and Its Impact on Surface Band Bending and the Charge Carrier

Department of Chemical Physics, University of Science and Technology of China,. 11 ... INTRODUCTION. 35. Enhancing photoexcited electron-hole pair sep...
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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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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

293 294

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.

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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

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

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

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

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

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

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

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cation photoreduction.

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

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

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show that charge transfer during a photoelectrochemical reaction at a semiconductor

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

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

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CONCLUSION

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

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

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

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photoelectrochemical reaction achieved under high vacuum conditions, alters the surface

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

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

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

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

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sample.

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

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

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

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holes at TiO2 surfaces.

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

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

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consequences of cation photoreduction at a semiconductor interface.

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ASSOCIATED CONTENT

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Supporting Information

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

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

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AUTHOR INFORMATION

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Corresponding Authors

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

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ORCID

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Shiliang Ma: 0000-0003-1478-2771

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Ian Harrison: 0000-0002-3103-6247

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Notes

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The authors declare no competing financial interest

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ACKNOWLEDGEMENTS

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

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

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

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

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