Sensitization of TiO₂ nanotube array photoelectrodes with Mn_xCd_ySe

Abstract

The photoanode obtained by the deposition of Mn_xCd_ySe nanocrystals onto TiO₂ nanotube arrays using a successive ionic layer adsorption and reaction technique realized a significant improvement in the charge transport and current generation compared with a pristine TiO₂ nanotube based anode. The concentration of manganese was determined to be 10% of the concentration of cadmium, and thus x = 0.1y. The widening of absorption spectra and the magnitude of photocurrent density were found to be significantly affected with a variation in the number of SILAR cycles and the annealing temperature. A stable photocurrent density of ∼8 mA cm⁻² under AM 1.5 illumination (1 sun) was achieved for Mn_xCd_ySe nanocrystals-embedded TiO₂ nanotube arrays heterostructure based photoelectrodes prepared through 9 cycles of SILAR deposition, followed by annealing at 400 °C under a nitrogen atmosphere. The results obtained suggest the versatility of the Mn_xCd_ySe-sensitized TiO₂ nanotube matrix as an efficient electrode for photovoltaic applications.

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Introduction

The pioneering work by Fujishima and Honda in 1972 ¹ along with the interesting structural, thermal and electronic properties of TiO₂ spurred the interest among various scientists for research on the titanium oxide semiconductor as a photoanode material for solar energy conversion applications. A significant amount of research has been directed towards the modification of the size, shape and crystal structure of TiO₂ in various forms. Masuda and Fukuda demonstrated that highly ordered arrays of the oxide material could be grown with optimum anodization conditions.² Zwilling and co-workers furthered this concept and demonstrated the self-ordered growth of TiO₂ nanotube arrays (NTAs) by electrochemical anodization of titanium (Ti) in a fluoride containing electrolyte.³ Following these studies, many advances were made towards the optimization of length, thickness and aspect ratio of TiO₂ nanotubes by Schmuki et al.^4,5 These studies attracted wide interest, and numerous efforts have been devoted to the synthesis of titania nanostructures (nanorods, nanowires and nanotubes) using the anodization technique in different electrolytes.^6–10 The nanotubular geometry provides open structure for an easy access of electrolyte and facilitates the hole transfer through the pores. Titania based electrodes exhibit a large band gap around 3.2 eV or 3.0 eV for anatase and rutile phases, respectively, and this results in the generation of charge carriers upon illumination through the absorption of energies above the band gap.^11,12 However, since the solar spectrum consists of photons with energies ranging from about 0.5 to 3.5 eV, the photoconversion efficiency achieved through TiO₂ is very low as the photons with energies below the semiconductor band gap are not absorbed. Therefore, sensitization of titania can lead to a panchromatic absorption of visible light and render it eminently suitable for photovoltaic applications.

Most common approaches for improving the light harvesting ability of large band gap semiconductors are either based on chemisorption of dye molecules onto the surface of titania nanoparticles^13–15 or on the incorporation of quantum dot semiconductors as sensitizer materials.^16–21 In case of dye sensitization, charge separation originates through efficient injection of electrons from the excited state of the photoexcited dye molecules into the conduction band of TiO₂. For the quantum dot sensitized approach, quantum dots serve the sensitization purpose instead of dye molecules. Quantum dots offer several advantages over dye molecules such as tunable optical properties with size, formation of better heterojunctions with solid hole conductors and a better charge transport. The innate ability of alloy nanocrystals to exhibit varied physical and optical properties of the bulk semiconductor based on the size and composition offer the possibility of their use as efficient photosensitizers with wide band gap semiconductors (e.g. TiO₂ and ZnO) as scaffold in photovoltaic (PV) systems. In the past, binary sensitizers such as CdSe,¹⁷ CdS,²¹ CdTe,¹⁹ PbS,²⁰ InAs¹⁶ and Ag₂S¹⁸ have been widely employed for the sensitization of TiO₂. Unfortunately, binary quantum dot sensitized PEC solar cells generally suffer from severe photostability issues. Chemical reactions of photogenerated charge carriers with the electrolyte or the bulk material leads to photocorrosion in general. Pseudobinary semiconductor alloys (AB_xC_1−x) scaled to nanometer regime as sensitizer materials have been relatively less exploited and might offer more suitable conditions for the application of inorganic semiconductor quantum dot sensitizers in the development of stable solar cells. For example, pseudobinary semiconductors such as CdSSe,^22,23 CdSeTe²⁴ and ZnCdSe^25,26 have been effectively employed as sensitizers for TiO₂ nanostructures. Fast charge transfer from the quantum dot semiconductors to the neighboring large bandgap semiconductor materials is crucial to subdue the photocorrosion phenomena that are generally observed in quantum dot sensitized semiconductors. One dimensional (1D) nanostructured materials offer a direct and efficient pathway for the photogenerated charge carriers.^27–29 Therefore, the presence of one dimensional nanostructured semiconductors along with quantum dot sensitizers would be an efficient approach to overcome the charge transport issues. Considering the potential, it is important to exploit and investigate the photoelectrochemical performance and durability characteristics of new pseudobinary semiconductor quantum dots as sensitized one-dimensional nanostructured large band gap semiconductors. In this report, for the first time, we have studied the photovoltaic performance of the Mn_xCd_ySe sensitized TiO₂ nanotubes (NTAs). The Mn_xCd_ySe sensitized TiO₂ NTAs were synthesized using the SILAR technique, and the effect of the varying number of deposition cycles and the post annealing temperature were optimized for performance.

Experimental

Materials and methods

Titanium foil (0.005 inches thick, 99% purity) was purchased from STREM chemicals. Ethylene glycol (EG, 99%, Sigma Aldrich), ammonium fluoride (NH₄F, 98%, Acros Organics), cadmium acetate (Cd(OAc)₂·4H₂O, 98%, GFS), manganese chloride (MnCl₂·4H₂O, J.T.Baker), sodium borohydride (NaBH₄, 98+%, Acros Organics), selenium (Se, BKH chemicals), sodium sulfite (Na₂SO₃, 98+%, STREM chemicals) and sodium sulfide (Na₂S, Alfa Aesar) were purchased and used without further purification. The organic solvents used, namely acetone, isopropyl alcohol (IPA) and ethanol, were of ACS grade. De-ionized water was used throughout the study. The power supply used was supplied by Circuit Specialists (model number CSI12001X).

Synthesis of Mn_xCd_ySe-sensitized TiO₂ NT array photoelectrodes

TiO₂ NT arrays were synthesized as described previously.²⁵ Briefly, titanium (Ti) foils of size ∼2.5 cm² were cleaned ultrasonically in a mixture of acetone and isopropyl alcohol for 15 min, prior to anodization. The cleaned Ti strips were anodized at a constant potential of 40 V (D.C.) for 1 h in a fluorinated solution of ethylene glycol containing 0.5 wt% NH₄F and 3 wt% water in a two electrode configuration with a coiled platinum wire as the counter electrode. Anodized films were washed with IPA and annealed at 450 °C under air for 2 h (at a slow heating rate of 4 °C min⁻¹) to obtain the desired crystallization.

Mn_xCd_ySe nanocrystals were deposited on TiO₂ NTAs using the facile and convenient SILAR technique. The precursor solutions were made in ethanol/H₂O (50/50 vol) solvent, and the deposition was conducted under ambient temperature and pressure. The use of ethanol as a solvent lowers the surface tension, which leads to a better penetration of the solution inside TiO₂ NTAs. One SILAR cycle comprised of an immersion of annealed TiO₂ NTA film in 0.02 M MnCl₂ solution for 45 seconds, followed by rinsing in ethanol/water solvent and subsequent immersion in 0.02 M Cd(OAc)₂ solution and finally in Na₂Se solution (generated in situ by the reduction of Se metal with 0.04 M NaBH₄) for the same amount of time. The samples were prepared for 5, 7, 9 and 12 cycles of deposition and annealed at either 200 °C or 400 °C under N₂ atmosphere for 1 hour to analyze the effect of post-deposition thermal annealing on the photoelectrochemical properties of TiO₂ NTA.

Characterization

UV-visible absorption spectra of prepared photoelectrodes were obtained with a Shimadzu UV-2401PC UV-Vis diffuse reflectance spectrophotometer. BaSO₄ was used as the reflectance standard in the wavelength range of 200–800 nm. The surface morphologies of TiO₂ NTAs and Mn_xCd_ySe deposited TiO₂ NTA films were examined using a Hitachi S-4800 scanning electron microscope equipped with an energy dispersive spectrometer (Oxford EDS system). The crystalline phases were investigated using a Philips 12045 B/3 X-ray diffractometer with a Cu target (Kα radiation, λ = 1.54 Å). The diffraction patterns were recorded at 35 kV and 25 mA in the range of 2θ = 20° to 70°. The stoichiometry of films was estimated using Perkin Elmer Optima 8000 inductively-coupled plasma optical emission spectrometer (ICP-OES). Samples were analyzed using a PHI 5600 model X-ray photoelectron spectrometer (XPS) equipped with a monochromatic A1 anode (1486.6 eV). The spectrometer was calibrated to the Ag 3d_5/2 line at 368.27 ± 0.05 eV. The XPS spectra were recorded at 14 kV and 300 W with an analysis area of 800 μm². The survey spectra were acquired at a pass energy of 29.35 eV and narrow scans at 23.95 eV. Charging effects were corrected using the adventitious C 1s line at 284.6 eV. After 5 point smoothing of data, the peaks were fitted using SDP v4.6 Gaussian fitting software from XPS International. The photoelectrochemical (PEC) responses of the samples were measured in a conventional three-electrode system with TiO₂ NTAs (on Ti foil) or Mn_xCd_ySe deposited TiO₂ NTAs as the working electrode, a coiled platinum wire as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode in an aqueous solution containing 0.35 M Na₂SO₃ and 0.25 M Na₂S. The measurements were conducted under ambient conditions in a custom made quartz window cell with Gamry® Reference 600 potentiostat. Photoanodes were illuminated using a 300 W Xe lamp solar simulator supplied by Newport. The incident light intensity through standard AM 1.5G filter was tuned to 100 mW cm⁻² measured using Newport Thermopile Sensor 919P-003-10.

Results and discussion

Fig. 1 shows the absorption spectra of TiO₂ NTAs and Mn_xCd_ySe/TiO₂ NTAs heterostructures over the spectral range of 250–900 nm.

Fig. 1 Absorption spectra of pure nanocrytalline TiO₂ NTA film and the films sensitized with (a) TiO₂ NTA/Mn_xCd_ySe (5); (b) TiO₂ NTA/Mn_xCd_ySe (7); (c) TiO₂ NTA/Mn_xCd_ySe (9); (d) TiO₂ NTA/Mn_xCd_ySe (11) at annealing temperatures of 200 °C and 400 °C, respectively. Absorption edge was found to undergo red shift with an increase in the number of deposition cycles.

Fig. 1a–d depict the absorption spectra of the Mn_xCd_ySe/TiO₂ NTA(n) film annealed at various temperatures, where n indicates the number of deposition cycles. Pristine TiO₂ NTA film exhibited a fundamental absorption edge around 380 nm corresponding to the inter-band charge transfer.^30,31 In addition, a broad absorption profile in the visible region can be noted around 480 nm, which is a characteristic of oxygen-deficiency of the film.³² Another reason might be the surface plasmon resonance arising from Ti nanoparticles trapped within the nanotubes during anodization process. With incorporation of Mn_xCd_ySe, two key observations were noted. First, the absorption was improved significantly in the visible region, and second, an upward shift (towards longer wavelength) in the absorption edge position was noticed. As the number of deposition cycles increased from 5 to 11, enhanced particle loading resulted in more pronounced absorption in the visible region. With annealing at 200 °C under a nitrogen atmosphere, absorption improved in visible region, and the absorption edge experienced red shift compared to the as-deposited sample. With an increase in the annealing temperature to 400 °C, the absorption intensity decreased, but the absorption edge was further shifted towards longer wavelength in all the cases. This indicates that the inclusion of Mn_xCd_ySe nanocrystals within the TiO₂ NTA framework leads to a more effective utilization of solar spectrum through the sensitization effect.

To determine the fundamental gap of the composite and to identify its nature, Tauc equation was employed which can be given as,^33,34

αhν = A(hν − E_g)ⁿ

where α is the absorption coefficient, A is the constant, and n indicates the indirect (n = 1/2) or direct (n = 2) band gap material. The band gap energy (E_g) is determined using optical absorption coefficient (α) from the experimental absorbance. Band gap (E_g) values are obtained by extrapolation of the linear region of the curve to the abscissa (α = 0). Fig. S1† shows the Tauc plots, i.e., variation of (αhν)² versus photon energy (hν) for the TiO₂/Mn_xCd_ySe composite film with various number of deposition cycles annealed at 400 °C. A better fit was obtained with n = 2 for the composite films indicating that the deposited Mn_xCd_ySe nanocrystals have direct band gap. The calculated band gap values for 5, 7, 9 and 11 cycles of deposition are 3.22, 3.19, 3.27 and 3.36 eV, respectively.

The crystal structure of TiO₂ NTAs before and after modification with Mn_xCd_ySe was confirmed using XRD analysis. The diffraction pattern of pristine TiO₂ NTA film has been depicted in Fig. 2a.

Fig. 2 (a) Pristine TiO₂ NTAs and (b) stacked XRD pattern of TiO₂ NTA/Mn_xCd_ySe (9) unannealed and samples annealed at 200 °C and 400 °C. The patterns reveal that CdSe phase appears after annealing. The absence of a peak due to Mn indicates low concentration of Mn.

The pattern shows prominent peaks corresponding to anatase TiO₂ phase (JCPDS File no. 00-021-1272) and Ti metal foil substrate (JCPDS File no. 00-044-1294). The XRD patterns of the annealed Mn_xCd_ySe/TiO₂ NTA films were compared with the pattern of as-deposited (unannealed) Mn_xCd_ySe/TiO₂ NTA and are shown in Fig. 2b. The signal from the unannealed sample is similar to that from the pristine TiO₂ NTA film. However, after thermal annealing under nitrogen atmosphere, additional peaks corresponding to the CdSe phase start appearing. It is apparent that before thermal treatment, Mn, Cd and Se species were adsorbed physically on the surface of the film, and sensitized quantum dots manifest into the crystalline CdSe phase only upon annealing. Another important observation is the enhancement in the intensity of the peaks corresponding to the CdSe phase with the increase in annealing temperature. It indicates that the crystalline nature of the film is improved with the annealing temperature. No significant change is observed for the CdSe phase upon the incorporation of Mn in Mn_xCd_ySe crystallites. The absence of any signal due to Mn in Mn_xCd_ySe crystal lattice indicates the low dopant concentration of Mn or the presence of Mn below the detection limit. These results are in agreement with the previous studies conducted by authors on Zn_xCd_1−xSe where low concentrations of Zn dopant were not observed in XRD.²⁵

The morphology of the pristine TiO₂ NTAs has been shown in Fig. 3a.

Fig. 3 Scanning electron microscopy images (top view) of (a) unmodified TiO₂ NTAs, (b) TiO₂ NTA/Mn_xCd_ySe (5)-400 °C, (c) TiO₂ NTA/Mn_xCd_ySe (7)-400 °C, (d) TiO₂ NTA/Mn_xCd_ySe (9)-400 °C, (e) TiO₂ NTA/Mn_xCd_ySe (9)-unannealed, and (f) TiO₂ NTA/Mn_xCd_ySe (9)-200 °C. The images depict that the Mn_xCd_ySe nanocrystals are uniformly attached to the surface of TiO₂ NTAs.

The image illustrates the formation of tubular morphology with diameter and length in the ranges of 80 nm and 5–6 μm, respectively. The formation of the TiO₂ with nanotubular morphology is attributable to the electrolyte-assisted dissolution and oxidation of the Ti films during anodization, followed by the annealing process.³⁵ As observed in the case of sensitized films (Fig. 3b–f), the size of Mn_xCd_ySe clusters vary significantly with the number of deposition cycles and the annealing temperature. As demonstrated by images Fig. 3b–d, with the increase in number of deposition cycles from 5 to 9, the coverage of nanocrystals on the nanotube surface was increased. In case of the unannealed sample (Fig. 3e), tiny nanoparticles were found distributed at the mouth of nanotubes. After annealing (Fig. 3b–d and f), the Mn_xCd_ySe nanoparticles aggregate and form nanoclusters. Further, the cross-sectional SEM images (Fig. 4) depict that the Mn_xCd_ySe nanocrystals were deposited within the entire NTs network, including both external and internal regions.

Fig. 4 Cross-sectional view of (a and b) unsensitized TiO₂ NTAs and (c) sensitized TiO₂ NTA/Mn_xCd_ySe (9) film. The sensitized sample was subjected to a thermal treatment under a nitrogen atmosphere at 400 °C for 1 hour. Mn_xCd_ySe deposits are clearly visible on the walls of the nanotubes.

The chemistry of the MnCdSe quantum dots adhering to TiO₂ NTAs was confirmed to be Mn_xCd_ySe by the compositional XPS analysis of the sensitized TiO₂ film. Fig. 5 shows the spectra of the Cd 3d, Se 3d and Mn 2p species on the surface of the Mn_xCd_ySe/TiO₂ NTA film.

Fig. 5 X-ray photoelectron spectra for the TiO₂ NTA/Mn_xCd_ySe composite film showing (a) Cd 3d peak (b) Se 3d peak and (c) Mn 2p peak. The compositional analysis confirms the tethering of Cd²⁺, Se²⁻ and Mn²⁺ species on the surface of the sensitized film.

Sharp photoelectron peaks appear at binding energies of 404.7 eV (Cd 3d_5/2) and 411.5 eV (Cd 3d_3/2) for Cd 3d spectrum (Fig. 5a). The binding energy values, separation energy and degeneracy of states correspond to the formation of the CdSe bond.³⁶ The signal due to Se 3d presented a well-defined doublet associated with spin–orbit interaction (Fig. 5b). The two components obtained at 53.5 eV and 54.3 eV with a separation of 0.8 eV correspond to Se²⁻ species associated with the CdSe crystal. Canava et al. studied the effect of chemical treatments on the Se 3d signal and demonstrated that the shift between the two components of the Se 3d peak is highly sensitive to the chemical preparation techniques.

Moreover, depending upon the chemical treatment of the sample, the shift assigned to the second component of Se 3d can vary up to 1.5 eV.³⁷ In a previous study by the authors on Zn_xCd_1−xSe (ref. 25), this shift was observed to be 0.9 eV.²⁵ The core-level spectrum of the Mn 2p peak is shown in Fig. 5c. High noise level obtained for the signal indicates the nominal doping concentration of Mn. The Mn 2p spectrum exhibited two peaks at ∼641.8 eV and ∼640.7 eV for Mn 2p_3/2, indicating the existence of Mn²⁺ species in Mn_xCd_ySe crystals, as the peaks for metallic Mn and Mn⁴⁺ ion are located at 637.7 eV and 642.4 eV, respectively as demonstrated by Lin et al.³⁸

The stoichiometry analysis of Mn_xCd_ySe nanocrystals deposited on the sensitized TiO₂ NTA film was estimated using ICP-OES analysis, and the results obtained are tabulated in Table 1. As observed from stoichiometry calculations, Mn atoms substitute around 10% of Cd atoms in the CdSe lattice structure.

Table 1 Stoichiometry calculations of Mn_xCd_ySe-sensitized TiO₂ NTA based photoanodes from ICP-OES analysis

Ions	Conc (ppm)	Stoichiometry
Cd^2+	76.59	0.95
Se^2−	56.48	1.00
Mn^2+	3.75	0.095

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Such a low concentration of Mn (with x = 0.1) is in agreement with the previous studies on Zn_xCd_1−xSe, where Zn was found in a similar concentration.²⁵

The dependence of photocurrent on the applied potential for Mn_xCd_ySe-sensitized TiO₂ NTA films in Na₂S and Na₂SO₃ electrolyte is shown in Fig. 6, and the values have been summarized in Table 1.

Fig. 6 i–V characteristics of the sensitized films as working electrodes, measured under 100 mW cm⁻² intensity (AM 1.5 global filter) (a) TiO₂ NTA/Mn_xCd_ySe (5) (b) TiO₂ NTA/Mn_xCd_ySe (7), (c) TiO₂ NTA/Mn_xCd_ySe (9) and (b) TiO₂ NTA/Mn_xCd_ySe (11). A 5-fold increase in photocurrent density was achieved compared to the pristine TiO₂ NTA film.

Sulfide ions in Na₂S act as hole scavengers as discussed by Kamat et al. in 2011.³⁹ Na₂SO₃ reduces the disulfide species to sulfide again.⁴⁰ The photocurrent density achieved for pristine TiO₂ NTA film was ∼1.39 mA cm⁻² under one sun illumination. The current density obtained was 2.24 mA cm⁻² for the as-deposited film sensitized with 5 cycles, which increased to 4.67 mA cm⁻² for the film with 7 deposition cycles (Fig. 6a and b). This observation is in agreement with SEM results which depicted an increased crystal loading with the increasing number of deposition cycles. With even more sensitizer loading (up to 9 cycles), no significant variation in current density (4.42 mA cm⁻²) was noticed (Fig. 6c). However, a higher loading of 11 (Fig. 6d) and 12 cycles (not shown here) led to a decrease in the observed photocurrent. These aggregated Mn_xCd_ySe nanocrystals in turn act as recombination centers for photoinduced electron–hole pairs, rendering them less effective as sensitizers.^41–43 Moreover, increased loading leads to the blockage of nanotubes pores resulting into a poor electrolyte penetration. Thus, a variation in the number of deposition cycles was found to have a profound effect on the current density. Thermal treatment under a nitrogen atmosphere was also found to alter the photoresponse of the films under illumination. Mn_xCd_ySe/TiO₂ NTA samples prepared through 5, 7 and 9 SILAR deposition cycles followed by annealing at 200 °C, exhibited noticeable enhancement in the photocurrent density compared to the as-deposited film. The ratio of the photocurrent density to dark current density (I_light/I_dark)⁴⁴ has been represented in Table 2.

Table 2 Summarized photocurrent values from the i–V analysis of TiO₂ NTA/Mn_xCd_ySe films synthesized with a varied number of deposition cycles and annealing temperatures

Electrode	Annealing temperature	Ratio of the photocurrent density to dark current density (Ilight/Idark)
TiO2 NTA/MnxCdySe		No. of deposition cycles
		5 cycles	7 cycles	9 cycles	11 cycles
	UA	2.23	4.65	4.42	3.27
	200 °C	5.22	6.11	6.66	4.47
	400 °C	6.36	6.90	8.79	6.17

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In addition, increasing the annealing temperature from 200 °C to 400 °C resulted in a further increase in the current density in all the cases (Fig. 6a–d). These results are in accordance with the XRD analysis, since with an increase in the annealing temperature, the films become more crystalline in nature. However, the films were no longer stable at temperatures more than 400 °C (data not shown). Therefore, 9 cycles of SILAR deposition followed by annealing at a temperature of 400 °C under N₂ atmosphere provides the optimum set of conditions to obtain the highest photocurrent density for Mn_xCd_ySe-sensitized TiO₂ NTAs. The higher photoresponse generated by sensitized films is attributable to the greater charge collection efficiency of the composite film, facilitated by fast electron injection of excited electrons from the conduction band (CB) of Mn_xCd_ySe to CB of TiO₂. The incorporation of Mn_xCd_ySe nanocrystals into TiO₂ matrix enables thermodynamically favourable electron transfer from Mn_xCd_ySe to TiO₂ NTAs, as depicted in Scheme 1. The holes produced are scavenged by the electrolyte.

Scheme 1 Schematic representation of photoinduced charge transfer in the TiO₂/Mn_xCd_ySe composite system. Photoinduced electrons transfer occurs from the conduction band (CB) of Mn_xCd_ySe to the CB of TiO₂, and the holes produced are scavenged by the electrolyte.

More importantly, a better charge conduction and photocurrent density can be attributed to the presence of Mn species (even at such a low concentration), since for the CdSe/TiO₂ NTA film synthesized under similar conditions, the current generated was found to be lower than that for the Mn_xCd_ySe/TiO₂ NTA film (Fig. 7).

Fig. 7 i–V characteristics of TiO₂ NTA/CdSe (9)-400 °C and TiO₂ NTA/Mn_xCd_ySe (9)-400 °C films measured under 100 mW cm⁻² intensity (AM 1.5 global filter).

The photocurrent values obtained in the present study are higher compared to other sensitized-TiO₂ photoelectrodes studied before.^40,45–48 The increase in the photocurrent density with sensitization highlights the fact that Mn_xCd_ySe helps to effectively harvest a greater part of the solar spectrum. It should be noted that although the photocurrent values for films prepared from the repeated set of experiments may vary by a factor of ±10% due to a variation in the experimental conditions, the trend remains the same.

The photocurrent on–off cycles recorded in a rapid sequence (every 5 s) with respect to zero external bias are shown in Fig. 8.

Fig. 8 i–t characteristics of the sensitized films as working electrodes, measured under 100 mW cm⁻² intensity (AM 1.5 global filter) (a) TiO₂ NTA/Mn_xCd_ySe (5) (b) TiO₂ NTA/Mn_xCd_ySe (7), (c) TiO₂ NTA/Mn_xCd_ySe (9) and (b) TiO₂ NTA/Mn_xCd_ySe (11). The figure shows that the current density increases instantaneously upon illumination, confirming the stability of photoanodes.

Photoresponses recorded under intermittent light illumination indicate that the current produced is due to illumination, and the responses are stable as well as reproducible. An instantaneous rise in the photocurrent when the light source was turned on and a prompt recovery to the original value once the illumination was turned off were observed. It demonstrates the minimal mass transport at the electrode–electrolyte interface. It also illustrates the corrosion resistance of the photoanode under visible light illumination.⁴¹ The abrupt response of the film to illumination demonstrates that the photoactivity of the Mn_xCd_ySe/TiO₂ NTA photoanode is stable and reproducible.

Conclusions

The present study demonstrates a facile technique for the deposition of Mn_xCd_ySe nanocrystals on TiO₂ NTAs employing SILAR technique, followed by annealing at a high temperature. The composite films were employed as photoelectrodes in the Na₂S/Na₂SO₃ electrolyte, and the effect of the number of deposition cycles and the annealing temperature on optical and photoelectrochemical properties was unravelled for their possible use, similar to that of the photoanodes for solar cell applications. The incorporation of Mn_xCd_ySe nanocrystals within the TiO₂ NTA structure showcases a wider absorption range and a better light harvesting ability. A 5-fold increase in the photocurrent density achieved for the Mn_xCd_ySe/TiO₂ NTA film compared to the pristine TiO₂ film is a clear manifestation of these aspects. The film was synthesized with 9 cycles of deposition, followed by annealing at 400 °C under a nitrogen atmosphere and demonstrated the highest photocurrent density in this study, thereby confirming the role of the number of SILAR cycles and the annealing temperature in improving the photoresponse of the electrode under visible light illumination.

Acknowledgements

The authors sincerely thank Dr York Smith, University of Utah, for SEM analysis. We thank Dr Wen-Ming Chien for the technical assistance regarding XRD measurements. This project is funded by the Department of Energy under the contract DE-FC36-06-GO86066. Dr David Peterson and Dr Eric Miller act as the program manager and the technical manager, respectively.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06345g

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