Enhanced Photocatalytic Hydrogen Evolution with TiO2âTiN

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C: Plasmonics; Optical, Magnetic, and Hybrid Materials

Enhanced Photocatalytic Hydrogen Evolution with TiO-TiN Nanoparticle Composites 2

Edwin B. Clatworthy, Samuel Yick, Adrian T. Murdock, Morgan Charles Allison, Avi Bendavid, Anthony F. Masters, and Thomas Maschmeyer J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b09221 • Publication Date (Web): 17 Jan 2019 Downloaded from http://pubs.acs.org on January 19, 2019

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The Journal of Physical Chemistry

Enhanced Photocatalytic Hydrogen Evolution with TiO2-TiN Nanoparticle Composites † Samuel Yick, ‡ Adrian T. Murdock, ‡ Morgan C. Allison, † Avi Bendavid, ‡ Edwin B. Clatworthy, † † Anthony F. Masters and Thomas Maschmeyer* †School

of Chemistry, The University of Sydney, Sydney, NSW 2006, Australia Scientific and Industrial Research Organisation (CSIRO), Nanostruct Manufacturing, 36 Bradfield Rd, West Lindfield, NSW 2070, Australia ‡Commonwealth

Supporting Information Placeholder 5-6 work functions when compared to TMNs noble such metals. as ABSTRACT: Metal nitrides have potential in energy applications

TiN and ZrN can absorb UV light due to interband tra due to their physical and optical properties. Nanoparticle well as visible and near-IR light due to their pla composites of titanium nitride (TiN) and titanium dioxide (TiO 2) Additionally, their electronic properties can be t were investigated for their photocatalytic ) evolution hydrogen (H 2 their metal/nitrogen stoichiometry (e.g. TiN and/or x, 0.6 < x 3.2 eV, for anatase) limit utilization tofrom only the contribution the TiN plasmon. We propose, th near-UV spectrum %% % < 390 nm). Strategies such as the addition visible light excitation of the TiN plasmon, UV of noble metal co-catalysts (Pt, Au, Ag) and reagents to reduce downward band be TiO band gap is necessary 2 sacrificial (e.g. MeOH, soluble biomass waste) have been to the employed TiO and promote hot electron migr 2/TiN interface dramatically increase activity by, for from example, reducing the2TiO /TiN interface for H 2 evolution. electron/hole recombination rates. In addition, visible light sensitization by localised surface plasmon resonance (LSPR) has 2,3 However, the relatively low terrestrial crustal been a strategy. 2. EXPERIMENTAL abundance of noble metals reduces their potential for applications at scale. Recently, transition metal nitrides (TMNs) have emerged as attractive materials in photocatalytic applications for several key Materials. TiN(COM), 99.2+%, 20 nm, cubic (US Nano Mat 4 TMNs possess superior corrosion resistance, very high reasons. Inc.) and Aeroxide® (Evonik) were used as receive 2 P25 TiO melting points (TiN m.p. = 2950 °C) and typically exhibit smaller Milli-Q (Type 1) water was prepared using Millipor

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CSIRO was prepared using a transferred arc plasma method based Dynamic light scattering (DLS) measurements were 10 described on a gas tungsten arc welding setup as elsewhere. a Zetasizer Nano ZS, Malvern in 10 mM NaCl using th Titanium metal was purchased from Plasmaterials, Inc. and was index value (P25) TiO of 2.49. pH measurements we 2of used as received. High purity He and N by BOC. In performed with a PHM210 Standard pH Meter, Radio 2 were supplied order to generate the nanoparticles, the chamber housing the area measurements were perfo Analytical. Surface -3mbar.to plasma system was evacuated A 1:1 8×10 mixture ofMicromeritics He Accelerated Surface Area and Porosim and N assembly. Each sample was degassed 2020 instrument. at 200 2 was then introduced from the cathodic trigger 2 The chamber pressure was elevated to 173 mbar and stabilised. To adsorption/desorption isotherms were collected a initiate the plasma, the target material which sits onarea the was anode was BET surface calculated from the adsorption struck by the tungsten trigger cathode at 210 A to induce an arcthe Micromeritics software. XPS isotherm by using plasma. The process was continued for a predetermined period. were performed using a Specs SAGE 150 spectroscope After, the chamber was cooled and the nanoparticles physically %% excitation at 1%2%%%% %%% TEM, HRTEM and EDS a extracted. were performed on a JEOL 1400 microscope operating with images recorded on a Gatan Erlangshen camera, Preparation of the Nanoparticle Composites. Aqueous colloidal 2200 microscope The operating at 200 kV with the imag suspensions of P25 and TiN were prepared by ultrasonication. a Gatan Ultrascan camera. Images w initial pH of the water was set to 5 withdigitally 0.1 M HCl.on After using Gatan Digital Micrograph and ImageJ softwa ultrasonication (1 h) the colloidal suspension of TiN was added to the colloidal suspension of P25 with vigorous stirring. The weight to liquid volume ratio of the initial suspensions and the combined suspension was ~ 1.0–2.0 mg/mL, and the maximum totalAND volume 3. RESULTS DISCUSSION of the combined suspensions was 120 mL. The combined suspensions were allowed to stir for 24 h. After stirring the Aeroxide® P25in was used as the titania photocatal suspension was centrifuged to afford a material ranging color 2TiO its excellent activity and prevalence in industr from pale to dark blue, depending on TiN loading, indicating 11 We employed reference titania-based photocatalytic acti mixing of the two nanoparticle samples. The materialfor was washed two (E100) different samples of TiN that we three times with de-ionized water and ethanol andnanoparticle then by plasma-based techniques. The first was a com dried overnight at 60 °C. from US Research Nanomaterials Inc., prepared by Photocatalytic H2 Evolution Setup. 5 mg of TiO 2-TiN composite, phase synthesis, distinguished by the P25 or TiN was ultrasonicated in a 3:1 mixture (20 vapor of H 2O:MeOH TiN(COM). The second sample was prepared by the Thi mL) at a pH of 7–8 for 15 min. The suspension was then Nanostructured Materials team of CSIRO by the tra deoxygenated for 30 min with stirring under Ar. The double-walled 10 Unlike plasma method, and is designated as TiN(CSIRO). the quartz reactor was irradiated with a polychromatic 350 W mercury preparation of transition-metal nanoparticle co arc lamp (Newport) (Diagram S1) at a distance of 6 cm from the preparation of TiN nanoparticles is more chall light source. Different glass cut-off filters were employed with the normally involves nitridation (NH metal, metal 3, N 2) of a pure mercury arc lamp including % > 305, > 420, > 455, > 495 and > 610 oxide, urea-gel or metal complex precursor, some nm (Newport) (Fig. S1). A combination of the 305 nm cut-off filter under an inert atmosphere. To achieve s and a UG11 glass filter %% = 250–390 nm) waspreparation employed for (asglass determined experiments using only UV irradiation. The UG11 filterby wasXRD) nitridation temperatures between 800–2000 employed for double lamp experiments. A 300 W (ozone free) °C, depending on the nature of specifications of the experimental se xenon arc lamp was equipped with the 455 nmand cut-off filter for 12-15 configuration, gas flow-rate). Such chemical procedures ca double lamp experiments. For experiments involving platinum, an induce severe and aggregation of the nan amount of chloroplatinic acid (equal to 1 wt% Pt) was added sintering before the%% high temperatures. Alternatively, physical p de-oxygenation. The reactor was then irradiated > 305 nm) for 1 nanoparticles such as those using plasma tech hour for photodeposition to occur before measurements of H 2 over morphology and stoichiometr evolution under visible light %% > 455 nm).better During control photocatalysis 16-17TiO-TiN important for optical and catalytic applicati 2 the suspension was maintained under Ar, continuously agitated composites of different TiN loading (0.5, 1, 2, 10, with stirring, and the reactor was maintained at constant were prepared by physically mixing separate ul temperature of 220 gas °C. was H carried in Ar at a controlled flow colloidal suspensions Ultrasonication TiO 2 and TiN. of rate of 30 mL/min to a GC (Shimadzu GC-2014,aqueous sample loop 1 treatment has been previously demonstrated to eff mL) equipped with a discharge ionization detector (Vici pulsed P25 aggregates to in primary particle sizes for up discharge ionization detector D-4-I-SH14-R; detection limit the 18 To aid combination of the two phases, the suspension. low ppb range)2 quantification. for H Error bars represent the colloidalTiO suspension was adjusted to 5 to promo standard deviation of three or more repeat experiments. 2 alone attraction, as the points of zero TiN charge occur of at TiO pH 2 and was measured several times to determine baseline activity before 19-20After combination of % 6.2 and 3.6 respectively. and the TiO 2 testing each series composites. TiO The2 rates of H 2-TiNof TiN suspensions, the resultant colloidal suspen evolution of -TiN the TiO composites were normalized to the same 2 24 h followed by centrifugation and three washings amount of2.TiO Light intensity measurements were conducted water and EtOH respectively. The material was then using a Solar Light Co. Dual-Input Data Logging Radiometer. % 60 °C for at least 12 h. Preparation of nanopartic Characterization. Powder XRD patterns were collected using a thermal procedures such as partial oxid employing PANalytical X’Pert Pro MPD X-ray diffractometer using nitridation Cu %% %% to make partial of TiO mixed phase TiO were 2 2-TiN = 1.5419 Å) on zero-background Si (100) plates. XRD patterns of deliberately avoided. This is because of the conc TiN were refined using FullProf software. Solid state UV/Vis of oxy-nitride species of which the stoichiometr diffuse reflectance spectroscopy was performed on a Varian Cary 21 are difficult to control. 5 UV/Vis spectrophotometer (lamp change at 350 and 800 nm) and The TiO (P25), TiN samples, and composites TiO were 2 2-TiN band-gap values were extracted using the Tauc and Kubelka-Munk analysed by XRD to identify the crystallographic equations. Solution UV/Vis spectroscopy was performed on an The crystalline composition of P25 consists of a Agilent Technologies Cary 60 UV-Vis spectrophotometer.


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moleculesphase and hydroxyl groups existing at the int of anatase (73–85%), rutile (14–17%) and an amorphous (0– 2 23 and TiN nanoparticle surfaces cannot be ruled out. 13%)

Figure 1. HRTEM images and electron diffractionFigure patterns 2. HRTEM of images (a) of TiO showing 2-TiN(CSISO)-33wt% TiN(COM) and (b) TiN(CSIRO). (a) clusters of small amorphous TiN nanoparticles one or more particles of (b, TiO c, d) clusters of TiN cubes i 2 and (Fig. 11 S2). Both TiN samples exhibit diffraction patterns contact with clusters of TiO 2. consistent with the %%%%%%% rocksalt (FCC) structure (Fig. S3). Due to the ultrasonic preparation of the composit Pattern refinement and analysis revealed TiN(COM) possesses scattering (DLS) were performed at t 0.944 Ti occupancy (5.6% Ti defect) and a lattice parameter (a)measurements of reaction mixture, i.e. pH 7–8, to determine if th 4. 240 Å (bulk value = 4.239 Å), whereas TiN(CSIRO) possesses 2-TiN 0.816 Ti occupancy (18.4 % Ti defect) and a = 4.227 Å (Figs. S4, composites remained agglomerated at a pH value le electrostatic attraction. Thus, preparation S5). The decrease in a of TiN(CSIRO) is consistent with Ti 22 XRD patterns of the photocatalytic reactions involved the dispersio sublattice vacancies. composites 2-TiNTiO were dominated 2by reflections, TiO but the presence TiN could byof ultrasonication (50 Hz) for 15 min at % pH 7–8 be established by observing the (200) reflection at % 42.75–43 DLS (12.5 mg/L) were treated by ultrasonication fo degrees 2% (Figs. S6, S7). 10 mM NaCl) before measurements were taken. The distribution and hydrodynamic 2diameter each revealed for TiO High resolution transmission electron microscopy (HRTEM) was 19 while only a single peak previous both rep performed to identify the morphologies of the TiN samples and (consistent with TiN samples exhibited two peaks (Fig. 2-TiN S17a). A TiO 2-TiN composites. Although images of the TiN samples composites measured exhibited intensity distribu revealed different morphologies, both exhibited agglomeration indicating TiN TiO remained agglomerated 2 and the (Fig. 1). The TiN(COM) nanoparticles werepeak observed to be that (Figs. S17b–d).upDue :TiN1:1 ratio, TiO the TiO 2primarily 20–50 nm in diameter but also contained particles to to 2the TiN(CSIRO)-50wt% 200 nm in size (Figs. S8–10). Their shapes were predominantly sample was measured with the indexthe (RI) value of both TiNTiO separately (Fig. S17e) 2 and irregular spheres and ellipsoids. In comparison, TiN(CSIRO) single peak was observed when using the RI value 2,while nanoparticles were primarily 10–18 nm cubes but also contained two peaks observed when applying the RI va particles up to 32 nm in size (Fig. S11). In addition towere the 10–18 similar the peaks observed for TiN alone. These nm cubes, particles between 5–10 nm in size with to amorphous that at high S12). TiN loadings some agglomerates of Ti morphology could also be observed in the sample (Fig. are no longer in contact with ultrasonication TiO treat 2 after HRTEM images of2 (P25) TiO revealed particles resembling as evidenced by two peaks being present (Fig. S17e rectangular prisms with well-defined edges, primarily 20–30 nm in UV/Vis spectroscopy size, but with range between 10–50 nm, (Figs. S13a–c). Compositein solid-state diffuse-refl samples with 33 wt% TiN were used for HRTEM in spectroscopy order to easilywere used to investigate the electr TiN, and the TiO composites. TiO a spectrum identify the nature of the contact between TiO the phases. For the 2,two 2-TiN 2 gave an absorption TiO TiN particles in beginning at % % 400 nm, corres 2-TiN(COM-33wt%) composite, individualwith indirect value (E of 3.28 eV, but with no featu contact with TiO be observed (Fig. S14). the TiO bandg)gap 2 could 2- For TiN(CSIRO-33wt%) composite, scattered TiNvisible clustersspectrum of various (Fig. 3a). Both TiN samples di sizes including the smaller amorphous particles absorptions and well across defined mid/near UV to visible wavel cubes could be observed in contact with individual solid state and multiple and aqueous colloidal suspension sp TiO Electrons of the in TiO the partially filled conduction b 2 nanoparticles (Fig. 2). EDS measurements 2TiN(COM-33wt%) and 2the -TiN(CSIRO-33wt%) TiO samples intraband transitions up to % 3 eV %% % 413 nm), resu 24 The solid profile. contribution to the UV/Vis absorption state confirmed that the morphology of the nanoparticle composites is of TiN(COM) displays an asymmetric absor that of clusters TiN TiO phases (Figs S15, S16).spectrum Due to the 2 andof from % %during 400–800 nm, which is ascribed to LSPR mode relatively low-temperature drying procedure used the maximum water at % = 495 nm. For TiN(COM), as an a nanocomposite preparation, the presencepeak of surface


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colloidal suspension, the absorption of the %12) LSPR thatbecomes correspond redto a direct optical transition shifted to % = 690 nm and broadens out to % = 800 S18). nm25(Fig. In comparison, 3). The the solid state spectrum of broadness of the reveals a direct optical transition of 3.10 eV (Fi solid state and aqueous colloidal suspensio TiN(CSIRO) display a less intense response of th suspension a red-shift of the beginning of the in flank %% = 466 nm) and of the LSPR absorption peak compared to TiN(COM) (Figs. 3a–b).The experime compare well with reports on the effect of size a optical properties of TiN. Calculations based o nanoparticles reveal increasing near-field inten % = 700 nm for particles < 50 nm with maximum enhan 7 Simulation = 777 nm for particles with a 75 nm radius. of the optical properties of isolated TiN cubes (10–50 n red-shifting (from % = 560 nm) and increased ext 26The first intensity with increased cube size. optical measuremen of cubic TiN nanoparticles (6.5–7.6 nm) prepared b displayed a plasmon peak that shifted from % = 693 t 16 More state) with increasing N during preparation. 2 gas flow recent results obtained from TiN nanoparticles, p thermal plasma techniques, have shown cubic TiN nm) with a lower nitrogen and higher oxygen conten and a red-shift of the plasmon absorption (from % = 27 The difference in UV/Vis spec when recorded in MeOH). between the TiN(COM) and TiN(CSIRO) samples app attributable to the difference in particle size a the surface composition (identified by XPS, Fig. Combination of TiN2 with resulted TiO in a broad absorptio feature in the visible spectrum (starting at % > loading 0.5–50 wt%) in addition to the UV absorpt TiO 2 (Figs. 3(c), S20, S22). This absorption featu spectrum of the TiO composites was more 2-TiN(COM) pronounced than that for the TiO composites of t 2-TiN(CSIRO) same TiN loading. This is consistent with the d plasmon resonance between the two TiN samples. both series2-TiN of TiO composites revealed a small decr the value of theg indirect level of TiO increasing TiN 2 Ewith content (Figs. S21, S23, S24, Table S2). Composite resulted in a greater decrease than thedid E composites g levelof TiN(CSIRO). BET surface area analyses revealed that TiNTiO samples 2 and both exhibited type 2 isotherms (Figs. S25–S27). The BE 2/g, of TiO 56consistent m with19reports on 2 was found to be The BET specific surface areas of TiN(COM) and Ti 2/g 2/g were found to be 50 and m 151 m respectively. From TEM images, the particle size of TiN(COM) is not too of P25, which would account for the similar speci values. The higher specific surface area of the Ti is likely a consequence of the smaller particle s area to volume ratio), compared toTiN(COM). those of TiO 2 and The photocatalytic evolution H activities of the Ti 2 2-TiN composites were investigated by photocatalyti MeOH (3:1 H under UV, visible, and combined U 2O/MeOH) irradiation. The2average evolution H rates are reported in Tab Under combined UV/Vis irradiation (Hg arc lamp, % > TiO highes 2-TiN(CSIRO) composites displayed the 2 Figure 3. (a) Solid state UV/Vis2, spectra TiN(CSIRO) of TiO and evolution, up to 5-fold greater than observed (Fig. for 2 alone TiN(COM), (b) UV/Vis spectra of aqueous colloidal suspensions 4). For the TiO composites, an increasing Ti 2-TiN(COM) of TiN(CSIRO) and TiN(COM), and (c) solid state UV/Vis resulted inspectra an increasing activity, however, th of TiO and TiO 2, TiO2-TiN(COM)-33wt% 2-TiN(CSIRO)-33wt%. wholly consistent. The 33 wt% TiN sample gave enhancement (3x activity of bare S28–S29). TiO For the 2) (Figs. LSPR absorption is the result of several contributions including the TiO -TiN(CSIRO) composites a clear trend between 2 variable particle sizes/shapes and inter-particle coupling, giving and the H 16 The differences 2 evolution activity was observed (Figs. 4a– rise to multiple resonance modes. between with 33wt% TiN(CSIRO) also gave the best improve solid and aqueous colloidal suspension spectra are due to the that of alone) TiO and demonstrated stable activity f 2 different responses of the plasmon to different dielectric media (air before the activity began to decrease (Fig. S30) and water). The absorbance at % < 449 nm is due to interband activity after 10 h of continuous irradiation w transitions of electrons moving orbitals from N-p to %% to Ti-d t 2g

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agglomeration and deposition of the photocatalyst Figure 4. (a)on evolution the wall of activity of TiO 2H 2 and TiO 2-TiN(CSIRO) the quartz reactor (Fig. S31). Sonication of composites the reactionand suspension (b) average H rate of TiO 2 evolution 2-TiN(CSIRO) for 15 min recovered the activity to at least 90% of the initial value,irradiation, composites under UV/Vis λ > 305 nm. Al 2 evolution however, rates are normalized to the same 2. amount of TiO Table 1. Average H rates of TiO and2-TiN TiO 2 evolution 2, TiN, the activity of the photocatalyst continued nanoparticle composite samples under UV/Vis and UV irradiation. subsequent sonication treatment (Fig. S30). C

solid state UV/Vis spectra of the sample before a activity testing revealed minor changes in the sp the optical properties of the catalyst were not s (Fig. S32). A slight reduction in the absorbance the plasmonic response %% = 600–800 nm) was obser fresh sample 2of -TiN(CSIRO)-33wt% TiO between 1 h o irradiation and 20 min in the dark under argon purgi TiO 1.8 ± 0.1 1.4 ± 0.03 2 times demonstrating a negligible change in th 2 evolution TiN(COM) 0.53 ± 0.03 – activity over 6 h (Fig. S33). The individual TiN (C TiN(CSIRO) 0.40 ± 0.09 – samples were tested under the same condition nanocomposites and displayed 2relatively evolution low TiO 3.3 ± 0.2 – 2-TiN(COM)-0.5wt% activities under combined UV/Vis irradiation (F TiO 2-TiN(COM)-1wt% 3.2 ± 0.4 – demonstrates that the combination of TiO 2 and TiN nanoparticles results in a synergistic effect, which has previo TiO 2-TiN(COM)-2wt% 2.6 ± 0.3 – for photocurrent generation and photocatalytic ac TiO 3.9 ± 0.08– 2-TiN(COM)-33wt% bilayers of TiO TiN under UV/Vis irradiation, but not 2 and 2 TiO 2.9 ± 0.07 – evolution. The highest photocurrent density an 2-TiN(CSIRO)-0.5wt% degradation rate of rhodamine blue were reported f TiO 4.0 ± 0.1 – 2-TiN(CSIRO)-1wt% bilayers with the highest (21 and 28 28%). The 2 ratioTiN:TiO TiO 5.1 ± 0.3 – 2-TiN(CSIRO)-2wt% improved activity of the thin TiOfilm bilayers was esti 2-TiN to be a result of reduced electron-hole recombinat TiO 7.0 ± 0.2 3.7 ± 0.2 2-TiN(CSIRO)-10wt% of TiN alone to facilitate photocatalytic from MeOH H 2 evolution TiO -TiN(CSIRO)-20wt% 8.8 ± 0.2 6.0 ± 0.1 2 reformation under combined UV/Vis irradiation may TiO 9.4 ± 0.3 6.4 ± 0.1 2-TiN(CSIRO)-33wt% by electron interband transitions orbitals from the N-p as to T t 2g TiO -TiN(CSIRO)-50wt% 8.0 ± 0.5 5.3 ± 0.2 mentioned earlier. However, it is also possible t 2 a Using a UG11 glass filter combined with a 305 and oxy-nitrides may contribute to the observed ac nm cut-off filter. samples. The lower activity of TiN compared be due to TiO 2 may to several factors, such as the direct nature o transitions of TiN. Typically, irradiation of i semiconductors generates charge carriers with lo than those with direct band gaps due to the requirem to mediate the recombination of 29 an In electron-hole addition, recent DFT calculations have shown that TiN ha adsorption free energy of hydrogen relative to P 30HfN, transition metal nitrides (ScN, YN, DFT NbN and calculations comparing a range of Ti-ceramics has molecular hydrogen dissociation is the most endo 31 This suggests that TiN surface (TiN TiS TiO 2 >> 2 > TiC > TiP). the relatively low H activity of the indivi 2 evolution samples is also a consequence of the difficulty to hydrogen atoms on the TiN surface.gvalues Comparison of of the TiN(CSIRO) composite2 evolution samples with activity H did not reveal a correlation between the energy of t gap and2 H evolution activity (Fig. S35). Low l TiN(CSIRO) %% 10 wt%) revealed a negligible g, but at chang higher loadings %% 20 wt%) a slight decrease of g was observed. The trend of decreasing E2 evolution H g and increasing activity was not consistent as the TiO 2-TiN(CSIRO)-50wt% sample displayed lower activity than did the TiO 2-TiN(CSIRO)33wt% sample. This shows that a smaller of the TiO E g value 2-TiN composite does not explicitly correlate with t activity of the composite. For example, gold-loa 2 exhibited a decreasing E (down to 2.6 eV) with increasin g value loading (up to 10 wt%), however the best performin photocatalytic H from reformation of ethanol h 2 evolution loading of 32 1 wt%. Following irradiation under combined UV/Vis light the TiO 2-TiN composites was examined under separate visible light irradiation. Under UV light %% = 305 observed that the TiO composites exhib 2-TiN(CSIRO) enhanced activity of a similar factor to that Sample

H2 evolution rate (μmol/h), λ > 305 nm

H2 evolution rate (μmol/h), λ = 305–390 nma


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To explore potential contribution of the visi irradiating under combined UV/Vis light (Figs. 5, S36,the Table S3). response of TiN, cut-off filters to remove UV ra This shows that the enhanced activity composites of TiO 2-TiN applied (Hg lamp, % > 420, > 455 and > 610 nm). N occurs essentially for UV-driven photocatalysis. The arc enhanced TiO evolution in activity of2-TiN the TiO composites, relative to a TiO 2-TiN composites displayed any2 improvement 2, may be relative withto the TiO removal of UV light. Additi consequence of a combination of factors. activity As mentioned 2earlier, no activity was observed for either of the TiN s DFT calculations irradiation with visible light only, even with t photodeposition. The absence of activity of the visible light alone infers that charge carriers ge plasmon (e.g. hot electrons) are unable to be util 2 for H2 evolution. This is in contrast to the recent rep of the plasmon of TiN nanoparticles on a can TiO 2 photoanode generate hot electrons that2 as migrate determined into TiO by 9 However, a bias was applied to photocurrent measurements. photoanode to aide collection of plasmonically from TiN into Instead of using an applied bias, ex 2. TiO the TiO 2 band gap and photogeneration of electrons creates a surface photovoltage which reduces ban 38 Concomitant excitation of the TiN pla TiO 2 surface. TiO 2 photovoltage may help to promote hot electron migrate away from 2/TiN the TiO interface. To investigat hypothesis dual lamp photocatalytic experiment Thus, a photocatalytic reaction involving UV li = 250–390 nm) combined with visible light wavele lamp, % > 455 nm) was conducted using the best perf Figure 5. Average2 H evolution rates of TiO 2 and 2TiO 2-TiN(CSIRO)-33wt%. Switching between UV and TiN(CSIRO) nanoparticle composites (10, 20, 33 and 50 wt% TiN) UV/Vis light demonstrated a repeatable but modes under UV/Vis (orange) and UV only (purple) irradiation. in the H activity of between 2–5% relativ 2 evolution under only on UV TiN light have shown that the adsorption free energy of hydrogen is (Table 2, Fig. 6). high relative to Pt and other metal nitrides, and that molecular Reacting under TiO the same conditions and switching 2 30-31 on TiN than TiO hydrogen dissociation is more endothermic 2. UV and UV/Vis light resulted in no significant d 2 In addition, first principle calculations have shown that the energy evolution activity (Table 2, Fig. S37). These of the activation barriers on the anatase (101) andthe rutile (110) support proposition that hot electron generat surfaces for diffusion of hydrogen atoms across the surface into from TiN to2 TiO for or can be facilitated wh 2Hevolution the bulk are lower than for molecular33-34 hydrogen It desorption. composite is then conceivable that an excess of hydrogen atoms migrating Table 2. Average2 H evolution rates of TiO 2 and 2across the TiO may result in a reverse spill-over onto the 2 surface TiN(CSIRO)-33wt% switching between UV and TiN nanoparticles,2promoting evolutionHdue to the instability of irradiation. 35 Conventional hydrogen atoms on the TiN surface. hydrogen spillover occurs when H on a metal surface, such as Pt, Irradiation, 2 dissociates Sample H2 evolution rate and the hydrogen atoms migrate from the metal to the metal oxide λ (nm) (μmol/h) support, however, the reverse mechanism has also been extensively 35 TiO 250–390 1.4 ± 0.03 2 discussed. Another factor potentially contributing to the enhanced activity -TiN of composites TiO is that TiN possesses 2 250–390 + > 455 1.5 ± 0.04 interband transitions in the UV spectrum that can generate 250–390 1.5 ± 0.03 additional charge carriers. Non-plasmonic noble metal 250–390 + > 455 1.5 ± 0.02 nanoparticles of Rh, Pd, Ir and Pt supported on metal oxides have been shown to enhance the catalytic activity of certain 250–390 reactions 1.4 ± 0.03 when irradiated with UV and visible light. The enhanced activity TiO -TiN(CSIRO)250–390 7.4 ± 0.04 was reported to be attributed to the absorption 2of light by bound 33wt% 36It is transitions. 250–390 + > 455 7.8 ± 0.07 electrons, exciting interband unlikely that TiN acts as a sink for photogenerated electrons generated in TiO 2 due 250–390 7.6 ± 0.03 to the lower work function of TiN compared to anatase and rutile 250–390 + > 455 7.7 ± 0.06 (discussed below). 250–390 7.4 ± 0.04 The difference in activity between the series of composites with different TiN samples (COM vs CSIRO) is likely a consequence of the different TiN morphologies. The smaller size and higher surface area of the TiN(CSIRO) sample allows for greater contact with the2 TiO nanoparticles. In addition, the experimentally determined mean free path of an electron in stoichiometric TiN thin films has been measured as 45 nm, agreeing well with the 37of calculated value It 49 would nm. be conceivable that with increasing TiN particle size, fewer electrons generated from interband transitions would reach interface. the TiO Larger 2/TiN TiN(COM) particles may also excessively shade the TiO 2, reducing the number of photogenerated charge carriers.


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the interface (Scheme 1, top left and right). The determined work functions of rutile and anatase are 41 however the value of the TiN work funct respectively, occasionally quoted without consideration of treatment history, which can strongly affect t 42 An early investigation of the bulk Fermi le observed. TiN was reported as 3.74 eV, and from deposition substrate as 3.59 eV, measured by thermionic em 43 Two separate investigations of TiN as an vacuum. field emitter array material revealed first a value second, a value of 3.8 eV (referenced against Mo were extracted from Fowler-Nordheim analysis, h 6, 44 second larger value was attributed In toaddition, surface oxide first-principles studies reported the work functio 45 More eV and 3.03 eV for the relaxed and unrelaxed surfac recent determinations of the TiN work function voltage/capacitance and Kelvin probe data of Ti metal oxides report values ranging46-50 from These 4.2 to 5.15 Figure 6. H2 evolution rate2-TiN(CSIRO)-33wt% of TiO values switching between UV and UV/Vis2 irradiation. evolution All Hreported are effective work functions, as the work fun is influenced by metal induced gap states when rates are normalized to the same 2. amount of TiO interface with high-permittivity dielectric meta 2), 46, 48,The 51 and extrinsic defects from the deposition procedu absence 2of evolution H activity under only visible infers that hot electrons cannot migrate away from to downward band bending 2 from towards TiO the interface (Scheme 1, bottom left). Due to the accumulation 2 surface, an electric field at the interface woul directed from TiN 2to (Scheme TiO 1, top right). Initi excitation of thegap TiOin the presence of electron2 band surface adsorbates prior to UV irradiation, e.g. H in 2O, will result migration of electrons to the bulk and holes to th 2/electrolyte interface. Holes reaching the TiO interface and e 2/electrolyte electrons tunnelling across interface the TiO to the LUMO o 2/TiN TiN can reduce the degree of band bending and imp 38It TiN transfer of hot electrons2from . is proposed to TiO that the excitation of the gap TiO and generation of photovo 2 band reduces band bending2/TiN at the interface TiO (Scheme 1, bot right). This promotes the migration of hot electro from the 2TiO /TiN interface to the TiO interface t 2/electrolyte chemically useful. This interpretation is consi photoelectrochemical investigation of TiN nan 2 nanowires. There, instead of creating photovolta the TiO 2 band gap (this work), an external bias was ap direct electrons generated from excitation of the T 9 from the2TiO /TiN interface.

4. CONCLUSIONS

Scheme 1. Energy band diagrams of the TiO and TiN 2 (anatase) interface, before contact (top left), afterIn contact at equilibrium (top conclusion, a series of TiO composites 2-TiN nanoparticle right), after contact with visible light (bottom left), and after contact been prepared, characterised and tested as aque with UV and visible light f(bottom , TiN fermi right). level, E suspensions, E TiN f demonstrating their enhanced pho 2 , TiO , TiN % work function, , TiO % TiO2 2 fermi level, TiN TiO2 2 work evolution activity due to their synergistic effe function, , ETiO conduction TiO g TiO2 2 indirect band gap.2CBM, that smaller (10–18 nm) TiN nanoparticles enha 2 band minimum, VBM, TiO band maximum. The width of 2 valence evolution activity of TiO to a greater exten 2 nanoparticles the TiO for the anatase 2 CBM represents a range of reported values larger TiN (20–50 nm) nanoparticles, and this enh CBM based on flatband and ionization potential measurements (0.3 maximised at a relatively high loading (33 wt%) u V to −0.39 V vs SHE, −4.74 to −4.05 39-40 eV vs vacuum). While the enhanced activity of the composites o

for comparison, UV-driven photocatalysis, it was also shown t is irradiated by both UV and visible light. For gold the2 H evolution activity the TiO composite is irradia 2 whenof nanoparticles (~ 1 wt%) loaded onto by a TiO sol2 (P25) immobilization procedure demonstrated H with both UV and visible light. It is proposed th 2 evolution activity activity observed, under visible light only %% = 400–700 nm), attributed to thewhen combining visible wit consequence of hot electron generation from the T plasmonic response of gold, that was 5% of the activity compared promoting migration of hot electrons away from t 2/TiN to irradiation with UV light only %% = 320–500 nm). Theoretically, interface the TiO These results demo 2 photovoltage. contact between TiN and TiO result in electron transfer due to 2 should that nanoparticles of TMNs can be employed to from TiN to2, TiO creating a negative charge accumulation layer photocatalytic activity of wide band gap semic within the TiO and downward band bending 2 surface 2 to from TiO


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Naldoni, A.; Guler, U.; Wang, Z.; Marelli, M.; Mala TiO for solar fuel 2 under UV and combined UV/visible light9. Besteiro, V.; Govorov, A. O.; Kildishev, A. V.; generation. It is clear TMNs will have a future role toL.play in Broadbanddesign Hot Electron photocatalytic materials through improved composite on theCollection for Solar Water Sp Plasmonic Titanium Nitride. Adv. Opt. Mater. 2017, 5, 1601031 nanoscale and the preparation of more sophisticated TMNs.

10. Yick, S.; Murdock, A. T.; Martin, P. J.; Kennedy, D. T.; Bendavid, A. Tuning the Plasmonic Response of TiN ASSOCIATED CONTENT Synthesised by the Transferred Arc Plasma Technique. 2018, 10, 7566-7574. Supporting Information 11. Ohtani, B.; Prieto-Mahaney, O.; Li, D.; Abe, R. W (Evonik) P25? Crystalline Composition Analysis, Rec Isolated Particles The Supporting Information is available free of charge Pure on the ACS and Photocatalytic Activity Photobiol., A 2010, 216, 179-182. Publications website. 12. Li, J.; Gao, L.; Sun, J.; Zhang, Q.; Guo, J.; Yan Nanocrystalline Titanium Nitride Powders by Direct Contains information pertaining to materials, sample preparation, Titanium Oxide. J. Am. Ceram. Soc. 2001, 84, 3045-3047. photocatalytic H setup, and instrumentation and results 2 evolution 13. Giordano, C.; Erpen, C.; Yao, W.; Milke, B.; Anton for XRD, UV/Vis, DLS, surface area, XPS, TEM andNitride EDS (PDF). and Metal Carbide Nanoparticles by a Soft U Chem. Mater. 2009, 21, 5136-5144. 14. Kaskel, S.; Schlichte, K.; Chaplais, G.; Khanna Characterisation of Titanium Nitride Based Nanopart AUTHOR INFORMATION Chem. 2003, 13, 1496-1499. 15. Giordano, C.; Corbiere, T. A Step Forward in Metal N Corresponding Author Carbide Synthesis: From Pure Nanopowders to Nanoco *E-mail: [email protected] Colloid. Polym. Sci. 2013, 291, 1297-1311. Phone: +61-2-93512581 16. Reinholdt, A.; Pecenka, R.; Pinchuk, A.; Runte, S ORCID Weirich, T. E.; Kreibig, U. Structural, Composition Edwin B. Clatworthy: 0000-0002-7204-2213 Colorimetric Characterization of TiN-Nanoparticles 2004, 31, 69-76. Adrian T. Murdock: 0000-0002-0642-799 17. Ishii, S.; Sugavaneshwar, R. P.; Nagao, T. Tit Avi Bendavid: 0000-0002-2454-9714 Nanoparticles as Plasmonic Solar Heat Transducers. J. Anthony F. Masters: 0000-0001-7857-8759 2016, 120, 2343-2348. Thomas Maschmeyer: 0000-0001-8494-9907 18. Sato, K.; Li, J. G.; Kamiya, H.; Ishigaki, T. Ultra Tio2 Nanoparticles in Aqueous Suspension. J. Am. Ceram. Notes 91, 2481-2487. The authors declare no competing financial interests. 19. Suttiponparnit, K.; Jiang, J.; Sahu, M.; Suv Charinpanitkul, T.; Biswas, P. Role of Surface Area, Size, and Crystal Phase on Titanium Dioxide Nanopart ACKNOWLEDGMENT Properties. Nanoscale Res. Lett. 2010, 6, 27. The authors wish to acknowledge the facilities andZ.; theXiong, scientific 20. Guo, J.; Yang, M.; Xiong, S.; Chen, J. Sun, at L.;the Wang, J.; Wang, H. Dispersion of Nano-Ti and technical assistance of Microscopy Australia Australian Aqueous Media. of J. Alloys Compd. 2010, 493, 362-367. Centre for Microscopy & Microanalysis at the University 21. Moriya, Y.; Takata, T.; Domen, K. Recent Progre Sydney, specifically Dr Hongwei Liu for his assistance with Development HRTEM and EDS. The authors wish to acknowledge Dr Xiaobo Li of (Oxy) Nitride Photocatalysts for Wate Visible-Light Irradiation. Coord. Chem. Rev. 2013, 257, 1957for his valuable discussions. E. B. C. acknowledges the receipt of 22. Hojo, J.; Iwamoto, O.; Maruyama, Y.; Kato, A. De an Australian Postgraduate Award. This work was financially Thermal and Electrical Properties of Ti Nitride and V Nit supported by the Australian Research Council. J. Less-Common Met. 1977, 53, 265-276. 23. Wu, C.-Y.; Tu, K.-J.; Deng, J.-P.; Lo, Y.-S.; Wu Enhanced Surface Hydroxyl Groups of TiO with 2 Nanoparticles Superior Water-Dispersibility for Photocatalysis. M REFERENCES 566. 24. Patsalas, P.; Kalfagiannis, N.; Kassavetis, S. O Plasmonic Performance of Titanium Nitride. Materials 2015, 3154. 1. McKone, J. R.; Lewis, N. S.; Gray, H. B. Will25. Solar-Driven Patsalas,WaterP.; Logothetidis, S. Optical, Electro Splitting Devices See the Light of Day? Chem. Mater.Properties 2014, 26, 407of Nanocrystalline Titanium Nitride Thin 414. Phys. 2001, 90, 4725-4734. 2. Ni, M.; Leung, M. K.; Leung, D. Y.; Sumathy, K. A Review and Recent 26. Cortie, M.; Giddings, J.; Dowd, A. 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Saito, Y.; Kawata, S.; Nakane, H.; Adachi, H. Emission Characteristics Difference of Photocatalytic Activity Among Anat of Niobium Nitride Field Emitters. Appl. Surf. Sci. 1999, 146, 177-181. Brookite TiO PCCP 2014, 16, 20382-20386. 2. 7. Guler, U.; Naik, G. V.; Boltasseva, A.; Shalaev, V. M.; Kildishev, A. 30. Abghoui, Y.; Skúlason, E. Hydrogen Evolution React V. Performance Analysis of Nitride Alternative Plasmonic MaterialsNitrides. J. Phys. Chem. C 2017, 121, 24036 Transition-Metal for Localized Surface Plasmon Applications. Appl. Phys. B 2012, 31. He, Y.; 107, Laursen, S. Trends in the Surface and Catalyt 285-291. Transition-Metal Ceramics in the Deoxygenation of a 8. Koutsokeras, L.; Hastas, N.; Kassavetis, S.; Valassiades, O.; Compound. Charitidis, Pyrolysis Model ACS Catalysis 2017, 7, 3169-3180. C.; Logothetidis, S.; Patsalas, P. Electronic32. Properties of Binary and Jovic, V.; Chen, W.-T.; Sun-Waterhouse, D.; Black Ternary, Hard and Refractory Transition Metal Nitrides. 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Enhanced Photocatalytic Hydrogen Evolution with TiO2âTiN

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