A Photoluminescence Study Under High Vacuum Conditions

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 2

Photoreduction of Hydrogen Cations on TiO and Its Impact on Surface Band Bending and the Charge Carrier Recombination Rate: A Photoluminescence Study Under High Vacuum Conditions Shiliang Ma, Zhen Zhang, and Ian Harrison J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b12624 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 3, 2018

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The Journal of Physical Chemistry

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Photoreduction of Hydrogen Cations on TiO2 and its

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Impact on Surface Band Bending and the Charge Carrier

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Recombination Rate: A Photoluminescence Study Under

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High Vacuum Conditions

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Shiliang Ma1, Zhen Zhang2, Ian Harrison1, *

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1.

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Department of Chemistry, University of Virginia, McCormick Road, Charlottesville, Virginia 22904-4319, USA

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Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui, 230026, P. R. China

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Corresponding Author

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* To whom correspondence should be addressed. Email: [email protected]

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Tel.: +1 (434) 924-3639

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1 ACS Paragon Plus Environment

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ABSTRACT

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The interaction between hydrogen species and TiO2 surfaces is important because of its

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relevance to hydrogen production in the photocatalytic splitting of water on TiO2. In this

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study, the effect of the photocatalytic reduction of hydrogen cations, a half

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photoelectrochemical reaction run under high vacuum conditions, on the surface band

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bending of TiO2 was investigated by photoluminescence (PL) spectroscopy. Exposure of

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TiO2 to molecular hydrogen had no effect on the PL emission from TiO2 but the PL

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intensity increased under UV irradiation when TiO2 was exposed to hydrogen cations

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produced by ionization of molecular hydrogen by a cold cathode pressure gauge. The PL

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intensity increase, signaling a decrease in the upward band bending and increased

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electron-hole radiative recombination rate, is caused by charge transfer during

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photoreduction of hydrogen cations. Residual photoexcited holes left in TiO2 due to the

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transfer of photoexcited electrons to hydrogen cations tend to accumulate at the TiO2

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surface, balancing the originally trapped electrons at the TiO2 surface and thereby

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lowering the original upward band bending. These unusual observations point out that

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charge transfer during a photoelectrochemical reaction at a semiconductor interface alters

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the surface band bending and photoexcited electron-hole recombination rate in ways that

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are likely to impact the efficiency of photocatalytic devices.


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INTRODUCTION Enhancing photoexcited electron-hole pair separation and suppressing their

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recombination in TiO2 have attracted vast research interest due to their central roles in

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improving the efficiency of photocatalytic hydrogen production, pollutant decomposition,

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and photovoltaic cells based on TiO2.1-3 Though it has been rarely studied, tuning the

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surface band bending of TiO2 to more efficiently promote electron-hole pair separation

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and actively sweep desired charge carriers to the surfaces is a promising method to

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improve photocatalytic activity. Upward band bending is usually observed at TiO2

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surfaces because negative charges accumulate at its surfaces upon the equilibration of the

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bulk and surface Fermi energies.4-5 The upward band bending towards the surface directs

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photoexcited electrons and holes formed near the surface to migrate in opposite directions,

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and thus improves charge separation.6-8 Sheppard et al.9 showed by surface photovoltage

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measurements that surface band bending introduced by doping niobium at different

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concentrations influences charge separation rates in TiO2 during illumination. Zhang et

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al.7 reported that a 100-fold difference in the hole-mediated photodesorption rate of O2

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from TiO2(110) surface can be achieved by modulating the band bending of TiO2 through

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donor or acceptor molecule adsorption. In addition, the effects of doping various cations

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into TiO2 on photocatalytic hydrogen production rates, attributed to the band bending

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changes by Karakitsou et al.,10 indicates that band bending change is a physical process

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broadly applicable rather than a molecule-site specific chemical process which is only

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valid for certain molecules or sites. Thus, studying band bending may provide

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fundamental insights to better understand and improve a broad range of photocatalytic

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processes and devices involving TiO2.

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Photoluminescence (PL) spectroscopy which observes the radiative

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recombination of photoexcited charge carries (electron-hole pair) serves as a contactless

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and nondestructive method to characterize the band bending change of TiO2 based on a

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dead-layer model in which there is little PL emission from the band bending region

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(depletion layer) underneath the surface where the local potential drives the photoexcited

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electrons and holes in opposite directions.4-6, 11-13 Hence, the measured PL emission

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originates mainly from the TiO2 bulk. Because the 320 nm UV irradiation only penetrates 3 ACS Paragon Plus Environment


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~ 25 nm into the TiO2 sample,14 which is comparable to the length of the depletion layer,

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PL intensity change due to the band bending change in TiO2 can be easily observed under

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high vacuum conditions.5-6 An increase in upward band bending that increases the depth

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of the depletion layer and reduces the dimensions of the flat-band bulk region where

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radiative electron-hole pair recombination can take place, causes PL emission intensity to

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decrease. Therefore, a decrease in PL intensity indicates an increase in the upward band

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bending of TiO2 and vice versa.5

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The interaction between hydrogen and TiO2 has been extensively studied due to

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its relevance to the final step in hydrogen production through photocatalytic water

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splitting. Although it was reported that H2 only interacts with TiO2 weakly through van

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der Waals interactions at room temperature,15-17 heating TiO2 in H2 at elevated

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temperature introduces a new visible light response to TiO2 and improves its

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photoactivity. These high temperature H2/TiO2 changes were attributed to hydrogen atom

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incorporation and subsequent OH and TiH formation in the TiO2 lattice.18-20 Studies have

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also shown that at room temperature, atomic hydrogen can diffuse into the TiO2 lattice

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and form OH and TiH species, during which the Ti3+ concentration is increased due to the

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donation of electrons from hydrogen atoms, making TiO2 a more n-type doped

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semiconductor.15, 21-23 A key reaction, of unsurpassed interest, between hydrogen and

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TiO2 is the reduction of protons by photoexcited electrons to form hydrogen molecules.24

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Nevertheless, little attention has been paid to the band bending of TiO2 that modulates the

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electron-hole pair separation efficiency and which may change during photochemical

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reaction because charge transfer is involved.5

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In this study, we use PL spectroscopy to report that the upward band bending of

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TiO2 is decreased due to charge transfer incurred during the photoreduction of hydrogen

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cations at a TiO2 nanopowder sample held at 90 K in a high vacuum chamber. High

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vacuum is employed to eliminate possible band bending changes caused by adsorption of

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unknown species that might otherwise occur in air or aqueous solutions. The low sample

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temperature rules out the possibility that any atomic hydrogen present might be able to

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thermally diffuse into the lattice and thereby change the band bending. Similar PL

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behavior observed during photoreduction of gas phase helium cations suggests that the 4 ACS Paragon Plus Environment

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band bending change is a general physical outcome that will apply to other photocatalytic

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processes at semiconductor interfaces where charge transfer is involved.



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EXPERIMENTAL Experiments were conducted with the equipment illustrated in Fig. 1, which has been described elsewhere in detail.5-6 Briefly, the stainless-steel vacuum chamber of ~1.5 L

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volume was evacuated with a turbopump followed by a mechanical pump to achieve high

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vacuum with a base pressure of 5×10-9 Torr. The chamber pressure was measured by a

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cold cathode gauge (MKS series 423 I-MAG) which also serves as a gas ionization

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source as discussed later. A CaF2 window allowed both UV and visible light transmission

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into the chamber for PL measurements. To prepare the sample, TiO2 nanoparticles were

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pressed onto a tungsten grid (W-grid) with square openings of 0.22 mm to produce a

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circular disk with a diameter of 7 mm and a thickness of ~ 0.095 mm. The tungsten grid

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was put close to the CaF2 window by clamping it to an oxygen-free high-conductivity

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(OFHC) copper sample holder which was mounted to a feedthrough. The temperature of

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the sample was measured by a K-type thermocouple spot-welded onto the W-grid at the

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periphery of the sample. The sample temperature could be precisely controlled within 0.1

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K in the range of 82 – 1000 K by adjusting the combination of electrical heating across

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the W-grid and liquid-nitrogen cooling conducted through the feedthrough. Research

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grade (≥99.99%) O2, H2, and He gas bulbs were connected to the gas line.

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A gas dosing control module with a trap volume of 51.2 cm3, whose pressure was

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continuously monitored by a Baratron pressure gauge, was used to control the number of

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molecules dosed into the chamber (Fig. 1). The number of molecules dosed can be

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calculated using the ideal gas law based on the gas pressure reading and trap volume. The

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trapped gas pressure can be controlled with a precision of 0.001 Torr by adjusting the

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pressure control valve shown in Fig. 1. A quadrupole mass spectrometer (SRS RGA 200)

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was installed between the chamber and the turbo pump in order to analyze the purity of

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gases and to detect molecules desorbed from the sample surface.

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PL of TiO2 was measured by a PerkinElmer FL 55S fluorescence spectrometer

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equipped with a pulsed Xe light source and a photomultiplier capable of detecting light

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from 200 – 900 nm. The excitation of TiO2 sample was by 320-nm UV irradiation with a

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continuous intensity of ~ 1.5 × 1014 cm-2 s-1, which was selected by gratings inside the


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spectrometer and an additional 320-nm band-pass filter. A 390-nm cutoff filter was used

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to reduce the reflected UV from interfering the collected PL signal.

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The TiO2 used in this study is P25 (Evonik Industries, formerly Degussa), a

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nanopowder that is widely used in research as a reference standard. The TiO2 samples

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were heated in ~ 0.50 Torr O2 at 600 K for 30 min and then pumped out for 10 min

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before each set of experiments to remove adsorbates from the TiO2 surfaces and to make

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their properties consistent. To study the effects of gases, a selected amount of gas

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molecules was dosed into the chamber and isolated from the turbopump by closing the

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gate valve. To study the effect of atomic hydrogen, a tungsten filament connected to the

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chamber through an elbow was heated to 2300 K, which was measured by an optical

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pyrometer (Pyrometer Instrument Co.), to dissociate molecular hydrogen into hydrogen

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atoms.25

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Figure 1. Configuration of the high vacuum system employed in this study.



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RESULTS AND DISCUSSION

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Figure 2. Effects of hydrogen dose on the PL of TiO2. (A) PL spectra of TiO2 collected at

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different states. (B) PL peak intensity change caused by H2 dose (red) and chamber

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pressure measured by cold cathode gauge (purple).

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PL of P25. The PL spectra of P25 TiO2 depicted in Fig. 2A are collected at the three

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different states marked in Fig. 2B. As discussed in other reports,5 the 390 nm and 780 nm

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peaks, which are invariant between different scans, are due to reflection from the sample

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and CaF2 window, and therefore can be neglected. The PL emission of TiO2 ranges from

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450 nm to 650 nm with a peak at ~530 nm and only intensity changes were observed

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within this study. Although the P25 TiO2 contains both anatase (~75%, 85 nm dia.) and

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rutile (~25%, 25 nm dia.) phases,26 our previous work has shown that the PL emission of

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P25 under our conditions comes exclusively from the anatase phase.5 Therefore, the

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results presented here should be valid for the surface of anatase TiO2.


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Effect of H2 dosing on PL of TiO2. Fig 2B shows the effect of dosing H2 on the PL

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peak intensity of TiO2. Before dosing H2, the chamber pressure increased from 10-8 to 10-

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7

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shows no impact on the PL trend. However, it is clearly shown that the dose of 7.1 × 1016

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H2 molecules causes PL intensity to increase rapidly. The fast pressure drop after the H2

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dose is mainly due to the adsorption of H2 onto the inner chamber walls experiencing

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liquid N2 cooling, while the gradual drop of pressure is due to the equilibration of gas

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consumption due to ionization by the cold cathode gauge, leakage through the gate valve,

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as well gas adsorption/desorption on the chamber walls. A comparison between the

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chamber pressure and the PL increase rate suggests that the PL increase rate is correlated

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with the chamber H2 pressure.

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Torr due to the isolation of the chamber from the vacuum pumps. This pressure increase

TiO2 is an n-type semiconductor and naturally develops upward band bending

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underneath its surfaces4-5, which can be treated as due to trapped negative charges on its

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surfaces for simplicity. The upward band bending of TiO2 can be reduced when electron

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donor molecules are adsorbed on its surfaces because the donor molecules which donate

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electrons into the TiO2 lattice possess positive charges on TiO2 surfaces and therefore

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balance the originally trapped negative charges which causes the upward band bending.

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Similarly, the adsorption of electron acceptor molecules increases the upward band

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bending of TiO2.4, 6-7 It might be argued that the increase of PL upon H2 dose should be

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attributed to H2 acting as electron donor at the TiO2 surfaces. However, various studies

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have shown that molecular hydrogen interacts only weakly with both anatase and rutile

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TiO2.16-17, 27-28 Lin et al.16 reported only van der Waals interactions between H2 molecules

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and anatase TiO2 such that the extent of charge transfer between H2 and TiO2 is

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negligible. Consequently, the observed increase of PL intensity caused by H2 dosing on

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TiO2 may be more complicated.



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Figure 3. The role of the CCG in PL intensity increase caused by hydrogen dosing.

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Role of Cold Cathode Gauge (CCG) as ionizer. Fig. 3 shows evidence that the

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increase of PL emission is not solely due to the introduction of hydrogen molecules.

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Instead, the CCG, which measures the chamber pressure, also plays an important role. As

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depicted, switching off the CCG at 1100 s shows no impact on PL of TiO2 in vacuum.

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However, when the CCG is turned off, the effect that H2 dosing increases PL emission

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from TiO2 shown in Fig. 2 is absent, as revealed by the dose of 1.7 × 1016 H2 molecules

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at ~ 2800 s in Fig. 3. The effect begins immediately after the CCG is switched on. These

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results indicate that the PL increase due to H2 dosing is caused by species formed during

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the interaction between the CCG and H2 molecules, instead of molecular hydrogen,

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which is consistent with reports that molecular H2 interacts only weakly with TiO2.16-17,

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27-28

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TiO2 or hydrogen doped TiO2 for which a processing temperature above 600 K is usually

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required.18-19, 29 Fig. 3 also shows that the PL intensity does not flatten out immediately

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after pump out but continues to increase with gradually decreasing slope. This behavior is

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mainly because the TiO2 nanoparticle sample is porous and therefore gaseous species can

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be trapped by the pores.30 The overall PL increase from ~ 5 to ~ 50 (arb. unit) in Fig. 3

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indicates that a 10-fold difference in the photoexcited electron-hole radiative

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recombination rate is caused by interaction between TiO2 and the active hydrogen species

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produced by the CCG.

It is worth noting that our finding at 90 K does not conflict with reports of black


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The CCG operates at a voltage of ~ 5 kV, wherein electrons extracted from the

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cathode are accelerated towards the anode. When gas molecules are present, the

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accelerated electrons can strike and ionize the gas molecules. The ions formed are

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collected by the cathode and the ion current provides a measure of the pressure. Based on

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the working mechanism of the CCG, it is reasonable to assume that the active species

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causing the PL increase are hydrogen cations formed during the ionization of H2

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molecules.27 The fact that the CCG used to measure the chamber pressure also serves as

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an ionization source of H2 molecules points to the attention that should be paid to the

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possibility that a measurement process may influence an experiment.

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One possible but unlikely interference to the above postulation is that oxygen, which

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may adsorb on the walls of the CCG during sample pretreatment in oxygen, can desorb

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by electron-stimulation from the CCG and cause changes to the PL of TiO2. To rule out

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this possibility, the data shown in Fig. 3 was pretreated by 0.500 Torr of H2 for 20

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minutes after cleaning the sample in oxygen at 600 K to exchange the oxygen adsorbed

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on the inner walls of the chamber. Due to the adsorption of hydrogen to the inner walls of

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the chamber, the starting chamber pressure in Fig. 3 (> 10-8 Torr) is higher than that of

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Fig. 2 (< 10-8 Torr). This also causes a slower pressure drop rate and a significantly

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higher chamber pressure before pump out (~ 10-5 Torr) than Fig. 2 (< 10-6 Torr). This

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pretreatment by H2 significantly decreases the possibility that the PL increase is caused

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by electron-stimulated oxygen desorption from the CCG. In addition, the result shown in

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Fig. S2 that oxygen desorption decreases PL intensity of TiO2 completely rules out the

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possible effect by oxygen since oxygen works in the reverse way.



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Figure 4. UV dependence of PL increase caused by interaction between CCG and H2

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molecules.

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UV induced photoreaction. Fig. 4 illustrates the influence of UV irradiation on the

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PL increase caused by active species produced during the interaction between the CCG

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and H2 molecules, which are most likely to be hydrogen cations as discussed above. The

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green curve in Fig. 4 shows that there is no PL increase without UV irradiation, even

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though the CCG is turned on and active species contributing to the PL increase can be

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produced. Instead, the purple curve collected under UV irradiation shows normal PL

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increase as observed before. The indispensability of UV irradiation for PL to increase

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elucidated in Fig. 4 indicates that the PL increase is caused by reaction between

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photoexcited charges and the active hydrogen species generated by the CCG, which is

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most likely to be hydrogen cations.

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Figure 5. Effect of atomic hydrogen on PL development with the CCG off Ruling out possible effects of atomic hydrogen. Although the active species causing

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PL to increase is most likely to be H2+ based on the working mechanism of the CCG,

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which agrees well with the pressure dependent measurements shown in Fig S1, it is also

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possible that atomic hydrogen (H·) may be produced by collision or ionization of H2

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molecules by high energy electrons, or by collision of H2+ with H2 molecules.31-32 The

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latter two processes allow for the production of several kinds of Hn+ (n=1, 2 or 3) cations.

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It can be deduced that atomic hydrogen adsorption onto TiO2 should increase PL

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intensity based on reports from the literature. Although it was reported that atomic

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hydrogen can diffuse into TiO2 lattices, 21-22 due to its small diffusion coefficient at low

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temperature,15, 21-22 it is reasonable to assume that hydrogen atoms adsorbed onto TiO2 at

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90 K stay mainly on the surface. It was found that atomic hydrogen donates delocalized

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electrons into the TiO2 lattice and thus atomic hydrogen acts as an electron donor.23

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Consequently, atomic hydrogen adsorbed on TiO2 surfaces should be positively charged

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after donating electronic charge into the TiO2 lattice, which will decrease the upward

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band bending of TiO2 and increase PL. Hence, it is necessary to differentiate whether the

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PL increase due to molecular hydrogen gas exposure with CCG on is caused by hydrogen

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cations or atomic hydrogen.

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Fig. 5 illustrates the effect of atomic hydrogen, which was introduced to the TiO2

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sample by dissociating H2 through a hot tungsten filament at 2300 K,25 on the PL of TiO2.

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The finding that atomic hydrogen exposure increases PL is consistent with the

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expectations expressed above. But, unlike the effect observed in Fig. 4, atomic hydrogen

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not only causes PL to increase under UV irradiation, but also causes PL to increase

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without UV irradiation. The difference in the PL behavior between dosing atomic

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hydrogen and the active species produced by molecular hydrogen with the CCG on

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indicates that the active species produced by the CCG is unlikely to be atomic hydrogen.

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Hence, we propose that the active species that causes the PL to increase during molecular

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hydrogen dosing are hydrogen cations (Hn+, with n most likely 2 33) which are produced

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by the ionization of hydrogen molecules by the CCG.



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Figure 6. Schematic of processes occurring on TiO2 surface. The energy level diagrams

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are for electrons where the direction of increasing energy is reversed for holes.

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Band bending and charge recombination rate change model. A proposed

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mechanism explaining the PL increase caused by hydrogen cations under UV irradiation

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is illustrated in Fig. 6. Hydrogen cations are produced during the measurement of

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chamber pressure through the CCG which ionizes hydrogen molecules. Under UV

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irradiation photoexcited electron-hole pairs are generated in TiO2. Because the

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photoexcited electrons are strong reducing agents, hydrogen cations that arrive at TiO2

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surfaces are reduced by the photoexcited electrons (process 1). The photoexcited holes

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left in the valence band tends to migrate toward the surface of TiO2 (process 2) under the

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electric field in the band bending region (depletion layer). The accumulation of holes at

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the surface of TiO2 balances the originally trapped negative charges under TiO2 surfaces

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and thus causes the band bending to be less upward. The reduction of upward band

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bending decreases the volume of the band bending (depletion) region and accordingly the

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radiative recombination rate of photoexcited charge carriers increases as monitored by

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the PL intensity. The overall photoreaction results in positive charge transfer from

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hydrogen cations to TiO2 nanoparticles, making TiO2 particles positively charged.

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Because the 320-nm UV penetration depth into TiO2 is only ~ 25 nm,14 the observed PL

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only comes from the first layer of TiO2 particles, which are insulated from the tungsten

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grid by thousands of layers of TiO2 particles. Hence, it is reasonable that the observed top

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layer TiO2 can gain some excess positive charges without quasi-instantaneous discharge.

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In addition, the fact that hydrogen cations cause no change to the PL of TiO2 in dark

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without the assistance of photoexcited electrons suggests that the positive charge held by 14 ACS Paragon Plus Environment

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the hydrogen cations cannot be directly injected into the valence band of TiO2 (process 3,

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which cannot occur spontaneously) because the photoexcited holes are strong oxidants

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and occupy an energy level higher than that of hydrogen cations, which can be inferred

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from other studies in which H2 acts as hole scavenger.15

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Figure 7. PL intensity of TiO2 decay after turning off UV irradiation (other figures are

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with continuous UV irradiation except this one).

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Quantitative analysis Although reaction between photoexcited electrons and

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hydrogen cations will increase PL because of the associated band bending change as

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discussed above, it is worth considering if consumption of photoexcited electrons by

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reaction could substantially quench the PL intensity. As detailed below, the latter effect is

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negligible. The number of photons incident on each TiO2 particle at the surface during

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the period from 3500 s to 11000 s in Fig. 3, wherein the PL changes from ~ 4 to ~ 35

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intensity units, is about 6.3 × 107 photons with the assumption that the particle size of the

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PL-active, anatase particles in P25 is ~ 85 nm.26 Essentially all of the incident UV

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photons are absorbed to produce e/h pair excitations given the 25 nm UV absorption

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length of TiO2. According to the dead-layer PL model we developed earlier,5 it only

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requires at most ~ 150 leftover holes per TiO2 particle to reduce the upward band bending

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sufficiently to recover 88% of the maximum PL (appropriate for a flat band condition)

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given a TiO2 volume defect density of 1018 cm-3. Therefore, in this study, the ratio of the

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number of photoexcited electrons reacting with hydrogen cations to leave leftover holes 15 ACS Paragon Plus Environment


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to the number of incident photons absorbed is at most ~ 0.00024%, which is negligible.

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Non-radiative and radiative (ca. 0.25% quantum efficiency)34 e/h pair recombination

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dominate the photoexcitation relaxation. Therefore, the side-effect of PL quenching

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caused by reaction between photoexcited electron and hydrogen cations via consumption

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of photoexcited electrons available for recombination, is negligible in this study.

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If the number of photoexcited electron-hole pairs is much larger than the number

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of inhomogeneously positioned charges establishing the band bending potential gradient,

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a possible concern is whether the band bending could be electrostatically scrambled by

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charge redistribution of the photoexcited electrons and holes over the TiO2 particle.

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However, the short life time of photoexcited electron-hole pairs observed as ~ 17 µs in

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Fig. 7 indicates that there are only ~ 0.15 photoexcited electron-hole pairs per TiO2

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particle on average at any time during continuous UV irradiation as in Fig. 3. Sufficient

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upward band bending to quench the PL to 12% of its maximal (flat band) value would be

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established with 150 immobilized electrons at the surface and the counter positive

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charges distributed across defects in the depletion zone at a defect density of 1018 cm-3.

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Consequently, modulation of the TiO2 band bending by the steady-state photoexcited

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charge density in our experiments should be negligible.

327 328

Figure 8. Effect of helium on PL intensity of TiO2. Influence of CCG on PL development.

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Because the decrease in upward band bending, signaled by a PL increase, is

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caused by positive charge transfer from the hydrogen cations into TiO2 through 16 ACS Paragon Plus Environment

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photoinduced reduction, it is plausible that photoinduced reduction of other cations

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should also induce a similar band bending change. Fig. 8 shows similar PL intensity

333

changes caused by exposure to helium gas with the CCG off or on. As depicted in Fig. 8,

334

neutral helium atoms have no impact on PL of TiO2 with the CCG off. But the PL

335

increases when the CCG is turned on and ions are formed. The finding that helium cation

336

exposure also causes PL of TiO2 to increase lends further support to the idea that the

337

active species in the hydrogen dosing PL experiments are hydrogen cations, rather than

338

hydrogen atoms, and that a band bending decrease is a general physical consequence of

339

cation photoreduction.

340

These unusual PL observations, enabled by low temperature, high vacuum, and a

341

TiO2 nanopowder sample not subject to prompt charge neutralization in its environment,

342

show that charge transfer during a photoelectrochemical reaction at a semiconductor

343

interface alters the photoexcited electron-hole pair recombination rate and surface band

344

bending in ways that are likely to impact the efficiency of photocatalytic devices.

345

CONCLUSION

346

By employing PL spectroscopy to monitor the photoexcited electron-hole radiative

347

recombination and the dead-layer model to analyze the band bending of TiO2, it was

348

found that the reduction of hydrogen cations by photoexcited electrons, which is a half

349

photoelectrochemical reaction achieved under high vacuum conditions, alters the surface

350

band bending and charge recombination rate of TiO2. Main findings include:

351

(1) Molecular hydrogen exposure had no effect on the band bending of TiO2 at 90 K.

352

(2) Molecular hydrogen could be ionized into hydrogen cations by the cold cathode

353

gauge measuring the chamber pressure and these cations could reach the TiO2

354

sample.

355

(3) The reduction of hydrogen cations by photoexcited electrons decreases the

356

upward band bending of TiO2, signaled by increased PL intensity (electron-hole

357

radiative recombination rate), because of the accumulation of positively charged

358

holes at TiO2 surfaces.



359

(4) The band bending and charge carrier recombination rate changes observed for

360

TiO2 due to photoreduction of both hydrogen and helium cations are physical

361

consequences of cation photoreduction at a semiconductor interface.

362

ASSOCIATED CONTENT

363

Supporting Information

364

Dependence of PL increase rate on H2 pressure (Fig. S1) and mean free path of H2+ in H2

365

(Table S1). The different effect of O2 (Fig. S2)

366

AUTHOR INFORMATION

367

Corresponding Authors

368

Ian Harrison: Email: [email protected]. Telephone: +1(434)924-3639.

369

ORCID

370

Shiliang Ma: 0000-0003-1478-2771

371

Ian Harrison: 0000-0002-3103-6247

372

Notes

373

The authors declare no competing financial interest

374

ACKNOWLEDGEMENTS

375

We gratefully acknowledge support of this work by the Department of Energy under

376

grant number DE-SC0002365 and the Army Research Office under grant number

377

W911NF-13-1-0101, as well AES fellowships for Shiliang Ma from AES Corp. and the

378

University of Virginia. Z. Zhang acknowledges support by NSFC (No. 21773047).

379


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A Photoluminescence Study Under High Vacuum Conditions

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