Solid-state NMR of active sites in TiO2 photocatalysis: a critical review
, Abstract
Titanium dioxide (TiO2) is one of the optimal semiconductor metal oxide photocatalysts with a wide range of application fields, such as heterogeneous catalysis, energy science, and environmental science. Solid-state nuclear magnetic resonance (NMR) spectroscopy is a powerful tool for characterizing both structure and dynamics at an atomic-molecular level in heterogeneous catalysts. In this review, we first provide a brief discussion on the progress in investigating the structures of titanium and oxygen in bulk and on the surface of TiO2 by using various solid-state NMR techniques. Advances in the understanding of electronic structure and properties of TiO2 with distinct surface features, including various crystal facets and heteroatomic adsorption by chemical probe-assisted NMR techniques, are secondly presented. The solid-state NMR characterization of heteroatom active sites (such as 13C, 15N, 11B, 27Al) and their function in TiO2 photocatalysts is described in detail. Finally, a critical discourse assesses the current limitations and prospects of solid-state NMR in its application to the optimization and design of advanced TiO2 photocatalysts.
Keywords
INTRODUCTION
Titanium dioxide (TiO2) has emerged as a leading candidate in the field of photocatalysis owing to its unique confluence of desirable attributes: exceptional optical and electronic properties, robust thermal and chemical stability, environmental benignity, and economic viability. This advantageous combination has propelled TiO2 to the forefront of research in diverse applications, including solar energy harvesting[1-3], photocatalytic hydrogen generation[4-6], CO2 conversion[7-9], and organic pollutant degradation[10,11]. Since 2000, the statistical analysis reported that publications on TiO2 photocatalysis crossed 10,000 per year, suggesting that this field has been intensively interested in research and development.
TiO2 exists in three crystalline phases: anatase, rutile, and brookite, and the former two can be used as semiconductor photocatalysts. The structure of both rutile and anatase TiO2 consists of chains of [TiO6] units, where six O2- ions surround one Ti4+ ion to form an octahedron[12]. However, due to the differences of each [TiO6] octahedron distortion, the Ti–Ti and Ti–O distances, octahedron chain assemblies, the energy band structure and mass density between two crystalline phases of TiO2 are different. Besides the crystalline phases, the differences in crystallite size and specific surface area, crystalline plane and morphology also cause change of properties in TiO2 due to the subsequent change in structures and content of microscopic Ti and O sites on TiO2 surface. For example, using K-edge X-ray absorption near edge structure (XANES) techniques, Chen et al. found that compared to the octahedron Ti sites in TiO2 with big size (50 nm), severe distortion of the Ti site environment exists in TiO2 nanoparticles with small size (1.9 nm)[13]. Due to the truncation of the lattice, the distorted Ti sites should be a penta-coordinate square pyramidal geometry, located mainly on the nanoparticle surface, responsible for the chemisorption of organic molecules. It was also found that more distorted bond angles of bridging oxygens (O–Ti2) and more unsaturated Ti sites (pentacoordinated Ti4+) are present on the (001) facet compared to the (101) facet[14,15]. All that lead to more active bridging O centers, easier O2 adsorption, and higher surface energy on the (001) facet[16,17]. Yu et al. reported that the higher photocatalytic ability of TiO2 thinner films was due to lesser opacity and more surface active sites[18]. Therefore, these microscopic active sites on the TiO2 surface should play a crucial role in improving photocatalytic activities.
The semiconductors-based photocatalytic reactions should involve three steps [Figure 1][6,12,19-21]: Firstly, the electron-hole pairs (carriers) are excited by photons with an energy more than the band gap of TiO2. Secondly, the photogenerated carriers separate or recombine during migration. Finally, the photogenerated carriers react with surface-adsorbed molecules through active sites on the TiO2 surface. As well known, owing to the wide band gap (rutile of 3.0 eV, and anatase of 3.2 eV), ultraviolet (UV) radiation is a prerequisite to facilitating the formation of the carriers during the photocatalytic reaction. Since ca. 50% of the solar radiation on earth is in the visible (vis) region, UV light makes up only ca. 5% of the natural light spectrum; improving the absorption of solar light by TiO2 has become one of the most urgent tasks in photocatalytic research and development. To solve this problem, some common and promising modification approaches, such as heteroatoms (ions) doping[1,22-32] and loading[1,19,32-41], have been utilized to narrow the band gap and enhance the separation efficiency of photogenerated carriers. Additionally, the reaction of photogenerated carriers on the TiO2 surface is also crucial in photocatalysis, in which the photogenerated carrier transfers to surface active sites to form active intermediates, and then the active intermediates react with surface molecules[42-48]. For example, surface hydroxyl (OH)/oxygen (O) sites and adsorbed H2O on TiO2 can trap photogenerated holes to form active paramagnetic intermediates (such as
Figure 1. Semiconductors-based photocatalytic mechanism. Reproduced with permission from[12]. Copyright 1995, American Chemical Society.
An in-depth understanding of active centers, including heteroatoms (ions) and surface active sites is the key to establishing structure-activity relationships, which can facilitate the rational design of highly efficient TiO2 photocatalysts. Thus, numerous techniques, such as high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Infrared (IR) spectroscopy, Electron paramagnetic resonance (EPR), solid-state nuclear magnetic resonance spectroscopy (NMR), etc., have been employed to study the structure of photocatalysts and the relevant reaction mechanisms. Generally, the crystal structure is determined by HRTEM, XRD, and Raman spectroscopy[51-56], and the electronic structures can be studied by X-ray photoelectron and X-ray absorption and emission spectroscopies[57-60]. EPR has been used to detect defect sites [Ti3+ and oxygen vacancy (Vo)], and surface paramagnetic species (O2•-, O- species, and •OH)[42,50,61], while IR spectroscopy has been used to detect surface OH and reaction intermediates of EPR and NMR silence[17,53,62].
Among them, solid-state NMR is a powerful tool for characterizing the structure of solid-state materials at the atomic-molecular level[63-70]. Although research on solid-state NMR techniques for photocatalyst active-site structures and photocatalytic reactions is rapidly increasing [Figure 2], the research interest in the application of solid-state NMR techniques is still relatively low compared to other techniques in the field of photocatalysis. To the best of our knowledge, this article presents the first examination of the application of solid-state NMR to TiO2 photocatalysis. Specifically, we conduct an overview of the advances made utilizing solid-state NMR techniques to analyze TiO2 photocatalysts. Through selected examples from the literature, we demonstrate how solid-state NMR has been utilized to reveal the atomic structures and interactions between various sites or components within TiO2. The review also discusses both the current limitations of solid-state NMR methodology in photocatalyst research and promising future directions for this technique.
Figure 2. Yearly number of publications and stepwise development for photocatalysts studied by NMR spectroscopy. NMR: Nuclear magnetic resonance.
TITANIUM AND OXYGEN ACTIVE SITES IN TiO2
The past few decades have witnessed a surge in efforts dedicated to unraveling the structural and electronic intricacies of TiO2, employing both experimental and theoretical approaches. However, the structure and coordination environments of Ti and O sites on the TiO2 surface are very different from those in bulk and thus remain poorly understood due to the complex surface structures. A wide variety of active sites exist on the TiO2 surface, including distorted Ti sites, low-coordinated Ti sites, bridging O sites, tri-coordinated O sites, etc. The distorted Ti sites, such as the double-bonded titanyl (Ti=O) groups on the (011) surface of reconstructed rutile TiO2, can promote the dissociation of part of adsorbed water[71,72]. The coordinatively unsaturated Ti atoms, including the penta-coordinated Ti sites (TiO5) on the surface and the tetra-coordinated Ti sites (TiO4) on the edge/corner, are important anchoring sites for the adsorption of targeted molecules (such as H2O, HCHO, and ethylene)[73-76] and for maintaining the high stability of the cocatalysts (such as Pd, Ru, Au, etc.)[77-79]. Theoretical calculations predicted that the bridging O sites (OTi2) of TiO2 can interact with the proton and facilitate the dissociation of H2O, which are closely related to catalytic mechanisms[80-82]. Furthermore, the coordinatively unsaturated Ti and O sites serve as active sites for charge carrier trapping and molecular adsorption during CO2 reduction and methanol conversion[83-85]. Thus, determining the structure and distribution of the Ti and O sites on TiO2 is a prerequisite for understanding the structure-property relationship.
Titanium active sites in TiO2
47/49Ti NMR can be utilized to detect the local environments of Ti sites in TiO2 photocatalysts[86-88]. However, the isotopes have a low natural abundance, with 47Ti at 7.28% and 49Ti at 5.51%. They also exhibit low Larmor frequencies, measuring 22.55 MHz at a magnetic field of 9.4 T. Additionally, the Ti atoms in TiO2 photocatalysts are quadrupole nuclei, with spin quantum numbers of I = 5/2 for 47Ti and I = 7/2 for 49Ti. These nuclei possess a relatively low gyromagnetic ratio and experience significant quadrupolar interactions, which lead to low sensitivity and resolution of these spectra due to the wide signal lineshape[89-91]. The 47/49Ti static NMR spectra were used to monitor the transition of the crystalline phases of brookite, anatase, and rutile in as prepared TiO2 at different annealing temperatures from 400 to 850 °C, and 47/49Ti NMR parameters were extracted from the spectral simulations of the corresponding components of bulk polycrystalline phases [Figure 3][92,93]. The size, crystallinity, and crystal phase of TiO2 can sensitively be reflected on the lineshape of the corresponding NMR signal. For example, the 49Ti NMR parameters, including chemical shift (δiso), quadrupole coupling constant (CQ), and asymmetry parameter (ƞ), for anatase were determined to be -67 ppm, 4.6 MHz, and 0.1, respectively, whereas for rutile the corresponding 49Ti NMR parameters are -15 ppm, 13.4 MHz, and 0.2, respectively. It was found that the narrow peak should be the 49Ti NMR signal in TiO2 bulk and the other relatively broad peak should be the 47Ti NMR signal in TiO2 bulk. With the increase of annealing temperature to 700 °C, the quadrupole linear patterns of 47Ti and 49Ti static NMR signals become more pronounced, indicating that the crystallinity of the anatase becomes better. With the further increase of annealing temperature, two NMR signals corresponding to 47Ti and 49Ti in rutile bulk increase gradually at the expense of the 47Ti and 49Ti NMR signals in anatase bulk, indicating the transition of crystal phase from anatase to rutile.
Figure 3. Experimental and simulated static Hahn-echo 47/49Ti NMR spectra of TiO2. The TiO2 nanoparticles were annealed before solid-state NMR experiments at variable temperatures. Reproduced with permission from[93]. Copyright 2001, American Chemical Society. NMR: Nuclear magnetic resonance.
However, a long-time (ca. 20 h) acquisition at 33.81 MHz using a CMX Infinity 600 spectrometer and large sample quantity (9.5 mm rotor) was necessary to ensure the signal-to-noise ratio of the spectra, and it is difficult to differentiate and assign the seriously overlapped resonances due to the large chemical shift anisotropy (CSA) and quadrupolar interactions. The advanced NMR techniques have been developed to optimize the signal excitation and acquisition. Bräuniger et al. use fast amplitude-modulated (FAM) radiofrequency (RF) pulse trains to enhance the sensitivity of the signal via transferring spin population from the satellite transitions to central transition[94]. In comparison with Hahn-echo acquisition, the intensity of the 47/49Ti central-transition line has increased by more than twice in the magic angle spinning (MAS) NMR spectra of TiO2.
As well known, the 47Ti and 49Ti isotopes exhibit almost identical Larmor frequencies and natural abundances[89]. Thus, to analyze the Ti sites of TiO2, it is important to distinguish the 47Ti and 49Ti signals in the NMR spectra. When these isotopes occur in sites with a significant electric field gradient (EFG), the different nuclear spin quantum numbers would result in varying effective RF fields for the central transition nutation frequencies. As such, Larsen et al. proposed isotope-selective quadrupolar Carr-Purcell Meiboom-Gill (QCPMG) pulse sequence to selectively excite the 47Ti or 49Ti powder linear[95]. The authors performed solid-state NMR experiments and numerical simulations on the anatase and rutile of TiO2. The 47Ti and 49Ti isotopes for anatase, with different quadrupolar interaction between the EFG at the titanium site and their nuclear quadrupole moments (Q), were separated at the field of 21.1 T, and their EFG and CSA tensors were accurately determined [Figure 4].
Figure 4. Experimental static 47Ti and 49Ti NMR spectra of anatase using the QCPMG pulse sequence. (A) The ordinary QCPMG pulse sequence; (B) the devised 49Ti-selective pulse sequence; and (C) the devised 47Ti selective pulse sequence. Reproduced with permission from[95]. Copyright 2006, Elsevier. NMR: Nuclear magnetic resonance; QCPMG: quadrupolar Carr-Purcell Meiboom-Gill.
Precisely discerning the diverse Ti environments in TiO2 presents a significant analytical challenge in static 47/49Ti NMR due to the significant overlap and inherent broadness of their spectral signatures. To overcome this problem, Epifani et al. used the MAS method at 12 kHz. This approach effectively improved the resolution of their NMR signals, allowing for a more detailed and accurate characterization of the titanium environments in both pristine TiO2 and V2O5-loaded TiO2 samples[96]. As shown in Figure 5, two resonances at 213 and -245 ppm were present in the 47/49Ti MAS NMR spectrum of amorphous TiO2 before high-temperature heating (Figure 5, Left), which correspond to surface Ti bonded to OH and lattice Ti sites in TiO2, respectively. A new dominant 49Ti resonance at 792 ppm appeared after heating at 400-500 °C. The resonance at 792 ppm was even more intense in the 47/49Ti MAS NMR spectrum of TiO2-V2O5 nanocrystals (Figure 5, Right). The new signal of 792 ppm should be ascribed to surface Ti sites, having a unique distorted tetrahedral environment unlike that of bulk anatase TiO2. The V2O5 loading facilitates the rearrangement of surface Ti sites to distorted tetrahedral geometry.
Figure 5. 47/49Ti MAS NMR spectra. (Left) pristine TiO2 samples are examined as prepared, and after heating at 400 and 500 °C. (Right) V2O5-loaded TiO2 samples were examined under identical conditions. Reproduced with permission from[96]. Copyright 2015, Elsevier. MAS: Magic angle spinning; NMR: nuclear magnetic resonance.
Oxygen active sites in TiO2
Oxygen is another most important constituent atom of TiO2 and plays a critical role in various chemical processes that occur on the catalyst surface. Thus, an understanding of the structure and distribution of the O sites helps develop more efficient and effective catalytic systems for energy storage and conversion. As previously reported[67-70,97-102], 17O MAS NMR can be utilized to distinguish the local structures of oxygen sites in oxygen-containing materials. However, the 17O quadrupolar nucleus (I = 5/2) exhibits a relatively low gyromagnetic ratio (γ = -5.774 MHz·T-1) and low 17O abundance (0.037%). Thus, it is difficult to use 17O NMR to study the oxygen-containing materials in conventional magnetic fields (≤ 14.1 T), and the 17O isotopic entailment is necessary to acquire the 17O MAS NMR spectra[103]. In the earlier study, the 17O enriched TiO2 was commonly prepared by hydrolysis of organic titanate [such as Ti(Oi-Pr)4] using 17O enriched H2O[101] or high-temperature calcination of TiO2 using 17O enriched O2[104]. Accordingly, the 17O NMR signal of bulk oxygen, which is predominant in TiO2, was solely observed. The 17O MAS NMR spectrum of anatase and rutile TiO2 exhibits a single resonance at 562 and 596 ppm, respectively, which was assigned to tri-coordinated oxygen (OTi3) in the corresponding crystal phase. Besides the resonance in the anatase (562 ppm) or rutile (596 ppm) domain, three resonances at 516, 543, and 572 ppm, corresponding to distorted OTi3 or tetra-coordinated oxygen (OTi4) sites on the interface between anatase and rutile, were observed in the spectra of mixed-phase TiO2 [Figure 6].
Figure 6. 17O MAS NMR spectra of anatase TiO2 and mixed-phase TiO2 of anatase and rutile with 17O enrichment. Reproduced with permission from[104]. Copyright 2014, Elsevier. MAS: Magic angle spinning; NMR: nuclear magnetic resonance.
Recently, Li et al. used 90% 17O-enriched H2O to realize surface-selectively 17O-labeled anatase TiO2 with dominant exposed (001) facets [TiO2(001)] and anatase TiO2 with dominant exposed (001) facets [TiO2(101)] [Figure 7][105]. Based on the 17O MAS NMR spectra and theoretical calculations, surface O sites on TiO2 exposing different facets were roughly distinguished. The signals at high frequencies (600-780 ppm) can be attributed to bridging O sites (OTi2); the peaks at 460-580 ppm should be due to tri-coordinated oxygen (OTi3) species; the resonances at lower frequencies (100-250 ppm) can be assigned to hydroxyl groups (Ti–OH); the signals at -200-30 ppm can be ascribed to adsorbed H2O. The 17O NMR parameters, including quadrupole coupling constant (CQ) and asymmetry parameter (ƞ) [Table 1]. In addition, it has been accepted that the type, content, and structure of oxygen sites on different crystal surfaces vary greatly due to the difference in surface defects and reconstruction, which can be studied by 17O MAS NMR spectroscopy. The 1H → 17O cross-polarization (CP) MAS and two-dimensional (2D) heteronuclear correlation (HETCOR) NMR have been utilized to probe the spatial proximity of the H and O atoms, which, however, is time-consuming and exhibits low detection efficiency for the quadrupolar nucleus with low γ and low surface abundance[106-108]. These conventional correlation NMR techniques validated the assignment of surface OH and adsorbed H2O on the TiO2 surface[106]. To date, it is a great challenge to identify the atomic-level structures of surface O species (including O sites, OH groups, and interfacial H2O) and the detailed interactions between them on TiO2 due to the complexity of the interfacial environments, the high mobility of interfacial H2O, and the interference from outer-layer H2O[109,110].
Figure 7. (A) Experimental and simulated 1D 17O MAS NMR spectra of TiO2 (001). The simulated spectra are based on DFT calculations on different structures; (B) The structure model of TiO2 (101); (C) Experimental and simulated 1D 17O MAS NMR spectra of the fully dried surface-selectively 17O-labeled TiO2 (101) (black line). The simulated spectra (colored lines and peaks) by using parameters obtained from DFT calculations. Reproduced with permission from[105]. Copyright 2017, Springer Nature. 1D: One-dimensional; MAS: magic angle spinning; NMR: nuclear magnetic resonance; DFT: density functional theory.
Summary of the 17O MAS NMR signals occurring in the spectra of TiO2, their chemical shifts (δiso), quadrupole coupling constants (CQ), asymmetry parameters (ƞ), and assignments according to literature
| δiso (ppm) | CQ (MHz) | ƞ | Assignment | Ref. |
| 600-750 | < 1.7 | 0.3-0.8 | OTi2 on anatase surface | [105] |
| 535-570 | 1.1-1.5 | 0.2-0.8 | OTi3 in anatase | [105] |
| 530-550 | ca. 1.0 | 0.5-1.0 | OTi3 on anatase (001) facet | [105] |
| 500-560 | 1.1-1.7 | 0.5-0.8 | OTi3 on anatase (101) facet | [105] |
| 572 | 2.0 | 0.1 | OTi3 in low-ordered TiO2 | [104] |
| 596 | 1.8 | 0.6 | OTi3 in rutile | [104] |
| 543 | 1.6 | 0.6 | Distorted OTi3 or OTi4 near interface between anatase and rutile | [104] |
| 516 | 1.8 | 0.8 | Distorted OTi3 or OTi4 near interface between anatase and rutile | [104] |
| 100-300 | 6.2-7.0 | 0.1-0.5 | OH on anatase surface | [105] |
| 21 | 8.37 | 0.71 | H2O absorbed at step-edge Ti5C in OA | [105] |
| 7 | 8.58 | 0.7 | H2O absorbed at step-edge Ti5C in OB | [105] |
SURFACE TITANIUM AND OXYGEN SITES STUDIED BY SURFACE-ENHANCED NMR SPECTROSCOPY
Due to the high price of isotope reagents, low detection sensitivity, and low surface atomic content, it is difficult to acquire the 47/49Ti and 17O NMR spectra in a short time. Dynamic nuclear polarization (DNP) transfers the polarization of paramagnetic centers to nearby nuclei by microwave irradiation, which can enhance NMR signals in the ratio of the gyromagnetic ratio of the electron and the polarized nucleus. The DNP NMR spectroscopy instruments need to set up microwave sources and cryogenic probes (T < 120 K) to achieve efficient polarization transfer via cross effect, solid effect, and Overhauser effect[111-118]. The NMR signal enhancement techniques have been used to examine the surface structure of a variety of inorganic and hybrid materials[119-122], known as DNP-surface-enhanced NMR spectroscopy (SENS).
Direct DNP transfers polarization directly to target nuclei[123-125], and indirect DNP transfers polarization first to 1H nuclei and then to target nuclei by CP[126-129]. Both of them can be realized for TiO2 photocatalysts[130,131]. Chen et al. prepared 17O-enriched TiO2 by ball milling (BM) with 17O-enriched
Figure 8. (A) 17O MAS NMR spectra recorded at 14.1T on 17O enriched TiO2; (B) 17O MAS NMR spectrum recorded at 35.2 T on a TiO2 phase enriched in 17O; (C) 17O DNP NMR spectrum of a TiO2 phase enriched in 17O; (D) 17O DNP CPMAS NMR spectra in comparison to the DFS-enhanced echo spectrum. Reproduced with permission from[130]. Copyright 2020, American Chemical Society. MAS: Magic angle spinning; NMR: nuclear magnetic resonance; DNP: dynamic nuclear polarization; CPMAS: cross-polarization magic angle spinning; DFS: double-frequency sweep.
The long-standing obstacle of acquiring well-resolved 17O and/or 47/49Ti NMR spectra of TiO2 photocatalysts at natural abundance has been overcome with the implementation of DNP techniques. To measure the surface O and Ti structures, Nagashima et al. developed a novel pulse sequence of refocused insensitive nuclei enhanced by polarization transfer (RINEPT)-
Figure 9. (A) Pulse sequence of RINEPT-
The 1H → 47/49Ti DNP-enhanced RINEPT-
The 1H → 17O RINEPT-
Figure 10. DNP-enhanced 1H → 17O RINEPT-
The water adsorption and dissociation on the surface of metal oxide is a subject of immense importance in various fields such as photocatalysis, energy science, and material science[2,132-138]. This is because water plays a critical role in various chemical processes that occur on the surface of these materials[42,139-143]. A detailed understanding of the adsorption and dissociation of interfacial H2O on these surfaces can help researchers develop more efficient and effective catalytic systems and materials for energy storage and conversion. It has been confirmed that a water molecule could react with the oxygen vacancy or rupture over the low coordination surface Ti sites of TiO2, and form hydroxyls[141,144-147]. However, water adsorption and dissociation on nondefect titanium sites have been disputed for decades[82,148-152]. It is difficult to distinguish the Ti–OH groups formed by H2O dissociation on nondefect TiO2 surface from either the Ti–OH groups generated by H2O reaction with defect sites or the original Ti–OH groups present on TiO2. The DNP-SENS technique would provide the possibilities for exploring the detailed mechanism of water adsorption and dissociation on metal oxide.
SURFACE ELECTRONIC STRUCTURE AND PROPERTIES STUDIED BY PROBE-ASSISTED NMR TECHNIQUES
Surface structural features (including oxygen vacancies, cations, anions, and hydroxyl groups) play crucial roles in the development of efficient catalysts, especially metal oxides, and have been widely investigated[17,153-164]. The differences in these surface features result in the nano-sized particles with different physical/chemical properties. Taking anatase TiO2 nanocrystallite as an example, each facet [including (101) and (001) facet] possesses distinctive chemical properties due to the differences in both the content and electronic structure of the surface species from facet to facet. Probe molecules, including 13C-carbon monoxide, 15N-pyridine, and 31P-trimethylphosphine (TMP), can be adsorbed onto catalysts, and their different NMR chemical shift values can reflect the various microenvironments of catalyst surfaces. Thus, the chemical probe-assisted NMR has been used to characterize the electronic structure in catalyst structures. Among them, TMP is a sensitive and reliable chemical probe to clarify qualitative and quantitative information on the adsorbed sites of the various catalysts[165-168]. In general, the 31P chemical shift of -2~-5 ppm is ascribed to the TMP interacting with surface H+ (Brönsted acid, BA site), while the 31P chemical shift of -20~-58 ppm corresponds to the TMP interacting with surface exposed metal sites (i.e., Lewis acid, LA site), and a linear correlation between the 31P chemical shift and the LA strength (or the binding energy) was found[169,170].
For TiO2 nanoparticles, TMP, as the nucleophilic probe molecule, can strongly interact with the unsaturated coordinated Ti sites on the surface (that is, the surface TMP-Ti complex). Based on the 31P chemical shift of the TMP-Ti complexes, surface Ti sites on various facets with different strengths of LA, surface energies, and spatial structure can be identified. Recently, Peng et al. prepared high-quality anatase TiO2 nanocrystals with different exposed facets using hydrothermal synthesis with variable hydrogen fluoride (HF, 0-6 mL), labeled as TiO2 powder with 90% (101) facet, (101)-dominated TiO2 with 80% (101) facet, and (001)-dominated TiO2 with 75% (001) facet [Figure 11A-C][162,164]. Probe-assisted 31P solid-state NMR spectroscopy was employed to study the surface features and provided extraordinary sensitivity to their chemical states [Figure 11D-F]. There was almost no TMP-H+ complex at -2~-5 ppm present on the TiO2 powder but the TMP-LA complex had a main signal at -36 ppm and a small shoulder at -29 ppm. According to a previous report[171], the major peak and the shoulder with the integrated area ratios of 89.8% and 10.2% were attributed to the interaction between TMP and surface five-coordinate Ti sites (Ti5C) on (101) and (001) facets, respectively, which is consistent with the density functional theory (DFT) calculation [Figure 11G and H]. When the TMP interacts with the Ti5C sites on the reconstructed (1 × 4) (001) facet [(001)RC], the chemical shift of 31P NMR should be 50 ppm [Figure 11I]. Thus, the TMP-assisted NMR experiment can differentiate between facets of decreasing energy through their chemical shift values: (001) > (101) > (001)RC [Figure 11G-I]. Noteworthily, F ions are retained on the (101) and (001) facets when the TiO2 samples were prepared with HF. Owing to the electronic withdrawing effect of surface F ions exerted on Ti5C on these two facets, around 5-7 ppm downshift in chemical shift of the (101) and (001)-dominated TiO2 retained F ions from the corresponding facets of -36 ppm (101) and -29 ppm (001) in the powder sample to -31 and -22.5 ppm, respectively. The NMR signal of -42.5 ppm was ascribed to the formation of F-containing surface oxygen vacancies on unstable (001) facets. Additionally, the significant increase of the Brønsted acid signal (-2~-3 ppm) was rationalized by the interaction of protons with the fluorine (F). The surface F on the (001) and (101) facets significantly enhanced the LA strength of Ti5C sites by reflecting a downshift of 31P chemical shift. On the other hand, the post calcination of the prepared TiO2 led to partially replacing F with OH, rendering an upshift of 31P chemical shift, suggesting the LA strength of Ti5C decreased. The TiO2 surfaces tend to adsorb various surface impurity groups (including F, OH, and SO4) to relax surface energy, and they have a substantial influence on LA strength of Ti5C sites on TiO2 surfaces, which is closely related to photocatalytic activity.
Figure 11. HRTEM images of as-prepared TiO2 with (A) 90% and (B) 80% (101) facet, and (C) 75% (001) facet, and (D-F) their corresponding 31P MAS NMR spectra of TMP-adsorbed TiO2. Theoretical models and calculated adsorption energy (Ead) between TMP and Ti5C sites on various TiO2 facets, including (G) (001) facet, (H) (101) facet, and (I) the reconstructed (1 × 4) (001) facet (Ti: light grey; O: red; P: orange; C: grey; H: white). Reproduced with permission from[164]. Copyright 2017, Springer Nature. HRTEM: High-resolution transmission electron microscopy; MAS: magic angle spinning; NMR: nuclear magnetic resonance; TMP: trimethylphosphine.
It has been shown that the surface features play a crucial role in photocatalytic H2 evolution[160]. According to the TMP-assisted solid-state NMR spectroscopy, it was found that the electron density of surface Ti5C sites strongly decreased near the F ions, forming a dipole electric field (Fδ- ← Tiδ+). The photogenerated holes and electrons can be efficiently separated under the action of the F-induced surface dipole electric field, which greatly prolongs the lifetime of the photogenerated carriers and, consequently, enhances photocatalytic activities. This point was further validated using a series of surface functional groups (–O–, F, OH, and SO4), which were systematically investigated by TMP-assisted 31P NMR spectroscopy and DFT calculations[162,164]. The NMR spectra are sensitive to variations in the surface electronic properties for the series of TiO2 samples [Figure 12A-C], and accordingly, a rational theoretical model is proposed. The highly sensitive 31P chemical shift values correlate linearly and positively with the electron-withdrawing capacity of the surface functional groups to the Ti5C sites (OH < –O– < SO4 < F), indicating that these functional groups can provide fine-tuning of LA and BA sites on TiO2 surface [Figure 12D and E]. Furthermore, the adsorption energies on surfaces modified by these functional groups (OH < –O– < SO4 < F) show a linear relationship with 31P chemical shift in the solid-state NMR spectra and the activity of photocatalytic H2 evolution[162]. Moreover, the transfer process of photogenerated electrons is more efficient if the reactants are pre-adsorbed on the TiO2 surface. Thus, the TMP-assisted 31P NMR spectroscopy can sensitively probe surface electronic structure and properties, thus facilitating the design and development of efficient photocatalysts.
Figure 12. Summary of the electronic effect (chemical shift) imposed by different adsorbates during sequential treatments/modifications on (A) TiO2 PD, (B) F-capped (101) facet [F-(101)], and (C) F-capped (001) facet [F-(001)]; (D) Illustration of interaction between TMP and surface features on TiO2 facet with various treatments/modifications; (E) The summary of 31P chemical shift of TMP-adsorbed Ti5C on (001)/(101) facets with different treatments and modification. Reproduced with permission from[164]. Copyright 2017, Springer Nature. PD: powder; TMP: trimethylphosphine.
HETEROATOM ACTIVE SITES AND THEIR FUNCTION IN PHOTOCATALYSTS
The heteroatom (ions) doping should be one of the most promising methods to narrow the band gap and extend UV-light adsorption to the vis-light region. However, metallic doping (such as Al, V, and Ag) tends to be thermally unstable and inevitably introduces the recombination centers of photogenerated carriers[172-177]. On the other hand, the non-metallic doping (such as C, B, N, etc.) can introduce impurity bands located at 0.8-0.9 eV (i.e., below the conduction band bottom) due to the formation of localized oxygen vacancies[24,177-179], which results in a low electron mobility in anatase TiO2. As such, although both metallic and non-metallic implantations can achieve vis-light absorption, they do not warrant enhanced photocatalytic activity of the doped TiO2 photocatalysts. An in-depth understanding of heteroatoms (ions) sites and their mechanism is the prerequisite to establishing structure-activity relationship, which can promote the rational design of more efficient TiO2 photocatalysts.
13C-carbon
Carbon-doping of TiO2 can efficiently enhance photocatalytic H2 production, CO2 reduction, and degradation of dyes and some small organic molecules under visible irradiation[180-185]. C-doped TiO2 can be prepared by a simple sol-gel method using various carbon sources, including glucose, sucrose, and the titanium alkoxide precursor itself[186-191]. It has been confirmed that the structure and distribution of carbon species are closely correlated with the photocatalytic activity of C-doped TiO2.
Rockafellow et al. achieved 13C enrichment of carbon species in C-doped TiO2 with 13C-labeled glucose[192]. According to the one-dimensional (1D) 13C MAS NMR spectra with spectral editing and 2D 13C–13C correlation NMR spectrum, the detailed six-carbon fragments were present in C-doped TiO2 before annealing (13C6-TiO2-0, Figure 13A and B). After annealing, the aromatic species were the main component of carbon species in the hybrid TiO2 materials. Interestingly, when a washing step is added between the initial drying and annealing (13C6-TiO2-5W), the major carbon species are transformed into an orthocarbonate structure, with C substituting Ti sites inside the TiO2 [Figure 13C]. However, the reason for this change was still ambiguous. The authors found that the presence or absence of aromatic-carbon species on the TiO2 surface was unrelated to the rate of photocatalytic degradation of quinoline, but had a significant effect on the product distribution. Instead, the reason for the high reactivity is presumed to be the formation of orthocarbonate centers.
Figure 13. (A) 13C NMR spectra of 13C6-TiO2-0. The spectrum of glucose for reference. Spectra of 13C6-TiO2-0 with spectral editing; (B) 2D 13C–13C correlation spectrum of 13C6-TiO2-0. Inserts: two structural fragments consistent with the observed cross peaks; (C) 13C NMR spectra of 13C6-TiO2-5W: quantitative (DP) spectrum of all C and corresponding spectrum of nonprotonated C, J-modulated dephasing spectra, and selection of sp3-hybridized C by a five-pulse CSA filter. Reproduced with permission from[192]. Copyright 2009, American Chemical Society. NMR: Nuclear magnetic resonance; 2D: two-dimensional; DP: direct polarization; CSA: chemical shift anisotropy.
We also reported a study of a minute quantity of carbon species doping on TiO2 by using the titanium alkoxide precursor itself[190]. According to the 13C MAS NMR experiments, the detailed structural changes of surface carbon species have been clarified [Figure 14]. After the C doping, four types of carbon species were observed, including carboxylate (182.9 ppm), graphite-like C (129 ppm), aromatics C (128.3 ppm), alkyl C (7.6-29.8 ppm). After washing the C-doped TiO2 with HCl solution, the graphite-like C species should be the main carbon-containing component in the photocatalyst. It was found that graphite-like C species should be the active site to promote the separation of photogenerated carriers, resulting in high photocatalytic efficiency. In contrast, surface alkoxy and carboxylate C species would poison severely the
Figure 14. 13C MAS NMR spectra of various C-doped TiO2 samples (Right). Proposed hole and electron transfer mechanism in the C-doped TiO2 photocatalyst (Left). Reproduced with permission from[190]. Copyright 2018, American Chemical Society. MAS: Magic angle spinning; NMR: nuclear magnetic resonance.
15N-nitrogen
Nitrogen doping of TiO2 can efficiently promote vis-light adsorption[24,194-199]. Reyes-Garcia et al. reported the detailed structure of nitrogen species in the N-doped TiO2 prepared from different dopant precursors and methods through 15N solid-state NMR analysis[200]. As shown in Figure 15, three types of amino species (at
Figure 15. (A) 15N MAS NMR spectra of 15N-doped TiO2 prepared by sol-gel methods; (B) 15N MAS NMR spectra of 15N-doped TiO2 monolayer and powders by direct nitridation. Reproduced with permission from[200]. Copyright 2007, American Chemical Society. MAS: Magic angle spinning; NMR: nuclear magnetic resonance.
11B-boron
The boron-doping can not only narrow the band gap of TiO2 effectively to extend the absorption band to the vis-light region but also facilitate the separation of photogenerated carriers to promote photocatalytic activities[23,201-205]. Thus, the categories and structure of boron species have been extensively studied to gain their correlations with the photocatalytic properties. The conclusions of the aforementioned research were mostly obtained by XPS, which, however, remain controversial. For example, the XPS peaks at 190.5-
To gain more insights into the structure-activity relationship in photocatalytic reactions, we incorporated FAM RF pulse trains into the MQ MAS sequence, namely the so-called 3QZ-FAM MAS NMR technique, to improve the sensitivity of the 11B NMR spectroscopy and investigate the detailed chemical environments of boron in B-doped and (B, Ag)-codoped TiO2 photocatalysts [Figure 16A and B][202]. Up to five B sites were distinguished, corresponding to surface sites (B5), small B polymer (B2 and B3), interstitial T* (tri-coordinated, B4), and Q* (pseudo-tetrahedral coordinated, B1) sites. Noteworthily, substituted B sites were absent in the materials. A 2D 11B-11B double-quantum (DQ) MAS NMR technique was first used to reveal the spatial distributions of the B sites in the B-doped and (B, Ag)-codoped TiO2 photocatalysts [Figure 16C and D]. Accordingly, we found that only the tri-coordinated interstitial boron (T*) species was near the substitutional Ag species to form [T*–O–Ag] structural units. Combined with the evolution of the chemical states of the B and Ag dopants revealed by in-situ XPS experiments, a unique intermediate structure was formed by the [T*–O–Ag] units trapping the photogenerated electron in the (B, Ag)-codoped TiO2 during the irradiation as shown in Figure 17. To date, the developed 2D 11B-11B DQ Correlation NMR technique has been used to detect the spatial correlation of the B species in various B-containing materials, including boron nitride (BN), activated carbon impregnated with boric acid (B/OAC), boron-substituted MCM-22 zeolite (B-MWW) and silica-supported boron oxide (B/SiO2)[39,212,213,217].
Figure 16. 2D 11B 3QZ-FAM MAS NMR spectra (sheared) of (A) 10% B-doped and (B) (B, Ag)-codoped TiO2 samples. 2D 11B DQ MAS NMR spectra of (C) 10% B-doped and (D) (B, Ag)-codoped TiO2 samples. Possible Boron Species in B-doped and (B, Ag)-codoped TiO2 were shown at the bottom. Reproduced with permission from[202]. Copyright 2013, American Chemical Society. 2D: Two-dimensional; FAM: fast amplitude-modulated; MAS: magic angle spinning; NMR: nuclear magnetic resonance; DQ: double-quantum.
Figure 17. Electron/hole transfer mechanism for the (B, Ag)-codoped TiO2 photocatalyst under irradiation from a solar-light source. Reproduced with permission from[202]. Copyright 2013, American Chemical Society.
27Al-aluminum
Although aluminum (Al)-doping cannot promote vis-light absorption, it would affect the crystal growth, cation diffusivity, and conductivity of TiO2[176,218-220]. For photocatalysis, several contradictory results occurred due to the ambiguity of the structure-activity relationship of the Al sites in the Al-doped TiO2 photocatalysts. Some reports found that Al doping could promote the separation of photogenerated carriers and thus improve photocatalytic activity[221-223]. On the other hand, others proposed that Al doping might introduce recombination centers of photogenerated carriers, which could have a negative impact on photocatalytic activity[220,224]. Thus, it is essential to identify the active Al species in the Al-doped TiO2.
To unravel the origins of the exceptional activity of Al-doped TiO2 with dominant (001) facets [Al-TiO2-xFx(001)], we employed advanced solid-state NMR methods to elucidate the fine structural details of the dopants, specifically F and Al species [Figure 18][225]. Notably, we first applied the 2D 19F–27Al dipolar heteronuclear multiple-quantum coherence (D-HMQC) NMR technique to probe F–Al proximity, enabling definitive confirmation of the F–Ti2Al structural motif within the Al-TiO2-xFx(001) samples. According to the quantitative 19F NMR measurements, the content of the F–Ti2Al structure rises with greater Al doping, while that of other structures (including Ti–F–Ti, F–Ti3, and Ti–F–Al) hardly increases. Combined with the spin-trapping electron spin resonance (ESR) results, it was found that the formation of the F–Ti2Al structure promotes the separation and transfer of photogenerated carriers in the Al-TiO2-xFx(001) photocatalyst and is, therefore, considered to be the active site for photocatalytic reactions. However, with the further increase of Al doping, the oxygen vacancies occur, which should be the recombination centers of photogenerated carriers as reported in the previous work[176,220].
Figure 18. (A) 1D 27Al MAS NMR spectra of TiO2-xFx(001) and Al–TiO2-xFx(001) catalysts; (B) 2D 27Al 3Q MAS NMR spectrum of the
CONCLUSION AND OUTLOOK
Solid-state NMR spectroscopy can provide detailed information about the nature of the active centers and their structure-activity relationships in the TiO2 photocatalysts. The local structure and coordination of different sites or species in the TiO2 framework or on the TiO2 surface can be identified by NMR chemical shift, owing to the high sensitivity of the NMR technique to the surrounding electronic environment. For example, bulk titanium and surface titanium can be distinguished by 47/49Ti chemical shifts, and bulk oxygen, surface oxygen sites, hydroxyl groups, and adsorbed H2O with different coordination states can be identified by 17O chemical shifts. Surface electronic structure and properties, including the type, strength, and concentration of diverse adsorption sites, can be detected by probe-assisted NMR techniques. The correlation and connectivity of different sites or species in TiO2 can be extracted from the specific internuclear interactions, including dipolar-dipolar and J-coupling interactions. Various advanced homonuclear/heteronuclear correlation NMR techniques based on dipole–dipole interaction or J-coupling have been developed to detect the dipolar-dipolar interactions for identifying internuclear proximities or chemical bonding, respectively[226,227]. For example, the spatial proximity between different active sites on hetero-atom (such as C, B, Al, etc.)-modified TiO2 can be probed by 2D homonuclear (13C–13C/11B–11B) and heteronuclear (19F–27Al) correlation NMR spectra, while the 2D 1H–11B/17O/47/49Ti/95Mo correlation NMR can identify the surface B/O/Ti/Mo site in the TiO2 photocatalysts, which facilitate the solution of the complex surface structure.
Although remarkable progress has been achieved on the application and development of solid-state NMR methods, considerable challenges remain in TiO2 photocatalysis for solid-state NMR characterization. It is difficult to detect dilute heteroatom sites and low-content surface/interface species (such as surface O and Ti sites) at high resolution. The intrinsic low sensitivity of solid-state NMR limits its application for this aspect, especially for some infamous nuclei with low natural abundances and low-γ features. All these active centers (including surface active sites and heteroatom sites) are fundamentally important in TiO2 photocatalysis. Moreover, the complexity of complex surface structures, heteroatom distributions, and various covalent and non-covalent interactions in the TiO2 photocatalysts leaves a huge space for advanced solid-state NMR techniques.
Our understanding of the complex structure of TiO2 photocatalysts remains limited by the capabilities of existing solid-state NMR hardware and methodologies. An in-depth understanding of their structure-function relationships is essential for the further development of these fields. The increasing availability of ultra-high-field magnets and cryoprobes is improving solid-state NMR techniques to a higher level of detection sensitivity and resolution. Further development of sensitivity-enhanced 2D NMR techniques would enable the identification of the microstructure, distribution, and interaction of different active sites. Notably, the typical dopant elements used to modify TiO2 photocatalysts, such as 51V, 67Zn, 71Ga, 93Nb, and
DECLARATIONS
Authors’ contributions
Prepared and revised the manuscript: Feng N
Revised the manuscript: Feng N, Xu J, Deng F
Availability of data and materials
Not applicable.
Financial support and sponsorship
This work was supported by the National Natural Science Foundation of China (22372177, 22127801, 22225205, 22320102002, 22161132028), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB0540000), Natural Science Foundation of Hubei Province (S22H120101), Hubei International Scientific and Technological Cooperation Program (2022EHB021), and International Collaborative Center for Sustainable Catalysis and Magnetic Resonance (SH2303).
Conflicts of interest
Xu J served as editorial member of Chemical Synthesis, and he is Guest Editor of the Special Issue: “Advanced Characterization Techniques and Applications for Catalytic Materials”, while the other authors have declared that they have no conflicts of interest.
Ethical approval and consent to participate
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Consent for publication
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Copyright
© The Author(s) 2024.
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Feng, N.; Xu, J.; Deng, F. Solid-state NMR of active sites in TiO2 photocatalysis: a critical review. Chem. Synth. 2024, 4, 43. http://dx.doi.org/10.20517/cs.2024.12
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