Deposition of CdS nanoparticles within free-standing both-side-open stretched TiO2 nanotube-array films for the enhancement of photoelectrochemical performance
Graphical abstract
A free-standing both-side-open TiO2 nanotube-array (TiNT) film makes the nanochannels more amenable to CdS deposition compared with the close-ended structures, thus preventing entry clogging. The high-dispersive and uniform CdS deposition within the tubes allows easy access of the outer light and the electrolyte to the nanochannels, as well as provides superior electron-collection efficiency. This results in significant enhanced incident-photon-to-current conversion efficiency in the CdS/both-side-open TiNT compared with the CdS/one-side-close TiNT.
Highlights
► A facile route to obtain a free-standing both-side-open stretched TiO2 nanotube-array film. ► Uniform CdS deposition within such both-side-open TiO2 nanotubes. ► The maximum incident-photon-to-current conversion efficiency is increased from 41% to 68%.
Introduction
The growth of highly ordered, stretched TiO2 nanotube (TiNT) arrays using electrochemical anodization of Ti foils has attracted a significant amount of interest because of the impressive properties of such arrays for applications in supplying clean and recyclable energy [1], [2], [3], [4]. Compared with the disordered TiO2 nanoparticle film, the tubular structure features highly decreased intercrystalline contacts and vectorial charge transport along the nanotube axis [1], [2], [5], thereby reducing the charge-carrier recombination possibilities considerably. However, TiNT arrays are limited by their poor utilization of solar energy attributable to the wide band gap of TiO2. Efforts to shift this band gap through the coupling of CdS [3], [5], [6], [7] or CdTe [8] or, most recently, multi-heterojunction (e.g. CdS and CdSe [9]) have successfully extended the photoresponse to visible light and exhibited impressive photoconversion efficiencies in PEC studies. However, the nature of TiNT arrays attached to an opaque Ti foil and having a closed bottom has restricted the uniform deposition of narrow band-gap semiconductors, thereby presenting a technically challenge.
Several detachment methods have recently been investigated to flake the TiNT arrays off the underlying Ti substrate. Such methods include ultrasonic agitation [10], [11], methanol evaporation [12], selective dissolution in various solutions [13], [14] and, more recently, self-detaching multi-step electrochemical process [15]. Our previous study [14] deposited CdS into free-standing TiO2 nanotube array films as solar light-driven photocatalysts. The CdS/TiNT arrays were directly struck by the incident light from a favorable illumination direction, largely reducing the light loss through the solution. In this case, however, the tube-ends are closed at the bottom, and air might be trapped inside the tube, making inner surface of the tube less accessible to CdS deposition or even plugging the entrance. This condition will decrease the heterojunction contact area and block the electrolyte diffusion.
In this communication, pre-crystallized annealing, followed by the chemical removal of the bottom caps, was used to obtain free-standing, both-side-open TiNT films. The successive ionic layer adsorption and reaction (SILAR) process was then employed to deposit CdS within the tubes. Such open-ended structures make the nanochannels easily amenable to CdS deposition while minimizing deposition at the tube entry, thus preventing pore clogging. The obtained heterostructure CdS/both-side-open TiNT film showed superior light-harvesting efficiency and reduced the distance that electrons must travel to reach the underlying conducting substrate. These findings were confirmed by the incident photon-to-electron efficiency (IPCE) measurements under back-side and front-side illumination. These results clearly demonstrate that significant enhancement in the PEC cell efficiency can be achieved through the coupling of CdS with both-side-open TiNT films as the photoanodes.
Section snippets
Experimental Section
The TiNT arrays were prepared on Ti foil (TiNT/Ti) using the electrochemical anodization of Ti foil at 80 V in an electrolyte composing 0.5 wt% NH4F and 3 wt% H2O in ethylene glycol for 8 h. We then modified our previous method reported elsewhere [16] to obtain a free-standing, both-side-open TiNT film. First, the resulting TiNT/Ti was pre-annealed at 300 °C under a mixture of 10% N2 and 90% O2 flow for 1 h, followed by immersing in 33 wt% H2O2 solution for 20 s, so that the lifting of the entire TiNT
Results and Discussion
Highly oriented TiNT arrays were flaked off the underlying Ti foil using pre-crystallized annealing, followed by immersion in an H2O2 solution. The lifting of TiNT film off the Ti foil, is shown in Fig. 1(a). The dimension of the film is 1 cm × 2 cm, with nearly the entire area of Ti foil anodized in the electrolyte. Fig. 1(b)–(e) exhibit the typical morphology of the 36 μm long TiNT arrays, examined by FESEM. The average inner diameter at the top of the nanotube is ~ 110 nm with a wall thickness of ~
Conclusions
In summary, conformal and uniform CdS depositions within the both-side-open TiO2 nanotubes were obtained using pre-crystallized annealing, followed by solution-based methods. The obtained structures significantly enhanced CdS/TiO2 and CdS/electrolyte contact areas and reduced the distance that electrons must travel to reach the underlying conducting substrate, resulting in a significant increase in photocurrent generation. Although numerous factors related to photoelectrochemical performance
Acknowledgement
The authors would like to thank the National Science Council of the Republic of China for financially supporting this research.
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