Enhanced Photocatalytic Hydrogen Evolution with TiO2–TiN

Article Cite This: J. Phys. Chem. C XXXX, XXX, XXX−XXX

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

School of Chemistry, The University of Sydney, Sydney, New South Wales 2006, Australia Nanostructured Thin Film Materials Manufacturing, Commonwealth Scientific and Industrial Research Organisation (CSIRO), 36 Bradfield Rd, West Lindfield, New South Wales 2070, Australia

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S Supporting Information *

ABSTRACT: Metal nitrides have potential in energy applications because of their physical and optical properties. Nanoparticle composites of titanium nitride (TiN) and titanium dioxide (TiO2) were investigated for their photocatalytic hydrogen (H2) evolution activity via methanol reformation. Physical mixing of the nanoparticulate TiO2 and TiN was employed to prevent the oxy-nitride formation and particle aggregation observed in thermal preparations. This convenient combination of TiO2 and TiN demonstrated a substantial synergistic effect with enhanced activity (9.4 μmol/h TiO2−TiN vs 1.8 μmol/h TiO2) under combined UV/vis light. Irradiation under only UV light resulted in a similar enhancement factor compared to using combined UV/vis light, demonstrating that the enhanced activity of the composites occurs essentially for UV-driven photocatalysis. No activity/enhancement was observed with only visible light irradiation; however, minor enhancement was observed when switching between UV and UV/vis irradiation, suggesting a contribution from the TiN plasmon. We propose that the plasmonic contribution is dependent on the band gap excitation of TiO2, which reduces the degree of band bending at the TiO2/TiN interface. This promotes the migration of hot electrons from TiN away from the TiO2/TiN interface to be used for H2 evolution.

1. INTRODUCTION The development of efficient, robust, and scalable photocatalysts for solar hydrogen (H2) evolution is an attractive target to help satisfy future energy demands.1 Transition metal oxides, particularly titania, are promising candidates. However, their large band gaps (e.g., >3.2 eV, for anatase) limit utilization to only the near-UV spectrum (≈λ < 390 nm). Strategies such as the addition of noble metal co-catalysts (Pt, Au, and Ag) and sacrificial reagents (e.g., MeOH and soluble biomass waste) have been employed to dramatically increase activity by, for example, reducing electron/hole recombination rates. In addition, visible light sensitization by localized surface plasmon resonance (LSPR) has been a strategy.2,3 However, the relatively low terrestrial crustal 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 reasons.4 TMNs possess superior corrosion resistance, very high melting points (TiN mp = 2950 °C), and typically exhibit smaller work functions when compared to noble metals.5,6 TMNs such as TiN and ZrN can absorb UV light because of interband transitions, as well as visible and near-IR light because of their plasmonic properties. Additionally, their electronic properties can be tailored by altering their metal/ © XXXX American Chemical Society

nitrogen stoichiometry (e.g., TiNx, 0.6 < x < 1.2) and/or by the introduction of additional metals.7,8 Recent work exploring the plasmonic enhancement of TiO2-based photoelectrocatalysts by TiN nanoparticles reported photocurrent measurements of TiO2 nanowire photo-anodes covered with TiN nanoparticles.9 The results demonstrated an enhanced photocurrent when applying a bias and illumination following the TiN plasmon profile. Here, we investigate the ability of TiN nanoparticles to enhance the photocatalytic H2 evolution activity of a photocatalyst via MeOH reformation using aqueous colloidal suspensions of TiO2−TiN nanoparticle composites. We employ convenient physical mixing of the nano-sized particles as a facile and cost-effective approach for composite preparation, avoiding the need for inert-atmosphere handling, and avoiding particle aggregation and oxy-nitride formation from thermal procedures. Under combined UV/vis irradiation, the composites afford up to a 5-fold improvement in the rate of H2 evolution via photocatalytic MeOH reformation compared to TiO2 (P25) or TiN nanoparticles alone. That the H2 Received: September 21, 2018 Revised: December 21, 2018 Published: January 17, 2019 A

DOI: 10.1021/acs.jpcc.8b09221 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C evolution activity (μmol/h) of the TiO2−TiN composites was found to be greater than the sum of the activity of the two separate materials, tested under identical conditions, indicates a synergistic interaction. Irradiation of the composites under only UV light revealed a similar enhancement factor to irradiation with UV/vis light, demonstrating that the enhanced activity of the composites occurs essentially for UV-driven photocatalysis. No activity/enhancement was observed when irradiating with only visible light, however, switching between UV and UV/vis light revealed a minor improvement in the activity, indicating a contribution from the TiN plasmon. We propose that in addition to visible light excitation of the TiN plasmon, UV excitation of the TiO2 band gap is necessary to reduce downward band bending at the TiO2/TiN interface and promote hot electron migration away from the TiO2/TiN interface for H2 evolution.

A 300 W (ozone free) xenon arc lamp was equipped with the 455 nm cutoff filter for double lamp experiments. For experiments involving platinum, an amount of chloroplatinic acid (equal to 1 wt % Pt) was added before deoxygenation. The reactor was then irradiated (λ > 305 nm) for 1 h for photodeposition to occur before measurements of H 2 evolution under visible light (λ > 455 nm). During photocatalysis, the suspension was maintained under Ar, continuously agitated with stirring, and the reactor was maintained at constant temperature of 20 °C. H2 gas was carried in Ar at a controlled flow rate of 30 mL/min to a GC (Shimadzu GC-2014, sample loop 1 mL) equipped with a discharge ionization detector (Vici pulsed discharge ionization detector D-4-I-SH14-R; detection limit in the low ppb range) for H2 quantification. Error bars represent the standard deviation of three or more repeat experiments. TiO2 alone was measured several times to determine baseline activity before testing each series of TiO2−TiN composites. The rates of H2 evolution of the TiO2−TiN composites were normalized to the same amount of TiO2. Light intensity measurements were conducted using a Solar Light Co. Dual-Input Data Logging Radiometer. 2.4. Characterization. Powder X-ray diffraction (XRD) patterns were collected using a PANalytical X’Pert Pro MPD X-ray diffractometer using Cu Kα (λ = 1.5419 Å) on zerobackground Si(100) plates. XRD patterns of TiN were refined using the FullProf software. Solid-state UV/vis diffuse reflectance spectroscopy was performed on a Varian Cary 5 UV/Vis spectrophotometer (lamp change at 350 and 800 nm) and band gap values were extracted using the Tauc and Kubelka−Munk equations. Solution UV/vis spectroscopy was performed on an Agilent Technologies Cary 60 UV−Vis spectrophotometer. Dynamic light scattering (DLS) measurements were performed on a Zetasizer Nano ZS, Malvern in 10 mM NaCl using the refractive index (RI) value of TiO2 (P25) of 2.49. pH measurements were performed with a PHM210 Standard pH Meter, Radiometer Analytical. Surface area measurements were performed using a Micromeritics Accelerated Surface Area and Porosimetry System 2020 instrument. Each sample was degassed at 200 °C and N2 adsorption/ desorption isotherms were collected at −196 °C. The Brunauer−Emmett−Teller (BET) surface area was calculated from the adsorption branch of the isotherm by using the Micromeritics software. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Specs SAGE 150 spectroscope with Mg Kα excitation at 1253.6 eV. Transmission electron microscopy (TEM), high resolution TEM (HRTEM), and energy-dispersive X-ray spectroscopy (EDS) analyses were performed on a JEOL 1400 microscope operating at 120 kV with images recorded on a Gatan Erlangshen camera, and a JEOL 2200 microscope operating at 200 kV with the images recorded digitally on a Gatan Ultrascan camera. Images were processed using Gatan Digital Micrograph and ImageJ software.

2. EXPERIMENTAL SECTION 2.1. Materials. TiN(COM), 99.2+%, 20 nm, cubic (US Nano Materials, Inc.) and Aeroxide TiO2 P25 (Evonik) were used as received. Milli-Q (type 1) water was prepared using Millipore Elix 10. TiN-CSIRO was prepared using a transferred arc plasma method based on a gas tungsten arc welding setup as described elsewhere.10 Titanium metal was purchased from Plasmaterials, Inc. and was used as received. High purity He and N2 were supplied by BOC. In order to generate the nanoparticles, the chamber housing the plasma system was evacuated to 8 × 10−3 mbar. A 1:1 mixture of He and N2 was then introduced from the cathodic trigger assembly. The chamber pressure was elevated to 173 mbar and stabilized. To initiate the plasma, the target material which sits on the anode was struck by the tungsten trigger cathode at 210 A to induce an arc plasma. The process was continued for a predetermined period. After that, the chamber was cooled and the nanoparticles physically extracted. 2.2. Preparation of the Nanoparticle Composites. Aqueous colloidal suspensions of P25 and TiN were prepared by ultrasonication. The initial pH of the water was set to 5 with 0.1 M HCl. After 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 total volume of the combined suspensions was 120 mL. The combined suspensions were allowed to stir for 24 h. After stirring, the suspension was centrifuged to afford a material ranging in color from pale to dark blue, depending on TiN loading, indicating mixing of the two nanoparticle samples. The material was washed three times with de-ionized water and ethanol (E100) and then dried overnight at 60 °C. 2.3. Photocatalytic H2 Evolution Setup. The TiO2−TiN composite (5 mg), P25 or TiN was ultrasonicated in a 3:1 mixture of H2O/MeOH (20 mL) at a pH of 7−8 for 15 min. The suspension was then deoxygenated for 30 min with stirring under Ar. The double-walled quartz reactor was irradiated with a polychromatic 350 W mercury arc lamp (Newport) (Diagram S1) at a distance of 6 cm from the light source. Different glass cutoff filters were employed with the mercury arc lamp including λ > 305, >420, >455, >495, and >610 nm (Newport) (Figure S1). A combination of the 305 nm cutoff filter and a UG11 glass filter (λ = 250−390 nm) was employed for experiments using only UV irradiation. The UG11 glass filter was employed for double lamp experiments.

3. RESULTS AND DISCUSSION Aeroxide TiO2 P25 was used as the titania photocatalyst because of its excellent activity and prevalence in industry and literature as a reference for titania-based photocatalytic activity.11 We employed two different nanoparticle samples of TiN that were both prepared by plasma-based techniques. The first was a commercial sample from US Research Nanomaterials Inc., prepared by plasma-arc vapor phase B

DOI: 10.1021/acs.jpcc.8b09221 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C synthesis, distinguished by the abbreviation, TiN(COM). The second sample was prepared by the Thin Film and Nanostructured Materials team of CSIRO by the transferred-arc plasma method, and is designated as TiN(CSIRO).10 Unlike the preparation of transition−metal nanoparticle co-catalysts, chemical preparation of TiN nanoparticles is more challenging. This normally involves nitridation (NH3, N2) of a pure metal, metal oxide, urea gel, or metal complex precursor, some of which require preparation under an inert atmosphere. To achieve single phase TiN (as determined by XRD), nitridation temperatures may vary between 800 and 2000 °C, depending on the nature of the precursor and specifications of the experimental setup (furnace configuration, gas flow-rate).12−15 Such chemical procedures can induce severe sintering and aggregation of the nanopowders because of the high temperatures. Alternatively, physical preparations of TiN nanoparticles such as those using plasma techniques can offer better control over morphology and stoichiometry, which are important for optical and catalytic applications.16,17 TiO2−TiN composites of different TiN loadings (0.5, 1, 2, 10, 20, 33, and 50 wt %) were prepared by physically mixing separate ultrasonicated aqueous colloidal suspensions of TiO2 and TiN. Ultrasonication treatment has been previously demonstrated to effectively disperse P25 aggregates to primary particle sizes for up to 15 vol % suspension.18 To aid the combination of the two phases, the pH of each colloidal suspension was adjusted to 5 to promote electrostatic attraction, as the points of zero charge of TiO2 and TiN occur at pH ≈6.2 and 3.6, respectively.19,20 After combination of the TiO2 and TiN suspensions, the resultant colloidal suspension was stirred for 24 h followed by centrifugation and three washings with deionised water and EtOH, respectively. The material was then dried in air at ≈60 °C for at least 12 h. Preparation of nanoparticle composites employing thermal procedures such as partial oxidation of TiN or partial nitridation of TiO2 to make mixed phase TiO2−TiN were deliberately avoided. This is because of the concomitant formation of oxy-nitride species of which the stoichiometry and homogeneity are difficult to control.21 The TiO2 (P25), TiN samples, and TiO2−TiN composites were analyzed by XRD to identify the crystallographic phases present. The crystalline composition of P25 consists of a variable mixture of anatase (73−85%), rutile (14−17%), and an amorphous phase (0−13%) (Figure S2).11 Both TiN samples exhibit diffraction patterns consistent with the δ-phase rocksalt (fcc) structure (Figure S3). Pattern refinement and analysis revealed TiN(COM) possesses 0.944 Ti occupancy (5.6% Ti defect) and a lattice parameter (a) of 4.240 Å (bulk value = 4.239 Å), whereas TiN(CSIRO) possesses 0.816 Ti occupancy (18.4% Ti defect) and a = 4.227 Å (Figures S4 and S5). The decrease in a of TiN(CSIRO) is consistent with Ti sublattice vacancies.22 XRD patterns of the TiO2−TiN composites were dominated by TiO2 reflections, but the presence of TiN could be established by observing the (200) reflection at ≈42.75−43° 2θ (Figures S6 and S7). HRTEM was performed to identify the morphologies of the TiN samples and TiO2−TiN composites. Although images of the TiN samples revealed different morphologies, both exhibited agglomeration (Figure 1). The TiN(COM) nanoparticles were observed to be primarily 20−50 nm in diameter, but also contained particles up to 200 nm in size (Figures S8− S10). Their shapes were predominantly irregular spheres and ellipsoids. In comparison, the TiN(CSIRO) nanoparticles were

Figure 1. HRTEM images and electron diffraction patterns of (a) TiN(COM) and (b) TiN(CSIRO).

primarily 10−18 nm cubes but also contained particles up to 32 nm in size (Figure S11). In addition to the 10−18 nm cubes, particles between 5 and 10 nm in size with an amorphous morphology could also be observed in the sample (Figure S12). HRTEM images of TiO2 (P25) revealed particles resembling rectangular prisms with well-defined edges, primarily 20−30 nm in size, but with range between 10 and 50 nm (Figure S13a−c). Composite samples with 33 wt % TiN were used for HRTEM in order to easily identify the nature of contact between the two phases. For the TiO2− TiN(COM)-33 wt % composite, individual TiN particles in contact with TiO2 could be observed (Figure S14). For the TiO2−TiN(CSIRO)-33 wt % composite, scattered TiN clusters of various sizes including the smaller amorphous particles and well-defined cubes could be observed in contact with individual and multiple TiO2 nanoparticles (Figure 2). EDS measurements of the TiO2−TiN(COM)-33 wt % and the TiO2−TiN(CSIRO)-33 wt % samples confirmed that the morphology of the nanoparticle composites is that of clusters of TiO2 and TiN phases (Figures S15 and S16). Because of the relatively low-temperature drying procedure used during the nanocomposite preparation, the presence of surface water molecules and hydroxyl groups existing at the interface of the TiO2 and TiN nanoparticle surfaces cannot be ruled out.23 Because of the ultrasonic preparation of the composites, DLS measurements were performed at the pH of the reaction mixture, that is, pH 7−8, to determine if the TiO2−TiN composites remained agglomerated at a pH value less favorable for electrostatic attraction. Thus, preparation of samples for photocatalytic reactions involved the dispersion of the composites by ultrasonication (50 Hz) for 15 min at ≈pH 7−8. Samples for DLS (12.5 mg/L) were treated by ultrasonication for 15 min (pH 7, 10 mM NaCl) before measurements were taken. The intensity distribution and hydrodynamic diameter for TiO2 each revealed only a single peak (consistent with previous reports),19 while both TiN samples exhibited two peaks (Figure S17a). All TiO2−TiN C

DOI: 10.1021/acs.jpcc.8b09221 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 2. HRTEM images of TiO2−TiN(CSISO)-33 wt % showing (a) clusters of small amorphous TiN nanoparticles in contact with one or more particles of TiO2 and (b−d) clusters of TiN cubes in contact with clusters of TiO2.

composites measured, exhibited intensity distributions with a single peak indicating that the TiO2 and TiN remained agglomerated (Figure S17b−d). Because of the 1:1 TiO2/TiN ratio, the TiO2−TiN(CSIRO)-50 wt % sample was measured with the RI value of both TiO2 and TiN separately (Figure S17e). A single peak was observed when using the RI value of TiO2, whereas two peaks were observed when applying the RI value of TiN, similar to the peaks observed for TiN alone. These results suggest that at high TiN loadings some agglomerates of TiN nanoparticles are no longer in contact with TiO2 after the ultrasonication treatment, as evidenced by two peaks being present (Figure S17e). UV/vis spectroscopy in solid-state diffuse-reflectance and solution spectroscopy was used to investigate the electronic properties of TiO2, TiN, and the TiO2−TiN composites. TiO2 gave a spectrum with an absorption beginning at λ ≈400 nm, corresponding to an indirect-band gap (Eg) value of 3.28 eV, but with no features in the visible spectrum (Figure 3a). Both TiN samples displayed strong absorptions across mid/near UV to visible wavelengths in both solid state and aqueous colloidal suspension spectra (Figure 3a,b). Electrons in the partially filled conduction band can undergo intraband transitions up to ≈3 eV (λ ≈ 413 nm), resulting in a strong contribution to the UV/vis absorption profile.24 The solid-state spectrum of TiN(COM) displays an asymmetric absorbance profile from λ ≈ 400−800 nm, which is ascribed to LSPR modes, with a peak maximum at λ = 495 nm. For TiN(COM), as an aqueous colloidal suspension, the absorption of the LSPR becomes redshifted to λ = 690 nm and broadens out to λ = 800 nm (Figure 3). The broadness of the LSPR absorption is the result of several contributions including the variable particle sizes/ shapes and interparticle coupling, giving rise to multiple resonance modes.16 The differences between the solid and aqueous colloidal suspension spectra are due to the different responses of the plasmon to different dielectric media (air and

Figure 3. (a) Solid-state UV/vis spectra of TiO2, TiN(CSIRO) and TiN(COM), (b) UV/vis spectra of aqueous colloidal suspensions of TiN(CSIRO) and TiN(COM), and (c) solid-state UV/vis spectra of TiO2, TiO2−TiN(COM)-33 wt %, and TiO2−TiN(CSIRO)-33 wt %.

water). The absorbance at λ < 449 nm is due to interband transitions of electrons moving from N p to Ti dt2g orbitals (Γ25 to Γ12) that correspond to a direct optical transition of 2.86 eV (Figure S18).25 In comparison, the solid-state spectrum of TiN(CSIRO) reveals a direct optical transition of 3.10 eV (Figure S18). Both the solid state and aqueous colloidal suspension spectra of TiN(CSIRO) display a less intense response of the LSPR, and in suspension a red-shift of the beginning of the interband absorption flank (λ = 466 nm) and of the LSPR absorption peak (λ = 738 nm) compared to TiN(COM) (Figure 3a,b). The experimental results compare well with reports on the effect of size and shape on the optical properties of TiN. Calculations based on spherical TiN nanoparticles reveal increasing near-field intensity efficiency from λ = 700 nm for particles 305 nm) the TiO2−TiN(CSIRO) composites displayed the highest rates of H2 evolution, up to 5-fold greater than observed for TiO2 alone (Figure 4). For the

plasmon peak that shifted from λ = 693 to 761 nm (solid state) with increasing N2 gas flow during preparation.16 More recent results obtained from TiN nanoparticles, prepared by nonthermal plasma techniques, have shown cubic TiN particles ( 367−420 nm, TiN loading 0.5−50 wt %) in addition to the UV absorption shoulder of TiO2 (Figures 3c, S20, and S22). This absorption feature in the visible spectrum of the TiO2−TiN(COM) composites was more pronounced than that for the TiO2−TiN(CSIRO) composites of the same TiN loading. This is consistent with the difference of the plasmon resonance between the two TiN samples. Tauc plots of both series of TiO2−TiN composites revealed a small decrease in the value of the indirect Eg level of TiO2 with increasing TiN content (Figures S21, S23, and S24, Table S2). Composites of TiN(COM) resulted in a greater decrease of the Eg level than did the composites of TiN(CSIRO). BET surface area analyses revealed that TiO2 and both TiN samples exhibited type 2 isotherms (Figures S25−S27). The BET surface area of TiO2 was found to be 56 m2/g, consistent with reports on P25.19 The BET specific surface areas of TiN(COM) and TiN(CSIRO) were found to be 50 and 151 m2/g, respectively. From TEM images, the particle size of TiN(COM) is not too dissimilar to that of P25, which would account for the similar specific surface area values. The higher specific surface area of the TiN(CSIRO) sample is likely a consequence of the smaller particle size (higher surface area to volume ratio), compared to those of TiO2 and TiN(COM). The photocatalytic H2 evolution activities of the TiO2−TiN composites were investigated by photocatalytic reformation of MeOH (3:1 H2O/MeOH) under UV, visible, and combined UV/vis irradiation. The average H2 evolution rates are reported in Table 1. Under combined UV/vis irradiation

Figure 4. (a) H2 evolution activity of TiO2 and TiO2−TiN(CSIRO) composites and (b) average H2 evolution rate of TiO2−TiN(CSIRO) composites under UV/vis irradiation, λ > 305 nm. All H2 evolution rates are normalized to the same amount of TiO2.

TiO2−TiN(COM) composites, an increasing TiN content resulted in an increasing activity, however, the trend was not wholly consistent. The 33 wt % TiN sample gave the greatest enhancement (2× activity of bare TiO2) (Figures S28 and S29). For the TiO2−TiN(CSIRO) composites, a clear trend between the TiN content and H2 evolution activity was observed (Figure 4a,b). The sample with 33 wt % TiN(CSIRO) also gave the best improvement (5-fold that of TiO2 alone) and demonstrated stable activity for up to 10 h before the activity began to decrease (Figure S30). The decrease in activity after 10 h of continuous irradiation was attributed to agglomeration and deposition of the photocatalyst on the wall of the quartz reactor (Figure S31). Sonication of the reaction suspension for 15 min recovered the activity to at least 90% of the initial value, however, the activity of the photocatalyst continued to decrease after subsequent sonication treatment (Figure S30). Comparison of the solid-state UV/vis spectra of the sample before and after extended activity testing revealed minor changes in the spectrum, indicating the optical properties of the catalyst were not significantly altered (Figure S32). A slight reduction in the absorbance corresponding to the plasmonic response (λ = 600−800 nm) was observed. Cycling a fresh sample of TiO2−TiN(CSIRO)-33 wt % between 1 h of irradiation and 20 min in the dark under argon purging multiple times demonstrates a negligible change in the H2 eolution activity over 6 h (Figure S33). The individual TiN (COM and CSIRO) samples were tested under the same conditions as the nanocomposites and displayed relatively low H2 evolution activities under combined UV/vis

Table 1. Average H2 Evolution Rates of TiO2, TiN, and TiO2−TiN Nanoparticle Composite Samples under UV/Vis and UV Irradiation sample TiO2 TiN(COM) TiN(CSIRO) TiO2−TiN(COM)-0.5 wt % TiO2−TiN(COM)-1 wt % TiO2−TiN(COM)-2 wt % TiO2−TiN(COM)-33 wt % TiO2−TiN(CSIRO)-0.5 wt % TiO2−TiN(CSIRO)-1 wt % TiO2−TiN(CSIRO)-2 wt % TiO2−TiN(CSIRO)-10 wt % TiO2−TiN(CSIRO)-20 wt % TiO2−TiN(CSIRO)-33 wt % TiO2−TiN(CSIRO)-50 wt %

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

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

± ± ± ± ± ± ± ± ± ± ± ± ± ±

1.4 ± 0.03

1.8 0.53 0.40 3.3 3.2 2.6 3.9 2.9 4.0 5.1 7.0 8.8 9.4 8.0

0.1 0.03 0.09 0.2 0.4 0.3 0.08 0.07 0.1 0.3 0.2 0.2 0.3 0.5

3.7 6.0 6.4 5.3

± ± ± ±

0.2 0.1 0.1 0.2

Using a UG11 glass filter combined with a 305 nm cutoff filter.

a

E

DOI: 10.1021/acs.jpcc.8b09221 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C irradiation (Figure S34). This demonstrates that the combination of TiO2 and TiN nanoparticles results in a synergistic effect, which has previously been observed for photocurrent generation and photocatalytic activity in thin-film bilayers of TiO2 and TiN under UV/vis irradiation, but not for H2 evolution. The highest photocurrent density and photocatalytic degradation rate of rhodamine blue were reported for thin-film bilayers with the highest TiN/TiO2 ratio (21 and 28%).28 The improved activity of the TiO2−TiN thin-film bilayers was estimated to be a result of the reduced electron− hole recombination. The ability of TiN alone to facilitate photocatalytic H2 evolution from MeOH reformation under combined UV/vis irradiation may be explained by electron interband transitions from the N p to Ti dt2g orbitals as mentioned earlier. However, it is also possible that surface oxides and oxy-nitrides may contribute to the observed activity of the TiN samples. The lower activity of TiN compared to TiO2 may be because of several factors, such as the direct nature of the interband transitions of TiN. Typically, irradiation of indirect-band gap semiconductors generates charge carriers with longer life times than those with direct band gaps due to the requirement of a phonon to mediate the recombination of an electron−hole pair.29 In addition, recent density functional theory (DFT) calculations have shown that TiN has the highest adsorption free energy of hydrogen relative to Pt and other TMNs (ScN, YN, HfN, NbN, and TaN) .30 DFT calculations comparing a range of Ti−ceramics has also shown that molecular hydrogen dissociation is the most endothermic on the TiN surface (TiN > TiO2 > TiS2 > TiC > TiP).31 This suggests that the relatively low H2 evolution activity of the individual TiN samples is also a consequence of the difficulty of reducing protons to hydrogen atoms on the TiN surface. Comparison of the Eg values of the TiN(CSIRO) composite samples with H2 evolution activity did not reveal a correlation between the energy of the indirect band gap and H2 evolution activity (Figure S35). Low loadings of TiN(CSIRO) (≤10 wt %) revealed a negligible change in Eg, but at higher loadings (≥20 wt %) a slight decrease of the Eg was observed. The trend of decreasing Eg and increasing H2 evolution activity was not consistent as the TiO2−TiN(CSIRO)-50 wt % sample displayed lower activity than did the TiO2−TiN(CSIRO)-33 wt % sample. This shows that a smaller Eg value of the TiO2−TiN composite does not explicitly correlate with the photocatalytic activity of the composite. For example, gold-loaded TiO2 exhibited a decreasing Eg value (down to 2.6 eV) with increasing gold loading (up to 10 wt %); however, the best performing catalyst for photocatalytic H2 evolution from reformation of ethanol had a gold loading of 1 wt %.32 Following irradiation under combined UV/vis light, the activity of the TiO2−TiN composites was examined under separate UV and visible light irradiation. Under UV light (λ = 305−390 nm), it was observed that the TiO2−TiN(CSIRO) composites exhibited enhanced activity of a similar factor to that recorded when irradiating under combined UV/vis light (Figures 5 and S36, Table S3). This shows that the enhanced activity of TiO2−TiN composites occurs essentially for UVdriven photocatalysis. The enhanced activity of the TiO2−TiN composites, relative to TiO2, may be a consequence of a combination of factors. As mentioned earlier, DFT calculations have shown that the adsorption free energy of hydrogen on TiN is high relative to Pt and other metal nitrides, and that molecular hydrogen dissociation is more endothermic on TiN

Figure 5. Average H2 evolution rates of TiO2 and TiO2− TiN(CSIRO) nanoparticle composites (10, 20, 33, and 50 wt % TiN) under UV/vis (orange) and UV only (purple) irradiation.

than TiO2.30,31 In addition, first principle calculations have shown that the energy of the activation barriers on the anatase (101) and rutile (110) surfaces for diffusion of hydrogen atoms across the surface or into the bulk are lower than for molecular hydrogen desorption.33,34 It is then conceivable that an excess of hydrogen atoms migrating across the TiO2 surface may result in a reverse spill-over onto the TiN nanoparticles, promoting H2 evolution due to the instability of hydrogen atoms on the TiN surface.35 Conventional hydrogen spillover occurs when H2 dissociates on a metal surface, such as Pt, and the hydrogen atoms migrate from the metal to the metal oxide support; however, the reverse mechanism has also been extensively discussed.35 Another factor potentially contributing to the enhanced activity of TiO2−TiN composites is that TiN possesses interband transitions in the UV spectrum that can generate additional charge carriers. Nonplasmonic noble metal nanoparticles of Rh, Pd, Ir, and Pt supported on metal oxides have been shown to enhance the catalytic activity of certain reactions when irradiated with UV and visible light. The enhanced activity was reported to be attributed to the absorption of light by bound electrons and exciting interband transitions.36 It is unlikely that TiN acts as a sink for photogenerated electrons generated in TiO2 because of the lower work function of TiN compared to anatase and rutile (discussed below). 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 the TiO2 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 calculated value of 49 nm.37 It would be conceivable that with the increasing TiN particle size, fewer electrons generated from interband transitions would reach the TiO2/TiN interface. Larger TiN(COM) particles may also excessively shade TiO2, reducing the number of photogenerated charge carriers. To explore the potential contribution of the visible light plasmonic response of TiN, cutoff filters to remove UV radiation were applied (Hg arc lamp, λ > 420, >455, and >610 nm). None of the TiO2−TiN composites displayed any improvement in H2 evolution activity relative to TiO2 with the removal of UV light. Additionally, no activity was observed for either of the TiN samples when irradiating with visible light only, even with the addition of Pt via photodeposition. The F

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The Journal of Physical Chemistry C absence of activity of the composites under visible light alone infers that charge carriers generated by the TiN plasmon (e.g., hot electrons) are unable to be utilized by TiO2 for H2 evolution. This is in contrast to the recent report that excitation of the plasmon of TiN nanoparticles on a TiO2 photoanode can generate hot electrons that migrate into TiO2 as determined by photocurrent measurements.9 However, a bias was applied to the photoanode to aid the collection of plasmonically generated electrons from TiN into TiO2. Instead of using an applied bias, excitation of the TiO2 band gap and photogeneration of electrons and holes creates a surface photovoltage, which reduces band bending at the TiO2 surface.38 Concomitant excitation of the TiN plasmon and TiO2 photovoltage may help to promote hot electrons from TiN to migrate away from the TiO2/TiN interface. To investigate this hypothesis, dual lamp photocatalytic experiments were conducted. Thus, a photocatalytic reaction involving UV light (Hg arc lamp, λ = 250−390 nm) combined with visible light wavelengths (Xe arc lamp, λ > 455 nm) was conducted using the best performing sample TiO2−TiN(CSIRO)-33 wt %. Switching between UV and combined UV/ vis light demonstrated a repeatable but modest improvement in the H2 evolution activity of between 2 and 5% relative to the activity under only UV light (Table 2, Figure 6).

evolution can be facilitated when the composite is irradiated by both UV and visible light. For comparison, gold nanoparticles (∼1 wt %) loaded onto TiO2 (P25) by a sol-immobilization procedure demonstrated H2 evolution activity under visible light only (λ = 400−700 nm), attributed to the plasmonic response of gold, that was 5% of the activity compared to irradiation with UV/vis light (λ = 320−500 nm). 41 Theoretically, contact between TiN and TiO2 should result in electron transfer from TiN to TiO2, creating a negative charge accumulation layer within the TiO2 surface and downward band bending from TiO2 to the interface (Scheme 1, top left and right). The experimentally determined work Scheme 1. Energy Band Diagrams of the TiO2 (Anatase) and TiN Interface, before Contact (Top Left), after Contact at Equilibrium (Top Right), after Contact with Visible Light (Bottom Left), and after Contact with UV and Visible Light (Bottom Right)a

Table 2. Average H2 Evolution Rates of TiO2 and TiO2− TiN(CSIRO)-33 wt % Switching between UV and UV/Vis Irradiation sample TiO2

TiO2−TiN(CSIRO)-33 wt %

irradiation, λ (nm) 250−390 250−390 250−390 250−390 250−390 250−390 250−390 250−390 250−390 250−390

+ >455 + >455

+ >455 + >455

H2 evolution rate (μmol/h) 1.4 1.5 1.5 1.5 1.4 7.4 7.8 7.6 7.7 7.4

± ± ± ± ± ± ± ± ± ±

0.03 0.04 0.03 0.02 0.03 0.04 0.07 0.03 0.06 0.04

Ef TiN, TiN fermi level, Ef TiO2, TiO2 fermi level, φTiN, TiN work function, φTiO2, TiO2 work function, Eg TiO2, TiO2 indirect band gap. CBM, TiO2 conduction band minimum, VBM, TiO2 valence band maximum. The width of the TiO2 CBM represents a range of reported values for the anatase CBM based on flatband and ionization potential measurements (0.3 to −0.39 V vs SHE, −4.74 to −4.05 eV vs vacuum).39,40

a

functions of rutile and anatase are 4.9 and 5.1 eV, respectively;42 however, the value of the TiN work function is occasionally quoted without consideration of the sample’s treatment history, which can strongly affect the Fermi levels observed.43 An early investigation of the bulk Fermi level value for TiN was reported as 3.74 eV, and from deposition on a Mo substrate as 3.59 eV, measured by thermionic emission under vacuum.44 Two separate investigations of TiN as an alternative field emitter array material revealed first a value of 2.92 eV, and second, a value of 3.8 eV (referenced against Mo). Both values

Figure 6. H2 evolution rate of TiO2−TiN(CSIRO)-33 wt % switching between UV and UV/vis irradiation. All H2 evolution rates are normalized to the same amount of TiO2.

Reacting TiO2 under the same conditions and switching between UV and UV/vis light resulted in no significant difference in H2 evolution activity (Table 2, Figure S37). These results strongly support the proposition that hot electron generation and migration from TiN to TiO2 for H2 G

DOI: 10.1021/acs.jpcc.8b09221 J. Phys. Chem. C XXXX, XXX, XXX−XXX

The Journal of Physical Chemistry C

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were extracted from Fowler−Nordheim analysis; however, the second larger value was attributed to surface oxides.6,45 In addition, first-principle studies reported the work function of TiN to be 3.25 and 3.03 eV for the relaxed and unrelaxed surfaces.46 More recent determinations of the TiN work function in air from voltage/capacitance and Kelvin probe data of TiN deposited on metal oxides report values ranging from 4.2 to 5.15 eV.47−51 These values reported are effective work functions, as the work function is influenced by metal-induced gap states when TiN creates an interface with high-permittivity dielectric metal oxides (e.g., HfO2), and extrinsic defects from the deposition procedure.47,49,52 The absence of H2 evolution activity under only visible irradiation infers that hot electrons cannot migrate away from the interface because of downward band bending from TiO2 toward the interface (Scheme 1, bottom left). Because of the accumulation layer in the TiO2 surface, an electric field at the interface would have field lines directed from TiN to TiO2 (Scheme 1, top right). Initially, excitation of the TiO2 band gap in the presence of electronacceptor surface adsorbates, e.g., H2O, will result in migration of electrons to the bulk and holes to the TiO2/electrolyte interface. Holes reaching the TiO2/electrolyte interface and excess electrons tunneling across the TiO2/TiN interface to the lowest unoccupied molecular orbital of TiN can reduce the degree of band bending and improve the transfer of hot electrons from TiN to TiO2.38 It is proposed that the excitation of the TiO2 band gap and generation of photovoltage reduces band bending at the TiO2/TiN interface (Scheme 1, bottom right). This promotes the migration of hot electrons from TiN away from the TiO2/TiN interface to the TiO2/electrolyte interface to be chemically useful. This interpretation is consistent with the recent photoelectrochemical investigation of TiN nanoparticles on TiO2 nanowires. There, instead of creating photovoltage from exciting the TiO2 band gap (this work), an external bias was applied to help direct electrons generated from excitation of the TiN plasmon away from the TiO2/TiN interface.9

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b09221. Contains information pertaining to materials, sample preparation, photocatalytic H2 evolution setup, and instrumentation and results for XRD, UV/vis, DLS, surface area, XPS, TEM, and EDS (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +61-293512581. ORCID

Edwin B. Clatworthy: 0000-0002-7204-2213 Thomas Maschmeyer: 0000-0001-8494-9907 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to acknowledge the facilities and the scientific and technical assistance of Microscopy Australia at the Australian Centre for Microscopy & Microanalysis at the University of Sydney, specifically Dr Hongwei Liu for his assistance with HRTEM and EDS. The authors wish to acknowledge Dr Xiaobo Li for his valuable discussions. E.B.C. acknowledges the receipt of an Australian Postgraduate Award. This work was financially supported by the Australian Research Council.

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REFERENCES

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4. CONCLUSIONS In conclusion, a series of TiO2−TiN nanoparticle composites has been prepared, characterized and tested as aqueous colloidal suspensions, demonstrating their enhanced photocatalytic H2 evolution activity because of their synergistic effects. It was observed that smaller (10−18 nm) TiN nanoparticles enhanced the H2 evolution activity of TiO2 nanoparticles to a greater extent than larger TiN (20−50 nm) nanoparticles, and this enhancement was maximized at a relatively high loading (33 wt %) under UV light. While the enhanced activity of the composites occurs essentially for UVdriven photocatalysis, it was also shown that TiN enhanced the H2 evolution activity of TiO2 when the composite is irradiated with both UV and visible light. It is proposed that the enhanced activity observed, when combining visible with UV light, is a consequence of hot electron generation from the TiN plasmon, and promoting migration of hot electrons away from the TiO2/TiN interface due to the TiO2 photovoltage. These results demonstrate that nanoparticles of TMNs can be employed to improve the photocatalytic activity of wide band gap semiconductors such as TiO2 under UV and combined UV/visible light for solar fuel generation. It is clear TMNs will have a future role to play in photocatalytic materials through improved composite design on the nanoscale and the preparation of more sophisticated TMNs. H

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