Yangyang Yu,Kejing Wu,Shiyu Lu,Kui Ma,Shan Zhong,Hegui Zhang,Yingming Zhu,Jing Guo,Hairong Yue,,Changjun Liu,,Siyang Tang,Bin Liang,,*

1Low-Carbon Technology and Chemical Reaction Engineering Laboratory,School of Chemical Engineering,Sichuan University,Chengdu 610065,China

2Institute of New Energy and Low-Carbon Technology,Sichuan University,Chengdu 610207,China

3Institute for Clean Energy&Advanced Materials,Faculty of Materials and Energy,Southwest University,Chongqing 400715,China

4Shanxi Province Key Laboratory of Higee-Oriented Chemical Engineering,North University of China,Taiyuan 030051,China

Keywords:TiO2pigments Pulsed chemical vapor deposition Ultrathin layer Weatherability Lightening power Photocatalytic suppression mechanism

ABSTRACT TiO2pigments are typically coated with inert layers to suppress the photocatalytic activity and improve the weatherability.However,the traditional inert layers have a lower refractive index compared to TiO2,and therefore reduce the lightening power of TiO2.In the present work,a uniform,amorphous,2.9-nm-thick TiO2protective layer was deposited onto the surface of anatase TiO2pigments according to pulsed chemical vapor deposition at room temperature,with TiCl4as titanium precursor.Amorphous TiO2coating layers exhibited poor photocatalytic activity,leading to a boosted weatherability.Similarly,this coating method is also effective for TiO2coating with amorphous SiO2and SnO2layers.However,the lightening power of amorphous TiO2layer is higher than those of amorphous SiO2and SnO2layers.According to the measurements of photoluminescence lifetime,surface photocurrent density,charge-transfer resistance,and electron spin resonance spectroscopy,it is revealed that the amorphous layer can prevent the migration of photogenerated electrons and holes onto the surface,decreasing the densities of surface electron and hole,and thereby suppress the photocatalytic activity.

1.Introduction

Powdered TiO2is the mostly used white pigment in papermaking,synthetic fibers,paints,cosmetics,and plastic industries[1-3]due to its excellent physicochemical properties.However,the high photocatalytic activity often leads to the degradation of surrounding organics and/or polymer molecules when exposed to ultraviolet (UV)radiation,which weakens the weatherability of paints[4].Therefore,suppression of photocatalytic activity is required for practical application of TiO2pigments.Commercially,TiO2pigments are typically coated with inert materials(Table S1),such as Al2O3[5,6],SiO2[4,7],ZrO2[8,9],NiO,CoO,CeO2[10,11],aluminum alkoxide [12],and SiO2-Al2O3double layers[13,14],using wet chemical deposition techniques.The wet deposition is a process where the inert grains precipitate or crystallize onto the surface of TiO2nanoparticles(NPs).However it is difficult to precisely control the uniformity and thickness of coating layers in the size range of sub-nanometer[15].The formation of coating layers severely depends on the experimental conditions,including the deposition rate,pH,precursor concentration,and temperature [16].Additionally,the wet chemical deposition usually requires subsequent operations,such as washing,separation and drying[16],making the process more complicated and meanwhile producing more waste discharges.

To overcome these drawbacks,the methods chemical vapor deposition(CVD)[17,18],atomic layer deposition(ALD)[19-21],and molecular layer deposition(MLD)[12]have been proposed to coat SiO2,Al2O3,or aluminum alkoxide layers onto TiO2NPs.During the CVD process,the inert NPs deposit onto the surfaces by the reaction of two gaseous chemicals,which presents the same shortcomings encountered during the wet processes[15].The ALD technology can be used to grow uniform films with accurate thickness in the sub-nanometer size range[15].However,the growth-per-cycle (GPC)of ALD is approximately 0.1-0.2 nm at 180°C.Such low GPC is not desirable in practical situations [20,21].The MLD technology can achieve GPCs varying from 0.5 nm at 100 °C to 0.35 nm at 160 °C,but the aluminum alkoxide films are not stable in the water[12].The experimental steps of pulsed CVD are similar to that of ALD,however,the unreacted precursor is not completely removed before the vaporous water is introduced into the reactor,and it is an ALD process with a minor partial CVD process[22].Pulsed CVD technology is not able to achieve complete selflimiting,but it can produce a uniform and dense coating layer with high GPC.

In our previous work,room temperature pulsed CVD was used to grow amorphous SiO2or SnO2layers onto TiO2NPs,according to the reaction of SiCl4or SnCl4with the hydroxide groups on particle surface,coating the inert components uniformly in a controllable way[16,23].Thus,the photocatalytic activity of TiO2NPs is sufficiently suppressed using one-cycle-coating.However,the amorphous SnO2and SiO2films decrease the lightening power of anatase TiO2due to their lower refractive indexes(2.0 and 1.5,respectively)compared to the anatase TiO2(2.6).The refractive index of amorphous TiO2ranges from 2.2 to 2.3[24],greater than those of SnO2and SiO2,therefore the TiO2pigments coated with amorphous TiO2could present higher lightening power.

Furthermore,the mechanism of photocatalytic suppression is important,but still controversial.Hughes et al.[25]studied the suppression effect of Al2O3layers on the photocatalytic activity of TiO2NPs,and proposed that Al2O3coating layers acted as electron acceptors to restrain the formation of hydroxyl or peroxide free radicals.Wiseman[25]argued that there are a large number of surface hydroxyl groups in Al2O3layers,and proposed that compact,coherent SiO2or Al2O3coating layers reduce the photocatalytic activity of TiO2NPs because of their lower porosity and surface areas.Durrant [26]suggested that SiO2,Al2O3,and ZrO2coating layers acted as physical barriers to restrain the photogenerated electrons and holes from migrating from the bulk of TiO2NPs to the surface due to their more positive valence bands and more negative conduction bands compared to those of TiO2.However,this hypothesis could not explain the suppression mechanism of CoO[10]and SnO2[23]coating layers.Recently,we reported the suppression effect of an amorphous SnO2coating layer and proposed that photocatalytic suppression was due to its high charge-transfer resistance[23].Amorphous SnO2coating layers have low electron mobility,and the photogenerated electrons and holes in TiO2recombine in amorphous SnO2shells during their migration from the bulk of TiO2to the surface.Therefore,the oxidation-reduction reactions induced by electrons and holes on the surface are prevented,and hence the photocatalytic activity of TiO2is suppressed.This suggests that the amorphous layer can resist the migration of photogenerated electrons and decrease the photocatalytic activity of TiO2NPs.To further validate this mechanism and simultaneously obtain the TiO2pigments with low photocatalytic activity and high lightening power,the layer of amorphous TiO2with high refractive index was tested in this work.

In this work,TiCl4was used to form an amorphous TiO2coating layer by room-temperature pulsed CVD since it is volatile and can react with hydroxyl groups at room temperature[27].The mechanism of the photocatalytic activity suppression was systematically investigated using time-resolved photoluminescence(PL),photocurrent measurements,electrochemical impedance spectroscopy(EIS),and electron spin resonance (ESR)spectroscopy.The photocatalytic activities of uncoated TiO2,amorphous TiO2-coated TiO2NPs (TiO2/TiO2core/shell NPs(TiO2/TiO2CSNs)),amorphous SnO2-coated TiO2NPs (TiO2/SnO2CSNs)and amorphous SiO2-coated TiO2NPs (TiO2/SiO2CSNs)were compared for a better understanding of the photocatalytic suppression mechanism.

2.Experimental

2.1.Materials

The TiCl4(99.9%metals basis)and tetrabutyl titanate(analytical reagent(AR))were provided by Aladdin Chemistry Co.(Shanghai,China).The Rhodamine B(RhB)(AR)was provided by Cologne Chemicals Ltd.(Chengdu China),and the anatase TiO2powder of 100-300 nm in diameter was provided by Taihai Technology Co.,Ltd.(Panzhihua,China).

2.2.Preparation of TiO2/TiO2CSNs

The reaction was conducted in a vessel approximately 3 L in volume(d=0.16 m and L=0.15 m)(Fig.1a).The TiO2powder was preheated at 120°C for 2 h,and then spread on a porous tray to form an approximately 2-mm-thick layer,allowing the gaseous TiCl4to diffuse well inside the layer of powder during the deposition time of 60 min [28].Valve V1 was connected to a 50 mL glass bottle containing liquid TiCl4,and valve V3 was linked to a mechanical pump.The sealed reactor was vacuumed to 50 mbar(valves V1 and V2 closed,valve V3 open)to produce a pressure difference between the glass bottle and the reactor.Subsequently,close valve V3 and open valve V1,the TiCl4was vaporized and fed into the reactor,where it reacted with the hydroxyl groups on the surface of TiO2NPs to form a superficial coating layer(Fig.1b-d and Section S1.1 in the Supporting Information).After 60 min,a washing bottle containing sodium hydroxide solution was added between valve V3 and the vacuum pump,and the by-products(i.e.,HCl)and unreacted precursor were removed and washed by opening V3.Finally,open valve V2 and introduce moist air(2.8 wt%water)into the reactor,and the reaction of water with the superficial Ti-Cl formed Ti-OH groups on the surface(Fig.1e and Section S1.1 in the Supporting Information).As products,part of the TiO2/TiO2CSNs were dried at 105°C,referred to as uncalcined TiO2/TiO2CSNs,while other TiO2/TiO2CSNs were calcined at 500°C for 2 h,referred to as calcined TiO2/TiO2CSNs.The thickness of the amorphous TiO2layer was increased by repeating this process.In addition,TiO2/SnO2and TiO2/SiO2CSNs were also prepared as described in our previous paper[23].

2.3.Preparation of amorphous TiO2

The amorphous TiO2sample was prepared in a 250 mL three-necked flask by hydrolyzing tetrabutyl titanate.5 mL of tetrabutyl titanate was dropwise added into 50 mL of deionized water under vigorously agitation.The produced white flocculent precipitate was aged for 4 h to ensure complete hydrolysis.Finally,the suspension was centrifuged and the cake was washed five times using deionized water and ethanol,respectively.The obtained product was dried at 105°C,and the amorphous TiO2sample was denoted as Am-TiO2in the present work.

Fig.1.(a)Schematic diagram of room temperature pulsed CVD reactor,(b)-(e)growth of the amorphous TiO2layers.

2.4.Characterization

The surface morphologies of the samples were characterized using a transmission electron microscope(TEM)(JEM 2100F),and the crystalline phases were characterized using a high-resolution transmission electron microscope(HRTEM)(JEM 2100F).The surface compositions of all NPs were detected using X-ray photoelectron spectroscopy(XPS)(Thermo Scientific ESCALAB 250Xi),and calibration was performed with respect to the C 1s peak at 284.8 eV.The PL lifetime of NPs was tested using a Hitachi F-4700 time-resolved PL spectrophotometer,and the photoelectric properties of NPs were characterized using photocurrent density spectroscopy,electrochemical impedance spectroscopy (EIS)(CIMPS-2)and electron spin resonance spectroscopy(ESR)(JESFA200).The lightening power of NPs was measured using a Datacolor 800 V colorimeter.

2.5.Photocatalytic activity measurements

The photocatalytic activities of the samples were measured by degrading RhB under UV irradiation.,150 mL RhB solution(6 mg L?1)was photodegraded using 30 mg powder sample as catalyst under the irradiation of a xenon lamp(300 W,UV).The reacted slurry was separated by centrifugation,and the final concentration of RhB in the solution was measured using UV-visible spectrophotometry.The physical adsorption of the RhB over the catalyst was detected by performing tests in the dark,and the adsorption equilibrium data was collected before the UV irradiation.

3.Results and Discussion

3.1.Morphology and composition of coating layer

The surface morphologies of the NPs were observed using TEM.Fig.2a is the image of the uncoated TiO2NPs.The coating layers are observed on the surface of TiO2/TiO2CSNs,and the thickness obviously increases with the number of coating cycles(Fig.2b-e).The thicknesses of the films are(1.3±0.8),(2.1±0.8),(2.9±0.9)and(3.6±0.9)nm for the samples that underwent one,two,three,and four coating cycles respectively(Table S2).As the number of deposition cycles increases,the coating becomes more and more uniform (Section S1.2 in the Supporting Information),as expected due to the increase in photocatalytic suppression with increasing number of coating layers.By comparison,the coating layers of the TiO2/SnO2and TiO2/SiO2CSNs are(2.4±0.8)and(7.8±1.8)nm thick(Fig.2f and g,and Table S2)after three cycles of coating.The GPC order of coating layers with different reagents is as follows:SiCl4(2.6 nm)＞TiCl4(0.9 nm)＞SnCl4(0.8 nm).

In our previous work[23],the SiO2and SnO2coating layers of uncalcined samples were considered as amorphous.The crystal form of TiO2coating layer was confirmed by the HRTEM results in this work,as shown in Fig.2h and i.It can be clearly observed that the uncalcined TiO2/TiO2samples do not show any lattice fringes inside their coating layers,and the morphologies of their coating layers are more similar to that of the Am-TiO2powder (Fig.2i).By contrast,lattice fringes with lattice spacing of 0.35 nm,assigned to the anatase(101)lattice,are clearly observed in the core areas of the samples[29-31],suggesting that the anatase TiO2core is coated with an amorphous TiO2layer(Fig.2h).

Fig.2.TEM images of(a)uncoated TiO2;(b)-(e)uncalcined TiO2/TiO2CSNs prepared after one,two,three,and four coating cycles,respectively;(f)uncalcined TiO2/SnO2;and(g)TiO2/SiO2CSNs prepared after three deposition cycles.HRTEM images of(h)TiO2/TiO2CSNs prepared after four deposition cycles and(i)Am-TiO2.

The surface composition and chemical states of the coating layers were detected using XPS analysis (Fig.3).The O 1s spectrum of uncoated TiO2consists of two peaks with binding energies(BEs)centered at 529.6 and 530.7 eV(Fig.3a),which are attributed to the Ti--O bonds and surface hydroxyl groups(--OH)of TiO2,respectively[16,32].For Am-TiO2,BEs of O 1s spectrum shift to higher value(Fig.3a),indicating a larger electron resistance of Am-TiO2than that of anatase TiO2(-Section S1.3 in the Supporting Information)[33].The O 1s spectrum of the uncalcined TiO2/TiO2CSNs is similar to that of Am-TiO2,confirming the existence of the amorphous TiO2coating layer of uncalcined TiO2/TiO2CSNs.The O 1s spectrum of the uncalcined TiO2/SnO2CSNs presents three peaks with BEs of 530.3,530.7,and 532.2 eV (Fig.3a),which are attributed to Ti--O,Sn--O bonds[34,35],and the surface-OH groups of SnO2,respectively [36].The BE of Ti--O bonds shifts from 529.6 to 530.3 eV(Fig.3a),which can be resulted from the resistance of amorphous SnO2thin film or the greater electronegativity of Sn within the interface bonds of Ti--O--Sn (Section S1.3 in the Supporting Information)[37].For the uncalcined TiO2/SiO2CSNs,the peaks at 530.3,532.6,and 533.7 eV (Fig.3a)are attributed to Ti--O,Si--O bonds [38],and the surface--OH groups of SiO2,respectively[39].The BE of Ti--O bonds shifts from 529.6 to 530.3 eV because amorphous SiO2has a higher resistance than anatase TiO2,or that Si with greater electronegativity bonds to the surface of TiO2via Ti--O--Si bond(Section S1.3 in the Supporting Information)[4].

The Ti 2p spectrum of uncoated TiO2consists of two peaks with BEs of 464.1 and 458.4 eV(Fig.3b),which are attributable to Ti4+2p1/2and Ti4+2p3/2,respectively[40-43].Similar with the O 1s spectrum,these BEs shift to 464.6 and 458.9 eV in the spectrum of the uncalcined TiO2/TiO2CSNs,to 464.9 and 459.1 eV in the spectrum of the uncalcined TiO2/SnO2CSNs,and to 464.8 and 459.0 eV in the spectrum of the uncalcined TiO2/SiO2CSNs(Fig.3b).The amorphous TiO2coating layer makes the BEs of Ti 2p of anatase TiO2increase,suggesting that the resistance of the amorphous TiO2layer is larger than that of anatase TiO2.The BEs of Ti 2p of the TiO2/SnO2and TiO2/SiO2CSNs are higher than those of the TiO2/TiO2CSNs(Table S3),suggesting that the resistances of the amorphous SnO2and SiO2films are larger than that of the amorphous TiO2film,or that of Sn or Si,which exhibits greater electronegativity than Ti,connects to the surface of TiO2NPs via Ti--O--Sn or Ti--O--Si bonds and further increases the BEs of anatase TiO2.The Sn 3d spectrum of the uncalcined TiO2/SnO2CSNs consists of two peaks with BEs of 495.8 and 487.4 eV(Fig.3c),which are attributed to Sn4+3d3/2and Sn4+3d5/2[44],and the Si 2p spectrum of the uncalcined TiO2/SiO2CSNs consists of two peaks with BEs of 103.7 and 102.3 eV(Fig.3d),which are attributed to Si 2p of SiO2and SiO1.19,respectively[45,46].

The BEs of the uncalcined TiO2/TiO2CSNs obtained after different coating cycles are compared and the results are shown in Table S3.The BEs of Ti 2p and O 1s increase with increasing number of coating cycles,and become similar to that of Am-TiO2after four coating cycles,indicating that the thicknesses,uniformity,and compactness of the coating layers increase as the number of coating cycles increases.Moreover,these results suggest that after four coating cycles,a uniform and compact amorphous TiO2coating layer is obtained,and the thickness of the film is similar to the penetration depth of XPS(Section S1.3 in the Supporting Information).These results are consistent with the data obtained from TEM tests(Fig.2b-e and Table S2).

3.2.Suppression effect on the photocatalytic activity

Fig.3.XPS spectra of(a)O 1s,(b)Ti 2p,(c)Sn 3d,and(d)Si 2p for uncoated TiO2,uncalcined CSNs,and Am-TiO2.All the coated samples were prepared after three coating cycles.

The photocatalytic activities of NPs were measured during the degradation of RhB.As shown in Fig.4a,in the absence of NPs,the concentration of RhB does not change in the dark,however,it slowly decreases under UV irradiation because of the self-degradation of RhB.In the presence of anatase TiO2NPs,the concentration of RhB significantly decreases after 10 min under vigorous stirring in the dark due to the adsorption of RhB by TiO2.The adsorption stopped after 10 min.When the slurry is under the irradiation of UV light,the concentration of RhB decreases rapidly,and approximately 90% of the RhB degrades within 15 min.However,when anatase TiO2powder coated with amorphous TiO2of one coating cycle is used in the photocatalytic reaction,the degradation rate of RhB is significantly slower compared to the reaction using uncoated TiO2powders,yet it is significantly faster compared to the reaction using Am-TiO2powder or that of the self-degradation of RhB,indicating that the coating is thin or the TiO2NPs are incompletely covered.The degradation rates of RhB in the presence of uncalcined TiO2/TiO2CSNs that underwent one to four coating cycles keep decreasing as the coating cycles increase,and finally become similar to the degradation rate of the reaction using Am-TiO2powder and the selfdegradation rate of RhB.

The reaction rate constant is estimated according to an apparent first-order kinetic equation[47]:where kappis the apparent first-order rate constant,t is the time of UV irradiation,C0is the concentration of RhB before irradiation,and C is the concentration of RhB when UV irradiation time is t.

The kappvalues are obtained from the slope of the linear fit(Fig.4b)and listed in Table S4.When using TiO2/TiO2CSNs that underwent four coating cycles,kappis similar to that of the reaction using Am-TiO2powder and the self-degradation reaction of RhB.These results suggest that the amorphous TiO2layer can sufficiently suppress the photocatalytic activity of anatase TiO2.

The photocatalytic activities of the uncalcined CSNs are compared with the calcined CSNs,as shown in Fig.4c and d,where all the samples underwent three coating cycles.The kappvalues for the uncalcined and calcined coated TiO2CSNs are listed in Table S5.The uncalcined TiO2/TiO2,TiO2/SnO2,and TiO2/SiO2CSNs exhibit very similar kappvalues,which are significantly smaller than that of uncoated TiO2(Table S5),indicating that all the amorphous TiO2,SnO2,and SiO2coating layers present excellent suppression effects on the photocatalytic activity of TiO2.

When using CSNs that underwent three cycles of coating,the reaction rates are 0.0072,0.0048,and 0.0085 for the TiO2,SiO2,and SnO2coatings,respectively(Table S5),while the GPCs of the coating layers for different reagents decrease as follows:SiCl4(2.6 nm)＞TiCl4(0.9 nm)＞SnCl4(0.8 nm)(Fig.2),indicating that the ability to suppress the photocatalytic activity of the coating layers is not only related to the thickness of the coating layers but also related to the types of coated materials.

The kappvalue of the calcined TiO2/TiO2CSNs is similar to that of uncoated anatase TiO2because calcination converts the amorphous TiO2coating layer into anatase TiO2,making the TiO2coating layer lose its suppressive effects(Table S5).The calcined TiO2/SnO2CSNs show significantly larger kappvalues than the uncalcined ones,but slightly lower than the uncoated TiO2NPs(Table S5).Calcination process converts the amorphous SnO2coating layers into rutile SnO2,and the band gap of rutile SnO2(3.56 eV)is larger than that of TiO2(3.18 eV),reducing the absorption of UV light onto TiO2[48].Furthermore,the compact rutile SnO2coating on the surface of anatase TiO2also results in the photocatalytic activity decrease of anatase TiO2(Section S1.4 in the Supporting Information).Moreover,kappof the calcined TiO2/SiO2CSNs is very similar to that of the uncalcined ones because of the high band-gap of SiO2(8-8.9 eV);in fact,SiO2acts as an insulator(Table S5)[26].

3.3.Photocatalytic suppression mechanism

Fig.4.(a)Photodegradation of RhB using uncalcined TiO2/TiO2CSNs that underwent different coating cycles,and(b)corresponding kinetic constants;(c)uncalcined and calcined CSNs that underwent three coating cycles and(d)corresponding kinetic constants.

Fig.5.Time-resolved PL of(a)uncalcined TiO2/TiO2CSNs that underwent different coating cycles;and(b)uncalcined and calcined CSNs that underwent three coating cycles.The curves are obtained by fitting the original curves with an exponential function of two parameters.

Under photoirradiation,excited electrons generated from the TiO2semiconductors enter conductive bands,resulting in positive holes formed in valence bands.The majority of the photogenerated electrons recombine with holes inside the crystalline bulk,and as a result,only a few of the electrons and holes migrate and arrive at the surface where the redox reactions occur[49].The recombination of photogenerated electrons and holes produces PL[50],and therefore the PL lifetime depends on the recombination rate.When the rate is slow,PL lifetime is long,and therefore the density of the photogenerated electrons is high on the surface,leading to a high photocatalytic ability[51].

The PL lifetime was measured using time-resolved PL (Fig.5).To compare the PL lifetimes more directly,the PL profiles are fitted using a two-parameter-exponential function.The estimated amplitude(Ai,i=1,2)values and time constants(τi,i=1,2)are shown in Tables S6 and S7.The average PL time,τave,is obtained according to the following equation[52]:

where τ1represents the PL lifetime generated by the self-trapped excitons at the intrinsic TiO6octahedral sites in the TiO2lattice,and τ2represents the PL lifetime generated by the self-trapped excitons at surface defect sites[50].

As depicted in Fig.5a and Table S6,the uncalcined TiO2/TiO2CSNs exhibit obvious drops in PL lifetimes from 3.63 to 2.39,2.20,1.95,and 1.92 ns as the coating cycle increases.The PL lifetimes are similar for the samples that underwent three and four coating cycles.From Fig.5b and Table S7,it is obvious that the PL lifetimes of the uncalcined TiO2/TiO2,TiO2/SnO2,and TiO2/SiO2CSNs are much shorter than that of uncoated TiO2.However,the calcined TiO2/TiO2and TiO2/SnO2CSNs possess similar PL lifetimes to the uncoated TiO2,which is resulted from transformation of amorphous TiO2and SnO2coating layers to anatase TiO2and rutile SnO2,respectively.On the contrary,the calcined TiO2/SiO2CSNs exhibit a much shorter PL lifetime than uncoated TiO2,because the calcined SiO2acts as an insulator with a band-gap in the 8-8.9 eV range [26].The results confirmed that small PL lifetime is highly related to low photocatalytic activity(Fig.4).

Fig.6.(a)Photocurrent densities of uncalcined TiO2/TiO2CSNs that underwent different coating cycles and(b)comparison of uncalcined and calcined samples.(c)Nyquist plots of TiO2,uncalcined and calcined CSNs that underwent three coating cycles and(d)the corresponding equivalent circuit.

Photocurrent density is conducted under the intermittent UV irradiation at zero applied bias voltage(Fig.6a and b)to characterize the surface density of photogenerated electrons.As shown in Fig.6a,the photocurrent density of uncoated TiO2is 1.10 μA·cm?2under UV light and becomes zero in the dark.The photocurrent density of TiO2/TiO2CSNs is much lower than that of uncoated TiO2and decreases with coating thickness.The photocurrent density of the TiO2/TiO2CSNs that underwent three coating cycles is similar to that of Am-TiO2,indicating the complete coating after three cycles.The low photocurrent density is highly related to slow transfer or poor generation of electrons to the surface.Thus,the amorphous coating layers prevent the photogenerated electrons from migrating to the surface,as indicated similarly by PL lifetimes(Fig.5)and photocatalytic activity measurements(Fig.4).

The effect of calcination on photocurrent densities is shown in Fig.6b.The photocurrent densities of the calcined TiO2/TiO2and TiO2/SnO2CSNs are 1.08 and 1.19 μA·cm?2,respectively,which are much higher than those of the uncalcined TiO2/TiO2(0.06 μA·cm?2)and TiO2/SnO2CSNs (0.09 μA·cm?2).This suggests that the crystallized layer exhibits better conductivity for electrons than amorphous layer,which is confirmed by the fact that the photocurrent density of calcined TiO2/TiO2CSNs(1.08 μA·cm?2)becomes similar to the value for uncoated TiO2(1.10 μA·cm?2).However,the calcined TiO2/SnO2CSNs(1.19 μA·cm?2)show slightly higher photocurrent density than uncoated TiO2.This increase is attributed to the formation of heterojunction between rutile SnO2and anatase TiO2,which promotes the separation of electrons and holes.[53]Because of the low electron conductivity of SiO2,the photocurrent density of calcined TiO2/SiO2CSNs(0.11 μA·cm?2)is still much lower than uncoated TiO2.

Fig.7.(a)ESR spectra of samples containing 40 mmol?L-1 TEMPO before and after 10 min of UV irradiation,(b)primary integral and(c)quadratic integral spectra of samples containing 40 mmol?L-1 TEMPO after 10 min of UV irradiation.(d)ESR spectra of samples before and after 5 min of UV irradiation at 123 K,(e)primary integral,and(f)quadratic integral spectra of samples after 5 min of UV irradiation at 123 K.All samples underwent three coating cycles.

To further investigate the influence of the amorphous coating layers on the recombination of the photogenerated electrons and holes in the samples,EIS plots of the uncalcined and calcined CSNs are compared using Nyquist plots(Fig.6c and d).In the Nyquist plots(Fig.6c),the radii of the semicircles correspond to their charge-transfer resistance[54].An equivalent circuit(Fig.6d)is used to simulate the resistance measurements,where Rs,R1,C1,and ZWrepresent the resistance of the solution,charge-transfer resistance,capacitance,and Warburg impedance,respectively[55,56].The charge-transfer resistances of uncalcined TiO2/TiO2,TiO2/SnO2,and TiO2/SiO2CSNs are 854,1013 and 933 kΩ,respectively,which are much higher than that of uncoated TiO2(290 kΩ).The high resistances of the amorphous TiO2,SnO2,and SiO2layers prevent the migration of photogenerated electrons to the surface,resulting in suppressing photocatalytic activity of TiO2.

The amorphous TiO2layer of the TiO2/TiO2CSNs converts to anatase TiO2after calcination,and its charge-transfer resistance becomes 294 kΩ,which is very similar to that of uncoated TiO2(290 kΩ).The calcined TiO2coating layer loses its resistance to the migration of electrons,as well as its ability to suppress photocatalytic activity.The calcined TiO2/SnO2CSNs exhibit a charge-transfer resistance of 269 kΩ,slightly lower than that of uncoated TiO2(290 kΩ),indicating that heterojunction structure between rutile SnO2and anatase TiO2can reduce the charge-transfer resistance.In addition,the charge-transfer resistance of the calcined TiO2/SiO2CSNs is as high as 695 kΩ.These results show that high charge-transfer resistance is always accompanied with low photocatalytic activity (Fig.4c and d),short timeresolved PL(Fig.5b),and small photocurrent density(Fig.6b).Therefore,the amorphous coating layers of TiO2,SnO2,and SiO2suppress the photocatalytic activity of TiO2by preventing the photogenerated electrons and holes from migrating to the surface.

Electron spin resonance (ESR)spectroscopy combined with spin probes and spin traps is an effective tool to study the behavior of electron-hole pairs [57].In this study,2,2,6,6-tetramethylpiperidine-1-oxyl(TEMPO)is used as spin probe to monitor the generation and migration of photogenerated electrons [58].Meanwhile,the photogenerated holes migrate to the surface of TiO2to form oxygen vacancies[59],which can be detected by ESR.

As shown in Fig.7a,the ESR spectra of the samples containing 40 mmol?L-1TEMPO show three peaks with 1:1:1 intensities in the dark[58].When irradiated for 10 min,the signal intensity of TEMPO drops sharply for the TiO2sample,indicating that TEMPO captures photogenerated electrons to form ESR-silent TEMPOH.However,in the case for uncalcined TiO2/TiO2,TiO2/SnO2,or TiO2/SiO2CSNs,the TEMPO signal intensities under UV irradiation are similar with those obtained in the dark,indicating that the amorphous TiO2,SnO2,and SiO2layers sufficiently prevent the migration of photogenerated electrons to the surface.However,for the calcined TiO2/TiO2and TiO2/SnO2samples,sharp decreases in the TEMPO signals are observed under UV irradiation,suggesting the poor electron resistance of anatase TiO2and rutile SnO2layers.When the TiO2/SiO2CSNs are calcined,the TEMPO signal intensity under UV irradiation only slightly decreases compared with the signal obtained in the dark,because SiO2,as an insulator,could prevent the migration of excited electrons.To compare the effects of different coating layers on electron migration,the TEMPO signals of different samples after 10 min of UV irradiation are compared(Fig.S2a).The integration absorption spectra are obtained(Fig.7b),and their integral areas representing the amounts of TEMPO are shown in Fig.7c.The integral areas are 0.96,0.89,and 1.05 for the uncalcined TiO2/TiO2,TiO2/SnO2,and TiO2/SiO2CSNs,respectively,which are much larger than that of anatase TiO2(0.25).Reasonably,the areas become 0.33,0.38,and 0.91,respectively,after calcination,confirming the results of the photocurrent density experiments(Fig.6b)and the EIS plots(Fig.6c).

As shown in Fig.7d,all samples exhibit ESR signals at g=2.001,characterized as oxygen vacancies[60],before and after UV irradiation,respectively.As for TiO2,the ESR signal of oxygen vacancies after 5 min of UV irradiation is much higher than that in the dark,indicating that photogenerated holes migrate to the surface to produce oxygen vacancies.The ESR signals of oxygen vacancies for the uncalcined TiO2/TiO2,TiO2/SnO2,and TiO2/SiO2CSNs under UV irradiation are the same as those obtained in the dark,suggesting that the amorphous TiO2,SnO2,and SiO2layers sufficiently prevent the photogenerated holes from migrating to the surface.Obviously,the calcined TiO2/TiO2and TiO2/SnO2CSNs exhibit higher ESR signal of oxygen vacancies after UV irradiation than those in the dark.The ESR signal spectra for different samples after 5 min of UV irradiation are compared(Fig.S2d),and their primary integral and quadratic integral spectra are shown in Fig.7e and f.The integral areas of the absorption spectra of the uncalcined TiO2/TiO2and TiO2/SnO2CSNs are 0.58 and 0.61,and they become 1.26 and 1.31 after calcination,respectively.These results are consistent with the measurements of photogenerated electrons.

The migration and recombination process of photogenerated electrons and holes are illustrated in Fig.8,according to the results of time-resolved PL,photocurrent density,EIS,and ESR.Photoexcited TiO2semiconductors produce electrons and holes under UV light,which are transferred to the amorphous coating layer where recombination occurs.The large electrical resistance of the amorphous coating layer prevents the electrons and holes from moving to the surface and greatly reduces the surface density of electrons and holes,and therefore it suppresses the photocatalytic activity of TiO2.

Fig.8.Migration and recombination process of photogenerated electrons and holes.

3.4.Pigment properties

The additional coating layer on TiO2NPs suppresses their photocatalytic activity.Coating might also increase the weather duration of paintings.To measure the influence of coating on the color properties of TiO2pigment,lightening power measurements of the coated samples are performed,and the results are shown in Fig.9.The lightening power represents the tinting strength of a white pigment.The amorphous TiO2,SnO2,and SiO2coating layers decrease the lightening power of anatase TiO2due to their lower refractive indexes (in the 2.2-2.3 range,2.0,and 1.5,respectively)compared to anatase TiO2(2.6).However,the lightening power of the uncalcined TiO2/TiO2CSNs is higher than those of the uncalcined TiO2/SnO2and TiO2/SiO2CSNs due to the higher refractive index of amorphous TiO2(in the 2.2-2.3 range)compared to those of amorphous SnO2(2.0)and SiO2(1.5).These results suggest that the amorphous TiO2coating layer has better pigment properties than the SnO2and SiO2coating layers.

Fig.9.Lightening power of uncoated TiO2and uncalcined CSNs prepared using three coating cycles.

3.5.Industrialization prospect

The designed pulsed CVD reactor in our experiments can only handle a small amount of the samples (～2.5 g)due to its limitation on mass diffusion.However,this pulsed CVD process can be easily magnified with mechanical vibration fluidized bed reactors,impeller agitation fluidized bed reactors,and rotary reactors (Section S1.5 in the Supporting Information).Due to the limitation of mass diffusion,sufficient precursors in the gas are required to achieve the 1-2 mm TiO2accumulation,resulting in a low conversion of TiCl4(30.4%,as shown in Section S1.5 and Table S8 in the Supporting Information).However,in fluidized bed reactor,we can recycle the carrier gas to reuse the precursor,and increase the conversion.In the rotary reactor,since the different surfaces of the particles are constantly exposed to the precursor atmosphere,the precursors can be consumed maximally,leading a highest conversion of the precursors.In addition,this process can achieve good economic benefits,as shown in Section S1.5 of the Supporting Information.

4.Conclusions

In summary,we engineered a uniform,(2.9±0.9)nm thick amorphous TiO2protective layer onto the anatase TiO2pigments via room temperature pulsed CVD,using TiCl4as titanium precursors.Compared with other amorphous layers(e.g.,SnO2and SiO2),the TiO2layer presents higher lightening power.Moreover,the photocatalytic activity is substantially suppressed after coating process,which leads to a boosted weatherability of TiO2pigments.Intrinsically,the high electrical resistance of the TiO2layer inhibits the migration of electrons and holes to the surface,thereby suppressing the photocatalytic activity of the samples.Such passivation mechanism is also applicable for other amorphous layers.The present work demonstrates the feasibility and mechanism of the interface engineering for the weatherability enhancement,and simultaneously maintains the high lightening power of TiO2pigment.The proposed fundamental method also provides guidance for the design of tunable layers on the oxide surface.

Supplementary Material

Supporting Information (TiO2coating layer growth mechanism,Supplementary analysis of film thickness,Supplementary analysis of XPS,and charge transfer in calcined TiO2/SnO2CSNs;additional characterization results)to this article can be found online at https://doi.org/10.1016/j.cjche.2019.04.002.