catalysts
Review Titanium-Dioxide-Based Visible-Light-Sensitive Photocatalysis: Mechanistic Insight and Applications
Shinya Higashimoto Department of Applied Chemistry, Faculty of Engineering, Osaka Institute of Technology, 5-16-1 Omiya, Asahi-ku, Osaka 535-8585, Japan; [email protected]; Tel.: +81-(0)6-6954-4283
Received: 15 January 2019; Accepted: 14 February 2019; Published: 22 February 2019
Abstract: Titanium dioxide (TiO2) is one of the most practical and prevalent photo-functional materials. Many researchers have endeavored to design several types of visible-light-responsive photocatalysts. In particular, TiO2-based photocatalysts operating under visible light should be urgently designed and developed, in order to take advantage of the unlimited solar light available. Herein, we review recent advances of TiO2-based visible-light-sensitive photocatalysts, classified by the origins of charge separation photo-induced in (1) bulk impurity (N-doping), (2) hetero-junction of metal (Au NPs), and (3) interfacial surface complexes (ISC) and their related photocatalysts. These photocatalysts have demonstrated useful applications, such as photocatalytic mineralization of toxic agents in the polluted atmosphere and water, photocatalytic organic synthesis, and artificial photosynthesis. We wish to provide comprehension and enlightenment of modification strategies and mechanistic insight, and to inspire future work.
Keywords: Titanium dioxide (TiO2); visible-light-sensitive photocatalyst; N-doped TiO2; plasmonic Au NPs; interfacial surface complex (ISC); selective oxidation; decomposition of VOC; carbon nitride (C3N4); alkoxide; ligand to metal charge transfer (LMCT)
1. Introduction
Titanium dioxide (TiO2) is one of the most practical and prevalent photo-functional materials, since it is chemically stable, abundant (Ti: 10th highest Clarke number), nontoxic, and cost-effective. In recent years, a great deal of attention has been directed towards TiO2 photocatalysis for useful applications such as photocatalytic mineralization of toxic agents in the polluted atmosphere and water, photocatalytic organic synthesis, and artificial photosynthesis [1–20]. 3+ The TiO2 involving Ti sites that are oxygen-deficient at the impurity level exhibits n-type semiconductor. The photocatalytic activities of TiO2 strongly depend on crystal structures (anatase, brookite, and rutile), crystallinity, crystalline plane, morphology, particle sizes, defective sites, and surface OH groups. The valence band (V.B.) and conduction band (C.B.) of TiO2 consist of O 2p and Ti 3d orbitals, respectively, and their band gap (forbidden band) is circa ~3.0–3.2 eV (~410–380 nm). Photo-irradiation (hv > 3.2 eV) of the TiO2 photocatalyst leads to band gap excitation, resulting in charge separation of electrons into the C.B. and the holes in the V.B. These photo-formed electrons and holes simultaneously work as electron donors and acceptors, respectively, on the photocatalyst surface, thus enabling the photocatalytic reactions. Details are given in other articles and reviews [21–25]. UV light reaching the earth surface represents only a very small fraction (4%) of the solar energy available. Therefore, many researchers have endeavored to design several types of visible-light-responsive photocatalyst. In particular, TiO2-based photocatalysts operating under visible light should be urgently designed and developed, in order to take advantage of the unlimited solar light available. In the late 1990s, Anpo et al. first reported that TiO2 doped with Cr, V, and Fe cations by ion implantation operates under visible light irradiation. They exhibited red shift of the band-edge of the
Catalysts 2019, 9, 201; doi:10.3390/catal9020201 www.mdpi.com/journal/catalysts Catalysts 2019, 9, 201 2 of 22
Catalysts 2019, 9, x FOR PEER REVIEW 2 of 22 TiO2, resulting in decomposition of NO into N2,O2, and N2O[26]. This work accelerated subsequent TiOworks2, resulting for the in design decomposition and development of NO into of N visible-light-responsive2, O2, and N2O [26]. This photocatalysts. work accelerated Recently, subsequent much worksattentionattention for hasthe has beendesign been paid and toto development visible-light-responsivevisible-light-responsive of visible-li TiO ght-responsiveTiO2 prepared2 prepared by: photocatalysts.by: doping doping with with nitrogen Recently, nitrogen (N), muchcarbon (N), carbon(C), and (C), sulfur and sulfur (S) ions (S) etc.; ions surface etc.; surface plasmonic plasmonic effects witheffects Au with or Ag Au nanoparticles or Ag nanoparticles (NPs); the (NPs); interfacial the interfacialsurface complex surface (ISC); complex coupling (ISC); with coupling visible-light-sensitive with visible-light-sensitive hetero-semiconductors hetero-semiconductors (cadmium sulfide, (cadmiumcarbon nitride sulfide, etc.); andcarbon dye-sensitized nitride etc.); photocatalysts. and dye-sensitized In fact, some photocatalysts. photocatalysts areIn consideredfact, some to photocatalystswork under similar are considered principles. to work under similar principles. AlongAlong thesethese backgrounds,backgrounds, this reviewthis review focuses onfocuses the recent on advancesthe recent of the visible-light-sensitiveadvances of the visible-light-sensitiveTiO2 photocatalyst. TiO These2 photocatalyst. advances have These been advances classified have by been the originclassified of by charge the origin separation of chargephoto-induced separation in photo-induced (1) the bulk impurity in (1) the (N-doping), bulk impurity (2) hetero-junction (N-doping), (2) of hetero-junction metal (Au NPs), of metal and (3) (Authe NPs), interfacial and surface(3) the complexinterfacial (ISC) surface (See Figurecomplex1). They(ISC) have(See beenFigure well 1). characterizedThey have been by several well characterizedspectroscopic by techniques, several spectroscopic and applied techniques, for mineralization and applied of volatile for mineralization organic compounds of volatile (VOC), organic water compoundssplitting to (VOC), produce water H2, andsplitting fine organic to produce synthesis. H2, and fine organic synthesis.
Figure 1. Visible-light-sensitive TiO photocatalyst modified by (1) nitrogen-doping, (2) plasmonic Au Figure 1. Visible-light-sensitive TiO2 2photocatalyst modified by (1) nitrogen-doping, (2) plasmonic Aunanoparticles nanoparticles (NPs), (NPs), and and (3 ()3 interfacial) interfacial surface surface complex complex (ISC). (ISC).
2. Nitrogen-doped TiO2 Photocatalysts 2. Nitrogen-doped TiO2 photocatalysts In 1986, Sato and co-workers first explored the photocatalytic activity of nitrogen-doped TiO2 In 1986, Sato and co-workers first explored the photocatalytic activity of nitrogen-doped TiO2 (N-doped TiO2) photocatalysts for the oxidation of gaseous ethane and carbon monoxide [27]. (N-doped TiO2) photocatalysts for the oxidation of gaseous ethane and carbon monoxide [27]. They They found that N-doped TiO2 photocatalyst exhibited a superior photocatalytic activity to pure TiO2 found that N-doped TiO2 photocatalyst exhibited a superior photocatalytic activity to pure TiO2 under visible light irradiation. Later, in 2001, Asahi et al. demonstrated visible-light-induced complete under visible light irradiation. Later, in 2001, Asahi et al. demonstrated visible-light-induced photo-oxidation of gaseous CH3CHO (one of VOCs) to CO2 with an N-doped TiO2 photocatalyst [28]. complete photo-oxidation of gaseous CH3CHO (one of VOCs) to CO2 with an N-doped TiO2 In this section, fundamental synthetic routes, characterizations, and application of photocatalytic photocatalyst [28]. In this section, fundamental synthetic routes, characterizations, and application reactions are highlighted. of photocatalytic reactions are highlighted.
2.1. Synthesis of N-doped TiO2 Photocatalyst 2.1. Synthesis of N-doped TiO2 photocatalyst N-doped TiO2 was prepared by employing several procedures and materials. Details are given N-doped TiO2 was prepared by employing several procedures and materials. Details are given in Reference [13]. Preparation methods for N-doped TiO2 photocatalysts can be classified into two incategories: Reference dry[13]. processes Preparation and methods wet processes. for N-doped TiO2 photocatalysts can be classified into two categories: dry processes and wet processes.
2.1.1. Dry Processes
Typically, N-doped TiO2 powder can be prepared by the nitrification of TiO2 in an ammonia (NH3) gas flow at high temperature [28,29]. The amount of N doping into the TiO2 can be controlled Catalysts 2019, 9, 201 3 of 22
2.1.1. Dry Processes
Typically, N-doped TiO2 powder can be prepared by the nitrification of TiO2 in an ammonia Catalysts 2019, 9, x FOR PEER REVIEW 3 of 22 (NH3) gas flow at high temperature [28,29]. The amount of N doping into the TiO2 can be controlled ◦ by annealing temperatures in the range of 550−600 C under an NH3 flow. However, a large number by annealing temperatures in the range of 550−600 °C under an NH3 flow. However, a large number of O vacancies are introduced into the N-doped TiO2 with increasing annealing temperature, since the of O vacancies are introduced into the N-doped TiO2 with increasing annealing temperature, since NH3 decomposes into N2 and H2 at high temperature, and TiO2 is simultaneously reduced by H2 [30]. the NH3 decomposes into N2 and H2 at high temperature, and TiO2 is simultaneously reduced by H2 Figure2 shows schematics of N-doping into TiO 2, accompanied by the formation of oxygen vacancies [30]. Figure 2 shows schematics of N-doping into TiO2, accompanied by the formation of oxygen to exhibit the n-type semiconductor. vacancies to exhibit the n-type semiconductor.
2- O2- O2- O2- O 4+ 4+ 4+ Ti4+ TiO2 Ti Ti Ti 2- O2- O2- O2- O
NH3 + h 2- N3- O2- O2- O 4+ (a) Ti4+ Ti4+ Ti4+ Ti 2- O2- O2- O2- O (p-type)
H2
- 2- 3- 2- 2 e O h+ N O (b) Ti4+ Ti4+ Ti4+ Ti4+ 2- O2- O2- O2- O
- 2- N3- O2- e O (c) Ti4+ Ti4+ Ti4+ Ti4+ 2- O2- O2- O2- O (n-type) N-doped TiO 2 3− 2− + Figure 2. When the N is replaced with lattice O ions in the TiO2 lattice, the hole (h ) is formed in Figure 2. When the N3− is replaced with lattice O2− ions in the TiO2 lattice, the hole (h+) is formed in order to compensate for the charge balance (p-type semiconductor) (a). However, an oxygen vacancy order to compensate for the charge balance (p-type semiconductor) (a). However, an oxygen vacancy is produced by the reduction with H2, which is formed by the decomposition of NH3 to produce an is produced by the reduction with H2, which is formed by the decomposition3− of NH3 to produce an oxygen vacancy and excess electrons (b). As a consequence, N doped into TiO2 (N-doped TiO2) oxygen vacancy and excess electrons (b). As a consequence, N3− doped into TiO2 (N-doped TiO2) involves electrons located at N 2p and Ti 3d sites at impurity levels (n-type semiconductor) (c). involves electrons located at N 2p and Ti 3d sites at impurity levels (n-type semiconductor) (c). 2.1.2. Wet Processes 2.1.2. Wet Processes A sol-gel method can be employed for the preparation of N-doped TiO2 powder. Typically, NH3 aq. (NHA sol-gel4OH) method is added can to be a solution employed of for titanium the preparation (IV) isopropoxide of N-doped (TTIP) TiO2 [ 31powder.–33] to Typically, form titanium NH3 ◦ hydroxideaq. (NH4OH) involving is added N-species. to a solution The precipitateof titanium was (IV) dried, isopropoxide followed (TTIP) by calcination [31–33] atto ~400–450form titaniumC in airhydroxide to obtain involving a yellowish N-species. TiO2 powder. The precipitate was dried, followed by calcination at ~400–450 °C in
air to obtain a yellowish TiO2 powder. 2.2. N-states in N-doped TiO2
2.2. N-statesOne of in the N-doped major TiO concerns2 is to understand the physico-chemical nature of the N species in N-dopedOne TiOof the2, which major are concerns responsible is to for understand the visible lightthe sensitivity.physico-chemical They were nature characterized of the N byspecies density in functional theory (DFT) calculations, X-rap photoelectron spectroscopy (XPS), Ultraviolet-visible N-doped TiO2, which are responsible for the visible light sensitivity. They were characterized by (UV-vis)density andfunctional electron paramagnetictheory (DFT) resonance calculations, (EPR) spectroscopy.X-rap photoelectron spectroscopy (XPS), Ultraviolet-visible (UV-vis) and electron paramagnetic resonance (EPR) spectroscopy.
2.2.1. DFT calculations
DFT calculations demonstrated the electronic structures of the N-doped TiO2 photocatalyst (see Figure 3). The substitution of N with lattice O of the N-doped TiO2 exhibits band gap narrowing (circa 0.1 eV) caused by mixing orbitals of N 2p with O 2p, resulting in the negative shift of the valence band edge. On the other hand, the interstitial N is localized to impurity states (N 2p levels)
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2.2.1. DFT Calculations
DFT calculations demonstrated the electronic structures of the N-doped TiO2 photocatalyst (see Figure3). The substitution of N with lattice O of the N-doped TiO 2 exhibits band gap narrowing (circa Catalysts 2019, 9, x FOR PEER REVIEW 4 of 22 0.1 eV) caused by mixing orbitals of N 2p with O 2p, resulting in the negative shift of the valence band edge. Onabove the the other V.B. hand, (circa the0.7 eV) interstitial in the mid-band N is localized gap. Therefore, to impurity the oxidation states (N power 2p levels) of photo-induced above theV.B. holesCatalysts on 2019 the, 9N, x 2pFOR is PEER lower REVIEW than on the O 2p in the TiO2 lattice. 4 of 22 (circa 0.7 eV) in the mid-band gap. Therefore, the oxidation power of photo-induced holes on the N 2p is lowerabove than the on V the.B. O(circa 2p 0.7 in theeV) TiOin the2 lattice.mid-band gap. Therefore, the oxidation power of photo-induced Substitutional N-doped TiO2 Interstitial N-doped TiO2 holes on the N 2p is lower than on the O 2p in the TiO2 lattice.
Substitutional N-doped TiO2 Interstitial N-dopedN TiO2 N O O O N Ti TiTi Ti TiTi N O O O O O O O Ti Ti Ti Ti Ti Ti O CB O O CBO
CB CB 3.06 eV 2.46 eV N 2p 3.06 eV N 2p 2.46 eV N 2p VBN 2p VB VB VB
Figure 3. Schematic illustration of structures and their corresponding energy bands for substitutional Figureand 3.FigureSchematic interstitial 3. Schematic illustration N species illustration in of the structuresof N-dopedstructures and TiOand2 their, together co correspondingrresponding with photo-indu energy energy bandsced bandselectronic for substitut for processes. substitutionalional
and interstitialand interstit N speciesial N species in the in N-doped the N-doped TiO TiO2,2 together, together withwith photo photo-induced-induced electronic electronic processes. processes. 2.2.2. XPS spectra 2.2.2. XPS2.2.2. Spectra XPS spectra XPS analysis can confirm the oxidative states of the N species and bonding states in the XPS analysis can confirm the oxidative states of the N species and bonding states in the XPSN-doped analysis TiO can2 (See confirm Figure the 4I). oxidative N 1s XPS states peaks of at the a binding N species energy and bondingin the range states of in~396 the–400 N-doped eV showedN-doped different TiO2 (See oxidative Figure 4I).states N 1sof the XPS N peaks species. at aBy binding the combination energy in theof the range DFT of calculations ~396–400 eV [31], it TiO2 (See Figure4I). N 1s XPS peaks at a binding energy in the range of ~396–400 eV showed different washoweds identified different that oxidative the N 1s states XPS peaksof the Nat species.~396–397 By eV the are combination due to the of substituti the DFT oncalculations of N with [ 31the], itlatti ce oxidativewas states identified of the that N the species. N 1s XPS By peaks the combination at ~396–397 eV of are the due DFT to the calculations substitution [ 31of N], itwith was the identified lattice that O of TiO2 [13,34], while those at ~399–400 eV are due to the interstitial N in the form of NOx or NHx the N 1sO XPS of TiO peaks2 [13,3 at4], ~396–397 while those eV at ~ are399 due–400 toeV theare due substitution to the interstit of Nial withN in the the form lattice of NO O ofx or TiO NH2x [13,34], [13,31,35,36 ]. while those[13,31, at35 ~399–400,36 ]. eV are due to the interstitial N in the form of NOx or NHx [13,31,35,36].
[I-a] 400 eV [II-a] 396 eV
.
a.u N-doping function
- TiO2
KM Intensity Intensity / 2.8 eV 3.1 eV 390 395 400 405 2 3 4 5 Photoenergy / eV Binding energy / eV 1.5 [I-b] 399 eV [II-b] TiO2 . a.u 1 N-doping
function -
Intensity Intensity / 0.5 KM 2.3 eV 3.1 eV
0
390 395 400 405 2 2.5 3 3.5 4 Binding energy / eV Photoenergy / eV
Figure 4.FigureXPS 4 [I]. XPS and [I] UV-visand UV- absorptionvis absorption spectra spectra [II][II] of of (a (a)) N N-doped-doped TiO TiO2 nanoball2 nanoball film [3 film4], and [34 (b],) and (b) Figure 4. XPS [I] and UV-vis absorption spectra [II] of (a) N-doped TiO2 nanoball film [34], and (b) N-dopedN- TiOdoped2 prepared TiO2 prepared by the by the sol-gel sol-gel method method[ [3366].]. N-doped TiO2 prepared by the sol-gel method [36].
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The UV-vis absorption spectra of the N-doped TiO2 are shown in Figure 4II. The N-doped TiO2 2.2.3. Optical Propertiesproperties with the substitution of N exhibited band gap narrowing from 3.1 to 2.8 eV. On the other hand, the N-dopedThe UV-vis TiO2 prepared absorption by spectrathe sol-gel of the method N-doped exhibited TiO2 are visible shown light in Figureabsorption4II. The up N-dopedto 540 nm TiO (2.32 witheV), due the substitutionto the electronic of N transition exhibited from band lo gapcalized narrowing N doping from level 3.1 to 2.8the eV.C.B. On of thethe otherTiO2, hand,while theband-narrowing N-doped TiO was2 prepared not observed. by the These sol-gel results method are exhibited in good agreement visible light with absorption the DFT upcalculations. to 540 nm (2.3 eV), due to the electronic transition from localized N doping level to the C.B. of the TiO2, while band-narrowing2.2.4. Electron Paramagnetic was not observed. Resonance These (EPR) results spectra are in good agreement with the DFT calculations.
− • 2.2.4.N Electron species Paramagneticin the N-doped Resonance TiO2 are present (EPR) Spectra at either diamagnetic (N ) or paramagnetic (N ) bulk centers, which are responsible for the visible light sensitivity [31,37]. The EPR measurements can − • detectN the species paramagnetic in the N-doped (N•) TiObulk2 arecenters present (see at Figure either 5). diamagnetic One type,(N of three) or paramagnetic lines with a (Nhyperfine) bulk centers,tensor (g which = 2.006 are and responsible A = 32.0 G) for splitting the visible by nu lightclear sensitivityspin of nitrogen [31,37 (].I = The 1), was EPR observed. measurements The signal can • detectintensity the of paramagnetic N• radicals (N increased) bulk centers when (seethe Figurelight 5was). One turned type, on, of three while lines the with signal a hyperfine intensity tensorsignificantly (g = 2.006 decreased and A = when 32.0 G) the splitting light bywas nuclear turned spin off. of In nitrogen general, (I =the 1), paramagnetic was observed. interaction The signal intensity of N• radicals increased when the light was turned on, while the signal intensity significantly between N species and O2 makes EPR signals disappear. However, they were remarkably enhanced decreased when the light was turned off. In general, the paramagnetic interaction between N species in the presence of O2 under λ > 420 nm, while its signal intensity still remained to some extent even and O makes EPR signals disappear. However, they were remarkably enhanced in the presence of after the2 light was turned off. These results suggest that N-species are located in bulk inside the TiO2, O under λ > 420 nm, while its signal intensity still remained to some extent even after the light was and2 visible light irradiation of the N-doped TiO2 exhibits effective charge separation to form holes turned(N• radicals) off. These and resultselectrons, suggest which that participate N-species in are the located oxidation in bulk and insidereduction the TiOof reactant2, and visible molecules, light • irradiationrespectively. of the N-doped TiO2 exhibits effective charge separation to form holes (N radicals) and electrons, which participate in the oxidation and reduction of reactant molecules, respectively.
4 [III] in vacuum in Ar in O2 light on off on off on off 3 N0 I / N I 2
1 0 50 100 Time / min
Figure 5.5. Schematic illustration of [I][I] formationformation ofof paramagneticparamagnetic ··NN byby thethe excitationexcitation ofof diamagneticdiamagnetic − 2 N species.species. Electron Electron paramagnetic paramagnetic resonance resonance (E (EPR)PR) signal signal [II] [II] of of ·N·N radicals on N–TiO2, and thethe relative signalsignal intensityintensity ofofI INN/IIN0N0 [III][III] under under vacuum, vacuum, in in the the presence presence of of argon argon (Ar) or O 2 (400(400 Pa) Pa) [37]. [37]. IIN0 andand IINN showshow the the intensity intensity due due to to ·N·N radicals radicals at at the the initial initial and and measured measured time, time, respectively. respectively.
2.2.5. Photo-ElectrochemicalPhoto-electrochemical properties Properties
Nakamura etet al.al. investigatedinvestigated the the photo-electrochemical photo-electrochemical oxidation oxidation power power of of the the N-doped N-doped TiO TiO2 by2 employingby employing several several electron electron donors donors [38]. [38]. Figure Figure6 shows 6 shows that thethat photo-induced the photo-induced hole onhole the on N the 2p levelN 2p − - − - − -− - canlevel directly can directly oxidize oxidize only Ionlyions I under ions under visible visible light light illumination, illumination, while while I , SCN I , SCN, Br ,, Br and, and H2O H are2O oxidizedare oxidized by theby holethe hole on theon V.B.the V.B. under under UV lightUV lig illumination.ht illumination. Therefore, Therefore, the oxidationthe oxidation power power of the of holesthe holes induced induced on the onN the 2p N level 2p level is lower is lower than thatthan of th thoseat of onthose the on O 2pthe on O the2p V.B.on the Tang V.B. et al.Tang studied et al. thestudied dynamics the dynamics of photogenerated of photogenerated electrons andelectrons holes and on the holes N-doped on the TiO N-doped2 using TiO transient2 using absorption transient spectroscopyabsorption spectroscopy [39]. They concluded [39]. They that concluded the lack that of activity the lack of of nanocrystalline activity of nanocrystalline N-doped TiO 2N-dopedfilm for photocatalyticTiO2 film for photocatalytic water oxidation water is due oxidation to rapid is electron–holedue to rapid electron–hole recombination. recombination. On the other hand, Higashimoto et al. investigated the photo-electrochemical reduction power of the N-doped TiO2 (see Figure 7) [33]. When the N-doped TiO2 was photo-excited under visible Catalysts 2019, 9, 201 6 of 22
On the other hand, Higashimoto et al. investigated the photo-electrochemical reduction power of the N-doped TiO2 (see Figure7)[ 33]. When the N-doped TiO2 was photo-excited under visible Catalysts 2019, 9, x FOR PEER REVIEW 6 of 22 light irradiation, the photo-induced electrons were accumulated on the oxygen vacancies of TiO2. lightCatalysts irradiation, 2019, 9, xthe FOR photo-induced PEER REVIEW electrons were accumulated on the oxygen vacancies of6 TiO of 222 . Subsequently,Subsequently,light whenirradiation, when various various the photo-induced kinds kinds of of redox redox electrons species species we asre accumulated electron acceptors acceptors on the wereoxygen were introduced vacancies introduced intoof TiO intothe2. the 4+ + 3+ photo-chargedphoto-chargedSubsequently, N–TiO N–TiO2 ,when the2, the accumulatedvarious accumulated kinds of electrons redoxelectrons species could could as reduceelectronreduce O acceptors22 molecules,molecules, were Pt introduced Pt4+, Ag, Ag+, and into, and Au the3+ Au 2+ + 2+ 4+ + 3+ ions, butions, not butphoto-charged MVnot MV,H2+, ,HN–TiO and+, and Cu2, Cuthe2+ accumulatedions. ions. InIn principle,principle, electrons the the could N-doped N-doped reduce TiO O TiO2 2molecules, has2 has the the potential Pt potential, Ag ,to and reduce to Au reduce + + ions, but not MV2+, H+, and Cu2+ ions. In principle, the N-doped TiO2 has the potential to reduce H /HH2,/H but2, but many many oxygen oxygen vacancies vacancies involved involved in the bulk bulk TiO TiO2 2couldcould influence influence the thedrastic drastic charge charge H+/H2, but many oxygen vacancies involved in the bulk TiO2 could influence the drastic chargeγ recombination.recombination. In particular,In particular, photo-induced photo-induced electronselectrons trapped at at the the oxygen oxygen vacancies vacancies (mainly (mainly γ recombination. In particular, photo-induced electrons trapped at the oxygen vacancies (mainly γ region) could reduce O2 molecules to form such active oxygen species as hydrogen peroxide (H2O2), region) could reduce O2 molecules to form such active oxygen species as hydrogen peroxide (H2O2), resultingregion) in further could reduceoxidation O2 molecules of organic to substrates. form such active oxygen species as hydrogen peroxide (H2O2), resulting inresulting further in oxidation further oxidation of organic of organic substrates. substrates.
Figure 6. SchematicFigure 6. Schematic illustration illustration of proposed of propos energyed energy bands bands for for the the N-dopedN-doped TiO TiO2, together2, together with with some some Figure 6. Schematic illustration of proposed energy bands for the N-doped TiO2, together with some photo-induced electronic processes. E: equilibrium redox potentials for one electron transfer [38]. photo-inducedphoto-induced electronic electronic processes. processes.E E:: equilibrium equilibrium redox redox potentials potentials for for one one electron electron transfer transfer [38]. [38].
E / V vs. NHE (pH = 2.5) E / V vs. NHE (pH = 2.5)
-0.5 C.B. C.B. -0.5 α + α 2H / H2: -0.15 V 0 β + × 2H / H2: -0.15 V 0 β e-× × Cu2+ / Cu0: +0.334 V e- 0.5 γ × 2+ 0 Cu / Cu : ++0.334 V Visible light O2 + 3H / H2O + •OH : +0.64 V 0.5 γ [PtCl ]4- / Pt + 6Cl- : +0.742 V O + 3H6+ / H O + •OH : +0.64 V Visible light 2 Ag+ / Ag0 :2 +0.7991 V [PtCl ]4- / Pt + 6Cl- : +0.742 V 1 3.1 eV [AuCl6 ]- / Au + 4Cl- : +1.002 V Ag+ / Ag40 : +0.7991 V 1 2 H O -and/or organic- compounds 3.1 eV [AuCl2 4] / Au + 4Cl : +1.002 V h+ 2 H Ooxidative and/or organic products compounds N-doping level 2 h+ oxidative products N-doping level
3 V.B.
3 V.B.
Figure 7. Energy levels for sub-band structures of N-doped TiO2 and photo-induced charge transfer into various kinds of redox species under visible light irradiation. The energy levels of sub-bands at Figure 7. FigureEnergythe 7. Energα, β, levelsandy levels γ potential for for sub-band sub-band regions structures structures(oxygen vacancies) of N-doped N-doped and TiO N- TiOdoping2 and2 and photo-induced levels photo-induced are also chargeshown. charge transfer Oxygen transfer intointo various variousvacancies kinds kinds were of redoxof estimatedredox species species from under under the photo-electroc visiblevisible lig lightht hemicalirradiation. irradiation. measurements. The The energy energy Signs levels levels of of circle sub-bands of and sub-bands cross at at the αthe, β ,α and, standβ, andγ forpotential γ energeticallypotential regions regions favorable (oxygen (oxygen and unfavora vacancies)vacancies)ble electronand and N- N-doping dopingtransfers, levels respectively levels are arealso [33]. also shown. shown. Oxygen Oxygen vacanciesvacancies were were estimated estimated from from the photo-electrochemicalthe photo-electrochemical measurements. measurements. Signs Signs of of circle circle andand crosscross stand for energeticallystand for energetically favorable favorable and unfavorable and unfavora electronble electron transfers, transfers, respectively respectively [33 [33].]. Catalysts 2019, 9, 201 7 of 22 Catalysts 2019, 9, x FOR PEER REVIEW 7 of 22
2.3. Application to photocatalytic Photocatalytic decompos Decompositionition of of volatile Volatile organic Organic compounds Compounds (VOC) (VOC)
Time profileprofile forfor the the photocatalytic photocatalytic decomposition decomposition of gaseousof gaseous acetaldehyde acetaldehyde onthe on N-dopedthe N-doped TiO2 TiOis shown2 is shown in Figure in Figure8. The 8. N-doped The N-doped TiO 2 exhibitedTiO2 exhibited photocatalytic photocatalytic activity activity 5 times 5 times greater greater than TiOthan2 TiOunder2 under visible visible light light irradiation, irradiation, while while they exhibitedthey exhibi similarted similar activities activities under under UV light UV light irradiation irradiation [28]. [28].
Figure 8. Photocatalytic decomposition of gaseous acetaldehyde on the N-doped TiO photocatalyst. Figure 8. Photocatalytic decomposition of gaseous acetaldehyde on the N-doped TiO2 photocatalyst. Evolved CO concentration ( , •, N-doped TiO ; , , TiO )[28]. Evolved CO2 concentration (○, ●, N-doped TiO22; □, ■, TiO2)2 [28]. # Table1 shows that the N-doped TiO 2 exhibited photocatalytic activity for the decomposition of Table 1 shows that the N-doped TiO2 exhibited photocatalytic activity for the decomposition of several kinds of VOC into CO2 under visible light irradiation (λ > 420 nm). It was observed that the several kinds of VOC into CO2 under visible light irradiation (λ > 420 nm). It was observed that the N-doped TiO2 exhibited photocatalytic activity for the decomposition of aldehydes, but little activity N-doped TiO2 exhibited photocatalytic activity for the decomposition of aldehydes, but little for alcohol, acid, ketone, and halogene compounds. The vanadium species was deposited on the activity for alcohol, acid, ketone, and halogene compounds. The vanadium species was deposited N-doped TiO2 (VCl3/N-doped TiO2) by impregnation method. As shown in Table1, VCl 3/N-doped on the N-doped TiO2 (VCl3/N-doped TiO2) by impregnation method. As shown in Table 1, TiO2 showed higher photocatalytic activity for the decomposition of all VOC, in particular, acetic acid VCl3/N-doped TiO2 showed higher photocatalytic activity for the decomposition of all VOC, in or acetone by ~13–16 times more than N-doped TiO2. Therefore, it was confirmed that vanadium particular, acetic acid or acetone by ~13–16 times more than N-doped TiO2. Therefore, it was species worked as the effective co-catalyst. confirmed that vanadium species worked as the effective co-catalyst.
Table 1. Yields of CO2 for the photocatalytic decomposition of various kinds of volatile organic Table 1. Yields of CO2 for the photocatalytic decomposition of various kinds of volatile organic compounds (VOC) in aqueous solutions with N–TiO2 and VCl3/N-doped TiO2 under visible light compounds (VOC) in aqueous solutions with N–TiO2 and VCl3/N-doped TiO2 under visible light irradiation (λ > 420 nm) for 3 h [40]. irradiation (λ > 420 nm) for 3 h [40].
Yields of CO2/µmol Entry Reactant Molecules Yields of CO2 / μmol entry Reactant molecules N-doped TiO VCl /N-doped TiO N-doped TiO2 2 VCl3/N-doped3 TiO22 a 1 methanol1a methanol 0.20.2 1.1 1.1 a 2 ethanol2 a ethanol 0.30.3 0.5 0.5 3 formaldehyde a 4.6 21.6 a 3 formaldehyde4 acetaldehyde a 4.64.1 21.6 35.0 4 acetaldehyde5 formic a acid a 4.10.7 35.0 4.8 5 formic6 acid acetica acid a 0.71.2 4.8 17.0 a 6 acetic7 acid aacetone 1.20.7 17.0 11.4 8 ethyl acetate a 1.3 10.6 7 acetone a 0.7 11.4 9 dichloromethane b 2.4 4.1 a 8 10ethyl acetatetrichloromethane b 1.31.5 10.6 4.1 9 11dichloromethane1, 1-dichloroethane b b 2.40.7 4.1 4.8 10 12trichloromethanetrans-1, b 2-dichloroethylene b 1.51.0 4.1 5.7 11 1, 1-dichloroethaneConcentrations b of VOC are (a)0.7 0.5 M and (b) 50 mM. 4.8 12 trans-1, 2-dichloroethylene b 1.0 5.7 Concentrations of VOC are (a) 0.5 M and (b) 50 mM. Furthermore, effects of co-catalysts (48 metal ions using nitrate, sulfate, chloride, acetate, and oxide precursors) deposited on the N-doped TiO2 for the photocatalytic activities were examined (See Figure 9) [40]. The bars marked in yellow exhibited higher photocatalytic activities than the
Catalysts 2019, 9, 201 8 of 22
Catalysts 2019, 9, x FOR PEER REVIEW 8 of 22 Furthermore, effects of co-catalysts (48 metal ions using nitrate, sulfate, chloride, acetate, and oxideN-doped precursors) TiO2 by deposited itself. In on particular, the N-doped N-doped-TiO TiO2 for the2-deposited photocatalytic Cu, Fe, activities V, and Pt were oxides examined exhibited (Seehigh Figure photocatalytic9)[ 40]. The activities. bars marked The local in yellow structures exhibited of the higher co-catalysts photocatalytic were characterized activities than by XPS. the It N-dopedwas observed TiO2 by itself.that Cu In particular,loaded N-doped N-doped-TiO TiO2 involves2-deposited cuprous Cu, Fe, oxide V, and (Cu Pt2O) oxides or Cu exhibited hydroxides, high Fe photocatalyticloaded N-doped activities. TiO2 involves The local clusters structures cont ofaining the co-catalysts Fe–O bonds were or Fe characterized2+ hydroxide [41], byXPS. and Pt It wasloaded 4+ 2+ observedN-doped that TiO Cu2 loadedinvolves N-doped Pt /Pt TiOspecies2 involves [36]. The cuprous redox oxide potentials (Cu2O) of orco-catalysts Cu hydroxides, such as Fe V loaded (+IV/+V), 2+ N-dopedFe (+II/+III), TiO2 involves Cu (+I/+II), clusters and containingPt (+III/+IV) Fe–O were bonds in the or range Fe hydroxide of circa +0.6 [41 to], and+1.0 Pt V loadedvs. SHE, N-doped while the 4+ 2+ + − TiOmulti-electron2 involves Pt reduction/Pt species of O2 [ 36leads]. The to the redox formation potentials of ofactive co-catalysts oxygen species such as via V (+IV/+V),O2 + 2H + Fe 2e / (+II/+III),H2O2 (E Cu0 = +0.687 (+I/+II), V vs. and SHE). Pt (+III/+IV) Therefore, were the in co-catalysts, the range of such circa as +0.6 Pt, toFe, +1.0 Cu, Vand vs. V SHE, species, while enhance the + − multi-electronthe photocatalytic reduction activity of O 2dueleads to the to the effective formation electron of active transfer oxygen to O2 species (O2 reduction), via O2 + resulting 2H + 2e in/ the H2Oformation2 (E0 = +0.687 of active V vs. oxygen SHE). species. Therefore, the co-catalysts, such as Pt, Fe, Cu, and V species, enhance the photocatalytic activity due to the effective electron transfer to O2 (O2 reduction), resulting in the formation of active oxygen species.
VCl3 N-TiO2 NH4VO3
NaCl MgCl2 FeCl3
K2CrO4 CuCl2 KCl CaCl2 4 -1 KMnO Al(NO3)3 CoCl2 Ni(NO3)2 RbCl Zn(Ac)2 SrCl2 H2MoO4 PdCl2 Ga(NO3)3 YCl3 ZrCl2O RuCl3 RhCl3
mol h 20 AgNO3 CsCl BaCl2 H2PtCl6 μ Cd(NO3)2 SnCl2 H2WO4 SbCl3 15 HAuCl4 TeCl4 10 Pb(NO3)2 BiCl3 Ce(SO4)2 5 Nd(NO3)3 La(Ac)3 Pr(Ac)3 Sm(NO3)3 EuCl3 0 Tm(Ac)3 Gd (NO3)3 TbCl3 DyCl3 HoCl3
evolution evolution / ErCl3 Yb Cl3 2 Lu(Ac)3 CO Several kinds of metallic salts for co-catalyst
Figure 9. Photocatalytic activities for the decomposition of acetic acid under visible light irradiation (λ >Figure 420 nm) 9. onPhotocatalytic N-doped TiO activities, modified for bythe various decomposition kinds of of metal acetic species acid under as co-catalysts. visible light Each irradiation metal λ 2 salt( used > 420 in nm) this studyon N-doped is shown TiO [402, modified]. by various kinds of metal species as co-catalysts. Each metal salt used in this study is shown [40]. 2.4. C3N4-Modified TiO2 Compared with N-doped TiO2 2.4. C3N4-modified TiO2 compared with N-doped TiO2 Several nitrogen sources such as urea, cyanamid, cyanuric acid, and melamine were employed for theSeveral preparation nitrogen of N-containing sources such as TiO urea,2 photocatalyst, cyanamid, cyanuric i.e., the acid, TiO 2andsurface melamine is modified were employed with polymerizedfor the preparation carbon nitride of N-containing (C3N4)[42– 51TiO].2 Thephotocatalyst, structures ofi.e., the the C, TiO N-species2 surface strongly is modified depend with onpolymerized their concentrations. carbon Ifnitride the C, (C N3 speciesN4) [42–51]. are present The structures in only a of small the amount,C, N-species they actstrongly as a molecular depend on photosensitizer.their concentrations. At higher If the amounts C, N species they form are apresent C3N4 crystallinein only a small semiconductor, amount, they which act as chemically a molecular bindsphotosensitizer. to TiO2. The C 3AtN 4higher–TiO2 wasamounts systematically they form synthesized a C3N4 crystalline by thermal semiconductor, condensation ofwhich cyanuric chemically acid on thebinds TiO to2 surfaceTiO2. The [51 ].C3 InN4 fact,–TiO H2 2waswas systematically evolved from TEAsynthesized aq. on the by Cthermal3N4–TiO condensation2 photocatalyst of undercyanuric visibleacid light on the irradiation, TiO2 surface while [51]. the In N-doped fact, H2TiO was2 didevolved not exhibit from TEA H2 production. aq. on the C From3N4–TiO characterization2 photocatalyst of Cunder3N4–TiO visible2 by Fourierlight irradiation, transformed-infrared while the (FT-IR),N-doped XPS, TiO electrochemical2 did not exhibit measurements, H2 production. and DFT From calculations,characterization the band of structures C3N4–TiO and2 photo-inducedby Fourier transformed-infrared charge separation mechanisms (FT-IR), wereXPS, demonstratedelectrochemical (Figuremeasurements, 10). The C and3N4 –TiODFT2 calculations,was found tothe exhibit band photo-inducedstructures and chargephoto-induced separation charge through separation the hetero-couplingmechanisms were of semiconductors demonstrated between(Figure 10). C3 NThe4 and C3N TiO4–TiO2 on2 was the surface.found to Onexhibit the otherphoto-induced hand, N-dopedcharge TiOseparation2 was photo-sensitized through the hetero-c by bulkoupling impurity of ofsemiconductors the N-doping. between It can be assumedC3N4 and that TiO many2 on the surface. On the other hand, N-doped TiO2 was photo-sensitized by bulk impurity of the N-doping. It can be assumed that many oxygen vacancies promoted the charge recombination, resulting in weak reduction power in the N-doped TiO2.
Catalysts 2019, 9, 201 9 of 22 oxygen vacancies promoted the charge recombination, resulting in weak reduction power in the
N-dopedCatalysts 2019 TiO, 92, .x FOR PEER REVIEW 9 of 22
Pt species C.B. H e- 2 C.B. 0.6 eV Photo-charging 2 H+ Visible-light 2.5 eV alcohol h+ V.B. oxidative products V.B.
TiO C N 2 3 4
FigureFigure 10. 10.Photo-induced Photo-induced charge charge separation separation on on the the C C3N3N44 depositeddeposited TiOTiO22 surfacesurface [[51].51].
3. Plasmonic Au NPs Modified TiO2 3. Plasmonic Au NPs modified TiO2 3.1. What Is Localized Surface Plasmon Resonance (LSPR)? 3.1. What is localized surface plasmon resonance (LSPR)? Localized surface plasmon resonance (LSPR) is an optical phenomenon generated by light when it interactsLocalized with surface conductive plasmon nanoparticles resonance (NPs) (LSPR) that is are an smaller optical than phenomenon the incident generated wavelength. by light The LSPRwhen is it induced interacts by with the conductive collective oscillations nanoparticles of delocalized (NPs) that electrons are smaller in response than the toincident an external wavelength. electric field.The LSPR The resonance is induced wavelength by the collective strongly oscillations depends onof delocalized the size and electrons shape of in the response NPs, the to interparticle an external distance,electric field. and theThe dielectric resonance property wavelength of the surroundingstrongly depends medium. on the The size Au andand Agshape NPs of exhibit the NPs, unique the plasmoninterparticle absorption distance, [52 and,53]. the The dielectric plasmonic property Ag NPs of are the considered surrounding to be medium. unstable The under Au illumination, and Ag NPs andexhibit could unique be applicable plasmon toabsorption multi-colored [52,53]. rewritable The plasmonic devices. Ag In NPs this section,are considered we focused to be on unstable stable plasmonicunder illumination, Au NPs exploited and could for be a visible-light-sensitive applicable to multi-colored photovoltaic rewritable fuel celldevices. or photocatalyst In this section, [54,55 we]. focused on stable plasmonic Au NPs exploited for a visible-light-sensitive photovoltaic fuel cell or 3.2.photocatalyst Preparation [54,55]. and Characterization of Au–TiO2 Photocatalyst
3.2.1.3.2. Preparation Photodeposition and characterization (PD) Methods of Au–TiO2 photocatalyst
By using the photocatalysis of TiO2, metallic Au was deposited on the TiO2 surface, accompanied 3.2.1. Photodeposition (PD) methods by the oxidation of methanol [56,57] or ethanol [58]. Typically, TiO2 powder was suspended in a 50 vol.By % aqueoususing the methanol photocatalysis in the presence of TiO of2 HAuCl, metallic4·6H 2AuO, purgedwas deposited of air with argon.on the The TiO suspension2 surface, wasaccompanied photoirradiated by the with oxidation UV light of under methanol magnetic [56,57] stirring. or ethanol The temperature [58]. Typically, of the suspensionTiO2 powder during was photoirradiationsuspended in a was50 vol. maintained % aqueous at 298K.methanol The Au/TiOin the presence2 photocatalyst of HAuCl was4·6H centrifuged,2O, purged washed of air withwith distilledargon. The water, suspension dried at was 393K, photoirradiated and ground in with an agate UV mortar.light under magnetic stirring. The temperature of the suspension during photoirradiation was maintained at 298K. The Au/TiO2 photocatalyst was 3.2.2.centrifuged, Colloid washed Photodeposition with distilled Operated water, in dried the Presence at 393K, ofand a Hole ground Scavenger in an agate (CPH) mortar. Colloidal Au NPs were prepared using the method reported by Frens [59]. In brief, mixtures of 3.2.2. Colloid photodeposition operated in the presence of a hole scavenger (CPH) an aqueous tetrachloroauric acid (HAuCl4) solution and sodium citrate were heated and boiled for 1 h. The colorColloidal of the Au solution NPs were changed prepared from deepusing blue the method to deep red.reported The citrateby Frens plays [59]. a roleIn brief, in the mixtures reduction of ofan Au aqueous ions, and tetrachloroauric the capping agent acid (HAuCl in suppressing4) solution the and aggregation sodium citrate of Au NPs.were Theheated suspension and boiled of TiOfor 21 inh. anThe aqueous color of solution the solution of colloidal changed Au NPs from and deep oxalic blue acid to wasdeep then red. photo-irradiated The citrate plays at λ a> role 300 nmin the at 298reduction K under of argon Au ions, (Ar). Theand solidsthe capping were recovered, agent in washed,suppressing and driedthe aggregation to produce Au–TiOof Au 2NPs.. Details The aresuspension given in Referenceof TiO2 in [60 ].an aqueous solution of colloidal Au NPs and oxalic acid was then photo-irradiated at λ > 300 nm at 298 K under argon (Ar). The solids were recovered, washed, and dried to produce Au–TiO2. Details are given in Reference [60].
3.2.3. Deposition precipitation (DP) method Deposition–precipitation (DP) methods were employed for the deposition of a gold (III) species on the TiO2 surface [61,62]. The [AuCl(OH)3]−, main species present at pH 8, adjusted by NaOH aq., reacts with hydroxyl groups of the TiO2 surface to form a grafted hydroxyl–gold compound. The
Catalysts 2019, 9, 201 10 of 22
3.2.3. Deposition Precipitation (DP) Method Deposition–precipitation (DP) methods were employed for the deposition of a gold (III) species − on the TiO2 surface [61,62]. The [AuCl(OH)3] , main species present at pH 8, adjusted by NaOH aq., reacts with hydroxyl groups of the TiO2 surface to form a grafted hydroxyl–gold compound. The catalyst was then recovered, filtered, washed with deionized water, and dried. Finally, the powder was calcined at ~473–673 K in air.
3.2.4. Characterization of the Au–TiO2 Photocatalyst
The Au–TiO2 photocatalysts were typically characterized by the transmittance electron microscope (TEM) for the particle sizes, and UV-vis absorption for optical properties (See Table2).
Table 2. Particle sizes of Au nanoparticles (NPs) and optical properties of the Au–TiO2 prepared by several techniques.
Entry Au Deposition Methods Particle Sizes/nm Top Peak/nm Ref. 1 PD ~10–60 ~530–610 [56–58] ~12–14 ~550–560 [60,63–66] 2 CPH 13 ~550–620 [67] ~2–6 ~550–560 [61] 3 DP < 5 550 [68–70]
Kowalska et al. [56,57] reported that Au–TiO2 photocatalysts with different Au particle sizes (~10–60 nm) were prepared by photo-deposition (entry 1). The particle sizes of Au strongly depend on the particle sizes of the TiO2 polycrystalline structure. The top peak of plasmonic absorption was in the range of ~530–610 nm, depending on the particle sizes of the Au NPs. Tanaka and Kominami et al. [60,63–66] reported unique CPH methods for the preparation of Au-TiO2 (entry 2). The particle sizes were uniformed to be ~12–14 nm, which exhibits plasmonic absorption at ~550–560 nm. Thus, colloidal Au NPs were successfully loaded onto TiO2 without change in the original particle size. Furthermore, the top peak of Au plasmon absorption was found to extend towards 620 nm by simple calcinations of the samples. This phenomenon is due to high contact area between TiO2 and Au NPs without change of particle size [66]. Additionally, Naya et al. [67,68] and Shiraishi et al. [69] employed precipitation deposition methods to deposit small Au NPs (~2–6 nm) on TiO2 (entry 3).
3.3. Application of LSPR of Au–TiO2 to Several Photocatalytic Reactions
Au NPs deposited on TiO2 have been used as visible-light-responsive photocatalysts for several chemical reactions: decomposition of VOCs, selective oxidation of an aromatic alcohol, direct water splitting, H2 formation from sacrificial aqueous solutions, and reduction of organic compounds (see Table3). Several research groups concluded that photocatalytic activities are induced by LSPR of the Au NPs. Some research indicates that small Au NPs (~5 nm) effectively work for the reactions [61,69]. Tanaka and Kominami et al. suggest that two types of Au particles of different sizes loaded onto TiO2 exhibit different functionalities. That is, the larger Au particles contribute to strong light absorption, and the smaller Au particles act as a co-catalyst for H2 evolution [63]. Catalysts 2019, 9, 201 11 of 22
Table 3. Applications to several photocatalytic reactions on the Au–TiO2 photocatalyst.
Entry Photocatalytic Reactions Au Deposition Methods References oxidations of 2-propanol and ethanol PD [56–58] 1 oxidation of formic acid CPH [60] oxidation of thiol to disulfide DP [67] oxidation of amine to imine DP [68] 2 CPH [66] oxidation of aromatic alcohol to aldehyde DP [69] oxidation of benzene to phenol PD [70]
H2 formation from alcohols CPH [63,71] 3 DP [61] water splitting into H and O 2 2 CPH [64,72] 4 reduction of nitrobenzene to aniline CPH [65] Catalysts 2019, 9, x FOR PEER REVIEW 11 of 22 3.4. Application to a Photovoltaic Fuel Cell Operating under Visible Light Irradiation 3.4. Application to a photovoltaic fuel cell operating under visible light irradiation TheThe Au–TiO Au–TiO2 2filmsfilms were foundfound toto exhibit exhibit the the behavior behavior of aof photovoltaic a photovoltaic fuel cellfuel [54cell,55 [54,55].]. An anodic An anodicphotocurrent photocurrent was yielded was yielded on the on Au the−TiO Au2−filmTiO2as film the as visible the visible light waslight irradiated, was irradiated, while thewhile current the currentwas observed was observed neither neither on a TiO on2 filma TiO under2 film visible under lightvisible irradiation, light irradiation, nor on the nor Au on− theTiO 2Aufilm−TiO when2 film the whenlight the was light turned was off. turned The short-circuitoff. The short-circuit photocurrent photocurrent density (J scdensity) was strongly(Jsc) was influenced strongly influenced by kinds of 2+ bydonors, kinds andof donors, the photocurrent and the photocurrent efficiency was ef maximizedficiency was in maximized the presence in of the Fe presenceions. Furthermore, of Fe2+ ions. the Furthermore,photocurrent the action photocurrent spectra were action closely spectra fitted were with closely the absorption fitted with spectrum the absorption of the Au spectrum NPs deposited of the Auon NPs the TiOdeposited2 film (Seeon the Figure TiO 211 film). (See Figure 11).
Figure 11. Short-circuit photocurrent densities [I] vs. apparent formal potential of different donors on
Figurethe Au 11.− TiOShort-circuit2 photoanode photocurrent in acetonitrile/ethylene densities [I] vs. apparent glycol (v/v formal 60/40) pote containingntial of different 0.1 M donors LiNO3 onand the0.1 Au M− donors;TiO2 photoanode IPCE [II] ofin theacetonitrile/ethylene Au−TiO2 film in gl a Nycol2-saturated (v/v 60/40) acetonitrile containing and 0.1 ethyleneM LiNO3 glycoland 0.1 (v/v: M donors;60/40) IPCE solution [II] containingof the Au− 0.1TiO M2 film FeCl in2 and a N 0.052-saturated M FeCl acetonitrile3 [55]. and ethylene glycol (v/v: 60/40) solution containing 0.1 M FeCl2 and 0.05 M FeCl3 [55]. 3.5. Mechanisms of Charge Separation 3.5. MechanismsThe mechanism of charge for separation the Au plasmon-induced charge separation is shown in Figure 12. Visible lightThe irradiation mechanism generates for the the Au photo-excited plasmon-induced state of charge the Au separation NPs by LSPR. is shown The photo-excited in Figure 12. electronsVisible lightare injectedirradiation into thegenerates C.B. of TiOthe 2,photo-excited while the holes state abstracted of the electronsAu NPs from by LSPR. a donor The in the photo-excited solution. The electronsAu NPs behaveare injected like aninto intrinsic the C.B. semiconductor, of TiO2, while andthe theholes Fermi abstracted levels ofelectrons Au NPs from and TiOa donor2 are leveledin the solution.out, resulting The Au in theNPs formation behave like of Schottkyan intrinsic barrier semiconductor, at Au–TiO2 junctions. and the Fermi This bandlevels model of Au seems NPs and to be TiOsimilar2 are withleveled dye-sensitized out, resulting photo-anodic in the formation electrodes. of Schottky barrier at Au–TiO2 junctions. This band model seems to be similar with dye-sensitized photo-anodic electrodes. Recently, Furube et al. studied the plasmon-induced charge transfer mechanisms between Au NPs and TiO2 by means of femtosecond visible pump/infrared probe transient absorption spectroscopy [73]. The electron transfer from the Au NPs to the C.B. of TiO2 was confirmed to occur within 50 fs, and that the electron injection yielded 20–50% upon 550 nm laser excitation.
[I] [II]
Figure 12. Schematic illustration [I] and its energy band levels [II] for the photo-induced charge
separation on the Au–TiO2 in the presence of donors [55]. Catalysts 2019, 9, x FOR PEER REVIEW 11 of 22
3.4. Application to a photovoltaic fuel cell operating under visible light irradiation The Au–TiO2 films were found to exhibit the behavior of a photovoltaic fuel cell [54,55]. An anodic photocurrent was yielded on the Au−TiO2 film as the visible light was irradiated, while the current was observed neither on a TiO2 film under visible light irradiation, nor on the Au−TiO2 film when the light was turned off. The short-circuit photocurrent density (Jsc) was strongly influenced by kinds of donors, and the photocurrent efficiency was maximized in the presence of Fe2+ ions. Furthermore, the photocurrent action spectra were closely fitted with the absorption spectrum of the Au NPs deposited on the TiO2 film (See Figure 11).
Figure 11. Short-circuit photocurrent densities [I] vs. apparent formal potential of different donors on
the Au−TiO2 photoanode in acetonitrile/ethylene glycol (v/v 60/40) containing 0.1 M LiNO3 and 0.1 M
donors; IPCE [II] of the Au−TiO2 film in a N2-saturated acetonitrile and ethylene glycol (v/v: 60/40)
solution containing 0.1 M FeCl2 and 0.05 M FeCl3 [55]. 3.5. Mechanisms of charge separation The mechanism for the Au plasmon-induced charge separation is shown in Figure 12. Visible light irradiation generates the photo-excited state of the Au NPs by LSPR. The photo-excited electrons are injected into the C.B. of TiO2, while the holes abstracted electrons from a donor in the solution. The Au NPs behave like an intrinsic semiconductor, and the Fermi levels of Au NPs and TiO2 are leveled out, resulting in the formation of Schottky barrier at Au–TiO2 junctions. This band model seems to be similar with dye-sensitized photo-anodic electrodes. Recently, Furube et al. studied the plasmon-induced charge transfer mechanisms between Au NPs and TiO2 by means of femtosecond visible pump/infrared probe transient absorption Catalystsspectroscopy2019, 9, 201[73]. The electron transfer from the Au NPs to the C.B. of TiO2 was confirmed to12 occur of 22 within 50 fs, and that the electron injection yielded 20–50% upon 550 nm laser excitation.
[I] [II]
FigureFigure 12.12. Schematic illustration [I] and its energy band levels [II][II] forfor thethe photo-inducedphoto-induced chargecharge
separationseparation onon thethe Au–TiOAu–TiO22 inin thethe presencepresence ofof donorsdonors [[55].55].
Recently, Furube et al. studied the plasmon-induced charge transfer mechanisms between Au NPs and TiO2 by means of femtosecond visible pump/infrared probe transient absorption spectroscopy [73]. The electron transfer from the Au NPs to the C.B. of TiO2 was confirmed to occur within 50 fs, and that the electron injection yielded 20–50% upon 550 nm laser excitation.
4. Photo-Induced Interfacial Charge Transfer
4.1. Dye-Sensitized TiO2 Photocatalysis 2+ Dye sensitized TiO2 photocatalysis was studied in the late 1990s. The Ru complex, [Ru(bipy)3] grafted on the TiO2 surface exhibits visible light absorption [74,75]. In this system, the excitation of the Ru complex induces electron transfer via metal–ligand charge transfer (MLCT). The photo-induced electrons are then transferred onto TiO2, resulting in photocatalytic water splitting to produce H2. The platinum-chloride-modified TiO2 system was reported by Kisch et al. [76,77]. Photo-irradiation 4+ − of Pt(IV) chloride exhibits visible-light absorption to generate the active center, (Pt (Cl )4 + hν → 3+ 0 – 3+ Pt Cl (Cl )3. The photo-induced electrons are transferred from Pt to C.B. of TiO2 as reductive sites, while the Cl0 work as the oxidative sites, resulting in the redox photocatalytic reactions. Important strategies to develop these types of photocatalysts are to design robust sensitizers adjusted with HOMO-LUMO levels.
4.2. Visible-Light-Responsive TiO2 Photocatalyst Modified by Phenolic Organic Compounds
Strong interaction of phenolic groups in organic compounds with Ti–OH of the TiO2 surface probably forms two types of interfacial surface complexes (ISC, Figure 13I), which exhibits visible light absorption via LMCT. The photocatalysis of the ISC is strongly influenced by the electronic structures of the ISC (Figure 13II): the ISC with EWG exhibits strong oxidizability under visible light irradiation, and it can favorably oxidize the TEA, together with H2 evolution from deaerated TEA aqueous solutions [78]. The visible light response of the ISC is attributed to electronic excitation from the donor levels (0.7 V above V.B.) to the C.B. of TiO2 (see Figure 14). Therefore, the electronic structures of sensitizers strongly influence the photocatalytic activities. Ikeda et al. [79] demonstrated that a 0 0 TiO2 photocatalyst modified with 1,1 -binaphthalene-2,2 -diol (bn(OH)2) exhibited photocatalytic H2 evolution from deaerated TEA aq. under visible light irradiation. Kamegawa et al. [80] designed a 2, 3-dihydroxynaphthalene (2,3-DN)-modified TiO2 photocatalyst for the reduction of nitrobenzene to aminobenzene under visible light irradiation. Catalysts 2019, 9, x FOR PEER REVIEW 12 of 22
4. Photo-induced interfacial charge transfer
4.1. Dye-sensitized TiO2 photocatalysis
Dye sensitized TiO2 photocatalysis was studied in the late 1990s. The Ru complex, [Ru(bipy)3]2+ grafted on the TiO2 surface exhibits visible light absorption [74,75]. In this system, the excitation of the Ru complex induces electron transfer via metal–ligand charge transfer (MLCT). The photo-induced electrons are then transferred onto TiO2, resulting in photocatalytic water splitting to produce H2. The platinum-chloride-modified TiO2 system was reported by Kisch et al. [76,77]. Photo-irradiation of Pt(IV) chloride exhibits visible-light absorption to generate the active center, (Pt4+(Cl−)4 + hν → Pt3+Cl0(Cl–)3. The photo-induced electrons are transferred from Pt3+ to C.B. of TiO2 as reductive sites, while the Cl0 work as the oxidative sites, resulting in the redox photocatalytic reactions. Important strategies to develop these types of photocatalysts are to design robust sensitizers adjusted with HOMO-LUMO levels.
4.2. Visible-light-responsive TiO2 photocatalyst modified by phenolic organic compounds
Strong interaction of phenolic groups in organic compounds with Ti–OH of the TiO2 surface probably forms two types of interfacial surface complexes (ISC, Figure 13I), which exhibits visible light absorption via LMCT. The photocatalysis of the ISC is strongly influenced by the electronic structures of the ISC (Figure 13II): the ISC with EWG exhibits strong oxidizability under visible light aqueousirradiation, solutions and it [78].can favorablyThe visible oxidize light response the TEA, of together the ISC iswith attributed H2 evolution to electronic from deaeratedexcitation fromTEA the donor levels (0.7 V above V.B.) to the C.B. of TiO2 (see Figure 14). Therefore, the electronic structures of sensitizers strongly influence the photocatalytic activities. Ikeda et al. [79] demonstrated that a TiO2 photocatalyst modified with 1,1′-binaphthalene-2,2′-diol (bn(OH)2) exhibited photocatalytic H2 evolution from deaerated TEA aq. under visible light irradiation. Kamegawa et al. [80] designed a 2, 3-dihydroxynaphthalene (2,3-DN)-modified TiO2 photocatalyst for the reduction of nitrobenzene to aminobenzene under visible light irradiation. On the other hand, the phenolic compounds were degraded on the TiO2 in the presence of O2 under visible light (λ > 420 nm) illumination, producing Cl– and CO2 [81]. The ISC formed by the interaction of phenolic compounds with TiO2 exhibited self-degradation. It was proposed that an Catalystselectronic2019, 9transition, 201 occurs from the ISC to the C.B. of TiO2 to form active oxygen species, which13 of also 22 participate in the oxidative degradation of phenolic compounds.
R R 35
H H OH OH O O O O 30
OOTi O Ti O mol 25
2 H2O μ /
R 2 R 20 (d) 15 H H (f) O O OH OH O O
Evolved H Evolved 10 TiO Ti TiO Ti (e) 2 H2O 5 (c) R: 0 (a, b)
-OCH3 (MC), -C(CH3)3 (BC) :EDG 0246 Time / h -COOH (BA), -CN (BN), -SO3Na (TN) :EWG [I] [II]
FigureFigure 13. 13.Schematic Schematic illustration illustration for for the the formation formation of of two two types types of of ISCs ISCs [I], [I], and and photocatalytic photocatalytic H 2H2 evolutionevolution [II] [II] from from aq. aq. TEA TEA (10 (10 vol. vol. %) %) on on (a) (a) BC/TiO BC/TiO2,2, (b)(b) MC/TiO MC/TiO22, (c) CA/TiOCA/TiO22, ,(d) (d) BA/TiO BA/TiO2, 2(e), (e)BN/TiO BN/TiO2, 2and, and (f) (f) TN/TiO TN/TiO2 [78].2 [78 ].BC: BC: 4- 4-t-butylt-butyl catechol, catechol, MC: 3-methoxy3-methoxy catechol,catechol, CA:CA: catecol, catecol, BA: BA: Catalysts 2019, 9, x FOR PEER REVIEW 13 of 22 2,3-dihydroxy2,3-dihydroxy benzoic benzoic acid; acid; BN: BN: 3,4-dihydroxy 3,4-dihydroxy benzonitrile; benzonitrile; TN: TN: tiron. tiron.
E / V vs. Ag/AgCl (pH 10)
e- C.B. H2
Efb -0.94 Pt + E(H /H2) -080 2 H+ Visible-light
TEA/TEA• ○ Donor level +1.58 + h O 0.68 eV Ti O O +2.26 Ti O N V.B. O (ISC) TiO 2 Figure 14. Schematic illustration of photo-induced charge separation on the BN/TiO for H evolution Figure 14. Schematic illustration of photo-induced charge separation on the2 BN/TiO2 2 for H2 fromevolution TEA aq.from in TEA the presence aq. in the of presence Pt as co-catalyst of Pt as co-catalyst under visible under light visible irradiation light irradiation [78]. [78].
On the other hand, the phenolic compounds were degraded on the TiO2 in the presence of O2 4.3. Interfacial-surface-complex-mediated visible-light-sensitive TiO– 2 photocatalysts under visible light (λ > 420 nm) illumination, producing Cl and CO2 [81]. The ISC formed by the interactionThe interfacial of phenolic surface compounds complex with (ISC)- TiOmediated2 exhibited visible-light-sensitive self-degradation. ItTiO was2 photocatalyst proposed that was an electronicapplied to transition selective occursoxidation from of the several ISC to aromatic the C.B. ofalcohols TiO2 to [82–88]. form active Unlike oxygen to the species, ISC in whichFigure also 14, participatereactant molecules in the oxidative adsorbed degradation onto the TiO of phenolic2 surface compounds.(ISC) is activated under visible-light irradiation, and they are converted into products. Figure 15 shows reaction time profiles for the oxidation of 4.3.benzyl Interfacial-Surface-Complex-Mediated alcohol in an acetonitrile solution Visible-Light-Sensitive suspended with TiO TiO2 2photocatalystPhotocatalysts in the presence of O2 under visible light irradiation (λ > 420 nm). This reaction does not proceed without TiO2 or The interfacial surface complex (ISC)-mediated visible-light-sensitive TiO2 photocatalyst was appliedirradiation. to selective It was found oxidation that of the several amount aromatic of benzyl alcohols alcohol [82– 88decreased]. Unlike with to the an ISC increase in Figure in the14, irradiation time, while the amount of benzaldehyde increased. Neither benzoic acid nor CO2 were reactant molecules adsorbed onto the TiO2 surface (ISC) is activated under visible-light irradiation, andformed they as are oxidative converted products. into products. The yield Figure of benzaldehyde 15 shows reaction reached timecirca profiles95%, and for the the carbon oxidation balance of in the liquid phase was circa 95% after photo-irradiation for 4 h.
50 100
40 (e) 80 %
mol (b)
μ 30 60
20 40
Amounts / / Amounts (a) 10 20 / balance Carbon (c), (d) 0 0 0 601 1202 1803 2404 Time / h
Figure 15. Selective oxidation of benzyl alcohol on TiO2 (50 mg) under visible light irradiation [82]. The initial amount of benzyl alcohol was 50 μmol. Amounts of: benzyl alcohol (a); benzaldehyde (b);
benzoic acid (c); CO2 (d); and percentage of total organic compounds in solution (e).
Photocatalytic oxidation of benzyl alcohol and its derivatives into corresponding aldehydes was carried out with TiO2 under visible light irradiation. Benzyl alcohol and its derivatives substituted by –OCH3, –Cl, –NO2, –CH3, –CF3, and –C(CH3)3 groups were successfully converted to
Catalysts 2019, 9, x FOR PEER REVIEW 13 of 22
E / V vs. Ag/AgCl (pH 10)
e- C.B. H2
Efb -0.94 Pt + E(H /H2) -080 2 H+ Visible-light
TEA/TEA• ○ Donor level +1.58 + h O 0.68 eV Ti O O +2.26 Ti O N V.B. O (ISC) TiO 2
Figure 14. Schematic illustration of photo-induced charge separation on the BN/TiO2 for H2 evolution from TEA aq. in the presence of Pt as co-catalyst under visible light irradiation [78].
4.3. Interfacial-surface-complex-mediated visible-light-sensitive TiO2 photocatalysts
The interfacial surface complex (ISC)-mediated visible-light-sensitive TiO2 photocatalyst was
Catalystsapplied2019 to ,selective9, 201 oxidation of several aromatic alcohols [82–88]. Unlike to the ISC in Figure14 of 14, 22 reactant molecules adsorbed onto the TiO2 surface (ISC) is activated under visible-light irradiation, and they are converted into products. Figure 15 shows reaction time profiles for the oxidation of benzyl alcohol in anan acetonitrileacetonitrile solutionsolution suspendedsuspended withwith TiOTiO22 photocatalystphotocatalyst in thethe presencepresence ofof OO22 under visiblevisible light light irradiation irradiation (λ >(λ 420 > nm).420 Thisnm). reactionThis reaction does not does proceed not withoutproceed TiO without2 or irradiation. TiO2 or Itirradiation. was found It that was the found amount that of the benzyl amount alcohol of benzyl decreased alcohol with decreased an increase with in the an irradiationincrease in time, the whileirradiation the amount time, while of benzaldehyde the amount increased. of benzalde Neitherhyde benzoicincreased. acid Neither nor CO benzoic2 were formedacid nor as CO oxidative2 were products.formed as Theoxidative yield ofproducts. benzaldehyde The yield reached of benzaldehyde circa 95%, and reached the carbon circa 95%, balance and inthe the carbon liquid balance phase wasin the circa liquid 95% phase after was photo-irradiation circa 95% after for photo-irradiation 4 h. for 4 h.
50 100
40 (e) 80 %
mol (b)
μ 30 60
20 40
Amounts / / Amounts (a) 10 20 / balance Carbon (c), (d) 0 0 0 601 1202 1803 2404 Time / h
Figure 15. Selective oxidation of benzyl alcohol on TiO2 (50 mg) under visible light irradiation [82]. Figure 15. Selective oxidation of benzyl alcohol on TiO2 (50 mg) under visible light irradiation [82]. The initial amount of benzyl alcohol was 50 µmol. Amounts of: benzyl alcohol (a); benzaldehyde (b); The initial amount of benzyl alcohol was 50 μmol. Amounts of: benzyl alcohol (a); benzaldehyde (b); benzoic acid (c); CO2 (d); and percentage of total organic compounds in solution (e). benzoic acid (c); CO2 (d); and percentage of total organic compounds in solution (e). Photocatalytic oxidation of benzyl alcohol and its derivatives into corresponding aldehydes was CatalystsPhotocatalytic 2019, 9, x FOR PEERoxidation REVIEW of benzyl alcohol and its derivatives into corresponding aldehydes14 of 22 carried out with TiO2 under visible light irradiation. Benzyl alcohol and its derivatives substituted by was carried out with TiO2 under visible light irradiation. Benzyl alcohol and its derivatives –OCH3, –Cl, –NO2, –CH3, –CF3, and –C(CH3)3 groups were successfully converted to corresponding substitutedcorresponding by –OCHaldehydes3, –Cl, with –NO 2a, –CHhigh3 , co–CFnversion3, and –C(CH and high3)3 groups selectivity were successfullyon TiO2, while converted no other to aldehydes with a high conversion and high selectivity on TiO2, while no other products were observed products were observed (See Table 4). However, the phenolic compound (entry 9) was deeply (See Table4). However, the phenolic compound (entry 9) was deeply oxidized, since it strongly oxidized, since it strongly adsorbed on the TiO2 surface [82]. adsorbed on the TiO2 surface [82].
Table 4. Chemoselective photocatalytic oxidation of different kinds of benzylic alcohols on TiO2 [82]. Table 4. Chemoselective photocatalytic oxidation of different kinds of benzylic alcohols on TiO2 [82]. OH O TiO2, O2 R1 R1 Visible-light (4h) R2 R2
Entry EntryR1 R1 R2 R2 ConversionConversion (%) (%) Selectivity Selectivity (%) (%) 1 1H HH H > 99 > 99>99 2 H C(CH3)3 > 99 > 99 2 H C(CH3)3 > 99 > 99 3 H OCH3 > 99 > 99 3 H OCH3 > 99 > 99 4 H CH3 > 99 > 99 4 5H HCH3 Cl > 99 > 99>99 5 6H HCl NO2 > 99 > 99>99 7 H CF > 99 > 99 6 H NO2 3 >99 >99 8 CH3 H > 99 > 99 7 9H HCF3 OH > 8599 23 >99 8 CH3 H >99 >99 4.3.1.9 What is the originH of the visibleOH light response? >85 23 4.3.1. What Is the Origin of the Visible Light Response? The interaction of benzyl alcohol with TiO2 was analyzed by FT-IR spectroscopy (See Figure 16). CharacteristicThe interaction features of benzyl of the alcoholISC are withas follows: TiO2 was (i) a analyzed remarkable by FT-IR downward spectroscopy negative (See band Figure at 3715 16). −1 Characteristiccm−1 attributed features to theof O–H the ISCstretching are as follows: of the terminal (i) a remarkable OH group; downward (ii) a new negative band band appeared at 3715 at cm circa −1 attributed1100 cm−1, to which the O–H is attributed stretching to of the the C–O terminal stretching OH group; of the (ii) alkoxide a new bandspecies appeared formed at by circa the 1100interaction cm , of benzyl alcohol with TiO2, while that of benzyl alcohol by itself is 1020 cm−1. When the TiO2 was treated by diluted HF (aq), the IR band at 3715 cm−1 on the HF–TiO2 drastically decreased, while the photocatalytic activity significantly decreased. The active sites were confirmed to be alkoxide by the interaction of benzyl alcohol with the terminal OH groups of TiO2.
OH v(C-O) ca.1020 cm-1
alkoxide
v(C-O) H 1100 cm-1 O O-H 3715 cm-1 Ti O Ti
Ti-OH groups
[I] [II]
Figure 16. FT-IR spectra [I] of benzyl alcohol by itself and benzyl alcohol adsorbed on TiO2; and [II] their peak identification [82].
TiO2 by itself exhibited absorption only in the UV region, which is attributed to the charge transition from V.B. to C.B. When the benzyl alcohol was adsorbed on TiO2, absorption in the visible region could be observed. This absorption in the visible light region is assignable to the ISC through the LMCT (See Figure 17). The action spectra of apparent quantum yield (AQY) plots were fitted Catalysts 2019, 9, x FOR PEER REVIEW 14 of 22
corresponding aldehydes with a high conversion and high selectivity on TiO2, while no other products were observed (See Table 4). However, the phenolic compound (entry 9) was deeply oxidized, since it strongly adsorbed on the TiO2 surface [82].
Table 4. Chemoselective photocatalytic oxidation of different kinds of benzylic alcohols on TiO2 [82].
OH O TiO2, O2 R1 R1 Visible-light (4h) R2 R2
Entry R1 R2 Conversion (%) Selectivity (%) 1 H H >99 >99 2 H C(CH3)3 > 99 > 99 3 H OCH3 > 99 > 99 4 H CH3 >99 >99 5 H Cl >99 >99 6 H NO2 >99 >99 7 H CF3 >99 >99 8 CH3 H >99 >99 4.3.1.9 What is the originH of the visibleOH light response? >85 23
The interaction of benzyl alcohol with TiO2 was analyzed by FT-IR spectroscopy (See Figure 16). Characteristic features of the ISC are as follows: (i) a remarkable downward negative band at 3715 cm−1 attributed to the O–H stretching of the terminal OH group; (ii) a new band appeared at circa 1100 cm−1, which is attributed to the C–O stretching of the alkoxide species formed by the interaction Catalystsof benzyl2019 alcohol, 9, 201 with TiO2, while that of benzyl alcohol by itself is 1020 cm−1. 15 of 22 When the TiO2 was treated by diluted HF (aq), the IR band at 3715 cm−1 on the HF–TiO2 drastically decreased, while the photocatalytic activity significantly decreased. The active sites were which is attributed to the C–O stretching of the alkoxide species formed by the interaction of benzyl confirmed to be alkoxide by the interaction of benzyl alcohol with the terminal OH groups of TiO2. −1 alcohol with TiO2, while that of benzyl alcohol by itself is 1020 cm .
OH v(C-O) ca.1020 cm-1
alkoxide
v(C-O) H 1100 cm-1 O O-H 3715 cm-1 Ti O Ti
Ti-OH groups
[I] [II]
Figure 16. FT-IR spectra [I] of benzyl alcohol by itself and benzyl alcohol adsorbed on TiO2; and [II] Figure 16. FT-IR spectra [I] of benzyl alcohol by itself and benzyl alcohol adsorbed on TiO2; and [II] theirtheir peakpeak identificationidentification [[82].82].
−1 WhenTiO2 by the itself TiO 2exhibitedwas treated absorption by diluted onlyHF in the (aq), UV the region, IR band which at 3715 is attributed cm on to the the HF–TiO charge2 drasticallytransition from decreased, V.B. to while C.B. When the photocatalytic the benzyl alcohol activity was significantly adsorbed on decreased. TiO2, absorption The active in sitesthe visible were confirmedregion could to bebe alkoxideobserved. by This the absorption interaction in of the benzyl visible alcohol light withregion the is terminal assignable OH to groups the ISC of through TiO2. the LMCTTiO2 by (See itself Figure exhibited 17). The absorption action spectra only inof theapparent UV region, quantum which yield is attributed(AQY) plots to were the charge fitted transition from V.B. to C.B. When the benzyl alcohol was adsorbed on TiO2, absorption in the visible regionCatalysts could 2019, 9 be, x FOR observed. PEER REVIEW This absorption in the visible light region is assignable to the ISC through15 of 22 the LMCT (See Figure 17). The action spectra of apparent quantum yield (AQY) plots were fitted with thewith photo-absorption the photo-absorption of TiO 2of-adsorbed TiO2-adsorbed benzyl benzyl alcohol, alcohol, suggesting suggestin that visibleg that lightvisible absorption light absorption directly participateddirectly participated in the photocatalytic in the photocatalytic reactions. reactions.
60 0.6
(c) Visible-light 40 0.4 h+ H C 2 O Absorbance Absorbance
AQY /AQY % alkoxide 20 (b) 0.2 e- [Ti] [O] (a)
0 0 LMCT in the alkoxide 350 450 550 650 Wavelength / nm
Figure 17. UV-vis absorption spectra of TiO2 (a), TiO2 adsorbed with benzyl alcohol (b), and Figure 17. UV-vis absorption spectra of TiO2 (a), TiO2 adsorbed with benzyl alcohol (b), and apparent quantum yield (AQY) for the formation of benzaldehyde (c); and schematic illustration apparent quantum yield (AQY) for the formation of benzaldehyde (c); and schematic illustration of of photo-induced charge transfer through LMCT in the alkoxide [82]. photo-induced charge transfer through LMCT in the alkoxide [82]. DFT calculations [87] indicated the interaction of benzyl alcohol with surface hydroxyl groups on DFT calculations [87] indicated the interaction of benzyl alcohol with surface hydroxyl groups the TiO2 surface, resulting in the formation of alkoxide species. The electron density contour maps for on the TiO2 surface, resulting in the formation of alkoxide species. The electron density contour maps for the alkoxide species are shown in Figure 18. The orbital #212 at −0.80 eV forms the V.B. of TiO2, while #218 at +2.25 eV forms the C.B. One type of surface state consisting of the orbital (#215) originates with the alkoxide species ([Ti]–O–CH2–ph) hybridized with the O2p AOs in the V.B. of the TiO2. The energy gap between #215 and #218 (2.8 eV) was confirmed to be the origin for the visible light response.
Figure 18. Photo-induced electron transfer from the hybridized orbital to the C.B. of TiO2 under visible light irradiation [87]. Density maps of V.B., C.B., and hybridized orbital are shown here.
Catalysts 2019, 9, x FOR PEER REVIEW 15 of 22
with the photo-absorption of TiO2-adsorbed benzyl alcohol, suggesting that visible light absorption directly participated in the photocatalytic reactions.
60 0.6
(c) Visible-light 40 0.4 h+ CH 2 O Absorbance Absorbance
AQY /AQY % alkoxide 20 (b) 0.2 e- [Ti] [O] (a)
0 0 LMCT in the alkoxide 350 450 550 650 Wavelength / nm
Figure 17. UV-vis absorption spectra of TiO2 (a), TiO2 adsorbed with benzyl alcohol (b), and apparent quantum yield (AQY) for the formation of benzaldehyde (c); and schematic illustration of photo-inducedDFT calculations charge [87] transfer indicated through the LMCT interaction in the alkoxideof benzyl [82]. alcohol with surface hydroxyl groups Catalystson the2019 TiO, 9,2 201 surface, resulting in the formation of alkoxide species. The electron density 16contour of 22 maps for the alkoxide species are shown in Figure 18. The orbital #212 at −0.80 eV forms the V.B. of TiO2, while #218 at +2.25 eV forms the C.B. One type of surface state consisting of the orbital (#215) the alkoxide species are shown in Figure 18. The orbital #212 at −0.80 eV forms the V.B. of TiO , while originates with the alkoxide species ([Ti]–O–CH2–ph) hybridized with the O2p AOs in the V.B.2 of the #218 at +2.25 eV forms the C.B. One type of surface state consisting of the orbital (#215) originates with TiO2. The energy gap between #215 and #218 (2.8 eV) was confirmed to be the origin for the visible thelight alkoxide response. species ([Ti]–O–CH2–ph) hybridized with the O2p AOs in the V.B. of the TiO2. The energy gap between #215 and #218 (2.8 eV) was confirmed to be the origin for the visible light response.
Figure 18. Photo-induced electron transfer from the hybridized orbital to the C.B. of TiO under visible Figure 18. Photo-induced electron transfer from the hybridized orbital to the C.B.2 of TiO2 under lightvisible irradiation light irradiation [87]. Density [87]. Density maps of maps V.B., C.B.,of V. andB., C.B., hybridized and hybridized orbital are orbital shown are here. shown here. 4.3.2. What Makes the High Selectivity for the Photocatalytic Reactions?
It was observed that benzyl alcohol is adsorbed on TiO2 more favorably than benzaldehyde in a mixture of benzyl alcohol and benzaldehyde under dark conditions. This result indicates that the interaction between benzaldehyde and TiO2 is fairly weak. According to DFT calculations [87,88], the interaction of benzyl alcohol with the TiO2 surface formed a hybridized orbital, while benzaldehyde did not form orbital mixing. Therefore, once benzaldehyde was produced by the oxidation of benzyl alcohol, benzaldehyde was immediately released into the bulk solution, and was not oxidized further to benzoic acid or CO2.
4.3.3. Reaction Mechanisms behind the Selective Photocatalytic Oxidation of Benzyl Alcohol The photocatalytic activities for the oxidation of benzyl alcohol or α, α-d2 benzyl alcohol were investigated. The kinetic isotope effect (KIE) [=kC-H/kCD] was estimated to be 3.9 at 295 K. This result suggests that the process for the α-deprotonation is the rate determining step (RDS) for the overall reaction. From the experimental and theoretical studies by DFT calculations, one of the favorable reaction paths is depicted in Figure 19. When benzyl alcohol interacts with Ti–OH of the TiO2, the alkoxide species (ISC) is formed on a Ti site (3). The ISC was photo-excited under visible light irradiation via LMCT of the ISC, which induces holes (h+) and electrons (e−). Subsequently, the electrons are transferred to O2 to form superoxide anions (the bonding distance between O–O becomes longer), which induces α-deprotonation of the benzyl alcohol (4-5TS). Such hydro-peroxide species would further induce the de-protonation from another benzyl alcohol to form benzaldehyde (7-8TS), resulting in regeneration of the surface terminal OH groups. The consecutive generation of the terminal OH groups would, thus, be one of the key factors for the photocatalytic reactions. Catalysts 2019, 9, x FOR PEER REVIEW 16 of 22
4.3.2. What makes the high selectivity for the photocatalytic reactions?
It was observed that benzyl alcohol is adsorbed on TiO2 more favorably than benzaldehyde in a mixture of benzyl alcohol and benzaldehyde under dark conditions. This result indicates that the interaction between benzaldehyde and TiO2 is fairly weak. According to DFT calculations [87,88], the interaction of benzyl alcohol with the TiO2 surface formed a hybridized orbital, while benzaldehyde did not form orbital mixing. Therefore, once benzaldehyde was produced by the oxidation of benzyl alcohol, benzaldehyde was immediately released into the bulk solution, and was not oxidized further to benzoic acid or CO2.
4.3.3. Reaction mechanisms behind the selective photocatalytic oxidation of benzyl alcohol The photocatalytic activities for the oxidation of benzyl alcohol or α, α-d2 benzyl alcohol were investigated. The kinetic isotope effect (KIE) [=kC-H/kCD] was estimated to be 3.9 at 295 K. This result suggests that the process for the α-deprotonation is the rate determining step (RDS) for the overall reaction. From the experimental and theoretical studies by DFT calculations, one of the favorable reaction paths is depicted in Figure 19. When benzyl alcohol interacts with Ti–OH of the TiO2, the alkoxide species (ISC) is formed on a Ti site (3). The ISC was photo-excited under visible light irradiation via LMCT of the ISC, which induces holes (h+) and electrons (e−). Subsequently, the electrons are transferred to O2 to form superoxide anions (the bonding distance between O–O becomes longer), which induces α-deprotonation of the benzyl alcohol (4-5TS). Such hydro-peroxide species would further induce the de-protonation from another benzyl alcohol to form benzaldehyde
(7-8TS),Catalysts 2019 resulting, 9, 201 in regeneration of the surface terminal OH groups. The consecutive generation17 of of 22 the terminal OH groups would, thus, be one of the key factors for the photocatalytic reactions.
300 4-5TS 1 7-8TS CH 4-5TS O H O HC Ph-CH2OH Visible light O H 200 H H O H O O H Ti O Ti H O H 7-8TS Ti Ti O 100 Ti O 6 4 -1 CH2 2-3TS H HO O O 0 H 1 Visible light Ti O 2 5 -100 3 6 7
Energy / kJ mol kJ / Energy 3 5 7 8 CH OH2 -200 H HC O HH CH H CH2 H O H O O O H O O O O H H H O H -300 Ti O Ti Ti O Ti Ti Ti O alkoxide Ti Ti O alkoxide -400
8 -500
Figure 19. Possible reaction path for the selective oxidation of benzyl alcohol in the presence of O2 on
Figurethe TiO 19.2 under Possible visible reaction light irradiationpath for the [ 87selective]. oxidation of benzyl alcohol in the presence of O2 on
the TiO2 under visible light irradiation [87]. 4.4. Photocatalytic Oxidation of Benzyl Amine into Imine 4.4. PhotocatalyticImines are importantoxidation of intermediatesbenzyl amine into for imine the synthesis of pharmaceuticals and agricultural chemicals.Catalysts 2019 Selective, 9, x FOR PEER photocatalytic REVIEW oxidation of benzyl amine into N-benzylidenebenzylamine17 takes of 22 Imines are important intermediates for the synthesis of pharmaceuticals and agricultural place in the presence of O on the TiO at room temperature (Scheme1)[ 89,90]. Several kinds of chemicals.benzylic amines Selective were photocatalytic examined,2 and oxidation they2 were of benzyl converted amine into into the N-benzylidenebenzylamine corresponding imines, yielding takes benzylic amines were examined, and they were converted into the corresponding imines, yielding circa placecirca 38–94%in the presence [89]. The of origin O2 on of the the TiO visible2 at roomlight retemperaturesponse is due (Scheme to formation 1) [89,90]. of amine Several oxide kinds (ISC) of 38–94% [89]. The origin of the visible light response is due to formation of amine oxide (ISC) through through the interaction of benzylic amine onto the surface of TiO2, and the ISC exhibits electronic the interaction of benzylic amine onto the surface of TiO2, and the ISC exhibits electronic transition transition from the localized N 2p orbitals of the amine oxide (ISC) to the C.B. of TiO2. The from the localized N 2p orbitals of the amine oxide (ISC) to the C.B. of TiO2. The photo-induced redox photo-induced redox catalysis produces benzaldehyde in the presence of O2. Subsequently, the catalysis produces benzaldehyde in the presence of O . Subsequently, the condensation reaction of condensation reaction of benzaldehyde with2 another benzyl amine forms benzaldehyde with another benzyl amine forms N-benzylidenebenzylamine under dark conditions. N-benzylidenebenzylamine under dark conditions.
TiO2, O2 N 2 NH2 visible-light
benzyl amine N-benzylidenebenzylamine
Scheme 1. Selective oxidation of benzyl amine into N-benzylidenebenzylamine on the TiO2 Scheme 1. Selective oxidation of benzyl amine into N-benzylidenebenzylamine on the TiO2 photocatalyst under visible light irradiation. photocatalyst under visible light irradiation. 5. Conclusions 5. Conclusions This review focused on some fundamental issues behind the visible-light-sensitive TiO2 photocatalysts,This review highlighting focused on the some bulk and/orfundamental surface issues electronic behind structures the visible-light-sensitive modified by doping withTiO2 nitrogenphotocatalysts, anions; highlighting plasmonic the Au bulk NPs, and/or and interfacial surface electronic surface structures complexes modified (ISC) and by theirdoping related with photocatalysts.nitrogen anions; Tailoring plasmonic the interface Au NPs, and and bulk interfacial properties, surface including complexes surface band(ISC) bending,and their sub-band related structure,photocatalysts. surface Tailoring state distribution, the interface and charge and separation,bulk properties, significantly including reflects surface on the band photocatalysis. bending, Wesub-band hope that structure, this review surface has provided state distribution, some useful and contributions charge separation, for the future significantly design and reflects development on the ofphotocatalysis. novel photocatalytic We hope systems that this employing review has TiO prov2 asided well some as non-TiOuseful contributions2 semiconductor for the materials future withdesign nanoscale and development levels. The of applicationsnovel photocatalytic of such photocatalyticsystems employing systems TiO could2 as notwell onlyas non-TiO convert2 semiconductor materials with nanoscale levels. The applications of such photocatalytic systems could not only convert unlimited solar energy into chemical energy, but also protect our environment, leading to sustainable green chemistry.
6. List of abbreviations
No. Abbreviation Full Name 1 NPs nanoparticles 2 ISC interfacial surface complex 3 VOCs volatile organic compounds 4 V.B. valence band 5 C.B. conduction band 6 XPS X-ray photoelectron spectroscopy 7 EPR electron paramagnetic resonance 8 UV-vis Ultraviolet-visible 8 LSPR localized surface plasmon resonance 9 PD photodeposition 10 CPH colloid photodeposition by hole scavenger 11 DP deposition precipitation 12 TEM transmittance electron microscope 13 JSC short-circuit photocurrent 14 IPCE incident photo to current efficiency 15 DFT density functional theory 16 MLCT metal to ligand charge transfer 17 FT-IR Fourier transformed-infrared 18 KIE kinetic isotope effect 19 LMCT ligand to metal charge transfer
Funding: This research received no external funding
Catalysts 2019, 9, 201 18 of 22 unlimited solar energy into chemical energy, but also protect our environment, leading to sustainable green chemistry.
Funding: This research received no external funding. Conflicts of Interest: The authors declare no conflict of interest.
Abbreviations
NPs nanoparticles ISC interfacial surface complex VOCs volatile organic compounds V.B. valence band C.B. conduction band XPS X-ray photoelectron spectroscopy EPR electron paramagnetic resonance UV-vis Ultraviolet-visible LSPR localized surface plasmon resonance PD photodeposition CPH colloid photodeposition by hole scavenger DP deposition precipitation TEM transmittance electron microscope JSC short-circuit photocurrent IPCE incident photo to current efficiency DFT density functional theory MLCT metal to ligand charge transfer FT-IR Fourier transformed-infrared KIE kinetic isotope effect LMCT ligand to metal charge transfer
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