High Pressure Resonant X-ray Emission Studies of WO3 and Hydrogenated WO3

We performed two high pressure resonant x-ray emission experiments (RXES) on tungsten oxide (WO3), and a mixture of WO3 and ammonia borane (NH3BH3), respectively. In the first experiment, WO3 (confined by a Be gasket) was pressurized and RXES spectra were collected from ambient pressure up to 54.8 GPa via incident x-ray excitation of the W LIII line at 10.207 keV and collection of emission spectra from the Lα1 line near 8.398 keV. The spectra display significant alterations, in particular, a diminishment of one of the spectral lines with pressure suggesting some electronic state changes with pressure in WO3 which were largely reversible upon decompression of the sample to ambient pressure. For the second experiment, a powder mixture of WO3 and NH3BH3 was pressurized and irradiated to release H2. An RXES pattern was recorded at 4 GPa which displayed little difference between the RXES spectrum recorded of pure WO3 in the first experiment. However, at 10 GPa, a dramatic change occurred in the RXES spectrum suggesting that the H2 reacted with and/or intercalated into the WO3 lattice. These studies suggest the possibility of harnessing useful hard x-ray photochemistry as a novel means to dope semiconductors.


Introduction
A recent study successfully intercalated N2 into the unit cells of WO3 via chemical synthesis methods which included the application of high temperatures up to 420 ˚C [1].The goal here entailed adding a dopant into the WO3 lattice with the aim of modifying electronic energy levels via intercalation stress which lowered the 2.6 eV band gap of WO3 to 1.9 eV to improve solar light collection efficiency [1].As high temperature can cause the loss of intercalated dopants via thermal migration out of the unit cell, we sought to examine the possibility of using an alternative method to dope semiconductors to stimulate intercalation and/or chemical reaction with WO3 that does not add heat : useful hard x-ray photochemistry [2][3][4][5].Here, we take advantage of the highly penetrating, highly ionizing, and highly focused properties of hard x-rays (> 7keV) to initiate novel decomposition [2] and synthetic chemistry [3][4] all in situ inside a diamond anvil cell with little or no addition of heat [3].We have successfully produced H2, O2 (e.g. via KClO4 → KCl + 2O2 [5]), N2, Cl2, and F2 via irradiation of various compounds that were initially loaded as powders or liquids into a diamond anvil cell (DAC) sample chamber and then irradiated with synchrotron hard x-rays.For the purposes of this brief paper, we chose to produce H2 in situ as hydrogen is small relative to other molecular diatomic species that we can typically produce and thus, in theory, would be more mobile and thus have a better chance to be intercalated into the WO3 lattice.We chose to use Resonant X-ray Emission Spectroscopy (RXES [6]) as the means to interrogate valence level electronic changes and hybridization in the W atom postulating that if hydrogen intercalated and/or reacted with WO3, this would significantly modify valence electronic states/oxidation number in tungsten.We sought to perform two experiments: The first would be to obtain RXES patterns from virgin WO3 as a function of pressure.The second experiment would examine an irradiated and pressurized mixture of WO3 and NH3BH3 to ascertain if any intercalation and/or chemical reaction occurred in WO3, which would then confirm useful hard x-ray photochemistry as a novel means to dope semiconductors.

Experimental Methods
For sample confinement, pre-machined 3 mm diameter beryllium gaskets with 100 μm thick dimples were pre-indented to ~50 μm thickness using a Paderborn-style diamond anvil cell (DAC) with diamonds each having 300 μm flats.A 120 μm diameter hole was drilled via electric discharge machining in the Be gasket.In the first experiment, tungsten oxide powder (Alfa Aesar®, >99% purity) was manually introduced into the empty gasket hole with no additional pressure medium.One thermally-relieved ruby sphere (~20 µm in diameter) was used for pressure measurement.The two diamond cell halves were brought together and closed to seal the sample.For the second experiment, tungsten oxide (WO3) powder (Alfa Aesar®, >99% purity) and ammonia borane (NH3BH3) (Alfa Aesar®, >99% purity) were individually pulverized and then gently mixed with a spatula in a 1:1 ratio by volume.The empty gasket hole of the second gasket was then filled with the mixed powder along with one thermally-relieved ruby sphere (~20 µm in diameter) for pressure measurement and pressurized.Two experiments were conducted at the High Pressure Collaborative Access Team's (HP-CAT's) 16 ID-D beamline at the Advanced Photon Source (APS).Synchrotron hard x-rays were filtered to excite the W LIII line at 10.207 keV.The filtered x-rays were then focused down to ~30 μm in the vertical direction and 60 μm in the horizontal direction at the sample using Kirkpatrick-Baez mirrors.Emission spectra were collected from the Lα1 line near 8.398 keV.The inelastically scattered x-rays were collected by a curved 1 m Rowland circle analyzer in the vertical plane [Si (4 4 4) reflection] and set at an angle of 90º relative to the incident x-ray beam.The analyzer array refocused the inelastic signal onto a Peltier-cooled Si detector (Amptek® XR 100CR) for data collection.The analyzer was fixed in energy and the incident beam energy scanned with a monochromator.Data collection took roughly 1-2 hours/spectrum, (60 seconds per energy channel).Multiple scans were collectedroughly 8 for each pressure and summed.The overall resolution of the spectrometer was ~1 eV.The DAC was mounted perpendicularly in the x-ray beam so that penetration of the sample occurred through the beryllium gasket instead of through the diamonds (side in side out geometry).This was done because the incident 10.207 keV x-rays [8] would be otherwise strongly attenuated if they passed through the diamond-diamond axis.Further experimental details on the experimental setup can be found in ref. 7.All measurements were conducted at room temperature.

Results
In resonant x-ray emission spectroscopy (RXES), the element under study (in this case W), is irradiated to create a core hole and then the x-ray emission caused when an electron fills the hole is collected.Both the incident and received x-rays are monochromatized [9].For our experiment, we chose the LIII (2p3/2) transition for the excitation energy which is at 10.207 keV [8].This experimental arrangement is often used for interrogating pressure-induced effects on the d-orbitals [8-9], in particular for our system, the 5d orbitals.As W has a [Xe]4s 2 5d 3 electronic configuration,

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there are empty 5d orbitals that can accept electrons from the 2s shell.The collection energy was set for the Lα1 line which is at 8.398 keV [8].Though we conducted full 2-dimensional RXES studies, we present the 1-dimensional partial fluorescence yield x-ray absorption curves in Figure 1 which displays the results of both experiments.In the first set of experimental results (left figure), the RXES spectra of WO3 are plotted versus incident x-ray energy as a function of pressure.The ambient pattern displays two peaks which are related to W5d→W2p3/2 (peak on the left) and W4f→W2p3/2 (smaller intensity peak roughly 35 eV to the right) transitions [9].Perusing the other patterns taken at higher pressure, it is immediately apparent that the higher energy W 4f orbital feature (which is reduced somewhat in intensity) is quenched up to the highest pressure studied (54.8 GPa) where it nearly disappears.Upon decompression, the second spectral line is recovered with some probable hysteresis.
For the second part of Fig. 1 (right plot), RXES spectra were acquired of a mixture of NH3BH3 and WO3 that was initially pressurized to 4 GPa.We took an initial pattern right after loading and then another RXES pattern after some 8 hours of x-ray irradiation at 10.207 keV at the 16 ID-D beamline.Both spectra appear very similar to each other indicating that the released hydrogen has not interacted with WO3.The sample was further pressurized to 10 GPa and a dramatic change in the RXES spectrum is observed which does not change significantly after another 8 hours of irradiation.The W4f→W2p3/2 peak has disappeared and another peak (O2s-W5d→W2p3/2) [9] appears mixed in with the W 5d→W 2p3/2 peak.Upon completion of our experiments, the sample color was noted to have changed color to a deep royal blue after irradiation.

Discussion
The quenching of the W4f→W2p3/2 peak in the virgin WO3 sample with pressure was likely due to reduction in unit cell volume and concomitant increase in energy associated with altering the spinorbit coupling [9].For the WO3 + NH3BH3 mixture, we propose that molecular hydrogen, which was released after 8 hour irradiation at 4 GPa, diffused throughout the sample due to increasing pressure gradients at 10 GPa (see Fig. 2 for visual evidence of diffusion of hydrogen).During this process, the hydrogen likely chemically reacted with the oxygen in the WO3 lattice which opened up electronic holes that enabled O2s-W5d→W2p3/2 transitions [9].This suggests that hydrogen has incorporated itself into the WO3 unit cell.

Conclusion
We have performed the first reported high pressure RXES study of WO3 up to 54.3 GPa.We have observed a quenching of the W4f→W2p3/2 line at higher pressure which we attribute to increased energies of holes in the 4f level.We have also performed a RXES study of a mixture of WO3 + NH3BH3.After irradiation of the sample to release molecular hydrogen, no significant changes were observed in the RXES spectrum at 4 GPa.However, after further pressure increase to 10 GPa, we likely observed chemical reaction with hydrogen and WO3 such that hydrogen incorporated itself into the WO3 unit cell.Our studies give support to further development of doping methods for semiconductors harnessing useful hard x-ray photochemistry.

Fig. 2 .
Fig. 2. (Left) Photo (looking through one diamond in a microscope) of a sample of NH3BH3 that was irradiated at low pressure (3 GPa) for 8 hours and then pressurized to 55 GPa.Molecular hydrogen has diffused toward the rim of the gasket as evidenced in the greyish ring on the rim of the hole.(Right) Raman spectrum of the irradiated sample taken at 55 GPa.A strong hydrogen vibron was observed after irradiation of this sample near 4270 cm -1 .