Mo5+ Codoping on

Subscriber access provided by IDAHO STATE UNIV

C: Surfaces, Interfaces, Porous Materials, and Catalysis 3+

5+

Structural and Microstructural Effects of Mo /Mo Codoping on Properties and Photocatalytic Performance of Nanostructured TiO Thin Films 2

Zhiyuan Liu, Wen-Fan Chen, Xichao Zhang, Ji Zhang, Pramod Koshy, and Charles C. Sorrell J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 17 Apr 2019 Downloaded from http://pubs.acs.org on April 17, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The Journal of Physical Chemistry

Structural and Microstructural Effects of Mo3+/Mo5+ Codoping on Properties and Photocatalytic Performance of Nanostructured TiO2 Thin Films Zhiyuan Liu1, Wen-Fan Chen1*, Xichao Zhang1, Ji Zhang1, Pramod Koshy1, and Charles Christopher Sorrell1 1School

of Materials Science and Engineering, UNSW Sydney, Sydney, NSW 2052, Australia

* Corresponding author: [email protected] Abstract Mo3+/Mo5+ equally codoped TiO2 thin films (0.050-0.100 mol% individually) were fabricated by spin coating on polished fused silica glass substrates followed by annealing at 450°C for 2 h. The highly transparent (>80%) films consisted of poorly crystallized single-crystal anatase, where interstitial solid solubility decreases the lattice parameters, with a reversed trend at the highest codoping level from supersaturation and V•• O formation. The crystallinities correlate with the alternating trends of peak intensities and shifts in both the GAXRD and Raman data. XPS data reveal the absence of any valence changes, indicating that intervalence charge transfer is not relevant. Although there are four potential midgap states that may alter the Eg (Mo••••• , Mo•••• i i , ••• Moi , and V′′′′ Ti ), any effects from them on the performance would be mitigated by the annihilation of the charge-compensating V•• O . Since the Eg values correlate with the lattice parameters, then the principal driver of the Eg is the residual lattice compressive strain. However, the photocatalytic performance is principally a function of the crystallinity. 1

Introduction

Owing to the toxicity and carcinogenic nature of synthetic persistent organic pollutants (POPs), such as dyes and their biodegradation products, approaches to degrade these materials have attracted increasing attention.1 During recent decades, several methods, including adsorption,2 photocatalytic degradation, sonolysis, and radiolysis,3 have been used accordingly. The efficiency and rapidity of photocatalytic materials to form simpler molecules (typically CO2 and H2O) for air purification4 and wastewater treatment5 has focussed attention on semiconducting oxides, including ZnO, CeO2, WO3.6-10 Titanium dioxide (titania, TiO2) is considered to be the leading such candidate owing to its superior properties in terms of chemical stability, optical properties, nontoxicity, economy, and photocatalytic capacity.11 However, this material has a wide band gap (~3.2 eV), thus requiring near-UV light irradiation (~8% of the solar energy spectrum12) for photoactivation,13 leading to relatively poor photocatalytic performance under visible light, thereby limiting its applicability. Further, TiO2 tends to show relatively high electron-hole recombination rates, leading to reduction in the photocatalytic efficiency.14 To overcome these disadvantages, research has focused on doping to produce suitable midgap energy levels within the band gap for effective band gap reduction and to reduce the recombination time by promoting faster transfer of charge carriers15,16 to the conduction band. Both metallic and non-metallic dopants, such as La3+, Fe3+, Cu2+, Ag+, S2-, C4±, and N3-, have been investigated for their potential to improve the photocatalytic performance.17-25 Compared to other metallic dopants, molybdenum (Mo) is one of most attractive owing to its multiple valence states and the associated, potential, multiple, shallow-donor levels within the band gap.26 Table 1 summarizes work on Mo single doping and codoping of TiO2 from literature. Wang et al.27 reported that 2.0 mol% Mo5+doped TiO2 nanoparticles fabricated by sol-gel showed superior photocatalytic performance due to the enhancement of visible light absorption owing to lowering of the conduction band minimum of 1

ACS Paragon Plus Environment

The Journal of Physical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 21

anatase. Khan and Berk28 observed that 0.25 mol% Mo6+-doped anatase/brookite nanoparticles fabricated by sol-gel improved the photocatalytic performance due to the effects of band gap narrowing and decreased recombination rate of photogenerated electron-hole pairs. Chen et al.29 reported that 0.01 mol% Mo5+-doped anatase thin film fabricated by sol-gel exhibited maximal visible-light photocatalytic activity due to the effects of nucleation and recrystallization, semiconduction, dopant size, and minimized lattice distortion. Jiang et al.30 reported that 0.10 wt% Mo5+-doped anatase thin film composites with conjugated polyvinyl alcohol exhibited optimal photocatalytic performance owing to recrystallization of the anatase and/or the modification of the semiconducting properties induced by Mo-doping. Cui et al.31 reported that 0.05 mol% Mo3+doped or 0.05 mol% Mo5+-doped anatase thin films produced by sol-gel gave optimized photocatalytic performance owing to the synergistic effects of crystallinity, surface area, and band gap Table 1. Survey of experimental work on Mo-doped and Mo-codoped TiO2 photocatalysts Dopants

Morphology

Method

Doping Levels

Nanoparticulate

Single Doped Sol-Gel

≤3.00 mol%

Mo6+

Nanoparticulate

Sol-Gel

≤2.00 mol%

Mo5+ Mo5+ Mo3+ Mo5+

Thin Film Thin Film

Spin Coating Sol-Gel Dip Coating

Thin Film Nanoparticulate Nanoparticulate Thin Film

Mo5+

Mo3+/Fe3+ Mo6+/Al3+ Mo3+/W6+

Annealing Conditions

Ref. 27

≤1.00 mol% ≤0.20 mol%

450°C for 2 h 300°C for 1 h 500°C for 1 h 450°C for 2 h 180°C for 4 h

Spin Coating

≤1.00 mol%

450°C for 2 h

31

Codoped Sol-Gel Hydrothermal Sol-Gel Dip Coating

≤2.00 mol% ≤2.00 mol% ≤0.50 mol%

450°C for 2 h 150°C for 10 h 500°C for 3 h

32 33 34

28 29 30

In addition to single doping of Mo, other authors have reported codoped TiO2 with Mo and a second metal dopant. Guo et al.32 reported that 2 mol% Fe3+/2 mol% Mo3+ codoped anatase nanoparticles fabricated using sol-gel exhibited enhanced photodegradation of methylene blue solution owing to the enhancement of visible light absorption by the codoping. Khan et al.33 reported that 4 mol% Ag3+/4 mol% Mo6+-codoped anatase nanoparticles showed the best photocatalytic performance due to reduced crystallite size and associated increased surface area. Akbarzadeh et al.34 reported that 1 wt% W6+/1 wt% Mo3+ codoped anatase thin film exhibited superior photocatalytic performance owing to band gap narrowing. Although these data suggest that the valence state of Mo may have an impact on the photocatalytic performance, there appears to be only a single study comparing the effects of different Mo valence states, which showed that the initial and final valences of the individual dopants had a dominant effect on the performance.30 The present work aims to elucidate any synergistic effects of dopant valence during codoping by investigating the mineralogical, optical, topographical, and photocatalytic properties of anatase codoped thin films using Mo3+ and Mo5+. 2

Experimental

The detailed fabrication process of the TiO2 thin films is described elsewhere.35-41 Table 2 summarizes the materials, instrumentation, characterization methods, and parameters. 2


Page 3 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 2. Summary of experimental details Materials and Instrumentation Precursor Solutions  Titanium tetraisopropoxide (TTIP, Reagent Grade, 97 wt%, Sigma-Aldrich)  Isopropanol (Reagent Plus, ≥99 wt%, Sigma-Aldrich)  MoCl3 (Reagent Grade, 95 wt%, Sigma-Aldrich) MoCl5 (Reagent Grade, 95 wt%, Sigma-Aldrich) Substrate  Polished fused SiO2 (99.9% purity, Sunray Oil, Singapore, 20 x 10 x 1 mm3) Spin Coating  Spin coater (Laurell Technologies WS-65052) Annealing Muffle furnace Characterization Parameters  Glancing angle X-ray diffraction (GAXRD, PANalytical  45 kV, 40 mA Empyrean Thin-Film XRD)  Laser Raman microspectroscopy (Raman, Renishaw inVia  Green argon ion laser (514 nm, 25 Raman Microscope) mW, 50X, spot size 1.5 mm  X-ray photoelectron spectroscopy (XPS, Thermo  13 kV, 12 mA, 2-5 nm beam Scientific ESCALAB 250Xi X-ray Photoelectron penetration, spot size 500 mm Spectrometer Microprobe)  Atomic force microscopy (AFM, Bruker Dimension Icon  Tapping mode, scan size 1 µm x 1 Scanning Probe Microscope) µm  Ultraviolet-visible spectrophotometer (UV-Vis,  Dual-beam, 300-800 nm PerkinElmer Lambda 35 UV-Visible Spectrometer). Photocatalytic Performance Materials  Methylene blue (MB, M9140, dye content ≥82 wt%, Sigma-Aldrich) Instruments  UV lamp (3UV-38, 8 W, UVP)  UV-Vis spectrophotometer (UV-Vis, PerkinElmer Lambda 35 UV-Visible Spectrometer) Photocatalyst Preparation Precursor solutions were prepared using TTIP dissolved in isopropanol at 0.1 M titanium concentration (2.84 g of TTIP was diluted to 100 mL volume with isopropanol). Mo3+ and Mo5+ dopant levels were varied in the range 0.050-0.100 mol% (individual metal basis) by adding MoCl3 and MoCl5 to the solution (total codoping level 0.100-0.200 mol%). Precursor solutions were mixed by magnetic stirring for 10 min in Pyrex beakers without heating. Spin coating was done by depositing ~0.2 mL of precursor solution onto a polished fused silica substrate spinning at 2000 rpm in nitrogen over a period of ~10 s. Films were dried by spinning for an additional 15 s and the overall process was repeated six more times (~1.4 mL) Annealing in air was done in a muffle furnace at 450°C for 2 h; the heating rates were 0.5°C/min from room temperature to 200°C and 1°C/min from 200°C to 450°C, followed by natural cooling. Photocatalytic Performance MB solutions were prepared by dissolving MB in deionized water at 10-5 M concentration. Solutions were magnetically stirred in a Pyrex beaker for 1 h without heating. Samples were placed 3



Page 4 of 21

in fresh MB solution in a dark container for saturation for ~12 h prior to testing. Samples were placed in separate small beakers, filled with ~10 mL of MB solutions, and exposed to UV radiation (365 nm) for 24 h. The vertical lamp-liquid and liquid-sample distances were ~6 cm and ~4 cm, respectively. After irradiation, solutions were analyzed by UV-Vis spectrophotometry (664 nm) to determine the extent of degradation. 3

Results and Discussion

GAXRD spectra of codoped TiO2 thin films as a function of total doping level are shown in Figure 1. The patterns show that anatase is the only phase present in all samples. The following quantitative data must be considered in light of incomplete recrystallization, which is indicated by the GAXRD intensities and peak widths. No rutile, brookite, or secondary phase was detected since these phases were absent or present at levels below the level of detection of the instrument.42

Figure 1. GAXRD spectra of undoped and codoped TiO2 thin films with different codoping levels annealed at 450°C for 2 h: (a) full spectra, (b) enlargements of major peak The intensity of the main (101) anatase peak in Figure 1(b) did not show any significant change with different doping levels. The asymmetric peak (blue) for the 0.100 mol% codoped TiO2 thin film appears to exhibit a small shift to lower angle, indicating lattice expansion. This shift could have resulted from lattice distortion and associated destabilization37 or the formation of oxygen vacancies (V•• O ) for charge compensation. Table 3 summarizes these data relevant to a range of structural features. These include variations in crystallite sizes (D), lattice strains (ε), dislocation densities (δ), stacking fault probabilities (α), and microstresses (σstress) of undoped and codoped TiO2 thin films as a function of codoping level after annealing at 450°C for 2 h. The relevant equations that are given in Appendix A43,44 are estimated based on broadening of (101) peak and these data are correlated graphically in Figure 2. Table 3. Variation in crystallite size (D), lattice strain (ε), dislocation density (δ), stacking fault probability (α), microstress (σstress), and number of crystallites per unit surface area (N) of undoped and codoped TiO2 thin films with different codopant levels after annealing at 450°C for 2 h Parameter

D

ε

δx

1015

α

σstress

Nx

1016

4


Lattice Parameters a c c/a

Volume axaxc

Page 5 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


mol% Undoped 0.050 0.075 0.100

nm ~10 ~12 ~11 ~14

-- lines/m2 1.582 10.000 1.322 6.944 1.439 8.264 1.139 5.102

-1.602 1.337 1.456 1.153

GPa 223.731 186.840 203.393 161.034

-30.000 17.361 22.539 10.933

5


nm 0.3888 0.3861 0.3849 0.3862

nm 0.9713 0.9642 0.9628 0.9645

-2.498 2.497 2.501 2.497

nm3 0.1468 0.1437 0.1426 0.1439


Figure 2

Page 6 of 21

Scaled dependence of structural changes for TiO2 thin films with different codoping levels after annealing at 450°C for 2 h based on GAXRD data: (a) structural features, (b) crystallographic data 6


Page 7 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The data in Figure 2(a) demonstrate that all of the structural features are inversely proportional to the crystallite size. It is clear that there should be an inverse correlation between the crystallite size (D) and the number of crystallites per unit surface area (N), so the data for 1 – D are given. The residual stress (σstress), stacking fault probability (α), dislocation density (δ), and lattice strain (ε) represent structural parameters that depend on both the crystallite size and the doping level. Since the doping addition levels are linear but the structural data are not, then the correlations between the inverse crystallite size and structural features demonstrates that the crystallite size is the dominant factor in the structural-mechanical response of the structure to the doping. Similar inverse correlations between the crystallite size and structural features have been observed by others.45,46 These correlations are to be expected since they reflect increasing structural stability resulting from the decreasing surface area/bulk ratio, where the surface area consists of highly defective grain boundaries. Further, increasing stability with grain growth reflects increasing stability associated with the standard stages of densification, these being nucleation, grain growth, and coalescence.47 In contrast, as shown in Figure 2(b), there is a direct correlation between the crystallite size and the c/a axial ratio. This is significant because an increase in the c/a axial ratio has been suggested to reflect lattice expansion.48-50 The consistency between the trends in Figure 1(a) against the c/a axial ratio indicates that the c/a axial ratio represents a suitable indicator of the structural response to the dopant dissolution. In contrast, although it has been reported that the lattice volume may correlate to the crystallite size,51,52 this does not appear to be the case, which is shown by the non-correspondence between the trends in the a axial length, the c axial length, and the unit cell volume in Figure 1(b) against the inverse crystallite size and c/a axial ratio. In addition to the determination of the lattice strain from the Scherrer equation53 using only the (101) GAXRD peak, the effects of internal stress from dopants and external stress from mechanical effects also can be considered through Williamson-Hall plots54 using all of the GAXRD peaks. The relevant calculations using the equations that are given in Appendix B are estimated based on the locations of the peaks. However, the nature of GAXRD and the low crystallinities of the thin films are such that the calculations do not provide definitive data, as revealed by the very low coefficients of determination r2 for 0.180 for the undoped TiO2 thin film in Figure 3(a) and 0.273, for all of the codoped TiO2 thin films in Figure 3(b). Despite these limitations, the data suggest positive slopes, indicating that the thin films exhibit lattice expansion.55,56 Although V•• O are 37 considered to be an intrinsic feature of undoped TiO2, hence TiO2-x, the assumption of their effect on the lattice volume is contradictory, with both expansion and contraction57-60 being reported. The reported single-crystal lattice parameters for anatase are a = 0.3799 nm and c = 0.9539 nm (c/a axial ratio = 2.511)61 while those of the undoped TiO2 thin film from Table 3 are a = 0.3888 nm and c = 0.9713 nm (c/a axial ratio = 2.498). In summary, the data for lattice strain from the Scherrer equation, Williamson-Hall plots, and lattice parameters support that view that all of the TiO2 thin films have expanded. These data support the conclusion that V•• O causes lattice expansion. In contrast, the reduction in c/a axial ratio indicates lattice contraction.

7



Figure 3

Page 8 of 21

Williamson-Hall data for TiO2 thin films annealed at 450°C for 2 h: (a) undoped and (b) codoped

The Raman data in Figure 4(a) show that the trend in crystallinity, as indicated by the 145 cm1 peak height, is consistent with inverse crystallite size, which, again, is a reflection of the effects of surface area/bulk ratio and densification. The maximal intensity of the 145 cm-1 peak for the 0.100 mol% codoped TiO2 thin film is consistent with maximal crystallinity, which negates the possibility that lattice distortion is responsible for the lattice expansion indicated by the GAXRD data. Thus, the formation of oxygen vacancies (V•• O ) for charge compensation is likely to play the dominant role in lattice expansion. The blue shift in Figure 4(b) indicates that the undoped and codoped TiO2 thin film have undergone compressive strain (relative to the reported theoretical position of the main Eg mode for anatase at 144 cm-1)62, which is consistent with the lattice contraction63,64 shown in Figure 2(b). At 0.050 mol% codoping level, the compressive strain increased; at 0.075 mol% codoping level, the extent of compressive strain decreased; and, at 0.100 mol% codoping level, there was a slight increase in the extent of compressive strain. These alternating shifts in strain are paralleled in the structural and inverse crystallite size data of Figure 2(a) and the c/a axial ratio data of Figure 2(b).

Figure 4

(a) Raman spectra and (b) Raman shifts at 144 cm-1 for TiO2 thin films with different codoping levels annealed at 450°C for 2 h; all patterns scaled identically, with maximal peak intensity (0.100 mol% Mo3+/Mo5+) of 100000 counts) 8


Page 9 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In order to consider the potential for substitutional and/or interstitial solid solubility, Table 4 summarizes the relevant crystal radii for Ti and Mo, the size of the two interstices adjacent to the central Ti position in the elongated TiO6 octahedron in the c axis direction37, and unreported crystal radii determined by analogy. Table 4. Relevant Shannon crystal radii65 for substitutional and interstitial solid solution formation (TiO6 interstitial site radii = 0.0782 nm)37 Substitutional (Sixfold Coordination) Radius Difference Crystal with Ti4+ in Cation Coordination Radius Sixfold Coordination (nm) (%) 4+ Ti 0.0745 N/A 3+ Ti VI 0.081 + 8.72 2+ Ti 0.100 +34.23 Ti4+ --3+ Ti V --2+ Ti --6+ Mo 0.073 – 2.01 5+ Mo 0.075 + 0.67 VI 4+ Mo 0.079 + 6.04 Mo3+ 0.083 +11.41 6+ Mo --5+ Mo --V 4+ Mo --3+ Mo --Solid Solubility

Interstitial (Fivefold Coordination) Radius Difference Crystal with TiO6 Interstices in Radius Fivefold Coordination (nm) (%) ------0.065 –16.88 0.071* – 9.21 0.088* +12.53 --------0.064 –18.16 0.066* –15.60 0.069* –11.76 0.073* – 6.65

* Determined by analogy by assuming 12.75% size reduction for fivefold relative to sixfold coordination on the basis of Ti4+ According to Hume-Rothery's rules for metallic substitutional solid solubility,29,66 substantial solid solubility is favored when the crystal radius of the dopant varies by 15%, then only partial solubility is expected. Comparison of the data in Table 4 supports the conclusion that all four Mo valences would be highly soluble. From size considerations alone, the tightly packed a-b plane would be affected significantly while any effect on the c axis would be minimal owing to the adjacent large interstices.37 Consequently, Mo3+, Mo4+, and Mo5+ would be expected to cause slight a-b plane expansion and Mo6+ would cause slight contraction. In contrast, from valence considerations alone, since the electronegativity increases with increasing valence,67 then Mo3+, Mo4+, Mo5+, and Mo6+ would be expected to decrease both the a-b plane and c axis in this order. Since the a-b axis is tightly packed but the c axis is more loosely packed, it is probable that a-b plane distortion is dominated by size effects and the c axis distortion is dominated by valence effects. Hume-Rothery’s rules for metallic interstitial solid solubility require the solute ion to be substantially smaller than that of the solvent ion.29,66 Since the Mo ions are only slightly smaller than Ti, this suggests that substantial interstitial solid solubility is less likely. However, these rules are for close-packed metallic structures, not ionic-covalent oxides, so the actual radius of the interstices is more relevant. Since this is 0.0782 nm37 and all of the Mo crystal radii are smaller, 9



Page 10 of 21

then substantial interstitial solid solubility would be expected. In this case, mutually reinforcing size and valence effects alone suggest that the principal effect would be c axis contraction in the order Mo6+ > Mo5+ > Mo4+ > Mo3+; slight a-b plane contraction would follow in the same order. However, these distortions are likely to be mitigated by stress minimisation from off-site ionic shifts from integrated solid solubility.68 In summary, substitutional solid solubility would be expected to be dominated by size effects in the a-b plane, with expansion in the order Mo3+ > Mo4+ > Mo5+ > Mo6+, and little effect along the c axis In contrast, interstitial solid solubility would be expected to be dominated by both size and valence effects along the c axis, with contraction in the order and valence effects alone suggest that the principal effect would be c axis contraction in the order Mo6+ > Mo5+ > Mo4+ > Mo3+, and slight a-b plane contraction. Since Figure 2(b) shows that the c axis contraction with doping is greater than a-b plane contraction, then it is concluded that the solid solubility mechanism is interstitial. Figure 5 shows the XPS spectra (Ti 2p and Mo 3d) of undoped and codoped TiO2 thin films. The binding energies were obtained by referencing the C 1s line to 284.4 eV. In Figure 5(a), the two main peaks for Ti 2p3/2 (458.8 eV) and Ti 2p1/2 (464.5 eV) are assigned to Ti4+ in TiO2.69 However, the two main peaks for Ti 2p3/2 (457.55 eV) and Ti 2p1/2 (463.30 eV), which represent Ti3+ in TiO2,69 were not observed, indicating the absence of this species or its levels being below the detection limit. The Ti 2p binding energies remain the same in all samples, indicating no alteration of the Mo–O–Ti bonding configuration,70 which supports the conclusion of interstitial solid solubility, provided the tightly packed a-b plane bonding dominates that of the loosely packed c axis bonding. More relevantly, nil Ti peak shifts indicate the absence of Ti valence state change upon codoping. In Figure 5(b), the two main peaks for Mo were located between those for Mo4+ (~230.56 eV and ~233.78 eV)69 and Mo5+ (~232.18 eV and ~235.28 eV)69, indicating the nonintegral valence Mo5-x; no peaks for Mo3+ (~230.56 eV and ~233.78 eV)69 was observed. The absence of valence changes with codoping indicates that It can be seen that there is no valence changes with different codoping levels, suggesting that intervalence charge transfer (IVCT) between matrix and dopant ions or between dopant ions did not occur.

Figure 5

XPS spectra for TiO2 thin films with different codoping levels after annealing at 450°C for 2 h, showing representative peaks for (a) Ti 2p and (b) Mo 3d energy regions (shaded areas show range of reported binding energies for Ti) 10


Page 11 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The XPS data showing consistent reduced binding energy (viz., valence) for Mo5+ and the GAXRD and Raman data showing lattice contraction suggest that doping occurred through the following process: 4+ (5-x)+ (post-annealing) 2Ti(4-x)+ + Mo3+ + Mo5+ + 𝑉•• 𝑂 (pre-annealing) → 2Ti + 2Mo

(1)

where Ti(4-x)+ denotes the starting material TiO2-x, with major Ti4+ and minor Ti3+. The annihilation of 𝑉•• 𝑂 by interstitial solid solubility would not be expected to enhance the photocatalytic performance as this defect is recognised as a midgap state that can lower the band gap (Eg)71 and as a surface-active site at which photocatalytic redox can take place.37 The Kröger-Vink defect equilibria72 for Mo interstitial solubility in TiO2 are as follows: Ionic Charge Compensation x 2Mo2O5 → 4Mo••••• + 5V′′′′ i Ti + 10OO

(2)

x MoO2 → Mo•••• + V′′′′ i Ti + 2OO

(3)

x 2Mo2O3 → 4Mo••• i + 3V′′′′ Ti + 6OO

(4)

Electronic Charge Compensation: ••••• Mo2O5 + V•• + 8e′ + 4OxO O → 2Moi

(5)

MoO2 → Mo•••• + 4e′ + 2OxO i

(6)

1

Mo2O3 + 2O2 (g) → 2Mo••• + 6e′ + 4OxO i

(7)

where: Mo••••• 𝑖 Mo•••• 𝑖 Mo••• 𝑖 ′′′′ VTi Ox𝑂 𝑒′

= = = = = =

Mo ion interstitial on the Ti lattice site (quintuple positive charge) Mo ion interstitial on the Ti lattice site (quadruple positive charge) Mo ion interstitial on the Ti lattice site (triple positive charge) Titanium vacancy (quadruple negative charge) Oxygen ion on the oxygen lattice site (neutral charge) Electron (single negative charge)

Consequently, there are four possible midgap energy states that have the potential to affect the Eg): ••• , Mo•••• Mo••••• i i , Moi , and V′′′′ Ti . A second major effect that affects the Eg derives from the difference between amorphous and crystalline TiO2. Amorphous bulk and grain boundaries have a higher Eg (≥3.4 eV)73 than that of the crystalline bulk (3.2 eV).74 Figure 6 shows AFM images of the surfaces of the codoped TiO2 thin films; the crystallite and grain sizes and RMS surface roughness are summarized in Table 5. The similarities of the crystallite and grain sizes show that the grains are single-crystal. It also can be seen that the grain sizes follow a trend that is converse to those shown in Figure 2(a) while the surface roughnesses decreased consistently with codoping. The inconsistency with the surface roughnesses is likely to be due to the non-closest packing of the grains, where surface roughness is a measure of the depth 11



Page 12 of 21

of the grain boundary groove, which would differ when grains impinge or are bordered by a pore/void. This has been observed in other.31

Figure 6

AFM images of codoped TiO2 thin films with different codoping levels after annealing at 450°C for 2 h

Table 5. Variation of crystallite sizes, grain sizes, and surface roughnesses of codoped TiO2 thin films with different codoping levels after annealing at 450°C for 2 h Sample mol% Undoped 0.050 0.075 0.100

Crystallite Size nm 10 12 11 14

Average Grain Size nm 13.73 15.29 13.34 14.16

RMS Surface Roughness (Rq) nm 0.696 0.633 0.595 0.577

UV-Vis transmission spectra of the codoped TiO2 thin films at different doping levels are shown in Figure 7 and indirect band gaps are shown in Table 6. All of the samples are highly transparent (~90%) and flat, smooth, and microstructurally homogeneous, as confirmed by the presence of interference fringes.75 All of the codoped TiO2 thin films exhibit a blue shift in the absorption edge, demonstrating a decrease in the extent of absorption. Inconsistent trends in the Eg are observed commonly37 because this parameter depends on a number of factors, including 12


Page 13 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


structural (e.g., crystallinity), microstructural (e.g., grain size), and semiconducting (e.g., midgap energy states) effects.

Figure 7. UV-Vis transmission spectra of codoped TiO2 thin films with different codoping levels after annealing at 450°C for 2 h Table 6.

Optical indirect band gaps of codoped TiO2 thin films with different codoping levels annealed at 450 °C for 2 h

Indirect Band Gap (eV)

Undoped 3.34

0.050 mol% 3.39

0.075 mol% 3.41

0.100 mol% 3.38

Another concurrent effect on the Eg is the residual lattice stress, the effects of which have generated contradictory calculations in the literature. Yin et al.76 considered uniaxial and hydrostatic tension and compression, concluding that compression increases the Eg. In contrast, Sun et al.77 determined that uniaxial compression (c axis contraction) caused a slight increase in the Eg and biaxial compression (a-b plane contraction) caused a significant decrease in the Eg. Figure 2(b) shows that the principal structural effects from the interstitial solid solubility are contraction of both the a-b plane and the c axis, which would cause the observed increase in the Eg, in agreement with Sun et al.77. Alternatively, the increase in the Eg could be caused by hydrostatic compression (a-b plane and c axis), also in agreement with Yin et al.76. Thus, the present data are not sufficient to support one model over the other. More relevantly, the direct correlation between the Eg and lattice parameters suggests that the principal driver of the Eg is the residual lattice strain rather than the effects of crystallinity, grain size, or introduction of midgap states. The slight reversed trend for the residual lattice strain at the highest codoping level of 0.100 mol% Mo suggests that saturation solubility was achieved at ≥075 mol% Mo and that subsequent supersaturation and associated V•• O formation caused the lattice expansion. The photocatalytic performances of the undoped and codoped TiO2 thin films are shown in Figure 8. These data indicate the following:  

The codoped TiO2 thin films outperform the undoped TiO2 thin films. The performance of the TiO2 thin films is the best at the highest codoping level. 13





Page 14 of 21

The trend in performance is inversely proportional to the structural and inverse crystallite size data of Figure 2(a) and the c/a axial ratio data of Figure 2(b).

Figure 8. Photocatalytic performance of codoped TiO2 thin films with different codoping levels after annealing at 450°C for 2 h Since the performance does not exhibit a monotonic trend, more than one factor is responsible. It is notable that the performance data correlate with the Raman peak intensities in Figure 4(a) and the crystallite sizes in Figure 1 and Table 5; these intensities reflect the extent of crystallinity, which itself is reflected in the crystallite size. Thus, the photocatalytic performance appears to be dominated by the crystallinity, which has been observed before.30 Since the data in Figure 2(a) show consistent inverse relations with the Raman peak intensities, it is likely that these parameters also reflect the trends in crystallinity. However, other factors also may be important, including recombination time,78 diffusion distance,79 mobility,80 density of surface-active sites,81 and exposed crystallographic planes.82 Figure 9 summarizes the general trends of the relevant data. These plots show correlations between the Raman peak intensity and shift as well as the grain and crystallite sizes. The Raman shift is a reflection of the Eg, which does not dominate the performance. The grain and crystallite sizes are not consistent with the performance since smaller grains would be expected to have greater concentrations of surface-active sites. This analysis confirms that the main effect on the performance is the crystallinity. However, the reasons for the irregular trends in the crystallinity are not obvious although they appear to derive from the structural and inverse crystallite size data shown in Figure 2(a).

14


Page 15 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 9. Summary of general trends of relevant data (shaded data are consistent with the performance data) 4

Conclusions

Mo3+/Mo5+ codoped TiO2 thin films were deposited on polished fused silica glass substrates by spin coating and annealing at 450°C for 2 h. GAXRD data indicate that the anatase is not highly crystalline and that doping decreases the lattice parameters, with a reversed trend at the highest codoping level (0.100 mol%) owing to supersaturation and V•• O formation. The lattice parameter data suggest that the solubility mechanism is interstitial. The peak intensities and shifts of both the GAXRD and Raman data correlate with the crystallinities, as reflected also by the crystallite sizes, which reflect the extent of crystallinity. The XPS data reveal no valence changes, indicating that IVCT is not a relevant effect. The use of Mo3+ and Mo5+ as raw materials, the detection of Mo4+ and Mo5+, and the interstitial solid solubility indicate that there are four potential midgap states that ••• may alter the Eg: Mo••••• , Mo•••• i i , Moi , and V′′′′ Ti . However, their effects on the performance would •• be mitigated by the annihilation of the VO , which would eliminate their role as a midgap state and a surface-active site. The equivalence of the grain and crystallite sizes from the AFM and GAXRD data, respectively, confirm that the grains are single-crystal. The UV-Vis data reveal that the thin films are highly transparent. The Eg values calculated from these data correlate with the lattice parameters and hence the compressive lattice strain, suggesting that the principal driver of the Eg is the residual lattice strain rather than the effects of crystallinity, grain size, or introduction of midgap states. The photocatalytic performance is principally a function of the crystallinity, which also is reflected in the crystallite size. Supporting Information 15



Page 16 of 21

Equations relevant to a range of structural features, including the variations in crystallite sizes (D), lattice strains (ε), dislocation densities (δ), stacking fault probabilities (α), and microstresses (σstress). Acknowledgements The authors acknowledge the financial support from the Australian Research Council (ARC) and the characterization facilities provided by the Australian Microscopy & Microanalysis Research Facilities (AMMRF) node at UNSW Australia. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

Kannan, N. and Sundaram, M.M., 2001. Kinetics and mechanism of removal of methylene blue by adsorption on various carbons-a comparative study. Dyes Pigments, 51(1), pp. 25-40. Karagöz, S., Tay, T., Ucar, S. and Erdem, M., 2008. Activated carbons from waste biomass by sulfuric acid activation and their use on methylene blue adsorption. Bioresour. Technol., 99(14), pp. 6214-6222. Stock, N.L., Peller, J., Vinodgopal, K. and Kamat, P.V., 2000. Combinative sonolysis and photocatalysis for textile dye degradation. Environ. Sci. Technol., 34(9), pp. 1747-1750. Ren, H., Koshy, P., Chen, W.-F., Qi, S. and Sorrell, C.C., 2017. Photocatalytic materials and technologies for air purification. J. Hazard Mater., 325, pp. 340-366. Dong, H., Zeng, G., Tang, L., Fan, C., Zhang, C., He, X. and He, Y., 2015. An overview on limitations of TiO2-based particles for photocatalytic degradation of organic pollutants and the corresponding countermeasures. Water Res., 79, pp. 128-146. Li, D. and Haneda, H., 2003. Morphologies of zinc oxide particles and their effects on photocatalysis. Chemosphere, 51(2), pp. 129-137. Channei, D., Phanichphant, S., Nakaruk, A., Mofarah, S., Koshy, P. and Sorrell, C., 2017. Aqueous and surface chemistries of photocatalytic Fe-doped CeO2 nanoparticles. Catalysts, 7(2), p. 45. Liu, Z., Li, X., Mayyas, M., Koshy, P., Hart, J.N. and Sorrell, C.C., 2018. Planar-dependent oxygen vacancy concentrations in photocatalytic CeO2 nanoparticles. CrystEngComm, 20(2), pp. 204-212. Mehmood, R., Ariotti, N., Yang, J.L., Koshy, P. and Sorrell, C.C., 2018. pH-Responsive Morphology-Controlled Redox Behavior and Cellular Uptake of Nanoceria in Fibrosarcoma. ACS Biomater. Sci. Eng., 4(3), pp. 1064-1072. Kwong, W.L., Koshy, P., Hart, J.N., Xu, W. and Sorrell, C.C., 2018. Critical role of {002} preferred orientation on electronic band structure of electrodeposited monoclinic WO3 thin films. Sustain. Energ. Fuels, 2(10), pp. 2224-2236. Chen, D., Jiang, Z., Geng, J., Wang, Q. and Yang, D., 2007. Carbon and nitrogen co-doped TiO2 with enhanced visible-light photocatalytic activity. Ind. Eng. Chem. Res., 46(9), pp. 2741-2746. Burda, C., Lou, Y., Chen, X., Samia, A.C., Stout, J. and Gole, J.L., 2003. Enhanced nitrogen doping in TiO2 nanoparticles. Nano Lett., 3(8), pp. 1049-1051. Xu, A.-W., Gao, Y. and Liu, H.-Q., 2002. The preparation, characterization, and their photocatalytic activities of rare-earth-doped TiO2 nanoparticles. J. Catal., 207(2), pp. 151– 157. Linsebigler, A.L., Lu, G. and Yates Jr, J.T., 1995. Photocatalysis on TiO2 surfaces: principles, mechanisms, and selected results. Chem. Rev., 95(3), pp. 735-758.

16


Page 17 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[15] Wu, G. and Chen, A., 2008. Direct growth of F-doped TiO2 particulate thin films with high photocatalytic activity for environmental applications. J. Photochem. Photobiol A, 195(1), pp. 47-53. [16] Zhou, P., Wu, J., Yu, W., Zhao, G., Fang, G. and Cao, S., 2014. Vectorial doping-promoting charge transfer in anatase TiO2 {0 0 1} surface. Appl. Surf. Sci., 319, pp. 167-172. [17] Mekprasart, W., Khumtong, T., Rattanarak, J., Techitdheera, W. and Pecharapa, W., 2013. Effect of nitrogen doping on optical and photocatalytic properties of TiO2 thin film prepared by spin coating process. Energy Procedia, 34, pp. 746-750. [18] Liqiang, J., Xiaojun, S., Baifu, X., Baiqi, W., Weimin, C. and Honggang, F., 2004. The preparation and characterization of La doped TiO2 nanoparticles and their photocatalytic activity. J. Solid State Chem., 177(10), pp. 3375-3382. [19] Khan, H. and Swati, I.K., 2016. Fe3+-doped anatase TiO2 with d–d transition, oxygen vacancies and Ti3+ centers: synthesis, characterization, UV–vis photocatalytic and mechanistic studies. Ind. Eng. Chem. Res., 55(23), pp. 6619-6633. [20] Gupta, K., Singh, R.P., Pandey, A. and Pandey, A., 2013. Photocatalytic antibacterial performance of TiO2 and Ag-doped TiO2 against S. aureus. P. aeruginosa and E. coli. Beilstein J. Nanotechnol., 4(1), pp. 345-351. [21] Yoong, L.S., Chong, F.K. and Dutta, B.K., 2009. Development of copper-doped TiO2 photocatalyst for hydrogen production under visible light. Energy, 34(10), pp.1652-1661. [22] Lin, C.Y.W., Channei, D., Koshy, P., Nakaruk, A. and Sorrell, C.C., 2012. Multivalent Mndoped TiO2 thin films. Physica E Low Dimens. Syst. Nanostruct., 44(10), pp. 1969-1972. [23] Lin, C.Y.W., Channei, D., Koshy, P., Nakaruk, A. and Sorrell, C.C., 2012. Effect of Fe doping on TiO2 films prepared by spin coating. Ceram. Int., 38(5), pp. 3943-3946. [24] Nakaruk, A., Chen, H., Waibel, A., Koshy, P. and CC, S., 2012. Surface Modification of Titanium Dioxide Thin Films via Manganese Doping. e-J. Surf. Sci. Nanotechnol., 10, pp. 103-106. [25] Nakaruk, A., Lin, C.Y.W., Koshy, P. and Sorrell, C.C., 2012. Iron doped titania thin films prepared by spin coating. Adv. Appl. Ceram., 111(3), pp.129-133. [26] Gai, Y., Li, J., Li, S.S., Xia, J.B. and Wei, S.H., 2009. Design of narrow-gap TiO2: a passivated codoping approach for enhanced photoelectrochemical activity. Phys. Rev. Lett., 102(3), p. 036402. [27] Wang, S., Bai, L.N., Sun, H.M., Jiang, Q. and Lian, J.S., 2013. Structure and photocatalytic property of Mo-doped TiO2 nanoparticles. Powder Tech., 244, pp. 9-15. [28] Khan, H. and Berk, D., 2014. Synthesis, physicochemical properties and visible light photocatalytic studies of molybdenum, iron and vanadium doped titanium dioxide. React. Kinet. Mech. Cat., 111(1), pp. 393-414. [29] Chen, W.-F., Chen, H., Koshy, P., Nakaruk, A. and Sorrell, C.C., 2018. Effect of doping on the properties and photocatalytic performance of titania thin films on glass substrates: Singleion doping with Cobalt or Molybdenum. Mater. Chem. Phys., 205, pp. 334-346. [30] Jiang, Y., Chen, W.-F., Koshy, P. and Sorrell, C.C., 2019. Enhanced photocatalytic performance of nanostructured TiO2 thin films through combined effects of polymer conjugation and Mo-doping. J. Mater. Sci., 54(7), pp. 5266-5279. [31] Cui, Y., Chen, W.-F., Bastide, A., Zhang, X., Koshy, P. and Sorrell, C.C., 2019. Effect of precursor dopant valence state on the photocatalytic performance of Mo3+-or Mo5+-Doped TiO2 thin films. J. Phys. Chem. Solids., 126, pp. 314-321. [32] Guo, J., Gan, Z., Lu, Z., Liu, J., Xi, J., Wan, Y., Le, L., Liu, H., Shi, J. and Xiong, R., 2013. Improvement of the photocatalytic properties of TiO2 by (Fe+ Mo) co-doping-A possible way to retard the recombination process. J. Appl. Phys., 114(10), p. 104903. [33] Khan, M., Yi, Z., Fawad, U., Muhammad, W., Niaz, A., Zaman, M.I. and Ullah, A., 2017. Enhancing the photoactivity of TiO2 by codoping with silver and molybdenum: the effect of 17



[34] [35] [36] [37]

[38] [39] [40]

[41] [42] [43] [44] [45] [46] [47] [48] [49] [50]

Page 18 of 21

dopant concentration on the photoelectrochemical properties. Mater. Res. Express., 4(4), p. 045023. Akbarzadeh, R. and Javadpour, S., 2014. W/Mo-doped TiO2 for photocatalytic degradation of Methylene blue. Int. J. Eng. Sci. Innovative Tech, 3, pp. 621-629. Chen, W.-F., Koshy, P., Adler, L. and Sorrell, C.C., 2017. Photocatalytic activity of V-doped TiO2 thin films for the degradation of methylene blue and rhodamine B dye solutions. J. Aust. Ceram. Soc., 53(2), pp. 569-576. Chung, L., Chen, W.-F., Koshy, P. and Sorrell, C.C., 2017. Effect of Ce-doping on the photocatalytic performance of TiO2 thin films. Mater. Chem. Phys., 197, pp. 236-239. Chen, W.-F., Koshy, P., Huang, Y., Adabifiroozjaei, E., Yao, Y. and Sorrell, C.C., 2016. Effects of precipitation, liquid formation, and intervalence charge transfer on the properties and photocatalytic performance of cobalt-or vanadium-doped TiO2 thin films. Int. J. Hydrog. Energy, 41(42), pp.19025-19056. Chen, W.-F., Koshy, P. and Sorrell, C.C., 2016. Effects of film topology and contamination as a function of thickness on the photo-induced hydrophilicity of transparent TiO2 thin films deposited on glass substrates by spin coating. J. Mater. Sci., 51(5), pp. 2465-2480. Chen, W.-F., Koshy, P. and Sorrell, C.C., 2015. Effect of intervalence charge transfer on photocatalytic performance of cobalt-and vanadium-codoped TiO2 thin films. Int. J. Hydrog. Energy, 40(46), pp. 16215-16229. Chen, W.-F., Koshy, P., Zhu, B. and Sorrell, C.C., 2015. Effect of film thickness on the photocatalytic performance of TiO2 thin films deposited by spin coating. In Ceramic Materials for Energy Applications IV: A Collection of Papers Presented at the 38th International Conference on Advanced Ceramics and Composites, January 27-31, 2014, Daytona Beach, FL (Vol. 595, p. 51). John Wiley & Sons. Chen, W.-F., Chen, H., Koshy, P. and Sorrell, C.C., 2015. Photocatalytic Performance of Vanadium-Doped and Cobalt-Doped TiO2 Thin Films. J. Aust. Ceram. Soc., 51(1). Klug HP, Alexander LE. X-ray diffraction procedures for polycrystalline and amorphous materials. New York: Wiley; 1997. Solomon D.H. and Hawthorne D.G., 1983. Chemistry of pigments and fillers. New York: Wiley. Sreedhar, M., Brijitta, J., Reddy, I.N., Cho, M., Shim, J., Bera, P., Joshi, B.N. and Yoon, S.S., 2018. Dye degradation studies of Mo-doped TiO2 thin films developed by reactive sputtering. Surf. Interface Anal., 50(2), pp. 171-179. Malliga, P., Pandiarajan, J., Prithivikumaran, N. and Neyvasagam, K., 2014. Influence of film thickness on structural and optical properties of sol–gel spin coated TiO2 thin film. J. Appl. Phys., 6, pp. 22-28. Vidhya, R., Sankareswari, M. and Neyvasagam, K., 2016. Effect of annealing temperature on structural and optical properties of Cu-TiO2 thin film. Relation, 2, p.1. Bensaha, R. and Bensouyad, H., 2012. Synthesis, characterization and properties of zirconium oxide (ZrO2)-doped titanium oxide (TiO2) thin films obtained via sol-gel process. In Heat Treatment-Conventional and Novel Applications. InTech. Yin, W.J., Chen, S., Yang, J.H., Gong, X.G., Yan, Y. and Wei, S.H., 2010. Effective band gap narrowing of anatase TiO2 by strain along a soft crystal direction. Appl. Phys. Lett., 96(22), p. 221901. Santara, B., Giri, P.K., Imakita, K. and Fujii, M., 2014. Microscopic origin of lattice contraction and expansion in undoped rutile TiO2 nanostructures. J. Phys. D, 47(21), p.215302. Aidhy, D.S., Liu, B., Zhang, Y. and Weber, W.J., 2015. Chemical expansion affected oxygen vacancy stability in different oxide structures from first principles calculations. Comput. Mater. Sci., 99, pp. 298-305. 18


Page 19 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


[51] Swamy, V., Menzies, D., Muddle, B.C., Kuznetsov, A., Dubrovinsky, L.S., Dai, Q. and Dmitriev, V., 2006. Nonlinear size dependence of anatase TiO2 lattice parameters. Appl. Phys. Lett., 88(24), p. 243103. [52] Yurtsever, H.A. and Çiftçioğlu, M., 2017. The effect of rare earth element doping on the microstructural evolution of sol-gel titania powders. J. Alloy Compd., 695, pp. 1336-1353. [53] Klug, H.P. and Alexander, L.E., 1997. X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials Wiley, New York [54] Ayed, S., Belgacem, R.B., Zayani, J.O. and Matoussi, A., 2016. Structural and optical properties of ZnO/TiO2 composites. Superlattice Microst., 91, pp. 118-128. [55] Bharti, B., Barman, P.B. and Kumar, R., 2015. August. XRD analysis of undoped and Fe doped TiO2 nanoparticles by Williamson Hall method. In AIP Conference Proceedings (Vol. 1675, No. 1, p. 030025). AIP Publishing. [56] Murugesan, C. and Chandrasekaran, G., 2015. Impact of Gd3+ substitution on the structural, magnetic and electrical properties of cobalt ferrite nanoparticles. RSC Adv., 5(90), pp. 7371473725. [57] Dolgonos, A., Mason, T.O. and Poeppelmeier, K.R., 2016. Direct optical band gap measurement in polycrystalline semiconductors: A critical look at the Tauc method. J. Solid State Chem., 240, pp. 43-48. [58] Wu, X., Ng, Y.H., Wang, L., Du, Y., Dou, S.X., Amal, R. and Scott, J., 2017. Improving the photo-oxidative capability of BiOBr via crystal facet engineering. J. Mater. Chem. A, 5(17), pp. 8117-8124. [59] Dang, Y. and West, A.R., 2019. Oxygen stoichiometry, chemical expansion or contraction, and electrical properties of rutile, TiO2±δ ceramics. J. Am. Ceram. Soc., 102(1), pp. 251-259. [60] Pan, X., Yang, M.Q., Fu, X., Zhang, N. and Xu, Y.J., 2013. Defective TiO2 with oxygen vacancies: synthesis, properties and photocatalytic applications. Nanoscale, 5(9), pp. 36013614. [61] Mooney, P., 2005. RRUFF Project: Developing an integrated database of Raman and infrared spectra, X-ray diffraction and chemistry data for minerals. In 2005 Salt Lake City Annual Meeting. [62] Ohsaka, T., Izumi, F. and Fujiki, Y., 1978. Raman spectrum of anatase, TiO2. J. Raman Spectrosc., 7(6), pp. 321-324. [63] Santara, B., Giri, P.K., Imakita, K. and Fujii, M., 2014. Microscopic origin of lattice contraction and expansion in undoped rutile TiO2 nanostructures. J. Phys. D, 47(21), p. 215302. [64] Choudhury, B. and Choudhury, A., 2012. Dopant induced changes in structural and optical properties of Cr3+ doped TiO2 nanoparticles. Mater. Chem. Phys., 132(2-3), pp. 1112-1118. [65] Shannon, R.T. and Prewitt, C.T., 1969. Effective ionic radii in oxides and fluorides. Acta Crystallogr. B, 25(5), pp. 925-946. [66] Hume-Rothery W, Smallman RE, Haworth CW. Structure of metals and alloys. London: Belgrave Square, 1969. [67] Smith, D.W., 1990. Electronegativity in two dimensions: Reassessment and resolution of the Pearson-Pauling paradox. J. Chem. Educ, 67(11), p. 911. [68] Chen, W.-F., Mofarah, S.S., Hanaor, D.A.H., Koshy, P., Chen, H.K., Jiang, Y. and Sorrell, C.C., 2018. Enhancement of Ce/Cr codopant solubility and chemical homogeneity in TiO2 nanoparticles through sol–gel versus Pechini syntheses. Inorg. Chem., 57(12), pp. 7279-7289. [69] Naumkin, A. V.; Kraut-Vass, A.; Gaarenstroom, S. W.; Powell, C. J. NIST X-Ray Photoelectron Spectroscopy Database, NIST Standard Reference Database 20, Version 4.1; U.S. Department of Commerce: Washington D.C., 2012. [70] Pan, L., Zou, J.J., Zhang, X. and Wang, L., 2010. Photoisomerization of norbornadiene to quadricyclane using transition metal doped TiO2. Ind. Eng. Chem. Res., 49(18), pp. 85268531. 19



Page 20 of 21

[71] Kim, D.H., Hong, H.S., Kim, S.J., Song, J.S. and Lee, K.S., 2004. Photocatalytic behaviors and structural characterization of nanocrystalline Fe-doped TiO2 synthesized by mechanical alloying. J. Alloy Compd., 375(1-2), pp. 259-264. [72] Kröger, F.A. and Vink, H.J., 1956. Relations between the concentrations of imperfections in crystalline solids. In Solid state physics (Vol. 3, pp. 307-435). Academic Press. [73] Nguyen, C.V. and Hieu, N.N., 2016. Effect of biaxial strain and external electric field on electronic properties of MoS2 monolayer: A first-principle study. Chem. Phys., 468, pp. 9-14. [74] Hanaor, D.A. and Sorrell, C.C., 2011. Review of the anatase to rutile phase transformation. J. Mater. Sci., 46(4), pp. 855-874. [75] Swanepoel, R., 1984. Determination of surface roughness and optical constants of inhomogeneous amorphous silicon films. J. Phys. E, 17(10), p. 896. [76] Yin, W.J., Chen, S., Yang, J.H., Gong, X.G., Yan, Y. and Wei, S.H., 2010. Effective band gap narrowing of anatase TiO2 by strain along a soft crystal direction. Appl. Phys. Lett., 96(22), p. 221901. [77] Sun, Y., Thompson, S.E. and Nishida, T., 2007. Physics of strain effects in semiconductors and metal-oxide-semiconductor field-effect transistors. J. Appl. Phys., 101(10), p. 104503. [78] Ozawa, K., Emori, M., Yamamoto, S., Yukawa, R., Yamamoto, S., Hobara, R., Fujikawa, K., Sakama, H. and Matsuda, I., 2014. Electron–hole recombination time at TiO2 single-crystal surfaces: Influence of surface band bending. J. Phys. Chem. Lett., 5(11), pp. 1953-1957. [79] Li, W., Ni, C., Lin, H., Huang, C.P. and Shah, S.I., 2004. Size dependence of thermal stability of TiO2 nanoparticles. J. Appl. Phys., 96(11), pp. 6663-6668. [80] Simon, J. and Andre, J.J., 2012. Molecular semiconductors: photoelectrical properties and solar cells. Springer Science & Business Media. [81] Liu, G., Jimmy, C.Y., Lu, G.Q.M. and Cheng, H.M., 2011. Crystal facet engineering of semiconductor photocatalysts: motivations, advances and unique properties. Chem. Commun., 47(24), pp. 6763-6783. [82] Liu, M., Piao, L., Zhao, L., Ju, S., Yan, Z., He, T., Zhou, C. and Wang, W., 2010. Anatase TiO2 single crystals with exposed {001} and {110} facets: facile synthesis and enhanced photocatalysis. Chem. Commun., 46(10), pp. 1664-1666.

20


Page 21 of 21 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC Graphic

21


Mo5+ Codoping on

Recommend Documents