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Black TiO2 nanomaterials have recently emerged as promising candidates for solar-driven photocatalytic hydrogen production. Despite the great efforts to synthesize highly reduced TiO2, it is apparent that intermediate degree of reduction (namely, gray titania) brings about the formation of peculiar defective catalytic sites enabling cocatalyst-free hydrogen generation. A precise understanding of the structural and electronic nature of these catalytically active sites is still elusive, as well as the fundamental structure–activity relationships that govern formation of crystal defects, increased light absorption, charge separation, and photocatalytic activity. In this Review, we discuss the basic concepts that underlie an effective design of reduced TiO2 photocatalysts for hydrogen production such as (i) defects formation in reduced TiO2, (ii) analysis of structure deformation and presence of unpaired electrons through electron paramagnetic resonance spectroscopy, (iii) insights from surface science on electronic singularities due to defects, and (iv) the key differences between black and gray titania, that is, photocatalysts that require Pt-modification and cocatalyst-free photocatalytic hydrogen generation. Finally, future directions to improve the performance of reduced TiO2 photocatalysts are outlined.
Solar energy storage in the form of chemical bonds is of paramount relevance in the modern energy economy to increase the share of renewable energy utilization at zero-carbon emission.
The ideal energy vector envisioned to store solar energy is molecular H2. It has high energy density and can be obtained from water splitting, a very well-known chemical reaction that has inspired the development of several technologies such as electrolyzers, photoelectrochemical (PEC) cells, and photocatalytic reactors for powdered catalysts in aqueous suspensions. (1−7)
In particular, photocatalysis with powdered semiconductor catalysts has been greatly developed in the last 50 years finding application in pollutants removal, (8−10) CO2 photoreduction, (11,12) N2 fixation, (13) and indeed in H2 production from water splitting or photoreforming of H2O/alcohol (i.e., methanol and ethanol) mixtures. (14)
Photocatalytic chemical transformations consist of several consecutive steps, thus limiting the overall photoconversion efficiency. A typical photocatalytic process starts with the generation of electron–hole pairs in the semiconductor bulk following light irradiation, and their subsequent migration toward the surface where reaction with molecular substrates occurs.
TiO2 is the most diffuse photocatalyst providing a set of material properties such as an outstanding stability toward photocorrosion, nontoxicity, low cost, and conduction and valence band edges (CB and VB, respectively) straddling the redox potentials of many sustainable chemical transformations (Figure 1). However, TiO2 efficiency has been hampered by its wide bandgap of ∼3.2 eV that limits light absorption to the UV region of the solar spectrum (∼4% of the total solar irradiance).
Furthermore, TiO2 shows high recombination of photogenerated charge carriers. To mitigate this limit, different approaches have been explored for the material’s assembly such as engineering of nanocrystals’ shape and facets, formation of heterojunctions with other semiconductors, and the deposition of noble metal (Au or Pt) cocatalysts to enhance charge separation through the formation of an interfacial Schottky barrier. (15,16)
Historically, the limited light absorption has been tackled, instead, by doping TiO2 nanomaterials with foreign atoms. Doping of TiO2 nanomaterials produces colored TiO2, which are materials with modified electronic structure due to the introduction of suitable “intra-bandgap” electronic states that modify TiO2 light absorption and optical properties.
Early work on doped TiO2 employed transition metals (e.g., V, Cr, Mn, Fe, and Cu) introduced as substitutional atoms inside the crystalline habit to generate 3d electronic states lying in the range 0.5–1.5 eV below the CB of TiO2 and thus providing visible light absorption and photocatalytic efficiency (Figure 1). However, metal doping showed an unfavorable trade-off between absorption and photocatalytic activity, being often responsible for increased charge recombination via newly formed deep electronic levels. (16)
In contrast, nonmetal (e.g., N, C, and S) doping has shown great potential in forming efficient visible light active TiO2 photocatalysts, typically due to the formation of 2p electronic states above the VB capable of producing efficient charge transfer electronic transition to the 3d CB of TiO2 and thus providing high photocatalytic activities (Figure 1).
For example, N-doping was reported to yield yellow TiO2 powders exhibiting a red shift of the optical absorption onset up to ∼500 nm. (17) However, in some cases, N or C species induce only a surface modification (18) of TiO2 rather than bulk doping.
TiO2 powders of various other colors have also been reported. Liu et al. (19) developed an alternative version of nitrogen doping to produce red TiO2 anatase microspheres; the reported methodology relied on predoping TiO2 with interstitial boron atoms. The predoping process improved the solubility of substitutional N atoms in the lattice of anatase TiO2 and limited at the same time the formation of Ti3+ centers as extra electron from B atoms compensating the charge difference between lattice O2– and substitutional N3–. Red TiO2 was found to absorb the full visible light spectrum and exhibited an optical Eg that varied from ∼1.9 eV on the surface to 3.2 eV in the core, as a consequence of the introduced boron concentration gradient. (19,20) Interestingly, the finding did not generate follow-up work in photocatalysis.
Tian et al. (21) reported the preparation of green TiO2. The green color originated from a charge-transfer complex involving hydrazine groups linked to surface Ti4+ centers; green anatase powders showed a broadband light absorption in the visible region that extended also to the near-infrared (NIR) range (∼1100 nm), with an optical band gap of 1.05 eV. These examples are based on doping (or codoping) of TiO2 with extrinsic donor or acceptor species.
However, colored TiO2 can also be formed by intrinsic doping, namely by the introduction of oxygen vacancies (VOs) and formation of Ti3+ centers in the TiO2 lattice.
The synthesis of these materials is usually carried out by a high-temperature treatment of TiO2 in various reducing atmospheres (e.g., vacuum, Ar, H2/Ar, and pure H2). (22−24) TiO2 can be obtained with a gray, blue, brown, or black color depending on the utilized conditions. The resulting color is ascribed to the formation of various amounts of Ti3+ and VOs. Increasing the “level” of reduction leads, in general, to a higher density of defects (e.g., Ti3+ and VO concentration) and consequently to “darker” TiO2 powders.
In 2011, Chen et al. reported the first black TiO2 nanomaterial for photocatalysis. (25) Upon a thermal treatment at 200 °C under high pressure of H2, stoichiometric anatase TiO2 was converted in defective nanocrystals with high visible light absorption (Figure 2a) and high photocatalytic activity for H2 generation during photoreforming of water/methanol mixture if a Pt cocatalyst was used. The enhanced performance of black TiO2 was related to the increased light absorption due to introduction of lattice disorder and H-doping, which consequently narrowed the optical bandgap of black TiO2 to 1.54 eV by introducing electronic states forming significant VB and CB tailing (Figure 2b). An interesting feature of this type of core–shell black nanocrystals was the sharp optical absorption band edge shown at ∼1000 nm.
A myriad of material designs, from H-doping to defect engineering, have appeared since 2011 and have produced reduced TiO2 nanomaterials with enhanced (or not) photocatalytic performance and having various colors from green, yellow, blue, and back to black, (26−28) as well as an important “gray” version. (29,30)
Despite early studies on reduced TiO2 that started in the 50s (31−33) and significantly progressed with the advent of scanning tunneling microscopy (STM) and surface science, (34) the renewed interest in the past decade on defective oxides for photocatalysis has enabled the discovery of new materials and phenomena, holding great promise to further boost the advances toward solar fuel generation.
Recently, the synthesis of gray TiO2 nanomaterials showing high photocatalytic H2 production by operating under cocatalyst-free conditions has been reported, opening new opportunities in the design of catalytic sites for photocatalysis. (29,35−37)
In particular, Liu et al. introduced several preparation methods to reduce TiO2 nanopowders (29,30,38) and nanotubes (35,39) to form partially reduced materials with stable and very high photocatalytic hydrogen production without the use of any noble metal cocatalyst. The high-temperature, high-pressure (500 °C, 20 bar) hydrogenation of anatase or mixed anatase/rutile TiO2 produced unique catalytic sites that enabled cocatalyst-free hydrogen production rate that were 2 orders of magnitude higher than those observed for stoichiometric powders, reaching values of more than 200 μmol g–1 h–1. Later on, other synthetic procedures, such as high energy ion implantation, (39) hydride ball milling, (38) and partial oxidation of TiN powders, (40) have been shown to produce similar reduced TiO2 varieties producing hydrogen without any addition of noble metals.
The use of various forms and structures of hydrogenated TiO2, black TiO2, or more generally reduced TiO2 have meanwhile also shown superior activity when used as photoanodes for photoelectrochemical water splitting. (41−49) Recent findings have unveiled that a major contribution to the working mechanism of reduced titania electrodes is given by the enhanced conductivity, (50) while others argue a higher donor density of black TiO2, resulting in higher band bending and therefore charge separation at the electrode/electrolyte interface. (51) Further studies are needed and might reveal specific roles of defects in addressing photoelectrochemical selectivity during water splitting (i.e., two-electron vs four-electron oxidation). These aspects have been recently reviewed and thus they will not be covered in this Review. (52)
Nevertheless, the common line that underlies black TiO2 research is the mismatch among the amount of increased absorption or band gap narrowing and the corresponding photocatalytic activity; although light in the visible range may be absorbed, no visible light reaction activity may be observed.
Relevant questions that emerge from a literature survey are the following: (i) Where are visible photons lost during the photocatalytic process? (ii) What is the best material design to increase both absorption and photocatalytic activity? (iii) What is the subtle connection among crystal defects, structural and electronic singularities, light absorption, charge separation, and photocatalysis?
In this Review, we provide some answers to these questions with the aid of current knowledge and give a brief overview on defective TiO2 nanomaterials for photocatalysis, drawing relationships that interlink structural and electronic features in TiO2 to photoconversion efficiency.
In particular, we will cover the type of crystal defects in TiO2 and the consequences that they bring on lattice geometry as well as on electronic DOS. We present electron paramagnetic resonance spectroscopy (EPR) as a useful technique that provides plenty of information both on crystal structure and extra electrons hosted in TiO2 due to creation of crystal defects.
With this in mind, we will discuss several surface science studies on TiO2 photocatalysis that are, in our opinion, fundamental to tackle a rational design of defective black TiO2 for photocatalysis.
We will describe the different types of defects formed in various black TiO2 nanocrystals and review how they influence the photocatalytic hydrogen production by using water splitting or photoreforming of alcohols. This last section is dedicated to photocatalysis with black TiO2 and covers with a critical view the development of photocatalysts from core–shell nanocrystals and phase nanojunctions to cocatalyst-free black TiO2.
2.1. Types of Crystal Defects and Relation with Photocatalysis
A large variety of physical properties of crystalline material is regulated by the presence of different types of defects and imperfections. Defects engineering indeed plays a prominent role in tailoring electronic, magnetic, optical, mechanical, and quantum properties as well as it is crucial for activation of heterogeneous (photo)catalytic processes. (53)
The classification of crystal defects is generally made according to the dimensionality of the defect. (54,55) Zero-dimensional (0D) defects are related to a single or a few atomic positions and hence are called point defects (Figure 3a). In any TiO2 nanocrystal, we may find therefore several point defects such as Ti vacancy TiV (rare), Ti interstitial Tii (common and important for photocatalysis), O vacancy VO (often observed in reduced TiO2), and interstitial (e.g., hydrogen or nonmetal dopants) or substitutional impurity (e.g., metal or nonmetal dopants).
The introduction of point defects produces structural rearrangements that may create significant distortions in the local symmetry of Ti octahedra, thus influencing the charge transport and recombination during photocatalysis. This topic will be discussed in more detail in Section 2.2.
Similarly, the direct consequence of the introduction of 0D defects on electronic DOS of TiO2 depends on their specific nature, and several excellent reviews provide in depth discussion on this aspect. (10,15)
Substitutional metal dopants generally contribute to DOS through additional 3d states forming below the CB of TiO2 (Figure 1). A similar electronic effect is found when a significant amount of VOs is generated, with the mutual formation of Ti3+ charged sites producing electronic states 0.8–1.2 eV below the CB. (56) Otherwise, nonmetal dopants (e.g., N, C, S) and interstitials (e.g., H or Ti) present electronic features that populate the DOS of defective TiO2 in the region above the VB, providing a more effective strategy than metal doping to modify TiO2 electronic structure (Figure 1 and 2b). (25,26,56,57) Electronic transitions from 2p states due to nonmetal dopants to CB are usually very efficient and produce a positive trade-off between optical absorption and photocatalytic efficiency. Otherwise, metal centers producing excess of electronic states below the CB, once exceeding a specific threshold, behave like recombination centers for photogenerated charges, thus being detrimental for photocatalytic reactions. (58)
The physical location of point defects is another important feature, which has great influence in the electronic DOS of TiO2 and its photoreactivity. Point defects can be positioned at three different locations: (i) at the surface, defined as the first atomic layer of a nanocrystal; (ii) at the subsurface, defined as the crystal slab contained between the second layer from the surface and including few nanometers in depth; and (ii) in the bulk of the nanocrystal.
Linear or 1D defects are generally called dislocations and produce lattice strain (Figure 3b), which has been reported to be beneficial for photocatalytic activity with TiO2. Dislocations are the result of plastic deformation of crystal lattice and identify the area where the crystallographic registry is lost. (55) The 2D defects that appear in crystals can be usefully classified into three groups: free surfaces exposing uncoordinated atoms (see Figure 3c), interphases within a crystal such as stacking faults and antiphase boundaries, and other various types of boundaries, for instance, grain boundaries and two-phase boundaries (interphases). For TiO2 photocatalysts, 2D defects are especially important and dictate surface reactivity as well as charge transport and separation in the bulk, for example by creating anatase/rutile nanojunctions (Figure 3c). (60)
Volume defects (3D) mainly cover inclusions, crack, voids, and pores. Here, we consider more relevant to our purpose the discussion on voids created by the clustering of VOs and their effects on reactivity of TiO2. Many reports have shown that clustering of vacancies is a prominent phenomenon observed in a wide range of metal oxides ranging from CeO2(61,62) and simple perovskites (i.e., SrTiO3) (63,64) to double perovskites, where it is responsible for a large change in magnetoresistance response. (65,66) A relevant case is CeO2 that is considered a prototypical reducible oxide due to its high “oxygen storage capacity”. This property makes CeO2 a fundamental component of modern automotive exhaust treatment, with VOs and their linear clustering, determining reactivity of CeO2 catalysts (Figure 3d). (61,62) Ensembles of VOs have been also observed at the interface of SrTiO3 and LaAlO3 thin films, being responsible for the formation of electron gas at the interface between these two perovskites. (67) Although there is no clear evidence so far for the formation of such 3D defects in reduced TiO2, all these examples suggest that they may play prominent role in the physics and chemical reactivity of TiO2.
Finally, an interesting case study is the situation when more than one type of defects forms at the same time. This particular “defects pairing” is often overlooked in photocatalysis, mostly due to the challenge behind its structural and electronic characterization. A seminal contribution from Diebold et al. provided evidence for such defect pairing through STM images and density functional theory (DFT) calculations. (68) A reduced anatase (101) crystal showed ordered subsurface VOs in STM, consistent with DFT results predicting that VOs at subsurface and in bulk have a lower formation energy than those on the surface; here it is noteworthy that defects on rutile remain on the surface. (68) Therefore, defective polymorphs may behave entirely differently in photocatalysis. The formation of ordered subsurface defects invokes a Frenkel hop mechanism (Figure 4). It relies on the formation of VOs that induces the migration of a neighboring Ti atom to an interstitial site (Ti), leaving behind a TiV. This process is repeated producing a series of Tii-TiV pairs. (68) In photocatalysis with reduced TiO2, VOs are usually considered, but defect pairing should be also taken into account, and more efforts should be put to elucidate the connection between different types of defects to shine light on important aspects of photocatalytic processes.
2.2. Defects in Reduced TiO2: An Atomistic View from Electron Paramagnetic Resonance (EPR) Spectroscopy
Continuous wave (CW) X-band EPR spectroscopy has been used extensively to address nature and stability of the spin centers present in the TiO2 photocatalyst, before, during, and after light irradiation experiments. The most common paramagnetic centers encountered in various TiO2 preparations, from nanoparticles to nanotubes, are those associated with the presence of Ti3+ sites (3d1, S = 1/2) from the diamagnetic Ti4+ sites and oxygen-based radicals (S = 1/2, – O–•, – O2•–). These “spin-active defects” can be embedded in the lattice or formed on the material’s surface. The most studied polymorphs of TiO2 for the water splitting processes are anatase and rutile. Both systems have tetragonal crystal structures. However, anatase shows space group I41/amd (unit cell, a = 3.7845, c = 9.5143 Å), while rutile expresses the space group P42/mnm (unit cell a = 4.5937, c = 2.9587 Å). The magnetic moments arising from formation of the spin-containing centers, Ti3+ and oxygen-based radicals, are very sensitive probes that screen even minor alterations of their surroundings (ligand-field). The contribution of orbital angular momentum to the spin angular momentum shifts their geff values away from the free electron value, ge = 2.0023. The extent of the shift from ge is proportional to the spin–orbit coupling constant, and anisotropic resonances (gx,y,z) arise from distortions in the crystal-field symmetries, which lift the frontier orbitals degeneracies. Therefore, in TiO2 materials the geff values observed for the spin-defects can be grouped into two categories; (i) spin-containing centers that feature larger g-values than the free-electron in the vacuum and (ii) those with smaller g-values. This implies that from EPR measurements a rich set of information regarding both the structural deformation and the electronic features introduced by formation of defects is obtained. Formation of Ti3+ sites in TiO2 gives geff < 2.0023, with resonance signals that are most often described in term of S = 1/2 center embedded in a tetragonal-field (D4h). The g-value observed for oxygen-based radical in TiO2 falls at geff > 2.0023, and the observed g-tensor components are consistent with axial or rhombic S = 1/2 systems. Figure 5 collects a se