 O ¼ O2 þ 4Hþ þ 4e ; Eox ¼ þ1:299 V vs: NHE
from sulfite oxidation and water splitting, respectively. The water splitting efficiency parameter which compares photocurrent generation due to water splitting to that from sulfite oxidation is suitable for performance evaluation of such PEC devices. The STH efficiency parameter, specific to water splitting could not be compared to the commonly reported sulfide based electrolytes that utilize sulfite oxidation for current generation in PEC devices. The IPCE spectrum of the CdS/CdSe modified nanotubes indicated utilization of a broader range of the incident solar spectra up to around 709 nm (~1.75 eV), which correspond closely to the theoretical cutoff wavelength of CdSe. The IPCE spectrum also helped to identify the 400e513 nm wavelength region, where CdS and CdSe actively absorb radiation, to result in a better photon to current conversion compared to the wavelength region beyond 513 nm where active absorption takes place only from CdSe. A slightly improved PEC performance was observed using aqueous acidic electrolytes compared to aqueous alkaline electrolytes. The deposited chalcogenide particles were found to be unstable in the aqueous solutions due to oxidation or anodic dissolution. The absence of hole quenching agents in aqueous alkaline solution accelerates the corrosion of the nanoparticles, leading to the anodic dissolution that limits the scope of these SILAR deposited CdS and CdSe nanoparticles towards splitting water. This study also sheds light on the fact that a partial coverage of the TiO2 nanotube surface can be effective to produce high cell photocurrents. Implementation of such strategies to boost photocurrent could be promising if material stability issues can be overcome, perhaps by the use of very thin, conformal layers of stable materials.
(12) Fig. 8aec shows the LSV plots for 11 mm long Li:TiO2 nanotubes coated with CdS15/CdSe15 using (i) 1 M NaOH solution, (ii) phosphate buffer solution and (iii) 0.2 M Na2SO4 aqueous solution, respectively. The recorded photocurrent densities at 1.23 V vs. RHE for the 11 mm long nanotube electrodes in 1 M NaOH (pH ¼ 13.3), phosphate buffer (pH ¼ 7.4) and aqueous Na2SO4 aqueous (pH ¼ 5.6) solutions were 0.58 mAcm2, 0.66 mAcm2 and 0.91 mAcm2, respectively. The OERs under alkaline and acidic conditions are shown in Eqs. (11) and (12), respectively. The use of longer Li:TiO2 nanotubes (11 mm) did not yield a boost in photocurrent for the cells compared to the 5.7 mm long Li:TiO2 nanotubes. However, comparing PEC performance for the 11 mm long nanotubes using electrolytes with varying pH values from 13.3 to 5.6 showed a better PEC performance using a more acidic electrolyte. Reports of PEC performance of CdS, CdSe and/or CdS/CdSe modified TiO2 nanotubes relating to water splitting are few and to the best of our knowledge this trend of improved photocurrent response in more acidic solution has not been reported for the TiO2/CdS/CdSe system nor, to our knowledge for any other PEC system [25,27]. The increase in photocurrent is accompanied by a more negative shift of onset potentials for the LSV plots from 0.35 V vs. RHE (NaOH) to 0.58 V vs. RHE (phosphate buffer) and finally to 0.69 V vs. RHE for Na2SO4 electrolyte, confirms improved catalytic activity at lower pH or the occurrence of complex reaction paths. It is also noteworthy that the onset potential for the TiO2/CdS15/CdSe15 photoanode using an Na2S/Na2SO3 electrolyte (pH ¼ 13.1) was observed to be around 0.35 V vs. RHE which remained constant in 1 M NaOH solution (pH ¼ 13.3). Overall, the observed behaviour is not yet understood and will be focus of our next efforts. 4. Conclusion Our study of SILAR deposited CdS/CdSe nanoparticles on TiO2 nanotubes using Na2S/Na2SO3 and NaOH aqueous solutions has clearly distinguished the difference in the PEC performances arising
Acknowledgement We acknowledge financial support from the University of Virginia through the MAXNET Energy Partnership. Appendix A. Supplementary data Supplementary data related to this chapter can be found at https://doi.org/10.1016/j.electacta.2017.12.088. References [1] J. Ager, M. Shaner, K. Walczak, I. Sharp, S. Ardo, Experimental demonstrations of spontaneous, solar-driven photoelectrochemical water splitting, Energy Environ. Sci. 8 (2015) 2811e2824. [2] M. Walter, E. Warren, J. McKone, S. Boettcher, Q. Mi, E. Santori, N. Lewis, Solar water splitting cells, Chem. Rev. 110 (2010) 6446e6473. [3] Y. Li, D. Gao, S. Peng, G. Lu, S. Li, Photocatalytic hydrogen evolution over Pt/ Cd0.5Zn0.5S from saltwater using glucose as electron donor: an investigation of the influence of electrolyte NaCl, Int. J. Hydrogen Energy 36 (2011) 4291e4297. [4] Y. Li, G. Lu, S. Li, Photocatalytic hydrogen generation and decomposition of oxalic acid over platinized TiO2, App. Catal. A Gene 214 (2001) 179e185. [5] Y. Li, Y. Xie, S. Peng, G. Lu, S. Li, Photocatalytic hydrogen generation in the presence of chloroacetic acids over Pt/TiO2, Chemosphere 63 (2006) 1312e1318. [6] J. Schneider, D. Bahnemann, Undesired role of sacrificial reagents in photocatalysis, J. Phys. Chem. Lett. 4 (2013) 3479e3483. [7] D. Scanlon, C. Dunnill, J. Buckeridge, S. Shevlin, A. Logsdail, S. Woodley, Catlow, M. Powell, R. Palgrave, I. Parkin, G. Watson, T. Keal, P. Sherwood, A. Walsh, A. Sokol, Band alignment of rutile and anatase TiO2, Nat. Mater. 12 (2013) 798e801. [8] L.-K. Tsui, T. Homma, G. Zangari, Photocurrent conversion in anodized TiO2 nanotube arrays: effect of the water content in anodizing solutions, J. Phys. Chem. C 117 (2013) 6979e6989. [9] R. Noufi, P. Kohl, S. Frank, A. Bard, Semiconductor electrodes, J. Electrochem. Soc. 125 (1978) 246e252. [10] L.-K. Tsui, G. Zangari, The influence of morphology of electrodeposited Cu2O and Fe2O3 on the conversion efficiency of TiO2 nanotube photoelectrochemical solar cells, Electrochim. Acta 100 (2013) 220e225. [11] L.-K. Tsui, G. Zangari, Modification of TiO2 nanotubes by Cu2O for photoelectrochemical, photocatalytic, and photovoltaic devices, Electrochim. Acta 128 (2014) 341e348.
Please cite this article in press as: R. Ahmed, et al., Water splitting vs. sulfite oxidation: An assessment of photoelectrochemical performance of TiO2 nanotubes modified by CdS/CdSe nanoparticles, Electrochimica Acta (2017), https://doi.org/10.1016/j.electacta.2017.12.088
R. Ahmed et al. / Electrochimica Acta xxx (2017) 1e9 [12] L.-K. Tsui, J. Huang, M. Sabat, G. Zangari, Visible light sensitization of TiO2 nanotubes by Bacteriochlorophyll-C dyes for photoelectrochemical solar cells, ACS Sustain. Chem. Eng. 2 (2014) 2097e2101. [13] B. Streetman, S. Banerjee, Solid State Electronic Devices, Prentice Hall, 2005. [14] W.-T. Sun, Y. Yu, H.-Y. Pan, X.-F. Gao, Q. Chen, L.-M. Peng, CdS quantum dots sensitized TiO2 nanotube-array photoelectrodes, J. Am. Chem. Soc. 130 (2008) 1124e1125. [15] Y.-L. Lee, C.-F. Chi, S.-Y. Liau, CdS/CdSe Co-Sensitized TiO2 photoelectrode for efficient hydrogen generation in a photoelectrochemical cell, Chem. Mater. 22 (2010) 922e927. [16] B. Mukherjee, Y. Smith, V. Subramanian, CdSe nanocrystal assemblies on anodized TiO2 nanotubes: optical, surface, and photoelectrochemical properties, J. Phys. Chem. C 116 (2012) 15175e15184. [17] S. Chen, M. Paulose, C. Ruan, G. Mor, O. Varghese, D. Kouzoudis, C. Grimes, Electrochemically synthesized CdS nanoparticle-modified TiO2 nanotubearray photoelectrodes: preparation, characterization, and application to photoelectrochemical cells, J. Photochem. Photobiol. Chem. 177 (2006) 177e184. [18] Y. Yin, Z. Jin, F. Hou, Enhanced solar water-splitting efficiency using core/ sheath heterostructure CdS/TiO2 nanotube arrays, Nanotechnology 18 (2007). [19] L. Yang, S. Luo, R. Liu, Q. Cai, Y. Xiao, S. Liu, F. Su, L. Wen, Fabrication of CdSe nanoparticles sensitized long TiO2 nanotube arrays for photocatalytic degradation of anthracene-9-carbonxylic acid under green monochromatic light, J. Phys. Chem. C 114 (2010) 4783e4789. [20] H. Zhang, X. Quan, S. Chen, H. Yu, N. Ma, “Mulberry-like” CdSe nanoclusters anchored on TiO2 nanotube arrays: a novel architecture with remarkable photoelectrochemical performance, Chem. Mater. 21 (2009) 3090e3095. [21] Z. Shao, W. Zhu, Z. Li, Q. Yang, G. Wang, One-step fabrication of CdS nanoparticle-sensitized TiO2 nanotube arrays via electrodeposition, J. Phys. Chem. C 116 (2012) 2438e2442. [22] X.-F. Gao, W.-T. Sun, G. Ai, L.-M. Peng, Photoelectric performance of TiO2 nanotube array photoelectrodes cosensitized with CdS/CdSe quantum dots, Appl. Phys. Lett. 96 (2010), 153104-153101-153104-153103. [23] S. Cheng, W. Fu, H. Yang, L. Zhang, J. Ma, H. Zhao, M. Sun, L. Yang, Photoelectrochemical performance of multiple semiconductors (CdS/CdSe/ZnS) cosensitized TiO2 photoelectrodes, J. Phys. Chem. C 116 (2011) 2615e2621. [24] L. Liu, J. Hensel, R. Fitzmorris, Y. Li, J. Zhang, Preparation and photoelectrochemical properties of CdSe/TiO2 hybrid mesoporous structures, J. Phys. Chem. Lett. 1 (2010) 155e160. [25] K. Xie, Z. Wu, M. Wang, J. Yu, C. Gong, L. Sun, C. Lin, Room temperature synthesis of CdS nanoparticle-decorated TiO2 nanotube arrays by electrodeposition with improved visible-light photoelectrochemical properties, Electrochem. Commun. 63 (2016). [26] Y. Xie, G. Ali, S. Yoo, S. Cho, Sonication-assisted synthesis of CdS quantum-dotsensitized TiO2 nanotube arrays with enhanced photoelectrochemical and photocatalytic activity, ACS Appl. Mater. Inter. 2 (2010) 2910e2914. [27] C.-J. Lin, Y.-T. Lu, C.-H. Hsieh, S.-H. Chiena, Surface modification of highly ordered TiO2 nanotube arrays for efficient photoelectrocatalytic water splitting, Appl. Phys. Lett. 94 (2009), 113102-113101-113102-113103. [28] J. Fernandes, S. Khan, F. Baum, E. Kohlrausch, J. Lucena dos Santos, D. Baptista, S. Teixeira, J. Dupont, M. Santos, Synergizing nanocomposites of CdSe/TiO2 nanotubes for improved photoelectrochemical activity via thermal treatment, Dalton Trans. 45 (2016) 9925e9931. [29] S. Banerjee, S. Mohapatra, P. Das, M. Misra, Synthesis of coupled semiconductor by filling 1D TiO2 nanotubes with CdS, Chem. Mater. 20 (2008) 6784e6791. [30] S. Rühle, M. Shalom, A. Zaban, Quantum-dot-sensitized solar cells, ChemPhysChem 11 (2010) 2290e2304.
9
[31] H. Lee, H. Leventis, S.-J. Moon, P. Chen, S. Ito, S. Haque, T. Torres, F. Nüesch, €tzel, M. Nazeeruddin, PBS and CdS quantum T. Geiger, S. Zakeeruddin, M. Gra dot-sensitized solid-state solar cells: “old concepts, new results”, Adv. Funct. Mater. 19 (2009) 2735e2742. [32] R. Ahmed, G. Will, J. Bell, H. Wang, Size-dependent photodegradation of CdS particles deposited onto TiO2 mesoporous films by SILAR method, J. Nanoparticle Res. 14 (2012). [33] L.-K. Tsui, Y. Xu, D. Dawidowski, D. Cafiso, G. Zangari, Efficient water oxidation kinetics and enhanced electron transport in Li-doped TiO2 nanotube photoanodes, J. Mater. Chem. 4 (2016) 19070e19077. [34] L.-K. Tsui, G. Zangari, Titania nanotubes by electrochemical anodization for solar energy conversion, J. Electrochem. Soc. 161 (2014) D3066eD3077. [35] R. Ahmed, L. Zhao, A. Mozer, G. Will, J. Bell, H. Wang, Enhanced electron lifetime of CdSe/CdS quantum dot (QD) sensitized solar cells using ZnSe coreeshell structure with efficient regeneration of quantum dots, J. Phys. Chem. C 119 (2015) 2297e2307. [36] H. Choi, Y. Jung, S. Kim, Size effects in the Raman spectra of TiO2 nanoparticles, Vib. Spectrosc. 37 (2005) 33e38. [37] A.V. Baranov, Y.P. Rakovich, J.F. Donegan, T.S. Perova, R.A. Moore, D.V. Talapin, A.L. Rogach, Y. Masumoto, I. Nabiev, Effect of ZnS shell thickness on the phonon spectra in CdSe quantum dots, Phys. Rev. B 68 (2003), 165306165301-165306-165307. [38] M. Abdulkhadar, B. Thomas, Study of raman spectra of nanoparticles of CdS and ZnS, Nanostruct. Mater. 5 (1995) 289e298. [39] J.-H. Yum, B.E. Hardin, S.-J. Moon, E. Baranoff, F. Nüesch, M.D. McGehee, €tzel, M.K. Nazeeruddin, Panchromatic response in solid-state dyeM. Gra sensitized solar cells containing phosphorescent energy relay dyes, Angew. Chem. Int. Ed. 48 (2009) 9277e9280. [40] N. Buehler, K. Meier, J. Reber, Photochemical hydrogen production with cadmium sulfide suspensions, J. Phys. Chem. 88 (1984) 3261e3268. [41] Z. Chen, H. Dinh, E. Miller, Photoelectrochemical Water Splitting, Springer, New York, 2013. [42] Z. Chen, T. Jaramillo, T. Deutsch, A. Kleiman-Shwarsctein, A. Forman, N. Gaillard, R. Garland, K. Takanabe, C. Heske, M. Sunkara, E. McFarland, K. Domen, E. Miller, J. Turner, H. Dinh, Accelerating materials development for photoelectrochemical hydrogen production: standards for methods, definitions, and reporting protocols, J. Mater. Res. 25 (2010) 3e16. [43] E. Rabinovich, G. Hodes, Effective bandgap lowering of CdS deposited by successive ionic layer adsorption and reaction, J. Phys. Chem. C 117 (2013) 1611e1620. [44] K.-H. Lin, C.-Y. Chuang, Y.-Y. Lee, F.-C. Li, Y.-M. Chang, I.P. Liu, S.-C. Chou, Y.L. Lee, Charge Transfer in the heterointerfaces of CdS/CdSe cosensitized TiO2 photoelectrode, J. Phys. Chem. C 116 (2012) 1550e1555. [45] X.-Y. Yu, J.-Y. Liao, K.-Q. Qiu, D.-B. Kuang, C.-Y. Su, Dynamic study of highly efficient CdS/CdSe quantum dot-sensitized solar cells fabricated by electrodeposition, ACS Nano 5 (2011) 9494e9500. [46] D.J. Fermín, E.A. Ponomarev, L.M. Peter, A kinetic study of CdS photocorrosion by intensity modulated photocurrent and photoelectrochemical impedance spectroscopy, J. Electroanal. Chem. 473 (1999) 192e203. [47] L. Xi, J. Lek, Y. Liang, C. Boothroyd, W. Zhou, Q. Yan, X. Hu, Stability studies of CdSe nanocrystals in an aqueous environment, Nanotechnology 22 (2011). [48] B. Zachau-Christiansen, K. West, T. Jacobsen, S. Atlung, Lithium insertion in different TiO2 modifications, Solid State Ionics 30 (1988) 1176e1182. [49] R. van de Krol, A. Goossens, J. Schoonman, Spatial extent of lithium intercalation in anatase TiO2, J. Phys. Chem. B 103 (1999) 7151e7159. [50] L.-K. Tsui, M. Saito, T. Homma, G. Zangari, Trap-state passivation of titania nanotubes by electrochemical doping for enhanced photoelectrochemical performance, J. Mater. Chem. 3 (2015) 360e367.
Please cite this article in press as: R. Ahmed, et al., Water splitting vs. sulfite oxidation: An assessment of photoelectrochemical performance of TiO2 nanotubes modified by CdS/CdSe nanoparticles, Electrochimica Acta (2017), https://doi.org/10.1016/j.electacta.2017.12.088
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Water splitting vs. sulfite oxidation: An assessment of photoelectrochemical performance of TiO2 nanotubes modified by CdS/CdSe nanoparticles Rasin Ahmed, Yin Xu, Giovanni Zangari* Department of Materials Science and Engineering, University of Virginia, Charlottesville, VA 22904, USA
a r t i c l e i n f o
a b s t r a c t
Article history: Received 6 November 2017 Received in revised form 29 November 2017 Accepted 11 December 2017 Available online xxx
A major portion of the existing literature on TiO2 nanotubes modified by CdS and CdSe report an enhanced photoelectrochemical (PEC) performance using electrolytes with inorganic sacrificial electron donors where a dominant sulfite oxidation occurs. In this study, we distinguish the PEC performance arising entirely from water splitting to that originated by sulfite oxidation in the TiO2 nanotube system modified by CdS/CdSe nanoparticle depositions. The photocurrent density measured under the AM1.5G simulated spectra, 1 sun (100 mWcm2) intensity for the TiO2 nanotubes modified by CdS/CdSe nanoparticles and using an aqueous alkaline electrolyte (1 M NaOH) was observed to be 0.92 mAcm2 at 1.23 V vs. RHE which was 8-fold lower than in the case for sulfite oxidation (Jph/SO ¼ 7.4 mAcm2 at 1.23 V vs. RHE) using an Na2S/Na2SO3 aqueous electrolyte. A maximum STH efficiency of 0.71% and water splitting efficiency of 12.4% was determined for water oxidation using an aqueous alkaline electrolyte. To rationalize such device efficiency, we propose that the ‘water splitting efficiency’ parameter is much more meaningful since it correlates the PEC activity specifically to water splitting and not to other pathways that produce an artificially enhanced photocurrent through the use of sacrificial reagents. The incident photon-to-current conversion efficiency spectrum measured at 1.23 V (vs. RHE) bias for the TiO2 photoanodes modified by CdS/CdSe nanoparticles revealed that the current conversion efficiency is lower (~17% or less) when absorption occurs solely from the external CdSe layers indicating higher recombination during charge transport. The increase of dark current observed in the linear sweep voltammetry plots for the voltage range of 0.25 V to þ0.15 V vs. RHE was attributed to anodic dissolution of the photoanodes in aqueous electrolytes containing no sacrificial reagents. The photocurrents and onset potentials for the CdS/CdSe modified TiO2 photoanodes improved under acidic conditions showing a slightly increased PEC activity and faster reaction kinetics. © 2017 Elsevier Ltd. All rights reserved.
Keywords: Water splitting Sulfite oxidation Photoelectrochemical CdS CdSe TiO2 SILAR Solar-to-hydrogen (STH) efficiency
1. Introduction Hydrogen production from solar energy driven water splitting is an emerging technology which enables storage of the abundant incident sunlight in the form of hydrogen as an energy vector, via direct conversion of the radiant energy into chemical energy, through a carbon neutral pathway. The stored hydrogen could be successively distributed and used on site to drive a stationary or mobile fuel cell, as well as to power industrial sites [1]. In essence, solar water splitting converts electromagnetic energy from the
* Corresponding author. E-mail address: [email protected] (G. Zangari).
incident solar spectrum by utilizing a photoelectrochemical (PEC) cell where one or both of the electrodes are semiconducting and capable to absorb photons to generate electron-hole pairs (EHPs) that are injected at the electrode/electrolyte interface to perform electrochemical reactions, i.e. producing hydrogen at the cathode and oxygen at the anode. The research efforts focused in the area of photoelectrochemical water splitting relying on renewable resources are aimed at developing a system that utilizes only water and sunlight, without the use of sacrificial species [2]. The realization of a commercially viable PEC water splitting system has been held back due to the lack of suitable materials that could sustain long term water oxidation at the photosensitive anode and produce around 1.7e2.4 eV of energy per EHP necessary to drive the water splitting reaction. The difference between the
https://doi.org/10.1016/j.electacta.2017.12.088 0013-4686/© 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: R. Ahmed, et al., Water splitting vs. sulfite oxidation: An assessment of photoelectrochemical performance of TiO2 nanotubes modified by CdS/CdSe nanoparticles, Electrochimica Acta (2017), https://doi.org/10.1016/j.electacta.2017.12.088
2
R. Ahmed et al. / Electrochimica Acta xxx (2017) 1e9
figure above and the thermodynamic water splitting energy (1.23 eV per formula unit) is necessary to provide an overvoltage sufficient to run the reaction at a rate of around 10 mAcm2. The material properties that are commonly taken into consideration to design a photosensitive device are extinction coefficients, bandgaps and correct band alignments of the photoactive materials as well as the transport properties of the photogenerated charges. These factors determine the light harvesting efficiency, spectral response and charge injection efficiency of the cells, respectively, all of which are of great importance to produce a high photocurrent required for the fast electrolysis of water. Aside from charge generation and efficient transport of the separated charges, the chemical pathways for redox reactions at the electrolyte/electrode interface can also play a crucial role in achieving higher cell photocurrents under simulated or natural radiation. The use of sacrificial reagents which act as electron donors has been known to enhance the photocurrents, while in some cases the actual photocurrents arising from water oxidation remain unreported [3e6]. These data are essential for designing PEC systems that can run on water and sunlight only. An investigation of the PEC activity of a photoanode or photocathode system using electrolytes with and without sacrificial agents, as well as under different electrolyte pH, would shed light on the maximum conversion efficiency achievable for different chemical pathways, PEC performance trends and stability limits for the semiconductors under study. TiO2 is one of the best studied and commonly used photoanode materials, one of the few that exhibit long term stability under operation. TiO2 may exhibit three distinct phases, of which the most common are anatase and rutile [7,8]. Anatase TiO2 has a conduction band close to the hydrogen reduction potential (Eþ H/ H2 ¼ 0 V vs. NHE) and a much lower valence band level compared to the water oxidation potential (EH2O/O2 ¼ 1.23 V vs. NHE), allowing a variety of oxidation processes to occur [9]. However, its wide bandgap of 3.2 eV limits light absorption within the UV wavelengths (100e400 nm) which comprise less than 5% of the incident solar spectrum. While we attempted to widen the absorption spectrum of TiO2 nanotubes by modification with Cu2O, Fe2O3 or bacteriochlorophyll-C, sensitization with Group IIeVI compounds appears to be more effective [10e12]. In fact, CdS and CdSe, which have direct bandgaps of 2.42 eV and 1.73 eV, respectively, will result in significant broadening of the spectral response of the photoanode well into the visible wavelength region of 400e770 nm [13]. The TiO2 nanotube array modified by CdS and CdSe have been reported by a number of groups following a variety of deposition techniques such as electrodeposition, chemical bath deposition (CBD), sequential CBD, successive ionic layer adsorption and reaction (SILAR), magnetron sputtering, emulsion-based bottom-up self-assembly (EBS) or solvothermal process [14e29]. Amongst the solution-processed techniques, CBD and SILAR are known for their ability to achieve direct deposition of nanoparticles at the electrode surface [30]. Previous studies on SILAR aimed at nanoparticle depositions on porous TiO2 films (~20 nm particles) have reported to cover around 20% of the surface at saturation using 7 deposition cycles of CdS quantum dots and around 30e40% of surface using 6 deposition cycles of PbS quantum dots [31,32]. Thus, the TiO2 nanotubes modified by SILAR deposited CdS and CdSe would benefit from a partial surface coverage which will function as efficient light absorbers in the visible region of the solar spectrum and boost photocurrents owing to a shorter path (approximately half the nanotube thickness) for the photogenerated carriers to be injected in the electrolyte. In this paper, we study a well-known photoanode system such as TiO2 nanotubes, modified by direct deposition of CdS and CdSe nanoparticles using the SILAR technique and focus on the difference observed between PEC activities specific to water splitting and
sulfite oxidation. These figures can in fact vary widely, with sulfite oxidation at the anode enabling a much larger hydrogen current which is not warranted in a sustainable process. We further discuss the efficiency parameters that best describes the energy conversion pathway for water splitting systems, including the stability of CdS/ CdSe nanoparticles in aqueous electrolyte and report a trend of PEC performance improvement for the TiO2/CdS/CdSe photoanodes using a slightly acidic aqueous electrolyte. 2. Experimental section 2.1. Preparation of TiO2 nanotubes Anatase TiO2 nanotubes were prepared on Ti foils using a double anodization method [33,34]. Briefly, the foils were initially cleaned by sonicating for 15 min each in acetone, isopropanol and methanol solutions. A KEPCO BOP-100 power supply unit controlled with LabVIEW software was operated at 50 V to anodize the Ti foils in a 0.3 wt % NH4F (Sigma Aldrich, 99.99% þ trace metal basis) in ethylene glycol (Sigma Aldrich, 99.8% anhydrous) and 2 vol % water solution for one hour. The TiO2 nanotubes grown in the first anodization process were removed by using an adhesive tape and sonicated in ethanol for 10 min to detach excess adhesives from the peeled off nanotube surface. A second anodization was carried out using the same solution and voltage for 30 or 60 min to prepare 5.7 mm and 11 mm long nanotubes, respectively. The prepared samples were left overnight in deionized water to remove excess fluoride ions and later annealed in air at 350 C for 3 h to crystallize TiO2 in the anatase phase. In the final stage of preparation, just prior to SILAR depositions, some of the TiO2 nanotubes underwent electrochemical Li intercalation by applying a potential of 1.55 V vs. SCE for 3 s in 1 M LiClO4 (Sigma Aldrich, ACS Reagent) solution. 2.2. SILAR deposition of CdS and CdSe In order to modify the TiO2 or Li:TiO2 electrodes with CdS and CdSe, two different Cd, S and Cd, Se-precursor solutions pairs were prepared [35]. A 0.02 M Cd(NO3)2$4H2O (Sigma Aldrich, 99.0%) in methanol and 0.02 M Na2S$9H2O (Alfa Aesar, 98.0%) in methanol/ water (1:1 v/v) were used as Cd and S precursor solutions, respectively for the CdS depositions. The Ti foils were masked using single sided Kapton polyimide tapes (Ted Pella Inc.) before deposition so that the CdS could be deposited only onto the TiO2 nanotubes. These masked electrodes were dipped for one minute in the 0.02 M Cd2þ precursor solution followed by one minute of rinsing in a separate methanol solution and one minute of drying in Ar gas flow. The electrodes were then dipped for one minute in the 0.02 M S2 precursor solution for the sulfide ions to grow onto the adsorbed cadmium ions and form CdS nanoparticles. This completed one cycle of SILAR deposition of CdS. By repeating the number of SILAR deposition cycles, the SILAR deposited particles grow in size. A second solution pair of 0.03 M Cd(NO3)2$4H2O in ethanol (abs) and 0.03 M SeO2 (Alfa Aesar, 99.4%, metals basis), 0.06 M NaBH4 (Alfa Aesar, 98.0%) in ethanol (abs) were used as the Cd and Se precursor solutions, respectively for the CdSe depositions. The 0.03 M Se2 precursor solution was prepared in a three neck flask reaction chamber kept under constant Ar gas flow to prevent oxidation of the selenium products. The electrodes were dipped for 30 s alternatively in the Cd and Se precursor solutions followed by one minute of rinsing and drying for the deposition of CdSe nanoparticles on the TiO2 nanotubes. 2.3. Material and device characterizations Chemical composition of the materials was assessed with an
Please cite this article in press as: R. Ahmed, et al., Water splitting vs. sulfite oxidation: An assessment of photoelectrochemical performance of TiO2 nanotubes modified by CdS/CdSe nanoparticles, Electrochimica Acta (2017), https://doi.org/10.1016/j.electacta.2017.12.088
R. Ahmed et al. / Electrochimica Acta xxx (2017) 1e9
Energy Dispersive Spectroscopy (EDS) instrument attached to an FEI Quanta 650 Scanning Electron Microscope (SEM). Besides the FEI Quanta 650, additional SEM images for the CdS/CdSe deposited on TiO2 nanotube surfaces were acquired using a Zeiss Crossbeam 340. A Cary Varian 5E UV-VIS-NIR spectrophotometer was used to record the absorbance spectra for the CdS and CdSe modified photoanodes using the double beam, VW technique. The calibration was performed by using two movable mirrors to record the maximum and minimum baselines in the V and W positions, respectively. The reflectance spectrum of our sample was measured with respect to a low reflectance reference by positioning the Wbeam setup in the reference holder. The Raman spectra for the deposited samples were obtained using a 514 nm LASER as the excitation source coupled with a Raman Spectrometer (Renishaw). The photoelectrochemical performance for the CdS/CdSe-modified working electrodes were measured using a three electrode system consisting of a Pt-mesh counterelectrode and standard calomel electrode (SCE) as reference connected to a potentiostat (Biologic SP-150). All the electrode potentials were referred to the RHE electrode. A 150 W Xenon lamp coupled to an Oriel (Sol1A) class ABB solar simulator was used to produce an AM1.5G spectrum. Linear sweep voltammetry (LSV) plots were recorded under chopped illumination conditions for the electrodes. The incident photon-to-current conversion efficiency (IPCE) for the cell was measured using a monochromator (Princeton Instruments, Acton SP2150) coupled to a 250 W QTH lamp powered by a Princeton Instrument TS-428 PSU. The monochromator output light power was calibrated using a standard Si photodetector (Thorlabs S120B) in the 400e1000 nm wavelength range. The photocurrents for the IPCE test were measured using the Biologic SP-150 potentiostat at an applied potential of 1.23 V vs. RHE. The electrolyte solutions used in this study were (i) 0.35 M Na2SO3, 0.24 M Na2S$9H2O in water (pH ¼ 13.1), (ii) 1 M NaOH in water (pH ¼ 13.3), (iii) 0.08 M K2HPO4, 0.02 M KH2PO4 in water (pH ¼ 7.4), also called the phosphate buffer solution and (iv) 0.2 M Na2SO4 in water (pH ¼ 5.6). 3. Results 3.1. Characterization of the TiO2/CdS/CdSe photoanode The fabricated TiO2 nanotubes had an average inner diameter of 76 nm and average lengths of 5.7 mm and 11 mm, as determined from the SEM images (Fig. S1 in Supplementary material). The deposition of Cd, S and Se on the TiO2 nanotubes were confirmed by EDS where the Cd:S and Cd:Se atomic fraction were found to be very close to 1:1. Raman spectroscopy was performed to verify the formation of CdS and CdSe. The observed Raman peaks at 146 cm1 (Eg), 513 cm1 (A1g) and 637 cm1 (Eg) in Fig. 1a corresponds to three of the six allowed vibrational modes in anatase TiO2 [36]. The 207 cm1 and 407 cm1 peaks observed can be attributed to the longitudinal optical (LO) mode and its overtone, 2LO phonon modes for CdSe [37]. The remaining two peaks at 299 cm1 and 600 cm1 matched the characteristic CdS Raman peaks and were attributed to the 1LO and 2LO modes, respectively [38]. The normalized UV-VIS absorbance plot for the CdS and CdSe deposited (15 cycles each) onto the photoanode is shown in Fig. 1b. The extrapolation of the long wavelength edge revealed the absorption cutoff at 712.2 nm corresponding to the 1.74 eV bandgap of CdSe. Fig. 1c and d shows SEM images for the TiO2 surface and inner cross-section of the nanotube, respectively. The particle size distributions of SILAR deposited nanoparticles have found to be highly dispersed in our previous work [32]. Aside from isolated nanoparticles, there are larger clusters probably due to agglomeration, which were neglected to estimate the particle sizes for each SILAR
3
deposition cycles. In this paper, the deposited CdS/CdSe particles sizes (15 cycles each) were similarly well dispersed within the 7e16.5 nm range (Fig. S2 in the Supplementary material shows the size distribution) and an average particle size of about 10.4 nm could be estimated which corresponds well with the growth rates observed from our earlier study. The available TiO2 surface area, both at the mouth and within the tubes, was found to be partially covered with CdS/CdSe nanoparticles as shown in Fig. 1c and d. EDS measurements confirmed a homogeneous distribution of Cd, S and Se deposition on the TiO2 nanotube arrays (EDS mapping data are included in Supplementary material, Fig. S3eS6). 3.2. Photoelectrochemical response in Na2S/Na2SO3 electrolytes A majority of the studies on TiO2 nanotubes modified by CdS and/or CdSe depositions reported PEC performance using Na2S or Na2S/Na2SO3 electrolytes where the observed photocurrent densities varied in the range of 0.55e9.5 mAcm2 for TiO2/CdS, 0.6e27.5 mAcm2 for TiO2/CdSe and 4e14.9 mAcm2 for TiO2/CdS/ CdSe photoanode systems [14,15,17e26,29,39]. These large variations of the observed photocurrents could be specific to the deposition processes used in the above reports and may also depend on a number of factors such as the amount of semiconductor loading on the TiO2 nanotubes, quality of deposited material that might present a different behaviour with respect to interfacial and trap state recombination, different PEC test conditions, concentration of sacrificial reagents, etc. In this work, we chose an electrolyte consisting of 0.24 M Na2S and 0.35 M Na2SO3 (pH ¼ 13.1) to study sulfite oxidation in the TiO2/CdS/CdSe system following the report by Bühler et al. [40]. Photoanodes (nanotube length of 5.7 mm) with two different CdS to CdSe deposition ratios (1:1 and 1:2) were initially studied using the Na2S/Na2SO3 electrolyte. A 1:1 deposition ratio of CdS and CdSe was achieved by performing 15 cycle SILAR depositions of CdS followed by 15 cycle depositions of CdSe, labelled CdS15/CdS15 in this paper. Similarly, CdS10/CdSe20 would represent a 1:2 SILAR deposition ratio. The photocurrent densities for CdS10/CdSe20 and CdS15/CdSe15 modified electrodes were observed to be 5.5 mAcm2 (Fig. S7 in Supplementary material) and 7.4 mAcm2 (Fig. 2a) at 1.23 V vs. RHE, respectively. The onset potentials for both the photoanode configurations were found to be around 0.35 V vs. RHE. Clearly, the performance claimed in sulfite solutions is highly inflated and is not related to water splitting. Photocurrent transient were observed in the cyclic photoresponse at the onset of the illumination semi-cycle; such transients consisted in a fast current upshot followed by a decay, and were more pronounced at higher applied potentials. In fact, for lower applied potentials in the range of 0.3 V to þ0.3 V vs. RHE, these transients were either absent or showed a small peak intensity, but increased in intensity at higher applied potentials. Specifically, above 0.5 V the observed transients were accentuated which indicated enhanced electron recombination. We hypothesize that this recombination behaviour may be due to charge accumulation and recombination under conditions of higher applied bias, resulting in faster charge carrier migration. The presence of Na2S in the aqueous electrolytes introduces sulfide ions, [S2] and as a consequence sulfide oxidation becomes the dominant anodic process for charge transfer and photocurrent generation. The function of the reducing agent, Na2SO3, consists in the reintroduction of the [S2] in the electrolyte (Eqs. (1) and (2)) and the enhancement of the oxidation rate in order to sustain the hydrogen evolution reaction (HER) at the cathode (Eq. (3)), which would otherwise diminish over time due to disulfide, [S2 2 ] formation (Eq. (1)) [40]. Clearly this test is useful to determine the intrinsic kinetics of charge transfer at the interface and therefore the maximum possible rate of HER at the cathode, but does not
Please cite this article in press as: R. Ahmed, et al., Water splitting vs. sulfite oxidation: An assessment of photoelectrochemical performance of TiO2 nanotubes modified by CdS/CdSe nanoparticles, Electrochimica Acta (2017), https://doi.org/10.1016/j.electacta.2017.12.088
4
R. Ahmed et al. / Electrochimica Acta xxx (2017) 1e9
Fig. 1. (a) Raman peaks detected for TiO2 (anatase) and deposited CdS, CdSe using a 514 nm LASER. (b) Absorbance spectrum for the CdS15/CdSe15 deposited photoanodes. SEM images of CdS15/CdSe15 deposited on (c) the TiO2 nanotube surface and (d) inside of the nanotube columns.
Fig. 2. LSV plot under chopped illumination for photoelectrochemical cells employing TiO2 nanotube electrodes modified by CdS15/CdSe15 SILAR depositions using (a) Na2S/Na2SO3 and (b) NaOH aqueous electrolyte.
provide information on the rate of water oxidation (WO) that may occur at the photoanode, i.e. the reaction of interest. To summarize, the sulfide acts as a sacrificial species with fast kinetics generating electrons that flow at the cathode to enable HER (Eqs. (1)e(3)). Under these conditions, the higher photocurrent density observed in Fig. 2a arise from sulfite oxidation (Jph/SO) and does not reflect the performance of the photoanode system for water splitting.
Anodic reaction : 2S2 ¼ S2 2 þ 2e
(1)
2 2 Anodic reaction : 2S2 2 þ SO2 3 ¼ S 2 O3 þ S
(2)
Cathodic reaction : 2H2 O þ 2e ¼ H2 þ 2OH
(3)
Please cite this article in press as: R. Ahmed, et al., Water splitting vs. sulfite oxidation: An assessment of photoelectrochemical performance of TiO2 nanotubes modified by CdS/CdSe nanoparticles, Electrochimica Acta (2017), https://doi.org/10.1016/j.electacta.2017.12.088
R. Ahmed et al. / Electrochimica Acta xxx (2017) 1e9
3.3. Photoelectrochemical response in NaOH electrolytes Since a higher PEC performance was observed for the CdS15/ CdSe15 modified photoanode, this photoanode configuration was used to perform further PEC activity tests using a 1 M NaOH aqueous electrolyte (pH ¼ 13.3) where the PEC activity and photocurrent contribution would be due solely to water splitting (water oxidation at the anode and hydrogen evolution at the cathode). This electrolyte did not include any reagents that would allow the occurrence of spurious reactions. Fig. 2b shows the LSV plot for TiO2 photoanode (5.7 mm long) modified by CdS15/CdSe15. The photocurrent density due to water splitting, Jph/WS was observed to be 0.92 mAcm2 at 1.23 V vs. RHE which was 8-fold lower compared to the Jph/SO value of 7.4 mAcm2 at 1.23 V vs. RHE measured under similar conditions using the Na2S/Na2SO3 electrolyte. The large variation between our observed photocurrent densities using Na2S/Na2SO3 electrolyte and the NaOH aqueous electrolyte revealed a much slower oxidation rate for the water splitting compared to sulfite oxidation. The enhanced photocurrents using sacrificial reagents commonly reported in literature would thus lead to misleading conclusions about PEC activity of the TiO2/CdS/CdSe system when reporting water splitting efficiencies. The solar-to-hydrogen or STH efficiency (hSTH) is one of the most frequently reported parameters to benchmark the efficiency of water splitting devices; this parameter refers to the energy converted from the net hydrogen produced, divided by the irradiated energy from the full spectrum solar energy. This efficiency can be calculated using Eq. (4) where the parameters J, Vbias and Pin denote the output current for the cell, applied voltage bias and power density of input light, respectively [2]. A maximum hSTH of 6.55% at 0.244 V vs. RHE was calculated for the TiO2 photoanode modified by CdS15/CdSe15 using the Na2S/Na2SO3 electrolytes while the maximum hSTH using NaOH aqueous solution was determined to be 0.71% at 0.36 V vs. RHE. A comparison of the STH efficiencies for the TiO2 photoanodes modified with CdS15/CdSe15 using NaOH and Na2S/Na2SO3 electrolytes (Fig. 3a) at first sight will portray a more efficient hydrogen generation for the latter electrolyte. However, the Na2S/Na2SO3 solutions undergo sulfite oxidation and not water splitting; the use of STH efficiency in this particular case would be erroneous since the process is sustained only by a sacrificial species that should be replenished periodically. To accurately assess the water splitting ability, the ‘water splitting efficiency’ (4ws) is a much more meaningful parameter. 4ws is the ratio between the photocurrent generated from water splitting (Jph/WS) to that originated from sulfite oxidation (Jph/SO). The rationale for this ratio is as follows: the photocurrent generated by a PEC system could be modelled as a product of the maximum theoretical photocurrent
5
(Jmax), the efficiency of charge separation (4CS) and efficiency of the oxidation process (4sulfite for sulfite, 4ws water splitting) as shown in Eq. (5) [33]. The water splitting efficiency for the TiO2 nanotubes (5.7 mm long) modified by CdS15/CdSe15 deposition is shown in Fig. 3b and was found to be around 12.4% at an applied potential of 1.23 V vs. RHE.
hSTH ¼ Jph=WS Jph=SO IPCE ¼
Jð1:23 Vbias Þ Pin
¼
Jmax fCS fws ¼ fWS Jmax fCS fsulfite
Jph hc Jph ¼ 1240:8 e Pin l Pin l
IPCE ¼ he=h htransport hinterface ¼ he=h APCE
(4)
(5)
(6)
(7)
The incident photo-to-current conversion efficiency (IPCE) or external quantum efficiency (EQE) (Eqs. (6) and (7)) is an important diagnostic parameter that describes how well a photoresponsive system can convert light to electricity in the region of UV-VIS-IR wavelengths and this quantity can be experimentally determined using Eq. (6) [41]. This parameter depends on the efficiency of photon absorption (he/h) at the photoactive electrode, efficiency of charge transport towards the electrode/electrolyte interface (htransport) and the efficiency of interfacial charge transfer (hinterface) shown in Eq. (7) [42]. The product of the last two factors are known as the absorbed photon-to-current conversion efficiency (APCE) or the internal quantum efficiency (IQE) of the device, a term which highlights the fraction of charges that contribute to photocurrent generation after absorption occurs. The changes in the IPCE spectrum could be attributed to variations in either the absorption spectrum of the photoanode or the recombination of charges that limits the device photocurrent. The IPCE for the TiO2 electrode (5.7 mm long) modified by CdS15/CdSe15 depositions was measured at an applied potential of 1.23 V vs. RHE and is shown in Fig. 4. The spectrum shows a higher current conversion (~17e44%) in the 400e450 nm range where visible light absorption occurs from both the CdS and CdSe nanoparticles. Beyond the theoretical 512.7 nm (~2.42 eV bandgap of CdS), the photocurrents are produced solely from absorption in the outer CdSe layer. Thus, despite the fact that the current produced from light absorption beyond 510 nm (J>510), which is 58.1% of the total cell current calculated from the IPCE spectrum (JIPCE), the lower IPCE values (<17%) in the wavelength range beyond 510 nm is indicative of a lower APCE
Fig. 3. (a) STH efficiencies using NaOH and Na2S/Na2SO3 added electrolytes and (b) Water splitting efficiency for the TiO2 nanotubes modified by CdS15/CdSe15 depositions.
Please cite this article in press as: R. Ahmed, et al., Water splitting vs. sulfite oxidation: An assessment of photoelectrochemical performance of TiO2 nanotubes modified by CdS/CdSe nanoparticles, Electrochimica Acta (2017), https://doi.org/10.1016/j.electacta.2017.12.088
6
R. Ahmed et al. / Electrochimica Acta xxx (2017) 1e9
reported to dominate over the size quantization effect which caused the apparent lowering of the bandgap. 3.4. Evidence for CdS/CdSe oxidation and dissolution
Fig. 4. IPCE plot for the photoelectrochemical cells employing TiO2/CdS15/CdSe15 photoanodes.
arising from either a lower charge injection or charge collection efficiency for the cell (Eq. (7)). The photons beyond the 513 nm wavelength edge excite electron generation solely in the outer CdSe layer which most likely experiences a higher recombination at the CdS/CdSe, CdS/TiO2 interfaces and at the TiO2 trap sites during electron transport. The IPCE spectrum shows an extended tail beyond the 717 nm theoretical cutoff point of CdSe which has been observed by Rabinovich and Hodes for SILAR deposited nanoparticles [43]. With increase in the crystal size the ‘tail effect’ was
Higher SILAR cycle depositions of CdS30/CdSe30 on TiO2 nanotubes (5.7 mm long) did not improve the photoresponse (Jph ¼ 0.39 mAcm2 at 1.23 V vs. RHE) which is most likely due to optical loss and/or localized and interfacial recombination centres that affect electron transport negatively through a thicker nanoparticle layer (LSV plot included in Supporting material, Fig. S8). For a reverse order deposition of CdSe30/CdS30 a photocurrent density of 0.27 mAcm2 at 1.23 V vs. RHE is shown in Fig. 5a. The slightly reduced PEC performance of this photoanode (CdSe30/CdS30) compared to the CdS30/CdSe30 deposition order could be attributed to Fermi level alignment and favorable electron injection in the TiO2/CdS/CdSe system [22,44,45]. The interesting discrepancy to note in the LSV plot shown in Fig. 5a is the increase of current density between the voltages 0.25 V (vs. RHE) and þ0.15 V (vs. RHE). This increase was attributed to the photo dissolution of CdS and CdSe by the electrolyte. Previous studies on CdS and CdSe stability have reported formations of Cd2þ and elemental S and Se due to photocorrosion in aqueous electrolytes (Eqs. (8)e(10)) [46,47]. This was confirmed by observing the dark current density of the device as shown in Fig. 5b, where the peak current increased with prolonged exposure to the 1 M NaOH electrolyte.
CdX ðX ¼ S; SeÞ þ hy/e þ hþ
(8)
Fig. 5. LSV plot for PEC cell with TiO2 nanotubes (5.7 mm long) modified by CdSe30/CdS30 and using 1 M NaOH aqueous electrolyte under (a) simulated AM1.5G and (b) dark conditions.
Fig. 6. EDS mapping of TiO2 nanotube surface after exposure to 1 M NaOH aqueous electrolyte for over an hour shows Cd and O rich particle coverage along the surface.
Please cite this article in press as: R. Ahmed, et al., Water splitting vs. sulfite oxidation: An assessment of photoelectrochemical performance of TiO2 nanotubes modified by CdS/CdSe nanoparticles, Electrochimica Acta (2017), https://doi.org/10.1016/j.electacta.2017.12.088
R. Ahmed et al. / Electrochimica Acta xxx (2017) 1e9
2e þ 2H2 O/H2 þ 2OH CdX/Cd2þ þ X þ 2e
(9) (10)
A pronounced decrease of Se atomic fraction accompanied with lower Cd and S atomic fractions in the photoanodes after prolonged
Fig. 7. Possible reactions relating to the corrosion of the Cd-compounds.
7
exposure to the 1 M NaOH aqueous electrolyte was confirmed by EDS measurements indicating dissolution of these elements in the solution (Table S1 in Supporting material). SEM images in Fig. 6 show that the nanotube surface was covered by newly formed orthorhombic micron sized particles after one hour of exposure to the 1 M NaOH electrolyte. EDS mapping for these particles (inset in Fig. 6) show a high intensity for Cd and O, suggesting CdS (Se) oxidation, forming on the nanotubes either Cd oxides or hydroxides. The dark current response due to anodic dissolution in the LSV plots were observed between 0.25 V and 0.15 V vs. RHE, a region of potential where the STH efficiency might be overestimated. Since this dark current was proportional to the exposure time in the electrolyte leading to an enhanced pseudo-photocurrent, care should be taken to avoid such overestimations by determination of STH efficiency either by taking LSV measurements with minimum exposure to corrosion conditions or avoiding such potential regions. Fig. 7 shows the relative band alignments under alkaline conditions and possible reactions that could occur at the photoanode in the absence of sacrificial species, resulting in the corrosion of the Cd-compounds. The elemental Se and S formations reported in the literature most likely transform to H2SeO3 and partially to H2SO3, respectively resulting from oxidation at the photoanode which is consistent with the dissolution of Se and S from the photoanodes.
Fig. 8. LSV plots for Li incorporated TiO2 nanotubes (11 mm long) modified with CdS15/CdSe15 SILAR depositions using (a) 1 M NaOH aqueous electrolyte, (b) phosphate buffer solution and (c) 0.2 M Na2SO4 aqueous solution.
Please cite this article in press as: R. Ahmed, et al., Water splitting vs. sulfite oxidation: An assessment of photoelectrochemical performance of TiO2 nanotubes modified by CdS/CdSe nanoparticles, Electrochimica Acta (2017), https://doi.org/10.1016/j.electacta.2017.12.088
8
R. Ahmed et al. / Electrochimica Acta xxx (2017) 1e9
3.5. Photoelectrochemical performance after Li intercalation Li intercalation into the vacant octahedral sites of TiO2 nanotubes (length ¼ 5.7 mm) have been reported to boost PEC performance by passivation of the TiO2 trap states [33,48e50]. Due to trap state passivation, the photogenerated charges experience reduced recombination within the nanotube structure resulting in an unpinning of the Fermi level which is responsible for an increasing photocurrent trend at higher applied potentials. The photocurrent density for the 5.7 mm long Li:TiO2 nanotube modified by CdS15/ CdSe15 using NaOH electrolyte, was observed to be around 1 mAcm2 at 1.23 V vs. RHE which gradually increased to 1.21 mAcm2 at 2 V vs. RHE (Fig. S9 in Supporting material). The onset potential for both the doped and undoped electrodes were 0.36 V vs. RHE which is similar to the 0.35 V observed in the case of sulfide oxidation using Na2S/Na2SO3 electrolyte. The absence of photocurrent spikes at higher applied potentials due to cyclic illumination in the LSV plot is consistent with the trap state passivation and decrease in defect density of TiO2, as discussed in our previously published reports.
OER under alkaline condition : 4OH ¼ O2 þ 2H2 O þ 4e ; Eox ¼ þ0:401 V vs: NHE (11) OER under acidic condition : 2H2 O ¼ O2 þ 4Hþ þ 4e ; Eox ¼ þ1:299 V vs: NHE
from sulfite oxidation and water splitting, respectively. The water splitting efficiency parameter which compares photocurrent generation due to water splitting to that from sulfite oxidation is suitable for performance evaluation of such PEC devices. The STH efficiency parameter, specific to water splitting could not be compared to the commonly reported sulfide based electrolytes that utilize sulfite oxidation for current generation in PEC devices. The IPCE spectrum of the CdS/CdSe modified nanotubes indicated utilization of a broader range of the incident solar spectra up to around 709 nm (~1.75 eV), which correspond closely to the theoretical cutoff wavelength of CdSe. The IPCE spectrum also helped to identify the 400e513 nm wavelength region, where CdS and CdSe actively absorb radiation, to result in a better photon to current conversion compared to the wavelength region beyond 513 nm where active absorption takes place only from CdSe. A slightly improved PEC performance was observed using aqueous acidic electrolytes compared to aqueous alkaline electrolytes. The deposited chalcogenide particles were found to be unstable in the aqueous solutions due to oxidation or anodic dissolution. The absence of hole quenching agents in aqueous alkaline solution accelerates the corrosion of the nanoparticles, leading to the anodic dissolution that limits the scope of these SILAR deposited CdS and CdSe nanoparticles towards splitting water. This study also sheds light on the fact that a partial coverage of the TiO2 nanotube surface can be effective to produce high cell photocurrents. Implementation of such strategies to boost photocurrent could be promising if material stability issues can be overcome, perhaps by the use of very thin, conformal layers of stable materials.
(12) Fig. 8aec shows the LSV plots for 11 mm long Li:TiO2 nanotubes coated with CdS15/CdSe15 using (i) 1 M NaOH solution, (ii) phosphate buffer solution and (iii) 0.2 M Na2SO4 aqueous solution, respectively. The recorded photocurrent densities at 1.23 V vs. RHE for the 11 mm long nanotube electrodes in 1 M NaOH (pH ¼ 13.3), phosphate buffer (pH ¼ 7.4) and aqueous Na2SO4 aqueous (pH ¼ 5.6) solutions were 0.58 mAcm2, 0.66 mAcm2 and 0.91 mAcm2, respectively. The OERs under alkaline and acidic conditions are shown in Eqs. (11) and (12), respectively. The use of longer Li:TiO2 nanotubes (11 mm) did not yield a boost in photocurrent for the cells compared to the 5.7 mm long Li:TiO2 nanotubes. However, comparing PEC performance for the 11 mm long nanotubes using electrolytes with varying pH values from 13.3 to 5.6 showed a better PEC performance using a more acidic electrolyte. Reports of PEC performance of CdS, CdSe and/or CdS/CdSe modified TiO2 nanotubes relating to water splitting are few and to the best of our knowledge this trend of improved photocurrent response in more acidic solution has not been reported for the TiO2/CdS/CdSe system nor, to our knowledge for any other PEC system [25,27]. The increase in photocurrent is accompanied by a more negative shift of onset potentials for the LSV plots from 0.35 V vs. RHE (NaOH) to 0.58 V vs. RHE (phosphate buffer) and finally to 0.69 V vs. RHE for Na2SO4 electrolyte, confirms improved catalytic activity at lower pH or the occurrence of complex reaction paths. It is also noteworthy that the onset potential for the TiO2/CdS15/CdSe15 photoanode using an Na2S/Na2SO3 electrolyte (pH ¼ 13.1) was observed to be around 0.35 V vs. RHE which remained constant in 1 M NaOH solution (pH ¼ 13.3). Overall, the observed behaviour is not yet understood and will be focus of our next efforts. 4. Conclusion Our study of SILAR deposited CdS/CdSe nanoparticles on TiO2 nanotubes using Na2S/Na2SO3 and NaOH aqueous solutions has clearly distinguished the difference in the PEC performances arising
Acknowledgement We acknowledge financial support from the University of Virginia through the MAXNET Energy Partnership. Appendix A. Supplementary data Supplementary data related to this chapter can be found at https://doi.org/10.1016/j.electacta.2017.12.088. References [1] J. Ager, M. Shaner, K. Walczak, I. Sharp, S. Ardo, Experimental demonstrations of spontaneous, solar-driven photoelectrochemical water splitting, Energy Environ. Sci. 8 (2015) 2811e2824. [2] M. Walter, E. Warren, J. McKone, S. Boettcher, Q. Mi, E. Santori, N. Lewis, Solar water splitting cells, Chem. Rev. 110 (2010) 6446e6473. [3] Y. Li, D. Gao, S. Peng, G. Lu, S. Li, Photocatalytic hydrogen evolution over Pt/ Cd0.5Zn0.5S from saltwater using glucose as electron donor: an investigation of the influence of electrolyte NaCl, Int. J. Hydrogen Energy 36 (2011) 4291e4297. [4] Y. Li, G. Lu, S. Li, Photocatalytic hydrogen generation and decomposition of oxalic acid over platinized TiO2, App. Catal. A Gene 214 (2001) 179e185. [5] Y. Li, Y. Xie, S. Peng, G. Lu, S. Li, Photocatalytic hydrogen generation in the presence of chloroacetic acids over Pt/TiO2, Chemosphere 63 (2006) 1312e1318. [6] J. Schneider, D. Bahnemann, Undesired role of sacrificial reagents in photocatalysis, J. Phys. Chem. Lett. 4 (2013) 3479e3483. [7] D. Scanlon, C. Dunnill, J. Buckeridge, S. Shevlin, A. Logsdail, S. Woodley, Catlow, M. Powell, R. Palgrave, I. Parkin, G. Watson, T. Keal, P. Sherwood, A. Walsh, A. Sokol, Band alignment of rutile and anatase TiO2, Nat. Mater. 12 (2013) 798e801. [8] L.-K. Tsui, T. Homma, G. Zangari, Photocurrent conversion in anodized TiO2 nanotube arrays: effect of the water content in anodizing solutions, J. Phys. Chem. C 117 (2013) 6979e6989. [9] R. Noufi, P. Kohl, S. Frank, A. Bard, Semiconductor electrodes, J. Electrochem. Soc. 125 (1978) 246e252. [10] L.-K. Tsui, G. Zangari, The influence of morphology of electrodeposited Cu2O and Fe2O3 on the conversion efficiency of TiO2 nanotube photoelectrochemical solar cells, Electrochim. Acta 100 (2013) 220e225. [11] L.-K. Tsui, G. Zangari, Modification of TiO2 nanotubes by Cu2O for photoelectrochemical, photocatalytic, and photovoltaic devices, Electrochim. Acta 128 (2014) 341e348.
Please cite this article in press as: R. Ahmed, et al., Water splitting vs. sulfite oxidation: An assessment of photoelectrochemical performance of TiO2 nanotubes modified by CdS/CdSe nanoparticles, Electrochimica Acta (2017), https://doi.org/10.1016/j.electacta.2017.12.088
R. Ahmed et al. / Electrochimica Acta xxx (2017) 1e9 [12] L.-K. Tsui, J. Huang, M. Sabat, G. Zangari, Visible light sensitization of TiO2 nanotubes by Bacteriochlorophyll-C dyes for photoelectrochemical solar cells, ACS Sustain. Chem. Eng. 2 (2014) 2097e2101. [13] B. Streetman, S. Banerjee, Solid State Electronic Devices, Prentice Hall, 2005. [14] W.-T. Sun, Y. Yu, H.-Y. Pan, X.-F. Gao, Q. Chen, L.-M. Peng, CdS quantum dots sensitized TiO2 nanotube-array photoelectrodes, J. Am. Chem. Soc. 130 (2008) 1124e1125. [15] Y.-L. Lee, C.-F. Chi, S.-Y. Liau, CdS/CdSe Co-Sensitized TiO2 photoelectrode for efficient hydrogen generation in a photoelectrochemical cell, Chem. Mater. 22 (2010) 922e927. [16] B. Mukherjee, Y. Smith, V. Subramanian, CdSe nanocrystal assemblies on anodized TiO2 nanotubes: optical, surface, and photoelectrochemical properties, J. Phys. Chem. C 116 (2012) 15175e15184. [17] S. Chen, M. Paulose, C. Ruan, G. Mor, O. Varghese, D. Kouzoudis, C. Grimes, Electrochemically synthesized CdS nanoparticle-modified TiO2 nanotubearray photoelectrodes: preparation, characterization, and application to photoelectrochemical cells, J. Photochem. Photobiol. Chem. 177 (2006) 177e184. [18] Y. Yin, Z. Jin, F. Hou, Enhanced solar water-splitting efficiency using core/ sheath heterostructure CdS/TiO2 nanotube arrays, Nanotechnology 18 (2007). [19] L. Yang, S. Luo, R. Liu, Q. Cai, Y. Xiao, S. Liu, F. Su, L. Wen, Fabrication of CdSe nanoparticles sensitized long TiO2 nanotube arrays for photocatalytic degradation of anthracene-9-carbonxylic acid under green monochromatic light, J. Phys. Chem. C 114 (2010) 4783e4789. [20] H. Zhang, X. Quan, S. Chen, H. Yu, N. Ma, “Mulberry-like” CdSe nanoclusters anchored on TiO2 nanotube arrays: a novel architecture with remarkable photoelectrochemical performance, Chem. Mater. 21 (2009) 3090e3095. [21] Z. Shao, W. Zhu, Z. Li, Q. Yang, G. Wang, One-step fabrication of CdS nanoparticle-sensitized TiO2 nanotube arrays via electrodeposition, J. Phys. Chem. C 116 (2012) 2438e2442. [22] X.-F. Gao, W.-T. Sun, G. Ai, L.-M. Peng, Photoelectric performance of TiO2 nanotube array photoelectrodes cosensitized with CdS/CdSe quantum dots, Appl. Phys. Lett. 96 (2010), 153104-153101-153104-153103. [23] S. Cheng, W. Fu, H. Yang, L. Zhang, J. Ma, H. Zhao, M. Sun, L. Yang, Photoelectrochemical performance of multiple semiconductors (CdS/CdSe/ZnS) cosensitized TiO2 photoelectrodes, J. Phys. Chem. C 116 (2011) 2615e2621. [24] L. Liu, J. Hensel, R. Fitzmorris, Y. Li, J. Zhang, Preparation and photoelectrochemical properties of CdSe/TiO2 hybrid mesoporous structures, J. Phys. Chem. Lett. 1 (2010) 155e160. [25] K. Xie, Z. Wu, M. Wang, J. Yu, C. Gong, L. Sun, C. Lin, Room temperature synthesis of CdS nanoparticle-decorated TiO2 nanotube arrays by electrodeposition with improved visible-light photoelectrochemical properties, Electrochem. Commun. 63 (2016). [26] Y. Xie, G. Ali, S. Yoo, S. Cho, Sonication-assisted synthesis of CdS quantum-dotsensitized TiO2 nanotube arrays with enhanced photoelectrochemical and photocatalytic activity, ACS Appl. Mater. Inter. 2 (2010) 2910e2914. [27] C.-J. Lin, Y.-T. Lu, C.-H. Hsieh, S.-H. Chiena, Surface modification of highly ordered TiO2 nanotube arrays for efficient photoelectrocatalytic water splitting, Appl. Phys. Lett. 94 (2009), 113102-113101-113102-113103. [28] J. Fernandes, S. Khan, F. Baum, E. Kohlrausch, J. Lucena dos Santos, D. Baptista, S. Teixeira, J. Dupont, M. Santos, Synergizing nanocomposites of CdSe/TiO2 nanotubes for improved photoelectrochemical activity via thermal treatment, Dalton Trans. 45 (2016) 9925e9931. [29] S. Banerjee, S. Mohapatra, P. Das, M. Misra, Synthesis of coupled semiconductor by filling 1D TiO2 nanotubes with CdS, Chem. Mater. 20 (2008) 6784e6791. [30] S. Rühle, M. Shalom, A. Zaban, Quantum-dot-sensitized solar cells, ChemPhysChem 11 (2010) 2290e2304.
9
[31] H. Lee, H. Leventis, S.-J. Moon, P. Chen, S. Ito, S. Haque, T. Torres, F. Nüesch, €tzel, M. Nazeeruddin, PBS and CdS quantum T. Geiger, S. Zakeeruddin, M. Gra dot-sensitized solid-state solar cells: “old concepts, new results”, Adv. Funct. Mater. 19 (2009) 2735e2742. [32] R. Ahmed, G. Will, J. Bell, H. Wang, Size-dependent photodegradation of CdS particles deposited onto TiO2 mesoporous films by SILAR method, J. Nanoparticle Res. 14 (2012). [33] L.-K. Tsui, Y. Xu, D. Dawidowski, D. Cafiso, G. Zangari, Efficient water oxidation kinetics and enhanced electron transport in Li-doped TiO2 nanotube photoanodes, J. Mater. Chem. 4 (2016) 19070e19077. [34] L.-K. Tsui, G. Zangari, Titania nanotubes by electrochemical anodization for solar energy conversion, J. Electrochem. Soc. 161 (2014) D3066eD3077. [35] R. Ahmed, L. Zhao, A. Mozer, G. Will, J. Bell, H. Wang, Enhanced electron lifetime of CdSe/CdS quantum dot (QD) sensitized solar cells using ZnSe coreeshell structure with efficient regeneration of quantum dots, J. Phys. Chem. C 119 (2015) 2297e2307. [36] H. Choi, Y. Jung, S. Kim, Size effects in the Raman spectra of TiO2 nanoparticles, Vib. Spectrosc. 37 (2005) 33e38. [37] A.V. Baranov, Y.P. Rakovich, J.F. Donegan, T.S. Perova, R.A. Moore, D.V. Talapin, A.L. Rogach, Y. Masumoto, I. Nabiev, Effect of ZnS shell thickness on the phonon spectra in CdSe quantum dots, Phys. Rev. B 68 (2003), 165306165301-165306-165307. [38] M. Abdulkhadar, B. Thomas, Study of raman spectra of nanoparticles of CdS and ZnS, Nanostruct. Mater. 5 (1995) 289e298. [39] J.-H. Yum, B.E. Hardin, S.-J. Moon, E. Baranoff, F. Nüesch, M.D. McGehee, €tzel, M.K. Nazeeruddin, Panchromatic response in solid-state dyeM. Gra sensitized solar cells containing phosphorescent energy relay dyes, Angew. Chem. Int. Ed. 48 (2009) 9277e9280. [40] N. Buehler, K. Meier, J. Reber, Photochemical hydrogen production with cadmium sulfide suspensions, J. Phys. Chem. 88 (1984) 3261e3268. [41] Z. Chen, H. Dinh, E. Miller, Photoelectrochemical Water Splitting, Springer, New York, 2013. [42] Z. Chen, T. Jaramillo, T. Deutsch, A. Kleiman-Shwarsctein, A. Forman, N. Gaillard, R. Garland, K. Takanabe, C. Heske, M. Sunkara, E. McFarland, K. Domen, E. Miller, J. Turner, H. Dinh, Accelerating materials development for photoelectrochemical hydrogen production: standards for methods, definitions, and reporting protocols, J. Mater. Res. 25 (2010) 3e16. [43] E. Rabinovich, G. Hodes, Effective bandgap lowering of CdS deposited by successive ionic layer adsorption and reaction, J. Phys. Chem. C 117 (2013) 1611e1620. [44] K.-H. Lin, C.-Y. Chuang, Y.-Y. Lee, F.-C. Li, Y.-M. Chang, I.P. Liu, S.-C. Chou, Y.L. Lee, Charge Transfer in the heterointerfaces of CdS/CdSe cosensitized TiO2 photoelectrode, J. Phys. Chem. C 116 (2012) 1550e1555. [45] X.-Y. Yu, J.-Y. Liao, K.-Q. Qiu, D.-B. Kuang, C.-Y. Su, Dynamic study of highly efficient CdS/CdSe quantum dot-sensitized solar cells fabricated by electrodeposition, ACS Nano 5 (2011) 9494e9500. [46] D.J. Fermín, E.A. Ponomarev, L.M. Peter, A kinetic study of CdS photocorrosion by intensity modulated photocurrent and photoelectrochemical impedance spectroscopy, J. Electroanal. Chem. 473 (1999) 192e203. [47] L. Xi, J. Lek, Y. Liang, C. Boothroyd, W. Zhou, Q. Yan, X. Hu, Stability studies of CdSe nanocrystals in an aqueous environment, Nanotechnology 22 (2011). [48] B. Zachau-Christiansen, K. West, T. Jacobsen, S. Atlung, Lithium insertion in different TiO2 modifications, Solid State Ionics 30 (1988) 1176e1182. [49] R. van de Krol, A. Goossens, J. Schoonman, Spatial extent of lithium intercalation in anatase TiO2, J. Phys. Chem. B 103 (1999) 7151e7159. [50] L.-K. Tsui, M. Saito, T. Homma, G. Zangari, Trap-state passivation of titania nanotubes by electrochemical doping for enhanced photoelectrochemical performance, J. Mater. Chem. 3 (2015) 360e367.
Please cite this article in press as: R. Ahmed, et al., Water splitting vs. sulfite oxidation: An assessment of photoelectrochemical performance of TiO2 nanotubes modified by CdS/CdSe nanoparticles, Electrochimica Acta (2017), https://doi.org/10.1016/j.electacta.2017.12.088