On the Morphology of Nanostructured TiO2 for Energy Applications: The Shape of the Ubiquitous Nanomaterial

Abstract

Nanostructured titania is one of the most commonly encountered constituents of nanotechnology devices for use in energy-related applications, due to its intrinsic functional properties as a semiconductor and to other favorable characteristics such as ease of production, low toxicity and chemical stability, among others. Notwithstanding this diffusion, the quest for improved understanding of the physical and chemical mechanisms governing the material properties and thus its performance in devices is still active, as testified by the large number of dedicated papers that continue to be published. In this framework, we consider and analyze here the effects of the material morphology and structure in determining the energy transport phenomena as cross-cutting properties in some of the most important nanophase titania applications in the energy field, namely photovoltaic conversion, hydrogen generation by photoelectrochemical water splitting and thermal management by nanofluids. For these applications, charge transport, light transport (or propagation) and thermal transport are limiting factors for the attainable performances, whose dependence on the material structural properties is reviewed here on its own. This work aims to fill the gap existing among the many studies dealing with the separate applications in the hope of stimulating novel cross-fertilization approaches in this research field.

Keywords: dye-sensitized solar cells; nanofluids; nanophase TiO2; perovskite solar cells; photoelectrochemical water splitting.

Figure 1

Schematics of a DSSC (left) and detail of the photoanode (right). A comprehensive review of the operating principles of a DSSC is reported in [20].

Figure 2

Schematic of the light trapping phenomenon in the nanostructured DSSC photoanode.

Figure 3

SEM images of examples of DSSCs PAs based on different TiO₂ nanostructures. (a,b) adapted with permission from [43]. Copyright 2008, Wiley & Sons; (c,d) adapted with permission from Ref. [48], Copyright 2010, American Chemical Society; (e,f) adapted with permission from Ref. [53], Copyright 2010, American Chemical Society.

Figure 4

SEM (top) and AFM (bottom) images of two examples of different titania patterned electrodes in PSCs. (a,b) adapted with permission from [59], Copyright 2016, Wiley and Sons; (c,d) adapted with permission from Ref. [63], Copyright 2019, Elsevier B. V.

Figure 5

Schematic of formation process of hierarchical TiO₂ nanostructures. Reproduced from 354 Reference [97] under the Creative Commons CC-BY-NC-ND license.

Figure 6

Scheme of the key steps of the photocatalytic reaction in a titania nanoparticle. If the incoming photon has an energy hν greater than the value of Eg (energy gap or band gap), an electron (e⁻) can be promoted from the valence band (VB) to the conduction band (CB), leaving behind a hole (h⁺). The electronic structures of VB and CB were determined from X-ray Absorption and Emission Spectroscopy. Adapted with permission from [132]. Copyright 2014, American Chemical Society.

Figure 7

Schematics of a photoelectrochemical cell for water-splitting. The light impinging on the photoanode generates hole and electron pairs; the first migrate to the surface and promote the oxygen evolution, while the second, after travelling to the cathode, drive the hydrogen evolution.

Figure 8

SEM images of hierarchical titania nanostructures (a,b), and diffuse reflectance spectra (c) of electrodes made of commercial titania nanoparticles (black curve) and of hierarchical nanostructures (red curve). Adapted with permission from [142]. Copyright 2014, American Chemical Society.

Figure 9

SEM images of different titania nanostructures for PEC electrodes: rough nanobeads (a) adapted with permission from [135], Copyright 2015, American Chemical Society, columnar (b) adapted with permission from [160], Copyright 2008, American Chemical Society, and tubular structures (c) adapted with permission from [144], Copyright 2009, American Chemical Society.

Figure 10

SEM images of branched tubular titania structures. Adapted with permission from [184], Copyright 2018, Wiley.

Figure 11

Schematic picture of the aggregate structure formed by nanoparticles arranged in a linear chain (black) and sidechains (gray). The conductivity of the whole aggregate (k_g) is calculated by considering the conductivity of the backbone nanoparticles embedded in a medium with an effective conductivity k_sc, due to the surrounding presence in the fluid of the sidechain nanoparticles. Adapted with permission from [231], Copyright 2006, AIP Publishing.

Figure 12

Influence of the fractal dimension on the thermal conductivity enhancement in TiO₂-based NFs using Equation (13) and the parameters described in [220]. Adapted with permission from [220], Copyright 2014, Wiley and Sons.